Submarine slope angles, drowning unconformities, and self-erosion of limestone escarpments

Abstract

We measured angle and height of carbonate platform slopes in the Pacific and the Atlantic and of siliciclastic continental slopes in the Atlantic. We also determined depositional regime and sediment budget of these slopes by using seismic profiles, morphology, and sediment cover. Slopes of carbonate platforms are significantly steeper than submarine slopes in siliciclastic terrains. Carbonate slopes are concave in profile; siliciclastic slopes are slightly convex or straight. The angle of carbonate slopes increases with height; the angle of siliciclastic slopes does not. The increase in slope angle of upward-growing carbonate platforms changes the sediment budget and depositional regime from accretionary slopes that have positive budget to erosional slopes that have negative budget. Oversteepening of carbonate platforms has important implications. (1) Drowning and burial of carbonate platforms by siliciclastics can produce a “drowning unconformity” that simulates a fall in sea level with seaward shift in onlap. (2) Upper slopes of high-rising platforms can be so steep that accretion is impossible, and instead, the slope is being eroded by slumps and turbidity currents from the platforms9 own sediment. This self-erosion is the prime cause of the conspicuous limestone escarpments in modern oceans.