We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The forma tion consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meterscale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.
The Tonto Group of the southwestern U.S. has played a key role in the understanding of Cambrian Earth history for over a century. It traditionally consists of three formations that were originally defined in the central portion of the Grand Canyon—the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone (Fig. 1; Gilbert, 1875; Powell, 1876; Noble, 1914, 1922). The names of the upper two of these units have recently been modified to reflect the heterogeneity of their lithologies (Bright Angel Forma tion and Muav Forma tion; Karlstrom et al., 2020), and two “new” rock formations have been added to the succession. At the base of the Tonto Group is the uppermost Sixtymile Forma tion, a heterolithic unit that conformably underlies the Tapeats Sandstone (Karlstrom et al., 2018, 2020). At the top of the Tonto Group is the oftoverlooked and previously temporally unconstrained Frenchman Mountain Dolostone (FMD), which is the subject of this paper.
The FMD has been challenging to decipher for multiple reasons: (1) its cliff-forming exposures are difficult to reach, (2) it has not yet yielded datable fossils, (3) its upper and lower contacts manifest themselves differently across the basin, and (4) its thickness varies greatly across a large geographic region. These characteristics made it difficult to confidently map and study by early stratigraphers in the Grand Canyon region. Noble (1922) distinguished this succession of dolostones as “subdivision A” of the Muav Limestone (Fig. 2A). Schenk and Wheeler (1942) lumped Noble’s “subdivision ‘A’” dolostones with the upper portions of the Muav as “Cambrian(?) and post-Cambrian limestones and dolomites” (Fig. 2A). McKee (1945) then divided the Muav Limestone into seven members, but he did not include Noble’s (1922) “subdivision A;” he referred to that interval simply as “Cambrian undifferentiated dolomites” (Fig. 2A). This dolomitic interval, which ranges in thickness from just a few meters in eastern Grand Canyon to 371 m at Frenchman Mountain near Las Vegas, Nevada, has been ignored in most subsequent treatments of Grand Canyon stratigraphy. It has yielded no datable fossils, so its precise age has long been unknown. In stratigraphic columns that include the Tonto Group, McKee’s “undifferentiated dolomites” are commonly included with the Muav Formation (e.g., Beus and Morales, 2003; Blakey and Middleton, 2012).
Brathovde (1986) examined the sedimentology and stratigraphy of this unnamed unit, primarily in Grand Canyon exposures. He searched unsuccessfully for conodonts and other biostratigraphically useful fossils and was therefore unable to constrain the age of this interval. He proposed that the unit be named the “Grand Wash Dolomite.” That name was never published in the peer-reviewed literature, however, and Elston (1989) indicated that it violated the North American Stratigraphic Code because the name “Grand Wash” was already in use for a different stratigraphic unit in the same area—the Grand Wash Basalt. Korolev (1997), using a sequence-stratigraphic approach, extended the analysis of this unit westward to the thick exposure at Frenchman Mountain, where these supra-Muav dolostones are overlain by the sparsely fossiliferous but well-dated Furongian Nopah Formation (Miller et al., 1981). That section thus constrained, for the first time, the age of McKee’s undifferentiated dolomites to Cambrian Series 3 (Miaolingian), except for the uppermost portion, as discussed below. Korolev (1997) proposed the name Frenchman Mountain Dolomite, which has subsequently been used on two geologic maps (Castor et al., 2000; Garside et al., 2012), a stratigraphic note (Rowland and Korolev, 2011), and—in the slightly modified form, Frenchman Mountain Dolostone—in a journal article (Karlstrom et al., 2020) and a published field guide (Rowland, 2022). However, this name is not yet in compliance with the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005) because a detailed description of the strata has not been published in a recognized scientific medium and no stratotype section has been formally established. We fulfill those criteria with this paper.
Frenchman Mountain is a regionally important exposure, not only of the eponymous Frenchman Mountain Dolostone (FMD) described here, but for the Tonto Group as a whole. At Frenchman Mountain, the complete succession is readily accessible and extremely well exposed. The section dips ~50° to the east, and the Tonto Group here displays a thickness transitional (~860 m) between that of cratonal sections of the Tonto Group in the eastern Grand Canyon (~250 m) and of shelf sections of correlative strata in the southern Great Basin region (~1500 m). Hence, the Frenchman Mountain exposure helps to constrain the position of the hingeline of the Cambrian shelf. It is within the upper plate of a west-translated Miocene detachment fault that restores ~60 km to the east to a position near Gold Butte, Nevada (Longwell, 1974; Fryxell and Deubendorfer, 2005), within a few tens of kilometers of Grand Wash Cliffs sections in Arizona examined by Brathovde (1986) and McKee (1945). The inclusion of the FMD in the Tonto Group, together with the recognition of the presence of the Tonto Group at Frenchman Mountain, is thus stratigraphically appropriate, and it establishes a valuable link between Grand Canyon Cambrian sections and sections in the southern Great Basin.
Because Frenchman Mountain lies within the Basin and Range province, adjacent to the southern Great Basin, the first generation of stratigraphic terminology applied to this section was Great Basin terminology (Longwell et al., 1965; Gans, 1974)—a tradition that some workers continued into the 1990s and beyond (Matti et al., 1993; Montañez and Osleger, 1993; Osleger and Montañez, 1996; Osleger et al., 1996; Webster, 2011b). Thus, the strata that we refer to the FMD and Muav Formation in this paper were formerly considered to be portions of the Bonanza King Formation (Fig. 2), whereas the strata we refer to the Bright Angel Formation were originally assigned to the Pioche Shale, Lyndon Limestone, and Chisholm Shale. The transition from southern Great Basin terminology to Colorado Plateau terminology, beginning in 1990 (Rowland et al., 1990; Castor et al., 2000), was driven by the growing recognition that Frenchman Mountain is fundamentally a cratonal section with palinspastic proximity to the Colorado Plateau.
The FMD was deposited in equatorial latitudes on the northfacing margin of Laurentia. The continent was oriented ~90° clockwise in comparison with its present orientation (Rowland and Shapiro, 2002; Torsvik and Cocks, 2017). Seaward of the craton-margin region where the FMD was being deposited was the “Great American Carbonate Bank” (Fritz et al., 2012; Taylor et al., 2012), the most extensive carbonate platform accumulation on Earth at that time (Kiessling et al., 2003).
The purposes of this paper are to (1) formally name the Frenchman Mountain Dolostone, (2) establish its age and regional stratigraphic relationships, and (3) describe and interpret the sedimentology, chemostratigraphy, and regional biostratigraphic framework of this unit, with an emphasis on the stratotype locality at Frenchman Mountain.
This paper is based primarily on traditional field measurements, using a Jacob staff calibrated in decimeters, and field descriptions of nine stratigraphic sections. The studied localities lie on an 85-km-long eastwest transect that straddles the boundary between the Colorado Plateau and Basin and Range provinces. Sections extend from the Palisades of the Desert in eastern Grand Canyon to Frenchman Mountain adjacent to Las Vegas (Fig. 1). We also include stratigraphic data from the Nopah Range in California and from a tenth Tonto Group section, Blacktail Canyon in Arizona, which was measured and described by Rose (2003, 2006, 2011); the Blacktail Canyon locality is significant because it was proposed by Rose (2011) as the type section of the Tonto Group (as originally defined). Detailed locality data and detailed descriptions of measured sections in the western Grand Canyon and Lake Mead regions are available in Korolev (1997, his appendix A), with the exception of sections at Palisades of the Desert and Fern Glen Canyon. The Palisades of the Desert section was measured near the top of a talus cone on the east side of the Colorado River at 36.14226°N, 111.80489°W. Stratigraphic details of the Fern Glen Canyon section are in Rowland et al. (1995). Petrographic thin sections of representative facies were examined and described by Korolev (1997; see also Brathovde, 1986).
Hand samples for carbon isotope chemostratigraphy were collected every 1 m in the Lyndon Limestone Member of the Bright Angel Formation and every 1.5 m within the Muav Formation, FMD, and Nopah Formation. These samples were petrographically screened by comparing the least-altered phases in thin section to their counterpart billets so that we could microdrill the least-altered phases from the billets, avoiding late-stage spar, crusts, fossils, and allochems. In dolostones, including the FMD, dolomicrite (1–4 μm grain diameter), dolomicrospar (5–20 μm), and dolomitic oolite (0.5–2 mm) were drilled, yielding ~1 mg of powder. Stable isotope ratios (δ18O and δ13C) were measured on ~300 μg of powder using two different systems at the University of Arizona Environmental Isotope Laboratory. Some samples were measured using an automated carbonate preparation device (Kiel III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252). In this system, powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70 °C. Some samples were measured using a GasBench continuous flow headspace sampler connected to a Thermo-Finnigan Delta-Plus mass spectrometer. These samples were reacted overnight at room temperature with dehydrated phosphoric acid in He-filled vials, after which the evolved CO2 was transferred to the mass spectrometer using a He-carrier gas. The isotope ratio measurement was calibrated based on repeated measurements of the NBS-19 and NBS-18 reference materials and of CAR2, an in-house carbonate standard. The following isotope ratios are assigned to these standards (δ18O followed by δ13C): NBS-19, −2.20‰, +1.95‰; NBS-18, −23.20‰, −5.01‰; and CAR2, −1.41‰, +2.03‰. Analytical precision is ±0.10‰ for δ18O and ±0.08‰ for δ13C (1σ) for the Kiel III system; in the GasBench system, analytical precision is ±0.15‰ for δ18O and ±0.10‰ for δ13C (1σ). Replicate samples were analyzed, one in each system, to ensure similarity of results. The complete data set, including precision for each analytical run, is available as Table S1 in the Supplemental Material1.
