Sediment 2007 Vortragsabstracts Sediment 2007 MARINOAN GLACIAL AND POSTGLACIAL SUCCESSIONS AT THE SOUTHERN RIFTED MARGIN OF THE CONGO CRATON (NEOPROTEROZOIC, NORTHERN NAMIBIA): FACIES, PALAEOGEOGRAPHY AND HYDROCARBON PERSPECTIVE Thilo Bechstädt1,2, Guy Spence1, Hartmut Jäger1,2, and Georg Werner1 1 Institute of Geology and Palaeontology, University of Heidelberg, 69120 Heidelberg, Germany 2 Steinbeis-Transfer Centre GeoResources, 69120 Heidelberg, Germany; [email protected] In northern Namibia continental rifting occurred at The post-glacial cap carbonate sequence (Maieberg about 750 Ma (Nosib Group). It is followed in the late Neo- Fm.) is much more widely developed than the Ghaub Fm. proterozoic by the deposition of predominantly shallow- The succession consists of a lower part, up to about ten water platform carbonates (Otavi Group). These are up to metres thick of often laminated, micritic Cap Dolomite. As 5000 metres thick, and were deposited on the southern in many other parts of the world, where possibly contem- “passive” margin of the Congo craton. The Otavi Group is poraneous glacial intervals have been found (e.g. South- exposed in a fold belt from the Kaokoveld in the West to west Namibia, Central Australia, Mackenzie Mts. in Cana- the Otavi Mountainland (OML) in the East. Deposition oc- da, East Greenland and East Svalbard Caledonides) a curred at sub-tropical to tropical latitudes, although at shaly-marly interval overlies the Cap Dolomites and forms that time climate might have been partly colder in the the middle and upper part of the cap carbonate sequence. tropics than today. The Otavi Group is thought to repre- During this interval, the climate might have distinctly im- sent a time span of around 200 million years. A complex proved. palaeogeographic setting with small, restricted basins oc- As in the other areas mentioned, the cap carbonate se- curred. Frequent submarine mass movements with a wide quence above the buff coloured Cap Dolomites changes in variety of clasts derived from the underlying Otavi and colour from brown to grey and/or black. It is often rich in Nosib Groups, suggest repeated uplift of partly several pyrite (distributed within the sediment in small aggre- thousands of metres and deep erosion of the former car- gates but also filling veins), if the Ghaub diamictite is pre- bonate platform/ramp. sent in the underlying rocks. This marly-shaly succession is Two discrete glaciogenic units are known in the Neo- locally more than 200 m thick and contains organic-rich proterozoic of northern Namibia. The older one (Chuos intervals, potential hydrocarbon source rocks. Strong Fm.) occurred during the Nosib rifting episode, the thickness differences indicate, that the shaly succession younger one (Ghaub Formation, zircon dating of associat- partly fills the preexisting slope and basin settings. In the ed tuffs: 635.5 ± 1.2 Ma) is intercalated in the middle part central OML, the Maieberg Fm. distinctly increases in of the Otavi Group. Ghaub diamictites often reach consid- thickness westward, from 70 m to 230 m, in line with the erable thickness (>100m) and are restricted to slope set- westward thickening of the diamictites. Debris flows and tings. Different intervals of diamictite sheddings (from ap- turbidite intervals several metres thick are intercalated proximately East to West) have been outlined in the central within the marls. OML. The clasts consist mostly of reworked Otavi and Nosib Two types of organofacies have been found: Type 1 is sediments; only a minor percentage is derived from local dominated by strongly degraded acritarchs and is oil crystalline basement. Diamictites seem to be lacking on prone, typical for lighter, medium grey rocks. Type 2 con- top of the shallower parts of the carbonate ramp, an argu- tains inertinite dominated kerogen with few degraded ment against “grounded ice”. As the assumed glaciers can- acritarchs and AOM, typical for dark grey to black rocks. not transport the basement clasts for thousands of metres The latter type represents higher anoxic levels. Fluid inclu- upwards, uplift of large areas that became glaciated is a sions and thermoluminescence indicate short lived higher likely scenario, largely corresponding to the one given by temperatures (150 to 200°C, hydrothermal pulses related Eyles and Januszczak (2004) (Zipper-Rift-Earth). Most of with ore emplacements) than the ones indicated by the the diamictites represent debris flows, and no prominent organic material. The degree of maturation might fluctu- striations occur on the clasts. The only valid argument for ate laterally. The described setting shows a potential for glaciations at that time is given by a few dropstones at the hydrocarbons, especially “tight