Stratigraphy from the Latin stratum and the Greek graphia has traditionally been considered the descriptive science of rock strata. In the last few decades, the critical value to stratigraphy of the information provided by nonlayered rock bodies – sedimentary as well as intrusive ignéous and massive metamorphic rocks of undetermined origin - has become evident.
Nonlayered rock bodies may be the source of geochronometric (numerical) ages determined by isotopic methods, but they also provide crucial age information through their cross-cutting and boundary relationships with layered and/or nonlayered rocks with which they are associated. Stratigraphy therefore should include the description of all rock bodies forming the earth’s crust and their organization into distinctive, useful, mappable units based on their inherent properties and attributes.
Stratigraphy is concerned not only with the original succession and age relations of rock bodies but also with their distribution, lithologic composition, fossil content and geophysical and geochemical properties and all other properties that contribute to the interpretation of their mode of origin, environmental setting and above all their geologic history. Therefore all classes of rocks - sedimentary (consolidated and unconsolidated), igneous and metamorphic - fall within the scope of stratigraphy and stratigraphic classification.
2. Stratum (plural-strata)Edit
A geologic stratum is a layer (a generally tabular body) of rocks characterized by certain lithologic properties or attributes that distinguish it from adjacent strata by visible planes of bedding or by less perceptible boundaries of changes in lithology.
3. Stratigraphic ClassificationEdit
The systematic organization of the earth’s rock bodies as they are found in their original relationships into units based on any of the properties or attributes that rocks may possess. Many different properties and attributes of the rocks may be useful as the basis for stratigraphic classification and consequently there are many different categories of stratigraphic classification.
4. Stratigraphic UnitEdit
A body of rock recognized as a distinct entity in the classification of the earth’s rocks based on any of the many properties or attributes that rocks may possess. Stratigraphic units based on one property will not necessarily coincide with those based on another. It is therefore, essential that different terms be used for each so that their named units can be distinguished from each other. Clear definition of a stratigraphic unit is of paramount importance.
5. Stratigraphic TerminologyEdit
The unit-terms used in stratigraphic classification such as formation, stage, biozone, is either formal or informal.
a. Formal stratigraphic terminology uses unit-terms that are properly defined and named according to an established or conventionally agreed scheme of classification; for example, Pugo Formation, Cretaceous System. The initial letter of the rank or unit term of named formal units is capitalized.
b. Informal stratigraphic terminology uses unit terms only as ordinary nouns without the units necessarily being named and without its being a part of a specific scheme of stratigraphic classification; for example, a chalky formation, the sandy zone, an oyster bed. The initial letter of an informal unit term is printed in lowercase.
Informal terms may be appropriately used for aquifers, oil and/or gas reservoirs, coal beds, quarry layers based on wireline logs or reflection seismic profiles, etc. as long as it is clear that they are not formal terms, e.g. "NW volcanics" referring to the volcanic rocks in northwestern Leyte (Pilac, 1965); "Formation I, II, III" referring to the formations of the Caraballo Group in Northeastern Luzon (MMAJ-JICA, 1976). However, the informal use of formal stratigraphic terms (formation, member, biozone, series, etc.) in published documents is strongly discouraged.
In the course of surface and subsurface mapping all geologists use provisional informal terms, for example, Carcar-type limestone. They are essential but when the results of the investigation are published it is preferable not to introduce informal terms in print unless they are formally proposed and described. Furthermore, in conversations and oral presentations, it is not clear if a unit term is being capitalized or not. An intended informal term may become formal, even though it has not been properly proposed and defined. For example, the term "Nido Reef Complex", based on the name of a well (Nido I) in northwestern Palawan was described by Hatley (1977) and subsequently defined formally as the Nido Limestone by Sali and others (1981). However, it should be noted that the nearby Nido Island is underlain by a Middle Permian limestone which is older than the Nido Limestone defined on the basis of the Nido well log.
If a stratigraphic unit merits a name, it merits proper definition and description that reduces the risk of unsound units being created, leading to confusion in the literature.
6. Stratigraphic NomenclatureEdit
The system of proper names given to specific classes of stratigraphic units; Mirador Limestone (lithostratigraphic); Cretaceous System (chronostratigraphic), and Discoaster calcaris Zone (biostratigraphic).