To evaluate the level of order present in the arrangement of FMD strata, and specifically the presence of cyclicity, we used the nonparametric statistical method of Burgess (2016). This method has two components. The first component—the Markov order metric—uses the observed facies succession pattern to evaluate the orderliness of the succession. The value of the metric m is between 0 (for perfectly disordered strata) and 1 (for perfectly ordered strata). The second component—the runs order metric—uses the pattern of increasing or decreasing thickness of the strata to evaluate their orderliness. The runs order metric r typically takes a value of 1 < r < 2 (for perfectly disordered strata thicknesses) and 2 < r < 3 (for perfectly ordered thicknesses). For each of these metrics, the corresponding P-value was obtained using a Monte Carlo approach based on 5000 random shuffles of the observed data. All of the analyses were run using the OrderID Matlab code available at https://csdms.colorado.edu/wiki/Model: OrderID. For the purposes of this analysis, the uppermost 52 m of the Frenchman Mountain section was not included. This interval consists of a single facies—glauconitic, bioclastic grainstone—which occurs only in this uppermost interval.
RESULTS AND INTERPRETATIONS
McKee (1945, p. 77–78) described three types of dolostone within his undifferentiated dolomites: “(1) a white to buff, granular, hard, massive dolomite; (2) a white to yellow, aphanitic (porcelain-textured), thin-bedded dolomite showing fine, irregular laminae on weathered surfaces; and (3) steelgray, fine-grained, thick-bedded dolomite which weathers with olive, silty surfaces and forms resistant cliffs.” These lithologies are consistently distinct from the limestones and dolostones of the underlying Muav Formation, which is why McKee decided to exclude them from the Muav. The carbonates at the top of the Muav are typically darker and more resistant to erosion than those at the base of the FMD. However, the contact between the Muav and the FMD is not simply a contact between limestone and dolostone. The uppermost Muav is also dolostone in many Grand Canyon sections, and it is consistently dolostone in the western Grand Canyon and Lake Mead region (Korolev, 1997) (Fig. 1). In Marble Canyon, in a fault-controlled side canyon (informally named “50-Mile creek”), on the west side of the Colorado River, 49.7 river miles below Lee’s Ferry, the carbonates of the Muav Formation transition into a siliciclastic facies below the dolostone of the FMD (Fig. 1). At some western localities, such as Diamond Bar Ranch, Arizona, the top of the Muav is characterized by karstic features: cracks with brecciated and terra rossa–like infill. This surface is capped by laminated to stromatolitic dolostones of the basal FMD. Together, these characteristics intimate the presence of a subaerial exposure surface and flooding event at the Muav-FMD contact.
In the Grand Canyon, the FMD forms westward-thickening vertical cliffs between the top of the Muav Formation and the base of the Devonian Temple Butte Formation (Fig. 3). In most sections examined in this study, the Havasu Member of the Muav, which directly underlies the FMD, is also dolomitized, but the boundary is usually unambiguous and sharp in the field. In western Grand Canyon and Lake Mead region sections, we identify the base of the FMD as an interval dominated by dololaminite, with or without stromatolites. This interval is lighter in color and less resistant to erosion than are the underlying beds of the Muav Formation. Also, in most western sections, beginning with those exposed in Quartermaster Canyon and the informally named “269-Mile canyon” (located on the north side of the Colorado River, 269.3 river miles downstream from Lee’s Ferry), and continuing westward, chert nodules are conspicuously present within the basal few meters of the FMD (Figs. 1 and 4). In the eastern Grand Canyon, the presence of meter-scale stratigraphic couplets is a distinguishing feature of the FMD.
On the basis of lithostratigraphic characteristics, we can correlate the FMD 280 km westward from the eastern Grand Canyon to the edge of the Cambrian craton and an additional 100 km onto the shelf (Fig. 1). Southern Great Basin localities have had a complex structural history. Craton-margin sections experienced Neogene westward extension (Wernicke et al., 1988), and shelf sections experienced both eastward Cretaceous thrusting (Levy and Christie-Blick, 1989) and Neogene extension. Thus, the relative Cambrian positions of some of these sections are not precisely known. The pre-extension position of Frenchman Mountain, however, has been reconstructed with a high degree of confidence (Fryxell and Deubendorfer, 2005).
The base of the FMD correlates with a third-order sequence boundary recognized by Keller et al. (2012) ~130 m above the base of the Banded Mountain Member of the Bonanza King Formation in the Nopah Range (Fig. 1). Montañez and Osleger (1993) did not recognize discrete sequence boundary surfaces within the Bonanza King Formation. Rather, they identified “sequence boundary zones,” which are intervals of thin, tidal flat–dominated parasequences that exhibit evidence of multiple episodes of exposure. In the Nopah Range, and also at Indian Ridge, Nevada, one of their sequence boundary zones occurs ~130 m above the base of the Banded Mountain Member. This interval is hypothesized to be correlative with the 130 m sequence boundary recognized by Keller et al. (2012).
A stratigraphic interval that has proven to be helpful in constraining the portion of the Bonanza King Formation that correlates with the FMD is a distinctive, reddish, silty interval at the base of the Banded Mountain Member. This reddish silty interval was recognized by Hazzard and Mason (1936), who informally named it “Member 2” in their original description of the Bonanza King Formation in the Providence and Marble Mountains of California. Barnes and Palmer (1961), working at the Nevada Test Site, later used this same interval to formally subdivide the Bonanza King Formation into two members: the Papoose Lake Member below and the Banded Mountain Member above. The silty carbonate interval defines the basal portion of the Banded Mountain Member. Osleger and Montañez (1996) referred to this interval as their “mixed unit,” interpreting the abundant quartz silt to represent eolian dust that was blown onto carbonate tidal flats from the non-vegetated Laurentian craton. Keller et al. (2012) referred to this interval as the “silty interval,” a term we use as well (Fig. 1). We interpret this silty interval to represent a lowstand systems tract that overlies the sequence boundary identified by Keller et al. (2012) in the Nopah Range (Fig. 1). We follow Osleger and Montañez (1996) and Keller et al. (2012) in employing this reddish siltstone as a geologically isochronous interval.
At Frenchman Mountain and Diamond Bar Ranch, a prominent reddish, silty interval occurs within the upper portion of the Kanab Canyon Member of the Muav Formation and also in the overlying Gateway Canyon Member (Figs. 1 and 3). The red color continues less prominently into exposures of the overlying Havasu Member. These lowstand-systems-tract red silts were presumably removed by erosion in more cratonward sections. We correlate this interval with the silty interval at the base of the Banded Mountain Member of the Bonanza King Formation, permitting a robust correlation between the shelf and the sparsely fossiliferous and unfossiliferous strata on the craton at this stratigraphic position. Its position is also bracketed by distinctive chemostratigraphic trends in these successions, observed by Montañez et al. (2000), Lin et al. (2019), and this study (see below). Together, such evidence supports correlation of the boundary between the Muav Formation and the Frenchman Mountain Dolostone with the third-order sequence boundary located a few tens of meters above the top of the reddish silty interval of the Banded Mountain Member, as identified by Osleger and Montañez (1996) and Keller et al. (2012).
From a thickness of 371 m at the stratotype section at Frenchman Mountain, the FMD thins eastward to 362 m at Tramp Ridge and 274 m at Devils Cove (Fig. 1). East of the Grand Wash fault, thicknesses decrease sharply (Fig. 1) to 117 m at Quartermaster Canyon, 106 m at 269-Mile canyon, and 77 m near Diamond Bar Ranch. Eastward into the Grand Canyon, the thickness gradually decreases to 52 m in Fern Glen Canyon (Rowland et al., 1995) and 20 m at Blacktail Canyon (Rose, 2003, 2011). The abrupt change in thickness across the Grand Wash fault suggests that this fault was active during the Cambrian Period.
In its easternmost exposures, in Marble Canyon, the FMD varies from 30 m in thickness at 50-Mile creek to just 8 m at Palisades of the Desert (Fig. 5). The thinness of the FMD in the Palisades of the Desert section is due to the presence of an incised valley filled with the Devonian Temple Butte Forma tion. Pre–Temple Butte incision of this valley removed at least 12 m of the FMD (Fig. 5).
Facies and Depositional Environment Interpretations
Brathovde (1986) found the FMD in Grand Canyon exposures to be pervasively dolomitized—predominantly composed of dolomicrite—with the original textures obliterated beyond recognition. However, in exposures in the Lake Mead region, including the Frenchman Mountain stratotype section, the original sedimentary textures are mostly preserved, despite dolomitization.
