ABSTRACT: Shelf Versus Shoreline Sandstones: Sedimentology and Sequence Stratigraphy

WALKER, ROGER G., McMaster University, Hamilton, Ontario, Canada

The distinction between shelf and shoreline sandstones is particularly difficult where both occur as long, narrow sand bodies. All active linear sand bodies on the shelf today appear to have formed originally at or very close to the shoreface. They are now being modified as a result of transgression. Even minor shifts in the position of a sand body during transgression results in reworking of all of the original shoreface sedimentary structures. At the same time, offshore marine faunas can be incorporated into the sand body, masking its shoreface origin. Shoreface deposits can form in prograding, lowstand, and transgressive settings. Lowstand and transgressive shorefaces can be surrounded by marine mudstones and hence mimic the positions of offshore sandbodies. The distinction of offs ore vs. shoreface may therefore depend largely on the stratigraphic position of the sandbody, with particular reference to bounding erosional discontinuities. On theoretical grounds, it is difficult to envisage how sand can be transported more than 100 km from a shoreface and subsequently concentrated into linear bars with coarsening-upward facies successions. It is therefore important to re-examine the sedimentology and stratigraphic position of "bars" such as those in the Shannon Sandstone of Wyoming and the Cardium and Viking formations of Alberta for evidence of (1) bounding discontinuities and (2) progradation onto offshore mudstones even after the establishment of a lowstand incised shoreface. This results in a gradationally based coarsening-upward succession that, if truncated by ubsequent transgression, may be extremely difficult to identify as a lowstand shoreface.

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