Paleotopographic Controls on Fluvial Architecture of Pre-Vegetated Braided Fluvial Strata in a Basal Cambrian-Ordovician Sandstone: Potsdam Group of the Ottawa Embayment and Quebec Basin

Abstract

Basal Cambrian-Ordovician siliciclastic strata are currently being considered targets for CO₂ sequestration in North America, including the Potsdam Group in the St. Lawrence Lowlands of Quebec. Significantly, depositional stratal architectures and textures, which represent first-order controls on porosity and permeability distribution in Potsdam strata, are largely unknown. Work reported here suggests that braided fluvial strata, which make up about 80% of the Potsdam isopach, form two distinct end-member stratal geometries, which in turn appear to be controlled by variations in basin topography. The first geometry consists of a thick (∼50-550 m), areally-extensive (∼1,300-12,000 km²) pile of coarse-grained, locally pebbly braided fluvial sandstone deposited in broad, structurally simple basins with wide (∼60-120 km), low-relief floodplains. Strata are dominated by 0.4-2.2 m thick, downstream-accreting compound bar deposits that formed in shallow but wide braided channels. These, in turn stack to form 1.3-3.3 m thick, laterally extensive (> 2 km) channel belt sand bodies capped with thin (<10 cm, but rarely up to 0.8 m) fine-grained overbank deposits consisting of a mixture of fine sand, silt and locally mud, illuvial matrix and pseudomatrix. The second geometry comprises thin (5-60 m), areally-restricted (∼30-200 km²) outliers of coarse to very coarse braided fluvial sandstone with local cobble- and boulder conglomerate deposited on narrow (≤10 km) floodplains bounded by local basement highs formed above deep-seated basement structures. Channel deposits consist of 1.2-3.5 m thick compound bar deposits with steep accretion surfaces (20-35°). Channels then stack to form channel belt successions that range from 2.2-7 m with rare ∼1-2 m deep, steep-sided (15-25°) confluence scours at their base. Significantly, fine-grained strata are generally absent. The first stratal geometry would form regionally extensive reservoirs with good lateral connectivity, but because of regularly-spaced, although thin fine-grained overbank deposits, vertical connectivity would be more limited. The second geometry, however, is a comparatively better reservoir unit being more lithologically isotropic with a paucity of overbank fines, predominance of high angle fabrics, and higher porosity owing to its coarser grain size. Moreover, given its spatial relationship with basement structures it would be easy to locate geographically, but reservoirs are more limited in area and thickness.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016