Geological Column

Principles and Development

The standard geological column represents an ideal succession containing rocks from all ages, the earliest rocks on Earth at the bottom of the column and the youngest ones at the top. The construction of the geological column is based on the underlying principles of stratigraphy, first proposed by Nicolaus Steno around 1669. According to his principle of superposition, the oldest strati-graphic units are located at the bottom of the column and the youngest at the top, with dips adjusted to the horizontal. The resulting geological column indicates the relations between the stratigraphie units and the subdivisions of geologic time, and their relative positions to each other. The principle of superposition is the basis for establishing the relative ages of all strata and the fossils that they contain.

The geological column was developed largely during the early 19th century, and its origin probably begins with the story of the first geological map of England, published by William Smith in 1815. Smith was the first to realize that fossils were arranged in order and regularly in strata, always in the same order from the bottom to the top of a section, each stratum being characterized by particular types of fossils. These observations led him to propose the principle of faunal succession. Furthermore, the relative order of the formations was proved to be the same even in distant locations of Great Britain. The application of these two simple principles (superposition and faunal succession) led to the construction of the first geological column. In addition, the geological column was based on the uniformi-tarian principles (the present is the key to the past, i.e., processes operating in the past were constrained by the same laws of physics that operate today) first proposed by James Hutton in the mid-18th century and further developed by Charles Lyell.

The standard geological column aims to establish a classification system to organize systematically the rocks of the earth's crust into formal units corresponding to intervals of geologic time. Such units must be of global extent to allow correlation. Among the formal units for stratigraphie classification, chronostratigraphic unitsunits based on the time of formation of the rock bodiesoffer the greatest potential for worldwide application because they are based on their time of formation, and are therefore the most accepted units to mark positions in the stratigraphie column. Other units such as lithostratigraphic, bio-stratigraphic, and unconformity-bounded units are all of limited areal extent and thus unsatisfactory for global synthesis. The biostratigraphic units are nevertheless unique in the sense that the fossils they contain show evolutionary changes through geologic time that are not repeated in the stratigraphie record. Due to the irreversibility of evolutionary change, biostratigraphic units are indicative of geologic age. However, owing to the imperfection or incompleteness of the fossil record, and the dependence of the fossil-producing organisms on biogeography and depositional facies, the boundaries of the biostratigraphical units commonly lie at different stratigraphie horizons and, similar to unconformity-bounded units, they may be diachronous and represent all or parts of one or several chronostratigraphic units. Magnetostratigraphic polarity units approach synchronous horizons because their boundaries [Page 569]record the rapid reversals of the earth's magnetic field. Although magnetostratigraphic polarity units may be useful guides for chronostratigraphic position and have a potentially worldwide extent, they have relatively little individuality because one reversal looks like another, and they can usually be identified only by supporting age evidence. Therefore, magnetostratigraphic polarity units require extrinsic data such as biostratigraphic data or stable isotope analyses for their recognition and dating. All these stratigraphie units are based on one property each, and they will not necessarily coincide with those based on another.

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