Abstract:
The Bethany Beach borehole was the seventh continuously cored and logged onshore hole drilled as part of the Middle Atlantic Coastal Plain Drilling Project and the fourth drilled as part of ODP Leg 174AX. This 1470-ft-deep hole targeted Miocene sequences near their maximum regional thickness in the Salisbury Embayment, yielding insights on sea level, tectonics, and sediment supply not addressed by earlier New Jersey holes. The borehole penetrated strata ranging from Oligocene to Pleistocene. The Oligocene section is composed of dark gray foraminiferal clays and glauconitic clays and sands and interpreted to include at least two distinct sequences. The Miocene section is predominantly silt and sand. Sequence boundaries are recognized where significant flooding events coincide with unconformities, producing a stack of highstand systems tracts that reflect limited accommodation in a shelfal setting. However, the Miocene section in southern Delaware is thicker and more fossiliferous than the coeval inner neritic to non-marine section in New Jersey. This reflects higher accommodation and hence higher subsidence at this site due to its proximity to the hinge zone of the Baltimore Canyon Trough. We have identified two sequence boundaries in the middle Miocene Calvert Formation, three in the middle to upper Miocene Choptank Formation, and two in the upper Miocene St. Marys Formation. Facies and fossil evidence indicate a stronger marine influence in the upper middle Miocene to upper Miocene interval at Bethany Beach compared to the same interval in New Jersey, and the cores contain sufficient shell material suitable for Sr-isotope stratigraphy. Other facies contrasts are evident that in part reflect differences in sediment supply and/or tectonics. For example, there is a predominance of silts and a paucity of clays in the middle parts of the southern Delaware sequences versus thick silty clays in New Jersey sequences. The results of in-progress Sr-isotope analyses will allow us to assess the age and regional significance of these sequences, especially the more poorly constrained upper middle to upper Miocene sequences that lack datable material at the New Jersey sites.