We recognize the following eight lithofacies in the FMD, representing peritidal and shallow subtidal depositional environments: (1) cryptomicrobial dololaminite (with or without associated chert nodules and with or without mudcracks and/or tepee structures), (2) silty dololaminite, (3) flatpebble conglomerate, (4) rippled dolostone, (5) burrow-mottled mudstone and wackestone, (6) stromatolitic boundstone, (7) oolitic grainstone, and (8) glauconitic bioclastic grainstone (Fig. 6). Carbonate lithologic terminology follows Dunham (1962). Each of these lithofacies is briefly described and interpreted in the following sections.
(1) Cryptomicrobial Dololaminite (Figs. 6A and 6B)
This facies consists of planar-parallel and wavy-parallel, light gray, finely laminated (0.5–2.0 mm) dolomitic mudstone-wackestone and interbedded flat-pebble conglomerate. It occurs in all sections, most commonly in the lower portion of the FMD as lithologically distinct, recessive intervals, ranging from 5 to 20 m in thickness. Buff to yellowish brown chert lenses are common, typically with laminae preserved within the chert (Fig. 6A).
In some cases, the laminae display occasional disruptions, desiccation features, and evidence of traction deposition, such as low-angle truncations. Thin, 2- to 8-cm, intraclastic beds are common. Desiccation features, such as mudcracks, micro-erosion surfaces, and tepee structures, are commonly present (Fig. 6B). This facies is typically closely associated with low-profile, laterally linked stromatolites (Fig. 6C). This facies is a ubiquitous component of carbonate platform deposits (e.g., Barnaby and Read, 1990; Zhang et al., 2015; Guo et al., 2018a; Pratt and Rule, 2021).
We interpret this facies to be the product of sediment trapping and binding by microorganisms and/or biologically induced direct precipitation of calcium carbonate from the water column on periodically exposed tidal flats (Assereto and Kendall, 1977; Shinn, 1983; Burne and Moore, 1987; Noffke and Awramik, 2013).
(2) Silty Dololaminite (Fig. 6D)
This is a mixed facies of interlaminated quartz silt and dolomicrite occurring in centimeter- and millimeter-scale couplets, displaying wavy-parallel and planar-parallel laminations. Asymmetric, climbing ripples are common, with centimeter-scale amplitudes and wavelengths in the range of 5–8 cm (Fig. 6D). This facies forms a significant component of the “silty interval” at the base of the Banded Mountain Member of the Bonanza King Formation (Fig. 1) (Osleger and Montañez, 1996). In the FMD, it is present only sporadically, and is rarely thicker than a few decimeters.
The quartz silt in this facies probably arrived in the carbonate peritidal environment as eolian dust from the craton, as suggested by Osleger and Montañez (1996). The silt was then intermingled with lime mud and reworked by tidal currents. It was ultimately deposited on tidal flats as laminated and rippled laminae.
(3) Flat-Pebble Conglomerate (Fig. 6E)
The flat-pebble conglomerate facies typically occurs as relatively thin (5–30 cm) beds of poorly sorted, partially imbricated, flat pebbles of chert and dolomicrite. Beds as much as 1 m thick in some cases occur. Pebbles are typically 2–5 cm long and <0.5 cm thick with rounded margins.
Flat-pebble conglomerates are common in Proterozoic and lower Paleozoic shallow-water carbonate deposits resulting from multiple processes (Lee and Kim, 1992; Myrow et al., 2004). In the case of the FMD, we interpret this facies to record the episodic occurrence of high-energy storm waves that ripped up and displaced portions of the intertidal and shallow-subtidal seafloor.
(4) Rippled Dolostone (Fig. 6F)
This facies consists of thin (1–8 cm), rippled, graded dolostone beds. The bases of the beds are coarse grained, and the rippled tops are composed of silty carbonate. These even, wavy-parallel beds are separated by very thin laminae (<2 mm) of non-resistant silty carbonate. The bottom of each bed reflects the morphology of the underlying rippled surface. Ripples with symmetrical crests are common, and ripple heights are 1–2 cm with an average wavelength of 6–7 cm. In some instances, a secondary crest, nearly perpendicular to the primary trend, is superimposed on the primary ripple crests, producing ladderback ripples on the bedding surface. Some ripples exhibit desiccation cracks on their crests. Some beds in this facies are moderately burrow mottled. This facies is commonly closely associated with the silty dololaminite facies. A similar facies has been described in other carbonate platform deposits (e.g., Chaudhuri, 2005).
We interpret this facies to record deposition in a peritidal setting, as indicated by the abundance of desiccation features, ladderback ripples, and a close association with the silty dololaminite facies. The conspicuous symmetrical ripples may record the influence of storm waves or possibly waves generated under slackwater conditions at high tide.
(5) Burrow-Mottled Mudstone and Wackestone (Figs. 6G and 6H)
The burrow-mottled facies, consisting of medium to dark gray, thick-bedded, burrow-mottled dolomitic mudstone and wackestone, is the volumetrically dominant facies within the FMD. It is common in Grand Canyon sections as well as those in the Lake Mead area. In Lake Mead region sections, this facies typically composes ~50% of the entire thickness of the formation. It is commonly intercalated with other facies; however, in Lake Mead region exposures, it commonly occurs in continuous intervals as much as 30 m in thickness. For the Muav Formation, McKee (1945) used the terms “marbled” and “mottled” to describe this facies.
In moderately burrowed examples of this facies, individual burrows are typically 1.5–2 cm in diameter and 7–10 cm long. They are commonly oriented vertically to subvertically. However, in more intensively burrowed examples, horizontal burrows are common (Fig. 6H). In addition to these large burrows, narrower burrows, 2–3 mm in diameter and 3–5 cm long, also occur.
The burrows, which were originally filled with lime mud, are now composed of coarse calcite mosaic, which makes them distinctly lighter in color than the surrounding matrix. As described by Brathovde (1986), the burrow-mottled facies most commonly has a crystalline mosaic texture represented by xenotopic dolomite and calcite crystals with irregular and curved intercrystalline boundaries and undulatory extinction. In most exposures, this facies is fairly homogeneous and exhibits no primary structures other than the burrows. The burrowing organisms presumably destroyed pre-existing sedimentary structures.
The predominance of lime mud and burrow mottling, the rarity of cross-stratification, and the absence of exposure features all indicate deposition of this facies in a quiet, subtidal setting that was occasionally interrupted by storm events that produced interbedded flat-pebble conglomerates.
(6) Stromatolitic Boundstone (Fig. 6I)
This facies includes relatively large, domal stromatolites (Fig. 6I) as well as lower-profile forms. In Grand Canyon sections, the laminae rarely form domes. In more distal craton-margin sections, such as at Frenchman Mountain and Tramp Ridge, the stromatolites more commonly take the form of 5- cm- to 2-m-tall domes or slightly branching columns. Within this facies, orange to dark brown silicification is common, resulting in abundant chert nodules and lenses and silicified portions of stromatolites.
We distinguish two subfacies of this facies: one with high-profile domal forms and the other with low-profile, laterally linked forms that grade into parallel laminae. The former is most commonly associated with the burrow-mottled and oolitic grainstone facies.
We interpret the domal stromatolites to have formed in a quiet, subtidal setting where micro-organisms baffled the water current and trapped sedimentary particles in a tangle of filaments (Noffke and Awramik, 2013). The lower-profile subfacies, in contrast, is more commonly associated with flat-pebble conglomerate; we interpret these low-profile stromatolites to have developed in a shallower, more energetic, peritidal environment. In such settings, in Cambrian platform deposits, stromatolites are very common (e.g., Zhang et al., 2015).
(7) Oolitic Grainstone (Fig. 6J)
Oolitic grainstones are not abundant in the FMD, although they become a dominant facies in the uppermost hundred meters of the correlative Bonanza King Formation in southern Great Basin sections (Montañez and Osleger, 1993). Montañez and Osleger (1993, their figure 12) reported an interval ~40 m thick of their oolitic-skeletal shoal facies at Frenchman Mountain. Our Frenchman Mountain measured section, which we presume to be a different section than the one measured by Montañez and Osleger (1993), contains several meter-scale intervals of oolite but no interval that is tens of meters in thickness (Fig. 7). The thickest occurrence we encountered in this study is an 8-m-thick, laterally continuous interval in our Devils Cove section. More commonly, ooid-rich intervals occur within individual stromatolite beds and in burrow-mottled intervals.
Oolitic beds are composed of oolitic-intraclastic grainstone and packstone. The grains are poorly sorted, with ooids ranging from 0.5 to 2.0 mm in diameter and intraclasts ranging from subrounded 2 mm grains to elongate clasts as much as 1 cm in length (Fig. 6J). All ooids display concentric laminae, and most have micritic envelopes. Paleocurrent data from this facies, based on measurements of cross-stratified beds at Quartermaster Canyon and at Whitney Ridge, Nevada, indicate bimodal paleoflow with preferential NE-SW orientation (Korolev, 1997).
We interpret the depositional environment that produced this facies to have been subtidal, oolitic shoals that were open to the sea. Open-marine conditions favor the growth of tangentially oriented calcite crystals (Heller et al., 1980), such as those present in the FMD. The NE-SW bimodal orientation of paleocurrents is at a high angle to the Miaolingian shoreline of the Colorado Plateau region. Thus, we infer that the paleocurrents that are preserved as cross-bedding in the oolite grain-stone record the action of tidal currents reworking ooid-shoal grains.
(8) Glauconitic Bioclastic Grainstone (Figs. 6K and 6L)
The glauconitic bioclastic grainstone facies is restricted to distal craton-margin sections where it forms very distinct, weakly stratified, reddish orange–to buff-weathering cliffs at the very top of the FMD. At Frenchman Mountain, this interval is 52 m thick. Its easternmost occurrence is at Tramp Ridge (Fig. 4), although it may have been present at more cratonward locations as well prior to the truncation of those sections by pre-Devonian erosion. But this facies is much less prominent, or completely absent, in shelf sections to the west. Gans (1974) reported no such facies in his stratigraphic study of Bonanza King Formation exposures in the upper plate of the Keystone thrust in southern Nevada, nor did Fenton (1980) report this facies in the Bonanza King Formation in the Desert Range, Nevada. Montañez and Osleger (1993) reported an ooid-skeletal-shoal facies at the correlative position at the top of the Bonanza King in sections to the west but not in thicknesses comparable to those at Frenchman Mountain.
Bioclasts in this facies are predominantly flat, millimeter-scale plates (Fig. 6L). In thin section, in polarized light, the plates exhibit uniform extinction and a perforate fabric typical of stereom. These characteristics unequivocally identify the bioclasts as echinoderm ossicles (Bathurst, 1972). Although we cannot rule out the possibility that these ossicles represent stylophorans, they probably represent eocrinoids, a group of mostly stalked, high-level suspension feeders that were abundant in the Miaolingian and Furongian of western North America (Robison, 1965; Sprinkle and Kier, 1987). Eocrinoids commonly occurred in gregarious associations (Sprinkle and Kier, 1987) and could produce encrinite-like facies such as this one. Extant echinoderm taxa have a low tolerance for salinity variations (Sprinkle and Kier, 1987), and it is commonly inferred that this limitation applies to the phylum as a whole. Thus, the abundance of echinoderm skeletal grains in this facies implies normal marine salinity. Rare elongate trilobite fragments are present in thin sections. Bioclastic grainstone is the dominant lithology in this facies, but wackestone and packstone also occur. Pore space is occluded with a fine, microspar mosaic.
Deep green, rounded, millimeter-scale peloids of the micaceous mineral glauconite are abundant at the base of the thick interval composed of this facies and are uniformly dispersed among the hypothesized eocrinoid plates (Fig. 6L). The glauconite grains become progressively less abundant upsection within the 52 m interval, and they are rare to absent at the top. In modern seas, glauconite forms in relatively deep water (Cloud, 1955). However, it is conspicuously abundant in Cambrian shallow-water sediments. Peters and Gaines (2012) attributed this anomaly to enhanced chemical weathering of continental crust globally, associated with the development of the Great Unconformity (see also Rowland, 2022).
Glauconite peloids, such as those in this facies, are commonly interpreted to be altered fecal pellets. Filter-feeding organisms, such as eocrinoids, remove clay and silt-size particles from the water column, producing fecal pellets of clay aggregates that are diagenetically altered to glauconite (Chafetz and Reid, 2000; see also El Albani et al., 2005). We suggest that the glauconite peloids in this facies probably resulted from that process. The low faunal diversity suggests a restricted environment, however the abundance of echinoderms implies normal marine salinity such as might occur in a coastal embayment.
The lack of desiccation features and absence of distinct bedding suggest a subtidal environment, while the fragmented bioclastic grains suggest the occasional presence of moderate- to high-energy storm waves. We suggest that this facies records the presence of an extensive, long-lived, subtidal, eocrinoid-rich embayment.
Sequence Stratigraphy, Stacking Patterns, and Facies Couplets
We recognize six depositional sequences within the FMD at Frenchman Mountain (Fig. S1). As is typical of shallow carbonate platforms, however, each sequence is not bounded by a single, unambiguous erosion surface (Strasser, 2015). Rather, there are sequence boundary “zones,” each consisting of stacked amalgamations of minor erosion surfaces, dololaminites, and stromatolitic facies (Fig. 7; Fig. S1). Sufficient ambiguity exists regarding where to define a sequence boundary in such situations that different researchers may identify sequences differently. Montañez and Osleger (1993), for example, recognized just three sequences within the FMD at Frenchman Mountain (their Banded Mountain Member of Bonanza King Formation), in contrast to our six.
Figures 4 and 5 show suprameter-scale occurrences of facies within the FMD at different localities. In contrast, a higher-resolution, meter-scale representation of facies within the Frenchman Mountain stratotype section is presented in Figure 7. At the supra-meter scale, intervals dominated by intertidal facies and those dominated by shallow-subtidal facies roughly correlate from one section to the next (Figs. 4 and 5). There is a no conspicuous pattern of lithologic cyclicity at this scale.
Supra-Meter-Scale Alternation of Subtidal Facies
Intervals of alternating subtidal facies are common in the FMD (Fig. 7). Each alternating couplet is typically several meters thick, composed of dark to medium gray, highly bioturbated, thick-bedded, dolomitic mudstone, gradationally overlain by one or another of three facies: (1) less-bioturbated, thinner-bedded, medium gray dolomitic mudstone, (2) a layer of laterally linked stromatolites, or (3) silty dololaminite (Fig. 7). The upper facies is then overlain by intensively burrow-mottled mudstone, which begins the next couplet. Evidence of erosion or subaerial exposure is lacking in these intervals.
Meter-Scale Peritidal Facies Couplets
Meter-scale couplets of alternating peritidal facies are ubiquitous components of carbonate platforms (e.g., Osleger and Read, 1991; Montañez and Osleger, 1993; Meng et al., 1997; Wilkinson et al., 1998; Lehrmann and Goldhammer, 1999; Burgess, 2006; Zhang et al., 2015; Guo et al., 2018a, 2018b), They conspicuously occur within the FMD (Figs. 7 and 8A–8F) as well as within correlative portions of the Bonanza King Formation. Distinctive dark- and- light bands exposed on Banded Mountain in Nye County, Nevada, provided the name of the Banded Mountain Member of the Bonanza King Formation, the unit that is partially correlative with the FMD (Fig. 1).
In the FMD, each dark- and- light couplet is typically one to two meters thick (Figs. 8A–8F). The dark interval is medium to dark gray, intensively burrowed mudstone (burrow-mottled facies), in some cases interbedded with silty dololaminite (Fig. 8E). High-relief, laterally linked stromatolites as well as rippled dolostone rarely occur in the basal portion of this dark band. Flat-pebble conglomerate commonly dominates the upper portion of this band.
Over a transition zone of one to two decimeters, the dark band typically grades into a light-colored band (Figs. 8B and 8C). The light-colored band is typically thinner than the dark band. It consists of silty dololaminite and/or dololaminite with mud cracks. Micro-erosional surfaces, karst features, and low-relief stromatolites are common in the lighter interval. In Grand Canyon exposures, where the bedding is horizontal, the meter-scale couplets typically form prominent cliff- and- slope topographic cycles. The burrow-mottled, darker portion of each couplet is less resistant to erosion than is the lighter portion, so the darker portion typically erodes more readily, forming a recess beneath the more resistant lighter interval.
There is a long history of discussion and debate concerning the origin of meter-scale variation in peritidal facies on carbonate platforms, with glacio-eustatic sea-level change commonly being the preferred mechanism (e.g., Osleger and Read, 1991; Montañez and Osleger, 1993; Meng et al., 1997; Strasser, 2015; Zhang et al., 2015; Guo et al., 2018a, 2018b). Inferred processes generating meter-scale cycles range from nearly random to deterministic in their temporal frequency (Lehrmann and Goldhammer, 1999), and some of the proposed processes may not be sensitive to water depth (Rankey, 2004) or wave height (Purkis et al., 2015). It has proven to be very challenging, therefore, to discern between the influence of externally driven, allocyclic forcing and that of inherent, autocyclic processes in the creation of alternating meter-scale couplets of peritidal carbonate facies preserved in the rock record (Wright and Burchette, 1996; Hill et al., 2009, 2012).
Peritidal couplets, such as those in the FMD, may not record actual changes in water depth; they may be a function of autocyclic processes that are unrelated to episodes of sea-level change. Such meter-scale couplets can be a product of random or nearly random patterns of lateral sediment transport as tidal flat islands accrete vertically and migrate laterally during relatively steady, passive-margin subsidence (Pratt and James, 1986; Wilkinson et al., 1998; Dyer and Maloof, 2015).
Decimeter-Scale Facies Couplets
Decimeter-scale couplets, 0.1 to 0.5 m thick, also occur in the FMD (Figs. 8G and 8H). They occur in light-colored, dololaminite-dominated intervals, but the suite of facies from which these cycles are constructed is variable. At Frenchman Mountain, the decimeter-thick couplets typically consist of silty dololaminite or flat-pebble conglomerate at the base, capped by cryptomicrobial laminite (Fig. 8H). At 269-Mile canyon and Quartermaster Canyon, the decimeter-scale couplets more commonly begin with a thin interval of burrow-mottled or nodular facies, capped by a thicker, cryptomicrobial interval with mud cracks. Low-relief stromatolites may occur at the bases of these couplets.
