Non-technical Summary
New trilobites from the Ophir Formation of the western Uinta Mountains in northern Utah represent some of the first identifiable and biostratigraphically useful fossils in the range, outside of Dinosaur National Monument. The fossils indicate that the Ophir Formation is similar in age to the Lodore Formation.
Introduction
The Uinta Mountains, locally known as the Uintas, are an east-west trending Laramide uplift in northeastern Utah and northwestern Colorado. Cambrian strata in this region include the Tintic Quartzite and overlying Ophir Formation in the west and the Lodore Formation in the east (Fig. 1.1). Each succession lies unconformably atop the Neoproterozoic Uinta Mountain Group and is disconformably overlain by the Mississippian Madison Limestone (Hansen et al., Reference Hansen, Rowley and Carrara1983; Bryant, Reference Bryant1992; Sprinkel, Reference Sprinkel2006), except where local depositional remnants of Devonian strata are preserved (e.g., Herr, Reference Herr1979; Soule, Reference Soule1992; Myrow et al., Reference Myrow, Hasson, Taylor, Tarhan and Fike2023a, Reference Myrow, Hasson, Taylor, Tarhan, Ramirez, Fowlkes and Chenb). Tintic Quartzite and Ophir Formation outcrops in the western part of the Uinta Mountains are separated from Lodore outcrops in the east by a ~75 km gap in which there are no Cambrian strata preserved and the Madison lies directly on the Uinta Mountain Group (Fig. 1.1, 1.2; Stokes and Madsen, Reference Stokes and Madsen1962; Sprinkel, Reference Sprinkel2006). Tintic-Ophir strata thin eastward toward this gap and the Lodore Formation thins westward toward it. This gap is hypothesized to result from erosion of Cambrian sediments before deposition of the Mississippian Madison Limestone (Williams, Reference Williams1957; Robison, Reference Robison1964; Hansen, Reference Hansen1965; Lochman-Balk, Reference Lochman-Balk1972). In the Uinta Mountains, the upper Tintic and Ophir units are Miaolingian (Williams, Reference Williams1957; Robison, Reference Robison1964) and the Lodore Formation was previously considered mostly upper Cambrian (Furongian; Untermann and Untermann, Reference Untermann and Untermann1949, Reference Untermann and Untermann1954; Williams, Reference Williams1953; Halgarth, Reference Halgarth1959; Robison, Reference Robison1964; Herr et al., Reference Herr, Picard and Evans1982), although recent data indicate that the middle portion is Miaolingian (Myrow et al., Reference Myrow, Hasson, Taylor, Tarhan, Ramirez, Fowlkes and Chen2023b).
Figure 1. Context of fossils studied herein. (1) Location of study area in northeastern Utah, western United States. (2) Map of Uinta Mountains and Cambrian localities studied here (stars). Outcrop exposures in black lines around southern rim of mountains. Western outcrops mapped as Tintic Quartzite but include Ophir Formation studied here; eastern outcrops assigned to the Lodore Formation. Outcrops based on geologic maps by Bryant (Reference Bryant1992), Hansen et al. (Reference Hansen, Rowley and Carrara1983), and Sprinkel (Reference Sprinkel2006). (3, 4) Photographs of Iron Mine Ridge section of upper 74 m of Cambrian rocks (Ophir Formation): (3) looking west-southwest diagonally down dip and across strike, with approximate Ophir-Madison contact labeled; white arrow indicates approximate level of fossil layers; (4) looking southeast showing upper Ophir Formation layers and with fossil layers level shown by white arrow. (5) Stratigraphic section of Ophir Formation at Iron Mine Ridge locality showing distributions of fossil taxa. (6) Stratigraphic section of Tintic Quartzite and Ophir Formation at Rock Creek showing distributions of fossil taxa.
These Uinta Mountains Cambrian rocks contain no datable volcanic units, lack unique populations of detrital zircons, and historically have lacked biostratigraphically useful fossils. Thus, it has been challenging to test the hypothesis that the Tintic-Ophir and the Lodore successions might be similar in age, or that they represent erosional remnants of the same depositional system. To help resolve this knowledge gap, here we present a specimen-rich but low diversity fauna from the Tintic-Ophir succession of the Uinta Mountains and employ it to refine the age and biostratigraphic position of these strata.
