Opal and Silicified Fossils

The Paleozoic epicontinental seas consisted of the highest silica concentrations in comparison to the later geologic epochs of the Phanerozoic Eon. Studies show a transition from Cambrian carbonates to predominantly aragonitic marine skeletons beginning in the early Triassic Period as the ocean chemistry and earth's climate changed after the Permian Extinction. In a mutually beneficent relationship, marine organisms use the silica in oceanic waters to build their skeletal shells. Siliceous ooze forms as organisms die and sink to the seafloor. When opal silica accumulates faster than it dissolves, the skeletal fragments are recrystallized and cemented forming chert. 

Pictured here is a beautiful fossil coral I found along the Niagara Escarpment near Sturgeon Bay, Wisconsin. The Silurian rocks have resisted glacial meltwaters and erosion as evidenced by the extensive outcroppings, picturesque rocky beaches, and archeological ship wrecks along the peninsula. A first glance and the coral appears as a slice of Brie cheese: sandy-colored dolomitic rock and a white chert crust; the skeleton is almost completely silicified. Associated faunas in these layers were scarce, but produced a few pockets of (mostly) Pentamerid brachiopods, also silicified. These fossils have gone through an incredibly complex diagenetic process and yet they yield remarkable fidelity even after 430 million years. For some perspective on what the world looked like, pictured below is a paleogeographic map of the Silurian Period where the coral was found. The white dot is Sturgeon Bay. (There is a fun interactive map online called dinosaurpictures.org that lets you type in your address and  time period. The map is pretty general, as it is missing some of the islands of Laurentia, 

Silicification is a diagenetic process which requires dissolution of the original calcium carbonate skeletal material to be precipitated by silica. Taphonomy, environmental and oceanic climate obviously mediate these processes, including, organic soft tissues, skeletal mineralogy, carbonate solubility; contrasting calcite and aragonite seas, greenhouse/icehouse effects. 

Upon close examination using a digital zoom camera, we can observe spheroidal beekite which forms by Liesgang diffusion in conditions of limited silica supply or fluctuating carbonate dissolution. Dissolution of calcium carbonate and precipitation of silica occur in conditions where pH declines from an intital stage of pH in environments supersaturated with silica and undersaturated in calcite. In carbonate sediments which are typically alkaline and Magnesium enriched, Opal forms during early dolomite diagenesis. Organic matter decomposition by sulfate-redusing bacteria creates low pH , and pyrite is also sometimes observed in silicified skeletal material. Following precipitation, silica goes through a well understood diagenetic process: opal-A > opal CT > cryptocrystalline quartz or chalchedony. Chert forms when opal silica accumulates faster than it dissolves and is buried on the seafloor (siliceous ooze). Skeletal fragments of siliceous organisms are recrystallized and cemented during this process.