A zone is a stratigraphic unit in many different categories of stratigraphic classification. Thus there are many kinds of zones, depending on the stratigraphic properties under consideration - lithozones, biozones, chronozones, mineral zones, metamorphic zones, magnetic-polarity zones, etc. When used formally, the term zone is given an initial capital letter (Zone) to distinguish it from its informal use. The kind of zone being used should be clearly indicated.
A stratigraphic horizon is an interface indicating a particular position in a stratigraphic sequence. In practice the term "horizon" has often been applied to a distinctive bed. There may be many kinds of stratigraphic horizons, depending on the stratigraphic properties involved lithohorizons, biohorizons, chronohorizons, seismic horizons, electric log horizons, and so on.
Stratigraphic horizons include not only the boundaries between two stratigraphic units, but also specific markers within these units that may be particularly useful for correlation purposes.
To correlate in a stratigraphic sense is to show correspondence in character and/or in stratigraphic position. There are different kinds of correlation depending on the feature to be emphasized.
Lithologic correlation (lithocorrelation) demonstrates correspondence in lithologic properties and lithostratigraphic position; a correlation with two fossil-bearing beds (biocorrelation) demonstrates correspondence in their fossil content and in their biostratigraphic position, and chronocorrelation demonstrates correspondence in age and in chronostratigraphic position.
The science of dating and determining the time sequence of events in the history of the earth.
11. Geochronologic UnitEdit
A unit of geologic time (time determined by geologic methods). It is not a body of rocks, and therefore not a stratigraphic unit although it may correspond to the time span of a stratigraphic unit.
The branch of geochronology that deals with the quantitative (numerical) measurement of geologic time, usually in thousands or million of years. The abbreviations Ka for thousand (1O3), Ma for million (106) and Ga for billion (109) years are now generally used to express the length of time before the present (years ago), not the duration of a past geological time interval.
In stratigraphy, the term facies can mean aspect, nature, or manifestation of character (usually reflecting conditions of origin) of rocks or specific constituent of rocks. Probably no geological term has been used more indiscriminately for a wider range of concepts than "facies".
As defined by Gressly (1838) the term "facies" was meant to express lateral change in lithologic aspect (change in lithologic aspect in the lateral extent of a stratigraphic unit). Its use has been broadened however, to express environment of deposition or formation (deltaic facies, marine facies, volcanic facies, shallow-water facies); lithologic composition (sandstone facies, limestone, red-bed facies); geographic or climatic association (Tethyan facies, boreal facies, tropical facies), fossil content (graptolitic facies, shelly facies); tectonic association (orogenic facies, geosynclinal facies) and metamorphic grade. "Facies" is also used as a noun for a rock body distinctive in aspect, appearance, or character. If the term "facies" is used, it is desirable to make clear the specific kind of facies to which reference is made (lithofacies, biofacies, metamorphic facies, tectonofacies, etc.).
Note: Sometimes, to stress the contemporaneous nature of variations in sedimentary attributes of a stratigraphic unit, facies may be defined as local expressions of contemporaneous deposition within a designated stratigraphic unit (be it lithostratigraphic or biostratigraphic).
14. Caution Against Preempting General Terms for Special Meanings.Edit
The preempting of general terms for special restricted meanings has been a source of much confusion and responsible for many of the controversies in stratigraphic terminology. For example, "stratigraphy" should not be confined to age relations of strata or rock bodies; "correlation" is not necessarily time-correlation; "geochronology" should not refer exclusively to isotopic dating; "zone" can be applied to other than fossil zones; a "biozone" is not a specific kind of biostratigraphic zone; and "interval" may refer to either time or space intervals. The preferable procedure is to conserve the original meaning of a term and to seek a more precise and less ambiguous word for the special meaning.
B. PROCEDURES FOR ESTABLISHING AND REVISING STRATIGRAPHIC UNITS.Edit
The proposal of a new formal stratigraphic unit requires a statement of intent to introduce the new unit as well as the reasons for doing so and should be published in a recognized scientific medium. To be valid and useful, a new unit must be duly proposed and duly described, and should include the following:
- A clear and complete definition, characterization and description of the unit, so that any subsequent investigator can recognize it unequivocally.
- The proposal of the kind, name, and rank of the unit.
