Late Cenozoic volcaniclastic deposits, Andean foredeep, Colombia

Abstract

Volcaniclastic deposits preserved in the upper Magdalena Valley, Colombia, were derived from stratovolcanoes on Cordillera Central during and after the late Cenozoic Andean orogeny. Andesitic lava clasts and suites of mineral grains, commonly mixed with plutonic and metamorphic rock fragments, predominate in proximal debris flows and in torrential channel lenses; reworked dacitic pyroclastic material is more common in distal debris flows, sheeted overbank sands, and flood-plain muds. Earliest Colombian volcanism is recorded by montmorillonite in the mid-Cenozoic La Cira Formation (1,000 m). During early phases of Andean orogeny in middle and late Miocene time (20 to 10 m.y. ago), abundant volcanic clasts and volcanic mineral grains accumulated in coarse channel and flood-plain sediments of the Honda Group (3,000 m), along with a few thin sheets of reworked pyroclastic debris. The overlying conglomeratic Neiva Formation (200 m) records renewed uplift and a fresh supply of lava. In latest Miocene time (8 to 9 m.y. ago) the climax of explosive activity swamped the Andean foredeep with debris that accumulated on coalescing fans of the Gigante Formation (750 m). About 7 to 5 m.y. ago, the final Andean orogenic phase deformed both Cordillera Central and its foredeep. Subsequent excavation of the uplifted eastern lowland was interrupted at least ten times by local incursions of volcanic detritus 10 to 75 m thick. These deposits include polymictic debris flows and torrential sediments that filled valleys and formed large fans, and reworked pumiceous debris concentrated in fluvial valley fill and in debris flows that spread widely across the Magdalena lowland. Older deposits are preserved in high-level mesas, valley terraces, and deeply dissected fans; younger deposits form low, relatively undissected fans and inner terraces along major streams.