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38-504: The formation comprises black shales , mudstones , siltstones , dolomite beds, gypsum and halite layers and a layer of tuff within the formation. With a thickness between 50 and 70 metres (160 and 230 ft) and present in an area of 600 by 600 kilometres (370 by 370 mi), the formation is considered the primary target for shale gas potential in the Southern Karoo. Total Organic Carbon (TOC) values average 4.5% with

76-414: A hard, fissile, metamorphic rock known as slate . With continued increase in metamorphic grade the sequence is phyllite , then schist and finally gneiss . Shale is the most common source rock for hydrocarbons ( natural gas and petroleum ). The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin. These might have oxygenated

114-467: A larger basinal area, 150 million years before the break-up of Pangea . The abundance of Glossopteris and Mesosaurus fossils are characteristic of the Gondwanan correlation across present-day South America, Africa, Antarctica and Australia. The Whitehill Formation has provided fossil reptiles, insects , fish and flora. The Whitehill Formation is an extensive Lower Permian unit, cropping out in

152-692: A range from 0.5 to 14.7%, placing the formation in the same range as the well-known Barnett Shale and Marcellus Formation of the United States. The Whitehill Formation of the Karoo and Nama or Kalahari Basin is contemporaneous with the Huab Formation of the Huab Basin and is correlated with a series of formations in the Pelotas and Paraná Basins in southeastern Brazil, deposited in

190-838: A thin band stretching from the Western Cape in South Africa through southeastern Namibia and the Northern Cape in the north to the Eastern Cape of South Africa in the east. The formation is found at the edge of the Karoo Basin at distances of 600 kilometres (370 mi) north to south and east to west. The formation is part of the Karoo Supergroup , more precisely the Ecca Group , where it overlies

228-429: Is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin , Al 2 Si 2 O 5 ( OH ) 4 ) and tiny fragments ( silt -sized particles) of other minerals, especially quartz and calcite . Shale is characterized by its tendency to split into thin layers ( laminae ) less than one centimeter in thickness. This property

266-1201: Is accompanied by telogenesis , the third and final stage of diagenesis. As erosion reduces the depth of burial, renewed exposure to meteoric water produces additional changes to the shale, such as dissolution of some of the cement to produce secondary porosity . Pyrite may be oxidized to produce gypsum . Black shales are dark, as a result of being especially rich in unoxidized carbon . Common in some Paleozoic and Mesozoic strata , black shales were deposited in anoxic , reducing environments, such as in stagnant water columns. Some black shales contain abundant heavy metals such as molybdenum , uranium , vanadium , and zinc . The enriched values are of controversial origin, having been alternatively attributed to input from hydrothermal fluids during or after sedimentation or to slow accumulation from sea water over long periods of sedimentation. Fossils , animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces. Shales may also contain concretions consisting of pyrite, apatite , or various carbonate minerals. Shales that are subject to heat and pressure of metamorphism alter into

304-422: Is called fissility . Shale is the most common sedimentary rock. The term shale is sometimes applied more broadly, as essentially a synonym for mudrock , rather than in the narrower sense of clay-rich fissile mudrock. Shale typically exhibits varying degrees of fissility. Because of the parallel orientation of clay mineral flakes in shale, it breaks into thin layers, often splintery and usually parallel to

342-638: Is composed of about 58% clay minerals, 28% quartz, 6% feldspar , 5% carbonate minerals, and 2% iron oxides . Most of the quartz is detrital (part of the original sediments that formed the shale) rather than authigenic (crystallized within the shale after deposition). Shales and other 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. Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe ) and sulfur (S ). Amorphous iron sulfide , along with carbon, produce

380-407: Is conformably overlain by white weathering shales, with intermittent chert lenses and pyritic stringers; the latter rarely exceeding 20 millimetres (0.79 in) in thickness. The sedimentary structure is generally massive, however laminations do occur that resemble algal lamellae. The formation appears white due to weathering of pyrite ( sulfide ) at surface to sulfate ( gypsum ). This section

418-660: Is considered the prime focus for potential shale gas prospects in the Southern Karoo. TOC levels average at 4.5% TOC, and range from 0.5 to 14.7%, comparable to known shale gas producing formations as the Marcellus Formation and the Barnett Shale . The formation is considered to have economic potential in an area around Beaufort West to Graaff-Reinet . Results from Rock-Eval pyrolysis , vitrinite reflectance measurements, open pyrolysis and thermovaporization analyses carried out on core samples drilled through

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456-953: Is correlated with the Irati Formation of the Paraná and Pelotas Basins in Rio Grande do Sul , Brazil , the Black Rock Member of the Falkland Islands , and with the Huab Formation in the Huab Basin of northwestern Namibia. The lower section correlates with the Palermo and Rio Bonito Formations of the Paraná Basin and the Prince Albert Formation of the Karoo. The Permian sequence in

