Evidence for a cosmogenic origin of fired glaciofluvial beds in the northwestern Andes: Correlation with experimentally heated quartz and feldspar

Abstract

Fired sediment, considered equivalent to the ‘Black Mat’ impact of 12.9 ka, has been located and analyzed in the Andes of northwestern Venezuela. The ‘Black Mat’ refers to possible fallout from the Encke Comet airburst presumed to have occurred over the Laurentide Ice Sheet, the impact spreading ejecta over large portions of North America and Europe, making it an interhemispheric event of considerable magnitude. These possible equivalent beds in the northern Andes, first considered to result from a lightning-induced conflagration adjacent to the retreating Late Wisconsinan (Mérida Glaciation) ice, are now known to have undergone intense heating upon impact to a temperature much higher than what would occur in a wet, first-stage, successional tundra. Analyses carried out by SEM and FESEM, in SE and BSE modes, show massive micro-disruption on grain surfaces, fractures diminishing with depth toward grain interiors and C welded onto quartz and plagioclase minerals. Bubbles on some grains, possibly the result of exclusion of water-of-crystallization, are seen on some samples, principally quartz. The presence of copious monazite in the carbonaceous coatings is considered part of the incoming ejecta, as it is not a common indicator mineral in the local lithology. Analysis by SEM and FESEM of quartz and plagioclase subjected experimentally to temperatures ranging from 500 to 900 °C shows that intense heating affects resident mineralogies to differing extents, with grain disruption more prevalent along cleavage planes deep into grain interiors. The intergrowth of carbonaceous “black mat” material with thermally disrupted and fragmented quartz and feldspar, a “welded” patina of 100–400 nm thickness could only occur with temperatures in excess of 900 °C, the event interpreted here to be of cosmogenic origin.