The geologic history of Wyoming’s Hanna Basin is still being written. Surprisingly, here appeared an opportunity to share insights from previously accomplished work with that conducted anew by other scholars. The area of study was in the southeastern quadrant of Wyoming, which exhibits the state’s most complex history with respect to the Laramide orogeny. Especially important for present purposes were the tectonic conditions of the late Paleocene and earliest Eocene, recorded within the Hanna Formation. Of central focus is the 2020 publication by Dechesne and her six co-authors. Geographically, the landscape they covered was a thin, synclinal slice of the northeastern margin of the Hanna Basin. Key goals for the present publication have been to illustrate positive linkages and to highlight discrepancies between Dechesne et al. (2020) and relevant prior geological work. A concern that permeates all facets of this approach is the ability to verify viability of brand-new geologic descriptions, data, and resulting conclusions. Essential graphical elements were introduced first into this present publication. Once that package of background information was available, more focused analyses were rigorously pursued on diverse issues within the Dechesne et al. (2020) publication. Dechesne’s team presented a significantly modified but adequately defended approximation of the Paleocene–Eocene boundary. Data from fossil plants (macro- and palynofloras), continental mollusks, and bulk organic-carbon isotopes all agree within one measured section (of five sections studied) with an approximated Paleocene–Eocene boundary along with a ‘carbon isotope excursion’ (CIE). Strength of available evidence seems questionable, however, in that the inordinately high variability in bulk organic carbon (characteristic of a CIE) has been demonstrated only in the Hanna Draw Section. Although fluvial, paludal, and lacustrine facies are considered in several contexts, in no sense does the publication’s organizational form provide a ‘detailed stratigraphic framework.’ One zircon-based U–Pb depositional date (54.42 ± 0.27 Ma) came from this study that matched early Wasatchian time. Participants in the Dechesne et al. (2020) project are to be commended in that their resulting paper ranged broadly across the geologic setting, stratigraphy, paleocurrents, paleobotany, continental mollusks, zircon geochronology, associated lithofacies, and paleogeography. Despite that breadth, there exists a plethora of unexpected and wholly avoidable inconsistencies, strong contradictions within what should be homogeneous datasets, and seemingly inexplicable omissions of obviously necessary and sometimes clearly existing but unutilized data, one must question the reliability of much of the information presented in their paper.
A love-affair with the general vicinity of Wyoming’s Hanna Basin began when my geographer colleague and friend (the late Lawrence M. Ostresh, Jr.) and I studied the complex sequence of Cretaceous shoreline migrations and drainagepattern evolution across what is now the state of Wyoming from latest Cretaceous through Paleogene time (Lillegraven and Ostresh, 1988 and 1990). Completion of those two reviews helped us to understand the centrality of the Hanna Basin to Wyoming’s Laramide topographic history. Especially important were tectonic details of the late Paleocene and earliest Eocene. That relatively brief geologic interval also takes a central position within the present publication.
Subsequent published results of my focus on the Hanna Basin are listed in Appendix 1 of the present paper. Now, more than three decades after starting, I continue to offer sincere enthusiasm for any geologically oriented new research that is relevant to general vicinity of the Hanna Basin. Its history is still being written, and I openly wish to share benefits from already accomplished work with that conducted anew by other scholars. For example, the present publication focuses on the 2020 paper by Marieke Dechesne and her six co-authors. Geographically, the landscape they covered is a thin, synclinal slice of the northeastern margin of the basin (Fig. 1). Our respective research efforts hopefully will, at least in the long run, be mutually beneficial. The key goals of this report have been to illustrate positive linkages and clarify shortcomings between the new study and relevant geological work that already has been proposed.
Figure 2 is a ‘thumbnail’ image of the entire margin of the eastern half of the Hanna Basin (including Simpson Ridge and the Carbon Basin). Take note of information in the caption to Figure 2 that explains how to retrieve the individual four maps composing the graphic when printed at full scales (i.e., one map is at 1:12,000 and three are at 1:24,000). The black-and white dashed rectangle (centered on T. 23 N., R. 81 W.) marks the boundaries of Figure 3 (i.e., of the present paper, as well as of a modified version of fig. 3 as published by Dechesne et al., 2020). Throughout the present publication, references to its own figures, tables, and appendices are capitalized (e.g., Fig. 1). Elements referred to in previous publications, in contrast, are recognizable by the use of lower-case titles (e.g., table 2). Context should make clear the intended citation.
TOWARD AIDING INITIAL ORIENTATIONS
The Basic Process
A concern that permeates all facets of this research is the ability to verify pre-existing and brand-new geologic descriptions, data, and resulting conclusions. All of that information is portrayed in some variety of spatial construct, whether it be in the form of geologic maps, cross sections, stratigraphic columns, or even tabular-data presentations. All discussion of that range of information in the present text is dependent upon support provided by eleven figures, five tables, and two appendices. Care has been applied to specifications of physical location, scale, and verifiability of features presented on each graphic. Graphical elements are introduced first, sequentially identified via figure and table numbers. Relevant introductory information is presented both within the main text and in figure captions. Once that pertinent background information is introduced, more focused analyses and discussions are pursued on diverse issues raised within the Dechesne et al. (2020) publication.
Figure 1 — Geographic Reference Maps
The southeastern quadrant of Wyoming (Fig. 1B) most assuredly holds the state’s most complex record of the Laramide orogeny. Notice in map B that the red rectangle covers the northeastern edge of the Hanna Basin. Today, the basin terminates to the south at the northern edge of Elk Mountain, which came into existence by deep-centered faulting late in the Laramide orogeny (see Lillegraven, 2015, figs. 6 and 7, and cross-sections F′–G′, H′–I′, and F–G). Prior to that surprisingly late uplift of Elk Mountain, the southern boundary of the Hanna Basin was at the northernmost wall of the Medicine Bow Mountains, at the south end of what is now known as the Pass Creek Basin. The northern boundary of the Hanna Basin is set directly upon southern anticlinal folds from the Freezeout and Shirley Mountains. Those anticlinal folds represented older uplift histories than Elk Mountain, dating back at least to the early Paleocene (see Lillegraven et al., 2004, figs. 11, 13–14, 16B–18, and appendix 1).
Figure 2 — Geologic Maps of Eastern Margins of Hanna Basin
Welcomed land access is a practical concern related to the conduct of mapping as shown in Figure 2. The combined map shown here forms a highly irregular, reversed capital-letter ‘C’ (or a ‘⊃’). With minor exceptions, I had nothing but cooperation in gaining permissions for access to all the areas reproduced here in color. That was not the case, however, within the white area internal to the colored edges of the “⊃.’ The general problem was that mining corporations themselves had little legal authority to provide citizen permissions for research access. And the actual (mostly out-of-state) landowners generally were more than reluctant to provide access.
The basic map within Figure 3 of the present paper is reproduced here, nearly unedited from its original appearance as figure 3 of the open-access publication by Dechesne et al. (2020). I have supplemented the map, however, by: (1) addition of numbered section-, township-, and range-boundaries; (2) addition of leg numbers for the field-measured, stratigraphic segments where used; and (3) connection of thrust-fault lines and symbols for the Dragonfly and Owl Ridge structures in the northeastern corner of the map. A more thorough statement of the nature of this map exists in the caption for Figure 3. The purpose of Figure 4 is to clearly show how the present figure relates to the adjacent and partly overlapping, prior geologic mapping to the north and east.
The map that is figure 3 as presented by Dechesne et al. (2020) is the most important source for locality information linked to the authors’ new stratigraphic framework. Five tables are introduced here, with each presenting elements within relevant localities. The tables will be used extensively in support of discussions later in the present publication. Table 1 will help in communicating the extents to which adjacent ends of stratigraphic legs forming the Hanna Draw Section are aligned or laterally offset. Table 2 attempts to promote better understanding of stratigraphic occurrences of relevant paleobotanic macrofossils. Similarly, Table 3 attempts to promote better understanding of stratigraphic occurrences of relevant palynologic microfossils. Table 4 compares stratigraphic occurrences of traditionally numbered, major coal layers as they are alternatively recorded in: (1) supplement S1 (of Dechesne et al., 2020); versus (2) the historical time of numbering (i.e., by Dobbin et al., 1929, pl. 27). Finally, Table 5 compares the paleo-hydrological data as alternatively recorded by Dechesne et al. (2020) among: (1) their figure 3 (map view); (2) their figure 4 (page-sized measured sections); and (3) supplement S1 (almost four-fold enlarged measured sections, with many substantive differences from their figure 4).
These macrofossil taxa are mostly found stratigraphically below the Big Channel sandstone complexes —They principally represent early Paleogene time throughout the Rocky Mountain basins:
Records of the following two species are restricted to Paleocene time:
Figure 4 — Reduced Opacity Facilitates Visualization of Structural and Stratigraphic Links to Prior Mapping
Upon reduction of opacity of the basic map as shown in Figure 3, one can gain nearly perfect linkage in Figure 4 to edges and overlaps within prior geologic mapping to the north and east. In Figure 4, the following individual capital letters identify the measured sections: A, ‘Doug’; B, ‘Big Channel Lateral’; C, ‘Hanna Draw Section’ (with 15 legs); D, ‘The Breaks’ (with 3 legs); and E, ‘Beer Mug Vista no. 2.’ Note the marked differences in interpreted positions of the synclinal axial trace that separates measured sections A-B-C from sections D-E. Also, note that virtually all of the landscape shown on this composite map resides on components of the highly asymmetric ‘defining syncline of northeastern Hanna Basin.’
Figure 5 — Another Attempt at Identifying Accurate Placement of the Five Sets of Measured Sections
Figure 5 would be the most likely platform by which to recognize inaccurate placements by me of any of the five sets of measured sections presented in figure 3 by Dechesne et al. (2020). Advantages of plotting the original datasets on U.S. Geological Survey standard 1:24,000 topographic maps would have been many. All would have enhanced accuracy of user measurements. In any case, the topographic control provided by the TE Ranch, Difficulty, Como West, and Elmo quadrangle maps, combined with axial traces of the defining syncline of northeastern Hanna Basin and Hanna syncline, will prove useful. Most obvious is the striking clarity of topographic asymmetry in basin-margin expression of the defining syncline of northeastern Hanna Basin.
Figure 6 — Stratigraphic Positions of Numbered Coals Differ Among Dechesne et al. (2020, fig. 4 and supplement S1) and Dobbin et al. (1929, pl. 27)
The coal beds of interest to this research were numbered by Dobbin et al. (1929) and displayed in their plate 27. It is genuinely worthwhile to carefully read their following statement:
“Purpose of the survey.—The geologic investigations that form the basis for this report were undertaken primarily for the purpose of classifying and valuing the public land with regard to coal. In order to do this it was necessary to locate accurately with respect to land lines the outcrops of all coal beds that were considered to be of workable thickness. The thickness and the character of the coal beds were determined in all accessible mines and prospects, and in the absence of openings sufficient prospecting was done to obtain the desired information. The quality of the coal was determined by analyses of fresh samples taken from working mines, or from prospects that had been recently opened. The geologic structure of the field was worked out with considerable accuracy in order to determine the depth of the coal beds below the surface and their availability for mining.”
My personal conversations with locally employed mining engineers, as well as working miners themselves, universally praised the reliability of documentation of coal resources across the Hanna Mining District as provided within U.S. Geological Survey Bulletin 804. The fieldwork itself along with the writing benefitted from more than a dozen, well-qualified professional assistants, full access to the Union Pacific Coal Company’s maps, drill-hole records, and mine-development data, and from well logs produced by the Producers & Refiners Corporation.
All subsequent professional mining activities across the Hanna Mining District thereafter used the basic data generated by the 1929 coal-layer description and its numerical-ordering system. That includes the research program developed by Dechesne et al. (2020). Figure 6 of the present publication provides an excerpted tracing by me of coal layers 77 through 89 from plate 27 by Dobbin et al. (1929). That traced map also includes: (1) all bedding-plane attitudinal measurements provided by the original investigators; (2) partial axial traces of the Hanna syncline and the defining syncline of northeastern Hanna Basin; and (3) the much younger addition of coal layer 90 and five measured sections from Dechesne et al. (2020, fig. 3).
Adding to confusion, however, most coal levels in both figures (i.e., fig. 4 and supplement S1) from Dechesne et al. (2020) do not harmoniously match with coal-based stratigraphy shown in Dobbin et al. (1929, pl. 27; or in the present publication’s Fig. 6). Once again, one must ask which positions are closest to the truth? Inconsistencies in the newer publication makes previous reports a more reliable default. Right or wrong, I now favor the data-rich, detailed field studies conducted in 1913 (and published in 1929). Table 4 of the present publication summarizes the key comparisons between supplement S1 (or this publication’s Fig. 7C) and Dobbin et al. (1929, pl. 27; or this publication’s Fig. 6). An originally single dataset (from 1929) of distributed coal levels now exists as three (i.e., 1929, fig. 4 and supplement S1 from Dechesne et al., 2020) quite different compilations of data. The differences in reported thicknesses of between-coalbed strata are not trivial. Because we must now retain degrees of uncertainty about which dataset is most accurate, confusion can hardly be avoided.
Figure 7 — Simplification and Redrafting of Supplement S1
The origin of Figure 7 in the present publication requires background information in explanation of the Figure’s origin and reason for existence. The necessary information directly involves figure 4 and supplement S1 of Dechesne et al. (2020).
Dechesne et al. (2020) provided their favored stratigraphic framework for the upper Hanna Formation (including coals 77–90) in their figure 4. That figure is contained within their publication’s main text. The figure 4 from Dechesne et al. (2020) can be considered to be a structurally similar version (although providing considerably different content) of their more extensively data-bearing ‘supplement S1.’ That supplement, along with two other forms of supplementary materials, all require separate downloading from the Internet.
Supplement S1 (in contrast to Dechesne et al.’s, 2020, fig. 4) includes fully readable, metric-scaled stratigraphic columns, GPS-controlled locality data for sampled bulk organic carbon, paleobotanical highlights, and specific data related to the local effort in zircon dating. In short, the ‘supplementary’ information provides unique data that must be taken into account. Inexplicably, however, certain components of important, homologous data are displayed at differing stratigraphic levels in supplement S1 than exists in the simpler (but more difficult to read due to crowding) figure 4. Interested users, as a result, have no way to know which alternatives of stratigraphic-level placements are more correct. In any case, the ‘supplements’ must be considered as integral and indispensable components of the complete publication.
Significant differences in content also occur between the explanatory legends for the above-mentioned pair of similarly structured figures (i.e., Dechesne et al., 2020, fig. 4 and supplement S1). The present publication’s Figure 7 was intentionally crafted by me to favor contents of Dechesne et al.’s (2020) supplement S1 over contents of their figure 4. Also, the newly conceived Figure 7 has reduced contents (compared with supplement S1) that restrict all stratigraphically plotted data to: (1) clearly readable, metrically scaled stratigraphic columns; (2) significant paleobotanical (macrofloral and palynological) information; (3) analytical results from variously datable zircon samples; (4) comparisons of stratigraphic positions of specific coal beds 77–90 as proposed between Dechesne et al. (2020, supplement S1) and Dobbin et al. (1929; also see present paper’s Fig. 6, Tables 1–3, and relevant passages in the main text for details); and (5) boundaries of stratigraphic sequences said to provide new evidence for delimiting the ‘Paleocene–Eocene Thermal Maximum Zone’ (PETM) within the present study area. Please note that many forms of data that might logically have been intended, but failed to be present on supplement S1, also will not be presented in Figure 7.
Dechesne et al. (2020) considered coal-based stratigraphy in the northeastern Hanna Basin most closely within the Hanna Draw Section. The Dechesne et al. (2020) section as recognized in The Breaks is presented here as Figure 8. That placement uses figure 4 from Lillegraven et al. (2004) as the base-map. That base-map also was the source of identification of coal seams 77–87, along with all stratigraphic attitudinal measurements provided in Figure 8.
In the present context of Figure 7, Dechesne et al. (2020) provided no new strike and dip information from relevant strata involved in any of the 15 measured legs that constitute the Hanna Draw Section. Thus, there exists little opportunity for users of their publication to verify the stratigraphic-thickness measurements. Accuracy of thicknesses of the involved stratigraphic legs cannot be verified in light of the inevitably varying stratigraphic dips within the defining syncline of northeastern Hanna Basin.
Dobbin et al. (1929) provided only a few attitudinal measurements of strata in their plate 27 that are relevant to present interests. As an example of important concern over such matters, notice in leg 9 of the present Figure 6 that the path of the measured section through most of that leg’s mapped extent nearly parallels the strike of the nearby (1929) attitudinal symbol of ca. N. 9° W., 12° NE. Also, as suggested by the sharp bend of coal 88, it appears that most of leg 13 of the Hanna Formation is actually older than leg 12. In short, the data available to this project’s readers are inadequate to show that (with exception of Figure 8) the reported thickness of any given stratigraphic leg in Figure 7 is accurate. Also, just how conformable are the individual relationships shown in Figure 7 between tops and bottoms of ostensibly superposed stratigraphic legs? As shown in this paper’s Table 1, most lateral jumps between successive legs on the maps of Figures 3–6 reach a half-kilometer to more than a full kilometer. Almost no information is provided about orientations of relevant strata or about lithologic correlation from one leg to the next.
Because this segment of discussion involves both figure 4 and supplement S1 from Dechesne et al. (2020), this seems a reasonable place to briefly consider three miscellaneous items relevant to both graphics. (Item 1) The term ‘Super Swamp’ uniquely exists in figure 4. The words are placed halfway between the Big Channel Lateral and Hanna Draw measured sections. No explanation or any form of description of the structure called Super Swamp appears in the article by Dechesne et al. (Item 2) Both in figure 4 and supplement S1 of Dechesne et al. (2020) appears ‘Hanna Syncline’ halfway between the Hanna Draw and The Breaks measured sections. As considered in the present publication’s caption to Figure 4, the northernmost extension of the Hanna syncline’s axial trace is too short to reach a point of visibility anywhere near the place indicated by Dechesne et al. (Item 3) Again both in figure 4 and supplement S1 of Dechesne et al. (2020), just east of leg 2 of The Breaks measured section a fault appears. Uniquely in supplement S1, the words ‘syn-sedimentary deformation’ appears directly adjacent to the ostensible fault. Up-section from that ‘fault,’ the overlying strata are deformed to vertical at the leading edge of the fault. There exists no known faulting or major folding this far west in the local landscape (also in the present paper see the next section, ‘Figure 8’).
Figure 8 — Relevant Features in Vicinity of The Breaks
Most geologists familiar with the Hanna Basin refer to ‘The Breaks’ as an irregularly shaped area that encompasses roughly seven sections of rapidly eroding, tectonically spectacular badlands located at the northeastern extreme of the basin. Its approximate limits are constrained at the:
northwestern corner — part of sec. 31, T. 24 N., R. 80 W.;
northeastern corner — part of sec. 3, T. 23 N., R. 80 W.;
southeastern corner — part of sec. 10, T. 23 N., R. 80 W.; and
southwestern corner — part of sec. 7, T. 23 N., R. 80 W.
The axial trace of the defining syncline of northeastern Hanna Basin is completely outside (i.e., to the southwest) of the area usually referred to as ‘The Breaks.’
Dechesne et al. (2020), in contrast to above, used the term ‘The Breaks’ as a much smaller place, dominating section 8, T. 23 N., R. 80 W. (Fig. 8) along with adjacent corners of sections 5, 6, and 7. Its total area is about one section. Compared with the 20-leg measured section of the total Hanna Formation in the northeastern corner of the basin (Lillegraven et al., 2004, appendix 1), the more limited version of ‘The Breaks’ used by Dechesne et al. (2020) involves equivalents of the upper half of leg 15 and all of legs 16 and 17. Almost all of the paleobotanical information from The Breaks available to date was gained from leg 3 (Fig. 7D) of the present paper. The fossil records within localized parts of the upper and lower lacustrine units are ripe for thorough studies.
Figure 9 — On the Basin-marginal Position of the Defining Syncline of Northeastern Hanna Basin
Figure 9 A (modified from Lillegraven, 2015, fig. 9) illustrates the course of an almost south-north cross section (steps 1–2–3) that runs completely across the eastern realm of today’s Hanna Basin. Figures 9B and 9C, respectively, provide a simplified Paleocene precursor followed by a present-day view along that cross-sectional trace. The purpose of those cross sections (which have no vertical exaggeration) is to provide spatial perspective of the defining syncline of northeastern Hanna Basin (i.e., the principal structural feature of this publication) relative to the approximate breadth of the basin. Although not pursued in the present document, northeasterly displacement by the Dragonfly and Owl Ridge thrust faults was intimately involved in development of the defining syncline of northeastern Hanna Basin. Notice that the axial trace of that syncline is no less than eight kilometers north of any reasonable concept of the ‘central Hanna Basin.’ Additional detail relevant to tectonic relationships between the Dragonfly/Owl Ridge thrusts and the defining syncline exists in Lillegraven et al. (2004, figs. 4, 17A–D, and 18).
Figure 10 — Carbon Isotopes, Pollen, Paleocene— Eocene Boundary, and PETM
The stratigraphic column shown in this screenshot represents leg 7 from the Hanna Draw Section (Dechesne et al., 2020) of the Hanna Formation. The purpose of the column is to link paleobotanic evidence with values of bulk carbon-isotopic data determined from samples collected from within, above, and below the laterally extensive, ‘Big Channel’ sandstone complexes. The unusually high degree of variation in values among these data was considered by Dechesne et al. (2020) to reflect the carbon-isotope excursion (CIE), which is a global indicator of the Paleocene–Eocene Thermal Maximum (PETM).
Figure 11 — Pure Chance, or Something More?
The identical pair of digital models exhibiting the junction between the northeastern-most Hanna Basin and southern parts of the Sweetwater arch is interesting and thought provoking. The basic concept here came to A.W. Snoke and me as we were developing the field-trip guidebook for the northern Hanna Basin and bordering mountainous fragments of the Sweetwater arch (Lillegraven and Snoke, 1996, pp. 36–37, figs. 4 and 26). I have no explanation for the apparent radial ‘focusing’ of eight anticlinal axial projections. Is the seeming radial array the result of pure chance, or is this something of substance that is trying to transmit important clues about structural controls held in common between the defining syncline of northeastern Hanna Basin and basement-involved, fault-controlled folding expressed nine or more kilometers outside of the basin? What would the north-central half of Wyoming look like today (as seen from space) if we knew enough to completely ‘unfold the basement-involved accordion’ of the southern Sweetwater arch to its pre-Laramide comparative simplicity? The anticlinal folds themselves most certainly are fault controlled, but how deeply into the crustal sequences do these faults penetrate?
I don’t think anybody knows the answers to those questions, but at least I intentionally include their mention here to help make a point. No-matter how narrowly we might focus our initial field-geological research, we should be required to stretch our limits of observation and try to see if surrounding structures have important things to tell us.
CLOSER FOCUS ON THE ‘NEW STRATIGRAPHIC FRAMEWORK’
The just-introduced figures, tables, and appendices relate to the most important issues approached by Dechesne et al. (2020). Those mostly graphical elements will now be put to work.
Several passages in the text by Dechesne et al. (2020) specify that parts of the defining syncline of northeastern Hanna Basin represent central-basin (or axial) facies. For example, as a result of rapid subsidence from late Cretaceous through early Eocene time, the northeastern corner of the Hanna Basin developed:
“… a thick, relatively continuous succession of shallow marine, fluvial, paludal and lacustrine strata [that] is preserved in the center of the basin” (Dechesne et al., 2020, p. 1).