- The designation of a stratotype (type section) or type locality on which the definition and description of the unit is based
The revision and redefinition of a previously proposed and named unit requires a statement of intent to revise the unit, the reasons for doing so, and a discussion of the history of the unit - author, original reference, previous treatment. If necessary, it should include a comprehensive description of the unit and designation of a new type section or type locality (or revision of the old one). The revision of a unit, to be valid, must also be published in a recognized scientific medium.
1. Definition, Characterization and DescriptionEdit
A comprehensive definition, characterization, and description of a new stratigraphic unit should include a clear account of its boundaries, diagnostic properties, and other attributes.
For lithostratigraphic and biostratigraphic units, emphasis should be on lithologic and paleontologic properties, respectively.
For unconformity - bounded units, emphasis should be placed on the description of the bounding unconformities.
For chronostratigraphic units emphasis should be placed on features bearing on age and time correlation. In addition, the definition, characterization, and description should include the following topics as pertinent.
a. Name (see Chapter 2-B-3)
b. Stratotypes and Other Standards of Reference
Geographic and geologic designation of stratotypes is essential in establishing a new stratigraphic unit. The geographic designation should include detailed location maps and indication of the means of access to the type section or type locality. Also important are provisions for artificial markers and the relation of boundary stratotypes to boundaries of other stratigraphic units and to other significant horizons in the section.
For units of the type for which it is impractical to utilize stratotypes as standards, reliance is placed on the accurate description and illustration of those features that constitute diagnostic criteria of reference for the unit, for example, biostratigraphic units.
c. Description of Unit at Stratotype or Type Locality
The description is a statement summarizing the total content of a stratigraphic unit. It should include the lithologic properties, the thickness or size of the rock body, biostratigraphic character, structural attitude, geomorphic expression, unconformities or hiatuses, nature of the boundaries of the unit (sharp, transitional, unconformable, etc.) and distinguishing and identifying features characterizing the unit at the stratotype or type locality.
d. Regional Aspects
Geographic extent; regional variations in thickness in lithostratigraphic, biostratigraphic or other properties, or in geomorphic expression, regional stratigraphic relations; relations to other kinds of stratigraphic units, nature of boundaries (sharp, transitional, unconformable, etc.); relations of the boundaries of the unit to the boundaries of other stratigraphic units of the same or different kind; criteria to be used in identifying and extending the unit geographically away from the stratotype or type locality.
e. Geological Age
Relative age in terms of position within the Standard Global Chronostratigraphic (Geochronologic) Scale and/or numerical age determined or inferred by isotopic or other methods.
f. Correlation with other units
Discussion of the kind of correlation and information on which it is based.
g. Genesis (where appropriate)
Conditions of origin of the rocks of the unit, significance with respect to paleogeograpy or geologic history.
h. References to the Literature.
2. Special Requirements for Establishing Subsurface Units.Edit
Many useful stratigraphic units are based on subsurface sections (well, mine or tunnel). Many more subsurface units will doubtless be proposed as the sediments of offshore areas become better explored. For example, the Upper Oligocene-Lower Miocene Nido Limestone in Palawan took its name from the Nido well (Sali and others, 1981). If adequate sample information is available, such subsurface sections can be used legitimately for proposing, defining, and describing new stratigraphic units.
The same general rules of procedure used for outcrop sections apply to subsurface units defined on the basis of exposures in mines or tunnels or from sections penetrated in wells. In proposing a new subsurface unit, the well or mine in which the type section is present becomes the type locality.
In well sections, stratotypes need to be designated by well depths and on well logs rather than by markers at the surface and geological information for these stratotypes will be based largely on well samples and well logs. Subsurface parastratotypes and hypostratotypes (see -6) may be useful in supplementing poorly exposed surface stratotypes or type localities. The following data are desirable for establishing subsurface units.
a. Designation of Well or Mine
- Name of type well or mine;
- Location of the well or mine by written description, map, exact geographic coordinates, farm or lease block, or any geographic feature suitable for location, identification;
- Name of the operating agency or individual.
- For mines, depth level where exposed.
- For wells, date of drilling, total depth and surface elevation.
