Contourite depositional system after the exit of a strait: Case study from the late Miocene South Rifian Corridor, Morocco

Idealized facies of bottom current deposits (contourites) have been established for fine-grained contourite drifts in modern deep-marine sedimentary environments. Their equivalent facies in the ancient record however are only scarcely recognized due to the weathered nature of most fine-grained deposits in outcrop. Facies related to the erosional elements (i.e. contourite channels) of contourite depositional systems have not yet been properly established and related deposits in outcrop appear non-existent. To better understand the sedimentary facies and facies sequences of contourites, the upper Miocene contourite depositional systems of the South Rifian Corridor (Morocco) is investigated. This contourite depositional system formed by the dense palaeo-Mediterranean Outflow Water. Foraminifera assemblages were used for age-constraints (7.51 to 7.35 Ma) and to determine the continental slope depositional domains. Nine sedimentary facies have been recognized based on lithology, grain-size, sedimentary structures and biogenic structures. These facies were subsequently grouped into five facies associations related to the main interpreted depositional processes (hemipelagic settling, contour currents and gravity flows). The vertical sedimentary facies succession records the tectonically induced, southward migration of the contourite depositional systems and the intermittent behaviour of the palaeo-Mediterranean Outflow Water, which is mainly driven by precession and millennial-scale climate variations. Tides substantially modulated the palaeo-Mediterranean Outflow Water on a sub-annual scale. This work shows exceptional examples of muddy and sandy contourite deposits in outcrop by which a facies distribution model from the proximal continental slope, the contourite channel to its adjacent contourite drift, is proposed. This model serves as a reference for contourite recognition both in modern environments and the ancient record. Furthermore, by establishing the hydrodynamics of overflow behaviour a framework is provided that improves process-based interpretation of deep-water bottom current deposits

Radiolarian contourites record Eocene AABW circulation in the equatorial East Atlantic

50 m of Middle Eocene pure radiolarian ooze were drilled at ODP Site 660 in the equatorial East Atlantic, 80 km northeast of the Kane Gap. The oozes comprise a 10 m high and 2 km broad mound of seismic reverberations, covered by manganese-rich sediment, and contain trace amounts of sponge spicules and diatoms, negligible organic carbon (0.15%), clay, and variable amounts of pyrite. The yellow to pale brown silty sediments are relatively coarse-grained (30–45% coarser than 6 μm), little bioturbated, and commonly massive or laminated on a cm-scale. The unlithified radiolarian ooze may indicate an interval of high oceanic productivity, probably linked to a palaeoposition of Site 660 close to the equatorial upwelling belt during Middle Eocene time. The absence of organic matter, however, and both the laminated bedding and the mound-like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen-rich deep water, which passed through the Kane Gap, winnowed the fine clay fraction, and prevented the preservation of organic carbon. The ooze may be either a contourite-lag deposit, or a contourite accumulation of displaced radiolarians, originating south of the Kane Gap and being deposited in its northern lee, thus documenting the passage of a strong cross-equatorial bottom-water current formed near Antarctica. These Eocene contourites may be an analogue for ancient radiolarites in the Tethyan Ocean

Distribution of contourite drifts on convergent margins: Examples from the Hikurangi subduction margin of New Zealand

Contourite drift systems form a significant component of the marine clastic sedimentary record. Although contourites form in all tectonic settings, few studies have described their development along convergent margins; such characterization is needed to underpin oceanographic and palaeoenvironmental studies in active settings. This study is the first to document contourite drift development along the Hikurangi subduction margin of New Zealand. Integration of bathymetric, seismic and well data enables five classes of drift to be recognized around the subduction wedge, occurring in three principal associations: (i) an upper slope drift association of giant elongate mounded (ca 150 km long, 50 km wide and up to 1100 m thick) and plastered drifts (ca 300 km long, 8 km wide and <600 m thick), which occurs upon and inboard of a major intrabasinal thrust‐cored high, whose long axis parallels the coast; shallow bottom currents disperse sub‐parallel to this axis; (ii) a spatiotemporally discontinuous association of confined and mounded hybrid drifts (ca 500 m long, <2 km wide and up to 500 m thick) that occurs along the mid‐to‐outer slope domain of the wedge, recording the interaction of along‐slope and downslope currents within trench‐slope basins; and (iii) a trench fill assemblage that implies the passage of abyssal bottom currents across a 40 km reach of the trench‐axial Hikurangi Channel‐levée, with associated modification of the channel form and of overbank sediment waves. The fundamental presence of contourites along this margin appears to depend on the orientation and strength of oceanographic bottom currents. However, drift type and evolution vary depending on the slope gradient and the presence of irregular seafloor topography created by tectonic structures. The documented drifts are generally smaller, less continuous, and develop more intermittently than similar styles of drifts documented on passive margins; this mode of occurrence may be characteristic of contourite development on convergent margins