NEWLY MAPPED PALEOZOIC STRATIGRAPHIC UNITS IN THE HUANUCO REGION OF CENTRAL PERU; TECTONIC IMPLICATIONS FOR THE MARAÑON FOLD-THRUST BELT

The Marañon fold-thrust belt (MFTB) in the Huanuco region (around Margos, Jesus and Cauri) of central Peru contains stratigraphic sequences from Precambrian to Quaternary. Paleozoic, Mesozoic and Cenozoic units are all represented. Basement units to the Andean sequence on the eastern margin of the belt were previously defined as the Neoproterozoic Marañon Complex, Permo-Triassic Mitu Group, and Triassic-Jurassic Pucara Group. But recent mapping has identified a significant interval of other Paleozoic units and unconformities that have tectonic implications for the belt's margin.

Near Cauri we defined the Cauri Volcanics: basic-volcanics intercalated by thin sedimentary units that discordantly overlie the Marañon schist, and underlie the Contaya Fm. This suggests a pre-Ordovicio-Silurian age of the volcanics predated by orogenesis and erosion. Discordantly overlying these volcanics is the Ordovicio-Silurian Contaya Fm: fine facies intercalated by sandstones, but metamorphosed into a schist. The uncleaved Mitu Group overlies this schist unconformably, and constrains the upper age of the cleavage-forming orogenic event to Siluro-Permian. Although the Carboniferous Ambo Group is missing near Cauri, regionally this uncleaved sequence provides a Carboniferous upper age on the orogenic event affecting the Contaya Fm. This Siluro-Carboniferous orogenic event has not been recognized on the MFTB's eastern margin, but corresponds in age to e.g. the Marcano (392Ma) event in the Arequipa Massif of Peru, and the Upper Devonian Chanica orogenic phase in Bolivia. The continental sediments and volcanics that characterize the Ambo Group discordantly overlie the Marañon Complex. This unconformity reflects a renewed Carboniferous depositional system following a deformational event in the Siluro-Devonian.

We have identified a significant interval of formerly unrecognized Paleozoic rocks on the eastern margin of the Andean MFTB that belong to two Paleozoic sequences separated by orogenic episodes in the Neoproterozoic-Ordovician and Siluro-Carboniferous. The first episode corresponds to an early event in the development of the Terra Australis Orogen (Cawood), and probably to the 440Ma Atico event in Peru. The second episode marks the final Gondwanide Orogeny in this orogen in this area.