Abstract

The Paleogene, nonmarine Gualanday Group is part of a Cenozoic molasse sequence exposed in the Upper Magdalena Valley of Colombia. In the Neiva basin area, where the average thickness is about 1,400 to 1,500 m, this group is divided into five informal lithostratigraphic units, the lower conglomerate, the lower to middle transition, the middle conglomerate, the middle to upper transition, and the upper conglomerate. The lower, middle, and upper conglomerate units are predominantly close-packed, chert-pebble conglomerate; the two transition units are interbedded lithic sandstone, mottled siltstone, and conglomerate. The sandstone and conglomerate of the upper part of the group are more quartzose than those of the lower part.

The Gualanday Group marks the initiation of uplift of the Central Cordillera of the Colombian Andes. The sediments forming this group were transported eastward out of the Central Cordillera. Most if not all of this clastic material was supplied by erosion of Cretaceous strata that covered the site of the newly rising Cordillera. Cretaceous chert provided coarse clasts to the conglomerate, with the bedding thickness and fracture block size of the chert controlling clast size. Shale supplied abundant detrital kaolinite to the Gualanday siltstone. With increasing depth of erosion through time, the quartzose lower part of the Cretaceous sequence supplied an increasing amount of detritus to conglomerate and sandstone in the upper part of the Gualanday Group. The Central Cordillera was uplifted in three pulses. Each pulse exposed fresh Cretaceous bedded chert to erosion and resulted in deposition of one of the three major conglomerate units of the Gualanday Group.

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