Unitary Association Method Research Articles

The Unitary Association Method of Relative Dating and Its Application to Archaeological Data

The primary objective of relative dating techniques is to determine a reliable sequence of archaeological deposits. This task becomes more difficult whenever our research steps beyond individual sites to the study of intercommunity relationships because we need to develop some means of associating unconnected deposits in time. Radiocarbon dating, as a stand-alone method, cannot always be used to draw reliable correlations between sites. The relevance of archaeological dates, including absolute dates, relies ultimately on the determination of artifact or sample associations and their respective superpositional relationships. The Unitary Association Method of Relative Dating is an alternative to seriation methods that is less susceptible to spatial variation and offers analytical strengths needed for regional chronological analyses.

A UPb age from the Toarcian (Lower Jurassic) and its use for time scale calibration through error analysis of biochronologic dating

The Early Jurassic is the most poorly constrained part of the geochronologic scale within the Mesozoic, owing to the scarcity of calibration points. We report a new UPb zircon age of 181.4 ± 1.2 Ma (2σ) from an ash layer within the type section of Crassicosta Zone, a North American regional standard ammonite zone in the Middle Toarcian. For time scale calibration, the isotopically dated level needs to be tied to the Jurassic chronostratigraphic framework established in northwest European successions. Three major contributing factors (difference between local observed and true ranges, uncertainties of correlation, and taxonomic noise) are assessed to determine the maximum biochronologic dating error. The local ammonite distribution data are robust in that 95% confidence level extensions of observed ranges of common ammonite taxa would not significantly modify the zonal assignment. The computer-assisted unitary association (UA) method is used for world-wide biochronologic correlation. Representative North and South American, western Tethyan, and northwest European sections were selected to establish the global maximum ranges for 103 Middle Toarcian ammonite taxa and a sequence of 40 UA. Maximum permissible correlatives of the Crassicosta Zone lie between the northwest European standard Semipolitum Subzone (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone). In particular, the dated tuff layer cannot be older than the Semipolitum Subzone or younger than the Illustris Subzone (Variabilis Zone). Taxonomic inconsistencies are present in the source data but their effect in corrupting UA-based correlation is shown to be negligible. The precise UPb age (2 σ error < ± 1%) that is constrained at the (sub)zonal level furnishes a useful new time scale calibration point. It warrants adjustments to previous Toarcian stage boundary age estimates and allows a significant reduction of their associated uncertainties.

Early permian (Sakmarian)brachiopods from southeastern Oman

The Lower Permian succession of the Huqf area (Sultanate of Oman) represents the uppermost part of a mega-sequence which includes the Late Carboniferous to? Sakmarian tillites of the Al Khlata Fm. and the overlying transgressive marine Saiwan Fm. of Early Permian age. The Saiwan yields a rich brachiopod fauna associated with bivalves, gastropods, crinoids, conularids, cephalopods and bryozoans. The brachiopod fauna of the Saiwan includes Derbyia haroubi nov. sp., Arctitreta cf. bioni ( Reed), Reedoconcha permixta ( Reed), Neospirifer aff. hardmani ( Foord), Neospirifer sp., Trigonotreta sp., ? Cyrtella aff. nagmargensis ( Bion), Subansiria sp., Punctocyrtella spinosa Plodowski, Punctospirifer sp., Callispirina sp., Fletcherithyris sp., ? Gilledia sp. This brachiopod assemblage indicates a Late Sakmarian age. Application of the Unitary Association method (Guex 1991) to the brachiopod data of the Saiwan leads to establish a local biochronological sequence of three faunal associations.

Early Cretaceous radiolarians of the Northeast Indian Ocean (Leg 123: Sites 765, 766 and DSDP Site 261): The Antarctic-Tethys connection

During ODP Leg 123, abundant and well-preserved Neocomian radiolarians were recovered at Site 765 (Argo Abyssal Plain) and Site 766 (lower Exmouth Plateau). Assemblages are characterized by the numerical dominance of a small number of non-tethyan forms and by the scarcity of tethyan taxa. Remarkable contrasts exist between radiolarian assemblages extracted from claystones of Site 765 and reexamined DSDP Site 261, and faunas recovered from radiolarian sand layers, only found at Site 765. Clay faunas are unusual in their low diversity of apparently ecologically tolerant (or solution resistant?), ubiquist species, whereas sand faunas are dominated by non-tethyan taxa. Comparisons with Sites 766 and 261, as well as sedimentological observations, lead to the conclusion that this faunal contrast resulted from a difference in provenance, rather than from hydraulic sorting or selective dissolution. The ranges of 27 tethyan taxa from Site 765 were compared to the tethyan radiolarian zonation by Jud (1992) by means of the Unitary Associations Method. This calculation allows to directly date the Site 765 assemblages and to estimate the amount of truncation of ranges for tethyan taxa. Over 70% of the already few tethyan species of Site 765, have truncated ranges during the Valanginian-Hauterivian. Radiolarian assemblages recovered from claystones at Sites 765 and 261 in the Argo Basin apparently reflect restricted oceanic conditions during the latest Jurassic-Barremian. Neither sedimentary facies nor faunal associations bear any resemblance to what we know from typical tethyan sequences. We conclude that the Argo Basin was paleoceanographically separated from the Tethys during the Late Jurassic and part of the Early Cretaceous by its position at higher paleolatitudes and/or by enclosing land masses. Assemblages recovered from radiolarian sand layers are dominated by non-tethyan species that are interpreted as circumantarctic. Their first appearance in the late Berriasian-early Valanginian predates the oceanization of the Indo-Australian breakup (M11, late Valanginian), but coincides with a sharp increase in margin-derived pelagic turbidites. The Indo-Australian rift zone and the adjacent margins must have been submerged deeply enough to allow an intermittent influx of circumantarctic cold water into the Argo Basin, creating increased bottom current activity. Cold-water radiolarians carried into the Argo Basin upwelled along the margin, died, and accumulated in radiolarite layers due to winnowing by bottom currents. High rates of faunal change and the sharp increase of bottom current activity are thought to be synchronous with possible pronounced late Berriasian-early Valanginian lowstands in sea level. Hypothetically, both phenomena might have been caused by a tendency to glaciation on the Antarctic-Australian continent, which was for the first time isolated from the rest of Gondwana by oceanic seaways as a result of Jurassic-Early Cretaceous sea-floor spreading. The absence of most typical tethyan radiolarian species during the Valanginian-Hauterivian is interpreted as reflecting a time of strong influx of circumantarctic cold water following oceanization (M11) and rapid spreading between Southeast India and West Australia. The reappearance and gradual abundance/diversity increase of tethyan taxa, along with the still dominant circumantarctic species are thought to result from overall more equitable climatic conditions during the Barremian-early Aptian and from the establishment of an oceanic connection with the Tethys Ocean during the early Aptian.

Unitary associations method: use of graph theory and computer algorithm

Abstract Two species are defined as compatible if their chronologic cooccurrence has been observed (= real association) or can be deduced from biostratigraphic data (= virtual association). An unitary association (U.A.) is a maximal set of compatible species. Each U.A. is characterized by a set of species and/or by a set of species pairs: these characteristic elements are used to identify the U.A. in fossiliferous beds. The U.A.'s which can be identified in a large geographical area are said to be reproducible. A biochronological scale is an ordered sequence of reproducible U.A.'s. The problem of constructing such a discrete “time” scale is approached from a graph-theoretic point of view.