IDENTIFYING METAZOAN ACTIVITY IN MICROBIALITE FACIES FROM THE EARLY TRIASSIC OF NW PANGEA

Microbialite morphology is incredibly varied and diverse in the Early Triassic. After the end-Permian mass extinction, a dearth of metazoans and subsequent activity by the benthos, both sessile and motile, gave way to increases in microbial buildups and increased microbe and algal presence in the reef niche. Interactions of metazoans may influence the formation and growth of microbialites, especially in this period of microbialite resurgence. Significance of metazoan activity on the growth and health of microbial buildups is seen through the fossil record, as the two are intimately linked: decline of microbes in reef systems is coeval with increased metazoan pressure (abundance and diversity), while the opposite is true. Admittedly, this is one piece of a very complex puzzle spanning the history of life on Earth—other factors, like changes in ocean chemistry, may be at play, though not included here.

Microbialites, as well as body and trace fossils, were studied from the Early Triassic NW Pangean margin carbonates of Lost Cabin Springs, NV and Mineral Mountains, UT localities. Similarities in facies were compared for microbialite irregularity, metazoan skeletal assemblage, identification of trace fossils within microbialite facies, as well as meters-thick heavily bioturbated vermicular limestone (primarily Planolites) immediately underlying microbialite facies at each locality. This was accomplished with a combination of micro-textural identification of microbialites and metazoan skeletal material in thin sections as well as identifying macro-textural trends using detailed polished slab mapping. On many occasions, microbialites have obvious laminations with steep vertical orientation, or characteristic stromatolitic structure. Putative burrow-like structures cross-cut laminations in several examples. Metazoan skeletal material includes shell fragments within micrite fill surrounding microbialite zones. Future goals from these results include modelling active metazoan alterations and influence on microbialite morphology and microbe growth, to determine a mechanism for disruption.