20 CRETACEOUS DEPOSITS OF THE! EASTERN GULF REGION. 



conformable series, although there are local unconformities of slight time importance in some of 

 the shallow-water phases of the series. 



The Upper Cretaceous deposits throughout the extent of their occurrence hi the eastern 

 Gulf region are overlain unconformably by Eocene strata. 



LOWER CRETACEOUS. 



The oldest Cretaceous strata of the region constitute a terrane which in areal distribution 

 extends from the Alabama River valley, north of Montgomery, Ala., eastward through Alabama 

 to the Chattahoochee River valley, at and immediately south of Columbus, Ga., and thence 

 northeastward through Georgia, intersecting the Savannah River valley at and south of 

 Augusta, Ga. 



The materials consist of irregularly bedded, coarse, arkosic, more or less micaceous sand, 

 with subordinate lenses of usually massive clay of greater or lesser purity. The terrane rests 

 upon a basement of crystalline rocks and is separated from the overlying Eutaw and other 

 Upper Cretaceous and Tertiary formations by an unconformity. A short distance west of 

 Alabama River the formation passes unconformably beneath and is completely buried by 

 overlapping strata of the Tuscaloosa formation. 



Fossil plants have been found in the terrane at one locality in Alabama. 



UPPER CRETACEOUS. 

 TUSCALOOSA FORMATION. 



The Tuscaloosa formation consists of irregularly bedded sands, clays, and gravels having an 

 estimated total thickness of 1,000 feet. The sands, even well down toward the base of the 

 formation, arc in places slightly glauconitic; the clays are in part massive and in part thinly 

 laminated and are not uncommonly carbonaceous and hgnitic; both the sands and clays 

 vary in color from light drabs and grays to dark greens or grays, and are in many places 

 varicolored, being blotched with purples, reds, pinks, yellows, and browns; the gravels occur 

 chiefly in the basal beds of the formation near their contact with the underlying basement rocks. 



The formation rests unconformably upon a basement which consists chiefly of Paleozoic 

 rocks, but in part, toward the eastern end of the area, of crystalline rocks of probable pre- 

 Cambrian age, and in part, at the extreme eastern end of its occurrence, of Lower Cretaceous 

 strata. The terrane is overlain conformably by the Eutaw formation. The Tuscaloosa forma- 

 tion has yielded a large number of fossil plant species. 



EUTAW FORMATION. 



Typical beds. — The Eutaw formation consists predominantly of more or less glauconitic 

 sand, massive to cross-bedded in structure, with, in parts of the terrane, irregularly interbedded 

 fine laminae and laminated layers of dark clay, the latter usually containing comminuted plant 

 fragments and some lignite. In places the beds are calcareous, this being especially true of the 

 basal part of the formation in the Chattahoochee region and of the uppermost beds of the 

 division throughout the greater part of its linear extent. The total thickness of the formation 

 is estimated to be 400 to 500feet. The beds are believed to be entirely of marine origin, although 

 much of the formation was doubtless laid down in very shallow marine waters. On the basis 

 of the distribution of massive as opposed to laminated beds two members described below are 

 differentiated within the formation. 



The terrane rests with conformable relations upon the underlying Tuscaloosa formation, 

 and is overlain conformably in part by the Selma chalk and in part by the Ripley formation. 



Marine invertebrate fossils occur abundantly in the basal 100 or 150 feet of the formation 

 in the Chattahoochee region. Fossils also occur abundantly in places in the Tombigbee mem- 

 ber of the formation, as explained below. Fossil plants have been found at two horizons below 

 the Tombigbee member in the Chattahoochee region. 