In our 50-Mile creek section, there is a striking example of centimeter- and decimeter-scale couplets nested within meter-scale alternations of facies. That section of the FMD consists of meter-scale couplets of siltstone and silty limestone with a very regular alternation of lithologies with respect to the abundance of silt (Fig. 5). Within the meter-scale fluctuations in silt abundance are centimeter-scale alternations; centimeter-scale limestone beds are interbedded with centimeter-scale siltstone intervals.
Statistical Analysis of Facies Succession Patterns and Thicknesses
The logged strata within the Frenchman Mountain stratotype section consist of 222 intervals of seven of the eight lithofacies described above (excluding the glauconitic bioclastic grainstone facies, which occurs in one continuous interval at the top of the Frenchman Mountain section) (Fig. S1, see footnote 1). The average stratum thickness is 1.36 m.
The null hypothesis is that there is no order in the sequence and thicknesses of these strata. Following Burgess (2016), we derived the Markov metric m from an analysis of the vertical succession of FMD facies, and the runs metric r from an analysis of the thicknesses of the units. This analysis involved a comparison of the values derived from the documented occurrences and thicknesses of strata in the measured stratotype section at Frenchman Mountain with 5000 randomly shuffled versions of the same set of strata. This provided a way to generate a suite of similar but disordered strata for comparison with the measured section.
The results are displayed in Figure 9. Figure 9A shows the vertical succession of facies. Figure 9B is a transition probability matrix of the facies, used to calculate the Markov metric. For example, there is a 38% probability of transitioning from facies 7 to facies 1. Figure 9C shows the frequency of the facies, irrespective of thickness. Figure 9D is a graph of the Markov order metric, with the red line indicating the observed value m = 0.081 (P-value 0.8274); the blue area shows the probability distribution of the metric based on 5000 random shuffles of the observed data. Figure 9E is a graph of the runs order metric r, with the red line indicating the observed value r = 1.283 (P-value 0.5324); the blue area shows the probability distribution of the metric based on 5000 random shuffles of the observed data.
P-values of 0.05 or less are generally considered to be statistically significant, which would providing support for the alternative hypothesis that there is order in the sequence and/or thickness of the succession of strata. The very high P-values obtained here do not permit the rejection of the null hypothesis of no order. We conclude, therefore, that there is no evidence for order, including cyclicity, in the stratigraphy of the FMD at Frenchman Mountain based on the applied metrics. This conclusion is visually reinforced by the graphs in Figures 9D and 9E, which show that the calculated Markov metric and runs metric values lie close to the center of the respective distributions generated from 5000 random shuffles of the observed data.
A study that is peripherally relevant to our study was conducted by Burgess (2008), in which he used data from Montañez and Osleger (1993) to analyze carbonate thickness distributions at Frenchman Mountain, among many other localities, in the Muav Formation and FMD (considered by Montañez and Osleger to be the Bonanza King Formation). Facies succession patterns, such as cyclicity, were not addressed in the Burgess (2008) study, only the distribution of bedding thickness. Carbonate platforms commonly exhibit an exponential distribution of lithofacies thicknesses, with a high number of thin units and exponentially fewer intermediate and thick beds. However, Burgess found that the distribution of stratigraphic thicknesses in the Frenchman Mountain section, among other sites, did not conform to the predicted exponential distribution; there were too few thin units and too many intermediate and thick beds. A possible explanation suggested by Burgess (2008) is a measuring bias on the part of the stratigrapher; thin beds, especially of the same lithofacies, tend to be grouped together in the stratigraphic column. In our study of the FMD, such groupings of thin lithofacies units doubtless occurred. We measured our sections with a 1.5 m Jacob staff, and it was not practical, within a reasonable amount of time, to record decimeter-scale facies changes within a section that is hundreds of meters thick.
Autocyclic versus Allocyclic Processes in the Deposition of the FMD
There is general agreement that there was an absence of polar ice caps, sea ice, and continental ice in the Cambrian Period (Hearing et al., 2018; Wotte et al., 2019; but see Runkal et al., 2010, and response by Landing, 2011). Coupled with extremely high (~4500 ppm) atmospheric CO2 (Berner and Kothavala, 2001), this indicates that the Cambrian Period, and the Miaolingian and Furongian Epochs in particular, experienced “supergreenhouse” climatic conditions. This term is used in a paleo-climate context to characterize geologic intervals of extraordinary warmth (Stephens et al., 2016). This in turn argues against glacioeustatic fluctuations as drivers of repetitive facies transitions within the FMD. Under supergreenhouse conditions, orbitally driven fluctuations of sea level are thought to be minimal or completely absent (Catuneanu et al., 2009). We conclude, therefore, that conspicuous meter-scale alternations of peritidal facies seen in some portions of the FMD (Fig. 8) were primarily autocyclic and were not driven by glacio-eustatic sea-level change or other allocyclic forces.
Research on aquifer eustasy in epicratonic Cretaceous supergreenhouse settings suggests that geologically shortterm shifts between humid to arid climate conditions can produce variations in aquifer charge and discharge that are volumetrically similar to the impact of waxing and waning glaciers (Hay and Leslie, 1990; Wendler and Wendler, 2016). Such variations can drive cyclic patterns in shallow carbonate systems (Wendler et al., 2016), and such patterns may occur in synchrony with Milankovitch- and sequence stratigraphic–scale cycles. In settings where geochronologic control is superb, it may be possible to link cyclic carbonatehosted proxies like weathered clays to climate drivers and processes, such as increased chemical weathering under humid lowstand conditions, and vice versa (see reviews in Sames et al., 2020).
Examination of the FMD in craton-margin exposures typically involves hiking up narrow canyons or along narrow ridge crests. For that reason, it is difficult to trace the lateral continuity of individual beds, lithofacies, and stacking patterns. However, two studies of Furongian portions of the Great American Carbonate Bank in central Nevada and western Utah were specifically designed to test the lateral continuity of individual facies couplets (Zeiza, 2010; Widiarti, 2011). Both of these studies revealed that couplet boundaries disappear within tens to hundreds of meters parallel to bedding and that within individual couplets, the component facies pinch out or interfinger with other facies or with non-alternating intervals. In both cases, the researchers concluded that autocyclic processes, similar to those inherent in the aggrading-tidal-island model of Pratt and James (1986), were the agents responsible for the alternation of facies rather than allocyclic processes.
Autocyclic processes provide the simplest explanation for the succession of facies in the FMD. This conclusion is supported by our Markov order and runs order statistical analyses (Fig. 9). Aquifereustatic influences on these rocks are worth investigating through proxies such as weathering-sensitive minerals and derivative byproducts such as kaolinite and illite (e.g., Hallam et al., 1991; Bergaya et al., 2006).
THE STRATOTYPE AT FRENCHMAN MOUNTAIN
We establish a stratotype for the FMD at Frenchman Mountain that is accessible and on public land managed by the Bureau of Land Management (Fig. 10A). A detailed stratigraphic column, from Korolev (1997), is presented in Figure S1 (see footnote 1). The stratotype is located in a prominent unnamed canyon 0.7 km south of East Lake Mead Boulevard on the western face of the mountain (Figs. 10A and 11). The mouth of this canyon is at 36°11′29″ N, 115°0′28″ W. We define the base of the FMD in the stratotype as the sharp contact between dark gray, burrow-mottled, erosion-resistant dolo-stone of the Muav Formation and the overlying, light gray, less resistant dololaminite (Fig. 7). We define the top of the formation as the sharp contact between resistant, cliff-forming, reddish-weathering, glauconitic, bioclastic, echinoderm plate–rich grainstone and the overlying, brownish gray–weathering, ledge-forming, interbedded shale and dolostone of the Dunderberg Shale Member of the Nopah Formation (Fig. 7).
Accessing the FMD stratotype exposure requires bouldering skills, and the uppermost portion is very steep. Another exposure where the FMD and associated units may be more easily and safely examined is located 2 km to the south of the stratotype canyon, east of the intersection of East Bonanza Road and Los Feliz Street (Fig. 10B). There, a moderately steep, unmarked hiking trail, beginning at 36°10′33″ N, 115°0′54″ W, switches back and forth up the west face of the mountain through the Muav Formation, FMD, and overlying units. We measured the stratotype section prior to discovering the more easily accessible exposure.
Most of the stratotype lies on the Las Vegas NE 7.5′ quadrangle map, as does the more southerly exposure. The uppermost portion of the stratotype is on the Frenchman Mountain 7.5′ quadrangle map. Geologic maps of these quadrangles are available from the Nevada Bureau of Mines and Geology (Matti et al., 1993; Castor et al., 2000). The FMD was mapped as the Banded Mountain Member of the Bonanza King Formation by Matti et al. (1993) and as the Frenchman Mountain Dolomite by Castor et al. (2000). On the 1:100,000-scale Las Vegas 30′ × 60′ quadrangle geologic map (Page et al., 2004), it is mapped as Bonanza King Formation.
