Organic facies variations in the Valanginian-mid-Hauterivian interval of the Agrio Formation (Chos Malal Area, Neuquén, Argentina): local significance and global context

Abstract

Abstract Marine shales and marls of the Valanginian-Hauterivian Agrio Formation have been studied at five localities in order to assess lateral variations over a 100 km S-N, shelf to basin transition. The two main organic-rich intervals at the base of the Pilmatue Member (Valanginian) and the base of Agua de la Mula Member (late early Hauterivian) have been characterized using a combination of bulk organic chemistry and palynofacies. Except for the former at the southern end of the transect, both intervals have mean total organic carbon (TOC) contents of 2–3% and are dominated by marine amorphous organic matter, suggesting a similar dysoxic genetic organic facies. The mean hydrogen indices determined from the slope of S2 v. TOC are 174 in the Pilmatue Member, but 387 in the basal part of the Agua de la Mula Member, a difference that mainly reflects the range in thermal maturity (late v. early oil window, respectively). Significant lateral variation occurs in the Pilmatue Member, with dark organic-rich intervals being rare in the south but dominant at the northern (distal) end of the transect; this trend is matched by a progressive increase in the peak or mean carbonate-free TOC and hydrogen indices, the latter reaching 6% and 297, respectively, near Estancia Pampa Tril. The bulk of the Agua de la Mula Member in the south is developed in organic-poor oxic facies, with a predominance of terrestrial phytoclasts and type IV kerogen, but dysoxic-anoxic conditions apparently predominate in the northern area. Valanginian-Hauterivian black shale facies appear generally rare on a global basis, but their occurrence can be related to the combination of the progressive rise in sea level during the Early Cretaceous and locally more restricted conditions.