Silurian Time Scale Research Articles

High-precision U–Pb zircon age constraints on the duration of rapid biogeochemical events during the Ludlow Epoch (Silurian Period)

Precise determinations of the rates and durations of Palaeozoic biogeochemical events are largely unavailable. Here, we present two new high-precision U–Pb (zircon) dates from volcanic ash deposits from the Ludlow Series (Silurian System) of Podolia, Ukraine, that yielded weighted mean 206 Pb/ 238 U dates of 424.08 ± 0.20 (0.29) [0.53] Ma and 422.91 ± 0.07 (0.21) [0.49] Ma (analytical, tracer and total uncertainties). These new dates bracket the largest post-Cambrian global carbon cycle perturbation (Lau Excursion) and constrain the ‘Ludlow Rise’ in 87 Sr/ 86 Sr. These chronostratigraphically well-controlled dates improve the calibration of the Silurian time scale and provide the first determinations of the rates of biogeochemical change during the Ludlow Epoch. Supplementary material: U–Pb geochemical methods, data and CL imagery are available at http://www.geolsoc.org.uk/SUP18798 .

High-resolution, early Paleozoic (Ordovician-Silurian) time scales

For much of the geologic time scale, resolving power appears to be limited by the duration of biostratigraphic zones and subzones. Yet these zones exploit the appearance and disappearance events of only a small fraction of the species they span. Computer algorithms can sequence an order of magnitude more events and make explicit the uncertainties that arise when calibrating the resulting time line. An illustrative case history builds an Ordovician and Silurian time scale from the geologic record of the entire graptolite clade—over 1900 species from more than 400 localities and sections worldwide. The same approach can be applied to any stratigraphic interval with detailed biostratigraphic observations. It provides the foundation for time scales and paleobiologic time lines. Using logic similar to graphic correlation, optimizing algorithms search for a composite sequence of species first- and last-appearance events that minimizes the implied shortcomings of all the field observations. The algorithms minimize the number of species coexistences that are implied, but never observed, and the net adjustments needed to bring local range charts into agreement with a single composite sequence of events. After total section thicknesses have been rescaled to help normalize for variation in depositional rate, mean stratigraphic thicknesses are used to scale the intervals between adjacent events in the composite sequence. The resulting scaled composite sequence is converted to a relative geologic time scale by identifying stage and zone boundaries within the sequence of graptolite events. This relative scale is, in turn, calibrated by dated volcanic ash beds that were incorporated in the search for the optimal sequence. These dated events are also used to test for linearity of the scaled composite. The final time scale has a potential resolving power of 0.02–0.1 m.y., more than ten times better than can be achieved by traditional zones. Graptolite zones vary widely in duration from as short as 0.1 m.y. to nearly 5.0 m.y. The mean duration of zones or zonal groupings calibrated here is 1.44 m.y. in the Ordovician and 0.91 m.y. in the Silurian. The average uncertainty in locating zone boundaries in a single composite sequence is about one-fourth of the mean zone duration. The variance resulting from differences in time scales developed from differing numbers of field observations and in response to changes in the optimization criteria gives a very conservative measure of the overall robustness of the method. These differences indicate an average uncertainty in the age of graptolite zone boundaries that is more nearly equal to the mean zone duration. For zone duration, the mean uncertainty amounts to about one-half of the length of an average zone.

Implications of a new, biostratigraphically well-controlled, radio-isotopic age for the lower Telychian Stage of the Llandovery Series (Lower Silurian, Sweden)

Radio-isotopic analysis of single zircons from two early Telychian K-bentonites, one of which is among the most widespread Lower Paleozoic volcanic ash falls in northern Europe, yields overlapping weighted mean 206 Pb/ 238 U ages of 438.7 ± 1.0 Ma and 437.8 ± 0.5 Ma, respectively. The former age is from zircons of the Osmundsberg K-bentonite from the type locality at Osmundsberget in the Siljan area of central Sweden where it occurs in the lower part of the Spirograptus turriculatus Graptolite Zone and in the lower part of the Angochitina longicollis Chitinozoan Zone. Zircons giving the latter age are from a bed previously identified as the Osmundsberg K-bentonite at the Kallholn Quarry in the same area. Based on new biostratigraphic data, the latter bed is now considered to be slightly younger than the Osmundsberg K-bentonite. The dated stratigraphic level of the ash layers is slightly younger than the base of the Telychian Stage and thus represents a minimum age for the Aeronian/Telychian Stage boundary. A U-Pb age of > 438 Ma for the base of this stage, however, is older and in conflict with estimates in the most recent compilation of the Silurian time scale. In view of the fact that only three radio-isotopic dates from the entire Llandovery have been previously accepted, this new and biostratigraphically exceptionally well-controlled radio-isotopic date fills an important gap in the Lower Silurian geochronology.

Dating of graptolite zones by sedimentation rates: implications for rates of evolution

Preliminary determinations of ancient pelagic sedimentation rates agree with modern rates at about 4 meters per million years. By combining data on the thickness of graptolite zones from the North American Cordillera with data from other parts of the world, we have refined the Early Silurian time scale and obtained much better resolution than is possible for radiometric dates. The new Early Silurian time scale allows estimation of true rates of change in graptolite diversity. The Llandoverian diversity explosion is twice as rapid as was previously thought. The brevity of diversity lows and rapidity of speciation support modern theories of quantum evolution.

Comment and reply on ‘Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale’

Research Article| March 01, 1979 Comment and reply on ‘Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale’: COMMENT W. R. Church W. R. Church 1Department of Geology, University of Western Ontario, London, Ontario, N6A 5B9 Canada Search for other works by this author on: GSW Google Scholar Author and Article Information W. R. Church 1Department of Geology, University of Western Ontario, London, Ontario, N6A 5B9 Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1979) 7 (3): 114. https://doi.org/10.1130/0091-7613(1979)7<114a:CAROSO>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. R. Church; Comment and reply on ‘Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale’: COMMENT. Geology 1979;; 7 (3): 114. doi: https://doi.org/10.1130/0091-7613(1979)7<114a:CAROSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Comment and reply on ‘Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale’

Comment and reply on ‘Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale’

Subdivision of Ordovician and Silurian time scale using accumulation rates of graptolitic shale

Sections of graptolitic shale within the Cordilleran foldbelt are very condensed. An average accumulation rate of 4 ∓ 1 m/m.y. calculated for these shales falls within the range of 2 to 5 m/m.y. for modern deep-sea clay and siliceous ooze. The Deep Sea Drilling Project has shown that once an age is determined for a stratigraphic horizon within a pelagic section, it is possible to estimate the ages of other horizons by assuming constant rates of sediment accumulation. This method, used on graptolitic shale, suggests that the time span represented by a graptolite zone is proportional to its thickness. The thickest zone analyzed is that of Nemagraptus gracilis , which is calculated to have spanned about 8 m.y., whereas the thinnest zone, Glyptograptus persculptus , took less than 1 m.y. to accumulate. The readily recognized zones of Nemagraptus gracilis and Climacograptus bicomis , which together consistently form thick biostratigraphic intervals, are the zones most commonly found, whereas the G. persculptus zone is very rare. A new subdivision of the Ordovician and Early Silurian time scale has been made on the basis of the calculated time duration of graptolite zones.