Geological Time Systems - a UML model and XML encoding

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• Paper in Geosphere: A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards

Also see GeologicTimeDiscussion

Introduction

The GeoSciML Geologic Timescale model and encoding is described in detail in the paper A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards published in Geosphere.

The diagrams on this page have been updated to reflect changes to the Observations and Measurements model, and to the GeoSciML model. The GeologicTime model is included as a package in the GeoSciML model template -- SimonCox - 19 Oct 2007

The classic "geological time scale" is a hierarchical ordinal system, in which the eras are ranked: "stages" nest within "series" within "systems" within "eras" within "eons" (in the most common version of the ranking system).

• Explanation of Era/Boundary-Node in terms of Stratigraphic Chart:
  

The time positions of the start and end points or geological eras are not known precisely, except for most of the Precambrian, where the boundaries are defined chronometrically. Typically there will be a number of estimates available, based on dating specimens retrieved from particular localities believed to correspond to the boundary of interest. A locality ratified by ICS is known as a "Global Boundary Stratotype Section and Point (GSSP)" and is indicated by a "golden spike" on the chart.

The complete definition of a geological time scale requires

1. a description of the hierarchical structure of named eras
2. the temporal positions of the boundaries between the eras.

This is an interpretation of the model implied by the guidelines from the IUGS International Commission for Stratigraphy.

ICS Stratigraphic Chart

The ICS publishes a representation of the geologic timescale as a coloured stratigraphic chart, showing the order and names of the elements of the time scale, and the dates of the boundaries as fixed through GSSP where available. Since the last formal publication of the timescale in 2004, the chart has been updated a number of times reflecting the progression of work in ICS. Previous versions are not available from the ICS website, but can be obtained from web-archives, for example the "Wayback Machine".

We have retrieved all the variants from the archive and provide them here as a convenience - see attachments below. There appears to be four significant versions, shown here (click icon for full version).

2004-04-06	2005-12-10	2006-04-09	2008-04-01

GeologicTime XML

Schema documents

• http://schemas.opengis.net/gml/3.1.1/base/temporal.xsd
• http://schemas.opengis.net/gml/3.1.1/base/temporalTopology.xsd
• http://schemas.opengis.net/gml/3.1.1/base/temporalReferenceSystems.xsd
• http://www.geosciml.org/schemas/geotime/2.0_rc1/geologicTime.xsd

Example Instance

• https://www.seegrid.csiro.au/subversion/GeoSciML/trunk/instances/Timescale_ICS_GeoTime.xml

Complete representation of 2008 ICS Stratigraphic Chart.

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Theory

Temporal Reference System

A temporal reference system supports the ordering of events in time.

ISO model for temporal reference systems

A model for Temporal Reference Systems is described in ISO 19108.

• ISO 19108 Temporal Reference Systems:
  

All temporal reference systems derive from a common TM_ReferenceSystem. This has a mandatory property domainOfValidity which describes the spatio-temporal scope of the reference system (e.g. "Common Era, global" or "Cambrian, Australia").

Four concrete specializations are defined as follows.

• TM_Calendar is a reference system based on years, months and days.
• TM_Clock is based on hours, minutes and seconds in a particular day.
• TM_CoordinateSystem provides the basis for describing temporal position numerically. This uses two properties to define a timeline: an origin, which ties the scale to an external temporal reference position, and interval, which provides the basic unit or precision, such as seconds, or millions-of-years.
• TM_OrdinalReferenceSystem (TORS) provides the system required for a timescale based on named intervals. A TORS is composed of an ordered sequence of one or more component TM_OrdinalEra (TOE) elements. A TOE may be recursively decomposed into ordered member TOE elements, thus allowing a hierarchical system to be constructed. Each era is characterised by its name (inherited from Definition). Note that the term "era" is used generically, and is used for component intervals of all ranks.

ISO 19108 is implemented in the GML schema documents temporal.xsd, temporalTopology.xsd, temporalReferenceSystems.xsd.

Modification required for the geological timescale

• UML representation of Temporal Ordinal Reference System used as the basis for the model described here:
  

In the definition of TOE we have found it necessary to introduce a variation to the ISO definition.

In the standard model [ISO 19108:2003] the limits of an era are defined precisely by attributes of type DateTime. However, in historic, archeologic contexts, and certainly in the geologic timescale, while the order of eras within a TORS is known, the positions of the boundaries are often not precisely known and can only be estimated.

We suggest that standard practice is better represented by a model using an explicit TimeOrdinalEraBoundary (TOEB) element to carry information concerning the transition between two TOEs. The temporal position of the era boundary is given by an associated TM_Instant, but the TOEB exists in its own right even its position is not known. In the context of the geological timescale, the TOEB is central, since it is the temporal concept that is associated with the boundary stratotype.

GeologicalTimeScale

• UML for geologic timescale - top level:
  

• GeologicTimescale is a kind of TimeOrdinalReferenceSystem that is composed of one or more ordered TimeOrdinalEras together with two or more TimeOrdinalEraBoundarys. The GeologicTimescale is thus a temporal complex that includes as first-order elements both (a) the eras and (b) the boundaries composing the reference system.

Geochronologic specializations of each are provided.

• GeochronologicEra is a kind of TimeOrdinalEra with boundaries defined by geologic evidence. It specializes TimeOrdinalEra by adding a rank attribute, whose value is one of the standard terms Eon, Era, Period etc. Some additional properties are discussed in a following section.

Two specializations of TimeOrdinalEraBoundary are introduced.

• GeochronologicBoundary represents an era boundary defined with reference to some geologic evidence. Its properties are discussed in detail in the following section.
• NumericEraBoundary is provided for those boundaries defined chronometrically. It has no additional associations, but a "status" attribute is provided.

GeochronologicEra is substitutable for TimeOrdinalEra, so can appear within the description of a TimeOrdinalReferenceSystem or TimeOrdinalEra. A GeochronologicEra might have a name like "Phanerozoic", "Callovian" etc.

GeochronologicBoundary is substitutable for TimeOrdinalEraBoundary, so can appear within the description of a TimeOrdinalEra. An GeochronologicBoundary might have a name like "Base of Silurian", "Base of Adelaidean" etc., but see note on names below.

Correlations and evidence in the geological record

• Correlations from timescale elements to geological record - complete:
  

A ChronostratigraphicUnit is the (notional) feature composed of all the rocks formed during the associated GeochronologicEra. Both ChronostratigraphicUnit and the commonly used LithostratigraphicUnit are kinds of GeologicUnit. A ChronostratigraphicUnit? carries a "rank" attribute, whose value is one of the standard terms System, Stage, Zone etc.

Similarly, a ChronostratigraphicBoundary is the (notional) compound surface marking the upper or lower bound of a unit. Both ChronostratigraphicBoundary and LithostratigraphicBoundary are kinds of StratigraphicBoundary.

In practice, the complete shape of any ChronostratigraphicUnit and ChronostratigraphicBoundary instance will not be precisely known, so while the existence of a unit and its boundaries is a fact, they will never be fully described. The ChronostratigraphicUnit is the complete body of rock formedDuring a GeochronologicEra, and the ChronostratigraphicBoundary correlatesWith a GeochronologicBoundary.

Conventional samples of both units and boundaries may be defined, with a spatial dimensionality two orders less than the parent. For a solid unit, this sample is a section (whose shape is a curve), while for a surface the sample is a point. These are shown in the model as StratigraphicSection and StratigraphicPoint, respectively. A StratigraphicPoint is always contained within a StratigraphicSection, which is its hostSection.

In principle, a StratigraphicSection may host any number of StratigraphicPoints. While an unlimited number of samples of the concrete geological unit or boundary may be made, a single instance provides the reference locality or stratotype for the associated era or era-boundary, respectively.

Considering all these associations, there is a triangular relationship between the conceptual object (chronologic era or boundary), its physical realisation in the geologic record (chronostratigraphic unit or boundary), and samples of the record (stratigraphic sections or points), one of which may act as the stratotype for the conceptual object.

A further important concept is the StratigraphicEvent. In general, events are associated with time primitives of either zero or a finite extent (i.e. time instants or time periods). However, in the context of the geological timescale, a useful event has negligible duration, and is associated with a boundary and characterised by a StratigraphicPoint.

