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Chapter 7 ¡ Properties of Faults
Fig. 7.1. Geologic map of the Ensley area, Alabama. The topography is nearly flat, making the map close to a horizontal section. Units (oldest to youngest): Cambrian: Cc: Conasauga Ls., Ckt: Ketona dolomite; Cambro-Ordovician: COk: Knox dolomite; Silurian: Srm: Red Mountain Formation; Devonian-Mississippian: DMctfp: Chattanooga Shale, Tuscumbia Limestone, Fort Payne Chert; Mississippian-Pennsylvanian: MPpw: Parkwood Formation; Pennsylvanian: Ppv: Pottsville, containing the following marker units (oldest to youngest): by: Boyles sandstone, bc: Black Creek coal, ml: Mary Lee coal, nc: Newcastle (upper Mary Lee) coal, pt: Pratt (American) coal, cb: Cobb coal. OVT: Opossum Valley thrust, JVT: Jones Valley thrust. Contact relationships: A: offset of strike traces, B: truncation of units at contact, C: unit crosses contacts, D: missing section, E: missing section, F: older over younger. (After Butts 1910 and Kidd 1979)
verse fault drag (Sect. 7.2.7). The contact indicated by C and D is a reverse fault. Once a contact has been identified as a fault, it is usually shown as a heavier line on the map. The juxtaposition of different units across a fault favors some form of topographic expression for the fault because of the likelihood that the two units will not weather and erode identically. The boundary between the juxtaposed units is likely to form a topographic lineament. The fault-zone material (Sect. 7.2.6) may erode differently from any of the surrounding country rocks and thus cause a topographic valley or ridge along the fault. 7.2.2 Discontinuities on Reflection Profile The primary criterion for recognizing a fault on a seismic- or radar-reflection profile is as a break in the lateral continuity of a reflector or a group of reflectors (Fig. 7.2a, arrows A). Picking faults can be difficult because sometimes the reflectors hang over the actual fault