STRATIGRAPHY. – S., also called stratigraphic geology or historical geology, reconstructs the chronological order of the deposition of materials on the Earth’s surface and correlates the remains of strata even when they are found at great distances, separated by erosion or displaced by tectonic movements (v. TECTONICS; GEOLOGY).
The basic stratigraphic unit is called a “stratum,” which may be imagined as material deposited continuously upon a subaqueous or subaerial surface. Its bounding surfaces are called the “faces” of the stratum: the lower, or older, face is called the “bed”; the upper face is the “roof.” The thickness of a stratum may range from a fraction of a millimeter to several decimeters; if greater, the stratum is called a “bench.” A succession of strata constitutes a “pile” or “series of strata” (fig. 1).
The end of the astral or pre-geological phase of the Earth (characterized by the preponderant effect of forces operating within the geoid) and the beginning of its historical or geological phase are generally considered to coincide with the formation of the first solid crust (lithosphere) and the initiation of its evolution under the action of exogenous agents, that is, with the establishment of the first sedimentary cycles. Life probably appeared shortly thereafter; however, its earliest representatives have not been preserved in the fossil state, both because the oldest sediments have almost everywhere undergone intense metamorphism and also because, presumably, the first organisms lacked parts capable of fossilization (v. PALEONTOLOGY).
I. ABSOLUTE AND RELATIVE AGE OF GEOLOGICAL FORMATIONS
For the absolute age of the Earth and its astral phase, V. UNIVERSE. Regarding the determination of the absolute age of the Earth’s geological phase and its subdivisions (eras, periods, etc.; V. table), some methods serve exclusively for the chronology of the most recent formations (Pleistocene and Holocene), while others apply indistinctly to all preceding geological periods. Among the former, proceeding backward in time, the method of “dendrochronology” (counting and interpreting the annual rings of trees) may be cited; in some regions, this has allowed secure dating in the absence of other data up to more than 3,000 years ago. The “radiocarbon” method, developed by American scholars in recent years, has dated historical, protohistoric, and prehistoric formations up to about 20,000 years ago. The interpretation of postglacial climatic phases through the analysis of “varves” in postglacial deposits covers a period of the last 15,000 years or so; finally, the correlation between established astronomical cycles and the alternation of glacial and fluvial phases (“Milan curve”) has, though not yet with full certainty, permitted the determination of the absolute chronology of the entire ancient Quaternary (Pleistocene) over a span estimated at roughly 600,000 to one million years (v. MAN) (fig. 4).Among the methods based on various criteria for determining the absolute chronology of eras and periods prior to the Quaternary, those that may be classified as paleontological (presumed constant rate of evolution of certain animal groups), geological (assessment of the rate of sedimentation, weathering, etc.), geochemical (presumed constant increase in the salinity of the oceans), and geophysical (duration of the Earth’s cooling) have generally yielded fallacious or unreliable results, especially when generalized. Conversely, methods based on the phenomenon of radioactivity (v. RADIOACTIVITY; EARTH, age of) have proven invaluable in providing absolute chronology data. Recently, a new radioactive geochronological method, the “rubidium-strontium” method, has been developed. It is based on the radioactivity of a natural isotope of rubidium (Rb 87; half-life ca. 60 × 10^8 years), which in its radioactive decay produces a stable atom of strontium (Sr 87); the strontium-radiogenic-rubidium ratio is then determined in minerals containing rubidium. This method has confirmed, in various cases, the geochronological data obtained by the lead method, which remains the most important method.
By the lead method, ages for eras and geological periods have been revealed that are staggering, amounting to hundreds of millions of years. Thus, the beginning of the Tertiary Era is placed at about 70 million years ago; the Secondary and Primary Eras are estimated to have lasted 130 and 310 million years, respectively; and the Cambrian period is thought to have begun about 510 million years ago. An even greater duration must be assigned to the Primary Era.
For the geologist, it is especially important to establish the relative age, that is, the position that a stratum or formation occupies within a stratigraphic series. Such research is based on the following general criteria: stratigraphic, lithological, and paleontological; in addition, some indirect methods are used.
The stratigraphic criterion, which appears the most straightforward, assumes that in two or more regularly superimposed strata that have not undergone major displacement, the one lying above is more recent than those beneath. This principle of “superposition” is reliably applied where the order of succession has remained unaltered (fig. 1). If the pile of strata is part of an overturned fold, the order is reversed in the lower part of the fold (inverted limb), where, from top to bottom, the strata proceed from older to younger (fig. 5). Other complex cases fall under particular tectonic styles (v. TECTONICS), such as overthrust sheets, diapir folds, etc.
