ABSTRACT: The Origin of Discrete Shallow Marine Sandbodies

NUMMEDAL, DAG, and GREGORY W. RILEY, Louisiana State University, Baton Rouge, LA

The origins of shallow and marginal marine sandbodies encased in marine shale remain controversial. The leading contending ideas are that they formed as (1) shelf sand ridges, (2) incised valley fills, or (3) lowstand shorefaces. To help focus this debate we present here stratigraphic models of these three alternatives. The models are based on published seismic and core studies of late Pleistocene and Holocene strata from North American and European shelves.

(1) Shelf sand ridges are shore-oblique sandbodies, tens of kilometers long, overlying a generally flat ravinement surface. The ravinement surface truncates valley fills as well as interfluves. It is a diachronous surface where it cuts valley fill or other coastal plain strata and may coincide with the (late Pleistocene) subaerial sequence boundary on interfluves. Shelf sand ridges remain active in water depths of many tens of meters, but as they ultimately drown (become "moribound") they are stabilized by marine bioherms. (2) Valley fills are shore-normal or oblique, tens to hundreds of kilometers long, and bounded by a sequence boundary at their base and a ravinement surface on top. The fill is heterolithic and may consist of estuarine, flood-tidal delta, tidal inlet, and fluvial ch nnel deposits. (3) Late Pleistocene "lowstand" shorefaces are extensive, shore-parallel sandbodies. They may either be capped by, or overlie, the seaward terminus of the subaerial exposure surface. Sandbodies emplaced by forced regression will overlie a condensed section and are capped by the subaerial erosion surface, the sequence boundary. These deposits belong in the highstand systems tract. In contrast, sandbodies that prograde during and shortly after relative sea level lowstand will onlap and downlap on the sequence boundary. They represent the lowstand systems tract and are capped by a ravinement surface. Most shelf margin sandbodies probably consist of superimposed late highstand and lowstand deposits.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)