Anderson, B.J. Witzke, and J.B. Hartung

Meteoritics, v. 28, no. 3, p. 317


In a recent drilling program 12 cores totaling over 1,200 m were recovered from the Manson Impact Structure. Four principal impact rock types were encountered: (1) Sedimentary Clast Breccia (SCB), (2) Crystalline Clast Breccia with Sandy Matrix (CCB-S) and Melt-Rock Matrix (CCB-M), (3) Central Peak Igneous and Metamorphic Rocks (CP), and (4) an overturned flap of Impact Ejecta (IE). The SCB is dominated by clasts of Cretaceous marine shale and mudstone, with subordinate Cretaceous sandstones, Paleozoic carbonates, minor Proterozoic Red Clastics, and rare crystalline rock and impact melt-rock clasts in a medium gray, calcareous, sandy shale matrix. Parallel deformation features (PDFs) and other evidence of impact metamorphism are extremely rare. The SCB reaches a maximum thickness in excess of 200 m and is interpreted as a post-impact debris flow that originated at the crater margins. The abundance of large clasts (up to 75 m) and the pervasive occurrence of SCB (cored in all regions of the Manson Impact Structure including the Central Peak pit) suggest a high energy emplacement mechanism, possibly water rushing into the crater following an impact in a shallow marine environment. The uppermost unit on the Central Peak, the CCB-M, displays abundant clasts, dominated by quartz grains, most displaying PDFs, shock isotropism, and/or partial melting. Some clasts display accretionary mantling by melt materials, apparently while airborne, with subsequent mixing into the CCB-M. An isotropic melt matrix frequently displays flow-banding and devitrification textures. The CCB-M is interpreted as an impact melt layer, derived primarily from crystalline basement rocks, and may represent CCB-S that experienced sufficient heat to melt the matrix grains. A central zone of the CCB-M displays clusters of sanidine crystals, recrystalized from impact melt in a region that apparently cooled more slowly. The CCB-S is dominated by clasts of basement gneiss and granite in a matrix of sand- to silt-size grains of crystalline rocks and mineral grains derived from disaggregated crystalline rocks. Mineral grains display abundant impact deformation features, and almost all quartz grains display abundant multiple intersecting PDFs. Rare grains of melt-rock and sedimentary rock fragments imply minor mixing of overlying materials. The CCB-S is interpreted as the impact-brecciated floor of the Transient Crater, uplifted on the Central Peak. The CPs are Proterozoic crystalline basement rocks that form the core of the Central Peak. They are dominated by gneisses and granites, but also include minor mafic and ultramafic rocks. In thin section, CPs display abundant PDFs and other shock deformation features. They commonly occur as meters-scale blocks separated by zones of CCB-S and display thin veins of melt material, apparently pseudotachylite. The CPs, CCB-S, and CCB-M display the effects of post-impact hydrothermal alteration. The IE, encountered in one core in the Terrace Terrane, totaled over 200 m of Proterozoic Red Clastics and Paleozoic carbonates and clastics in overturned stratigraphic position. Pre-impact units occur, in reduced thickness, and the sequence is interpreted as an ejecta flap, only partially penetrated by core drilling. Deeper coring in this region would penetrate to the structurally-preserved impact surface, a tempting target for future drilling in this well-preserved impact structure.