STRATIGRAPHY. - Stratigraphy, also called stratigraphic geology or historical geology, reconstructs the chronological order of deposition of materials on the earth's surface, and links together the remains of terrains even when they lie at great distances, dismembered by erosion or displaced by tectonic movements (v. TECTONICS; GEOLOGY).
The stratigraphic unit is precisely called a "stratum," which can be imagined as formed by material deposited, without interruption, on a subaqueous or subaerial bottom. Its boundary surfaces are called the "faces" of the stratum: the lower, or older, one is called the "floor"; the upper one the "roof." The thickness of a stratum may range from a fraction of a millimeter to a few decimeters; if greater, the stratum takes the name of "bank." 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 the forces operating within the geode) and the beginning of its historical or geological phase are generally made to coincide with the formation of the first solid crust (lithosphere) and with the beginning of its evolution through the action of exogenous agents, that is, with the realization of the first sedimentary cycles. A little later life too must have appeared; its first representatives, however, have not been handed down in a fossil state, both because of the vigorous metamorphism undergone almost everywhere by the most ancient sediments, and also because, probably, the first organisms did not possess parts susceptible of being preserved as fossils (v. PALEONTOLOGY).
I. ABSOLUTE AGE AND RELATIVE AGE OF GEOLOGICAL FORMATIONS
For the absolute age of the earth and of its astral phase, v. UNIVERSO. As regards the determination of the absolute age of the geological phase of the earth and of its subdivisions (eras, periods, etc.: v. table), some methods serve exclusively for the chronology of the more recent formations (Pleistocene and Holocene), while others serve for all the preceding geological periods without distinction. Among the former one may cite, going back in time, the method of "dendrochronology" (counting of the annual rings of trees and its interpretation), which in some regions has made it possible to carry out, in the absence of other data, reliable datings back to more than 3000 years ago; the so-called "radiocarbon" method, developed by American scholars in recent years, by which historical, protohistorical and prehistoric formations have been dated, for now to about 20,000 years ago; interpretation of postglacial climatic phases through the analysis of post-Pleistocene "varves" for a period covering the last 15,000 years or so; finally, correlation between ascertained astronomical cycles and the alternation of glacial and fluvial phases ("Milankovitch curve"), the latter being a method that has made it possible to determine, it is not yet known with what reliability, the absolute chronology of the entire early Quaternary (Pleistocene) for a span of time estimated at from 600,000 to about one million years (v. 1000) (fig. 4).
Among the methods developed on the basis of quite varied criteria for determining the absolute chronology of eras and periods prior to the Quaternary, those that can be classified as paleontological (presumed constant rate of evolution of certain animal groups), geological (assessment of the rate of sedimentation, of meteoric degradation, etc.), geochemical (presumed constant increase in the salinity of the oceans) and geophysical (duration of the cooling of the earth) have generally given results that are either fallacious or unreliable, especially if generalized. Conversely, the methods based on the phenomenon of radioactivity (v. RADIOATTIVITÀ; TERRA, età della) have proved valuable in providing data of absolute chronology. Recently a new radioactive method of geochronology called the "rubidium-strontium" method has been perfected. It is based on the radioactivity of a natural isotope of rubidium (Rb 87; mean life about 60 - 10⁸ years) which in its radioactive decay generates a stable atom of strontium (Sr 87); the ratio of radiogenic strontium to rubidium is then determined in minerals containing rubidium. This method has confirmed, in various cases, the geochronological data obtained by the lead method, which latter remains always the most important method.
With the lead method, unsuspected durations, assessable at hundreds of millions of years, have been revealed for the geological eras and periods. Indeed, the beginning of the Tertiary era is dated back to about 70 million years ago; while the Secondary era and the Primary era must be assigned a duration of 130 and 310 million years respectively: the beginning of the Cambrian period would therefore date back to 510 million years ago. An even greater duration must be attributed to the pre-Cambrian era: the age, in fact, of the two oldest minerals known so far, a uraninite and a lepidolite, both coming from a Canadian pegmatite, measured respectively by the lead method and by the rubidium-strontium method, has been concordantly estimated at 2.1 × 10⁸ years! And this, as is logical, is a minimum limit of the duration of the geological age of the earth, still far lower, as it seems on various grounds, than the maximum value of it which still eludes direct measurement, but which should be around, according to Holmes, 3,350 - 10⁸ years!
