Siliciclastic Sedimentary Rocks

Introduction

Siliciclastic sedimentary rocks are sedimentary rocks composed mainly of silicate particles derived by the weathering breakdown of older rocks and by pyroclastic volcanism. Sandstones, conglomerates, and shales are the three members of this group. ^[1]

Siliciclastic sedimentary rocks make up roughly three-fourths of all sedimentary rocks in the geologic record, and they are present in sedimentary successions ranging in age from Precambrian to Holocene. ^[1]

Their abundance across geological time reflects the fundamental dominance of silicate minerals in the crust. Wherever rock is exposed to weathering and erosion, it produces silicate detritus; wherever that detritus is deposited and buried, siliciclastic rock eventually forms. The three-billion-year continuity of siliciclastic rocks in the record makes them the primary archive for reconstructing the history of ancient mountains, drainage systems, climates, and ocean basins.

Geological Significance

Siliciclastic rocks are of special interest to geologists as indicators of Earth history. Their textures and structures provide important information about ancient sediment transport and depositional conditions. The minerals and rock fragments they contain are the most definitive clues available to the nature and location of ancient mountain systems - including vanished ranges such as the ancestral Rocky Mountains and Appalachian Mountains - that have since been eroded away. ^[1]

Petroleum geologists are especially interested in sandstones because more than half of the world’s reserves of oil and gas occur in these rocks. Shales are likewise of great importance because the organic matter in shales is believed to be the source material of oil and gas. ^[1]

Mineralogy vs. Bulk Chemistry

Particle and chemical composition are fundamental properties of sedimentary rocks that allow one kind to be distinguished from another and that provide additional information about the history of the rocks. Geologists commonly use the term mineralogy to refer to the identity of all the particles or crystals making up a rock. Bulk chemistry is a different but related aspect of overall composition, directly tied to the mineralogy of the rock. ^[1]

Sedimentologists have traditionally been more interested in mineralogy than in bulk chemistry, considering mineralogy more useful for characterising and classifying sedimentary rocks and for interpreting their geological history. That perspective is changing, and an increasing number of studies are now concerned with sedimentary geochemistry. ^[1]

The practical reason mineralogy dominated for so long is straightforward. Mineral grains can be identified and counted under a petrographic microscope on a thin section, giving directly interpretable provenance information. Bulk chemical analyses require expensive instrumentation and give results that blend the contributions of many different minerals, making interpretation more complex. Now that tools such as X-ray fluorescence and electron probe microanalysis are routine, geochemistry is increasingly complementing mineralogy rather than competing with it.

The Three Members

Sandstones are the coarsest and most studied member of the group, composed of sand-size (1/16-2 mm) silicate framework grains, with variable amounts of cement and fine-grained matrix. They are the primary reservoir rocks for petroleum and are powerful indicators of depositional environment and provenance.

Conglomerates are coarser still, dominated by gravel-size (>2 mm) clasts. They are less abundant than sandstones but form in high-energy environments and in proximal settings close to sediment sources.

Shales are the finest-grained member, composed predominantly of clay minerals and silt. They are volumetrically the most abundant sedimentary rock type and serve as the primary source rocks for hydrocarbons, as well as important seals for petroleum reservoirs.

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