This geologic study is focused on a less than 5 square-mile (ca. 13 km2) tract of public land in northwestern Wyoming, 8 miles (12.9 km) south-southwest of the small town of Clark in Park County. The study area is south of Clarks Fork of Yellowstone River along the eastern base of the topographic feature called Bald Ridge, also known structurally as Dead Indian monocline. Since the Middle Eocene, the study area has been along the northwestern margin of the Bighorn Basin. Prior to that time, the study area existed near the west–east center of the basin. Bald Ridge became elevated late in the Laramide orogeny (no older than the Early Eocene) through east-directed faulting of basement rocks via the extensive Line Creek–Oregon Basin thrust system. As that active faulting occurred, the overlying Phanerozoic strata (Lower Cambrian through Lower Eocene) responded with numerous west-directed, out-of-the-basin thrusts as a new western-basin margin developed along the eastern realm of the newly born Absaroka volcanic field. Most of that deformation occurred after deposition of uppermost levels of the Lower Eocene Willwood Formation.
The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin. The stratigraphic column in a north–south band along the eastern flank of the Beartooth Mountains and continuing southward into the present study area was markedly deformed and deeply eroded late during the Laramide orogeny. The present small, more southerly study area is structurally and erosionally simpler than its more northerly equivalent. Thus, its study adds important geological information to the history of the northern Cody Arch, a convex-westward string of related basement-involved uplifts extending southward to southwest of the city of Cody. Progressively steepening eastward dips of strata characterize a west-to-east transect from the summit of Bald Ridge (capped by the shallowly dipping, Mississippian Madison Limestone) to the western edge of strongly overturned outcrops of the Eocene Willwood Formation. The Upper Cretaceous Meeteetse Formation is the stratigraphic horizon at which the dips attain vertical or slightly overturned orientations.
All consequential faults within the newly mapped area are thrusts, and they show generally westward (out-of-the-basin) displacements. Despite those west-directed displacements, their primary cause was tectonic shortening at depth below Bald Ridge that was directed to the northeast or east-northeast. During the Laramide orogeny, certain thrust planes within the east-dipping Phanerozoic rock column cut down-section stratigraphically (but uphill relative to Earth’s surface) and thereby placed younger strata upon older. The cumulative result, as recognized at several levels within the present area of study, was marked thinning of the total section. For example, surface exposures of the mostly Paleocene Fort Union Formation, 4,000 feet (1,219 m) thick only 7 miles (11.3 km) to the east, was completely eliminated from the local surface stratigraphy by that means.
The northern end of Bald Ridge is formed by the highly asymmetric Canyon Mouth anticline. That structure differs strongly in the attitude of its hinge line from the general east-northeast dip of strata cloaking Bald Ridge. The Canyon Mouth anticline’s hinge line plunges steeply to the southeast, and dips on its northeastern flanks are vertical to partly overturned. Surprisingly, hinge lines and flanks of all other anticlinal/synclinal structures recognized within the present map area share those same orientations with Canyon Mouth anticline. These consistent but unexpected differences in orientation from unfolded strata may represent very late events in the history of Laramide strain vectors across the study area.
Working in northern parts of this study area, an independent group determining radiometric ages of detrital-zircon grains reported close agreements in age with their host localities in the Early Cretaceous Mowry Shale and Frontier Formation. However, under the present paper’s interpretation of the local stratigraphy, the other workers misidentified formational hosts for all three samplings. That resulted in age-determination errors of depositional history within the Upper Cretaceous section of as much as 28.8 million years.
Subject, Purpose, Relevant Prior Mapping, and a Primary Goal
The subject of this research is a less than 5 square-mile (ca. 13 km2) tract of Wyoming landscape (Park County) administered by the U.S. Bureau of Land Management. The setting is just southeast of the canyon mouth through which Clarks Fork of Yellowstone River enters the modern Bighorn Basin (Figs. 1 and 2). Today’s setting of the study area undeniably is a geologically characteristic ‘basin margin.’ However, the study area exposes sedimentary rocks that, prior to the Middle Eocene, represented strata that were deposited close to the west–east midpoint of an earlier configuration of the Bighorn Basin that was roughly twice its present width. The much grander original basin became markedly narrowed west–east early in the Eocene. That came about through effects of deep-crustal, east-directed faulting that also resulted in strong uplifting of all rocks above the basement-rooted faults. Remnants of those markedly uplifted parts are geographically expressed today as ‘Bald Ridge’ (see cover image for this issue of Rocky Mountain Geology).
Rock exposures across the northern half of the study area (Figs. 3 and 4) are especially informative. Indeed, they provide extraordinary opportunity for study of deformational interaction between two components of the structural story. The first component is the uplift and eastward thrusting of Dead Indian monocline late in the Laramide orogeny. That monocline bears a structurally relevant name, but it is the same physical feature as the geographic entity known as Bald Ridge. The second structural component is the resulting, comparatively passive deformation from effects of uplift and eastward translation of the monocline. Principally affected were strata just to the east of the present (and then ‘new’) northwestern margin of the Bighorn Basin. I proposed the fundamental components of that tectonic interaction in Lillegraven (2009). The present publication should be considered as a continuation of the 2009 effort—as a more focused follow-up to that geographically more extensive investigation.
The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin. The principal focus is adjacent to the eastern base of Bald Ridge (= Dead Indian monocline). Geographic and stratigraphic boundaries of the present project are shown in Figure 3. My interpretations of relatively shallow subsurface relationships via 6 representative cross sections are shown in Figure 4 along with excerpted figures 3, 10, 11, and 32 from Lillegraven (2009), now incorporated as parts of Figure 4.
The most important previous geologic mapping surrounding this general area of study exists through the efforts of Love and Christiansen (1985), Love et al. (1993), Pierce (1965a, 1965b, 1966, and 1978), Pierce and Nelson (1968), and Wise (1983). I consider all of those works to be genuinely heroic in scope and of inestimable value. Nevertheless, all are at scales (e.g., 1:62,500 and 1:250,000) small enough to demand re-mapping at a larger scale (1:12,000) to make possible the more detailed examination attempted here. My own prior work of relevance to this particular area (i.e., Lillegraven and Ostresh, 1988 [figs. 1, 4–11 and table 2] along with Lillegraven, 2009 [figs. 2, 3, 6–13, 29–30, and 32 and table 1]) also required significant updating and greater focus on specific formational boundaries, as seen in present Figure 3. The topographic and geographic basemaps used here were: Anonymous (1991) and Anonymous (1987a and 1987b).
A primary goal of this paper is to assure that any interested reader will benefit from unhindered opportunity to re-occupy any or all points of data referred to on the geologic map (Fig. 3), interpretive cross sections (Fig. 4), associated with photographs and a chart (Figs. 5–41), locality data for relevant detrital-zircon collecting sites (Table 1), Appendices 1 and 2, and general text. Both appendices are placed following the References Cited. Appendix 1 presents values of eastings and northings as Universal Transverse Mercator data for all numbered measurements and non-attitudinal observations appearing on Figures 3 and 4. Also listed are page numbers within field notes, all of which are publicly available for use in the University of Wyoming’s American Heritage Center, listed under the archival collection from Jason A. Lillegraven. Appendix 2 lists, formation-by-formation within the study area, the: (1) litho- and biostratigraphic criteria by which each formational unit was identified; (2) locations of the most informative outcrops; and (3) figure numbers in which representative photographs of formations are provided. Finally, Table 1 lists diverse yet spatially equivalent notations of geographic coordinates. The coordinates will help in locating each of the three outcrops discussed in the text that yielded samples of detrital zircons used by May et al. (2013a, 2013b) as they attempted to age-date the host strata.
Photographic Scales for Screen Visualization Versus Hard-Copy Printing
Here are special notes (as pertinent to each of the 41 figures) relevant to scales for screen visualization as contrasted to hard-copy printing. First, all online figures presented here are within the limits of standard page size for Rocky Mountain Geology. Those figures, reproduced at default viewing scales, should be considered as ‘thumbnail images,’ although all can be viewed or printed at any desired scale. Second, although the following use is optional, exclusively scale-configured commands for hard-copy printing of Figures 3 and 4 are provided immediately following the ‘References Cited’ section of this paper. Reproduction is specified in that context to yield accurate printouts at 1:12,000 scale, both for the geologic map and for the sheet of interpretive cross sections.
The reference map for the state of Wyoming (Fig. 1) puts the minuscule nature of this project’s area of study into perspective. Almost all mapped outlines of basin/mountain boundaries across the map stem from tectonism and deposition during the Laramide orogeny (Snoke, 1993). The principal exceptions include pre-Laramide emplacement of parts of the Wyoming–Idaho fold-thrust belt and post-Laramide uplift/subsidence of the Teton Range and Jackson Hole, evolution of the Absaroka volcanic field, uplift of the Yellowstone Plateau, collapse of the central Sweetwater Arch, and opening of the Saratoga Basin.
The enlarged reference view (in Fig. 2) shows that this study area (i.e., Figs. 3 and 4) is close to the northwestern-most edge of today’s Bighorn Basin. The study is south and slightly east of today’s extreme basinal edge as seen directly east of the Beartooth Mountains (diverse geological features reviewed in Lillegraven, 2009). All of the light-green features in Figure 2 exist within recognized boundaries of the present Bighorn Basin. In sharp contrast, most of the montane areas of today (i.e., represented on the western half of the map by the light-purplish color) were occupied, during all but latest intervals of the Laramide orogeny, by an originally much more westward-extensive Bighorn Basin (in Fig. 4, see fig. 32, excerpted from Lillegraven ).
Note the bright-yellow band in Figure 2 that extends southward from along the modern eastern edge of the Beartooth Mountains. The yellow band represents a coherently recognizable sequence of Paleozoic to within early Eocene strata (see Pierce, 1965b). That stratigraphic sequence initially was marine, but beginning in mid-Jurassic time became progressively more terrestrial. Happily, the present area of study (bounded within Fig. 3) is stratigraphically more coherent and less complexly folded and faulted than the correlative sequences east of the Beartooth block. Most of the expected stratigraphic components of the present study area are adequately exposed (and illustrated within Figs. 3–40) and more readily identifiable than in sections to the north of Clarks Fork of Yellowstone River. Also, most of the principal Laramide deformation affected all of its formations simultaneously, during latter parts of the Early Eocene (i.e., during the late history of deposition of the Willwood Fm.). That late-Laramide deformation stemmed from east-directed faulting at deep crustal levels, expressed in what was originally just west of the west–east center of the Bighorn Basin (in Fig. 4, see excerpted figs. 10 and 11 from Lillegraven ). That deep faulting resulted in montane-scale uplift (i.e., the Dead Indian monocline) and prodigious erosion west of, and covering, the entire study area. That faulting, uplifting, and erosion resulted in the obliteration of a large part of the original Bighorn Basin’s western mass.
The area of present study (bounded in Fig. 3) represents the northern element of a shared pattern of Laramide orogenic history characteristic of the entire ‘Cody Arch’ (name coined by Sundell, 1990). Relevant parts of that structurally related arch consist of (from north to south; Fig. 2): Bald Ridge; Dead Indian Hill; Pat O’Hara Mountain; Rattlesnake Mountain; and Cedar Mountain. All of that convex-westward arch’s map area and montane nature (along with edges of the post-Laramide Absaroka volcanic field immediately to the west) remained structurally undeveloped prior to Eocene time and was simply part of the western Bighorn Basin’s depositional environment until late into the Early Eocene.
As becomes obvious in Figure 5, the flatirons of eastern Bald Ridge would dominate any scenic comparison with the restricted area of the present study. Figure 5 presents a view to the west as photographed from within far-western parts of today’s Bighorn Basin. The arrow-emphasized white rectangle shows the limits of slightly more than the northern two-thirds of the geologic map (Fig. 3). This panorama is intended to help the reader visualize the intermediate nature of the study area as set between genuinely mountainous topography (i.e., Bald Ridge plus Beartooth Mountains) and today’s westernmost surfaces of the Bighorn Basin. Consequential uplift of Bald Ridge (becoming the structural feature of Dead Indian monocline) occurred no earlier than during late phases of deposition of the Willwood Formation (i.e., late Early Eocene; Robinson et al., 2004).
The east-dipping Dead Indian monocline most definitely is an appropriate, structurally based name for that imposing uplift, and Lillegraven’s (2009, caption for fig. 32, p. 74) reference to it as the “Bald Ridge anticline” was in error. The Precambrian basement (of igneous and metamorphic rocks) underlying that structure is exposed only near the course of Clarks Fork of Yellowstone River along the base of the western flank of that deeply eroded monocline (Foose et al., 1961, fig. 1 of pl. 2; Pierce, 1965b). Starting above those basement outcrops is a remarkably complete sequence of Phanerozoic strata. That sequence begins with the Cambrian Flathead Sandstone and continues stratigraphically upward into the Lower Eocene. The mid-Jurassic Gypsum Spring Formation represents the base of documentation for the present study. Bald Peak (elevation of 8630 ft [2630 m] above sea level) is the topographically highest point on the monocline. Strata at Bald Peak are the Mississippian Madison Limestone, which near the crest dips shallowly at about 16 degrees to the east-southeast. Stratigraphically higher parts of the section dip progressively more steeply to the east as the monocline’s surface topographically descends eastward into the Bighorn Basin. Upon reaching the stratigraphic level of the Upper Cretaceous Meeteetse Formation, some of the strata are overturned (in Fig. 4, see cross sections C–D and E–F along with excerpted fig. 10 from Lillegraven ).
TECHNICAL ASPECTS OF THE GEOLOGIC MAP AND CROSS SECTIONS
This paper’s geologic map (Fig. 3, scale 1:12,000) of the study area was enlarged from the original 1:24,000 scale topographic base published in 1991. The doubled enlargement eased entry of new data, both on the map and in the geologic cross sections presented in Figure 4. Values for strikes and dips, accompanied by measurement numbers, are geographically specified in Appendix 1 with Universal Transverse Mercator (UTM) metric data (using the North American Datum of 1927 for the Continental United States [NAD 27 CONUS]). Parallel information for numerous non-attitudinal observations (recognizable by presentation of identifying italicized numbers enclosed by brackets) are pinpointed on the map using small red dots. Additional to boundaries for township, range, and section in Figure 3, kilometer-spaced UTM grids (in green) could allow users to readily introduce future data onto this map.
Six broad yellow lines across Figure 3 mark the locations of geologic cross sections, presented at the scale of 1:12,000 in Figure 4. Note that all faults shown on the map are interpreted as thrust faults, and all of them show apparent relative displacement in generally westward (including southwestward and northwestward), out-of-the-basin directions. Despite the apparent nature of westward convergence, more probably the active thrusting at depth below Dead Indian monocline was dominantly northeastward to eastward (note arrays of structural arrows in excerpted fig. 10 within Fig. 4). Although the symbol Jsm identifies ‘Sundance and Morrison Formations undifferentiated,’ nowhere within the study area has definitive Sundance Formation been recognized. Three localities also are shown (in red) from which May et al. (2013a, 2013b) gathered samples used in detrital-zircon dating.
Each cross section in Figure 4 uses the vertically scaled range of 4,000–5,200 feet (1,219–1,585 meters) above mean sea level, drafted with no vertical exaggeration. Most of the shown formational elements dip eastward or northeastward, reflecting influences of late-Laramide uplift of Bald Ridge (= Dead Indian monocline). Notice, however, that beginning within the Meeteetse Formation and proceeding stratigraphically upward, much of the bedding becomes essentially vertical to overturned. Initially west-directed, down-section thrust faulting placed younger strata upon older. The expected Fort Union Formation of Paleocene age (Lofgren et al., 2004) apparently was completely cut out of the outcrop assemblage by faulting, even though only a few miles to the east that rock unit is seen to be several-thousand feet thick (Lillegraven, 1993, fig. 4E, p. 428 and 2009, fig. 3 and p. 71–72).
The excerpted figures 10 and 11 from Lillegraven (2009) within Figure 4 present a cross-sectional model with much greater depth (base at ca. –14,000 ft [ca. –4.2 km] below mean sea level). Cross section A–A’ of the 2009 publication closely parallels the map placement of cross section E–F of the present paper. The excerpted ‘figures 10 and 11’ are presented here to illustrate: (1) how the Meeteetse Formation and younger strata became overturned; and (2) just why so much of the original Mesozoic and Paleogene stratigraphic section was lost through faulting and erosion along the new basin margin during tectonic uplift of Bald Ridge. The uplift of Bald Ridge was generally via northeasterly to easterly directed basement faulting within the deep Line Creek–Oregon Basin thrust system, and the originally overlying sedimentary sequence of the then-much-larger Bighorn Basin responded passively via out-of-the-basin faulting plus erosion, as illustrated in the model.
The excerpt of Lillegraven’s (2009) figure 11 within the present paper’s Figure 4 diagrammatically illustrates “… how little of original stratigraphic section has been retained following late-Laramide uplift.” The excerpt of Lillegraven’s (2009) ‘figure 32’ in Figure 4 interprets a much greater west–east extent of the Bighorn Basin (prior to late in the Early Eocene) than exists today. Although the three localities sampled by May et al. (2013a, 2013b) for detrital zircons are not represented directly on the mapped cross sections, they are centered on the basal Cody Shale (loc. 5 (30)), upper Mesaverde Formation (loc. 6 (31)), and basal Lance Formation (loc. 7 (32)). Physically, the sampled strata are closest to cross section C–D, and the approximate stratigraphic levels of sampling (as projected onto cross section C–D) are indicated by three red squares.
MISCELLANY RELEVANT TO LANDSCAPE DIRECTLY NORTH OF STUDY AREA
The northern boundary of the study area (Fig. 3) is roughly 1.3 mi (2.1 km) southeast of the northern end of Bald Ridge. As indicated on the geologic map by Pierce (1965b) and reinforced by Wise (1983, p. 77), the Phanerozoic stratigraphy seen on almost all of the Dead Indian monocline (i.e., Bald Ridge) continues the southward trend of strata as present along the southeastern corner of the Beartooth front.
Oddly enough, however, as shown in Figures 6 and 7, a short segment of the northern end of Bald Ridge is formed by a highly asymmetric structure known as the Canyon Mouth anticline. The orientation of that anticlinal hinge line (Davis and Reynolds, 1996, p. 384–387) differs strongly from the basic orientation of bedding within the Dead Indian monocline. According to Wise (1983, p. 78), the Canyon Mouth anticline plunges southeast
“… at increasingly steeper angles ranging from about 10 to 60 degrees … [and] its axial plane dips about 45 degrees to the southwest. Basic geometry demands that the strike of the axis must be N45W, parallel to the strike of the essentially vertical north limb of the fold. This strike contrasts markedly with the N10E strike of the adjacent Beartooth Front and the Dead Indian Monocline.”
The southeastern end of exposed parts of the Canyon Mouth anticline is less than a half mile (< 800 m) north of the area of present study. Although far less spectacular in appearance, all other folds recognized within the study area share the Canyon Mouth anticline’s features of having southeastern-plunging hinge lines and asymmetrically greater dips on their northeastern flanks.
Thus, the extraordinarily well-exposed nature of the Canyon Mouth anticline provides a sound model for interpretation of smaller, less well-exposed contractional features within the bounds of Figure 3. The line drawing by Wise (1983, fig. 3, p. 80) and the photograph by Boyd (1993, frontispiece, p. 164) label all formalized stratigraphic elements that compose the Canyon Mouth anticline. Wise proposed, as well, that a southerly splay of the Beartooth fault (which near the southeastern corner of the Beartooth block trends north-northeast–south-southwest) penetrates the core of the anticline and exhibits roughly 1,500 ft (457 m) of vertical separation (up on the northwestern side; see Foose et al., 1961, fig. 5, p. 1,157).
Figure 8 is intended principally to help users identify key stratigraphic units of the northern map area as seen from a distance.
GENERAL FEATURES OF THE LOCAL STRATIGRAPHIC COLUMN
Purpose of this Section
Recall that Appendix 2 provides formation-by-formation summaries of the litho- and biostratigraphic criteria by which each stratigraphic unit was identified. The present section, written in close conjunction with captions to the photographic figures, is intended to go beyond the scope of Appendix 2 with provision of a variety of additional items of interest about each formation in the study area. Most of those items reflect local aspects of the formation that would supplement or provide information not recorded in previously published maps or text. Also keep in mind that Appendix 1 provides added quantitative locality information for all attitudinal measurements as well as a diversity of non-attitudinal observations, numbered on Figures 3 and 4.
Gypsum Spring Formation and Older Rocks
The Middle Jurassic Gypsum Spring Formation is the stratigraphically lowest mapping unit in this project. Note that only a narrow band of the red mapping color along the western margin of Figure 3 actually represents the Gypsum Spring Formation.
The most common lithologic content of the Gypsum Spring Formation is thin-bedded, fine-grained sandstone. Figures 9, 10, and 12 provide distant views of this formation. From a paleontological point of view, this is the most profusely represented formation within the study area. The tan-colored, thin, sandy bedding planes are pelecypod-rich, or best referred to as fragmentary shelly to coquinoid, worm-burrowed, or generally bioturbated. Occasional bits of petrified wood exist, and uncommonly there exist bands of carbonaceous shale or even low-grade lignite.
Both intra- and inter-formational, out-of-the-basin (west-directed) minor faults are shown on Figure 3, and Figure 12 highlights a remarkably intact fault–fold complex. Because of the many excellent exposures and usual thin bedding of strata in the Gypsum Spring Formation, varying degrees of disharmonic deformation and chaotic fault gouge can be seen clearly.
Sundance and Morrison Formations Undifferentiated
The Upper Jurassic Morrison Formation is the most colorful stratigraphic unit within the study area (Fig. 3). Figures 9 and 10 show the formation in southern parts of the map, Figures 11 and 12 represent the middle area, and Figures 15–17 and 21 illustrate the formation’s nature in northern parts of the study area. Although the Morrison is highly variable in color from one outcrop to the next, the following stratigraphic sequence seems to hold true along the north–south transect of exposures within Figure 3:
uppermost reaches — highly variegated, mostly gray, tan, or reddish;
upper levels — barn red in fresh exposures;
lower levels — purple; and
basal levels — gray to tan.
Lithologically, the Morrison Formation is dominantly mudstone, although usually thin, fine- to medium-grained sandstone beds are common. Basal parts of the Morrison section just north of Newmeyer Creek are shot through with soft-pebble conglomerates (composed of detrital-mudstone clasts, usually less than an inch in diameter).
