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Allen, 1995). Conversely, the location of the
peripheral bulge can become relatively 'fi xed'
along lateral lithospheric variability in the fore-
land (Waschbusch & Royden, 1992a,b) or when
pre-existing structural weaknesses in the crust are
reactivated (Dorobek
et al
., 1991).
Whether the Cincinnati Arch was a positive
structural feature that infl uenced deposition
in Kentucky and Tennessee during the Late
Ordovician Taconic Orogeny is contentious.
Interpretations range from the arch having little
or no infl uence on sedimentation (Gustadt,
1958; Weiss
et al
., 1965; Cressman, 1973; Weir
et al
., 1984), to limited uplift that affected sedi-
mentation primarily during the Cincinnatian
(Ettensohn, 1992, 1994), to geographical uplift
only on the domes with an intervening saddle
(Borella & Osborne, 1978). However, integration
of outcrop and core data (Wilson, 1962; Hrabar
et al
., 1971; Mackey, 1972; Pope & Read, 1997a,b,
1998), favours uplift or decreased rates of subsid-
ence along the Cincinnati Arch in Kentucky and
Tennessee during much of the Late Ordovician and
that this affected sedimentation, a view predicted
by stratigraphic modelling (Quinlan & Beaumont,
1984). During this time a shallow-water carbon-
ate platform developed on the Cincinnati Arch
and was surrounded by deeper water, storm-
infl uenced carbonate and shale (Keith, 1988a,b;
Holland, 1993; Holland & Patzkowsky, 1996, 1997;
Pope & Read, 1997a,b; McLaughlin
et al
., 2004).
This paper combines the detailed sequence strati-
graphic framework of the Upper Ordovician strata
of the Cincinnati Arch (Holland, 1993; Holland &
Patzkowsky, 1996, 1998; Pope & Read, 1997a,b,
1998) to decipher the geographical position and
depositional history of the peripheral bulge in this
basin during the Taconic Orogeny. This history is
compared with that predicted by several types of
fl exural models to gain insight into the mechan-
ical behaviour of the Appalachian foreland basin
system during its initial formation.
may deform under a load in two distinct modes,
depending on whether the lithosphere is modelled
as a thin sheet of elastic or viscoelastic material
overlying an asthenosphere modelled as an invis-
cid fl uid. In elastic models (Fig. 1), the foreland
basin adjacent to the thrust load undergoes imme-
diate subsidence proportional to the mass of the
load, with the amount of subsidence decreasing
exponentially with distance (Nádai, 1963). Distal
areas experience broad, but minor uplift on the
peripheral bulge. For an elastic plate, the amount
of uplift or subsidence at any location will change
over time only if the distribution or magnitude of
the load changes. In contrast, the uplift or subsid-
ence at any location will change for a viscoelastic
plate even if the distribution or magnitude of the
load does not change. For a viscoelastic rheology
(Fig. 1), the plate will deform initially as if it were
elastic, but over time it will behave as if it were a
weaker elastic plate. In effect, this will cause the
basin to deepen and narrow, with the peripheral
bulge becoming higher and narrower, while
moving towards the thrust load.
In both of these cases, the location of the
peripheral bulge will shift if the location of load-
ing shifts. For example, in the case of a peripheral
foreland basin, in which the edge of a continent
undergoes attempted subduction, such as the
modern-day Persian Gulf, the location of loading
shifts considerably as the continent approaches
the subduction zone (Jacobi, 1981). Relative to a
fi xed point on the continental interior, the loca-
tion of loading initially lies over the adjacent,
attached oceanic lithosphere. As the continent
approaches the subduction zone, the location of
loading moves progressively toward this fi xed
point. Ultimately, the loading takes place at
the edge of the continental lithosphere, before
subduction is aborted, owing to the buoyancy of
continental lithosphere. As a result, the forebulge
migrates from the oceanic lithosphere across the
edge of the continent to the interior of the con-
tinent. In the case of a retro-arc foreland basin,
located on the craton side of a volcanic arc, such
as in the Precordillera of Argentina or the Western
Interior Basin of North America, thrust-sheet
motion is limited to a relatively defi ned area of
shortening (Allen & Allen, 2005). As a result, the
peripheral bulge experiences less overall motion
towards the craton. Additionally, if the litho-
sphere beneath the foreland basin varies later-
ally in its rigidity, the position of the peripheral
bulge may become 'fi xed' for extended periods of
PERIPHERAL BULGE BEHAVIOUR
Models of foreland basin stratigraphy indicate
that peripheral bulge behaviour is controlled by
several factors, including whether the lithosphere
displays an elastic or a viscoelastic response,
whether the lithosphere is mechanically continu-
ous or broken, and whether the locations of loads
are relatively static or mobile. The lithosphere
