GEOLOGY OF THE INTERMOUNTAIN WEST THE LOWER CRETACEOUS IN EAST-CENTRAL UTAH—THE CEDAR MOUNTAIN FORMATION AND ITS BOUNDING STRATA A Field Guide Prepared For SOCIETY OF VERTEBRATE PALEONTOLOGY (original) (raw)
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The Medial Cretaceous in East-Central Utah – the Cedar Mountain Formation and its Bounding Strata
2017
Although only recognized as a discrete stratigraphic unit since 1944, the Cedar Mountain Formation represents tens of millions of years of geological and biological history on the central Colorado Plateau. This field guide represents an attempt to pull together the results of recent research on the lithostratigraphy, chronostratigraphy, sequence stratigraphy, chemostratigraphy, and biostratigraphy of these medial Mesozoic strata that document the dynamic and complex geological history of this region. Additionally, these data provide a framework by which to examine the history of terrestrial faunas during the final breakup of Pangaea. In fact, the medial Mesozoic faunal record of eastern Utah should be considered a keystone in understanding the history of life across the northern hemisphere. Following a period of erosion and sediment bypass spanning the Jurassic–Cretaceous boundary, sedimentation across the quiescent Colorado Plateau began during the Early Cretaceous. Thickening of these basal Cretaceous strata across the northern Paradox Basin indicate that salt tectonics may have been the predominant control on deposition in this region leading to the local preservation of fossiliferous strata, while sediment bypass continued elsewhere. Thickening of overlying Aptian strata west across the San Rafael Swell provides direct evidence of the earliest development of a foreland basin with Sevier thrusting that postdates geochemical evidence for the initial development of a rain shadow.
The Lower Cretaceous in east-central Utah—The Cedar Mountain Formation and its bounding strata
Geology of the Intermountain West
Although only recognized as a discrete stratigraphic unit since 1944, the Cedar Mountain Formation represents tens of millions of years of geological and biological history on the central Colorado Plateau. This field guide represents an attempt to pull together the results of recent research on the lithostratigraphy, chronostratigraphy, sequence stratigraphy, chemostratigraphy, and biostratigraphy of these medial Mesozoic strata that document the dynamic and complex geological history of this region. Additionally, these data provide a framework by which to examine the history of terrestrial faunas during the final breakup of Pangaea. In fact, the medial Mesozoic faunal record of eastern Utah should be considered a keystone in understanding the history of life across the northern hemisphere. Following a period of erosion and sediment bypass spanning the Jurassic–Cretaceous boundary, sedimentation across the quiescent Colorado Plateau began during the Early Cretaceous. Thickening of t...
Late Cretaceous to Early Tertiary tectonostratigraphy of southwestern Utah
Museum of Northern Arizona Bulletin, v. 59, p. 181-188, 1993
The Upper Cretaceous to Paleogene sedimentary rocks of southwestern Utah record three temporally overlapping tectonic phases: 1) active Sevier-style thrust activity and foreland sedimentation; 2) cessation of thrust activity; and 3) active Laramide-style folding and intermontane sedimentation. The formations recording this tectonic evolution are, from oldest to youngest: the Iron Springs (and eastern equivalents), Kaiparowits, Canaan Peak, Grand Castle (new informal name), Pine Hollow, and Claron formations. Tbe Santonian to lower Campanian(?) upper part of the Iron Springs and mid-to-upper Campanian Kaiparowits formations represent synorogenic, fluvial sedimentation derived from the Sevier fold and thrust belt. The Iron Springs Formation received sediment from Precambrian to Upper Paleozoic strata exposed in the Wah Wah and Blue Mountain thrust sheets of southwestern Utah. The upper Campanian(?) to lower Paleocene Canaan Peak Formation was deposited in an east-to-northeast-directed, braided fluvial system. Petrographic and geochemical analysis of volcanic and siliceous clasts indicate that the Canaan Peak and Kaiparowits formations were derived from the Jurassic Deifonte Volcanics of southeastern California and the Mississippian Eleana Formation of southern Nevada. The early Paleocene Grand Castle formation represents an east- to southeast-flowing braided river system. Clast and sandstone lithologies indicate that the Grand Castle formation had the same provenance as the Iron Springs Formation. Conglomerate of the Grand Castle formation overlaps the easternmost Sevier thrust faults, suggesting a post-Sevier origin. The lower Pine Hollow Formation records active Laramide partition of the foreland basin associated with the development of the Johns Valley anticline and possibly the Circle Cliffs uplift. Fluvial, deltaic, and lacustrine deposits of the Claron Formation overlap paleotopographic highs of the Pine Hollow basin and indicate cessation of Laramide deformation by the middle Eocene.
