Benthic foraminiferal assemblages from the Lower Cretaceous of the Neuquén Basin, Argentina: paleoecological and paleoenvironmental constraints

CARATELLI MARTINA; ARCHUBY FERNANDO

Perugia

Simposio; FORAMS 2023; 2023

The Agua de la Mula Member of the Agrio Formation (Neuquén Basin, Argentina) represents shallow marine environments and consists of high-frequency sedimentary sequences characterized by mixed siliciclastic-carbonate deposition. Five stratigraphic section encompassing the Agua de la Mula Member were analyzed across the Neuquén Basin, revealing differences between proximal and distal sectors. An integrated approach based on the benthic foraminiferal record (species composition, morphogroups, alpha diversity analysis, epifaunal/infaunal ratio) and multivariate statistical methods (cluster analysis, RDA, partial-RDA, PERMANOVA, PCoA), allowed to evaluate the paleosynecological and paleoenvironmental significance of the foraminiferal fossil associations (FFAs) identified. Relative sea-level changes influenced variations in sedimentation rate, oxygenation and food availability, and are here considered as main controlling factors in the distribution of benthic FFAs. A total of 7,709 specimens of benthic foraminifera were identified, distributed into 52 species in 31 genera, 15 families and 5 suborders. Calcareous tests dominate the foraminiferal assemblages with hyaline forms being the most common throughout the Agua de la Mula Member and Suborder Lagenina the most abundant, while Robertinids dominate among aragonitic tests. Agglutinated benthic foraminifera represent 18% of the total fauna. Ten FFAs were defined based on benthic foraminiferal species composition. Samples were collected from poor lithified pelites of high-frequency (6th-order) sequences. Four associations (Guttulina sp., Polymorphinids, Spiroplectammina sp.2- Haplophragmoides sp.1, and Planularia madagascariensis) are mostly made of foraminifera with planispiral tests and lenticular, flattened and elongated morphotypes, related to shallow infaunal life habit and bacterial and detritivorous scavenger trophic strategies. The other six FFAs (Epistomina hechti-Epistomina australis, Reinholdella hofkeri, Haplophragmoides sp.1, Patellina subcretacea Trocholina infragranulata, Trochammina depressa, and Trochammina depressa-Reinholdella hofkeri) are mainly represented by active herbivorous and detritivorous epifaunal foraminifera with opportunistic behavior, as suggested by high percentages of planoconvex, biconvex and conical tests forms. In order to assess the paleoenvironmental significance of the benthic FFAs, these paleosynecological entities were evaluated in relation to sampled geographic localities (i.e. latitude and longitude), ammonite biozones, sedimentary facies, and position into 3rd and 4th-order depositional sequences. Shallow infaunal-dominated FFAs largely occur in coarser facies within 3rd-order regressive systems tracts, and they were considered indicative of well oxygenated conditions and food availability in sediments below the sediment-water interface, as well as associated with high sedimentation rates. Such conditions are well developed in the most proximal sections of the basin where shallow infaunal FFAs are common. Associations dominated by epifaunal foraminifera occur in fine-grained, dark shales within 3rd-order transgressive systems tracts, and were interpreted as related to oxygen and food stressed environments with organic matter likely confined at the seafloor due to long exposure favored by low sedimentary input within the more distal position. This paleoecological and paleoenvironmental study revealed that the high frequency sequences (6th-order) of the Agua de la Mula Member of the Neuquén Basin, as well as distribution of foraminiferal assemblages, were likely influenced by major relative sea-level changes. Such assessment can also be applied to other basins where comparable shallow marine conditions were developed.