Sedimentary paleoenvironments of the Toro Negro Formation (Neogene), Andean broken foreland, northwest Argentina.

Abstract

The Toro Negro Formation (Neogene) records the sedimentation in the broken-foreland Vinchina Basin during the Andean Orogeny, in northwestern Argentina. This unit is composed of conglomerates, sandstones and mudstones together with some beds of breccias and tuffs deposited mainly in fluvial environments. Eleven facies associations (FA) were identified in this unit, eight of them corresponding to the lower member and three to the upper member. FAI is composed of massive intraformational megabreccias and cross-bedded conglomerates deposited in braided fluvial systems that filled a deep fluvial paleovalley formed in the north of the studied region. FAII was only identified in marginal positions of the paleovalley and is composed of intraformational breccias and sandstones forming channel belts incised within alluvial plains deposits. FAIII comprises gravelly channel belts and sandy floodplains dominated by crevasse splays interpreted as deposited in semiarid anastomosing fluvial systems. FAIV shows coarsening-upward successions with thick muddy alluvial plain deposits including thin crevasse splays and gravelly single channels. Encapsulated channel complexes composed of coarse-grained sandstones and scarce conglomerates, form FAV. FAVI covers a low-relief erosive surface and is mainly composed of conglomerates and gravelly sandstones deposited in broad alluvial plains. This fluvial complex evolved to a lower-energy system (FAVII) characterized by an increase of muddy floodplain deposits and the existence of fine-grained sandstone lobes with gravelly lenses. FAVIII is dominated by mudstones deposited in a playa lake environment. In the upper member, FAIX corresponds to deep gravel-bed braided streams. FAX is composed of fine-grained deposits of a highly aggradational fluvial plain. Finally, FAXI mainly consists of breccias and conglomerates deposited in streamflow-dominated piedmonts. Three stages were recognized in the paleoenvironmental evolution of the Toro Negro Formation. Stage I (Early Miocene) corresponds to the generation and infill of the paleovalley, by fluvial systems coming from the west (FAI-V) in the northern-central part of the basin. Stage II (Medium to Late Miocene) consists of gravell y-sandy fluvial systems (FAVI-VII) prograding from the north-northwest towards the playa lake (FAVIII) developed in the south. Finally, Stage III (upper member, Late Miocene-Early Pliocene) corresponds to pulses of clastic-wedge progradation (FAIX-XI) from the west (Precordillera).