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