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Eschard, Rémi1, Philippe Joseph2
(1) Institut Français du Pétrole, Rueil Malmaison, France
(2) Institut Francais du Petrole, 92852 Rueil Malmaison Cedex, France

ABSTRACT: Comparing Basin-floor Fan and Slope-fan Depositional Processes and Architecture in the Pab Outcrops and Other Examples

In the classical models of turbiditic systems, basin-floor fans are deposited during relative lowstands of the sea-level and slope-fans during relative sea-level rises. An important implication of this model is to consider basin-floor fan as sand-rich systems, sediment by-passing the platform to accumulate in the basin, and slope-fans as mud-rich systems, sediments being mostly stored in the platform setting. However, examples of mud-rich high-transport efficiency basin floor fans are common in the west-african margin and sand-rich low-transport efficiency slope fans are frequently identified associated to prograding clastic ramps. It is now accepted that the basin physiography has a major influence on the deep-water depositional processes and therefore, on the architecture of the fans.
We will illustrate the differences between “so-called” basin-floor fans and slope-fans in several examples, both in outcrop and subsurface. In the Pab outcrops for example, (Maastrichtian, Pakistan), turbiditic channels and lobes of a major basin-floor fan extended far away in the abyssal plain and are overlaid by a sand-rich slope fan whose extension was restricted to the base of the slope. In the lower system, turbulence of the gravity flow was probably a dominant mechanism to explain the deep basal channel incision, levee development and high-transport flow efficiency. In contrast, the slope-fan above was connected to a sand-rich delta prograding over the shelf edge. Hyperpycnal flows related to fluvial floods probably explain the sand-sheet geometry, the high sand-shale ratio and the limited extension of the slope fan.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.