THE THORNTON REEF (SILURIAN) ILLINOIS: A KEY TO UNDERSTANDING THE PALEOZOIC HISTORY OF A PRIMARY ANCIENT MARINE ECOSYSTEM

The Chicago area Thornton Reef is one of the most important geologic features of the American Midwest. Exposed in one of the largest quarries in the country along with a 2.5 km long I-80 roadcut, this large reef, (105 meters thick and 2.4 km diameter) exhibits a comprehensive view of a Silurian reef that is unique worldwide. Although quarrying at Thornton began in the 1830’s, it wasn’t until the end of the century that its expanding exposures attracted the attention of geologists. During the twentieth century, research at Thornton provided information that answered fundamental questions about the origins of the strongly dipping strata of the region. One of the earliest and most important observations was that the dipping beds were depositional, representing large reefs at specific locations, and not related to tectonic activity. Later in the century, utilizing continuously changing and new quarry exposures, it was possible to recognize temporal patterns of reef growth along with complex patterns of deposition and biotic zonation. Continuing research in the twenty first century has shown that the reef also exhibits prominent sequence boundaries related to glacial eustatic seal level changes. Through a combination of continuing excavations at the site and almost 150 years of study, the Thornton reef provides a unique three-dimensional view of an early Paleozoic carbonate build up found nowhere else.

The reef also has been an important education tool. Numerous professional geologic organizations have conducted field trips to Thornton while industry groups have visited the site to gain perspectives on hydrocarbon and aggregate sources. Educational institutions beginning with the University of Chicago have regularly visited the site since the early 1900s. The general public can also visit the site through the Thornton Historical Society whose tours bring up to 500 visitors a year and has been booked for up to 5 years in advance. If the 150,000 vehicles that cross this site daily are also included in the yearly total, this reef is regularly seen by more people than most other geological features in the region. These visits vehicular inspire enough public interest in the reef and quarry that at least once every few years a Chicago area newspaper will run an article about their science and economic importance.