Chapter 4. Stratigraphy and Age of Pocomoke Sound Sediments

Introduction

The stratigraphy and sedimentary history of many regions of the mainstem channel of Chesapeake Bay (Goldberg and others, 1977; Officer and others, 1984; Cooper and Brush, 1991; Cronin and others, 2000, 2003) and marshes around the margins of the bay (Brush, 1984; Kearney, 1996; Brush and Hilgartner, 2000) has been well studied using various types of sediment cores. In contrast, the sedimentary record and depositional and environmental history of Pocomoke Sound is almost completely unstudied. This chapter describes the general stratigraphic framework of Chesapeake Bay based on prior studies and the stratigraphy and age of sediments cored in 2001 in Pocomoke Sound.

Previous Studies of Chesapeake Bay Quaternary Geology

The Neogene and Quaternary history of Chesapeake Bay recently has been discussed by Hobbs (2004) in a review of the literature of both onshore and submarine geological units. Our study concerns only the Holocene sediments from the Pocomoke Sound region, which like other parts of Chesapeake Bay and the Delmarva Peninsula, is the product of late Quaternary sea-level oscillations caused by glacial-interglacial climatic cycles (Shideler and others, 1984; Colman and Hobbs, 1987; Colman and Mixon, 1988). Colman and others (1988) used geophysical and borehole data to reconstruct the development of a series of paleochannels and channel infilling in Chesapeake Bay and the Delmarva Peninsula. They recognized three generations of paleo-channel development, which are referred to as the Exmore, Eastville and Cape Charles paleochannels. These channels formed during glacial periods when global sea level was about 100-120 m below its present level. Estuarine sediment infilled each channel during the subsequent period of interglacial high sea level, and each sequence of paleo-channel fill is separated from the underlying sediments by an erosional unconformity. Recent long sediment coring studies indicate that in some regions, fluvial and marsh sediments underlie estuarine sediments at the base of each channel fill sequence (Cronin, 2000).

The youngest channel to develop, the Cape Charles paleochannel, formed during the low sea level of the last glacial maximum, around 20 ka BP. In the Pocomoke Sound region, geophysical studies suggest that approximately 10 to 25 m of Holocene sediment was deposited above the Cape Charles Erosional Surface (CCES) (Colman and Hobbs, 1987). Based on the well-dated sedimentary record from the mainstem channel of Chesapeake Bay (Cronin, 2000; Colman and others, 2002; Bratton and Colman, 2003), it is presumed that most of the Pocomoke Sound sediments lying above the CCES represent Holocene estuarine sediments deposited during the past 8 ka years, although the lowermost sediments may represent pre-8 ka year fluvial sediments deposited before the final phase of global sea level rise flooded the bay. The current study focuses on the uppermost five meters of late Holocene sequence lying above the unconformity of the CCES.

Pocomoke Sound Core Site Descriptions

Core sites in Pocomoke Sound were selected on the basis of geophysical records of Colman and Hobbs (1987) (Figure 4.1). The coring strategy for May 2001 was designed to take transects of short cores (1-2 m core length) across different parts of the deep Pocomoke Sound channel in order to obtain basic information on sediment characteristics of Pocomoke Sound sediment in shallow and deeper water regions. In transects where three cores could be obtained (i.e., cores PC-2A-C; PC-3A-C, PC-6A-C) the "B" sites in each transect represented locations in the deeper main channel of the sound, the "A" and "C" sites were situated on the adjacent shallow water flanks. The initial results from these cores demonstrated that the deeper channel was the site of fine-grained sediment deposition, and the flanks of the channel consisted mostly of coarse-grained sands. This pattern is similar to that found elsewhere in the mainstem of Chesapeake Bay. On the basis of their physical characteristics, lithology, X-radiographs, micropaleontology, and chronology, two sites, PC-2B and PC-6B, were selected as having the greatest potential to obtain a high-resolution paleoecological and sedimentary record and long cores (~ 5m long) were taken at these sites in September 2001.

Lithological Descriptions of Cores

Lithological descriptions and X-radiographs for seven cores (PC-2B, PC-3A, PC-3B, PC-4B, PC-4C, PC-6A and PC-6B) from the central sound and lithological descriptions for three cores in the northern part of the sound (TR-1D, TR-2D, TR-5D) are provided at this url: http://geology.er.usgs.gov/eespteam/Atlantic/index.htm.

Sediments in cores taken in the Pocomoke Sound channel are similar to those found in the main channel of Chesapeake Bay, consisting predominantly of methane-rich muds, with occasional shells and light-dark laminae (Figures 4.2, 4.3). These lithologies are ideal for detailed paleoecological and chronological study due to the relatively continuous fine-grained sedimentation and common fossilized remains of ecological indicators. In contrast to the fine-grained sediments in the channel, sediments blanketing the flanks of Pocomoke Sound consist mainly of sands typical of shallow regions throughout most of the Chesapeake Bay (Figure 4.4). These probably represent lag deposits in which the fine-grained size fractions have been transported into the deeper channel. Such sandy deposits are difficult to recover using gravity and piston coring methods. Moreover, they usually do not contain material suitable for either radiocarbon dating, (i.e., mollusk shells are broken and transported), or lead-210 dating (little fine-grained material). For these reasons, the shallow sandy facies of Pocomoke Sound were not used for paleoecological analysis.

