--> Abstract: Characterization of a Sequence Boundary: An Integrated Study of the Unconformity between the Mannville and Colorado Groups in the Subsurface of the Cessford Field, Southern Alberta, Canada, by I. Banerjee, S. K. Ghosh, and H. J. Abercrombie; #91012 (1992).

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ABSTRACT: Characterization of a Sequence Boundary: An Integrated Study of the Unconformity between the Mannville and Colorado Groups in the Subsurface of the Cessford Field, Southern Alberta, Canada

BANERJEE, INDRANIL, Institute of Sedimentary and Petroleum Geology, Calgary, Alberta, Canada, SANTOSH K. GHOSH, Petrosan Inc., Calgary, Alberta, Canada, and HUGH J. ABERCROMBIE, Institute of Sedimentary and Petroleum Geology, Calgary, Alberta, Canada

The Late Albian Joli Fou marine transgression in southern Alberta is recorded by the tidally deposited Basal Colorado Sandstone (BCS), which occupies broad lenticular depressions incised within the paralic Upper Mannville strata. Time represented by this unconformity, equivalent to three missing megafauna zones, may be up to several million years. Sedimentology, paleontology, organic and inorganic geochemistry, mineralogy, and petrography of strata above and below the unconformity have been studied in approximately 100 samples taken from 30 wells over a 3600 sq. kilometers area.

The unconformity is marked by a chert pebble conglomerate lying above the incised surface. A structureless, heterogeneous conglomerate containing coal pebbles and armored mud balls documents an erosional episode prior to transgression and deposition of the BCS. Early, spherulitic siderite in the upper few meters of Mannville strata indicates meteoric influence at the unconformity.

Paleontological data show an abrupt change in diversity of dinoflagellates and foraminifers from brackish Mannville sediments to open marine BCS and Joli Fou sediments. Organic material is dominantly humic (Type III) in the Mannville while organic material in the BCS is characteristic of mixed humic and sapropelic origin (Types II and III). Absence of oxidized zones and the presence of green mudstones,

coal pebbles, framboidal pyrites, and early siderite near the unconformity can all be ascribed to the reducing swampy condition prevailing during Upper Mannville time.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)