U-Pb ages of detrital and volcanic zircons of the Toro Negro Formation, northwestern Argentina: Age, provenance and sedimentation rates

Abstract

The Toro Negro Formation is a foreland sequence in western La Rioja province, Argentina, which records the late-stage tectonic evolution of the Vinchina Basin. Together with the underlying VinchinaFormation, these two units represent one of the thickest and longest continually exposed foreland sections in northwest Argentina. The Vinchina basin is uniquely situated between the Toro Negro and Umango blocks of the Western Sierra Pampeanas to the north and south, the Precordillera to the west, and the Sierra de Famatina to the east. New U-Pb dating of volcanic tephra providesimproved age constraints on the pace of sedimentation, and U-Pb ages of detrital zircons serve to strengthen existing provenance interpretations. We show that deposition of the Toro Negro Formation spans roughly 6.9 to 2.3 Ma: Late Miocene to Early Pleistocene. A high-relief, erosional unconformity with the underlying VinchinaFormation developed sometime between 9.3-6.9 Ma, although stratigraphic considerations suggest it spanned only the later part of this time interval (perhaps 7.5-6.9 Ma). Above this unconformity, undecompactedsedimentation rates are remarkably high at ~ 1.2 mm/yr, slowing to ~ 0.3 mm/yrafter ~6 Ma.An unconformity in the upper part of the section is constrained to occur sometime between 5.0-3.0 Ma, probably beginning not long after 5.0 Ma. The timing of both unconformities broadly matches the timing of inferred tectonic events in the Sierra Famatina ~50 km to the east, the Fiambalá basin to the north, and the Bermejo basin to the south, suggesting they may record regional tectonism at these times. Provenance interpretations of detrital zircon spectra are consistent with previous interpretations based on sediment petrography. They show that provenance did not change significantly during the course of Toro Negro deposition, precluding major tectonically-induced drainage reorganization events. Sediments were derived primarily from the north (Toro Negro Block) and west (Precordillera). The data are consistent with a subtle increase in sediment supply from the Precordillera beginning around 6.5 Ma.

Using basin thermal history to evaluate the role of Miocene–Pliocene flat‐slab subduction in the southern Central Andes (27° S–30° S)

“…() and Amidon et al . () with a range of Precambrain grains between ca . 1400 and 1000 Ma, major populations between 600 and 420 Ma, 380 and 250 Ma, and a Neogene population that decreased in age upsection.…”

“…AFT and AH e thermochronometers are plotted relative to depositional age to distinguish between samples that have been reset by basin burial (younger than the depositional age) and those that have not been reset by burial (older than the depositional age). Existing data sets are indicated by numbers where 1 = Collo et al ., ; 2 = Amidon et al ., ; 3 = Carrapa et al ., ; 4 = Johnson et al ., ; 5 = Reynolds et al ., ; 6 = Fosdick et al ., ; 7 = Safipour et al ., . (b) A kernel density distribution and histogram of total AH e analyses shown in (a).…”

“…(). In the Vinchina depositional area, we augment existing geochronological constraints (Ciccioli et al ., ; Amidon et al ., ; Collo et al ., ) to better determine the depositional age of the strata. In the Valle Hermoso depositional area, we use detrital zircon U–Pb geochronology to provide the first stratigraphic controls on the age of this section.…”

“…18 Ma and 12 Ma (Reynolds et al ., ), in the Vinchina area between ca . 19 Ma and 2 Ma (Dávila et al ., ; Ciccioli et al ., ; Amidon et al ., ; Collo et al ., ), and in Fiambalá depositional area between ca . 9 Ma and 3 Ma (Carrapa et al ., , ) (Figs and ).…”

Mio-Pliocene aridity in the south-central Andes associated with Southern Hemisphere cold periods

“…S2). The Toro Negro section lacks suitable deposits for paleomagnetic dating and has been previously dated to ∼7-2.3 Ma by U-Pb zircon dating of tephra (40). Samples for measurement of cosmogenic 10 Be were collected from sandstones and conglomerates exposed at the base of 3.5-to 6-m-high road cuts to minimize modern cosmogenic nuclide production that could skew paleo-erosion rate estimates.…”