Puzzlingly, even though several sections of the Morrison Formation are well exposed along their basal strata, there exists no sign of the expected Sundance Formation between the Morrison and the Gypsum Spring Formations. Innumerable minor faults show up clearly through their effects upon beds of sandstone (e.g., see Fig. 15). Fragmentary dinosaur bones exist at multiple outcrops and at differing stratigraphic levels.
Outcrops of the Lower Cretaceous Cloverly Formation provide the most laterally extensive and dependably complete array of ridge-formers within the area of study (Fig. 3). The formation’s dominant lithology is nonmarine, strongly indurated, fine-grained, commonly rippled sandstone. Almost universally, those sands are affected or deformed by a wide and interesting diversity of joints, kink bands, and spaced complexes of step faults or shatter zones, regularly accompanied by robust slickensides. Traditional kinds of macrofossils are limited in the Cloverly.
The following graphics provide a solid overview of the general nature of occurrences of the formation: Figures 3–4, 6, 8–11, 13–17, 19, 21, 23, and 28A. As emphasized in the Cloverly section of Appendix 2, this strongly cemented formation provides a useful stratigraphic place-keeper for those engaged in field mapping across landscapes of otherwise poorly exposed rock.
In southern parts of the study area (i.e., Fig. 3, sec. 33), most measurable outcrops of the Cloverly Formation are limited to sandstones and siltstones within the stratigraphically upper third of that unit’s mapped extent. Even the strongest bedding planes within the Cloverly, however, tend to be markedly offset and sometimes folded. Perhaps in explanation of the deformation, the sandstone bodies in basal parts of the overlying Thermopolis Shale probably have been displaced westward, and at least locally they have physically overridden upper strata of the Cloverly Formation. The general sense of faulting direction was just south of west, out-of-the-basin.
More powerful evidence for deformation of the Cloverly Formation via out-of-the basin thrusting exists in the western half of the SE1/4 of section 33 (see Fig. 3 and cross sect. K–L of Fig. 4). The west–southwest-directed thrust fault shown there is a miniature version of the geographically far more extensive process recognized in the northeastern corner of south-central Wyoming’s Hanna Basin by Lillegraven et al. (2004, figs. 4 [parts B and C], 17, and 18). Although referred to informally here as a ‘great wall’ (see Appendix 1, meas. nos. 4748–4751, incl. ‘’), the model structure in the Hanna Basin holds the name ‘Dragonfly fault’ (Lillegraven, 2015, cover image plus caption, p. 30). The amount of throw achieved along this intraformational, out-of-the-basin Cloverly thrust is unknown.
Farther north, in the center of section 28, stratigraphically lower parts of the Cloverly Formation serve as the hanging wall for a short, west-directed thrust fault upon the Morrison Formation in the footwall (Fig. 3, cross sect. G–H of Fig. 4, and Fig. 11). That thrust fault allowed development of a highly asymmetrical anticline. Closely following the course of Figure 3’s yellow line indicating alignment of Figure 4’s G–H cross section, a highly asymmetrical anticlinal fold plunges east–southeastward at about 40º. The northeastern flank of that anticline dips up to 85° (measured perpendicular to the anticlinal hinge line), whereas the dip of the southwestern flank nearly matches the course of the anticline’s plunge. Although not shown on Figure 3 (due to inadequate space to draft graphical details), the lower strata of the Cloverly Formation’s contribution to the anticline are separated into at least three hanging wall blocks. Note in Figure 3 that nearby attitudinal measurements in the Thermopolis Shale and Mowry Shale dip to the southeast, whereas almost all other dips within the northern half of section 28 dip to the northeast. That distinction may well reflect impact at depth of the southeast-plunging anticline.
As true for the Cloverly Formation in section 33, its outcrops in section 21 are restricted to near the stratigraphic top of the formation (Figs. 3 and 14). The most northerly available attitudinal measurement (no. 8213) exhibits sedimentary rocks that hold fossil-plant debris and were heavily worm-burrowed prior to lithification.
The marine Lower Cretaceous Thermopolis Shale is very poorly exposed through most of the present study area. In Figures 13 and 14, for example, the landscape facing the camera (i.e., stratigraphically above the Cloverly Fm.) illustrates the most basic of problems—a near absence of usable outcrops.
With exception of section 21 (Fig. 3 and cross sect. A–B in Fig. 4), much of the mapping of the Thermopolis Shale has amounted to the following process. First one would plot the extent of sagebrush that occurs stratigraphically above the strongly indurated Cloverly Formation. Then one would stratigraphically do the opposite action by plotting the extent of sagebrush occurring below the compact and reliably fossil-fish-bearing Mowry Shale. Literally, simple faith would then tell the struggling field geologist that the intervening, sagebrush-covered landscape must hold the Thermopolis Shale. More substantively, the dominant lithology of the Thermopolis Shale through the southern two-thirds of the study area (i.e., in sec. 33 and across most of sec. 28) is soft clay shale, usually with a concentration of comparatively tough sand bodies in the stratigraphically lower bedding. Nevertheless, at least in southern parts of the study area (i.e., in sec. 33), there does exist (as discussed above in relation to the Cloverly Fm.) adequate evidence for out-of-the-basin, west-directed thrust faulting that brought the Thermopolis Shale to partly override the Cloverly Formation.
In marked contrast to the realistically difficult situation described in the preceding paragraph, the quality of exposure of strata in the Thermopolis Shale in the northern third of section 28 and across section 21 (Fig. 3) dramatically improves (see Figs. 15–19, 21, 25, and 28A). Those northerly strata, however, crop out exclusively on steep slopes and along a precipitous ridge, thus presenting investigative hazard. Also note in the photos that those northerly outcrops are in large part constructed of well-indurated sandstone beds. Although this is only conjecture, it may be that the striking reduction in the Thermopolis Shale’s stratigraphic thickness through section 28 (see Fig. 3) was due to northwardly progressive loss of the formation through faulting of nearly all of the original upper, more shale-rich beds.
Probably the most important local aspect of the Thermopolis Shale is within the north-central edge of section 21 (Fig. 3 and cross sect. A–B of Fig. 4). The generally thin nearby outcrops of that formation abruptly widen their appearance to the northeast in map view by way of at least three steps of west-directed thrust faults. That sequence of faults acted to multiply repeat most (if not all) correlative outcrops of the Thermopolis Shale seen just to the southwest (Fig. 19). Tectonism of uncertain nature also seems to have significantly deformed the poorly exposed Mowry Shale immediately south of the fault-widened outcrop area of the Thermopolis Shale. Furthermore, similar fault-based repetitions of outcrops were generated within the Frontier Formation just to the southeast (Fig. 22). It seems reasonable to suggest that all of these disturbances shared a common cause of W–E- or, more probably, SW–NE-shortening very late in the Laramide orogeny.
The marine, Lower Cretaceous Mowry Shale is a readily recognizable rock unit within the area of study (Fig. 3; Appendix 2). The Mowry Shale is lithologically sturdy enough that it almost always forms outcrops, if ever so small, even within landscapes otherwise dominated by vegetation (e.g., Fig. 13). The following graphics provide geographic and lithologic context for that formation within the area of current interest: Figures 3–4, 8, 13, and 15–25. The Mowry Shale reliably is to be seen overlying the Thermopolis Shale or sometimes almost appressed to the Cloverly Formation.
Although the summary for the Mowry Shale in Appendix 2 was developed using locally observed characteristics, almost everything presented there also holds true across the full north–south extent of Wyoming and into adjacent states. Nevertheless, confusion is possible as derived from the Frontier Formation in that both units are dominated by tan or gray to jet black, extremely fine-grained sedimentary rocks. The Mowry, however, typically involves highly fissile shale, whereas most of the Frontier generally exhibits more homogeneous mudstone facies. The Frontier Formation conformably overlies the Mowry Shale, so an interval of uncertainty invariably exists when attempting to mark a definitive contact.
Also, even though both the Mowry Shale and Frontier Formation may exhibit bands of bentonite, the levels of volcanic contribution and resulting weathered bentonitic strata are much greater in the Frontier. Related to that air-fall volcanic input, both the Mowry Shale and the Frontier Formation tend to deform by impressively malleable bending. The siliceous Mowry Shale, however, is more rigid (e.g., much like a cooled stick of butter), and it has higher probability of deforming via sharply defined, small kinks and faults. The Mowry Shale also contains more regularly occurring strata of siltstone and/or fine-grained sandstone, whereas mudstone is truly dominant in the local Frontier Formation. Although this is exaggeration, the Frontier tends to deform more like toothpaste (e.g., see Fig. 22). Very probably representing a local phenomenon, both the uppermost Mowry Shale and basal Frontier Formation of the study area commonly are slickensided and replete with cone-in-cone structures (Fig. 20).
Aside from its reliably expected stratigraphic position, the most important identifying feature of the Mowry Shale is paleontological. Almost universally, it exhibits fossilized fish remains (especially disarticulated vertebrae, ribs, cranial and mandibular elements, teeth, body scales, and coprolites) as three-dimensional objects located between adjacent, extraordinarily thin layers of the shale. Importantly, the natural hard-parts (i.e., bones, teeth, and bony scales) and even soft elements (originally fecal masses) occur as undistorted, three-dimensional objects that commonly deform multiple, exceedingly thin layers of shale both below and above the between-layered, disarticulated death-positions of what are now fossils. That characterization is opposed to two-dimensional impressions of biological materials upon the surfaces of individual layers of shale.
The style and astonishingly high quality of fossilization exhibited within the Mowry Shale required a most unusual, geographically widespread, persistent, and super-hospitable chemical environment in the continental-interior seaway to allow such perfection in preservation. My own observations of most bony fragments in the local Mowry Shale were gained from fishes of tiny body size. Nevertheless, I can report uncommon discoveries of vastly larger body parts that document the contemporary existence—albeit of much more uncommon preservation—of huge fishes within the present area of study.
Useful outcrops of the Frontier Formation within the study area (Fig. 3) are essentially restricted to: (1) south of the southeastern bay of Hogan Reservoir (cross sect. K–L of Fig. 4); and (2) the northern half of section 21 (cross sect. C–D of Fig. 4). The local Frontier Formation holds, by far, the greatest concentrations of volcanic air-fall materials existing among the array of rock units within the study area. Black mudstones dominate the lithology, and a remarkably thick surface veneer exhibits ‘popcorn weathering’ typical of bentonite-rich, fine-grained strata.
Siltstone and sandstone beds are minor, and they are mostly seen near the Mowry–Frontier and Frontier–Cody Shale formational transitions. Most of the sandy beds are limited to very fine-grained facies. Uniquely, however, stringers of coarse-grained sandstones do exist south of Hogan Reservoir along with a quite unexpected bed of quartzite-pebble conglomerate. Those coarse-clastic stringers (meas. no. 8270 of Appendix 1) are otherwise embedded within the typical bentonitic mudstones characteristic of the local Frontier Formation. The section in Appendix 2 dedicated to the Frontier Formation along with the discussion of Mowry–Frontier comparisons in the preceding text provide additional lithologic information. Figures 3 and 4 give map and cross-sectional views of several parallel, southeast-plunging anticlinal and synclinal folds, and the following photographs are intended to help in grasping their geographic distribution and surficial impacts: Figures 8, 16–18, and 21–29.
The following graphics provide geographic and geologic context for the marine, Upper Cretaceous Cody Shale: Figures 3–4, 8, 18, 21, and 23–29. The Cody Shale is conformable upon the Frontier, so again the contact between the two formations is somewhat arbitrary. In comparison with the underlying Frontier Formation, however, the Cody Shale has markedly reduced levels of black mudstone, it is obviously less bentonitic, and it is richer in sandstones (generally thin-bedded and all are very fine-grained) and siltstones. The dominant lithology is shale. Some of the shale layers within the Cody are silvery, and a small proportion yield fossilized disarticulated fish parts, thus potentially leading to formational misidentification as the Mowry Shale. However, existing knowledge about much lower parts of the stratigraphic ordering ordinarily would readily eliminate that hazard.
The Cody Shale for the most part underwent less deformation than the Frontier Formation. The local Cody does, however, host a small but strongly developed, southeast-plunging syncline in the SW1/4 of the NE1/4 of section 21. Only the northeastern flank of that syncline is cut through by a series of small faults, thus introducing asymmetry to the fold. As is also the case for the Frontier Formation, the present-day erosional surface of the Cody Shale (again near the center of the NE1/4 of sec. 21) holds five large blocks of granite, two of which are marked with red triangles adjacent to the letter ‘G’ on Figure 3 and on cross section A–B of Figure 4.
The stratigraphically lowest of the three collecting sites for detrital zircons developed by May et al. (2013a) was originally given the locality number of ‘5,’ and it was judged by them to have sampled the Mowry Shale. The physical setting of the site is shown in the present paper in Figures 3, 4 (cross sect. C–D), 21, and 25–28A. A month after their first publication on the subject, May et al. (2013b) provided the same site with a new locality number (‘30’) and declared that it sampled the Frontier Formation, thus shifted from the Mowry Shale. As shown in Table 1 and the various figures, locality 5 (30) is considered in the present study to have sampled strata low in the Cody Shale.
The type sections for the Mesaverde Formation and Lewis Shale were established during the late 19th Century in the San Juan Basin of southwestern Colorado. Since then, those same names have been applied to Upper Cretaceous strata across the state of Wyoming. As discussed by Lillegraven and McKenna (1986, p. 9), however, “… use of the names Mesaverde Formation and Lewis Shale for strata in Wyoming represents a long-standing, although generally understood, error.” Wyoming strata bearing those names are now recognized to be significantly younger than rocks in their type areas in Colorado, and the sedimentary rocks represent wholly different transgressive–regressive sequences that occurred within the Western Interior Seaway. For specifics, using figure 6B in Lillegraven and Ostresh (1990, p. 15), compare polygon ‘3’ (type Lewis Shale) with polygons ‘8’ and ‘9’ (transgression and regression, respectively, of Lewis Shale as recognized in Wyoming and northern Colorado). Despite these issues of miscorrelation, consistent use of the terms ‘Mesaverde Formation’ and ‘Lewis Shale’ across the breadth of Wyoming has been general, even by the U.S. Geological Survey and Wyoming State Geological Survey. Thus, the present paper also will help perpetuate a generally understood error through continued use of the stratigraphically and temporally misleading term ‘Mesaverde Formation.’
Exposures of the shallow marine, Upper Cretaceous Mesaverde Formation within the study area are limited to the northern half of the geologic map (Fig. 3 and cross sects. C–D and E–F of Fig. 4). Lithologically, those outcrops are extensively slickensided, thin- to medium-bedded, gray to brown, fine-grained sandstone and lesser beds of siltstone. Figure 32 provides particularly good representation of the local Mesaverde’s clastic nature. Other photos showing the formation within the study area include: Figures 8, 21, 23–27, 29–31, and 33. Stratigraphic equivalents among those beds usually are readily traceable back and forth along the outcrop pattern, and almost all of the attitudinal measurements gained in the present research are restricted to within the stratigraphic top third of outcrops. Even though much more restricted in thickness, the strata within the study area closely match nearby, reliably mapped occurrences of the Mesaverde Formation (e.g., Lillegraven, 2009, figs. 12 and 13).
As shown on the geologic map (Fig. 3), the northern half of the outcrop pattern for the Mesaverde Formation is significantly narrower than its more southerly parts. Also, the northern parts descend northward down a rather precipitous ridge, and virtually the entirety of that ridge exhibits all manner of semi-chaotic deformation. Probably the sharp thinning of the Mesaverde’s outcrop pattern is due to not-yet-understood tectonic causes. North of measurement 4730 ( Appendix 1), near the center of the eastern half of section 21, the Mesaverde Formation becomes impossible to follow. That is because of sheetwash cover derived from the more westerly, adjacent Cody Shale. Within that occluding debris, however, automobile-sized blocks of clearly detached Mesaverde sandstones exist, suggesting a history of jointing or faulting otherwise hidden by Quaternary alluvium.
The local Mesaverde Formation also shows effects of intersections with two quite separate fold systems. The more northerly fold is a southeastern continuation of the syncline that was mentioned above in the Cody Shale (Fig. 28). Measurements 5461 and 5462 ( Appendix 1), both taken in the overlying, basal levels of the Meeteetse Formation, pinpoint the hinge line of that syncline and show exactly where its southeast-directed axis crosses the Mesaverde Formation. Development of the previously described, asymmetric northeastern flank of that syncline appears to have been linked to a southwest-directed thrust fault adjacent to the syncline. In any case, the syncline and adjacent thrust fault caused major distortion (including a miniature, nearly vertical anticline at observation ) at its intersection with the Mesaverde Formation.
At the uppermost crest of the ridge near the center of the SE1/4 of section 21 (Figs. 3 and 32) exists a parallel, southeast-plunging anticline and syncline set. These are extraordinarily tight folds. The hinge line of the anticline is just north of measurement 5464 ( Appendix 1), and the hinge line of the syncline is pinpointed between measurements 5461 and 5462 (both taken in the Meeteetse Fm.). Both of those folds disappear to the southeast within the central mass of the Meeteetse Formation, and strata of the Meeteetse along the northeastern flank of the anticline are overturned.
The second relevant collecting locality for detrital zircons as used by May et al. (2013a) was originally given the locality number of ‘6’ and was said by the authors to be within the Mowry Shale. In the present paper, Figures 3, 4 (cross sect. C–D), 21, 25, and 27 provide information about the physical location of the sampling site. In a closely following second paper, May et al. (2013b) gave that site a new locality number of ‘31.’ And in that same publication they shifted identification of the formational host from Mowry Shale to the Frontier Formation. As shown in Table 1, locality 6 (31) is considered within the present study to be in the northernmost unburied strata of the Mesaverde Formation.
After having viewed the exposures, in all probability most visitors to this study area would rank the local, lithologically soft and readily molded Upper Cretaceous Meeteetse Formation as the most interesting of the 11 mapped rock units. Of the 15 structural measurements made in the Meeteetse, 10 of the bedding planes were overturned, usually requiring measurements to be effected on undersides of the strata. Notice in cross sections C–D and E–F of Figure 4 (as well as the excerpted fig. 10 from Lillegraven, 2009 in the present paper’s Fig. 4) that the Meeteetse Formation serves as a transition between significantly eastward-dipping strata coming off the eastern flank of Bald Ridge and consistently overturned strata east of the base of Bald Ridge. An interpretive explanation of that relationship is provided in the caption to the excerpted figure 10 (from Lillegraven, 2009).
At least two anticlinal/synclinal-fold pairs terminate by plunging steeply southeastward into the structurally chaotic mass of the Meeteetse Formation. Extraordinarily tight folds, distinct faults, slickensided sandstones, and cone-in-cone structures exist throughout the formation’s thickness. Structurally chaotic bedding abounds, and individual stratigraphic sequences unexpectedly terminate sharply or become duplicated. One can gain a sound impression of the nature of the local Meeteetse Formation from the following graphics: Figure 3; cross sections C–D and E–F of Figure 4; and Figures 8, 25–27, 29–31, and 33–38.
Sedimentary rocks of the Meeteetse Formation are almost universally weakly cemented and compositionally diverse. The generally soft strata vary from usually thin lignite beds to unstructured black mudstone to coarse-grained arkose, and often they are shot through with small, well-rounded metamorphic pebbles. Almost the entire formation commonly has dense concentrations of disseminated particles of lignite. Stringers of small cobbles do exist, but they are uncommon. Many of the sandstone beds at multiple levels are very coarse-grained and, with only a few exceptions in underlying formations, are the oldest rock unit seen within the study area to commonly hold coarse-grained sands.
The general paleoenvironmental setting of the Meeteetse Formation is estuarine along with low wetlands adjacent to the greatly varying positions of the Western Interior Seaway’s edge. The Meeteetse Formation is temporally correlative with the Wyoming version of the Lewis Shale as seen farther to the south. The Meeteetse Formation and Lewis Shale interleave broadly with one another across the vastness of Wyoming’s Upper Cretaceous Western Interior plain, so consistently recognizing firm lithologically correlative boundaries between these two constantly shifting formations is virtually impossible. Reliable assignment of geologic age for the local Meeteetse Formation has been elusive, but most probably it would fit somewhere between zones 27 (Baculites baculus) and 30 (Sphenodiscus) on the scale based upon ammonite biostratigraphy (see column for 45° N latitude in Lillegraven and Ostresh, 1990, fig. 6B).
Paleontological field exploration of the Meeteetse Formation as exposed in the study area has been minor. Nevertheless, the limited outcrops of that formation (Fig. 3) are highly deserving of careful prospecting. Worm burrows abound in several different facies of fine-grained strata, and giant oyster shells along with other pelecypods, gastropods, and coquinoid shelly-bits have been located but not collected. What appear to be fragments of a dinosaur limb bone also were discovered but not collected. No known close prospecting for remains of mammalian or other small vertebrates has been conducted.
The following graphical elements will help explain the degree of uncertainty about the lithologic nature of what is identified here as the Upper Cretaceous Lance Formation: Figure 3; cross sections C–D and E–F of Figure 4; and Figures 8, 29–31, 35–38, and 40-41. The landscape now identified as representing Lance Formation has not been justified on the basis of uniquely diagnostic lithologic or paleontological features. Indeed, all four of the overturned attitudinal measurements were made on lithologically common, tan or gray, generally thin-bedded, fine-grained sandstones surrounded by mudstones. Their outcrops were, however, less carbonaceous than the underlying Meeteetse Formation, and the strata appear to have been deposited under fluvial settings.
Thorough mapping by Pierce (1965a and 1966) adjacent to this study area used the term Lance Formation. That name was derived from the vicinity of Lance Creek in Wyoming’s southeastern Powder River Basin. In the present area of study, the term was intended to apply to latest Cretaceous, fluvial strata overlying the Meeteetse Formation and stratigraphically underlying the Paleocene Fort Union Formation. As pointed out in the caption to Figure 36, the landscape containing the presently recognized Lance Formation provides very limited outcrops. Nevertheless, in the absence of sound evidence that would justify some other action, I propose that the present map (i.e., Fig. 3) is the first to provide recognition of the Lance Formation, even if only tentatively so, within any area covered by the Deep Lake geologic quadrangle.
Basal parts of the presently recognized Lance Formation also hold collecting-locality number 7 as initially recognized by May et al. (2013a) in their search for datable detrital zircons. In that publication, they identified the strata yielding the site to be part of the Frontier Formation. In their follow-up paper, May et al. (2013b) re-numbered locality 7 to become number 32 but retained its formational identification as Frontier Formation. May et al.’s collecting-locality 7 (32; see Table 1) unquestionably is stratigraphically the highest of their three collecting sites within the present study area. Graphics showing the position of locality 7 (32) include: Figures 3–4 (cross sect. C–D), 29, 31, and 36.
Fort Union Formation
The locally nonmarine Fort Union Formation of Paleocene age almost certainly was deposited across the entirety of the present area of study (i.e., Fig. 3). Nevertheless, today it exists nowhere within that landscape, either as outcrops or in the known subsurface. Indeed, no record exists of exposures of the Fort Union Formation in Wyoming to the northwest of the northwestern flank of Heart Mountain (Lillegraven, 2009, p. 71–72). That absence from the study area typically comes as a surprise in light of the fact that a mere 7 miles (11.3 km) to the southeast of the study area the Fort Union Formation is at least 4,000 ft (1,219 m) thick (Pierce, 1966, cross section A–A’).
The excerpts of figures 10 and 11 from Lillegraven (2009) as presented in this paper’s Figure 4 suggest an explanation for the Fort Union’s absence in the study area. Both figures propose that a major, west-directed, out-of-the-basin thrust fault cut down-section and thereby served to place younger strata (e.g., Willwood Fm.) onto older strata (i.e., Lance Fm.). That extensive process of stratigraphic thinning thus completely eliminated preservation of the Fort Union Formation ahead of the thrust’s plane, because its mass was uplifted to the land’s surface and gradually eroded away. The 2009 paper also recognized 18 additional examples (via its table 1) along the northwestern edge of the Bighorn Basin of west-directed, out-of-the-basin thrust faults that cut down-section and thereby placed younger onto older strata. Interestingly, those examples involved a variety of quite different stratigraphic horizons in their hanging walls and footwalls. That is, this brand of faulting functioned across most, if not all, of the existing stratigraphic column. The cumulative effect of completed disturbance by way of those faults was to severely thin the total stratigraphic section from what still persists today only a few miles east of the present study area. It was noted above in discussions of the individual formations within the area covered by Figure 3 that most are markedly thinner today than are their stratigraphic correlates only a little farther to the east.
It is important to keep in mind that all of these west-directed thrust faults took place at depths, minimally a mile below the Middle Eocene land surface. As emphasized in the excerpted figure 11 in the present paper’s Figure 4, the extent of erosion during such tectonically active intervals of the Laramide orogeny was prodigious. Indeed, that model emphasizes how little of the total original stratigraphic section was preserved after the late-Laramide interval of active tectonism and coeval erosion.
In respect to exposed section, the mostly Lower Eocene Willwood Formation is the most intensively studied rock unit in the Bighorn Basin. The compendium edited by Gingerich (2001) provides useful summaries of the formation’s depositional and biostratigraphic groundwork that has been accomplished across the basin through the past century.
The typically red-, purple-, yellow-brown-, and white-banded Willwood Formation is a genuine caprock in a physical as well as a conceptual sense. Physically, the formation is a caprock in that, except for Tertiary volcanic sequences restricted to west of Bald Ridge, there exists a great hiatus in the geologic record that began at the top of the Willwood Formation. At least anywhere near the vicinity of this study, a profound gap in the local record represents at least 40 million years (m.y.) of Wyoming history. Specifically, that hiatus spans the interval from late in the early Eocene until the Quaternary.
The Willwood Formation also is a conceptual caprock that can be understood following geographic analysis of the post-depositional deformation of its strata. As summarized in the following two paragraphs, geographic trends in the nature and magnitude of deformation within the Willwood Formation along the northwestern margin of the Bighorn Basin show clearly that strongly contractional tectonism, characteristic of the Laramide orogeny more generally, persisted at least into the end of the formation’s deposition.
Particularly instructive in this discussion would be comparisons of the Willwood Formation’s post-depositional deformation along an essentially east-to-west transect that runs from the southwestern quarter of T. 56 N., R.101 W. (see coordinate system in Fig. 2) into the presently mapped area of Figure 3 (and, more specifically, terminating at the outcrop shown in Fig. 40). To aid in intelligibility, three specific areas are designated here that roughly follow that desired east-to-west alignment of deformed Willwood Formation:
Area 1. ‘Eastern.’ See Pierce (1965a), Clark geologic quadrangle, east of the southern part of Chapman Bench (which supports a segment east of Wyoming State Highway 120) in the NE1/4 of section 31, T. 56 N., R. 101 W.;
Area 2. ‘Intermediate.’ See Pierce (1965a), Clark geologic quadrangle, west of the southernmost part of Chapman Bench, west of Wyoming State Highway 120, in all of section 2 and eastern edges of section 3, T. 55 N., R. 103 W.; and
The comparative orientations of bedding reported for today’s Willwood Formation as drawn from those three sampling areas are as follow:
Area 1. Strike is to the northwest, dip is 3° to the southwest (other measurements in nearby sections are comparable);
Area 2. Strike is universally to the northwest (8 measurements considered), dip is variable, but mostly more than 30° (and up to 70°) to the northeast; and
Area 3. Strike also is universally to the northwest, and dip (proximal to contact with Lance Fm.) is variable, from vertical to overturned (stratigraphic ‘up’ was to the northeast during earlier stages of deformation).
In area 1, even though it is less than a quarter of the distance from west to east across the present-day Bighorn Basin, the Willwood Formation is essentially flat lying. Thereby, it emulates a structural attitude expected within a ‘central basin.’ All three areas share strikes in common to the northwest, but areas 2 and 3 differ from area 1 in consistently expressing dips to the northeast. That statement includes the overturned attitudes, all of which in earlier stages of deformation began with a status of stratigraphic ‘up’ being to the northeast. Dips from all three areas reflect differing magnitudes of post-depositional deformation of the Willwood Formation. Also, the specified transect from east to west records westward, progressively intense deformation. That kind of trend would be fully expected along any newly evolving northwestern edge of the Bighorn Basin. In this particular case, the trend in greater deformation westward was being established as Bald Ridge was uplifted via basement-involved faulting at depth. The essential point now being made, however, is the conceptual surprise that such important elements of the evolution of the basin’s northwestern edge would have occurred so late in the Laramide orogeny. Essentially, that tectonic evolution occurred at the temporal brink of termination of the entire orogeny (probably Bridgerian North American Land Mammal Age, roughly mid-Eocene; Robinson et al., 2004).
Scope and Scale
As emphasized in the pair of introductory reference maps (Figs. 1 and 2) combined with the new geologic map (Fig. 3), the formal study area represents only a tiny edge of northwestern Wyoming’s Bighorn Basin. Less than 5 square miles (ca. 13 km2) are involved. Nevertheless, when the bulk of Bald Ridge is added to the picture, representatives of nearly all expected Phanerozoic rock units are included, ranging from the Cambrian Flathead Sandstone to the Lower Eocene Willwood Formation. More important are changes in structural relationships that occurred within the local formational assemblage. Prior to the middle of Eocene time, the present area of study was positioned in a ‘mid-basinal’ depositional and structural setting, not in a basin-margin situation as is the case today. Prior to Eocene time, the east–west breadth of the Bighorn Basin was essentially twice that of today.
As stated in the present paper’s ‘Introduction,’ this is a follow-up, more detailed investigation of just one area reported on by me in 2009. The initial purpose of the present publication was simply to improve the accuracy and thoroughness of that prior mapping. The new map is constructed at the formational level, beginning with the Middle Jurassic sequence on the eastern base of Bald Ridge (= Dead Indian monocline). The effort concluded within the lower Eocene strata roughly a mile (ca. 1.6 km) to the east. Six new geologic cross sections (each extending from 4,000 to 5,200 ft [1,219–1,581 m] above sea level) were developed.
Formational and Evolutionary Approach
Although the effort was not entirely satisfactory, I attempted to summarize diagnostic physical and paleobiological characteristics of each formation in Appendix 2. Also, as can be seen on the 36 photographic figures, I placed emphasis on defining the approximate stratigraphic boundaries between successive formations as they appear in the field. The Lower Cretaceous Thermopolis Shale and uppermost Cretaceous Lance Formation both exhibit very limited exposures within the study area. Neither the Upper Cretaceous Meeteetse Formation nor the Lance Formation had been previously recognized within the designated study area. Also, the present study recognized no evidence within the area of study for expected outcrops (or existence at all) of the Upper Jurassic Sundance Formation or the Paleocene Fort Union Formation.
The new geologic map (Fig. 3) and cross sections (Fig. 4) were intentionally scaled at 1:12,000 to allow rendering of greater detail than would have been possible by using the original Bald Peak topographic quadrangle’s scaling at 1:24,000. To aid in verifiability by future workers, all field measurements and documentation of special observations are provided in Appendix 1, using North American Datum of 1927 (NAD27 CONUS), metric Universal Transverse Mercator, Zone 12 data in aid of relocation. The digital format underlying all figures as presented here allows on-screen viewing at any scale. The URLs presented at the end of the References Cited section facilitate hard-copy printouts of Figures 3 and 4 at the default scale of 1:12,000.
Almost all mapped boundaries between mountains and basins as shown in Figures 1 and 2 stem from tectonic uplift and/or deposition linked to subsidence during the course of the Laramide orogeny. On the left side of Figure 2, almost all of the purplish (presently montane) areas prior to the early Eocene constituted a western half of the Bighorn Basin. Today’s study area is shown in Figure 2 by the small red rectangle just to the right of Bald Ridge. The white rectangle in Figure 5 shows northern elements of the study area. Both graphical sources show that today’s study area is genuinely on the boundary between mountains developed during the orogeny and newly restricted elements of an originally much more extensive (to the west) Bighorn Basin.
Relative lowlands along eastern flanks of Bald Ridge (= Dead Indian monocline) and the Beartooth Mountains hold much in common in terms of Laramide geologic history. For example, the north–south oriented yellow band in Figure 2 that connects basinal edges adjacent to eastern flanks of those two uplifts is intended to represent an originally uninterrupted, correlative sequence of stratigraphic development. With that in mind, the northeastern edge of the Deep Lake geologic quadrangle (Pierce, 1965b) should be studied closely. It clearly shows that the area east of the Beartooth block underwent major Laramide folding and faulting, secondarily complicated by prodigious burial and secondary erosion within late Cenozoic erosional debris derived from mountains just to the west. That combination, seen north of Clarks Fork of Yellowstone River, has resulted in a much more complicated picture than exists in less deformed strata of the present study, south of the river.
Thus, despite the small field area involved in the present study, it presents comparatively coherent stratigraphic configurations, especially as viewed across section 21 of Figure 3. The present study area (paralleling the north end of the Cody Arch) can now be thought of as a conceptually useful link between the stratigraphic section east of the Beartooth block and the geologic setting north of the city of Cody.
Stratigraphic linkages immediately to the east of the present study are much more difficult to grasp. With that in mind, I have included on Figure 4 of the present paper the excerpt of figure 10 from Lillegraven (2009). The pervasive fault systems within that cross-section A–A’ may help in understanding stratigraphic relationships with the main Bighorn Basin just to the east of that cross section’s limits.
