Contourites within a deep-water sequence stratigraphic framework

Abstract

Sequence stratigraphy has proven to be an extremely useful predictive tool in the search for hydrocarbons along the continental margins. However, of the several models in use, none includes the effects of alongslope processes in deep-water. This paper, therefore, is a first attempt to place contourite depositional systems (CDSs) firmly within a sequence stratigraphic framework, based on detailed examination of over 20 CDSs worldwide. It also presents a new view of how sea level variation influences bottom current generation and intensity. Two key controls on contourite drift formation are identified: sediment influx and bottom current velocity. Sea level directly influences the sediment influx to a basin and, therefore, the contourite response fits nicely into the downslope sequence stratigraphy model. Bottom current velocity variations in response to sea level are more complex, and two key controls are identified: (1) oceanic gateways can effectively constrict and accelerate water masses and are therefore closely associated with CDS evolution; fluctuating sea level will affect the water exchange through a gateway; (2) changing rates of bottom-water generation: some water masses appear more vigorous during periods of lowstand, whereas others appear more sluggish. In order to accommodate this variation, two new sequence stratigraphy models are herein presented, comprising both downslope and alongslope processes. The first model reveals a CDS where bottom current activity is markedly more vigorous during times of sea level highstand, whereas the second model indicates margin evolution where bottom-water currents are most vigorous during times of sea level lowstand. It is recognised that there are additional controlling factors linked to sea level variation which can significantly modify the distribution and development of contourite elements.