Geology Reference
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of this study is the recognition that organic-rich
sediments are transported from one compartment
to another within the intracoastal system.
The shallowing-upwards sediment package
follows the increase in accommodation space
that becomes available initially close to shore, in
the coastal lakes, and later, further inland in the
marshes. The fi rst geomorphical features to fi ll
in were small ponds directly behind the beach
ridges (the Southern Lakes). Second was the large
coastal lagoon that lies slightly more inland (Lake
Ingraham). Finally, with sea-levels overtopping
the marl ridge, the interior former freshwater
marsh fi rst collapsed to become a back-barrier
lagoon, and recently started to infi ll with muds
carried across the marl ridge by fl ood tides and
storms. With steadily rising sea level, marine
fl oodwaters probably will spill more frequently
across the ridge, feeding a progressively larger
tidal volume (prism). Generally, a larger tidal
prism results in a larger cross-sectional area of the
channels through which the water fl ows (O'Brien,
1931), and thus increased channel margin erosion
can be expected, as the main channels already
reach to bedrock.
single sea-level excursion may well be incorrect.
During a single episode of platform fl ooding,
multiple upward-coarsening carbonate cycles,
deposited during the infi lling of Florida Bay by
repeated aggradation of mudstone to packstone
successions, have been described by Tedesco &
Wanless (1991). Drummond & Wilkinson (1993)
also create, in a one-dimensional computational
forward model of carbonate accumulation, mul-
tiple shallowing-upwards cycles during a single
sea-level fl uctuation. These results from both fi eld
and modelling studies serve as a caution that a
simple one-to-one relationship between sea-level
change and cycle response may not always exist,
particularly with scenarios of low-amplitude
sea-level change.
Recent research on late Holocene sea level
suggests small sea-level oscillations or steps
embedded within the overall sea-level rise.
This study of sedimentation in response to the
latest small rise along the southwest coast of
Florida shows that these embedded small, accel-
erated sea-level jumps can trigger rapid sedimen-
tation and generation of a shallowing-upwards
cycle within the coastal system. This recent rise,
combined with the sea-level oscillation between
3200 and 2400 yr BP, occurs within a single
overall sea-level high-stand. These embedded
oscillations are generating multiple shallowing-
upwards sediment successions. These multiple
successions can either be vertically stacked
or spatially offset, and completely, partially
or not eroded by the subsequent succession.
Importantly, the nature of each embedded suc-
cession may be quite different, controlled by the
nature and pattern of pre-existing sedimentary
environments and topography, and the details of
the sea-level oscillation.
The data and interpretations described in this
paper provide new insights into the generation
of small-scale shallowing-upwards successions.
Observations of modern environments such as
Cape Sable reveal that each pulse of sea-level rise
can result in a deposition of a shallowing-upwards
facies succession and, since transgression is step-
wise, multiple shallowing-upward successions
may develop. Applying these fi ndings to the rock
record, a composite set of metre-scale peritidal
carbonates, in which each unit commonly is inter-
preted to be deposited during a prolonged period
of stable or slowly rising sea level, might indeed
have been the depositional result of a series of
rapid, small pulses of sea-level rise, within an
overall high-stand.
Geological signifi cance
It has proved diffi cult to determine from the
stratigraphic record whether shallowing-upwards
carbonate rock associations are driven by autocy-
clicity or allocyclicity (Lehrmann & Goldhammer,
1999). Mechanisms responsible for formation
of peritidal lithological successions have been
a focus of debate between supporters of the two
end-member models (Burgess
et al
., 2001). The
main issue is whether the stacked shallowing-
upwards successions were deposited due to exter-
nal forcing such as relative sea-level oscillations
(cf. Goodwin & Anderson, 1985; Goldhammer
et al
., 1990; Osleger & Read, 1991; Yang
et al
., 1995),
or whether intrinsic dynamics, such as chang-
ing rates of carbonate productivity or sediment
supply, could be a plausible alternative to create
such stratal patterns (cf. Ginsburg, 1971; Pratt &
James, 1986; Drummond & Wilkinson, 1993;
Burgess, 2001). However, in both end-member
models, the shallowing-upwards successions
represent a distinct high-stand cycle of sedimen-
tation with the peritidal deposits representing
coastal progradation (Laporte, 1967; James, 1979;
Wilkinson, 1982; Pratt & James, 1986).
However, the general assumption that each
shallowing-upwards
cycle
represents
one