Calibration of the timescale using observations

• UML for age determination of timescale boundaries:
  

A DateMeasurement is a kind of measurement, whose result is a (numeric) value with reference to a TimeCoordinateSystem. In common with all observations, it relates to a physical target or featureOfInterest, usually either (i) a specimen, or (ii) a sampling site such as a StratigraphicPoint in its context within its host StratigraphicSection. Note, however, that the feature of interest might be another specimen or station, not collected directly from the stratotype point, but from another locality holding material that is more amenable to date measurements, but which can be correlated with the stratotype.

The measurement uses a DatingProcedure, preferably a precise numeric method such as one of the radiometric methods or based on astronomical cycles. If these are not suitable for the physical evidence, then less precise methods are used.

The GeochronSpecimen is sampled on a site, either a point or over a small interval bracketing the point of interest (i.e. a short section). If the material at the stratotype itself is unsuitable for date determination, then a specimen may sample a different locality that is correlated with the stratotype. (In this case the UML class diagram is potentially confusing since it shows relationships and cardinalities involving classes, but does not make clear that multiple distinct instances of each class may be involved. This could be shown for a particular instance using a UML object diagram.)

Finally a StratigraphicDateEstimate specializes TimeInstant to represent an interpretation based on one or more DateMeasurements. The inheritance relationship means that StratigraphicDateEstimate is substitutable for TimeInstant. Thus the association labelled "Geometry" between TM_Instant and TimeOrdinalEraBoundary usually refers to a StratigraphicDateEstimate when the GeochronologicBoundary specialization is involved

Components for the GSSP

• Profile of model showing the components required to represent the GSSP:
  

This diagram shows a "complete" model, constructed by combining elements introduced above, but suppressing some classes and particularly associations that either conflict with or are not used by current practice as described in the guidelines [Remane et al. 1996]. Furthermore, in this diagram most of the required class attributes are also shown. For example, a number of attributes describe details of points and sections, some of which are inherited from parent classes as indicated by the annotation.

We may summarize the story told in this model as follows.

A GeologicTimeSystem is a specialised TimeOrdinalReferenceSystem which is composed of an ordered sequence of TimeOrdinalEra elements, but also records the TimeOrdinalEraBoundary elements as reference points. GeochronologicEra and GeochronologicBoundary are specializations of the standard eras and boundaries.

One StratigraphicPoint plays the role of stratotype for a GeochronologicBoundary, which record a StratigraphicEvent. The relationship of the StratigraphicSection that hosts the point to the GeochronologicEra that is bounded by the boundary records the stratigraphic position of the section but does not associate a unique section with an era. Note that, although unit-stratotypes are still used for some regional and local purposes, their use is deprecated for specification of the global timescale.

DateMeasurements are made on either

1. the StratigraphicPoint itself in its context (e.g. for determinations based on astronomical cycles), or
2. on a GeochronSpecimen (e.g. for radiometric date determinations).

The specimen is sampled either

1. at the StratigraphicPoint that is the stratotype,
2. at another StratigraphicPoint that is correlated with the stratotype, or
3. on an interval bracketing the point of interest (i.e. a short StratigraphicSection).

A StratigraphicDateEstimate provides the preferred value of the position of the GeochronologicBoundary. The estimate is usually based on one or more DateMeasurements, but may be derived from some other basis. Note that StratigraphicDateEstimate has a quality associated with it, which allows the estimated error to be recorded, and StratigraphicDateEstimate along with StratigraphicPoint have status attributes that can be used to record whether these are ratified through GSSP.

A suitable StratigraphicPoint has an association with one or more StratigraphicEvents, which result in observable evidence in the section that defines the point such as the appearance or disappearance of particular fossil taxa, or the beginning or end of some climatic phenomenon. Note, however, that in the ICS approach, it is the StratigraphicPoint ("Golden Spike") that provides the ultimate reference for the boundary, so its position will remain unchanged even if new evidence modifies the interpretation of the stratigraphic event [Walsh et al. 2004].

StratigraphicEvent inherits from the Event class (not shown) a time association with a notional TM_Object. However, as used here, it is assumed that the position of a StratigraphicEvent is not available directly, but may be recovered by tracing the association with a boundary or prototype point, for which estimates of the position are available.

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Notes

Theory: Ordinal Reference Systems vs. general Topological Complexes

Each era within an ordinal reference system may be subdivided into an ordered sequence of sub-eras, resulting a hierarchy. To ensure there is no ambiguity in the relative positions of eras, only a single hierarchy is allowed.

For example, in the temporal topological complex shown graphically above, we show edges representing eras as arrows, between nodes represented as filled circles. The eras have various ranks implied by the thickness of the line, and are labelled "B", "C1" etc. Some of the nodes are labelled "B_C", "B4_B5", etc.

The parts of the graph colored green represent a valid ordinal reference system. For example, a feature assigned the age "B22" is unambiguously "Earlier" than a feature of age "B4", and is "During" the life of a feature of age "B". These relationships are clear even if the numerical positions of the end points of some or all of the eras are not known, or not known precisely.

The parts of the graph colored blue contain an alternative subdivision of era B, labelled "b1", "b2", etc. Note that, unless the positions of the nodes are well calibrated, it is not (for example) possible to determine the relative temporal positions of features whose ages are b3 and B24. The order of components is ambiguous, so the more generalised complex does not qualify as a valid reference system.

Thus, an ordinal temporal reference system is effectively a temporal topological complex constrained so that only one of any pair of parallel branches may be subdivided.

The blue subset may, however, comprise a different reference system, for example having a different scope. Note that the temporal relationship between objects characterised using different reference systems is in general indeterminate.

Correlation projects attempt to resolve this by discovering, or asserting, relationships between elements of time systems defined originally for different domains of validity. If successful, this may result in a merging of different systems to form a single system (hierarchy) with a domain of validity that is the union of the domains of the contributing systems.

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Variation from standard model

The use of gt:TimeOrdinalEraBoundary to delimit gml:TimeOrdinalEra is a variation from ISO 19108, where the begin and end posistions of a TM_OrdinalEra use the standard ISO 8601 timescale, which only applies to the historical era. This adjustment has been discussed with Charles Roswell, the editor of ISO 19108 (and a geologist!). I've put in a change request to 19108 proposing this change.

Names on time nodes

Like all GML "Objects", an xmml:StratNode may carry several names but only one id (see LabelsAndHandles). Thus a single node may be labelled as all of "Base of Cenozoic", "Base of Paleogene", "Base of Paleocene" and "Base of Danian", but may only carry a single handle, e.g. "MZ_CZ". The handle (i.e. value of gml:id) may be opaque. If a mnemonic code is used, for consistency I recommend this refer to the highest rank eras terminated by this node (in this example: Mesozoic and Cenozoic). The depth of the lowest order era varies between parts of the overall timescale, its naming is less consistent and varies regionally, and the coarser units are more likely to be generally known and used.

Other geological timescale projects

• Geowhen
• CHRONOS

Some comments from RobertRohde of Geowhen:

In designing the GeoWhen database, my goal was to provide information and age estimates on how the variety of archaic and regional stage nomenclatures relate to the modern conception of geologic time. Of course, the relations between different naming schemes are at best approximate. If I am understanding your project correctly, your goal is to provide a consistent scheme for the transmission and handling of individual time scales, while largely aiming to avoid the issue of how alternative nomenclatures are interrelated.

-- RobertRohde - 02 May 2004

Yes - you are correct re. design goals. The discussion of ordinal time reference system vs. general time topological complex makes the case for defining a particular OTRS by its unique sequence of eras - only one hierarchy allowed! So while the XMML formulation does support direct recording of multiple estimates of the position of nodes within a particular OTRS, it does not allow for the recording of alternative sequences based on different boundaries within a particular OTRS.

Now one can imagine various ways of composing a long OTRS from segments of shorter OTRS's, which would then support slotting in alternative decompositions of one or more eras, probably defined for a local region. However, the current ISO 19108 formulation (which I've attempted to stay faithful with) has a somewhat artificial distinction between decomposition of a TRS (which has a "domain of validity") into "components" and decomposition of an era (which is not scoped by domain) into "members". So in order to support your requirements I think we would have to

1. add a domainOfValidity property to the redefined Time Ordinal Era
2. allow TOE's to act as top-level objects with identity, so that they could be composed into alternative OTRS's as required.

-- SimonCox - 03 May 2004