The complete Cambrian section at Frenchman Mountain is ~900 m thick (Rowland, 1987; Rowland et al., 1990), accounting for more than half of the strata exposed on the mountain’s west face. Approximately half of the Cambrian section is composed of the 371-m-thick FMD, which is conformably overlain by the Furongian Nopah Formation. The Nopah Formation, which is 28 m thick at Frenchman Mountain (Sundberg, 1979), is disconformably overlain by the Devonian Mountain Springs Formation (Figs. 10, 11). This unit is disconformably overlain by 6.7 m of Middle Devonian strata of member C of the Mountain Springs Formation (Miller and Zilinsky, 1981). The Mountain Springs Formation is in turn overlain by the Devonian Sultan Limestone, which is correlative with the Temple Butte Formation in the Grand Canyon.
The FMD has yielded no biostratigraphically useful fossils (McKee, 1945; Brathovde, 1986; Korolev, 1997; Middleton and Elliott, 2003). Microbialites and burrow-mottled dolomicrites are abundant, but these biosedimentary structures have no bio-stratigraphic value except when considered in an evolutionary or intrabasinal framework larger in scope than this study (e.g., Shapiro and Awramik, 2000).
In two of our measured sections in the Lake Mead region, the uppermost lithofacies of the FMD is a bioclastic grainstone composed of glauconite peloids, echinoderm plates, and rare trilobite spines seen only in thin section (Figs. 4, 6K, and 6L). None of these fossils are biostratigraphically diagnostic. Fossil assemblages employed to address the age of portions of the FMD have been described from bounding strata at Frenchman Mountain (Pack and Gayle, 1971; Palmer and Halley, 1979; Sundberg, 1979; Webster, 2011a) and correlative or bounding sections in Nevada at Sheep Mountain (Hardy, 1986), Muddy Mountains (Resser, 1945), Desert Range (Palmer, 1979; Fenton, 1980), Groom Range (Webster et al., 2011), and Split Mountain (Sundberg et al., 2011; Webster, 2011c; Lin et al., 2019).
The sedimentological analysis of the Bonanza King Formation by Montañez and Osleger (1993) included a Bonanza King section at Frenchman Mountain, the upper portion of which we now place within the FMD. Their determination that the Bonanza King Formation represents a fully aggraded, flat-topped carbonate platform permits the inference that the top of the FMD is correlative with the top of the Bonanza King, as depicted in Figure 12.
Fenton (1980) studied trilobites in a section of the Bonanza King Formation in the Desert Range, 65 km north-northwest of Frenchman Mountain. His study showed that the Papoose Lake Member lies at least partially within the Glossopleura walcotti Zone. The boundary between the Glossopleura walcotti Zone and the Ehmaniella Zone is poorly constrained in the Bonanza King Formation due to a dearth of trilobites; it occurs roughly in the middle of the Papoose Lake Member, which is reinforced by our chemostratigraphic data, discussed below.
Above the “silty interval,” the Banded Mountain Member of the Bonanza King Formation contains Ehmaniella Zone taxa at its base, followed by Bolaspidella, Cedaria, and Crepicephalus Zone taxa, up to within ~10 m of the top (Fig. 12). In the Desert Range, Palmer (1979) documented trilobites that bracket the Crepicephalus-Aphelaspis Zone boundary up to 9 m below the top of the Bonanza King Formation. Similarly, Fenton (1980) recovered a diverse fauna of Aphelaspis Zone trilobites within the upper ten meters of the Bonanza King Formation, up to the top meter. He reported the first appearance of Dicanthopyge, marking the base of the Dicanthopyge Zone, in the top meter of this unit (Fig. 12).
Sundberg (1979) examined the biostratigraphy of the Dunderberg Shale and Halfpint Members of the Nopah Formation, which conformably overlies the Bonanza King Formation. He found no identifiable trilobites in Frenchman Mountain exposures; however, in other sections, he found Dicanthopyge Zone taxa within the lower few meters of the Dunderberg Shale, succeeded in turn by Prehousia Zone, Dunderbergia Zone, and Elvinia Zone taxa, as depicted in Figure 12. The presence of Dicanthopyge through Elvinia Zone fossils in the Dunderberg Shale and Halfpint Members of the Nopah Formation was also reported by Miller et al. (1981).
In sections examined in this study, we did not observe karstic features at the contact between the Frenchman Mountain Dolostone and the Dunderberg Shale. However, Keller et al., (2012) reported the common occurrence of such features at the top of the Bonanza King Formation in southern Great Basin exposures, recording emergence and exposure. Thus, we infer the presence of a depositional hiatus between the Frenchman Mountain Dolostone and the Nopah Formation (Fig. 12). This horizon defines the top of supersequence γ of Keller et al. (2012). Morgan (2012) and Taylor et al. (2012) used this same horizon to define the top of their Sauk II supersequence. The presence of Dicanthopyge Zone trilobites in the uppermost Bonanza King Formation and also in the base of the Dunderberg Shale (Fig. 12) suggests that this depositional hiatus was relatively brief.
The presence of Furongian Series trilobites, i.e., Aphelaspis Zone and Dicanthopyge Zone taxa, in the upper ten meters of the Bonanza King Formation documents that this formation, and therefore the FMD, extends across the Miaolingian-Furongian boundary (Fig. 12; cf. Karlstrom et al., 2018, 2020; Sundberg et al., 2020).
δ13Ccarb Isotope Profile
In order to place the δ13Ccarb chemostratigraphy of the FMD into a global context, we sampled all Cambrian carbonates at Frenchman Mountain from the Lyndon Limestone Member of the Bright Angel Formation through the Nopah Formation (Fig. 13; Table S1, see footnote 1). This suite provides a preliminary regional reference section for Miaolingian-to-Furongian δ13Ccarb chemostratigraphy of the central Basin and Range and permits identification of global and within-basin isotopic patterns.
There are five notable chemostratigraphic features in the succession, including, from oldest to youngest, two Wuliuan negative excursions, an unconformity- or condensation-revealing isotopic shift at the Wuliuan-Drumian boundary, a Drumian negative excursion or couplet of excursions, and a large positive excursion in the Paibian (Steptoean). These five chemostratigraphic features are preceded by trends toward nadir and/or acme values, are not represented by lone data points, and have been reproduced by replicate sampling of excursion-bearing and excursion-bounding samples from the Frenchman Mountain succession.
The lowest of these excursions is recorded in the Lyndon Limestone Member of the Bright Angel Formation (N5? in Fig. 13). It is actually a pair of negative excursions, the lower of which descends to a nadir of −5‰, followed by a return to values near 0‰. The upper excursion then descends to −2.6‰. This pair of excursions spans ~15 m of section and corresponds biostratigraphically with the lower Glossopleura walcotti Zone. A coeval negative excursion was documented by Montañez et al. (2000) in the Papoose Lake Member of the Bonanza King Formation at Jangle Ridge, Nevada, and elsewhere. A negative excursion at approximately this stratigraphic level also occurs in the Emigrant Forma tion at Split Mountain, Nevada, where Lin et al. (2019) termed it the N5 excursion. This stratigraphic interval corresponds to the Tincanebits and Meriwitica Tongues of the Bright Angel Formation, two distinctive, ridge-forming Grand Canyon units (McKee, 1945). These two tongues amalgamate westward within the Grand Canyon to form the Lyndon Limestone at Frenchman Mountain and elsewhere in the southern Great Basin. Given the possibility that the tops of the Tincanebits and Meriwitica Tongues represent maximum flooding surfaces or sequence boundaries, a regional examination of this pattern and potential diagenetic linkages is merited.
The next prominent negative excursion, to −2.0‰, occurs in the basal Spencer Canyon Member of the Muav Formation. This excursion, labeled N6? in Figure 13, is not at the unit contact, but is stratigraphically within the transition from the Glossopleura walcotti Zone to the Ehmaniella Zone. This is similar to an excursion documented in the Banded Mountain Member of the Bonanza King Formation at Indian Ridge, Nevada (Montañez et al., 2000), and also in the Emigrant Formation at Split Mountain, where Lin et al. (2019) termed it the N6 excursion. The co-occurrence of these negative excursions with trilobite extinctions, i.e., zone boundaries, suggests a causal relation, at least within the southern Great Basin.
Higher in the succession, in the Gateway Canyon Member of the Muav Formation, there is a sharp change from negative-trending values to positive values at or close to the Ehmaniella-Bolaspidella Zone transition, which translates globally to the Wuliuan-Drumian Stage boundary (Fig. 13). This offset suggests that an unrecognized unconformity is present in the Gateway Canyon Member of the Muav Formation or that this represents a condensed interval not captured by our relatively coarse (1.5 m) sampling of this unit. This unconformity or condensed interval occurs near the top of the silty interval of the Muav Formation and Bonanza King Formation. We suggest that this surface corresponds to the regionally extensive sequence boundary and sub-Drumian Carbon Isotope Excursion (DICE) unconformity that Howley and Jiang (2010) documented throughout the Great Basin, including in the Banded Mountain Member of the Bonanza King Formation in the nearby Desert Range. It is very close to the global Wuliuan-Drumian boundary and is similar in age and magnitude to a ~4‰ negative shift documented by Shembilu and Azmy (2021) near the boundary between the Hawke Bay and March Point Formations in Newfoundland, Canada.
