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Patterns and Timing of Synorogenic Sedimentation in Upper Cretaceous Rocks of Central and Northeast Utah
Patterns of sedimentation in central and northeast Utah along the western margin of the Late Cretaceous seaway were controlled by thrust-fold tectonic events in eastern and southern Nevada, and western and central Utah. The rocks can be divided into eight major lithofacies that define a depositional sequence extending eastward from central Utah down depositional dip to eastern Utah. Patterns of lithofacies and depositional facies, paleontology, and structural relations indicate at least four regional unconformities that represent major hiatuses in both the time and rock record over most of the report area. The position of each major hiatus marks a major contrast in lithofacies between units that bound the unconformities.
The rocks were divided into eight major lithofacies that are (ordered proximal to distal): 1) the sheet conglomerate facies formed from debris flow and sheet flood sedimentation on coalesced alluvial fans, 2) the scour-fill conglomerate facies formed by sedimentation in braided channels on an alluvial fan or braidplain, 3) the pebbly trough-crossbedded facies formed principally by deposition in straight to moderately sinuous bedload to mixed load streams on the lower part of an alluvial braidplain, 4) the accretion and scour-fill facies formed on a poorly drained alluvial plain crossed by moderately sinuous and sinuous streams and dotted with swamps and lakes, 5) the channel–form sandstone and coal facies formed on the lower part of a coastal plain, 6) the coarsening-upward tabular sandstone facies formed at or near the marine shoreline, 7) the dark-gray siltstone and claystone facies formed in a variety of open-marine settings, and 8) the calcareous claystone and carbonate rock facies formed in an internally-drained nonmarine closed-basin system.
Paleontologic data from the rocks provide a reasonably accurate estimate of the principal periods of orogenic activity. Four principal periods of thrust-fold activity are indicated in and adjacent to the region. The time-spans are: 1) early and/or middle Cenomanian, 2) middle Campanian, 3) late Campanian, and 4) latest Campanian and/or early Maestrichtian. Additional tectonic-sedimentation episodes occurred but are less well defined.
Middle Cenomanian to middle Santonian formations formed from sedimentation in response to eustatic sea-level fluctuations and thrust-fold events at the extreme west margin and west of the study area. Preserved rocks of middle Santonian (86 and 87 Ma) to early middle Campanian time (near 79 Ma) represent alluvial fan, coastal plain, deltaic and open-marine sedimentation (including density-flow). In mid-Campanian time (near 79 Ma), new tectonism in the west and northwest part of the region along the Meade-Crawford thrust sheets and their central Utah extensions initiated a new cycle of sedimentation that continued into the latter part of the late Campanian (approximately 74 Ma). Thrusting in late Campanian time (approximately 74 Ma–a principal period of movement on the Absaroka thrust system in north-central Utah) resulted in a new cycle of alluvial fan to coastal plain deposition, latest Campanian or early Maestrictian time (from 71 to 73 Ma) marks the disappearance of the marine foreland depositional basin in Utah and the initiation of the development of one or more internally-drained basins (Laramide deformation and sedimentation). Maestrichtian rocks formed from lithic detritus and chemically precipitated sediments deposited in a basin(s) that lay between the Sevier orogenic belt on the west and the San Rafael structural element on the east. The basin extended from central Utah north and northeast into the region of the western and central parts of the present day Uinta depositional basin.
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