Geochemical and Microwear Analysis of an Obsidian Artifact from the Brown Bluff Site (3WA10), Arkansas

Article excerpt

REPORTS

ABSTRACT

A bipolar flake of obsidian was recovered from the Brown Bluff Site, Arkansas. This specimen, made from volcanic glass (obsidian) of the Malad chemical type of southern Idaho, is the first recovered from any professional excavation in Arkansas, and currently is the most distant geochemically documented occurrence of Malad obsidian. Microwear analysis revealed no convincing evidence for use of the artifact as a tool.

Keywords: obsidian; x-ray fluorescence source analysis; microwear analysis

In 1994, archaeologists from the Arkansas Archeological Survey, under the overall direction of Green, conducted excavations at the Brown Bluff site (3WA10), located about 12 miles south of Fayetteville in the Ozarks region of northwestern Arkansas (Figure 1). Excavations in Test Unit 71 at this site produced a single obsidian flake, recovered from water screening of the 10-20 cm level fill following removal of an overlying rock slab. This obsidian flake is the first recovered from any professionally excavated site in Arkansas. The specimen was analyzed using non-destructive x-- ray fluorescence to determine the geologic source (chemical type) of the obsidian used in its manufacture, and was subjected to microwear analysis in an attempt to determine the likely prehistoric uses) this flake may have served.

THE SPECIMEN

The obsidian flake (Figure 2) is 21.3 mm long, 10 mm wide, 3.1 mm in maximum thickness, and weighs 0.64 grams. It is a bipolar flake with lateral edge damage on the ventral side. The main striking platform, which also is edge damaged on the dorsal surface, probably disintegrated in flake production.

USE-WEAR ANALYSIS

The specimen was analyzed by Kay using a differential-interference binocular microscope (see Kay [1996] for general details). Examination at intermediate range magnifications (i.e., 100-400X) was done of the ventral (interior) surface and edge because of the possibility that observed microscopic scratches (abrasion) on this surface could be directly attributed to post-production damage (if not use). The most obvious microscopic damage consists of fresh-looking striae, including ones caused by a particle that wobbled back and forth as it streaked across the ventral surface (Figure 2b). Although we cannot be absolutely certain, it is likely that they were produced during the excavation and/or recovery of the artifact, which was found in a wet-screened sediment sample.

To verify the likely cause of the surface abrasion on the artifact, a series of 13 obsidian replicas of similar size and shape were produced by bipolar percussion; one was held out as a control, the others were subjected to individual "natural" abrasion experiments. All experiments were conducted by Kay on an individual bipolar flake or chunk that was subjected, along with other substances, to discrete abrasive activities designed to mimic natural events that might have either affected the sediments of the site or the recovery of the obsidian specimen. These ranged from kneading, crushing and shaking moist dead leaves, to trampling in moist sandy loam, to various water slurry mixtures of sandy foams with sand and gravel, to dry screening or shaking sand, or sand and gravel. Some of the experiments produced highly individualized, or discrete, abrasive wear while others were quite similar; the latter could be subsumed within larger groupings, or patterns, of abrasion. Only one experiment, however, produced comparable results to that observed on the bipolar obsidian artifact from Brown Bluff; this was a trampling experiment against a moist sandy loam substrate. With the exception of the "rockered" striae observed on the Brown Bluff artifact (Figure 2b), there was virtually no difference in the appearance, size and orientation of the surface abrasion on the experimental replica. Consequently, the most likely explanation for the abrasion and edge damage on the Brown Bluff artifact is post-depositional trampling within the sandy loam substrate. …