Stratigraphy
Stratigraphy, a branch of
geology, is basically the study of
rock layers and layering (
stratification). It is primarily used in the study of
sedimentary and layered
volcanic rocks. The subject was essentially invented and first rigorously applied by
William Smith in
England in the
1790s and early
1800s. Smith, known as the
Father of English Geology, created the first
geologic map of England and first recognized the significance of
strata or rock layering.
Stratigraphy includes two related subfields: lithologic or
lithostratigraphy and biologic stratigraphy or
biostratigraphy.
See also: LithostratigraphyLithostratigraphy, or lithologic stratigraphy, is the most obvious. It deals with the physical
lithologic or rock type change both vertically in layering or bedding of varying rock type and laterally reflecting changing environments of deposition, known as
facies change. Key elements of stratigraphy involve understanding
how certain geometric relationships between rock layers arise and
what these geometries mean in terms of depositional environment. One of stratigraphy's basic concepts is codified in the
Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
Chemostratigraphy is based on the changes in the relative proportions of trace elements and
isotopes within and between lithologic units.
Carbon and
oxygen isotope ratios vary with time and are used to map subtle changes in the
paleoenvironment This has led to the specialized field of
isotopic stratigraphy.
Cyclostratigraphy documents the often cyclic changes in the relative proportions of
minerals, particularly
carbonates, and fossil diversity with time, related to changes in
palaeoclimates.
See also: BiostratigraphyBiostratigraphy or
paleontologic stratigraphy is based on
fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are correlatable in time. Biologic stratigraphy was based on William Smith's
principle of faunal succession, which predated, and was one of the first and most powerful lines of evidence for,
biological evolution. It provides strong evidence for formation (
speciation) of and the
extinction of
species. The
geologic time scale was developed during the 1800s based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of
radiometric dating, which gave it and the stratigraphy it was based on an absolute time framework, leading to the development of
chronostratigraphy.
One important development is the
Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of
hydrocarbon-bearing reservoir rocks, seals and traps in
petroleum geology.
See also: Archaeological stratigraphyIn the field of
archaeology, soil stratigraphy is used to better understand the processes that form and protect
archaeological sites. The law of superposition holds true, and this can help date finds or features from each
context, as they can be placed in sequence and the dates interpolated. Phases of activity can also often be seen through stratigraphy, especially when a trench or feature is viewed in
section (profile). As pits and other features can be dug down into earlier levels, not all material at the same absolute depth is necessarily of the same age, but close attention has to be paid to the archeological
layers. The
Harris-matrix is a tool to depict complex stratigraphic relations, as they are found, for example, in the contexts of
urban archaeology.
*
Important publications in stratigraphy*
Key bed*
Sequence stratigraphy*
Sedimentary basin analysis*
University of South Carolina Sequence Stratigraphy Web*
Front Range stratigraphy*
International Commission on Stratigraphy*
University of Georgia (USA) Stratigraphy Lab*
CHRONOS - Geoscience Database
