Stratigraphy is the science of rock layering, with particular concern for composition, geographic distribution, and geological and chronological importance. This discipline also involves the interpretation of rock strata in terms of mode of origin and geologic history. As a main branch of sedimentology, stratigraphy generally relates the large-scale vertical and lateral similarities between units of rock layering to environment of deposition. Those relationships are defined by lithologic and physical properties, geographic position, distribution, paleontological characteristics, and age relationships. Stratigraphers, using these properties and details of a sediment’s composition, structure, and texture can then synthesize aspects of environmental geology and, moreover, interpret the broader aspects of Earth’s geologic history.
Any stratigraphic investigation only becomes productive with a proper understanding of sedimentology, as stratigraphy is more a progeny than sibling of sedimentology. Formally, sedimentology is the study of natural sediments, both lithified (sedimentary rocks) and unlithified, and of the processes by which they form. Sedimentology includes all processes that give rise to sediment or modify it after deposition. These processes may include weathering, which breaks up or dissolves preexisting rocks; transportation; deposition; and diagenesis, which chemically and physically modifies sediment after deposition and burial, converting it into sedimentary rock. Sediments such as mud, sand, and gravel deposited by mechanical processes are known as clastic sediments, whereas those deposited predominantly by chemical or biological processes (limestones, dolomites, rock salt, chert) are known as chemical sediments.
Sedimentologists classify sedimentary rocks according to origin and size of included particles. However, the vast array of conditions by which sediments accumulate has borne a great number of sedimentary classification schemes giving rise to hundreds of sedimentary types, which invariably overlap. To complicate this system, most sedimentary rocks are, at least in part, made of several types of sedimentary rocks.
In order to make greater sense of the complexities of sedimentary relationships, classifications and hierarchal organizations were produced to give clarity and uniformity to stratigraphic analysis. Lithostratigraphic units in descending order are the supergroup, group, formation, member, and bed or stratum. The formation is the fundamental unit of stratigraphic geology, just as the cell is fundamental to biology. A formation must be lithologically distinct and large enough in scale as to be mappable. These classifications are defined strictly on the basis of lithology. Boundaries between units, including formations, are placed at the position of distinctive rock change or a distinctive erosional surface. In this manner, the divisions between units have the least possible ambiguity.
All rocks, whether metamorphic, igneous, or sedimentary, weather at the surface of the Earth.
Metamorphic and igneous rocks are formed at temperatures and pressures that are not seen at the surface so when these rocks are brought to the surface they become unstable. In the absence of air, most minerals in these rocks would remain intact for millions of years; however, because of Earth’s climate, water, and atmosphere, these minerals break down and convert in a predictable sequence. The new minerals that will eventually form sedimentary rocks are stable at lower temperatures and pressures and are very slow to convert.
Sometimes difficult to distinguish from sedimentology, stratigraphy is specifically concerned with the layering and orientation of sediments. This outgrowth of sedimentology is based predominantly on the law of superposition, which presumes that in a normal sequence of rock layers not heavily disturbed or overturned since their deposition, the youngest rocks will lie above older rocks. Stratigraphy is primarily concerned with stratigraphic nomenclature, lithostratigraphy, chronostratigraphic and biostratigraphic series, and correlating specific successions between regions. From studying stratigraphic sequence of depositional layers, the geological history of a region or outcrop can be reconstructed. Stratigraphers must account for such factors as the average rate of deposition of the various sediments, their composition, the extent of the strata and any incorporated fossils to construct a proper analysis. These sequences are then correlated to those of similar age in other regions with the ultimate aim of establishing a consistent geochronologic sequence for the entire Earth. In areas where the strata have undergone folding, faulting, and erosion, stratigraphic techniques are used to determine their correct sequence.
In order to properly reconstruct a stratigraphic sequence of rock layers, uniformity among the scientists performing fieldwork must exist so that each sequence can be arranged in approximately the same manner. To this end, and to ensure uniform usage of stratigraphic nomenclature and classification within the field, a stratigraphic code compiling the principals and practices of stratigraphy was developed. The latest version of this code, known as the North American Stratigraphic Code, was published in May, 1983 by the North American Commission on Stratigraphic Nomenclature and is widely accepted by North American geologists.
Great efforts are made in stratigraphy to reconstruct and interpret ancient environmental conditions through stratigraphic correlation. Stratigraphers use correlation for most practical applications; there is indeed a great monetary benefit from understanding where fossil fuels and other raw materials can be located based on the lithology of sediments. Thus, correlation of sediments becomes vital to any future geologic endeavors as globalization necessitates greater material allocation. The location of fossil fuel reservoirs is intimately tied to the ancient environments in which they were originally deposited. For this reason, reconstruction of ancient environments and paleoclimates has become a foremost concern for many geologists. Most readily available hydrocarbons used in the modern age are extracted from sedimentary rocks, as fossil fuels are, on occasion, the last stop in the decomposition of rocks yielding their primary constituents. Sedimentary rocks are also responsible for aquifers and are host to a wide variety of metallic and nonmetallic ores.
