Deep-Water Turbidites and Submarine Fans
Depositional environments of deep-water deposits commonly are complex and consequently do not neatly fit any single facies model. Rather than developing specific models we discuss these deposits within the context of depositional elements and first principles of process sedimentology. Depositional elements are described using 3D seismic as well as outcrop data. Detailed facies descriptions from outcrops are then integrated with these depositional elements. Following the theme of this publication, we emphasize facies and depositional environments rather than the mechanics of turbidity currents and related processes.
The spatial and temporal distribution of depositional elements is determined largely by characteristics of the shelf-edge staging area. Such factors as grain-size distribution, sediment caliber, frequency of flow events, and magnitude of flows are all a function of conditions at the shelf edge and upper slope. Sediments are supplied from the staging area to the slope and basin floor beyond. Turbidity currents traverse the slope through canyons and slope channels. When these flows reach the basin floor they continue to remain confined by levees for a certain distance. This distance is a function of grain-size distribution in the flow, flow magnitude, and flow velocity. Levee height diminishes seaward, and eventually where levees can no longer effectively confine the basal sand-rich part of the flow the leveed channel transitions into a frontal splay or lobe.
Relative sea-level change plays an important role in turbidite deposition, in that sea level is a major factor controlling conditions in the outer shelf and upper slope. During relative sea-level lowstands, shorelines and consequently depocenters tend to be located at the shelf edge. This sets up conditions favorable for delivery of sediments to the slope and basin floor. Conversely, relative sea-level highstands commonly are associated with depocenters at the inner to middle shelf, resulting in a paucity of coarse sediments being actively delivered to the shelf edge and ultimately to the slope and basin floor. Variations in grain size delivered to the shelf edge during a cycle of sea-level change can vary predictably hence the temporal and spatial distribution of depositional elements in linked deep-water environments can likewise be better understood within this context.