James G. Ogg

A Mesozoic time scale

We present an integrated geomagnetic polarity and stratigraphic time scale for the Triassic, Jurassic, and Cretaceous periods of the Mesozoic Era, with age estimates and uncertainty limits for stage boundaries. The time scale uses a suite of 324 radiomenc dates, including high-resolution 40 Ar/ 39 Ar age estimates. This framework involves the observed ties between (1) radiometric dates, biozones, and stage boundaries, and (2) between biozones and magnetic reversals on the seafloor and in sediments. Interpolation techniques include maximum likelihood estimation, smoothing cubic spline fitting, and magnetochronology

Geologic Time Scale 2004 – why, how, and where next!

A Geologic Time Scale (GTS2004) is presented that integrates currently available stratigraphic and geochronologic information. The construction of Geologic Time Scale 2004 (GTS2004) incorporated different techniques depending on the data available within each interval. Construction involved a large number of specialists, including contributions by past and present subcommissions officers of the International Commission on Stratigraphy (ICS), geochemists working with radiogenic and stable isotopes, stratigraphers using diverse tools from traditional fossils to astronomical cycles to database programming, and geomathematicians. Anticipated advances during the next four years include formalization of all Phanerozoic stage boundaries, orbital tuning extended into the Cretaceous, standardization of radiometric dating methods and resolving poorly dated intervals, detailed integrated stratigraphy for all periods, and on-line stratigraphic databases and tools. The geochronological science community and the International Commission on Stratigraphy are focusing on these issues. The next version of the Geologic Time Scale is planned for 2008, concurrent with the planned completion of boundary-stratotype (GSSP) definitions for all international stages.

On the Geologic Time Scale

This report summarizes the international divisions and ages in the Geologic Time Scale, published in 2012 (GTS2012). Since 2004, when GTS2004 was detailed, major developments have taken place that directly bear and have considerable impact on the intricate science of geologic time scaling. Precam brian now has a detailed proposal for chronostratigraphic subdivision instead of an outdated and abstract chronometric one. Of 100 chronostratigraphic units in the Phanerozoic 63 now have formal definitions, but stable chronostratigraphy in part of upper Paleozoic, Triassic and Middle Jurassic/Lower Cretaceous is still wanting. Detailed age calibration now exist between radiometric methods and orbital tuning, making 40Ar-39Ar dates 0.64% older and more accurate. In general, numeric uncertainty in the time scale, although complex and not entirely amenable to objective analysis, is improved and reduced. Bases of Paleozoic, Mesozoic and Cenozoic are bracketed by analytically precise ages, respectively 541 0.63, 252.16 0.5, and 65.95 0.05 Ma. High-resolution, direct age-dates now exist for base-Carboniferous, base-Permian, base-Jurassic, base-Cenomanian and base-Eocene. Relative to GTS2004, 26 of 100 time scale boundaries have changed age, of which 14 have changed more than 4 Ma, and 4 (in Middle to Late Triassic) between 6 and 12 Ma. There is much higher stratigraphic resolution in Late Carboniferous, Jurassic, Cretaceous and Paleogene, and improved integration with stable isotopes stratigraphy. Cenozoic and Cretaceous have a refined magneto-biochronology. The spectacular outcrop sections for the Rosello Composite in Sicily, Italy and at Zumaia, Basque Province, Spain encompass the Global Boundary Stratotype Sections and Points for two Pliocene and two Paleocene stages. Since the cycle record indicates, to the best of our knowledge that the stages sediment fill is stratigraphically complete, these sections also may fulfill the important role of stage unit stratotypes for three of these stages, Piacenzian, Zanclean and Danian

Status of Divisions of the International Geologic Time Scale

Each chronostratigraphic unit of the International Geologic Time Scale will be defined at its base by a Global Stratotype Section and Point (GSSP) or Global Standard Stratigraphic Age (GSSA). Nearly 50 GSSPs and 10 GSSAs have now been ratified. Ideally, the GSSP coincides with events having a global correlation potential. The international stage divisions of some systems, such as the Jurassic or Neogene, are similar to traditional usage in European geology. However, in order to utilize global correlation horizons, the international stage divisions of other systems, such as the Ordovician or Permian, have required assembling new stage nomenclatures or hybrids of different regional stages. A reference table by the International Commission on Stratigraphy itemizes the current or potential GSSP and GSSA definitions of all international geologic time units.