We investigate the regional climatic effects of the formation of the ‘‘Nevadaplano’’ plateau during the Sevier Orogeny in an overall warming world. Paleohydrology was reconstructed from 590 individual measurements of phosphate O isotopes in continental faunas of the Lower Cretaceous Cedar Mountain Formation, Utah, U.S.A. Semi-aquatic (turtles, crocodiles) and terrestrial (dinosaurs) taxa are compared to coeval pedogenic carbonates to interpret changing water sources over time. Samples were grouped into four stratigraphic faunas (lower Yellow Cat, upper Yellow Cat, Ruby Ranch, and Mussentuchit members). Resulting isotopic values were converted to d18Ow values using established d18Op–d18Ow and d18Oc–d18Ow relationships. At a formation scale, turtles (d18Op = 14.1 to 15.7% V-SMOW) and crocodiles (d18Op = 15.0 to 19.2%) document water compositions of 28.1 to 26.1% and 27.7 to 24.2%, respectively, within the zonal range for formation-scale meteoric water at 34u N paleolatitude (27.1 to 24.8%) established by pedogenic carbonates (d18Oc = 22.0 to 23.5% V-SMOW). These data suggest that, like soil carbonates, turtle and crocodile phosphate isotopes can be used as proxies for meteoric water isotopic composition. Dinosaur d18Op (sauropods: 19.7 to 21.9%, ornithischians: 16.6 to 21.7%, small theropods: 16.9 to 18.2%, and large allosauroids: 19.1 to 20.3%) values generally exceed those of semi-aquatic taxa. Using mass-balance equations for modern terrestrial animals adjusted for size and inferred dinosaur physiology, ingested water is calculated for the above dinosaur groups. On a member scale, when meteoric-water values are compared with calculated dinosaur drinking water, values are equal to or lighter than meteoric water for most herbivorous groups (as low as 215.5% for ornithischians) and equal to or heavier than meteoric water for most carnivorous groups (as high as 22.0% for allosauroids). Changes in d18Ometeoric water, d18Odinosaur ingested water, faunal assemblages, and sedimentology, from member to member, correlate to thrusting events of the Sevier Orogeny. High elevations in the orogeny attenuated the influences of Pacific moisture, causing rainshadow-induced aridity on the leeward foreland basin during upper Yellow Cat time, and hosted seasonal snow accumulation by the end of Ruby Ranch time, as suggested by 18O-enriched water (e.g., up to an average of 22.0% from an allosauroid tooth) and extremely 18O-depleted water (e.g., 215.5% for ornithischians) in the Ruby Ranch Member. By Mussentuchit-time, delivery of the Western Interior Seaway–dominated moisture to the region, despite continued rise of the Sevier Mountains.
The Lower Cretaceous section exposed in northeastern Utah includes the Cedar Mountain and Dakota Formations. The Cedar Mountain Formation consists of fluvial-lacustrine and pedogenic carbonate beds and includes important dinosaur sites. Its age in northeastern Utah is early to late Albian from a radiometric age (detrital U-Pb zircons) of 104.46 ± 0.95 Ma associated with a well-preserved sauropod skull, che¬mostratigraphic analysis, and palynology in the overlying Dakota Formation. Part of the Cedar Mountain Formation was deposited during the Kiowa–Skull Creek depositional cycle. The Dakota Formation consists of fluvial sandstone and mudstone beds; however, locally it includes a thin interval of marine mudstone and shale beds. Dinoflagellate cysts re¬covered from this basal marine interval represent peak sea level during the Kiowa–Skull Creek depositional cycle and the first marine incursion of the Cretaceous Western Interior Seaway into Utah. The age for this event is middle late Al¬bian. Only nonmarine palynomorphs were recovered from beds above the marine interval. An ash bed in the middle Dakota yielded a U-Pb zircon age of 101.4 ± 0.4 Ma, which correlates to the newly defined Muddy-Mowry depositional cycle. The Mowry Shale consists of siliceous marine shale that represents widespread open-marine conditions for the area. The radiometric age of the Mowry is between 98.5 ± 0.5 Ma and 97.2 ± 0.7 Ma (40Ar/39Ar sanidine) from bentonite beds in Wyoming. However, the biostratigraphic age is contro¬versial because of downward revision to key neogastroplitid ammonite zones, revision of the Albian-Cenomanian bound¬ary age to 99.6 Ma, and recently published palynostrati¬graphic work.