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Abstract:

Microcrystalline intergranular material of varying compositions occurs as well-developed geopetal structures, cutans, and massive pore fillings within subsurface samples of the fluvial, volcanogenic sandstones of the Comodoro Rivadavia Formation. These intergranular materials occur either as mixtures of chlorite, smectite and iron oxides, or as mixtures thereof that contain abundant quartz. Electron microprobe analyses of the material show that the quartz-rich material contains from 71 to 91% SiO2.

The textures are interpreted to be the result of infiltration. It is suggested that the microcrystalline quartz-rich material is the alteration product of originally fine-grained volcanic glass, which was infiltrated into the sands from suspended loads of the "Comodoro Rivadavia" rivers during the Cretaceous. This inclusion of fine-grained glassy material is attributed to the erosion of airfall ashes that intermittently blanketed the drainage basin.

The formation of the quartz-rich material requires both a siliceous source and additional silica derived from hydration reactions of adjacent volcanogenic sediment. The erosion of volcanic-ash deposits provides for intermittent, concentrated, suspended loads, resulting in a variable source of solutes during early burial. These variations of suspended-load composition are potentially overlooked features of arid and semiarid floodplain deposits, which are proximal to contemporaneous volcanism (e.g., rift, intermontane, forearc, and intraarc settings). Infiltrated glassy materials are a potential source of compositional and textural heterogeneity in volcanogenic sandstones that influences the path of later diagenetic alteration and the change of permeability and porosity of those sandstones.

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