Geographic features of the Mason Impact Structure

The Manson impact structure appears at the bedrock surface, but there is no visual evidence of the crater on the surface land because it is concealed under 70 - 300 feet of glacial till. Like other complex impact craters, the Manson impact structure has three distinct features. The central peak is the mound of rock found at the center of the crater. Surrounding the central peak is an area known as the crater moat. The terrace terrane is the outermost crater feature, which forms a ring around the crater moat.

(The word "terrain" refers to the land surface, usually the topographic relief. The word "terrane" refers to the entire rock package (the total crust) in an area. Terrane is an appropriate term for the Manson crater regions because it refers to the characteristics of the entire rock package, not just the surface.)

The 12 cores obtained in the early 1990s penetrated the three major regions of the Manson impact structure: the terrace terrane, crater moat and central peak. In-depth studies of the core samples determined that the Manson impact structure was comprised of the following six types of impact rocks.

Phanerozoic-Clast Breccia: Breccia clasts in a shale matrix that was originally stripped from the land surface, mixed with ejecta in a ground surge and quickly transported back into the crater by seawater. Found above all crater regions.

Ejecta: An overturned sequence of clastic sedimentary rocks thrown from the crater during formation. Found only on the terrace terrane.

Keweenawan Shale Clast Breccia: Broken and partially melted billion-year-old black shale and melt-rock grains. Found only on the central peak.

Crystalline-Clast Breccia with a Melt-Rock Matrix: Similar to crystalline-clast breccia with a sandy matrix, except the sandy matrix has been melted and is dominated by powdered crystalline rocks and impact melt-rocks. Found only on the central peak.

Crystalline-Clast Breccia with a Sandy Matrix: Smaller blocks of granite and gneiss in a matrix of silt to sand-size rock fragments. Found only on the central peak.

Proterozoic Basement Blocks: Large blocks of granite and gneiss which form the core of the central peak and extend below much of the crater floor. Found only on the central peak.

The last photo is a photomicrograph, a photograph through a microscope, of a thin-section of the Crystalline-Clast Breccia with a Sandy Matrix from the Manson 2-A core taken on the Central Peak. The field of view is 3 mm wide. The rock was originally a granite that altered and uplifted about 15,000 feet by impact energy. The light brown colored mineral in the image is quartz, a common constituent of granite, but unaltered quartz usually appears white to gray with few features. The dark brown parallel lines that can be seen crossing the quartz grains are known as parallel deformation features (or PDFs). These thin, parallel sets of lines are actually thin zones of melted quartz that are produced when the impact shock wave causes atoms in the quartz to slide slightly along certain crystallographic planes. The energy produced by this sliding creates thin zones of melting. The pressure required to produce these features is much greater than can be generated by any natural force near the surface of the Earth. In fact, except for a few very high pressure test facilities, only nuclear bomb blasts are known to produce similar features. Therefore, the presence of multiple intersecting sets of PDFs in quartz is considered to be definitive evidence of meteorite impact. The PDFs in Manson impact rocks have been enlarged by the etching action of hot acidic fluid that circulated in the structure shortly after the impact. Because of this etching action, the Manson PDFs are among the most prominent that can be seen in any impact materials.