Petrofacies

The concept of employing petrofacies as a method of provenance exploration began with Lombard’s efforts to locate the source of sand temper for Hohokam ceramics. Once a petrofacies model has been developed, it significantly contributes to petrographic analyses by allowing investigators to assign ceramic sherds to probable zones of raw material procurement. This then allows for the interpretation of ceramic production locales through discussions regarding the movement of pottery within the river basin.

Results similar to those in Arizona would allow ceramic petrographers working in the Angelina River basin in East Texas to connect areas of raw material procurement with specific ceramics in known Woodland and Caddo (ceramic – bearing) assemblages. This manner of investigation might then yield results that allow us to elucidate upon the inter and intra-site relationships of prehistoric occupants, providing an invaluable asset to use in framing our research questions regarding Woodland and Caddo cultures in East Texas.

Why should we invest our time and effort into testing a model of ceramic petrofacies?

While instrumental neutron activation analysis (INAA, or more commonly, NAA) has been successful at demarcating between local and non-local ceramics at a regional scale throughout the East Texas region of the Caddo area, the lack of systematic sampling for raw materials and the perceived homogeneous geochemical signatures within that dataset have let to significant interpretive challenges. Although the geochemistry may appear homogeneous at the elemental scale, the geologic variability within the lower Angelina River basin is ample, and provides promise for substantive gains in provenience resolution, making it an ideal location to test the petrofacies method (where latitudinal variability occurs at a higher frequency than its longitudinal counterpart due wholly to the nature of the coastal geology in which deep sands were deposited incrementally as sea level dropped).

In short, I am collecting a variety of sand samples from within the Angelina River basin, and plan to run a variety of tests to explore whether those same sands (or combination of sands) can be linked with ceramic assemblages from nearby archaeological sites.

Can you see a N-S variation in ceramic composition within the Angelina River basin?

Short answer, yes. As I look at what I assume to be locally-manufactured ceramics from sites along the Angelina River, various compositional differences present themselves; however, whether those can be linked to a specific geologic formation (or a specific peripheral drainage of the Angelina River) remains unknown. This observation serves as  the basis for a test of ceramic petrofacies.

How apparent are the geologic changes along the Angelina River?

Over the course of the last three years, I have been traversing different regions of the Angelina River by kayak. In doing so, I have gained a working familiarity with the local geology, and the stratigraphic variability that occurs throughout much of its’ course. That said, I am far from an expert – which is where it helps to have friends that are. There are a number of graduate students, professors, and researchers at SFA, Texas A&M, Texas State, and the University of Texas that have been very helpful throughout the development of this project.

PETROFACIES2

(click to enlarge)

During my outings, if I came across a particular strata that I was unfamiliar with (there were a lot in the beginning), I would collect a sample and bring it back to the lab. It took some time before my eyes finally cued into the many differences in sand (and clay) composition that are visible under a standard microscope.

PETROFACIES3

(click to enlarge)

The one thing that became apparent quite early on in this effort is that a high degree of variability does exist within the local geology of the Angelina River basin. That’s not to say that it wouldn’t be nice to have more (most of our variability is limited to sedimentary composition – nearest outcrop of igneous rock is the Llano Uplift), but we should have enough to begin parsing out the various nuances.

While we won’t be able to take samples out of the main drainage, a variety of intersecting drainages (like the one below) exist at different elevations within the channel. This means that it will be possible to collect samples at a variety of different water levels – which helps, particularly with the fluctuations in water levels over the last few years.

PETROFACIES4

Sampling Strategy

The sampling strategy does vary with water level (can only get sand samples from those drainages above it), and is guided by a model of predicted petrofacies boundaries that is based upon geologic boundaries defined in the Geologic Database of Texas. While this resource was used to create the predictive petrofacies model (theory), it will be the samples themselves (empirical data) that highlight the actual shifts in local geology, which we can then use as a check against the ceramics.

There are four geologic groups (see below) that occur in the location of this test, and I expect that we will be able to successfully demarcate between those. However, there are also multiple formations that occur within each of these groups, and I am skeptical whether this will provide the resolution needed to segregate between the various drainages..

Basis for Predicted Petrofacies

This predictive model will guide the sampling strategy, in which 10 samples will be collected from selected areas of the drainage basin. By definition, the boundaries of a petrofacies are a created construct, since abrupt changes in composition rarely occur in adjacent drainages. Boundaries for this predictive model–dubbed “Lombard Lines” in the context of this project–are names for Dr. James P. Lombard, who pioneered this method. The Lombard Lines illustrate those areas where divisions in sand composition zones are expected to occur.

Predicted ARB Petrofacies Boundaries

The Take-Away

In archaeological application, petrofacies are considered “temper resource procurement zones whose sand compositions are distinct from one another at a relevant scale of investigation.” Petrofacies models have been employed successfully in archaeological contexts in various basins throughout Arizona, in the San Pedro Valley, Tonto basin, Tucson basin, Perry Mesa and Agua Fria, Tanque Verde Wash, and the Gila and Phoenix basins; however, despite the high level of success in applying these models within the greater Southwest, the technique has not yet been employed in the Southeastern U.S. (to include the ancestral Caddo region).

I am proposing to test a model of petrofacies in the Angelina River basin, using Caddo ceramic assemblages recovered from East Texas, in an archaeological region where archaeologists have struggled to identify ceramic production provenance through more traditional means. Should this test of the method prove successful, it may just give us the information we need to begin pursuing more complex research questions concerned with craft specialization, local ceramic economies, cultural transmission, and–as an ancillary benefit–may help us to better discriminate between Woodland and Caddo ceramic traditions.

CARAPICE

(Angelina River shoreline near Lake Sam Rayburn)

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