Minnesota at a Glance Paleozoic History of Southeastern Minnesota-Ancient Tropical Seas

Abstract

Imagine a sandy, tropical seashore extending across southern Minnesota—part of a vast, shallow sea that covers much of North America. The sandstone, shale, and limestone rock layers exposed across much of southeast Minnesota (Figs. 1 and 2) are a geologic record of such conditions that existed hundreds of millions of years ago, during the early Paleozoic Era. Although many people are not aware of the geologic history of the Paleozoic bedrock in Minnesota, the rocks are familiar to anyone who has visited southeastern Minnesota. The bluffs along the St. Croix, Minnesota, and Mississippi Rivers, and their tributaries, are composed of layers of Paleozoic-aged rock such as the St. Peter Sandstone and the Prairie du Chien Group (Fig. 2). Paleozoic rocks lie beneath glacial sediments across much of southeast Minnesota, from as far north as Taylors Falls, southwest to Mankato (Fig. 1). They extend south into Iowa and east into Wisconsin. To understand the detailed history of the rock in southeastern Minnesota, you need only be familiar with the processes of deposition and erosion. These Paleozoic rock layers are sedimentary in origin. They are composed of particles of pre-existing rocks or minerals, or are precipitated by biological or chemical processes. Deposition is the accumulation of particles into layers, or beds. Small grains are dropped by wind or settle in water to form sandstone and shale. Elements, such as calcium, magnesium, carbon, and oxygen precipitate from seawater or are left as biological remains, such as shells, to form what we call calcareous, or carbonate sediments and later rocks —either calcium-rich limestone largely consisting of the mineral calcite, or more magnesium-rich dolostone largely consisting of the mineral dolomite—with much of the magnesium commonly added later by percolating water