Significance of molluscan shell beds in sequence stratigraphy: an example from the Lower Cretaceous Mannville Group of Canada1

Abstract

ABSTRACTDetailed 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.