A geobiological investigation of the Mazon Creek concretions of northeastern Illinois, mechanisms of formation and diagenesis
Master of Science
Dr. Gordon Southam
Dr. Cameron Tsujita
Siderite concretions from the mid-Pennsylvanian Francis Creek Shale of the Mazon Creek area, in northeastern Illinois, contain exceptionally preserved soft-tissue fossils. Scanning electron microscopy, energy dispersive spectroscopy, time of flight-secondary ion mass spectrometry and micro-x-ray diffraction on polychaete worm, jellyfish and holothurian samples highlighted detrital clay and quartz grains in siderite cement. The worm and jellyfish are pyritised and surrounded by pyrite halos, whereas the holothurian is preserved as a weathered impression. Specimens possessed either microbial textures or framboidal pyrite, in carbon-enriched zones. The carbon-enriched zones contained organic compounds, indicating that extensive degradation associated with high temperature diagenesis did not occur in this region. Laboratory diagenesis experiments using sulphate- and iron-reducing bacteria revealed the challenges of soft-tissue preservation; bacteria rapidly degrade buried labile organics. Concretion and model analysis suggest rapid burial and early diagenetic mineralisation is essential for the preservation of soft tissues, including microbial remains.
Fernandes, Andrea S., "A geobiological investigation of the Mazon Creek concretions of northeastern Illinois, mechanisms of formation and diagenesis" (2012). Electronic Thesis and Dissertation Repository. 398.