Laurel M. Bybell

Closure of the Isthmus of Panama: The near-shore marine record of Costa Rica and western Panama

The final closure of the Isthmus of Panama at ∼3.5 Ma divided the American tropical ocean into two separate and different oceanographic regions. Consequences for the marine biota were profound, but, hitherto, correlation of the Pacific and Caribbean coastal sections has not been precise enough to track biologic patterns. We present here a correlation of 31 sections from the Pacific and Caribbean coasts of Costa Rica and western Panama. Using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation, we estimate that the Caribbean section ranges from 8.2 Ma to 1.7 Ma; and the Pacific sequence, from 3.6 Ma to <1.7 Ma. These intervals bracket postulated dates for final closure of the Isthmus and provide the first well-dated record of middle and late Pliocene faunas from the region. The Caribbean and Pacific sections include very different environments of deposition, yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons. On the Caribbean side, formations tied together by the overlap of the upper Pliocene markers Sphenolithus abies and Pseudoemiliana lacunosa (3.5 Ma to 3.6 Ma) range from very shallow to shallow inner shelf (<200 m) and upper slope (200-800 m). The Pacific coast sections were mostly deposited in a trench slope environment, which is absent on the Caribbean side. These sections fortuitously include abundant thick intra-formational slumps containing shallow-water fauna more appropriate for biological comparison with the Caribbean biota. Similarly, the ∼1.9 Ma to 1.5 Ma interval, well constrained by various taxa, includes middle- to outer-shelf, and inner-shelf to upper-slope deposits on the Caribbean side, and marginal-marine to inner-shelf deposits on the Pacific coast. Using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections, we show that evolutionary divergence of the Pacific and Caribbean near-shore marine faunas had occurred by 3.5 Ma. This strongly suggests that the Isthmus was effectively closed by this time.

Shelf and open-ocean calcareous phytoplankton assemblages across the Paleocene-Eocene Thermal Maximum: Implications for global productivity gradients

Abrupt global warming and profound perturbation of the carbon cycle during the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma) have been linked to a massive release of carbon into the ocean-atmosphere system. Increased phytoplankton productivity has been invoked to cause subsequent CO2 drawdown, cooling, and environmental recovery. However, interpretations of geochemical and biotic data differ on when and where this increased productivity occurred. Here we present high-resolution nannofossil assemblage data from a shelf section (the U.S. Geological Survey [USGS] drill hole at Wilson Lake, New Jersey) and an open-ocean location (Ocean Drilling Program [ODP] Site 1209, paleoequatorial Pacific). These data combined with published biotic records indicate a transient steepening of shelf-offshelf trophic gradients across the PETM onset and peak, with a decrease in open-ocean productivity coeval with increased nutrient availability in shelf areas. Productivity levels recovered in the open ocean during the later stages of the event, which, coupled with intensified continental weathering rates, may have played an important role in carbon sequestration and CO2 drawdown.

Latest Paleocene lithologic and biotic events in neritic deposits of southwestern New Jersey

