WILCOX GROUP. 



37 



Gonforinity ])etween the Miihvuy and Wilcox, 

 I am awaro of only one or possibly two local- 

 ities where liiroct ])hysical evidouce of an 

 erosion interval is available. The fii-st locality 

 is in the vicinity of Fort Gaines, Ga., wiiere 

 numerous pothole-like depressions in limestone 

 of the Midway formation, in places 20 feet 

 m depth, are filled with Wilcox deposits. A 

 second locality widely removed from the pre- 

 eedin<r is alona; tlie Rio Grande, where, how- 

 ever, tlie dejxjsits have not ])een positively 

 correlated. In southwestern Maverick ('ounty, 

 aloni; (he Riotirantie between ^Vlut(> Blud' and 

 the line between Maverick and Wet)b coun- 

 ties, according to information communicated 

 hy L. W. Stephenson, marine fossiliferous beds 

 of limestones, clays, and glauconitic sands 

 of Midway age are separated by a marked 

 erosional unconformity from tlie o\-erlying 

 beds provisionally regarded as of Wilcox age. 

 The Wilcox consists of 200 to 2.50 feet of 

 u-regularly bedded medium to coarse grained 

 sandstone, with subordinate tliin laminated 

 layers and laminar of gray clay, many thick 

 massive lenses of sand, more or less lignitic, 

 and much fragmentary vegetable material. 

 At one place a well-developed basal conglom- 

 erate 2 to o feet tliick, is largely made up of 

 pebbles of h'on carbonate derived from the 

 underlying Midway. In so vast an area, where 

 all the studies have been of a reconnaissance 

 nature, breaks in the sedimentation will prob- 

 ably not be easily recognizable in the field, par- 

 ticularly when the general lilhologic similarity 

 bet<\eeu shallow water and littoral sediments 

 of different ages is borne iu mind. Besides the 

 faunal changes that mark the transition from 

 Midway to W^ilcox, wliich are considerable, and 

 the floral changes, which are inade(juatcly 

 known because of the paucity of the Mid- 

 way ( ?) flora, it may be noted tluit succeeding 

 the Midway, during which time mariiu; faunas 

 penetrated northward at least into Tennessee, 

 there was preserved at the base of the Nana- 

 falia formation an extensive bed of lignite 

 from .") to 7 feet in thickness. That this was 

 formed in place (autochthonous) by terrestrial 

 vegetation and that tlur marine waters had 

 withdrawn southward beyond tlur present out- 

 croj) is almost certainly estaldished. It may 

 also be noted tlial northward along the contact 

 of the outciop of the Wilcox beds with the 

 Midway successively younger Wilcox betls rest 



on the Midway, so that the middle Wilcox 

 (Holly Springs sand) of Oxford and IIoUv 

 Springs, Miss., several huiulred feet above the 

 base of the Wilcox in that latitude, are the ex- 

 treme basal deposits of the beds of Wilcox age 

 in Henry County, Tenn. These horizons can 

 be traced i)y the lithology and are strikingly 

 confirmed l)y tiie distribution of the flora in 

 the eiLstern Gulf area. In addition the well 

 records available for studj' show that the Wil- 

 cox as a whole becomes tliicker down tlie dip, 

 a sure indication of either erosion or of deposi- 

 tion (luring an ailvance and subsequent retreat 

 of the Gulf waters. 



In tlur western (iulf area the floras ar(> not 

 sulhciently rei)resented for exact correlation. 

 Nevertheless, as shown in the discu.ssion of the 

 local sections and of correlation, all the floras 

 across Arkansas and Louisiana westward t(j 

 Wilson County, Tex., are not older than the 

 Holly Springs sand (middle Wilcox). The de- 

 posits containing these floras commoidy lie but 

 a short distance above the top of the dejiosits 

 of Midwaj' age, as at Benton and Malvern in 

 iVi-kansas or along Calaveras Creek in 'Wilson 

 Count}', Tex. Tbe well records in the Naborton 

 oil field of western Louisiana show that thick 

 be<ls, representing all of the lower Wilcox and 

 most of till! middle Wilcox of the eastern Gulf 

 region, were extensively transgressed by the 

 late middle Wilcox and nowhere reach the sur- 

 faces as an outcropping formation. The lignites 

 mined in Burleson and Wood counties, Tex., 

 probably represent the middle Wilcox. The 

 floras preserved are sufficient to render conclu- 

 sive the statement that the WUcox deposits of 

 the western Gulf area are either of Holly 

 Springs (middle Wilcox) or Gremxda (upper 

 Wilcox) age. In other words, the Ackernum 

 or lower Wilcox of the eastern Gulf area tloes 

 not outcrop west of Mississippi River. 



These facts clearly demand an interval of 

 enun-gence and erosion between the Midway 

 and the Wilcox, an mterval of considerable 

 duration but of not very great change in level. 



The proof of a similar interval between the 

 Wilcox and the overlying Claiborne; is not so 

 conclusive. It rests on the physical evidence 

 of an erosional interval observed by Veatch 

 and Sti>phenson ' at sev(>ral localities in western 

 Georgia; on tlie littoral character of tiie basal 

 beds (Tallahatta buhi-stone) of the Claiborne 



' Veatch, otto, and Stephenson, L. W., Georgia Gool. Survej' Bull. 26, 

 p. 228, 1911. 



