POSTER PRESENTATION GIVEN BY DAVID DOVE AT THE 2019 PECOS CONFERENCE IN CLOUDCROFT NEW MEXICO
DM Dove, S Di Naso, W Hurst, W Lucius
THE SCIENCE BEHIND THE METHOD
USING PXRF, NAA, AND ICP-MS TO IDENTIFY CERAMIC PRODUCTION AND TRADE OF SAN JUAN REDWARE
Geochemical sourcing studies conducted on San Juan Redware pottery and clay from Montezuma, Recapture, Cottonwood, and Alkali Canyons in southeast Utah and several large villages in southwestern Colorado have revealed insightful information about this widely exchanged ware.
We present our latest discoveries; new evidence of intra-, inter-, and extra-canyon movement of redware pottery from a major production center in central Montezuma Canyon to numerous communities within and outside the production zone. We also introduce two potential production zones in Recapture and Cottonwood Canyon as defined by new additions to the assemblage of PXRF-analyzed clay and ceramics.
Montezuma Canyon is a 73 km-long erosional incision through Cretaceous and Jurassic rocks. It was home to numerous Puebloan habitation sites during the Basketmaker III to PIII era. Canyon sites dating to the AD 750-1050 period contain abundant San Juan Redware pottery. The clay resources selected for their production can be geochemically sourced to specific locales where pedogenic processes have reduced mudstones to weathered clay, some of which can be used to make pottery. Questions that we hope to answer through these ongoing analyses follow:
Can red ware found in Montezuma canyon be linked to the geological landscape? Which sites in southeastern Utah were producing San Juan Redware? What mechanism was used to exchange red ware pottery across the MV region; direct exchange from producer to end user or down-line trading where pots moved from site to site until reaching a final destination?
Using a systematic sampling strategy, clay samples were collected from the Jurassic Morrison Formation and include additional samples gathered by the Blanding Redware Project. Thus far, the geological reconnaissance and clay sample array covers approximately 2,380 square kilometers. All clays were subjected to technological ceramic analysis. Refiring analysis was used to evaluate usability and identify those clays that matched the Munsell values of our archaeological sherd assemblage.
Clay and sherd samples were analyzed using Inductively Coupled Plasma - Mass Spectrometry (ICP-MS), and Portable X-RAY Fluorescence (PXRF). Analytical data selected from our database includes 108 clay and 220 sherd samples, including a subset of 59 sherd samples from 163 sherds analyzed using Instrumental Neutron Activation Analysis (INAA). RQ-Mode Principal Components Analysis (RQ-PCA) and ESDA (Exploratory Spatial Data Analysis) were used to examine the point patterning (cluster morphology) of samples within PCA compositional space.
RESULTS & DISCUSSION
Some of the clays that are exposed throughout Montezuma Canyon can be used to make viable pots. Pottery found on these sites refire to a comparable Munsell color to refired clays that were collected from outcrops found proximal to them. Gray clay associated with the lower Brushy Basin and Westwater Canyon facies at the Nancy Patterson site are viable, however, their refired clay color is not observed in the analyzed red ware assemblage. Not all clays were found to be useable. Samples with relatively high percentages of 2:1 type clay (e.g. montmorillonite) commonly deformed; those containing excessive silt or sand often crumbled or broke easily.
The resulting PCA biplots produced similar punctiform (point) patterning in independent analyses of elemental data for sherds using both PXRF and INAA. Despite the lower detection limits of INAA compared to PXRF, most of the elements analyzed using PXRF were within two-sigma of their certified values for SRM U.S.G.S Cody Shale Sco-1. Measurements above two sigma of the certified values nonetheless demonstrated good precision. An analysis of a subset of sherds (n-59) by PXRF and INAA using the same elements was used to confirm that BOTH methods produce impressive results. Unarguably, whether using INAA or PXRF, and using the same or even different variables (elements), the two methods produced identical ceramic groups (Figures 3, 4, and 5) demonstrating the viability of PXRF as an analytical sourcing method provided it is implemented properly.
Although our clay database for the various members of the Morrison Formation is extensive, no San Juan Redware found at villages within Montezuma Canyon was linked to clays located outside of the canyon. Clay geochemistry and geostatistical analyses demonstrate that the lower Morrison exposures found within central Montezuma Canyon are chemically different from the upper Morrison exposures that predominate in the canyons to the west and the upper Morrison Fm exposed in Montezuma Canyon.
Our growing geochemical database consists of clays found proximal to production villages and sherds recovered from them. Analysis of the PXRF data using PCA indicates that ceramics found on production sites have complimentary geochemical signatures with the surrounding landscape. Clays analyzed with ICP-MS and PXRF, and subsequent PCA analyses, indicate that both methods produce complimentary results and can readily identify unique geochemical signatures geographically (Di Naso, S.M. 2018).
PCA analyses of NAA data on 163 sherds, and a subset of this assemblage using PXRF, produces a densely clustered morphology we identify as the ‘Nancy Patterson’ agglomeration. The Nancy Patterson agglomeration is dissimilar from the clays sampled elsewhere in southeastern Utah (Figure 2). This highly-clustered group consists of clay found proximal to the site and sherds from Nancy Patterson, Cave Canyon Village, Monument Village, Champagne Spring, and Mitchell Springs (Figures 3 and 4).
This analysis has successfully revealed the origin of many of the sherds in our sample subset recovered from sites in Montezuma Canyon and southwestern Colorado. Through systematic sampling of the geologic landscape the origin of the pottery found in this canyon and others will be revealed as we further examine the material consequence of a cultural relationship that existed between and among sites in southeastern Utah and southwestern Colorado (Dove, D.M., Di Naso, S.M., Hurst, W., and Lucius, W.A., manuscript in preparation 2019).