Microscopic biomorphic structures in Upper Cretaceous marine sedimentary pyrite

Abstract

The purpose of this work is to provide a description of biomorphic structures found in sedimentary pyrite present in the organic-rich marine limestone of the Agua Nueva Formation (Upper Cenomanian, Upper Cretaceous), central Mexico, using scanning electron microscope images. Six types of biomorphic structures were found and interpreted as coccoids (type 1), stomatocysts (types 2 and 3), pollen (types 4 and 5) and tubular structures corresponding to bacterial sheaths (type 6). Energy-dispersive X-ray spectroscopy analysis revealed iron and sulfur within the analyzed structures, indicating that they were replaced by pyrite. Furthermore, the high content of elemental carbon in the biostructures, in contrast to the surrounding pyrite in which this element is absent, suggests that the biostructures are of biotic origin. The coccoids and tubular elements were interpreted as autochthonous bacterial fossils or remains of bacterial activity associated with pyrite formation. In contrast, the stomatocysts and pollen represent allochthonous elements imported from continental areas.