Abstract

BORRERO PENA, Carlos Alberto; ROSERO CESPEDES, Juan Sebastián; VALENCIA M, Julián David  and  PARDO TRUJILLO, Andrés. THE VOLCANICLASTIC SEQUENCE OF ARANZAZU: RECORD OF A NEOGENE FLUVIAL SYSTEM IMPACTED BY VOLCANISM IN A MIDDLE SECTOR OF THE CENTRAL CORDILLERA, COLOMBIA. bol.geol. [online]. 2008, vol.30, n.2, pp.61-77. ISSN 0120-0283.

The volcaniclastic sequence of Aranzazu (VSA, late Pliocene - early Pleistocene?) was sourced from the northernmost sector of the Machín - Cerro Bravo volcanic complex. The volcaniclastic accumulations filled the pre-existing fault-bend depressions in the surroundings of Aranzazu town (Caldas department, Colombia). A new classification of volcaniclastic deposits is proposed, in which the lahars are defined as volcaniclastic re-sedimented deposits, and differentiated from the primary volcaniclastic and epiclastic deposits. The updating the sedimentology and rheology of the deposits related with the laharic events is aimed. The VSA stratigraphy is based on the lithofacies identification and the definition of the architectural elements for syn- and inter-eruptive periods. The VSA lower member corresponds to the successive aggradation of syn-eruptive lahars (SV and SB elements) resulted from re-sedimentation of pumice-rich pyroclastic deposits and transported as debris and hyperconcentrated stream/flood flows. The VSA middle and upper members defined by coal contents were formed during the dominion of inter-eruptive (FF element) over the syn-eruptive (SV and SB elements) periods. They were formed during the reestablishment of the fluvial condition after the syn-eruptive laharic activity. Once the fluvial deposition was strengthened, the necessary conditions for the peat formation were propitious and the coal-bearing bedsets were developed.

Keywords : volcaniclastic deposits; lahars; aggradation; explosive volcanism; syn-eruptive and inter-eruptive periods.

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