Dr. Robinson is a visiting researcher at TARL who spends a good deal of time here working on various collections from around Texas. This article is from TARL’s December 2016 newsletter.
Recent research in the TARL microscopy lab has placed a highly magnified focus on a small section of a prehistoric Caddo structure from 41LR2, the Sanders site, a mound site in the Caddo country of northeast Texas. The material is silty clay layers of two colors which came from the West mound.
The specimen was found by Tim Perttula, Mark Walters, and Bo Nelson, archeologists with ongoing research interests in the Sanders site. They noticed that the item was in two colors, a yellowish brown (10YR 7/4; very pale brown) of the immediate floor about one cm thick; and a darker, grayish brown (10YR 3/2; very dark grayish brown) color above the floor surface, starting about 2.3 cm thick in the specimen (this is not the full thickness of the original deposit). The specimen has a surface with rain cracks and a clear stick impression near one edge. This may or may not indicate a structural floor surface.
All the microscope work was accomplished in the TARL microscopy lab on the Olympus BH2 polarizing light stereoscopic microscope. The initial examination and transect counting were made at 100X magnification.
Results
As of this writing, the microscope work has been finished except for follow-up checks as needed. All the counts need to be added to a spreadsheet that will facilitate the making of graphs and statistical comparisons.
The analysis returned a wealth of data on the micromorphology and mineral composition of the sample. Particles and bodies identified in the specimen include silt quartz, hematite, hornblende, micas, pyroxene, feldspar, voids, and organic materials.
Assessment
The specimen shows no petrographic difference between the materals of different colors. The material is technically a clayey silt rather than a clay, but it is rich in additional particles and bodies. These additional minerals and organic bodies may provide additional information on the structures in the mound.
Acknowledgements
The work reported here has been carried out entirely at the TARL microscopy laboratory, a facility that is proving flexible to address a variety of research topics. Marilyn Shoberg of TARL manages the microscopy lab under the direction of Dr. Brian Roberts. The outsized sample thin section was the result of customized work by National Petrographic Service of Houston, Texas.
Reference Cited
Folk, R.L.
1980 Petrology of Sedimentary Rocks. Hemphill Publishing Company. Austin.