Integrated Ocean Drilling Program (IODP) Expedition 313 continuously cored Lower to Middle Miocene sequences at three continental shelf sites off New Jersey, USA. The most seaward of these, Site M29, contains a well-preserved Early and Middle Miocene succession of planktonic diatoms that have been independently correlated with the geomagnetic polarity time scale derived in studies from the equatorial and North Pacific. Shallow water diatoms (species of Delphineis, Rhaphoneis, and Sceptroneis) dominate in onshore sequences in Maryland and Virginia, forming the basis for the East Coast Diatom Zones (ECDZ). Integrated study of both planktonic and shallow water diatoms in Hole M29A as well as in onshore sequences in Maryland (the Baltimore Gas and Electric Company well) and Delaware (the Ocean Drilling Program Bethany Beach corehole) allows the refinement of ECDZ zones into a high-resolution biochronology that can be successfully applied in both onshore and offshore regions of the East Coast of the United States. Strontium isotope stratigraphy supports the diatom biochronology, although for much of the Middle Miocene it suggests ages that are on average 0.4 m.y. older. The ECDZ zonal definitions are updated to include evolutionary events of Delphineis species, and regional occurrences of important planktonic diatom marker taxa are included. Updated taxonomy, reference to published figures, and photographic images are provided that will aid in the application of this diatom biostratigraphy.
Marine diatoms have been employed for biostratigraphic correlation of Lower and Middle Miocene sediments deposited on the Atlantic Coastal Plain and offshore between Florida and New Jersey for more than 50 years. Biostratigraphic zonations developed during the 1970s and early 1980s by George W. Andrews (1976, 1978, 1988a) and William H. Abbott (1978, 1980, 1982) relied primarily on benthic and shelf-dwelling diatoms (Actinoptychus, Delphineis, Rhaphoneis, Sceptroneis) that are common in these deposits. Limited age control was provided by sporadic occurrences of oceanic planktonic diatoms, calcareous nannofossils, and planktonic foraminifera (Abbott, 1980, 1982) and Sr-isotopic ages (Sugarman et al., 1993).
In both the Pacific and Southern Oceans, however, very reliable biochronologies have been developed for Miocene and younger sediments using evolutionary events of planktonic marine diatoms (Barron, 2003; Scherer et al., 2007), many of which have been tied directly to paleomagnetic stratigraphy. Unlike planktonic foraminifers and calcareous nannoplankton, diatoms are diverse and common in cool waters that are characteristic of both higher latitude regions and coastal regions dominated by coastal upwelling. Planktonic diatoms are common in diatomaceous sediments deposited along the margins of the North Pacific, facilitating the application of planktonic diatom biochronology (Yanagisawa and Akiba, 1998; Scherer et al., 2007). However, planktonic diatoms are scarcer in onshore Miocene diatomaceous sediments of the Atlantic Coastal Plain where the continental shelf-slope gradient is much more gentle than that of Pacific margins, resulting in poorly refined diatom biochronologies (Abbott, 1978, 1980). Abbott (1978, 1980) incorporated some more cosmopolitan planktonic diatoms such as Annellus californicus, Coscinodiscus lewisianus, C. plicatus ( = Thalassiosira grunowii), and Denticula hustedtii ( = Denticulopsis simonsenii) into his biostratigraphic studies; however, their ranges were often found to be sporadic in nearshore sequences, limiting correlation with the geologic time scale.
Andrews (1977, 1988b) recognized an evolutionary succession of Delphineis species that are common in onshore sediments of the Atlantic Coastal Plain. Other studies (Abbott, 1978, 1980; Andrews, 1988a) showed that Delphineis species are very useful for biostratigraphic correlation. However, only two taxa, D. penelliptica and D. ovata have been incorporated into diatom zonations (Abbott, 1978; Andrews, 1988a). Typically, the biostratigraphy of Abbott (1978) and Andrews (1988a) has been pieced together from numerous onshore sections that were limited in duration and correlated by lithology, using Shattuck’s (1904) lithologic units, (updated by Ward, 1984). In particular, Andrews (1988a) proposed an East Coast Diatom Zonation (ECDZ) that has been widely used. A limited number of continuous onshore and offshore coreholes have been studied for diatom biostragtigraphy (Abbott, 1978, 1982; Burckle, 1998), most notably, the Baltimore Gas and Electric Company (BG&E) corehole (Calvert Cliffs, west shore of Chesapeake Bay, Maryland; Fig. 1; Abbott, 1982); however, detailed documentation of the diatom assemblages has been limited.
