Lowstand Deep-Water Clastic Fans and Related Depositional Systems: Terminology, Characteristics, Processes, and Variability

KOLLA, V., and R. MARTIN, Elf Exploration, Inc., Houston, TX

As presently used in sequence stratigraphic practice, lowstand clastic deep-water fans and related depositional systems consist of mainly basin floor fan, slope fan, and shingled turbidites. "Basin floor fan" refers to fan-shaped turbidites and related sedimentary gravity flow deposits in the intraslope basins and basins of bathyal and deeper paleowater depths; the basin floor fan is expected to have unchannelized and channelized sandy turbidites but should be devoid of channel-overbank deposits. "Slope fan" implies, in principle, fan-shaped, channel-overbank turbidites and related deposits that built the continental slopes and slopes of the intraslope basins, outward and upward. The slope fan system may also occur on the basin floors of intraslope basins. The basin floor fan is thoug t to be deposited during relative sea level fall and maximum lowstand (basin floor fan time or basin floor fan systems-tract time), and the slope fan is thought to be deposited during subsequent sea level stillstand and slow rise (slope fan time or slope fan systems-tract time). "Shingled turbidites" are turbidites deposited at the toes of clinoforms of the prograding wedge following the time of slope fan deposition. Shingled turbidites may include massive sands like those in basin floor fans but are deposited closer to the sources of supply than basin floor fans.

Outside the realm of strict sequence-stratigraphic terminology, "deep-sea fan" usually refers to fan- or cone-shaped morphology or organized systems of turbidites and related deposits and related deposits in basinal settings. The concept of the slope fan in sequence stratigraphy extends the deep-sea fan usage to continental slopes also. In actuality, fans of any type may or may not be fan-shaped sedimentary bodies, depending upon the tectonic and physiographic setting of the basins, but must consist largely of organized systems of turbidites and related deposits.

In addition to basin floor fan and slope fan systems or systems tracts, mass-transport complexes and slope or base-of-slope aprons (slides and slumps, debris flow, and some turbidity current deposits) should be recognized as separate depositional systems. In the past, these depositional systems were included under slope fan or slope fan complex. However, the characteristics of aprons and mass-transport complexes, although highly variable and frequently gradational into turbidites, are sufficiently different form the slope fan and therefore need separate identity.

We discuss and illustrate in our presentation the sedimentological, well-log, and seismic characteristics of fans, slope aprons, and related systems and their variabilities with some examples from the Gulf of Mexico and elsewhere. Outside the mold of sequence-stratigraphic practice, sedimentologists and geologists use other terminologies for deep-sea fan systems. We attempt to point out the equivalencies of these terminologies to the ones used in sequence stratigraphy.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)