Sequence stratigraphy and evolution of Middle to Upper Devonian Beaverhill Lake strata, south-central Alberta

Abstract

Abstract Beaverhill Lake strata of late Givetian to early Frasnian age in the subsurface of south-central Alberta consist of shelfal limestones, dolostones and some anhydrites as well as basinal limestones and shales, and range in thickness from 75 to 240 m. These strata were deposited in the Central Alberta Basin and on banks (platforms) or isolated reefs on both sides of the basin. An integrated physical stratigraphic/sedimentological and conodont biostratigraphic study of these strata reveals that Beaverhill Lake strata form most of one overall (first-order) transgressive-regressive (T-R) cycle of sedimentation that also includes coastal plain and marginal marine deposits of the underlying Watt Mountain Formation (conodont zone of Lower subterminus Fauna) and shallow-marine carbonates of the lower part of the overlying Cooking Lake Formation on the east side of the Central Alberta Basin and the lower part of the overlying Leduc Formation on the west side of the basin. This overall T-R cycle consists of two major (second-order) T-R cycles. The lower, second-order cycle extends from the sub-Watt Mountain subaerial unconformity up to a widespread subaerial unconformity (R4 or Upper C) on banks and isolated reefs on both the east and west sides of the Central Alberta Basin and its equivalent, conformable transgressive surface in the basinal succession and includes Beaverhill Lake strata deposited during conodont zone of Upper subterminus Fauna to upper MN2 Zone. The upper, second-order T-R cycle extends from this subaerial unconformity or its basinal equivalent up to the conformable transgressive surface at the top of the Waterways Formation in the Central Alberta Basin or up to an equivalent position in the lower part of the Cooking Lake and Leduc formations on both sides of the basin. Strata in the upper, second-order cycle were deposited from the upper MN2 to MN4 conodont zones. The Beaverhill Lake portion of each of these second-order cycles includes seven third-order sequences, the basic stratigraphic unit of this study. T-R cycles were produced by the interaction of base-level changes, extrabasinal tectonism and depositional controls. The interaction of these controls resulted in different stacking styles of growth on banks and isolated reefs on the east and west sides of the Central Alberta Basin. Areally widespread progradation of carbonate banks on the east side of the basin was directly related to the delivery of argillaceous sediment to the basin from the northeast. The most likely terrigenous source areas were an uplifted portion of the Caledonian orogenic belt, exposed along the east coast of Greenland, or the eastern part of the Franklinian orogenic belt, exposed in the Canadian Arctic islands. The deposition of basinal argillaceous strata in the study area was episodic. The onset of episodes of argillaceous sediment deposition corresponds to transgressive facies shifts and was triggered by increased rates of base-level rise and not to shelfal incision and sediment bypass during intervals of base-level fall. Clay-rich sediment was carried by either surface, or near-surface, marine currents, or by episodic storm-generated currents or episodic storm-generated sediment-gravity flows. The southwesterly transport of this sediment was due to the southeasterly paleo-trade winds or to storm tracks that moved parallel to these winds. During some intervals, this fine terrigenous sediment appreciably infilled portions of the basin proximal to coeval carbonate banks on the east side of the basin. Westerly, leeside shedding of lime mud from these carbonate banks by storm-induced currents resulted in even further infilling of the basin. As a result, these portions of the basin became sufficiently infilled to promote the westerly, leeside bank progradation during intervals of either slower base-level rise or base-level fall. Banks and most isolated reefs on the west side of the Central Alberta Basin were far removed from the entry of clay-rich sediment into the basin. Therefore, portions of the Central Alberta Basin proximal to these banks and isolated reefs were not significantly infilled by coeval argillaceous successions. Lacking appreciable argillaceous sediment infill of these portions of the basin, banks and most isolated reefs on the west side of the Central Alberta Basin prograded basinward only by building out over their own debris and finer bank- or reef-derived sediment. This severely limited the extent of bank or isolated reef progradation. As a consequence, banks and isolated reefs on the west side of the Central Alberta Basin display an overall retreating or backstepping style of growth. The major episodes of isolated reef growth occur at different times on the east and west sides of the Central Alberta Basin and reflect the difference in the prior evolution of carbonate banks in both areas. Trends, inherent in both stratigraphic cross-sections and in cumulative isopach maps, also suggest that most of the micrite in basinal limestones was shed from upslope portions of carbonate ramps, from areas to the northeast of the sub-Cretaceous subcrop unconformity.