A LOOK AT POSSIBLE UPDATES TO THE GASTROPOD FAUNA OF THE LATE CRETACEOUS COLERAINE FORMATION OF MINNESOTA (HILL ANNEX MINE STATE PARK)

The Coleraine formation, located in northern Minnesota along the Mesabi Iron Range, is thought to be an estuarial deposit of near shore sands, gravels and clays. This formation was laid down during a transgression/regression episode in conjunction with the rise of the Western Interior Seaway about 90 million years ago during the Late Cretaceous Period. It lays between the relatively recent surface glacial till deposit and the much older iron-rich Biwabik Formation that is the focus of open pit iron mining. Fortunately, historical mining operations at the Hill Annex Mine site stockpiled Cretaceous sediments from the Coleraine Formation into segregated stockpiles at the Hill Annex Mine. This presents us the opportunity to examine the level of diversity of life that existed in northern Minnesota at this time.

Gastropods (snails) are one of the most common fossils found in the Coleraine Formation at the Hill Annex Mine State Park in Minnesota. These fossils mainly exist in the form of internal molds (mud and sand filling the interior of the shell) and external impressions of the outside of the shell in the matrix. The original shell was not preserved or re-mineralized except in rare cases. However, by comparing the shape of the internal and exterior molds, one can identify several different species of gastropods. In 1944, Harlan Bergquist described 12 gastropod species from the Hill Annex Mine. More recently, preliminary work by members of the Hill Annex Paleontology Project suggests there are many more gastropod species to be classified.

The diversity of gastropods is an indicator of the environment. According to a 1997 laboratory guide book in Earth Science, a high diversity of marine gastropods indicates shallow, warm, well-lite water, with some agitation. Under these conditions gastropods shells are typically robust, thick and ornamental. While a low gastropod diversity, with thin shells and limited ornamentation indicates deeper and/or cooler waters.