Lithostratigraphy is the process of classifying rock bodies based on their physical characteristics, forming a standardized system for geologists worldwide. This classification is essential for mapping rock layers, correlating rock sequences across different geographical areas, and reconstructing the complex history of the Earth. A lithostratigraphic unit is a body of rock, defined solely by its observable lithologic properties and its stratigraphic position relative to other rock bodies. The system is built upon physical rock features, such as mineral composition or color, and is intentionally independent of the rock’s inferred age or the time it took to form.
Defining the Bed and the Smallest Units
The smallest formally recognized unit in the hierarchy of lithostratigraphic classification is the Bed, or its equivalent for volcanic rocks, the Flow. A Bed is a single, distinct layer in a sequence of stratified sedimentary rocks, clearly distinguishable from the layers directly above and below it. This distinction is typically marked by well-defined bedding planes, which are surfaces separating individual layers. Beds commonly range in thickness from a centimeter up to a few meters, though layers thinner than a centimeter are usually referred to as laminae.
A Flow is the corresponding smallest formal unit for extrusive volcanic rocks, which formed from cooling lava on the Earth’s surface. It represents a discrete body of volcanic rock, such as a lava flow, identified based on its texture, composition, or layering order. Both the Bed and the Flow are at the lowest level of the classification system, making them the most detailed formal subdivisions.
While the Bed and Flow are formal units, they are only assigned a proper geographic name when they possess unique scientific or economic importance. These named layers are often called “marker beds” or “key beds” because their distinct features allow geologists to trace them over wide areas, providing a valuable reference point for correlation. Most individual beds and flows remain unnamed and are simply described as part of the larger unit that contains them.
Criteria for Distinguishing Rock Units
The definition of any lithostratigraphic unit, including the smallest Bed or Flow, relies entirely on the observable physical and chemical characteristics of the rock, collectively known as lithology. These characteristics include the rock’s mineral and chemical composition, color, grain size and arrangement (texture), and primary structures like cross-bedding or ripple marks. Fossils may also contribute to the definition, but only if they are rock-forming particles, such as the shells in a coquina limestone.
The boundaries of a lithostratigraphic unit are placed where a significant change in lithology occurs, which can be either a sharp, distinct contact or a gradual transition. When the change is gradual, geologists must arbitrarily designate a boundary within the zone of transition, often where the rock type shifts to a dominant composition. This practice ensures that the defined unit has a clear, if sometimes artificial, limit.
Every formal lithostratigraphic unit is defined by a designated reference location called a Stratotype, which is typically a Type Section. The Type Section is a specific, well-exposed geographic location where the unit’s characteristic lithology and its upper and lower boundaries are clearly visible. The geologist establishing the unit must describe this section precisely so that other scientists can accurately recognize and trace the unit elsewhere. If the unit is not fully exposed or varies significantly across a region, additional Reference Sections may be defined to better illustrate its extent and variability.
Understanding the Lithostratigraphic Hierarchy
The smallest units are organized into a tiered hierarchy that establishes the scale and relationship of rock bodies, starting from the smallest and ascending to the largest. The conventional hierarchy begins with the Bed or Flow, which are then grouped into a Member. A Member is the formal unit next in rank and represents a named subdivision of a larger unit, possessing lithologic properties that distinguish it from other parts.
The fundamental unit in the entire lithostratigraphic classification is the Formation. A Formation is a body of rock distinctive enough in its lithology and thick enough to be mapped at the surface or in the subsurface, making it the primary unit for geological mapping. Formations can be composed of one or more Members, or they may not be divided into Members at all, depending on the complexity and variability of the rock.
Above the Formation, the next rank is the Group, which is a succession of two or more associated Formations that share significant lithologic properties. Groups are defined to simplify the classification of rock sequences in a region but are not a mandatory subdivision. The largest formal unit is the Supergroup, a collection of two or more associated Groups or Groups and Formations related by a common characteristic. This hierarchy demonstrates that while a Bed is the smallest formal layer, the Formation serves as the essential, mappable building block of the Earth’s rock record.