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Keywords:

  • Debris flow;
  • deepwater basin floor;
  • Karoo basin;
  • slide initiation;
  • slide termination;
  • syn-sedimentary deformation

Abstract

Submarine mass movement deposits exposed in the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, provide a rare opportunity to analyse and interpret their emplacement history and deformation processes at a scale comparable to seismic examples. An up to 80 m thick slide deposit, continuously exposed in two 2 km long sub-parallel sections, passes from extensionally deformed material (clastic dykes and down-dip facing low-angle shear surfaces) down-dip into a compressional toe zone with large (tens of metres amplitude) folds dissected by steep, up-dip facing thrust planes. The compressional shear planes sole out onto a highly sheared décollement and cross-cutting relationships indicate an up-depositional dip younging in the timing of fold dissection. Lithofacies characteristics and detailed correlation of volcanic ash and other marker beds over more than 500 km2 in the bounding undeformed stratigraphy indicate a low-gradient (<0·1°) basin floor setting. The slide is abruptly overlain by an up to 50 m thick debrite with sandy clasts supported by an argillaceous matrix. Shear loading of the debris flow is interpreted to have driven large-scale deformation of the substrate through the generation of high shear stresses at a rheological interface due to: (i) the abrupt contact between the slide and the debrite; (ii) the coincident thickness distributions of the debrite and slide; (iii) the distribution of the most intense folding and thrusting under the thickest parts of the debrite; (iv) the preservation of fold crests with only minor erosion along fold limbs; (v) the presence of the debrite under overturned folds; (vi) the presence of laterally extensive marker beds directly above deformation units indicating minimal depositional topography; and (vii) the demonstrably local derivation of the slide as individual folded beds are mapped into undeformed strata outside the areas of deformation. The debrite is directly overlain by fine-grained turbidite sandstone beds that show widespread vertical foundering into the debrite. This case study demonstrates that intensely deformed strata can be generated by negligible amounts of down-dip movement in a low-gradient, fine-grained basin floor setting with the driver for movement and deformation being the mass imbalance resulting from emplacement of episodic debris flows. Simple interpretation of an unstable slope setting based on the presence of such deformed strata should be treated with caution.