By Martin Keller
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Additional info for Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America (Special Paper (Geological Society of America))
1985; Sarmiento, 1986, 1990; Albanesi, 1991; Lehnert, 1995a, 1995b). These studies revealed a marked difference in age between the sections around Guandacol (Fig. 3) and those to the south of the Río Jáchal. , 1985). From a section a bit farther south (Cerro Potrerillo), Albanesi (1991) described conodonts and graptolites from the Arenig-Llanvirn boundary. South of the Río Jáchal, however, the top of the San Juan Formation and the transition into the overlying formations is always of early Llanvirnian age (Eoplacognathus suecicus zone: Sarmiento, 1986, 1990; Lehnert 1995a, 1995b).
Within the reef succession they are thick bedded, but near the base of the formation at Cerro La Silla they are thin bedded. There, mudstones with abundant pseudomorphs after gypsum have been observed alternating with the birdseye limestones. Whole-fossil mudstones and wackestones (Wilson, 1975) are present in medium- to thick-bedded intervals and show a diverse fauna with brachiopods, trilobites, crinoids, gastropods, nautiloids, ostracods, and sponge spicules. Among the algae Nuia and Girvanella are the most prominent; in places, almost monomictic Nuia wackestones were deposited.
Zone) from more than 50 shallowing-upward cycles, found evidence of a sequence boundary at the transition from the middle to the upper third of the exposed succession. The sequence boundary is located in the uppermost Crepicephalus zone. Both sequences, below and above the sequence boundary, are third-order sequences. Lithologically, the bed above the sequence boundary shows lithoclasts with coarse quartz grains. 31 Together with abundant microbial laminites and mudcracks in this interval, the terrigenous input is taken as evidence of a type 1 sequence boundary.