The Pavón Formation as the Upper Ordovician Unit Developed in a Turbidite Sand-Rich Ramp. San Rafael Block, Mendoza, Argentina

Abstract

The Pavón Formation crops out in the central-east region of the San Rafael block; it is composed of massive green-reddish-grey sandstones, wackes, quartz sandstones, siltstones and shales. The sand-dominated facies show tabular bedding with sharp contacts and scarce syndepositional deformational structures, and were deposited in a turbidite sand-rich ramp. The graptolite fauna, in particular the presence of Climacograptus bicornis Biozone indicates a Sandbian age (Upper Ordovician ). Sandstone petrography records a stable craton or a faulted continental basement as probable source areas. Illite crystallinity index suggest anchimetamorphic conditions underwent by the sequence. Relatively high CIA values and K/Cs ratios indicate intermediate to advanced weathering. Geochemical provenance proxies display detrital compositions derived from an average upper continental crust, but relatively high abundances of compatible elements (Cr, Ni, V, Ti and Sc) along with low Th/Sc ratios suggest mixing with a less fractionated component. The Zr/Sc ratios indicate that recycling was not important. An ophiolitic source can be neglected based on Y/Ni and Cr/V ratios. Chemical analyses of detrital chromian spinels indicate that they were formed within mid-ocean ridge basalts and continental flood basalts. Nd model ages of Pavón sandstones scatter around 1.4 Ga, ε Nd(t) (t = 455 Ma) values range from −0.4 to −4.1 and ƒSm/Nd is of −0.40 ± 0.06 on average, indicating an affinity to Grenvillian-age crust. Detrital zircon grains dated confirmed a main Mesoproterozoic source (with peaks at 1.1 and 1.4 Ga), with subordinated inputs from Neoproterozoic and Paleoproterozoic crystalline rocks. The complete provenance dataset suggest the basement of the San Rafael block (Cerro La Ventana Formation) as the main source of detritus, but derivation from the Western Pampeanas Ranges was also probable. The siliciclastic sequence was deposited in a foreland basin at latitude of around 26°S, and linked to the accretion of the Cuyania terrane towards west of Gondwana; this accretion caused uplift by thrusting of the Mesoproterozoic crust to the east at ca. 460 Ma.