Jurassic magnetostratigraphy, 1. Kimmeridgian-Tithonian of Sierra Gorda and Carcabuey, southern Spain

Abstract Two coeval sections of red to white ammonite-rich pelagic limestones spanning the complete Kimmeridgian and most of the Tithonian were sampled in detail. All samples were treated by progressive thermal demagnetization to remove a present field overprint. Characteristic magnetization is carried primarily by magnetite. Polarity intervals are easily identified and correlate well between the two sections. The Tithonian polarity sequence can also be correlated to sections in northern Italy. The similarity between the polarity sequence and the M-sequence of marine magnetic anomalies, coupled with the precise biostratigraphic control, allows assignment of the following ages to the M-sequence: the Late/Early Tithonian boundary is correlated to the end of M-20, the Tithonian/Kimmeridgian boundary to the end of M-23, the Late/Early Kimmeridgian boundary to the latter part of M-24, and the Kimmeridgian/Oxfordian boundary within or slightly after M-25. The mean directions of characteristic magnetization have α 95 s less than 3° and demonstrate extensive differential block rotation within the Subbetic province. Paleolatitudes during the Kimmeridgian/Tithonian are in the range of 16–24°N.

Jurassic magnetostratigraphy, 2. Middle-Late Oxfordian of Aguilon, Iberian Cordillera, northern Spain

Abstract A reproducible magnetic polarity pattern has been obtained from four overlapping stratigraphic sequences of ammonite-rich gray pelagic limestones, spanning part of the Middle and Late Oxfordian (G. transversarium toE. bimammatum ammonite zones). Progressive thermal demagnetization removed a present field overprint. Fold tests, both between- and within-outcrops, are positive. The mean direction of magnetization is 322°, 45° (α95 = 6°), which implies a paleolatitude of 26°N. Frequent changes of polarity were observed. The magnetostratigraphic sequence contains the ammonite zones, subzones, and horizons of the Tethyan faunal realm. Using the reference horizons provided by this detailed ammonite zonation, the polarity zones can be readily correlated among all sections. The composite polarity pattern is predominantly of reversed polarity, punctuated by a number of relatively shorter duration normal polarity intervals. In order to compare this sequence to that of the marine magnetic anomalies, we have revised the age estimates for the pre-M25 marine magnetic normaly pattern according to the most recent time scale. Based on those ages, the sedimentary record at Aguilon suggests a much greater frequency of polarity changes than does either model derived from the two sets of marine magnetic anomalies of this time. This sedimentary record probably is a truer approximation of the actual geomagnetic polarity history because of the continuity of sedimentation in these sections and the inherent ambiguities involved in determining a polarity pattern from the very low amplitudes that everywhere characterize this part of the marine record.

Early Triassic magnetic polarity time scale—integration of magnetostratigraphy, ammonite zonation and sequence stratigraphy from stratotype sections (Canadian Arctic Archipelago)

Abstract Stratotypes defining the stages of the Early Triassic (Griesbachian, Dienerian, Smithian and Spathian) are located on Ellesmere and Axel Heiberg islands in the northern Canadian Arctic. Ammonite-rich horizons are within a clastic outer shelf-to-slope facies of thick progradational wedges of mudstones and siltstones. Three sections were sampled for magnetostratigraphy and interpreted for transgressive and regressive pulses of sedimentation. Using the ammonite zonation as a guide, the transgressive-regressive cycles and magnetostratigraphies have been correlated among the sections and to the published Triassic sequence stratigraphy time scale, thus enabling definition of the magnetic polarity pattern for the upper Griesbachian to Smithian stages in multiple sections. The magnetic polarity and associated sequence stratigraphy pattern for the lower Griesbachian and for the Spathian were derived from single sections. The Griesbachian and Dienerian stages each have two pairs of normal- and reversed-polarity chrons; the Smithian is predominantly of normal polarity, and the Spathian is predominantly of reversed polarity. This magnetic polarity time scale may help to resolve age correlations of North American redbed facies and to define the Permian-Triassic boundary. After correction for variable structural orientations, the mean directions of magnetization from the three sites converge at 296° declination, 57° inclination (k = 60,α95 = 16.5°; equivalent pole = 41°N, 161°E; paleolatitude = 38°N), which is consistent with the pole derived from nearby Early Permian volcanics and supports a postulated post-Early Triassic, pre-Tertiary counterclockwise rotation of this region with respect to cratonic North America.

