--> Abstract: Facies and Stratal Patterns in Incised Valley Complexes: Examples from the Recent Gironde Estuary (France), and the Cretaceous Viking Formation (Canada), by G. P. Allen and H. W. Posamentier; #91004 (1991)

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Facies and Stratal Patterns in Incised Valley Complexes: Examples from the Recent Gironde Estuary (France), and the Cretaceous Viking Formation (Canada)

ALLEN, GEORGE, P., Total Exploration Laboratory, Saint-Remy-les-Chevreuse, France, and HENRY W. POSAMENTIER, Esso Resources, Calgary, Canada

Incised valleys form during a relative fall in sea level and fill during lowstand and relative rise. They constitute favorable exploration targets, forming regionally continuous sand ribbons up to 20-50 m thick and 10-15 km wide, sealed by transgressive marine muds.

Modern estuaries furnish analogs to study facies and stratal patterns within incised valleys. Studies in the Gironde estuary indicate that they are filled with a tripartite upward transgressive section comprising basal fluvial sands, overlain by estuarine sands and muds, blanketed by marine muds.

The fluvial sands in the valley thalweg accumulate by aggradation of the fluvial profile during lowstand time, and represent the updip equivalent of the lowstand prograding wedge. During transgression, fluvial aggradation ceases, and the backstepping transgressive tract onlaps the fluvial profile. In a vertical section, the lowstand-transgressive tract contact is recognizable by a major fluvial-estuarine facies boundary associated with the landward migration of the bayline over the fluvial plain. These estuarine deposits are usually truncated by a wave ravinement surface, overlain by nearshore sand and shelf muds. In high tide settings, a tidal ravinement surface also forms, resulting in an increased thickness of estuarine sand.

These characteristic facies and stratal patterns have been recognized in incised valleys of the Cretaceous Viking Formation. These valleys formed during third order (1-3 Ma) eustatic cycles, punctuated by higher frequency eustatic cycles (<1 Ma), with short-lived base level falls accompanied by renewed stream incision and fluvial sand deposition. This has resulted in complex multistory valley fills comprising several superposed sharp-based upward transgressive units.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)