Discoveries of Chicxulub impact ejecta of the Albion Formation in road cuts and quarries in southern Quintana Roo, México and Belize, broaden our understanding of ejecta depositional processes in large impacts. There are numerous new exposures of ejecta near the Río Hondo in Quintana Roo México, located at distances of 330–350 km from the center of the Chicxulub crater. A single ejecta exposure was discovered near Armenia in central Belize, 470 km from Chicxulub. The Albion Formation is composed of two lithostratigraphic units: the spheroid bed and diamictite bed, originally identified at Albion Island, Belize. The new spheroid bed exposures range from 2 to 5 m thick and are composed of altered glass fragments, accretionary lapilli, and pebble-sized carbonate clasts in a fine-grained calcite matrix. The base of the spheroid bed is exposed at Ramonal South in México and at Albion Island and Armenia in Belize, and at all three locations, the spheroid bed was deposited on a weathered karst land surface that had emerged in the Late Cretaceous. The new diamictite bed exposures are composed of altered glass fragments and carbonate clasts up to 9.0 × 3.2 m in size. In all but one of the new exposures, the diamictite bed extends to the surface with observed thicknesses up to 8 m. At Agua Dulce in México, the weathered top of the diamictite bed is overlain by thin-bedded Tertiary carbonates. No diamictite bed is found in Armenia, where the spheroid bed is overlain with a limestone conglomerate containing altered glass shards and shocked quartz. These discoveries indicate that ejecta are emplaced in large terrestrial impacts by at least two distinct flows: (1) an initial flow involving a volatile-rich cloud of fine debris similar to a volcanic pyroclastic flow, which extends >4.7 crater radii (the spheroid bed), and (2) a later flow of coarse debris that may not extend much beyond 3.6 crater radii (the diamictite bed). The former deposit we attribute to material entrained in the impact vapor plume, and the latter to the turbulent collapse of the ejecta curtain.

Full article available in PDF version.

At Wetumpka impact structure, the surficial polymict impact breccia crops out discontinuously over a relatively small area within the intrastructure terrain. This polymict breccia unit contains a significant number of shocked quartz grains that represent a slightly lower shock pressure regime than the previously documented shocked quartz population obtained from a separate subsurface impact breccia unit. Specifically, the shocked quartz grains found in the polymict breccia unit display two distinct types of planar microstructures (named herein P1 and P2), and the host grains range in size from fine sand to pebbles and cobbles. P1 elements closely resemble planar fractures (PFs) or planar cleavage in quartz, which occur in multiple sets of open, parallel, fl at to curviplanar planes aligned with distinct crystallographic orientations. P2 elements are much shorter, much thinner, and more closely spaced than P1 planes, and they resemble in part closed, partly decorated to nondecorated, classic, longer planar deformation features (PDFs). However, these P2 elements are not PDFs. Sets of P2 elements are commonly developed off of, or are crosscut by, through-going P1 planes, which form “feather features” (FFs). P1- and P2-type planar microstructures are associated with low shock pressures (~7–10 GPa), whereas more typical planar deformation features previously described in Wetumpka's subsurface impact breccias are associated with shock pressures of 10–16 GPa. We attribute the difference in Wetumpka's shock levels (i.e., between quartz grains in the polymict impact breccia and previously described quartz grains in the deeper, subsurface impact breccias) to differences in provenance of these grains from within the impact structure's transient crater. Proximal ejecta deposits, which were derived from more shallow reaches of the target materials, are the most likely candidate for sources for the P1- and P2-bearing grains in the surficial polymict impact breccia unit. We interpret the present distribution and occurrence of the surficial impact breccia unit to be best explained by late-modification-stage slumping of proximal ejecta from the impact structure's rim.