If all the data needed cannot be obtained from one well, two or more wells should be used, with one being designated as the holostratotype and the others as parastratotypes or hypostratotypes (see -A-6).
b. Geologic Logs
Lithologic and paleontologic logs of the well or wells and maps and cross sections of the mine, in written and graphic form are needed. The boundaries and subdivisions of a new unit should be indicated clearly on logs or charts.
c. Geophysical logs and Profiles
Electrical or other wireline logs (preferably from several adjacent wells) and seismic profiles are desirable. The boundaries and subdivisions of the units should be marked and shown at a scale large enough to permit full appreciation of detail.
It is essential that sets of cores, cuttings or other samples, fossil material, logs and so on covering the type section of a subsurface stratigraphic unit be readily available for study. Such materials may be placed at geological surveys, universities, museums, or other institutions with proper curatorial facilities. The location of the depository should be given.
3. Naming of Stratigraphic UnitsEdit
The names of formal stratigraphic units are compound. For most categories, they should consist of a geographic name combined with an appropriate term indicating the kind and rank of the unit (Klondyke Formation, Cretaceous System) or descriptive term (Angat Limestone). The derivation of the geographic component of the name should be explained and the kind and rank of the unit should be specified.
The formal name of a biostratigraphic unit is formed from the names of one or more appropriate fossils combined with the appropriate term to the kind of biostratigraphic unit (Exus albus Assemblage Zone). The choice of the fossils selected to name a biostratigraphic unit should be discussed.
Some chronostratigraphic units of the standard global chronostratigraphic scale bear long established nongeographic names of diverse origin (Cretaceous System, Tertiary System, Miocene Series).
The name of a new stratigraphic unit should be unique. Therefore, before attempting to establish a new formal stratigraphic unit, authors should refer to national records of stratigraphic names to determine whether a name has been used previously. Reference should be made to the Lexicon of Geologic Names if available.
a. Geographic Component of Names of Stratigraphic Units
Geographic names should be derived from permanent natural or artificial features at or near the place where the stratigraphic unit is present. Names derived from such impermanent sources as farms, churches, schools, crossroads, and small communities are not entirely satisfactory but are acceptable if no others are available.
Appropriate names may be selected from those shown on provincial, topographic, hydrographic or comparable maps. If a name that does not meet this test has to be used, the place from which the name is derived should be described and identified precisely and shown on a map accompanying the description of the new unit. Use of such a geographic name for the stratigraphic unit should be subject to approval by the standing stratigraphic committee or by the national organization of place names.
The geographic name after which a stratigraphic unit is named should not be changed or abbreviated (e.g. "Saint Paul Limestone" should not be changed to "Paul"; "San Pascual Formation" to "Pascual"). On the other hand, the feature term of a geographic name, e.g. river, lake, mountain, barrio should be omitted from the name of stratigraphic units, unless required to distinguish between two otherwise identical names. For example, Mt. Madanlog Formation (Santos-Yñigo, 1944) was renamed Madanlog Formation by Santos and others (1962).
The name of a high-ranking unit may appropriately, though not necessarily, be derived from a more extensive geographic feature or area rather than names of its lower-ranking components. For example, the use by Durkee and Pederson (1961) of the name "Bued River Group" for Neogene Formations exposed along the Bued River that traverses Benguet and La Union Province.
ii. Spelling of Geographic Names
Spelling of geographic components of the name of a stratigraphic unit should conform to the usage of the country that contains the geographic locality from which the name has been taken.
iii. Changes in Geographic Names
Change in the name of a geographic feature does not entail change of the corresponding name of a stratigraphic unit. The original name of the unit should be maintained; e.g. Montalban Formation, should not be changed to Rodriguez Formation, because the former town of Montalban is now called Rodriguez.
iv. Duplication of Geographic Names
Duplication of geographic names should be avoided. A name previously applied to any unit should not later be applied to another unit unless geographic separation precludes confusion. For example, the stratigraphic name Angat has been applied to a predominantly limestone unit of Early to Middle Miocene age on the west flank of Southern Sierra Madre Range in Luzon. Nevertheless, the term Angat Ophiolite was used by Karig (1983) for an ophiolitic sequence in the same region.
v. Names for Subdivision of Stratigraphic Units
When a unit is divided into two or more formal component units, the geographic name of the original unit should not be employed for any of the subdivisions. For example, Cebu Coal Measures and the Cebu Orbitoid Limestone, both of which are members of the Cebu Formation (Corby and others, 1951).
