Aspects of the sedimentology, stratigraphy, and diagenesis of Lower Mississippian shelf margin carbonates : Souris Valley-Lodgepole interval of the Williston Basin
Sereda, Richard D.
The Lower Mississippian Souris Valley Beds and Lodgepole Formation were deposited during transgressive and regressive phases of the Mississippian seas. They represent strata which were laid down in depositional settings ranging from basinal to upper shelf slope. The transgressive phase is represented by glauconitic limestones of the Transition beds, and thinly laminated and massive nodular cherty mudstones of a basin to lower shelf slope setting. Thinly bedded mudstones and graded wacke-to packstones represent the deposits of the regressive phase. These are interpreted to represent a sequence of distal to proximal tempestites, the product of storm depositional processes acting on a carbonate ramp. Along the northern margin of the Williston Basin, in a basin to lower shelf slope setting, a series of Waulsorttan-type bioherms are deduced from subsurface cores and reflection seismic profiles. The bioherms are divided into core, flank, and supra-bioherm facies based on their distinctive lithologies and diagenetic features. These facies record the evolution of bioherms growing in changing water depth and wave energies. corresponding to the transgressive and regressive phases of the Mississippian seas. Bioherm growth was initiated on paleo-highs in the underlying Upper Devonian Birdbear Formation, which in turn is the result of multi-stage dissolution of Middle Devonian salt horizons, associated with sub-Phanerozoic basement tectonics. Three major diagenetic environments are preserved in the basin, shelf slope, and bioherm strata. Syn-sedimentary diagenesis in the form of fibrous calcite spar is unique to the lower bioherm core of the Waulsortian-type mounds, whereas, all other diagenetic phases are non-facies selective. Shallow burial diagenesis in a mixed seawater -freshwater zone is represented by rare isopachous dolomite. A variety of morphologies of calcite spar, which display luminescence zoning, are believed to represent progressive burial of both bioherm and non-bioherm sediments. The last diagenetic phase recorded is a complex mineralogical assemblage associated with the Sub-Mesozoic Unconformity. The basin, shelf slope and bioherm transgressive -regressive cycles correspond to transgressive -regressive cycles in shallow shelf carbonate sequences of the Lodgepole Formation in Manitoba. This shelf-slope-bioherm-basin sequence is divisible into six depositional events, which are grouped into three major sequence-stratigraphic packages. Within each sequence-stratigraphic package both conformable and unconformable relationships exist between the strata of the shelf and basin. This newly recognized stratigraphy and associated bioherms offers new economic opportunities in a relatively mature exploration basin.