Clay minerals evolution and thermal burial history of the Neogene Bermejo foreland basin, south-Central Andes

COLLO, G., DÁVILA, F.M., ASTINI, R.A.

Congreso; International Sedimentological Congress; 2010

The Bermejo basin, in the Eastern Precordillera region (30º-33º LS), records part of the Andean synorogenic wedge accumulated in a segment for which it is inferred a current flat subduction regime. This stratigraphy is partly exposed in the Precordillera thrust belt immediately to the west and remains in the subsurface within the Bermejo basin, located between the thin-skinned thrust belt and the present-day broken foreland to the east. Stratigraphic analysis has allowed recognition of a three-formation division: Huachipampa Fm, Quebrada del Cura Fm and Rio Jachal Fm. The Bemejo foreland basin migrated gradually to the east during the last ~20 Ma from a position approximately aligned with the present position of the Western Precordillera to its current position acquired by the Late Miocene-Early Pliocene. Previous work has divided the stratigraphic history of these successions in three main stages linked to the Andean Orogeny. The first two developed between 20 and 7 Ma have been associated with the evolution of an asymmetric flexural foreland (simple foreland), controlled by the thrusting and loads of the Cordilleran tectonic sheets. The third stage corresponds to a broken foreland and extends from 7 Ma to present. The stratigraphy in several deep wells within the Bermejo basin, a frontier exploration area, shows as much as 6000 m thick record. As such, a detailed knowledge of its thermal history and burial trajectory of this, still unroofed stratigraphy, is extremely relevant for applied purposes. Here we present a preliminary thermal study based in clay mineralogy characterization from a deep well within the Bermejo basin, the Pozuelos borehole (YPF.SJ.P.es-1; ~5150 mbs). Samples were analyzed with a PANalytical X´Pert PRO X-Ray Diffractometer. Orderings in interstratified illite/smectite (I/S) were established through decomposition of detailed X-ray diagrams. Clay mineral assemblage from the sallower sample (250-255 mbs) is dominated by R0, R1 and illite with absence of R3 ordering. Between 1175 and 5128 mbs, R0, R1, R3 and illite are present, with a clear decrease in R0 content with depth, but without disappearing even in the deepest levels of the well. The distribution of I/S phases along the borehole allows us to suggest a progressive illitization process (R0 → R1 → R3 → I) related to the burial history of the Tertiary succession. This is clearly related to a gradual increase of ordering in the I/S and illite content from the top to the base of the unit and with a strong correlation between the degree of illitization and stratigraphic age of the rocks. The presence of R0 even in deeper levels shows that the basal filling from the basin does not exceed the diagenetic field, with maximum temperatures bracketed between 100 ± 20°C. These results are consistent with the thermal gradients of 20°C/km estimated from borehole data (121°C at ~5132 mbs), suggesting that the illitization is related to a continuous burial under the present thermal regime and allows interpreting that the Bermejo basin is a cold and immature depozone. The low-thermal regime could be related to a very-low heat flow, compatible with the presence of the flat subduction in this segment of the Andes. Moreover, progress of illitization is comparable to that recorded for exhumed correlative deposits in the Vinchina Basin to the north, although even lower thermal conditions were established for that basin (~17°C/km, Collo et al., this congress). Our studies allow suggesting some prospective potential in the immediately underlying stratigraphy.