ANALYSIS OF IRONSTONE CONCRETIONS OF THE MAZON CREEK LAGERSTÄTTE USING XRF

Mazon Creek is a Konservat-Lagerstätte that represents a Pennsylvanian-aged tropical deltaic environment with an exceptional assemblage of both fauna and flora. Located across several counties in north-central Illinois, fossils from this site are known for their exquisite soft-body preservation that occurred through early formation of siderite in likely a pervasive-growth around the decaying organism. Previous studies of fossil distributions show a transition from preservation of predominantly terrestrial and freshwater organisms to marine organisms, 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. However, concretions collected from both Braidwood and Essex faunas may often be “duds” - those with no discernable fossils present once opened. While these two biofacies are mainly distinguished by their fossil assemblages, only recently geochemical analyses have been used to further the understanding of concretion formation at Mazon Creek. Through the use of x-ray fluorescence spectrometry (XRF), this work provides further evidence for a gradational depositional environment change from delta mouth to front during the formation of Braidwood and Essex concretions. Binary plots of Al, Ca, K, Rb, Si, Sr, and Ti generated consistently indicate two groupings of data: one consisting almost entirely of Essex fauna concretions, and another a mix of mostly Braidwood and some Essex concretions, which is often subdivided into two parts. Braidwood concretions typically exhibit elevated abundance of elements present while Essex concretions are relatively depleted. Results of XRF analyses suggest a relationship between a concretion’s proximity to a sediment source during the time of its formation and the elemental composition of the concretion.