Journal of the Geological Society, London, Vol. 163, 2006, pp. 671–682. Printed in Great Britain. Structure and evolution of hydrothermal vent complexes in the Karoo Basin, South Africa HENRIK SVENSEN1,BJØRNJAMTVEIT1, SVERRE PLANKE1,2 & LUC CHEVALLIER3 1Physics of Geological Processes (PGP), Department of Physics, PO Box 1048, Blindern, University of Oslo, Norway (e-mail: [email protected]) 2Volcanic Basin Petroleum Research (VBPR), Oslo Research Park, 0349 Oslo, Norway 3Council for Geoscience, PO Box 572, Bellville 7535, Cape Town, South Africa Abstract: The Karoo large igneous province, formed at c. 183 Ma, is characterized by the presence of voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders. Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, slightly predating the onset of the main phase of flood volcanism. Large igneous provinces, such as the North Atlantic and the have provided constraints on the spatial relationship between sill Karoo–Lesotho provinces, are characterized by the presence of intrusions and hydrothermal vent complexes (Planke et al. 2005). an extensive network of sills and dykes emplaced in sedimentary Generally, the hydrothermal vent complexes represent conduit strata (e.g. Du Toit 1920; Walker & Poldervaart 1949; Chevallier zones up to 8 km long rooted in contact aureoles around sill & Woodford 1999; Berndt et al. 2000; Brekke 2000; Bell & intrusions, where the upper part of the vent complexes comprise Butcher 2002; Smallwood & Maresh 2002; Trude et al. 2003; eyes, craters or mounds, up to 10 km in diameter (Fig. 1; Planke Planke et al. 2005). An important consequence of intrusive et al. 2005). Field observations from the Karoo Basin show that activity in sedimentary basins is that the magma causes rapid similar hydrothermal vent complexes commonly comprise brec- heating of the intruded sediments and their pore fluids, causing cias of sedimentary rocks with a varying degree of magmatic expansion and boiling of the pore fluid (Jamtveit et al. 2004), material (e.g. Dingle et al. 1983; Woodford et al. 2001; Jamtveit and metamorphic dehydration reactions. These processes may et al. 2004). lead to phreatic volcanic activity by the formation of cylindrical The aim of this paper is to document the structure, composi- conduits that pierce sedimentary strata all the way to the surface. tion and evolution of sediment-dominated hydrothermal vent The hydrothermal vent complexes thus represent pathways for complexes in the Karoo Basin, South Africa (Fig. 2a). Here, we gases produced in contact aureoles to the atmosphere, with the have chosen to focus on two representative complexes that were potential to induce global climate changes (Svensen et al. 2004). selected after reconnaissance fieldwork on more than 10 com- Consequently, constraints on processes leading to the formation plexes in the Dordrecht–Rossow area in the Eastern Cape of hydrothermal vent complexes in sedimentary basins, their Province (Fig. 2b). Specifically, we want to address the following abundance and structure may lead to a better understanding of questions that can be tested by detailed fieldwork and petro- the causes of the abrupt climate changes that are associated with graphic analysis. (1) Do sediment-dominated hydrothermal vent many large igneous provinces (e.g. Wignall 2001; Courtillot & complexes represent phreatic explosion events? (2) What is the Renne 2003). involvement of igneous material in phreatic sediment-dominated Hydrothermal and phreatomagmatic vent complexes are recog- vent complexes? (3) What is the degree of hydrothermal altera- nized from several sedimentary basins associated with large tion in and around short-lived phreatic systems? Quantitative igneous provinces, including the Vøring and Møre basins off aspects of the formation of hydrothermal vent complexes and mid-Norway (Skogseid et al. 1992; Svensen et al. 2003; Planke processes in volcanic basins are addressed elsewhere (Jamtveit et et al. 2005), the Faeroe–Shetland Basin (e.g. Bell & Butcher al. 2004; Malthe-Sørenssen et al. 2004). 2002), the Karoo Basin in southern Africa (e.g. Du Toit 1904, 1912; Gevers 1928; Stockley 1947; Dingle et al. 1983; Jamtveit Terminology et al. 2004), in the Karoo-equivalent basins of Antarctica (Grapes et al. 1973; Hanson & Elliot 1996; White & McClintock 2001), Volcanic basins are sedimentary basins with a significant amount and the Tunguska Basin in Siberia, Russia (e.g. Zolotukhin & of primary volcanic rocks (e.g. sills and dykes). Al’mukhamedov 1988). Two- and three-dimensional seismic data Pierced basins are sedimentary basins with many piercement 671 672 H. SVENSEN ET AL. and aeolian (upper part of the Stormberg Group) (Catuneanu et al. 1998). At the time of flood basalt eruption, up to 400 m of fine-grained sand had been deposited by aeolian, fluvial and lacustrine processes (Smith 1990; Veevers et al. 1994). The Inner zone sedimentary rocks forming the southwestern parts of the basin Outer zone were gently folded during the Cape Orogeny (278–215 Ma; Crater and Upper Catuneanu et al. 1998), whereas the rest of the basin is surface deposits part essentially undeformed. Both southern Africa and Antarctica experienced extensive volcanic activity in the early Jurassic Period. Dolerites and lavas Inward dipping in the Karoo Basin were emplaced and erupted within a strata relatively short time span (183 Æ1 Ma; Duncan et al. 1997). Sills and dykes are present throughout the sedimentary succession, and locally form up to 70% of the basin volume (e.g. Rowsell & De Swardt 1976). Hydrothermal vent complexes and volcanic phreatic complexes are numerous in the Karoo Basin, and primarily exposed within the Stormberg Group sediments. The complexes comprise a range of different rock types, ranging from lava and pyroclastic rocks to sediment breccia and sand- Lower stone (e.g. Du Toit 1904, 1912; Gevers 1928; Taylor 1970; part Dingle et al. 1983). The formation of the complexes is related to Conduit the sill emplacement episode and the formation of the Karoo igneous province (e.g. Dingle et al. 1983; Woodford et al. 2001; Fluids Jamtveit et al. 2004). The hydrothermal vent complexes repre- (liquid/gas) sent a very prominent feature of the Karoo Basin, and more than 320 have been identified in the Stormberg Group. Heating/boiling Methods Sill Detailed mapping of two hydrothermal vent complexes, Witkop II and III ~300 m (Fig. 2b), was carried out during three field seasons, including an Contact aureole extensive sampling programme (c. 100 samples). The fieldwork focused on mapping the total extent of the vent complexes, identifying and mapping sedimentary facies units and their boundaries, logging of sections in the vent complexes, and identifying structural features such as Fig. 1. Idealized cross-section through a hydrothermal vent complex faults and pipes. Aerial photographs from the Chief Directorate of based on field observations from the Karoo Basin and seismic Surveys and Mapping in Cape Town (South Africa) were used for interpretation from the Norwegian Sea (Jamtveit et al. 2004; Planke et al. geological mapping because of the lack of detailed topographic sheets for 2005). We divide hydrothermal vent complexes into a lower and an upper the area. Length and height measurements were made by a Garmin Etrex part, and an inner and an outer zone. global positioning system (GPS), and used to calibrate maps and logs. Petrography of a large number of thin sections (.50) has been studied by optical and electronic microscopes at the Department of Earth structures such as mud volcanoes, dewatering pipes and hydro- Sciences, University of Oslo, with a JEOL JSM 840 SEM and a thermal vent complexes. CAMECA SX 100 electron microprobe. A microprobe analysis of zeolite was carried out with an accelerating voltage of 15 kV, a beam current of Sills are tabular igneous intrusions that are dominantly layer 5 nA and a 5 ìm raster size, using synthetic and natural standards. parallel. They are commonly subhorizontal. Sills may locally The Department of Water Affairs in South Africa drilled a percussion have transgressive segments, i.e. segments that cross-cut the borehole trough the inner zone of the Witkop III hydrothermal vent stratigraphy. complex in April 2003. The borehole started at 1908 m above sea level, Hydrothermal vent complexes are pipe-like structures formed and terminated at 300 m below the surface. Chips were collected every by fracturing, transport and eruption of hydrothermal fluids. The metre during drilling,