What is Formation Geology?

Stratum is a general term for all layered rocks, including metamorphic and volcanic-originated layered rocks. It is a layer or a group of rock layers with certain uniform characteristics and attributes and distinctly different from the upper and lower layers.

Histograms of rock series (usually accompanied by brief text descriptions) depicting a certain area or an outcrop or a drilling record according to the sequence of geological ages, also known as "stratigraphic columnar sections". Sometimes the order (upper-new, lower-old), interrelationship, lithology, fossils, and thickness are only described in text, and the strata are separated only by line segments, regardless of the proportion of the average thickness of the strata. However, usually the stratum surface is required to be drawn in proportion to the average thickness of the rock layer, and the thickness of each layer is marked in order to have a more intuitive impression.

Internationally, the strata are divided into three categories:

Lithologic strata based on lithology;

Biostratigraphy based on fossils;

Time stratigraphy or chronostratigraphy based on formation time.

Classes and are actually a type, and their division and comparison have global simultaneity. The chronostratigraphic units (boundary, system, system, and stage) are also based on fossils for division and comparison, just like biological strata. Strata generally refers to layered rocks and deposits, including sedimentary rocks, volcanic rocks, and metamorphic rocks that are metamorphic from sedimentary rocks and volcanic rocks.

Since the stratum has the concept of the age, the stratum has the so-called up-down or new-old relationship. This is called the stratigraphic sequence, which is equivalent to the pages of a book. If the stratum is not disturbed or reversed, the stratum in the lower part is older and the stratum in the upper part is newer. This stratigraphic sequence is called the normal horizon, and the relationship between the new and the old is called the stratigraphic sequence law. .

However, the strata that make up the crust are intricate, or the strata are missing due to crustal rise in a certain period, or the sequence is reversed due to tectonic changes such as inverse faults, or the appearance and appearance of the stratum are changed due to metamorphism. This is like the same ancient book has become fragmented, chaotic, and illegible. You must do some proofreading and research work to sort out the order, divide chapters, and conduct research.

The same is true for strata. Even if the sequence is sorted out, it is necessary to determine the stratum era and the stage of different levels. This kind of work is called stratigraphic division. For example, the stratum of an area can be divided into several boundaries, a certain boundary can be divided into several lines, and a certain line can be divided into several groups. This is like the same book contains many pages, which must be divided into several chapters. Subsections are the same.

Because the entire stratum of an area is the "book page" of the crustal history of the area. Stratigraphic division must be based on lithology, such as conglomerate, sandstone, limestone, and sand-shale interlayers; and second, based on biofossils, because the strata of different ages often contain different types of Paleontology or paleontology; third, based on sedimentary cycles, such as transgression to retreat in a certain period, sedimentary rock layers often appear from coarse to fine (such as from conglomerate, sandstone, shale to limestone, etc.) Rhythmic changes to coarse (such as from limestone, shale, sandstone to conglomerate, etc.), this phenomenon is called a sedimentary cycle. Sedimentary cycles can reflect changes in land and sea and changes in geographical environment. The fourth is based on stratigraphic contact, such as integration, parallel unconformity, and angular unconformity. ); Five is based on the isotope age of the stratum, especially for those ancient strata lacking fossils are more significant.

Stratigraphic division refers to the division of epochs in the strata of an area; if the stratigraphy of different regions is compared with the epoch, it is called stratigraphic comparison. According to the stratigraphic division of a region, you can only understand the development of geohistory in one region; according to the stratigraphic comparison of several regions, you can understand the commonness and difference of geohistory development in a wider range, and understand the characteristics of the stratigraphic region of the vast area In order to grasp the laws of stratigraphic spatial differentiation, in other words, it is possible to reshape the crustal development history and paleogeographical environment of larger regions or national or even worldwide.

Stratigraphic contrast is firstly a comparison of geological ages or horizons. The main basis of comparison is biological fossils, as shown in the figure. Those in the stratum of the three sites that contain the same fossils can be connected by dotted lines to indicate that their stratum grains (represented by A, B, C, D) or times are the same. Site 3 lacks the B layer, and may not have been deposited due to crustal uplift, or it may have been ablated due to sediment. Strata without fossils is generally referred to as "dumb formation". In this case, paleontological methods cannot be used for comparison, but comparison using lithology, sedimentary cycles, contact relationships between rock formations, and isotope age becomes an important method.

For example, the North China region (including the southeast of Northeast China) has similar stable sedimentary environments throughout the Early Paleozoic, so the lateral changes of lithology are small and often have similar characteristics, such as the Cambrian purple-red shale, the Middle Cambrian oolitic Like limestone, Upper Cambrian bamboo leaf-like limestone, Middle Ordovician pure thick block limestone, etc., can be one of the signs of contrast. However, in recent years, research on micro-organism fossils and spore pollen is getting deeper and deeper. Some so-called dumb formations are not "dumb", which can be used as an important basis for dividing and contrasting formations. Regardless of stratigraphic division or contrast, comprehensive methods should be used to obtain more reliable results. ^[1]