The Whitehill Formation is an extensive Lower Permian unit, cropping out in 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 north to south and east to west. The formation is part of the
Karoo Supergroup, more precisely the
Ecca Group, where it overlies 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 206Pb/238U age of 280.5 ± 2.1 Ma. Other authors use an estimated age for the whole formation of around 275 Ma. In the central part of the
Karoo Basin, the formation ranges in thickness from 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.
Lithologies 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 is conformably overlain by white weathering shales, with intermittent
chert lenses and
pyritic stringers; the latter rarely exceeding 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 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 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".
Depositional environments The Whitehill Formation 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. 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 the Dwyka and Ecca Groups propose a general source along the southern
Panthalassian margin of current southern Africa.
Correlations 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 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 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.
Fossil content 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 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: ;Reptiles •
Mesosaurus tenuidens •
Noteosaurus africanus •
Stereosternum tumidum ;Fish •
Palaeoniscum capensis ;Insects •
Afrochoristella maclachlani •
Afrocupes firmae •
Sharovia permiafricana ;Flora •
Lepidodendron whitehillianum == Economic geology ==