Interpreting post-depositional alterations in deep-marine sandstones: Butano Sandstone, central California USA

The Butano Sandstone forms part of an Eocene submarine fan deposited in the La Honda basin in central California. The Butano submarine fan has been subdivided into three main facies associations: southern inner-fan deposits, central middle-fan deposits, and northern outer-fan deposits. In spite of homogeneous quartzofeldspathic sandstone composition, these outcrop belts have several compositional and mineralogical differences, likely caused by post-depositional events related to the complex tectonic history of the La Honda basin, and expressed in deep-burial diagenesis and possible hydrothermal alteration. Both compaction and cementation reduced the intergranular volume (lower than 19 %) and the Compactional Index (average 0.9), indicating the major role of compaction in reducing original porosity. The southern proximal-fan and northern distal-fan outcrops exhibit different relative abundance of detrital quartz (Qm 43.5 %) than the middle-fan central outcrop (Qm 50.7 %). Primarily plagioclase and subordinately K-feldspar suffered leaching and intense dissolution during burial, particularly in the central part of the basin, even showing similar and quite constant distribution of alteration degree among the three outcrop belts (dominantly Fpc, Fpd, Fka, and Fkb grain types). Plagioclase abundance decreases in the middle-fan deposits proportionally with the increase of authigenic kaolinite, even where present in moderate amount; in contrast, authigenic kaolinite of inner-fan strata probably formed after partial dissolution of K-feldspar. Diagenetic changes are most abundant in the Butano Sandstone in the central part of the La Honda basin, likely primarily due to hydrothermal effects during deep burial.