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6 - The interpretation of massive rain-out and debris-flow diamictites from the glacial marine environment
- Edited by M. Deynoux, J. M. G. Miller, Vanderbilt University, Tennessee, E. W. Domack, Hamilton College, New York, N. Eyles, University of Toronto, I. Fairchild, University of Birmingham, G. M. Young, University of Western Ontario
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- Book:
- Earth's Glacial Record
- Published online:
- 06 July 2010
- Print publication:
- 19 May 1994, pp 83-94
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Summary
Abstract
Lithofacies and clast fabric analyses of four massive diamictites from the glacigene Dwyka Formation show two (Floriskraal and Douglas) to be primarily of rain-out origin and the other two (Elandsvlei and Kransgat River) to have formed by subaqueous sediment gravity flow. Ancient massive diamictites, which exhibit no bedding or other internal structures, thus may have a multiple origin as they can form directly by dense rain-out or indirectly by resedimentation of glacial material. Rain-out and resedimentation can occur simultaneously, and distinction between the two processes in ancient diamictites is problematical. For the interpretation of ancient massive diamictites a lithofacies analysis must therefore be a first priority, because results of fabric analyses and other criteria are often inconclusive. Diagenesis, compaction and dewatering can obliterate diagnostic features necessary for the successful interpretation of massive diamictites.
Introduction
Glacial sequences from Precambrian to Late Palaeozoic in age consist mostly of homogeneous diamictites lacking sedimentary structures as well as observable grain fabrics. Such sequences attain thicknesses of several hundreds of metres over large areas and their interpretation always poses a problem to glacial geologists.
A massive diamictite is here defined as being a homogeneous rock body without bedding or other internal structures on a macro- and mesoscale (Fig. 6.1). A diamictite sequence may, however, be thickly bedded although bedding planes are commonly absent in diamictites up to several tens of metres thick. According to Petti – john (1975) truly massive beds are probably very rare and radiography of such seemingly homogeneous beds has in many cases revealed internal bedding structures.
14 - A Permian argillaceous syn- to post-glacial foreland sequence in the Karoo Basin, South Africa
- Edited by M. Deynoux, J. M. G. Miller, Vanderbilt University, Tennessee, E. W. Domack, Hamilton College, New York, N. Eyles, University of Toronto, I. Fairchild, University of Birmingham, G. M. Young, University of Western Ontario
-
- Book:
- Earth's Glacial Record
- Published online:
- 06 July 2010
- Print publication:
- 19 May 1994, pp 193-203
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- Chapter
- Export citation
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Summary
Abstract
The basal part of the thick mudrock sequence of the Ecca Group overlying the glacigenic Dwyka Group (late Carboniferous to Artinskian in age) shows evidence of periglacial conditions during deposition. The dark-coloured to black mudrocks which attain a thickness of up to 250 m, contain organic-rich horizons, siltstone and fine-grained sandstone interbeds, marine fossils, and carbonate, siliceous and phosphate concretions, lenses and beds. Deposition was by suspension settling of mud, turbidity current activity and minor fall-out of airborne volcanic ash in a large sea with episodic anoxic bottom conditions. The climate probably varied from subpolar to temperate. The syn- to post-glacial mudrock sedimentation in the foreland basin was controlled by the rapid collapse of the marine ice sheet, basin tectonics (subsidence and rising foreland arc), oceanic circulation, relative sea-level change and a cool to temperate climate.
Introduction
The present Karoo Basin formed when the Gondwana supercontinent broke up during the Late Jurassic to Early Cretaceous. The beds of the Karoo Supergroup consist of remnants of upper Carboniferous to lower Jurassic strata which covered extensive parts of southwestern Gondwana. In southern Africa, Karoo beds also occur in several smaller, mostly fault-controlled, basins towards the north, but this paper deals exclusively with the postglacial sequence in the main Karoo Basin, which covers an area of just over 600000 km2 (Fig. 14.1). The glacigenic Dwyka Group which occurs at the base of the Karoo Supergroup, is conformably overlain by a predominantly mudrock sequence (Ecca Group) up to 3000 m thick (Tankard et al., 1982).