Geology Reference
In-Depth Information
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-Mississip-
pian: DMctfp: Chattanooga Shale, Tuscumbia Limestone, Fort Payne Chert; Mississippian-Pennsylvanian:
MPpw: Parkwood Formation; Pennsylvanian: Ppv: Pottsville, containing the following marker units (old-
est 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 ex-
pression 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 topo-
graphic 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
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