GRAIN SIZE ANALYSIS IN LACUSTRINE GREEN RIVER FORMATION (EOCENE) STROMATOLITES: IMPLICATIONS FOR STROMATOLITE BUILDING PROCESSES
Laminae of the LaClede stromatolites (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 some of the properties of the stromatolite-building community. Based on our observations of grain trapping and binding by living mats, we speculate that the trapping and binding abilities are more consistent with a cyanobacterial community vs. a eukaryotic community. The work presented here was conducted in association with the 2012 International Geobiology Summer Course.