Seismic phenomena on the earth have given sedimentologists and stratigraphers the opportunity to see below the surface and unlock architectural mysteries of subterranean bedding. However, scientists need not wait until an earthquake strikes near their research area to extract information about the subsurface. Artificially generated seismic waves are used to obtain information regarding geologic structure, stratigraphic characteristics, and distributions of different rock types. Seismic stratigraphy was created to study the seismic data for these purposes.
Sequence stratigraphy is another branch of stratigraphy in which the cycles of worldwide ocean level changes are studied in rocks, preserving these marine transgressions and regressions that are then correlated to other time-equivalent rocks around the world. The principal of sequence stratigraphy is intimately linked to the assumption of cosmopolitan eustatic sea level changes. A sequence is representative of a complete cycle of the rise and fall of base sea level. Sequence stratigraphy is truly a method of interpreting stratigraphic data initially deduced through seismic data, then made applicable to an outcrop or well log data. Well logs are produced from data obtained from measurements of electrical conductivity, transmissibility of sound waves, and emission of nuclear radiation in the rocks adjacent to the well bore. The variations in measurements reflect the gross lithology of the rocks surrounding the bore hole. An instrument known as a sonde that is designed to mea-sure velocity of sound waves, electrical resistivity, and natural or induced gamma radiation emitted by the rock then relays its measurements back to the surface and is recorded onto digital tape. As the sea level swings from highstand to lowstand, a progression of depositional tracts are laid down, being preserved in the rock and recorded in well logs. Typically, sequence stratigraphy focuses on clastic sediments deposited on continental margins because these sedimentary environments are particularly affected by cycles in relative sea level change. The sequence concept was a revolution for stratigraphy and although not all geologists agree that reliable global sea level charts can be assembled from sequence stratigraphic data, most agree that these changes do affect the sediment geometry and rock unit distribution on continental margins or within time-equivalent sedimentary basins.
A relatively new branch of stratigraphy, known as magnetostratigraphy, applies concepts of geomagnetism to old rocks, allowing stratigraphers to reconstruct a detailed magnetic polarity time scale for the Earth. The primary application of magnetostratigraphy is as a tool to correlate marine strata. This discipline becomes appreciated when paleontologic or lithologic correlation is difficult using traditional methods. Mostly, the phenomenon of polar reversal acts as a contemporaneous, synchronous event that provides a precise tool for chronostratigraphic correlation. However, magnetostratigraphy is not limited to marine sediments and its techniques have been expanded in recent times to correlation of on-land sections. Therefore, most work in magnetostratigraphy in the future will correlate older on-land strata, thereby extending the data for polarity shifts further back in geologic time. Combining magnetostratigraphy, sequence stratigraphy, and paleoclimatic data, anthropologists can determine the duration in which a fossil or artifact locality was used. The paleomagnetic data from the Peking man site of Zhoukoudian suggests that the deposits of human fossils and artifacts endured from approximately 500 to 230 thousand years ago.
The incorporation and catalog of fossil remains has played a key role in most analyses of sedimentary rock younger than the Cambrian explosion of life. Fossils have been a most important means of correlation because, as a result of evolution, rock strata of approximately equal age exhibit similar flora and fauna. Dating and correlation of stratified rocks by means of fossils is known as biostratigraphy. The benefit to rock strata and fossils is mutual; fossils localized in strata can be dated using relative and absolute dating, and in so doing provide a date to the sediment itself. The fundamental unit in biostratigraphy is the biozone. This biostratigraphic unit is a body of rock characterized by its fossil content, making it distinct from adjacent strata. The simplest form of biozone is the taxon range zone where the zone is defined between the first (lowest) and last (highest) occurrence of a single genus, species, or higher taxon. However, biozones can incorporate multiple taxonomic groups to define a range. In anthropology, most fossil specimens found around the world owe much of their importance to the date prescribed to them from stratigraphic paleontology or biostratigraphy.
All rocks, whether igneous, metamorphic, or sedimentary hold some clues to their origin and the environment. However, sedimentary rocks, with their structures, textures, fossils, and compositions provide a matchless insight into past environments, climates, ecosystems, orientation of ancient continents, and mountain systems that have long since vanished. The task of deciphering, compiling, understanding, and utilizing the evidence locked in strata is the stratigrapher’s primary charge.
References:
- Boggs, S., Jr. (2001). Principles of sedimentology and stratigraphy (3rd ed.). Upper Saddle River, NJ: Prentice Hall.
- Nichols, G. (1999). Sedimentology & stratigraphy. Malden, MA: Blackwell Science.
