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Abstract
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.
Highlights
From 2012 to 2015, the Servicio Geológico Colombiano (SGC) conducted detailed prospecting of 1600 km2 in the Tunja area, in the department of Boyacá, in a sector of Plate 191, Tunja (Renzoni et al, 1976); such prospecting (Terraza et al, 2016) made it possible to redraw the geologic map of this region of the Eastern Cordillera of Colombia into a new stratigraphic panorama of the Upper Cretaceous corresponding to the Guadalupe Group
The study area is located in the central region of the Eastern Andes (Eastern Cordillera) of Colombia, in the Tunja area, where two stratigraphic sections were measured that included the base of the Guadalupe Group: one in Alto del Gavilán in the municipality of Motavita, and the other in Vereda Salitre in the municipality of Soracá, where the lithostratigraphic and geochemical characteristics of this unit were described in detail
The results reported in the present article correspond to the detailed lithostratigraphic description of the base of the Guadalupe Group and to X-ray diffraction (XRD) and X-ray fluorescence (XRF) data of the samples collected in the two stratigraphic sections measured in this study
Summary
From 2012 to 2015, the Servicio Geológico Colombiano (SGC) conducted detailed prospecting of 1600 km in the Tunja area, in the department of Boyacá, in a sector of Plate 191, Tunja (Renzoni et al, 1976); such prospecting (Terraza et al, 2016) made it possible to redraw the geologic map of this region of the Eastern Cordillera of Colombia into a new stratigraphic panorama of the Upper Cretaceous corresponding to the Guadalupe Group. This new geological knowledge is partly reported in this article. The study area is located in the central region of the Eastern Andes (Eastern Cordillera) of Colombia, in the Tunja area, where two stratigraphic sections were measured that included the base of the Guadalupe Group: one in Alto del Gavilán in the municipality of Motavita, and the other in Vereda Salitre in the municipality of Soracá (see the location in the geological framework), where the lithostratigraphic and geochemical characteristics of this unit were described in detail
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