Print this pageStratigraphic and Biogeochemical Cycles in Ancient Lake Deposits
CARROLL, ALAN R., and KEVIN M. BOHACS
Lacustrine sequence stratigraphy is governed by the balance between tectonically-driven potential accommodation and sediment supply. Potential accommodation, defined as the difference between the lowest point in the lake basin and its drainage spillover point, represents the maximum space available for sediments to accumulate during highstands. Unlike shallow marine systems, sediment supply in lakes is strongly tied to local climate. The boundary conditions for lacustrine deposition may therefore be approximated by plotting tectonic subsidence against climatic humidity. "Underfilled" lake basins result when the precipitation/evaporation ratio is low relative to tectonic subsidence; aggradational desiccation cycles in ephemeral-lake and playa facies and hypersaline biomarkers are common. "Balanced-fill" lake basins occur when water+sediment inflows are sufficient to fill the lake-basin periodically, but significant outflows are absent. A combination of aggradational cycles and progradational parasequences typically results. Biomarker distributions reflect specific populations of algae and other aquatic organisms. "Overfilled" lake basins reflect an excess of sediment supply, due to high precipitation/evaporation ratios relative to subsidence. Clastic parasequences related to shoreline migration predominate; biomarkers reflect fresh water conditions and an increased input of terrestrial organic matter.
Long-term thickness successions of these lake types may help to distinguish tectonic from climatically-induced cyclicity, based on the limiting effects of either basin subsidence or available water+sediment supply on interval thickness. For example, increased basin subsidence may result in a succession of thin overfilled to balanced-fill intervals overlain by a thick series of underfilled lake deposits. Subsequent decreasing subsidence would reverse the succession. Such is the case in the Green River Formation in Wyoming, for example, where a thick hypersaline interval is encased in thinner fresh water and alkaline lake facies. In contrast, increased climatic humidity may result in a succession of thin underfilled intervals overlain by thick balanced-fill or overfilled lake deposits. This relationship is often expressed at the pansequence scale, due to shorter-term climatic fluctuations.