So Different, Yet So Similar: Comparing and Contrasting Siliciclastic and Carbonate Slopes

Carbonate slopes have a tendency to be steeper than their clastic counterparts. Commonly the stabilization potential by binding of slope sediment and early cementation of carbonates is evoked to explain this difference. However, differences and similarities between clastic and carbonate slope systems with respect to their gross development, curvature, and angle of dip are only expressed if one evaluates slope settings that are affected by comparable extrinsic and intrinsic processes. The likeness of clastic continental slopes and cool-water carbonate platforms is great where deep shelves, low slope angles and usually sigmoidal slope profiles are typical. Coarse-grained deltas compare with tropical carbonate platforms. Both have steep depositional slopes, exponential and linear slope profiles, and coarse sediments originating from shallow water depths. Exponential profiles are common on rimmed platforms because reefs are resistant to erosion and the platform edge therefore relatively stationary vertically. This also accounts for ice-covered margins because the grounding level of the ice limits vertical fluctuations. A special case for carbonates is the in situ accretionary slope factory with abiotic and biotically induced precipitates stabilizing and building carbonate slopes. However, in situ slope accretion and stabilization in itself does not necessarily explain the large-scale geometry of the platform flanks. It is more the reason the slope factory is in-sensitive to light and can therefore accrete during both low and highstands. Thus when a relative sea-level fall exposes the platform top and shallow-water carbonate production stops, in situ carbonate production continues in the slope realm. The combined effort of both types of sediment production and hence surplus allows the system to build up to the angle of shear and constantly prograde. A direct comparison are coarse-grained fjord and lake deltas where the inherent fast prograding system, which is dominated by a mixture of coarse sand and rubble, obtains steep, planar slopes. Clearly, while sediment properties vary greatly, stark similarities in gross development, curvature, and angle are observed in comparable settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California