A transect of holes cored on the shallow New Jersey shelf during Integrated Ocean Drilling Program (IODP) Expedition 313 provided an opportunity to further develop this East Coast diatom biostratigraphy and to refine its correlation with the geologic time scale. In particular, coring at Site M29, the most seaward of a transect of three coreholes, recovered a nearly continuous succession of diatom-rich sediments between ∼695 and 325 m subbottom depth that preliminary strontium and planktonic microfossil assigned to the late early and middle Miocene. Reconnaissance examination of samples from Hole M29A revealed numerous planktonic diatom taxa that have proven useful for diatom biostratigraphy in the equatorial and North Pacific (Barron, 1985, 2003; Yanagisawa and Akiba, 1998), promising an improved correlation of the ECDZ diatom biostratigraphy of the east coast of the United States. Study of the diatom assemblages of Hole M29A along with key stratigraphic successions from onshore drill cores, the Baltimore Gas and Electric well, and the Ocean Drilling Program Leg 174AX Bethany Beach corehole have been initiated with the purpose of refining diatom biostratigraphy. After the diatom biostratigraphy has been refined, it will be applied to IODP Expedition 313 Holes M27A and M28A, as well as to some additional onshore coreholes.
MATERIALS AND METHODS
IODP Expedition 313 Hole M29A, at 39°31.170′N, 73°24.792′W, 36 m water depth, cored to a depth of 754.44 m lower Miocene to Pleistocene sediments, mostly silts and sandy silts with preliminary estimates provided by calcareous nannofossils, planktonic foraminifers, and dinoflagellates (Mountain et al., 2010) (Fig. 1) and strontium isotopes measured on mollusks and foraminifers (Mountain et al., 2010; Browning et al., 2013). Common diatoms were routinely encountered in smear slides of sediments prepared from Lower and Middle Miocene sediments for routine lithologic descriptions by the IODP Expedition 313 science party. Consequently, material was sent to one of us (J. Barron) for diatom biostratigraphic study.
The BG&E well was drilled to a depth of 103.237 m (338.8 feet) at the company’s atomic reactor site at Calvert Cliffs, near the west shore of Chesapeake Bay, Maryland (38.43°N, 76.44°W) (Stefansson and Owens, 1970). The BG&E well was cored continuously from a depth of 3.84 m (12.6 feet) and was studied for diatoms by both Andrews (1978) and Abbott (1982), and therefore is considered a key stratigraphic reference section for Miocene diatom biostratigraphy in the Atlantic Coastal Plain. Microscope slides prepared by Abbott (1982) were obtained from the California Academy of Sciences (by J. Barron) and examined under the light microscope. Andrews (1978) named Miocene Lithologic Units (MLU) after the 24 stratigraphic zones of Shattuck (1904). These include: MLU 1-MLU 3, the Fairhaven Member of the Calvert Formation; MLU 4-MLU 13, the Plum Point Marl Member of the Calvert Formation; MLU 14-MLU 16, the Calvert Beach Member of the Calvert Formation; and MLU 17-MLU 20, the Choptank Formation (Ward, 1984).
The Bethany Beach corehole contains a nearly continuous record of Oligocene-Pleistocene sediments along the coast of eastern Delaware (38.548°N, 75.0625°W) (Miller et al., 2003; Browning et al., 2006; McLaughlin et al., 2008). McLaughlin et al. (2008) provide detailed lithologic descriptions and biostratigraphic correlations utilizing calcareous nannofossil, dinoflagellates, planktonic foraminifers, radiolarians, diatoms, and strontium isotopes, making the Bethany Beach corehole a key stratigraphic reference section for the Atlantic Coastal Plain. Because this corehole contains an extended succession of Lower and Middle Miocene diatom-bearing sediments, it was selected for detailed diatom biostratigraphic study.
