Late Paleozoic coal beds and coaly mudstones in northwestern basins of Argentina: Paleoenvironmental context and paleoclimatic significance

Abstract

Coals and coaly mudstones are found in the late Paleozoic Paganzo Basin, forming a key stratigraphic level deposited during the Bashkirian – Moscovian interval of relative tectonic quiescence. Coal beds vary from bituminous high volatility type C to sub-bituminous type A and, the coaly mudstones are kaolinitic-rich and presently used in the ceramic industry. Taking in account the stratigraphic position, facies associations, and the characteristics of the organic-rich levels, three domains were recognized within the basin: 1) The western domain (Precordillera) where paralic coals formed in transitional (mainly estuarine) environments, 2) The central domain (western Sierras Pampeanas) with limnic coals formed in floodplain areas, and 3) The eastern domain (eastern Sierras Pampeanas and Nevados del Famatina), in which coals are almost absent and are replaced by kaolinitic-rich coaly mudstones. Sea-level changes and climatic conditions related to the end of the Gondwanan glaciation played critical roles in the formation of coals and coaly mudstones. Regarding sea-level changes, three main transgressive events were identified in the Paganzo Basin. The oldest one corresponds to the “Namurian” postglacial transgression that produced no coal beds. The second event is the Pennsylvanian transgression 1, which favored the formation of coals and coaly mudstones in both coastal (i.e estuarine) and inland (i.e fluvial) environments. The third transgression, Pennsylvanian transgression 2, produced only very thin and scarce coal seams. During the transition from glacial (late Serpukhovian) to early postglacial climates (early Bashkirian), the vegetation was scarce and not suitable for the development of extensive mires. The post-glacial (late Bashkirian- early Moscovian) climatic amelioration, characterized by humid and temperate conditions, facilitated the proliferation of the NBG flora composed of hygrophilous elements as equisetalean, arborescent to sub-arborescent lycophytes and pteridosperms fronds, together with some gymnosperm that produced large amounts of organic detritus. On the contrary, the transition from subhumid to semiarid climate (late Moscovian-Kasimovian) led to the declination of the arborescent floras and the disappearance of coal beds. Although climate can be assumed to have simultaneously affected the whole basin, fluvial systems needed time to adjust to the sea-level changes and, therefore, at least two possibilities should be considered regarding the chronostratigraphic significance of the coal horizon. Firstly, that sea-level rise effectively controlled the equilibrium profile of channels well inland and coals and coaly mudstones across the Paganzo Basin form a time-transgressive horizon with a lag time not yet quantified based on the available paleontological and radiometric data. Secondly, that the growing of the riparian vegetation led by the ameliorating climatic conditions resulted in the almost synchronic formation of coals in the three domains