LATE TRIASSIC BRACHIOPOD SHELL BEDS FROM THE AUGUSTA MOUNTAIN FORMATION, CENTRAL NEVADA

Researchers argue that the expansion of ecological guilds implies that an increase in total resource supply occurred throughout the Phanerozoic thus equating to an increase in biomass though time (e.g., Bambach, 1993). Although researchers predict an increase in biomass through time, biomass has not been quantified throughout the Phanerozoic in a rigorous manner because it is often difficult to obtain precise measurements. One possible solution to this problem is examining the abundance and degree of fleshiness through time within shell beds. Kidwell (1990) defines two different modes of shell bed concentrations - archaic mode and modern mode – using physical dimension, taxonomic composition and taphonomic attributes. An archaic shell bed is dominated by less fleshy brachiopods than the modern shell bed deposit which were dominated by mollusks, especially bivalves, with much fleshier bodies. This study examines Late Triassic shell beds from central Nevada to determine if the shell beds represent an archaic or modern mode by examining the geometry and the taxonomy of the deposits. Results indicate that Schwagerispira brachiopods comprise the majority of the shell beds within a complex micrite matrix. The shell beds are considered archaic, because their contained biota is relatively uniform, they are not thick beds, and they contain only brachiopods, which are all attributes of typical archaic mode deposits. The bi-modal size-sorting of the beds, which contain a wide range of shell lengths and thicknesses that align along a linear relationship, indicate a rapidly-deposited, moderately well-mixed concentration. This indicates heterogeneity in the sorting of these deposits, also affirming the hypothesis that these beds are event deposits. The results indicate that 1) the archaic shell bed mode continued into the Late Triassic 2) the shell beds are characterized as an event deposit and 3) finding deposits displaying the archaic mode from this time period indicates biomass is still lower in the Late Triassic than today.