ASSESSING THE PALEOECOLOGICAL IMPACT OF DIAGENETIC OVERPRINTS ON SHALLOW MARINE MACRO- AND MICROFOSSIL ASSEMBLAGES: A CASE STUDY THE LATE TRIASSIC LUNING AND GABBS FORMATIONS (WESTERN NEVADA, USA)

Diagenetic overprinting can make comparisons of fossil assemblages difficult; however, careful petrographic and geochemical analysis can provide insights into alteration and potential impacts on preservation of fossil assemblages. A common tool in determining diagenetic alteration in carbonates is the ratio of strontium to manganese. Here we present the results of a paleoecological analysis of shelly macrofaunal and vertebrate microfossils from two Late Triassic (Norian) marine carbonate deposits that exhibit significant differences in Sr/Mn both between and within the sampled localities. This work addresses potential diagenetic impacts on the abundance of microfossils (such as teeth and dermal denticles) and calcareous macrofossil assemblages from Norian carbonates.

Samples were collected from the Norian-age Luning Formation (LF) and Nun Mine Member (NMM) of the Gabbs Formation in western Nevada. Petrographic analysis was used to ensure that samples represented similar depositional environments and to verify the degree of diagenetic alteration using differential recrystallization of diagnostic shell fragments from oysters, burrowing clams, brachiopods, and echinoderms. Sample productivity for extracted microfossils was evaluated by the number of vertebrate microfossils (teeth and denticles) per cubic centimeter of residue. Productivity was then compared to Sr/Mn ratios, and ordination was used to compare macrofossil assemblages.

Preliminary results suggest that the variability in macrofossil assemblages amongst samples is not strongly influenced by the Sr/Mn ratio. The presence of infaunal aragonitic bivalves in low Sr/Mn NMM samples—largely absent in high Sr/Mn LF samples—are responsible for much of the difference between sites. Aragonitic shells are highly susceptible to diagenetic alteration or destruction. Their presence in low Sr/Mn NMM samples, and relative absence in high Sr/Mn LF samples, indicates that diagenetic effects are not different enough to produce a preservation bias that excludes thinner aragonitic shells from preservation in more highly altered low Sr/Mn samples. Initial results from analysis of microfossil productivity also show no strong correlation with the Sr/Mn ratios. This suggests sedimentation rate may have a more influential role in microfossil productivity.