Synorogenic conglomerates in a Late Paleozoic Gondwanan broken foreland, northwestern Argentina.

EZPELETA M., ASTINI, R.A. Y DÁVILA, F. M.

Banff, Canadá

Congreso; Alluvial Fans Meeting 2007; 2007

The westernmost Gondwana margin, in the south Central Andes, has faced ocean-continent subduction at least from Early Paleozoic times. In northwestern Argentina, the Gondwanic sedimentation is represented by the sedimentary sequences of the Paganzo basin, interpreted as a simple flexural foreland system. However, thick coarse alluvial conglomerates exposed in the Famatina Ranges, far from the deduced arc and hinterland regions (modern Cordillera), and interposed among typical fluvial and lacustrine systems that characterize the eastern part of Paganzo, allowed to reinterpretate the paleoenviroments, paleogeography and tectostratigraphic setting (Ezpeleta et al., 2007). This succession, known in the local geology as the Las Pircas Formation (LPF, Late Pennsylvanianearly Permian), is composed of a >400-m distinct coarse conglomerates. The LPF unconformably covers Lower Pennsylvanian glacial deposits of the Agua Colorada Formation and Ordovician granites. Four facies associations were recognized. A) Disorganized boulder conglomerates: thick-bedded matrix to clast-supported, largely massive with maximum outsized-boulders of as much as 15 m; interpreted as rock-falls, avalanche and debris flow deposits suggesting proximal alluvial fan development (colluvium deposits) with small headwater and high gradients. B) Organized conglomerates, mostly graded to stratified, well-rounded seldom imbricated conglomerates interpreted as gravely braided streams deposits. C) Lenticular cross-bedded and graded sandstones with frequent coal beds and pedogenetic features interpreted as fluvial, crevasse splay and overbank deposits. D) Graded sandstones to siltstones with frequent climbing ripples and mudstones interpreted as hyperpicnal flow deposits (turbidites) in temporary lacustrine environments. The clast size distribution and stratigraphic organization of the unit show a first order upward-fining megasequence. Gravity-flow deposits in the lower part of the section indicate that low-efficiency transporting processes were interrupted by temporary relatively-deep lakes in contrast with higherefficiency gravelly and sandy braided streams with recurrent mega-boulder beds in the middle and upper part of the unit. Three upward-coarsening lower-order cycles suggest consecutive phases of alluvial fan progradation separated by intraformational unconformities. These surfaces are compatible with repeated alluvial fan growth. A strong paleotopographic control in the development of relatively small depocenters is indicated by a complex facies mosaic, rapid transition between facies associations, intraformational unconformities and local onlap onto paleovalleys carved into basement inherited from a previous glacial landscape. Significant tectonic activity is indicated by recurrent mega-boulder beds. Clast composition suggests unroofing of Lower Pennsylvanian sandstones, and Ordovician granites and volcanic rocks. The fact that megaboulders are granitic suggests crystalline basement exhumation during deposition of LPF. Although hundred-meter upward-fining arrangements have been explained by progressive decreasing in the accommodation space in an extensional basin, a coeval basement-involved folding (Dávila et al., 2003) in the Famatina Ranges suggests development of upward-fining sequences during shortening. This indicates the development of a synorogenic conglomeratic wedge. Lithofacies distribution and paleocurrent data allow reconstruction of a westward dispersal system for the LPF that correlates with marine and deltaic units within the foredeep. However, dominant paleocurrents to the west cannot be related to a simple flexural foreland model (cf. DeCelles and Giles, 1996) by shifting of deformation from hinterland to craton. On the contrary, basement involvement, as shown from composition, indicates partitioning of the foreland comparable to the Miocene-modern Andean broken foreland.