Abstract
Five altered volcanic-ash beds have been correlated near the Cenomanian-Turonian stage boundary through much of the Western Interior Basin. These instantaneous events constitute independent chronostratigraphic marker-beds enabling the synchroneity of lithological, biological or geochemical records to be tested. In this way, the Greenhorn Sea is a unique place where the potentiality of this high-resolution stratigraphic tool is illustrated. The Cenomanian-Turonian interval is the ideal period for this sort of study because major oceanic changes, including the global expansion of the oxygen minimum zone, are recognised across OAE2 event, involving the disappearance of Rotalipora, complex keeled foraminifera which previously occupied deep oceanic waters.
Biostratigraphic data coupled with bentonite correlation, in several sections of Colorado, show the diachronism of the occurrences of R. cushmani and H. helvetica. Consequently, the extension of the W. archaeocretacea partial range zone is extremely variable at the regional scale. Thus, this association has a low temporal value. Planktonic foraminiferal analyses also reveal an A. multiloculata event in the M. mosbyense zone. During tens of thousands of years, this species, plentiful in the Western Interior Basin, multiplies and prevails on Rotalipora before a gradual decline. Rotalipora gave rise to Anaticinella by the atrophy of its keel. Thus, Anaticinella may stay in shallower habitats and avoid the expansion of the oxygen minimum zone. However, even if this adaptation allowed a return to the surface water, this selective advantage would not be enough for Anaticinella to survive the ecologically drastic modifications. Extinction of Anaticinella and its ancestor Rotalipora occurred contemporaneously.
Carbon stable isotope analyses show that main paleoceanographic events have occurred at Pueblo during the Cenomanian-Turonian stage boundary. The initial rapid increase and first peak of δ13C indicate the first anoxic event (event 1) before a decrease in values and a main manganese enrichment which are the evidence for a well-oxygenated environment (event 2). This assumption is supported by the occurrence of an abundant and diversified benthic community at the same interval. The second increase in δ13C proxies signs the rise of anoxia (event 3). The temporal distribution of these events with the bentonite marker beds and the ammonite biostratigraphy attest to the synchroneity of events 2 and 3 over 600 km between the Pueblo and Lohali Point sections. The absence of event 1 at Lohali Point in the S. gracile zone implies the existence of a hiatus.
Thus, thanks to the five bentonite marker beds, extending from the S. gracile to the M. nodosoides ammonite zone over 2 Ma, we have not only achieved precise regional correlation but, dealing with facies variations, our observations also indicate gaps in the sedimentary record. Thus, in detail, some thin levels expressed in Pueblo (reference section) cannot be correlated in any other eastern sections. Furthermore, supposed continuous sections may contain important hiatuses. An example is given at El Vado (New Mexico) where two bentonite marker beds are missing. In spite of the lack of significant planktonic foraminifera, the indications provided by bentonite geometries and by nannofossils reveal the existence of a hiatus of at least 850 kyr. It could be explained by the location of the area along the trend of a tectonic forebulge linked to the Sevier orogeny. This bathymetric high had great consequences on the currents flows in this large interior sea where tethysian and boreal water masses competed.
Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone
