Argentine Precordillera: Sedimentary and Plate Tectonic by Martin Keller

By Martin Keller

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

Sample text

Except for intervals of birdseye limestones, lagoonal deposits or the tidal flat are absent. The middle ramp is the zone between fair-weather wave base and storm wave base. Tempestites are common in this segment and proximal-distal trends are usually recognized (Aigner, 1982, 1985; Burchette and Wright, 1992). In the San Juan Formation, the wackestone-intraclast-packstone and the wackestoneoncolite associations were deposited on the middle part of the ramp (Fig. 25). The outer ramp shows much less evidence of storm influ- ence because it is located below storm wave base.

In addition, rounded crinoidal fragments and other bioclasts are present. Mid-ramp deposit. 5. 2: Bioclastic wackestone-packstone with gastropods, sponge fragments, and many other bioclasts. Many of these beds have tempestitic origin and were bioturbated after deposition. Mid-ramp deposit. 5. 3: Bioclastic packstone of upper reef interval. Note abundant gastropods and large sponge fragment. Distal inner ramp environment. 5. 4: Tempestitic packstone-grainstone with abundant intraclasts and oncoids (arrow).

The individual beds are often separated by dark gray to black shale partings. The fauna consists of abundant sponges or their remnants (spiculites) and fragments of small trilobites, brachiopods, crinoids, and Nuia. Ostracods and calcareous algae are present only to a minor extent. The dark matrix is often peloidal and locally shows small intraclasts. In some horizons, bioturbation is pervasive and responsible for the destruction of primary sedimentary structures as well as, at least in part, the fragmentation of the bioclasts.

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