A shocked quartz pebble from the Araguainha impact structure, Brazil, investigated by U-stage, CL, and EBSD

Quartz pebbles in the conglomerates of the Devonian Furnas Formation in the central uplift of the 40 km Araguainha impact structure, Brazil, are pervasively crosscut by shear fractures and shocked, showing sets of planar fractures (PFs), planar deformation features (PDFs) and feather features (e.g., von Engehardt et al., 1992). A recent reinvestigation of such pebbles interpreted the shear fractures as resulting from post-shock brittle deformation, but still impact-related (King et al., 2025).

A petrographic thin section and a polished mount from one of such pebbles were here investigated by Universal-stage (U-stage), cathodoluminescence (CL) and electron backscatter diffraction (EBSD) to characterize the shear fractures. The shear fractures are filled by angular clasts of quartz embedded in a fine-grained quartz matrix, showing neither crystallographic preferred orientation nor recrystallization. The host quartz is twinned and clearly dislocated along the shear zones. A network of thin pressure-solution lines occurs at ca. 60° from the main set of shear fractures. The distribution and orientation of (shock) planar fractures was compared with the orientation of the shear fractures, following the suggestion that they should show a preferred orientation, and this might be related with the shock wave propagation direction (e.g., Pittarello et al., 2020). Further investigations on oriented pebbles are planned to better constrain their deformation history in relation with the impact event.

Engelhardt v. et al. (1992) Meteoritics 27:442-457.

King et al. (2025) Meteoritics & Planetary Science 60:124-132.

Pittarello et al. (2020) Meteoritics & Planetary Science 55:1082-1092.