Shirley Plantation Sediment Coring Project

Carter, Saunders, and Kiser
operating the vibra-core.

Introduction
On September 9, 2010, the Shirley marine archaeology crew, Charles Carter, Chee Saunders, Taft Kiser, and Theresa Hicks collected three core samples (labeled Core 1, 2, and 4) of river sediments from randomly selected locations around the nineteenth century wharf complex. The goal was to determine stratigraphy and the degree of sediment accumulation since the first European colonists inhabited and farmed along the James River. The crew used a vibracore braced on an aluminum A-frame with the engine sitting within an anchored jon-boat. Two samples were extracted upriver and one sample just downriver from the dock complex. The 7 ft. depth cores were taken and stored in 3 in. wide by 10 ft. long aluminum casings. The crew quickly realized that while three cores were successfully harvested, there were others better equipped to collect and analyze sediment cores.

Carter brought in the Virginia Institute of Marine Science (VIMS) of the College of William & Mary, Williamsburg, Virginia, to collected four cores in the James River off of the Shirley Plantation Wharf complex. VIMS used a vibra-coring system deployed from a boat launched from Hopewell and collected four cores samples total on two field days: February 2, 2011 and April 6, 2011 (Kuehl 2011: 1).

Project Objectives
The sediment coring had six main objectives:

1. To determine the characteristics of the shallow stratigraphy and sediments in the immediate vicinity of the historic wharf at Shirley Plantation on the James River.
2. To determine if there is a measurable change in sediment characteristics pre- and post-wharf construction.
3. To estimate the amount of siltation which has occurred since the wharves were built.
4. To determine the feasibility of coring and/or geotechnical measurements in reconstructing the original river bottom bathymetry prior to wharf construction.
5. To determine if changes in sediment character are sufficient to enable a remote sensing approach (i.e., high-resolution seismic) in order to map the old river bottom surface.
6. To recover any artifacts contained in the cores to help develop a sediment chronology (Kuehl 2011: 1).

Upon return to W&M, VIMS analyzed the four cores for density using a Geotek multi-sensor core logger, and split and described for color, texture, and sedimentary structure. In addition, cores were imaged using digital x-radiography to obtain a measure of density variations and to identify and remove any suspected artifacts (i.e., pottery shards). Subsamples were also taken for water content measurements.  In addition to these cores, three additional cores were provided to the Institute by the Shirley marine archaeology group.  These cores were split, x-rayed, and described.



Figure 1. April 29, 2007 Google Earth map showing core locations
relative to the Shirley Wharf Complex. 

Results and Discussion

Generally, all of the cores contained high mud content (Silt+Clay), with some mixed sand and organic material in sections.  From visual and x-radiographic characterization, all seven cores were highly bioturbated in the top sections, with decreasing bioturbation and increasing stratification down core (Figure 2).  The two longest cores (Dock and Deep) penetrated a marked transition near 3 m sub-bottom depth, where significant changes in density structure and composition were observed (Figure 3).  An erosional (scour) surface was also evident at this depth. In these two cores, density increases at the transition, and varies below, as seen in the gamma density profile. Layers enriched in organic matter are common beneath the transition, and relatively rare below (Kuehl 2011:3). 

Figure 2.

Figure 3.

Significantly, artifacts were recovered in three of the four VIMS cores, which provide a temporal framework for the recovered sediments:

1) Mortar at 153 cm in the Shore core (Figure 4).

Figure 4.

2) Rosehead nail at 309 cm in the Dock core (Figure 5).

Figure 5.

3) Clinker (coal) at 221 cm in the Midpoint Core (Figure 6).

Figure 6.



The two longest cores, the Dock and Deep, provided key pieces of information that can be used to address the overall objectives of this project. This is confirmed by the descriptions, x-rays, density and water content, as well as the artifact found in the Dock core. The transition from highly layered, organic rich, dense sediments below about 3 m, to more muddy finely laminated to bioturbated sediments above. There appears to be an erosional unconformity separating the two sediment types (facies), and the nail was found at this boundary. A possible interpretation is that this transition reflects a major change in river siltation caused by early deforestation and agricultural practices. High silt runoff from human activity likely accelerated the input of fines and drastically increased the sedimentation rates in the area. Based on the position of the nail, the estimated accumulation rate for the upper section would be 1.5 cm/year [3 meters = 300 cm over 200 years = 1.5 cm/yr], which is quite a bit higher than would be expected under prehistoric sedimentation equilibrium conditions. It can also be presumed from this artifact that the water depths off the head of the wharves were about 3 m deeper than today. Independent age control (radiometric dating) can validate the dates of the strata (Kuehl 2011:7).

Conclusion and Recommendations
The results from this pilot coring study unambiguously provided a proof of concept relative to our original objectives. Such a marked sediment transition at 3-m depth is a legacy of rapidly changing environmental conditions.  Additional coring to trace the continuity of this surface, combined with acoustic mapping and radiometric dating and collection of additional artifacts is required to positively confirm the age and origin of the pre-19^th century river bottom surface (Kuehl 2011:8).

Works Cited
Kuehl, Steven A.

2011    Shirley Plantation Sediment Coring: Final Report. Virginia Institute of Marine Sciences College of William and Mary, Williamsburg, Virginia.