The stratigraphy and structural evolution of the late Proterozoic Gariep Belt in the Sendelingsdrif-Anisfontein area, northwestern Cape Province

Abstract

Abstract by author:

The stratigraphic, structural and metamorphic evidence is in agreement with a plate tectonic model involving the growth and oblique closure of a proto-Atlantic ocean. The southeastward obduction of the leading edge of a subduction complex onto the west facing margin of the Kalahari craton, prior to or during final closure of the intracrustal arm of the Damara Orogen, was followed by the continued SSE-ward migration of the uncollided part of the subduction zone parallel to the western margin of the craton

During the rifting stage of passive continental margin evolution, the basal clastic Stinkfontein Sequence was deposited in an alluvial fan environment. Fluviatile conglomerates and quartz arenites (Lekkersing Formation) interfinger with feldspathic arenites and minor volcanics (Vredefontein Formation), and prograde into marginal marine clastics and carbonates (Gurachavib Formation). Rift faulting produced local grabens into which mass-flow sediments (Kaigas Formation) and volcaniclastics (Rosh Pinah Formation) were shed. During the sea-floor spreading stage, a shallow-water carbonate-clastic unit, the Hilda Sequence, was deposited. Conglomerates, quartzites, and schists with resedimented gravity-flow characteristics (Wallekraal Formation) are sandwiched between lower and upper platform carbonates (Pickelhaube and Dabie River Formations). The Hilda is unconformably overlain by the Numees Sequence, consisting of a widespread glaciogenic diamictite (Sendelingsdrif Formation) and a near-basal banded iron formation (Jakkalsberg Formation). A deep-water clastic unit, the Holgat Sequence, was laid down during the early stage of the lower Nama transgression

Several generations of deformational features formed during a single SE- to SSE-directed tectonic event. A transpressive phase, D1, produced NNW-trending basement-involved thrust faults, mesoscopic ENE-facing F1 folds, and a penetrative S1 transposition foliation. A southward decrease in the wrench shear/thrust shear ratio is recorded by the structural fabrics and strain markers. The D1 structures were coaxially refolded by non-cylindrical WSW-facing F2 shear folds during a sinistral plane wrenching phase, D2. A late left-lateral transtensive phase (D3-D5) coincided with the emplacement of the Kuboos-Swartbank plutons and produced westerly-cascading gravity folds and east-west to WNW-ESE trending tensional features

The mineral parageneses and microtextures indicate that D1 was accompanied by regional lower greenschist facies M1 metamorphism, culminating in post-kinematic biotite porphyroblast growth. The later deformational phases are mostly associated with retrograde effects, although a local thermal metamorphic event, M2, reached amphibolite facies conditions during the Kuboos pluton intrusion