SEDIMENTOLOGIC DEFINITION OF THE LACUSTRINE PARASEQUENCE FOR SMALL (<5KM2) BASINS

Current trends in the study of climatically driven lake level change as a means of detecting trends in Holocene moisture availability have exposed the need to understand the sedimentological characteristics of lacustrine stratigraphy in greater detail. Simple littoral-profundal facies models do not provide the detail necessary to accurately characterize the sedimentologic variability encountered in core studies. A comparative study of over 350 modern surface samples from 7 lakes in a centrally located, east-west transect across Minnesota was conducted in order to sedimentologically define the lacustrine parasequence, the basic building block of the sequence stratigraphic approach to stratigraphic correlation. The modern parasequence is defined as a relatively conformable succession of genetically related sediments arranged in beds or bed sets. Sediment samples were collected at every meter of bathymetric change along wind parallel and wind perpendicular transects across each lake basin. The study design allows us to investigate trends in sedimentology at regional and local scales. Regionally, compositional differences and similarities are investigated in relation to the precipitation gradient cross-cut by our transect. Locally, the effects of wind exposure, substrate type, and ground water chemistry on the comparability of samples from like depths are examined. All lakes studied are located high in their respective watersheds and are of a simple circular morphology. Each lake is characterized in terms of its temperature profile, bathymetry, and catchment area:surface area ratio. Each sample studied is characterized using loss on ignition (LOI), smear slide observations, and basic grain size analysis. Results show regionally that eastern Minnesota lakes are characterized by high organic content while western lakes contain a higher proportion of carbonates. Within each lake greater variability in sedimentary characteristics exists between samples of like depth on wind parallel (exposed) transects.