gas” further to the north, base of the overlying postglacial interval. in the Owambo Basin. Geo.Alp, Vol. 4, 2007 3 Sediment 2007 MULTI-PHASE POROSITY-PRESERVING CHLORITE CEMENTATION IN SHALLOW MARINE VOLCANO- CLASTIC SANDSTONES, SAWAN GAS FIELD, PAKISTAN Anna Berger1, Susanne Gier1, and Peter Krois2 1 Department of Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria; [email protected] 2 OMV (Norge) AS, C.J. Hambros Plass 2c, 0164, Oslo, Norway Sandstones with anomalously high porosity and per- pore lining cement, comprising a 5–10 mm thick rim cov- meability at great burial depth and high temperatures are ering all detrital grains, clearly developed in two genera- of economic importance, as a significant amount of tions; an older, poorly crystallized and a younger better hydrocarbons have been discovered in these reservoirs. crystallized growth. The latter comprises euhedral, The Sawan gas field lies 500 km northeast of Karachi, in pseudohexagonal crystals, oriented with their faces per- the Middle Indus Basin. The reservoir rocks, Cretaceous pendicular to the host detrital grain surface. Rim precipi- volcanoclastic sandstones (Albian-Cenomanian) of the tation occurred after an initial stage of compaction but Lower Goru Formation, show anomalously high porosities early relative to other diagenetic phases. Both chlorite rim (20%) at depths of 3000 to 3500 m. generations grew by direct precipitation from pore waters, using products derived from volcanic rock frag- The sandstones are subarkoses to lithic arkoses and ments. In areas with no, thin, or discontinuous chlorite sublitharenites to feldspathic litharenites. Strongly al- rims, quartz cementation is common. Well-developed tered volcanic rock fragments are the most important chlorite rims inhibited quartz cementation, preserved lithic component. The clay fraction (<2 mm) consists of anomalously high porosities and permeabilities, but did Fe-rich chlorite (chamosite) and illite. Diagenetic features not inhibit carbonate cementation. such as compaction, quartz overgrowth cements, carbon- ate- and chlorite cements and feldspar dissolution can be The restriction of the porosity preserving diagenetic observed. processes to sandstones deposited in a very shallow marine environment with occasional influx of meteoric Authigenic chlorite occurs as two-phase pore lining water provides evidence for a primary depositional con- cement, pore filling cement and chloritized detrital com- trol during the burial process and could indicate an ponents, all having similar chemical compositions. The eogene tic chlorite precursor phase. 4 Geo.Alp, Vol. 4, 2007 Sediment 2007 THREE DIMENSIONAL GROUND-PENETRATING RADAR ANALYSIS OF ALLUVIAL FAN DEPOSITS (DEATH VALLEY, SW USA) Ulrich Bieg1, Jens Hornung1, Matthias Greb1, Nadja Fritschka2, Jörg Lang2, Anke Friedrich3, and Matthias Hinderer1 1 University of Technology Darmstadt, Schnittspahnstr. 9, 64287 Darmstadt, Germany; [email protected] 2 University of Hannover, Appelstraße 11A, 30167 Hannover, Germany 3 LMU Munich, Luisenstr. 37, 80333 München, Germany Altough the number of scientific papers on deposition- system or mass flows) and the prevailing depositional al units of alluvial fans is growing, only little is known processes. The delineated depositional systems, which are about the subsurface stacking pattern of these architec- separated by bounding surfaces, can be recognized in the tural elements. This study wants to test the utility of outcrops. Depositional packages have been interpreted ground-penetrating radar (GPR) in three dimensionally according to the morphometric base-level concept. imaging of alluvial fans, a landform that has had little at- tention by GPR users. So far four alluvial fans (Trail Canyon Fan, Hanaupah Canyon Fan, Warm Spring and Anvil Spring Fan) have The study combines high-resolution, high-frequency been investigated. The depositional record of all three GPR surveys with sedimentological studies at the cut- fans shows overall genetic units, which are either domi- faces of incised channels. Using scaled 2D outcrop photo nated by debris flows or fluvial processes. Irrespective of mosaics we quantitatively analysed lithofacies and archi- the lithology in the catchment area, the overall stacking tectural elements. Architectural elements are grouped to pattern shows increasing fluvial activity towards the top higher-order depositional systems (e.g. braided-fluvial of the successions, by coeval reduced debris flows. Geo.Alp, Vol. 4, 2007 5 Sediment 2007 MIXED SILICICLASTIC/CARBONATE ENVIRONMENTS FROM THE CENOZOIC NORTH ALPINE FORELAND BASIN: POSSIBILITIES AND LIMITATIONS OF INTERPRETING COMPLEX EPI-CONTINENTAL SEDIMENTS (UPPER MARINE MOLASSE, EARLY MIOCENE) Ulrich Bieg1, James