Lithostratigraphic

Lithostratigraphy is a branch of stratigraphy that studies and classifies rock layers, or strata, primarily on the basis of lithology—their rock type and physical characteristics—rather than on age alone. It focuses on describing the distribution, continuity, and relationships of rock units as observed in the field and in subsurface data.

The fundamental concepts include lithostratigraphic units, with the formation being the principal unit. Formations are defined by a distinctive lithology or combination of lithologies that can be mapped over a region, and they are bounded by changes in rock type or by unconformities, faults, or other discontinuities. Subunits such as members (subdivisions with recognizable lithologies) and beds (thin layers) are used to capture internal variability. Each unit is anchored by a type section and a type locality, which establish its reference description.

Lithostratigraphy is distinct from chronostratigraphy and biostratigraphy because it does not rely on time to define units. Correlation between areas is achieved by matching lithology, sedimentary structures, and, where available, fossil content and facies sequences, often supplemented by borehole logs and other geophysical data.

Field methods and data play central roles: mapped geology, measured sections, and detailed descriptions of color, grain size, mineralogy, sedimentary structures, thickness, and lateral extent. These descriptions are integrated with subsurface datasets to build coherent stratigraphic frameworks.

Limitations include potential diachroneity, where the same lithology occurs at different times, and the possibility that units pinch out or reappear across regions. Despite these challenges, lithostratigraphy is essential for hydrocarbon exploration, basin analysis, and regional geology, providing a practical basis for mapping and correlating sedimentary rocks.