Geologic setting and localities
Cambrian rocks are exposed along the southern flank of the Uinta Mountains and include the Tintic Quartzite and overlying Ophir Formation in the western part of the range, east of Salt Lake City (Bryant, Reference Bryant1992; Fig. 1.1, 1.2), and the Lodore Formation as far east as Juniper Mountain, Colorado (Sprinkel, Reference Sprinkel2006). Due to high-angle reverse faulting, Cambrian rocks are not exposed on the surface along the northern flank of the Uintas, except for a small outcrop of the Tintic Quartzite at the Long Park Reservoir dam in Daggett County, Utah (Sprinkel, Reference Sprinkel2006).
The Cambrian succession in the western Uintas is usually up to ~200 m thick and dominated by gray to brown quartzite and sandstone, which is locally conformably overlain by an up to ~70 m thick, easily weathered interval characterized by alternating sandstone and shale. Although these upper rocks are mapped as Tintic Quartzite, the upper ~70 m portion of the succession has been interpreted by some stratigraphers to represent the Ophir Formation (Williams, Reference Williams1953, Reference Williams1957; Lochman-Balk, Reference Lochman-Balk1955). This dichotomy between map unit names and stratigraphic assignment could be because, in the Uintas, the ~70 m of finer-grained strata that cap or overlie the Tintic are too thin and limited in geographic extent to be mapped separately as a distinct unit. For comparison, near its type area ~125 km to the southwest, the Ophir Formation alone ranges up to ~130 m in thickness (Hintze, Reference Hintze1988).
In the eastern Uinta Mountains, the Lodore Formation ranges up to 180 m in thickness and is dominated by feldspathic to quartzitic sandstone with minor shale and conglomerate (Hansen, Reference Hansen1977; Herr, Reference Herr1979). In places where the Lodore is not capped by Devonian or younger erosional remnants (Herr, Reference Herr1979; Soule, Reference Soule1992; Myrow et al., Reference Myrow, Hasson, Taylor, Tarhan and Fike2023a), the succession is similar in appearance to the Tintic-Ophir succession. The upper portion of the Lodore consists of alternating glauconitic shale, sandstone, and minor sandy dolostone—lithologies that are comparable to portions of the Dotsero Formation to the east in Colorado (see synthesis by Myrow et al., Reference Myrow, Taylor, Miller, Ethington, Ripperdan and Allen2003) or perhaps the ‘transition beds’ between the cliff-forming Tapeats Sandstone and the overlying Bright Angel Formation of the Grand Canyon region (McKee and Resser, Reference McKee and Resser1945; Karlstrom et al., Reference Karlstrom, Mohr, Schmitz, Sundberg and Rowland2020).
Iron Mine Ridge
The Iron Mine Ridge site (Fig. 1.2–1.5) is near Iron Mine Mountain in Wasatch County, Utah, at ~3,015 m elevation (Fig. 1.2). The succession consists of ~250 m of comparatively well-exposed dark-colored sandstones, well-indurated quartz sandstones, siltstones, and shales (Fig. 1.5). Outcrops are mapped as Tintic Quartzite at Iron Mine Ridge (Bryant, Reference Bryant1992), despite recognition that the upper portion of the succession is likely the Ophir Formation or transitional between the two (Williams, Reference Williams1953). This upper portion of the section consists of 38.6 m of tan to medium-brown siltstones and 15–30 cm of thick, orange, cross-bedded sandstones with thin, interbedded, light-green, platy shales, overlain by a 35.6 m covered interval immediately below the Mississippian-age Madison Limestone (Fig. 1.3). Fossils described below come from the upper portion of the mapped Tintic succession, in what is here considered the Ophir Formation.
Rock Creek 1 and 2 (Hell's Ten Miles and Dave's Brach Frags)
Two superjacent localities (Fig. 1.2, 1.6) ~1,300 m apart form a composite measured section through partially covered Cambrian strata, ~29–30 km east of Iron Mine Ridge. These exposures are in the vicinity of Rock Creek (a named topographic horizontal control station on a peak of ~3,464 m elevation, north of a Rock Creek stream), in Duchesne County, Utah. The sites near the Rock Creek station are in rocks mapped as Cambrian Undifferentiated (Bryant, Reference Bryant1992). The lower half of this succession is lithologically equivalent to the Tintic exposures near its type area, but the upper half of the succession contains substantial limestone interbedded with thin siltstone, shale, and sandstone (Fig. 1.6) and is here considered the Ophir Formation. In this respect, the Rock Creek 1 and 2 section differs from the majority of other Tintic exposures in the Uinta Mountains. The outcrops at Rock Creek occur between mapped Tintic outcrops immediately to the west and east (Bryant, Reference Bryant1992; Fig. 1.2). Lochman-Balk (Reference Lochman-Balk1955, fig. 1b, column 13) interpreted the Cambrian section here as consisting of Tintic Quartzite overlain by ‘cf. Ophir.’ We concur with her assessment and tentatively ascribe the upper portion of the section here to the Ophir Formation.
Biostratigraphy
Previous work and historical context
Most of the Tintic Quartzite in its type area, in the Tintic mining district ~125 km southwest of the western Uintas, has been considered early to medial Cambrian (Series 2 to Miaolingian) in age (Peterson, Reference Peterson1953; Lochman-Balk, Reference Lochman-Balk1976), whereas at its top in the Uinta Mountains it is thought to be younger, perhaps representing deposition during the early portion of the Miaolingian Epoch (early Wuliuan, Glossopleura Biochron; Robison, Reference Robison1976). This age determination was necessarily based on inference, and on fossils that were too poorly preserved or lacked sufficient stratigraphic control to be of biostratigraphic utility. For example, the only figured body fossil ascribed to the Tintic is a single partial trilobite (erroneously reported as Olenellus? Hall, Reference Hall1862; Peterson, Reference Peterson1953, fig. 5; Lochman-Balk, Reference Lochman-Balk1955) collected in float from the uppermost portion of the Tintic Formation at Long Ridge—a hillside that is littered with trilobite-bearing slabs sliding downhill from basal exposures of the overlying Ophir Formation. Trace fossils from the Tintic in the Wasatch and Lakeside ranges of Utah, although useful for regional correlation (Magwood, Reference Magwood1996), do not provide age control for the unit.
In the Uinta Mountains, unfigured specimens from the Ophir Formation include two unidentifiable trilobite fragments and a suite of ‘longuloid’ or ‘obolid’ brachiopods (Westonia Walcott, Reference Walcott1901) collected by Williams (Reference Williams1953) from exposures at Iron Mine Mountain. In the type area of the Ophir to the southwest, Robison (Reference Robison1976) stated that the Ophir Formation contains trilobites of the Glossopleura walcotti Biozone (Wuliuan, Miaolingian; Palmer, Reference Palmer1954; Lochman-Balk, Reference Lochman-Balk1955) and the overlying Ehmaniella Biozone, but did not figure or describe them. Although the Ophir Formation contains more fossils than the Tintic, few specimens have been figured that can be tied with any confidence to stratigraphy or locality, and most of these are syntypic materials of Glossopleura producta (Hall and Whitfield, Reference Hall and Whitfield1877) from the Oquirrh Range southwest of Salt Lake City (Hall and Whitfield, Reference Hall and Whitfield1877; Walcott, Reference Walcott1886, Reference Walcott1916; Palmer, Reference Palmer1954).
