The Vinchina Broken- Foreland Basin: Tectonic Controls in the evolution of the fluvial systems of the Toro Negro Formation (Neogene), NW Argentina

CICCIOLI, P.L.; MARENSSI, S.A.; LIMARINO, C.O.

Mendoza

Congreso; 18th International Sedimentology Congress; 2010

During the Andean Orogeny, inland foreland basins developed in NW Argentina under an arid to semiarid climate. Tectonics played a major control in the evolution of such basins. The Toro Negro Formation (Miocene- Early Pliocene) exhibits a complete record of the broken foreland stage of the Vinchina Basin when the uplift of the Famatina System and Toro Negro range segmented the foreland area. The Toro Negro Formation is composed of conglomerates, sandstones and mudstones together with some levels of breccias and tuffs deposited in fluvial environments. The control exerted by the intense tectonic activity over the fluvial systems of the Lower Member of the Toro Negro Formation is analyzed in this contribution.             Eight facies associations (FA) have been identified in the Lower Member of the Toro Negro Formation. FAI is composed of massive intraformational breccias and cross-bedded conglomerates deposited in braided fluvial systems that filled a deep paleovalley formed in the north of the studied region. FAII has been only identified in the margin of the paleovalley and is composed of conglomerates and sandstones forming channel belts incised within alluvial plains deposits. FAIII comprises multistorey sandy-gravelly channels with thin muddy alluvial plains interpreted as deposited in semiarid anastomosed fluvial systems. FAIV, also interpreted as formed by anastomosed rivers, shows thick alluvial plain successions including well developed crevasse splay deposits. Encapsulated channel complexes, integrated by coarse-grained sandstones and conglomerates, form FAV. Channels appear encased in thick muddy alluvial plains successions that include well developed sandy crevasse splay accumulations. FAVI covers a low-relief erosive surface and it is almost entirely composed of conglomerates and gravelly sandstones deposited in broad alluvial plains. This fluvial system evolved to a lower energy fluvial one (FAVII) characterized by the increase of muddy flood plain deposits and the existence of fine-grained sandstones lobes with conglomeradic lenses. Finally, FAVIII is dominated by fine grained rocks (claystones and silstones) with sporadic intercalations of sandstones. This unit is interpreted as sedimented in a playa lake environment that occupied the south and central parts of the basin. The genetic stratigraphy of the fluvial systems recognized in the Lower Member of the Toro Negro Formation is analyzed using two principal concepts: 1 accommodation space and 2. degree of incision of the fluvial systems. Using these concepts two depositional sequences have been recognized (Sequence I and II) as well as five major accommodation space stages: 1. Early confined low-accommodation space, 2. Confined low-accommodation space, 3. Confined high-accommodation space, 4. Unconfined high-accommodation space, 5. Unconfined low-accommodation space.  The intraformational breccias at the base of the FAI are interpreted as deposited during the early confined low-accommodation space stage belonging to Sequence I. Confined low-accommodation stage is represented in the middle and upper parts of the FAI (in the central part of the paleovalley) and FAII (in paleovalley marginal positions). FAIII and FAIV were deposited in confined high-accommodation space conditions that favored the formation and preservation of thick floodplain deposits. The loss of confinement of the fluvial system is well recorded in FAV (unconfined high-accommodation space). This facies association is characterized by the extent of sedimentation throughout the basin after the paleovalley was totally filled. FAVI, formed in braided alluvial plains, marks the base of Sequence II and represents unconfined low-accommodation conditions. Finally, a progressive increase in accommodation space is shown in FAVII and FAVIII.