Geology Reference
In-Depth Information
Similarly, wireline-log correlations across the
Taconic foreland basin from Kentucky to Virginia
indicate that the zone of shallow-water sediments
across the Cincinnati Arch is less than 180 km
wide (Hohman, 1998; Pope & Read, 1997a). This
suggests the Cincinnati Arch is far too narrow a
feature to be a peripheral bulge formed by fl ex-
ural loading of a continuous elastic or viscoelastic
plate. We suggest the Cincinnati Arch is so narrow
because it was forming as a response to the react-
ivation of basement features, where the fl exural
load stresses were transmitted to the same loca-
tion repeatedly over a prolonged period of time
(10-12 Myr).
The Sebree Trough also could be interpreted
as a back-bulge basin to the Cincinnati Arch.
However, the Sebree Trough is also too narrow a
feature to be a back-bulge basin, since the width
of the back-bulge basin should be comparable to
the width of the forebulge (Turcotte & Schubert,
2002). Likewise, the extremely small amount
of subsidence that could be generated from litho-
spheric fl exure in a back-bulge basin (<0.2% of the
subsidence at the load) also argues against such an
origin for the deep-water Sebree Trough.
amount of missing Blountian sediments indicate
that this structure experienced more wide-
spread erosion than the Jessamine Dome prior to
deposition of the overlying M5 strata.
Basement control on location of the
Cincinnati Arch
Evidence for the persistence of the Cincinnati
Arch indicates that it formed in the same location
repeatedly. Peripheral bulges may not migrate,
or can appear to reoccur in the same position
repeatedly (Waschbusch & Royden, 1992a,b),
owing to inherited structural weaknesses or
heterogeneities in the underlying lithosphere
(Fig. 1). In such a scenario, the weaker regions of
the lithosphere bend more easily than stronger
regions, so plate bending is concentrated within
the weak zone and the peripheral bulge becomes
'fi xed'. In this case, the width of the peripheral
bulge should refl ect the width of the weak zone, not
the fl exural wavelength of the plate (Waschbusch &
Royden, 1992a).
The most prominent heterogeneity in the
basement along the Cincinnati Arch (Fig. 9) is
the boundary between the Grenville front and the
Eastern Granite-Rhyolite Province (EGRP) and
the overlying East Continent Rift Basin (ECRB) that
parallels the eastern limit of the Cincinnati Arch
(Drazhoval et al ., 1992; Van Schmus et al ., 1993).
The Grenville front consists of high-grade metamor-
phic rocks that were thrust over the calc-alkaline
rocks of the EGRP and rift-related basalts and sedi-
ments of the ECRB (Drazhoval et al ., 1992; Hauser,
1993). In Kentucky the ECRB occurs in a restricted
zone that narrows to the south and is probably com-
pletely buried in Tennessee (Fig. 9). We suggest
that the ECRB was a zone of lithospheric weakness
and that stresses imposed on the eastern edge of the
Grenville lithosphere were transmitted westward
and concentrated in this area, producing increased
uplift or decreased subsidence along the overlying
Cincinnati Arch, adjacent to the boundary between
these two zones. This hypothesis explains the nar-
row width of this peripheral bulge (as manifested
in the north-south alignment of shallow-water fea-
tures and unconformities), and its occurrence in the
same location for such a prolonged period during
the Late Ordovician. A similar scenario, involving
isostatic uplift of low-density Proterozoic sedi-
ments to the west of the Grenville basement was
proposed for uplift along the Cincinnati Arch asso-
ciated with Acadian (Silurian-Devonian) deforma-
tion (Rast & Goodman, 1995).
Persistence of Cincinnati Arch
The localization of peritidal and shoal-water
facies, as well as buildups and unconformities
in the Upper Ordovician along the Cincinnati
Arch all indicate that the crest of the Cincinnati
Arch was a relatively high topographic feature
throughout much of the Taconic Orogeny. Such
topographic persistence does not necessarily
imply uplift and could refl ect lower subsid-
ence rates on the domes relative to surrounding
regions. The Jessamine and Nashville Domes
were the most common sites of shallow-water sed-
imentation on the arch, which was surrounded at
all times by deeper water. Additionally, while the
record of sedimentation on these domes is similar,
it is not identical and these variations in sedimen-
tation and stratigraphy probably provide informa-
tion about coeval tectonism and its sedimentary
response.
Erosion of the High Bridge and Nashville
Groups as well as the Millbrig and Deicke
K-bentonites indicates that the Jessamine and
Nashville Domes were topographically positive
features subjected to subaerial exposure and ero-
sion prior to Taconic phase deposition on these
domes. Erosion of the Deicke K-bentonite over
much of the Nashville Dome area and greater
Search WWH ::




Custom Search