Rock Fragments in Sandstones

Introduction

Rock fragments are pieces of ancient source rocks that have not yet disintegrated into individual mineral grains. They are present in sandstones whenever the source rock was fine-grained enough for a sand-sized chip to represent an intact multi-mineral piece of rock. A single rock fragment carries more provenance information than any individual mineral grain because it preserves the texture, mineralogy, and fabric of the source rock in one package - making them the most reliable source-rock indicators in sandstones.

Abundance and Variability

Rock fragments make up about 15-20 percent of framework grains in the average sandstone, but their content is highly variable, ranging from zero to more than 95 percent. ^[1]

The extreme upper end of that range - over 95 percent - occurs in immature sandstones deposited very close to their source, such as the coarse-grained deposits of an alluvial fan at the base of a fault scarp. In contrast, a quartz arenite deposited far from its source after multiple recycling cycles may contain essentially no rock fragments at all.

Types of Rock Fragments

Clasts of any kind of igneous, metamorphic, or sedimentary rock can occur in sandstones, but clasts of fine-grained source rocks are most likely to be preserved as sand-size fragments. ^[1]

Very coarse-grained source rocks such as granites typically yield clasts of coarse sand-size or larger; however, granite clasts are not common because granites usually disintegrate to yield individual mineral grains rather than intact multi-mineral clasts. ^[1]

The most common rock fragments in sandstones are clasts of volcanic rocks and volcanic glass (in younger rocks), and fine-grained metamorphic rocks such as slate, phyllite, schist, and quartzite. Sand-size fragments of silica-cemented siltstone, fine-grained sandstone, and shale are less common. Clasts of limestone or other carbonate rocks are also less common, probably in part because they do not survive weathering and transport. ^[1]

Chert as a Rock Fragment

Composite quartz grains consisting of very small quartz crystals - microcrystalline quartz - are called chert. Chert grains are actually rock fragments, derived by weathering of bedded chert or chert nodules in limestone, and can be abundant in some sandstones. ^[1]

The classification of chert as a rock fragment rather than a single mineral grain is important because it affects both the mineral classification of the sandstone and the provenance interpretation. A sandstone rich in chert plots in the rock-fragment field on a QFL diagram; it cannot be classified as a quartz-rich sandstone even though the chert grains are composed entirely of SiO₂. The chert also points to a specific source rock type - bedded siliceous sediments or silicified limestones - rather than the more generic igneous or metamorphic sources of ordinary quartz grains.

Provenance Reliability

Rock fragments are particularly important in sediment provenance studies. They are moderately easy to identify, and they are more reliable indicators of source rock types than individual minerals such as quartz or feldspar, which can be derived from many different source rock types. ^[1]

The reliability advantage of rock fragments over mineral grains is a matter of specificity. Quartz grains can come from granites, gneisses, vein quartz, quartzites, or recycled sandstones - a very broad range of sources. A volcanic rock fragment, by contrast, comes from a volcanic or volcaniclastic source. A phyllite or schist fragment points unambiguously to a low- to medium-grade metamorphic terrain. This specificity makes rock-fragment-rich sandstones - lithic arenites - among the most informative for basin analysis and tectonic reconstruction.

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