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Anatomy and growth pattern of Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA) and high-resolution seismic studies

John E. Damuth, Roger D. Flood, Renato O. Kowsmann, Robert H. Belderson and Marcus A. Gorini
Anatomy and growth pattern of Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA) and high-resolution seismic studies
AAPG Bulletin (August 1988) 72 (8): 885-911

Abstract

Imaging of the Amazon deep-sea fan with long-range side-scan sonar (GLORIA) has, for the first time, revealed the anatomy, trends, and growth pattern of distributary channels on this fan. Only one channel-levee system was active at any given time and extended from the Amazon Submarine Canyon downslope onto the lower fan (> 4,200 m). Formation of new channel-levee systems occurred when a currently active channel-levee system was cut off and abandoned through avulsion, and a new channel-levee system was established nearby. Through time, successive channel-levee formation and abandonment built two broad levee complexes consisting of groups of overlapping, coalescing segments of channel-levee systems across the present fan surface. These, plus older, now buried levee complexes, indicate that fan growth is radially outward and downslope through development of successive levee complexes. The most striking characteristic of the distributary channels is their intricate, often recurving, meanders with sinuosities of up to 2.5. Cutoffs and abandoned meander loops indicate that the channels migrate laterally through time. Channel bifurcation results predominantly from avulsion when flows breach a channel levee, thereby abandoning the present channel and establishing a new channel-levee segment nearby. No clear evidence of channel branching (i.e., division of a single channel into two active segments) or braiding was observed. The formation, maintenance, and modification of such extensive and intricate meander systems presumably requires large volumes of relatively continuous turbidity flow through channels for relatively long time periods. "Classic" sporadic turbidity-current events are apparently too infrequent to form the meandering channels. However, the bulk of sediments forming the channel-levee systems consists of fine terrigenous (hemipelagic) sediments, which were deposited relatively continuously throughout entire intervals ( approximately 100,000 years) of glacio-eustatic sea level lowering (e.g., Wisconsin). Although defining precisely what "continuous" means is not yet possible in terms of frequency of individual flow events (per year, decade, etc), this type of more continuous, high-volume flow is probably an important factor that promotes channel meandering. Another important factor that appears to cause channel meandering is changes in the slope of the fan surface (i.e., valley slope) encountered by a channel-levee system as it builds across the fan. The channel is apparently able to maintain a preferred gradient or slope along its thalweg, despite an increase in the slope of the fan surface, simply by meandering.


ISSN: 0149-1423
EISSN: 1558-9153
Coden: AABUD2
Serial Title: AAPG Bulletin
Serial Volume: 72
Serial Issue: 8
Title: Anatomy and growth pattern of Amazon deep-sea fan as revealed by long-range side-scan sonar (GLORIA) and high-resolution seismic studies
Affiliation: Mobil Res. and Dev. Corp., Dallas Res. Lab., Dallas, TX, United States
Pages: 885-911
Published: 198808
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
References: 58
Accession Number: 1988-062623
Categories: OceanographyApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sects., sketch maps
N02°00'00" - N10°00'00", W50°00'00" - W44°00'00"
Secondary Affiliation: Lamont-Doherty Geol. Obs., USA, United StatesCENPES/PETROBRAS, BRA, BrazilInst. Oceanogr. Sci., GBR, United KingdomUniv. Fed. Rio de Janeiro, BRA, Brazil
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 1988
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