Grain size analysis in Lacustrine Green River Formation (Eocene) Stromatolites: implications for stromatolite building processes

CORSETTI, FRANK A.; DURAN MEJIA, VERONICA; BOIDI, FLAVIA; ACHBERGER, AMANDA; COX, CAITLIN; MILLS, DANIEL B.; PETRYSHYN, VICTORIA A.; FRANTZ, CARIE M.; SHAPIRO, RUSSELL S.

Charlotte, North Carolina

Exposición; 2012 GSA Annual Meeting & Exposition; 2012

The Geological Society of America

Lacustrine stromatolites are commonly micritic and/or contain precipitated carbonate fabrics, making them closer textural analogues to many ancient stromatolites versus coarse-grained modern marine examples. Here, we present grain-size data from the lacustrine stromatolites of the La Clede Beds, Green River Formation (Eocene), to better understand the grain size distribution within the stromatolites, and discuss what the grain size distribution might imply for the process of stromatolite formation. Stromatolite laminae (50-500 microns in thickness) are predominantly micritic, with a lesser fraction composed of calcite precipitates. The intercolumn fill, on the other hand, is composed of very coarse sediment (e.g., ooids, ostracods, and detrital grains), typically between 0.3 and 2 mm. A small fraction of larger grains are present within the stromatolite. The majority of the grains between 500 and 700 microns typically reside in “micro-topography” (mm-scale depressions on the former surface of the stromatolite). Finer grains (60-500 microns) are present within some laminae, but only 5% of the grains were found in laminae that dip >20 degrees. Our study highlights the difference between grains “legitimately” trapped along stromatolite laminae versus those caught in micro-topography and subsequently bound along the stromatolite surface, allowing us to better constrain the process of stromatolite formation. If we assume a predominantly microbial origin for the stromatolites, then the microbial community was not able to trap and bind larger grains where the laminae dipped beyond 20 degrees, it was able to trap/precipitate micrite at all dip angles, and it was able to bind the grains trapped (by gravity) in microtopography. Thus, the grain size analysis allows us to constrain the some of the properties of the stromatolite-building community. We speculate that the trapping and binding abilities are more consistent with a cyanobacterial community vs. a larger, stickier, eukaryotic community.