Abstract

In the Countess oilfield of southern Alberta, Canada, the Glauconitic sandstone* of the Lower Cretaceous Mannville Group has been studied in the subsurface within a single township (100 km²), where cores from 60 closely-spaced wells are available. The Glauconitic sandstone occurs as a series of lenticular sandstone bodies (10 km × 3.5 km × 30 m), either discrete, shingled, or amalgamated and encased within marine shales. The Glauconitic sandstone is part of a regressive sequence marked by two regional markers: a limestone bed below and a coal above. Within this interval the Glauconitic sandstone grades upward to a sandstone-shale sequence with flaser, wavy and lenticular bedding typical of tidal flats. Examination of cores of the Glauconitic sandstone reveals characteristic tidal structures such as: i) mud-draped foresets, ii) double mud couplets and bundle sequences, iii) reactivation surfaces, and iv) sigmoidal foresets. The dominance of high-angle cross beds also indicates deposition by unidirectional currents. A wide-mouthed, mesotidal estuary filled with a flood tidal delta-channel complex seems to be the likely environment of deposition.