Origin, distribution, and significance of cements in the Miocene Vinchina Formation sandstones, Vinchina Foreland Basin, northwestern Argentina

Abstract

The Miocene Vinchina Formation (VFm) is the thickest sedimentary unit of the Vinchina Basin, an Andean broken-foreland basin in northwestern Argentina. Between 16 and 7 Ma, more than 10,000 meters of sediments accumulated in this basin. The sedimentology and compositional characteristics of the unit have been thoroughly studied. However, its diagenetic history is poorly understood. Based on the composition of authigenic minerals and their temporal relationships in seventy-eight sandstone samples, this study presents an interpretation of the diagenesis of the VFm sandstones. Petrographic observations allowed identification of authigenic minerals and their temporal and spatial distribution. X-ray diffraction (XRD) analyses helped differentiate zeolites and clay-mineral species. Using a scanning electron microscope (SEM) with an energy-dispersive spectrometer (EDS) permitted the identification of crystal morphology and the composition of the neoformed phases. Sandstones of the VFm exhibit a diverse arrangement of twelve authigenic minerals, calcite, zeolites, and gypsum being the most prevalent. Their distribution varies geographically throughout the basin due to paleoenvironmental facies distribution, framework clast composition, and diagenetic processes. Carbonate cements dominate the coarser-grained (fluvial) facies in the northern region. In contrast, gypsum is more conspicuous in the finer-grained (lacustrine and playa lake) facies prevalent in the basin’s southern sector. Accordingly, clay cements increase from north to south as sediment grain size decreases. The distribution of zeolite cements correlates with the quantity and nature of volcanic clasts. Analcime is abundant in the upper and lower sections and correlates with rhyolitic paleovolcanic clasts. Heulandite and laumontite are frequent in the central and northern areas linked to pulses of andesitic neovolcanic detrital contributions. Finally, deep burial and uplift control the alteration of the neoformed authigenic phases. That is the case of the gypsum to anhydrite, clinoptilolite–heulandite to laumontite, and smectite illitization transformations during the mesogenesis or the formation of secondary porosity during telogenesis. Compositional and textural characteristics of VFm sandstone cements were used to produce a diagenetic model to explain the various pathways, from eogenesis to telogenesis, which occurred in different parts of the basin. Altogether, diagenetic studies suggest that primary composition, depositional-facies distribution, and burial depth were major controlling factors during diagenesis.The occurrence of the laumontite–quartz mineral pair in sandstones of the lower part of the VFm suggests that maximum temperatures may have reached a range between 139 and 162 °C. Depending on the sedimentary thickness considered, these values are consistent with either a 13.9 °C/km or a 30 °C/km geothermal gradient.Thus, establishing a robust depth–time model that considers the effects of progressive unconformities is necessary to determine the basin’s paleogeothermal gradient accurately.