Siliciclastic Sedimentary Rocks

Siliciclastic sedimentary rocks are classified according to the grain size of the particles or grains that are cemented together to form the rock.

SEDIMENT PARTICLE DESCRIPTION ROCK NAME

Gravel
Rounded rock fragments. CONGLOMERATE

Gravel
Angular rock fragments. BRECCIA

Sand
Quartz predominant, visible grains, often thickly bedded, depositional structures such as cross-bedding common. SANDSTONE

Silt
Quartz predominant, grains barely visible, gritty feel. SILTSTONE

Clay
Thick beds >1cm blocky, fine mud, no particles discernable, may show polygonal cracks, composed predominantly of clay minerals and very fine quartz. MUDSTONE

Clay
Laminated mudstone, fissile, splits into thin sheets. SHALE

SEDIMENT PARTICLE	DESCRIPTION	ROCK NAME
Gravel	Rounded rock fragments.	CONGLOMERATE
Gravel	Angular rock fragments.	BRECCIA
Sand	Quartz predominant, visible grains, often thickly bedded, depositional structures such as cross-bedding common.	SANDSTONE
Silt	Quartz predominant, grains barely visible, gritty feel.	SILTSTONE
Clay	Thick beds >1cm blocky, fine mud, no particles discernable, may show polygonal cracks, composed predominantly of clay minerals and very fine quartz.	MUDSTONE
Clay	Laminated mudstone, fissile, splits into thin sheets.	SHALE

The strength of cementation is often an important characteristic in engineering terms. Well-cemented quartz sandstones can be very strong mechanically, whereas friable uncemented sandstones are relatively weak rocks. Siltstones, mudstones and shales are usually weak rocks because of the dominance of platy clay minerals that are soft and provide little frictional resistance.

Conglomerates and sandstones have relatively high volumes of voids (pores) and are economically important as aquifers for water supply and reservoir rocks for gas and petroleum.