Paper No. 9
Presentation Time: 10:30 AM
LACUSTRINE MICROBIALITES
AWRAMIK, Stanley M., Department of Earth Science, University of California, Santa Barbara, CA 93106 and BUCHHEIM, H. Paul, Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350, awramik@geol.ucsb.edu
Lacustrine microbialites have a rich and varied record in Earth history. Their record extends back at least 2,720 million years and they are forming in lakes today. Not all lakes develop microbialites. In any given lake, they are more likely to develop when calcium input is high, the lake is primarily balanced-filled to over-filled, clastic sediment supply is low or lacking, nutrient input is relatively low, turbidity is low, and shallow-water conditions are relatively stable. Lakes are spatially restricted relative to continental surface area. In addition, lakes are geologically short-lived. Hence, the likelihood of preserving such epicontinental deposits over long spans of geological time is sharply reduced. Despite these temporal, spatial, and limnological restrictions, the record of these microbialites is still rich. Although a systematic survey of lacustrine stromatolites through time and space has yet to be completed, preliminary patterns of their temporal distribution are no surprise: The Cenozoic has the richest record, followed by the Mesozoic and then the Paleozoic. The pre-Phanerozoic, although spanning more than seven times the duration of the Phanerozoic, has a sparse record.
Lacustrine stromatolites can have two or more characteristics that are useful to distinguish them from marine stromatolites: (a) morphological variability within a bioherm or biostrome, (b) macrolamination, (c) dark and light laminae usually composed of calci-/dolo-micrite and calci-/dolo-siltite, respectively, (d) moderate to sharp boundaries between light and dark lamina, (e) light laminae composed of fibrous carbonate, (f) intermediate colored/shaded laminae in addition to end-member, dark-light laminae couplets, (g) laminae and microstructure that are very complex, (h) laminae often have a high degree of inheritance within macrolaminae or other multiple successions of laminae, (i) exhibit bumpy, botryoidal upper surfaces, and (j) encrust solid substrates such as clasts, logs, or twigs. Occasionally they preserve calcimicrobes. Some of these features occur in marine stromatolites, but it is the combination of features that is potentially diagnostic of a lacustrine setting.
