Abstract

A high-resolution 3D seismic data set from Alberta, Canada, has revealed great detail in the morphology and architecture of a series of incised-valley fills from the Grand Rapids Formation (Albian, Lower Cretaceous). A large-scale well-log study was used to place the seismic stratigraphic architecture in regional context. Seismic facies were calibrated with numerous cores and wells. The data reveal that the Grand Rapids Formation was composed of shoreface parasequences cut by many incisions filled with fluvial to estuarine facies. The incisions contain one to several channel fills with a range of bar architectures, but dominated by lateral accretion.

Incised valleys are here defined as any fluvial incision that is bounded below by a sequence boundary. Incised valleys form in response to a relative fall in sea level and, unlike distributary channels, are not in facies relationship with the surrounding sediments into which they are incised. A range of direct and indirect evidence indicates that all of the studied incisions into the shoreface parasequences are incised valleys.

Thirty-two incised valleys were identified in the Grand Rapids Formation in the 3D seismic. They range in width from a few tens of meters to a few kilometers, and in depth from 5 to 60 m. There is no relationship between the size of a valley and its stratigraphic position. Composite sequence boundaries have many, intersecting incised valleys of a range of size. High-frequency sequence boundaries have only a few incised valleys.

None of the incised valleys have a dendritic drainage network; they mostly have straight to gently sinuous edges. They were interpreted to have formed from fluvial entrenchment rather than knickpoint migration. Valleys also exhibit large variations in depth of the thalweg over short distances. This is interpreted to result from local processes based on the application of an engineering model that predicts scour depth of modern channels.

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