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External Controls from the Formation and Preservation Potential of Outer Bank Bars: a New Intra-Channel Architectural Element within Sinuous Submarine Slope Channels

Nakajima, Takeshi 1; Peakall, Jeff 2; McCaffrey, William D.2; Paton, Douglas 2; Thompson, Philip 3
1 Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan.
2 School of Earth and Environment, University of Leeds, Leeds, United Kingdom.
3 Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, United Kingdom.

In this study of 3D seismic data from the Amazon slope channels we describe the size, geometry, facies, formative processes and external controls of a new intra-channel architectural element, “outer bank bars (OBBs)”, interpreted as being unique to submarine channels. OBBs comprise channel-fill deposits found in tight meander loops at outer corners of bend apices. In contrast to fluvial point bars, this architectural element is characterized by inward-dipping accumulations due to outer bank accretion at channel bends. OBBs may in some cases be continuous with point bars in along channel profile. They are developed only in the late stage aggradational channel-fill deposits of tight meander loops of sinuous channels. This architectural element is interpreted to have been formed as a result of a combination of enhanced deposition on the outsides of bends by flow super-elevation and strongly depositional flows caused by flow volume reduction during the aggradational periods of channel development. The continuity and preservation potential of OBBs are interpreted to be related to style of channel abandonment (i.e. transgressive or avulsive). These observations thus suggest the external controls on the formation and preservation potential of OBBs. Outer bank bar accretion resulted in the subsequent thalweg shifting towards the inner bend and eventually led to sinuosity reduction. This hitherto undescribed sand-prone architectural element may have significant implications for hydrocarbon reservoir prediction in submarine channels.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009