Sedimentology of the Rimac-Chillon alluvial fan at Lima, Peru, as related to Plio-Pleistocene sea-level changes, glacial cycles and tectonics

The Rimac and Chillon Rivers eroded deep valleys on the Lima coastal plain during the Late Miocene (before ca. 5.3 Ma), due to at least 485 m of uplift produced by the Nazca Ridge, combined with a sea level lowstand of around -50 m. The main paleo-Rimac channel along the southeastern boundary of the alluvial cone was apparently deflected by the Lima Anticline and reached the sea in the vicinity of Morro-Solar, whereas the paleo-Chillon ran largely parallel to the anticline, breaching it to enter the Pacific at present-day Magdalena. These valleys were filled by fine-grained sediments, possibly during marine transgression at 1.7 Ma, which was followed by uplift and regression to below present sea level. Meltwater surges from the Andean Cordillera during subsequent interglacial stades caused an accumulation of coarse, reworked glacial moraine in the Rimac and Chillon fans, forming the Lima Congolomerate and drowning the Lima Anticline. The Rimac and Chillon Rivers subsequently migrated north and westward, possibly in response to tectonic tilting of the landscape, causing silt and mud to accumulate in abandoned channels along the southeastern boundary of the fan.