Coeval development of extensional and contractional features along transform margins: insights from the Diaz Marginal Ridge

The Diaz Marginal Ridge (DMR), on the southern transform margin of South Africa, is a bathymetric feature parallel to the Agulhas Falkland Fracture Zone (AFFZ) that has long been considered an archetype marginal ridge; and yet its origin and evolution remains unconstrained. Using recently acquired seismic data we present a new structural interpretation of the DMR and its association with the evolution of both the AFFZ and the Southern Outeniqua Basin. In contrast to previous scenarios invoking thermo-mechanical explanations for its evolution, we observe a more straightforward structural model in which the genesis of the DMR results from the structural inversion of a Jurassic rift basin. This inversion resulted in the progressive onlap of latest Valanginian-Hauterivian aged stratigraphic units, important for the formation of stratigraphic plays of the recent Brulpadda discovery.Paradoxically, this contraction is contemporaneous with renewed extension observed in the inboard normal faults. The orientation of the DMR and inboard structures have been demonstrated to be controlled by the underlying Cape Fold Belt (CFB) fabric. The onset of motion across the AFFZ shear system led to east-west orientated maximum stress and north-south orientated minimum stress. We propose this stress re-orientation resulted in strain partitioning across existing structures whereby in addition to strike-slip on the AFFZ there was coeval extension and contraction, the nature of which was determined by fault orientation. The fault orientation in turn was controlled by a change in orientation of the underlying CFB. Our model provides new insights into the interplay of changes in regional stress orientation with basement fabric and localised magmatism along an evolving transform. The application of horizontal strain partitioning can provide an explanation of similar features observed on other transform margins.

The composition, geography, biology and assembly of the coastal flora of the Cape Floristic Region

The Cape Floristic Region (CFR) is globally recognized as a hotspot of plant diversity and endemism. Much of this diversity stems from radiations associated with infertile acid sands derived from sandstones of the geologically ancient Cape Fold Belt. These ancient montane floras acted as the source for most subsequent radiations on the Cape lowlands during the Oligocene (on silcretes) and Mio–Pliocene (on shales). The geomorphic evolution of the CFR during the Plio–Pleistocene led to the first large-scale occurrence of calcareous substrata (coastal dunes and calcarenites) along the Cape coast, providing novel habitats for plant colonization and ensuing evolution of the Cape coastal flora—the most recent diversification event in the Cape. Few studies have investigated the CFR’s dune and calcarenite floras, and fewer still have done so in an evolutionary context. Here, we present a unified flora of these coastal calcareous habitats of the CFR and analyze the taxonomic, biological and geographical traits of its component species to gain insights into its assembly. The Cape coastal flora, comprising 1,365 species, is taxonomically dominated by the Asteraceae, Fabaceae and Iridaceae, with Erica, Aspalathus and Agathosma being the most speciose genera. In terms of growth-form mix, there is a roughly equal split between herbaceous and woody species, the former dominated by geophytes and forbs, the latter by dwarf and low shrubs. Species associated with the Fynbos biome constitute the bulk of the flora, while the Subtropical Thicket and Wetland biomes also house a substantial number of species. The Cape coastal flora is a distinctly southern African assemblage, with 61% of species belonging to southern African lineages (including 35% of species with Cape affinity) and 59% being endemic to the CFR. Unique among floras from the Cape and coastal Mediterranean-climate regions is the relatively high proportion of species associated with tropical lineages, several of which are restricted to calcareous substrata of the CFR. The endemic, calcicolous component of the flora, constituting 40% of species, represents 6% of the Cape’s regional plant diversity—high tallies compared to other biodiversity hotspots. Most coastal-flora endemics emerged during the Plio–Pleistocene as a product of ecological speciation upon the colonization of calcareous substrata, with the calcifugous fynbos floras of montane acid substrata being the most significant source of this diversification, especially on the typically shallow soils of calcarenite landscapes. On the other hand, renosterveld floras, associated with edaphically benign soils that are widespread on the CFR lowlands, have not been a major source of lineages to the coastal flora. Our findings suggest that, over and above the strong pH gradient that exists on calcareous substrata, soil depth and texture may act as important edaphic filters to incorporating lineages from floras on juxtaposed substrata in the CFR.

Stratigraphic Architecture of the Karoo River Channels at the End-Capitanian

The main Karoo Basin of southern Africa contains the continental record of the end-Triassic, end-Permian, and end-Capitanian mass extinction events. Of these, the environmental drivers of the end-Capitanian are least known. Integrating quantitative stratigraphic architecture analysis from abundant outcrop profiles, paleocurrent measurements, and petrography, this study investigates the stratigraphic interval that records the end-Capitanian extinction event in the southwestern and southern main Karoo Basin and demonstrates that this biotic change coincided with a subtle variation in the stratigraphic architectural style ∼260 Ma ago. Our multi-proxy sedimentological work not only defines the depositional setting of the succession as a megafan system that drained the foothills of the Cape Fold Belt, but also attempts to differentiate the tectonic and climatic controls on the fluvial architecture of this paleontologically important Permian succession. Our results reveal limited changes in sediment sources, paleocurrents, sandstone body geometries, and possibly a constant hot, semi-arid paleoclimate during the deposition of the studied interval; however, the stratigraphic trends show upward increase in 1) laterally accreted, sandy architectural elements and 2) architectural elements that build a portion of the floodplain deposits. We consider this to reflect a long-term retrogradational stacking pattern of facies composition that can be linked to changes on the medial parts of southward draining megafans, where channel sinuosity increased, and depositional energy decreased at the end-Capitanian. The shift in the fluvial architecture was likely triggered by basin-wide allogenic controls rather than local autogenic processes because this trend is observed in the coeval stratigraphic intervals from geographically disparate areas in the southwestern and southern main Karoo Basin. Consequently, we propose that this regional backstepping most likely resulted from tectonic events in the adjacent Cape Fold Belt.

