PRELIMINARY XRF ANALYSIS OF IRONSTONE CONCRETIONS FROM THE MAZON CREEK LAGERSTÄTTE

Mazon Creek is a Pennsylvanian-aged Konservat-Lagerstätte located across several counties in north-central Illinois and represents a tropical deltaic environment with an exceptional assemblage of both fauna and flora. Fossils from this site are known for their exquisite soft-body preservation that occurred through early formation of siderite in what was likely a pervasive growth surrounding the decaying organism. It is also through examination of fossil distributions that a transition from preservation of predominantly terrestrial organisms to marine organisms is seen, indicating conditions for the formation of siderite concretions were ideal over a vast area of the delta. These areas, known as the Braidwood and Essex fauna, are thought to represent a more terrestrial to delta front and brackish water bay/estuary environment, respectively. Interestingly, concretions from both Braidwood and Essex faunas that are collected may often be “duds” - those with no discernable fossils present once opened. While these two faunas are mainly distinguished by their fossil assemblages, only recently geochemical analyses have been used to further the understanding of concretion formation at Mazon Creek. This study attempts to provide further evidence of depositional environment conditions through the use of x-ray fluorescence spectrometry (XRF) on concretions from both Braidwood and Essex faunas. Binary plots of Al, Ca, Rb, Si, Sr and Ti consistently indicate two groupings of data. One group consists almost entirely of Essex fauna concretions. The second group is a mix of mostly Braidwood and some Essex concretions, which is often subdivided in two parts. The results of XRF analyses suggest a potential relationship between proximity to a sediment source during the time of concretion formation.