Tallahatta Sandstone Petrology

Tallahatta Sandstone: Petrology


	Twenty four thin-sections were produced from Tallahatta Sandstone samples collected during the study. Photomicrographs of thin-sections confirm that Tallahatta Sandstone is composed of well-sorted, sub-angular to sub-rounded quartz grains. The difference between knappable quality Tallahatta and "waste" material is the percentage of intergranular cement versus porosity (artificially stained blue in plane polarized light images) that the sandstone contains. Cementation in the Tallahatta Formation is variable, but appears to have occurred in three phases. The first phase was the formation of pore-lining opal (c.f., Cabir & Pan horst, 1999). Other researchers have confirmed that opal-CT (a material ultimately produced from the alteration of volcanic ash; Schroeder and Harris, 2004) is a common component in finer-grained lithologies of the Tallahatta Formation, but we have not been able to confirm that the opal cement is this variety. Opal is a non-crystalline variety of silica and is recognized in thin section through its isotropic nature under polarized light (crossed Nichols). The second generation of cement was chert. Chert is a cryptocrystalline variety of silica and is recognized in thin section by a "speckled" appearance when viewed under crossed Nichols. Chert is a much more common variety of cement than is opal, but also has a pore-lining disposition. In some parts of the rock samples examined, chert is much more abundant and almost completely fills all of the intergranular pore space. It is this "patchy" distribution of chert in conjunction with the first phase of opal that is responsible for the "snowflake" appearance of many Tallahatta lithic materials. The distribution of chert/opal (as well as knappable material in general) is patchy suggesting that cementation was concretionary.

			The same representative thin section photomicrograph shown under crossed polarized light.
	Representative thin section photomicrograph of well-cemented Tallahatta Sandstone (plane polarized light).
		The third and most voluminous phase of cement was chalcedony. Chalcedony is also a cryptocrystalline phase of silica, but it is characterized by a bladed appearance under crossed Nichols (see photo above right). In well cemented (i.e., knappable quality) Tallahatta, chalcedony completely filled all remaining pore space. Limonite (hydrated iron oxide) was a final phase of cement in poorly cemented intervals of the Tallahatta Formation. It weakly lined pores that had not been filled by the early phases of silica cement. The amount of chert and chalcedony cements, relative to limonite and porosity in the quartz arenite, were quality-determining factors. Too little chalcedony/chert cement (alternatively too much porosity) prevented conchoidal fracturing meaning that it could not be shaped into useful lithic tools. In samples that we examined that contained significant amounts of porosity, pore space is concentrated in patches rather than evenly distributed throughout the rock. This is a remnant of the patchy distribution of the silica cements; areas with high porosity inevitably contain less early cement. It should be noted that selective dissolution of some grains (e.g., shells) has been known to produce a similar pattern of pores; however, the shapes of the voids do not resemble any of the most common shell types found in the Tallahatta Formation. Moreover, where present in the Tallahatta, the shells are preserved. They have been replaced by silica, but they do not show any indication of preferential dissolution. At the present time, we have not detected enough petrographic variation to be able to trace Tallahatta Sandstone artifacts back to a particular part of the formation or to a specific prehistoric quarry site; however, this might be possible as more petrographic data are collected.
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