The Cincinnati Arch: a stationary peripheral bulge during the Late Ordovician - Perspectives in Carbonate Geology - page 255

The proximal foredeep was bordered on the east
by tectonic highlands that served as the principal
siliciclastic source to the region for the remain-
der of the Ordovician. The western side of this
foredeep passed into a shallow carbonate ramp
developed over the Cincinnati Arch (Fig. 2). Major
structural elements associated with this arch
were the Nashville and Jessamine Domes on the
crest, the Sebree trough on the western side and
border faults of the Rome trough on the eastern
side (Borella & Osborne, 1978; Cable & Beardsley,
1984; Weir et al ., 1984; Diecchio, 1993; Kolata
et al ., 2001).
The Taconic foreland basin in the central and
southern Appalachians experienced its earliest
development during the Late Ordovician, and
its history commonly is subdivided into two dis-
tinct phases (Fig. 3). The fi rst phase, known as the
Blountian (W-b to M4 sequences), began in the lat-
est Llanvirn to earliest Caradoc and its depocentre
was in eastern Tennessee (Shanmugam & Walker,
1980, 1982; Shanmugam & Lash, 1982; Ettensohn,
1994). The second phase, known as the Taconic
(M5-C6 sequences), began in the middle to late
Caradoc and its depocentre was primarily in east-
ern Pennsylvania and northern Virginia (Ettensohn,
1994). Each of these phases produced a distinctive
second-order (10-15 Myr duration) depositional
sequence, characterized by pronounced rapid
deepening in the foredeep followed by prolonged
shallowing and progradation on the adjacent shal-
low-water carbonate ramp (Rodgers, 1971).
The Blountian phase (Fig. 3) records the
development of the rapidly subsiding Sevier
foredeep in Alabama, Georgia, Tennessee and
Virginia, which was subsequently fi lled with a
thick turbidite and deltaic package (Shanmugam &
Walker, 1980, 1982; Shanmugam & Lash, 1982;
Ettensohn, 1991, 1994). These deep-water facies
pass westwards and up-ramp into mid-ramp facies
of the Chickamauga Group in Tennessee and coeval
limestone units in Virginia (Read, 1980; Ruppel &
Walker, 1984; Pope & Read, 1998). Farther to the
west along the Cincinnati Arch, peritidal carbon-
ates of the High Bridge and Stones River Groups
accumulated under humid conditions on a nearly
fl at, muddy carbonate ramp in Kentucky, Virginia
and Tennessee (Cressman & Noger, 1976; Holland &
Patzkowsky, 1996, 1997; Pope & Read, 1998).
The Taconic phase (Fig. 3) is characterized
by the Martinsburg foredeep, best developed in
northern Virginia, eastern Maryland and eastern
Pennsylvania (McBride, 1962; Read, 1989; Lash,
1988; Ettensohn, 1991). Similar to the Sevier

foredeep, the Martinsburg foredeep was fi lled with
a thick turbidite to deltaic package. These coarse
siliciclastic sediments prograded westward, cul-
minating in the Queenston Delta (Dennison, 1976;
Kreisa, 1981; Diecchio, 1993). To the west, the
foredeep passed up-dip onto a mixed carbonate-
siliciclastic ramp developed over the Cincinnati
Arch.
These Taconic phase rocks along the Cincinnati
Arch record a fundamental change in sedimenta-
tion style from relatively pure peritidal carbonates
deposited in warm waters (Stones River and High
Bridge Groups), to cool, temperate water subtidal
carbonates interbedded with varying amounts of
shale (Brookfi eld, 1988; Patzkowsky & Holland,
1993; Lavoie, 1995; Pope & Read, 1998). Relatively
pure subtidal and peritidal carbonates were depos-
ited along the crest of the Cincinnati Arch and are
surrounded by interbedded carbonate and shale of
the Sebree trough and Appalachian basin foredeep
(Keith, 1988a). The cleaner carbonate facies of the
Lexington Limestone in Kentucky and Nashville
Group in Tennessee (Fig. 3) are laterally equivalent
to the well-described Trenton facies of the north-
ern Appalachian Basin (Keith, 1988a,b; Brett et al .,
2004). Biostratigraphic evidence suggests that
uppermost Ordovician (Gamachian) rocks are
not preserved on the Cincinnati Arch or in the
Appalachian foreland basin in Kentucky (Sweet,
1984). Such strata were probably not deposited
or were eroded during the eustatic fall associated
with short-lived terminal Ordovician glaciation
(Dennison, 1976; Brenchley et al ., 1994). Uppermost
Ordovician siliciclastics do occur in Virginia and
Tennessee (Dorsch et al ., 1993) and in the Queenston
delta farther north (Dennison, 1976).

EVIDENCE OF PERIPHERAL BULGE
POSITION

Several lines of evidence suggest that the
Cincinnati Arch was a persistent and relatively
fi xed site of shallow-water deposition throughout
the Late Ordovician. In particular, the distribu-
tion of peritidal facies, shoal-water facies, build-
ups and unconformities provide evidence that the
Cincinnati Arch was a shallow-water to emergent
feature throughout most of the Late Ordovician.

Peritidal facies

Peritidal facies are consistently associated with
the crest of the Cincinnati Arch, particularly the