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Fluvial and Tidal Channel Fill Sequences Beneath West-Central Florida Gulf Coast Barrier Islands

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
Attendees
Contact:
Chief Scientist
Megan V. FitzGerald, Department of Geology, University of South Florida, Tampa, FL
Richard A. Davis, Jr., Department of Geology, University of South Florida, Tampa, FL

Stratigraphic analysis of fluvial and tidal channel fill sequences can provide insight into the long-term morphodynamics and sea level history of the west-central Florida Gulf coast barrier island system. The location and study of fluvial and tidal channels potentially offers the most complete record of sediment accumulation along the Florida barrier coast because sediments deposited in these low-lying regions are more likely to be preserved during the rise and fall of sea level, especially under conditions of limited sediment availability such as have existed along the west-central Gulf Coast of Florida. Identification of pre-Holocene fluvial channels provides data that contributes to the development of a Pleistocene stratigraphic record along the Florida Gulf coast and a more complete understanding of high order sea level fluctuations.

The study sites are located in the barrier and backbarrier regions of Pinellas, Manatee and Sarasota counties. Specifically, the locations are the Caladesi Island, Treasure Island, Anna Maria Island, Buttonwood Harbor and Bowlees Creek. Old inlet channels were located with aerial photography and high resolution seismic surveys were utilized in order to locate paleochannels of fluvial/estuarine environments. These sites were subsequently cored. Stratigraphic analysis of vibracores, including detailed descriptions and interpretations of depositional environments, radiocarbon dating and biostratigraphy, suggests that the Florida Gulf coast Pleistocene sedimentary record is more extensively preserved than previously recognized. In locations such as Anna Maria and Treasure Islands, data show that a significant amount of the sediments infilling the channels are Pleistocene to early Holocene in age.

The identification of fluvial and tidal inlet fill sequences, which include both peat, clay layers, and other organic rich layers, indicates that the area now occupied by the barrier island system has experienced multiple changes in depositional environments during the Quaternary Period (Figure 1). Tidal inlets have opened, closed, and/or reopened. Tidal channel facies are characterized by thick, imbricated, well-sorted, coarse shell deposits that fine upward. Although the tidal channels examined in this study did not preserve the record of Pleistocene sedimentation along the west-central barrier coast, they offer significant data concerning Holocene morphodynamics regarding the style of channel fill. In contrast, fluvial/estuarine channels, now flooded by the Holocene rise in sea level, were more widespread in the Pleistocene and early Holocene than at present. Some of them have retained part of their Pleistocene sedimentary record. Radiometric dating of organic material suggests that fluvial/estuarine sedimentation along the west-central Florida peninsular Gulf Coast was prominent at least 30,000 YBP but diminished greatly by 3,000 YBP.

Sedimentary Facies
Sedimentary Environment
Max. Unit Thickness (m)
Dominant Fauna
Average Weight Percents
% Organic Content
Other
Gravel
Sand
Mud
MS
back-barrier
4.00
Cc, Cf, Aa, Ai
4
71
25
0
-
ims
washover
2.00
marine/ bay
10
84
6
0
Ophiomorpha
bms
tidally influenced
1.00
none
3
87
10
0
mud laminae
WS
barrier sands
3.00
Cc
0
100
1
0
-
SS
tidal inlet
2.50
fragmented
50
50
0
0
fining up sequences
OMS
marsh/ mangrove
0.90
none
0
85
6
10
bioturbated
BMS
estuarine
4.20
none
0
93
7
trace
distinct burrows
OBS
dune
1.00
none
0
93
0
7
fine organics
SAM
fluvial/ estuarine
1.20
none
0
80
20
0
-
MUD
bedrock
0.70
none
0
12
82
0
-

WS=Well-Sorted Sand OMS=Organic Muddy Sand
MS=Muddy Sand BMS=Bioturbated Muddy Sand
ims=interbedded muddy sand OBS=Organic-rich Brown Sand
bms=bioturbated muddy sand SAM=Sandy Mud
SS=Shelly Sand MUD=Mud

Figure 1: Description of sedimentary facies and interpreted environments of deposition.

Fluvial paleochannels Pleistocene and early Holocene sedimentation produced sequences recovered from these valley fills that are characterized by extremely well-sorted,
paleogeographic reconstruction

Figure 2: Paleogeographic reconstruction of channel-fill sequence near Treasure Island.

fine-grained and oxidized brown, organic-rich sand. In addition, sediments identified as Pleistocene lack body fossils, but are often bioturbated. In contrast, Holocene sediments are not oxidized, contain numerous body fossils and a variety of grain sizes. (Figure 2) is a reconstruction and stratigraphic section from the backbarrier of Treasure Island, and it shows a thick, organic rich layer 3.5 m down section. The cores that provided the data for this block diagram were taken at the mouth of an unnamed stream which empties into the backbarrier waters of Treasure Island. Dates obtained at the top and bottom of the unit are 7890+/- 60 yrs and 11750 +/- 120 yrs, respectively (Figure 3). Other cores from the same region which penetrate deeper into the underlying sediments contain dark brown, oxidized sand beneath the organic rich layer. The well-sorted brown sands are interpreted as Pleistocene dunes.

generalized stratigraphic sequence

Figure 3: Generalized stratigraphic sequence showing marine sediments unconformably overlying organic-rich, fine sand interpreted as vegetated dunes. These dune sediments unconformably overly bioturbated estuarine muddy sand of Pleistocene age.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Morphodynamics > Fluvial and Tidal Channel Fill Sequences Beneath West-Central Florida Gulf Coast Barrier Islands


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