HOLOCENE EVOLUTION OF THE DELAWARE ESTUARY

Estuaries are dynamic environments in which geomorphic changes are driven by the deposition and erosion of sediment, which may occur over a range of timescales, from almost instantaneous to gradual changes over thousands of years. Here we describe the influence of hurricanes and relative sea-level (RSL) rise on the development of the Delaware estuary using both proxy records and modeling. We use these data to draw a series of paleogeographic maps, to illustrate changes in the tidal regime during the Holocene.

Stratigraphic investigation of salt marsh deposits along the northern shore of the Delaware Estuary reveal the presence of at least 5 transgressive-regressive mud and marsh-peat sequences deposited during the past 2200 years. Stratigraphic position and thickness of mud units, overall horizontal facies boundaries within the regressive sequences, rapid post-erosion infilling and marsh recovery suggest that erosion occurred under storm-surge conditions and may be a sedimentary record of pre-historical hurricane strikes.

Rates of RSL change were highest during the early and mid Holocene and have decreased over time, due to the diminishing response of the Earth’s mantle to GIA and reduction of eustatic input. The sea-level history within the estuary is spatially variable. In the Inner Delaware RSL was -17 m at 6,000 years BP and rose until 4,000 years BP at 4.5 mm/yr, with a reduced rate of 1.8 mm/yr from 4,000 BP to AD 1900. The rate of sea-level rise in the Outer Delaware was diminished. RSL was -20 m at 8,000 years BP and rose by 2.8 mm/yr until 4,000 years BP and at a reduced rate of 1.7 mm/yr from 4,000 years BP to AD 1900.

RSL changes affect shelf width and bathymetric depths, and therefore the reflection and amplification of tidal waves and the distribution of frictional dissipation of the tidal energy that is transferred from the deep oceans to the shallow shelf regions. Tidal range in the Outer Delaware has remained fairly constant through the latter half of the Holocene while tidal range in the Inner Delaware has increased steadily from about 3 ka to present day; a nearly 100% increase in total.