The Lodore Formation has a similar history. Prior to recent work in the Dinosaur National Monument area (Myrow et al., Reference Myrow, Hasson, Taylor, Tarhan and Fike2023a, Reference Myrow, Hasson, Taylor, Tarhan, Ramirez, Fowlkes and Chenb), the formation was considered early late Cambrian (Dresbachian) or, in modern terms, latest Miaolingian in age. That age, however, was based on the occurrence of simple ‘ptychoparioid’ trilobites, brachiopods, hyoliths, and ‘tiny coiled gastropods,’ none of which were illustrated or tied to detailed measured sections (Untermann and Untermann, Reference Untermann and Untermann1954; Halgarth, Reference Halgarth1959). All were from the upper half of the unit but apparently lacked sufficient detail to permit biostratigraphic assignment. Lodore Formation trace fossils and K-Ar dating of glauconite in the unit also have not permitted accurate age assessment (Herr, Reference Herr1979; Herr et al., Reference Herr, Picard and Evans1982). Myrow et al. (Reference Myrow, Hasson, Taylor, Tarhan, Ramirez, Fowlkes and Chen2023b) reported several species of Elrathiella Poulsen, Reference Poulsen1927 tied to a stratigraphic section within Dinosaur National Monument, Utah, and assigned the Lodore Formation to the Ehmaniella Biozone and provisionally to the Altiocculus Subzone.
New fossils
There are two fossiliferous horizons in the Ophir Formation at our Iron Mine Ridge section (Fig. 1.4, 1.5). The lowest is a gray sandstone with heavy bioturbation and abundant white fossil fragments and rare valves of acrotheloid brachiopods and impressions of hyoliths and hyolithids (Fig. 2). Approximately 1.5 m above this horizon is a 2–5 cm thick, dark gray to tan sandstone with abundant trilobites plus rarer brachiopods and hyoliths (Fig. 1.5). Trilobites collected include Elrathiella aff. Elrathiella euthyopsis Sundberg, Reference Sundberg1994 and Trachycheilus aff. T. whirlwindensis Sundberg, Reference Sundberg1994. The majority of specimens are preserved as molds or casts on the bedding surfaces and within the beds. The combined sample of trilobites and other taxa from both beds at this site contained >615 specimens, with most from the upper bed and ~93.5% of specimens being trilobites.
Figure 2. Associated fossil material from the Ophir Formation at the Iron Mine Ridge locality (formerly upper Tintic Quartzite) and Rock Creek locality (formerly Cambrian Undifferentiated). (1–3) Fossils from Iron Mine Ridge (see Fig. 1.5): (1) brachiopod (FHPR 11151) from 40.5 m; (2) two hyoliths (FHPR 11155) from 40.5 m; (3) molds of two indeterminate hyoliths (FHPR 11153) from 39 m. (4–6) Fossils from Cambrian layers at Rock Creek (see Fig. 1.6): (4) mold of articulated dorsal exoskeleton of an indeterminate ‘ptychoparioid’ trilobite (FHPR 18457) from 114 m (Ophir Formation); (5) mold of a hyolithid hyolith (FHPR 18458) from 109 m (Ophir Formation); (6) linguloid brachiopod (FHPR 18460) from 77 m (Tintic Quartzite). Scale bars = 1 cm (1–5); 5 mm (6).
In the Rock Creek area, there are three productive horizons. The highest one (~114 m, Rock Creek 1; Figs. 1.6, 2.4) is in a bioturbated siltstone, where one indeterminate ‘ptychoparioid’ trilobite was found. Five meters below this horizon, a hyolith and several brachiopods were found (Fig. 1.6). Lower in the section, at the nearby Rock Creek 2 locality, abundant brachiopod fragments and at least one possible species of Lingulella Salter, Reference Salter1866 occurred in a medium to coarse reddish sandstone (~77 m; Figs. 1.6, 2.6).
Interpretation and implications
Species of Elrathiella, like those from the Ophir Formation at Iron Mine Ridge, were reported by Sundberg (Reference Sundberg1994) to range from the Elrathiella to basal Ehmaniella subzones of the Ehmaniella Biozone in Utah and Nevada. Elrathiella euthyopsis is also known from the Whirlwind Formation, Drum Mountains, and House Range, Utah. Species of Trachycheilus Resser, Reference Resser1945 range through the entire Ehmaniella Biozone (Proehmaniella–Altiocculus Subzones; Sundberg, Reference Sundberg1994). Trachycheilus whirlwindensis is from the Elrathiella to Ehmaniella subzones of the Whirlwind Formation, Drum Mountains; Shadscale Formation, Dugway Range, Utah; and Member E, Pole Canyon Limestone, Patterson Pass, Nevada. These two trilobite species suggest that, in the western Uintas, deposition of the Ophir Formation occurred during the middle portion of the Ehmaniella Biochron, Topazan Age (late international Wuliuan Age, Miaolingian Epoch), ~503 Ma (Karlstrom et al., Reference Karlstrom, Mohr, Schmitz, Sundberg and Rowland2020; Sundberg et al., Reference Sundberg, Karlstrom, Geyer, Foster, Hagadorn, Mohr, Schmitz, Dehler and Crossey2020).
