Abstract
The late Cambrian - late Tremadocian La Silla Formation is a carbonate unit of the eastern Precordillera in Argentina whose facies indicate a shallow platform environment. Until this moment, there were no studies that referred to the diagenetic evolution of these rocks. The present study involves the characteristics and distribution of the silicification that affects this unit and determines its different diagenetic stages through petrographic (with cathodoluminescence support) and stratigraphic analyses. An early diagenetic chert, in a stage previous to the compactation, was observed. This chert is related to silica-rich seawater in contact with permeable and porous sediments. A later pulse of chert, associated with fracturing, also occurs. The knowledge provided by the characteristics and distribution of chert in these carbonates is significant, especially when considering that the La Silla Formation in San Juan province is the most quarried unit for the elaboration of lime.
Highlights
Chert is a hard, dense or compact sedimentary rock, consisting of micro- or cryptocrystalline quartz crystals that contain amorphous silica and impurities such as calcite or iron oxide (Bates and Jackson 1980)
The fact that the sediments were affected by the silicification during early diagenesis may have caused the dissolution of these components (Maliva and Siever 1989); this means that, in these carbonates, one of the silica sources may be biogenic, there are no organisms’ remains
Gao and Land (1991), in an example similar to this study, inferred that chert could have been formed during early diagenesis in marine environments, and they propose silica enriched seawater as a source of it
Summary
Dense or compact sedimentary rock, consisting of micro- or cryptocrystalline quartz crystals that contain amorphous silica (opal) and impurities such as calcite or iron oxide (Bates and Jackson 1980). There are many references to chert associations in carbonates; comprising different depositional environments and ages (Geeslin and Chafetz 1982, Maliva and Siever 1989, Gao and Land 1991, Lawrence 1994, Giménez-Montsant et al 1999, Pope 2004). Chert can provide a window to the original composition of the sediment, the diagenetic history and the biota of the host rock due to its low susceptibility to further diagenetic alterations (Maliva 2001). The La Silla Formation (Keller et al 1994) of late Cambrian to late Tremadocian age, is a carbonate unit of the eastern Precordillera whose facies indicate a shallow platform environment (Cañas 1999, Keller 1999, M.M. Raviolo et al, unpublished data). It is free of terrigenous material and is extensively quarried to produce lime. The present study was carried out in the southern outcrops of the La Silla Formation in the Cerro Pedernal, Cerro Pedernal de los Berros, Loma Redonda, Sierra Chica de Zonda, and Sierra de Villicum (Fig. 1)
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