STROMATOLITES AS INDICATORS OF LAKE CLIMATE AND CHEMISTRY

Lacustrine stromatolites can provide substantial information about lake chemistry and climate. Studies of Eocene Green River Formation stromatolites in Wyoming provide a basis for a better understanding these factors.

Detailed mineralogic and isotopic analysis of laminae in a Laney Member stromatolitic biostrome presents a record of climate change and chemical variability: d¹⁸O and d¹³C show an upward light trend indicating wetter climate. Calcite/dolomite ratio increases upward, consistent with the lighter isotopic trend. Tracing the biostrome normal to shoreline, lateral salinity gradients are indicated by a decreasing calcite/dolomite ratio and more positive d¹⁸O and d¹³C values toward the more saline lake center.

Observations on the occurrence of stromatolites in two other members of the Green River Formation (Tipton and Wilkins Peak) illustrate, like in the Laney Member, that stromatolite occurrence is related to basic lake type and corresponding lake chemistry. In overfilled, calcium-rich, fresh-water lakes, stromatolites formed near basin margins with a vertical facies association of stromatolites alternating with near-shore, high-energy deposits (oolites, flat pebble conglomerate (fpc), and storm deposits). Balanced-filled, carbonate-rich lakes commonly produced a lithofacies association of, from base to top, dolostone, fpc with ooids/peloids, stromatolites, and kerogen-rich carbonates. Underfilled lakes developed the same lithofacies package, but lacked stromatolites because lake waters were severely under-saturated with respect to calcite.

We conclude that lake-type can be used to make predictions concerning the occurrence and facies relationships of stromatolites. For example, an underfilled saline-alkaline lake is generally deficient in Ca⁺⁺, and it would be predicted that stromatolites would not be common, except perhaps near fresh-water fluvial or spring inputs. Balance-filled, low-gradient alkaline lakes would have the best chance of producing an extensive stromatolite record. Overfilled alkaline lakes may deposit thick stromatolite successions that are primarily restricted to the basin margins.