| Summary: | The Twiggs Clay is an upper Eocene claystone found as discontinuous deposits downdip of the Fall Line in the Coastal Plain Province of eastern and central Georgia. A distinctive facies of the Twiggs is found in two lenses near the town of Wrens. The nature, origin, and potential commercial application of this resource are examined.
Four strata and two mineralogic assemblages are recognized in the Wrens deposit. The lower three strata are near white in color and very low in density. These strata are mineralogically similar and are composed predominantly of disordered silica and smectite. The smectite is dioctahedral, and the X-ray diffraction signature of the opaline phase is most like that of tridymite. The uppermost stratum is composed of kaolinite and smectite. This stratum lies above a prominent erosional surface and may represent a younger deposit.
The opaline assemblage is an altered diatomite and likely represents rapid accumulation of biosediments in a low-energy environment of normal marine salinity. Sustained high productivity was supported by favorable oceanic circulation bringing nutrient laden waters from the Caribbean through the Gulf Trough. Volcanic activity in west Texas and Central America contributed nutrients, and fluvial discharge locally enhanced nutrient levels.
Diagenetic transformation of amorphous silica to disordered, lepispheric silica occurred through in-situ dissolution and re-precipitation at relatively low temperatures. Reactions between clay minerals and pore waters modified the exchange complex and reduced the total exchange capacity of the clay. Authigenic smectite formed at the expense of lepispheric silica.
Cation exchange capacity of the Wrens deposit is controlled by smectite but is generally moderate due to significant silica and kaolinite impurities. However, the smectite may be an effective cation-exchanger. Variations in exchanged-cation population are significant and are explained by differences in pore water pH.
Sorption capacity is typically high for water and moderate for oil. Both smectite and opal are active in the sorption process, and microporosity is important. It is possible that different mechanisms are responsible for the sorption of oil and water. The Wrens deposit has good potential for a variety of sorbent applications and may be an effective filter or clarifying agent.
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