Lower Eocene Calcareous Nannofossil Biostratigraphy Beneath the Atlantic Slope and Upper Rise off New Jersey—New Zonation Based on Deep Sea Drilling Project Sites 612 and 613

Abstract

Lower Eocene calcareous nannofossil limestone cored at DSDP Site 612 on the middle slope off New Jersey represents an almost complete biostratigraphic sequence; only the lowest biozone (CP9a; NP10*) was not recovered. The thickness of the strata (198 m), the good preservation of the nannofossils, and the lack of long hiatuses justify the acceptance of this section as a lower Eocene reference for the western North Atlantic margin. The widely recognized and very similar nannofossil zonations of Martini (NP zones) and Bukry-Okada (CP zones) are emended slightly to make their lower Eocene biozones coeval; in addition, five new subzones are erected that subdivide zones CP10 and CPU (NP12 and NP13). Established biozone names are retained as they are altered little in concept, but alphanumeric code systems are changed somewhat by appending an asterisk (*) to identify zones that are emended. Zone CP10* (NP12*) is divided into two parts, the Lophodolithus nascens Subzone (CP10*a; NP12*a) and the Helicosphaera seminulum Subzone (CP10*b; NP12*b). Zone CPU* (NP13*) is divided into three parts, the Helicosphaera lophota Subzone (CPll*a; NP13*a), the Cyclicargolithuspseudogammation Subzone (CP1 l*b; NP13*b), and the Rhabdosphaera tenuis Subzone (CPll*c; NP13*c). At Site 612, a time-depth curve based on nannofossil datums dated in previous studies reveals a smoothly declining sediment accumulation rate, from 4.9 cm/10 3 yr in CP10* (NP12*) to 2.8 cm/10 3 yr. in CP12* (NP14*). The ages of first-occurrence datums not previously dated are approximated by projection onto this timedepth curve and are as follows: Helicosphaera seminulum, 55.0 Ma; Helicosphaera lophota, 54.5 Ma; Cyclicargolithus pseudogammation, 53.7 Ma; Rhabdosphaera tenuis, 52.6 Ma; and Rhabdosphaera inflata, 50.2 Ma. At nearby Site 613 on the upper rise, strata of similar age, 139 m thick, contain an unconformity representing Subzone CPll*b (NP13*b) and a hiatus of approximately 1.1 m.y. duration. The sediment accumulation rate in the lower part of this section (9.7 cm/10 3 yr.) is twice that observed for equivalent strata at Site 612. The hiatus and the heightened sediment accumulation rate at Site 613 probably represent the effects of episodic mass wasting on the early Eocene continental slope and rise. INTRODUCTIONDuring Deep Sea Drilling Project Leg 95, core holes at Sites 612 and 613 on the continental margin off New Jersey penetrated a thick section of richly fossiliferous Eocene calcareous nannofossil limestone (Fig. 1). Eocene strata are widespread beneath the U.S. Atlantic margin. Beneath the outer shelf and upper slope the section is 400 to 500 m thick, but it thins both to seaward beneath the rise and to landward beneath the coastal plain, where erosional unconformities are evident.The sections at Sites 612 and 613 represent an almost complete sequence of lower Eocene nannofossil biozones. In all, smear slides from 101 samples were examined in a 221-m section from Site 612 on the middle slope (water depth 1404 m) and from 55 samples in a 148-m s...