| Whitehill Formation | |
|---|---|
| Stratigraphic range:Artinskian-Kungurian ~282.6–275 Ma | |
| Type | Geological formation |
| Unit of | Ecca Group |
| Sub-units | Khabus Tuff |
| Underlies | Collingham &Tierberg Formations |
| Overlies | Prince Albert Formation |
| Area | 600 km × 600 km (370 mi × 370 mi) |
| Thickness | 50–70 m (160–230 ft) |
| Lithology | |
| Primary | Black shale,dolomite,mudstone |
| Other | Gypsum,halite,tuff,dolerite (intrusions) |
| Location | |
| Coordinates | 21°12′S14°06′E / 21.2°S 14.1°E /-21.2; 14.1 |
| Approximate paleocoordinates | 51°00′S30°30′W / 51.0°S 30.5°W /-51.0; -30.5 |
| Region | ǁKaras Region Eastern,Northern &Western Cape |
| Country | Namibia South Africa |
| Extent | Nama &Karoo Basins |
 Geologic map of Namibia with the Whitehill Formation partly cropping out in the southeastern area (orange) | |
TheWhitehill Formation, alternatively written asWhite Hill Formation and formerly known asWhite Band orWhitehill orWhite Hill Member, is a regionalEarly Permian (Artinskian toKungurian, dating to around 282 to 275 Ma) geologicformation belonging to theEcca Group in the southeasternǁKaras Region of southeasternNamibia andEastern,Northern andWestern Cape provinces ofSouth Africa.
The formation comprisesblack shales,mudstones,siltstones,dolomite beds,gypsum andhalite layers and a layer oftuff 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 forshale gas potential in the Southern Karoo.Total Organic Carbon (TOC) values average 4.5% with a range from 0.5 to 14.7%, placing the formation in the same range as the well-knownBarnett Shale andMarcellus Formation of the United States.
The Whitehill Formation of theKaroo andNama or Kalahari Basin is contemporaneous with theHuab Formation of theHuab Basin and is correlated with a series of formations in thePelotas andParaná Basins in southeastern Brazil, deposited in a larger basinal area, 150 million years before the break-up ofPangea. The abundance ofGlossopteris andMesosaurus fossils are characteristic of theGondwanan 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 a thin band stretching from theWestern Cape in South Africa through southeastern Namibia and theNorthern Cape in the north to theEastern Cape of South Africa in the east.[1] 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.[2] The formation is part of theKaroo Supergroup, more precisely theEcca Group, where it overlies thePrince Albert Formation and is overlain by theCollingham Formation,[3] and in the southern Karoo by theTierberg Formation.[4]
Single zirconU-Pb SHRIMP dating yielded an age of 279.1 ± 1.5 Ma for theUhabis River Tuff present in the upper strata of the underlying Prince Albert Formation.[5] The Khabus Tuff within the Whitehill Formation provided a weighted mean206Pb/238U age of 280.5 ± 2.1 Ma.[6] Other authors use an estimated age for the whole formation of around 275 Ma.[7]
In the central part of theKaroo 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 ofTotal Organic Carbon (TOC).[8] The formation is highly conductive and can be traced in seismic profiles across nearly the whole basin.[9]
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-greyshales andmudstones, which grade upward into more light brownish, buff weathering, slightly coarser grainedsiltstones. This zone is conformably overlain by white weathering shales, with intermittentchert lenses andpyritic 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 toweathering of pyrite (sulfide) at surface tosulfate (gypsum).[10] This section is grading upward into darkcarbonaceous, bluish-grey weathering shales. From a lithological point of view only the upper part of the succession consists of the Whitehill-characteristic carbonaceousblack shales.[11]
Atuffaceous 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 the Whitehill Formation in central southern Namibia.[12] In the Northern Cape, the formation is intruded bydolerite sills.[13] The sediments are considered to be deposited in ananoxic environment. Besides pyrite, the shales containdolomite lenses near the base. The rocks are highly folded and faulted by theCape orogeny forming theCape Fold Belt, and thus, interpreted as "decollement".[14]
The Whitehill Formation was deposited in a shallowinland sea with little to no connection to the open ocean. This sea, stretching from theParaná Basin in current southeastern Brazil to theKarasberg andKaroo Basins of southwestern Africa, probably represents the transition from marine to brackish or freshwater conditions, in sediment-starved, stratified and anoxic embayments.[3]
From outcrops in southernmost Namibia (Aussenkjer-Noordoewer area) it became evident that the boundary between the Prince Albert and the Whitehill Formation represents the turning point from aprogradational to aretrogradational succession.[15] 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,[16] although other interpretations of the tuffs of the Dwyka and Ecca Groups propose a general source along the southernPanthalassian margin of current southern Africa.[17]
The Whitehill Formation is time equivalent with thePietermaritzburg Formation of theWaterberg Basin of southern Namibia.[18] To the north in the Karoo Basin, the shales of the Whitehill Formation are laterally equivalent with theVryheid Formation.[19] The upper part of the formation is correlated with theIrati Formation of theParaná andPelotas Basins inRio Grande do Sul,Brazil, theBlack Rock Member of theFalkland Islands,[18] and with theHuab Formation in theHuab Basin of northwestern Namibia. The lower section correlates with thePalermo andRio Bonito Formations of the Paraná Basin and thePrince Albert Formation of the Karoo.[20] The Permian sequence in the Huab Basin is much thinner than those of the Paraná and Karoo Basins.[21] The fossil assemblages ofGlossopteris andMesosaurus are known from other parts ofGondwana; the Vryheid Formation of South Africa and coal deposits of the Lower Permian in Australia.[22]
The Whitehill Formation has provided a variety of fossil insects, rare flora andMesosaurus 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 the pink lower, more massive mudstone succession of the Whitehill Formation, whilecoprolites containing either palaeoniscoid scales or fragmentarycrustacean carapaces are preserved on bedding planes.[23]
The following fossils have been reported from the Whitehill Formation:[24][25][26][27][28][29][30][31]
The high concentration of organic matter in theblack shales of the Whitehill Formation make it an interesting target forshale gas exploration. The formation is considered the prime focus for potential shale gas prospects in the Southern Karoo.[33] TOC levels average at 4.5% TOC,[34] and range from 0.5 to 14.7%, comparable to known shale gas producing formations as theMarcellus Formation and theBarnett Shale.[35] The formation is considered to have economic potential in an area aroundBeaufort West toGraaff-Reinet.[36]
Results fromRock-Eval pyrolysis,vitrinite reflectance measurements, open pyrolysis and thermovaporization analyses carried out on core samples drilled through the formation show that organic matter has reached an advanced stage of kerogen development. These rocks can therefore be classified asovermature, likely because of the thermotectonic processes related to theCape orogeny forming theCape 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.[37]