The lithological criterion relies on comparing the petrographic nature of two or more strata, synchronizing those that exhibit identical characteristics in this regard. It can yield excellent results if applied with due caution, especially in regions with close geological affinities, one of which has a known chronology; it can also be applied in regions that are geographically close. However, it is well known that a given lithological type is not exclusive to a particular geological age; conversely, two coeval rocks formed in close proximity may display very different petrographic characteristics. It was once thought that crystalline rocks (gneiss, granites, etc.) were the oldest, but it has since been shown that they could have formed at any time. The application of this method therefore requires great care.
The paleontological criterion is more reliable because it is based on fossils, which are the remains of organisms that lived in the past and were deposited during sedimentation (v. PALEONTOLOGY). It has been established that many fossil species were present in localities far apart and that certain species occupied exclusively corresponding stratigraphic positions, thus documenting the synchronization of the strata in which they are contained—hence the concept of “index fossils.” It is of paramount importance, however, to ensure that fossils, after their initial deposition, have not undergone erosion, transport, and resedimentation (“reworked fossils”), as this would indicate an age older than the true one.
Indirect criteria. – Sometimes the age of a geological formation can be determined indirectly, which is particularly important when fossils are absent or, if present, are unusable because they belong to species of no stratigraphic value (persistent fossils, common fossils). Thus, a geological formation intercalated between two others of known age will have a date between that of the upper formation and that of the lower one. Indirect criteria, based especially on the action of contact metamorphism, are also used with regard to endogenous rocks, both among themselves and with sedimentary rocks. It is evident that an eruptive rock is more recent than another that has been metamorphosed by it; that a vein is more recent than the rocks it cuts through; whereas if a lava flow or a vein of eruptive rock is covered by sediments that show no traces of metamorphism, the latter are evidently more recent. In certain cases, another way to verify which of two strata or series of strata is more recent is based on the “sense of sedimentation” of the strata themselves, since, for example, a coarse-grained sediment is older than one that passes into others in the order of superposition with grains that become ever finer. Even the position of fossils of sessile benthos can provide useful indications about the original order of succession of strata; thus, the apices of thecae of archaeocyathids rooted on the same surface of a stratum indicate that the older strata lie in that direction.
II. THE CONCEPT OF “FACIES.” – When following a stratum on the surface, one often notes changes in its lithological composition and in the nature of its fossils. These different lithological and paleontological aspects offered by the same stratum are called facies, which reflect particular geographical and biological characters (habitat) of the place where the sedimentation occurred. Thus, there will be continental, lagoonal, and marine facies, and among the latter, littoral (or coastal), bathyal, and abyssal facies. Continental facies are characterized by terrestrial fossils, with sediments of aeolian origin (dunes, loess), volcanic (tuff), glacial (moraines), alluvial, and limnic origin (clays, sands, gravels, etc.). Particular importance is assumed among the latter by fossil fuels, generally referable to a swamp or marsh environment. Except in special cases, continental facies do not reach very considerable thicknesses. The remains of continental organisms can be terrestrial (subaerial, aerial, hypogeal) and aquatic (fluvial, limnic). In facies of lagoonal type, of estuary or delta, fossils of terrestrial and aquatic type are present, but generally adapted to special environments (fresh water, brackish water with varying salinity). Marine facies are characterized by fossils that are both benthic (sessile or mobile) and pelagic (nektic and planktonic); these, based on habitat, can be subdivided according to depth into: neritic facies (up to 200 m.), characterized by the numerical abundance of fossils of eurythermal organisms and by the prevalence of terrigenous sediments (i.e., transported by watercourses to the sea) and organogenic ones, both of accumulation of remains and derived from constructive activity (e.g., madreporic reefs); bathyal facies (200–1000 m.), of deeper sea, where muddy sediments prevail and the fauna is stenothermal; calcareous plankton (foraminifera, pteropods, etc.) prevails in warm seas and siliceous plankton (diatoms and radiolaria) in cold seas; abyssal facies (beyond 1000 m.), of a muddy environment where siliceous oozes of diatoms, radiolaria, and foraminifera (globigerinids, etc.) still prevail up to 5000 m. in depth, beyond which red clays occur, composed prevalently of volcanic ash and cosmic dust. Rocks of marine origin, which among sedimentary rocks are the most extensive and thick (raised by tectonic phenomena, they constitute the highest mountain ranges on Earth), are mainly due to terrigenous, benthic, and planktonic contributions.
The fossils that are proper to a given facies are called facies fossils. They are called