HIGH-RESOLUTION VARIABILITY OF STRATIGRAPHIC RECORDS CONSTRAINED BY VOLCANIC ASHES THE CENOMANIAN-TURONIAN BOUNDARY IN THE WESTERN INTERIOR BASIN

Stratigraphic records are commonly used as tools for correlations at various scales. That clearly supposes a synchronism of these records, generally justified by their similar evolutions. Nevertheless, independent temporal validation seems to be essential. In this way, the Cenomanian-Turonian boundary saw deep changes as geochemical accidents (d¹³C, Mn and ⁸⁷Sr/⁸⁶Sr) or biological stress.

Late Cretaceous strata of the Western Interior Basin contain many bentonites. Instantaneous events on the geological time scale, these faded volcanic ashes constitute widespread independent time lines allowing to test the synchronism of the stratigraphic records and to underline space and environmental modulations. Moreover, this seaway is an unique place where tethysian and boreal water masses competed and where we can precisely studied the properties of those two paleogeographic domains.

A preliminary mission permitted the study of sections on a transect from the western edge of the basin (Arizona) to this eastern stable margin (Kansas) while passing to its center. Thanks to five major bentonite marker beds, extending from the S. gracile to the M. nodosoides zone, nearly over 2 Myr, our observations have not only confirmed precise regional correlations but also indicate gaps in the sedimentary record. Thus, we didn’t found in Pueblo, "world reference section", some levels expressed in other sections. In the same way, at El Vado (New Mexico), we see important vertically facies variations. Whereas the lack of significant planktonic foraminifera, the indications provided by the bentonite marker beds and by nannoflore reveal a hiatus of at least 300 kyr. It could be explained by a forebulge zone due to the Sevier Orogeny. We have to study other sections in this region for validate this assumption. Thus, we could be able to determinate the paleogeographic extension of the forebulge, to apprehend the influence of the paleomorphology on the current’s flow and to specify the impact on water chemistry and on life.

A study carried out on planktonic foraminifera leads to discuss W. archeocretacea partial range zone duration. Indeed, this ecozone seem to be reduced because of R. cushmani persistence. Furthermore, by connecting Pueblo and Graneros sections, distant of about fifty kilometers only, the data relating to the microfauna reveal diachronisms in the occurrences of several taxa.