fl d006-01 page 1 of 26 Geological Society of America Field Guide 6 2005 Neoproterozoic Uinta Mountain Group of northeastern Utah: Pre-Sturtian geographic, tectonic, and biologic evolution Carol M. Dehler Department of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322, USA Douglas A. Sprinkel Utah Geological Survey, P.O. Box 14610, Salt Lake City, Utah 84114, USA Susannah M. Porter Department of Earth Science University of California Santa Barbara, Santa Barbara, California 93106, USA ABSTRACT The Neoproterozoic Uinta Mountain Group is undergoing a new phase of strati- graphic and paleontologic research toward understanding the paleoenvironments, paleoecology, correlation across the range and the region, paleogeography, basin type, and tectonic setting. Mapping, measured sections, sedimentology, paleontology, U-Pb geochronology, and C-isotope geochemistry have resulted in the further characteriza- tion and genetic understanding of the western and eastern Uinta Mountain Group. The Red Pine Shale in the western Uinta Mountain Group and the undivided clastic strata in the eastern Uinta Mountain Group have been a focus of this research, as they are relatively unstudied. Reevaluation of the other units is also underway. The Red Pine Shale is a thick, organic-rich, fossiliferous unit that represents a restricted environment in a marine deltaic setting. The units below the Red Pine Shale are dom- inantly sandstone and orthoquartzite, and represent a fl uviomarine setting. In the eastern Uinta Mountain Group, the undivided clastic strata are subdivided into three informal units due to a mappable 50–70-m-thick shale interval. These strata repre- sent a braided fl uvial system with fl ow to the southwest interrupted by a transgressing shoreline. Correlation between the eastern and western Uinta Mountain Group strata is not complete, yet distinctive shale units in the west and east may be correlative, and one of the latter has been dated (≤770 Ma). Regional correlation with the 770–742 Ma Chuar Group suggests the Red Pine Shale may also be ca. 740 Ma, and correlation with the undated Big Cottonwood Formation and the Pahrump Group are also likely based upon C-isotope, fossil, and provenance similarities. This fi eld trip will examine these strata and consider the hypothesis of a ca. 770–740 Ma regional seaway, fed by large braided rivers, fl ooding intracratonic rift basins and recording the fi rst of three phases of rifting prior to the development of the Cordilleran miogeocline. Keywords: Neoproterozoic, Uinta Mountain Group, intracratonic rift, vase-shaped microfossil, Bavlinella faveolata, Leiosphaeridia sp. Dehler, C.M., Sprinkel, D.A., and Porter, S.M., 2005, Neoproterozoic Uinta Mountain Group of northeastern Utah: Pre-Sturtian geographic, tectonic, and biologic evolution, in Pederson, J., and Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. 1–25, doi: 10.1130/ 2005.fl d006(01). For permission to copy, contact [email protected]. © 2005 Geological Society of America 1 fl d006-01 page 2 of 26 2 Dehler et al. INTRODUCTION tion indices (TAI) indicate 4.5–7 km of strata (Hansen, 1965; Stone, 1993; Sprinkel et al., 2002). This fi eld trip guide is a review and an update of the existing It is uncertain how the eastern and western Uinta Mountain data sets regarding Uinta Mountain Group geology and reports Group strata correlate; however, similar petrographic patterns are the latest ideas about depositional environments, correlation, evident across the range and throughout the group. Geochemical paleogeography, biologic evolution, and tectonic setting in north- and provenance studies show arkosic sandstone and shale in the eastern Utah during Neoproterozoic time. The focus of the fi eld north part of the range were derived from the Wyoming craton to trip will be on the previously understudied units (Red Pine Shale the north, and quartz arenite in the southern part of the range was and undivided eastern clastic strata), as well as a reevaluation of derived, in part, from a Paleoproterozoic source to the east (e.g., previous interpretations about the other better-studied units, and Wallace, 1972; Sanderson, 1978; Sanderson, 1984; Ball and how all of these units relate to one another in time and space. Farmer, 1998; Condie et al., 2001). The Uinta Mountain Group is interesting for many reasons: The age of the Uinta Mountain Group is likely entirely (1) it is one of few exposed strata in the region for understanding Neoproterozoic. It unconformably overlies the metamorphic the early tectonic evolution of the Late Neoproterozoic western quartzitic and schistose units of the Red Creek Quartzite and Laurentian margin; (2) it likely records the inception of climate Owiyukuts Complex (ca. 1.7 to ca. 2.7 Ga; Hansen, 1965; Sears change leading into the low-latitude glaciations of the Sturtian et al., 1982) and is unconformably overlain by lower Paleozoic episode; and (3) it contains a wealth of microfossils throughout strata. A 770 Ma detrital zircon population from the middle east- the succession that can inform us of pre-Sturtian biologic evolu- ern Uinta Mountain Group (Fanning and Dehler, 2005) indicates tion and how it may relate to (1) and (2) above. Lastly, (4) the that the majority of the Uinta Mountain Group is younger than Uinta Mountain Group is a “sleeping giant” in terms of being 770 Ma. The uppermost unit in the Uinta Mountain Group, the explored for its information on Precambrian geology. It has been Red Pine Shale, yielded a microfossil assemblage and C-isotope under the “curse of the Proterozoic sandstones” (Link et al., variability similar to that of the 742 Ma upper Chuar Group in 1993) for too long, and ongoing and future research will hope- Arizona, therefore putting a possible upper age limit on the fully lift the curse. Uinta Mountain Group (Vidal and Ford, 1985; Karlstrom et al., A general overview of the Uinta Mountain Group is pro- 2000; Porter and Knoll, 2000; Dehler, 2001; Dehler et al., 2006). vided fi rst, followed by a two-day road log that takes the reader Paleomagnetic data from the Uinta Mountain Group indicate clockwise around the Uinta Mountains. On Day 1, the western deposition in equatorial latitudes, and the Uinta Mountain Group and central strata of the north fl ank will be visited, and on Day paleopole sits right on the Chuar Group apparent polar wander 2 the easternmost strata and the strata of the south fl ank will be path, also suggesting a similar age (Weil et al., 2005). viewed and discussed. Western Uinta Mountain Group Stratigraphy UINTA MOUNTAIN GROUP STRATIGRAPHY The western Uinta Mountain Group has been subdivided The Neoproterozoic Uinta Mountain Group is a 4–7-km- several different ways (Williams 1953; Wallace and Crittenden, thick siliciclastic succession that is exposed only in the Uinta 1969; Wallace, 1972; Sanderson, 1984). The nomenclature used Mountains and makes up the core of the Uinta Mountain anti- here is mainly after Wallace (1972), in combination with what cline (Fig. 1). The strata exposed in the western Uinta Mountains has been considered mappable on a 1:125,000 scale by Bry- are characteristically different than those in the eastern Uinta ant (1992). These units include the lowermost formation of Mountains, perhaps due to structural subbasins imparting control Moosehorn Lake (including the basal undivided Uinta Mountain on depositional style. Hansen (1965) indentifi ed two structural Group), the formation of Red Castle, the formation of Dead Horse domes within the overall Uinta anticline, one in the western and Pass, the Mount Watson Formation, the formation of Hades Pass, one in the eastern part of the range, and these roughly correspond and the Red Pine Shale (Figs. 2 and 3). Only the Mount Watson to the changes in stratal character. Formation has been formally named following the Stratigraphic In the western Uinta Mountains, the Uinta Mountain Group Code (Sanderson, 1984). The Red Pine Shale was formalized comprises >4 km of sandstone and sedimentary quartzite, with by Williams (1953) prior to adoption of the Stratigraphic Code lesser shale and rare conglomerate (Wallace, 1972) (Figs. 1 (North American Stratigraphic Commission on Nomenclature, and 2). These strata show much lateral and vertical variability 1983). Other informal units proposed by Wallace (1972) may be and have undergone many subdivisions (see Sanderson, 1984). mappable at larger scales, but will not be featured in this paper. In the eastern part of the range, the Uinta Mountain Group is dominantly sandstone with lesser shale and a distinctive basal Basal Undivided Uinta Mountain Group and Formation of conglomerate and breccia (Jesse Ewing Canyon Formation; Moosehorn Lake Sanderson and Wiley, 1986). The base of the Uinta Mountain The lowermost 60 m of exposed Uinta Mountain Group in Group is exposed only in the eastern Uinta range, and calculated the western range is an undivided interval of white quartz arenite. thicknesses from air photos, seismic profi les, and thermal altera- It is likely a lateral equivalent of the formation of Red Castle, and O O O O O k 110 109 112 111 k k 108 r r r Manila WYOMING O o O 41 B o o 41 F F F SR Great e 43 Flaming Gorge Reservoir Zuj Salt 15 a s s s r r k h y Pz ve XWf t r a Lake c i Xr e k R n 1 B e a Zur 2 re iv l m e 3 C S 4 e B H 5 191 Dutch John W r Sheep Creek 3 B SR R 44 O Zuu Zuu WN A r S e COLOR D iv Zucl Zud S PA R A I N U R 80 80 r N T Cl TAH Salt Lake CityS e Zucu U K eb A M O 4 AO W Zuh U I N T Zur