Stratigraphy of lacustrine deposits

The major stratigraphic aspects of lacustrine rock units are geometry (thickness and lateral extent), facies patterns, and vertical sequence. Sizes and shapes of modern lakes show wide ranges, but many large ones are subcircular to elongate. In cross section most thick lacustrine units are broadly lenticular with maximum thickness near the center of the basin where subsidence is greatest. Bottom sediments of modern lakes encompass a wide variety of lithofacies. If clastic sediments dominate, there may be concentric belts of gravel, sand, sandy marly mud, and mud, which are controlled by wave base and overall energy gradients. Facies patterns in chemical and organic sediments are not so easily predicted. However, two carbonate models are recognized, one with increasing carbonate content toward the center of the lake and the other with higher carbonate concentrations near the margins. The former results from nearshore dilution by terrigenous sediment and the latter from greater carbonate productivity in shallower water. Similarly, two organic facies patterns predominate. Offshore increase in organic matter results from deposition and preferred preservation below wave base. In contrast, nearshore concentrations of organic matter are mostly caused by in-place accumulations of organic remains. Few ancient lacustrine sequences are either sufficiently well preserved or studied in sufficient detail for construction of even general facies maps. One obvious exception is the Green River Formation of Peleocene(?) to Eocene age, the most extensively studied lacustrine rock unit in the world. In the Green River Formation, the general facies pattern in northeast Utah and northwest Colorado is one of marginal coarse clastics and centralized organic-rich mudstone; a general basinward increase in carbonate rock is also notable. Most lakes pass through more than one cycle of expansion and retreat. The resulting vertical sequence is a composite of many complete and incomplete cycles. Lacustrine rocks display a variety of allocyclic sequences: glacial and nonglacial varves, transgressive-regressive cycles, and various composite groupings represented by bundles of varves or other cyclic deposits.