Patricia L. Ciccioli, Sergio A. Marenssi
2 012
AndGeo vol.39 no.3 Santiago set. 2012
La Formación Toro Negro (Neógeno) registra parte de la sedimentación de la cuenca de antepaís fracturado de Vinchina (Orogenia Andina) en el NW argentino. Esta unidad está compuesta por conglomerados, areniscas y pelitas junto con algunos niveles de brechas y tobas depositados principalmente en ambientes continentales. Once asociaciones de facies (AF) fueron reconocidas: ocho corresponden al miembro inferior y tres al superior. La AFI está compuesta por megabrechas intraformacionales y conglomerados depositados en ríos entrelazados gravosos que rellenaron el paleovalle fluvial en el norte del área. La AFII, formada por brechas intraformacionales y areniscas integrando fajas de canales fuertemente incisas dentro una planicie aluvial, fue identificada en las posiciones marginales del mencionado paleovalle. La AFIII, integrada por fajas de canales gravosos con planicies arenosas, corresponde a ríos anastomosados de clima semiárido que evolucionó a un sistema con espesas planicies fangosas con delgados desbordamientos arenosos y canales simples gravosos (AFIV). Por su parte, la AFV consiste de complejos de canales arenosos encapsulados con ciclos grano-crecientes. La AFVI está compuesta por conglomerados y areniscas guijarrosas depositadas en una planicie entrelazada que evoluciona a un sistema fluvial de menor energía (AFVII) caracterizado por un aumento de depósitos fangosos con lóbulos arenosos y lentes gravosas. La AFVIII está dominada por sedimentos finos depositados en un lago somero. En el miembro superior, la AFIX corresponde a un sistema fluvial entrelazado profundo gravoso. La AFX está compuesta por facies finas acumuladas en una planicie aluvial fuertemente agradante sobre la que progradan depósitos gruesos correspondiente a un piedemonte dominado por flujos canalizados (AFXI). En la evolución paleoambiental de la Formación Toro Negro se reconocen 3 etapas. La etapa I (Mioceno Temprano) corresponde a la formación de un paleovalle fluvial en el norte del área y posterior relleno por cursos fluviales gravo-arenosos provenientes del oeste principalmente (AFI-V). En la etapa II (Mioceno Medio a Tardío) se desarrollan sistemas fluviales no confinados areno-gravosos (AFVI-VII) que progradan sobre un lago somero (AFVIII) desarrollado en el sur del área. Finalmente, la etapa III (miembro superior, Mioceno Tardío-Plioceno Temprano) corresponde a pulsos de progradación de la cuña clástica (AFIX-XI) desde el oeste (Precordillera).
Sedimentary paleoenvironments of the Toro Negro Formation (Neogene), Andean broken foreland, northwest Argentina
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 gravelly-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).
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 gravelly-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).
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