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

GILDA COLLO; FERNANDO NIETO; JULIETA NÓBILE; RICARDO A. ASTINI

Castellaneta Marina

Congreso; XIV International Clay Conference; Castellaneta Marina, Italy; 2009

ASSOCIAZIONE ITALIANA PER LO STUDIO DELLE ARGILLE

The 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). Its Early Miocene-Pliocene depositional age (19-3.4 Ma) is coincident with the onset of the flat subduction at this latitude. 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 structural formula, 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 (~7 Km depth), as is evidenced by the AEM showing phases with Si contents of 3.91 a.p.f.u. and layer charges of -0.28. Given that R0 becomes unstable over temperatures of ~120ºC its presence even in the deepest levels allows estimating a maximum paleogeothermal gradient of 17ºC/Km. Although similar low-gradients (~20°C/Km) have already been found in the modern Andean “flat-slab” foreland (Hamza et al., 2005), a short residence time of the sequence at maximum burial conditions (~3.4 Ma) and a reduction of the thermal regime as consequence of high sedimentation rates (blanketing effect, cf. Husson and Moretti, 2002) should be considered as factors retarding the progression of the I/S.