Abstract

The Carnian sandstones of the Southern Alps are fluviodeltaic and lagoonal clastic wedges derived from volcanic source rocks changing with time from mesosilicic to persilicic lavas and pyroclastics. Two-thirds of the variance in detrital modes reflect this petrologic evolution of the source. The effect of sedimentary processes is never strong enough to obscure the fundamental relationship of sandstone composition to provenance, but it accounts for most short-term variability in quartz/feldspar and feldspar/ volcanic rock fragment ratios. Plagioclase and quartz grains are concentrated, respectively, in the very fine to fine fractions and in medium-grained sandstones. The quartz/feldspar ratio (Q/F) thus decreases through delta-plain upward-fining sequences, from channel-lag sediments (Q/F = 1/3) to overbank sands (Q/F = 1/8). The sedimentary control on the composition of fluviodeltaic sands results from the coupling of mechanical effects in high-gradient headwater streams and hydraulic sorting in low-gradient depositional environments. Assuming that the effectiveness of mineralogical segregation in different grain-size fractions during transport depends primarily on paleorelief, a decrease in altitude during Carnian times is inferred within the Triassic volcanic source, which lay south of the sedimentary depocenter. Reworking by weak traction currents in lagoonal settings has further increased the textural maturity and the mineralogical stability of the sands. Volcanic rock fragments were reduced by selective destruction in marine environments, but the Q/F ratio was not affected, and a common value of 1/6 characterizes all lagoonal strata.

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