“Formación La Luna”: expresión espuria en la geología colombiana

Abstract

The Hidrosogamoso Project in the Middle Magdalena Valley (around El Tablazo) presents upper Cretaceous strata refer as "La Luna Formation" (Morales and The Colombian Petroleum Industry 1958, 653-655) composed from base to top by the Salada, Pujamana, Galembo and La Renta formations. The contact with the underlaying El Salto Formation is conformable, and with the overlaying Umir Formation is disconformable (sensu Howe 1997, 135). The thickness of the stratigraphic interval is 316 m in the Aguablanca creek section, of which 118 m are from the Salada Formation, 57 m from the Pujamana Formation, 96 m from the Galembo Formation and 45 m from the La Renta Formation. In the Montebello ranch stratigraphic section, the measured thickness is 304 m, with 111 m (incomplete) for the Pujamana Formation, 177 m (incomplete) for the Galembo Formation and 16 m (incomplete) for the La Renta Formation; the Salada Formation and the lower part of the Pujamana Formation do not appear there; furthermore, the La Renta Formation is faulted and folded, and only the base was measured (16 m), but 693 m from the end of the section by the road to Montebello ranch in a monocline at the west flank of the Chucurí anticline, a measure track adds 61 m by which the La Renta Formation would have at least a thickness of 77 m. The thickening in the Pujamana, Galembo and La Renta formations in the Montebello stratigraphic section is explained by means of higher rates of tectonic subsidence and sediment supply in this area, related to normal faulting affecting the sedimentation in the Middle Magdalena Valley and the Hidrosogamoso project area. A new lithostratigraphic unit for the Middle Magdalena Valley it is formally proposed and is called the Formación La Renta, characterized by his phosphorite content and fino-granular siliceous rocks, with local concentrations of benthic foraminifera of the Siphogenerinoides genera.

Maastrichtian Microfossils of the Shallow Marine Umir Formation, Northeastern Colombia

“…We sampled sediments from the Aguablanca Creek section in the Middle Magdalena Basin (7° 7' N, 73° 18' W; Eastern Colombian Cordillera). This locality was previously studied by Terraza (2019), and we focused on the uppermost interval investigated by this author (320.8–344.2 m stratigraphic height), which comprises the uppermost part of the La Luna Formation (La Renta Formation sensu Terraza, 2019) and the lower segment of Umir Formation (Figs. 1–3).…”

“…The Umir Formation is composed of a monotonous succession of dark grey marlstones, mudstones, and siltstones. The La Luna-Umir contact was proposed at 322.8 m by Terraza (2019).…”

“…However, since most of these studies have focused on economically relevant coal-bearing strata of the middle and upper part of this unit, there is still debate on whether the lower part of the Umir Formation was deposited in a marine setting, and on the nature of its boundary with the underlying La Luna Formation. Previous works suggested the presence of an unconformity or a condensed section between the La Luna and Umir formations, as well as in coeval units in more proximal areas, such as in the Catatumbo and Cesar-Rancheria basins (Ward et al , 1973; Sarmiento et al , 2015; Terraza, 2019).…”

“…In addition, the latest Cretaceous was characterized by a global sea-level fall, which has been associated with the possible occurrence of intermittent ice sheets at high latitudes (Hardenbol et al , 1998; Kominz et al , 2008; Huber et al , 2018). Consequently, Campanian–Maastrichtian sedimentary successions of northern South America record a myriad of tectonic (basin deformation, local uplifts), stratigraphic (xenoconformities, diachronism of units), and sedimentary (abrupt lateral facies shifts, retrogradational stacking) features that make their depositional evolution highly complex (Ayala-Calvo et al , 2009; Bayona et al , 2011, 2020; Sarmiento et al , 2015; Montaño et al , 2016; Sarmiento-Rojas, 2019; Terraza, 2019; Valencia-Gómez et al , 2020; Gramal-Aguilar et al , 2021).…”

“…This is the case for the Maastrichtian Umir Formation, cropping out in the Middle Magdalena Valley (MMV), northeastern Colombia (Fig. 1), where depositional and chronostratigraphic interpretations have been traditionally based on microfossils, and eventually, integrated with sedimentological studies (Tchegliakova, 1993, 1995, 1996; Santos, 2012; Montaño et al , 2016; Navarrete et al , 2018; Terraza, 2019). However, since most of these studies have focused on economically relevant coal-bearing strata of the middle and upper part of this unit, there is still debate on whether the lower part of the Umir Formation was deposited in a marine setting, and on the nature of its boundary with the underlying La Luna Formation.…”