ABSTRACT

This outcrop investigation of fractures in the Devonian Ohio Shale identified three regional joint sets (Fig 1) and documented the fractography, geometry, and chronology of these sets. The three regional joint sets have similar orientations to the fractures in clastic rocks documented throughout Ohio by Ver Steeg (1942, 1944). All three joint sets have delicate fractographic features characteristic of tensile Mode I joints. NW-striking Set I joints are typically planar to curviplanar with a spacing frequency of 6.86 joints/ 10±4.26 feet. N-striking Set II joints are the least well developed of the three sets, show varying curving to planar geometry, and have a frequency of 7.49 joints/10±5.58 feet. NE-striking Set III joints are extremely planar, show well developed plume axes and arrest lines, and have a frequency of 3.7 joints/10±2.1 feet. The fracture chronology from outcrop observations in the Ohio Shale indicates that Set II is the oldest, followed by Set III, and finally Set I. In contrast, the fracture chronologies of Evans (1994) from Devonian Shales in cores from the Eastern Gas Shales Project and Engelder and Geiser (1980) for joints in Devonian rocks of New York have NW-striking Set I joints older than NE-striking Set III joints.

FIGURE 1:

Strike averages of Set I (a), Set II (b), and Set III (c). Devonian outcrop trace is shown.

The fracture patterns in the Ohio Shale were compared with the position and orientation of geophysically defined basement structures to determine if the joints formed by reactivation of older, subsurface fault zones. At one station in Gates Mills, Ohio, there was an increase in fracture frequency from 4 fractures/10 feet to 110 fractures/10 feet within a 30-foot wide zone. This highly fractured zone is northeast of and nearly parallel to the Middleburg Fault, suggesting that the increase in frequency may be related to Alleghanian reactivation of this fault zone. However, in general there was no systematic relationship between joint set trends and basement structures, and the propagation directions of joints interpreted from fracture plume patterns indicate that NW-striking Set I and NE-striking Set III joints propagated horizontally, and were not formed by vertical migration of fractures from units below the Ohio Shale.

One locally developed joint set is present in the area northwest of Columbus, Ohio, in Franklin and Delaware Counties. The main dip of the set is in an ESE direction. The fractographic features on this set show a very consistent morphology indicating Mode I extension. The joints all propagated from a single bedding plane, which generally consisted of a gray shale interbed in the black carbonaceous shales. The frequency of the joints at the original bedding plane was up to 16 fractures per foot, and became more widely spaced as some joints terminate downwards. Plume patterns always show a downward propagation pattern in the direction of dip. The high density of fractures in this set maybe of economic interest in locating shallow Ohio Shale gas wells in central Ohio.

Other types of deformation were also observed in the Ohio Shale. The oldest structures observed were clastic dikes in Scioto County in southern Ohio (Fig 2a). The dikes have a siltstone composition and were intruded into the Ohio Shale post-deposition and pre-jointing. Small-scale thrust faults were observed in the Chagrin Member within the northern outcrop belt of the Ohio Shale (Fig 2b). Typical faults have dips ranging between 10° and 30°, apparent vertical offset ranging between 6 and 32 inches, and breccia zones consisting of crushed siltstone and clay along the fault planes. Kinematic analysis of these faults yields a compression axis nearly parallel to the NE-striking contemporary maximum compression direction, indicating the faults may be neotectonic structures (Fig 2c). Pop-ups observed in streams in northern Ohio show a trend maximum perpendicular to the contemporary maximum compression direction and are also interpreted to be neotectonic (Fig 2d).

FIGURE 2:

(a) Equal-area lower-hemisphere diagram of clastic dikes in southern Ohio, (b) Equal-area lower-hemisphere diagram of orientations of small-scale thrust faults in the Chagrin Member in northern Ohio, (c) Fault plane solution for small-scale thrust faults in the Chagrin Member. Contemporary compression direction (Bold arrows)from (Plumb and Cox, 1987) (d) Rose diagram of trends of pop-up axes in northern Ohio.

The Ohio Shale provides a unique spectrum of deformation structures that can be used to construct a deformation history model for Ohio extending from the Late Devonian to the Present. Clastic dikes record the oldest deformation within the Ohio Shale in the Late Devonian-Early Mississippian. Set II joints formed approximately parallel to the Cincinnati Arch during Acadian relaxation and forebulge migration in the Appalachian Basin. Set III joints formed at depth in response to basin flexure during the Alleghany orogeny. Set I joints formed in response to Alleghenian compression during the development of the southern and central Appalachians. Small-scale, neotectonic thrust faults formed at depths where thrust conditions prevail in the Appalachian Basin. At the surface, pop-ups released stresses related to the neotectonic maximum contemporary compression direction.