Donnees nouvelles sur le stratotype du Berriasien; correlations entre magnetostratigraphie et biostratigraphie

A magnetic polarity time scale for the Early Cretaceous and Late Jurassic

“…If the later calpionellid zonation scheme of Remane [35] is used, the Valanginian/Berriasian boundary would fall in or just younger than M15. This revision is consistent with the results from the Berriasian stratotype section [20], where the boundary was in a covered interval above a negative magnetozone which may correspond to M15. In view of these observations we place this boundary just younger than M15 which gives it an estimated age of 131 Ma.…”

“…Although this correlation may be associated with a hiatus it agrees favourably with a correlation near M22 found in the magnetostratigraphy of DSDP site 534 [19]. Comparison with the magnetic polarity time scale suggests an age of 143 Ma for this boundary, reasonably close to the 141 Ma age proposed by Van Hinte [30] on the assumption of equal biozone durations.…”

“…However, a different correlation of the boundary close to M18 was found in limestones from the southern Alps [17] and in DSDP sediments from the northern Atlantic [18]. Investigations of the magnetic polarity sequence in the Berriasian type section in southern France [19,20] imply a Cretaceous/Jurassic boundary location older than that found in Umbria, but the stratotype section does not extend completely to the boundary. Discrepancies in the boundary correlation result in part from the use of different biostratigraphic schemes [21,22] and the lack of paleontological unity of the definition of the boundary.…”

The Jurassic-Cretaceous boundary: An age-old correlative enigma

“…Palynocysts in bed 148 at Berrias (grandis subzone) are the same as those in the Hard Cockle beds of Durlston Bay, assigned there to magnetochron M19n (Figure 4). Other species immediately beneath the lowest 'normal' chron recorded at Berrias (Galbrun et al, 1986), putatively M18 and in the grandis subzone, correlate with the Cypris freestones of Dorset (lower M19n). Palynocysts in bed 149 (subalpina subzone) at Berrias (no magnetochron assigned) correlate with the same in the Scallop bed at Durlston in M16n.…”

“…M18n was formerly the only 'normal' magnetozone identified in calpionellid zone B, then providing an approximation to a jacobi/grandis zone (Ogg et al, 1991: Channell & Grandesso, 1987. Initially numbering of chrons relied somewhat on Galbrun's original numbering at Berrias (Galbrun et al, 1986); as already mentioned a problematic section (and type area) to have as a standard. Given the nature of the sediments and apparent breaks in the grandis, subalpina and privasensis intervals there, it is perhaps not surprising that it is difficult to match its magnetostratigraphy conclusively with that in thicker and presumed more complete sequences, such as at Durlston or Bosso.…”