In the southwestern New Jersey Coastal Plain, four drill holes contain continuous neritic sedimentation across the Paleocene/Eocene boundary (calcareous nannofossil Zone NP 9/NP 10 boundary). Significant lithologic and biotic changes occur in these strata near the top of the Paleocene. Global warming, increased precipitation, and other oceanographic and climatic events that have been recognized in high-latitude, deep-oceanic deposits of the latest Paleocene also influenced mid-latitude, shallow-marine, and terrestrial environments of the western North Atlantic. The diverse, well-preserved calcareous nannofossil flora that is present throughout the entire New Jersey boundary section accurately places these events within the uppermost part of the upper Paleocene Zone NP 9. Several rapid but gradational changes occur within a 1.1-m interval near the top of Zone NP 9. The changes include (1) a change in lithology from glauconitic quartz sand to clay, (2) a change in clay mineral suites from illite/smectite-dominated to kaolinite-dominated, (3) a change in benthic foraminiferal assemblages to a lower diversity fauna suggestive of low-oxygen environments, (4) a significant increase in planktonic foraminiferal abundance, and (5) an increased species turnover rate in marine calcareous nannofossils. Pollen was sparse in the New Jersey drill holes, but terrestrial sporomorph species in Virginia exhibit increased turnover rates at a correlative level. Foraminiferal assemblages and lithology indicate that relative sea level rose in New Jersey at the same time as these late Paleocene events occurred in late Biochron NP 9. The higher sea levels influenced sediment type and absolute abundance of planktonic foraminifers in the deposits. Above the initial increase of kaolinite in the upper part of Zone NP 9, the kaolinite percentage continues to increase, and the maximum kaolinite value occurs in the uppermost part of Zone NP 9. There are few changes in either the sediments or the biota precisely at the Zone NP 9/NP 10 boundary in New Jersey. The clay-rich deposits with a high kaolinite clay mineral suite, the lowered diversity benthic foraminiferal assemblages, the abundant planktonic foraminiferal specimens, and the calcareous nannofossil assemblages continued essentially unchanged into the earliest Eocene Zone NP 10. Within the lower part of Zone NP 10, the kaolinite percentage decreased to very low values.

Eocene-Oligocene sea-level changes on the New Jersey coastal plain linked to the deep-sea record

We use magnetostratigraphy and Sr-isotope stratigraphy to improve stratigraphic control for the Eocene to Oligocene of the New Jersey coastal plain (ACGS4 borehole). Magnetostratigraphy in many cases is complicated in outcrop sections of shallow-water (

Age and correlation of emerged pliocene and pleistocene deposits, U.S. Atlantic Coastal Plain

Abstract Paleontologic and paleomagnetic investigations were conducted on several hundred Pliocene and Pleistocene marine samples from five regions of the emerged Atlantic Coastal Plain: (1) the Delmarva Peninsula, (2) eastern Virginia, (3) central and northern North Carolina, (4) southern North Carolina and northeastern South Carolina, and (5) the Charleston area, South Carolina. Molluscan and ostracode interval and assemblage zonations, which are the primary means of regional correlation, have been calibrated using planktic biochronologic, paleomagnetic, radiometric and amino-acid recemization data. These multiple dating criteria were used to determine the age and, where possible, the duration of marine transgressive/regressive sequences. A correlation chart illustrates the age relationships of 27 formations from five regions. One important conclusion is some of the Yorktown Formation of Virginia and North Carolina (including the “Duplin” Formation), and some of the Raysor of South Carolina are late Pliocene in age. The late Pliocene Chowan River Formation of North Carolina is older than the early Pleistocene Waccamaw Formation of South Carolina, which in turn may be older than the James City Formation of North Carolina. During the last 1.0 million years, multiple marine transgressions occurred in each region, but the age of these middle and late Pleistocene formations often may differ from one area to the next. A significant result of the study is the evidence for the lack of time equivalence of formations in the five different regions; that is, the sequence of marine transgressions in one region does not necessarily correspond to that in another. This appears to be the result of differing subsidence and uplift histories, the patchiness of the depositional record, and the limitations of the dating techniques in light of the rapidity and frequency of sea-level fluctuations.

Late Eocene to Early Oligocene Calcareous Nannofossils in Alabama and Mississippi: ABSTRACT

