Reconstruction of the Early Mérida, pre-LGM glaciation with comparison to Late Glacial Maximum till, northwestern Venezuelan Andes

Abstract

Valley glaciers have been well studied in the northwestern Venezuelan Andes over the last 20-year period with most sourcing reconstructions limited to the Last Glacial Maximum (LGM) and confined to the mapping of moraine positions and the detailed analysis of pedostratigraphic sequences reaching back into the Middle Wisconsinan (Weichselian) Glaciation. Using bedding macrofeatures, fabric analysis (magnetic azimuths) with mirror images, clast inclination and microtextural evidence determined with the FE-SEM (Field Emission Scanning Electron Microscopy) and EDS (Energy Dispersive Spectroscopy), and micromorphology of the PED5 composite section of Early to Late Mérida age was analyzed to reconstruct the till succession and glacial dynamics. The fabric is used to reconstruct the build-up of valley ice in the lower Mucuñuque Valley, the icecap spreading over the upper catchments of neighbouring drainage basins toward the headwaters of the Mucuchache Valley over a time frame estimated to be < 90 ka and lasting to ∼ 60 ka. Microtextural analysis was invoked to determine if physical differences on quartz and garnet mineral surfaces could be determined between various diamictons such as subglacial, flow and meltout tills, and outwash. Sample identification was made optically and with EDS. The evidence supports the theory that a substantial ice mass – a virtual icecap – existed during the Early Mérida stade of the last glaciation with the ice mass originating and decaying from the upper Mucuñuque Catchment south over some 40° of arc. This is the first evidence for an icecap complex in the northwestern Andes. Comparison between Early and Late Mérida glacial styles indicates that, in the Mucuñuque–Mucuchache corridor of the Mérida Andes glaciation in the LGM was catchment restricted with no evidence of icecap growth.