Anatomy of a Wave-Dominated, Tide-Influenced, Fluvial-Affected (Wtf) Mouthbar Element Complex - Evidence from Outcrop, Core and Wireline Data: Horseshoe Canyon Formation, Alberta, Canada
Ainsworth, Bruce; Vakarelov, Boyan; Lee, Changhwan; MacEachern, James A.
The Upper Cretaceous Horseshoe Canyon Formation is exposed in 3-D along the Red Deer Valley near Drumheller, Alberta. These clastic marginal marine sediments were deposited in mixed-process (wave, tide and fluvial influenced) depositional environments. The studied interval is subdivided into seven relatively thin (8m on average; low accommodation) allostratigraphic members (A to G). Each contains a transgressive and regressive phase, collectively forming T-R sequences. The overall succession represents a seawards-stepping system. The presence of four cored wells and 75 wireline logged wells surrounding the outcrop enable the architecture of each allomember to be extrapolated from the outcrop into the subsurface. This calibration to subsurface data is aided by hand-held gamma-ray data from some outcrop locations.
The northern section of Allomember C is subdivided into a Transgressive and Regressive Element Complex Assemblage Set (TECAS and RECAS, respectively). The RECAS in this area is up to 6m thick and is represented by two wave-dominated, tide-influenced, fluvial-affected (Wtf) Mouthbar Element Complexes (EC). Each Wtf-Mouthbar EC is subdivided into three Element Sets (ES) which are made up of individual Mouthbar Elements. The ES are upward coarsening facies associations, with rhythmically interlaminated fine- to very fine-grained sandstones and structureless mudstones and carbonaceous material characterizing the base. Each ES passes upwards into decimetre-scale trough cross-bedded sandstones that are overlain by parallel to low-angle laminated sandstones and a coal. The parallel laminated sandstones are interpreted as the intertidal wave swash zone. Paleocurrent directions suggest the trough cross-stratification can be attributed to shoaling wave processes. The rhythmic interlamination of coarse- and fine-grained material is interpreted as a tidal product. The structureless or fluid mudstones are attributed to tidal or fluvial processes and carbonaceous debris as evidence of fluvial influence. A very low-diversity suite of ichnogenera indicate a stressed marine environment. Subsurface mapping and outcrop architecture of the three ES in each EC show that each successive ES steps forwards and also offsets laterally.
Clinoform surfaces draped by mudstones or carbonaceous material exist at two scales in the EC; intra- and inter-Element Set scale. These dipping surfaces may form barriers or baffles to fluid flow if encountered in hydrocarbon reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013