TECTONICS AND SEDIMENTARY FACIES INTERPRETATION OF THE MESOZOIC GIRON GROUP, COLOMBIA (Invited Presentation)

Preliminary field investigations involving exposed Giron Group sediments in Colombia provided some interesting geologic information in terms of provenance, depositional environments, and development of the Mesozoic synrift sedimentary basins in Colombia. Overall, the synrift Triassic-Jurassic rift-related volcanic rocks, redbeds, and evaporites are prominent in Colombia, and a thick basin sequence of Cretaceous sandstones and shales dominates the central part of the Cordillera Oriental. These were mostly deposited in dominantly north-trending grabens. Identified sedimentary rock types were arkose, clast-and-matrix-supported conglomerate, red mudstone, variegated sandstone, siltstone and breccia. Extensive kaolinitization was noticeable in several sandstone samples. Among the notable sedimentary structures were cross bedding, mudcracks, raindrops, rootlets, and ripple marks. Thin section petrology revealed poorly sorted mono-and-polycrystalline quartz, altered K-spar, mica, and other accessory heavy minerals representing both unstable and stable constituents. Representative samples were evaluated for trace elements and variations in concentration of Ti, Zn, Cr, Ce, Y, Ba, Rb, Sr, and Cu were noted in these samples. Basin development began during the early Mesozoic and was dominated by a synrift sedimentary succession. Deposition of synrift sedimentary succession was primarily linked with the separation of North and South America in the proto-Caribbean and provenance of Giron sediments was most likely the Andean basement sources, recycled Paleozoics, and late Paleozoic igneous rocks. Based on field observations, stratigraphic relationships, sandstone petrology, and preliminary trace elements data, the synrift sedimentary facies initiated with deposition in a terrestrial arid environment that became paralic to lacustrine and shallow marine in the late Mesozoic.