Palaeoceanographic setting of the Callovian North Atlantic

Abstract

Summary DSDP/IPOD Site 534 located in the Blake Bahama Basin adjacent to the Florida coast off eastern USA exemplifies the early opening stages of the North Atlantic in Callovian (mid-Jurassic) time. The earliest sediments accumulated on a rugged ocean floor located near the edge of a WNW-ESE-trending fracture zone, not far from the spreading axis. At that time the ocean was c. 500 km wide with a borderland experiencing a tropical climate. Input of both inorganic and organic material was mostly of marine origin, but also included quartz, illite, chlorite and large quantities of fine plant material. Carbonate banks flourished during a time of raised sea-level, shedding large quantities of sediment, mostly peri-platform ooze, which was redeposited by turbidity currents to Site 534 after a marginal ridge (Blake Spur ridge) was finally breached during later Callovian time. The site subsided c. 300 m during the Callovian and for a time lay below the CCD which was still shallower than 3000 m. Distinctive hummocky seismic reflections and sedimentary structures in the cores suggest that sluggish bottom water circulation existed, giving rise to sediment-drifts (‘muddy contourites’). Hydrothermal precipitates drifted westwards from the ridge. Inferred trade wind patterns favoured extensive coastal upwelling along the African borderland, the closest margin to Site 534 during the Callovian. The favoured origin of numerous, but volumetrically minor, black shale intervals is that marine organic matter was concentrated where the oxygen-minimum-zone intersected the shelf-continental rise and that this material was subsequently redeposited by currents and/or gravity flows to form layers which readily became black and de-oxygenated during diagenesis.