(Plio-)Pleistocene alluvial-lacustrine basin infill evolution in a strike-slip active zone (Northern Andes, Western-Central Cordilleras, Colombia)

The (Plio)-Pleistocene Zarzal Formation was deposited in the Cauca Depression and Quindío-Risaralda Basin between the Western and Central Cordilleras (Northern Andes). This area is structurally located on the transcurrent Romeral Fault System (RFS). Because of the interaction between the Nazca plate and the Chocó-Panamá block (an active indenter), the RFS strike-slip component changes direction around the study zone (dextral in the south, senestral in the north). Zarzal sediments are the oldest ones not to have been affected by the Pliocene Andean tectonic phase. Their study aims at better understanding the subsidence of these two interandean sedimentary basins within a regional compressional regime. The two basins are separated by the the Serrania de Santa Barbara (SSB), made of Tertiary sediments thrusted during the Pliocene tectonic phase. Zarzal sediments are fluvio-lacustrine: diatomites are encountered on both sides of the SSB and are alternating with braided stream or alluvial deposits. Sedimentation was strongly influenced by volcanic processes which led to the deposition of the Quindío-Risaralda and Cartago volcaniclastic fans sourced from the Central Cordillera. These volcaniclastic mass flows mixed with the Zarzal sediments, and even dammed and infilled a lake in the Quindío-Risaralda Basin (east of the SSB). Numerous extensional features affect Zarzal sediments with a mean extensional trend subparallel to the SSW-NNE trending cordilleras. The syndepositional tectonic activity is demonstrated by numerous seismites. In the Cauca Depression, sediments are infilling the basin more rapidly than it subsides or than the relief lifts up, thereby drowning the topography. This might be related to the tectonically- induced, downstream damming of the Cauca River valley to the north by the Chocó-Panamá block.