A Reexamination of Facies Models for Clastic Shorelines

Currently accepted depositional models of coastal facies derive mostly from studies of modern depositional systems combined with interpretations of ancient successions. Two factors, however, can limit the efficacy of such facies models. First, Holocene coastal settings differ significantly from those in which many, if not most, ancient coastal sediments accumulated. Second, input to a model from the rock record commonly is specific to a particular tectonic and oceanographic setting (which may be poorly constrained) and is not fully exportable to other settings. This paper explores how these factors impact our interpretive capability.

Many, if not most, ancient shoreface deposits accumulated under conditions of progradation, a process relatively uncommon among the world’s present shorelines. Instead, many modern postglacial coasts experience rising sea level and reduced sedimentation, which enhances barrier-island development and influences the shoreface-to-shelf bottom profile and sand–mud distributions. Ignoring these differences promotes inaccuracy in our facies models.

Often overlooked in the application of coastal facies models are variations imposed by texture, energy level, and tectonism. Sedimentary structures on fine-grained sandy coasts differ substantially from those on a coast underlain by coarse-grained sediment. Deposits on a high-energy coast are unlike those in a low-energy setting. Differing degrees of accommodation influence the nature of the preserved succession. Some of the problems inherent in current facies models can be obviated by considering them as end members within a continuum of models that incorporate different energy regimes, textural characters, and preservational modes.