Source rock and environmental control on clay mineral associations, Lower Section of Paganzo Group (Carboniferous), Northwest Argentina

Abstract

The significance of clay mineral assemblages present in the Lower Section of the Paganzo Group (Carboniferous) is considered in this paper. The sequence was deposited in a large (140,000 km 2) foreland basin located in Northwest Argentina. The Lower Section of Paganzo Group is a classic Gondwanic sequence that includes alternating marine and non-marine deposits up to 1200 m thick. The existence of two marine episodes (here named Namurian and Stephanian transgressions) is the basis for subdividing the Lower Section of Paganzo Group into five depositional systems (DS). DS1 makes up the base of deposits and includes conglomeratic diamictites and sandstones of glacial origin. DS2 comprises mudstones, turbiditic sequences and marls of transitional (fjord?) environments, all deposited as a response to the sea level rise occurring during Namurian times. DS3 is mostly made up of fluvial sandstone channels that interfinger with carbonaceous and coal-rich flood basin deposits that originated under warm and humid climatic conditions. The Stephanian transgression, included in DS4, only reached the Precordillera area, where it is marked by mudstones, marls and fine sandstones of nearshore environments. To the east, in the Sierras Pampeanas area, thick coal and kaolinitic beds belonging to high-sinuosity river deposits were deposited. Finally, DS5 comprises a variegated section of sandstones and mudstones of fluvial origin deposited under a seasonally dry climatic regime. Four clay mineral assemblages are defined on the basis of X-ray diffraction analyses of 54 mudstones samples: illite+kaolinite (I+K) , illite+chlorite (I+Ch), kaolinite≫illite (K≫I) and illite, kaolinite and illite/smectite mixed layers (I+K+I/Sm). Because of the low degree of diagenesis, the strong dependence of clay mineral distribution on pre-depositional factors such as source area, sea level changes and paleoclimate can be recognized. In this manner, clay assemblages of the fluvial deposits from the Sierras Pampeanas area were mostly formed by illite and kaolinite, while illite and chlorite dominate in marine intervals that have the Precordillera as their main source area. Paleoclimate seems to have controlled the degree of mechanical and/or chemical weathering, as well as the significant kaolinite (and halloysite) accumulation that took place from feldspar and mica alteration under warm and humid conditions. Mixed layer clays of illite/smectite, illite/vermiculite and chlorite/smectite are generally subordinated phases tied to the degradation of previous clays.