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Barremian to lower Aptian interbedded pelagic and clastic limestones cored in the western Mid-Pacific Mountains (Deep Sea Drilling Project Site 463) show strong affinities with coeval deposits from a small Cretaceous basin, surrounded by "Urgonian" carbonate platforms, on the northern margin of the Tethys. We suggest that the depositional environments were nearly the same: a saddle on a structural high (Pacific) or a marginal basin (Tethys), both moderately deep, where pelagic muds were sporadically troubled by avalanches of bioclastic and oolitic sands, besides the usual mass flows or slides that involve deeper-water muds. So, it is indirectly proved that the guyots of the western MidPacific Mountains belonged to the world-encircling "Urgonian belt" as early as Barremian time, like many regions of the Tethyan domain. In and around the Vocontian Trough (French subalpine ranges), there is a good correlation between the occurrence and disappearance of bioclastic turbidites and, respectively, the subsidence and the drowning of the carbonate banks. Thus, the disappearance of carbonate elastics at Site 463 during the early Aptian suggests that the western guyots sank at that time, as did the Urgonian platforms in the French subalpine ranges. The limestones cored at Site 463 show that lithification in coarse-grained carbonates probably can be achieved (with slight residual porosity) without emersion, producing the diagenetic features encountered in exposed calcarenites of the Vocontian domain. Most astonishing are the cycles found especially in Aptian to Albian pelagic limestones of Site 463, which are like cycles known at some Atlantic sites and in the Vocontian Basin, where these alternations are under study. We think these cycles are related to climatic changes leading to fluctuations in planktonic productivity (with changes in nannofossil and even in micro fossil assemblages), and to fluctuations in the influx of clay. Calculations both for Site 463 and the Vocontian Trough show that the deposition time of a single couplet spans a time (about 10,000-50,000 years) consistent both with the calculations of Dean et al. (1978) for similar Oligocene chalk-marl cycles of the Atlantic and the duration of isotopic stages of the Pleistocene.

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The Early Cretaceous Environment at Deep Sea Drilling Project Site 463 (Mid-Pacific Mountains), with Reference to the Vocontian Trough (French Subalpine Ranges)