The Sedimentology, Stratigraphy and Depositional History of the Lower Cretaceous Viking Formation at Harmattan East and Crossfield, Alberta, Canada

Abstract

<p> The Lower Cretaceous (Albian) Viking Formation at Harmattan East and Crossfield, Alberta, contains two regionally extensive erosion surfaces, VE3 and VE4, separating three allomembers, A-B, D and E. These erosion surfaces can be mapped over large areas of the Alberta basin allowing for the creation of a Viking allostratigraphy. </p> <p> The allostratigraphic base of the Viking alloformation in the study area is informally designated BV. The BV log marker is overlain by allomember A-B, which in turn is overlain by the regionally extensive ravinement surface VE3. The VE3 surface is sharply overlain by allomember D, a northeastward thinning clastic wedge composed of storm dominated facies and nonmarine deposits. Allomember D is in turn overlain by the regionally extensive ravinement surface VE4. Allomember E, which overlies this unconformity is a complex succession of coarse grained facies interbedded with dark mudstones. The upper . part of allomember E is composed of dark mudstones bounded at the top by a regionally extensive condensed section (Base of Fish Scales) that informally marks the allostratigraphic top of the Viking alloformation in the study area. </p> <p> Viking sedimentation began with the deposition of basinal and offshore transitional mudstones, siltstones and sandstones of allomember A-B. A major drop in sea level allowed valleys to incise into these sediments. Nonmarine and upper shoreface deposits of allomember A-B were eroded at Harmattan East during the ensuing transgression that produced the VE3 ravinement surface. A second relative sea level lowering resulted in northeastward progradation of allomember D. Renewed transgression modified the older subaerial erosion surface on top of allomember D, forming the marine ravinement surface VE4 and the overlying deposits of allomember E. Multiple stillstands or slow rates of transgression produced the "steplike" southwestward climbing morphology on the VE4 surface. Fluvial systems supplied coarse sediment to each shoreface incision ("step"). During minor sea level falls, storm and tidal currents reworked sediment at these shorefaces and also transported sediment basinward over older "stepped" shorelines forming onlap markers EO to E5. Continued transgression blanketed the coarse grained interbeds with offshore dark mudstones (Colorado Shale). A major pause in basin deposition led to the formation of a condensed section of fish skeletal remains (Base of Fish Scales). The base of this unit marks the end of Viking depostion in the study area. </p> <p> The Harmattan East Viking oil field is producing from the coarse grained transgressive lag that overlies VE4. It is separated from Caroline field (along depositional strike) by a rise in the VE4 surface. </p>