A simplified geological map of the outcrops of Karoo Supergroup rocks in Southern Africa. The Ecca Group is represented by the orange key on the map.
Backgroundedit
During the time of the deposition of the Ecca Group, the depositional environment, with some exceptions, was predominantly marine. The Ecca sea was vast but shallow, reaching only around 500 m at its deepest in its west/northwestern and southern facies where the Tanqua and Laingsburg Depocenters are situated respectively. The marine environment ranged from deep pelagic, submarine fan systems in the lower deposits which grade steadily north-eastwards to shallow marine deposits including shelf (continental) marine and marginal marine facies, and finally to beach deposits in younger successions. Coal-bearing fluvial-deltaic, and peatbog settings are also well known from the Ecca Group.[3][4][5][6]
The Ecca Group was deposited in a vast retroarcforeland basin. This foreland system was caused by crustal uplift (orogenesis) that had previously begun to take course due to the subduction of the Palaeo-pacific plate beneath the Gondwanan Plate. This resulted in the rise of the Gondwanide mountain range in what is known as the Gondwanide orogeny. The mountain-building and erosion caused by the growing Gondwanide mountain range was the initial subsidence mechanism acting on the Karoo Basin. Flexural tectonics partitioned the Karoo Basin into the foredeep, forebulge, and backbulge flexural provinces. This resulted in deposition of the Karoo Basin.[7][8][9][10][11]
The Ecca Group comprises sixteen recognized geological formations.[14] These individual formations have been grouped into three geographical areas, which are the southern, western/northwestern, and northeastern facies successions. In the east of South Africa there are deposits of as of yet undifferentiated mudstone sequences attributed to the Ecca Group.[15][16][17][18][19]
With the exception of the Prince Albert, Whitehill, Collingham, and the uppermost Waterford Formations which are found in both the southern and western/northwestern facies, the geological formations of the Ecca Group can only be found in one of the three previously aforementioned facies successions. This is because each of these facies successions represents differing preserved environments that can be observed in their diagnostic geological features. The facies successions, along with their geological formations, are described below:[20][21][22][23][24]
Western/Northwestern Ecca facies
This facies succession is purely marine. The rocks contain a complete transition, grading laterally into one other, from basin-floor marine deposits through to channelized submarine slope to shelf, pro-delta and beach environment deposits. The deposits of the western/northwestern facies fall within the Tanqua Depocenter, one of the vast submarine fan systems known from the marine Ecca.[25] Associated formations are listed below (from oldest to youngest):
Collingham Formation: Is composed of thin beds of siliceous, dark grey mudstone alternated with softer, yellow-grey tuff beds. The fine-grained nature of the sediments indicate that the depositional environment was a low energy suspension setting in a submarine fan environment. The presence of tuff beds indicates that volcanic activity took place during the time of sediment deposition.
Tierberg Formation: Predominantly composed of blackish, planar, argillaceousshales. Tuff beds are found in the lower sections of this formation while calcareousconcretions and clay pellet conglomerates are found in the upper sections. Upwards-coarsening sequences of mudstones, siltstones, and sandstones that exhibit ball-and-pillow structures are also found in the uppermost sections. The presence of the shales in the lower Tierberg indicate a low energy marine environment that transitioned to pro-delta to distal delta depositional environments with the appearance of the upward coarsening sequences and other associated geological features. Various invertebrate fossils have been recovered, namely fish scales, sponge spiracles, and trace fossils of Planolites and Nereites.
Skoorsteenberg Formation: Only found in the southernmost area of the western facies. A lens-shaped formation composed of five sandstone-rich units that are interbedded with shale layers. These contain bouma turbidite sequences. The depositional environment is thought to have been an unstable delta-front slope of a fluvialdelta system. Trace fossils of worm feeding trails and Glossopteris fossils are common.
Kookfontein Formation: The lower section of this formation is composed of horizontally laminated dark-grey shales that are interbedded with clasticrhythmites. Minor upward thickening cycles are observed here which grow more prominent in the upper sections. The rock type changes in the upper sections to alternating siltstone and fine-grained sandstone beds. This formation is a continuation of the environmental facies of the Skoorsteenberg Formation where sediments were deposited in a pro-delta setting.
