FORMATION AND EMPLACEMENT OF IMPACT BRECCIAS IN CARBONATE TARGET ROCKS: EXAMPLES FROM SOUTHERN MISSOURI

Two widely accepted impact structures are located in southern Missouri: Decaturville (7 km dia.) and Crooked Creek (6 km dia.). The Weaubleau structure (8 km dia.) is another area of locally anomalous intense structural deformation. Planar lamellae in quartz grains from the “Weaubleau Breccia” are consistent with an impact origin and further analyses are underway. Collectively, these structures contain a variety of breccias that differ in the timing of their formation during the cratering process and in their modes of deposition or emplacement.

Principal breccia types include (1) fracture breccia, (2) megabreccia, (3) injection breccia, (4) dilation breccia, (5) crystalline basement breccia, and (6) ejecta/resurge breccia. The distinction between fracture breccia and megabreccia is arbitrarily placed at 1.0 m clast size, but the mode of formation and emplacement is essentially the same. Both are found in central uplift areas and in material-flow debris blankets in crater moat regions. Fracture breccia and megabreccia clasts rest in a comminution matrix or are surrounded by injection breccia. Injection breccia appears to have a fluid matrix that formed and was emplaced at depth during the excavation and modification stages. Dilation breccia, known only from the Weaubleau structure, essentially formed in situ during the excavation stage but may have been transported en mass during the modification stage. Granitic and pegmatitic clasts in the Decaturville and Weaubleau structures indicate deep penetration and uplift associated with formation of crystalline basement breccia. The uppermost part of the “Weaubleau Breccia” is a carbonate-matrix-supported breccia that exhibits coarse grading associated with deposition in a marine setting.

The primary factor in the preservation of different breccia types at each of Missouri structure is related to the depth of post-impact erosion. The ejecta/resurge facies of the Weaubleau Breccia was buried below limestones of the Meramecian Warsaw and Salem formations prior to subaerial exposure, karstification, dissolution of overlying limestones, and subsequent re-burial by Pennsylvanian strata. Most of the middle and late Paleozoic strata have been eroded from the Decaturville and Crooked Creek structures.