LATE QUATERNARY TURBIDITE EMPLACEMENT ON THE HORSESHOE ABYSSAL-PLAIN (IBERIAN MARGIN)

The Horseshoe abyssal plain (HAP) is located on the Iberian margin at approximately 4800 m mater depth, and is confined within topographic e levations with a relief of about 3000 m, with the exception of Gorring e 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, num bered H-2, H-3, H-3, and H-13, contrasting with sets of thinner beds o f the order of decimeters thick (H-4,H-5,H-6,H-7 and H-9,H-10,H-11,H-1 2), and pelagites centimeters in thickness, An intensive study of eigh t piston cares, using visual observation (color and thickness), relati ve stratigraphic position of units, magnetic susceptibility logs, calc ium carbonate content, and mineralogy of turbidite bases, concludes in a bed-by-bed correlation of all individual turbidites, The major sour ce of terrigenous material feeding the BAP is the Sao Vicente canyon, which incises the Portuguese shelf, while minor sources are the surrou nding seamounts. The elongate geometry of the abyssal plain with its s ingle dominant source of sediments produced laterally continuous depos its from both large and small flows, These covered the entire 356 km l ength of the plain, Grain-size analysis of the four thickest turbidite s (H-2, H-3, H-3, and H-9) demonstrates only slight downcurrent fillin g 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 t he plain, The thicker units all show the same pattern of thickness, wi th a maximum in the middle of the plain around a topographic constrict ion and bend, This is most plausibly explained as due to reduction in flow speed of an initially supercritical flow causing enhanced deposit ion, 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 t hat both thick (similar to 3 m) and thin (similar to 0.3 m) beds are d ue to supercritical hows a few tens of meters high, However, the thick beds resulted from high-concentration hows (C-v similar to 4% by volu me) whereas thinner beds require low concentration (C-v < 1%) to run o ut over the full length of the basin. The stratigraphy is tied into th e dated oceanic pelagic record by analysis of foraminifera in the pela gic layers above the turbidites and through recognition of two Heinric h layers (H-1 and H-2, ages 14.3 and 21 ka), The resulting age framewo rk shows higher turbidite frequency in the glacial (2.7/kyr) than inte rglacial (Holocene) (1.0/kyr), This also gives higher mass flux during the glacial, Emplacement of turbidites cannot be clearly related to s ea-level changes but may well be due to seismic activity. However, one of the largest earthquakes in human experience (Lisbon in 1755) trigg ered only a thin turbidite, invalidating the term ''seismite'' for thi ck turbidites.