Abstract

The provenance of middle Permian to Maastrichtian sandstones from the subsurface of the Marañon and Ucayali Basins was determined through U-Pb dating of 113 detrital zircon samples from 21 hydrocarbon exploration wells. An additional 52 samples representing many of the subsurface lithostratigraphic units drilled in the Marañon and Ucayali Basins were collected from 42 outcrop localities in the Huallaga Basin and one outcrop locality in the Pachitea sub-Basin for U-Pb dating. The exposed units were analyzed to determine whether the outcrop sandstones had the same provenance as their subsurface counterparts.

Analytical results show that profound temporal changes in long-term detrital zircon provenance were observed in all the basins; spatial changes in detrital zircon populations between and within the Marañon and Ucayali Basins appear to be less significant. Western pre-Andean “Peruvian” source areas were major contributors of detrital zircons to sandstones in all the basins, especially during the middle Permian to Late Jurassic. Zircons with ages that are contemporaneous with the deposition of these sandstones were contributed by active continental arcs or locally via erosion and recycling of Permo-Triassic plutons. Abundant Neoproterozoic–Cambrian zircons were likely derived locally from the Pampean arc and Puncoviscana Formation (now buried in Peru) that formed in western Amazonia prior to emplacement of the Marañon Complex. Conversely, the primary sources for Archean to middle Paleoproterozoic detrital zircons were through local erosion and recycling of Paleozoic and Mesozoic sandstones and from recycling of Solimões Basin sandstones. Direct provenance from more distant central and eastern Amazonian cratonic source areas is possible but is considered unlikely.

Regional Late Middle Jurassic to Early Cretaceous uplift of the ancestral Solimões Basin, the eastern Marañon and Ucayali Basins, and western Amazonia caused a major shift in regional detrital zircon provenance, from local and western “Peruvian” sources to proximal western Amazonian cratonic sources. The development of new fluvial drainage areas in western Amazonia rapidly replaced less important long-distance detrital zircon sources from central and eastern Amazonia. Sources of local and west-derived “Peruvian” detrital zircons diminished by the Late Cretaceous (Cenomanian) as the Pampean arc and Puncoviscana Formation were buried from north to south; contemporaneous zircons were largely trapped within a deep back-arc basin west of the Marañon Complex. Detrital zircons from local “Peruvian” source areas continued to be important for Upper Cretaceous sandstones in the Huallaga and southwestern Ucayali Basins but were replaced by proximal western Amazonian cratonic-sourced zircons by the end of the Cretaceous.

Similar detrital zircon-age populations observed in middle Permian to Lower Cretaceous sandstones suggest that subsurface correlations may be imprecise in certain areas. Local recycling and redeposition of zircons from older sandstones is regarded as a more important mechanism for the formation of key hydrocarbon reservoir sandstones than was previously known.