b. Unit-term Component of Names of Stratigraphic Units
The unit-term component of the name of a stratigraphic units indicates the kind and rank of the unit. The unit-term "formation" for example, indicates that the unit is a lithostratigraphic unit, "taxon-range biozone" that the unit is a biostratigraphic unit, "stage" that it is a chronostratigraphic unit, etc. A stratigraphic unit-term nay differ in various languages (stage, étage, Stufe, piano, piso) or it may be very nearly the same (system, systeme, sistema). If a useful term is difficult to translate into a particular language it may be desirable to "borrow" the term from the language of its origin, for example, range zone from English. Stratigraphic terms with Greek or Latin roots are desirable because they are understood in a wide range of languages, for example, — chronozone.
c. Relation of Names to Political Boundaries
Stratigraphic units are not limited by international frontiers, and effort should be made to use only a single name for each unit regardless of political boundaries.
d. Reduction in Number of Names through Correlation
If correlation has established the identity of two named stratigraphic units, the later name should be replaced by the earlier, other considerations being equal, in the interest of simplicity in nomenclature.
If a named subsurface unit can be correlated with a named surface unit of the same kind, and if the characteristics of both are so similar that the two names are unnecessary, preference should generally be given to the name of the surface unit. But other factors should also be considered, such as priority of publication, usage, completeness of section, accessibility, nature of exposures in surface sections, and availability of type material from subsurface sections. For example, the Late Oligocene - Early Miocene, offshore Nido Limestone corresponds to the Saint Paul Limestone, an onshore formation in Northern Palawan that has the same age and analogous stratigraphic position in the local sequence. The limestone on the geographic locality called El Nido, in Northern Palawan is of Middle Permian age and has been named Minilog Limestone (Hashimoto and Sato, 1973). However the term Nido Limestone has been adopted by most workers in the region for the widely distributed Late Oligocene - Early Miocene limestone formation in offshore northwestern Palawan since the formation’s name saw print in 1978 (Hatley, 1978) and formally proposed by Sali and others (1981).
e. Uncertainty in Assignment
If there is uncertainty with respect to the assignment of rock bodies to one or the other of two named units, it is always better to express this doubt rather than to make an arbitrary assignment. The following conventions may be used:
- Cretaceous? - doubtfully Cretaceous
- Aksitero? Formation - doubtfully Aksitero Formation
- Amlang - Klondyke Formation strata intermediate in position (horizontally or vertically) between beds assuredly assigned to either of the two formations, which share the characters of both but which cannot be assigned decisively to either one and may eventually be made into a new formation. In the example cited, this refers to the Labayug Limestone which has a transitional stratigraphic position with respect to the Klondyke Formation and Amlang Formation.
- Pliocene-Pleistocene - one part Pliocene and one part Pleistocene, e.g., Ilagan Formation in Cagayan Valley, northern Luzon.
- Eocene or Oligocene - questionably either Eocene or Oligocene.
- Cretaceous and Paleogene - both Cretaceous and Paleogene but no distinction (undifferentiated) yet possible between the two. This used to be the designation of the basement rocks in many parts of the Philippines, sometimes referred to as KPg.
The name of the older or lower unit, if this distinction can be made, should always come first when two units are hyphenated or used in combination.
f. Abandoned Names
The name of a stratigraphic unit, once applied and then abandoned, preferably should not be revived except in its original sense when it seems useful to refer to an obsolete or abandoned formal name. Its status should be made clear by referring to it as such or by using such a phrase as "Rosario Formation" (or Linao sandstone member) of Corby and others (1951). The term "Rosario Formation" has since been redefined by Lorentz (1984) who subdivided the original unit into the Amlang Formation and Cataguintingan Formation. To determine if a name has been abandoned or is obsolete authors should refer to the National Stratigraphic Lexicon, if available.
g. Preservation of Traditional and Well-established Names
Although it is strongly urged that all new stratigraphic units be named according to the recommendations of this Guide, it is realized that there are many well-established and traditionally used stratigraphic units, particularly lithostratigraphic units of long historical standing for which exception should be made.
a.A Recognized Scientific Medium
Establishment of a formal stratigraphic unit, or the revision of an existing unit requires publication of a statement of interest and an adequate description of the unit in a recognized peer-reviewed scientific medium.