Materials and methods
Fossils were collected by splitting weathered slabs at exposures on U.S.D.A. Forest Service land in the western Uinta Mountains near Iron Mine Mountain and near Rock Creek peak. Fossils were logged, collected, and reposited at the Utah Field House of the Natural History State Park Museum. Fossils occur as molds or casts on weathered sandstone bedding planes and were split out of the sandstones by splitting or breaking off corners. In total, ~575 trilobite cranidia, some pygidia, and 40 brachiopods and hyoliths were collected from Iron Mine Ridge; one articulated trilobite mold, a hyolith impression, and many fragments of brachiopods were collected from Rock Creek.
Illustrated trilobite specimens were coated with colloidal graphite and then ammonium chloride sublimate. Specimen orientation for photography and measurements was primarily with the cranidial anterior border and/or palpebral lobes, librigenal border, or pygidial border in a horizontal plane.
Repositories and institutional abbreviations
DMNS, Denver Museum of Nature and Science, Denver, Colorado; FHPR, Utah Field House of Natural History State Park Museum, Vernal, Utah; USNM, National Museum of Natural History, Smithsonian Institution, Washington, DC.
Systematic paleontology
Phylum Arthropoda Gravenhorst, Reference Gravenhorst1843
Class Trilobita Walch, Reference Walch1771
Order Unknown (see Sundberg and Webster, Reference Sundberg and Webster2022)
Family Alokistocaridae Resser, Reference Resser1939
Subfamily Alokistocarinae Hupé, Reference Hupé1955
Remarks
Sundberg (Reference Sundberg1994) recognized two subfamilies: Ehmaniellinae and Altiocculinae. With the recognition that Ehmaniellidae and Alokistocaridae are synonymous, the subfamily Ehmaniellinae is synonymous with Alokistocarinae (see Esteve et al., Reference Esteve, Sundberg, Zamora and Gozalo2012).
Genus Elrathiella Poulsen, Reference Poulsen1927
Type species
Elrathiella obscura Poulsen, Reference Poulsen1927, Pemmican River Formation, Inglefield, Greenland, by original designation.
Elrathiella aff. Elrathiella euthyopsis Sundberg, Reference Sundberg1994
Figure 3
Occurrence
Iron Mine Ridge, Ophir Formation (formerly upper Tintic Quartzite), Wasatch County, Utah.
Figure 3. Elrathiella aff. E. euthyopsis Sundberg, Reference Sundberg1994 from the Ophir Formation: (1) cranidium, FHPR 11094-3; (2) cranidium, FHPR 11094-2; (3) cranidium, FHPR 11148; (4) cranidium, FHPR 11093c; (5) librigena, FHPR 11147; (6–8) cranidium, FHPR 11132a, dorsal, anterior, and lateral views, respectively; (9–11) cranidium, FHPR 11098, dorsal, lateral, and anterior views, respectively; (12) cranidium, FHPR 11094a, b (smaller specimen); (13) cranidium, FHPR 11132-2; (14, 15) cranidium, FHPR 11101, dorsal and lateral views, respectively; (16) pygidium with articulated thoracic segment, FHPR 11146; (17) pygidium, FHPR 11093b; (18) pygidium, FHPR 11093a, questionably assigned to this species; (19–21) pygidium, FHPR 11147a, dorsal, posterior, and lateral views, respectively; (22) librigena, FHPR 11059, internal mold (not latex). All photos are of latex casts, unless otherwise noted. All specimens are from 40.5 m in the Iron Mine Ridge section (Fig. 1.5; Wasatch County, Utah).
Materials
Numerous cranidia and a pygidium.
