Abstract: Open-Coast Clastic Deposits: Theme and Variations

H. Edward Clifton

Depositional facies models distill the complexity of nature into relatively simple, easily grasped concepts that provide an important basis for reconstructing ancient depositional environments. The generalizations that allow models to be constructed, however, also inherently limit the range of their applicability. Models can be enhanced by documenting the variations that are likely to occur in natural settings. A consideration of open-coast clastic deposits shows how this approach can be used. Studies of modern coastal systems indicate that an upward-trending sequence of depositional facies (bioturbated or storm-dominated inner shelf; cross-bedded upper shoreface; planar-laminated foreshore; nonmarine deposits) characterizes most open-coast clastic successions. This gener lized model has been applied successfully to a variety of ancient coastal deposits, owing largely to the consistency of both processes and preservation through geologic time.

The model, however, must accommodate significant internal variability, imposed by differences in sediment texture, nearshore morphology, ambient energy, storm events, and relative sea level change. The influence of available texture is strong and commonly underappreciated. Nearshore morphology determines the nature and directional aspect of sedimentary structures, as well as the internal continuity of the section. The influence of ambient wave energy, typically considered to be of fundamental importance in shaping coastal facies, can be examined only in terms of textural connotations. Storm effects can be highly important, but their imprint differs as a function of facies. The nature of relative sea level change determines the nature of preservation of coastal successions. The variati ns about the basic theme are reasonably predictable and provide a basis for enhanced paleo-environmental interpretation. Ignoring them, however, and reliance on a general model can lead to misinterpretation of ancient coastal deposits.

AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers