Anglais scientifique

The origin of sedimentary rocks

Weathering decomposes bedrocks. Flowing water, wind, gravity, and glaciers then erode the decomposed rocks, transport them downslopes and finally deposit them on the sea coast. Finally, the loose sediment is cemented to form hard sedimentary rock. Sedimentary rocks make up only about 5 percent of the Earth’s crust, but cover about 75 percent of the surface of the continents.

Types of sedimentary rocks

Sedimentary rocks are divided into three main classes:

1.    Clastic sedimentary rocks: Clastic sedimentary rocks are composed of fragments of weathered rocks, called clasts. These are grains that have been transported, deposited, and cemented together. Clastic rocks includes conglomerates, sandstones, siltstones and shales and make up more than 85 percent of all sedimentary rocks Conglomerate and breccia are coarse-grained clastic rocks. Sandstone is a clastic sedimentary rock comprising mainly sand-sized grains. Claystone, shale, mudstone, and siltstone are all fine grained clastic rocks.

Clastic sediment is named according to particle size . Gravel includes all rounded particles larger than 2 millimeters in diameter. Angular particles in the same size range are called rubble. Sand ranges from 1/16 to 2 millimeters in diameter. Silt varies from 1/256 to 1/16 millimeter. Clay is less than 1/256 millimeter in diameter. Mud is wet silt and clay.

Table 1. Sizes and names of sedimentary particles and clastic rocks.

2.    Bio-chemical    sedimentary    rocks:    Bio-chemical    sedimentary rocks consist of the remains of plants or animals or form by direct

precipitation of minerals from solution. Limestone containing fossils, for example, is a bio-chemical sedimentary rock formed in ancient seas.

Rock salt is an example of chemical sedimentary rock forms when salt precipitates from evaporating seawater or saline lake water. Organic sedimentary rocks, such as chert and coal, form by lithification of the remains of plants and animals.

3.    Pyroclastic sedimentary rocks: Pyroclastic sedimentary rocks are a type of sedimentary rocks composed primarily of volcanic materials that has been transported and reworked through mechanical action, such as by wind or water. In other words we can define pyroclastic rocks as they are formed from lavas which are ejected into the air. They are also called volcaniclastic rocks. This group will be discussed in more details in unit 14.

Sedimentary structures

Nearly all sedimentary rocks contain sedimentary structures, features that developed during or shortly after deposition of the sediment. These structures help us understand how the sediment was transported and deposited.

The most obvious and common sedimentary structure is bedding, or stratification layering that develops as sediment is deposited. Bedding forms because sediment accumulates layer by layer. Nearly all sedimentary beds were originally horizontal.

Cross-bedding consists of small beds lying at an angle to the main sedimentary layering. Cross-bedding forms in many environments is where wind or water transports and deposits sediment.

Ripple marks are small, nearly parallel sand ridges and troughs that are also formed by moving water or wind. They are like dunes and sand waves, but smaller.

In graded bedding, the largest grains collect at the bottom of a layer and the grain size decreases toward the top.

Mud cracks are polygonal cracks that form when mud shrinks as it dries. They indicate that the mud accumulated in shallow water that periodically dried up.