CURCUEL FORMATION: A NEW SANTONIAN TO CAMPANIAN SEDIMENTARY DEPOSITS OVERLYING CCOP BASEMENT IN WESTERN COLOMBIA
We revisited the Ricaurte-Altaquer section located at Nariño department in the south-westernmost part of Colombian Western Cordillera. In this section was defined by Spadea and Espinosa (1996) the Ricaurte Arc, an island-arc complex composed by basalt to andesite suites and hornblende andesites. At the top of the sequence was described by the authors a sequence of Campanian radiolaria from cherty sedimentary rocks with only 12 m thick.
Stratigraphic survey in the Ricaurte section allowed us the definition of the Curcuel Formation, as a part of the Ricaurte Arc (Spadea and Espinosa, 1996). The area is not completely accessible by military security conflicts, but based in aerial photographs, we calculated a total thickness of about 400 m. The unit is divided in two informal members: the lower, Cabuyal composed by low metamorphosed chert and shales, and the upper San Miguel composed by fine to medium grained sub-lithic sandstones. The Curcuel Formation is proposed as an extension of the Santonian-Campanian megasequence in western Colombia proposed by Villamil et al. (1999) initially in southern and central Colombia, which include a widespread bedded cherts interbedded with fine to coarse grained clastics.
The main implications of Curcuel Formations in Western Colombia probably are that, these rocks constitute a new identifiable rock source of sediments for the Oligocene to Pliocene Tumaco forearc located to west of the Colombian Western Cordillera, the lower sediments in this forearc have a record of Precambian to Jurassic reworked zircons probably sourced in this formation; the probable age of the accretion of the CCOP remnants in western Colombia; and the migration of the northwestern margin in South America in order to explain the continuity of chert deposition and the generalized vigorous upwelling dominant in this epoch according to Villamil et al. (1999).