The first ⁴⁰Ar–³⁹Ar date from Oxfordian ammonite-calibrated volcanic layers (bentonites) as a tie-point for the Late Jurassic

Abstract

Eight volcanic ash layers, linked to large explosive events caused by subduction-related volcanism from the Vardar Ocean back-arc, interbedded with marine limestones and cherts, have been identified in the Rosso Ammonitico Veronese Formation (northeastern Italy). The thickest ash layer, attributed to the Gregoryceras transversarium ammonite Biozone (Oxfordian Stage), yields a precise and reliable 40 Ar-39 Ar date of 156.1 ± 0.89 Ma, which is in better agreement with GTS2004 boundaries than with the current GTS2012. This first biostratigraphically well-constrained Oxfordian date is proposed as a new radiometric tie-point to improve the Geologic Time Scale for the Late Jurassic, where ammonite-calibrated radiometric dates are particularly scarce.

Jurassic Monster Polar Shift Confirmed by Sequential Paleopoles From Adria, Promontory of Africa

“…For sections (or portions of sections) older than CM22, the chronostratigraphy of the sections (and related poles) was estimated using three key biostratigraphic events: first occurrence (FO) of Conusphaera mexicana minor , FO of Faviconus multicolumnatus , and last occurrence (LO) of Cyclagelosphaera wiedmanni , which have proven particularly useful to subdivide the Oxfordian‐Kimmeridgian interval in the Tethys realm, and two of which— C. mexicana minor FO and F. multicolumnatus FO—have been correlated to the M‐sequence at CM22 and CM25A, respectively, using magnetostratigraphy from the expanded S'Adde section from Sardinia (Muttoni et al, ; Figure ). The LO of C. wiedmanni is estimated at ~156 Ma and traced onto MHTC12 at CM30 (Figure ) based on Ar/Ar dating (156.1 ± 0.89 Ma) of an ash level at the top of the Rosso Ammonitico Medio unit at the Kaberlaba section (Pellenard et al, ) that was traced by means of lithostratigraphy to the nearby (1 km apart) Bombatierle section of this study, where the LO of C. wiedmanni is recorded also at the top of the Rosso Ammonitico Medio unit (Channell et al, ).…”

“…Chronology of Jurassic‐Early Cretaceous poles from Adria (acronyms as in Table ) derived from stratigraphic sections correlated to the M‐sequence marine magnetic anomalies of Malinverno et al (; MHTC12) by means of magnetostratigraphy (continuous solid lines) or correlated using key nannofossil events (dashed lines). These bioevents are the FO (=first occurrence) of C. mexicana minor and the FO of F. multicolumnatus , correlated to the MHTC12 sequence using magnetostratigraphy from the S'Adde section of Sardinia (Muttoni et al, ), and the LO (=last occurrence) of C. wiedmanni , estimated at ~156 Ma based on an Ar/Ar date (156.1 ± 0.89 Ma) on an ash layer at the top of the Rosso Ammonitico Medio at Kaberlaba (Pellenard et al, ), which is indicated. Also shown by gray bars are the age ranges of the 146.4 ± 1.4‐Ma Ithaca kimberlite pole (146Ik; Van Fossen & Kent, ), the 144 ± 5‐Ma Hinlopenstretet dikes pole (144Hi; Halvorsen, ), the 145.4 ± 1.4‐Ma Swartruggens‐Bumbeni kimberlites pole (145SB; Hargraves et al, ), and the 156.1 ± 1.6‐Ma Ontario kimberlites pole (156Ok; Kent et al, ).…”

Middle Jurassic age of basalts and the post-obduction sedimentary sequence in the Guevgueli Ophiolite Complex (Republic of Macedonia)

“…In the geologic time scale of Gradstein et al (2012), this age corresponds to the Late Oxfordian. If we consider the more recently documented radiometric age of 156.1 ± 0.89 Ma for the Middle Oxfordian Gregoryceras transversarium Ammonite Zone (Pellenard et al 2013), the emplacement age of the Fanos granite corresponds to the late Early or early Middle Oxfordian.…”

Calcite uranium–lead geochronology applied to hardground lithification and sequence boundary dating

“…The Jurassic is a period calibrated by only 17 radiometric ages (Geological Time Scale – GTS2012, Gradstein et al ., 2012), none of which come from marine series of the European domain where biostratigraphic calibration is taken as the reference (Gradstein et al ., 2012; Pellenard et al ., 2013). The majority of the ages come from continental strata and are only included as secondary guides in the GTS2012 (Gradstein et al ., 2012).…”