Strewn slides of diatoms and silicoflagellates were prepared by placing ∼1–2 cm3 of sediment in a small porcelain mortar and covering it with distilled water. The pestle was then used to disaggregate the samples. To prepare slides, a disposable pipette was used to extract a small amount of the suspension from near the top of the liquid. A drop was then transferred from the pipette to a 40 × 22 mm cover slip and dried at low heat on a hot plate. Slides were then mounted in Naphrax (index of diffraction = 1.74). The entire slide was examined at X920 magnification and the occurrences of stratigraphically important and ecologically significant diatoms were recorded. The overall abundance of diatoms in each sample was listed as abundant (A, >60%), common (C, 30%–60%), few (F, 5%–30%), and rare (R, <5%). The relative abundance of diatom species in an assemblage was estimated at X500 as follows: abundant (A), more than one specimen seen in each field of view; common (C), one specimen observed in two fields of view; few (F), one specimen present in each horizontal traverse of the coverslip; and rare (R), for sparser occurrences.
This study primarily focused on Site M29, which contained an excellent record of stratigraphically important diatoms and silicoflagellates. In general, samples were studied every 5–10 m with fine-grained lithologies preferentially selected. Qualitative estimates of diatom abundance and preservation are included (Table 1). The diatom and silicoflagellate taxonomy applied is shown in Appendix 1.
Fifteen Lower to lower-Middle Miocene (ca. 23–13 Ma) sequence boundaries (m5.8 through m4.1; Fig. 2) were previously recognized using criteria of onlap, downlap, erosional truncation, and toplap and traced through three generations of offshore multichannel seismic data (Monteverde et al., 2008; Mountain et al., 2010). The depths of these seismic sequence boundaries were predicted in the coreholes using a velocity depth function prepared prior to Expedition 313 (Mountain et al., 2010). Sequence boundaries were independently recognized in the cores based on core surfaces, lithostratigraphy (grain size, mineralogy, facies, and paleoenvironments), facies successions, benthic foraminiferal water depths, downhole and core gamma logs, and chronostratigraphic ages (Mountain et al., 2010). Initial correlations were adjusted (by tenss of centemeters to a few meters in vertical position) using revised velocity-depth function and synthetic seismograms based on drilling results (G. Mountain, 2013, written commun.). These new correlations were tested using additional lithologic, benthic foraminiferal, age, and downhole and core log data (Browning et al., 2013; Miller et al., 2013). The placement of the sequence boundaries follows Miller et al. (2013).
Diatom Biostratigraphy of Hole M29A
At Site M29, diatoms are concentrated between ∼693 and 336 m composite depth (mcd), with underlying and overlying intervals either being barren or containing only very rare, poorly preserved diatoms. The occurrences of stratigraphically important diatoms and silicoflagellates in Hole M29A are shown in Table 1. Samples studied from Hole M29A contain numerous diatom taxa, in particular species of Delphineis, which proved to be useful in the onshore biostratigraphic studies of Abbott (1978, 1980) and Andrews (1988a, 1988b). The succession of first and last occurrences of these diatom taxa in Site M29 (Table 2) has been used to refine the East Coast Diatom Zones (ECDZ) of Andrews (1988a) (Appendix 2).
The recognition of a number of important planktonic diatoms that have been successfully applied in Early and Middle Miocene diatom biostratigraphy in the equatorial and North Pacific (Barron, 1985; Tanimura, 1996; Yanagisawa and Akiba, 1990, 1998; Barron, 2003) allows biostratigraphic ages to be assigned to the section studied in Hole M29A (Table 2). These diatom datum levels are used in Figure 2 to construct an age versus depth plot for Hole M29A. First occurrence datum levels are preferentially used, because they provide constraint on the maximum age of a particular horizon. In the case of last occurrence datum levels, an arrow pointing down the paleomagnetic section has been added to indicate that they may represent reworking. Although both first and last occurrences may be controlled by differential preservation and/or regional ecology, the succession of diatom datum levels for Hole M29A displays a progressive trend of younger ages up section that can be represented by lines of accumulation on an age versus depth plot. Diatom datum levels suggest that M29A sediments at Site M29 accumulated at a rate of ∼28 m/m.y. between ca. 18.7 and 15.9 Ma (sedimentary sequences m5.6-m5.3; Miller et al., 2013) with sediment accumulation rates increasing to ∼70 m/.y. between ca. 15.9 and 14.6 Ma (sequence m5.2). An unconformity with a hiatus spanning the interval from ca. 14.6–13.8 Ma is inferred at ∼502 mcd, corresponding with sequence boundary m5. Above this horizon the interval of sediment containing sequences m5 to m4.1 to the top of preserved diatoms at 336 mcd appears to have been deposited between ca. 13.8 and ca. 13.0 Ma at a sediment accumulation rate close to 180 m/m.y. (Browning et al., 2013).
