ABSTRACT: Differentiating between Shelf and Shoreline Sand Bodies by the Use of Paleocurrent Information

SNEDDEN, JOHN W., Mobil Research and Development Corporation, Dallas, TX

Shoreline and shelf sand bodies can be distinguished from one another by careful analysis of their associated paleocurrent information. Examples drawn from modern and ancient coastal settings

demonstrate that these sand bodies display characteristic variations in bedform orientation, dip magnitude, and dip consistency. These variations relate to transport dynamics, bedform types, and small-scale environmental shifts.

Paleocurrent measurements from dune bedforms incorporated within ancient storm-built shelf sand-ridges show a characteristic alongshelf orientation. Paleoflow is often highly unimodal with low standard deviations in dip azimuth. Cross-strata set boundaries tend to be planar-horizontal and easily filtered out of the datasets. Flow directions are compatible with the interpreted direction for geostrophically balanced storm flows. The lack of bimodality in dip azimuths indicates that fairweather currents were weak and the dune bedforms were relatively inactive between storms.

In contrast, modern and ancient shoreline sands of wave and storm-deposited coastlines show more spread in paleoflow directions, although coherent in orientation with the shelf bedforms. Dune bedforms originating in nearshore bar-troughs tend to be slightly bimodal-bipolar and oriented alongshore. High standard deviations in dip azimuths relate to influence of rip channels, infrequent preservation of fairweather bedforms, and presence of dipping planar and curved cross-strata set boundaries. In addition, flat and low-angle dipping bedding generated by wave action can add to the dispersion. Unlike shelf sands, shoreline sand bodies can encompass a wide variety of subenvironments (e.g., foreshore, upper and lower shoreface, tidal inlet) with dissimilar bedding types and orientations.

These observations have important implications for interpretation of directional information provided by the dipmeter and formation microscanner. Analysis of dip azimuths, dip magnitude trends, and consistency of dip measurements derived from these tools is one means of discriminating between subsurface reservoirs formed in shelf and shoreline settings.

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