5 research outputs found
The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?
We briefly examine the possible antiquity of the supercontinental cycle while noting
the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual
change from a magmatically dominated Hadean Earth to a plate tectonically dominated
Neoarchaean system. A brief review of one of Earthâs oldest cratons, Kaapvaal,
where accent is placed on the lithostratigraphic and geodynamic-chronological history
of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The
ca. 3.1â2.8 Ga WitwatersrandâPongola (Supergroups) complex retroarc flexural foreland
basin developed while growth and stabilization of the craton were still underway.
Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc
complexes) from both north and west does not support the formation of a Neoarchaean
supercontinent, but may well have been related to a mantle plume which enhanced primary
gold sources in the accreted terranes and possibly controlled the timing and rate
of craton growth through plate convergent processes. Subsequent deformation of the
Witwatersrand Basin fill with concomitant loss of â€1.5 km of stratigraphy must have
been due to far-field tectonic effects, but no known mobile belt or even greenstone
belts can be related to this contractional event. At ca. 2714â2709 Ma, a large mantle
plume impinged beneath the thinned crust underlying theWitwatersrand Basin forming
thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with
subsequent thermal doming leading to graben basins within which medial bimodal volcanics
and immature sediments accumulated. Finally (possibly at ca. 2.66â2.68 Ga),
thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas
and sediments, with minor komatiites still present. Ongoing plume-related influences
are thus inferred, and an analogous cause is ascribed to a ca. 2.66â2.68 Ga dike swarm
to the north of the Ventersdorp, where associated rifting allowed formation of discrete
âprotobasinalâ depositories of the Transvaal (ca. 2.6â2.05 Ga Supergroup, preserved
in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above
these lowermost rift fills show an association with localized compressive deformation
along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence
of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma,
the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation
platform covering much of it and preserved in all three Transvaal Basins (TB).
During this general period, at ca. 2691â2610 Ma, the Kaapvaal Craton collided with a
small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield
tectonic effects are likely implicit in TB geodynamics, again there is no
case to be made for supercontinent formation. Following an 80â200 million years
(?) hiatus, with localized deformation and removal of large thicknesses of chemically
precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of
rifting associated with volcanism, and subsequent thermal subsidence within a sag
basin setting; an association of the second such event with flood basalts supports
a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern
Kaapvaal Craton and reflects a major plume, following which KaapvaalâCZ
collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a
small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence
in favour of active mantle (cf. plume) influences with subordinate and localized tectonic
shortening, implicit within the review of ca. 3.1â2.05 Ga geological history of the
Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated
thermal systems, at least for this craton.University of Pretoria and the National Research Foundation of South
Africa.http://www.tandfonline.com/loi/tigr20nf201
The importance of changing oceanography in controlling late Quaternary carbonate sedimentation on a high-energy, tropical, oceanic ramp: north-western Australia
The North West Shelf is an ocean-facing carbonate ramp that lies in a warm-water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long-period swells and large internal tides, resulting in preferentially accumulating coarse-grained sediments. Circulation is dominated by the south-flowing, low-salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward-flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea-level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid-ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4-12·7 Ka, in somewhat saline waters during initial stages of post-Last Glacial Maximum (LGM) sea-level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner-ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine-derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid-ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer-ramp sediment is mainly pelagic, veneering shallow-water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at â 200 m. water depth suggest periodic upwelling or Fe-redox pumping, whereas enhanced near-surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at â 140 m. water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner-ramp sediments can contain important heterozoan elements, mid-ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid-ramp sedimentation mimicking maximum flooding intervals, and outer-ramp planktic productivity can generate locally important fine-grained carbonate sediment bodies. Changing oceanography during sea-level rise can profoundly affect sediment composition, sedimentation rate and packaging. © 2004 International Association of Sedimentologists