Strontium isotope stratigraphy (Browning et al., 2013) suggests ages that follow the same up-section trend as the diatom datum levels (green x symbols, Fig. 2), especially for the interval above ∼600 mcd (ca. 16–13 Ma), although they indicate ages ca. 0.4 m.y. older than those suggested by diatoms. For detailed discussion of age versus depth relations in Hole M29A based on all of the microfossil groups and strontium isotope stratigraphy, see Browning et al. (2013).
Diatom Biostratigraphy of Key Onshore Sections
In order to test and refine ECDZ biostratigraphy, the BG&E well and the Bethany Beach core were studied (Fig. 1).
The occurrences of stratigraphically useful diatoms in the BG&E well are shown in Table 3, with ECDZ 2–6b being recognized. The Calvert Formation-Choptank Formation boundary, or MLU 16–17 boundary, is recognized at ∼34 m depth in the BG&E well (Abbott, 1982), where it coincides with the ECDZ Subzone 6a-6b boundary or first occurrence of Delphineis biseriata (Table 3).
The occurrences of stratigraphically useful diatoms in the Bethany Beach corehole are shown in Table 4. ECDZ 1–6b are recognized; the uppermost diatom-bearing sample from a depth of 202 m may represent ECDZ 7. Diatoms suggest that the age equivalent of the boundary between the Calvert and Choptank Formations as recognized by diatoms in the BG&E well (the ECDZ 6a-6b boundary or 13.3 Ma; Table 3) should be placed stratigraphically higher, at ca. 204 m than its reported position at 250 m in the Bethany Beach corehole (McLaughlin et al. (2008) (Table 4). McLaughlin et al. (2008) suggested that the Calvert/Choptank boundary appeared to be significantly older in the Bethany Beach corehole than in the Calvert Cliffs and attributed the difference to a time-transgressive change in the lithologic boundary between the generally finer-grained Calvert Formation and the more clastic-rich Choptank Formation.
In Figure 3 diatom datum levels are combined with strontium isotope stratigraphy (updated here) to suggest an age versus depth plot for the Bethany Beach corehole. We updated the detailed Sr-isotopic ages of Browning et al. (2006) to the Gradstein et al. (2004) time scale (see Browning et al., 2013, for discussion). The interval between ca. 440 m, the lowest sample containing diatoms, and ca. 274 m appears to have accumulated between ca. 20.2 and 17.5 Ma, implying an average sedimentation rate of ∼61 m/m.y. Although McLaughlin et al. (2008) suggested the possibility of an unconformity at ca. 351 m corresponding with an upsection shift from coarser to finer grained sediments, the updated strontium and limited diatom data do not reveal a hiatus in deposition. Up section, McLaughlin et al. (2008) identified a heavily burrowed surface upsection at 273.6 m that appears to be an unconformity corresponding with the interval between ca. 17.5 and 16.4 Ma. Above this horizon, the interval up to ca. 218 m coincides with ca. 16.4–15.0 Ma, suggesting a sediment accumulation rate of ca. 36 m/m.y. An unconformity between ca. 218 and 209 m separates ECDZ 4 below from ECDZ 6a above, corresponding to the interval between ca. 15.0 and 13.6 Ma. This unconformity is possibly between a shelly, heavily bioturbated, medium to fine silty sand from 214 to 213 m and a coarsening-upward sequence that begins at 211.6 m (McLaughlin et al., 2008). Above this unconformity, units 4 and 5 of McLaughlin et al. (2008) (ca. 212–185 m) contain diatoms and strontium isotopes ranging in age from ca. 13.6 and 13.0 Ma, implying a sediment accumulation rate of ∼48 m/m.y.