In terms of the principal contributions from their research:
“Stratigraphic sections were measured for an ~1250 m interval spanning the Paleocene–Eocene boundary across the northeastern syncline of the basin, documenting depositional changes between axial fluvial sandstones, basin margin, paludal, floodplain, and lacustrine deposits” (Dechesne et al., 2020, abstract, p. 1).
Those authors considered part of their study area to represent a ‘basin-axial’ (i.e., central-basin) geologic setting, as confirmed by the following:
“Sandstone, shale, and coal intervals are typical for fluvial, paludal, and lacustrine environments, and lateral facies changes are gradational and influenced by relative position within the basin, e.g., basin-axial (Hanna Draw Section [HD]) …” (Dechesne et al., 2020, p. 8).
Examination of the following graphics would be helpful here: Lillegraven et al. (2004, figs. 5 and 18); and Figs. 1–2 and 4–10 of the present publication. Each of those figures individually contributes some feature that helps drive home the concepts that all five measured sections are: (1) components of the defining syncline of northeastern Hanna Basin (i.e., the ‘northeastern syncline’ sensu Dechesne et al., 2020); and (2) that the syncline itself is part of the tectonic northeastern rim of the Hanna Basin. Even though it is true that the Hanna Draw measured section is closer to the basin’s center than is The Breaks, the axis of the syncline is no less than eight kilometers north of any reasonable concept of a ‘central (or axial) Hanna Basin.’ I suggest that this issue is not a mere nicety of preferred semantics. Rather, it is a central element of the firmly established stratigraphic and tectonic framework of the northeastern Hanna Basin’s edge. The defining syncline of northeastern Hanna Basin “.
.. now can be said to be an inter-basinal syncline that is the most extensive structural element known from the Hanna Basin” (Lillegraven, 2015, p. 46). The following pages in that reference (p. 46–48, 54, 58, 63–64, 68, 73, 82, 95, and 103) also trace that syncline’s total recognizable existence, which explains why its name (designated by Lillegraven et al., 2004, fig. 4B) has truly become inappropriate.
What is the maximum thickness of the Hanna Basin’s entire Phanerozoic sedimentary column? That is difficult to answer, because virtually all exposed sequences of Cambrian or Devonian formations through lower Eocene strata have been thinned by younger-on-older thrust faulting and/or Laramide episodes of erosion that reflected responses to neighboring tectonic uplifts. Dechesne et al. (2020, p. 3) answered the above question as follows:
“The Hanna Basin is especially noteworthy for its thick package of >12,500 m (40,000 ft) of Phanerozoic sedimentary strata ….”
Newly available evidence referred to below suggests that the Hanna Basin’s Phanerozoic section is significantly even thicker than that.
Developed across the eastern half of the Hanna Basin, I have provided much new information, centered on field-based measurements applied to four geologic maps, 25 geologic cross sections, and 36 stratigraphic columns (Lillegraven, 2015, figs. 4–7 and 8, respectively). All of those figures are constructed at, or adjusted to, 1:24,000 scales. Figure 8 from the 2015 paper is of special interest. It starts with the 36 stratigraphic columns as they would appear today, including field-recognized missing parts due to faults and/or erosional surfaces. Then, following a duplication of each column, if appropriate, the more junior columns were individually extended in thickness (in conservative palinspastic fashion) after comparison with nearby, clearly intact, correlative stratigraphic sections. As I summarized (Lillegraven, 2015, fig. 2, p. 41), a conservatively estimated total Phanerozoic thickness originally approached 15 km.
In my opinion, inclusion of figure 2 within the publication by Dechesne et al. (2020) was unfortunate. I say that because the figure provides only minimal, fragmented information that is better, and more thoroughly presented, later within that paper. The column shown in figure 2 of Dechesne et al. (2020) also is scaled only to roughly 6,800 m in total thickness. I suggest that this is misleading in light of the recently released measurements. The total thickness shown is closer to 10 km (again, see Lillegraven, 2015, fig. 2, p. 41).
The caption by Dechesne et al. (2020, p. 3) for their figure 2 states: “Stars in Coal 82 upper and Coal 83 show first occurrences of Platycarya.” According to information provided by Dechesne et al. (2020, p. 16), Platycaryaplaty-caryoides appears (as palynomorphs only) close to the level of coal 82 in the Hanna Draw Section. That represents the oldest known occurrence (i.e., first appearance) of the genus in the local section. Macrofossils of Platycarya sp. do not enter the picture until close to the (substantially younger) level of coal 83. Thus, the upper star in figure 2 of Dechesne et al. (2020) only represents a ‘first occurrence’ of the genus as compressed macrofloral fossils. Also, neither figure 4 nor supplement S1 from Dechesne et al. (2020) shows evidence (in the form of clear symbols) of actual occurrence of either species of Platycarya stratigraphically below the level of coal 83.
The caption for figure 2 from Dechesne et al. (2020, p. 3) also refers to the presence of ‘Miocene gravels’ above an erosional surface that cut into the highest remaining strata of the Hanna Formation. But as reported by me (Lillegraven, 1994, text p. 206 and fig. 6, p. 213), near the top of the Hanna Formation, lacustrine shales:
“… grade upward into a progressively more arkosic lithofacies. The arkoses include interbedded red granite boulders near the top of the section. The giant granitic clasts were derived from erosion of the Precambrian core of the immediately adjacent Shirley Mountains.”
I have never found Miocene gravels near the top of the Hanna Formation. But even the available information about Precambrian sources into the uppermost Hanna Formation would provide important insight toward restructuring the stratigraphic framework for the Hanna Basin.
At the bottom of the Hanna Formation’s relevant section, I heartily agree with Dechesne et al. (2020, p. 4) that “The base of the Hanna Formation is considered conformable with the Ferris Formation in the center of the Hanna Basin ….” Indeed, Clemens and Lillegraven (2013, p. 162–163) summarized detailed geographic, stratigraphic, and temporal evidence about the transition between the Ferris Formation and overlying Hanna Formation as also considered by Dobbin et al. (1929), Boyd and Lillegraven (2011a, 2011b), Hajek (2009), Hajek et al. (2010, 2012), and Wang et al. (2011).
Figure 7 by Clemens and Lillegraven (2013, p. 156–158) relates to the total thicknesses of the Hanna Formation in the northeastern Hanna Basin. The figure is a comicstrip-style series of five chronologically sequential cross sections, all rendered to the same scale. The transit line for the cross sections is aligned 12.4° west of north. Each cross section extends 160 km (at page scale) from the southeastern reentrant of the Wind River Basin generally southward across the Sweetwater arch (= Granite Mountains massif) into the central Hanna Basin just north of the town of Hanna. The series of five cross sections covers the temporal interval from Late Cretaceous to the present day. In terms of measurements in the vertical scale, the cross sections (vertical exaggeration = 2x) extend in the Hanna Basin from five kilometers below the Precambrian–Phanerozoic boundary to about 15 km above that boundary (i.e., to the modern land surface).
One principal intention in developing the cross sections in figure 7 (of Clemens and Lillegraven, 2013) was to show more than just the commonly drawn upward thrusting of the Archean basement of the Ferris, Seminoe, Shirley, and Freezeout Mountains against strata of the northeastern Hanna Basin. More specifically, our intention was to show how the vast Phanerozoic stratigraphic pile of the northern Hanna Basin during the Laramide orogeny filled new, subsidence-related accommodation space (Neuendorf et al., 2005, p. 4) contemporaneously with the next-door upward thrusting of basement masses. A result of the associated erosion of newly exposed Precambrian basement led to locally derived, arkosic sediments of the Ferris and Hanna formations. As alluded to by Dechesne et al. (2020, p. 4), this is how:
“… the basin margins show disconformable to angular contacts, because active underlying structures influenced accommodation and disrupted sedimentation ….”
Citing a series of authorities, Dechesne et al. (2020, p. 4) reported:
“… several maximum thickness values [for the Hanna Fm.] ranging from ~3500 m (11,000 ft) to 2150 m (7000 ft)….”
As pointed out by Dechesne et al. (2020, p. 4, who cited Perry and Flores, 1997; Dunn, 2003; and Lillegraven et al., 2004), the Hanna Formation’s general age calibration is dependent upon both palynomorph-based stratigraphy and distribution of lineages of fossil mammals. Their following sentence, however, is most puzzling to me, and it demands careful scrutiny (emphases added):
“P3 pollen are found at the base of the Hanna Formation in the center of the basin, indicating onset of Hanna sedimentation in the mid-Paleocene or later, and P5 pollen are found at the basin margin in The Breaks corroborating uplift below and during deposition ….”
I will paraphrase and divide that sentence into four numbered-component parts (each identified by underlining):
Part 1 (P3 pollen grains are found in basal Hanna Formation in the center of the Hanna Basin). I do not know where (or if) such a discovery in the center of the basin has been made. The stratigraphically lowest pair of P3- bearing sites presented by Lillegraven et al. (2004, fig. 16A, ‘Este Lado’ and unnamed site at Pats Bottom) sometimes are referred to as being in the ‘central basin.’ But those fossils were derived from the underlying Ferris Formation (Lillegraven and Eberle, 1999, figs. 3–7; Boyd and Lillegraven, 2011a, 2011b), not from the Hanna Formation. In terms of visualizing the stratigraphic connection between the uppermost sites in the Ferris Formation at Pats Bottom and the lowest palynomorph-bearing sites in The Breaks (Lillegraven et al., 2004, fig. 11) see Clemens and Lillegraven (2013, p. 162–163). The Hanna Draw Section, if perhaps that was intended in the sentence by Dechesne et al. (2020), is within a basin-margin structural setting. And that specific setting is at least eight kilometers removed from any ‘central basin’ position. See text and graphics related to the present paper’s Figure 9A–C.
Part 2 (Onset of deposition of Hanna Formation during ‘mid-Paleocene or later’ time). The stratigraphically lowest (and oldest) known palynomorph-bearing sites in the Hanna Formation are almost exactly two kilometers lower in the Hanna Formation’s column than the base of what Dechesne et al. (2020) consider to be The Breaks Section (compare Fig. 8 of the present paper with fig. 11 in Lillegraven et al., 2004). Fossil pollen that dominate more than 1,500 m of the lower Hanna Formation in vicinity of ‘The Breaks’ (sensu Lillegraven et al., 2004, fig. 11), along with well-studied fossil vertebrates that are relatively common between ca. 1,100 and 1,500 m above the base of the Hanna Formation, are set upon an intervening, erosional angular unconformity cut into steeply dipping, Upper Cretaceous Steele Shale. The fossil mammals collected within that ‘Vertebrate Fossil Bearing Zone’ are secure representatives of theTorrejonian and earliest Tiffanian ‘North American Land Mammal Ages’ (abbreviated ‘NALMAs’). As reviewed by Lofgren et al. (2004, fig. 3.2, p. 46–47), theTorrejonian NALMA represents an interval of the early Paleocene epoch that ranged from early 64 Ma through 62 Ma. The entire Paleocene represents an interval of Paleogene time that began early in 66 Ma and terminated near the beginning of 55 Ma. With those approximate numbers in mind, one can see that the Torrejonian NALMA represents about three million years within the earlier half of the 11-million-year-long Paleocene epoch. The youngest part of the Torrejonian is significantly older than the chronological mid-point of Paleocene history. And recall that more than 1,100 m of Hanna Formation underlie the ‘Vertebrate Fossil Bearing Zone.’ So, it is extremely improbable that the “… onset of Hanna sedimentation [began] in the mid-Paleocene or later.”
Part 3 (P5 pollen are found at the basin’s margin in The Breaks). As shown by Lillegraven et al. (2004, figs. 11 and 16A), two localities (EBP01, Hanna Fm. leg 4, ‘Beer Mug Vista’ and EBP04, Hanna Fm. leg 1, ‘WIPS Tips’) low in the Hanna Formation of the Breaks are surprising in hosting two palynomorphic species (Caryapollenites wodhousei and C. veripites) that are characteristic of, and elsewhere restricted to, zone P5 of the biostratigraphic scheme created by Nichols and Ott, 1978 (also see: Ott, 1964; Nichols and Ott, 2006; and Nichols, 2009), indicative of late Paleocene time. The above two localities deeply underlie the Hanna Formation’s stratigraphic level of the Torrejonian–Tiffanian ‘Vertebrate Fossil Bearing Zone.’ The WIPS Tips locality (in basal Hanna Fm.) is photographically shown in figure 16B of Lillegraven et al. (2004, p. 33). There is no question that the Hanna Formation at this locality was deposited directly upon eroded Steele Shale. Following close examinations, no faults could be seen to exist within the section of Hanna Formation shown in the photograph. Also, and very important to recognize, both the WIPS Tips and Beer Mug Vista localities, in addition to yielding two species of P5-characteristic form, exist in association with several species of palynomorphs characteristic of standard zone P3 (see summary in Lillegraven et al., 2004, fig. 16A, p. 32). For additional information about this situation, refer to the following part of Appendix 2 of the present paper, “Original Use of‘Beer Mug Vista’.”
Part 4 (The P5 pollen found at the basin margin in The Breaks corroborates uplift below and during deposition). I have no idea what process or mechanism those words attempt to champion. Nevertheless, my team of 2004 is inappropriately listed as proponents of some aspect of the fully cited statement. Just what is uplifted ‘below and during deposition’? In contrast, the scenario I do feel comfortable supporting is that Carypollenites wodehousei and C. veripites appeared on the planet (somewhere out of paleontological sight) long before the late Paleocene. Then, early in the Torrejonian, they burst into the sunlight within collections (of concern here) from the Hanna Basin. Such a scenario certainly does suggest weaknesses within the dominant theory of phylogenetic relationships involving Caryapollenites. Those weaknesses may devastate some paleontologists’ pet theories involving palynomorphs as firmly tested tools for dating Paleocene geologic and biologic events. But this sort of realization does happen, all too regularly, to complicate or even refute favored paleontological paradigms (see McKenna and Lillegraven, eds., 2006).
On a completely different paleontological subject, Dechesne et al. (2020, p. 4) stated that:
“… a single tooth of Hyracotherium grangeri was found near the top of the Hanna Formation (Lillegraven et al., 2004) ….”
Here, however, is what Lillegraven et al. (2004, p. 25) actually had to say on that matter:
“To date … no temporally diagnostic mammalian fossils have been discovered from strata equivalent to the top of leg 12 of the measured section to near the top of leg 19. But at that latter level, many bone fragments of a large mammal (probably a pantodont) and a few teeth of the primitive horse Hyracotherium grangeri were discovered and collected. As discussed by Lillegraven (1994), the latter is characteristic of early phases of the Wasatchian NALMA (c. 55 Ma), now considered to represent a very early part of the Eocene Epoch.”
Interestingly, Hyracotherium from near the top of the Hanna Formation is approximately 930 m stratigraphically higher than the level of coal 82 (as measured using fig. 11 of Lillegraven et al., 2004 and Fig. 8 of the present paper). Dechesne et al. (2020) have recognized the beginning of Eocene time at a level near coal 82. A mere ‘almost-kilometer’ of intervening strata raises little controversy here, because Gingerich and Clyde (2001, figs. 2–5) have provided unparalleled evidence across the complete Bighorn Basin (ibid., fig. 1) that Hyracotherium was long-lived. It appeared in strata identified as the earliest Wasatchian (ibid.; zone Wa-0) and it persisted into the highest level of that NALMA (Wa-7). The Wasatchian land-mammal age commenced at about 55 Ma and persisted through approximately the first third of the Eocene epoch (Woodburne, 2004, fig. 8.5, p. 325).
In plate 1 (of Lillegraven, 1994), interested readers may see three sets of enlarged stereo-photographs of teeth from Hyracotherium grangeri collected from nearly the highest reaches of the Hanna Formation. Illustrated are its right P3, left M2, and a fragmentary right M3 (or, less likely, M2).
Early text of the present paper (see ‘Figure 7 — Simplification and Redrafting of Supplement S1’) thoroughly compares the structure and data-contents of figure 4 and supplement S1, both graphics from Dechesne et al. (2020). The points were made that: (1) the ‘supplementary’ information contains unique and essential data that must be taken into account; and (2) homologous data are displayed at differing stratigraphic levels in supplement S1 than exists in the simpler (but more difficult to read due to crowding) figure 4. Largely because of the greater clarity and richness of essential data found in supplement S1, I selected that for further use in preference to what exists in figure 4. Figure 7 of the present paper was generated by me first to enhance usability (by way of less-cluttered graphics) and secondly as a decision to include a more limited range of variables. I took great care, however, to keep exactly the same stratigraphic levels for all data elements in Figure 7 that existed in supplement S1.
How thick is each sedimentary leg within each of the five measured sections? One would expect to have that answered by presentation in Dechesne et al. (2020) of a simple, fully verifiable listing of stratigraphically controlled metric values. Such orderliness of numbers does indeed exist for each leg in each measured section within supplement S1 (and with exactly the same values in the present paper’s Fig. 7). Now, restricting counts to the Hanna Draw Section, using the red letters/numbers in Figure 7, and adding up the numerical values from all 15 legs to get the cumulative total, the presented answer is 1,193m. But in the appropriate text of Dechesne et al. (2020, p. 4), the value quoted for Hanna Draw’s total thickness is 1,250 m. That is a difference of only 57 m, but why should there be any difference at all from what is stated in supplement S1? No reason for the difference has been provided. Similarly, what is the measured thickness of The Breaks Section? It has only three legs, the combined thickness of which total (as in supplement S1) 808 m. But again, the Dechesne et al. (2020, p. 4) publication’s text gives that thickness value of 940 m, an unexplained difference of 132 m. Such disparities do not herald the end of civilizations. But why should any user of data from this study need to worry about accepting hard-won yet undependable field measurements?
Dechesne et al. (2020, p. 4) seem to unintentionally provide yet another source of confusion by stating:
“The Breaks section approximately corresponds with the location of Leg 17, originally measured by Lillegraven (1994) ….”
That quotation is misleading, because research observations from the cited 1994 paper focus exclusively on stratigraphy and paleontology of the uppermost strata of the Hanna Formation as studied north of the Medicine Bow River. The northern area is geographically separated from The Breaks Section by roughly six kilometers. Also, the stratigraphic column of figure 6 in the 1994 paper was in a first stage of development compared to figures 4B, 4C, and 11 as published in 2004 by Lillegraven et al. Much new, more relevant and more accurate field-based information had been gained during the intervening decade.
As emphasized by Dechesne et al. (2020), the thickest and laterally most continuous coal beds in the studied area have been highly useful in mensuration and as stratigraphic markers. The geologic map (pl. 27) developed by Dobbin et al. (1929) has been a key source of information for such geologic uses (see caption to the present paper’s Fig. 6). Again, focusing only on the Hanna Draw Section, I encourage readers to compare the marked stratigraphic levels of specific-numbered coal beds between figure 4 and supplement S1 from Dechesne et al. (2020). Observers would recognize many examples of substantial differences in vertical assignment of the same stratigraphic levels of any given coal bed.
Dechesne et al. (2020) did not explain those differences, or even mention existence of the differences. I am not sure that my choice was correct, but I had assumed that supplement S1 from Dechesne et al. (2020) provided more accurate stratigraphic levels of coal beds than what is presented in figure 4. The supplement S1-derived positions are shown on the present publication’s Figure 7, using black ink and simple notation (e.g., ‘coal 81’). But now, Table 4 of the present paper provides yet another series of possibilities for stratigraphic positions. This time, the comparison would be between coal-level positions in supplement S1 and those coming directly from my tracing of numbered coal seams in Dobbin et al. (1929, pl. 27; = Figs. 6 and 7). The Dobbin et al. stratigraphic elevations are shown on the present paper’s Figure 7 rendered light-blue and with addition of ‘1929’ (e.g., ‘1929 coal 81’). So, among these alternatives, which would be closest to the truth? I’m not sure here either, but I do hold considerable faith in the quality of Dobbin et al.’s coal distributions as plotted in their plate 27.
Sound documentation of new paleocurrent (‘paleoflow’) data within upper parts of the Hanna Formation would be especially welcome. In that regard, the second conclusion of my 2015 monograph on the subject of late Laramide fragmentation of the eastern greater Green River Basin reads as follows (p. 102):
“2. Today’s Hanna and Carbon basins represent two relatively small, eastern fragments of an originally enormous, Cretaceous and Paleocene greater Green River Basin. During the post-Wasatchian Eocene, this part of Wyoming became tectonically subdivided into the Great Divide, Washakie, Hanna, Carbon, Pass Creek, and Laramie basins. The interval of greatest deformation involved, and probably was mostly limited to, the earlier half of Eocene time (i.e., late in the Laramide Orogeny). More specifically, most Laramide contractional tectonic activity within the greater Hanna Basin seems to have been restricted to very late in the depositional history of the Hanna Formation, continuing even after completion of its deposition.”
The effects of breakup of the original eastern Green River Basin would have been linked to profound rearrangements of fluvial and lacustrine drainage patterns related to the origin of Lake Gosiute and its eventual southward drainage into what is now northwestern Colorado (see restorations of early Eocene drainage patterns as developed by Lillegraven and Ostresh, 1988, figs. 7–11).
With that as introduction, I studied with considerable care (by developing Table 5) the prominently displayed paleoflow data presented in figures 3, 4, and supplement S1 by Dechesne et al. (2020). Rose diagrams were used by Dechesne et al. to display their paleoflow data in figures 3 and 4, although they did not explain how the counts of measurements that were assigned to rays of the diagrams are to be read. Thus, I used the descriptive terms of ‘no paleoflow,’ ‘weak,’ ‘limited,’ ‘good,’ or ‘strong’ to describe relative robustness within a continuum of available data. Supplement S1, in contrast, arrayed the data as compass-oriented, individual straight arrows. In that format, I assumed a ratio of 1 vector-flow measurement for each individual arrow. The robustness of gathered data is quantified simply as ‘n arrows’ (e.g., 7 arrows). Also, I assumed the data as applied to all three graphical elements came from the same database (i.e., fig. 3 as a map and fig. 4 along with supplement S1 as stratigraphic columns = measured sections). The authors of Dechesne et al. (2020) did not specify that differing databases were employed in the production of these three related graphics. The Hanna Draw Section (with 15 legs) and The Breaks Section (with 3 legs) assigned data to individual legs within the measured sections. The other three measured sections assigned data as reported measurements (in meters) above the base of the section.
The way Table 5 is set up, comparisons can be readily made between data within figures 3 and 4, or between figure 4 and supplement S1, or between figure 3 and supplement S1. To me, it was surprising how much data were not actually put to use. For example, the Doug Section’s figures 3 and 4 both had ‘good’ data, but ‘No paleoflow data shown’ was the case in supplement S1. Look for that situation yourself in Table 5, and you will find a total of 23 such cases. I see no sound reason for why any of the present datasets should have been ignored.
As another example for study, note the data for leg 5 of the Hanna Draw Section. In figure 3 of Dechesne et al. (2020), ‘strong’ data are shown (and flow was broadly to the east). In its figure 4, ‘weak’ data shows (with flow to northeast, revealing only 1 level with data). Supplement S1 shows two levels with data, and exhibits flows to north, northeast, and southwest. Virtually everything is different within that example, even though (with low counts of measurements) everything should be more or less the same among figures 3 and 4 and supplement S1. Look closely throughout Table 5, audit my evaluations against the published map and both versions of measured sections, and ask yourself if my analyses generally are awry or reasonable.