ABSTRACT The Eocene-Oligocene boundary in the central Gulf Coastal Plain has been placed traditionally at the contact of the Shubuta Member of the Yazoo Formation and the Red Bluff Formation, or the contact of the Shubuta and the facies equivalents with the Bumpnose Formation. Calcareous nannofossils were examined from six upper Eocene to lower Oligocene localities in Alabama and Mississippi. The Shubuta, Red Bluff and equivalents have a very similar calcareous nannofossil flora, and both are in Martinis Zone NP21. However, from the base of the Shubuta up through the Red Bluff, 10 calcareous nannofossil extinction horizons can be used to subdivide the lower part of Zone NP21. Discoaster saipanensis Bramlette and Riedel, D. barbadiensis Tan Sin Hok, and Reticulofenestra reticulata (Gartner and Smith) (which become extinct at or near the top of Zone NP20) are only rarely present in the 27 Shubuta samples examined, are poorly preserved and are assumed to have been reworked. Below the Shubuta lies the Pachuta Marl Member of the Yazoo, which was examined at one locality in Mississippi and two in Alabama, and although the flora is poorly preserved, it contains significant numbers of all three Eocene species. If the Eocene-Oligocene boundary is assumed to correspond to the Shubuta-Red Bluff contact, this boundary, at least in the Gulf Coastal Plain, cannot be recognized using traditional calcareous nannofossil markers, because of its inclusion within Zone NP21. This contact, however, appears to coincide with the last occurrence of the planktonic foraminifer Globorotalia cerroazulensis s.l.; the extinction of Hantkenina spp. may occur slightly below the contact. The extinction of Discoaster saipanensis below that of the planktonic foraminifers has also been observed on several legs of the Deep Sea Drilling Project, where this offset is no more than a few meters. At the Red Bluff type locality, the separation approximates 20 m. Clearly, in the study area, the extinctions of G. cerroazulensis and D. saipanensis do not define the same horizon.

Remarks on the calcareous nannofossil markers Rhomboaster and Tribrachiatus around the Paleocene/Eocene boundary

During the search for the Paleocene/ Eocene (P/E) boundary-GSSP, confusion has evolved concerning the events characterising the P/E boundary in terms of calcareous nannofossil biostratigraphy. Authors have variously reported the boundary as occurring in NP9 or NP10. This is essentially based on different morphologic interpretations of the nannoliths of the genera Rhomboaster and Tribrachiatus. It is in the interest of the scientific community that calcareous nannofossil specialists agree on a common interpretation of similar observations. Subdivision of the P/E boundary interval by calcareous nannofossils is based on a lineage of nannofossils in which the more or less rhombohedral Paleocene genus Rhomboaster evolved into the double-triradiate, Eocene genus Tribrachiatus. The first occurrence (FO) of R. bramlettei was chosen by Martini (1971) as defining the base of the calcareous nannofossil Zone NP10, which often was used as an approximation of the P/E boundary. The identity and systematic position of the marker species has been discussed by various authors and their different viewpoints have led to different interpretations of the same data, leading to confusion in the scientific community and even to the premature conclusion that calcareous nan-nofossil biostratigraphy is of little use in the P/E boundary interval. After the descriptions of the holotypes mainly in the 1960s and 70s (Fig. 1), attempts at synthesising observations have been made by producing line drawings by Romein (1979). Perch-Nielsen et al. (1978), Perch-Nielsen (1985), Angori & Monechi (1996), Bybell & Self-Trail (1995, 1997), and Wei & Zhong (1996) illustrated specimens in different orientations. Bybell & Self-Trail (1995, 1997) and Wei & Zhong (1996) produced and illustrated clay models while Aubry et al. (this volume) showed models at the conference in Göteborg. For our study 3-D computer models were developed (Fig. 2). These models were constructed using a simple geometric model in order to test the hypothesis that the range of morphotypes from a simple rhomb through R. cuspis to R. bramlettei and R. calcitrapa could be developed simply by altering a single parameter, spine length. The models as shown in Fig. 2 were developed on this basis (except the T. contortus model where the interlayer angle was changed). We believe these models are realistic representations of the observed morphologies and conclude that there is no significant difference in structure within the cuspis– bramlettei–calcitrapa continuum and that they should all be included in the same genus. The following morphotypes have been distinguished (Fig. 1): R. cuspis: “Specimens with edges indicating a rhombohedral symmetry, but with faces depressed, usually strongly concave and cuspate corners extented like spines” (Bramlette & Sullivan 1961, original description). R. bramlettei (including R. calcitrapa and R. bitrifida, both of which feature longer free rays than R. cuspis): Body, a deformed rhombohedron, with 6 (long) rays distributed on two levels with 3 rays each. 60° between all rays in top view and two “flattened corners of a rhombohedron”.

