EVIDENCE AND CONTEXT FOR THE FIRST OCCURRENCE OF MESOPROTEROZOIC CA. 1450 MA RHYOLITE MAGMATISM IN THE SOUTHWEST, MARQUEÑAS FORMATION, PICURIS MOUNTAINS, NEW MEXICO, USA
The lower contact of the Marqueñas Formation is interpreted to represent an unconformity that separates the Mesoproterozoic Marqueñas Formation from the Paleoproterozoic ca. 1700 Ma Vadito Group. Map relationships within the study area point to a corrugated contact representing 300 meters of paleorelief that was infilled by lower Marqueñas Formation conglomerates. In some areas, strike changes in the contact do not coincide with strike changes in compositional layering in the underlying Vadito Formation. The Vadito compositional layering exhibits a 14 degree angular discordance with the contact and overlying Marqueñas bedding along a 1.5 kilometer segment of the contact. In the same area, distinctive lithologic layers within the Vadito terminate against this contact. Locally, slickenlines can be found on the contact between the Marqueñas and Vadito; however, there is no evidence of faulting on this surface at other localities, suggesting this surface is not a regional fault contact. Our discovery of ca. 1450 Ma rhyolite clasts immediately above this unconformity further supports evidence for the 1480 to 1400 Ma Picuris orogeny.