Sediments in cores PC-4B and PC-4C exhibit notable differences from those found in other channel cores. Core PC-4C, recovered in 7.3 m water depth, is predominantly light gray mud, interspersed with concentrations of black organics, from the bottom of the core at 100 cm up to 25 cm (Figure 4.5). From 25 cm to 10 cm, a sandy, pebbly facies was deposited, and a large mollusk was present at 16 cm. Highly bioturbated silty-sandy muds were deposited in the upper 10 cm of the core. Core PC-4B was collected in 27.3 m of water but exhibits fairly unusual sediments for a low energy channel-type environment. From the bottom of the core at 118 cm up to a possibly planar unconformity at 75 cm, the core exhibits alternating packages of pebble beds overlain by light gray non-planar-wavy mud and sand laminations overlain by thick brown organics. These packages become increasingly more non-planar with depth. From 75 cm to 25 cm, poorly sorted mud and sand are interlaminated with mm to cm-scale gravel beds. In the upper 25 cm, organic mats are scattered throughout the dominant mud lithology. A bryozoan mat caps the upper 2 cm. Due to the complexity of the sedimentary history at these sites only the pollen was studied in any detail from these sites.

A final group of cores were taken in the northern region of Pocomoke Sound (Cores TR-1, TR-2, and TR-5), where sediments typically consist of coarse-grained sediment with abundant plant matter, especially in the upper tens of centimeters.

Radiocarbon Dates and Pollen Biostratigraphy

Core PC-6B-2

Seven radiocarbon dates were obtained from this 476 cm long piston core using an accelerator mass spectrometer (AMS); six dates were obtained on shells, and one was from a bryozoan (Table 4.1). Ages given below are calendar years (cal yrBP) obtained by converting radiocarbon ages into calendar years using the calibration program CALIB 4.4 (Stuiver and others, 1998). Four dates collected in the upper 205 cm were too young to calibrate and they probably represent sediments deposited during the last century. The other three dates from samples collected between 430 cm and 458 cm each indicate ages between 460-480 cal yrBP.

The radiocarbon dates are consistent with pollen biostratigraphic data showing the initial Colonial land clearance during the 17th century marked by the first increase in Ambrosia pollen >1% at 340 cm. This marker horizon is stratigraphically above the oldest radiocarbon dates (~460-480 yr BP, see Willard and others, this volume). The peak in Ambrosia pollen, which has been correlated to large-scale land clearance between ~1880 and 1910 AD, occurs at 280 cm in core PC-6B. This marker horizon lies stratigraphically below the young radiocarbon dates from material in the upper two meters. The estimated age based on pollen is also within the age uncertainty from the ²¹⁰Pb-based model for this same core (~ 1900 AD at 225 cm, see Holmes and Marot, this volume).

Core TR-2-D

One radiocarbon date was obtained on bivalves and oysters from 66-68 cm depth in this core, yielding an age of 520 cal yrBP. The extremely high abundance of Pinus pollen throughout the 72 cm length of this core indicates that the entire thickness was deposited within the last century. The apparent contradiction of pollen biostratigraphy and radiocarbon dating merits further examination.

Core TR-1-D

One radiocarbon date was obtained on an oyster fragment from 30-34 cm depth in core TR-1-D, which yielded an age too young too calibrate. This is consistent with pollen evidence, which indicates that entire 92-cm long core was deposited within the last century.

Site PC-2B

Several cores were obtained at this site, and radiocarbon dates were obtained from two cores (Table 4.1). In core PC-2B-1, bivalve fragments and gastropods from 196-198 cm yielded conventional radiocarbon dates of 108.4 pMC (percent modern carbon), indicating that they were deposited after ~1950. Likewise, in core PC-2B-3, Mulinia shells from 219 cm depth yielded a conventional radiocarbon age of 107.2 pMC, also suggesting a post-1950 AD age. Three other radiocarbon dates were obtained from the 450-cm long PC-2B-3 core, and they indicate that the entire core was deposited after the period of maximum land clearance in the mid to late 1800's (Table 4.1). These radiocarbon ages are consistent with pollen evidence showing abundant Ambrosia pollen (8%) at the base of the core (Willard and others, this volume, Chapter 7).

Site PC-3B

Two cores were obtained and dated from this site: PC-3A and PC-3B. Bivalve and gastropod shells from 40-42 cm depth in core PC-3A yielded ages too young to be calibrated. Bivalve fragments from 168-171 cm depth in core PC-3B yielded a conventional radiocarbon date of 126.9 pMC, which signifies a post-1950 age. Pollen evidence from PC-3B also supports the radiocarbon ages indicating that the entire 170 cm of sediment at these core sites was deposited during the past century.

Core PC-4B

One radiocarbon date was obtained from bivalve and gastropod fragments collected at 30-32 cm depth in core PC-4B and was too young to be calibrated. This is consistent with pollen evidence indicating that horizon representing large-scale land clearance in the late 19^th century is between 40 cm and 50 cm depth in the core.

Conclusions

Radiocarbon, lead-210, and pollen dating of sediment cores from Pocomoke Sound indicate relatively continuous deposition of fine-grained sediments in the main Pocomoke channel. The uppermost sequence of fine-grained sediments taken in cores from > 7 m water depth was deposited during the past few centuries, suggesting a relatively high mean sedimentation rate (>1 cm yr^-1). Cores from the sandier sediments blanketing the shallow (< ~ 7 m water depth) flanks of Pocomoke Sound were dated only in a preliminary fashion and suggest less continuous and coarser grained sedimentation than was found in the channel. Further discussion of the age models for the cores is provided in Chapters 5-7 in discussions about the paleoecological history of Pocomoke Sound.

Acknowledgements

Radiocarbon dating of samples were carried out by Beta Analytic Inc; lead-210 dating by C. Holmes and M. Marot, USGS. We thank Capt. Rick Younger and the crew of the R/V Kerhin for assistance in obtaining cores used in this study. X-radiographs were provided by Bill Panagoetou of the Maryland Geological Survey.

References

U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/2004/1350/chapters/chapter4.htm
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