ABSTRACT

Detailed study of shell beds in the Ostracode Zone of the Lower Cretaceous Mannville Group in south-central Alberta has been made in

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three wells in the Alberta Plains and four outcrop sections in the Foothills. Most of these shell beds occur within black shales except a few, which are included within lime mudstones. The thickness of these shell beds ranges in the centimetre to decimetre scale. These shell beds comprise disarticulated and broken pelecypods, and mostly entire gastropod shells, densely to loosely packed, with or without preferred orientation. Both early diagenetic framboidal and late diagenetic coarsely crystalline pyrite, as well as sideritic mud, are common constituents.

Four types can be recognized in these shell accumulations:

1. Shell concentrations above submarine hardground (minor hiatal accumulation).
2. Shell concentration below submarine hardground (major hiatal accumulation).
3. Scour-based, graded, and laminated shell bed (event bed).
4. Complex bioturbated shell accumulation (major hiatal accumulation?).

Sequence stratigraphic analysis of the Mannville Group suggests the existence of a maximum flooding surface within the Ostracode Zone, which has yielded an open marine, dinoflagellate assemblage. Stratigraphic analysis of the shell beds shows that Type 2 or Type 4 shell beds will provide the required documentation of such a flooding surface. Vertical variations of total organic carbon content, carbon/sulphur ratio and dinoflagellate diversity in the screen sections studied, provide additional data for stratigraphic analysis.

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