Classification of fine-grained sedimentary rocks

Abstract Classifications of fine-grained sediments and sedimentary rocks are based on combinations of several criteria. These include: (1) texture, (2) fissility, (3) tectonic association or environment of deposition, (4) mineral composition, (5) color, (6) chemical composition and (7) degree of metamorphism. None of the classifications are generally used, however. For field studies, texture is the most significant property. The proportions of clay, silt and sand are used herein to define four major groups: claystone (clay), siltstone (silt), mudstone (mud) and sandstone (sand). Claystone, siltstone and sandstone are rocks containing more than 50 percent of clay-, silt- or sand-sized material, respectively. Mudstone contains less than 50 percent of clay, slit or sand. Silty and sandy are useful modifiers of claystone when clay-sized material is less than 75 percent but more than 50 percent. Clayey and sandy are used to modify siltstone when silt-sized material is less than 75 percent but more than 50 percent. The expression good sorting is applied to fine-grained clastic rocks that contain more than 90 percent silt or clay-sized material. Fair-sorted rocks contain 75 to 90 percent silt or clay. Poorly sorted siltstone or claystone contains less than 75 percent silt or clay. If fissile, fine-grained clastic rocks are so designated. The mineral composition of most fine-grained clastic rocks can be determined reliably with the petrographic microscope and by X-ray diffraction. Therefore, where possible, they should also be named according to one of the sandstone classifications and on the dominant clay mineral present. Color, bedding types and sedimentary structures, fossil content, type of cement and fissility should be included in complete descriptions. An example of a good field and laboratory description is: clayey siltstone, illite-subarkose, poor sorting, grayish red (5R4/2), dolomite cement, slabby, horizontal lamination, pelecypods.