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73 ␥ comprises 26% of the bay area. Almost 38% is 6 m. Hillsborough Bay, the northeastern lobe, is approximately 15 km long by 7 km wide and comprises approximately 10% of the bay complex. Its depth distribution is similar to that of Old Tampa Bay. Boca Ciega Bay, located north of the mouth of Tampa Bay, is not technically part of the estuary but is a small lagoon behind the coastal barrier islands. Much of Boca Ciega Bay has been dredged and filled, resulting in a substantial decrease in estuarine habitat. Greater than 75% of Boca Ciega Bay is 4 m (Huang and Goodell 1967). The Tampa Bay drainage basin covers approximately 5700 km2 and delivers a total mean annual freshwater inflow of about 63 m3 /s into the estuary (Galperin et al. 1991). Four natural rivers, the Hillsborough, Alafia, Manatee , and Little Manatee, flow into Tampa Bay, contribTampa Bay and Charlotte Harbor are the 2 largest estuaries in the eastern Gulf of Mexico. They lie in close proximity to one another, separated by less than 200 km, along the westward-facing, barrier-island Gulf Coast of peninsular Florida (Fig. 5.1). They have similar dimensions , share the same regional geological setting, have a similar climate (humid subtropical), and share a similar oceanographic setting (tide and wave regimes). Geologic research over the past 50 years has developed slower for Charlotte Harbor than for Tampa Bay. Over the past 20 years, studies have focused on the recent geologic history and modern depositional units because interests and funding have concentrated more on anthropogenic impacts and environmental concerns. Setting Tampa Bay is a large multilobed system of interconnected bays and lagoons (Fig. 5.2). It covers over 1000 km2 , but despite its large aerial extent is rather shallow with an average depth of 4 m. It has been naturally divided into 5 physiographic subregions. Middle and lower Tampa Bay form the main body, which is 15–20 km in width, 30 km in length, and contains 58% of the total area. Fifty percent of middle and lower Tampa Bay is 2–6 m deep, and 30% attains depths >6 m. Almost all of the depths >6 m are in this part of the bay. Old Tampa Bay, the northwestern lobe, is approximately 25 km long, 5–10 km wide, and 5 Florida Gulf Coast Estuaries Tampa Bay and Charlotte Harbor Gregg R. Brooks 74 ~ Brooks ates and evaporites of Cretaceous–Tertiary age (Mitchum 1978). The region was dominated by carbonate deposition throughout the Paleogene, with a shift toward mixed carbonate-siliciclastic deposition during the Neogene. Underlying the unconsolidated surficial-sediment veneer are the complex Miocene deposits of the Hawthorn Group consisting of interbedded carbonate and siliciclastic sands and muds (Scott 1988). Overlying the limestone surface the unconsolidated sediment cover is thin (212 persons per square mile. Agriculture accounts for 800 km of linear transect data, coupled with 20 boreholes collected in and around Tampa Bay, Suthard (2005) identified an eastwest –trending subsurface trough in the Miocene limestone underlying the north-central portion of middle Tampa Bay. Reaching depths of approximately 30 m below the seafloor, this trough contains a series of post- to mid-Miocene infilling seismic sequences including: (1) a basal unit of siliciclastic sediments prograding from the south and east, (2) two sequences of undetermined origin , (3) a Pleistocene lacustrine sequence, and (4) modern estuarine sediments. South of the trough in the center of Tampa Bay, a broad carbonate-bedrock high is overlain by modern estuarine sediments (Suthard 2005). In the southern portion of the bay large-scale warping and karst development in the Miocene limestone surface creating another deep basin were evident from seismic data. This basin has been filled by 4 depositional sequences, the lowest of which exhibits progradation from the southeast and south. Deposits are interpreted to be late Miocene siliciclastics of the Peace River Formation. Basin infilling likely occurred during multiple mid- to late-Neogene and Quaternary sea-level fluctuations. The karst and warp deformation was attributed to spatially selective subsurface collapse initiated by deep-seated dissolution produced by focused hydrologic processes (Suthard 2005). underlain by a series of rectilinear northwest-to-southeast and northeast-to-southwest–trending troughs 40–50 m in depth. Troughs occur in middle Miocene limestones and are infilled with late Miocene, calcareous, sandy, phosphatic clays. They concluded that these troughs controlled the development of paleofluvial systems. They also concluded that the locations of modern and relict barrier systems have been controlled by...

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