Sedimentation on Feni and Gardar Sediment Drifts

Abstract

Feni and Gardar ridges are two major North Atlantic sediment drifts-positive sedimentary accumulations over 1600 m thick presumed to result from sediment distribution by bottom water circulation since Eocene-Oligocene time. At Deep Sea Drilling Project Sites 610 and 611 we penetrated the Miocene to Holocene sequences of these drifts, and we looked for depositional changes across sediment wave fields on their surfaces with a series of offset holes. The sediment waves, over 20 m in height and over 2 km in wavelength, are morphologically more complex than hitherto recognized. Around the drill sites, waves characterize the upper 100 to 200 m of seismic sections, but we found no evidence of wave migration in these records. Lithologies in the sediment waves are fundamentally pelagic. Few current-produced sedimentary structures or hiatuses were found. Glacial cycles allow detailed wave-to-trough correlations, which show that since 2.4 Ma, little or no wave migration could have occurred. Sedimentation rate differences between wave and trough locations suggest that wave migration did occur in the Pliocene at the Gardar Ridge site. A review of possible sedimentary processes and parameters indicates that internal waves in a stratified water column might be important for the initiation and migration of sediment waves. Changes in sedimentary regime on the drifts seem to be linked to interaction between bottom water masses from northern (Norwegian Sea Overflow Water-NSOW) and southern (Antarctic Bottom Water-AABW; and North Atlantic Deep Water-NADW) sources and not to changes in intensity of NSOW alone. Regional seismic reflectors drilled at the Feni Ridge site are linked to oceanographic changes: "R2" of early Miocene age appears to signify a silica productivity event; "Rl" of late Miocene age could be linked to the isolation of the Mediterranean, which may have affected the NADW slope current. Our interpretation of the sedimentary history of the drifts is that during the preglacial period NSOW was probably the dominant bottom water mass over the drifts and the sediment waves were actively migrating. We suggest that the influence of NSOW lessened with the onset of the glacial-interglacial periods. Since that time, bottom currents have simply "maintained" the morphology of the now largely relict wave fields.

Introduction

“…Waveforms observed at DSDP Sites 610 and 611 were found to be surprisingly stable, migrating on a million-year time scale (Kidd and Hill, 1987). However, that study sampled wave crests and troughs, not wave flanks.…”

Site 884

“…The resulting overview of drift deposition provided by these and prior drilling legs shows that the deposits are fundamentally pelagic in bulk composition, consist of nannofossils and foraminifers, are without sedimentary structures indicative of current-controlled deposition, and have high and relatively uniform sedimentation rates. Tracing the reflector surfaces underlying the drift deposits to drill holes where they could be dated made it possible to determine that deposition of drift deposits in the North Atlantic began at the time of the Eocene/Oligocene boundary (Kidd and Hill, 1987). The thick sediment deposit on the northeast side of the Obruchev Swell was first noted by Ewing et al (1968) in their general review of Pacific Ocean sediment thicknesses.…”

“…in the Miocene and Pliocene to 100 m/m.y. in the Quaternary (Ruddiman, Kidd, et al, 1987;Kidd and Hill, 1987). The resulting overview of drift deposition provided by these and prior drilling legs shows that the deposits are fundamentally pelagic in bulk composition, consist of nannofossils and foraminifers, are without sedimentary structures indicative of current-controlled deposition, and have high and relatively uniform sedimentation rates.…”

“…Drift sediments are almost always homogeneous and bioturbated and rarely display the sedimentary structures commonly associated with deep currents such as parallel laminae, sharp contacts, and scour marks or cross-bedding. Rates of deposition, often several tens of meters per million years, depend on supply and are noticeably higher in the drift deposit than in nearby deposits that have been accumulating under purely pelagic conditions (Kidd and Hill, 1987;Pickering et al, 1989).…”

“…The deposit is early Miocene in age at a depth of 723 mbsf, giving a sedimentation rate of more than 40 m/m.y. Kidd and Hill, 1987). Drilling at Site 611 encountered the Gardar Drift, a deposit 1000 km long, 130 km wide, and 1450 m thick.…”