Comparison of the pebbles of the Shinarump and Moss Back members of the Chinle formation

Abstract

Lithology, color, size, sphericity, and roundness of pebbles from the Shinarurnp and Moss Back members of the Chinle formation were analyzed and compared. The difference in the quartz:quartzite:chert ratios of the pebbles, the presence of limestone and siltstone pebbles, and to a lesser degree,the difference in color of pebbles serve to distinguish the Moss Back from the Shinarump. In areas where both the Moss Back and Shinarump are present, the average ratios of quartz, quartzite, a.nd chert are respectively about 12:37:51 and 82:16:2. Limestone and siltstone pebbles are commonly found in the Moss Back, whereas they are rarely found in the Shinarump. The colors of the Moss Back pebbles are generally darker than those of the Shinarump pebbles. The Moss Back contains more gray to black pebbles and fewer light-colored pebbles, such as red, orange, and white, than the Shinarump. Size, sphericity, and roundness of pebbles do not show a significant difference between the two units. Fossiliferous pebbles in the Moss Back and Shinarump were derived chiefly from sediments of Carboniferous and Permian ages and could have had common sources.