Age and correlation of the False Bay dolerite dyke swarm, south- western Cape, Cape Province

Abstract

Potassium-argon dates of four dykes from the False Bay dolerite swarm yield a mean age of emplacement of 132 ± 6 Ma. Dykes included within this particular swarm display coherent geochemical patterns that can serve to distinguish them from other swarms related to the older Cape Granite suite and the younger alkaline intrusions (such as the olivine melilitites).

The age of the False Bay dykes rules out their correlation with the Karoo dolerites and associated Drakensberg basalts (including the Kalkrand basalts in Namibia), which have been dated at 180 Ma. Rather the dyke swarm appears to be synchronous with the Etendeka Formation of Namibia, which includes both eruptive rocks and associated intrusions (central complexes, sills, and dyke swarms) dated at 130 - 135 Ma.

	View from the sea E towards Chapmans Peak. A prominent dyke of dolerite can bee seen cutting up through the granite and into the overlying strata of the Cape Supergroup.		Narrow dolerite dyke cutting Cape Granite, Froggy Pond, False Bay
	Highly weathered dolerite dyke exposed in road cutting, Llandadno - Hout Bay highway

Cretaceous dykes associated with the opening of the South Atlantic: the Mehlberg dyke, northern Richtersveld

Abstract

Dolerite dykes associated with the post-Karoo continental flood basalt provinces occur near the Atlantic coast in the northern Richtersveld and neighbouring southern Namibia. The most prominent(the Mehlberg dyke) intrudes in an en echelon manner along the regional contact between the late Precambrian Gariep Belt and its basement.

Geochemical characterization reveals the Mehlberg dyke to represent a less evolved example of the basaltic magma type that occurs in dolerite sills intruding Karoo strata in nearby sub-basins (Warmbad and Keetmanshoop), so a simple feeder dyke relationship is not supported. Correlation is further ruled out by the Cretaceous age of intrusion of the Mehlberg dyke (134 ± 3 Ma, determined by the 40Ar/39Ar method), significantly younger than the Jurassic (~l80 Ma) dolerite sills, but synchronous with other Atlantic margin dyke swarms such as False Bay in the southwestern Cape and Horingbaai in northern Namibia.

All these dyke swarms probably result from extensional tectonics associated with the opening of the South Atlantic during the Cretaceous. The age data reinforce the recognition of two distinct periods of late Mesozoic basaltic igneous activity in Namibia, namely the older Karoo province (represented by the Jurassic sills) and younger Etendeka province (represented by the coastal dyke swarms).

Mehlberg dolerite dyke cutting steeply dipping foliated metavolcanics of the 2000 Ma Orange River Group in the Gariep front zone, northern Richtersveld

Karoo basalts in the Ellisras sub-basin, Northern Province

Abstract

Jurassic basalts cap the Karoo stratigraphic sequence in the Ellisras sub-basin of the Northern Province. The 75 metre section of basalt flows, obtained from a borehole, is probably a remnant of a thicker accumulation of unknown thickness.

K-Ar ages of flows from top and bottom of the profile average 179 ± 5 Ma, which is similar to ages obtained for the Lebombo and Lesotho lavas.

Little geochemical variation can be detected in the lava section, and the basalts appear to represent another occurrence of the voluminous Low Ti Zr (or LTZ) type, which characterises the Springbok Flats, Central Botswana and Lesotho. None of the Ellisras basalts resemble those of Northern Lebombo, Nuanetsi, Tuli and the area east of Soutpansberg, thereby providing a western limit to the geochemically enriched magma type (High Ti-Zr or HTZ) that appeared to have been erupted from the Lebombo Soutpansberg/Tuli Nuanetsi triple junction.

The Ellisras basalts represent part of the margins of a vast basalt lava field consisting of the LTZ magma type that was erupted from a separate centre (or centres) in Lesotho and/or Botswana.

Drill Core showing the basal contact between the Ellisras basalts and the underlying Karoo sandstones. Conspicuous dark carbonaceous layer below lava suggests eruption over a heavily forested land surface

Drill core section through a single basalt flow, showing a thick amygdaloidal zone at the top, with a distinctive pipe amygdale near the base

Interesting Karoo basalt exposure near Kalkrand in southern Namibia

	Road cut on the national highway between Windhoek and Cape Town (near Kalkrand). The lower lava flow is overlain by red tuffaceous waterlain sediments, which in turn was invaded by a second flow.		The water-rich sediments and the base of the later flow are disturbed and brecciated, due to the explosive effects caused by hot lava coming into contact with water
	Close up of the red tuffaceous horizon between the lower basalt flow, which looks like it has lost its highly amygdaloidal top by erosion. Above the tuffaceous horizon can be seen the brecciated base of the overlying flow

Some interesting features of the Karoo sills and inclined sheets

Karoo sandstone incorporated into a dolerite sill exposed in the Barkly Pass.

Basal contact of the Hantam dolerite sill above Calvinia. The contact is at the top of the photo, with vertically jointed hornfelsed Karoo sediments beneath, the bedding in which only reappears about 2m below.