Geology Reference
In-Depth Information
Wilson, 1962; Holland & Patzkowsky, 1997). Like
the peritidal facies, shoal facies on the Nashville
Dome display a more organized pattern of distri-
bution in the M5-C5 sequences. Cross-bedded,
crinoidal to skeletal, phosphatic grainstones are
a prominent feature of the Bigby-Cannon, Catheys,
Leipers and Fernvale formations. Although such
grainstones are present locally at the base of several
parasequences, they are thickest and best developed
along a persistent north-south belt (Fig. 6) on the
western fl ank of the Nashville Dome (Wilson, 1949;
Holland & Patzkowsky, 1998). There, the cross-beds
display a strong bimodal pattern, suggesting tidal
infl uence and the development of a tidal-bar belt
(Ball, 1967) along the edge of the Sebree trough, just
west of the Nashville Dome. This belt of carbon-
ate sands passes westward into a thick package of
deep subtidal storm beds and passes eastward into
bioturbated shallow subtidal packstones.
biostrome-producing organisms is present than
during the Blountian phase and includes chaetetids
( Solenopora ; see Riding, 2004), stromatoporo-
ids, tabulates, colonial rugosans and bryozoans.
Several distinct biostromes are present, but all are
restricted to a few square kilometres in area or less
and do not repeatedly occupy the same region.
Shallow subtidal, shoal and peritidal facies also
all commonly contain these potential buildup-
constructors (Wilson, 1949; Weir et al ., 1984;
Holland & Patzkowsky, 1998; Pope & Read, 1998;
Patzkowsky & Holland, 1999 ).
On the Jessamine Dome, the Sulphur Well
Member of the Lexington Limestone is a bryo-
zoan biostrome up to 5 m thick, surrounded by a
grainstone shoal along the axis of the Cincinnati
Arch (Cressman, 1973; Ettensohn et al ., 1986b).
Deposition of the Sulphur Well Member is hypoth-
esized to be structurally controlled (Ettensohn
et al ., 2004). Stromatoporoids and chaetetids
locally form biostromes in the Lexington Limestone
(McLaughlin et al ., 2004).
Bryozoans also form small, localized bio-
stromes in shallow subtidal facies throughout
the Cincinnatian (Cuffey, 1998), but these are
typically less than a metre thick and less than a
few dozen square metres in area. A small patch
biostrome consisting of chaetetids is present in
one outcrop in the Cincinnatian near Winchester,
Kentucky (Weir et al ., 1984). The C5 sequence in
northern Kentucky and southeastern Indiana con-
tains a series of biostromes composed of colonial
rugosans and tabulates, including the Bardstown,
Kentucky 'reef' (Browne, 1964), the Otter Reef
Coral Bed near Richmond, Kentucky (Simmons &
Oliver, 1967), and various biostromes associated
with Saluda peritidal facies (Hatfi eld, 1968). In
most places, these biostromes are approximately
1 m thick.
On the Nashville Dome, few biostromes
are known from the Taconic phase. One low
(50 cm), small (100 m in length) biostrome com-
posed entirely of the chaetetid Solenopora is
present in the C1 sequence along the west-
ern fl ank of the Nashville Dome near Franklin,
Tennessee (Holland & Patzkowsky, 1998, their
Fig. 11). Large domal stromatoporoids are com-
mon in grainstone-rich facies of the M5 and M6
sequences, as they are on the Jessamine Dome,
and form only localized biostromes up to a
metre thick. Rugosan and tabulate biostromes
are absent in the Cincinnatian on the Nashville
Dome, unlike the Jessamine Dome and northern
Cincinnati Arch.
Buildups
Organic buildups (generally biostromes, less com-
monly low-relief bioherms) are not common on
the Jessamine and Nashville Domes and are rarely
more than a metre thick. Commonly, the organisms
capable of producing organic biostromes are associ-
ated with shoal and peritidal facies and have a rela-
tively fi xed distribution in the Late Ordovician.
Blountian phase
On the Jessamine and Nashville Domes, numer-
ous organisms capable of constructing biostromes
are present in the High Bridge and Stones River
Groups. Most commonly, these include stromato-
poroids and tabulate corals (favositids and tetra-
diids; Wilson, 1949; Patzkowsky & Holland, 1999).
However, in only a few cases do they occur in
concentrations dense enough to constitute bio-
stromes. For example, small, isolated buildups
developed in the Carters Limestone near the south-
ern margin of the Nashville Dome (Alberstadt
et al ., 1974). These biostromes consist of tabu-
lates, stromatoporoids and bryozoans and are up
to a few metres thick. More commonly, biostrome-
producing organisms are present but only sparsely
distributed through shallow subtidal bioturbated
wackestone and packstone facies (Cressman &
Noger, 1976; Patzkowsky & Holland, 1999).
Taconic phase
In the M5 through C6 sequences of the Jessamine
and Nashville Domes, a wider array of potential
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