Thermal burial history in the Chaco-Paraná Basin, Argentina

39.BONICH, M., COLLO, G., DÁVILA, F.M.

Congreso; International Sedimentology Congress; 2010

The Chaco-Paraná basin (locally known as the Pampas) between 64˚-61˚ WL constitutes the modern foreland system of central Argentina. Accommodation of deposits in this region has been related to a combination of tectonic loading of the Cordoba ranges and dynamic topography by subduction of the flat slab (Dávila et al., 2010). According to present-day temperature measurements, the geothermal gradient is nearly normal. But the sedimentary record is as old as Late Paleozoic, when the subsidence mechanics and ancient geotherms were distinctly different than today. The Upper Paleozoic and Mesozoic were largely associated to extension, within the cratonic scenario of the Gondwana supercontinent. The stratigraphy is mainly known from oil industry borehole cutting (Yacimientos Petrolíferos Fiscales, YPF during the 60’ and 70’) and patchy exposures along and across the Cordoba ranges. Upper Carboniferous and Lower Permian diamictites form the Upper Paleozoic (Winn & Steinmetz, 1998), which rests unconformably on crystalline basement. Cretaceous strata rest on a major discordance on Upper Paleozoic and locally basement. On top Cenozoic alluvial strata, interlayered by two shallow marine horizons (Marengo, 2006), lay conformably on Mesozoic. In this contribution we present a new basin thermal study along the Josefina borehole YPF.SF.J.es-1 (Santa Fe Province), ~100 km eastward of the Cordoba ranges, in order to understand the burial history of the Pampas since the Late Paleozoic. We assumed neoformed clay mineralogy is close related to subtle temperature changes during the ancient burial/exhumation of the basin and might assist to solve the thermal history of this region. We followed the laboratory protocol treatment recommended by Moore and Reynolds (1997) and analyze the samples with an X-Ray Diffratometer X PANalytical X'Pert PRO. The samples analyzed are: (a) the metamorphic basement located at ~4500 meters below the surface (mbs); (b) Upper Paleozoic located from the surface between ~4000 and ~2780 mbs; (c) Cretaceous rocks located between 2130 and 590 mbs; (d) a Cenozoic sample located at ~285 mbs. Two samples from Cretaceous exposures were collected along the western flank of the Cordoba ranges to compare the borehole results. Kübler Index from Upper Paleozoic to Lower Cretaceous (KCIS between 0.54 and 0.36Δ°2θ) suggests temperatures between 175° and 215°C in the ~3000-4000 mbs interval. The clay mineral assemblage of the Upper Cretaceous samples, between ~2100 and 2800 mbs, are dominated by R0, R1 and illite with absence of R3 ordering suggesting for these levels temperatures around 100-120°C. Samples between ~1200 and 300 mbs (Cretaceous to Cenozoic) are dominated by R0 and illite, with absence of R1 and R3 orderings and temperatures between 50-120°C. Modern geotherm in the Pampas is higher than in regions further west (Collo et al., in rev.), which support the hypothesis that flat subduction affects deeply the thermal structure of the lithosphere. These preliminary results suggest two cycles of burial interrupted by an exhumation. The first burial (or alternative higher ancient geotherm) affected the Paleozoic sections. The strong exhumation would have occurred previous to the Cretaceous sedimentation. This exhumation episode is consistent with apatite fission track cooling ages recorded in basement rocks along the Cordoba ranges (Jordan et al., 1989). Temperatures of ~100-120°C recorded at the base of the Cretaceous strata (~2800 mbs) suggests a geothermal gradient between 33 and 40°C/km, although a second exhumation episode with significant thickness loss should be also considered.