Grain-size characterization of Amazon Fan deposits and comparison to seismic facies units

Abstract

Prior to Ocean Drilling Program Leg 155, the architecture of the Amazon Fan, as well as those of other modern submarine fans, had been investigated primarily by seismic reflection profiles. The acoustic facies and stratal patterns observed on these profiles provided a wealth of information that allowed deciphering fan growth patterns and the geometry of fan deposits. However, lithofacies could only be inferred from the seismic reflection data. Here we analyze grain-size data from 13 sites drilled during Leg 155, and place the results in the context of the stratigraphic units interpreted from seismic reflection data. Clay, silt, and fine sand are the dominant grain sizes of all cores retrieved from the Amazon Fan. In this mud-rich fan, sand is concentrated within the channel thalweg, at the base of channel-levee systems within the middle fan, and forms a significant fraction of the lower fan deposits drilled. All channel-levee deposits drilled are characterized by a fining-upward trend and by an overall coarsening in the downfan direction. Downfan coarsening of the levee deposits probably results from very efficient sorting associated with the channelized flow of turbidity currents together with the overall decrease in channel relief downfan. Finingupward cycles within a channel-levee system are observed in the middle fan where multiple phases of levee development are stacked upon each other. Each growth phase is marked by an abrupt coarsening that marks a channel bifurcation occurring downslope, followed by a fining-upward sequence of levee aggradation. Channel bifurcation results in the reworking of older channel deposits, and the formation of laterally widespread units. High-amplitude reflections (HARs) beneath the channel axis and high-amplitude reflection packet (HARPs) units are composed of two distinct grain-size populations. HARs and HARPs are the coarsest units that form the fan, with sizes up to 0 ø. Both units display an overall coarsening downfan. These characteristics are likely the result of multistep transport of sand downfan. Thick acoustically transparent units on seismic profiles are correlated to large mass-transport deposits, displaying grain-size characteristics similar to levee deposits with an overall finingupward and downfan-coarsening trend.

Genesis and character of thin-bedded turbidites associated with submarine channels

“…Cronin et al, 2000;Kane et al, 2007). This has been interpreted either as a consequence of a temporal decrease in magnitude and /or frequency of the turbidity currents (Walker, 1985;Manley et al, 1997;Normark and Damuth, 1997;Beaubouef, 2004), or more simply as the result of the growth of the levee over time (e.g. (Hiscott et al, 1997).…”

“…Savoye et al 2009). They are a product of over-spill of turbidity currents from an adjacent channel and over time increase in relief above the channel floor, both due to levee aggradation and channel down-cutting; they also tend to prograde into the basin as the channel-lobe transition advances, so that levees commonly overlie lobes/frontal splays (or high amplitude reflection packets -HARPs -in seismic data (Manley et al, 1997)) (e.g. Buffington 1952;Piper & Normark 1983;Hiscott et al 1997;Normark & Damuth 1997;Piper & Deptuck 1997;Peakall et al 2000;Skene et al 2002;Deptuck et al 2003;Kane et al 2007;Meiburg & Kneller 2010;Kane & Hodgson 2011;Nakajima & Kneller 2012;Morris et al 2014).…”

Multiple bottom-simulating reflections in the Black Sea: Potential proxies of past climate conditions

“…The Danube fan is a large mud-rich fan, essentially composed of a succession of stacked channel-levee systems that are emplaced during successive sea-level lowstands (Wong et al, 1994;Popescu et al, 2001). Typically, channel-levee systems in mud-rich fans are lenticular sedimentary units with coarse-grained sediments at the channel axis, and finergrained, well-stratified alternations of sand and mud in the lateral levees (Manley et al, 1997;Normark et al, 1997). Only one channel-levee system in the Danube fan was active at a time.…”

Architecture and evolution of upper fan channel-belts on the Niger Delta slope and in the Arabian Sea

“…They act as conduits through which clastic sediment is transported into the deep sea, and they provide confining and sorting mechanisms that allow sand to reach the basin plain. Channel-levee systems also form important repositories for coarse-grained sediment deposited along channel-axes (Bouma et al, 1985;Manley, Pirmez, Busch, & Cramp, 1997) and for fine-grained sands and silts deposited on levees (Hiscott, Hall, & Pirmez 1997;Piper & Deptuck, 1997). For these reasons, the architecture and evolution of channel-levee systems are of interest to the petroleum industry.…”