Integration of Palynology and Sequence Stratigraphy in the Analysis of Miocene Siliciclastic Aquifers, Delaware, USA

McLaughlin, Peter P.¹ (1) Delaware Geological Survey, Newark, DE

Ground water is an important natural resource in the coastal plain of Delaware, where it comprises nearly the entire water supply. Miocene confined aquifers are an important part of this resource in central and southern Delaware. Biostratigraphy and sequence stratigraphy can be applied to problems in aquifer geology much in the same way they can be applied to reservoir problems.

The lower to middle Miocene section of the study area is characterized by alternating sands and muds. These represent a stack of transgressive and highstand systems tracts deposited in shallow- to marginal-marine environments. Aquifers are typically developed in the nearshore sand facies at the top of the sequences. Four aquifers are targets: the Cheswold, Federalsburg, Frederica, and Milford sands. However, precise correlation of these lithologically similar sands can be a challenge where well control is limited.

Palynology is a tool that can help resolve correlation issues in this succession. The flora is dominated by Quercus, with common Carya and Pinus, and several subtropical to tropical “exotic” taxa that do not presently live in or near the study area. The assemblages reflect subtropically influenced warm-temperate environments with a decrease in subtropical influences through time. Subtle stratigraphic changes are noted in the assemblages. Lower Miocene assemblages are mostly composed of temperate and warm-temperate arboreal forms, commonly with greater abundances of taxodiaceous pollen than in higher intervals. Lower Miocene exotics include common Engelhardia-type pollen and scattered Podocarpus. Middle Miocene assemblages are similar but have fewer exotics; Pinus is commonly more abundant than in lower intervals. Engelhardia-type pollen is the most common exotic in the middle Miocene but generally decreases upward, and Pterocarya, Podocarpus, and Symplocos are present.