AN EROSIONAL REMNANT OF A SILICICLASTIC DIAMICTITE IN BELIZE

In the Maya Mountains of Belize, a diamictite 1.5 x 1.0 km in aerial extent lies unconformably on siliciclastics of the Santa Rosa Group. The diamictite has a scour base, fill of matrix and lithified clasts, erosional top, and maximum exposed thickness of 9.2 meters. The basal diamictite scour is irregular with up to 1.5 meters of erosional relief, in contrast to the broad scour surfaces typical of channels in the underlying Santa Rosa. Matrix accounts for approximately 27% of the deposit and consists of coarse sand to clay-sized particles of lithics and minerals of plutonic, volcanic, sedimentary, and metamorphic origins. Granule to boulder size clasts are composed exclusively of sandstone, siltstone and shale lithologies from Santa Rosa units. Clasts are poorly sorted, sub-rounded, and oriented randomly. Many clasts contain alteration rinds probably related to post-depositional hydrothermal processes, as evidenced by near-vertical milky quartz veins that cross-cut the deposit. Typically, trace element concentrations are higher in the Santa Rosa, but Cu and Zn are elevated in the diamictite from hydrothermal processes. Relative age of the diamictite is determined by stratigraphic position of the underlying latest Pennsylvanian to middle Permian Santa Rosa Group. This deposit is of interest because diamictites are known to result from glacial, tectonic and/or impact-related processes, and are relatively rare. However, no evidence for glacial events is recorded locally, and no impact-related debris flows from pre-Cretaceous rocks are reported from Belize. The diamictite occurs in a region where both tectonic and impact histories are documented.