New U-Pb ages and stratigraphic relationships for the Toro Negro Formation in the northernmost section of the Chilean-Pampean flat slab (28°42´S): sediment provenance and basin reconstruction

The Sierra de Los Colorados a key area in the transition between the Chilean-Pampean flat slab (27°–33°S) and the Central Volcanic Zone (15°–27°S). It is located between the fold-and-thrust belt (Frontal Cordillera and Precordillera) and the broken foreland (Famatina and Pampean ranges) in Argentina.

At this locality, crops out the Miocene to Pliocene synorogenic sequences, like the Toro Negro Formation (6.9-2.37 Ma, Amidon et al. 2016), deposited after a major episode of uplift and denudation, and attributed to the onset of the thick-skinned broken foreland deformation. It is deposited over a basal erosional unconformity that separates it from the underlying Vinchina Formation (Limarino et al., 2010). According to Amidon et al. (2016), sediment supply for this unit came mainly from the north and west (Toro Negro Range, Precordillera, and Frontal Cordillera), with no significant supply coming from the east (Famatina Range). Contrarily, Stevens Goddard et al. (2020) point out that sediment supply from the Famatina Range was abundant during its deposition.

To evaluate the sources areas for the Toro Negro Formation and to compare it with previously obtained data, a provenance analysis (U-Pb in zircon via LA-MC-ICP-MS) was applied to four samples of the Toro Negro Formation at Quebrada de La Troya. Tuff samples TN-25 and TN-29 yielded maximum depositional ages of 7± 1 Ma (N=3) and 10 ± 1 Ma (N=5). The presence of zircon grains with older ages of 196, 223 and 226 Ma, and 479 Ma, respectively, point out that they are reworked tuffs. Sandstone TN-27 (N=68) shows a polymodal zircon distribution with main peaks at 449, 252, and 997 Ma, and sandstone TN-30 (N=57) also shows a polymodal zircon distribution with main peaks at 488, 367, and 283 Ma. In general, the age patterns show groups at the Mesoproterozoic, Early Cambrian, Ordovician, Devonian, and Carboniferous-Permian, that could correspond to the source areas of the Toro Negro Range, the Pampean Ranges, the Famatina System, and the Paleozoic sedimentary cover of Precordillera and Frontal Cordillera, respectively.

The detrital zircon provenance data were not conclusive to determine if the Famatina Range was a source area for the Toro Negro Formation at Quebrada de La Troya. Nevertheless, fieldwork carried out on the foothills of the Famatina Range shows that the basal unconformity of the Toro Negro Formation completely eroded the Vinchina Formation and it lies directly above the Carboniferous-Permian strata. Paleocurrent data and clast composition show imbricated subangular pebbles belonging to the Carboniferous-Permian strata of the Famatina Range, suggesting it constituted a close source area.

The fact that the Toro Negro Formation received sediment from all the surrounding mountain systems and that the entire Vinchina Formation is eroded at one locality points out to a conjunct uplift of the fold and thrust belt and the broken foreland. Paleogeographic reconstructions suggest that the position of the Juan Fernández Ridge and the onset of the flat slab subduction caused this last stage of mountain building and sedimentation in the foreland.

References

Amidon, W.H., Ciccioli, P.L., Marenssi, S.A., Limarino, C.O., Fisher, G.B., Burbank, D.W., Kylander-Clark, A., (2016). U-Pb ages of detrital and volcanic zircons of the Toro Negro Formation, northwestern Argentina: Age, provenance and sedimentation rates. Journal of South American Earth Sciences, 70, 237–250.

Limarino, C.O., Ciccioli, P.L., Marenssi, S.A. (2010). Análisis del contacto entre las formaciones Vinchina y Toro Negro (Sierra de Los Colorados, provincia de La Rioja, Argentina), sus implicancias tectónicas. Latin American Journal of Sedimentology and Basin Analysis (2), 113e132.

Stevens Goddard, A. L., Carrapa, B., & Aciar, R. H. (2020). Recognizing drainage reorganization in the stratigraphic record of the Neogene foreland basin of the Central Andes. Sedimentary Geology, 405, 105704.