EVIDENCE OF BENTHIC BIOLOGICAL ACTIVITY PRESERVED IN SILICICLASTIC SEDIMENTARY STRUCTURES, NEOPROTEROZOIC UINTA MOUNTAIN GROUP, HIGH UINTAS WILDERNESS, UTAH

Several kinds of unusual sedimentary structures discovered in siliciclastic sandstone beds of the Neoproterozoic Uinta Mountain Group, High Uintas Wilderness, Utah, are interpreted as indirect evidence of significant benthic biological activity that was possibly microbial or algal in nature. The most unusual of these structures are relatively small, centimeter-scale, nonlaminated, dome- to ridge-like or bulbous deposits of fine-grained quartz sand which commonly exhibit remarkably steep slopes on at least one side and typically seem to have been exceptionally protuberant above the surrounding coeval substrate. These Neoproterozoic sand stromatolites, or sandolites, typically are vertically continuous, although they lack internal layering and exhibit proportions of height and slope angle that are beyond the depositional limits of normal primary sedimentary structures such as ripples and hummocky cross-stratification. It is proposed that these Neoproterozoic structures formed by the adhesion and vertical accretion of sediment that was induced by biological activity at the sediment - water interface. The same kind of sediment subsequently was deposited between the protuberant sandolites, but here the deposits have laminations which onlap or terminate against the sides of the sandolites. The latter feature also supports the interpretation that the sandolites had developed to a stage where they were significantly protuberant and steep-sided prior to burial and that their morphology is essentially the result of original deposition.

Although the unusual sedimentary structures were only observed in displaced blocks of rock, adjacent geopetal primary sedimentary structures including cross bedding with smoothly asymptotic basal surfaces and erosionally truncated upper terminations, lag deposits, and graded beds, indicate the original direction of stratigraphically upward and thus reveal the true orientation of the features.

The fact that bounding cross-bedded units are preserved in seemingly pristine condition, without any obvious indications of structural or tectonic deformation, indicates that the biosedimentary features in the adjacent strata accurately preserve the record of interplay between biological colonization of the substrate and fine-grained siliciclastic sedimentation.