Remarks
The specimens assigned to Elrathiella aff. Elrathiella euthyopsis are similar to Elrathiella euthyopsis in having an elongated glabella (73–79% cranidial length vs. 70–80% described by Sundberg in 1994 in the Whirlwind Formation of central Utah) that is moderately to strongly tapered and with a strongly rounded frontal lobe; anterior border strongly convex, deep anterior border furrow; moderately tapered; possible moderately coarse granular ornamentation (new material preserved in sandstone, presence of granular ornamentation cannot be firmly established). The material from the Iron Mine Ridge locality differs from the material from the Whirlwind Formation in having an anterior border that is shorter (vs. 45–55% of the frontal area length vs. 55–65%) and evenly curved; slightly narrower fixigenal width (40–50% glabellar width vs. 55–60%); a pygidium with a longer axis, and less pronounced pleural furrows. These specimens are left in open nomenclature due to their preservation as molds or casts in a medium-grained sandstone and the lack of detailed features of the exoskeleton surface.
A single pygidium (Fig. 3.18) that co-occurred with the cranidia has a more transversely elongated outline with more anteriorly located anterolateral corners. This specimen is questionably assigned to this species.
Genus Trachycheilus Resser, Reference Resser1945
Type species
Trachycheilus typicale Resser, Reference Resser1945, upper portion of the Bright Angel Formation or lowermost Muav Limestone (USNM loc. 73b; loc. 15 of Resser, Reference Resser1945), Kwagunt Valley, Grand Canyon, Arizona, by original designation.
Trachycheilus aff. T. whirlwindensis Sundberg, Reference Sundberg1994
Figure 4
Occurrence
Iron Mine Ridge, Ophir Formation (formerly upper Tintic Quartzite), Wasatch County, Utah.
Figure 4. Trachycheilus aff. T. whirlwindensis Sundberg, Reference Sundberg1994 from the Ophir Formation: (1–4) cranidium, FHPR 11139a, dorsal internal mold, dorsal, lateral, and anterior views, respectively; (5–7) cranidium, FHPR 11102, dorsal, anterior, and lateral views, respectively. All specimens are from 40.5 m in the Iron Mine Ridge section (Fig. 1.5). All images are of latex casts unless otherwise noted.
Materials
Two cranidia.
Remarks
Cranidia referred to Trachycheilus possess the tapered glabella, convexity, curved relatively wide (tr.) and short (sag.) anterior border, glabellar furrows, relatively short palpebral lobes, and a small medial sulcus in the front of the glabella typical of that genus. The larger cranidium (Fig. 4.1–4.4) from the Iron Mine Ridge locality is similar to T. whirlwindensis in its tapered glabella with a rounded frontal lobe, narrow fixigena, relatively short palpebral lobes, and convex, curved anterior border. Distinction between the two forms is that the former has a more tapered glabella and a longer preglabellar area than the two subspecies of T. whirlwindensis. The two cranidia also show no surface granules typical of T. granulosus Sundberg, Reference Sundberg1994. The Iron Mine Ridge specimens also differ from the single cranidium of the type species T. typicale Resser, Reference Resser1945 in having a longer preglabellar area and lacking the medium swelling and inbend of the anterior border. The smaller specimen (Fig. 4.5–4.7) has a narrower prefrontal area, wider fixigena, and less tapered glabella with a medial sulcus in the frontal lobe. In these features, this smaller specimen is more like T. whirlwindensis, however, these features could be the result of allometric changes during ontogeny. These two cranidia are left in open nomenclature due to their preservation as molds or casts in sandstone, the lack of details on their exoskeleton surface, and the absence of additional sclerites.
Indeterminate ‘ptychoparioid’
Figure 2.4
Occurrence
Rock Creek 1, Ophir Formation (formerly Cambrian Undifferentiated), Duchesne County, Utah.
Materials
Natural external mold of a single articulated and nearly complete individual in bioturbated fine-grained sandstone.
Remarks
Preservation too poor to allow a more precise identification than indeterminate ‘ptychoparioid’ based on short glabella.
