The Evolution of the Mazagan Continental Margin: A Synthesis of Geophysical and Geological Data with Results of Drilling during Deep Sea Drilling Project Leg 79

Abstract

Combining the results of drilling during Leg 79 with other on-and offshore regional geological and geophysical data, we infer the following history for the evolution of the Mazagan sector of the Northwest African margin:1. Rifting began in the Triassic, by crustal thinning associated with listric faulting, creating a series of basins and intervening highs. The basins received continental clastic sediments from erosion of the granitic highs. By the close of Triassic time, halite with minor potash was being deposited in shallow marine salt pans across a rift-valley system, perhaps a little below global sea level, stretching from the base of the Mazagan margin (Site 546), to the base of the Nova Scotia margin, some 150-200 km northwestward.2. Marine waters quickly drowned the basin to a depth of few hundred meters during the Early Jurassic, by some combination of subsidence (perhaps w ith faulting) or basin flooding, and the shoreline advanced eastward, probably to a position not far west of Site 545. During the later part of the Early Jurassic and during Middle Jurassic time, shallowwater carbonate banks occupied parts of the fault block near Site 544 and supplied debris downslope to the area of Site 547, where hemipelagic radiolarian marls were being deposited in a slowly subsiding basin a few hundred meters deep, intermittently poorly ventilated.3. At about the end of the Bathonian, as the earliest seafloor spreading began in the central Atlantic, faulting and slightly accelerated subsidence-perhaps accompanying renewed crustal stretching-recommenced, this time especially affecting the more proximal parts of the margin near Site 545, creating the main structural relief of the Mazagan Escarpment. This faulting probably reached its maximum in Late Jurassic time, and was accompanied or immediately followed by a great transgression-the Atlantic transgression. Oxfordian carbonate reefs established themselves along the edge of the Mazagan Plateau and contributed reef talus to the slopes below. An isolated reef perched atop the fault block at Site 544 and shallow-water debris tumbled down the slopes to Site 547.4. Subsidence continued during Early Cretaceous times, but no sediments of this age were deposited (permanently) on the Mazagan Slope, perhaps because most terrigenous supplies were blocked by a submarine swell over an old granitic fault block about 15 km southeast of the Plateau edge. Huge quantities of terrigenous sand were delivered to the offshore Moroccan Basin by turbidity currents flowing down the continental slopes south of Mazagan.5. In Aptian time hemipelagic sediments (clayey nannofossil ooze) began depositing rapidly on the Mazagan Slope and accumulation continued through the Albian and most of the Cenomanian. The Slope sediments often slumped or were transported farther downslope in debris flows. Site 544, now a deep bank, stood clear of sediments.6. After a period of erosion and/or nondeposition in Turonian-Santonian time, debris flows were active again on the Slope during Campanian and Mae...

Early Jurassic North Atlantic sea‐surface temperatures from TEX86 palaeothermometry

“…The basal portion of this interval (932·5 to 891·0 mbsf) is a complex mixture of lithologies including mudstones, nodular limestones, limestones and dolomitic breccias. At the base (Hole 547B, cores 23 and 24) a laminated micrite resembling a stromatolite and breccias of stromatolite occur that could represent tidal‐flat deposits (Winterer & Hinz, ). The overlying sediments were interpreted as being deposited in a pelagic, marine environment (radiolaria and calcareous nannofossils are present) with water depths of ca 100 m and influenced by down‐slope transport (Winterer & Hinz, ).…”

“…At the base (Hole 547B, cores 23 and 24) a laminated micrite resembling a stromatolite and breccias of stromatolite occur that could represent tidal‐flat deposits (Winterer & Hinz, ). The overlying sediments were interpreted as being deposited in a pelagic, marine environment (radiolaria and calcareous nannofossils are present) with water depths of ca 100 m and influenced by down‐slope transport (Winterer & Hinz, ). Between 891 and 846 mbsf, the sediments are interbedded claystones and nodular micritic limestones that were deposited in a pelagic environment, with less influence from downslope transport.…”

Mesozoic radiolarites – accumulation as a function of sea surface fertility on Tethyan margins and in ocean basins

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The sedimentary expression of oceanic anoxic event 1b in the North Atlantic

“…, 1984) and is separated from the Seine Abyssal Plain by the almost 3000 m high Mazagan Escarpment. It constitutes the outer edge of an extensional passive margin formed by extension in the Triassic along west‐dipping listric faults, which seem to have been active well into the Jurassic (Winterer & Hinz, 1984). The block of this passive margin system that was closest to land may have formed the foundation of the platform margin while more seaward blocks drowned and were covered by hemipelagic and pelagic sediments in the progressively deepening margin.…”