THE SHOCK BEHAVIOR OF CARBONACEOUS GREYWACKE: SHOCK RECOVERY EXPERIMENTS RESULTS VS. OBSERVATIONS IN NATURAL ROCKS (L. BOSUMTWI IMPACT STRUCTURE, GHANA)

Shock barometry in complex impact structures is a tool to trace mass movements in the central uplift, including the collapse of this feature in a late stage of cratering. Shock pressures recorded in lithologies of the central uplift may be as high as 45 GPa, e.g., in the Vorotilovo core drilled into the center of the Puchezh-Katunki structure. In other craters, shock levels of 35-40 GPa have been found. At this pressure, quartz is totally converted to diaplectic glass in crystalline rocks. The central uplift of the Lake Bosumtwi structure was drilled in 2004 by ICDP. There, carbonaceous meta-greywackes are the dominant lithology, consisting of quartz, calcite, muscovite-illite and ferroan clinochlore in various proportions. Surprisingly, the shock levels recorded in these rocks are much less than the modeled 40 GPa. It is discussed whether this strange observation is related to a different shock behavior of the porous and fluid rich meta-sediments or due to yet not explained specific circumstances in the cratering process.

To tackle the problem, shock recovery experiments were carried out at 34 and 39.5 GPa with a conventional setup consisting of high-explosive (Composition B), a 4 mm thick flyer plate and ARMCO steel containers, embedded in momentum traps. The 0.5 mm thick sample-disks were carbonaceous greywackes of a composition similar to the L. Bosumtwi lithologies. In both experimentally shocked greywackes, all quartz xx are diaplectic, and calcite displays multiple shock-induced mechanical twins as well as several sets of planar fractures. These results indicate that the shock behavior of carbonaceous greywackes is not completely different to that one of quartz-rich metasandstones.