Geology Reference
In-Depth Information
Unconformities
surrounding the dome (Haynes, 1994), indicating
that erosion was limited to the topographic-
ally higher dome. Erosional fragments of the
Carters Formation occur locally in the basal
Hermitage Formation (Wilson, 1962; Holland &
Patzkowsky, 1998).
Unconformities separating third-order deposi-
tional sequences record their greatest erosion or
period of non-deposition on the Jessamine and
Nashville Domes along the crest of the Cincinnati
Arch in the Late Ordovician, and commonly pass
off the arch into downdip conformities.
Taconic phase
Blountian phase
Widespread subaerial unconformities in the
Lexington Limestone developed only above
the regionally developed tidal-fl at facies
(Perryville Member - Lexington Limestone; Dove
Beds - Nashville) on the Cincinnati Arch (Mackey,
1972; Holland & Patzkowsky, 1996, 1997; Pope &
Read, 1997a,b) and pass down-ramp into con-
formable subtidal surfaces. Along the arch the
unconformity above the Perryville tidal-fl at
facies has small karstic cavities up to 1.5 m deep
that are fi lled with phosphatic sand produced
during the subsequent transgression (Pope &
Read, 1997b). Locally developed tidal fl at facies
(cf. Devils Hollow Member - Lexington) show less
evidence for subaerial exposure and pass laterally
into phosphatic hardgrounds often capped by
phosphatic grainstone.
Subaerial unconformities are more easily recog-
nized over the Nashville Dome due in large part to
the greater abundance of peritidal lime mudstones
(Dove Beds) in this area (Wilson, 1962). However,
the most widespread unconformity occurs at the
M6 boundary and this surface is directly correlative
with the unconformity above the Perryville Member
in Kentucky (Holland & Patzkowsky, 1996, 1997).
Third-order sequence boundaries in the
Cincinnatian Series (C1-C6) on the Jessamine
Dome and northern Cincinnati Arch rarely dis-
play obvious evidence of differential truncation
of underlying strata, in part because of an absence
of marker horizons such as bentonites. The C6
sequence, however, is preserved only on the
northern half of the Cincinnati Arch and shows a
depositional pinchout to the south. This restric-
tion to the northern arch is consistent with the
distribution of peritidal facies in the Cincinnatian
(Figs 5 and 7), as well as the east-west orientation
of the grainstone shoals in the C2-C4. Collectively,
these features suggest a persistent northward
dip of Jessamine Dome and northern Cincinnati
Arch in the C2-C6 sequences, with persistently
shallower conditions to the south.
On the Nashville Dome, truncation at the
M4-M5 boundary, made visible by K-bentonites
as in central Kentucky, reveals a strong east-west
Regional truncation has not been documented
at the M2-M4 sequence boundaries over the
Jessamine Dome or the M1-M4 boundaries on
the Nashville Dome, at least in part owing to a
lack of marker horizons such as K-bentonites.
Nonetheless, these erosional surfaces provided the
material for multiple intraclastic beds at the bases
of these depositional sequences (Kuhnhenn
et al
.,
1981; Holland & Patzkowsky, 1996). Additionally,
the occurrence of a laterally continuous
dolostone (Oregon Formation) in this other-
wise limestone dominated unit, located on
the Jessamine Dome suggests that this area was
subaerially exposed for a protracted interval
following deposition.
K-bentonite ash beds are common in the upper
part of the Blountian succession and two of
these beds (Deicke and Millbrig) are regionally
widespread, facilitating correlations throughout
the eastern mid-continent and Appalachian Basin
(Haynes, 1994; Kolata
et al
., 1996). The Millbrig
(locally known as Mud Cave or T-4) K-bentonite,
which occurs immediately below the High Bridge
Group-Lexington Limestone contact, is eroded
over much of the Jessamine Dome and is pre-
served only in structural lows (Fig. 4; Haynes,
1994; Kolata
et al
., 1996, 1998). The erosional
relief of the post-High Bridge Group unconformity
(M4-M5) is localized over the Jessamine Dome on
the Cincinnati Arch where up to 5 m of the Tyrone
Limestone is missing (Cressman, 1973). Over
the Jessamine Dome, the basal Lexington
Limestone commonly contains clasts up to 10 cm
in diameter of Tyrone Limestone lime mudstone.
This unconformity passes down-ramp into a cor-
relative conformity (Pope & Read, 1997b).
Over the Nashville Dome, up to 10 m of the
Carters Formation, including both the Deicke and
Millbrig K-bentonites are locally eroded, with the
amount of erosion increasing towards the west-
ern fl ank of the Nashville Dome (Wilson, 1962;
Wilson & Stearns, 1963; Holland & Patzkowsky,
1997). However, the K-bentonites are present on
the eastern fl ank of the dome and in the subsurface
