Paleolimnology based on organic matter analysis and geochemical studies: Santa Clara Abajo Formation (Triassic), Cuyana Basin, Mendoza, Argentina

Abstract

This paper presents the results of an integrated study of organic-rich lacustrine mudstones from the Santa Clara Abajo Formation (Middle Triassic, Cuyana Basin, Argentina). Five lacustrine intervals (LI 1–LI 5), encompassing both lake-margin and lake-center facies, were analyzed using palynofacies, organic geochemistry (Rock-Eval pyrolysis), X-ray fluorescence (XRF), and X-ray diffraction (XRD). Palynofacies analysis reveals a dominance of amorphous organic matter (AOM), with variable contributions of phytoclasts and degraded palynomorphs, grouped into two distinct assemblages: Palynofacies A (LI 1–3 and 5) and Palynofacies B (LI 4). Although identifiable algal palynomorphs are absent, Rock-Eval data indicate the presence of Type I–II–II/III kerogen, with variable hydrogen index (HI) values, suggesting substantial algal/ bacterial input. Thermal maturity is interpreted as early mature (Tmax 439–445 ◦C). XRF data show enrichment in redox-sensitive elements (V, U, Mo, Fe, S) and elevated V/(V + Ni) ratios, supporting predominantly anoxic conditions. Mineralogical data (XRD), including illite and chlorite with high crystallinity, feldspars, calcite, and minor analcime, suggest clastic input from metamorphic sources and volcaniclastic alteration. AOM textural variation and geochemical signals indicate a complex OM origin, including lipid-rich algal/microbial precursors. Thermal patterns, inferred from previous taphonomic analyses, when integrated with facies and OM composition, suggest that warm and stable conditions (sustained stratified water columns) promoted better OM preservation and greater floral diversity, while cold or fluctuating conditions limited vegetation development. Finally, geochemical signals (HI, TOC, kerogen type) are consistent with overfilled lacustrine systems. These findings provide a complementary perspective to earlier sedimentological interpretations (balanced-fill) and show similarities with time-equivalent units in other depocenters of the Cuyana Basin.