Sedimentology, Stratigraphy, and Depositional History of the Lower Cretaceous Viking Formation at Willesden Green, Alberta, Canada

Abstract

<p>The Lower Cretaceous Viking Formation is a complex stratigraphic unit containing a variety of geographically separated sand bodies of varying orientation, thickness and lithology. Many of these sand bodies are prolific hydrocarbon reservoirs which have been extensively drilled by industry. Despite this fact, their origins and interelationships are largely unknown. Establishment of a Viking Allostratigraphy in the Willesden Green area (1) allows the recognition of distinct allomembers based on bounding discontinuities and facies associations, and (2) determines the exact stratigraphic relationships and depositional histories of the Viking oil and gas fields as defined by basin-wide sea level fluctuations. Detailed core and log correlations indicate that in the Willesden Green area, the Viking Formation is made up of 4 distinct packages of sediment separated by 3 major stratigraphic breaks. The lower most package is regional in extent, and consists of three gradational coarsening-upward cyclic offshore mudstone/siltstone sequences (Members A and B). At Willesden Green, Member B is erosively incised by a major unconformable scour surface (VE2 - Viking Erosion surface 2) filled with conglomerate-rich channel/estuarine sediments (Member C). The VE2 incision was carved during an initial Viking lowstand, and infilled during a temporary stillstand in the ensuing transgression. The top of Member C is erosively truncated by a regionally tracable pebble-mantled ravinement surface (VE3). Member D, a sandying-upward sequence of storm-dominated lower shoreface to transitional offshore sediments, overlies VE3, and records a second major Viking progradational event. This unit thickens southward, and can be traced laterally into correlative upper shoreface and nonmarine sediments at Caroline and Harmattan. The top of Member D is bevelled by a second basin-wide ravinement surface (VE4) which rises in a step-like fashion towards the south. This surface is mantled by a thin veneer of pebbles which locally accumulate to form the elongate shaleencased, conglomeratic shelf sand bodies of Member E. Member E gradationally passes upward into the silty shales of the Colorado Group and records a final transgressive flooding of the Viking basin.</p>