Interstratified illite/smectite in the MIoCene Central Andean foreland: evolution of clay minerals under very low-paleogeothermal gradients.

COLLO, G., F. M. DÁVILA, F. NIETO, J. NÓBILE, R. A. ASTINI

Italia

Conferencia; 14ICC - International Clay Conference; 2009

The Miocene to Pliocene Vinchina basin (Vinchina and Toro Negro formations) constitutes one of the thickest foredeep sequences (locally >10 km) of the Central Andes of Argentina (~ 28° SL). Distribution of I/S interstratified phases through this alluvial succession are consistent with a progressive burial smectite-illitization (R0 ® R1 ® R3 ® I; Collo and Dávila, 2008). The clay mineral assemblage of the shallowest sample (~1.25 Km depth) is dominated by R0 (~70%), R1 and illite, with absence of R3 ordering. The I/S structure, calculated from AEM analyses on the basis of 11 oxygens, show compositional variations with Si contents ranging from 3.41 to 3.94 a.p.f.u., layer charges between -0.25 and -0.74 and K, Na and Ca as interlayer cations. The appearance of R3 takes place at ~5 Km depth and, likewise R1 and illite, shows an increment towards the base of the unit. Although R0 clearly decreases to the deeper levels, significant proportions (~30%) are still present in the lowermost analyzed sample (depth of ~7 Km), as is evidenced by the AEM showing phases with Si contents of 3.91 a.p.f.u. and layer charges of -0.28. The presence of R0 even in the deepest levels allows estimating a maximum paleogeothermal gradient of 17ºC/Km for the Vinchina basin. Although similar low gradients have already found in the modern Andean foreland (Hamza et al., 2005) and associated to xxxxx, we propose it could be alternatively associated with a thermodynamic blanketing effects (cf. Husson and Moretti, 2002) or with factors like “Effective K+ concentration” (Cuadros, 2006) and “residence time” of the sequences at maximum burial conditions, which suggest a retard in the kinetics of the clay ordering.