40 Ar/ 39 Ar plateau age spectra of seven sanidine samples from the Fire Clay tonstein (Middle Pennsylvanian), collected along a 300-km traverse in the Appalachian basin, range from 310.3 to 311.4 Ma. All plateau ages agree, within the limits of analytical precision, with their respective total gas ages. This agreement, together with the reproducibility between samples, suggests the analyzed samples did not contain any significant contaminant feldspar. The mean of these seven plateau ages, 310.9 ± 0.8 Ma, is interpreted to represent a precise numerical estimate of time of eruption and deposition of this tonstein and the coal bed in which it is found. The lack of any discernible difference between the age of two samples of the Fire Clay tonstein collected from east of the Pine Mountain thrust fault, along with the age of five samples from west of this fault, suggests that the Fire Clay tonstein has been reliably correlated with a tonstein on the Cumberland overthrust sheet. This correlation, together with the age data presented in this paper, indicates that the Pine Mountain thrust fault must be younger than the 310.9-Ma age obtained for the Fire Clay tonstein. The Fire Clay tonstein is biostratigraphically correlated with the Trace Creek Shale Member of the Atoka Formation in the Midcontinent of North America and with a position near the Westphalian B-C boundary in Western Europe. Our age of 310.9 ± 0.8 Ma for the Westphalian B-C boundary represents a well-constrained point, useful for the numerical refinement of the geologic time scale.

Abstract The East Durant field is on a closed anticline beneath an overthrust fault sheet; the field was discovered with the use of a coordinated program of core-drill, seismic, and subsurface exploration. It is approximately 12 mi southeast of the Cumberland field, which is somewhat analogous in character. The surface rocks are Comanchean of the Cretaceous System and lie unconformably on beds from Springer to Simpson in age. The entire geologic section from Springer to Arbuckle is represented and in some instances is duplicated where thrust faulting has occurred. Productive zones are limited to the Bromide and McLish Formations of the Simpson Group. Eight wells have been completed, of which six are dual completions and two are single-zone completions, one in the Bromide and one in the McLish. There are six gas wells and two oil wells. Cumulative production to March 1965 was approximately 6,075,893 Mcf of gas and 105,984 bbls of oil.