On the Geologic Time Scale 2008

Abstract This report summarizes the international divisions of the geologic time scale and ages. Over 35 chronostratigraphic units have been formalized since 2000, with about one third of the almost 100 geologic stages of the Phanerozoic still awaiting international definition. The same numerical time scale is used as in Geologic Time Scale 2004 for the majority of stage boundaries. Exceptions are made if the definitions for stage boundaries are at a different level than the previous “working” versions (e.g., base of Serravallian, base of Coniacian, and bases of Ghzelian, Kasimovian and Serpukhovian). In most cases, numerical changes in ages are within GTS2004 age error envelopes. On-screen display and production of user-tailored time-scale charts is provided by the TimeScale Creator, a public JAVA package available from the ICS website (www.stratigraphy.org) and www.tscreator.com.

Magnetostratigraphy of the Jurassic-Cretaceous boundary interval—Tethyan and English faunal realms

Abstract Geomagnetic reversals and magnetic polarity chrons provide an important chronostratigraphic tool for global correlation. An integrated Tithonian-Berriasian biostratigraphic and magnetic polarity time scale for the Tethyan faunal realm for the Tithonian and Berriasian stages is compiled from 17 independent biomagnetostratigraphic sections. This time scale incorporates zones and first/last appearance datums from ammonites, calpionellids, calcareous nannofossils, and dinoflagellates. This database provides an estimate of the range of observed appearance datums or zonal boundaries relative to polarity chrons; such apparent “diachroneity” probably results from a combination of preservation of species and of paleontological methodology, rather than from migration. The lithologic transition from “Rosso Ammonitico” red marly limestone to “Maiolica” white limestone occurs at different times during the Tithonian among the various sections, ranging from polarity zone M22n (mid-Early Tithonian) in some Spanish “slope” and Italian basinal-facies sections to polarity zone M19n (mid-Late Tithonian) in the central Atlantic and some plateau-facies Italian sections. This widespread lithologic change is, therefore, probably a result of shifting local patterns of fertility overprinted on the main regional trend. Magnetostratigraphy from the Purbeck Limestone Formation in the English Boreal faunal realm was obtained from the “classic” section at Durlston Bay in Dorset. The Dorset section displays predominantly normal polarity with a minimum of three reversed-polarity zones, but distortion of the magnetic polarity pattern by variable rates of sedimentation in this marginal clastic environment coupled with lack of independent correlation methods currently precludes a unique correlation to the Tithonian-Berriasian magnetic polarity time scale. The Tithonian-Berriasian magnetic polarity time scale may eventually provide a global chronostratigraphic definition of the Jurassic-Cretaceous boundary.

Jurassic magnetostratigraphy, 3. Bathonian-Bajocian of Carcabuey, Sierra Harana and Campillo de Arenas (Subbetic Cordillera, southern Spain)

Abstract Four sections in Majocian-Bathonian (Middle Jurassic) pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences. Magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300°C to in excess of 450°C. The polarity patterns indicate that the majority of the Bajocian and Bathonian is characterized by quite frequent reversals of the magnetic field. Lengthy periods of constant polarity, particularly constant normal polarity, were not observed. The average frequency of reversals is about 6 per ammonite zone, which roughly may be interpreted as a frequency of a reversal every 260,000 years, a rate comparable to that of the Miocene-Pliocene. Paleolatitudes of these sites (25–28°) are about 10° south of their present positions; variable clockwise block rotations within the Subbectic region have rotated these sites relative to stable Iberia.