--> Diagenesis of a Condensed Section Atop a Flexural Arch, Western Canada Foreland Basin

AAPG ACE 2018

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Diagenesis of a Condensed Section Atop a Flexural Arch, Western Canada Foreland Basin

Abstract

Regional subsurface correlation of the Santonian Puskwaskau Formation over an area of 140,000 km2 has revealed discrete, but relatively short-lived localized flexural depocenters in the proximal foredeep of the Western Canada Foreland Basin. These flexural depocenters are rimmed by areas of minimal subsidence where stratal thinning and truncation have taken place. Regional subsurface correlation shows that, in southern Alberta, four metres of bioclastic sandstone are time-equivalent to 100 metres of mudstone in the coeval depocenter to the north. This anomalous stratigraphic interval is interpreted as a condensed section that received minimal clastic input over approximately 700 kyr. Moreover, this carbonate-rich package consists of abundant layers of reworked inoceramid bivalves and oysters, sand-size carbonate fragments, wood, phosphatic debris, and detrital silicate minerals. Most intriguing is the diagenetic assemblage of this package that consists of authigenic quartz, kaolinite, apatite, marcasite, pyrite, calcite, and dolomite. The diagenetic assemblage, with the exception of quartz and kaolinite, is interpreted to have been microbially-mediated and facilitated by the presence of abundant organic and phosphatic material. The source of aluminum and silica required for kaolinite and quartz precipitation is interpreted to have been detrital silicate minerals, that were degraded by meteoric flushing of shallowly-buried near-shore sediment. The orientation of framework grains indicates that pore-filling cements were emplaced prior to compaction. The authigenic minerals mostly fill intergranular pore space and exhibit a complex spatial relationship and paragenetic sequence that suggests authigenesis was controlled by closely-timed variations in aqueous pore water chemistry. Moreover, the formation of authigenic cements was most probably facilitated by the well-sorted texture of the sediment that enhanced porosity and permeability, thereby allowing ion transfer by interparticle flow. The stratigraphic context (i.e. a condensed section) of the analysed interval suggests that the sediment experienced prolonged residence time at a very shallow burial depth. Our detailed regional-scale subsurface correlation that has revealed basins and arches in apparently homogeneous mudrock successions may help contribute to the refinement of models pertaining to the timing and local variability of fluid flow in siliciclastic mudstone-dominated foredeep successions.