ANATOMY OF AN UNCONFORMITY

Unconformities are more than simple erosion surfaces. During a regressive-transgressive cycle caused by relative uplift, three major erosion surfaces sweep across a platform. The first is a regressive surface of marine erosion (RSE) followed by a subaerial unconformity (SU) and finally a transgressive surface of marine erosion (TSE). While the subaerial unconformity is generally sculpted by fluvial processes, marine erosion surfaces are cut by wave and tidal ravinement. Fluvial and tidal erosion result in channeled erosion surfaces of high relief in contrast to the planar geometry of a wave-cut platform. Including both wave and tidal ravinement associated with the RSE and TST, five separate erosion surfaces may be superposed in a single unconformity. In most cases it is only the last of these surfaces which survives into the geologic record as a planar TSE formed by wave erosion. However, in stratigraphic sequences with lowstand (LST) and, or falling-stage systems tracts (FSST) multiple, stacked erosion surfaces occur; the LST and FSST are the consequence of the preservation of segments of the SU and RSE beneath the TSE. Preservation is especially likely where incised fluvial or tidal channels protect the section from wave ravinement. At least four types of erosion surface survive as paraconformities in the Silurian, Red Mountain Formation of Alabama and illustrate the manner in which a single unconformity may subdivide around sharp-based shoreface deposits.