CRETACEOUS DEPOSITS OF THE EASTERN GULF REGION. 



By Lloyd William Stephenson. 



OBJECT AND SCOPE OF THE WORK. 



This paper is intended as a brief statement of the results of stratigraphic and paleon- 

 tologic investigations made by the writer during recent years in the Cretaceous areas of the 

 eastern Gulf region. The principal objects in view were to determine the lithologic units worthy 

 of recognition as formations or as members, the lithologic and paleontologic characters of these 

 units, and their stratigraphic and age relations. Although the results of the work have not been 

 in all respects as satisfactory as might have been desired, yet it is believed that the purposes 

 outlined have in large measure been accomplished. 



The major lithologic divisions present in the region have all been recognized more or less 

 clearly by previous investigators, and no sweeping changes in the generally accepted nomencla- 

 ture of the deposits have been found necessary. But one new name (McNairy sand member 

 of the Ripley formation) is proposed and this is given the rank of member; two old forma- 

 tional names Which have, during recent years, fallen into disuse, have recently been revived 

 by the writer 1 with the rank of members; and certain basal Cretaceous deposits in eastern 

 Alabama and in Georgia, previously regarded as the eastward continuation of the Tuscaloosa 

 formation, are shown with reasonable certainty to be of Lower Cretaceous age and therefore 

 probably separated from the Tuscaloosa formation by an unconformity^. However, more 

 definite knowledge has been gained of the character and the geologic and geographic boundaries 

 of the several units recognized. 



The chief additions to the knowledge of the region have been those furnished by a critical 

 study of the organic remains entombed in the deposits; for by this means much light has been 

 thrown on the age relations of the several lithologic units. On pages 23-40 it is shown that the 

 usual methods of mapping formational units fail properly to express true age relations when 

 applied to the deposits of this region, and that to show these relations it is necessary to employ 

 additional superimposed symbols. In places (see PI. IX, in pocket) the boundaries of litho- 

 logic units run obliquely to the general direction of the strike of the strata composing the Upper 

 Cretaceous formations of the region with absolute disregard to the boundaries of paleontolosrie 

 zones and subzones. Although records of such phenomena are not uncommon in the litera- 

 ture, the relative abruptness with which some of the lithologic units here treated merge hori- 

 zontally into others of different character furnishes unusually striking examples of this sort of 

 formational relationship. 



The deposits considered are in part of Lower Cretaceous and in part of Upper Cretaceous 

 age. Their area of surface occurrence embraces a belt of country extending from southern 

 Illinois, where it is only a few miles wide, southward with gradually increasing width through 

 Kentucky and Tennessee to Mississippi and western Alabama, where it reaches a maximum 

 width of 80 miles; thence sweeping around to the east in -a broad curve through Alabama, and 

 with gradually decreasing width extending northeastward into Georgia. The lower repre- 

 sentatives of the series continue northeastward in a narrow belt of surface outcrops through 

 Georgia and South Carolina, eventually connecting with similar deposits in North Carolina: 

 the higher representatives also extend northeastward in Georgia, but they pass beneath over- 



1 Stephenson, L. W., Cretaceous [Georgia]: Bull. Georgia Geol. Survey No. 26, 1911, PI. V, pp. 111-112. 