Components of Basinal Restriction
By what mechanisms did the originally much wider, pre-Eocene Bighorn Basin lose its western half? As alluded to in the first section of this paper’s Introduction, the relatively coherent stratigraphy in the area of present research provides an extraordinary opportunity for the study of deformational interaction between two components of the Laramide structural story. The complementary pair of considerations include the: (1) tectonically active eastward thrusting and uplift of Precambrian rocks deep below the Dead Indian monocline; and (2) resulting, relatively passive deformation of Phanerozoic strata at shallower depths from effects of the uplifted and eastwardly translated monocline. Both components of the model are shown within Figure 4, as excerpted figures 10 and 11 from Lillegraven (2009).
The active component involved tectonism along the Line Creek–Oregon Basin thrust system, initiated deep within the crustal basement. The resulting (i.e., more passive) deformation involved almost the entire Phanerozoic stratigraphic column within the eastern (right-hand) half of excerpted figures 10 and 11 (from Lillegraven, 2009). Notice that almost all of the action via faulting occurred beneath a sedimentary blanket, minimally involving a mile in thickness. Usually, however, the thickness of the overburden was much more than that, probably exceeding 2.5 miles (4 km) at the extreme end.
Also note that all displacement related to the actively induced faulting within the more easterly mass of basinal sedimentary rocks was generally to the west, out-of-the-basin. That westward displacement would have been expected because the active part of the deformation by faulting within the Line Creek–Oregon Basin thrust system was directed eastward.
Finally, note that most of the faulting within the more passive side of the system (i.e., within the eastern elements) was along general bedding planes until eventually they broke through, directed down-section. Even though that resultant faulting was ‘down-section’ stratigraphically, it could only have happened after the affected strata were tilted upward relative to Earth’s local surface. The results in this case were the steeply east-dipping (or overturned beyond east-dipping) strata that we see at the surface today. Also, because of the prevalence of stratigraphically down-section, out-of-the-basin faulting, there have been numerous major reductions within original total stratigraphic thicknesses.
Magnitude of Stratigraphic Losses During Uplift of Bald Ridge
How much of the original stratigraphic thickness has been lost from the Bald Ridge section as derived from Laramide faulting? The original thickness of the sedimentary pile (counting from the lowest existing level of Jurassic strata into lower levels of the Willwood Fm.) in the central Bighorn Basin prior to being cut in half is estimated to have been about 18,260 feet (ca. 5,565 m); values of individual formational thicknesses were taken from Lillegraven (2009, fig. 3) and combined here as an element of Figure 4. Measuring from the present paper’s cross section E–F in Figure 4, that same total correlative pile (i.e., base of Gypsum Spring Fm. upward into Willwood Fm.) is only about 4,495 feet (ca. 1,370 m).
Using those approximated comparative totals, one can say that the basin-margin sequence of today in Figure 4’s cross section has only about one-fourth the thickness of its nearby basinal correlative precursor just to the east prior to uplift of Bald Ridge very late in the Laramide orogeny. I suggest most of that loss of section resulted from effects of down-section, out-of-the-basin thrust faults as shown in Figure 4, excerpted figures 10 and 11 from Lillegraven (2009). The 2009 reconstruction in figure 11 suggests why so little of the stratigraphic section has been retained following late-Laramide uplift of Bald Ridge (= Dead Indian monocline). For another analysis of a conceptually related (but geographically separated situation), see the section titled ‘Generalizations About Stratigraphic Separations Along Breaks Fault System’ in Lillegraven et al. (2004), pages 37–43, including figures 17A–D and 18.
Significance of Canyon Mouth Anticline and its More Southerly Mimics
The aptly named ‘Canyon Mouth anticline’ is a well-described fold that forms part of the northern extreme of Bald Ridge (= Dead Indian monocline). The anticline is set south of Clarks Fork of Yellowstone River, a short distance downstream from the point at which the river bursts into the Bighorn Basin. The anticline is highly asymmetrical, having a nearly vertical northeastern flank and with a much more moderate dip to the southwest on its southwestern flank. The hinge line of the fold plunges steeply into the western edge of the Bighorn Basin, and the overall vector of its hinge line courses to the southeast. That vector differs sharply from the general orientation of bedding, both within the eastern flanks of Dead Indian monocline and the easternmost edge of the Beartooth block. In both of those uplifts the general bedding has a north–south strike with generally eastward dips.
The Canyon Mouth anticline is placed approximately a half mile (0.8 km) north of the northern boundary of the map in this paper’s Figure 3. Interestingly, all of the contractional folds so far recognized within boundaries of the present study express the same suite of unanticipated orientations as does the Canyon Mouth anticline. Specifically: (1) the folds’ hinge lines plunge steeply to the southeast, vanishing into depths at the western edge of the Bighorn Basin; (2) the northeastern flanks of the anticlines have a steeper dip than their southwestern flanks (and the northeastern flanks of the synclines also dip more steeply than do their southwestern flanks); and (3) deformation of the folds via minor faults is greater on the northeastern than on the southwestern flanks, both in anticlines and synclines.
Hinge lines of all of those folds cut right across the otherwise broad landscapes having attitudes of bedding planes that are dipping to the east or east-northeast. This unexpectedly different pattern of orientation between the assemblage of contractional folds and the general formational bedding would seem to have tectonic significance. Nevertheless, because I made no serious attempt at systematically collecting new kinematic data, I can provide no evidence for causative factors that might explain the apparently secondary alteration of stress-strain orientation.
Detrital-zircon Geochronology Requires Consideration
This final element of the Discussion section deals with research made possible by the technology known as ‘laser ablation inductively coupled plasma mass spectrometry’ (abbreviated as ‘LA-ICP-MS’). Its practical operation is admittedly outside my personal expertise. Nevertheless, quite independently from my own work, LA-ICP-MS age determinations have been applied by May et al. (2013a, 2013b) within the area of the present study. Those authors established a series of geological conclusions dependent upon lithostratigraphic identifications with which I disagree. Thus their work must be considered here.
For the physical locations of sampling sites used by May and his co-workers, see the northeastern part of the present paper’s Figure 3 (a geologic map), and note the small red squares on cross-section C–D of Figure 4. Expressed in simplest relevant terms, the technology of LA-ICP-MS has been applied to geochronologic purposes using rapid radiometric analyses of large suites of individual sand-sized grains of zircon as separated from common siliciclastic sedimentary rocks, especially sandstones. For grains younger than a billion years (< 1 Ga), the bits of detrital zircon are most reliably datable on the basis of 206Pb/238U ratios. See May et al. (2013a, p. 43–46) for a thorough expression of the applied methodology.
As summarized in Table 1 of the present paper, sandstone samples (from locs. 5, 6, and 7) involving Late Cretaceous formations within the study area of the present paper were temporally characterized by May et al. (2013a). One month later, the same authors, using the same suite of zircon grains and data already derived from them, published the same analytical dates in a chronologically and geographically more expansive paper (May et al., 2013b). Within the area covered by Figure 3, the identifying locality numbers (i.e., the original loc. nos. 5, 6, and 7) were changed (to 30, 31, and 32, respectively), and so too were identifications of the formational hosts for the zircon grains (in two of the three localities). The summary of formational changes provided here in Table 1 is verifiable by close study of raw data as provided by May et al., 2013c, which is the Geological Society of America Data Repository’s DR Table 1.
Examination of the present paper’s Figure 41 will help in understanding just why the detrital-zircon geochronology contributed by May et al. (2013a, 2013b) requires close consideration within the present study of very basic geologic mapping. Begin by observing the field-based formational identifications made by May et al. (2013a) by following the lower set of three dashed red lines in Figure 41. Those lines show that localities 5 (30) and 6 (31) were sampled from strata identified by May’s team as the Mowry Shale, and locality 7 (32) was identified by them as part of the overlying Frontier Formation. Reasons for the (above-mentioned) re-identification of the formational occurrences of two samples (i.e., those changed from Mowry Sh. to Frontier Fm.) were not explained by May et al., either in 2013b or 2013c. In any case, ages for all three localities from the present area of study were reported as having come from sampling the Frontier Formation.
May et al. (2013a, 2013b) reported that all three localities (i.e., 5 (30), 6 (31), and 7 (32)) provided detrital-zircon age-dates (see Fig. 41) within boundaries of the Cenomanian Age (representing geologic time early in the Late Cretaceous). In those same papers, May and his co-workers opined that all three of the host localities yielded detrital-zircon ages that closely approximated the age of deposition of these sampled sandstones. That is, the geochronologic age of the more westerly magmatic donor of the zircon grains was very close to the assumed depositional age of the downstream sandstone bodies from which the team’s samplings for zircon ages were conducted.
At first glance, most of the above information seemed convincing. For example, the results from May et al. (2013a, 2013b) held special credibility because of close agreements in ages between the newly analyzed detrital-zircon grains liberated from the host strata compared with ages of Mowry Shale and Frontier Formation from other sources outside the boundaries of the present study. Related to that, the newly gained detrital-zircon ages from the present area of study (i.e., the landscape within Fig. 3) identified by May et al. as the Mowry Shale and Frontier Formation agree with the global geochronology (scaled in Ma) as developed by Ogg and Hinnov (2012)—and shown in the left-hand columns of Figure 41. That is, it seemed reasonable that the age-dates using LA-ICP-MS technology upon detrital zircons identified as components of the Mowry Shale and Frontier Formation by May et al. (2013a, or all from the Frontier Fm. as altered in 2013b) could well also represent depositional ages of the sampled sandstones.
Positive aspects of the study by May and his co-workers also were highlighted by knowledge that technical aspects of the analyses were overseen by known leaders in that brand of research. Additionally, the sample sizes involved radiometric data from nearly 100 individual detrital grains from each of the three sites, thus enhancing statistical evaluations of the samplings. In the paper by May et al. (2013a), their figure 5 is especially instructive, because it clearly shows the probability distributions of ages (expressed in Ma) of detrital zircons around the minimum age peaks from each of the three relevant localities. The significant total range of temporal variation is roughly 10 m.y. around each site. Thus the dates derived through conduct of the project by May and his co-workers represent the state of the art, over which I have few qualms.
The fundamental problem I do perceive about the research published by May et al., however, is that their field-based identifications of Upper Cretaceous formations from which they derived samples for analysis are markedly incorrect. My basic criticism, therefore, is not with some complex technological glitch within the LA-ICP-MS system that has led to faulty results. Quite to the contrary, my concern is with human error in having misinterpreted critically important formational boundaries across the breadth of the local Upper Cretaceous stratigraphic column.
A good place to visualize the source of errors is to begin at the actual base of the Mowry Shale, as shown in Figure 3 (a little southwest from the center of section 21, T. 56 N., R. 103 W.), just west of attitudinal-measurement number 8214. Based upon that mapping, in my opinion the true base of the Mowry Shale is geographically located about 600 m (ca. 1,969 ft) west-southwest of zircon-bearing locality 5 (30). That locality was interpreted as being in the Mowry Shale by May et al. in 2013a and, instead, in the Frontier Formation in their 2013b paper. Stratigraphically, I posit the true base of the Mowry Shale to be about 270 m (ca. 886 ft) lower in the column than locality 5 (30) of May et al. (see the nearby cross section C–D in Fig. 4).
