SO DIFFERENT, YET SO SIMILAR: COMPARING AND CONTRASTING SILICICLASTIC AND CARBONATE SLOPES

ERWIN W. ADAMS
Sarawak Shell Berhad, Locked Bag No. 1, 98009 Lutong, Sarawak, Malaysia
e-mail: Erwin.Adams@shell.com

AND

JEROEN A.M. KENTER
Statoil ASA, Sandsliveien 90, 5254 Bergen, Norway

ABSTRACT: Carbonate submarine slopes have a tendency to be steeper than their siliciclastic counterparts. Commonly the stabilization potential by binding of slope sediment and early cementation of carbonates is invoked to explain this difference. However, differences and similarities between siliciclastic 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. Three basic types of slope profiles (planar, concave, and sigmoidal) are reviewed and their mathematical expressions (linear, exponential, and Gaussian, respectively) used to compare and contrast slope systems originating from various settings. Exponential slopes with sharp shelfbreaks develop if sedimentary base-level fluctuations are minor compared to slope progradation. Gaussian profiles develop as a result of rounding of the shelfbreak by significant base-level fluctuations, whereas linear profiles result from excess sedimentation creating an angle of repose system. Both carbonate and siliciclastic systems exhibit all three types of slope curvature and mutually comprise muddy and grainy as well as debris-dominated slopes.

Similarities between continental slopes of siliciclastic passive shelf margins flooded during the Holocene transgression and nonrimmed, cool-water carbonate platforms are evident where deep shelves, low slope angles, and usually Gaussian slope profiles are typical. Lacustrine and proximal, active marine deltas compare with tropical carbonate platforms. Both have steep, 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 is therefore relatively stationary vertically, thus forming a distinct platform–slope break. 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 dominated by microbial boundstone-dominated deep oligophotic “reefs” and linear slopes of rubble, boulders, and sand. However, in situ slope accretion and stabilization by itself does not necessarily explain the first-order linear profile. Because the slope factory is insensitive to light accretion by slope shedding occurs during both lowstands and highstands. In other words, when shallow-water carbonate production ceases, in situ carbonate production continues in the slope region, and the combined effort of sediment production and the resultant surplus allows the system to build up to the angle of shear and constantly prograde. Since the dominant sediment texture delivered by the slope factory is coarse rubble and boulders that yield high angles of repose, often the flanks are steep. A direct comparison are coarse-grained deltas, especially those that develop in fjords and Alpine lakes, where—because of its proximity to the sediment source—the inherent fast prograding system, which is dominated by a mixture of coarse sand and rubble, obtains steep, linear slopes.

Clearly, while sediment properties may vary greatly, stark similarities in gross development, curvature, and angle are observed in comparable settings. As a consequence, morphometric attributes captured from seismic data have to be put in the context of the entire depositional system and basin setting to fully comprehend and predict sediment properties and depositional processes.

KEY WORDS: slope environment, submarine, carbonate, siliciclastic, geometry, angle of dip

Adams EW, Kenter JAM. 2013. So different, yet so similar: Comparing and contrasting siliciclastic and carbonate slopes. In Verwer K, Playton TE, Harris PM (Editors). Deposits, Architecture, and Controls of Carbonate Margin, Slope, and Basinal Settings, Special Publication 105: SEPM (Society for Sedimentary Geology), Tulsa, OK. doi: 10.2110/sepmsp.105.14.