Have you ever wondered why there is less natural relief and hard bottom habitats in the Gulf off Charlotte Harbor compared to areas north and south? Well, somebody did, and they asked me. Not knowing the answer, I offered to do some research and write about my findings.
The first question I looked at: Is the habitat off Charlotte Harbor really different? That answer is, not surprisingly, yes. The FWC’s Fish & Wildlife Research Institute has been doing extensive side-scan mapping of the Gulf seafloor, and considerable data has been collected across the entire west Florida shelf (including off Charlotte Harbor) in water depths between 30 and 600 feet. The side-scan mapping provides a habitat component in support of reef fish surveys.
Because it’s not practical to side scan every inch of the west Florida shelf, the habitats mapped are randomly selected and then mapped along transect lines. Based on the mapping that has been done to date, the area off Charlotte Harbor likely wins for “most shelly sand.” In fact only about 3 percent of the substrate off Charlotte Harbor is anything other than shelly sand.
So back to the big question — why? The answer is not so simple. The type of geologic surveys that would be needed to answer this question have not been done for the area off Charlotte Harbor. What we know about the geology of the west Florida shelf for Southwest Florida is based on surveys off Tampa Bay and further south off Estero Bay.
But a very interesting paper sent to me by a geologist from the University of South Florida looked at the formation of Tampa Bay and Charlotte Harbor. This paper might be relevant, and since it’s pretty interesting, let me tell you about it.
Charlotte Harbor and Tampa Bay are big open-water estuaries. Big open-water estuaries are not unique, but what is unique is that these two estuaries don’t appear to have been formed in the same way as others.
Typically, large estuaries along the eastern coastal plains are formed by large rivers that rapidly erode sediments, creating incised valleys in response to a fall in sea level. As sea levels rise, sediments fill in the incised valleys and form nearshore sand bodies such as barrier islands.
However in the case of Charlotte Harbor and Tampa Bay, there isn’t a river large enough to have created an incised valley.
Seismic surveys conducted in both Tampa Bay and Charlotte Harbor show that both estuaries were likely formed around 10-15 million years ago during the mid to late Miocene. At the base of both estuaries lies the Arcadia Formation, a carbonate (limestone) layer formed by the skeletal remains of (mostly tiny) marine organisms.
Over time, this carbonate basement was deformed by collapses (likely due to dissolving of the carbonates during a period of low sea level) into what are characterized as folds, sags, warps and sinkholes. The result is a series of sub-basin complexes, with as much as 320 feet of relief.
During the late Miocene to early Pliocene (around 5 million years ago), the estuaries were filled in by mostly upper Peace River formation sediments. These sediments are mostly siliciclastic (quartz-rich).
Siliciclastic sediments originated from weathering of crystalline bedrock in the Appalachian Mountains and the Piedmont. These sediments were transported south and began to cover north-central peninsular Florida during the middle Oligocene period (around 28 million years ago).
During the late Miocene to early Pliocene, the siliciclastic sediments were remobilized and discharged to the estuaries by local rivers, likely due to increase rain and thunderstorm activity. As much as 90 percent of the open space in the surficial sub-basins of Tampa Bay and Charlotte Harbor were filled in by these sediments.
Seismic surveys show that between 30 and 200 feet of upper Peace River Formation (the siliciclastic sediments) sits on top of the Arcadia Formation in Tampa Bay, and between 30 and 320 feet sit on top of the Arcadia Formation in Charlotte Harbor.
Perhaps because more upper Peace River Formation was transported into Charlotte Harbor than Tampa Bay, more were also transported out onto the nearshore shelf, explaining why we have more shelly sand. But that’s just a theory of a biologist, who is in no way a geologist. Until seismic surveys are conducted off Charlotte Harbor, we can only speculate.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at firstname.lastname@example.org or 941-764-4346.