It is difficult to define what constitutes a "peer-reviewed scientific medium." The chief qualifications are scientific purpose and availability to the scientific public on request by purchase or through an accessible library. Regularly issued peer-reviewed scientific journals meet this requirement. Many independent or irregularly issued publications also meet this, although in such cases some notice of the proposal should also appear in a widely circulated and regularly issued scientific journal.
Names proposed in informal open-file or restricted media such as letters, company reports unavailable to the public, open-file release, unpublished presentations, thesis or dissertations, newspapers, and commercial, non-reviewed conference proceedings or trade journals do not qualify.
Publications of new stratigraphic names in abstract issued in advance of complete reports usually do not establish these names, because the essential conciseness of abstracts does not permit adequate description.
Informal reference, such as "the formation at Wawa Dam" or "the limestone cropping out near Binangonan" does not establish a new formal unit, nor its use in a table or columnar section or on a map.
Most fieldtrip guidebooks, with distribution limited to participants of the excursion, are not acceptable as a "recognized scientific medium." Although these publications do meet the test of scientific purpose and availability and therefore, constitute valid publication, other media are preferable.
Priority in publication of a properly proposed, named, and defined unit should be respected. However, the critical factors should always be the usefulness of the unit, the adequacy of its description, freedom from ambiguity, and suitability for widespread application. Priority alone does not justify displacing a well-established name by one not well known, or only occasionally used; nor should one inadequately established name be preserved merely on account of priority.
i. Capitalization — The first letters of all words used in the names of formal stratigraphic units should always be capitalized (except for the trivial terms of species and subspecies in the names of biostratigraphic units) for example, Bulimina-Bolivina Assemblage Zone, Upper Cretaceous Series, Devonian System. Informal terms are not capitalized (except in those languages which require all nouns to be capitalized)
ii. Hyphenation — Compound terms for most kinds of stratigraphic units in which two common words are joined to give a special meaning should be hyphenated: for example, concurrent-range zone, normal-polarity zone. Exceptions are adjectival prefixes or combining forms which should generally be combined with the term-noun without a hyphen, for example, biozone, chronozone, subsystem, biohorizon, supergroup.
iii. Repetition of Complete Name — After the complete name of a stratigraphic unit has been referred to once in a description or discussion, part of the name subsequently may be omitted to avoid cumbersome repetition if the omission is compatible with clarity. For example, the Guadalupe Formation may be referred to as the "Guadalupe" or "the Formation" and the Cenomanian Stage as "Cenomanian" or "the Stage." The use of capitals for formal unit-terms when these are not coupled with a proper name is discretionary, depending on needs for clarity or emphasis.
5. Revision or Redefinition of Previously Established Stratigraphic Units.Edit
Revision or redefinition of an adequately established unit without changing its name requires as much justification and the same kind of information as for proposing a new unit and generally requires the same procedures.
Redefinition may be justified to make a unit more useful or easier to recognize, map, and extend throughout the area of its occurrence. For example, Lorentz (1984) justified his redefinition of the Rosario Formation of La Union-Benguet Province (Corby and others, 1951) by raising the status of its lower and upper members to the Amlang and Cataguintingan formations, respectively, because of an unconformity between these two members which is evidenced by the difference in depositional environments represented by these two units as reflected in their lithologies and associated fossils (see also Bandy, 1963), and structural relationships.
Redefinition may also become desirable because of taxonomic changes of the fossil content of a biostratigraphic unit or because of errors in an earlier work. Names of long standing and common usage may be preserved even though their nomenclature may not conform to modern rules and procedures.
Change in rank of a stratigraphic unit does not require redefinition of the unit or its boundaries or alteration of the geographic part of the name. Thus a stage may be raised to Series rank or reduced to a Substage, or a Formation may be raised to a Group or reduced to a Member, without changing its name. For example, the Amlang Member of the Rosario Formation was raised to the rank of formation (Lorentz, 1984). The rank of any stratigraphic unit, however, should be changed only for substantial reasons and after careful consideration.
Changes in major chronostratigraphic units of international scope should be made only after consultation with appropriate stratigraphic organizations.