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

Portilla Castañeda M.R.; Flores Bustamante A.; Tejada Malon N.J. · Proceedings of the LACCEI international Multi-conference for Engineering, Education and Technology (2026) · DOI: 10.18687/laccei2026.1.1.2321

Resumen

This research focused on the study of six 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 objective 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). The infilling of these fractures is composed exclusively of calcite in Groups I and II, while in Group III the septa contain kaolinite and chlorite, resulting from the diagenetic alteration of silicates, feldspars, and micas. The presence of relict pyrite indicates early reducing conditions, and the iron oxides (hematite, jarosite, goethite) that stain the crusts and margins derive from the late supergene alteration of ferrous ankerite. Three morphological groups were distinguished: Group I with regular polygonal septa, Group II with two populations of fractures, and Group III with fine chaotic septa. 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 septaries originated during early diagenesis through differential cementation and circulation of carbonate fluids for Groups I and II, and silicate fluids for Group III, with syneresis fracturing associated with compaction. This study contributes to the knowledge of diagenetic processes in Jurassic basins of northern Peru and highlights the value of these structures as paleoenvironmental and fluid evolution indicators.

Instituciones: Universidad Privada del Norte