40. SEDIMENTARY FACIES AND ASSOCIATED DEPOSITIONAL ELEMENTS OF THE AMAZON FAN1William R. Normark,2 John E. Damuth,3 and the Leg 155 Sedimentology Group4 |
Fifteen sedimentary facies are recognized in the 4 km of sediment cored at the 17 Leg 155 drill sites on the Amazon Fan. The Amazon Channel, the most recently active channel-levee system, was the primary focus of drilling (10 of the 17 sites) and provides the basis for the association of specific facies with levee, channel, and lobe elements of the fan. Twelve of the facies are terrigenous sediments and comprise the majority of the sections recovered from the four major submarine fan elements: (1) levee/overbank deposits; (2) channel-fill deposits (high amplitude reflection [HAR] units); (3) depositional lobe and crevasse splay deposits (high amplitude reflection packet [HARP] units); and (d) mass-transport deposits (MTDs). Levee systems and their overbank deposits are composed predominantly of six mud and silt facies that are differentiated by internal structure (e.g., cross-stratification), bed thickness, grain-size variations within silt beds, interbedded silt and mud, and clayey sediment. A seventh facies composed of organized sand beds is less common in levee deposits. In contrast, this organized sand facies, plus two other, more prevalent sand and gravel facies, form thick-bedded sand and gravel units in channel-fill deposits (HAR units), laterally extensive deposits at the bases of levee systems (HARP units), and lobe deposits on the lower fan. When the fan is active during glacial lowstands, the channel-levee systems (e.g., Amazon Channel) rapidly advance downfan as much as 20 m/yr prograding over coeval lobe deposits formed at the mouth of the advancing channel-levee system. This progradation rate is about three orders of magnitude greater than the average sediment aggradation rate on levee crests, as shown by stratigraphic correlation studies presented elsewhere in this volume. Consequently, even levees of relatively low relief on the lower fan are effective in confining sandy turbidity and related gravity-controlled flows within the active channel until the lobe area beyond the channel mouth is reached. Three distinctive muddy facies (including deformed mud with contorted or folded beds, sheared zones, and mud clasts; disorganized mud with scattered pebbles and granules; and homogeneous, structureless mud) characterize thick, volumetrically important, MTDs of regional extent throughout the fan. Three additional facies, biogenic mud, biogenic sand, and chemogenic sediment, are rare and volumetrically compose only ~1% of the total core recovered. The biogenic mud commonly forms very thin pelagic or hemipelagic drapes on older, abandoned, or inactive levee systems of the fan, and is deposited across the entire fan during interglacial highstands of sea level, such as the present Holocene, when the fan is temporarily inactive. |
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