Late-diagenetic clay mineral assemblages in carbonatized ash beds of the Vaca Muerta Formation (Neuqu′en Basin, Argentina): Insights into the diagenetic formation of chlorite

The mineralogical and geochemical composition of carbonatized tephras of the Vaca Muerta Formation (Tithonian-Berriasian) in the Neuque acute accent n Basin (Argentina), allows retracing the diagenetic history and the origin of clay minerals in order to establish the sequence of diagenetic transformations and to explain the presence of chlorite in the felsic/intermediate ash beds. The carbonatized ash beds were studied in three sections located in the central part of the Neuque acute accent n Basin: El Trapial (ET), Puerta Curaco (PC) and Chacay Melehue (CM). Rock-Eval analyses performed in the enclosing marls revealed that ET section belongs to the oil window, whereas PC and CM to the dry gas window. The clay mineralogy of ET section is mostly constituted by R1/R3 mixed-layer illite/ smectite (I/S) type, whereas in PC and CM sections, by highly-crystalline, aluminum-, magnesium-rich chlorite (sudoite) and R1/R3 I/S type. The high-ordered I/S (80-90% illite layers) in the oil window stage indicates that the central part of the Neuque acute accent n Basin was characterized by a geothermal gradient higher than previously reported, with an estimated value of 57 degrees C/km. The absence of any sudoite precursors in ET section and the pervasive distribution of sudoite aggregates in the matrix, suggests that sudoite have precipitated in situ from pore waters. The overmaturation of organic matter in PC and CM sections suggests that precipitation of sudoite occurred during late diagenetic stages, between 150 and 200 degrees C. The aluminum, magnesium and silicon, essential to sudoite's precipitation were available in the pore waters after the dissolution/recrystallization of minerals and volcanic glass shards present in the tephra. The formation of sudoite was related to the development of the Chos Malal Fold and Thrust Belt, which generated overpressure, deformation and fracturing of the rocks, thus facilitating the circulation of hot waters.

Automated summary

The study of carbonatized tephras of the Vaca Muerta Formation in the Neuque acute accent n Basin, Argentina, revealed the mineralogical and geochemical composition, diagenetic history, and clay mineral origin, providing insights into the sequence of diagenetic transformations and the formation of chlorite in the felsic/intermediate ash beds. The presence of sudoite in the studied sections was related to overpressure, deformation, and fracturing of rocks caused by the Chos Malal Fold and Thrust Belt, facilitating the circulation of hot waters and explaining the late diagenetic precipitation of sudoite.