Abstract

The Horseshoe abyssal plain (HAP) is located on the Iberian margin at approximately 4800 m water depth, and is confined within topographic elevations with a relief of about 3000 m, with the exception of Gorringe Bank, which rises to only 20 m below sealevel. The plain is built up by an alternation of turbidites of the order of meter-thick beds, numbered H 2 , H 3 , H 8 , and H 13 , contrasting with sets of thinner beds of the order of decimeters thick (H (sub 4,5,6,7) and H (sub 9,10,11,12) ), and pelagites centimeters in thickness. An intensive study of eight piston cores, using visual observation (color and thickness), relative stratigraphic position of units, magnetic susceptibility logs, calcium carbonate content, and mineralogy of turbidite bases, concludes in a bed-by-bed correlation of all individual turbidites. The major source of terrigenous material feeding the HAP is the Sao Vicente canyon, which incises the Portuguese shelf, while minor sources are the surrounding seamounts. The elongate geometry of the abyssal plain with its single dominant source of sediments produced laterally continuous deposits from both large and small flows. These covered the entire 356 km length of the plain. Grain-size analysis of the four thickest turbidites (H 2 , H 3 , H 8 , and H 9 ) demonstrates only slight downcurrent fining and vertical grading, expressed best in the ratio of coarse silt to fine silt plus clay. A small amount of sand is carried the length of the plain. The thicker units all show the same pattern of thickness, with a maximum in the middle of the plain around a topographic constriction and bend. This is most plausibly explained as due to reduction in flow speed of an initially supercritical flow causing enhanced deposition. Some of the beds appear to have a double coarse layer in the base, which may indicate partial reflection of flows from the side of the basin. It is suggested by application of equations for flow behavior that both thick ( nearly equal 3 m) and thin ( nearly equal 0.3 m) beds are due to supercritical flows a few tens of meters high. However, the thick beds resulted from high-concentration flows ( C v nearly equal 4% by volume) whereas thinner beds require low concentration ( C v < 1%) to run out over the full length of the basin. The stratigraphy is tied into the dated oceanic pelagic record by analysis of foraminifera in the pelagic layers above the turbidites and through recognition of two Heinrich layers (H-1 and H-2, ages 14.3 and 21 ka). The resulting age framework shows higher turbidite frequency in the glacial (2.7/kyr) than interglacial (Holocene) (1.0/kyr). This also gives higher mass flux during the glacial. Emplacement of turbidites cannot be clearly related to sea-level changes but may well be due to seismic activity. However, one of the largest earthquakes in human experience (Lisbon in 1755) triggered only a thin turbidite, invalidating the term "seismite" for thick turbidites.

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