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Title: Controls on slope channel-levee evolution in the Amazon fan
Author: Silva, Carlos Manuel de Assis
ISNI:       0000 0004 2742 6933
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2011
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Submarine channel-levee architecture is a result of interaction between turbiditic flows and the bathymetry that they encounter on the slope and that they themselves construct. However, the spatio-temporal evolution of channellevee systems, i.e., their manner of spatial accretion is not fully understood. In addition, the controls on the patterns of channel distribution and stacking remain relatively poorly understood. The results presented in this thesis are based on the interpretation of a 3D seismic survey located in water depths of about 1000 to 2000 m on the upper slope of the Amazon Fan. The analysis of the data includes two different approaches: on a larger scale, the interpretation of the tectono-stratigraphic analysis of the data; and on a smaller scale, the characterization of the evolution of the channel-levee systems and associated deposits. The methods of analysis used included the mapping of significant horizons and the extraction of sequences horizon slices across the channel-levee systems. The horizon slices allowed visualisation of the vertical distribution of the channel-levee elements and interpretation of their evolution. The seismic data can be divided in two main packages, separated by an unconformity. The pre-unconformity package is characterized by a predominant sub-parallel seismic facies (characterized by continuous, homogeneous and sub-parallel reflections), and is strongly folded and faulted due to gravity tectonics. In this interval, no channel-levee systems were identified, but canyonlike channels with dimensions as large as 2600 m wide and 220 m deep were observed. Conversely, the post-unconformity package comprises a heterogeneous arrangement of seismic facies, and is less strongly deformed. Three upslope-stacked channel-levee systems of Middle Pleistocene age can be distinguished in this interval. Each channel disperses obliquely down the slope, resulting in levee size asymmetry, with the downslope levee being of greatest size. The upslope stacking of channel-levee systems results from a sequence of avulsions on the upslope levees. Bathymetric influence of the older channel levee system is interpreted to dictate the slope-oblique orientation of the new channel. Two main architectural styles were identified, each one associated with different models of spatia-temporal development of channel-levees. A pattern of upstream accretion of the aggradational (channel-levee) component of the channel is associated with the channel narrowing and becoming more sinuous upward. This style is characterized by an onlapping pattern of channel and levee reflections. Downstream accretion of channel-levee is associated with a trend of channel widening and becoming less sinuous upwards. This style is characterized by a downlap pattern of the channel and levee reflections. Each style is related to different evolutionary histories of the turbidite flows and thalweg configuration in relation to the equilibrium profile. Three styles of channel distribution on slope can be distinguished: structurally controlled erosive channels, aggradational channels obliquely oriented to the slope and agradationally confined channels that are vertically stacked. The pre-unconformity erosive channels are diverted and possibly owe their erosive character to active anticlines that kept the paleo-slope above the equilibrium profile. The oblique direction of each channel in the postunconformity interval was inherited from the previous one in a sequence of upslope stacking after systematic avulsion through the left-hand levee. The vertical stacking of channel-levee elements is due to the confinement of the channel between the outsize downslope levee and an upslope anticline which inhibited avulsion in the area. It is probable that each stacked channel-levee element corresponds to a downstream channel avulsion. This study provided important information about the architecture and distribution of channels on the upper-slope of submarine fans subject to gravitational tectonics. Therefore, it can be used as an analogue and help the prediction of sand prone areas in other submarine fans which are not so well imaged.
Supervisor: Paton, D. ; McCaffrey, B. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available