Drowning Unconformities--A Peculiar Response of Carbonate Platforms to Rising Sea Level

Wolfgang Schlager

The slopes of modern carbonate platforms are generally steeper than siliciclastic ones. Furthermore, carbonate slopes steepen as they grow higher, whereas siliciclastic slopes do so only at very early stages of upbuilding. These conclusions are drawn from measurements of over 500 slope profiles in the Atlantic and Pacific. A survey of seismic profiles suggests that the greater declivity of carbonate slopes also holds for the more distant geologic past.

A consequence of the oversteepening of carbonate slopes is the "drowning unconformity." When carbonate platforms of a certain height are drowned and buried by siliciclastics, the clastics will onlap the carbonates simply because they cannot assume the steep slope angle. The resulting unconformity geometrically will resemble a lowstand unconformity even though it is caused by the demise of the platform, not by a drop in sea level.

Well-known unconformities for which the interpretation of drowning unconformities (rather than lowstand unconformities) should be considered include the Early Cretaceous unconformity off west Africa and off eastern North America as well as the mid-Cretaceous unconformity in the Gulf of Mexico.

Erosional truncation of the platform is common with drowning unconformities. This erosion need not be related to a lowstand of sea level but rather to the upbuilding stage of the platform. As the platform grows higher, slopes steepen and turbidity currents shift from deposition to erosion, turning the upper slopes of high-rising platforms into bare and erosional features.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.