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A major carbonate sequence occurs within the lower part of the Talladega slate belt in Chilton, Coosa, and Talladega Counties. The carbonate units are overlain by a major regional unconformity known as the pre-Lay Dam Formation unconformity. The carbonate sequence below the unconformity is represented in different areas by the Jumbo Dolomite, the Marble Valley carbonates, and the Sylacauga marbles. At the type location in Chilton County, the Jumbo is a 67-m-thick, predominantly thickly bedded dolostone. The contact with the underlying slates of the Wash Creek Slate (Mount Zion Formation) is an interlayered zone of dolostone and fine, commonly graphitic, clastic rock. This zone grades upward into a dolostone that contains a few pelitic layers. Near the base, the Jumbo contains intraclasts and recrystallized fragments up to 6 cm in length. Rounded quartz grains are disseminated in the lower section of the Jumbo. Near the middle of the massively bedded dolostone is a layer that contains intraclasts of massive and laminated carbonate as much as 12 cm in length. Just below the unconformity in the type section, the upper part of the Jumbo contains laminations of fine-grained clastic rock. Along strike to the northeast and southwest the unconformity appears to have erosional relief. Less than 1 km west of the type location the unconformity truncates the carbonate sequence completely. To the northeast the pre-Lay Dam Formation unconformity appears to rise in the section in the Marble Valley carbonates and the Sylacauga marbles, exposing a very thick carbonate sequence. Stratigraphic and structural relationships of the Jumbo, Marble Valley, and Sylacauga marbles are not yet resolved. Initial data indicate that the Jumbo occurs stratigraphically below and to the northwest of the Marble Valley carbonates, suggesting that the Jumbo is the oldest carbonate unit in the sequence.

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