From the Shores of an Ancient Sea
January 09, 2002
Over 500 million years ago, at the beginning of the Ordovician Period, shallow seas repeatedly formed and receded covering much of what is now central North America. Sediments deposited beneath these paleo oceans included sand, silt and carbonates which form the bluffs of southwestern Wisconsin today. Found within this relatively soft rock are patches of what geologists call “authigenic silica fabrics” – durable rock composed of the mineral silica (quartz) that formed at the same time the sediments were turning to rock. Silica is mobilized, transported and concentrated by infiltrating groundwater, so the presence of these fabrics indicates prolonged periods of exposure at the surface between ocean episodes. In some places, mats of blue-green algae/bacteria growing on the shores of the ancient seas collected silt and sediments in successive layers. Called stromatolites, these mounds of mud were later replaced by silica when the seas dried and have survived to this day as part of the silica fabric. The sample above is a chalcedony-quartz aggregate which weathered out of softer sediments in southwestern Wisconsin, and may be part of a stromatolite. Chalcedony is a form of microcrystalline quartz. The red color in the quartz crystals is staining from iron oxides and hydroxides.