Abstract: Subsurface to Surface: Use of Outcrop Gamma-ray Logs to Interpret Structurally Complex Exposures of Appalachian Devonian Strata

DENNISON, JOHN M., JONATHAN K. FILER, LARRY J. CAVALLO, ROBERT A. DRAKE, and JEREMY D. MIDDLETON

Three examples illustrate how gamma-ray logs constructed from scintillometer measurements of outcrops can clarify structural interpretations of monotonous strata lacking distinct marker beds. Scintillometer readings were taken every two stratigraphic feet and compared with gamma-ray logs from nearby wells in horizontal strata.

The Brallier Formation near Dailey, WV is folded into three anticlines and two synclines in monotonous turbidites. Stratigraphic position differed by hundreds of feet from the first field estimate. This outcrop was tied to a nearby borehole reaching the Oriskany Sandstone, and structure of the Elkins Valley anticline was clarified.

Brallier Formation strata near Bluefield, WV are anomalously thin and are cut by numerous extensional faults in overturned beds, which cause small amounts of thinning. An outcrop gamma-ray log reveals that a major omission of 2,000 feet of strata occurs in one covered interval, which is the major offset of the Lindside normal faulting zone.

Chattanooga Shale at Hagan, VA, was interpreted by Miller and others (1954) to contain an underthrust, duplicating much Chattanooga Shale on the upturned, southern edge of the Pine Mountain block (also called a reverse fault in Brallier Shale upthrust to the south). The presumed duplication is a couplet of very black shale overlying lighter gray-weathering shale. The strata are intensely contorted and slicken-sided in places. The two dark shales interpreted as zones of fault duplication have distinctly different signatures, and these two dark zones can be clearly identified in the normal sequence of middle Chattanooga. Underthrusting duplication is rejected, in favor of bedding surface slippage in the shale toward the axis of the Powell Valley anticline.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky