12.2: Lithostratigraphy
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Lithostratigraphy is a particular type of stratigraphy that uses the lithologic character of the rock as the basis for subdividing and classifying the rocks. Our ability to do lithostratigraphy is based on the fundamental concepts outlines by Steno’s Principles and the concepts of relative and absolute dating.
Lithostratigraphic Relationships
Vertical contacts are most commonly described as sharp, gradational, or interbedded. Lateral transitions are most often described as gradational, intertonguing, or pinching out.
Lithostratigraphic Units
We are governed by the North American Stratigraphic Code which lays out a comprehensive set of rules for naming and recognizing units. In order to have a unit be formally named (they are proper nouns) an author must provide all of the required information (inc. justification, boundaries, type sections, history, etc.) in a peer-reviewed publication that is widely available. The type section for a named lithostratigraphic unit must be accessible, well-exposed, and characteristic of the unit.
Lithostratigraphic units are given a two part name; the first part refers to a place where the unit is well exposed and the second part tells the reader what the rank of the unit is. This, the Mauch Chunk Formation occurs near and is a lithostratigraphic formation named for the town of Mauch Chunk (now renamed Jim Thorpe , after one of the greatest athletes of the last century). In descending order, the different ranks of lithostratigraphic units include:
Supergroup – is the largest formal lithostratigraphic unit. They are composed of collections of named formations and or groups.
Group – the unit above formation, it may be composed entirely or partially of named formations.
Formation – the fundamental unit of lithostratigraphy. They must be mappable at the surface or in the subsurface, usually across an area that is at least the size of a 1:24k USGS topographic map.
Member – the lithostratigraphic unit below a formation in rank. All members must occur within formations. Formations need not be divided (in whole or in part) into members. Members can extend laterally from one formation into another. Geographically restricted members are often referred to as members or tongues.
Bed (or Flow) – the smallest formal lithostratigraphic unit. It is a discrete and distinctive unit that is easily recognized.