SIGNIFICANCE OF MOLLUSCAN SHELL BEDS IN SEQUENCE STRATIGRAPHY - AN EXAMPLE FROM THE LOWER CRETACEOUS MANNVILLE GROUP OF CANADA

Detailed study of marine shales (the Ostracod zone) within a Cretaceous, third-order transgressive-regressive sequence in the Alberta Foreland Basin reveals a systematic association between shell beds and parasequence-scale flooding surfaces, including surfaces of maximum flooding. The Ostracod zone (a subsurface lithostratigraphic unit known as the Calcareous Member in outcrop) consists of 10-20 m of black shale and bioturbated sandstones with many thin, fossiliferous limestones. Parasequences (shallowing-up cycles 2-3 m thick) were delineated within this transgressive unit based on lithology, sedimentary structures, degree of bioturbation, dinoflagellate diversity, total organic carbon and carbon/sulphur ratios; many flooding surfaces are firmgrounds or hardgrounds. Shell-rich limestones occur in three different positions relative to these flooding surfaces, and each has a distinctive bioclastic fabric and origin. (i) Base-of-parasequence shell beds (BOPs) lie on or just above flooding surfaces in the deepest water part of a parasequence; they are thin (up to a few centimetres), graded or amalgamated skeletal packstones/wackestones composed of well-sorted granular shell, and are interpreted as hydraulic event concentrations of exotic shell debris. (ii) Top-of-parasequence shell beds (TOPs) are capped by flooding surfaces at the top, shallowest water part of a parasequence; they typically are several decimetres thick, are physically amalgamated packstones/grainstones or bioturbated wackestones, and contain abundant whole as well as comminuted shells; these are composite, multiple-event concentrations of local shells. (iii) Mid-sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds, and are intercalated between parasequences in the deepest water part of the larger sequence; they are laterally extensive lime mudstones a few decimetres thick, with sparse shells in various states of dissolution, recrystallization and replacement; these beds are terrigenous-starved hiatal concentrations and record maximum flooding within the Ostracod zone. Offshore sections of the Ostracod zone typically contain several starved mid-sequence shell beds, underscoring the difficulty of identifying a single 'maximum flooding surface' within a third-order sequence.