GRAIN TRAPPING IN STROMATOLITES: IMPLICATIONS FOR STROMATOLITE-BUILDING COMMUNITIES

Modern marine stromatolites are typically coarse-grained, whereas Precambrian examples are largely composed of micritic laminae, sparry precipitated fabrics, or a combination. Lacustrine stromatolites, however, are commonly micritic and/or contain precipitated carbonate fabrics, providing a closer textural analogues to many ancient stromatolites. Here, we present grain-size data from the lacustrine stromatolites of the LaClede Beds, Green River Formation (Eocene), complemented by a study of modern cyanobaterical mats, to better understand the grain size distribution within the stromatolites and its significance to stromatolite construction.

Stromatolites were collected in association with the 2013 International Geobiology Course and examined petrographically. The intercolumn fill is composed of very coarse sediment, but the stromatolite laminae are predominantly micritic. A small fraction of larger grains are present within the stromatolite, either “legitimately” trapped along lamina or found in areas of microtopography. Of those legitimately trapped, no coarser grains were found trapped beyond the angle of repose (~ 40 degrees), despite their presence in the original depositional environment. To complement the petrographic study, we studied modern cyanobacterial mats from Catalina Harbor at the USC Wrigley Institute for Environmental Studies, refining results from previous courses (2011 and 2012). Coupons of mats were placed at various angles, sediment of known mass and grain size was delivered, and the amount that was trapped and bound was quantified. Mats of different composition (Anabaena vs. Oscillatoria) and maturity (length of time spent in the tanks) were investigated. Low angles of inclination trapped all grain sizes, whereas high angles only trapped the finest fraction. The cyanobacterial community did not affect the results. Our results suggest that the LaClede stromatolite-building communities had trapping and binding capabilities similar to modern cyanobacterial mats and could not trap coarse grains. Although we cannot conclude that all fine-grained stromatolites were formed by cyanobacteria, our results suggest that coarse-grained stromatolites may require an additional component (likely eukaryotic) in order to trap coarse grains beyond the angle of repose.