Lithostratigraphy

Lithostratigraphy is a branch of stratigraphy that studies and organizes rock sequences on the basis of lithology and stratigraphic position, rather than by age. It focuses on the physical characteristics of rocks, their lateral continuity, and their vertical arrangement within a region or basin.

The fundamental units of lithostratigraphy are lithostratigraphic units, which are defined by distinctive lithology or combinations of lithologies and their geographic extent. The primary unit is the formation, a mappable body of rock with a characteristic lithology that can be traced over a region. Within formations, subunits called members may be distinguished when there is a persistent lithology or distinctive facies. The smallest unit commonly used is the bed, a single layer with relatively uniform properties. Lithostratigraphic practice also recognizes lithosomes, blocks of rock defined by lithology and stratigraphic coherence that may cross-cut structural features.

Key concepts in lithostratigraphy include the identification of boundaries at lithologic changes and unconformities, the construction of measured sections and stratigraphic columns, and the use of type sections and type localities to anchor units. Correlation between areas relies on lithologic similarity, thickness, and facies relationships, rather than explicit age equivalence. Units are named for geographic location or characteristic rock types.

Lithostratigraphy is distinct from chronostratigraphy and biostratigraphy, which define time-based or fossil-based units. In practice, lithostratigraphy provides the rock framework used in conjunction with other stratigraphic methods; absolute or relative dating is integrated through chronostratigraphy and fossil evidence. Correlation and interpretation explain how lithologic units relate to depositional environments, tectonics, and basin history.