Rocks from sediments

Sediment or sedimentary rock covers most ocean floor and three-quarters of the land. On land this skin is usually a few miles thick; but layers up to 19mi (30 km) thick collect on offshore basins. Most sedimentary rock comes from scraps of older (igneous or other) rocks eroded from the land, carried into lakes or seas by rivers, deposited, and then consolidated in a solid mass. When parent rock breaks up its minerals behave in different ways. Some of the silicates (the main mineral ingredients of igneous rocks) dissolve; others- quartz, for one- endures; and weathering creates new minerals- especially the clays that bulk large in most sedimentary rock. Besides the clastic sedimentary rocks (rocks made from fragments) others come from chemical precipitates or the remains of living things.

Processes converting sediment to rock are known as diagenesis. Two main processes occur. As sediments pile up their pressure squeezes water from the sediments below and packs their particles together. Then, some minerals laid down between grains cement a mass of sediment together.

Changes converting sediment to rock leave traces in the finished product. Transportation of eroded sediments abrades and rounds their particles, sorts these by density or size, “rots” unstable minerals, and concentrates resistant minerals, including diamonds and gold.

Deposition lays down sediments in broadly horizontal sheets called beds or strata, each separated from the next in the pile by a division called a bedding plane. Beds with ripple marks reveal ancient currents. Graded bedding (beds with grain size graded vertically) may hint at turbidity currents – sediment rich water sliding soupily down a continental slope. Cross bedding (sands laid down at an angle between two bedding planes) show features such as old dunes and sand-bars.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

Fig. 29 Breccia