HOME
        TheInfoList






Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.

In general, strata are primarily igneous or sedimentary relating to how the rock was formed. Sedimentary layers are laid down by deposition of sediment associated with weathering processes, decaying organic matter (biogenic) or through chemical precipitation. These layers are often distinguishable as having many fossils and are important for the study of biostratigraphy. Igneous layers occur as stacks of lava flows, layers of lava fragments (called tephra) both erupted onto the Earth's surface by volcanoes, and in layered intrusions formed deep underground. Igneous layers are generally devoid of fossils and represent magmatic or volcanic activity that occurred during the geologic history of an area.

There are a number of principles that are used to explain the appearance of stratum. When an igneous rock cuts across a formation of sedimentary rock, then we can say that the igneous intrusion is younger than the sedimentary rock. The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed stratum is younger than the one beneath and older than the one above it. The principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds.

(A) Correlation scheme indicates which layers penetrated at different locations belong to the same body

Geological correlation[7] is the main tool for reconstructing the geometry of layering in sedimentary basins. The lithological correlation is a procedure, decisive

Geological correlation[7] is the main tool for reconstructing the geometry of layering in sedimentary basins. The lithological correlation is a procedure, decisive what layers (strata) in geological cross-sections located in different places belong to the same geological body now (or belonged in the past).[8] The identification is based on comparison of physical and mineralogical characteristics of the rocks, and on general assumptions known as the Steno's principles:

1. The sedimentary strata occurred sequentially in time: the youngest at the top.
2. The strata are originally horizontal.
3. The stratum extends in all directions until it thins out or encounters a barrier.

The results are presented as a correlation scheme (A). Practical correlation has a lot of difficulties: fuzzy borders of the layers, variations in composition and structure of the rocks in the layer, unconformities in the sequence of layers , etc. This is why errors in correlation schemes are not seldom. When the distances between available cross-sections are decreasing (for example, by drilling new wells) the quality of correlation is improving, but meanwhile the wrong geological decisions could be made that increases the expenses of geological projects.

Lithodemic stratigraphy1. The sedimentary strata occurred sequentially in time: the youngest at the top.
2. The strata are originally horizontal.
3. The stratum extends in all directions until it thins out or encounters a barrier.

The results are presented as a correlation scheme (A). Practical correlation has a lot of difficulties: fuzzy borders of the layers, variations in composition and structure of the rocks in the layer, unconformities in the sequence of layers , etc. This is why errors in correlation schemes are not seldom. When the distances between available cross-sections are decreasing (for example, by drilling new wells) the quality of correlation is improving, but meanwhile the wrong geological decisions could be made that increases the expenses of geological projects.

The law of superposition is inapplicable to intrusive, highly deformed, or metamorphic bodies of rock lacking discernible stratification. Such bodies of rock are described as lithodemic and are determined and delimited based on rock characteristics. The 1983 North American Stratigraphic Code adopted the formal terms lithodeme, which is comparable to a formation; a suite, which is analogous to a group, and a supersuite, similar to a supergroup. A lithodeme is the fundamental unit and should possess distinctive and consistent lithological features, comprising a single rock type or a mixture of two or more types that distinguishes the unit from those around it. As with formations, a lithodemic unit is given a geographical name combined with either a rock name or some term describing its form. The term suite is deprecated. Also formalized is the term complex, which applies to a body of rock of two or more genetic classes (sedimentary, metamorphic, or igneous). This establishes two hierarchies of lithodemic units:[9]

Similar rules have been adopted in Sweden.[10] However, the 1994 International Stratigraphic Guide regards plutons and non-layered metamorphic rocks of undetermined origin as special cases within lithostratigraphy.[9]


See also