Atlantic City Site Report

Abstract

The Atlantic City borehole was the second site drilled as part of the New Jersey coastal plain drilling project, Leg 150X. It focused on middle middle Miocene to Oligocene "Icehouse" and middle-upper Eocene "Doubthouse" sequences known from previous rotary and cable tool wells. Recovery was not as good as at Island Beach (60% vs. 87%) because of hole stability problems; however, recovery was excellent for most of the critical lower-middle Miocene interval (390-937 ft; 81%). The surficial Cape May Formation (uppermost Pleistocene-Holo- cene; 123 ft thick) contains nearshore gravelly sand and clay at the top and fluvial deposits at the base that apparently correlate with the Cape May Formation at Island Beach. The ?middle Miocene Cohansey Formation (96 ft thick) sand and sandy clay represents fluvial deposits not present at Island Beach. The ages of both units are uncertain. Recovery of the uppermost part of the 706-ft-thick Kirkwood Formation was poor (no recovery from 293 to 390 ft), but recovery for the Kirkwood between 390 and 937 ft was excellent. The sand, silts, and clay facies expressed in the Kirkwood Formation at Atlantic City represent diverse fluvial, nearshore, and neritic (including pro- delta) environments. Several upward-coarsening sequences can be recognized on the basis of lithofacies breaks, gamma-log changes, and hiatuses, corresponding with confining units at the base and aquifer units at the top. These lithostratigraphic and geohydrologic units correspond with similar units at Island Beach, and we suggest that they correlate. Numerous shell beds in the Atlantic City borehole allow preliminary dating of these sequences with Sr-isotopic stratig- raphy, including the middle middle Miocene Kirkwood 3 sequence (13.3-13.5 Ma; from at least 401.7 to at least 470 ft), the upper lower Miocene Kirkwood 2 sequence 17.0-17.9 Ma; 512-666 ft), and the uppermost Oligocene to lower Miocene Kirkwood 1 sequence (20.3- 25.8 Ma; 666-937 ft). The Kirkwood 1 sequence may be divided into several additional sequences that have dramatic shell beds at their base and distinct ages determined by Sr isotopes: 20.3-21.9 Ma, 23.6-23.7 Ma, and -25.8 Ma. A sharp lithologic and gamma-log break at 741 ft may indicate another sequence boundary between 20.3 and 20.8 Ma, although Sr-isotopic resolution is not sufficient to document this hiatus unequivocally. The upper Eocene-Oligocene may be divided further into 3 se- quences based on lithologic and gamma-log changes that were dated with Sr isotopes: upper Oligocene (27.4-28.7 Ma), lower Oligocene (-33.4 Ma), and upper Eocene (36.6-37.7 Ma). Biostratigraphy is consistent with the Sr-isotopic ages and indicates additional lower upper Eocene and upper middle Eocene sequences. The systems tracts are generally well developed for these middle Eocene through middle Miocene sequences, with a basal shell or glau- conite sand at the base and sands at the top. Further biostratigraphic and Sr-isotopic studies should refine the ages of the sequences, whereas lithostratigraphic and benthic foraminifer biofacies studies should reveal details of the depositional environments and systems tracts of these well-developed sequences.