STRATIGRAPHIC INDICATORS TO THE CAUSE OF LAKE BASIN EVOLUTION IN MARGINAL LACUSTRINE AND FLUVIAL DEPOSITS

Transitions from overfilled to balanced-fill or underfilled lake basin type are caused by either a decrease in the sediment plus water fill rate, an increase in the potential accommodation rate, or both (Carroll and Bohacs, 1999). We envision four unique scenarios leading to this transition that may be differentiated by the vertical succession of facies and stratal surfaces within the near lake margin stratigraphy. 1) A reduction in the sediment plus water fill rate due to climate change or drainage rerouting would lead to an initial decrease in water depth marked by a basinward step in facies and formation of an exposure surface as the lake surface dips below the elevation of the outflow sill. This initial shoaling would be followed by deepening in areas where sediment supply is outpaced by basin subsidence. 2) An increase in the rate of basin subsidence would also lead to a basinward shift in facies and formation of an exposure surface. However, since the volume of sediment entering the basin is unchanged there should be thicker and more extensive coarse clastic material above the transition boundary compared to scenario 1. These may be fluvial or marginal lacustrine deposits. Deepening upwards facies successions will occur where sediment supply is outpaced by basin subsidence. 3) A reduction in the sediment fill rate by formation of an upstream sediment trap (basin) would lead to a basinward step in facies, formation of an exposure surface, and a decrease in coarse clastic material. The stratigraphic record in the basin may be similar to scenario 1; however, the record in upstream fluvial or lacustrine basins would vary significantly. 4) Unlike in the other scenarios, uplift of the outflow sill would lead to an initial backstep of facies and formation of a flooding surface. Backstepping may also be recorded in the underlying overfilled deposits prior to the transition depending on the rate and magnitude of sill uplift.

In addition to stratigraphic analysis, provenance data such as sandstone petrography, detrital clay mineralogy, and both stable and radiogenic isotope stratigraphy may aid in determining which scenario occurred. In the basin center, where mudstone facies dominate, geochemical proxies may differentiate the four scenarios in the transition from overfilled to balance-fill or underfilled basin types.