Integrated stratigraphic analysis of a Miocene-age sandy shallow marine depositional system in southern Delaware

Abstract

This study examines the stratigraphy and palynology of three Miocene geologic units that occur in the Delaware Coastal Plain: the Calvert, Choptank, and St. Marys Formations. The goal of this investigation is to examine the stratigraphic record of these ancient shallow-marine units in detail in a nearly-continuous cored borehole at Abbott’s Mill Historical Site, in Milford, Delaware. This study utilizes lithostratigraphy, fossil palynomorphs, and sequence stratigraphy to understand the age, depositional environments, and regional correlation of these units. ☐ Pollen and spores were analyzed to interpret the paleoclimate. Vegetation is dominated by Quercus, Pinus, and Carya. The lower to middle Miocene Calvert Formation is interpreted to be a warm-temperate to subtropical environment, whereas the middle Miocene Choptank Formation represents a decline in temperature and moisture. The middle to upper Miocene St. Marys Formation represents a warm-temperate, slightly moist climate regime. Three zones were recognized using cluster analysis of the pollen data. Zone 1 (middle Choptank to St. Marys) is characterized by little to no exotic taxa, with high abundances of Carya, Cyperaceae, and Quercus. Zone 2 (middle Calvert to lower Choptank) has abundant Pinus, Tsuga, and Picea, with decreased Quercus and Carya. Zone 3 (lower to middle Calvert) contains the greatest abundances of exotic species including Engelhardia. Dinoflagellates cysts allow a refined subdivision of the Miocene stratigraphy, with zones identified from dinoflagellate Zone DN2 (lower Miocene) through Zone DN9 (upper Miocene). ☐ Nine stratigraphic sequences were identified in this borehole, representing cycles of sea-level rise and fall in an overall wave-dominated, sandy, shallow-marine shoreline environment. The Calvert Formation is characterized by several coarsening-upward sections, with offshore clays transitioning to shallower silty sands. The Choptank Formation sediments are shelly, foreshore sands with breaks of muddier, finer-grained sections. The St. Marys Formation consists of interbedded silty sands and clays, representative of estuarine to marsh environments. Sequences identified here, constrained by the pollen and dinoflagellate biostratigraphy, correlate well with sequences identified at Marshy Hope and Bethany Beach. Results from this integrated stratigraphic analysis assist in better understanding the subsurface geology and hydrostratigraphy of southern Delaware.