494-430: Is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts. The fine particles that compose shale can remain suspended in water long after the larger particles of sand have been deposited. As a result, 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 below

532-581: Is grading upward into dark carbonaceous , bluish-grey weathering shales. From a lithological point of view only the upper part of the succession consists of the Whitehill-characteristic carbonaceous black shales . A tuffaceous zone occurs within the Whitehill Formation a few metres below the stratigraphic interval, which contains several dolomitic limestones. This limestone interval forms a mappable unit in all outcrops of

570-455: Is more likely to form nonfissile mudstone than shale. On the other hand, black shales often have very pronounced fissility ( paper shales ) due to binding of hydrocarbon molecules to the faces of the clay particles, which weakens the binding between particles. Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together. Pressure solution contributes to cementing, as

608-452: Is reduced. In addition to this physical compaction, chemical compaction may take place via pressure solution . Points of contact between grains are under the greatest strain, and the strained mineral is more soluble than the rest of the grain. As a result, the contact points are dissolved away, allowing the grains to come into closer contact. It is during compaction that shale develops its fissility, likely through mechanical compaction of

646-647: The Prince Albert Formation and is overlain by the Collingham Formation , and in the southern Karoo by the Tierberg Formation . Single zircon U - Pb SHRIMP dating yielded an age of 279.1 ± 1.5 Ma for the Uhabis River Tuff present in the upper strata of the underlying Prince Albert Formation. The Khabus Tuff within the Whitehill Formation provided a weighted mean Pb/U age of 280.5 ± 2.1 Ma. Other authors use an estimated age for

684-399: The U.S. Gulf Coast . As sediments continue to accumulate, the older, more deeply buried sediments begin to undergo diagenesis . This mostly consists of compaction and lithification of the clay and silt particles. Early stages of diagenesis, described as eogenesis , take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in

722-669: The wave base . Thick deposits of shale are found near ancient continental margins and foreland basins . Some of the most widespread shale formations were deposited by epicontinental seas . Black shales are common in Cretaceous strata on the margins of the Atlantic Ocean , where they were deposited in fault -bounded silled basins associated with the opening of the Atlantic during the breakup of Pangaea . These basins were anoxic, in part because of restricted circulation in

760-669: The Dwyka and Ecca Groups propose a general source along the southern Panthalassian margin of current southern Africa. The Whitehill Formation is time equivalent with the Pietermaritzburg Formation of the Waterberg Basin of southern Namibia. To the north in the Karoo Basin, the shales of the Whitehill Formation are laterally equivalent with the Vryheid Formation . The upper part of the formation

798-1005: The Huab Basin is much thinner than those of the Paraná and Karoo Basins. The fossil assemblages of Glossopteris and Mesosaurus are known from other parts of Gondwana ; the Vryheid Formation of South Africa and coal deposits of the Lower Permian in Australia. The Whitehill Formation has provided a variety of fossil insects, rare flora and Mesosaurus fossils, typical for the Permian of Gondwana. The mesosaurids are preserved as molds filled with gypsum crystals; they are commonly disarticulated. More rarely, casts of mesosaurids are found. Plant stems are similarly rare, and most commonly occur as chloritized fragments floating in

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836-596: The Whitehill Formation in central southern Namibia. In the Northern Cape, the formation is intruded by dolerite sills. The sediments are considered to be deposited in an anoxic environment . Besides pyrite, the shales contain dolomite lenses near the base. The rocks are highly folded and faulted by the Cape orogeny forming the Cape Fold Belt , and thus, interpreted as "decollement". The Whitehill Formation

874-421: The black coloration. Because amorphous iron sulfide gradually converts to pyrite , which is not an important pigment, young shales may be quite dark from their iron sulfide content, in spite of a modest carbon content (less than 1%), while a black color in an ancient shale indicates a high carbon content. Most shales are marine in origin, and the groundwater in shale formations is often highly saline . There

912-463: The boundary between the Prince Albert and the Whitehill Formation represents the turning point from a progradational to a retrogradational succession. The tuffs within the Whitehill Formation, as well as other tuffaceous beds found in the underlying and overlying formations, were possibly sourced by volcanoes located in present-day South America, although other interpretations of the tuffs of

950-506: The clumps of clay particles produced by flocculation vary in size from a few tens of microns to over 700 microns in diameter. The floccules start out water-rich, but much of the water is expelled from the floccules as the clay minerals bind more tightly together over time (a process called syneresis ). Clay pelletization by organisms that filter feed is important where flocculation is inhibited. Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along

988-408: The color of the rock. 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). The color shifts from reddish to greenish as iron in the oxidized ( ferric ) state is converted to iron in the reduced ( ferrous ) state. Black shale results from