Stratigraphically higher, in the lower FMD, there is a robust trend toward negative δ13Ccarb values that is interpreted to occur in the lower Bolaspidella Zone. Like the possible N5 negative excursion in the Lyndon Limestone Member of the Bright Angel Formation, it is also a couplet of negative values, spanning ~18 m of strata. The lower nadir of the couplet is −2.7‰ and the upper nadir of the couplet is −2.5‰. We tentatively interpret this couplet to represent the global DICE event. Its interpreted position in the lower Bolaspidella Zone is internally consistent with documentation of the DICE in the basal portion of the Drumian Stage throughout the Great Basin (Brasier and Sukhov, 1998; Montañez et al., 2000; Babcock et al., 2007; Howley and Jiang, 2010). The proximity of these excursions to the unconformity at the Muav Formation–FMD contact suggests that this isotopic shift is associated with a change in environments, although there is no covariance in δ18O values (R2 = 0.04; Fig. S2, see footnote 1) that would suggest pervasive meteoric diagenesis.
Near the top of the Frenchman Mountain succession is the only conspicuous positive excursion (to 3.5‰). This event begins in the upper FMD and continues upward into the Dunderberg Shale and Halfpint Members of the Nopah Formation. The excursion spans ~28 m of strata, including six trilobite zones from the Aphelaspis to Taenicephalus Zones.
The δ13Ccarb chemostratigraphy of the FMD is best considered in the context of both local basin-scale Cambrian chemostratigraphy and emerging global Cambrian chemostratigraphic compilations (Zhu et al., 2004; Barili et al., 2018; Cramer and Jarvis, 2020). Overall, the chemostratigraphy of the succession exhibits a remarkably similar pattern to both global and local (i.e., Great Basin) chemostratigraphic records. There are five notable features in the succession, including, from oldest to youngest, the N5 and N6 negative excursions, an unconformity- or condensation-revealing isotopic shift at the Wuliuan-Drumian boundary, the DICE event, and the Steptoean Positive Carbon Isotope Excursion (SPICE) event.
The positive excursion (~3.5‰) that spans the uppermost FMD and the Nopah Formation is similar in stratigraphic position and magnitude to the global SPICE event identified by Saltzman et al. (1998, 2000) at Shingle Pass in eastcentral Nevada, and elsewhere in the Great Basin. We interpret this excursion in the Frenchman Mountain section as the SPICE event. Interestingly, although the Elvinia and Taenicephalus Zones are present in the Smoky Member of the Nopah Formation at Frenchman Mountain, the δ13C values do not return to values between 0‰ and 1‰, as they do in SPICE-bearing successions at Shingle Pass, Nevada, and elsewhere in Laurentian basins (Saltzman et al., 2004). Perhaps this could be because the signal is condensed or telescoped at Frenchman Mountain because we did not sample at high enough resolution, or because the time-transgressive nature of this extinction within the Great Basin is poorly constrained (sensu Schiffbauer et al., 2017; Pulsipher et al., 2021; Shembilu and Azmy, 2021). If the SPICE is time transgressive in the southern Great Basin, it is possible that the trend away from the SPICE’s isotopic zenith is truncated by the unconformity at the base of the Devonian Mountain Springs Formation. As a cautionary note, the Cambrian-Devonian disconformity is cryptic at Frenchman Mountain as well as in other Lake Mead–region sections. The chemostratigraphic trend with the Devonian Mountain Springs Formation and Sultan Limestone coincidentally reveals a negative trend that appears to be remarkably in line with the trending-toward-zero values of the underlying Nopah Formation. The conformance of this trend is particularly relevant for exposures to the east of Frenchman Mountain because they commonly lack clastics or pronounced lithologic changes that help visually define the unconformity in the field.
Finally, we acknowledge that our chemostratigraphy for this succession is preliminary and that postdepositional processes could be driving some of the signals in this record. For example, meteoric diagenesis can be a major driver of regional- and global-scale trends, particularly negative δ13C excursions (see also Geyman and Maloof, 2019). Comparison of δ18O to δ13C for the Cambrian succession at Frenchman Mountain reveals no strong covariance (R2 = 0.44) of values when all units are considered as a group (Fig. S2). Yet, cross-plotting of values within each member or unit reveals that certain stratigraphic intervals, especially the Lyndon Limestone (Bright Angel Formation; R2 = 0.67), Rampart Cave (Muav Formation; R2 = 0.59), Spencer Canyon (Muav Formation; R2 = 0.81), and Dunderberg Shale (Nopah Formation; R2 = 0.71) Members, exhibit notable covariance. Because the Lyndon Limestone, Rampart Cave, and Spencer Canyon Members are punctuated by the N5 and N6 excursions, higher-resolution sampling coupled with Mn/Sr analysis, cathodoluminescence, and trace element analysis (e.g., Shembilu and Azmy, 2021) would likely yield insightful results.
Despite these as-yet-unconstrained preservational and diagenetic risks, it is remarkable how similar the timing, scope, and magnitude of excursions in the δ13C record of the Frenchman Mountain succession are to the global and regional δ13C records cited. This similarity suggests that the section is at least recording global to regional changes in carbon cycling.
AN AGE MODEL FOR THE TONTO GROUP
Integration of biostratigraphic, chemostratigraphic, and sequence stratigraphic data for the Frenchman Mountain Dolostone along with chemostratigraphic, biostratigraphic, and U-Pb-based geochronologic data from the rest of the Cambrian succession (Karlstrom et al., 2020) and the global syntheses of Peng et al. (2012, 2020) permits us to propose a working-hypothesis age model for the Tonto Group at Frenchman Mountain, as summarized in Figure 12. This age model will be tested and modified as new dates are obtained and fossils discovered.
Starting at the base of the succession at Frenchman Mountain, the age of the Tapeats Sandstone, which contains no body fossils, remains poorly constrained. Karlstrom et al. (2020) reported a U-Pb chemical abrasion–isotope dilution thermal ionization mass spectrometry (CA-IDTIMS) maximum-depositional age (MDA) of 507.7 Ma on detrital zircon for a sample collected ~30 m above the base of the Tapeats at Frenchman Mountain (Matthews et al. 2018), where this formation is 48 m thick (Rowland et al., 1990). An MDA of 508.2 Ma was also determined for a sample collected near the base of the Tapeats in the eastern Grand Canyon (Karlstrom et al., 2020). Based on these ages, Karlstrom et al. (2020) concluded that the Tapeats Sandstone was deposited within a two-million-year interval, ca. 508–507 Ma. We tentatively place the base of the Tapeats Sandstone at Frenchman Mountain closer to 510 Ma, based primarily on the supposition that it is correlative in part with the Zabriskie Quartzite of the Death Valley, California, region (Fig. 12). The geochronologic age of the top of the Zabriskie is not tightly constrained, but it is biostratigraphically constrained by trilobites in the overlying Carrara Formation (Palmer and Halley, 1979) (Fig. 12).
Dyeran trilobite biostratigraphy within the Bright Angel Formation at Frenchman Mountain has been investigated by Webster (2011a, 2011b). As reflected in Figure 12, the lowest trilobites in this section are olenellids of the Peachella iddingsi Zone (Webster 2011a; see also Sundberg et al., 2020; Foster, 2021; Rowland, 2022). These are overlain by olenellids of the Bolbolenellus euryparia and Nephrolenellus multinodus Zones, respectively. The top of the N. multinodus Zone marks the extinction of olenellids, which also traditionally defines the top of the Dyeran Stage. At Frenchman Mountain, this boundary lies ~25 m above the base of the Bright Angel Formation (Webster, 2011b; Sundberg et al., 2020; Rowland, 2022). In Laurentia, this horizon is dated at ca. 506.5 Ma (Sundberg et al., 2020) (Fig. 12). The age of the base of the Bright Angel Formation is not tightly constrained, but it must be older than 506.5 Ma and younger than 507.7 Ma, the MDA of detrital zircons in the Tapeats Sandstone (Fig. 12). We tentatively place the age of the base of the Bright Angel Formation at Frenchman Mountain at ca. 506.7 Ma and the age of the Bright Angel Formation–Muav Formation contact at ca. 505.5 Ma (see below). This implies that the Bright Angel Formation was deposited over a relatively brief interval of ~1.2 m.y. This interval will be better constrained with future work.
Our results indicate that the Muav Formation does not span as much time as previously proposed. Assignment of the basal contact of the Muav to the base of the Ehmaniella Zone, as proposed by Karlstrom et al. (2020), cannot be reconciled with new data; such placement is too young, at least for Frenchman Mountain and western Grand Canyon exposures. The lowest two units of the Muav, the Rampart Cave and Sanup Plateau Members, lie within the Glossopleura walcotti Zone in Peach Springs Canyon of the western Grand Canyon (Sundberg, 2011) and presumably also at Frenchman Mountain. Rose (2011) suggested that this zone might extend as high as the Peach Springs Member of the Muav; however, the biostratigraphy and chemostratigraphy presented here and also by Lin et al. (2019) require placement of the upper limit of this zone lower in the succession (at excursion N6 in Fig. 13), at least in the Frenchman Mountain and western Grand Canyon sections. We recognize that the base of the Muav Formation probably becomes younger toward the east, as does the underlying Bright Angel Formation—hypotheses internally consistent with our data and with data from McKee (1945), Resser (1945), and subsequent authors (e.g., Karlstrom et al., 2018, 2020).
Moving stratigraphically higher in the Muav, a shale horizon at the top of the Peach Springs Member in some Grand Canyon exposures contains specimens of the trilobite Glyphaspis, which is an Ehmaniella Zone taxon (Rose, 2011). Glyphaspis also occurs in the Gateway Canyon Member (McKee, 1945). Sundberg (2011) reported the occurrence of upper Ehmaniella Zone trilobites in the Gateway Canyon Member in Peach Springs Canyon. Such fossil occurrences are internally consistent with extension of the Ehmaniella Zone well up into the Gateway Canyon Member to the first appearance of bolaspidellid trilobites (McKee, 1945).
We hypothesize that the Ehmaniella-Bolaspidella Zone boundary occurs at our observed chemostratigraphic truncation of the global terminal-Wuliuan negative δ13C trend within the Gateway Canyon Member, toward the top of the “silty interval” (Fig. 13). We further hypothesize that this boundary also coincides with the regional unconformity identified by Howley and Jiang (2010).
The Havasu Member—the uppermost member of the Muav—has not yet yielded fossils. We conservatively interpret this member to lie within the lower portion of the Bolaspidella Zone (Figs. 12 and 13), an interpretation consistent with the biostratigraphy and chemostratigraphy of coeval strata in the southern and eastern Great Basin, including the silty interval (Howley and Jiang, 2010; this study).
The top of the Muav Formation is also older than previously hypothesized. Karlstrom et al. (2020) proposed that the Havasu Member lies within the Cedaria Zone, high in the Marjuman Stage, based on the presence of the trilobite Spencella (formerly Solenopleurella), which occurs several meters below the bottom of the Havasu Member. Spencella may possibly range into the Bolaspidella Zone, but it is primarily an Ehmaniella Zone taxon (Sundberg, 1991). Thus, there is no compelling biostratigraphic evidence to support the placement of the top of the Muav within the Cedaria Zone, nor do the chemostratigraphic data support such a placement. We suggest that the top of the Muav Formation occurs near the bottom of the Bolaspidella Zone (Fig. 12). This placement is based on our interpretation of the age of the FMD, the correlation of the “silty interval” and its capping unconformity throughout the Great Basin, and the presence of the DICE in the basal potion of the FMD.
Our age model suggests that the base of the Muav Formation is ca. 505.5 Ma, low in the Wuliuan Stage, while the age of its top is ca. 502.8 Ma, low in the Drumian Stage (Fig. 12). Thus the Muav Formation is hypothesized to represent ~2.7 m.y. of deposition.
Based primarily on the chemostratigraphic record, including the DICE, we infer the base of the FMD at Frenchman Mountain to be ca. 502.8 Ma, with its top at ca. 495.5 Ma (Fig. 12). Thus, the FMD was deposited over an interval of ~7.3 m.y.
Our age model suggests that the interval of the Tonto Group represented at Frenchman Mountain was deposited over a time interval spanning between 14 and 15 m.y. The FMD (along with the correlative Banded Mountain Member of the Bonanza King Formation) represents the waning stage of Sauk II—the second of the three commonly recognized phases of the Sauk Transgression.
Figure 14 shows time-thickness bar graphs (cf. Mayer and Dickinson, 1984) and net sediment accumulation rates for the formations of the Tonto Group at Frenchman Mountain, based on the ages indicated in our age model. As indicated on these graphs, the Tapeats Sandstone has by far the lowest net sediment accumulation rate within this suite of formations, ~16 m/m.y. The rate increases dramatically upsection into the Bright Angel Formation; at ~122 m/m.y., this formation has the highest rate of net sediment accumulation within the Tonto Group. This is a minimum rate, however, because compaction of the fine-grained siliciclastic sediments in this unit was not taken into account and within-unit unconformities cannot be constrained. The rate of net sediment accumulation progressively decreased during deposition of the Tonto Group; the rate for the Muav Formation is ~88 m/m.y., while the rate for the FMD is ~50 m/m.y.
The 50 m/m.y. sediment accumulation rate for the FMD at Frenchman Mountain records a relatively low rate of net accumulation, which we interpret as representing the rate of passive-margin subsidence. Carbonate platforms commonly grow at rates of 100 m/m.y. and above (Schwab, 1976; Bosscher and Schlager, 1993). Cook and Taylor (1977) reported a sediment accumulation rate of 80 m/m.y. in the Miaolingian Whipple Cave Formation carbonate platform, ~200 km north of Frenchman Mountain. A slightly older (Terreneuvian to Miaolingian) carbonate platform in the Canadian Rockies accumulated sediment at a rate of 80 m/m.y. (Bond and Kominz, 1984).
CONCLUSIONS AND SUGGESTIONS FOR FURTHER RESEARCH
In 1945, Eddie McKee distinguished—but did not formally name—a cliff-forming interval of carbonate rocks of uncertain age overlying the Muav Formation in the Grand Canyon. After more than three-quarters of a century of obscurity and anonymity, these strata finally have a formally recognized name, a stratotype section, an age model constrained by litho-, bio-, and chemostratigraphy, a depositional environmental framework, and a stratigraphic connection with global events that were occurring half a billion years ago. The FMD is the capping formation of the Tonto Group. It was deposited in equatorial latitudes as a flat-topped, fully aggraded carbonate platform on the margin of the Laurentian craton, inboard of the “Great American Carbonate Bank.” It is constructed of shallow-subtidal and peritidal facies. It is 371 m thick at its Frenchman Mountain stratotype locality, and it thins dramatically eastward into the Grand Canyon, largely due to pre–Middle Devonian erosion. Although meter-scale couplets of dark- and light-colored facies are visually conspicuous in portions of the formation, statistical analysis of the succession of facies and their thicknesses in the stratotype section provide no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest explanation for the succession of facies in this formation. The FMD spans ~7.3 m.y. of Miaolingian and Furongian time, including portions of five trilobite zones. It is correlative with all but the lower 130 m of the Banded Mountain Member of the Bonanza King Formation, one of the thickest and most recognizable rock formations in the southern Basin and Range province.
The relative ease of access, thickness, and unvegetated exposure of the FMD at Frenchman Mountain and other Lake Mead–region sites offers diverse opportunities for future research. For example, the unit is ripe for higher-spatial-resolution δ13Ccarb chemostratigraphic or trace element analysis of the DICE, 87Sr/86Sr chemostratigraphic analysis to assess the regional position of the Drumian-Guzhangian boundary, and magnetic susceptibility or clay analyses of cyclic facies to reconstruct a Cambrian aridity-humidity record. Similarly, correlation and geochronologic hypotheses presented herein would benefit from an array of biostratigraphic and geochronologic work, such as a targeted search for identifiable trilobite fragments in the bioclastic grainstones, dissolution of samples for phosphatic microfossils, and maximum depositional age–focused U-Pb analysis of windblown detrital grains throughout the FMD’s siltier intervals.
The Frenchman Mountain exposure of the FMD provides a felicitous opportunity for more rigorously pursuing the question of the generation of meter-scale and sub-meter-scale cyclicity on carbonate platforms. Meter-scale and sub-meter-scale rhythmic alternations of facies are well exposed and accessible.
Sub-FMD units within the Frenchman Mountain section also contain signals of potentially regional or global significance that merit further scrutiny. For example, the cryptic unconformity and δ13Ccarb excursions (N5, N6) in the Muav and Bright Angel Formations that we identified are hypothesized to be tied to trilobite extinctions and/or regional flooding or exposure events as represented by the Lyndon Limestone Member, Tincanebits Tongue, and Meriwitica Tongue of the Bright Angel. Such hypotheses are testable through focused, high-resolution chemostratigraphy and biostratigraphy of cratonic and basinal exposures of these intervals.
We gratefully acknowledge financial support from patrons of the Denver Museum of Nature and Science, the University of Nevada Las Vegas (UNLV) paleontological research fund, the David B. Jones Foundation, and U.S. National Science Foundation (NSF) grant EAR-1954634. Korolev acknowledges the UNLV Graduate and Professional Student Association. Thanks to F. Bachhuber for the use of his motorized canoe. K. Karlstrom and L. Crossey provided critical logistical support and useful discussions regarding Cambrian stratigraphic nomenclature. Grand Canyon fieldwork in 2019 was conducted under research permits of K. Karlstrom with Grand Canyon National Park. Frenchman Mountain samples for chemostratigraphic analysis were collected under Bureau of Land Management (BLM) research permit N-100230 to Rowland. M. Holland, C. Dehler, and students assisted in the measurement of sections in the eastern and central Grand Canyon and with stratigraphic and chemostratigraphic interpretations. F. Sundberg provided suggestions regarding biostratigraphy and the construction of Figures 12 and 13, and R. Metcalf assisted with photomicrographs. D. Dettman and the team at the University of Arizona’s Environmental Isotope Laboratory are thanked for their assistance with geochemical analyses. K. Azmy, F. Corsetti, C. Dehler, G. Jiang, K. Karlstrom, and B. Pratt provided helpful reviews of an early draft. Thanks to reviewers Galen Halverson and Bruce Wilkinson, Associate Editor Greg Hoke, and Science Editor David Fastovsky for insightful reviews that helped improve this paper.