Murca Formation

The Murca Formation (Formación Murca, Kim) is a geological formation of the Altiplano Cundiboyacense, Eastern Ranges of the Colombian Andes. The predominantly subarkose sandstone with claystones and siltstones formation dates to the Early Cretaceous period; Valanginian epoch and has a maximum thickness of 924 m.

Etymology
The formation was defined and named in 1990 by Moreno after the Murca River, Cundinamarca.

Lithologies
The Murca Formation has a maximum thickness of 924 m, and is characterised by a sequence of subarkose coarse-grained, locally cross-bedded sandstones with intercalating siltstones and claystones. The basal part of the black and grey feldspar bearing sandstone beds contains large pyrite crystals. The matrix is formed by cements of calcite, iron oxide, sericite and chlorite.

Fossils of Berriasella colombiana and Pseudoosterella ubalaensis have been found in the Murca Formation.

Stratigraphy and depositional environment
The Murca Formation, belonging to the Cáqueza Group, underlies the Trincheras Formation, lowermost unit of the Villeta Group. The contact with the underlying unit has not been observed. The age has been estimated to be Valanginian. Stratigraphically, the formation is time equivalent with the Cumbre, Rosablanca and Útica Formations. The formation has been deposited in a marine environment characterised by turbidites. A mid submarine fan and outer fan setting have been observed in the Murca Formation. A possible source for the sandstones were Precambrian sandstones, gneisses and granites, belonging to the Guiana Shield.

Outcrops
The Murca Formation is apart from its type locality, found near Nimaima and Guayabal in the western flank and Ubalá and Labranzagrande in the eastern flank of the Eastern Ranges.