Sequence stratigraphy: Eustasy or tectonic imprint?

Abstract

Sequence stratigraphy is viewed by many geologists as the expression in the stratigraphic record of the history of sea level changes. Sequence boundaries are its most distinctive elements and are used for global correlations. Until now, correlations between unconformities and postulated lowerings of sea level have been seen primarily in terms of cause and effect relationships, the postulated fall providing an explanation for the presence of an unconformity. They have not been truly perceived in terms of stratigraphic correlations which would imply the establishment of synchrony between the cause and the effect. To decipher the presence of a global sea level signal in the stratigraphic record, the synchrony of sea level related unconformities in widely separated basins must be established, which can be achieved by comparing the degree of overlap between hiatuses in different stratigraphic sections. Outcrop sections, wells, and the deep‐sea record reveal an apparently worldwide unconformity(ies) across the lower/middle Eocene boundary, characterized by intensive reworking and/or relatively rapid shallowing in some areas (e.g., Cyrenaica, NE Libya; California; Egypt). The lower‐middle Eocene intervals in sections on the North Atlantic margins have been correlated through magnetobiostratigraphy, and the durations of the hiatuses have been precisely determined. This record is compared with that in sections in other basins where shallowing and intensive reworking occur. This study shows that there are two groups of unconformities and associated hiatuses around the lower/middle Eocene boundary. The older hiatuses, in the latest early Eocene, are generally short, even on the shelf (∼1 m.y.); the younger hiatuses, in the early middle Eocene, are somewhat longer (>2 m.y.). The stratigraphic record around the lower/middle Eocene boundary suggests that there may be two mechanisms acting simultaneously to produce unconformities. Unconformities of a given age on the shelf may result from a global fall in sea level, while their stratigraphically correlative unconformities on the slope and rise may result from tectonic instability. Unconformities related to the third‐order cycles in sea level changes in Vail's model primarily may reflect tectonoeustasy.