Abstract

Synorogenic, cyclic, braided-stream deposits of the Cretaceous/Tertiary North Horn Formation in central Utah contain horizons with abundant biogenic sedimentary structures. Trace fossils are restricted to the finest grained rocks (very fine- to medium-grained sandstone). Bioturbation has disrupted all primary structures in floodplain and sandy channel-fill facies. Trace fossiliferous horizons contain densely packed (up to 70 to 80 traces/1,000 cm 2 ), horizontal and vertical endichnial traces with two distinct size modes. 1) Larger burrows have cylindrical to elliptical cross sections (mean width from 71 measurements = 1.5 mm; mean length = 98.0 mm). 2) Smaller burrows exhibit a weakly developed meniscate back-filling pattern or contain structureless fill. Both are circular in cross section (mean width from 76 measurements = 0.57 mm; mean length = 18 mm). The meniscate, back-filled burrows are identified as Muensteria sp. and the burrows with a structureless filling as Skolithos sp. Orientations of long axes of horizontal components of burrows are approximately perpendicular to paleocurrent directions determined from cross-stratified sandstone. Apparent movement of the trace-making organisms in the sand was away from the channel. Rhizoliths in the formation include root casts, rhizotubules, and rhizoconcretions. They are associated with reduced zones and burrows in a complex pattern in all sandstone facies. Tap-root morphology (greater than 30 cm in length; 1.5 to 3.0 cm in diameter) suggests a relatively low or fluctuating water table (i.e., seasonal or long-term discharge variations).--Modified journal abstract.

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