The Holocene History and Stratigraphy of Palustrine and Estuarine Wetland Deposits of Central Delaware

Abstract

Unsaturated floodplains and riverine, estuarine, and palustrine wetlands of coastal plain river valleys of central Delaware have mean loss-on-ignition (LOI) values of 2-9%, 9-13%, 17-21 %, and 43-46%, respectively. These distinctive LOI signatures are used in conjunction with radiocarbon dates and Delaware's local relative sea-level curve to reconstruct the Holocene history of the valleys of the study area. During the early Holocene, the sediment yield of the- Delaware coastal plain was too low for rivers to build extensive subaerially exposed floodplains. Instead, organic-rich muds and peat accumulated in perennially inundated, non-tidal palustrine wetlands. These environments existed as long ago as 11,000 YBP, well before any possible influence of sea-level rise. The elevation of these deposits and Delaware's sea-level history suggest that tidal influence began 1,000-2,000 years ago. The transition from fluvial to estuarine conditions occurs without any lithologic change or discontinuity near the top of a uniform palustrine peat. Therefore, the base of the Holocene transgressive sequence is conformable in these valleys, except where lateral migration by tidal streams has subsequently removed the conformable transition. Outside of the valleys, however, the base of the transgressive sequence is a major unconformity. Thus, the transgressive sequence preserved landward of the Delaware Bay coast is bounded at its base by a complex stratigraphic surface which is primarily conformable in valley fills but which is unconformable on paleointerfluves.