Stacked Sandstones — Litharenite Seal and Quartzarenite Reservoir: Synsedimentary Pedogenic and Groundwater Diagenesis in Jurassic Paleovalley Sandstones, Medicine River Jurassic "D" Pool, Medicine River Area, Alberta, Canada
Krause, Federico F.; Mellor, Andrea F.; Wiseman, Andrew; Debuhr, Chris
Stacked Jurassic sandstones in the Medicine River Jurassic "D" pool are valley-filling deposits contained in a karstic paleovalley cut into Carboniferous carbonates. The Jurassic sandstones filling the upper half of the paleovalley are separated by unconformities and are identified by industry as the J2 and J3 units. The J3 unit overlies the J2 unit and is a chert litharenite, whereas the underlying J2 sandstone is a quartz arenite. Both sandstones are cemented by mega- and micro-quartz, kaolinite and, in the case of J3 sandstones, abundant illite.
Diagenetic modification of the J3 sandstone is extensive and spans the interval. Notably, J3 sandstone cores are destratified or poorly stratified as they have few, if any, well-known physical and biogenic sedimentary structures. Instead they are horizonated with horizons being variably thick and cyclical. Typically horizon pairs are drab and dusky coloured and are accompanied by abundant pyrite and marcasite. Silica and clay plasma is most abundant in drab horizons where grains often float in this material. In contrast, dusky layers are grain-supported and plasma is common between grains. Also common are illuvial clay and cutanic deposits (argillans, ferrans and silans), root structures, continental burrows, peds, microfracture sets, and abundant sulfides. The observed structures and deposits are typical of silcretes where cyclical pedogenesis under acidic, reducing and poorly draining conditions resulted in the mobilization and reprecipitation of silica and alumina.
Because of this synsedimentary alteration the J3 sandstone is an effective seal to the underlying J2 sandstone reservoir. At first glance this condition is counterintuitive, not only because sandstones are sealing an underlying sandstone reservoir, but also because extensive quartz and illite precipitates elsewhere in the geologic record are thought to be products of intermediate to late stage burial diagenesis. In this instance, however, the diagenetic alteration of the J3 sandstone is the product of early, near surface paleosolic diagenesis. This alteration resulted in sufficient reduction of permeability and development of ineffective porosity to transform the J3 sandstone into a seal for the underlying J2 sandstone reservoir.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013