Basal contact of the Frontier Formation is conformable upon the Mowry Shale and, except through the inter-formational transition zone, the two rock units are readily separable lithologically. I have mapped the relevant stratigraphic contact just below attitudinal measurement 8215. Again, the zircon-bearing locality 5 (30) is far above the base of the Frontier Formation, actually held in lower parts of the Cody Shale (see Figs. 25–28). Continuing upward within the stratigraphic section, zircon-bearing localities 6 (31) and 7 (32) are in far younger strata (Figs. 25–27, 29, 31, and 36–38) than those considered by May et al. (either in 2013a or 2013b; Fig. 41).
Thus the basic problem as I see it is rather simple. As stated in the caption to Figure 41, none of the three sampling localities is older than strata near the base of the local Cody Shale. The solid green lines in Figure 41 serve to more confidently position localities 5 (30), 6 (31), and 7 (32) within the local lithostratigraphic column. The following relationships are reflected in Figure 41, and the geochronologic extents of the local formations agree with the stated sources of information in the two footnotes. In agreement with original dating of sampled detrital zircons by May et al. (2013a, 2013b), their youngest significant peaks from relative age probability values for each locality are shown in bold-faced type.
Strata bearing detrital zircons (calculated age 99.4 Ma) in locality 5 (30) are actually in near-basal parts of the Cody Shale (dated latest Turonian; ca. 90.3 Ma) not as stated by May et al. (2013a) in the Mowry Shale (Early Cenomanian) or in the Frontier Formation (2013b; Late Cenomanian).
Strata bearing detrital zircons (calculated age 98.3 Ma) in locality 6 (31) are actually in a thin northern remnant of the Mesaverde Formation near that formation’s stratigraphically uppermost levels (dated latest Campanian; ca. 72.5 Ma) not as stated by May et al. (2013a) in the Mowry Shale (2013a; Early Cenomanian) or Frontier Formation (2013b; Late Cenomanian).
Strata bearing detrital zircons (calculated age 96.5 Ma) in locality 7 (32) probably are in basal parts of the Lance Formation (dated Late Maastrichtian; ca. 67.7 Ma) not as stated by May et al. (2013a, 2013b) in the Frontier Formation (2013b; Late Cenomanian).
A clear implication of what is proposed immediately above is that the dates of deposition of all three levels of sandstones sampled for detrital-zircon ages differ strongly from the much older dates of magmatic origin of the detrital particles that eventually accumulated within the host-sand bodies. First-order estimations of the interval of time (scaled in ‘millions of years,’ m.y.) between magmatic genesis of the later-transported zircon grains and their deposition within the eventually sampled sandstone bodies can be calculated as: (1) for locality 5 (30), 99.4 Ma – 90.3 Ma = 9.1 m.y.; (2) for locality 6 (31), 98.3 Ma – 72.5 Ma = 25.8 m.y.; and (3) for locality 7 (32), 96.5 Ma – 67.7 Ma = 28.8 m.y. Rest assured that the rather high level of temporal precision implied here is used only for purposes of illustrating the process of analysis.
Magnitudes of the above discordances between dates of deposition of the host sandstones and age of magmatic origin of the detrital zircons captured within those sandstones should not be considered as ‘trivial.’ Also, be sure to keep in mind that the analytical accuracies of the detrital-zircon samples themselves are not being challenged in this discussion. It is only the inordinately ancient ages (and of course the identifications) of the individual parts of formations from which the detrital-zircon-bearing sandstones were collected that are presently under question. Procedurally, I developed most elements of the geologic map (Fig. 3) using litho- and biostratigraphic criteria prior to involuntarily becoming driven into this line of inquiry. Today’s ready access to the map itself, however, along with the associated criteria used to separate and identify the diverse rock units, I hope will be helpful to future workers as they accept the challenges of discovering more exactly where any additional errors may persist.
It is not unusual for an author, at the end of his or her present work, to propose ideas for relevant future research. As a less common variation on that practice, today I am responding to insight from reviewers of the present manuscript; they were way ahead of me. All three reviewers were justified in expressing disappointment that I stopped short of what should be considered as the next logical steps in structural analysis. The following paragraph provides examples of the most important tasks they proposed for present completion.
My text mentions existence of numerous occurrences of slickenlines within bedded strata and in cone-in-cone, post-depositional structures. Nevertheless, I left kinematic data that might have aided in recognition of key vectors of movement to future workers. Perhaps more fundamentally, I should have committed effort toward documentation of major tectonic features through development of a series of down-plunge projections of folds constructed atop diverse, ‘Google-style,’ full-color topographic images. Also, a series of equal-area stereonet plots using my own strike–dip measurements could be of value in determining more about the structural history of the project area.
Most assuredly, I do not challenge the concept that my paper would have gained value had I accepted those suggestions and completely reordered my presentation to bring them into effect. But, in defense of my conservatism, I plead the following. Most importantly, as I stated in the ‘Introduction’ to this research:
“The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin.”
My primary emphasis was development of very basic descriptive stratigraphy as applied to a complex area of minuscule dimensions. Nevertheless, I firmly believe that efforts completed there do provide generally verifiable, fundamentally relevant information that will be central to future progress in more quantitatively oriented structural analyses. In my mind, the ‘A-B-C’s of stratigraphic documentation in this difficult area had to come first.
In relation to those thoughts, here is what I promote as next steps. My suggestions would apply to a manageably larger geographic area that would lend itself to higher probabilitity for reliable success in quantitative structural analyses. One should return, wearing sturdy field boots, to the north–south band of terrain (highlighted in bright yellow in Fig. 2) that extends northward from within Bald Ridge and Figure 3 to just short of the Wyoming–Montana state boundary. Take pains in re-mapping and re-evaluating complexities of the Phanerozoic sections exposed immediately adjacent to the Beartooth block’s eastern margin. Then, right from the beginning of that new study, apply the kinds of thinking and procedures that reviewers suggested to me as I was crossing the line and breaking the tape at the limit of my own capabilities in such research. Working and thinking at that scale from the outset, in my opinion, would result in much broader and more reliable conclusions than whatever might have come from a project limited to less than 5 square miles. If I were a younger man—and better trained—I would pursue that expanded project with passion!
Covering a wide diversity of professional, practical, and personal contributions, each of the following individuals († deceased) contributed to the work’s completion: Barry Albright; †Don Blackstone; Tom Bown; Don Boyd; Tim Brewer; Steve Cather; J.-P. Cavigelli; Winston, Beryl, and Todd Churchill; Bill and †Dorothy Clemens; †Bill Cobban; Ken Driese; Skip Eastman; Jeff Eaton; Jaelyn Eberle; Andrea and Joe Erickson-Quiroz; B. Ronald Frost; Carol Frost; Bill Gern; Dave Gillette; Phil Gingerich; †Doug Hart; Tom Hauge; †Jim and Jeannine Honey; †Bob Houston; Steven M. Jackson; Dennis Knight; Lisa Kolste; Mary Kraus; Dave Lageson; Melissa Lawton; George and Val LeFebre; Don Lofgren; David Loope; †Dave and †Jane Love; †Malcolm and Priscilla McKenna; Jim Nielson; Brendon Orr; †Larry Ostresh; Anne Pendergast; Brian and Carrie Peters; Bob Raynolds; Bill Reiners; Larry Schmidt; Scott Smithson; Art and Judy Snoke; Maria Sonett; Kent Sundell; Dave Taylor; Enid Teeter; Robert Waggener; Anne Young; Page and Pearré Williams; Don Wise; and Mike Woodburne.
Formal reviewers of this paper’s manuscript included David R. Lageson (Montana State University, in Bozeman), Donald U. Wise (University of Massachusetts Amherst), and RMG Science Co-Editor Art Snoke (University of Wyoming, in Laramie).
Special thanks go to Barry Albright, Dave Gillette, and Melissa Lawton for their joint permissions to employ selected figures from the (2009) ‘Woodburne Festschrift’—Museum of Northern Arizona Bulletin, Number 65 (reproduced here within Fig. 4).
The research could not have been done at all without benefit of the wonders provided by my humor-rich bride, Linda Lillegraven.
Commercial software www.EarthPoint.us (tools for Google Earth), when applying its ‘Convert Coordinates’ features, converts latitude/longitudes into a wide variety of other geographic equivalents.
Scientific Editor: Arthur W. Snoke
Full-scale versions of Figures 3 (geologic map) and 4 (interpretive cross sections) are intended for printing at 1:12,000 scale, and they are available for download either as data supplements accompanying the full-text version of this article on GeoScienceWorld or directly via the following addresses:
Originally scaled (i.e., 1:12,000) Figure 3, https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/54.2/RMG-54.2-Lillegraven-Figure3.pdf
Originally scaled (i.e., 1:12,000) Figure 4, https://geobookstore.uwyo.edu/sites/default/files/downloads/rmg/54.2/RMG-54.2-Lillegraven-Figure4.pdf
Appendix 1. Listing of Data Points.
Thumbnail listing of data points presented on geologic map (Fig. 3) and interpretive cross sections (Fig. 4), based upon 7.5-minute topographic ‘Bald Peak, Wyoming’ quadrangle (1991). All measurements were taken within T. 56 N., R. 103 W., Park County, Wyoming, using North American Datum of 1927, Universal Transverse Mercator, Zone 12. Each bracketed and italicized measurement number presented below (e.g., ‘’) is associated on the geologic map with a small red dot that shows the position of a non-attitudinal feature. On attitudinal (i.e., strike–dip) measurements, any value of dip greater than 90° represents structurally overturned bedding. ‘JAL Notes Page’ refers to author’s field notes (free to public access) archived in University of Wyoming’s American Heritage Center. ‘JAL Meas. No.’ refers to author’s serially numbered entries of data points relevant to this study into field notes.
Appendix 2. Bases of Formational Identifications.
The following summaries specify most of the litho- and biostratigraphic criteria by which individual formations were identified in the field during this research. The characteristics are intentionally specific to the study area, even though a broader geographic examination of virtually each formation would include a much-expanded array of diagnostic features and/or features developed in parallel among different formations. Arrangement of the units is formation-by-formation, progressing from lowest to highest within the local stratigraphic column. Also included for each formation are the: (1) formational abbreviations used on Figures 3 and 4 along with most of the photographs; (2) geographic designations to the most informative outcrops of each formation within the area of study (all are in T. 56 N., R. 103 W.); (3) references to representative photographs of those specified outcrops, if available; and (4) presence of at least one published reference that could help in identifications.
Gypsum Spring Formation (Jgs) — Middle Jurassic (Picard, 1993)
Key outcrop area: North-central part of NW1/4 of section 33 and south-central part of SW1/4 of section 28, on both the southern and northern sides of Newmeyer Creek, close to the well-marked Bald Ridge Trail (Fig. 9).
Locally expressed features: Shallow-marine outcrops of gray, mostly fine- to medium-grained quartz sandstone, siltstone, and claystone dominate. Outcrops are rich with shattered gypsum beds and bits of pelecypod shells, locally coquinoid, worm-burrowed, or generally bioturbated. Characteristic outcrops are thin-bedded with weakly expressed ripples and well-developed cross-bedding. Although uncommon, there do exist occasional carbonaceous shales or even highly deformed, low-grade lignitic beds. Medium-bedded channels commonly cut downward through thin-bedded sandstone layers.
Sundance and Morrison Formations undifferentiated (Jsm) — Upper Jurassic (Picard, 1993)
Key outcrop area: Just west of center of section 28, one ridge east from the Bald Ridge Trail (Fig. 11).
