Department of Earth Sciences

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Browsing Department of Earth Sciences by Author "Alchin, Dennis John"

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    The structural and stratigraphic setting of the Rosh Pinah zinc-lead deposit within the Gariep trough
    (Stellenbosch : Stellenbosch University, 1993) Alchin, Dennis John; Halbich, I. W.; Stellenbosch University. Faculty of Science. Department of Earth Sciences.
    ENGLISH ABSTRACT: A geological investigation in the Rosh Pinah, Obib and Pickelhaube Peaks and Namuskluft-Dreiqratberg areas established the structural and stratigraphic setting of the Pinah zinc-lead deposits within the Gariep Trough. The sedimentary evolution of the Pan-African Gariep Belt was initiated by widespread rifting and opening of an ocean (the Adamastor Ocean) along a continental margin. Deposition of the basal clastic Stinkfontein Sequence as broad alluvial fans strandline sediments took place in the transgressive sea. The mass flow Kaigas Formation diamictites and volcaniclastics of the Rosh Pinah Formation were subsequently deposited in locally developed grabens. Syn-sedimantary ore beds of hydrothermal origin (includinq Rosh Pinah orebodies) were precipitated on the sea floor while reef carbonates formed along the coast line during the final stages of transgression. These sediments were eventually covered by a marginal marine clastic and carbonate facies with widespread intrusion of basic sills and dykes following uplift of the craton. These rocks are here grouped within the Rosh Pinah Formation. Durinq the sea floor spreading stage of continental evolution the elastic-carbonates of the Hilda Sequence were deposited in a shallow to moderately deep-water environment. The Wallekraal conglomerates were deposited as local submarine fans with minor debris flows and are overlain by the Dabie River Formation carbonates. During regression banded iron formations were deposited in shallow depressions and the glaciogenic Numees Sequence diamictites were deposited unconformably above the Hilda Sequence and iron formations. These in turn are overlain by the deepwater clastics of the Holgat Sequence. The Witputs sequence, which uncontormably overlies the Numees Sequence along the basement in the east is seen as a proximal shallow water equivalent of the distal Holqat Sequence. Gravity tectonics played an important role during the early structural evolution of the Gariep Belt. Mass flow and slump structures featured prominently in half graben structures formed during the rifting staqe. However, the closing of the Pan- African Adamastor Ocean as a result of subduction and continental collision led to a single, very intensive and protracted period of SE directed transpression, which largely obliterated all gravity deformation characteristics. The D1 transpressive phase produced NNW-trendinq basement involved oblique ramps during a major SB-directed tectonic event, which culminated in several deformation pulses. Arcuate nappe structures were formed, e.g. the Marmora, Schakalsberg and possibly the Rosh Pinah Nappes as a result of the oblique emplacement of thrust slices over the cover rocks. Thrusting was accompanied by east-northeast-verging F1 folds with axes rotated sub-parallel to the D1 transport direction, and a penetrative westerly dipping S1 transposition cleavage. The master Schakelsberg Thrust emplaced a thick slice or dense basic rock with ophiolitic affinities onto a foreland situated in the Schakalsberg mountains during the D1 transpressive phase. Gravitational instability was thereby caused in the crust, and an imbricate fan of listric splay faults formed in front of the overlying spreading mass. Several deformation paths for the complicated tectonite containing three partly non-colinear phases of deformation can be envisaged from here which includes oblique ramping, backthrusting and backfoldinq. The Annisfontein anticlinorium may have formed as a D1 basement thrust ramped up section and folded due to continued pressure from the W. During this pulse D1 structures were sub-coaxially refoldled by non-cylindrical WSW-verging F2 shear folds because of E-directed compression across NNW trending ramps. This is displayed in the Namuskluft-Dreigratberg area and is also inferred for the deeper levels of the western areas, and leads to backfolding over most of the area. The semi-allochthonous Rosh Pinah Formation containing the Rosh Pinah orebodies in the type area was refolded during D2 into a hinterland-verging anticlinorium because of its proximity to the major oblique ramps, which are also responsible for the steeply dipping thrusts in the Rosh Pinah-Namuskluft-Dreigratberg area, and which probably represent highly deformed earlier qraben and horst structures. On the eastern limb of the Annisfontein anticlinorium, a hinterland-dipping duplex of imbricate fan sheets developed on the deformed and overthrust sediments (the Pickelhaube Duplex Structure) as a result of ramping and steepening against the basement. Continued upward bulging of the Annisfontein anticlinorium due to either antiformal thrust stacking or multiple ramping along an initial basement irregularity on the footwall resulted in east-dipping parts of the Rosh Pinah Nappe Thrust being inactivated. Late, limited gravitational gliding of the overthrust sediments along the Rosh Pinah Nappe Thrust towards the east may have followed contractional deformation to enhance backfolding of thrust slices of Hilda lithologies i the duplex zone . The oblique ramplng ot litholoqies against the cratonic margin and possible deeper seated ramps finally also accounts for D3 structures. Due to differential movement these F3 folds may be slightly overturned towards the southeast, with slip occurring along cleavage planes to outline minor thrusting oblique to the previous events and trending NE-SW. This folding phase resulted in the present day north-south undulation of the structures and the landscape. The late lateral transtensive phase D4-D5 resulted from the emplacement of the Kuboos-Swartbank plutons and are not well represented in this area. The Gariep Belt has been subject to a protracted period of lower qreenschist facias metamorphism (M1), followed by a second local contact-thermal event (M2) which resulted from the emplacement of the Kuboos and Swartbank plutons in the Richtersveld.