Types of stratigrpahy
There are several types of stratigraphy that are described below.
Geochronology – Radiometric Stratigraphy
Geochronology, the science of absolute dating of rocks and determining the time sequence of geological events in Earth's history, particularly by radiometric dating, developed largely at the turn of the 20th Century and during its first three decades with the advent of atomic and nuclear physics and quantum theory (e.g. Holmes, 1911; see also Hole, 1998, for a review). It provided the framework of absolute time within which the relative chronostratigraphic scale could be calibrated.
Magnetostratigraphy
The work of Rutten (1959) presented a chronological scale of polarity reversals of the Earth’s magnetic field based on K–Ar radiometric dating in a sequence of volcanic rocks, and gave birth to the new science of magnetostratigraphy. Harrison and Funnel (1964) discovered that magnetic polarity reversals (chrons) are also recorded in marine deposits, which further improved the applicability of the technique. Subsequent works aimed to match and calibrate the reversals with conventional stratigraphic tools (ISSC, 1979; Tarling, 1983; Galbrun, 1984), and use the unique nonperiodic pattern of reversals to date and correlate diferent rock sequences. In combining the marine magnetic anomalies measured over the sea-floor record in the South Atlantic spreading profile with their dates of chrons on land, Jim Heirtzler and colleagues in 1968 laid the foundation for the modern timescale based on Cretaceous through Paleogene marine magnetic anomalies, also known as the Geomagnetic Polarity Time Scale (GPTS). The Cande and Kent (1995) GPTS is the currently accepted timescale that is in most widespread use.
Stratigraphic Classification, Terminology and Procedure
In 1976 the International Subcommission on Stratigraphic Classification (ISSC) of the International Commission on Stratigraphy (ICS) published the first edition of the International Stratigraphic Guide (edited by Hollis D. Hedberg),as a means to promote international agreement on the principles of stratigraphic classification, terminology, and rules of procedure. In 1983 the North American Commission on Stratigraphic Nomenclature proposed a version of the stratigraphic code, which expanded considerably its original scope. The standard international stratigraphic classification was finally approved in 1987 by the ISSC, and updated in 1994 in a second edition of the Guide (Salvador, 1994). An abridged version was published by Murphy and Salvador (2000), and made available on the ICS website. General comments on stratigraphic principles and procedures have also been presented by various authors, such as Reading (1978), Ager (1984), Blatt et al. (1991),and Whittaker et al. (1991), among others.
Facies Stratigraphy
The term and concept of stratigraphic facies (from Latin: appearance, aspect, face, form), meaning the combined lithological and paleontological characteristics of a stratigraphic section, were introduced in 1838 by the Swiss geologist and paleontologist Amanz Gressly (1814–1865) from his studies in the Jura Mountains. Gressly's pioneer contributions on the genesis and applications of sedimentary facies, stratigraphic correlations, and paleogeographic reconstructions are fundamental to modern stratigraphy (Cross and Homewood, 1997). Later in the course of the 19th Century the term was assigned to a variety of descriptive meanings by geologists, paleontologis and ecologists, which somehow confused the original definition. Facies analysis in the modern sense restored the concept to its original meaning, aiming at the description, interpretation and reconstruction of the depositional and paleogeographic setting of sedimentary units, combining lithological and paleontological data (Reading, 1978, 1996; Walker, 1979, 1992; Walker and James, 1992). It provides the basic framework to reconstruct the environmental evolution of the stratigraphic record through time.
Quantitative Stratigraphy
Various graphical, numerical and experimental methods applied to refining stratigraphic resolution and basin modelling studies, have been continually developed since the 1960's (e.g. Shaw, 1964; Ager, 1973; Miller, 1977; Van Hinte, 1978, 1982; Gradstein et al., 1985; Mann and Lane, 1995; Harbaugh et al., 1999; Paola et al., 2001). These techniques have jointly the greatest potential to achieve the finest biostratigraphic resolution possible in correlating different rock sequences, in studies of regional versus global correlation of geological events, in helping to reconstruct the geological history of sedimentary successions, and in petroleum reservoir correlation and modelling. The methods are greatly assisted by the universal adaptation of microcomputers to digital programming with colour graphics output.
Sequence Stratigraphy
Modern stratigraphy had a major impetus by the mid-20th Century, with the increase of petroleum exploration activities,the development of new technologies (e.g. of seismic reflection data in the 1970s) and the application of stratigraphic models to petroleum research (e.g. Sloss, 1962). In 1949 L. L. Sloss and coworkers coined the term sequence to represent a set of sedimentary cycles limited by unconformities. The notion of unconformity-bounded stratigraphic units received further support in the late 1950s with the works of H. E. Wheeler (1958, 1959a and b), who also introduced the concept of the chrono-lithostratigraphic chart. In 1963 Sloss consolidated the term stratigraphic sequence and its usage in regional chronostratigraphic correlations. These studies provided the basic framework to the later formulation of the sequence models, which was to incorporate the use of high-quality seismic-reflection data in modelling subsurface stratal patterns and general geometry, and the expected seismic reflection features of different lithofacies asssociations. In a series of publications starting in the late 1970s, Peter Vail, coworkers and colleagues presented a revolutionary stratigraphic method of basin analysis for what became known as "Sequence Stratigraphy" (e.g. AAPG Memoir 26, edited by Payton, 1977; Vail, 1987; Van Wagoner et al., 1987, 1988, 1990, 1991; Posamentier et al., 1988, 1992; Posamentier and Vail, 1988; Vail et al., 1991; Schlager, 1992; Walker and James,1992; and Posamentier and Allen,1994,among others). Sequence models constitute a powerful tool for unraveling basin-fill history, and as such have been applied to most stratigraphic studies of basin modelling. The method is based on the study of the relationships between global relative sea-level changes and large-scale sedimentary cycles within time-equivalent depositional successions bounded above and below by a significant gap in the stratigraphic record, i.e., by surfaces of erosion (unconformity-bounded units) or nondeposition. Suess (1906) was the first to propose that sealevel changes could be global. The global eustatic sea-level variation curve proposed by Vail et al. (1977a, b), and later refined by Haq et al. (1987, 1988) and Ross and Ross (1988), for the Phanerozoic sequences, was based on the approximate correlation of seismic sequences from a number of passive continental margins. In 1989 Galloway proposed the model of genetic sequences bounded by maximum-flooding surfaces,which implied a certain discrepancy with the unconformity-bounded depositional sequences of Vail et al. (1977a) and Van Wagoner et al. (1987), based essentialy on seismic stratigraphy. Galloway's approach, based mostly on sedimentological interpretation of depositional systems, facies relationships and geometries, is particularly significant in stratigraphic successions with little or no available seismic data, due to difficulties in marking and tracing regional unconformities. Despite some controversies behind the main theoretical basis for the sequence stratigraphy paradigm (e.g. Miall, 1991, 1994, 1997), the method brought about a major revolution in the science of stratigraphy, leading to new research to be carried out on complex clastic and carbonate successions around the world. By gathering within a single stratigraphic framework information derived from diverse disciplines of sedimentary geology, such as seismic stratigraphy, biostratigraphy, paleoecology, paleogeography, and sedimentology, among others, the sequence models permitted a much broader, integrated and sharper research approach in basin analysis.
Episodic and Cyclic Sedimentation: Event Stratigraphy and Cyclostratigraphy
In the past decades of the 20th Century new theories developed in the geoscience community which represent a synthesis of Lyell's Uniformitarism and Gradualism combined with a revival of Cuvier's Catastrophism, recognizing that both play a significant role in geological processes and the evolution of life. Theories such as the actualistic catastrophism (Hsü, 1983), the punctualism (Gould and Elderedge, 1977; Gould, 1984; Goodwin and Anderson, 1985), and the episodic sedimentation (Dott, 1983), are fundamented on the assumption that most of the stratigraphic record was produced during episodic events, and that abrupt environmental changes have modulated speciation and mass extinctions (e.g. Signor and Lipps, 1982; Flessa, 1986; Hallam, 1989a, b). Major catastrophic events, such as extraterrestrial impacts (e.g. Alvarez et al., 1980; McLaren and Goodfellow, 1990; Becker et al., 2001) and cataclysmic volcanic activity (e.g. McLean, 1985; Courtillot, 2000; Wignall, 2001) are also thought to have greatly affected the evolution of life on Earth. In 1982, G. Einsele and A. Seilacher discussed extensively the processes of cyclic and event sedimentation, introducing the principles of what would be later known as Event Stratigraphy (e.g. Kauffman, 1987, 1988; Walliser, 1996; Einsele, 1998). The method deals with the integrated study of episodic and short-term sedimentary and biotic processes in the stratigraphic record, and has the potential to improve substantially the resolution of geological correlations. Rhythmic stratigraphic cycles observed in pelagic siliciclastic and carbonate sequences have been related to the so-called "Milankovitch cycles", after the Serbian astrophysicist Milutin Milankovitch (1879–1958) who in 1941 presented a firm mathematical basis that related periodic variations in Earth's rotational and orbital motions (eccentricity, obliquity, precession) to long-term climate changes. However, the hypothesis of astronomically forced climate cycles was advanced already in the 19th Century to the Pleistocene ice ages by the French mathematician Alphonse Joseph Adhémar (1797–1862), in his work Les Revolutions de la mer (1842), and by the Scottish geologist James Croll (1821–1890), who in the 1860's and '70s proposed an Astronomical Theory of the Ice Ages,subsequently published in his Climate and Time (1875) and Climate and Cosmology (1885). These orbital-forced cycles control the intensity of seasonal and latitudinal distribution of solar radiation (insolation) reaching the planet's surface, and directly influence global climate, depositional processes and biotic productivity (e.g. Fischer and Arthur, 1977; Bottjer et al., 1986; Fischer, 1986, 1991; Schwarzacher, 1987; Fischer and Bottjer, 1991; Weedon, 1993; Satterley, 1996; Perlmutter et al., 1998). The detailed investigation of regular cyclic patterns in the stratigraphic record produced by the interaction of tectonic and Milankovitchtype climatic processes is the study of a new branch of stratigraphy named Cyclostratigraphy (Schwarzacher, 1993; Fischer, 1993, 1995; Gale, 1998). The method allows a way for estimating the time span of biozones and the magnitude of unconformities, improvement of the stratigraphic framework, and for a better understanding of sedimentary and climatic processes (e.g. Perlmutter and Matthews, 1989, 1992). For instance, the modern Neogene timescale now depends on precise orbital tuning of marine and continental cyclic sequences,and evolved into an astronomically tuned (polarity) timescale (APTS), which proved to be far more precise and accurate (e.g. Hilgen, 1991a, b; Wilson, 1993; Shackleton et al., 1995; Hilgen et al., 1995; Zachariasse, 1999).
Chemostratigraphy
Chemostratigraphy is a relatively new technique, developed mainly during the last decade (e.g. Humphreys et al., 1991; Ehrenberg and Siring, 1992; Pearce and Jarvis, 1992,1995; Racey et al.,1995; Pearce et al.,1999). It uses the primary geochemical variation in the whole-rock elemental composition of siliciclastic sediments and sedimentary rocks to correlate stratigraphic sequences,as well as to gather inferences on basin paleotectonic history, source rock lithologies,depositional pathways,and paleoclimates.
Isotope Stratigraphy (Sr, C, O)
Over the last decade there have been a substantial number of works concerned with defining the Sr isotopic evolution of the oceans during the Jurassic, Cretaceous and Cenozoic (see Hess et al., 1986, and McArthur, 1998, for a review). Such a highresolution Sr-isotope curve can be used as a global correlation tool and, over some intervals, have a stratigraphic resolution superior to that of biostratigraphy. Recent research on isotope stratigraphy has also been forefront in defining and refining carbon- and oxygen-isotope curves for the Mesozoic and Cenozoic, based on the analysis of carbonate rocks and fossils and of terrestrial organic matter (e.g. Holser, 1984; Faure, 1986; Shackleton, 1985; Holser and Margaritz, 1989). The oxygen-isotope curve has been primarily used for estimating the Cenozoic record of water-mass temperatures (e.g. Frakes et al., 1992; McCauley and DePaolo, 1997). When of characteristic shape and form,the carbon-isotope curve can be used for intercontinental correlation (e.g. for sections across the Cenomanian–Turonian boundary; Kuhnt et al., 1990; Gale et al., 1993; Pratt et al., 1994), as well as to allow inferences on patterns of the long-term organic carbon cycle (e.g. Scholle and Arthur, 1980; Arthuret al.,1985),and hence to indicate important periods of petroleum source-rock deposition.