Physical stratigraphy, paleontology, and magnetostratigraphy of the USGS-Santee Coastal Reserve core (CHN-803), Charleston County, South Carolina

4 Introduction 4 Acknowledgments 6 Unit conversions 6 Methods 6 Physical stratigraphy and lithology 6 Paleontology 6 Calcareous nannofossils 6 Palynology 6 Foraminifera 7 Strontium-isotope measurements 7 Paleomagnetic measurements 7 Results and stratigraphic discussions 7 Stratigraphy 7 Paleontology 11 Strontium-isotope results 14 Paleomagnetic results 14 Donoho Creek Formation (Black Creek Group) 14 Physical stratigraphy and lithology 14 Paleontology 15 Magnetostratigraphy 15 Peedee Formation 15 Physical stratigraphy and lithology 15 Paleontology 17 Strontium-isotope stratigraphy 21 Magnetostratigraphy 21 Rhems Formation (Black Mingo Group) sensu stricto 21 Physical stratigraphy and lithology 21 Paleontology 23 Magnetostratigraphy 23 Upper part of the Rhems Formation (Black Mingo Group) sensu Bybell and others (1998) 23 Physical stratigraphy and lithology 23 Paleontology 24 Magnetostratigraphy 26 Lower Bridge Member of the Williamsburg Formation (Black Mingo Group) 26 Physical stratigraphy and lithology 26 Lower beds 26 Upper beds 27 Paleontology 27 Magnetostratigraphy 27 Chicora Member of the Williamsburg Formation (Black Mingo Group) 27 Physical stratigraphy and lithology 27 Paleontology 28 Magnetostratigraphy 28 Mollusk-bryozoan limestone 28 Physical stratigraphy and lithology 28 Paleontology 29 Magnetostratigraphy 29 Wando Formation 29 Physical stratigraphy and lithology 29 Paleontology 30

Neogene biostratigraphy and paleoenvironments of Enewetak Atoll, equatorial Pacific Ocean

Abstract Micropaleontologic analyses of Neogene sediments from Enewetak Atoll, Marshall Islands, provide data on the age of lagoonal deposits, stratigraphic disconformities and the paleoenvironmental and subsidence history of the atoll. Benthic foraminifers, planktic foraminifers, calcareous nannofossils and ostracodes were studied from six boreholes, the deepest penetrating 1605 feet below the lagoon floor into upper Oligocene strata. The Oligocene-Miocene boundary occurs at about 1200 ft below the lagoon floor. The early and middle Miocene is characterized by brief periods of deposition and numerous hiatuses. Ostracodes and benthic foraminifers indicate a shallow-marine reefal environment with occasional brackish water conditions. Upper Miocene and lower Pliocene deposits placed in calcareous nannofossil Zones NN9–15 and in planktic foraminifer Zones N16–19 contain species-rich benthic microfaunas which indicate alternating reefal and brackish water mangrove environments. The upper Pliocene contains at least two major depositional hiatuses that coincide with a major faunal turnover in benthic foraminiferal and ostracode assemblages. The Quaternary is characterized by benthic microfaunas similar to those of modern atoll lagoons and is punctuated by at least 11 disconformities which signify periods of low sea level. Atoll subsidence rates during the last 10 Ma averaged 30 to 40 m/m.y.

Age of the Comfort Member of the Castle Hayne Formation, North Carolina

The biostratigraphic and chronostratigraphic position of the Comfort Member of the Castle Hayne Formation has been the subject of much debate. At the Martin-Marietta Quarry at Castle Hayne, New Hanover County, North Carolina, the planktic foraminifers indicate an assignment within an interval of the uppermost Turborotalia frontosa Zone to the Turborotalia pomeroli Zone. The calcareous nannofossils indicate an assignment to the middle part of the Chiasmolithus solitus Zone. The dinocyst data indicate placement in the upper part of the Kisselovia coleothrypta Zone of Costa and Downie. These zonal units are considered to be within the middle Eocene of international usage, and, on the basis of the time scale used in this paper, the Chiasmolithus solitus Zone represents a time interval of 42.1 to 45.4 megaannums (Ma). This differs significantly from a Rb/Sr glauconite date of 34.8 ± 1.0 Ma previously obtained at the same locality.