Abstract: Regressive-Delta Model with Example from Upper Devonian--Lower Carboniferous of Southern Ireland

Piers R. Gardiner

Current studies of modern and ancient deltas have highlighted process-response factors, leading to categorization within a ternary framework of fluvial, wave, or tide-dominated deltas. When considered as three-dimensional systems, the effect of structural controls long has been recognized, but implicit in any delta definition has been the concept of initial progradation into the receiver basin. On theoretical grounds this premise is considered unsound, for tectonic factors fundamentally affect process controls and delta evolution. Against a primary structural background, deltaic systems can be classified as prograding, regressive, or multiple with reference to their evolutionary trends. The regressive-delta model is one in which there is no initial progradation, regional basin subsidence always matching or exceeding sediment input. There is thus a continuous transgression during the life of the depositional system, which is characterized by de tructive facies. Within such a structural framework bulk sequence types reflecting temporally variable process controls can be recognized.

The upper Paleozoic delta of southern Ireland is proposed as an example of the regressive-delta model. In the Upper Devonian (Fa2d-Tn1b) an internally subsiding alluvial basin of at least 250 sq km, flanked by northeast-trending lineaments, was breached in the south. During the continuing northerly marine transgression there was considerable terrigenous input, with deltaic sequences (locally > 2,000 m) accumulating in depocenters within the southern part of the basin. These show an evolutionary trend from a tidal or wave-tidal delta type to a wave-dominated one, being terminated by carbonate-shelf Tn3 deposition.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma