Geology Reference
In-Depth Information
Rapid recycling of organic-rich carbonates during transgression: a
complex coastal system in southwest Florida
BRIGITTE M. VLASWINKEL
*1
and HAROLD R. WANLESS
†
*
Marine Geology and Geophysics, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA
†
Department of Geological Sciences, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33124, USA
ABSTRACT
Coastal and shallow marine environments are actively responding to an accelerated
rise in sea level, eroding in some areas and rapidly accreting in others. Cape Sable in
southwest Florida, with several natural and anthropogenic triggering events in the
past century, illustrates the nearly instantaneous response that can occur in a sed-
iment-rich coastal system. Results are presented of a fi eld study that documents a
shallowing-upwards sediment package as a response to a transgressive phase. Patterns
and rate of sedimentation are reported, the different sediment sources identifi ed and
the governing processes that control sedimentation style are determined. The study
integrates sedimentological and geochemical data with hydrodynamic time-series
measurements of water level, currents, suspended sediment concentration and salin-
ity. Results show a rapid, sequential infi lling of the intertidal zone from the most
seaward marine, to transitional marine-freshwater subenvironments, as accommo-
dation space becomes available due to sea level rise, increased fl ood tidal volume
and collapse of interior freshwater marshes. Average
in situ
sedimentation rates of
6.2 cm yr
−1
are reported on the intertidal mudfl ats and daily tides are the most impor-
tant agent responsible for the erosion, transportation and deposition of the fi ne-grained
sediment. A signifi cant amount of sediment is sourced from one coastal compartment
and transported to another within the intracoastal system. The shallowing-upwards
sediment package contains organic-rich carbonates with 15-35% total organic matter.
In contrast to common stratigraphic wisdom, the shallowing-upwards peritidal
sediments as recorded in southwest Florida are the depositional response to several
small, rapid pulses of sea-level rise instead of being diagnostic of a single high-stand
lithofacies such as commonly described in ancient epeiric sequences.
Keywords
Recycling, organic-rich carbonates, shallowing-upward, process dynamics,
sea-level rise, southwest Florida.
INTRODUCTION
(Scholl, 1964; Fairbridge, 1974; Carter, 1988;
Wanless
et al
., 1994). Stratigraphically, this recy-
cling and deposition during transgression can lead
to facies complexity within littoral and shallow-
marine sediments (Lobo
et al
., 2005). In addition to
sea level, sediment availability exercises a strong
control over changes in coastal morphology
and the preservation potential of a sedimentary
package (Hine
et al
., 1988; Wanless & Tagett,
1989; Otvos, 2004). Contrasting modes of coastal
evolution are commonly linked to variations in
sediment supply (Hine
et al
., 1988).
Southwest Florida includes a low-wave-energy
coastal system that is responding to changes in
sea level and sediment supply, as well as minor
anthropogenic modifi cations. The late Holocene
Low-energy coasts represent dynamic systems
that respond to a number of forcing mechan-
isms, including relative changes in sea level
and changes in sediment supply. Subtle relative
changes in sea level can result in pronounced
sedimentological, geomorphological and ecologi-
cal responses (Stapor
et al
., 1991; Allen, 2000;
Rankey & Morgan, 2002). Specifi cally, rising sea
level commonly is accompanied by substantial
sediment release, transport and redeposition
1
Current address: Shell International Exploration and
Production B.V., Kessler Park 1, 2288 GS, Rijswijk, The
Netherlands (E-mail: B. Vlaswinkel@shell.com).
