Alluvial to lacustrine sedimentation in an endorheic basin during the Mio-Pliocene: The Toro Negro formation, central Andes of Argentina

KYLANDER-CLARK, A; MARENSSI, S.A; LIMARINO C,; CICCIOLI, P; AMIDON, W

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2018 vol. 64 p. 69 - 87

0895-9811

A 2400 meter-thick sedimentary column belonging to theToro Negro Formation was recorded along the Del Yeso creek, Sierra de LosColorados, La Rioja province, Northwestern Argentina. Sevenfacies associations are described and interpreted to represent fluvial, lacustrineand alluvial environments developed in the southern part of the Vinchina basin fromthe Late Miocene until the earliest Pleistocene. The depositional evolution ofthe formation was divided in four phases. Phase I (~7-6.6 Ma) representssedimentation in medial (FA I) to distal (FA II) parts of a southward directeddistributive fluvial system (DFS) with a retrogradational pattern. During phaseII (6.6-6.1Ma), the DFS was replaced by a mixed clastic-evaporitic shallow lake(FA III) in a high aggradational basin. In phase III (~6.1-5 Ma) the eastwardprogradation of a fluvial system (FA IV) was recorded as a distal clasticwedge. Finally, phase IV (~5-2.4Ma) records two depositional cycles of proximalclastic wedge progradation of fluvial-dominated piedmonts (FAV, FAVII) from thesouthwest (Umango range) and/or the west (Precordillera) with an interveningplaya lake (FA VI).Two new U-Pb ages obtained from zircons in volcanicash layers confirm the Late Miocene age of the Lower Member of the Toro NegroFormation and permit a tight correlation with the central part of the basin (LaTroya section). The sedimentation rate calculated for the datedlacustrine-fluvial interval is higher than the corresponding one in La Troyaarea suggesting a higher subsidence in the southern part of the basin.Overall, the Vinchina basin is a good example ofa closed basin surrounded by the Precordillera fold and thrust belt to the westand basement-cored blocks to the north, south (Western Sierras Pampeanas) andeast (Famatina Range). During the Late Miocene (~7-6.6Ma) the ephemeraldrainage was controlled by an arid to semiarid climate and initially dissipatedmostly internally as terminal fan/distributive fluvial systems descending fromthe north. A thick lacustrine interval developed in the southern part of thebasin between ~6.6 and 6.1 Ma during a period of high subsidence and closeddrainage. Besides, this interval coincides with increased aridity recorded inother basins in the Northwest of Argentina. By ~6.1 Ma the area started toreceive the first coarse-grained sediments heralding the progradation of aclastic wedge from the southwest-west (Umango range and Precordillera) whichfully developed during the rest of the Pliocene to the earliest Pleistocene (~5-2.4Ma). This interval records ameliorating climate conditions