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Extra resources for Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America (GSA Special Paper 341)

Sample text

At Cerro La Chilca, there is a prominent hardground developed with vertical burrows penetrat- ing several centimeters into the uppermost bed of the San Juan Formation. Nodular packstones together with some wackestones and marlstones are present at the top of the limestone succession in Las Chacritas section. These rocks are not typical of the San Juan Formation. They are dark gray and thin to medium bedded, and 40 M. Keller some of the beds are graded. The individual beds are often separated by dark gray to black shale partings.

The middle unit is dark gray and sandwiched between the lower and the upper units, which both show light gray and white rocks. This subdivision is also reflected in the sediment composition. Unit 1 begins with 20–25 m of peloidal grainstones, dolomitic limestones, and microbial laminites (Fig. 20). Upsection, peloidal grainstones with minor intercalations of microbial laminites form the bulk of unit 1. The rocks are thin bedded but rapidly pass to massively bedded successions upsection. Toward the top of unit 1, which is made up of the microbial-boundstone association, chert becomes increasingly abundant and commonly masks the microbial structures.

Mudstones and microbial boundstones show abundant fenestral fabrics. Wackestones and packstones with gastropods and nautiloids are also present (1 in Fig. 18). Their matrix is muddy or, in places, consists of abundant peloids. Other bioclasts include trilobites, crinoids, sponges, and algae (2 in Fig. 18). Peloidal grainstone (4 in Fig. 18) is a very common rock type in the La Silla Formation; oolites are less abundant. Ooids are commonly replaced by dolospar and only ghost structures are preserved.

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