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Upper Devonian of South-East Ireland— Anatomy of a Constricted Alluvial Fan: Abstract
Three dimensional cross-sections permit detailed evaluation of this localised sand-deficient alluvial fan sequence. In bulk terms three valley fill episodes are recognised. Initially an 8 km wide NE-trending feeder canyon, some 150 m deep, was partially plugged by proximal gravelly material deposited by debris flows and sheet floods. This resulted in a broader, shallower canyon profile. Plugging was completed by conglomerates dominantly deposited by sheet and streamflood action.
Overstep onto lateral divides resulted in the third phase valley fill on a larger scale (> 20 km width) in the context of a stable distal fan - flood plain setting. Fine grained members, in part possibly wind transported, comprise some 50% of this 200+ m thick fill, with immature caliche profiles locally developed. Interleaved gravel deposits occur as thick braid bars, sheetflood crevasse splay units, and low sinuosity channel fills from ephemeral rivers. Low palaeocurrent variance and facies associations are considered suggestive of continued lateral constriction. Localised ponds with associated vegetation are interpreted to have been present in the central parts of the fan fringe.
The geometry and facies of this sequence is contrasted with other known alluvial fan models. The constricted model recognised here may prove to be relatively common in post-orogenic alluvial retreat situations.
Acknowledgments and Associated Footnotes
1 Geological Survey of Ireland, 14 Hume Street, Dublin 2, Ireland
Copyright © 2009 by the Canadian Society of Petroleum Geologists