Development of a Diatom Biochronology
Diatom biostratigraphic study of Hole M29A, the BG&E well, and the Bethany Beach corehole has allowed refinement of the ECDZ (see Appendix 2). The construction of age-depth plots for Site M29 and the Bethany Beach corehole as constrained by diatom datum levels and strontium isotope stratigraphy (Figs. 2 and 3) makes it possible to propose a diatom range chart for the East Coast of the United States (Figure 4). As concluded by Abbott (1978, 1980) and Andrews (1988a), the ranges of benthic and shallow shelf-dwelling diatoms (Actinoptychus, Delphineis, Rhaphoneis, Sceptroneis) are most useful for biostratigraphic correlations of onshore strata, and they form the basis for the ECDZ zonations (Appendix 2). However, the ranges of planktonic diatom taxa that have proven to be useful for biochronology in the equatorial and North Pacific (Barron, 1985, 2003; Yanagisawa and Akiba, 1998) in Hole M29A provide a means of correlation (Barron, 2003) with the paleomagnetic time scale of Gradstein et al. (2004). These age assignments are largely supported by strontium isotope stratigraphy in Hole 29A of Site M29 and the Bethany Beach corehole (Figs. 2, 3; Browning et al., 2013). Stratigraphically useful diatoms and silicoflagellates for the ECDZ zonation are illustrated in Plates 1–4.
Correlation of Other IODP 313 Holes and Onshore Sections
Diatom preservation is not as good or consistent at Sites M27 and M28, which contain coarser-grained sediments than Site M29; 34 samples were examined for diatoms from Hole M27A, 23 of which yielded diatoms that were preserved well enough to be assignable to ECDZ 1 (482.34 mcd) to ECDZ 6b (210.535 mcd) (Supplemental Table 11). Of the 44 samples that were examined for diatoms between cores 7 and 171 of Hole M28A, only 23 could be assigned to ECDZ zones (Supplemental Table 22), ranging from ECDZ 1 at 668.65 mcd to ECDZ 6b 248.84 mcd. Initial reconnaissance study of samples with finer-grained lithologies was supplemented by the study of additional samples that were chosen with the aim of refining zonal boundaries.
The revised ECDZ zones can also be applied to other onshore cores that were previously studied for microfossil biostratigraphy and strontium isotope stratigraphy (Fig. 1). With this in mind, 17, 13, and 20 samples were examined for diatoms, respectively, from the Cape May (Miller et al., 1996), Cape May Zoo (Sugarman et al., 2007), and Ocean View (Miller et al., 2001) cores, respectively (Supplemental Table 33). Table 5 provides a summary of the ECDZ biostratigraphy of Holes M27A, M28A, and M29A, the Bethany Beach corehole, the BG&E well, and cores from Cape May, Cape May Zoo, and Ocean View Site.
The Lower and Middle Miocene section cored at Site M29, the outermost of the IODP 313 sites on the shallow New Jersey shelf, contains a succession of planktonic marine diatoms that allows detailed correlation with diatom biochronologies developed in the equatorial and North Pacific. Coupled with the regular occurrence of shallow water diatoms of the genera Delphineis, Rhaphoneis, Sceptroneis, and Actinoptychus that have been used for correlation of onshore strata in Maryland, Virginia, and New Jersey, detailed study of the diatom biostratigraphy of Site M29 makes possible the refinement of East Coast Diatom Zones (ECDZ). Detailed study of key onshore reference sections, the BG&E well in Maryland and the Bethany Beach corehole in Delaware, is used for further refinement of this diatom biostratigraphy for the interval from ca. 20–13 Ma. Regionally along the New Jersey to Virginia coastal margin of the United States, diatoms are most common in sediments younger than ca. 20 Ma and older than ca. 13 Ma. Age-depth models for Site M29 and the Bethany Beach corehole are constructed and compared with strontium isotope stratigraphy, which mostly supports the age assignment of the refined ECDZ diatom biochronology. Hole M29 provides the best opportunity yet to determine the age of the margin-wide sequence boundary m5 (Monteverde et al., 2008). Diatom biostratigraphy suggests this m5 sequence boundary corresponds with an unconformity at Site M29 dated at ∼14.6–13.8 Ma. A correlative unconformity in the Bethany Beach corehole is dated by diatoms at ∼15.0–13.6 Ma.
