Coquina and Coastal Geology — Sebastian, Florida

Coquina is the shell rock of Florida's Atlantic coast. The Florida Geological Survey defines it as a type of limestone made up almost entirely of cemented shell fragments. At Sebastian Inlet, on the barrier island between Brevard and Indian River counties east of Sebastian, coquina shows up in two ways: in the rock beneath the inlet and the nearshore bottom, and in the inlet's own construction history, since the first jetties that held Sebastian Inlet open in 1923 were built of coquina rock.

Geologists place this rock in the Anastasia Formation. The U.S. Geological Survey Geolex record credits E.H. Sellards (1912) with applying the name to the extensive deposit of coquina rock found along the east coast of Florida, typically exposed on Anastasia Island, near Ormond and along the coast at Rockledge in Brevard County, the county that meets Indian River County at Sebastian Inlet.

Coquina began as loose shell on beaches and shallow sea floors much like the ones off Sebastian today. The Florida Atlantic University geosciences field guide to the Anastasia Formation places it in the Pleistocene, citing an estimated age of 130,000 to 100,000 years, and describes it as a multicyclic deposit formed during several rises of the sea. Its shell fragments are well worn, which the guide reads as a high-energy setting such as the crest of an offshore bar or the breaker zone of a beach, the same surf environment that runs along the Indian River County shore south of Sebastian Inlet.

The National Park Service explains the cementing step at Fort Matanzas National Monument: beds of Donax, the coquina clam, were exposed when ice age sea levels dropped, rainwater percolating through decaying vegetation became weakly acidic, and it dissolved calcium carbonate from the shells and redeposited it as calcite that glued the shells and sand into rock. The NPS adds that coquina can be found from St. Johns to Palm Beach Counties along Florida's east coast, a span that includes Brevard and Indian River counties at Sebastian Inlet.

Closer to Sebastian, the NPS geodiversity atlas for Canaveral National Seashore, in northeast Brevard County, states that the Anastasia Formation was originally deposited in ancient beach and shallow marine environments where sand grains and shell fragments cemented together to form coquina. The USGS Geolex record also notes a 1929 redefinition by Cooke and Mossom that extended the Anastasia to all Pleistocene marine deposits under the lowest plain bordering the east coast north of southern Palm Beach County, a definition broad enough to include the coastal lowland around Sebastian and the Indian River County shore.

Most of the rock near Sebastian Inlet is underwater. The Sebastian Inlet District describes the nearshore hardbottom south of the inlet as exposed limestone bedrock in coastal waters within 400 meters from shore, in water depths up to about 4 meters, or 13 feet, and notes that the Florida peninsula is rimmed with this exposed limestone, with a greater prevalence on the east coast. Nearshore hardbottom typically occurs as shore-parallel bands of exposed rock, and it goes through natural cycles of burial and exposure as sand shifts along the Indian River County beaches.

A University of Florida-hosted page from the South Florida multi-species recovery plan identifies the rock formations on which east coast reef communities develop as the Anastasia formation, which forms the backbone of the Atlantic coastal ridge, and notes that worm reefs grow in the high energy surf zones between Martin and Brevard counties. The District says these hardbottom reefs dissipate wave energy and contribute to shoreline stability by acting as a natural breakwater for the beaches south of Sebastian Inlet.

Coquina was the first building stone of Sebastian Inlet. According to the Sebastian Inlet District history, the inlet was opened in June 1923 at its current location, and small jetties were completed using coquina rock carried on barges and transported by a Model T Ford converted by Roy O. Couch, the inlet's longtime advocate. A photograph in the District's photo galleries shows the converted Model T used to haul the coquina rock for those first Sebastian Inlet jetties.

Coquina also lies in the floor of the inlet. In 2014, the District history records, Orion Marine Group dredged 50,000 cubic yards of coquina rock and sand from the bottom to increase capacity and create a more uniform, 42-acre sand trap, the basin that now collects sand drifting into Sebastian Inlet before it is pumped to the beaches to the south. The District FAQ gives today's north jetty as 1,000 feet long and the south jetty as 575 feet, far longer than the small structures of 1923.

Because sand placed on the beach can bury nearshore rock, the Sebastian Inlet District monitors the hardbottom whenever it moves sand from the inlet to the south beaches. For its 2019 project, the District reported placing an estimated 110,000 cubic yards of sand on a 1.5-mile stretch from near McLarty Treasure Museum to just south of Ambersand beach access, between FDEP monuments R10 and R17. Biologists from CSA Ocean Sciences dove nine transects at 1,000-foot intervals, measured sediment depth and hardbottom relief, and mapped the western edge of the nearshore hardbottom off the Indian River County shore.

The shoreline itself is mobile above the rock. A 1994 aerial photograph in the University of South Florida's Richard A. Davis collection documents washover south of Sebastian Inlet, where storm waves pushed sand across the barrier island. Between storms and sand bypassing, the coquina hardbottom south of Sebastian Inlet is alternately buried and exposed, which is why the District frames monitoring around a pre-project baseline, an immediate post-project survey and annual surveys after that.