For the geologist it is above all interesting to establish the relative age, that is, the position that a stratum or a formation occupies in a stratigraphic series. These investigations are based on the following general criteria: stratigraphic, lithological, and paleontological; besides the use of certain indirect methods.
The stratigraphic criterion, which presents itself as the most spontaneous, has as its premise that of two or more strata, regularly superimposed, and which have not undergone strong dislocations, the one that lies above is more recent than those below. This principle of "superposition" is of sure application wherever the order of succession has remained unaltered (fig. 1). If the pile of strata is part instead of an overturned fold, their order is reversed in the lower part of the fold (inverse flank), where, from top to bottom, one goes from the oldest to the most recent terrains (fig. 5). Other complex cases fall within particular tectonic styles (v. TETTONICA), such as overthrust sheets, diapiric folds, etc.
The lithological criterion makes use of the comparison of the petrographic nature of two or more strata, synchronizing those that present, in this regard, identical characters. It can render excellent services if applied with the necessary reservations, especially in regions that have close geological affinities, one of which is of known chronology; it can moreover be applied in regions very close to one another. But it is known that a given lithological type is not exclusive to a determined geological age; while, on the other hand, two coeval rocks, formed at a short distance, can present very different petrographic characters. It was once held that crystalline rocks (gneisses, granites, etc.) were the oldest, whereas it has since been ascertained that they could have formed in any age. The application of this method therefore demands much caution.
More reliable instead is the paleontological criterion because it is based on fossils, which are the remains of organisms that lived in the past and were deposited while sedimentation was taking place (v. PALEONTOLOGIA). It was possible to establish that many fossil species were present in localities even distant from one another, and that some of them occupied exclusively corresponding stratigraphic positions, thus documenting the synchronization of the terrains in which they are contained: whence also the indication of guide fossils. But it is of the utmost importance to make sure that the fossils, after their first deposition, have not undergone erosion, transport and redeposition ("reworked" fossils), indicating in this case an age earlier than the true one.
Indirect criteria. - Sometimes the age of a geological formation can be deduced indirectly, which proves particularly important when fossils are lacking or, if present, are however unusable, because they are species of no stratigraphic value (persistent fossils, commonplace fossils). Thus a geological formation intercalated between two others of known age will have a date comprised between that of the upper formation and that of the lower one. Indirect criteria, based above all on the action of contact metamorphism, are also used with regard to endogenetic rocks, whether among themselves or 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; while if a 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 the two strata or series of strata is more recent is that based on the "sense of sedimentation" of the strata themselves, since, for example, a coarse-grained sediment is older
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that pass to others in the order of superposition with ever finer grain. The position of the fossils of fixed benthos can also give useful indications on the original order of succession of the strata; thus the apices of the thecae of archaeocyathids planted on the same surface of a stratum indicate that the older strata lie in that direction.
II. CONCEPT OF "FACIES"
Following a stratum along the surface, one often notices changes in lithological composition and in the nature of the fossils. These different lithological and paleontological aspects offered by one and the same stratum take the name of facies, which reflect particular geographic and biological characters (habitat) of the place where 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 (dunes, loess), volcanic (tuffs), glacial (moraines), alluvial, and limnic (clays, sands, iron ores, etc.) origin. Among the limnic ones, fossil fuels, generally referable to a swamp or marsh environment, take on particular importance. Except in special cases, continental facies do not reach very considerable thicknesses. The remains of continental organisms can be terrestrial (subaerial, aerial, subterranean) and aquatic (fluvial, limnic). In facies of lagoonal, estuarine, or deltaic type, fossils of terrestrial and aquatic type are present, but generally adapted to special environments (fresh water, brackish waters of varying salinity). Marine facies are characterized by fossils both benthic (fixed or mobile) and pelagic (nektonic and planktonic); these, on the basis of 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 (that is, transported by watercourses to the sea) and organogenic ones, whether from accumulation of remains or derived from constructive activity (e.g., madrepore 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 radiolarians) in cold seas; abyssal facies (beyond 1000 m.), of muddy environment where diatom, radiolarian, and foraminiferal oozes (globigerinae, etc.) still prevail down to 5000 m. of depth, in addition to "red clays", composed predominantly of volcanic ashes and cosmic dust. Rocks of marine origin, which among sedimentary ones are the most extensive and thick (carried upward by tectonic phenomena they constitute the highest mountain chains of the Earth) are mainly due to terrigenous, benthic, and planktonic input. The fossils that are proper to a determined facies are called facies fossils. They are called
Two deposits that show the same facies are isopic or isotopic; heteropic, if they have a different facies, even though they are contemporaneous; isomesic are two deposits that were formed in identical environments (in two lagoons; in two lakes, etc.), heteromesic if formed in different environments (in a lagoon and in a deep sea); two deposits that were formed in the same "geographical province" are called isotopic (for example, a marine sand and a continental volcanic tuff in the Mediterranean basin), heterotopic, if formed in two different provinces.
III. STRATIGRAPHIC ANOMALIES
If two or more strata deposited successively without interruption of sedimentation are superimposed regularly one above the other, they are called conformable (fig. 1). Sometimes, however, it has happened that, after the deposition of a group of strata, through a gradual or abrupt retreat of the sea (regression) the sea floor emerged, that is, entered a phase of continentality, during which its surface was variously eroded. If after a certain lapse of time this region is again covered by the sea (ingression), the strata deposited there, even in the case that a parallelism with the older ones persists, are called unconformable (stratigraphic unconformity; fig. 2), while the absence of marine deposition constitutes a "gap in sedimentation." The deposits that follow one another regularly superimposed or that are comprised between an ingression and a regression belong to one and the same cycle of sedimentation.
IV. STRATIGRAPHIC SCALE AND ITS DIVISIONS
By coordinating the results of paleontological studies and of sedimentary cycles, and taking into account all the regions of the Earth and also other paleogeographic events of a general character, it has been possible to construct a succession of terms known as the stratigraphic scale. The earliest attempt at chronological classification appears to be due to Lehmann (1756), followed by that of A. G. Werner (1789), based on lithological characters. However, before Werner, G. Arduino had established, as early as 1759, a geological classification divided into four orders which he called: primary, secondary, tertiary, quaternary; these denominations are still preserved to indicate the four great eras: Paleozoic, Mesozoic, Cenozoic and Neozoic, and they correspond fairly well, as regards northern Italy, to the actual stratigraphic succession of that region, as indicated by Arduino. At geological congresses the nomenclature to be adopted for the divisions of the stratigraphic scale was established, whose terms are the group, the system, the series, the stage, the zone, and whose chronological equivalents are the era, the period, the epoch, the age, the hemera.
It should be noted that the boundaries between eras are established on a paleontological basis (disappearance and appearance of large groups of organisms), but they may also coincide with extensive stratigraphic unconformities (between the Archean and the Paleozoic, in some regions of North America) or with the beginning of great orogenic cycles (between the Mesozoic and the Tertiary: the beginning of Alpine orogenesis).
Geological periods may coincide, in given localities, with great cycles of sedimentation, bounded by partial retreats of the sea (Pliocene, in the Italian Peninsula). For the minor divisions, although cycles of sedimentation may correspond to them, the criterion of separation is based increasingly on characteristic fossils (genera, species, varieties). The "zone" is truly the paleontological unit: it may correspond to a single stratum and may be represented by a single species of characteristic fossil. Some periods (Carboniferous, Permian, Triassic), since their outcrops of both marine and continental origin are known over vast extents, have a double division and nomenclature. Naturally, there exists in no region of the Earth a natural section showing the entire stratigraphic series, from the Archean to the present; however, there are known localities where the complete successions of a whole era or period are represented (Paleozoic, in southwestern England; Mesozoic and almost all of the Cenozoic in some sectors of the Umbrian-Marches Apennines; the Devonian in the Meuse Valley, etc.). Finally, it is recalled that geological periods take their name either from geographical localities, sometimes Latinized, where the strata were first studied (e.g., Cambrian, from Cambria, the Latin name of Wales) or from the prevailing or characteristic type of rock (e.g., Carboniferous, in Western Europe, because of the presence of great quantities of fossil coal).