Date of Award
Master of Science - Geology
Dr. Chris Barker
Dr. Melinda Faulkner
Dr. R. LaRell Nielson
Dr. I-Kuai Hung
The Caddo Gap quadrangle is located on the southern margin of the Benton Uplift, the orogenic core of the Ouachita Mountains. Recent field mapping focused on delineating stratigraphic and structural relationships in the southern part of the quadrangle to improve existing reconnaissance-scale maps of the area. New structures were discovered that provide further information about the structural evolution of the Ouachita fold and thrust belt.
Field work included mapping the three informal members (Lower, Middle and Upper) of the Arkansas Novaculite. Major folds within the study area include the Nelson Mountain Anticline and two synclinal folds (“South Caddo Mountain” and “Arrowhead Mountain”) where south-dipping Novaculite has been folded around very sharply in map view, resulting in a “fishhook.” The backthrust interpretation for Strawn Mountain (Haley et al., 2009) was found to be more structurally complex than previously proposed.
Analysis of the Nelson Mountain Anticline shows it is steeply inclined and gently plunging: the calculated mean axial plane (n = 104) is 276°, 78°SW (N84°W, 78°SW) and mean fold axis is 22°, 101° (22°, S79°E), similar to the reconnaissance-scale findings of Evansin (1976). Two fishhook folds, both located at the west ends of east-west ridges of Arkansas Novaculite, have similar geometries. The mean axial planes are 290°, 71°SW (N70°W, 71°SW) and 289°, 81°SW (N71°W, 81°SW) and average fold axes are 37°, 125° (37°, S55°E) and 40°, 117° (40°, S63°E).
Previously unmapped, northeast-trending strike-slip faults in the Arkansas Novaculite were discovered on the east-west “South Ridge” immediately south of and adjacent to the South Caddo Mountain fold. This style of faulting differs significantly from previous regional interpretations (Haley et al., 2009) which instead show South Caddo Mountain juxtaposed against the north side of the South Ridge by thrust faulting. Field mapping found no evidence for thrust faulting on South Caddo or Arrowhead Mountains.
An alternative hypothesis is that late-stage, northwest-directed compression formed left-lateral strike-slip tear faults that cut obliquely through the regional structures of the Caddo Mountains. This style of faulting better explains the fishhook drag folds and the strike-slip faults on the South Ridge, with Strawn Mountain backthrusts accommodating the northwest-directed compression on the north limb of the Nelson Mountain Anticline.
Messmer, Martin, "Structural Geology of the Caddo Gap Area, Ouachita Mountains, Arkansas" (2018). Electronic Theses and Dissertations. 173.
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