Las Juntas Formation

The Las Juntas Formation or Las Juntas Sandstone ((Formación) Areniscas de Las Juntas, Kiaj, Kialj, K1j) is a geological formation of the Altiplano Cundiboyacense and Tenza Valley, Eastern Ranges of the Colombian Andes. The Las Juntas Formation is found in the departments Cundinamarca, Boyacá and Casanare. The predominantly sandstone formation dates to the Early Cretaceous period; Hauterivian epoch, and has a maximum thickness of 910 m.

Etymology
The formation was defined and named in 1979 by Rodríguez and Ulloa after Cerro Las Juntas, Guateque, Tenza Valley, Boyacá.

Lithologies
The Las Juntas Formation has a maximum thickness of 910 m and is characterised by a sequence of sandstones with interbedded shales.

Stratigraphy and depositional environment
The Las Juntas Formation, the uppermost unit of the Cáqueza Group, overlies the Macanal Formation and is overlain by the Fómeque Formation and the Apón Formation in the Sierra Nevada del Cocuy. The formation is subdivided into three members, from old to younger; Arenisca de El Volador, Lutitas Intermedias and Arenisca de Almeida. The age has been estimated to be Hauterivian. The formation has been deposited in a near shore deltaic environment, with as provenance areas the Santander High and the Guiana Shield. The formation represents a regressive sequence in the present-day Eastern Ranges, as the Rosablanca Formation in the Middle Magdalena Valley.

Outcrops
The Las Juntas Formation is apart from its type locality east of Guateque, found in Chingaza National Park, in the El Cochal Synclinal east of the Ocetá Páramo, between Lake Tota and Labranzagrande, other parts of the Tenza Valley such as close to Macanal and Almeida.

The Támara Fault thrusts the Las Juntas Formation southeastward on top of the Tertiary San Fernando and Diablo Formations, and the Chámeza Fault thrusts the older Macanal Formation on top of the Las Juntas Formation around Chámeza, Casanare.