Ole J. Martinsen1, Trond Lien1, Roger G. Walker2
(1) Norsk Hydro Research Center ASA, Bergen, Norway
(2) Roger Walker Consulting, Calgary
ABSTRACT: Deep-Water Deposits of the Upper Carboniferous Shannon Basin, Western Ireland
The Shannon Basin of Western Ireland exposes a more than 1250 m thick deep-water succession. The succession was probably controlled by differential, but decaying subsidence above the Shannon Lineament, a major structure that was extensionally reactivated. Significant thinning of the deep-water deposits onto the northern basin margin records transition onto the hangingwall dip slope of the major extensional structure.
The source-rock quality Clare Shales, the basal deep-water unit, were deposited after the main subsidence phase. The Clare Shales onlap the basin margins and thin dramatically onto the hangingwall dip slope where they are condensed.
The sand-rich, overlying Ross Formation records deposition in sinuous channel belts similar to those observed in Neogene turbidite systems off West Africa. The formation's sandy nature is attributed to trapping of sand in an enclosed basin floor setting.
The overlying, Gull Island Formation consists of a lower mud-rich turbidite system which thins onto and onlaps the northern hangingwall dip slope, and an upper prograding basin slope succession. The mud-rich character of the lower part is attributed to extensional fault movement, causing slumping of mud from the northern dip slope, and possibly also to spill to adjacent basins. The slumps are interbedded with various types of thin-bedded turbidite sandstones.
The final phase of deep-water deposition in the Shannon Basin was the progradation of a fine-grained basin slope, which is interpreted to downlap onto the lower basin-fill. This slope succession is capped by a deltaic succession.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado