The stratigraphic distribution of inertinite

Abstract

Abstract The paper presents the results of an investigation into the stratigraphic distribution of inertinite in humic coals, ranging from the Silurian to the Neogene periods. This range equals the time span during which both land plants and atmospheric oxygen evolved. The survey is based on a literature study of inertinite in coals covering a variety of depositional environments and climates in all continents. The inertinite percentages listed in this paper have been arranged in stratigraphic order. They are the mean values based on mineral-free, whole coal compilations. Maceral compositions listed in the literature for individual subsections have been averaged to represent complete coal seams/beds, although this might not have been always possible due to some ambiguity in the published record. Moreover, not all of the published and unpublished information could be used because of poor sample description, obscure geographic origin, or insufficient stratigraphic information. The results show that there have been times in the geological past, e.g. during the Late Permian, in which coals were particularly rich in inertinite compared with other times, e.g. during much of the Late Tertiary, in which many coals contained very little inertinite. Occasionally, such primary, global trends were modulated by a diffuse distribution of inertinite percentages that correspond to differences in climate, depositional settings, e.g. active orogenic margins versus tectonically inactive shelf margins, and other local or regional variations. Although various mechanisms for the generation of inertinite have been proposed in the past, the overall pattern in its stratigraphic distribution supports the notion of incomplete combustion as the main source of inertinite in coal. This points to secular variations in the atmospheric oxygen content as an explanation for the global changes in the inertinite distribution, while differential subsidence and climatically influenced reaction rates governed regional and local variations in inertinite percentages.