Locally expressed features: No exposures characteristic of the marine Sundance Formation were recognized within the area of study. The nonmarine, fluvial and lacustrine Morrison Formation is dominantly mudstone with many thin stringers of sandstone. The variegated mudstones on weathered surfaces are red-and-white banded, with banding thicknesses measurable in meters. When freshly exposed by digging, the initially reddish-weathering surfaces show that deeper strata are intensely barn red or purple. Similarly, the whitish surface bands are more of a greenish-gray when freshly excavated. Sandstone elements typically are light tan, thin-bedded, and obviously rippled when seen in cross section. Fragmentary dinosaur bones and diverse remains of other vertebrates are relatively common. Locally conglomeratic beds of rounded, soft-pebble, detrital-mudstone clasts (mostly less than an inch in diameter) exist.
Key outcrop area: The central area of section 33, west of the northern lobe of Hogan Reservoir (Fig. 10).
Locally expressed features: Outcrops are principally sandstone, mostly nonmarine in origin. Generally, the outcrops are fine-grained, light tan, yellowish, white, or brown in color, occasionally sideritic, and thin- to medium-bedded. Bioturbated, gray siltstones also are common. Clastic units generally are strongly indurated, exhibiting a multiplicity of minor fault slices, and often are channeled and obviously rippled. Commonly this unit is well exposed across the study area, or at worst it dependably exists as rubble within sagebrush cover and grasslands. Strata of the local Cloverly Formation almost always are obviously deformed, with ubiquitous kinks and small displacements. They also exhibit stronger forms of deformation including severe fracturing via out-of-the-basin faulting and associated slickensides. For mapping purposes, therefore, this is a consistently recognizable, almost always outcrop-forming, and informative rock unit despite its generally shattered nature. Its ubiquitous jointing, fracturing, and limited faulting would lend itself well to conducting detailed measurements linked to quantitative strain analyses. The very top of this unit regularly presents worm-burrowed siltstone containing debris of fossil plants.
Key outcrop area: In fault-repeated exposures dominating NE1/4 of NW1/4 of section 21 (Fig. 19).
Locally expressed features: The Thermopolis Shale is generally hidden within expanses of sagebrush and grassland across the study area. Indeed, the Thermopolis Shale is the least understood of all the formations within the current study area. It is dominated by soft, fine-grained, strongly bioturbated marine strata with relatively uncommon tan or rusty-brown, thin-bedded bands of broadly rippled, very fine-grained sandstone. The Thermopolis Shale unquestionably overlies numerous outcrops of the strongly indurated Cloverly Formation and underlies clear expressions of firmly packed, readily identifiable Mowry Shale. Through expression of usually thin-bedded sandstone bodies in the uppermost levels of the Thermopolis Shale, its contacts with Mowry Shale are reasonably well constrained. At about the north–south mid-length of the study area’s outcrops (Fig. 3), the mapped outcrop pattern of the Thermopolis Shale dramatically reduces in width northward, presumably reflecting influences of unexposed (and thus poorly understood) faulting. At the north end of the study area, there exist at least 3 fault-generated repetitions of sandy facies in the upper Thermopolis Shale (Fig. 19), leading to a dramatic and abrupt east–west widening of the formation’s mapped width.
Key outcrop area: Near the east-central edge of section 33, both along the north and south shores of Hogan Reservoir. Because no photograph is available for the stated key area, Figure 16 is substituted, centered on measurement 8214 [of Appendix 1], focused on strata close to the north end of SW1/4 of section 21.
Locally expressed features: Gross outcrops of Mowry Shale vary in color from light yellowish-tan to gray to jet black within this study area. Its dominant lithology is extremely fissile marine shale, commonly bearing delicately ornamented fish bones, three-dimensional vertebrae, dermal scales with clearly expressed growth lines, and tiny but well-preserved coprolitic bolides. There exist occasional layers of sideritic siltstone and very thinly bedded, fine-grained sandstone. Rarely, such beds are expressed as rounded-black and angular-white (‘salt-and-pepper’) quartz sandstone. There also exist examples of relatively indurated siltstone that encase abundances of gypsum set within more extensive thicknesses of typical, highly fissile, black Mowry Shale. Upper parts of the formation commonly are slickensided and bear strongly developed, cone-in-cone structures (Fig. 20). The Mowry Shale does contain thin but recognizable bentonite beds, but such layers are minor in comparison with richly bentonitic strata preserved in the overlying, more massively bedded Frontier Formation. The boundary between the Mowry and Frontier is a conformable, transitional continuum. The famous fish fossils preserved in the geographically widespread Mowry Shale typically represent species of tiny body size. Nevertheless, some isolated dermal scales measure more than an inch in diameter, suggesting existence of much larger, relatively rare species as well.
Key outcrop area: North-central half of section 21 (Fig. 25).
Locally expressed features: Dominant lithology is richly bentonitic, very incompetent gray to (more commonly) jet black, less fissile (than in the Mowry Shale) to massive mudstone or (less commonly) siltstone. Almost universally, the formation exhibits grossly folded, highly deformed bedding, sometimes leading to repeated elements involving substantial segments of the section. Sandstone layers (very fine-grained) and sideritic, rusty-stained beds are relatively uncommon compared to underlying and overlying formations. Siltstones and fine-grained sandstone, however, are well developed at the Mowry–Frontier contact and also increase as the conformable base of the Cody Shale is approached. South of Hogan Reservoir exist occasional stringers of coarse-grained sandstone and smoothly rounded quartzite-pebble conglomerates. In the northeastern quarter of section 21, atop the presently eroding surfaces of the Frontier Formation and Cody Shale, exist large blocks of displaced granite (marked with red triangles below the letter ‘G’ on Figs. 3 and 4 [on cross sect. A–B]).
Key outcrop area: Eastern half of SW1/4 of NE1/4 of section 21 (Fig. 26).
Locally expressed features: This unit differs markedly from the underlying Frontier Formation in being much less bentonitic and in having greater density of siltstone and unusually thin-bedded gray, tan, and white, fine-grained sandstone layers. The dominant lithology, however, continues to be shale, and multiple beds are expressed as silvery (siliceous), Mowry-like facies. That similarity sometimes includes fossilized remains of fishes (e.g., JAL meas. no. 8216 in Appendix 1), although they are species of considerably larger body size than generally seen in the Mowry Shale. The Cody Shale is markedly less deformed than the underlying Frontier Formation, although the uppermost reaches of the Cody Shale close to the overlying Mesaverde Formation are severely shattered, steeply east-dipping, and occasionally overturned. As shown in Figures 3 and 28, a southeast-plunging syncline deforms the Cody Shale, Mesaverde Formation, and stratigraphically lower parts of the Meeteetse Formation. Also, lower levels of the Cody Shale harbor the May et al. (2013a, 2013b) detrital-zircon collecting locality 5 (30).
Key outcrop area: Dead center of NW1/4 of SE1/4 of section 21 (Fig. 32).
Locally expressed features: Composition of the local Mesaverde Formation is dominated by gray to light brown, shallow-marine sheets of thin- to medium-bedded, fine-grained sandstone, inter-fingered with gray to dark siltstones. The strata commonly are slickensided, and two wholly separate, important folds of the Mesaverde Formation exist in section 21. The unit’s sandy bedding becomes severely shattered and thinned to extinction northward in the center of the NE1/4 of section 21. The present paper’s attitudinal measurement 4730 ( Appendix 1), involving strata here identified as Mesaverde Formation, is sited close to locality 6 of May et al. (2013a); they first identified the involved strata as Mowry Shale. Shortly thereafter, May et al. (2013b) re-identified strata as Frontier Formation from the same site (but with the locality’s identification number changed to 31). Stratigraphic evidence brought forth in the present work suggests that neither a formational identification as Mowry Shale nor Frontier Formation as sequentially offered by May et al. (first in 2013a and then in 2013b, respectively) is correct (closely study Fig. 21). Table 1 provides a summarized history of relevant formational identifications.
Key outcrop area: At the center of the N1/2 of the SE1/4 of section 21 (Fig. 34).
Locally expressed features: Local strata of the Meeteetse Formation represent estuarine to lowland-emplaced sequences. The formation is extraordinarily diverse lithologically, varying from structureless black mudstone to very coarse-grained arkosic sandstone (shot through with enormous numbers of tiny black, well-rounded pebbles composed of diverse metamorphic clasts) to disseminated particles of lignite spread through everything from mudstone to sandstone. Incredibly tight folding along with nearly ubiquitous slickensides and small faults characterize strong deformation and resulting sequences of chaotic bedding. Stringers of very coarse-grained sandstone and concentrations of pebbles may be observed at almost any stratigraphic level, from bottom to top of the section. Worm-burrows in fine-grained sandstones exist near the formation’s mid-section. Overturned beds stratigraphically high in the formation (e.g., Appendix 1’s measurement 8220 and other nearby sites) combine shells of giant oysters, other pelecypods, and gastropods, all overlain by cone-in-cone structures, in turn overlain by mixed shell bits. One fragile dinosaur limb-bone fragment exists at a non-measurement locality ‘’ of Appendix 1. Much of the Meeteetse Formation’s section in the study area visually mimics the many gray–white alternations of richly carbonaceous to lesser levels of carbon that characterize the type Meeteetse Formation (surrounding the Wyoming town of Meeteetse, south-southeast of Cody).
Key outcrop area: Useful exposures of the Lance Formation within the study area are limited to the southeastern margin of section 21 and the southwestern corner of section 22 (Fig. 36).
Locally expressed features: Exposures of the nonmarine Lance Formation are very limited within the mapped areas, and expressed lithology is restricted to gray or tan sheets of thin-bedded, rippled, fine-grained sandstone set within beds of soft mudstones. It is distinctly different from the Meeteetse Formation in the apparent paucity of consequential, carbon-rich horizons.
Represented neither in outcrop nor in subsurface within area of study: Most probably this ordinarily thick unit, widely exposed nearby to the east of the study area, was completely cut out by the major out-of-the-basin thrust fault shown on Figures 3 and 4 (cross sects. C–D and E–F along with the excerpted figs. 10 and 11 from Lillegraven ) at the existing contact between Lance and Willwood Formations (Fig. 40).
Locally expressed features: None at the land surface within the area of study (Fig. 3).
Locally expressed features: Characteristic strata of the Willwood Formation are common along the edge of the proposed thrust (Figs. 3, 4 [cross sects. C–D and E–F], 29, 36, and 40) that tectonically over-rode upper parts of the Lance Formation (also see excerpted fig. 10 from Lillegraven  in the present paper’s Fig. 4). Relevant strata strictly within the study area (Fig. 3) are vertical to overturned, with outcrops dominated by thin- to medium-bedded, fine-grained sandstone directly overlain by large-pebble to small-cobble conglomerates. Those conglomerates are totally encased in mudstone. The fine-grained fractions of exposed sections tend to be markedly variegated, with red or yellow, iron-rich components being obvious. Additional outcrops of the Willwood Formation just outside the boundaries of Figure 3 (shown in Figs. 9 and 39) are much more similar to the general appearance of that formation as seen across most of the expanse of the Bighorn Basin. The closely spaced, red and white candy-stripes of Eocene paleosol sequences are the real hallmark of the Willwood Formation.