Abstract

The city of Girardot is in a Tertiary syncline to the north, east, and west by rocks of the Upper Cretaceous, which comprise a complete series from the Upper Turonian to the Maastrichtian.

Two sections of the Upper Cretaceous have been described in detail in terms of their lithology and mollusk and foraminiferal fauna, one located to the west (Girardot-Nariño) and the other to the east (Girardot-Melgar). The age determination of the horizons is based, as far as possible, on ammonites, although valuable data were obtained from the study of foraminifera.

The Turonian and Coniacian are divided into some lithological horizons and are rich in ammonites and lamellibranches, while the microfauna is very poor, containing almost exclusively planktonic forms. The Upper Coniacian is presented by the "second lydite"; the loams and clays above it do not contain mollusk s, but a very rich and uniform microfauna, which we consider to be of Santonian age. The "Primera lidita" overlies these loans and clays and by sandy loams and clays, characterized by the presence of Campanian ammonites and by a microfauna very rich in several species of Siphogonerinoides. All horizons from the Turonian to the top of the Campanian are connected by lithological and microfaunal transitions, which indicate continuous sedimentation during these periods. In contrast, we observe a hiatus above the Campanian, followed at the beginning of the Maastrichtian by a new transgression, characterized by the appearance of Siphogenerinoides plummeri or bramtellei. Upwards, the Maastrichtian bears gradually become sandier and more continental. Some thin, clayey intercalations with Siphogenerinoides plummeri determine short periods of marine flooding.

Compared to the Upper Cretaceous around Bogotá, that of the Girardot region is richer in lime and less areonian than that of the Santonian and much richer in fossils. The thickness of the Santonian (and Coniacian?) is equal in both the Bogotá and Girardot regions, but the thickness of the Campanian and Maastrichtian n is much greater near Girardot than near Bogotá; this indicates that the zone of maximum sedimentation in the Cretaceous geosyncline moved at the beginning of the Campanian from a more eastern position towards the Magdalena River basin, where sediments of a greater thickness were deposited later in the Tertiary. This shift of the sedimentation axis reflects the initiation of the uplift of the Eastern Cordillera favored by a relative accentuation of the subsidence in the Magdalena Hova. The same subsidence phenomenon is observed on the eastern edge (Llanos) of the Eastern Cordillera from the Oligocene.