COMPOSITION AND GRAIN SIZE ANALYSIS OF COASTAL SANDS ALONG FLORIDA’S ATLANTIC AND GULF COAST BEACHES, A GEOLOGICAL STORY BENEATH OUR FEET

Millions of tourists visit Florida’s beaches each year, perhaps they also notice that the sand varies in appearance and texture at different beaches. Our objective is to quantitatively describe the coastal sands in terms of grain size and composition and propose some possible explanations for why the beach sands are different. We want to share our geological perspective, so that the general public can better appreciate and understand beach processes and sand origin.

In March 2020, a suite of ~ 100 sand samples were collected at 15 beach locations split between Florida’s Gulf Coast (i.e. Panama, Clearwater, Ft. Meyer) and Atlantic Coast (i.e. Cape Canaveral, Cocoa, Daytona). A total of five samples were collected at each location: from the active surf zone, the swash zone, locations up the beach (4 meters and 8 meters from the water) and from the coastal dune. Each sample was then dry sieved for grain size analysis and analyzed for composition by performing carbonate dissolution.

Our results highlight that Florida beach sand is a mixture of quartz and carbonate shell fragments and that the relative proportions can vary. For example, sand from the swash zone at Daytona is almost 100% quartz while sand at Flagler twenty miles north is almost all shell material. The presence of quartz is significant as most of the Florida carbonate platform was isolated from sources of quartz until 30 million years ago. At this time, filling of the Georgia Seaway allowed introduction of quartz from the eroding southern Appalachian’s, a fundamental change in depositional conditions. The grain size and composition at a particular beach can be influenced by many factors, including longshore currents, local outcrops, waves, and shoreface geomorphology. Complicating the story is that most beaches in Florida have been “nourished”, often from offshore dredging. In addition to different locations, we will also examine trends within an individual beach. For example, dune samples at a beach (i.e. Cocoa, Daytona, Honeymoon) are typically better sorted, finer grained, and more quartz rich than the nearby swash zone, highlighting the effect of wind transport and deposition. We’ll use photographs and grain size distribution curves to highlight visually and quantitatively differences between the coastal beach sands as we strive to share “what a geologist sees”.