Lithostratigraphy and sedimentology of the Middle Devonian Tra-Tra Formation, including the Grootrivier Member (Bokkeveld Group, Cape Supergroup), South Africa

The Tra-Tra Formation is a predominantly argillaceous, shallow marine to paralic sedimentary succession of Eifelian (Middle Devonian) age within the Bokkeveld Group (Cape Supergroup) that crops out extensively within the Cape Fold Belt of South Africa. It comprises three discrete lithofacies associations (termed E-G) which are interpreted as deposits of channelised tidal flat-lagoons, transgressive beach-barriers and wave-influenced prodeltas to distal delta-fronts. They accumulated within a series of incised coastal-plain valley-fill system along the palaeoshoreline of the Cape Basin following a protracted forced regressive phase during sedimentation of the underlying Hex River Formation. A discrete, geographically-extensive, 25 to 30 m thick, single or double, positive-weathering tabular sandstone within the Tra-Tra Formation is recognised herein as the Grootrivier Member. Palaeontologically, the Tra-Tra Formation comprises a restricted fauna of Malvinokaffric Realm invertebrates, fish and plant fossils that are of biostratigraphic importance in inferring its Eifelian age.

Biomes, geology and past climate drive speciation of laminate-toothed rats on South African mountains (Murinae: Otomys)

Mitochondrial DNA sequences (1137 bp) of the cytochrome b gene and craniodental and craniometric data were used to investigate the evolutionary relationships of six putative rodent taxa of Otomys (family Muridae: subfamily Murinae: tribe Otomyini) co-occurring in the Western Cape and Eastern Cape provinces of South Africa. Phylogenetic analysis of 20 new sequences together with craniodental and craniometric characters of 94 adult skulls reveal the existence of a unique lineage of Otomys cf. karoensis (named herein Otomys willani sp. nov.) from the Sneeuberg Centre of Floristic Endemism in the southern Drakensberg Mountain Range. Craniometric analysis distinguished O. karoensis from O. willani and identified a further four localities in the range of the latter species. We document southern range extensions of both Sloggett’s ice rat, Otomys sloggetti, and the vlei rat Otomys auratus to the Sneeuberg Mountain Range, in addition to appreciable genetic divergence between Sneeuberg and southern and central Drakensberg populations of O. sloggetti. Our results demonstrate parallel patterns of cryptic speciation in two co-occurring species complexes (Otomys irroratus s.l. and O. karoensis s.l.) associated closely with the boundaries of biomes (fynbos vs. grassland biomes) and geological formations (Cape Fold Belt vs. Great Escarpment).

South African Hydrostratigraphy: A conceptual framework

South African geology, geomorphology and climate are distinctly variable, resulting in a complex hydrological cycle superimposed on equally complex ground conditions. With fractured and karstic systems dominating the hydrogeology, thick vadose zones comprising soil and rock and at highly variable moisture conditions contribute to complex hydrostratigraphic systems comprising various confining and hydraulically connected units. This paper proposed standard terminology for basic concepts pertaining to the description of ground and water in the subsurface to eventually propose a hydrostratigraphic classification based on abiotic factors fairly constant over short periods of time (geology, geomorphology and climate), as well as those temporally highly variable (climate) and those introduced by human involvement (society). Ten major hydrostratigraphic units are eventually described, namely the Cape Fold Belt, Kalahari Desert, Witwatersrand Supergroup, Malmani Subgroup, Cenozoic Coastal Deposits, Saldanian Basement, Karoo Main Basin, Namaqua-Natal Metamorphics, Waterberg Group, and Archaean Granitoids.

Petrography, modal composition and tectonic provenance of some selected sandstones from the Molteno, Elliot and Clarens Formations, Karoo Supergroup, in the Eastern Cape Province, South Africa

The Late Triassic - Early Jurassic non marine clastic sediments of the Molteno, Elliot and Clarens Formations were studied to deduce their mineralogy and tectonic provenance. The study is based on road-cut exposures of the formations in the Eastern Cape Province of South Africa. Petrographic studies based on quantitative analysis of the detrital minerals shows that the clastic sediments (mostly sandstones) are predominantly made up of quartz, feldspars, and metamorphic and igneous rock fragments. Among the main detrital framework grains, quartz constitutes about 62-91%, feldspar 6-24% and 3-19% of lithic fragments. The sandstones can be classified as both sublitharenite and subarkose. Although, most of the sandstones (> 70 %) plotted in the sub-litharenite field. Petrographic and XRD analyses revealed that the sandstones originated from granitic and metamorphic rock sources. The QFL (Quartz-feldspar-lithic fragments) ternary diagrams indicate that the sandstones were derived from recycled or quartzose source rocks reflecting a craton interior or transitional continental setting which probably came from the Cape Fold Belt. This possibly revealed that most of the sandstones might have been derived as a result of weathering and erosion of igneous and metamorphic rocks in the Cape Supergroup. The study has revealed the depositional environments, and provide a basis for the description and interpretation of the sedimentology of the Molteno, Elliot and Clarens Formations.