UC Santa Barbara

The Nacientes del Teno Formation and the lowermost strata of the Rio Damas Formation in Maule, Chile (35.10°S) comprise a sequence of sedimentary and volcanic rocks deposited along the western margin of the Andean backarc basin during the Middle to Late Jurassic (174-145 Ma). Revised stratigraphy and new U-Pb geochronological data from this classic succession provide insights into the location, composition, and timing of coeval arc volcanic systems.

U-Pb geochronological data from detrital and primary volcanic zircons represent the first absolute ages (ca. 172-161 Ma) reported for the Nacientes del Teno Formation. Detrital zircon ages include a tightly clustered population ca. 165 Ma that coincides with a Middle Jurassic trough present in frequency histogram and relative probability plots of published datasets of U-Pb detrital zircon ages, challenging the notion of a lull in volcanic arc activity at this time.

The dominantly marine deposits of the Nacientes del Teno Formation interfinger with subordinate subaerial strata. The latter include channelized conglomerate with sub-rounded to sub-angular andesitic clasts up to 40 cm in diameter, suggesting a proximal alluvial fan and fan delta depositional environment with alternating continental and marine deposition. The Nacientes del Teno Formation includes a quartz rhyolite ignimbrite that yielded a U-Pb zircon age of ca. 160 Ma, the first Jurassic rhyolite reported for this portion of the Andes in central Chile, and in stark departure from the commonly assumed Miocene and Triassic ages for local rhyolites. Paleocurrent indicators from the W ± ~90° together with a general eastward decrease in grain size, decrease in volcanic content, and increase in carbonate content of marine deposits, suggest a sediment source to the west. The proximal volcaniclastic stratigraphy and tight distribution of U-Pb detrital zircon ages of the Nacientes del Teno Formation suggest nearby andesitic stratovolcanoes in the Middle Jurassic. These hypothesized volcanoes are located much further east than traditional models for the location of the “main magmatic arc” during the Jurassic, challenging the notion that Jurassic arc volcanism at this latitude was restricted to a single, narrow (a few tens of kilometers wide) volcanic chain located in the present day Coastal Cordillera.

Sedimentological features of lowermost Rio Damas Formation strata at 35.10°S are consistent with deposition in an alluvial fan environment. A maximum depositional age of ca. 151 Ma calculated from U-Pb detrital zircon ages agrees with previously published maximum depositional ages for the Rio Damas Formation. Sedimentological and geochronologic evidence suggest that the Late Jurassic regression represented by the formation resulted from tectonic uplift.