Geological study and characterization of septary concretions in the Aguas Calientes area, Eduardo Villanueva, San Marcos, Cajamarca 2025.

Abstract

This research focused on the study of five septate concretions found in fluvio-colluvial deposits of the Crisnejas River, in the Aguas Calientes area (Eduardo Villanueva district, San Marcos province, Cajamarca region), with the aim of determining their formation conditions and their relationship to the Chicama Formation (Upper Jurassic). Using a descriptive and analytical methodology, which included fieldwork with georeferencing, macroscopic characterization, and laboratory petrographic analysis, it was determined that these structures correspond to authigenic diagenetic concretions, formed in situ within deep-sea, anoxic marine black shales. The samples exhibit an ellipsoidal morphology (12–18 cm), a smooth external surface of primary origin, and radial internal fractures (septa) filled by carbonates (calcite, dolomite, and aragonite) in early stages, and subsequently by iron oxides (hematite and jarosite) during late supergene oxidation events. The bituminous crust and the presence of relict pyrite indicate initial reducing conditions with a high organic matter content. Geological mapping revealed that the specimens originate from the Chicama Formation, exhumed and concentrated by the seasonal dynamics of the Crisnejas River, which acts as an erosive and transport agent, preserving primary textures through short-distance transport. The results confirm that the septarian concretions originated during early diagenesis through differential cementation and the circulation of carbonate fluids, with fracturing by syneresis associated with compaction. This study contributes to the understanding of diagenetic processes in Jurassic basins of northern Peru and highlights the value of these structures as paleoenvironmental and fluid evolution indicators.