SEDIMENTOLOGY AND PETROLOGY OF THE FOUNTAIN FORMATION NEAR CANON CITY, COLORADO (ALLUVIAL, PROVENANCE, ARKOSE, PENNSYLVANIAN, DIAGENESIS)

The Fountain Formation is a coarse-grained arkosic clastic unit, generally attributed to Permo-Pennsylvanian alluvial-fan deposition. Data obtained from 28 measured and described stratigraphic sections, 82 petrologic samples, and over 600 paleocurrent direction measurements document some unusual aspects of the petrology, sedimentology, and stratigraphy of the Fountain in the Canon City area.;An anomalous basal interval, ranging in thickness from zero to over 100 meters, is here designated the Garden Park Member of the Fountain Formation. This unit consists of coarse-grained, channel-shaped lithosomes surrounded by finer and better-sorted sheetlike beds, and is attributed to deposition in a coastal setting by fluvial and restricted marine processes. The remainder of the Fountain consists of coarser and more tabular lithosomes and represents alluvial sediment deposited by bedload streams. Vertical cyclicity on three scales derives from individual sedimentational events, channel aggradation, and regional tectonic-geomorphic evolution. Carbonate rocks, of inferred algal-supratidal, lacustrine, and pedogenic origin, are minor but widespread constituents.;Quartzarenites, subarkoses and sublitharenites, derived from a combination of Precambrian crystalline rocks and Paleozoic sedimentary rocks, are the principal constituents of the Garden Park Member. Sediments of the upper, typical Fountain are arkosic and were derived entirely from crystalline and intraformational sources. Composition of conglomerate clasts varies downslope and along strike to define at least two alluvial fans with mutually distinct sediment sources. Mineralogy of sand-sized grains has been modified significantly through diagenesis, notably by early, pre-compactional kaolinization of feldspar grains. Authigenic clay minerals and quartz overgrowths resulted primarily from meteoric circulation at shallow depths.;The unconformable boundary between the Garden Park Member and the typical Fountain represents an episode of tectonism which produced (1) local deformation and partial erosion of the Garden Park Member, (2) widespread exposure of crystalline source rocks, and (3) increased rates of sediment supply and aggradation. Increase in climatic aridity appears to have combined with these factors to produce (4) higher detrital abundance of feldspars and mafic minerals and (5) slower transmission of less dilute pore waters during early meteoric diagenesis of the Fountain above the Garden Park Member.