Abstract

The Punta Negra Formation (Middle to Upper Devonian of the Precordillera of western Argentina) consists of a monotonous sequence of graywackes and shales. It increases in thickness from east to west reaching a maximum of about 1,700 m. The graywackes occur in beds that are generally 40 to 50 cms, but frequently more than 1 m thick, arranged in packets that may show thinning-upward, thickening-upward or symmetrical thickness patterns, separated by several meters of shale: beds within packets are generally amalgamated or separated only by a shale parting. Channels deeper than 1 m were not observed and few soles show erosional structures. Even in thick beds, sediment is not very coarse; most beds are medium grained, massive and grade up into shale; parallel lamination is common in the upper part of the bed, but ripples are rare. Many beds show two types of reverse grading: a thin, fine zone at the base, and a slight reverse grading above this, in the lower half of the bed. Paleocurrents are mainly from the south or southeast. Graywackes contain about 20% matrix, and 40% rock fragments indicating a derivation from Precambrian metamorphic rocks. During the Devonian sedimentation was controlled by two north-south oriented submarine rises, located between the stable shield to the east and the eugeoclinal (?oceanic) area to the west. The Punta Negra fan, which probably lacked a single main feeder channel, was about 100 kms wide by 50 kms across and was developed between the two rises. Sand was transported across the fan by high density turbidity currents. Just before deposition, the flows probably lost turbulence and passed through two stages of mass movement: (1) a stage of grain- and/or fluidized sediment flow, during which a slight inverse grading was developed in the lower half of the flow, and (2) a final stage in which matrix strength confined shearing to, and produced a strong inverse grading in, a narrow zone near the base of the flow (or bed).

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