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Detailed study of the Oquirrh Group in Tooele and Box Elder Counties, Utah, has revealed a history of basin development during the Late Pennsylvanian and Early Permian. Trace fossils are of paramount significance in recognizing depositional environments in this sequence (a) because of uniform composition and uniform grain size and (b) because most megafossils were redeposited several miles or more from their initial growth sites.
Middle Pennsylvanian littoral and inner-shelf limestones and quartz arenites form the base of the sequence studied. During Late Pennsylvanian time, the western Oquirrh basin area was primarily an outer-shelf environment. Trace fossils in shelf units are diverse, recording a variety of behavior patterns, and include Zoophycos, Teichichnus, and Helminthoida in the Hogup/Terrace Mountains and Zoophycos, Phycodes, Helminthoida, and Spirophycus(?) in the Grassy Mountains.
Rapid subsidence in latest Pennsylvanian-earliest Permian time is marked by conglomerates and arenites that carried shallow-water fossils into the bathyal environments.
Trace preservation is poor, but is typified by low diversity and high abundance. Uppermost Pennsylvanian turbidite deposits on Bovine Mountain contain Helminthoida and Chondrites; basal Wolfcampian rocks of the Grassy Mountains bear Helminthoida, Protopaleodictyon, and Lophoctenium.
Upper Wolfcampian fetid, soft-sediment-folded, arenaceous siltstones locally yield indigenous fauna that suggests an off-shelf origin. In the Hogup and Grassy Mountains, Helminthoida is preserved, but in general the trace-fossil record is scanty in this interval.
In Leonardian time, the Oquirrh basin filled to shelf depth, and a mixture of vertical and horizontal bioturbation again prevailed.
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