From Fire to Ice: The Geologic History of Illinois as Told Through Sand



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Sand is a commodity that impacts our lives in a great many ways. We enjoy it between our toes on a beach, yet we lament it when a wedge shot goes awry. Sand has many uses in our modern society, including abrasives, construction materials, hydraulic fracturing, and the making of glass. Many of Illinois’ sand deposits are important aquifers. Sand can be of many compositions and shapes and is transported by wind, water, and ice to distant sites of deposition, and with luck may be preserved as part of the geologic record. Thus, scientific observations of sand deposited at a particular place and time provide important clues about sedimentary provenance, ancient climates, weathering, sediment transport, paleogeography, and mountain building. Zircon is a mineral that is present in trace amounts in sand, and along with the ubiquitous quartz, is among the most valuable of the sand grains for scientific investigation. With the development of laser technologies and the refinement of mass spectrometry during the past 20 years, age determinations for large detrital zircon data sets are now possible. Zircon is the mineral of choice because it is durable both chemically and physically, is abundant in granitic magma, and captures much of the magma’s uranium as it crystallizes. Zircon is the perfect clock for deciphering Earth history. The sedimentary succession of Illinois ranges from a few thousand feet near Rockford to more than 10,000 feet in the deepest parts of the Illinois Basin near Mount Vernon. The basement rocks in Illinois consist of granite and rhyolite; they were formed over a billion years ago. These rocks were weathered and then cleaned off by the “Snowball Earth” glaciers 650-750 million years ago. The first succession of sand in Illinois was deposited beginning about 530 million years ago in rift

basins formed during the breakup of the supercontinent Rodinia. These early Cambrian sands are locally sourced and short traveled. By late Cambrian time, the rift basins were filled, and the local basement was buried as a marine incursion inundated the continental interior. Late Cambrian and Ordovician sands in Illinois were generated at distal source areas to the north and east and were transported by a combination of wind and water. This “sand factory” produced some of the purest deposits at any time or anywhere on Earth. The second major sand-bearing succession in Illinois, which formed after the vast inland seas receded, was deposited during the Carboniferous Period (300-360 million years ago), and it is here that much of Illinois’ energy resources reside. The Carboniferous time was dominated by receding seas, the development of mountains in the Appalachian region as Pangea was assembled, and continental glaciation in Gondwana. Carboniferous sandstones are a mixture of grains that were recycled from older sandstone and newly unroofed granite that became exposed as the Appalachian Mountains were uplifted. These sands were transported to Illinois through river systems that rival the largest modern rivers and deposited in a series of deltas and estuaries that existed here at that time.

Publication Date





5 p.m., Circus Room, Bone Student Center

From Fire to Ice: The Geologic History of Illinois as Told Through Sand