A new lithostratigraphic framework for the Anti-Atlas Orogen, Morocco
Robert James ThomasAbdelilah FekkakNasser EnnihEzzoura ErramiSusan LoughlinP. G. GresseL. ChevallierJean-Paul Liégeois
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Keywords:
Basement
Supergroup
Passive margin
Continental Margin
The Ventersdorp Contact Reef (VCR) is an Archaean conglomeratic gold placer, mined in the Carletonville, West Rand, and Klerksdorp goldfields of the Republic of South Africa. It belongs to a lithostratigraphic formation, the Venterspost Conglomerate Formation (VCF), which is considered to be stratigraphically independent of the underlying Witwatersrand Supergroup sediments and overlying Ventersdorp Supergroup volcanics. In this paper, the present formal stratigraphic placement of the Formation is compared with its alternative stratigraphic assignment to either of the underlying Witwatersrand or overlying Ventersdorp Supergroups. Evidence for a basin-wide, angular unconformity separating the VCF from Witwatersrand rocks, which were regionally lithified prior to deposition of the Formation, indicates that it is stratigraphically divorced from the Witwatersrand Supergroup. While similar angular, internal unconformities are present within the Witwatersrand Supergroup, a significant difference between these and the VCF unconformity is that the internal Witwatersrand unconformities are asserted to have basinward extents only to the tectonic hinge between continuous deposition and contemporaneous (Witwatersrand) foreland basin destruction. A genetic difference between the VCR and typical Witwatersrand placers is reflected by their differing mineralogical content and metamorphic grade. These differences, by implication, also reflect a substantial time gap separating their deposition. The fact that the conglomerates and sandstones of the VCF are locally interbedded with lava flows that are geochemically similar to the lava of the basal Westonaria Formation of the Ventersdorp Supergroup, indicates lithologic and genetic links to this Supergroup. These relationships are further supported by the presence of tuffaceous sandstone beds within the VCF. The occurrence of deformational load structures at the contact between the formation and the basal lava flows of the overlying Ventersdorp Supergroup indicates an unconsolidated, probably water-saturated state of the VCF sediments at the time of their envelopment by the lava, and implies a conformable relationship with the lower formations of the Supergroup. In conclusion, it is proposed that the VCF be stratigraphically assigned to the Ventersdrop Supergroup.
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Conglomerate
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Abstract The evolution of a passive margin to a foreland basin is generally assumed to entail early load‐induced up warping of the stable continental platform followed by foreland subsidence. This relatively straightforward elastic response of the continental platform, however, may be complicated if the colliding passive margin is irregular in outline. In a tectonic scenario in which an irregular margin is migrating toward a trench (A‐subduction), those areas of the margin which project seaward, the continental promontories, would be the first to ‘feel’ the approaching thrust terrane by flexing upward and eroding to form shelf unconformities. Those parts of the continental margin that are convex to the craton, the continental re‐entrants, however, would remain subsiding depocentres unaffected by load‐induced uplift at the promontories. Careful analysis of the geographic distribution of shelf unconformities in orogenic belts, then, may help to reveal the pre‐deformation morphology of the passive continental margin. An example of this may be found in the early phases of Ordovician foreland basin development in the central Appalachian orogen. Here, the shelf unconformities are most pronounced (greatest erosional relief) at the inferred Virginia and New York continental promontories. An adjacent inferred continental re‐entrant, the Pennsylvania re‐entrant, is characterized by an uninterrupted Ordovician sequence suggesting that the area of the proto‐North American platform, represented by this segment of the orogen, remained a depocentre during uplift in adjacent areas of the continental margin.
Continental Margin
Passive margin
continental collision
Diachronous
Obduction
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The Australian North West Shelf(NWS) and Southern passive continental margin are both main hydrocarbon accumulation areas in Australia.Similarities and differences exist in tectonic evolution history,fracture system,development time,hydrocarbon distribution,etc.Study on the differences from three respects: tectonic evolution controlling basin framework,source-controlling regional structure and trap-controlling fault.The results show that:(1) The NWS develops multi-period rifting,completed structures,and there appears oil lies upon gas vertical layered structure.The intensity of tectonic activity and differences of development period causes hydrocarbon distribution heterogeneity.The Southern margin develops rifting late and short duration.Basins in this area are all produce oil only.(2) Regional structure controls distribution of hydrocarbon generation center.The NWS develops source rock in all Mesozoic.The Southern margin develops in late Cretaceous and Cenozoic.(3) Differences of fracture system cause the differences of trap types.In the NWS,early-trap is destroyed by late faults,developed fault block traps.The Southern margin develops anticline traps and tilted fault block traps.The developed features of the two passive continental margins are similar to the South China Sea passive continental margin.The hydrocarbon distributions of the two areas have the guiding significance for the South China Sea passive continental margin.
Passive margin
Continental Margin
Anticline
Fault block
Hydrocarbon exploration
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Passive margin
Neogene
Cenomanian
Sequence (biology)
Sequence Stratigraphy
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The shape and classification of continental margins are in general dependent on style of continental splitting, rifting, subsidence and their proximity to the tectonic plate boundaries, at times the margins undergo for modifications by sediment deposition and volcanic activity. Worldwide continental margins are broadly categorized into two groups: passive (Atlantic) and active (Pacific) type margins. The main features of passive continental margin are shelf, slope, rise and abyssal plain. Analyses of marine geophysical data across the eastern and western continental margins of India show that both the margins are different in shape although both belong to passive margin group. While continental slope along the western continental margin is wider and provides scope for multiple picks of foot-of-slope, it narrows along the eastern continental margin and is clear for single pick of foot-of-slope. Continental slope and rise on western margin and rise on eastern margin were modified to a major extent by Deccan-Reunion hotspot volcanism and Bengal Fan sedimentation respectively. Volcanism has dominated on the western continental margin of India, thereby the margin had been turned into a volcanic passive continental margin, while eastern continental margin of India remained as nonvolcanic passive margin.
Continental Margin
Passive margin
Abyssal plain
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