Biostratigraphy and paleoenvironment of foraminiferal assemblages from neogene and quarternary deep-sea sediments collected during the nautimate cruise (Pacific Ocean, off Mexico, 18°N-21°N). Geodynamic implications

Abstract

Deep-sea dives with the submersible Nautile allow to reconstruct the lithostratigraphic series, and to collect samples of plutonic rocks and sediments along the inner wall of the Acapulco trench, off Mexico (Manzanillo area, 18°N-19°N, Pacific Ocean). Plutonic rocks are unconformably overlain by brown and grey sandstones and siltstones, constituting a continuous sequence of sediments, several hundreds meters thick. Assemblages of planktonic foraminifera give a late Miocene-early Pliocene age for the lowest part of the sedimentary sequence. Benthic foraminifera indicate that the minimum depth of deposition was located into the middle bathyal zone. Characteristic species are the following: Bolivina foraminata, Bulimina mexicana, Galliherina avigerinaformis, Gyroidina healdi, Hansenisca rotundimargo, Nodogenerina lepidula, Parafrondicularia miocenica, Fontbotia wuellerstorfi, Plectofrondicularia californica, Protoglobobulimina pseudotorta, Pyrgo murrhina, Siphonodosaria advena. These assemblages are dominated by endofaunic species (bolivinids, buliminids, uvigerinids and cassidulinids) and oxygen-minimum zone indicators (Buliminella subfusiformis, Epistominella smithi, Uvigerina subperegrina). These late Miocene-early Pliocene sediments were probably deposited into the oxygen-minimum zone, at the boundary between the lower and upper parts of the middle bathyal zone, corresponding approximately to a depth of 1000 m. Their present position (between 2500 and 4000 m) reveals a strong and rapid subsidence of this area (between 1500 and 3000 m) since the early Pliocene. These stratigraphic and paleoenvironmental results indicate the importance of these stepwise downward movements of this zone since Eocene. An additional study of late Miocene-Quarternary foraminiferal sediments collected during deep-sea dives conducted to the North (South of Tres Marias Islands, 20°N-21°N) gives complementary data on the stratigraphy and paleoenvironment of these deposits. It completes the understanding of the complex geodynamic history affecting the western boundary of the North America plate.