Facies distribution and textural variation in Faro Drift contourites: Velocity fluctuation and drift growth

Abstract

Detailed study of seismic profiles, piston cores and bottom photographs from the Faro Drift on the southern margin of Portugal has led to a better understanding of drift development and its relationship to bottom current circulation. Data on the contourite facies characteristics and the surface microphysiography have been published elsewhere; here, we concentrate on sediment distribution and geometry. Longitudinal trends in facies types, mean grain size, sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the Drift. These are generally more intense in the marginal channels and at the upstream or eastern end of the drift. Three different scales of vertical variation of facies can be identified. At the large scale, 300–500 m of sediment has accumulated over 4–6 Ma in a regular vertical succession due to the northward progradation of the Drift. At the medium scale, the upper 20–30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea-level stands respectively over the past 0.3 Ma. At the small scale, the topmost 2–3 m of sediment deposited in approximately 0.03 Ma shows three zones of coarser-grained sediments separated by finer-grained contourites. This sequence can also be interpreted in terms of long-term fluctuation in bottom current activity. Although the signal is clearly complex, this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns.