11.8: Identifying the Different Clastic Sedimentary Rocks
Identifying the Different Clastic Sedimentary Rocks
Sedimentary breccia Links to an external site. is an immature sedimentary rock with a poorly sorted mixture of clay, sand, and angular pebbles (gravel-sized) (Figure 11.17). The mineralogy of the clasts (sand and pebbles) often varies depending on the original source rock. Sedimentary breccias are typically found in continental environments, such as ancient landslides, or alluvial fans. The poorly-sorted, and angular nature of the gravel indicates the rock has only been transported a short distance, likely via gravity or inconsistent water.
Figure 11.17: Sedimentary breccia is a poorly sorted rock with angular large grains. (CC-BY 2.0, James St. John Links to an external site.)
Conglomerate Links to an external site. is an immature sedimentary rock, with a poorly sorted mixture of clay, sand, and rounded pebbles (gravel-sized) (Figure 11.18). The mineralogy of the clasts (sand and pebbles) varies depending on the original source rock. Conglomerates are typically found in continental environments, such as ancient landslides, alluvial fans, or pebble beds in rivers. The poorly-sorted nature of the gravel indicates the rock has only been transported a short distance, but the rounded grains suggest some transportation, generally via a consistent water source.
Figure 11.18: Conglomerate is a poorly sorted rock with rounded, large grains. (CC-BY 2.0, James St. John Links to an external site.)
A sandstone Links to an external site. consists of sand-sized clasts and is typically gritty to the touch (think about how sandpaper feels). These clasts vary in shape and mineralogy. Some sandstones may contain many minerals, while others may contain only quartz. Certain sandstone varieties, like arkose, are relatively immature and typically have more angular, moderately sorted, sand grains composed dominantly of the feldspar minerals (Figure 11.19). Arkose sandstone Links to an external site. is typically associated with continental environments, like alluvial fans. Quartz sandstone Links to an external site. is mature, as it contains well-rounded, well-sorted pure quartz grains. Quartz sandstone is typical of continental and transitional environments, like desert dunes or beaches. Lithic sandstones Links to an external site. contain an abundance of lithic (rock) clasts. The ternary diagram Links to an external site. in Figure 11.19, right, allows a sedimentary petrologist to carefully classify a sandstone based on its mineral composition.
Figure 11.19: Sandstone is composed of sand-sized grains. The ternary diagram to the right, illustrates the various sandstone types, which are determined based on the overall mineral assemblage of the sandstone. (Left, CC-BY 2.0, James St. John Links to an external site.; Right, CC-BY 4.0, Chloe Branciforte)
Mudrocks, which are composed of varying amounts of silt or clay-sized particles, include a number of different named rocks. Silts and clays are very small and will likely need magnification to be visible, perhaps beyond what your hand lens can provide. In general, silts may feel slightly gritty; a good test for this in the field is to rub the rock gently on your front tooth. Your teeth are surprisingly sensitive, and will pick up the slight grit of a silt in a siltstone Links to an external site.. If the rock is a mudstone Links to an external site. or shale Links to an external site. (contains layering, or fissility Links to an external site.), the rock will contain dominantly clay-sized sediment and will feel smooth to the touch and on your tooth (Figure 11.20). Should you want to try this with your samples, check with the instructor first. Overall, mudrocks are extremely mature, and are made from the smallest particles, carried by wind or barely moving water. Silt and clay-sized sediment are typically thousands of miles from their original source rock.
Figure 11.20: Shale is composed of mud-sized grains. (CC-BY 2.0, James St. John Links to an external site.)