Waterford Formation: The western/northwestern deposits of this formation are composed of a mix of shale, siltstone, rhythmite, and fine to medium-grained sandstones. The sandstones and siltstones are horizontally laminated and often exhibit wave-rippled surfaces. Alternating slump and slide and coarsening upward cycles are observed in the lower sections while in the upper sections ball-and-pillow structures and channel-fill deposits are observed. These features indicate that the depositional environment constituted delta front deposits in the lower sections which transitioned to a fluvialdelta-plain. Fossilized tree logs are frequently found in the sandstone layers, and in the other rock layers Cruziana and Skolithos burrow traces are found. Together with the underlying Tierberg, Skoorsteenberg, and Kookfontein Formation, the Waterford Formation forms an upward-coarsening deltaic megacycle.[31][32][33]
Southern Ecca facies
This facies succession is the largest of the three facies succession. Its lowermost formations are deep marine comprising basin floorpelagic sediments and submarine fan systems that grade upwards into channelized submarine slopes to shelf marine and beach environments. The Laingsburg Depocenter is found in this facies succession and include the following formations (from oldest to youngest):
Prince Albert Formation: Black to dark grey chert layers with yellowish grey weathering are only known from the southern facies. These rock sediments were deposited in a deep marine and distal submarine fan setting. Various invertebrate marine fossils, palaeoniscoid fishes, and coprolites have been recovered from this formation.
Whitehill Formation: In the southern facies the Whitehill Formation loses its diagnostic white band marker horizon. The rock type changes here to light brown, fine-grained sandstone and siltstone deposits that contain carbonateconcretions. Tuff deposits also occur intermittently. It is regarded as a distal, southern equivalent of the Vryheid Formation.
Collingham Formation: The southern facies deposits of this formation yield diagnostic pale yellowish-grey chert, often referred to in literature as the “Maijiesfontein Chert Bed”.
Vischkuil Formation: Dominated by dark coloured shale in its westernmost outcrops which grade into greywackesandstones in the east. The shale layers alternate with subordinate sandstones, siltstones, and minor yellow-grey tuff deposits. The shale layers often contain calcareous and phosphaticlenses, and also liesegang rings. The depositional environment is considered to have been deep marine within a basin-plain or outer basin-floor fan. The presence of tuff layers indicates that volcanic activity took place during the time of deposition.
Laingsburg Formation: Composed of four, massive sandstone units that are interbedded with planar-laminated shale and siltstones. Calcareousconcretions and clay pebble clasts are often found in the massive sandstones. Coalified plant fragments are frequently found in the shale and siltstone layers. It grades laterally into the upper Ripon Formation in the east.
Fort Brown Formation: Rhythmites of alternating sandstone/siltstone or siltstone/claystone layers, carbonaceousshales, and mudrocks, which are usually olive grey in color, are extremely common. The rhythmites and mudrock comprise the bulk of the lower deposits of the Fort Brown Formation. Minor impure sandstonewacke intercalations are found in its upper sections, displaying horizontal, wavy, ripple, and ripple-drift cross lamination. Tuffs are sometimes found due to the Cape Orogenic event taking place during the time of deposition. The mudrocks and shales are extremely fine-grained, finely laminated and are considered to represent shelf marinefacies that were deposited in a pro-delta setting. Fragmentary plant hash material and invertebratetrace fossils are found here.
Waterford Formation:This formation is exposed along a series of east-west trending synclines and anticlines. It is rich in sandstones, which are generally light in colour and are frequently interbedded with dark-grey to light yellowish grey siltstone and sometimes mudstone. Well-preserved symmetrical wave ripples are often found in the sandstone layers. Outcrops of this formation are extremely diagnostic as a result of the alternating “bands” of sandstone and siltstone This is thought to represent a marginal marine setting either within the intertidal zone or within a delta or beach setting.[18][34][35][36]
Northeastern Ecca facies
The northeastern facies is shallow marine in its lowermost and uppermost sections, and then changes to coal-bearing fluvial-deltaicpeatswamp settings in its central deposits. The northern facies often overlies unconformably on much older basement rocks unlike the other facies of the Ecca Group. It comprises three geological formations (from oldest to youngest):
Pietermarizberg Formation: Generally composed of dark colored carbonaceous and silt-rich shale rocks, siltstone, and mudstone with subordinate sandstone layers. This formation is only found in the easternmost section of the northeastern facies succession and is not well studied due to poor outcrops and exposures. The majority of the data collected from this formation have been from borehole cores. In addition exposed outcrops quickly erode out due to the sub-tropical climate of KwaZulu-Natal where deposits of this formation are only found. Sporadic fossil plant material and various invertebratetrace fossils have been found, and the depositional environment is thought to be shallow marine.
Volksrust Formation: Mainly comprises silt-rich, grey to black shale containing thin, bioturbatedsiltstone or sandstonelenses. Deposits of this formation interfinger laterally with the underlying Vryheid Formation and overlying Beaufort Group rock deposits. The rock sediments are fine-grained overall, indicating that the rock sediments were deposited in both lacustrine to lagoonal and shallow coastal settings. A large pelecypod marine bivalve has been recovered from this formation.[41][42][43]
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