2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 257-12
Presentation Time: 4:30 PM


FISHER, Keri A., Geology, University of Delaware, Newark, DE 19716 and MCLAUGHLIN Jr., Peter P., Delaware Geological Survey, University of Delaware, Newark, DE 19716, kafisher@udel.edu

The Marshy Hope borehole provides a relatively continuous record of the Calvert and Choptank Formations within southwestern Delaware. The borehole was drilled to 540-ft depth using wireline coring, providing a rich dataset on lithology, palynomorphs and environments in the lower and middle Miocene.

Lithologic analysis of the cores reveals a succession of sands and muds indicative of nearshore shallow-marine depositional environments. The lower part of Calvert Formation is composed of shelfal clay and silt and becomes a coarsening-upward alternation of sand and silt in the middle of the formation. An interval of clay and silt comprises the top of the formation. The overlying Choptank Formation is composed of mostly medium to coarse sands with interbedded silt and clay in the lower part, succeeded by an alternation of clay/silt packages and fine to coarse sands in the middle of the formation. The upper part of the Choptank Formation is characterized by a fining-upwards sand package and capped by clay and silt of the St. Marys Formation. The grain size trend from the Calvert Formation through the Choptank Formation is a general coarsening upwards from silt and clay to medium and coarse sands. The Calvert-Choptank interval is interpreted at this site as a stack of highstand-dominated stratigraphic sequences.

Palynomoph assemblages record the Miocene depositional history at the site and evolution of vegetation at nearby terrestrial areas. The variation in the types of palynomorphs, abundances, and diversity through the section helps to interpret depositional environments, biozones, ages and correlation to other sites in the region. The flora is dominated by Quercus, Carya, and Pinus. Taxodiaceae/Cupressaceae/Taxaceae, Ilex and Nyssa are common occurrences, and “exotic” taxa such as Engelhardia and Symplocos are also present. The overall change in palynomorphs from the Calvert Formation up through the Choptank Formation is a slight increase in Pinus and Carya, and a decrease in Quercus and “exotic” taxa. Dinoflagellate occurrences and strontium isotopes supplement the spore-pollen data for chronostratigraphic interpretation.