Data in the ‘Paleobotany’ section of the Dechesne et al. (2020) publication are difficult to verify. Fundamental problems are that: (1) discrepancies exist between stratigraphic levels of occurrences of species shown by plots on supplement S1 as compared with relevant statements in the text; (2) the starting points for thickness measurements commonly were unclear (e.g., from the top or from the bottom of a specific lithologic-marking body); (3) there exists widespread, ambiguous use of symbols and colored swatches that were intended to uniquely identify taxa or phenomena (e.g., what was the intended meaning of a red hexagon in supplement S1?); and (4) stratigraphic positions of fossils sometimes were declared to exist higher in the sedimentary column than the actual rock thicknesses extend (e.g., sample 16BCr-155 of Table 3).
Principally for the above reasons, I found the last three-quarters of page 16 in Dechesne et al. (2020) difficult to follow and use. Thus, to help future readers, I developed Tables 2 (paleobotanic macrofossil occurrences) and 3 (palynologic occurrences). The purpose of the text on page 16 was to summarize the biostratigraphic occurrences in the study area of the most important macrofloral and palyno-logical taxa. This was assisted by supplement S1’s large format and straight-forward metric calibration of the stratigraphic legs and undivided sections.
In the Hanna Draw Section, at least an approximate Paleocene–Eocene boundary has now been well established, around ‘coal 82 upper,’ near the base of leg 7. Prior to the work by Dechesne et al. (2020), my own team recognized placement of that boundary much higher in the section, close to the ‘88 coal,’ following recognition of paly-nomorphs of Platycarya by Tschudy, inGill et al., 1970 (reported by Lillegraven et al., 2004, fig. 11, p. 22).
A minor taxonomic inconsistency is present on pages 16 and 17 of Dechesne et al. (2020). On page 17 (in fig. 11 (I)) is presented a photomicrograph of a wrinkled pollen grain. It is identified in the caption to figure 11 as Corsinipollispsilatus. But on page 16, that same specimen is identified as Corsinipollenites psilatus. I am not sure which name is correct.
No mention was made of the amber and associated macrobiotic remains in the Hanna Formation as presented by Grimaldi et al. (2000). Sampling sites age-correlative with the present study area exist within The Breaks, and another collecting area exists just to the west, along the western side of the Hanna Draw Road (Carbon County no. 291). The sampling regimen in the Hanna Formation of The Breaks in part followed the 20-leg measured section established earlier by my Wyoming team (Lillegraven et al., 2004, fig. 4B).
Examination ofsupplement S1 shows that cataloged localities for nonmarine mollusks abound within the landscapes surrounding the Hanna Draw Section and The Breaks Section. No data from the other three measured sections are yet available. According to Dechesne et al. (2020, p. 7):
“The study of Hanna Formation continental mollusks is part of a larger project to revise and evaluate taxonomy and biostratigraphy of western United States Cretaceous and Paleogene taxa.”
Because of that commitment to an on-going study, only a few mentions are given of detailed stratigraphic occurrences of key species along the Hanna Draw Section. For example, the hydrobioid snail Micropygus minutulus, which is known elsewhere from late to latest Paleocene time, holds “conventional late Paleocene morphologies” (Dechesne et al., 2020, p. 18) when collected below coal 82 (i.e., along the currently recognized Paleocene–Eocene boundary).
More than 200 Paleogene continental molluscan localities are already known from throughout the Hanna Basin. Of that total count, 28 localities have been recorded in supplement S1 around the Hanna Draw Section, and 29 localities are recorded similarly around The Breaks Section. Late Paleocene and early Eocene occurrences of mollusks exist around both measured sections. Although not explained, the map (fig. 3 of Dechesne et al., 2020) provides symbols for seven mollusk-bearing localities in vicinity of the Hanna Draw Section and five localities in vicinity of The Breaks Section. Additionally, two groups of three localities each are shown dissociated from proximity to any measured sections. Thus, the symbols in figure 3 show a total of 18 mollusk-bearing localities as contrasted to the 57 sites plotted in supplement S1.
With all of those records in mind, and even though species richness is comparatively limited in the Hanna Basin, the molluscan fauna surely will be unusually helpful to biostratigraphic and paleoecological analyses once the larger project is available for general use. But, even now, the mollusk-oriented author’s following comment should be considered as revealing and highly important: “Mollusk occurrence in our section also indicates more lacustrine than fluvial or paludal conditions …” (Dechesne et al., 2020, p. 18).
Bulk Organic Carbon, Fossil Pollen, Paleocene–Eocene Epochal Boundary, and Paleocene–Eocene Thermal Maximum
Securing bulk organic carbon-isotope data for this part of the research was dependent upon highly technical instrumentation. Reliable quantitative information was nevertheless secured from four of the five measured sections (the Doug Section apparently was not sampled). When plotted, the finished data were almost universally seen to be within normally expected ranges. The sole exception was the interval between ‘coal 82 upper’ and ‘coal 83’ within the Hanna Draw Section. According to Dechesne et al. (2020, p. 19):
“A large shift in δ13Corg occurs between the last occurrence of Paleocene plants and the first appearance of earliest Eocene plants in the interval between Coal 82 upper and Coal 83. Both the most positive (-23.5‰ [VPDB, see supplement S3]) and most negative (-30.8‰) values occur in this part of the section, and likely reflect the carbon isotope excursion (CIE) globally found at the PETM.”
Whether that ‘likelihood’ reflects the truth or not, even the presently minimal available data almost seems compelling. Figure 10 of the present publication will help in characterizing these data.
Figure 10 is a modified excerpt from figure 5A (in Dechesne et al., 2020, p. 10). The sedimentary rock column shows essentially the entirety of leg 7 of the Hanna Draw Section. That column contains almost all of the rock between ‘coal 82 upper’ and ‘coal 83.’ Strata colored in yellow show the three stories of the ‘Big Channel’ sandstones. The sandstone masses are separated by intervening, generally finer-grained clastic sediments. The numbers in black (to the right of the column) indicate meters above the base of leg 7. The generic names to the left of the column represent specimens of the six plant species that are recognized in supplement S1 as ostensibly (see below) ‘first occurrences of Eocene taxa.’ The upper occurrence of Platycarya sp. is known by macrofloral fossils, but on this diagram the name is placed about 23 m too low on the columnar section (proper placement on Fig. 10 would be just above the top of the drafted column). The pair of occurrences of Platycarya platycaryoides near the base of leg 7 are properly placed, but they are known at this level only by fossil pollen. The broad, white zigzag line plots what was thought to be simplified values for δ13Corgalong the thickness of leg 7. The yellow dots (as well as the thin, zigzagging yellow dashed line), however, are copies of the actual data (of -n ‰ VPBD) as copied and re-scaled from figure 4 (of Dechesne et al., 2020, p. 6). Unfortunately, the δ13Corg-data were not also drafted onto the (relatively uncluttered) supplement S1. The swarm of vertical red lines is simply to help this paper’s users in correctly reading the values at each data-point.
Note that uniquely across the entire study area (exclusive of the unsampled Doug Section), recorded data in part of leg 7 actually exists well beyond the high and low extremes of background, measured negative values. It is solely upon this information in Figure 10, which reflects markedly unsettled negative bulk carbon-isotopic values, that Dechesne et al. (2020) postulated much of leg 7 reflects the carbon-isotope excursion (CIE) found around the world as part of the Paleocene–Eocene Thermal Maximum (PETM).
As originally mapped and reference-numbered by Dobbin et al. (1929, pl. 27; Fig. 6 of present paper), ‘coal 82’ had a dominating upper stratigraphic level and a lower level represented by a series of coaly lenses that are variable in thickness and lateral extensiveness. Late Paleocene fossil plants (occurring both as macrofossils and palynomorphs) are known as high stratigraphically in the Hanna Draw Section as the basal few meters of leg 7 (Dechesne et al., 2020, fig. 4 and supplement S1; present paper’s Figs. 7 and 10). A series of six ostensibly Eocene-indicating paleobotanic taxa occur directly above those Paleocene species. The first of those ‘Eocene’ species to appear (i.e., the lowest resident in the stratigraphic column) is pollen identified as Platycarya platycaryoides. The remaining five taxa follow in differing levels, beginning a few meters above the basal Platycarya and continuing almost to the top of the Hanna Draw Section. The botanic fossil record of that same, lithologically correlative sequence in The Breaks Section is depauperate, due to poor exposures of strata. Only one leafy dicotyledonous species (informally known as ‘Dicot sp. WW004’), previously known from other Wyoming basins having excellent biostratigraphic control, is representative of very early Eocene time (Dechesne et al., 2020, p. 16).
Unexpectedly, supplement S1 does not have the above-mentioned first appearance of Platycarya platycaryoides obviously plotted by name on the graphic. Rather, as readily seen in the present publication’s Figure 7, Platycarya is unforthcomingly represented within leg 7 by the three yellow hexagons labelled ‘Pollen/leaf localities.’ Once above leg 7 of the Hanna Draw Section, however, both macrofloral and pollen fossils of Platycarya are identified by name and by yellow circles in supplement S1.
It is also important that Dechesne et al. (2020) reported Paleocene fossils (right up into the base of leg 7 in the Hanna Draw Section) that remained morphologically perfectly typical. That is, the plant remains are not morphologically distinguishable from older, more generalized, upper-Paleocene plant fossils. That observation includes macrofloral and palynomorphic varieties, and it also applies to similarly typical, nearby Paleocene molluscan species. That is, there is no evidence supportive of very-latest Paleocene species exhibiting modifications that presage early evolutionary responses to a Paleocene–Eocene Thermal Maximum. Dominantly due to immigration of new forms and/or effects of local catastrophic processes, suddenly plant life of the stratigraphic column became characteristic of the Eocene.
So, what explains the special interest in Platycarya’s position within the local Hanna Basin stratigraphic section? Is there something special about its prominence as an indicator of Eocene time across the North American Western Interior? Probably the situation is more complex than traditionally considered. The genus Platycarya has long been known to have existed in European settings during both the late Paleocene and Eocene. Traditionally, it was thought that Platycarya entered North America as an immigrant during the earliest Eocene. As pointed out by Dechesne et al. (2020, p. 19):
“Platycarya pollen has historically been used as a marker taxon in defining early Eocene strata in the Western Interior (Nichols and Ott, 1978).”
But Dechesne et al. (2020, p. 19) also summarize essential, high-resolution information about occurrences of Paleocene and Eocene Platycarya in marine sedimentary drill-cores from the Gulf Coast and North Sea. They concluded, as well, that the genus actually had its first appearance in North America, including in the Bighorn and Powder River basins, during latest Paleocene, not earliest Eocene, time. Dechesne et al. (2020, p. 19) stated:
“In total, in the Hanna Draw section where the palynology is best constrained to date, we find the first appearances of six taxa known from the PETM or earliest Eocene between 24.4 m … and 123 m high in the section. Additionally, the presence of a Bighorn Basin PETM leaf morphotype [i.e., referring to ‘Dicot sp. WW004’] within the Big Channel sequence in The Breaks provides additional evidence that the PETM event is captured at this stratigraphic level in multiple places in the basin.”
Although not stated in that quotation, the strata referred to as being “… between 24.4 … and 123 m high in the section” is specific to leg 7 of the Hanna Draw Section. Also, the quote specifies that “… we find the first appearances of six taxa ….” Although it may be true that six taxa are, indeed, involved, only three are actually shown in supplement S1 to be known within that stated interval (i.e., they are Brosipollis striatus, Retristephanocolpites sp., and [as pollen referred to by the yellow-hexagon key indicating ‘Pollen/leaf locality’] Platycarya platycaryoides). Lygodium, Alnus, and Cnemidaria do appear, however, not long thereafter (see The Breaks Section). Finally, ‘Dicot sp. WW004,’ according to supplement S1, is known at the 125 m level (above the base of leg 3 in The Breaks Section), which is at least 5 meters below the channel sandstone complex.
