By Martin Keller
Read Online or Download Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America (Special Paper (Geological Society of America)) PDF
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Extra info for Argentine Precordillera: Sedimentary and Plate Tectonic History of a Laurentian Crustal Fragment in South America (Special Paper (Geological Society of America))
Org on September 24, 2011 34 M. Keller Fragments of trilobites and gastropods as well as peloids served as nuclei for the radially concentric ooids. Grapestones or coated grains are also present. In many oolite beds almost all ooids are diagenetically altered to microquartz or chalcedony. Individual beds rarely exceed 50 cm in thickness and many of them show cross-bedding. In places, onlap of oolite sands onto thrombolite mounds has been observed. Intraclast grainstones (3 in Fig. 18) and intraclast conglomerates are irregularly distributed throughout the sections.
In the eastern sections, abundant oncolite beds are present that are absent in the west. In addition, intraclasts become less abundant toward the west. In the Pachaco section (Fig. 3), no intraclasts or oncoids have been observed (Fig. 21). The tempestites there are thin layers (1–10 cm) of bioclastic wackestone and grainstone. org on September 24, 2011 Figure 23. Field aspects of San Juan Formation. 1: Large isolated stromatoporoid in main biostrome of Don Braulio section. Dark layers consist of pure chert.
Distribution of thrombolites and caliche horizons in type section of La Flecha Formation. Increasing abundance of caliche toward middle of section is taken as evidence for sequence boundary. Trilobite symbols mark biostratigraphic control. See text for discussion. org on September 24, 2011 32 M. Keller Zonda Formation (boundary 6). This would imply that most of the strata, both below and above the sequence boundary, in the La Angostura section are correlative and coeval to the Zonda Formation. An alternative interpretation is that the type 1 sequence boundary that separates the Zonda Formation from the La Flecha Formation in the San Juan subbasin (boundary 7) is the same type 1 sequence boundary that is present in the Guandacol subbasin.