Abstract

The Miocene Honda Group, more than 3000 m thick in the Upper Magdalena Valley of Colombia, was derived from the volcanically active igneous-metamorphic central range (Cordillera Central) of the Andes, which was mantled with sedimentary and volcanic rocks. The group was deposited by rivers flowing generally normal to the central range, indicating that major uplift of the eastern range occurred after accumulation of the Honda sediments.

Two major depositional environments of fluvial sedimentation are interpreted from sedimentologic and petrologic evidence: low gradient-high sinuosity, and high gradient-low sinuosity streams. The former type, favored during low tectonic and/or volcanic activity, resulted in red hematitic floodplain mudstones of montmorillonite, illite-montmorillonite and kaolinite, with subordinate thin tan, nonconglomeratic, subarkosic to arkosic, simple sand-bodies. The sandbodies, elongate with a low width: depth ratio, show the vertical arrangement of sedimentary features and fining-upward grain-size that characterize point bar deposits of low gradient, meandering streams. Directions of cross-stratification within the sandbody and the sandbodies themselves have a high variance.

The second type, operative during moderate to high tectonism and/or volcanism, was probably braided and deposited thick sandbodies, commonly multistory and multilateral, with a high width: depth ratio. Sedimentary structures have a predictable vertical arrangement but do not become finer in grain size upward. Directions of cross-strata in sandbodies and the sandbodies themselves have a lower variance than those of the first type. The gray, conglomeratic, arkosic to impure arkosic sandbodies are associated with subordinate drab mudstone of montmorillonite and illite-montmorillonite.

The Honda Group consists of two formations. The lower, La Dorada Formation (new name), was deposited during increasing tectonism an P volcanism, which were almost nil during accumulation of the conformably underlying La Cira Formation. The Puerto Salgar Member (new name) of the La Dorada is overlain by the Perico Member (new name), which is capped by an orthoconglomeratic interval. The upper formation of the Honda, the Villavieja (new name), accumulated during mild tectonic activity and episodic volcanism which yielded two sharply contrasting lithofacies: the Baraya Volcanic Member (new name) and the Cerro Colorado Redbed Member (new name).

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