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Shale is a fine-grained, detrital sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering or bedding less than one centimeter in thickness, called fissility. Shales are typically composed of variable amounts of clay minerals and quartz grains and the typical color is gray. Addition of variable amounts of minor constituents alters the color of the rock. Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment. Red, brown and green colors are indicative of ferric oxide (hematite - reds), iron hydroxide (goethite - browns and limonite - yellow), or micaceous minerals (chlorite, biotite and illite - greens). Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite, montmorillonite and illite. Shales and mudrocks contain roughly 95 percent of the organic matter in all sedimentary rocks. However, this amounts to less than one percent by mass in an average shale. The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain suspended in water long after the larger and denser particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lakes and lagoonal deposits, in river deltas, on floodplains and offshore from beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water. (source: Wikipedia) Public domain photo, above, courtesy of US Geological Society The Woodford Shale is a dark-colored, siliceous,Upper Devonian/Lower Mississippian shale which outcrops to the north-east and the south-west of the Arbuckle Mountains in Oklahoma. This shale also contains admixtures of bedded chert, phosphate nodules, and rare hydrocarbon-saturated lenses of dolostone. The Woodford Shale is considered to be one of the most important hydrocarbon source rocks in the Anadrako Basin Province. Return to List Back to Michael's Hobbies page Back to Michael Marcotte's HomePage