Sequence stratigraphy, micropaleontology, and foraminiferal geochemistry, Bass River, New Jersey paleoshelf, USA: Implications for Eocene ice-volume changes

摘要

Micropaleontological faunal studies coupled with foraminiferal geochemical analyses from the Bass River Site (Ocean Drilling Program [ODP] Leg 174AX; New Jersey, USA) reveal rapid changes in relative sea level due to million-year-scale glaciations during the early to middle Eocene, a time previously thought to have been mainly ice free. We examine benthic foraminiferal assemblages, stable isotopes (δ 18 O and δ 13 C), Mg/Ca, planktonic foraminiferal abundances, and ostracod abundances in eight lower to middle Eocene sequences at Bass River to reconstruct paleo–water depth and paleoceanographic changes within a sequence stratigraphic framework on the New Jersey paleo–continental shelf. Distinct benthic foraminiferal biofacies are identified and interpreted for paleodepth and environmental changes. Certain dominant species (e.g., Uvigerina spp., Cibicidoides eocaenus , Spiroplectammina alabamensis , Siphonina claibornensis , and Cibicidoides pippeni ) indicate changes in water depth and/or environmental conditions. We estimate middle to outer neritic (50–100+ m) paleodepths for much of the early to middle Eocene, with maximum water depths (～150 ± 25 m) occurring in the early Eocene. We integrate these results with ostracod abundances and diversity, planktonic foraminiferal abundances, lithofacies, downhole logs, and core erosional surfaces to create a sequence framework for the early Eocene to early late Eocene of the New Jersey coastal plain. We compare the relationships among these sequences to foraminiferal biofacies of coreholes of the New Jersey Coastal Plain Drilling Project (Island Beach, Atlantic City, and ACGS#4), showing coeval hiatuses associated with regional base-level lowerings. Benthic and planktonic foraminifera δ 18 O coupled with low-resolution Mg/Ca measurements provide a first-order correlation of sequence boundaries and δ 18 O seawater variations, indicating glacioeustatic changes associated with the growth and decay of small ice sheets on the order of 20–30 m sea-level equivalent during the Eocene.