Discussion
These Ophir Formation fossils provide the first upper-age constraints on deposition of the Tintic Quartzite in the Uinta Mountains, with the occurrence of Elrathiella and Trachycheilus requiring that local deposition of the Tintic ceased by or during early medial Cambrian time (Ehmaniella Biochron, Wuliuan Age, Miaolingian Epoch). These fossils also suggest that much of the Ophir Formation in the region was marine—a finding consistent with the presence of exclusively marine trace fossils in the unit and in the upper half of the conformably underlying Tintic Quartzite elsewhere (Magwood, Reference Magwood1996; personal communication, A. Ekdale, 2017).
A similar upper limit on deposition (Ehmaniella Biozone, Miaolingian) and environmental interpretation appears to exist in the eastern Uintas for the Lodore Formation (Myrow et al., Reference Myrow, Hasson, Taylor, Tarhan, Ramirez, Fowlkes and Chen2023b). Together with the eastward stratigraphic thinning of the Tintic and Ophir, the eastward disappearance of the typical cliff-forming quartzites of the Tintic, and the thermal exposure history (see review by Herr et al., Reference Herr, Picard and Evans1982), these fossil occurrences lend support to the hypothesis that the top of the Cambrian succession in this region was eroded and locally reflects a composite unconformity like those that occur atop the Sawatch Quartzite of Colorado (e.g., Myrow et al., Reference Myrow, Taylor, Miller, Ethington, Ripperdan and Allen2003). These data also permit us to hypothesize that the Tintic and lower Lodore are age equivalent and that, with comparable lithologies, they formed through similar depositional processes.
Based on the fossil material, the Ophir Formation of the Uinta Mountains appears to be broadly similar in age (Ehmaniella Biozone, Miaolingian) to the upper Lodore Formation of the eastern Uintas, as well as to the Wolsey Shale in Wyoming and Montana (Lochman-Balk, Reference Lochman-Balk1957; Schwimmer, Reference Schwimmer1973; Sundberg, Reference Sundberg1994), and the upper Bright Angel to lower Muav formations of the Grand Canyon (Karlstrom et al., Reference Karlstrom, Mohr, Schmitz, Sundberg and Rowland2020).
Future work
To test such hypotheses, we recommend focusing on the carbonate portion of the section at Rock Creek because it could have potential through crackout and dissolution-based treatments to identify additional fossils of biostratigraphic utility, and to couple fossils with δ13C chemostratigraphy. Similarly, the prevalence of young detrital zircon populations in Cambrian sandstones of this age in both northern Utah and adjacent areas in Colorado and Wyoming (Matthews et al., Reference Matthews, Guest and Madronich2017; Karlstrom et al., Reference Karlstrom, Hagadorn, Gehrels, Matthews and Schmitz2018, Reference Karlstrom, Mohr, Schmitz, Sundberg and Rowland2020; Cothren et al., Reference Cothren, Farrell, Sundberg, Dehler and Schmitz2022; Holm-Denoma, Reference Holm-Denoma, Matthews, Soar, Longman and Hagadorn2022) suggests that there is potential for obtaining a basal-age constraints for the Tintic Quartzite and the Lodore Formation. Such work would permit testing of the hypothesis that the units represent isochronous components of the same depositional system. By pairing such data with emerging biostratigraphy such as that presented here, there is opportunity to learn if/how deposition of the Tintic and Lodore is related to larger-scale Cambrian processes, e.g., Sauk-related deposition of other western epicratonic sandstones, ranging from the Flathead Sandstone of Wyoming to the Sawatch Quartzite of Colorado and beyond.
Acknowledgments
Field work was facilitated by U.S.D.A. Forest Service permit R4-ANF-MGM-FY20-001 and the Forest Service's D. Herron and J. Wilkins. Special thanks for assistance in the field to T. Howells, M.B. Bennis, D. Gray, and D. Herron. We are grateful for funding from the National Science Foundation (EAR 1954634, 1955115), David B. Jones Foundation, and patrons of the DMNS Department of Earth Sciences. We greatly appreciate the reviews of J. Taylor, G. Geyer, and an anonymous reviewer, whose comments greatly improved the manuscript.
Declaration of competing interests
The authors declare none.
