Felix M. Gradstein

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

A Cretaceous and Jurassic geochronology

An integrated geomagnetic polarity and geologic time-scale for the Jurassic and Cretaceous periods is presented, based on various methods according to the availability of definitive isotopic ages. An age-calibrated sea-floor–spreading model is used to interpolate the ages of the Kimmeridgian to Barremian, and the Campanian to Maestrichtian stages. Numerical age estimates for the Aptian to Santonian stage boundaries follow published isotopic age determinations. The hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the Hettangian to Oxfordian stages. The new scale results in ages of 208 Ma for the base of the Jurassic, 144 Ma for the Jurassic/Cretaceous boundary and 66.5 Ma for the top of the Cretaceous. The integrated biostratigraphic, magnetostratigraphic, and geochronometric record serves as a working hypothesis for geologic correlation of Jurassic and Cretaceous strata.

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.

The Ordovician Period

Abstract: Rapid and sustained biotic diversification reached its highest levels in the Paleozoic. A prolonged “hot-house” climate through Early Ordovician, cooling through Middle Ordovician and changing to “ice-house” conditions in Late Ordovician, global glaciation, oceanic turnover and mass extinction at end of period, strong fluctuations in eustatic sea level, appearance and diversification of pandemic planktonic graptolites and conodonts important for correlation, moderate to strong benthic faunal provincialism, re-organization and rapid migration of tectonic plates surrounding the Iapetus Ocean and migration of the South Pole from North Africa to central Africa all characterize the Ordovician Period. All seven Ordovician stages have formalized GSSPs.

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

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.

Algorithms for porosity and subsidence history

Abstract The FORTRAN 77 computer programs DEPOR (Depth and Porosity) and BURSUB (Burial and Subsidence) calculate depth-porosity functions and rates of compacted and decompacted burial and load-corrected subsidence, using both physical and stratigraphic borehole data. DEPOR requires as input: (1) measured porosities as a function of lithology and depth, or (2) lithology and sonic travel time as a function of depth. The porosity versus depth functions for each lithology are input in BURSUB which requires for each stratigraphic increment: (1) age in millions of years, (2) minimum and maximum water depth estimates, and if so desired (3) estimates of eustatic sealevel height. The programs present the result in a series of tables, but output can be adapted for graphics display, as demonstrated for two deep boreholes. Both DEPOR and BURSUB programs are listed.

Quantitative biostratigraphy of the Taranaki Basin, New Zealand: A deterministic and probabilistic approach

A quantitative biostratigraphic analysis of the Paleocene to lower Miocene of the Taranaki Basin has enabled high precision in correlation, zonation, and assessment of depositional history. Biostratigraphic range-end events, based on 493 taxa in cuttings samples from eight wells, representing foraminifera, nannofossils, dinoflagellates, and miospores, were culled to 87 range-top events that were then analyzed by deterministic (constrained optimization [CONOP]) and probabilistic (ranking and scaling [RASC]) techniques. All except 16 of the events are found to have relatively good biostratigraphic reliability. The RASC probable sequence and probabilistic zonation give the best estimate of the sequence of events and zones to be encountered in any new well in the basin and a precise biostratigraphic scale for future exploration. The CONOP composite section, which matches well with that derived by conventional graphic correlation (GRAPHCOR), is readily related to previous zonations based on maximum ranges of taxa but gives an order-of-magnitude greater precision. CONOP provides a precise correlation framework and reveals marked variation in thickness of stages across the basin. When the composite section is calibrated against the time scale, basinwide changes in depositional rate are revealed. The upper Eocene and Oligocene mark an interval of slow deposition, whereas the Miocene marks a sharp increase in deposition. The time-calibrated composite section enables unconformities and changes in depositional rate found in individual wells to be precisely estimated. Many new unconformities are indicated, particularly in the Paleocene and Eocene.

Early history of the Atlantic Ocean and gas hydrates on the Blake Outer Ridge: Results of the Deep Sea Drilling Project Leg 76

Leg 76 of the Deep Sea Drilling Project achieved two major scientific objectives. The first objective was met at Site 533, where on the Blake Outer Ridge, gas hydrates were identified by geophysical, geochemical, and geological studies. Gas-hydrate decomposition produced a volumetric expansion of 20:1 of gas volume to pore-fluid volume; this expansion exceeded by about a factor of four the volume of gas that could be released from solution in pore water under similar conditions. The gas hydrate includes methane, ethane, propane, and isobutane but apparently excluded normal butane and higher molecular weight hydrocarbons as predicted from gas hydrate crystallography. For the first time, marine gas hydrates were tested with a pressure core barrel. The second objective was achieved when coring at Site 534 in the Blake-Bahama Basin sampled the oldest oceanic sediments yet recovered. The sequence of oceanic basement and overlying sediments documents the geologic history of the early stages of the opening of the North Atlantic Ocean in detail. The oldest oceanic sediments are red claystones and laminated green and brown claystones of middle Callovian age. This finding supports the interpretation that the beginning of the modern North Atlantic occurred in the early Callovian (∼ 155 m.y. B.P.), as much as 20 m.y. later in time than often previously thought.

History, philosophy, and application of the Global Stratotype Section and Point (GSSP)

The history, philosophy, and application of the concept of the Global Stratotype Section and Point (GSSP) are reviewed. Geochronologic units defined by GSSPs serve as practical classificatory pigeonholes for the subdivision of geologic time. Accordingly, the main factor involved in the definition of GSSPs must be global correlatability. Early opposition to the GSSP concept centered around the desire for a traditional biochronologic time scale defined conceptually in terms of palaeobiological events, but such time scales are inherently unstable and thus unsuitable for the use of all geoscientists. The GSSP concept is also generally incompatible with the desire for ‘natural’ geochronologic boundaries. GSSPs have been defined mainly on the basis of biostratigraphic guiding criteria, but magnetic polarity reversals and chemostratigraphic and cyclostratigraphic horizons are now playing an important role. Most primary guiding criteria used to place a ‘golden spike’ will eventually become problematical in some way, so GSSPs should be defined so as to be correlatable by as many different lines of age-significant information as possible. The ‘Global Standard Stratigraphic Age’ (better renamed ‘Standard Global Numerical Age’) is a numerical analogue of the golden spike. Numerical definitions are currently appropriate for the formal subdivision of the Precambrian, and perhaps also for the Pleistocene/Holocene boundary. Recent suggestions to abandon chronostratigraphic terms (system, series, stage) in favor of geochronologic terms (period, epoch, age) are logically defensible, but could perpetuate the continuing confusion between various stratigraphic categories.