Time-stratigraphic aspects of a formation: Interpretation of surficial Pleistocene deposits by analogy with Holocene paralic deposits, southeastern Delaware

Abstract

Pleistocene shoreline deposits in close spatial and temporal proximity to Holocene deposits permit detailed interpretation of the development of Pleistocene coastal deposits when a modern-ancient analog approach is used. Peak sea levels during the past 1 m.y. or more have allowed the preservation of transgressive shoreline systems, here referred to as paralic units, along the inner continental margin. These Pleistocene paralic units are similar to Holocene coastal deposits now forming on Delmarva Peninsula. The Omar Formation (early to late Pleistocene age) incorporates at least four juxtaposed, transgressive, paralic units of different ages, each containing deposits of lagoonal sands and muds, tidal marsh, barrier sands and gravels, tidal delta sands and silty sands, and tidal channel sands and gravels. The successive paralic units become younger in the seaward direction. Thus, although the Omar Formation in the study area is almost entirely made up of transgressive deposits produced during rising sea levels, in a longer time frame it forms a progradational sequence. Only about 2% to 5% of the elapsed time during the deposition of the studied part of the Omar Formation is represented by sedimentary units. The internal time-stratigraphy of the Omar Formation is extremely complex. This makes the time-stratigraphic interpretation of any one outcrop or drill hole difficult. Any lithologic change may represent a wide variety of time gaps, from minutes to tens of thousands of years. The analog approach, in which sedimentary lithosome mapping is used, allows one to infer a time-stratigraphic framework of the deposits. These concepts can aid in the interpretation of depositional history of older paralic deposits and the nature of the development of certain types of clastic formations.