More information about sedimentary rocks (4500)

Organic sedimentary rocks are given this name because of their having been formed partly or wholly from organic material. The most widely spread are: limestones, chalk, dolomites, radio-larites, spongiolites, tripoli, caustobioliths.

Sedimentary rocks of chemical origin include a series of deposits owing to their origin processes that are chemical in character and formed chiefly by concentration through evaporating aqueous solutions, changes of temperature, loss of carbon dioxide, etc., aided more or less in some cases by the action of organic life (plants and animals) and resulting in precipitating in­soluble salts. These may be subdivided into carbonates, siliceous rocks, ferrugineous rocks (iron ores), sulphates and haloids.

Sedimentary rocks of mixed origin include the rocks which may be formed: 1) partly from clastic and partly from organic material; 2) from clastic material and that of chemical origin; 3) from the material of chemical or of organic origin; 4) from materials of clastic, organic and chemical origin. Their being of mixed origin results in some properties similar to those of rocks of organic, chemical and clastic origin.

The term "metamorphic", when broadly applied, includes any change or alteration that any rock has undergone. It involves changes that are both physical and chemical, and the rock so altered may have been originally of sedimentary or igneous origin.

The alteration includes changing in mineral composition or texture or both. This change being sometimes very great obscures the primary characters of the original rock, rendering the possi­bility of defining the source rock almost impossible.

Chemical composition of metamorphic rocks varies greatly because of the source material having been of widely different composition.

The chemical composition of many rocks is not greatly changed during the process of metamorphism; hence, metamorphosed igneous and sedimentary rocks frequently show the composition characteristic of their class. Chemical analysis therefore frequently forms an important criterion for discriminating between metamorphose sedimentary and igneous rocks.

Mineral composition of metamorphic rocks being dependent on chemical composition leads to wide variations in mineral content of metemorphic rocks. It has been shown that certain, minerals such as the feldspathoids (nepheline and sodalite) are characte­ristic of igneous rocks. Likewise there are certain minerals which are considered to be characteristic of metamorphic rocks.

Mineral composition often becomes at important criterion in distinguishing metamorphosed sedimentary from metamorphosed igneous rocks.

Metamorphic rocks include: marls, clayey limestones, arena­ceous limestones, and others.

As has been stated before, igneous rocks occur in the form of batholiths, laccoliths, stocks, dykes; etc. The mode of occurrence of metamorphic rocks depends on the kind of rocks they have been formed from. We shall speak mostly about sedimentary rocks because of their being of more practical importance.

The principal morphological units of sedimentary rocks are beds (layers). The layers may vary as to kind of material, colour, texture and thickness. Variations in thickness of individual layers may range from a very smell fraction of an inch up to one hundred feet and more. If we use the terms "layer" or "bed" which are synonym, we refer to thicker divisions. If the divi­sions were thinner we should use the term "lamina". "Stratum" is generally applied to a single bed or layer of rock while a group of beds deposited in sequence one above another and during the same period of geologic time is known as a formation or suite. The formation may include beds of both homogeneous and heteroge­neous rocks.

Every bed or suite of beds has thickness. We distinguish a true thickness true thickness, a horizontal thickness and vertical thickness. The true thickness is the length of the perpendicular line from any point on the top of the bed to the bottom of this bed. The horizontal thickness is the length of the horizontal line from any point on the top to the bottom drawn across. The vertical thickness is the length of the vertical line drawn from any point on the top to the bottom.

Beds of rocks may be observed in outcrops. The outcrops occur as artificial or natural ones. As a rule the primary occur­rence of sediments is almost horizontal. If there is any displacement as to primary position of beds, we call this displacement a dislocation or faulting.

If the dislocation is not accompanied by discontinuity, it is called a plicative dislocation should there-be any discontinuity, a disjunctive dislocation" would result.

Both plicative and disjunctive dislocations are the result of movements of the earth's crust. Had there been no crustal movements, no dislocations would have occurred. If the crustal movements are horizontal or nearly so, plicative dislocations result; if they are vertical or approximately vertical, the dislocations will be disjunctive. Plicative dislocations are often accompanied by disjunctive ones, or vice versa.

The occurrence of beds may be conformable and unconformable, of gentle dip and folded, transgressive etc.

The position of the bed in space is defined by the elements of its occurrence, that is, by strike and dip.

A strike of a bed is the direction of the line of its inter­section with a horizontal plane.

A dip of the bed is its inclination asto the horizontal plane.