Debate on identification of the Mio-Pliocene boundary and onset of the MOW recorded in IODP Site U1387C in the Gulf of Cádiz.

Abstract

Integrated Ocean Discovery Program (IODP) recovered sediments at Site U1387C in the Gulf of Cádiz on the middle slope of the Albufeira Margin at a current water depth of around 550 m. One scientific purpose of Expedition 339 was to get a better understanding of the opening of the Gibraltar Gateway and the onset of Mediterranean Outflow water (MOW) after the Messinian Salinity Crisis (MSC). At the end of the Miocene, during the MSC, the Atlantic-Mediterranean exchange was severely restricted, so that a sea-level drawdown occurred of more than a kilometer in the Mediterranean. The Zanclean flood ended the MSC and caused a change in depositional settings in the Mediterranean, where the Mio-Pliocene boundary is defined. In this study, the Upper Miocene to Earliest Pliocene sediments are analyzed to pin-point the Mio-Pliocene boundary and to trace the onset of the MOW in recovered sediments from Site U1387C.

Sediments in the studied interval of the record are dominated by dark greenish grey nannofossil mud deposited in hemipelagic settings. Bio-, magneto- or cyclostratigraphic data are not conclusive for exact identification of the Mio-Pliocene boundary, however it is clear that the Mio-Pliocene boundary is recovered in this interval with two likely possibilities. Also, the distinct seismic facies change from transparent to stacked high amplitude laterally continuous does not give enough precision without downhole logs to pin-point the boundary.

The transition from the Mio- to Pliocene is visible in grain size and sedimentation rate increase, XRF regular alternation amplifications and benthic oxygen isotopes tend to be lighter. These changes throughout the record possibly indicate the onset of the MOW, where bigger grain sizes indicate higher flow strength and the combination of amplified variations and lighter oxygen isotopes point out the influence of the Mediterranean basin. Downslope processes are more pronounced in younger sediments and are indicated by lower benthic/planktic ratios. Turbidites or debrites are not recognizable throughout this interval, since bioturbation is clearly visible.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015