OUTCROP STUDIES: VITAL TO UNDERSTANDING SHELF-MARGIN GROWTH, AND FOR THE CHARACTERIZATION OF DELTAIC SHELF RESERVOIRS

Deltaic and other shoreline units are the key lithosomes for the construction of shelves and shelf margins. Shelves aggrade and prograde by the back and forth transits of such shorelines, and the resultant transgressive-regressive deposits create the fundamental stratigraphic sequences of shallow-water successions. The most common elements of such sequences are estuarine or barrier-lagoon deposits, overlain by deltaic or strandplain deposits, though the initial transgressive deposits are commonly overlooked or underestimated.

Shelf transit time, the time taken by the system as it moves between shelf-edge to bayhead area and back to the outer shelf, is of some interest because it relates to the time scale of the transgrssive-regressive sequence. However, the landward-seaward extent of the generated sequences depends on the relative strength of the supply and base-level drivers, as well as on autogenic processes. Accommodation-driven transits generally create more extensive transgressions than supply-driven ones. In addition, outcrop studies of such delta-estuary sequences are suggesting that the dominant process, during the shelf transit from bayhead to shelf-edge sites, can change significantly, and ought to be incorporated into our models of stratigraphic sequences. These studies are also economically important for the correct characterization of shoreline reservoirs.

Outcrop study examples from a range of shelves and shelf margins illustrate improvements in our understanding of deltaic lithosomes, process change within base-level cycles in general, and shelf margin growth.