--> Abstract: Valley Fill Types and Dimensions: A Compilation from the Geological Record, by Martin Gibling; #90039 (2005)

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Valley Fill Types and Dimensions: A Compilation from the Geological Record

Martin Gibling
Dalhousie University, Halifax, NS

Valley fills constitute important hydrocarbon reservoirs. Based on a dataset of bedrock and Quaternary examples, they comprise: 1) Valley fills on bedrock; 2) Valley fills within coeval alluvial and marine strata; and 3) Valley fills in subglacial and proglacial deposits (mainly tunnel valleys). The examples are up to 1400 m thick and 105 km wide, and substrate strength and incision process strongly influenced their aspect ratios. Graphs of compiled valley-fill width and thickness may assist reservoir geologists in choosing realistic dimensions, where sub-surface valley fills are incompletely known. Many bedrock valley fills have low width:thickness ratio (W/T, 2-100, commonly <20) due to resistant valley walls and incision along tectonic lineaments. Their fills are largely alluvial, and reflect abundant sediment supply, episodic tectonism, discharge variation and river capture. In contrast, valleys within coeval strata have higher W/T (5-3640, commonly 100-1000), reflecting widening against less resistant valley walls. They typically contain alluvial and estuarine strata, and most reflect sea-level fluctuation, although some have basal angular unconformities and are tectonically enhanced. U.Paleozoic valley fills are on average considerably larger than Mesozoic examples, possibly reflecting large glacioeustatic fluctations in the U.Paleozoic. Tunnel valley fills are prominent in Pleistocene deposits, and formed mainly by catastrophic subglacial outbursts of meltwater (jökulhlaups). They are filled with glaciofluvial, lacustrine and marine strata, with mass-flows common. They are up to 400 m thick and 6 km wide, and their W/T values are relatively narrow (2.5 to 42), reflecting catastrophic incision and frozen substrates.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005