As already considered earlier in this publication, the work in the Hanna Basin by Dechesne et al. (2020) has very positively applied a commonly used means to at least approximate the Paleocene–Eocene boundary in its local stratigraphic context within the Hanna Draw Section. Following dispersal of Platycarya into the area of its fossilization and eventual recovery of paleontological remains (i.e., near the base of leg 7), a markedly altered level of the epochal boundary is now firmer and more readily verifiable. But the temporal boundary itself still is only approximated. Uncertainty should be emphasized because the Platycarya pollen at the 2-meter level of leg 7 (the ‘Big Channel’ segment, not the ‘Big Channel Axis’ segment seen in Fig. 7) shown in Figure 10, under conditions of existing technology, could either represent latest Paleocene or earliest Eocene time. That is, we cannot yet really know if that ‘2-meter Platycarya’ was a brave botanical pioneer immigrating into the global Paleocene (as experienced in North America), or if it was entering the global Eocene (as experienced in North America). Here is where we truly need solid radiometric dating to at least provide linkage to a global geochronology. Just where in the Hanna Draw composite section should one affix the Paleocene–Eocene boundary?
Zircon Geochronology and Ages
Sadly, the search for reliably datable grains of the mineral zircon (ZrSiO4) resulted in disappointingly minor returns (see supplement S2 for field and laboratory techniques applied to the concentrating and dating of zircon crystals, along with information on source localities). Uranium–lead zircon dating was applied to:
“… better constrain the age of the Hanna Formation and specifically the absolute age of strata interpreted to represent the PETM” (Dechesne et al., 2020, p. 7).
The target substance for dating was centimeter-scale, probable volcanic remains (‘tonsteins’), thought to be present in coaly beds and intervals of carbonaceous shale.
Once assorted causes of inappropriate results are set aside, one should consult Dechesne et al., 2020, supplement 2, figure S2-4 that states:
“The Concordia Age of 54.42 ± 0.27 Ma from the 4 youngest zircons (yellow ellipses) is interpreted as the best estimate of the age of ash deposition in this sample [RD0814-36; see near top of leg 3 of The Breaks Section].”
Then follows the detailed technical, internet-based information in Dechesne et al. (2020, supplement 2, table S2-1 CATIMS U–Pb zircon data). Sample RD0814–36 came from coal 88, which is close to the stratigraphic top of leg 3 (ca. 490 m level; far above strata considered to be involved in the PETM) of The Breaks Section. That locality also is well-separated laterally from the actual trace of the mapped and measured leg 3 itself (see Dechesne et al., 2020, supplement S1 and the present paper’s Figs. 6 and 7).
Five other sampling sites (each is indicated in supplement S1 and Fig. 7 by red, five-pointed stars) are all associated with the Hanna Draw Section. All of the five sites, however, are interpreted by Dechesne et al. (2020) as preeruptive and inherited zircons, and thus are not suitable for presently required, practical dating purposes. The following quotation from Dechesne et al. (2020, p. 19), although puzzling, speaks to what appears to be a sense of deep disappointment related to application of zircon geochronology in the northeastern Hanna Basin:
“Based on the zircon age distributions, it appears that there are very few if any ash-fall zircons, and it is possible that the sampled units are not directly derived from volcanic ash but rather fine-grained siliciclastic to clay-rich sediments in very low-energy environments, possibly fluvial floodplain deposits.”
Perhaps all is not lost, however, because at least we now have one highly reasonable 238U–206Pb radiometric date from near The Breaks. The 54.42 ± 0.27 Ma age is in strong agreement with the earliest Wasatchian, North American Land Mammal Age (= early Eocene Epoch) as summarized in figure 8.5 ofWoodburne (2004, p. 325).
Miscellany on Lithofacies
A major component of the Deschesne et al. (2020) publication deals with text, tabular summaries, graphical data, and photographic descriptions of many of the diverse varieties of lithofacies existing within the area of study. The orderly subheadings (within the major hierarchy of ‘Results,’ extending across pp. 7–16 of the 2020 paper) include: Stratigraphy; Fluvial Sandstones; Basin Margin; Paludal and Floodplain Deposits; and Lacustrine Sandstones and Shales. The photographs, in particular, help importantly in grasping the overall nature of the landscape, along with the problems that face researchers as they attempt to cope with of ten widely separated outcrops (which are mostly sandstone and inconveniently vegetated, fine-grained clastic sediments and coaly debris).
Although the included descriptions of diverse lithofacies from Dechesne et al. (2020) are necessary and useful in field studies, large proportions of that information already exist within highly similar geologic regimes in other Laramide-basin settings up and down the Rocky Mountain chain. With that in mind, my following comments focus principally upon what I consider to be variable errors of fact and/or interpretation that I see as important to the overall purposes of this study. For example, as already approached earlier in the present publication, the area of this research is restricted to elements of the geographically broad, defining syncline of northeastern Hanna Basin. And that structure is, in its entirety, a basin-margin feature, substantially distanced from any part of the Hanna Basin that should be considered as axial, or central-basinal.
Related to that last point, and contrary to emphasis placed by Dechesne et al. (2020, p. 8), the Beer Mug Vista no. 2 Section is no more ‘basin-marginal’ than is The Breaks Section. Those two sections share the same stratigraphic relationships to the 1929 coals 87 and 88 (see the present paper’s Fig. 6). It is only an illusion that the Beer Mug Vista no. 2 Section appears significantly closer to the basin margin than The Breaks Section. The illusion stems from the strong, out-of-the-basin thrusting that occurred just to the east of the Beer Mug Vista no. 2 Section via the Dragonfly and Owl Ridge fault complexes (see Lillegraven et al., 2004, figs. 17 and 18). Those faults put younger strata onto older and drew more westerly elements of the affected landscape moderately eastward.
While on the subject of the Beer Mug Vista no. 2 Section (‘BMV’), Dechesne et al., 2020, stated at the end of p. 8:
“Dips within BMV shallow upward and range from 30° at its base to 15° near its top, which supports the presence of an actively growing structure (Fig. 9B).”
That statement is incorrect in regard to the trend in dips. Immediately following this paragraph is a screenshot of an excerpt from Lillegraven (2015, fig. 4; that map can be downloaded, in its entirety and at scale [1:24,000], from the second citation provided directly following the ‘References Cited’ section of the present paper). To that small excerpt was added the approximate position of the path (the undulatory red line) for the measured section of BMV. The area shown is part of the NW 1/4 of sec. 31, T. 24 N., R. 80 W., in the Difficulty Quadrangle. Note the eight strike and dip symbols that, despite usual variations among dips, show no obvious trend from top to bottom within the section. As to the “… actively growing structure …” referred to in the same quotation, yes, the defining syncline of northeastern Hanna Basin did, indeed, require some unknown interval of time to become fully developed.
Still on the subject of the Beer Mug Vista no. 2 Section, Dechesne et al. (2020, p. 8) expressed interest in obviously locally derived clasts within the Hanna Formation of the BMV. They stated:
“Coarse grain size and angularity indicate a higher depositional gradient and limited transport distance because BMV is presently (after Laramide shortening) ~6 km from the Freezeout Mountains, the northern basin margin.”
While the Freezeout Mountains do, indeed, serve as part of the northern structural margin of the Hanna Basin, the straight-line distance from the base of the BMV to the exposed Freezeout Mountain anticline is only 3.5 km. Nevertheless, the overlying depositional extensions of upper Paleocene and Eocene strata almost certainly were continuous from the northeastern Hanna Basin northward (running through what is now the Shirley Basin) into the southeastern reentrant of the Wind River Basin (Lillegraven and Ostresh, 1988, figs. 6–9). The present-day environment of general erosion has removed that presumed original interbasinal connection of Paleocene and Eocene strata.
Dechesne et al. (2020, table 1, p. 9) presented an extensive list of quite specific lithofacies and biofacies found between coals 77 and 90 in the local Hanna Formation. The table has 30 colored boxes, each adjacent to a different facies’ descriptive name. The constructor of this table went to a goodly amount of trouble, but the entire publication’s only citation to the table (on p. 8) stated: “Lithofacies are summarized in Table 1.” So far as I can tell, table 1 remained unused within the text. Even in figure 8, roughly half of the items within the ‘generalized lithofacies key’ differ in multiple ways from what exists in table 1. Also, both table 1 and figure 8 are flawed with multiple misspellings, misused terms, and other technical issues. Figure 8 is referred to twice within the Dechesne et al. (2020) publication. It is unclear, however, what significance was intended with emphasis on the similarity in sandstone body sizes.
In figure 5 of Dechesne et al. (2020, p. 10), confusion exists between referred-to items in the photographs as compared to supplement S1. For example, figure 5A superimposes a stratigraphic column (actually it is leg 7 of the Hanna Draw Section) upon a panoramic photograph of relevant outcrops. The uppermost hexagonal symbol for the Eocene-associating genus Platycarya is approximately at level 155 m on the column. But in supplement S1, there exists no record of Platycarya at that level in the section (until one sinks downward to about 2 m above the base of leg 7). Beyond that clear conflict, there exists more generalized confusion in the use of inconsistent chart symbols. As an example, a yellow hexagon in supplement S1 means ‘Pollen/leaf locality’ and a yellow circle refers specifically to Platycarya. Those conventions are also the case in Dechesne et al.’s figure 4, but in figure 3,Platycarya is indicated by an orangish to reddish) hexagon (as here in fig. 5A). Below the base of the Big Channel sandstones, however, it is the case that pollen grains of Platycarya are specifically indicated in supplement S1 by the hexagonal symbol for ‘Pollen/leaf locality.’
Another probable flaw exists in figure 5A of Dechesne et al. (2020). On the right-hand part of the photo image is a dotted line that is identified in the caption as the trace of a ‘normal fault.’ No actual evidence is presented, however, to verify the dotted line as a fault trace. Another explanation is the compaction of ashy remnants of coal 84 sometime during the long-duration underground fire that led to development of the clinker beds that originally were strata located above the coal.
The caption to figure 6 in Dechesne et al. (2020, p. 11) states that The Breaks Section is ‘505 m thick.’ That number is the thickness only of the section’s leg 3. Ignored in the caption were the thicknesses of leg 2 (197 m) and leg 1(101 m) that complete The Breaks Section.
The Discussion section in Dechesne et al. (2020, pp. 20–21) dealing with ‘Paleogeography,’ in my opinion, exhibits insufficient appreciation for the importance of tectonics in interpreting the geologic history of the area under study. Tectonic evolution certainly does matter in this case. The area in question (a short segment of the defining syncline of northeastern Hanna Basin) was simultaneously being directly affected late in the Laramide orogeny by such surrounding structural features as the Dragonfly fault, Owl Ridge fault, the complex of right-lateral strike-slip faults in northeastern parts of The Breaks (sensu Lillegraven et al., 2004), Flat Top fold complex, Shirley fault, Freezeout Mountain anticline, Beer Mug anticlinal complex (see caption to Fig. 11 in the present paper), Troublesome Creek anticline, Sledge Creek anticline, Bald Mountain anticline, Shirley Mountains anticline, Smith Creek anticline, Simpson Ridge anticline, Hanna syncline, Elk Mountain anticline, and Dana Ridge anticline.
Every single one of those above-listed structural features was being formed and/or activated and/or partly dismembered by faulting along with being influential upon processes of deposition, erosion, and overall bedding orientations within all of the northeastern Hanna Basin before, during, and after the Paleocene–Eocene transition. Indeed, the geologic setting for the present study would rank among the most complex of basin-formational and basin-deformational histories within the entire Laramide chain. But in Dechesne et al.’s (2020) figure 13 (which is titled “Paleogeography of the Hanna Formation in the study area …”), not a single structural element or measurement linked to the above list of surrounding tectonic features is clearly identified, aside from the Freezeout Mountains and the defining syncline of northeastern Hanna Basin itself.
The first paragraph of the ‘Paleogeography’ section (Dechesne et al., 2020, p. 20) refers to dominance of east- and northeast-directed water flow in Paleocene–Eocene rivers of the northeastern Hanna Basin’s margin. That directional dominance has long been recognized, and I alluded to its significance in the above sections labelled ‘Introduction’ and ‘Paleocurrent Data.’ The most significant of information on drainage vectors pertains to the early (but not earliest) Eocene, when the principal development of freshwater Lake Gosiute occurred in the landscape west of the Rawlins uplift, extending beyond the Rock Springs uplift into the Green River Basin proper and Bridger Basin (Lillegraven, 1993, figs. 1A and 4U). Development of that enormous lacustrine system occurred during the latter half of Wasatchian time, continuing through nearly the entirety of the Bridgerian North American Land Mammal Age. Relatively young strata of the Hanna Formation in vicinity of the Hanna Basin had mostly been eroded away. The only exception is located just north of the Medicine Bow River in the southeastern extreme of figure 4 of Lillegraven (2015), south of the massive Cretaceous sequences. Strata of the highest known Hanna Formation in the northern Hanna Basin running along three sections both to the west and to the east of the junction between the TE Ranch and Difficulty topographic quadrangles define the setting of greatest interest for paleogeographic reconstruction in late Hanna Formation time.
Investigation of that defined area was not pursued by the Dechesne et al. team. Nevertheless, in the caption to figure 2 of Dechesne et al. (2020, p. 3) they asserted: “… the top of the Hanna Formation is erosional, and Miocene gravels overlie it (not shown in this figure).” However, there exists no sign of Miocene strata in that area of the Hanna Basin.
The uppermost three legs (of 20 legs total) of the composite Hanna Formation’s stratigraphic column are irregularly exposed along the following approximated coordinates and directions (see fig. 4 of Lillegraven, 2015):
leg 18 — starts close to the northwestern shore of the Medicine Bow River near the center of sec. 25, T. 24 N., R. 81 W. then runs to near the center of NW ¼ of sec. 25 (which is just below the 89 coal);
leg 19 — starts at the top of leg 18, then runs through the southwestern corner of sec. 24 and continues to its termination near the east-central edge of the SE ¼ of sec. 23; and
leg 20 — first it doglegs-west from the top of leg 19 to near the center of the SE ¼ of sec. 23 and then runs to termination near the center of sec. 23, T. 24 N.,R. 81 W.
As recorded by me in 1994 (p. 208 and pl. 1, p. 218), cheek teeth of Hyracotherium and large bone fragments (probably of a pantodont such as Coryphodon), have been collected and cataloged into the University of Wyoming’s vertebrate fossil collection of the Geological Museum. Museum data include: Hyracotherium grangeri Kitts, 1956; equid cheek teeth, UW 26001,26002, and 26003. The fossils came from the very top of the composite Hanna Formation’s leg 19.
Still a little higher above the Hyracotherium-bearing level are almost the stratigraphically highest remains of the Hanna Formation. The following two paragraphs hold what I consider to be important paleogeographic information about the youngest beds of the Hanna Formation (Lillegraven, 1994, p. 207–208):
“… the top kilometer of what I consider to be the Hanna Formation can be shown at many places to have been markedly deformed by localized compressional faulting. The same is true even above the stratigraphic level at which the fossils described in this paper occur. In fact, detailed mapping in the northern half of Section 22, T. 24 N., R. 81 W. shows that a major reverse fault cuts what is close to the very top of the Hanna Formation. Upper Cretaceous marine black shale was brought at that point into juxtaposition with arkosic/conglomeratic strata of essentially uppermost Hanna Formation. This fault has been recognized for decades, and usually has been identified as the “Shirley thrust,” a major Laramide-style reverse fault that helps define the northern structural border of the Hanna Basin. I agree with that interpretation but add that this is only one splay of a complex zone of compressional faulting that affected the entirety of the Hanna Formation as it occurs along the northern margin of the Hanna Basin.
“Localized faulting along the above-mentioned splay of the Shirley thrust was complex, as indicated by the presence of previously unrecognized, large, heavily deformed pods of Hanna Formation that are completely engulfed within faulted slivers of Cretaceous shales. The most important point, however, is that major reverse faulting in this area definitely post-dates even the youngest remaining parts of the Hanna Formation. Fossils recovered from high in the Hanna Formation, therefore, can provide invaluable information about the persistence of the Laramide orogeny in this part of the Rocky Mountains.”
A few of those ‘large, heavily deformed pods of engulfed Hanna Formation’ are large enough to have been mappable at the scale of 1:24,000. The hosting upper Cretaceous strata include (youngest-to-oldest) Medicine Bow Formation, Lewis Shale, Steele Shale, and Niobrara Formation. Pods of Hanna Formation visible on the map by me (2015, fig. 4) can be located with the following bit of geographic assistance (all directions start from the northernmost expanses of largely intact Hanna Fm.):
pod in Medicine Bow Formation — (Kmb, site 990) north of sec. 21, T. 24 N., R. 81 W.;
pod in Lewis Shale — (Kle, site 984) in northeastern corner of sec. 21, T. 24 N., R. 81 W.;
pod in Steele Shale — (Kst, site 969) in southernmost outcrop of Steele Shale; and
pod in Niobrara Formation — (Kni, near site 2384) near westernmost outcrops of Niobrara Formation.
Coordinates for all recorded sites can be downloaded directly by site number from Lillegraven (2015, appendix 1) or via my field notes (arranged by site number and date of visit). Regrettably, GPS records were not gathered into these resources until the 2001 field season.
The salient points here are at least twofold. First, the above-located pods of Hanna Formation within the tectonically highly deformed Cretaceous marine shales were not derived from the still-coherently bedded outcrops of today’s Hanna Formation. Rather, they were structurally ripped from younger strata of the Hanna Formation that were physically ground by complex webs of faulted Cretaceous strata. Almost all of those younger, ripped-up sedimentary chunks of Hanna Formation became disaggregated into residues of sand, silt, and clay bits and in the process were carried away by fluvial processes toward the Gulf of Mexico and/or Hudson Bay. But a few sizeable chunks nevertheless were preserved as today’s pods. What was the original thickness of upper levels of the local Hanna Formation? That appears to be unknowable with any certainty under limitations of presently available evidence. But as an admitted guess, yet an additional kilometer of sedimentary thickness may well have been involved.
Secondly, with little doubt, there existed (but as yet remain undiscovered) a total reversal of fluvial vectors in the northeastern-most Hanna Basin from originally northeast- to more recently west-southwest-directed flow patterns. That would have provided strong initial help in filling and later maintaining the existence of Lake Gosiute. But when did that major change in flow pattern happen? Upon closer inspection, sandstone architecture may lead to recognizable flow reversal somewhere within today’s legs 18, 19, or 20. Or perhaps the transition will forever need to be imagined within still-higher but already eroded-away levels of the Hanna Formation.
The history of Lake Gosiute (Lillegraven, 1993, fig. 4U) filled approximately the middle third of Eocene time. But everything known today from remnants of the Hanna Formation in the Hanna Basin represents only the early one-third of Eocene time, or older. Hyracotherium was morphologically stable through all of Wasatchian time as based on nonpareil biostratigraphic documentation from the Bighorn Basin (Gingerich and Clyde, 2001, figs. 2–5). But younger Eocene horses would definitely have been recognizable if collected from strata younger than that initial early-third of Eocene time.
CONCLUSIONS BY DECHESNE et al. AND RESPONSES FROM LILLEGRAVEN
The following post-publication, newly numbered conclusions are reproduced here verbatim from Dechesne et al. (2020, pp. 21–22). The associated responses are mine.
1. “This study constrains the Paleocene–Eocene boundary in the rapidly subsiding Hanna Basin ….”
I agree, Dechesne et al. (2020) present a significantly modified and adequately defended approximation of the Paleocene–Eocene-boundary.
2. This study “ … provides a detailed stratigraphic frame work of fluvial, paludal, and lacustrine facies across the upper part of the Hanna Formation.” The lithologic framework is in basic agreement between the Dechesne et al. figure 4 (p. 6) and supplement S1, but the much more detailed lithologic key presented in table 1 (p. 9) is not actually employed in any systematic way within their publication. Thus, although fluvial, paludal, and lacustrine facies are considered in several contexts, in no sense does the publication’s organizational form provide a consistently ‘detailed stratigraphic framework.’
3. “Plant fossils and palynology have so far proved to be the most reliable tools to constrain the PETM.”
As alluded to below in response to conclusion number 11, information related to stratigraphic occurrences of fossil molluscan taxa has led to precisely the same results as what has been determined independently from paleobotany. More to the point, however, the actual ‘evidence’ for recognition of a ‘carbon isotope excursion’ (CIE) of the ‘Paleocene–Eocene Thermal Maximum’ (PETM) is based upon the unusually high degree of variability observed among negative values of bulk carbon-isotopic data. That variability was determined from collected samples of shales within, above, and below the laterally extensive, ‘Big Channel’ sandstone complex. The uniquely high degree ofvariability was found in (and only in) leg 7 of the Hanna Formation’s Hanna Draw Section. The taxonomic exercise of confirming a particular stratigraphic level as the Paleocene–Eocene boundary on the basis of an immigrant taxon such as Platycarya cannot itself be considered as recognition of a CIE, or any other phase of a PETM. Indeed, there also exist obvious procedural dangers in attempting to expect precision in designating a globally applicable epochal boundary on the basis of first appearance of a probable intercontinental immigrant. Importantly, lateral correlatives of the Big Channel complex are: (1) ‘Big Channel Lateral,’ in which the carbon-isotopic data are completely ‘normal’ (i.e., showing no extremes in negative variability in fig. 4); and (2) ‘The Breaks’ (sensu Dechesne et al., 2020), in which figure 4 shows no isotopic data whatever all the way through the critical ‘PETM Zone.’
4. “Bulk δ13Corg isotopes from carbonaceous shales show most negative values between Coal 82 and 83 in Hanna Draw corresponding with the here-defined PETM;”
That statement is correct as related to recognition of stratigraphically minimal and maximal negative permil values of bulk organic carbon. The statement also is correct in that the Dechesne et al. (2020) team has defined (i.e., asserted) local recognition of the PETM. Strength of available evidence for those assertions, however, seems highly questionable in that the inordinately high variability in bulk organic carbon has been demonstrated only in the Hanna Draw Section.
5. However, “… bulk δ13Corg isotopes in The Breaks are harder to interpret and need denser sampling.”
As reminded in conclusion 3 above, isotope values in ‘The Breaks’ are lacking.
6. “The fluvial response to the PETM in the Hanna Basin as an increased pulse of coarse sediment coincides with sedimentary responses described in other Laramide basins where it is typically attributed to a more seasonal climate.”
Without pertinent references or a mechanistic explanation, I cannot visualize this assumption-rich set of inadequately documented circumstances as constituting a ‘conclusion’ by Dechesne et al. (2020).
7. “Besides the occurrence of a large sandstone at the
PETM, a repetitive pattern of similar large sand bodies alternating with coals exists throughout the Hanna Formation.”
I certainly accept as real the observation that the Hanna Formation exhibits stratigraphic repetitiveness of highly similar-looking, large sandstone masses. But, because of the lack of relevant evidence, I am not ready to accept that the repetitiveness has anything whatever to do with the PETM or other similar pale-oclimatic events. The widespread, deep-originating tectonism that was ongoing through the late Laramide orogeny in and near the northeastern quadrant of the Hanna Basin would seem to be a more effective cause of erosion-based and fluvially transported sandstone bursts than even major climatic events.
8. “It remains unknown if this cyclicity results from auto cyclicity common to avulsive systems, drainage capturing, climatic signals such as orbital forcing, or other causes.”
See the directly preceding response (to no. 7).
9. “The only date in our stratigraphic framework is the location of the PETM between Coal 82 and 83.”
At least in the present state of relevant knowledge, I certainly would not look for a geologic-dating tool that involves a PETM within the Hanna Formation. But most assuredly I would welcome the very convincing, early Wasatchian, zircon-based U–Pb depositional date that did come out of this study. I am astonished that such a date would be mentioned in the Dechesne et al. (2020, conclusion 10 of present series) study only as being unsuitable for presently required, practical dating purposes.
10. “Attempts to add time control with zircons remained inconclusive since the zircons were likely detrital and therefore report depositional ages, rather than absolute ages.”
Perhaps I misunderstand the intended distinction between ‘depositional’ and ‘absolute’ ages. But in the present context, the 54.42 ±0.27 Ma date (as ‘depositional’ following an unavoidable detrital interval) seems to be what the project actually sought. The authors seem to be using the term ‘absolute age’ not as an antonym of ‘relative age,’ but rather as a term for the age of initial crystallization of an analyzed zircon.
11. “Additional efforts to increase resolution in correlations and environmental interpretations, especially in the Eocene part of the section by using mollusks, have so far been unsuccessful due to the poor preservation of specimens and uncertainty tying into regional correlations.”
The molluscan studies conducted during the present project are part of a long-term research effort conducted from localities across the Rocky Mountain chain of basins and the high Great Plains. It is indeed unfortunate that more information could not have been released to benefit the present study. Nevertheless, the third paragraph in the left column of page 20 in Dechesne et al. (2020) points out the following: “Preserved morphologies of caenogastropods (Viviparidae, Pleuroceridae, and Hydrobiidae), veneroids (Sphaeriidae), and unionioids (Unionidae) are consistent with taxa for a late Paleocene age in localities below Big Channel. Likewise, sparse specimens from localities above Big Channel indicate lower Eocene from different taxa in same families.”
That basic stratigraphic information is precisely what was independently determined via paleobotany, so I would not agree with the above conclusion.
12. “More age control would better document timing and rates of sedimentation in the basin.”
That statement is a simple truism, not a ‘conclusion’ emanating from research by Dechesne et al. (2020).
13. “The identification of the Paleocene–Eocene boundary in the Hanna Basin adds an important spatial data point for comparison of PETM sections in terrestrial basins across the western United States.”
Refer back to my responses to conclusion numbers 1 and 3, above. Because of uncertainties inherent in designating an epochal boundary on the basis of first appearances of probable intercontinental immigrants (such as Platycarya), such boundaries can only be stratigraphically approximated. In the present study of the Hanna Basin, therefore, recognition of the first appearance of Platycarya cannot be depended on to confirm a specific stratigraphic level such as the Paleocene–Eocene boundary; it can only weakly corroborate hypotheses for the position of that boundary. Recall that the carbon isotope excursion (CIE) of the Paleocene–Eocene Thermal Maximum (PETM) is based upon wholly different geochemical criteria, and therefore the stratigraphic placement of oldest Platycarya cannot itself be considered to diagnose a CIE or any other phase of the PETM. With those linked concepts in mind, I cannot agree that the Hanna Basin’s approximation of the Paleocene–Eocene boundary can reliably serve as a spatial data point useful in inter-basinal correlation.
14. “The abundance of carbonaceous shales improves preservation of paleobotanical material and offers more detailed documentation of changes across the PETM compared to more arid basins, such as the Piceance and Bighorn Basins.”
This statement does not qualify as a ‘conclusion’ based upon actual research in the present project. Perhaps it does, however, hint to a possible future direction for productive research.
Participants in the Dechesne et al. (2020) project are to be commended in that their resulting paper ranged broadly across the geologic setting, stratigraphy, paleocurrents, paleobotany, continental mollusks, zircon geochronology, associated lithofacies, and paleogeography. Despite that breadth, there exists a plethora of unexpected and wholly avoidable inconsistencies, strong contradictions within what should be homogeneous datasets, and seemingly inexplicable omissions of obviously necessary and sometimes clearly existing but unutilized data such that one must question the reliability of much of the information presented in their paper.
Principal funding for my research contributions leading to this discussion derived from National Science Foundation research grants EAR 9506462 and EAR 9909354. Since my academic retirement in 2004, various forms of financial and personal support have come from my talented bride, Mrs. Linda E. Lillegraven. Personal assistance at many levels came by way of the lengthy list of individuals presented in my2015 Hanna Basin monograph (pp. 107–108). Reviewers of the manuscript leading to the present publication included Drs. Michael O. Woodburne, L. Barry Albright III, and Anthony Harper. Each of them, along with all members of RMGs editorial staff, contributed importantly to improvements in the manuscript and its graphics. Conceptually key was assistance in structural technicalities from Professor Arthur W Snoke. Despite all that help, I accept full responsibility for all failings within this publication.
I dedicate this paper to my colleague and friend Philip D. Gingerich (Professor Emeritus, University of Michigan) for his sustained investigations into the Paleocene–Eocene biological history of Wyoming’s Bighorn Basin.
Four geologic maps at original scale from previous publications (one from Lillegraven et al., 2004; and three from Lillegraven, 2015) might be useful in better understanding the present publication. Thus, the four maps are made available for download as data supplements accompanying the article on GeoScienceWorld and via the URLs below:
figure 4. From Lillegraven et al. (2004): Geology in vicinity of ‘The Breaks’ (scale 1:12,000), https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/55.2/Lillegraven-2004-Fig.-4.pdf
figure 4. From Lillegraven (2015): Geologic map of northern Hanna Basin (scale 1:24,000), https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/55.2/JAL-2015-Fig.-4.pdf
figure 5. From Lillegraven (2015): Geologic map of eastern Hanna Basin’s margin, Simpson Ridge Anticline, and Carbon Basin (scale 1:24,000), https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/55.2/JAL-2015-Fig.-5.pdf
figure 6. From Lillegraven (2015): Geologic map of southern Hanna Basin (scale 1:24,000), https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/55.2/JAL-2015-Fig.-6.pdf
Appendix 1. Listing of prior published research within the greater Hanna Basin area by Lillegraven (as sole author or with co-authors). Complete bibliographic citations are in the References Cited section of this publication. General topics are addressed here in italics.
Boyd, D.W., and Lillegraven, J.A., 2011a — Stratigraphy, invertebrate paleontology, paleogeography.
Boyd, D.W., and Lillegraven, J.A., 2011b — Correction of a section number in caption to Figure 2.
Clemens, W.A., and Lillegraven, J.A., 2013 — Structural geology, mammalian paleontology, and paleogeography.
Eberle, J.J., and Lillegraven, J.A., 1998a — Stratigraphy and mammalian paleontology.
Eberle, J.J., and Lillegraven, J.A., 1998b — Stratigraphy and mammalian paleontology.
Grimaldi, D.A., Lillegraven, J.A., Wampler, T.W., and two others, 2000 — Stratigraphy, paleobotany, invertebrate paleontology, geochemistry.
Lillegraven, J.A., 1993 — Comparative stratigraphy.
Lillegraven, J.A., 1994 — Stratigraphy, mammalian paleontology of youngest strata of Hanna Formation.
Lillegraven, J.A., 2015 — Stratigraphy, structural geology, tectonics, paleogeography.
Lillegraven, J.A., and Eberle, J.J., 1999 — Stratigraphy, vertebrate paleontology.
Lillegraven, J.A., and McKenna, M.C., 2008 — Stratigraphy, palynomorph evolution.
Lillegraven, J.A., and Ostresh, L.M., Jr., 1988 — Paleogeographic evolution.
Lillegraven, J.A., and Ostresh, L.M., Jr., 1990 — Stratigraphy, paleogeographic evolution, invertebrate paleontology.
Lillegraven, J.A., and Snoke, A.W., 1996 — Stratigraphy, tectonics.
Lillegraven, J.A., Snoke, A.W., and McKenna, M.C., 2004 — Stratigraphy, tectonics, paleogeographic evolution.
Lofgren, D.L., Lillegraven, J.A., Clemens, W.A., and two others, 2004 — Comparative stratigraphy, paleobotany, invertebrate paleontology, vertebrate paleontology.
McKenna, M.C., and Lillegraven, J.A., 2005 — Palynomorphic stratigraphy.
McKenna, M.C., and Lillegraven, J.A.,eds., 2006 — Paleontological evolutionary modeling.
Appendix 2. Salient information about two quite separate but closely spaced, stratigraphically important localities that inappropriately share identical names (i.e., ‘Beer Mug Vista’). The ‘Beer Mug’ itself is a dramatic anticlinal complex seen in sects. 17 and 20, T. 24 N., R. 80 W. (see photograph and geologic map in Lillegraven and Snoke, 1996, figs. 18–19; geology by Taylor, 1996).
Original Use of ‘Beer Mug Vista’
Original name: Beer Mug Vista, coined by R.E. Dunn (2003), p. 99 and fig. 2.3.
Curatorial numbers: Palynomorph field record EBP03 from fossil-leaf locality DMNH [Denver Museum of Nature & Science] I.2643.
Locality coordinates: In center of NE ¼ of SE ¼ of SE ¼ of sec. 4, T. 23 N., R. 80 W. in Carbon County, Wyoming. See Dunn (2003), figs. 2.2 (foldout) and 2.3 (p. 13). The site is within the eastern stratigraphic level of leg 4 in the measured section published by Lillegraven et al. (2004), fig. 4 (foldout in 2004) and Fig. 5F (in green) of present publication.
Relevant quadrangle map: Como West, 7.5-minute topographic quadrangle.
Scientific significance: In this and one other nearby locality, Caryapollenites wodehousei and C. veripites (characteristic of, and elsewhere restricted to, zone P5 of the zonation scheme by Nichols and Ott, 1978, indicative of late Paleocene) are found in association with standard paly-nomorphs characteristic of zone P3 (occurring much lower in the section, indicative of early Paleocene). According to Lillegraven et al. (2004, p. 34): “Dunn’s identifications of the pollen grains were confirmed by Dr. Douglas J. Nichols (in Dunn, 2003, appendix E).” In the words of Dunn (2003, p. 99): “The occurrence of palynomorphs characteristic of zone P5 below the numerous P3 samples in the FVBZ [Fossil Vertebrate Bearing Zone] is anomalous. There are two obvious possibilities that will serve as hypotheses: (1) the pollen zonation scheme of Nichols and Ott (1978) may not be valid, or (2) there is a previously unrecognized fault occurring in the section that places older strata on younger strata [sic].” Part 2 of that sentence is in error, because younger strata (i.e., P5) would need to have been somehow placed under older strata (i.e., P2 or P3). This situation is not considered in the potentially useful figure 2 (p. 3) in Dechesne et al. (2020), which inexplicably focuses exclusively on the Hanna Draw Section. Lillegraven et al. (2004) provided discussion of this situation within text associated with figures 11 and 16–18. Also see McKenna and Lillegraven (2005, p. 44–46), McKenna and Lillegraven, eds. (2006, p. 16–17), and Lillegraven and McKenna (2008, p. 36–40).
Why this situation should be addressed: Wholly unnecessary confusion would result from persistence of two different, scientifically important localities within close proximity to one another and holding identical names.
Recommended solution: Recast this original use of the name ‘Beer Mug Vista’ to ‘Beer Mug Vista no. 1.’ Subsequent Use of ‘Beer Mug Vista’
Subsequent name: Beer Mug Vista, coined by Dechesne et al. (2020), p. 4 and fig. 3.
Curatorial numbers: None.
Locality coordinates: The name applies to a 278-meter thick measured section (see supplement S1 of Dechesne et al., 2020) that occupies most of the NW ¼ of sec. 31, T. 24 N., R. 80 W. in Carbon County, Wyoming. Also see Figure 5E of the present publication.
Relevant quadrangle map: Difficulty, 7.5-minute topographic quadrangle.
Scientific significance: “The Beer Mug Vista (BMV) Section … captures proximal-to-distal facies relations from near the basin margin to the relatively more distal and shallow lacustrine section in The Breaks” (Dechesne et al., 2020, p. 4).
Why this situation should be addressed: Same reason as for the originally named site.
Recommended solution: Recast this subsequent use of the name ‘Beer Mug Vista’ to ‘Beer Mug Vista no. 2.’