The major stratigraphic aspects of lacustrine rocks are geometry, facies patterns, and vertical sequence. Together with information derived from petrography and paleontology, these attributes are useful for making a variety of detailed interpretations and reconstruction of ancient lakes and their surrounding areas.
Geometric relations include size, shape and thickness. To a large degree, these features are controlled by, and in turn reflect, the origin of the lake basin. As a result, the geometry of lakes is highly variable.
The bottom sediments of modern lakes encompass a wide variety of lithofacies. In clastic sediments, there may be concentric belts of gravel, sand, sandy marly mud, and mud, which are controlled by wave base and nearshore wave agitation. Facies patterns among chemical and organic sediments are not predicted so easily. However, two carbonate models are recognized, one with increasing carbonate content toward the center of the lake and the other with high carbonate concentrations near the margins. The former results from nearshore mixing by terrigenous sediment and the latter from greater carbonate productivity in shallow water. Similarly, there are two organic facies patterns. Offshore increases in organic matter result from deposition and preferred preservation below wave base. In contrast, nearshore concentrations of organic matter are caused by in-place accumulations of plant remains.
The vertical sequence of lacustrine rocks is not easily generalized. The predicted sequence as a lake basin fills and is replaced by fluvial environments is not closely approximated by known lacustrine rock units. Instead, most lakes have gone through more than one cycle of lake formation and filling and the resulting vertical sequence is a composite of many complete and incomplete lacustrine cycles.
Because lakes are responsive to local climatic or tectonic changes, a wide variety of sedimentary cycles has been identified in lacustrine rocks, including glacial and non-glacial varves, transgressive-regressive cycles, and various longer patterns represented by bundles of varves or other cyclic deposits.
