Edward H. Clifton1
(1) Consultant, Los Altos, CA
Abstract: Shoreface myths and misconceptions
Nearly all studies of ancient coastal successions invoke the concept of a shoreface, a term originally applied to the steeply inclined sea floor adjacent to a shoreline. Redefinitions, largely based on other attributes, have led to a number of myths and misconceptions about the shoreface, any one of which can significantly impair our interpretation of shallow-marine deposits.
Many studies describe individual shoreface successions 20 or more meters thick. In reality, shoreface successions are rarely likely to exceed 10 m or so in thickness. The base of the physiographic shoreface on prograding parts of modern high-energy coasts lies at a water depth of about 10 m. Greater depths seem to be limited to settings of transgression or erosion.
Many studies equate the shoreface to the presence of sand in the section. The distribution of sand on modern shelves conforms to the shoreface only by coincidence.
The shoreface is commonly thought to be related to storm or fair-weather wave base. In reality, it relates to neither.
Most studies subdivide that shoreface into upper, middle and lower segments, and some further subdivide these into proximal and distal components. In the absence of pebbles, only the upper shoreface is recognizable as a distinctive facies. With pebbles, a distinctive lower shoreface facies can be recognized.
The depth of erosion by ravinement is sometimes equated with the thickness of shoreface deposits in the same succession. The depth of the shoreface on a prograding coast, where the shelf profile is likely to be in equilibrium with the dominant wave field, may differ significantly from that of a transgressing coast, where the sediment supply may be insufficient to generate an equilibrium profile.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana