Sandstone dikes in dolerite sills: Evidence for high-pressure gradients and sediment mobilization during solidifi cation of magmatic sheet intrusions in sedimentary basins Henrik Svensen Ingrid Aarnes Yuri Y. Podladchikov Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway Espen Jettestuen Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway, and International Research Institute of Stavanger (IRIS), Prof. Olav Hanssensvei 15, 4068 Stavanger, Norway Camilla H. Harstad AGR Group, Karenslyst allè 4, 0278 Oslo, Norway Sverre Planke Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway, and Volcanic Basin Petroleum Research (VBPR), Oslo, Norway ABSTRACT aureoles on a short time scale, representing That study was based on fi eld examples from an intermediate situation between fl uid loss a sill emplaced in sediments during formation Sediment dikes are common within doler- during formation of microfractures and fl uid of the Central Atlantic Magmatic Province. The ite sill intrusions in the Karoo Basin in South loss during violent vent formation. role of pore fl uid boiling in causing high aureole Africa. The dikes are subvertical and as pressures and subsequent fl uid movement was much as 2 m wide, sometimes with abundant INTRODUCTION explored in more detail by Delaney (1982; and fragments of sedimentary rocks and dolerite. more recently, e.g., Jamtveit et al., 2004). The matrix consists of contact-metamorphic Subsurface sediment mobilization and fl u- Understanding sediment mobilization from sandstone. There is no petrographic evidence idization have been recognized from many contact aureoles may put important constraints for melting within the sediment dikes. The geological settings, ranging from overpressured on pressure evolution of aureoles. The past maximum temperature during heating is clastic reservoirs (Jolly and Lonergan, 2002; decade has seen an increasing interest in degas- restricted to the plagioclase and biotite stabil- Mazzini et al., 2003; Nichols et al., 1994) to sing of volatiles from sedimentary basins with ity fi eld, or above ~350 °C. Thermal model- contact metamorphism around magmatic sill magmatic intrusions, where high pore fl uid ing of a sandstone dike in a dolerite sill shows intrusions (Jamtveit et al., 2004; Svensen et al., pressure plays a key role (Ganino and Arndt, that a temperature of 350–450 °C is reached 2006). In sedimentary basins affected by mag- 2009; McElwain et al., 2005; Retallack and in the dike after a few hundred years of sill matic sill intrusions (i.e., volcanic basins), like Jahren, 2008; Svensen et al., 2004, 2007, 2009). cooling. The calculated pressure history of a the Karoo Basin in South Africa, sediment dikes Gas venting triggered by overpressure in con- cooling sill and its contact aureole shows that are reported from within doleritic sills (Van Bil- tact aureoles within shale has been proposed substantial fl uid pressure anomalies develop jon and Smitter, 1956). It is interesting that these to have caused global climate changes in the on a short time scale (1–15 yr) and are main- dikes comprise metamorphic sandstone, demon- end-Permian, Early Jurassic (Toarcian), and at tained for more than 100 yr. Calculated pres- strating that the sand intruded the dolerite while the Paleocene-Eocene boundary (Svensen et al., sure anomalies in the sill (−7 to −22 MPa) the sills were still hot. The importance of these 2004, 2007, 2009). and the aureole (4–22 MPa) are signifi cant observations is that they form direct evidence The aim of this study is to understand the and may explain sill fracturing and sediment for high pore fl uid pressure during sill emplace- formation of sandstone intrusions in dolerite mobilization from the aureole into the sill. ment and subsequent contact metamorphism. sills. We present several case studies of sedi- We conclude that sediment dikes represent In a classic study by Walton and O’Sullivan ment dikes and sediment breccias within sills common features of sedimentary basins with (1950), it was suggested that pressure drop in the Karoo Basin. However, the results can be sill intrusions in which fl uid pressure gra- during sill cooling and fracturing (i.e., thermal applied to other sedimentary basins where sedi- dients have been high. Sediment dikes thus contraction) led to boiling of aureole pore fl u- ments have been injected into magmatic sheet signify that pore fl uids may escape from the ids that ultimately led to sediment fl uidization. intrusions, including the Vøring Basin offshore Geosphere; June 2010; v. 6; no. 3; p. 211–224; doi: 10.1130/GES00506.1; 10 fi gures; 1 table. For permission to copy, contact [email protected] 211 © 2010 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/3/211/3338840/211.pdf by guest on 30 September 2021 Svensen et al. Norway, the Tunguska Basin of east Siberia, 2005), although volcanism in southern Africa sents a two-dimensional (2D) slice through the and the Amazonas Basin in Brazil. The process continued for several million years (Jourdan et dike. We then used image analysis techniques of sediment injections is addressed by adopting al., 2005). Sills and dikes are present throughout and a MATLAB (http://www.mathworks a new theoretical model for sill pressure evolu- the sedimentary succession in the Karoo Basin .com/) code to quantify the clast content (i.e., tion during cooling and crystallization (Aarnes (Fig. 1) (Chevallier and Woodford, 1999; Pol- area). Probability densities were calculated et al., 2008). teau et al., 2008b), where they locally compose using a smoothing procedure, where data were as much as 70% of the stratigraphy (Rowsell binned in either 10 consecutive areas (for sedi- GEOLOGICAL SETTING and De Swardt, 1976). ment clasts) or 5 consecutive areas (for dolerite clasts). The aspect ratio between the long and The Karoo Basin (Fig. 1) covers more than METHODS short axes of the fragments was also calculated. half of South Africa. The basin is bounded by Since our mapping analyses are done in 2D, the Cape Fold Belt along its southern mar- Sampling and Petrography and we only have one slice through the dike, gin and comprises as much as 6 km of clastic the results should be regarded as approximate. sedimentary strata capped by at least 1.4 km of Sediment dikes are common within thick Thin sections of collected samples were stud- basaltic lava (e.g., Johnson et al., 1997; Smith, (70–120 m) dolerite sills within the Beau- ied by optical and electron microscopes (scan- 1990). The sediments were deposited from the fort Group sediments. The depth of magma ning electron microscope, SEM) at the Depart- late Carboniferous to the Middle Jurassic, in an emplacement is estimated as 600–1000 m ment of Geology, University of Oslo. The SEM environment ranging from dominantly marine below the paleosurface, based on present-day is a JEOL JSM 840, and was also utilized for (the Dwyka and Ecca Groups) to fl uvial (the stratigraphic levels. We have done detailed cathodoluminescence (CL) imaging. Beaufort Group and parts of the Stormberg studies of three localities with sediment dikes Group) and eolian (upper part of the Stormberg in dolerites: (1) the Waterdown Dam area, Phase Stability Calculations Group) (Catuneanu et al., 1998; Veevers et al., (2) the Elandsberg roadcut (Nico Malan Pass), 1994). The Beaufort Group is a thick sequence and (3) the Golden Valley (Fig. 1). Many more We used Perple_X (Connolly, 2005) to com- of dominantly sandstones. The overlying Storm- localities with sediment dikes have been dis- pute phase diagrams for rocks with a pelite berg Group includes the Molteno Formation covered during our fi eld work in the Karoo composition to predict the temperature stabil- (coarse sandstone, shale, and coal), the Elliot Basin during the past decade (e.g., south of ity of the mineral assemblages identifi ed in the Formation (sandstone, shale; red beds), and the Cathcart), but the chosen localities are repre- sandstone dikes. The calculated phase diagram Clarens Formation (sandstone with occasional sentative. One of the sediment dikes from the is projected from an average pelite composi- siltstone horizons). Waterdown Dam locality contains numerous tion (Caddick and Thompson, 2008), with Both southern Africa and Antarctica under- fragments of sediments and dolerite. It was SiO2 = 59.8, Al2O3 = 16.6, FeO = 5.8, MgO = went extensive volcanic activity in early Juras- mapped in detail by covering it with transpar- 2.6, CaO = 1.1, Na2O = 1.7, K2O = 3.53, sic times, starting ca. 182.5 Ma. Dolerites and ent A4 plastic sheets and tracing individual TiO2 = 0.75, H2O = 5.0 (all in wt%). We calcu- lavas of the Karoo-Ferrar large igneous province clasts by hand. This method was preferred over lated the reactions using quartz saturation, which were emplaced within a relatively short time photo analysis due to better accuracy and the means that the phase assemblages obtained are span. The main phase of fl ood volcanism lasted benefi t of doing on-site interpretations on clast not dependent upon the bulk content of quartz. <1 m.y. (Duncan et al., 1997; Jourdan et al., type and clast outline. The resulting map repre- Hence the phase diagram is valid for sandstones Drakensberg Gr. Ecca Gr. Waterdown Dam Stormberg Gr. Dwyka Gr. A B Beaufort Gr. Basement Sill intrusion The Whitehill Fm. 30 Waterdown Dam 3 2 Farm Golden Valley 1 32 Road Cathcart Waterdown Dam (R 6 Nico Malan Pass 7) Cape Fold belt Indian Ocean Cape Town Dolerite dike 20 150 km 2km 24 28 Sediment dike Figure 1. (A) Simplifi ed geological map of the Karoo Basin in South Africa; black dots are sill intrusions and hydrothermal vent complexes.