Holly Olson of the US Geological Survey prepared excellent microscope slides. We thank Jean DeMouthe, collections manager of the diatom collection at the California Academy of Sciences, for the loan of William Abbott’s microscope slides from the BG&E (Baltimore Gas and Electric Company) well. Lucy Edwards and David Bukry of the US Geological Survey provided helpful comments and advice throughout this study. Funding was supplied by the Consortium of Ocean Leadership/U.S. Science Support Program, and samples were provided by the Integrated Ocean Drilling Program and the International Continental Scientific Drilling Program. We thank Scott W. Starratt and two anonymous reviewers for their reviews, as well as Greg Mountain (Associate Editor) and Carol Frost (Science Editor) for their comments.
APPENDIX 1. TAXA DISCUSSED–REFERENCE TO SYNONYMY AND FIGURES
Actinocyclus divisus (Grunow) Hustedt, as Coscinodiscus divisus Grunow (Abbott and Andrews, 1979, Plate 2, fig. 13; as Coscinodiscus curvatulus Grunow, Andrews and Abbott, 1985, Plate 7, fig. 9; Coscinodiscus rothii sensu Andrews, 1976, Plate 3, figs. 1, 2.
Azpeitia bukryi (Barron) Barron, Barron, 2006, Plate 8, fig. 4.
Azpeitia sp. cf. A. nodulifera (Schmidt) G. Fryxell & P.A. Sims, compare Coscinodiscus hirosakiensis Kanaya sensu Abbott and Andrews, 1979, Plate 2, fig. 15.
Azpeitia salisburyana (Lohman) P.A. Sims in Fryxell, Sims & Watkins, Barron, 2006, Plate 1, fig. 2; as Coscinodiscus salisburyanus Lohman, Lohman, 1974, Plate 2, figs. 5, 7; compare Coscinodiscus curvatulus sensu Abbott and Andrews, 1979, Plate 4, fig. 10. (Plate 2, image 6 herein)
Azpeitia vetustissima (Pantocsek) P.A. Sims in Fryxell, Sims & Watkins, as Coscinodiscus vetustissimus Pantocesek sensu Andrews, 1976, Plate 3, fig. 3.
Coscinodiscus gigas var. diorama (Schmidt) Grunow, Abbott and Andrews, 1979, Plate 2, fig. 14; Abbott, 1980, Plate 2, fig. 10; Barron, 1985, Plate 9, fig. 6; –compare C. apiculatus Ehrenberg, sensu Andrews, 1976, Plate 2, fig. 3
Craspedodiscus barronii Bukry, Barron, 2006, Plate 1, figs. 6a, 6b.
Craspedodiscus elegans Ehrenberg, Barron, 2006, Plate 1, fig. 7.
Crucidenticula paranicobarcia Akiba & Yanagisawa, Yanagisawa and Akiba, 1990, Plate 1, figs. 13–16. Note: First recorded occurrence in East Coast Miocene sections.
Delphineis penelliptica Andrews, Andrews and Abbott, 1985, Plate 8, figs 11–13; Andrews, 1988a, Plate 2, figs. 17–19. (Plate 3, images 3–5 herein) Note: ranges younger than recorded by Andrews, 1988a.
Denticulopsis hyalina (Schrader) Simonsen, Yanagisawa and Akiba, 1990, Plate 2, figs. 14, 33, 34; Plate 9, figs. 8, 9. (Plate 1, image 5 herein) Note: First recorded occurrence in East Coast Miocene sections.
Denticulopsis simonsenii Yanagisawa and Akiba, 1990, Plate 3, figs. 1–3; Plate 11, figs. 1, 5; as Denticulopsis hustedtii (Simonsen & Kanaya) Simonsen, Abbott and Andrews, 1979, Plate 4, fig. 4; Abbott, 1980, Plate 1, fig. 16; Andrews, 1988a, Plate 2, figs. 21–24. (Plate 1, images 6, 10, 11 herein)
Fragilariopsis maleinterpretaria (Schrader) Censarek & Gersonde, as Nitzschia maleinterpretaria Schrader, Barron, 2006, Plate 9, figs. 20, 21. Note: First recorded occurrence in East Coast Miocene sections.
Paralia sulcata (Ehrenberg) Cleve, Andrews, 1976, Plate 1, figs. 5, 6.
Rhaphoneis cf. adamantea Andrews, 1988a, p. 2, figs. 27–29, Plate 6, fig. 14.
Rhaphoneis fossile (Grunow) Andrews, Abbott, 1978, Plate 2, fig. 7; Andrews, 1988a, Plate 4, figs. 4–6; Wetmore and Andrews, 1990, Plate 1, fig. 8; as Dimerogramma fossile Grunow sensu Schrader and Fenner, 1976, Plate 5, figs. 12, 13, 22. (Plate 3, image 21; Plate 4, images 14, 15 herein)
Rhizosolenia norwegica Schrader in Schrader & Fenner, 1976, p. 996, Plate 9, figs. 4, 10.
Rouxia californica M. Peragallo in Tempere and Peragallo, Schrader, 1973, Plate 3, figs. 18–20, 22?, 26.
Sceptroneis grandis Abbott, Andrews, 1988a, Plate 4, figs. 33, 34.
Sceptroneis ossiformis Schrader in Schrader and Fenner, 1976, p. 998, Plate 2, figs. 14–17.
Thalassiosira grunowii Akiba & Yanagisawa, Andrews, 1988a, Plate 1, fig. 6?, 7; as Coscinodiscus plicatus Grunow, Abbott, 1978, Plate 1, fig. 1; Abbott, 1980, Plate 1, fig. 4; Abbott and Ernisee, 1983, Plate 4, fig. 1.
Thalassiosira irregulata Schrader in Schrader & Fenner, 1976, Plate 20, figs. 10–12.
Thalassiosira perispinosa Tanimura, 1996, p.181, Figs. 35–39, as Coscinodiscus lacustris Grunow sensu Abbott and Andrews, 1979, Plate 2, fig. 17; Andrews and Abbott, 1985, Plate 7, fig. 10. (Plate 2, image 2 herein)
Trinacria solnoceros (Ehrenberg) VanLandingham; Andrews, 1988a, Plate 8, figs. 6–8
Naviculopsis biapiculata (Lemmermann) Frenguelli, Perch-Nielsen, 1985, Plate 25, figs. 3, 4.
Naviculopsis lata (Deflandre) Frenguelli, Perch-Nielsen, 1985, Plate 25, fig. 21.
Naviculopsis navicula (Ehrenberg), Deflandre Wetmore and Andrews, 1990, Plate 1, fig. 3.
Naviculopsis ponticula (Ehrenberg) Bukry, Wetmore and Andrews, 1990, Plate 1, fig. 2.
Naviculopsis quadrata (Ehrenberg) Locker, Wetmore and Andrews, 1990, Plate 1, fig. 1.
APPENDIX 2. PROPOSED CHANGES IN EAST COAST DIATOM ZONES
ECDZ 1: Actinoptychus heliopelta Assemblage Zone
Andrews (1988a) defined the base of ECDZ 1 by the first occurrence of A. heliopelta and its top by the last occurrence of A. heliopelta. Current studies reveal that A. heliopelta, a rare, large diatom that is commonly fragmented, is commonly reworked into overlying ECDZ 2. It is proposed here that the first occurrence of Delphineis ovata be used to recognize the top of ECDZ 1. Therefore, ECDZ 1 can be recognized by the presence of A. heliopelta and the absence of D. ovata. The last occurrence of Rhaphoneis fossile also approximates the top of ECDZ 1. Two new subzones are proposed—the first occurrence of Delphineis lineata defines the top of subzone a and the base of overlying Subzone b.
ECDZ 2: Delphineis ovata Partial Range Zone
Andrews (1988a) defined the base of ECDZ 2 by the first occurrence of D. ovata and its top by the last occurrence of D. ovata. Abbott’s (1978) proposal to use the first occurrence of Delphineis penelliptica to define the top of ECDZ 2, however, is accepted here. ECDZ 2 thus becomes the Delphineis ovata Partial Range Zone (Abbott, 1978). Notes: D. ovata ranges slightly below the first occurrence of Crucidenticula sawamurae in Integrated Ocean Drilling Program Expedition 313 Site M29 and the Baltimore Gas and Electric Company (BG&E) well (Tables 1 and 3). In the Bethany Beach corehole, C. sawamurae is not present, but the first D. ovata is easily recognizable (Table 4).
Andrews (1988a) proposed the Rhaphoneis magnapuncta Range Zone to represent a combined ECDZ 3-4 interval. This zone was defined as the interval between first occurrence of R. magnapunctata and the last occurrence of R. magnapunctata.
ECDZ 3: Delphineis ovata-D. penelliptica Concurrent Range Zone
It is proposed here that ECDZ 3 be redefined to be equivalent to Abbott’s (1978) Delphineis ovata/D. penelliptica Concurrent Range Zone. This means that the base of ECDZ 3 is defined by first occurrence of Delphineis penelliptica and its top is the last occurrence of Delphineis ovata.
ECDZ 4: Delphineis penelliptica Partial Range Zone
The base of ECDZ 4 is defined by the last occurrence of D. ovata. However, rather than following Abbott (1978) in using the first occurrence of Coscinodiscus plicatus (Thalassiosira grunowii) to define the top of this zone, the top of ECDZ 4 and the base of overlying ECDZ 5 are redefined here to coincide with the first occurrence of D. novaecaesaraea, in keeping with Andrews’ (1977, 1988b) recognition of the stratigraphic usefulness of the evolutionary succession of Delphineis species.
ECDZ 5: Delphineis novaecaesaraea Partial Range Zone
Andrews (1988a) defined the base of ECDZ 5 as the last occurrence of Rhaphoneis magnapunctata and its top by the first occurrence of R. clavata. These large benthic diatoms are often rare and fragmented in diatom assemblages. It is proposed here that ECDZ 5 be redefined to be the interval from the first occurrence of D. novaecaesaraea to the first occurrence of Denticulopsis simonsenii (=D. hustedtii of Andrews, 1988a). ECDZ 5 therefore becomes the Delphineis novaecaesaraea Partial Range Zone.
ECDZ 6: Denticulopsis simonsenii Partial Range Zone
Andrews (1988a) defined the base of ECDZ 6 by first occurrence of Rhaphoneis clavata and its top by the last occurrence of R. gemmifera. It is proposed that the first occurrence of Denticulopsis simonsenii be used to define the base of ECDZ 6 and the first occurrence of Rhaphoneis diamantella be used to define its top. The first occurrence of Delphineis biseriata is proposed here to define the base of new subzone b and the top of new subzone a. Note: Figure 2 of Andrews (1988a) suggests that the first occurrence of Denticulopsis hustedtii (=D. simonsenii) occurs just above the base of his ECDZ 6. Andrew’s Figure 2 also shows that the first occurrence of Rhaphoneis diamantella coincides with the top of ECDZ 6.
Abbott (1978) proposed the Coscinodiscus plicatus Partial Range Zone as the interval coinciding with the range of C. plicatus (Thalassiosira grunowii) above the last Delphineis penelliptica; however, this biostratigraphic zone does not seem practical based on the long range of D. penelliptica in Hole M29 (Table 1). It is possible that D. penelliptica disappears earlier in more nearshore sections, such as the BG&E well (Table 3), because it is restricted by falling sea level.
ECDZ 7: Rhaphoneis diamantella Partial Range Zone
Andrews (1988a) defined the base of his ECDZ 7 by the last occurrence of Rhaphoneis gemmifera and its top by the last occurrence of R. diamantella.
The first occurrence of R. diamantella is proposed here to define the base of ECDZ 7. Increasing clastic debris, probably due to falling sea level, upsection characterizes ECDZ 7. Therefore, the top of ECDZ is not defined.
↵1Supplemental Table 1. Samples studied for diatoms from Hole M27A. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00864.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 1.
↵2Supplemental Table 2. Samples studied for diatoms from Hole M28A. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00864.S2 or the full-text article on www.gsapubs.org to view Supplemental Table 2.
↵3Supplemental Table 3. Samples studied for diatoms from the Cape May, Cape May Zoo, and Ocean View wells. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00864.S3 or the full-text article on www.gsapubs.org to view Supplemental Table 3.
- Received 12 September 2012.
- Revision received 8 July 2013.
- Accepted 24 July 2013.
- © 2013 Geological Society of America