R esumen Las exploraciones geológicas realizadas por el Servicio Geológico Colombiano (SGC) entre los años 2005 y 2008 muestran que las zonas potenciales para exploración y explotación de esmeraldas en el cinturón esmeraldífero occidental (CEOC) están estrechamente relacionadas a las formaciones geológicas Muzo (edad Hauteriviano-Barremiano) y Rosa Blanca (edad Valanginiano), y en el cinturón esmeraldífero oriental (CEOR), a las formaciones Santa Rosa y Chivor, de edad Berriasiano. La mineralización se encuentra asociada a diversas estructuras tectónicas de escala mesoscópica con alteraciones hidrotermales.
Abstract:The Apipucos Reservoir is located in Pernambuco-Brazil. Several districts of the metropolitan area use this reservoir to dispose of rubbish, waste and sewage. Dating sediments uses the 210 Pb from the atmosphere.
210Pb is a daughter of the 222 Rn, which emanates from the soil but is different from that contained in the sediment, which is in balance with the 226 Ra. The chosen model for dating sediments depends on certain conditions: in environments where the amount of sediment can vary, the Constant Rate of Supply (CRS) model is adopted. In environments where the sediments can be considered to be constant, a Constant Initial Concentration (CIC) model is applied. A 70 cm long and 5 cm internal-diameter wide core was used for sediment sampling. Samples were dried at 105 C, and about 5 g dry material was dissolved with acids. The 210 Pb and 226 Ra content was determined by their radioactive descendants' concentrations. For the second sampling point, both models could be used. The results showed an increase in sedimentation rate over the last 50-60 years. We could conclude that the top sediment interval had been there 30 years ago. We could decide that the CRS was the best applicable model.
The base of the Guadalupe Group, in the Tunja area of Colombia, contains cherts, porcellanites, mudstones, and siltstones with subordinate quartz arenites. The lithostratigraphic description of two stratigraphic sections showed that the dominant facies have fine granular textures and siliceous compositions, which considerably differ from those of the prevailing sandy terrigenous facies described in the type locality in the Eastern Hills of Bogotá, in the Arenisca Dura Formation, the basal unit of the Guadalupe Group in this sector.
The units that form the Guadalupe Group (base of the Guadalupe Group, Plaeners, and Arenisca Tierna) markedly differ from each other morphologically, which facilitates their mapping because the base and top units generate a steep morphology, and the intermediate units form surface depressions or valleys, similar to the morphology of the Guadalupe Group in its type locality in the Eastern Hills of Bogotá. The base of the Guadalupe Group consists of cherts and porcellanites toward the NW of the study area (Alto del Gavilán section), with mudstones, siltstones, quartz arenites, and to a lesser extent porcellanites and cherts prevailing toward the SE (Vereda Salitre section).
Geochemical analysis of total rock samples by XRD and XRF confirmed the primarily siliceous nature of the base of the Guadalupe Group, with SiO2 ranging from 62 to 98%, CaO less than 3.0%, and P2O5 peaking at 15.0%. Etayo-Serna (2015) conducted paleontological determinations of ammonites found in the stratigraphic section of Alto del Gavilán and assigned the base of the Guadalupe Group mainly to the Lower Campanian.
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