Channel-levee systems in submarine fans

CHANNEL-LEVEE SYSTEMS IN SUBMARINE FANS
WALKER, Roger G., Dept. of Geology, McMaster Univ., Hamilton, ON L8S 4M1, Canada. walkerr@mcmail.cis.mcmaster.ca
Modern fans can bebroadly subdivided into 1) a lower sand-rich part commonly termed a lowstand, or basin floor fan, and 2) an upper part that consists of a finer-grained channel-levee complexcomprising many individual channel-levee systems. Few channel-levee systems, of the same scale as the modern ones, have been described from the geological record.
Within the mainchannel or canyon there are commonly smaller thalweg channels, as shown in the 4-km wide Eocene Regencia Canyon, subsurface of Brazil. In the Lagoa Parda oilfield, 38channel-fills have been mapped. They are separated by overbank mudstones, and can be grouped upward in response to a long term Eocene rise of relative sea level. The sandstones have beenextensively cored, but not the levee facies.
In general, levee facies are characterized by thin-bedded turbidites, many of which may be lenticular as they pinch out against thelevee. The beds may contain climbing ripples, convolute lamination, scouring and presence of ripped-up mudstone clasts, and several beds may slide a short distance to produce asmall-scale slump. Behind the levees there may also be much larger slumped deposits (tens of metres thick), as in the modern Amazon, Mississippi and Rhone Fans. All of thesefeatures are superbly displayed in the Eocene Waitemata Group at Whangaparaoa Head, New Zealand. The thickness of slumped deposits exceeds 50 m (the height of the cliffs), the bedsinvolved are mostly thinner than about 50 cm and contain the structures noted above. Amalgamated sandstone beds up to 30 m thick represent associated channel deposits.