The Noreán Formation (, J1-2n, J1n) is a geological formation of the Eastern Ranges of the Colombian Andes, the Serranía de San Lucas and as basement underlying the southernmost Lower and northern Middle Magdalena Valleys. The formation consists of volcanic and pyroclastic lavas that range from andesites to rhyolites. Vitric, lithic and crystal tuffs and andesitic dikes and hypabyssal bodies are also present in the formation. The more than thick formation was deposited in a continental arc magmatic setting in an Early Jurassic graben that presently forms the basement of the Middle Magdalena Valley (VMM). A positive anomaly of Pb suggests a subduction-related genesis dominated by explosive volcanism. Etymology The Noreán Formation was first defined as the "Unidad Volcanoclástica de Noreán" ("Volcanoclastic Unit of Noreán") in 1995 and in the same year elevated to a formal formation by Clavijo in 1995 as part of the geologic mapping for Plancha 65 Tamalameque and named after the caserío Noreán, north of Aguachica, Cesar. The type locality of the Noreán Formation is along the road between Buturama and Bombeadero in Aguachica. Description The Noreán Formation is found in the northern part of the Eastern Ranges of the Colombian Andes, stretching from the Cesar Department in the north, towards the Serranía de San Lucas in Bolívar to the Santander Massif in Santander in the south. The formation forms the economic basement in the southern Lower Magdalena Valley (VIM) and the northern Middle Magdalena Valley (VMM). The formation is interpreted as characteristic of an important explosive volcanic phase, the materials of which were deposited in a graben setting of the Middle Magdalena Valley. The Early Jurassic basin was covered by a shallow sea and in part drained by rivers and lakes. The basin at time of deposition was bordered by a volcanic arc, characterized by basaltic to calc-alkaline magmas. The formation also comprises less than thin very fine to fine sandstone beds constituting quartz (90 to 60%), feldspars (10 to 40%) and lithic fragments (1-2 %). The volcanic and pyroclastic rocks of the Noreán Formation are composed of lavas that range from andesitic to rhyolitic, together with vitric, lithic and crystal tuffs. Mainly andesitic dikes and hypabyssal bodies are also present. Geochemically, the volcanic and pyroclastic rocks exhibit chemical similarities, belong to the calc-alkaline series and have negative anomalies of Nb, P and Ti and a positive anomaly of Pb, suggesting a subduction-related genesis. U-Pb