The Tampa Bay area is world-renowned for its silicified marine fossils from the Tampa Bay Member of the Arcadia Formation, dating to the Miocene Epoch of the Cenozoic Era. These fossils include extinct species of corals, both hard and head corals, as well as gastropods and pelecypods. Over millions of years, many of these remains have been replaced with chalcedony and common opal, creating stunning specimens in colors ranging from red, orange, amber, and blue to the rarest of all: black. These vibrant hues come from mineral impurities such as iron and manganese, along with organic materials that produce the elusive black tones.
Most fossil corals in the Tampa Bay region formed between 30 and 25 million years ago, during the Oligocene Epoch. At that time, slightly lowered sea levels allowed patchy reefs to develop, hosting a variety of marine life including sea snails and pelecypods. Environmental changes eventually led to the death of the corals, and over the next several million years, their remains were buried in silica-rich sediments delivered by rivers from the Appalachian Mountains and by decaying marine organisms. Through a process called permineralization, the original calcium carbonate of coral and shells was replaced, cell by cell, with silica. Hollow corals filled with silica gel eventually transformed into geodes lined with botryoidal chalcedony, and, where space permitted, drusy quartz crystals that sparkle beautifully.
Our team recently discovered a new locality in deeper waters, offering access to corals that have rarely been studied due to the technical challenges of collection. These deep-water specimens often feature rarer colors, including striking blues and blacks, influenced by organic breakdown, saltwater intrusion, and possibly offshore sulfur springs. These colors are exceptionally rare because most fossils found on land or in shallow nearshore deposits formed in groundwater at higher elevations during the Miocene. Inside geodized corals, common opal is also frequently present, and its layering interacts with light via the Tyndall effect to produce hues of white, blue, and gray.
In addition to chalcedony and opal, our dives have revealed secondary minerals in these geodes that were previously undocumented in the area. Pyrite, for instance, appears in some specimens, likely forming from sulfate-rich clay deposits derived from organic material breakdown. Hematite occasionally coats drusy quartz, producing a subtle iridescence. Many deep-water corals also phosphoresce under UV light, a result of organic acids embedded within layers of chalcedony and opal, while some display agate banding only visible under UV radiation.
This new deep open pocket provides a rare window into Tampa Bay’s prehistoric marine life. Its vibrant, diverse, and sometimes phosphorescent corals are a reminder of the complexity of fossilization and the treasures still waiting to be discovered beneath the waves.
-Rick
