Changes

===Source rocks===

===Source rocks===

Deposition, concentration, and preservation of organic matter to form source rocks require special environmental conditions. Fossils aid in the recognition of these unique settings, which are primarily defined by geochemical and petrological parameters. The Cretaceous [[Mowry Shale]] in Wyoming, for example, is an important source rock and contains from 1 to 5.2% organic carbon.<ref name=pt05r45>Davis, H. G., Byers, C. W., Pratt, L. M., 1989, Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming: AAPG Bulletin, v. 73, p. 1103–1110.</ref> The richest source rock is homogeneous pelagic mudstone that contains layers of siliceous radiolaria, kerogen, and fish debris. This mudstone was deposited in a restricted basin, where the water column was stratified and bottom waters were depleted in oxygen, allowing the preservation of organic matter. Anaerobic conditions are indicated by the presence of shallow water pelagic species and by the virtual absence of bottom-dwelling species or traces of animal activity. Lateral changes in the kinds and abundances of fossils and sedimentary structures also give clues to the oxygen gradient in the basin. Nearshore bottom-dwelling biota are found only at the margins of the Mowry sea.

Deposition, concentration, and preservation of organic matter to form source rocks require special environmental conditions. Fossils aid in the recognition of these unique settings, which are primarily defined by geochemical and petrological parameters. The Cretaceous [[Mowry Shale]] in Wyoming, for example, is an important source rock and contains from 1 to 5.2% organic carbon.<ref name=pt05r45>Davis, H. G., Byers, C. W., Pratt, L. M., 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0009/1100/1103.htm Depositional mechanisms and organic matter in Mowry Shale (Cretaceous), Wyoming]: AAPG Bulletin, v. 73, p. 1103–1110.</ref> The richest source rock is homogeneous pelagic mudstone that contains layers of siliceous radiolaria, kerogen, and fish debris. This mudstone was deposited in a restricted basin, where the water column was stratified and bottom waters were depleted in oxygen, allowing the preservation of organic matter. Anaerobic conditions are indicated by the presence of shallow water pelagic species and by the virtual absence of bottom-dwelling species or traces of animal activity. Lateral changes in the kinds and abundances of fossils and sedimentary structures also give clues to the oxygen gradient in the basin. Nearshore bottom-dwelling biota are found only at the margins of the Mowry sea.

==See also==

==See also==