From Chronology to StratigraphyInterpreting the Lower and Middle Eocene Stratigraphic Record in the Atlantic Ocean

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Abstract
Whereas it is well established that on a broad scale the deep-sea stratigraphic record is punctuated by unconformities involving hiatuses as long as an epoch or more, it is generally believed that on a fine scale the deep-sea sedimentary sequences bounded by these large-scale gaps are essentially continuous. I have examined stratigraphic sections which span the lower/middle Eocene boundary at over 100 deep-sea sites in the North and South Atlantic Ocean. Integrated magnetobiochronology provides a framework for deciphering the presence of unconformities and for estimating hiatus duration. Based on nine temporal maps representing the lower and middle Eocene sedimentary record in different sectors of the Atlantic Ocean, I show that, even at a fine scale, the deep-sea record is punctuated by multiple unconformities. Continuous sequences representing 5- to 10-my intervals of geological time alternate with hiatuses of 1 to several million years. The distribution of the hiatuses presents an uneven pattern such that, in many regions — but not all— the early Eocene Epoch is well represented whereas the latest early Eocene and the early middle Eocene Epochs are not. In addition, hiatuses in the deep sea overlap with hiatuses on the shelves in contrast with reports by some authors for the Oligocene and the Miocene Epochs. Whereas a discussion of the mechanism(s) responsible for this sedimentary architecture is beyond the scope of the paper, these findings suggest that the overall sedimentary architecture is controlled by (a) fundamental mechanism(s) other than those (glacio-eustasy) applicable to the Neogene, and in particular to the Pleistocene Epoch. This study further suggests that accumulation rates of pelagic sediments remained relatively steady (rather than highly fluctuating) over intervals of several hundred thousand years to several million years. The discontinuities observed in the deep sea suggest that its architecture may be most appropriately described in terms of allostratigraphic units.

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