Sedimentary Facies Patterns and Geologic History of a Holocene Marine Transgression

Abstract

Studies of Holocene sediments in coastal Delaware show complex sediment distribution patterns resulting from lateral and vertical movement of successive environments of deposition over a Pleistocene unconformity. These sediments are infilling a drowned topography with a local relief of 70 ft and possibly up to 125 ft eroded on highly variable Pleistocene sediments. Identification of the Pleistocene surface remains a problem. However, it may be recognizable at the unconformity as a soil zone or intermixture of firm marsh clay-silts with Pleistocene sands, as well as on the basis of radiocarbon dates. Larger depositional features forming around eroding Pleistocene headlands and infilling the estuaries include characteristic shoreline environments, such as spits, dunes, baymouth barriers, an intermeshing network of tidal deltas, nearshore marine erosional-depositional sands and gravels, and lagoons or estuaries with fringing Spartina, Distichlis, and Phragmites marshes, which form the westernmost edge of the transgressive units. The thickness and areal extent of the sedimentary bodies are to a large degree controlled by the morphology of the Pleistocene unconformity. A large portion of the Holocene sedimentary units is being eroded by the transgressing Atlantic Ocean. Cores of sediment under the shallow lagoons, such as Rehoboth, Indian River, and Assawoman Bays, and in the fringing marsh environment, show that the depositional units are thin, highly irregular in areal extent, extremely variable in thickness, and difficult to project. Sedimentary processes active in the shallow bays include shoreline marsh erosion and the formation of thin, possibly ephemeral, beach-dune washover complexes consisting of clean, well-sorted sand, with typical beach and washover sedimentary structures. These wash-over beaches are an anomaly completely surrounded by Spartina marshes on the landward side and extremely muddy sands grading into dark gray lagoonal muds on the bay side. It appears that distinctive sedimentary structures and sediment size-sorting relationships, such as those that characterize the larger, more common sedimentary units of the coastal area, may be formed in miniature at the very thin edge of transgression and may lead to considerable confusion in the interpretation of sediments of this type in the geologic record.