PALEOGENE CHRONOSTRATIGRAPHY, FACIES CHANGES, AND THE ENIGMATIC PETM IN CENTRAL DELAWARE

Our recent investigations of Paleocene-Eocene formations in the subsurface of the Coastal Plain of central Delaware have used geophysical well-logs, wireline core lithology logs, and biostratigraphy (planktonic foraminifera, calcareous nannofossils, dinoflagellate cysts) to clarify understanding of vertical and lateral stratigraphic variations. The upper Paleocene Vincentown Formation contains the regionally important Rancocas aquifer, the extent of which is limited by lateral facies changes. Thick, aquifer-quality, glauconitic quartz and carbonate aquifer sand/sandstone in the north changes southward, across an approximately five-mile-wide transition zone, to thinner, muddier, predominantly micaceous, glauconitic, sandy clay. Biostratigraphy indicates that all of the Rancocas aquifer is Paleocene in the area, does not extend into the Eocene, and limited to the Vincentown Formation. The lower Eocene Manasquan Formation contains calcareous clay, silt, and muddy fine sand with glauconite and abundant microfossils. The top and bottom of the Manasquan are burrowed unconformities marked by an upward shift to glauconite sand and interpreted as sequence boundaries; these produce correlatable spikes on gamma ray logs. The basal Manasquan unconformity corresponds to the Paleocene-Eocene boundary. The overlying Shark River Formation consists of dark glauconitic clayey silt and clay with some glauconite and quartz sand. Both the Manasquan and Shark River become slightly finer-grained and more clay-rich downdip to the south. Microfossils indicate the Shark River is lower to middle Eocene in the core samples studied. A heavily burrowed contact separates Shark River muds from muddy, burrowed glauconite sand at the bottom of the overlying middle Eocene Piney Point Formation. Because the Eocene section has steeper southeasterly dips than the overlying Miocene formations, the Eocene formations are progressively truncated northward under a basal Miocene unconformity, resulting in an unconformable contact between Rancocas aquifer sands and lower Miocene muds in the north. Neither the Marlboro Clay nor evidence of Paleocene-Eocene Thermal Maximum have been recognized in central Delaware. No evidence of significant fault control on the Paleocene-Eocene stratigraphy has been identified.