Geology Reference
In-Depth Information
Jessamine and Nashville Domes in the Upper
Ordovician.
N
OHIO
INDIANA
BLOUNTIAN
Blountian phase
50 km
WEST
VIRGINIA
On the Jessamine Dome in Kentucky, the basal
unit of the Blountian phase is the Wells Creek
Formation, a locally sandy dolostone deposited in
a peritidal setting. The bulk of the overlying High
Bridge Group consists predominantly of intensely
bioturbated shallow subtidal to intertidal lime-
stone (Camp Nelson and Tyrone Limestones) and
lesser amounts of dolostone (Oregon Formation;
Cressman & Noger, 1976; Kuhnhenn & Haney,
1986; Pope & Read, 1998). Parasequences in the
High Bridge Group commonly consist of thin,
intraclastic packstone bases that grade upward
into pellet packstone. This unit is in turn overlain
by skeletal/pelletal wackestone/mudstone capped
by fenestral mudstone (Pope & Read, 1998). Most
of these parasequences are diffi cult to trace later-
ally and probably formed by autocyclic processes
(Ginsburg, 1971; Pratt & James, 1986).
The Oregon Formation, a thin, diagenetic unit
of peritidal dolostone occurs in a roughly semi-
circular region (~1000 km
2
) over the Jessamine
Dome (Fig. 4) on the Cincinnati Arch within the
otherwise limestone-rich High Bridge Group
(Wolcott
et al
., 1972; Cressman & Noger, 1976;
Kuhnhenn
et al
., 1981; MacQuown
et al
., 1984;
Kuhnhenn & Haney, 1986). The Oregon Formation
contains abundant desiccation and prism cracks,
which suggest increasing aridity and emergence
of the platform during this time (MacQuown
et al
., 1984). The dolostone is fi ne-grained, fabric-
preservative, and probably formed by evaporative
pumping on a topographically high area (Horrell,
1981; Kuhnhenn
et al
., 1981; Kuhnhenn & Haney,
1986), indicating that this area of the arch was
emergent and probably surrounded by marine
waters when the dolostone formed.
On the Nashville Dome in Tennessee, peritid-
ally capped parasequences occur in the Blountian
phase in the Murfreesboro, Ridley, Lebanon and
Carters Limestones of the Stones River Group on
the Nashville Dome (Fig. 3; Holland & Patzkowsky,
1998). Most of these peritidal caps are biotur-
bated fenestral lime mudstones, suggesting humid
depositional conditions, but some, such as those
at the top of the Upper Carters, consist of desic-
cation-cracked, laminated, dolomitic mudstones,
indicative of more arid conditions. The distribu-
tion of all of these tidal-fl at deposits shows no
clear pattern over the Nashville Dome, suggesting
KENTUCKY
EROSION OF MILLBRIG
BENTONITE
VIRGINIA
TENNESSEE
SHALLOW
SUBTIDAL TO
PERITIDAL
LIMESTONE
OUTCROP
BELT
OREGON FM
PERITIDAL
DOLOSTONE
ALABAMA
Fig. 4.
Map of Blountian features along the Cincinnati
Arch. The distribution of the Oregon Formation indicates
that it formed along a topographic high (Wolcott
et al
.,
1972; Kuhnhenn
et al
., 1981). The Millbrig K-bentonite is
eroded over much of the Jessamine and Nashville Domes
(adapted from Scharpf, 1990; Haynes, 1994; Kolata
et al
.,
1998) suggesting that these areas were topographically high
or exposed during deposition of this widespread ash bed.
that the tidal fl ats developed as a series of isolated
islands (cf. Pratt & James, 1986) rather than from
a persistent localized high. However, the lack of
similar coeval peritidal facies in the Sequatchie
Valley to the immediate southeast of the Nashville
Dome indicates that the Nashville Dome was a
broad, shallow-water feature during the Blountian
phase of the Taconic Orogeny.
Taconic Phase
The Lexington Limestone and Nashville Group
(Fig. 5) record shallow to deep, predominantly
subtidal, cool-water carbonate and shale depos-
ition over the Cincinnati Arch (Cressman,
1973; Patzkowsky & Holland, 1993; Holland &
Patzkowsky, 1996; 1997; Pope & Read, 1997b).
Peritidal facies in these units are rare and consist
of bioturbated fenestral limestone. They are local-
ized over the arch (Fig. 6) and pass in all direc-
tions into deeper-water facies (Hrabar
et al
., 1971;
Mackey, 1972; Cressman, 1973; Ettensohn, 1992;
Holland & Patzkowsky, 1996, 1997, 1998; Pope &
Read, 1997a,b). The peritidal facies of the humid
Perryville tidal-fl at complex (Mackey, 1972)
were best developed along a north-south line
along the Cincinnati Arch, beneath a third-order
sequence boundary in the Lexington Limestone
(Pope & Read, 1997a, 1998). This tidal-fl at suc-
cession is correlative with similar facies in the
