The Wilcox Group is an important geologic group in the Gulf of Mexico Basin and surrounding onshore areas from Mexico and Texas to Louisiana and Alabama . The group ranges in age from Paleocene to Eocene and is in Texas subdivided into the Calvert Bluff , Simsboro and Hooper Formations , and in Alabama into the Nanafalia and Hatchetigbee Formations . Other subdivisions are the Lower , Middle and Upper Wilcox Subgroups , and the Carrizo and Indio Formations .
18-474: The group comprises mostly mudstone with secondary sandstone and lignite beds, ironstone concretions, as well as siltstones , conglomerates and limestones . The total thickness is about 1,400 to 1,800 feet (430 to 550 m). Sandstone modal compositions and detrital zircon U‐Pb analyses of the Wilcox Group indicate long‐distance sediment transport from primarily volcanic and basement sources to
36-751: A further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (63 μm). On December 13, 2016, NASA reported further evidence supporting habitability on the planet Mars as the Curiosity rover climbed higher, studying younger layers, on Mount Sharp . Also reported, the very soluble element boron was detected for the first time on Mars. In June 2018 , NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks approximately 3.5 billion years old. Lamination (geology) In geology , lamination (from Latin lāmina 'thin layer')
54-491: A variety of accessory minerals . In the Dunham classification (Dunham, 1962 ) system of limestones , a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most recently, this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component . A recent study by Lokier and Al Junaibi (2016) has highlighted that
72-476: Is a small structure, it is easily destroyed by bioturbation (the activity of burrowing organisms) shortly after deposition. Lamination therefore survives better under anoxic circumstances, or when the sedimentation rate was high and the sediment was buried before bioturbation could occur. Lamination develops in fine grained sediment when fine grained particles settle, which can only happen in quiet water. Examples of sedimentary environments are deep marine (at
90-527: Is a small-scale sequence of fine layers ( pl. : laminae ; sg. : lamina ) that occurs in sedimentary rocks . Laminae are normally smaller and less pronounced than bedding . Lamination is often regarded as planar structures one centimetre or less in thickness, whereas bedding layers are greater than one centimetre. However, structures from several millimetres to many centimetres have been described as laminae. A single sedimentary rock can have both laminae and beds. Lamination consists of small differences in
108-412: Is clay particles. Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant. Rock of this composition that does show laminations or fissility is sometimes described as mudshale rather than mudstone . The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in
126-473: Is distinguished from shale by its lack of fissility . The term mudstone is also used to describe carbonate rocks ( limestone or dolomite ) that are composed predominantly of carbonate mud. However, in most contexts, the term refers to siliciclastic mudstone, composed mostly of silicate minerals. The NASA Curiosity rover has found deposits of mudstone on Mars that contain organic substances such as propane , benzene and toluene . There
144-556: Is not a single definition of mudstone that has gained general acceptance, though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with a grain size less than 0.063 millimetres (0.0025 in). Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition, and mudstones have historically received less attention from petrologists than have sandstones . The simplest definition
162-420: Is that a mudstone is a fine-grained clastic sedimentary rock that is not laminated or fissile . Most definitions also include a requirement that the rock contain significant amounts of both silt- and clay-sized grains. One common requirement is that a mudstone is a mudrock (a rock containing more than 50% silt- to clay-sized particles) in which between a third and two-thirds of the mud (silt and clay) fraction
180-602: The Gulf of Mexico Basin . Structural and geochemical evidence indicates that hydrocarbons found in Wilcox reservoirs such as the Wilds sandstones were emplaced by vertical migration along wrench faults and associated fracture zones from underlying Cretaceous and Jurassic source rocks . Mudstone Mudstone , a type of mudrock , is a fine-grained sedimentary rock whose original constituents were clays or muds . Mudstone
198-454: The most common problems encountered when describing a mudstone is to incorrectly estimate the volume of 'grains' in the sample - in consequence, misidentifying mudstone as wackestone and vice versa. The original Dunham classification (1962) defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971 ) to a grain size of less than or equal to 30 μm. Wright (1992 ) proposed
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#1732772660442216-661: The region studied, whereas the Lower Wilcox shows significant heterogeneity. The drainage area for the Gulf of Mexico during the Paleocene and Eocene was larger than previously thought, encompassing not only the Laramide basement uplifts, but the volcanic province of northern Mexico and possibly Cordilleran tectonic regions along the westernmost North America. The Wilcox Group comprises important source and reservoir rocks in
234-467: The sandstones of the Wilcox Group reveal a complex grain assemblage derived from Laramide uplifted crystalline blocks of the central and southern Rocky Mountains , the Cordilleran arc of western North America, and arc‐related extrusive and intrusive igneous rocks of northern Mexico. Zircon ages also suggest possible recycling of older sediment through uplift related to accretion of arc terranes along
252-427: The seafloor) or lacustrine (at the bottom of a lake), or mudflats , where the tide creates cyclic differences in sediment supply. Laminae formed in glaciolacustrine environments (in glacier lakes) are a special case. They are called varves . Quaternary varves are used in stratigraphy and palaeoclimatology to reconstruct climate changes during the last few hundred thousand years. Lamination in sandstone
270-486: The sediment prior to lithification . Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit. When the mineral composition of mudstones has been determined, using such techniques as scanning electron microscopy , electron probe microanalysis , or X-ray diffraction analysis , they have been found to be composed primarily of clay minerals , quartz , and feldspars , with
288-671: The type of sediment that occur throughout the rock. They are caused by cyclic changes in the supply of sediment. These changes can occur in grain size , clay percentage, microfossil content, organic material content or mineral content and often result in pronounced differences in colour between the laminae. Weathering can make the differences even more clear. Lamination can occur as parallel structures (parallel lamination) or in different sets that make an angle with each other (cross-lamination). It can occur in many different types of sedimentary rock, from coarse sandstone to fine shales , mudstones or in evaporites . Because lamination
306-431: The west, northwest, and southwest. The Wilcox Group represents the earliest series of major post‐Cretaceous pulses of sand deposition along the western margin of the Gulf of Mexico. Laramide basement uplifts have long been held to be the source of Wilcox sediments, implying that initiation of basement uplift was the driving factor for the transition from carbonate sedimentation to clastic deposition. Zircon age‐spectra for
324-432: The westernmost North America, but that recycling of Cordilleran foreland basin sediments was not a major contributor. Comparison of Upper and Lower Wilcox zircon age-spectra indicate that Lower Wilcox sediments are richer in arc and volcanic material, whereas Upper Wilcox sediments are richer in basement material. Regional trends in detrital zircon age‐spectra indicate that Upper Wilcox sediments are moderately homogeneous over
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