SEDIMENTOLOGICAL AND PROVENANCE ANALYSIS OF THE JURASSIC LA QUINTA FORMATION IN THE WESTERN FLANK OF THE PERIJA RANGE

The northern edge of the South America plate in the Mesozoic presents several extensional events for the Triassic-Jurassic time, some of them associated with the breakup of Pangea, causing the separation between North American and South American plates and the opening of the Gulf of Mexico. The La Quinta Formation on the eastern flank of the Perija Range (PR) consists of ca 1500 m of red volcaniclastic sandstones and conglomerates interbedded with tuffs, siltstone and thin layers of limestone. In order to know the extent, lateral changes in litofacies associations and maximum age of accumulation, we studied this unit in 4 localities of the western flank of the PR along partial stratigraphic sections and we report the results of U/Pb age populations from detrital/volcanic zircons.

The most southern area (Codazzi), the uppermost 100 m of La Quinta Formation comprise calcareous mudstone and mixed lithologies with cross beds. Farther to the north (Manaure) this unit is ca 300 m thick and consists of red conglomerates and sandstones in fining-upward successions and thin interbeds of rhyolitic tuffs. The other two sections are in the northern plunge of the Perija Range, where this unit is > 2.5 km thick. A cored well drilled 144 m of the uppermost La Quinta Formation that includes breccias, red volcaniclastic sandstones, siltstones, mudstones, and toward the top conglomerates. In the northernmost segment this unit may be divided into a lower interval that comprises cross-bedded red sandstones and mudstone, a middle interval that is more conglomeratic and include levels of breccia, and an upper interval that is dominantly red fine-grained volcaniclastic succession. The lithofacies association of La Quinta Formation described above indicates more continental lacustrine deposition to the south, while to the north dominate fluvial and gravity flow deposition; volcanic rocks are almost absent on this side of the range.

U/Pb age populations of 3 samples in the northern PR indicate that the base of the unit includes Proterozoic (2150, 1890, 1444, 1018, 741, 600 Ma), Paleozoic (450 Ma) and Triassic-Jurassic (240, 174 Ma) populations, whereas for the top most of the population is middle Jurassic (160-180 Ma). For the southern segment, a sample collected to the top of the unit yielded two populations: 160-170 and 250-350 Ma.