Report of Investigations — No. 11
its outcrop. In its subsurface occurrence the Ellenburger has been defined as in cluding beds up to the base of the Simpson formation of Middle Ordovician age (Sel lards, 1933 a). Thus the Ellenburger in a portion of its occurrence may contain some beds younger than any occurring in the Honeycut formation in its thickest develop ment. The youngest known beds assigned to the Honeycut formation are overlain by rocks of Devonian age. It seems most likely that the hiatus of the unconformity includes beds present underneath Middle Ordovician in west and north Texas. Whether those Lower Ordovician beds younger than Honeycut as denned should be included in the Honeycut becomes a matter of judgment based on evidence from many sections. Especially, subsur face sections of north Texas should be compared with sampled sections of Lower Ordovician rocks present in the Arbuckle and Wichita Mountains of Oklahoma.
An understanding of the occurrence of the Ellenburger formations in the subsur face involves the use of dependable cor relative criteria common to both outcrop and subsurface data. Since faunal evidence obtained so far in well data is too rare to be of use, the physical or qualitative character istics of the Ellenburger subdivisions offer the only recourse for correlative data to be found in both outcrop and well samples. The major lithologic characteristics may serve to distinguish the group as a whole from other rocks but do not offer evidence for widespread correlation within the group. Cloud and Barnes found that grain size in the dolomites was a general guide for recognizing some subdivisions, and this is true to some extent in more wide spread subsurface correlation. The alter nation of limestone and dolomite has no stratigraphic significance, since one grades laterally into the other in an unpredictable manner.
The insoluble residues of the Ellenburger offer the best qualitative evidence found to date for recognizing subdivisions within the group. The possibility of subdividing the Ellenburger in certain areas of its sub surface extent on the basis of insoluble residues alone has been demonstrated (Cole, 1942; Crowley and Hendricks, 1945). Cloud and Barnes noted char
acteristics of the chert impurities in the limestones and dolomites on the outcrop
peculiar to certain formations. The char acteristics are evident in both the em bedded chert and the chert formed at the surface under the processes of erosion. It will be shown that samples taken from surface sections and digested in acid will yield residues that show a qualitative vari ance from one level of the Ellenburger to another. The change in residue quality does not coincide with formational bound aries in every case, but the points of change are sufficiently consistent that the residues offer a clue to formation identi fication, if not exact definition. The chief asset of the residues is the fact that varia tions in residue quality can occur with age despite little or no change in major type of sedimentation. The chief weakness of the insoluble residues as an aid to stratigraphic study of the Ellenburger is the fact that residue quality is an expression of facies and can change laterally with change in sedimentary conditions affecting deposition of insoluble material.
The term "insoluble residue" is here used in the same sense as others have used it (McQueen, 1931; Ireland, 1936). The samples were treated with dilute hydro chloric acid or, in some cases, with dilute acetic acid. The residual material is there fore known to be insoluble only in those acids.
One of the objectives of field work on the Ellenburger was to obtain sets of sam ples from measured sections whose insolu ble residues could be compared with those from subsurface sections. Therefore Cloud and Barnes selected and marked sections that offered the best opportunity for ob taining samples from beds in place. The very careful and painstaking field work of Cloud, Barnes, and their assistants, princi pally L. E. Warren and R. L. Heller, has made it possible to collect dependable sur face samples for comparison with subsur face samples. It is with great pleasure that this opportunity is used to express appre ciation for the excellent manner in which they clarified the field relationships of the various sections. Grateful acknowledgment is made for their generous cooperation in gathering samples from the field for com parison with samples from the subsurface.
Other sources of help on the subsurface study of the Ellenburger include various oil companies and geologists operating in north and west Texas. Their generous help