The late Triassic bivalve Monotis (Pacimonolis) aft. subcircularis Gabb indicative of a Norian (probably early late Norian) age has been collected from the southeastern end of Patranca Island (45°25'S-74°09'W) in the Chonos Archipelago, Southern Chile. Another fossil bivalve, Limidae gen. et sp. indet. has been collected from a small unnamed island (45°25'S-74° 10'W) 4 km to the north, and indicates a maximum Early Carboniferous, probably Mesozoic age. The fossil bearing unit is the Potranca Formation (or Potranca Unit) which forms part of the Chonos Metamorphic Complex, interpreted as a subduction complex developed on the southwestern margin of Gondwanaland. As these rocks have been previously considered to be of Paleozoic age (Late Silurian-Early Devonian), the geologic history of this part of the continental margin must be reinterpreted. These fossils provide clear evidence that marine depositional processes persisted within the Chonos Metamorphic Complex during Late Triassic time.
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