This contribution presents the diagenetic evolution of limestone deposits in the Copacabana Formation that occurs in the northern Altiplano, in the Lake Titicaca area of western Bolivia. The best-exposed stratigraphic succession of the Copacabana Formation occurs in the Yampupata section, and its division into five facies successions is based on petrographic analysis, cathodoluminescence, x-ray fluorescence analysis (chemical composition) and stable isotope data (δ18O and δ13C). The results showed that the carbonate rocks experienced early marine diagenetic processes such as micritization during or after the deposition (eogenesis). The initial burial event (mesogenesis 1), characterized by stabilization of temperature-water carbonates by freshwater, and represented by bladed calcite-cement, equant calcite cement, dissolution, dolomitization, neomorphism, silicification and compaction (physical), occurred in shallow burial conditions. During the second burial episode (mesogenesis 2), in deeper burial environment the processes include: compaction (physical and chemical) and neomorphism. Diagenetic processes have affected reservoir quality in the Copacabana Formation during the mesodiagenesis, and reduced the conditions for development of high-quality conventional hydrocarbon reservoirs. Depleted O and C stable isotope signatures indicate that these carbonate rocks deposits underwent both meteoric and burial diagenesis including moderate water-rock interaction.

We analyze diagenesis of carbonate rocks from the Late Permian Chutani Formation of western Bolivia (San Pablo de Tiquina section) in the southern Lake Titicaca zone, which is a sedimentary succession of semiarid tidal flat comprised of mixed carbonate and siliciclastic units. The diagenetic study includes petrographic analysis (conventional petrography and cathodoluminescence) and geochemical analysis (carbon and oxygen isotopes and minor element chemistry). An integrated study of lithofacies and isotope stratigraphy of carbonates shows a succession of five types of depositional environments: tidal barrier, tidal flat, shoal coastal and shoreface. The Chutani Formation was subjected to different diagenetic processes including micritization, cementation, mechanical compaction, dissolution, neomorphism, dolomitization and dedolomitization that occurred during marine to shallow burial stages. Carbon isotope (δ¹³C) values range between −7 and 2.9‰ (VPDB) with variations linked to stratigraphic changes. The transgressive stage of the basin exhibits an upwards decreasing trend of δ¹³C values whereas regression is marked by an increase in such values. The oxygen isotope values (δ¹⁸O) vary from −16.6 to −1‰ VPDB with lighter values towards the top of the stratigraphy. The transgressive trend may reflect mixing of meteoric water and/or volcanic-hydrothermal fluids with seawater or progressive oxygenation with enhanced circulation conditions. Heavier values during regression may reflect more evaporitic and anoxic conditions towards the Permian-Triassic boundary. Significant variation in isotope values among neighbouring samples is observed, especially during trangression, which may be the result of different diagenetic processes.

Trace metal (molybdenum, uranium, vanadium, zinc, and nickel) mass changes are used to investigate secular variations in oceanic redox conditions in the succession of Copacabana (Upper Pennsylvanian-Early Permian) and Chutani (Upper Permian) formations of the Titicaca sub-basin (western Bolivia). These trace metal mass variations display evidence of suboxic depositional conditions, with episodes of oxygenation in the shallow carbonate platform of the Titicaca Basin. These episodes are consistent with the unrestricted renewal of deep waters of the Late Pennsylvanian Midcontinent Sea via lateral advection of oxygen-deficient waters of the western tropical Panthalassic Ocean. Trace metals in the Chutani Formation also attest intermittent suboxic conditions with oxic periods being recorded. These results, compared to other Upper Permian sections worldwide, suggest the idea that shallower platforms had oxygen during the mass extinction events of that period.