ABSTRACT

Rock fragments which compose part of many sandstones furnish valuable sediment-source information, provided they can be correctly identified. In order to gain a better understanding of the problems and limitations of source-rock interpretation based on rock fragments, an experimental study was conducted on a suite of igneous, metamorphic, and sedimentary rocks, ranging in grain size from about 0 to less than +5, which had been artificially fragmented to five size fractions: granule, very coarse sand, coarse sand, medium sand, and fine sand. These size fractions were studied to determine changes in textural properties with progressive size reduction and the feasibility of identif ing parent-rock textures in fragments ranging from granule to fine sand size.

Petrographic analysis of these fragments indicates that diagnostic textures of all the rocks studied are preserved to essentially the same degree in granule-size fragments as in thin sections of the parent rocks. In fragments smaller than granule size, however, the degree to which textures are preserved, and thus the reliability of parent-rock identification, depends mainly upon the mean grain size and sorting of the parent rock. The textures of fine-grained rocks (< +5) such as shale, slate, and volcanic igneous rocks are recognizable in all fragment sizes analyzed, although preservation of some textures is poor in fine sand-size fragments; however, textures of the coarser-grained rocks (0, to 1) such as granite and gabbro are identifiable only in fragments of very coarse sand size or larger. Textures of foliated metamorphic rocks are preserved to varying degrees depending upon parent rock grain size and type of foliation. In the specimens studied, gneissic banding is recognizable only in fragments of very coarse sand size or larger; the lineation of fine-grain schist and phyllite, however, is generally preserved in all fragment sizes except fine sand size, and the lineation in slate is recognizable in all fragment sizes including fine sand size. In general, because of the distinctive lepidoblastic and nematoblastic arrangement of grains, fragments of foliated metamorphic rocks of a given mean grain size re easier to identify than similar-size fragments of nonfoliated rocks having comparable mean grain sizes.