SIGNIFICANCE OF NEARSHORE TRACE-FOSSIL ASSEMBLAGES OF THE CAMBRO-ORDOVICIAN DEADWOOD FORMATION AND ALADDIN SANDSTONE, SOUTH-DAKOTA

The Cambro-Ordovician Deadwood Formation and Aladdin Sandstone represe nt intertidal and subtidal, nearshore deposystems that contain few wel l-preserved body fossils, but contain abundant trace fossils. The pres ent study uses the much neglected trace-fossil fauna to describe the d iverse paleoenvironments represented in the Deadwood-Aladdin deposyste ms, and to better understand the environmental conditions that control led benthic life in the Early Paleozoic. The Deadwood-Aladdin ichnotax a can be separated into three distinct assemblages based on the changi ng sedimentologic and hydrodynamic conditions that existed across the Cambro-Ordovician shelf. Trace-fossil assemblages and corresponding li thofacies characteristics indicate that the Deadwood-Aladdin deposyste ms formed within an intertidal-flat and subtidal-shelf environment. Ba sed on the distribution and numbers of preserved ichnotaxa, the intert idal flat can be subdivided further into an ecologically stressful inn er sand-flat environment, and a more normal marine outer sand-flat env ironment, both of which belong to a mixed, Skolithos-Cruziana softgrou nd ichnofacies. The inner sand flat is characterized by low diversity, low numbers, and a general lack of complexly constructed ichnotaxa. T race fossils common to both assemblages tend to be smaller in the inne r flat compared to the outer sand flat. Taphonomic effects, such as su bstrate type and sediment heterogeneity, also aid in differentiating b etween the inner and outer sand-flat assemblages. The subtidal shelf e nvironment is categorized in the Cruziana ichnofacies. Ichnological ev idence of periodic tempestite deposition and hardground development wi thin this subtidal regime is manifested by high diversity and low abun dance of ichnogenera.