MAGNETOSTRATIGRAPHY OF THE LAS MANZANAS, PEÑA COLORADA, AND SOLAZUTI SECTIONS, SIERRAS SUBANDINAS, SALTA PROVINCE, ARGENTINA

A total of approximately 6,800 m of Neogene continental strata was measured and sampled from three sections in the Sierras Subandinas of Salta province, NW Argentina. All three sections are located in streams incised into actively rising, subtropical, jungle-covered ranges. Magnetostratigraphic ages of the strata, supported by 40Ar/39Ar dates, were determined at: 1) Arroyo Las Manzanas (2,400 m), 2) Arroyo Peña Colorada (3,030 m), and 3) Arroyo Solazuti (1,370 m). Arroyo Las Manzanas is located in the Sierra de Pintascayo, to the east of the Sierra Pescado, while the other two cut the Sierra Baja de Orán. The tops of Peña Colorada and Solazuti are cut by a thrust fault.

Five tuffs were dated at Las Manzanas, two at Peña Colorada, and another two at Solazuti. Twenty-four Class I polarity reversals were found at Las Manzanas, twenty-eight at Peña Colorada, but only six at Solazuti. Las Manzanas strata were deposited between 14-4 Ma with a notable increase in the sediment accumulation rate after 6 Ma. The Peña Colorada section was deposited between 9.5-4 Ma above the Tranquitas Formation and exhibit an increase in the sediment accumulation rate around 5Ma. The Solazuti section was probably laid down between 8.5-3 Ma. A large hiatus is present, however, between 8-5.5 Ma. Rapid sediment accumulation took place between 5.5-4.0 Ma.

The results suggest that the Sierra Pescado was rising by 6 Ma and depositing detrital strata in the Las Manzanas area. By 5 Ma, the Sierra de Pintascayo was rising and contributing sediment to the Peña Colorada and Solazuti sections. The Sierra Baja de Orán began to rise no earlier than 4 Ma. A reasonably complete chronology for uplift of the western two thirds of the Sierras Subandinas is established when these data are combined with existing magnetostratigraphic sections at Río Iruya to the west and Arroyo La Porcelana to the east.