Micropaleontologic record of late Pliocene and Quaternary paleoenvironments in the northern Albemarle Embayment, North Carolina, U.S.A.

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Abstract

Micropaleontological data provide a strong actualistic basis for detailed interpretations of Quaternary paleoenvironmental change. The 90 m-thick Quaternary record of the Albemarle Embayment in the mid-Atlantic coastal plain of the USA provides an excellent opportunity to use such an approach in a region where the details of Quaternary environmental change are poorly known.

The foraminiferal record in nine cores from the northern Outer Banks, east of Albemarle Sound, North Carolina, indicates the deposition of subhorizontal, mostly open-marine early to late Pleistocene units unconformably upon a basement of late Pliocene reduced-oxygen, fine-grained, shelf-basin deposits. Pollen data record several warm–cool fluctuations within the early to mid-Pleistocene deposits. Diatom data indicate that some fresh and brackish-water units occur within the generally open-marine Pleistocene succession.

A channel cut by the paleo-Roanoke River during the last glacial sea-level lowstand occurs in the northern part of the study area. Pollen indicates that the basal fluvial valley fill accumulated in cooler than modern climate conditions in the latest Pleistocene. Overlying silts and muds accumulated under cool climatic, estuarine conditions according to diatom and pollen data. Radiocarbon ages from the estuarine deposits indicate that the bulk of these sediments accumulated during the latest Pleistocene.

The estuarine channel-fill deposits are overlain by Holocene open-marine sands deposited as the rising sea transgressed into the estuary approximately 8.5 to 9.0 kyr BP. Within the barrier island drill cores of this study, fully marine sedimentation occurred throughout the Holocene. However, immediately west of the present barrier island, mid- to late Holocene estuarine deposits underlie the modern Albemarle Sound. The islands that currently form a continuous barrier across the mouth of Albemarle Sound have a complex history of Holocene construction and destruction and large portions of them may be less than 3 kyr old. The barrier island sands overlie open-marine sands of Colington Shoal in the north and to the south overlie fluvial and marine sand filling paleo-Roanoke tributary valleys.

The Pleistocene sediments underlying the northern Outer Banks study area are mainly of open inner to mid-shelf origin. If, as is likely, sea level continues to rise, a return to such environmental conditions is likely in the near future.

Introduction

Northeastern North Carolina's (NC) modern coastal system of rivers, estuaries, bays, lagoons, swamps, marshes, barrier islands, inlets and beaches (Fig. 1A) evolved to its current configuration during the past few thousand years as post-glacial sea-level rise reached within a few meters of its current elevation (e.g., Fairbanks, 1989, Bard et al., 1996). The earlier Quaternary history of this region, the northern Albemarle Embayment, is poorly known but the stratigraphic record here is among the thickest and most complete Quaternary sequences on the North American Atlantic margin (Riggs et al., 1992).

The purpose of this paper is to present micropaleontological data (foraminifera, diatoms and pollen) from nine deep coreholes drilled through the modern northern Outer Banks barrier island (Fig. 1B), to infer paleoclimatic conditions, and to reconstruct late Pliocene and Quaternary environments of deposition that can be incorporated into the seismic–stratigraphic framework developed for this region by Riggs et al. (1992), Sager and Riggs (1998), Boss et al. (2002), and Mallinson et al. (2005). The goal is then to reconstruct the chronology of late Pliocene and Quaternary paleoenvironmental changes represented in the sedimentary sequences in the northern Albemarle Embayment. The paleoenvironments represented in many of these sequences are the result of interglacial highstands and thus provide an understanding of the environmental shifts that will take place in the near future if sea level continues to rise.

Section snippets

Regional setting

The Albemarle Embayment is a Cenozoic regional depositional basin bounded to the north by the Norfolk Arch and to the south by the Cape Lookout High (Ward and Strickland, 1985, Ward et al., 1991). Within this basin Quaternary deposits are approximately 90 m thick but, in many places, Pleistocene sediments lie within a few meters of the current land surface (Popenoe and Ward, 1983, Riggs et al., 1992). As a result, the Holocene section is generally thin, thickening only in paleo-valleys and

Materials and methods

Nine cores (OBX-01 to OBX-09) were taken on the Outer Banks (Fig. 1B; Table 1) by rotosonic drilling. Core sites were selected based on geophysical data to further delineate the paleo-Roanoke valley and to penetrate the entire Quaternary stratigraphic section (Riggs et al., 1992, Sager and Riggs, 1998, Boss and Hoffman, 2001, Boss et al., 2002, Mallinson et al., 2005). Cores were logged and sampled for grain-size analysis, radiocarbon and amino acid racemization age determination, stable

Results and paleoenvironmental interpretations

Foraminiferal data are the primary basis for paleoenvironmental interpretations in this paper. In this section, the general patterns of foraminifera are described followed by the results of the cluster analysis of these data. The foraminifera, diatom, pollen, 14C and AAR data are then combined to describe paleoenvironmental change through time for each core. Cores are described and illustrated from NNW to SSE (OBX-02, OBX-03, OBX-06, OBX-05, OBX-01, OBX-04, OBX-07, OBX-09, OBX-08) but note that

Pliocene to late Pleistocene

The oldest strata penetrated in the OBX cores occur at the base of OBX-07 (Fig. 13A; Table 5). Planktonic foraminifera and pollen data suggest a likely late Pliocene (Yorktown Formation equivalent) age for this unit. The benthic foraminiferal assemblage (Bm) (Fig. 13B) indicates reduced-oxygen, open shelf conditions, and the presence of several percent planktonic foraminifera (Fig. 9) suggests mid-shelf depths. The pollen assemblage indicates warm conditions similar to those of today (Fig. 13B;

Conclusions

Foraminiferal, diatom and pollen data have proven to be valuable and, indeed, necessary tools for reconstructing paleoenvironmental change in the late Pliocene to Holocene sedimentary record preserved in the northern Albemarle Embayment of North Carolina. When placed in a temporal framework provided by 14C, AAR age estimates, planktonic foraminiferal biostratigraphic and pollen-derived climatic signals, we see that the preserved late Pliocene to Pleistocene record is dominated by predominantly

Acknowledgements

We thank R. Brooks, C.W. Hoffman, D. Merritt, D. Vance, I. Abbene, C. Smith, M. Dail, B. Landacre, C. Grand Pre, J. Smith, M. Robertson, J. Roberts and T. Sheehan for their help and support. M.A. Buzas and J. Jett kindly provided access to the Cushman Collection of foraminifera, Smithsonian Institution, J. Whittaker to the foraminiferal collections in The Natural History Museum, London. The rotosonic cores were drilled under the supervision of C.W. Hoffman (NC Geological Survey). We thank Tom

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