ELSEVIER Earth and Planetary Science Letters 121 (1994) 519-531 EPSL The Corte Blanco garnetiferous tuff: A distinctive late Miocene marker bed in northwestern Argentina applied to magnetic polarity stratigraphy in the Rio Yacones, Salta Province Jos~ G. Viramonte a, James H. Reynolds b, Cecilia Del Papa a, Alfredo Disalvo c a Universidad Nacional de Salta, Instituto GEONORTE and CONICET, Buenos Aires 177, 4400 Salta, Argentina b Department of Geoseiences and Anthropology, Western Carolina University, Cullowhee, NC 28723-9047, USA c Bridas Company, Buenos Aires, Argentina (Received August 10, 1993; revision accepted December 21, 1993) Abstract We introduce the Corte Blanco Tuff, a white garnetiferous air fall unit, as a distinctive Neogene marker bed. Three whole-rock K/Ar ages from rocks at the source of this unit indicate that it was erupted 8.73 + 0.25 Ma from the La Pava-Ramadas Caldera on the Argentine Puna. Ash spread eastward across the foreland provinces of the Eastern Cordillera and Sierras Subandinas. Recognition of this dated marker unit in these provinces provides the first, easily identified, late Miocene time line in the vast, densely vegetated region to the east of the Puna. We encountered the unit in seven localities from all three morphostructural provinces in NW Argentina. A depositional gap in the air fall material is present between 20 and 150 km to the east of the caldera. Recognition of the Corte Blanco Tuff in the Rio Yacones, near Salta, allowed at/interpretation to be made of a magnetic polarity stratigraphy section erected in marginally suitable Neogene detrital strata of the Rio Guanaco Formation. We interpret the strata we examined to have been deposited between 10.5 and 6.4 Ma and report a 10 ° clockwise rotation since the strata were deposited. This is the first numerically dated section in the Rio Guanaco Formation of NW Argentina. These results reveal that uplift of the Eastern Cordillera was in progress at this time. 1. Introduction Neogene tectonism in the central Andes in NW Argentina is recorded in the stratigraphic record by impressive accumulations of volcanic material near the magmatic arc and by a thick pile of detrital sedimentary rocks deposited in the foreland provinces on the east side of the moun- tains. Temporal constraints on the tectonic evolu- tion of the Andes in this part of Argentina are restricted to isotopic ages reported from volcanic strata by various workers in the magmatic arc Elsevier Science B.V. SSDI 0012- 821X(93)E0250-N [1-5]. Chronostratigraphic study of the foreland basin strata is hampered by several factors, in- cluding (1) access to the few good exposures is difficult in this densely vegetated region, (2) fos- sils are extremely rare in the basin-fi|ling strata, (3) few isotopic ages are reported from interca- lated volcanic air fall beds, and (4) basin-filling strata exhibit numerous lateral shifts in deposi- tional facies [6-9], making lithostratigraphic cor- relation a poor geochronometer. We introduce the Corte Blanco Tuff as a tem- poral datum applicable across a wide area in NW