Northwest African Continental Margin: History of sediment accumulation, landslide deposits, and hiatuses as revealed by drilling the Madeira Abyssal Plain

Abstract

ODP drill sites in the Madeira Abyssal Plain reveal sequences of organic‐rich turbidites derived from the northwest African margin, in which each turbidite has a volume of tens to hundreds of cubic kilometers. The frequency of turbidite emplacement has been combined with core and seismic data to show the volume of redeposited sediment. The basin began to fill about 22 Ma with numerous small turbidites, up to 100 per million years, each with volumes of a few cubic kilometers. The total volume of turbidites deposited increased between 16 and 11 Ma, as did their individual volumes, and then declined to 7 Ma. At 7 Ma, there was a dramatic increase in the amount of turbidite input to 768 km3/Myr and a rise in the average volume of each unit to 59 km3. These high values have been maintained to the present day. The variations in the amount of redeposited sediment most likely reflect the rates of sedimentation on the northwest African margin since high sedimentation leads to oversteepening of the slopes and eventual mass wasting. The dramatic changes at about 7 Ma may be due to a large increase in upwelling off northwest Africa caused by circulation changes associated with increased glaciation of the poles. Up to 20% of sediment may be remobilized by landslides, with each event leaving a hiatus. Each of these hiatuses extends over an average area of ∼4800 km2 and represents removal of sediment layers several tens of meters thick and of several hundred thousand years duration.

Turbidite record of frequency and source of large volume (>100 km3) Canary Island landslides in the last 1.5 Ma: Implications for landslide triggers and geohazards

“… Turbidite volumes are decompacted volumes calculated from ODP core based on the method of Weaver []. ?…”

“…Volumetrics of the deposits were calculated from the ODP core using the method of Alibés et al . [] and Weaver [].…”

Catastrophic events (hurricanes, tsunami and others) and their sedimentary records: Introductory notes and new concepts for shallow water deposits

“…Attempts to estimate paleo-intensities of recorded earthquakes have been proposed, based on sizes and volumes of remobilized sediments (Séguret et al, 1984;Hibsch et al, 1994;Rodriguez-Pascua et al, 2003). In addition, time-distributions of paleo-earthquakes are proposed for long periods (in late Quaternary or older accumulations) either in marine or in lacustrine settings (Sims, 1975;Adams, 1990;Marco and Agnon, 1995;Beck et al, 1996;Gorsline et al, 2000;Weidlich and Bernecker, 2004).…”

“…Off the African Mediterranean margin, El-Rabini (1995) discussed slumps generated by earthquake shocks. Over larger time scales, Weaver (2003) has discussed the interplay of slides and sediment accumulation in the Madeira Abyssal Plain. Although flood-generated turbidites generally have orders of magnitude smaller volumes, Normark and Reid (2003) have examined turbidites in the northeastern Pacific floor that were the products of the giant glacial lake (Lake Missoula) outbursts during the Late Pleistocene.…”

Frequent failure of the continental slope: The Gela Basin (Sicily Channel)

“…[23] The estimated value of 3 -4 ka is 2 orders of magnitude higher than the best estimates of failure recurrence from oceanic margins [e.g., Weaver, 2003], and it is comparable to failure frequency from alpine lakes [Schnellmann et al, 2002] and preliminary estimates of failure recurrence in Alaskan fjords (five major failures in the last 5 -7 ka (H. J. Lee, personal communication, 2009).…”