Morphology and Vertical Sedimentary Sequence Models in Holocene Transgressive Barrier Systems

Abstract

Studies of the transgressive barrier elements of the mid-Atlantic Delmarva Peninsula show four major types of morphologic and vertical sedimentary sequence models. Each model shows a response to present wave and current conditions as well as to the paleotopography inundated by the ongoing transgression. The geomorphic features of the shoreline areas of a transgressive barrier coast, as studied in the Delmarva Peninsula area, include numerous discreet geomorphic features that are in fact single sedimentary environment lithosomes. These sedimentary environment lithosomes are part of a dynamic condition in which geomorphic environment-sedimentary lithosomes are moving landwards and upwards in space and time with the presently continuing coastal transgression. As the present coastal transgression is in part a product of late Holocene relative sea level rise and in part the product of coastal erosion, vertical sequences and the sedimentary environment lithosomes of the late Holocene stratigraphic record at the edge of the transgression are in part truncated by erosion and, accordingly, are incomplete. However, detailed geomorphic and subsurface studies of the various elements of the estuarine and Atlantic Ocean barrier coast have shown that the linking of studies of coastal processes, surface geomorphic features, and subsurface stratigraphic sections make possible a relatively complete understanding of the sequence of geomorphic events that have occurred in the coastal zone during late Holocene time. Four major geomorphic-sedimentary environment sequence models established are as follows: Barrier Systems: barrier islands, baymouth barriers, and, barrier island chains vary in amount of separation from the mainland by lagoons, distance from other islands, and number of inlets. All are characterized by a vertical transgressive sequence of sedimentary lithosomes identical to the horizontal sequence in the direction of the transgression. 322 Beach against highland: beach system impinges on low lying (20–30 ft) pre-Holocene highlands formed of Pleistocene sediments which erode and provide a partial source of sands, gravels, and mud to the system. Estuarine barriers: long, arcuate barriers, small in width (150 ft) and thickness (10 ft) but of extreme length (up to 50–75 mi), occur along shorelines of large and small estuaries. Internal structures of these sand and gravel barriers are variable and complex; mainly composed of washover features. Four variants are identified. Spit complex: extends into open marine and bay areas. All expected coastal transgressive environments of the barrier systems are seen in addition to regressive spit accretion sands and gravels. Consequently, vertical sequences may be disrupted and out of order. Model studies of the variants of coastal morphologic units of the Delmarva Peninsula may be used to summarize the geologic history of the edge of the ongoing Holocene transgression. In addition, these models may be used to predict continuing changes in coastal morphology, assuming present and past processes will continue into the future. These predictions are useful to man in understanding and developing a more effective use of the coastal zone as man’s intensity of involvement with coastal geomorphic environments continues to increase.