Lower Cretaceous to Eocene sedimentary transverse ridge at the Romanche Fracture Zone and the opening of the equatorial Atlantic

A transform-parallel (transverse) ridge runs for several hundred kilometers along the northern side of the Romanche Fracture Zone (RFZ), being most prominent opposite to the eastern Mid Atlantic Ridge/transform intersection (RTI). Seismic reflection profiles and rock sampling indicate that the western part of the transverse ridge is made of uplifted slivers of oceanic lithosphere. In contrast, the eastern part (east of the RTI) consists of a thick (>4 km) sedimentary succession affected by faulting and thrusting. We call it the Romanche Sedimentary Sequence (RSS). If this sedimentary sequence were underlain by oceanic crust, its predicted age should be not more than ∼60 Ma. However, dolomitized Maiolica-type pelagic limestones containing calpionellids of early Cretaceous age (∼140 Ma) were sampled from the RSS, in addition to Paleocene/Eocene quartzs and stones, siltstones, claystones, diatomites, and limestones. We present a description of the rock samples and a reconstruction of their environment of deposition and of their age. The RSS may represent material deposited during initial continent/continent transform motion in a narrow, deep, E–W elongated basin that communicated with the central Atlantic. The RSS was later subjected to transpression and uplift due to transform-related tectonics. Gabbros, but not basalts, were sampled from the igneous basement underlying the RSS. It is not clear whether the RSS overlies oceanic crust or some sort of pre-oceanic, northern Red Sea-type crust. The presence of lower Cretaceous pelagic deposits near the Romanche eastern RTI implies an age for the initial stage of the opening of the equatorial Atlantic older than the generally assumed Aptian/Albian.