Paleocommunities of the Yorktown Formation (Pliocene) of Virginia

Figure 1: Map of eastern Virginia showing sampling localities.

Ideally, an initial survey of a potential collection site was done from a boat on the river. Large scale features, such as laterally extensive shell beds, burrowed horizons, indurated horizons, and large dissolution features are more easily seen from a distance than up close. The outcrop was then walked along its entire length, noting smaller scale features. Large non-molluscan fossils, such as large whale bones and bryozoan colonies, were noted so that they could be avoided in subsequent collection. The deep fossil burrows of Panopea, and other large, muscular clams, were also noted, so that they could also be avoided, if possible. Root systems of the modern flora were also noted, since plant root systems actively dissolve shelly material, and thus vegetative zones were avoided when possible.

Once the entire outcrop had been surveyed, an area to be sampled was chosen based on lack of groundwater alteration (e.g. dissolution and induration), decent vertical exposure, and access. At least two vertical sections within the area were measured, described, and marked off at one foot (.33 m) intervals (Fig. 2). Collections were taken adjacent to these foot markers. Initially, an area approximately 1-2 feet (.33-.66 m) wide and 1-3 inches high rectangle was carved using a hoe-pick. A volume of sediment beneath this collecting interval was excavated, undercutting the collection interval. The edges of the collection interval were also excavated. The undercut collection interval was then struck from above, using the hoe end of the hoe-pick, dropping the collection into a waiting plastic bin. Ideally, at least 6 lbs of sediment were collected from each horizon, although since it was somewhat difficult to judge the dry weight of water-saturated sediment and so the actual collections varied in weight from 4.5 lbs (xx kg) to 15 lbs (xx kg). The sample was then transferred to a bag labelled with the collection area and sampling interval for transport back to the lab. This procedure was repeated for each marked interval of each measured section. Replicate samples were taken from most sections. with three samples taken from a level adjacent to each foot marker.  

Figure 2: Sampling proceedure for the Yorktown Formation. Two sections at this locality were chosen for measurement. A sample was taken adjacent to each one foot startigraphic horizon. For sections from which replicate samples were taken, three collections were made at each foot marker in each section.

Once back in the lab, each sample was processed for shelly material (Fig. 3). First, five pounds of sample were washed through a #8 sieve. This sieve was placed in a tub of water, so that the finer portion which washed through the sieve was preserved for later examination. Gentle agitation was usually all that was required to remove most of the sediment from the shells, although in some cases, gentle scrubbing was also required. The largest shells (at least 1 cm in some dimension) were removed from the sieve and put aside. These larger shells proved to be rather fragile, and prone to disintegration if allowed to soak for too long. Smaller shells were not as easily damaged by either soaking or gentle agitation, and thus could be left in the sieve. After the sample had been washed through the #8 sieve, the coarse fraction was wet-washed through a #5 (4 mm) sieve. The fine fraction of this sieving was allowed to dry, and then labelled and placed in small plastic bags. The coarse fraction was also allowed to dry, and then was processed for fossil material.  

Figure 1: Processing procedure for Yorktown samples.

The molluscan remains in the coarse fraction were sorted into countable and uncountable shell fragments. Bivalve fragments were considered countable if they included enough of the hinge portion of the shell to allow positive identification of the bivalve species. Since one bivalve valve can contribute tens to hundreds of fragments to the fossil record, countable fragments must be defined so that each valve is only counted once. The hinge of most bivalves is relatively robust, easily recognized, and a unique, non-repeated element of each valve. Gastropod material, which was usually much less abundant than bivalve material was sorted based primarily on whether the fragment was identifiable to the species level. The bivalve and gastropod fragments were counted and tabulated.

Scaphopod, polyplacophoran, vertebrate, bryozoan, coral, inarticulate brachiopod, decapod, barnacle, and echinoderm material was also present in some samples. The presence of these groups was noted by recording a count of "1" in the appropriate data cell for each type of remain. Scaphopod material was identified to the genus level, while the other remains were simply noted.

In total, the data set consists of over 140 samples from 5 localities. More than 140 species were identified from these samples, and a total of 28,878 individual specimens were tabulated (Table 1)

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