Abstract

Elementary stratigraphic events, or parasequences, are generally understood as progradational events bounded by flooding surfaces. Major sediment accumulation and preservation occur during baselevel fall. Baselevel rise is recorded in the marine environment by a veneer of sediments that is generally neglected. Field evidence from the Late Proterozoic glacially related deposits of the West African craton suggests that this concept must be revised to be applied to a continental–marine transitional zone.

Strata deposited during periods of both baselevel fall and baselevel rise, and their volumetric proportions, vary as a function of their paleogeographic position relative to the shoreline. Accordingly, an elementary building block, or depositional genetic unit is defined (Fig. 10.5). It is composed of three kinds of architectural elements bounded by four kinds of erosional or non-depositional regionally correlative surfaces. Each architectural element (Ae) corresponds to a typical association of facies, which allow the distinction of a ‘progradational wedge’ (Ae1) made up of lower to upper shoreface wave- to storm-dominated shales and sandstones, a ‘continental wedge’ (Ae2) mostly composed of non-marine rocks comprising ephemeral fluvial deposits, eolian sand sheet and dune deposits, and lagoonal to backshore sandy or carbonaceous deposits, and a ‘transgressive wedge’ (Ae3) made up of high-energy, upper shoreface, clean, well-sorted sandstones. The distinction of the bounding surfaces is based on the geometric relationships and the nature of the facies tracts they bound. The non-depositional hiatal surface, or maximum flooding surface (MFS), forms the upper and lower limits of each genetic unit; it marks the maximum marine extension inland, and forms basinward a downlap surface for the ‘progradational wedge’ (Ae1) of the overlying unit.