STRUCTURAL ANALYSIS OF FOLD-RELATED JOINTS IN MCLEMORE COVE, WALKER CO., NORTHWEST GEORGIA

Pigeon and Lookout mountains are underlain by two broad synclines. McLemore Cove is the topographic expression of the joining anticline. As mapped by Cressler (1981, Georgia Geol. Surv., IC 29), this train of folds has a wavelength (syncline to syncline) of nearly 10 km and an amplitude of approximately 1500 m, which diminishes to the SW. Bedding orientations indicate that the McLemore Cove anticline plunges gently (5°) to the SW (az=209°) and is slightly asymmetric, consistent with NW vergence.

McLemore Cove provides more than 400 m of topographic relief and reveals Ordovician through Pennsylvanian stratigraphy. Escarpment-forming Pennsylvanian sandstone dips outward, away from the cove, along its W and SE rims and around its SW end (Dougherty Gap), beyond which Pigeon and Lookout Mountains are joined as a continuous plateau.

Joints (mode I fractures) in the McLemore Cove anticline are differentiated by orientation into two sets that are geometrically related to the fold. Cross-fold joints dip steeply to the NE. Poles to cross-fold joints are clustered around a mean of 4° 211° (plunge & trend), which coincides with the fold axis. Considered collectively, poles to axial joints are clustered around 3° 295°. Axial joints in the W limb of the fold dip steeply to the E and SE, while those in the SE limb dip steeply to the W and NW. At the nose of the fold, axial joints are nearly vertical. In each subdivision of the fold, intersections of mean orientations of bedding and axial joints (4° 204°, 6° 203°, and 7° 211°) nearly coincide with the fold axis, indicating that the fold axis is common to both planes. Cross-fold and axial joints indicate extension parallel and normal to the fold axis.

The plateau surface of Pigeon and Lookout mountains is drained by a rectangular pattern of streams, linear elements of which coincide with joint orientations. This suggests that joints like those observed in McLemore Cove pervade the adjacent plateau and exhibit control on its geomorphologic evolution. These joints are also likely avenues for subterranean water flow.

Geometric relationships between the McLemore Cove anticline and joints within it suggest coeval development of the structures. Alternatively, joints may have formed in response to fold-guided postorogenic stress, in which case they may be more recent.