A New Architectural Element for Modelling the Neogene Fluvial Basins Impacted by Volcanism in Central Colombia (South America)

Neogene fluvial basins on the flanks of the Central Colombian Cordillera (Colombia) were impacted by Miocene-Pliocene calc-alkaline volcanic activity. This ancestral volcanic activity corresponded to the initial formation of the northernmost volcanoes of the former Northern Volcanic Zone (NVZ). In this paper we present the results of the study of three fluvial basins located to the west and north of Manizales (Caldas Department, Colombia) where the Casabianca and Manizales Formations and Aranzazu Volcaniclastic Sequence were deposited.

By implementing analysis of architectural elements into the sedimentologic description, we defined three deposition intervals associated with inter-eruptives and syn-eruptive periods for the volcaniclastic sequences. In the inter-eruptive periods, fluvial deposition in meandering and anastomosed streams associated with alluvial fan systems was developed, while the syn-eruptive periods were characterized by thick volcaniclastic deposition (up to ~150 m).

To determine the environments of deposition we applied Miall`s, architectural elements classification for the inter-eruptive periods. But, Miall's categorization was limited to define the sedimentation during the syn-eruptive periods. Therefore, we proposed a new architectural element, named “SV”, to define the catastrophic sediment flows formed during rapid deposition of pyroclastic and volcaniclastic events. These events abruptly overpassed the geomorphic threshold and altered the fluvial style conditions of the main drainage network in the fluvial basins.

This research project also intended to narrow the existing gap between the traditional sedimentology focused on the epiclastic deposits and the volcanology overwhelmed by the effusive and pyroclastic deposits. During the revision of the available bibliography related to the terminology for volcaniclastic sedimentation, we found the necessity for proposing an appropriate nomenclature to name volcaniclastic deposits due to the lack of lithofacies coding in the existent literature to describe these hybrid deposits.