1026-430: The formation show that organic matter has reached an advanced stage of kerogen development. These rocks can therefore be classified as overmature , likely because of the thermotectonic processes related to the Cape orogeny forming the Cape Fold Belt overprint on lower Karoo rocks in the study area. It is possible that the maturity of the shales decreases farther north in the Karoo basin. Black shale Shale

1064-592: The mineral dissolved from strained contact points is redeposited in the unstrained pore spaces. The clay minerals may be altered as well. For example, smectite is altered to illite at temperatures of about 55 to 200 °C (130 to 390 °F), releasing water in the process. Other alteration reactions include the alteration of smectite to chlorite and of kaolinite to illite at temperatures between 120 and 150 °C (250 and 300 °F). Because of these reactions, illite composes 80% of Precambrian shales, versus about 25% of young shales. Unroofing of buried shale

1102-433: The narrow Atlantic, and in part because the very warm Cretaceous seas lacked the circulation of cold bottom water that oxygenates the deep oceans today. Most clay must be deposited as aggregates and floccules, since the settling rate of individual clay particles is extremely slow. Flocculation is very rapid once the clay encounters highly saline sea water. Whereas individual clay particles are less than 4 microns in size,

1140-415: The original open framework of clay particles. The particles become strongly oriented into parallel layers that give the shale its distinctive fabric. Fissility likely develops early in the compaction process, at relatively shallow depth, since fissility does not seem to vary with depth in thick formations. Kaolinite flakes have less tendency to align in parallel layers than other clays, so kaolinite-rich clay

1178-505: The otherwise indistinguishable bedding planes . Non-fissile rocks of similar composition and particle size (less than 0.0625 mm) are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones . Shales are typically gray in color and are composed of clay minerals and quartz grains. The addition of variable amounts of minor constituents alters

Whitehill Formation - Misplaced Pages Continue

1216-520: The pink lower, more massive mudstone succession of the Whitehill Formation, while coprolites containing either palaeoniscoid scales or fragmentary crustacean carapaces are preserved on bedding planes. The following fossils have been reported from the Whitehill Formation: The high concentration of organic matter in the black shales of the Whitehill Formation make it an interesting target for shale gas exploration. The formation

1254-817: The presence of greater than one percent carbonaceous material and indicates a reducing environment. Pale blue to blue-green shales typically are rich in carbonate minerals . Clays are the major constituent of shales and other mudrocks. The clay minerals represented are largely kaolinite , montmorillonite and illite. Clay minerals of Late Tertiary mudstones are expandable smectites , whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate. The transformation of smectite to illite produces silica , sodium , calcium , magnesium , iron and water. These released elements form authigenic quartz , chert , calcite , dolomite , ankerite , hematite and albite , all trace to minor (except quartz) minerals found in shales and other mudrocks. A typical shale

1292-444: The richest source rocks may contain as much as 40% organic matter. The organic matter in shale is converted over time from the original proteins, polysaccharides , lipids , and other organic molecules to kerogen , which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum. Before the mid-19th century, the terms slate , shale and schist were not sharply distinguished. In

1330-433: The sediments, with only slight compaction. Pyrite may be formed in anoxic mud at this stage of diagenesis. Deeper burial is accompanied by mesogenesis , during which most of the compaction and lithification takes place. As the sediments come under increasing pressure from overlying sediments, sediment grains move into more compact arrangements, ductile grains (such as clay mineral grains) are deformed, and pore space

1368-414: The waters and destroyed organic matter before it could accumulate. The absence of carbonate rock in shale beds reflects the absence of organisms that might have secreted carbonate skeletons, also likely due to an anoxic environment. As a result, about 95% of organic matter in sedimentary rocks is found in shales and other mudrocks. Individual shale beds typically have an organic matter content of about 1%, but

1406-731: The whole formation of around 275 Ma. In the central part of the Karoo Basin , the formation ranges in thickness from 50 to 70 metres (160 to 230 ft) and is highly organic with up to 14 weight percent of Total Organic Carbon (TOC). The formation is highly conductive and can be traced in seismic profiles across nearly the whole basin. The Whitehill Formation has been subdivided into two major subunits according to their weathering color in outcrops. The lower and thicker part consists mainly of bluish- to greenish-grey shales and mudstones , which grade upward into more light brownish, buff weathering, slightly coarser grained siltstones . This zone

1444-573: Was deposited in a shallow inland sea with little to no connection to the open ocean. This sea, stretching from the Paraná Basin in current southeastern Brazil to the Karasberg and Karoo Basins of southwestern Africa, probably represents the transition from marine to brackish or freshwater conditions, in sediment-starved, stratified and anoxic embayments. From outcrops in southernmost Namibia (Aussenkjer-Noordoewer area) it became evident that

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