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Autocyclic and Allocyclic Sequences in River- and Wave-Dominated Deltaic Sediments of the Upper Cretaceous Dunvegan Formation, Alberta: Core Examples
A regional study of the Upper Cretaceous Dunvegan Formation, in west-central Alberta, was undertaken in order to determine the nature of Dunvegan shorelines and in order to attempt to apply recent concepts in sequence stratigraphy.
The Dunvegan Formation correlates with a middle Cenomanian lowstand of global sea level and can be divided into seven allocyclic, sand-prone members (A to G), separated by regionally persistent, transgressive flooding surfaces. Sandstones within members show an offlapping relationship, produced as sandstone deposition stepped out into the Dunvegan sea as a succession of “shingles”.
The facies and sand body “geometries” mapped within the lower members (E to G) suggest deposition along river-dominated deltaic shorelines. Deposition of the upper members was mostly on storm- and wave-dominated prograding strandplains. This transition is marked by a probable sequence boundary which possibly correlates with a mid-Cenomanian sea-level drop.
Member bounding transgressive surfaces are regionally persistent, suggesting allocyclic controls that were probably tectonic in origin. Surfaces between individual “shingles” within members are probably autocyclic and may have resulted from avulsion or lobe switching.
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