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CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 24 (1976), No. 1. (March), Pages 92-116

A Possible Mechanism Relating Progradation, Growth Faulting, Clay Diapirism and Overthrusting in a Regressive Sequence of Sediments

Gerard C. Dailly


Genetic relationship between undercompacted clays at the base of a regressive sequence and growth faulting has been expressed in the literature, as has also the relationship between growth faulting and clay diapirism, between clay diapirism and overthrusting, and between undercompacted shale and overthrusting. Few publications have been devoted to the complete sequence of events relating progradation to undercompacted shale, growth faults, clay diapirism and finally to overthrusting.

This paper is based mainly on studies of three large regressive sequences of the Mississippi, Niger and Mackenzie Deltas. It presents a complete succession of interrelated phenomena, which are the consequence of the progradation of the regressive sequence of sediments. The succession of events is:

The progradation of a regressive sequence gives birth to a basal undercompacted layer, mainly shale, overlain by a compactible layer, mainly sands;

As the progradation continues, the isostatic subsidence introduces a systematic seaward thinning of the compactible layer: the "Sand Wedge" and a correlative thickening seaward of the uncompactible wedge; the "Shale Wedge";

The superposition of this wedge of compactible heavy sediments on light noncompactible sediments creates a disequilibrium which results in growth faulting.

But further disequilibrium is produced by compaction of the compactible wedge as the progradation continues. A continuous squeeze of the under-compacted shale in the direction of the lower pressure gradient, which is at sea bottom at the extremity of the sand wedge, occurs. Therefore, the toes of the more recent growth faults have to absorb more and more squeezed shale as the progradation continues. A progressive transition from growth fault to clay bulge and then to clay diapirism toward the offshore exists.

As the progradation continues, the size of the wedge and the importance of compaction is such that the quantity of squeezed shales increases and creates some initiatory lateral diapirism and thrusting at the seaward end of the Delta.

Manuscript received' Oct., 1975. Accepted: Nov., 1975. Published with permission: Elf R. E., 7 - Rue Nelaton, 75739 Paris, Cedex 15, France.

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