Abstract

An allostratigraphic unit is a mappable rock body, defined and identified on the basis of its bounding discontinuities. Such discontinuity-bounded depositional units commonly reflect relative sea-level changes and are recognized in the Upper Cretaceous Cardium Formation of Alberta and British Columbia. In shelf and nearshore sediments, depositional sequences typically comprise a conglomerate resting on a regional unconformity, overlain by a coarsening-upward sequence, erosionally overlain by another conglomerate. Each depositional sequence records: 1) shoreface progradation and concomitant aggradation of a shallow marine shelf; 2) relative sea-level fall causing subaerial erosion of the shelf and simultaneous fluvial supply of gravel to lowstand shorelines; and 3) marine transgression, involving erosional shoreface retreat and the landward redistribution of lowstand gravels as a veneer over the erosion surface. Distinctive facies and facies sequences, nonmarine units and conglomerates provide the basic criteria for matching coarsening-upward sequences in outcrop and subsurface. The resulting basinwide correlation has enabled us to make detailed reconstructions of depositional patterns and paleogeography, and permits a reasonable quantification of shoreline movements. The extent (hundreds of kilometres) and duration (average about 140,000 years) of each depositional sequence suggests a regional, relatively high-frequency (?)tectonic control of relative sea-level changes, perhaps superimposed on a more subtle global eustatic lowstand.