TASMANITES-ENRICHED UPPER DEVONIAN BLACK SHALES: ELECTRON MICROSCOPY AND CHEMICAL CHARACTERIZATION (SEM-EDX)

Upper Devonian black shales are characterized by large (up to 400 µm), spherical-shaped bodies of the microscopic marine green alga Tasmanites, observable on rock surfaces with a reflected light microscope. A major constituent of Paleozoic seas, this photoautotrophic alga dominated surface waters during times of organic-rich mud deposition, consequently contributing to the high concentrations of carbon within the sediments. This investigation aimed to determine if the algal bodies were chemically differentiated from the elemental composition of the background matrix and if so, of which element they were composed. Here we present relative abundances and distribution patterns of the elemental constituents of the matrix and fossilized algal bodies in three cored shale successions from high and low latitude epicontinental basins acquired using scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDX), and elemental mapping.

EDX analyses indicate that the matrix of all samples is composed of a suite of elements (Al, Si, O, Na, K, Mg, Fe, and S) derived from aluminosilicates and pyritic-sulfides. Minerals were observed as micron-sized crystals or enmeshed within the disseminated organics, heavy minerals, or clays. Dolomite and calcite were predominant in shales from the Iowa Basin. Micron-sized crystals of the secondary minerals sphalerite and a calcium sulfate, possibly gypsum, were observed in shales from the Illinois Basin.SEM analyses show that tasmanitid bodies cover the entire surface of the shale samples. EDX and elemental mapping revealed that carbon, although evenly disseminated throughout the matrix, was the major elemental constituent of the well-preserved microfossils. Elements other than carbon, e.g., Ni, P, Cu, Mg, etc., occurred at minor concentrations, or were below detection. Organic preservation is most likely due to the highly chemical-resistant structure of the algal cell wall, the fine-grained clay sediments, and the redox conditions at the water-sediment interface. The lack of concentrated elements in association with the algal bodies suggests that the Tasmanites were not sites of bacteria-induced mineral nucleation, particularly for pyrite.