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Flume Tank Study of Surface Morphology and Stratigraphy of a Fan Delta

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Flume Tank Study of Surface Morphology and Stratigraphy of a Fan Delta doi: 10.1111/ter.12131 Flume tank study of surface morphology and stratigraphy of a fan delta Junhui Wang,1,2 Zaixing Jiang,1,2 Yuanfu Zhang,1 Liming Gao,1 Xiaojie Wei,1 Wenzhao Zhang,3 Yu Liang4 and Haiying Zhang1,2 1School of Energy Resources, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, China; 2Institute of Earth Sciences, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, China; 3Research Center, CNOOC Ltd., Beijing 100027, China; 4Shanghai Branch of CNOOC Ltd., Shanghai 200030, China ABSTRACT The morphodynamics of a river flood on a fan delta and its finally transverse growth. A channel bifurcated in multiple resultant stratigraphic and sedimentary signatures have been stages by sequentially forming mouth bars or by simulta- studied by means of a flume experiment under controlled neously forming arrays of mouth bars. During the bifurcation, boundary conditions. The experiment revealed that deposition the diffluent point moved upstream, which resulted in chan- was dominant in flood periods when the channels were highly nel migration and the development of a delta lobe. Flood loaded with sediments. In contrast, erosion was dominant in events triggered fan-delta front slide-slump deposits. periods of low flow. Mouth bars were formed when a sub- aqueous channel began to backfill. The development of a Terra Nova, 27, 42–53, 2015 mouth bar began with progradation in the down-dip direction and proceeded by aggradation, then retrogradation and 2001; Sheets et al., 2002; Hickson surface morphology and stratigraphy Introduction et al., 2005; Kim and Paola, 2007; of the fan delta and the formation The potential of fan-delta strata to Martin et al., 2009; Kim et al., 2010; processes and dynamic relationships produce hydrocarbons has intensified Reitz and Jerolmack, 2012), and the of the associated facies, i.e. channel, interest in their morphodynamics experimental landscapes organize mouth bar and slide-slump deposit. and related stratal architecture. Pre- themselves in ways that are remark- The results suggest formative mecha- vious studies involved classification ably similar to what is observed in nisms related to fan deltas. schemes based on modern and the field (e.g. Cazanacli et al., 2002; ancient fan-delta systems (e.g. Eth- Paola et al., 2009). Therefore, physi- Method ridge and Wescott, 1984; McPherson cal experiments are important meth- et al., 1987; Nemec and Steel, 1988; ods for elucidating the sedimentary A fan delta was simulated in a flume Postma, 1990) and the sedimentary processes and sequence stratigraphy. where a three-dimensional coordinate and stratigraphic characteristics of Previous physical experiments have system was established to facilitate fan-delta deposits (e.g. Ethridge and examined the spatiotemporal distri- data acquisition and recording Wescott, 1984; Dabrio, 1990; Burns bution of sediments in an asymmetri- (Fig. 1). The initial slope was 0.1260 et al., 1997; Hoy and Ridgway, cally subsiding basin with three and the topography was recorded. 2003). However, little attention has different sediment sources (Connell Sediments were delivered from a been paid to the formation and evo- et al., 2012a,b), the formation of gauged barrel, mixed with the water lution of fan-delta sand bodies. delta-front fluxoturbidite (Yan et al., supply in the sand pool and then Sand-body genesis is usually 2004; Zhang et al., 2006), the distri- outflowed from the feeder outlet (15- explained by conjecture because it is bution of favourable reservoirs in cm width). Meanwhile, the water vol- difficult to reconstruct the original deltaic deposits (Zhang et al., 2000a; ume was kept constant by a pump sedimentary environment from field Xia et al., 2002; Wang et al., 2013), that drained water into the drainage and subsurface datasets (Postma the relative importance of factors pond at the same rate as the water et al., 2008). Physical experiments controlling shoreline migration (Kim discharge. offer an alternative means of under- et al., 2006) and delta formation The sediments were composed of standing how sedimentary systems (Zhang et al., 2000b; Muto and sand and silt & clay. The sands were change under well-controlled bound- Steel, 2001; Van Dijk et al., 2009, from a natural river and consisted of ary conditions and carefully moni- 2012) under controlled conditions quartz and feldspar with a density of tored surface topography (e.g. Paola and so on. However, the dynamic approximately 2580 kg mÀ3; they et al., 2001; Van Heijst and Postma, relationships among fan-delta facies, were sorted into coarser and finer which are important to understand grains. The silt & clay was unconsoli- Correspondence: Junhui Wang, PhD, the genesis of such sand bodies, have dated and cohesive argillaceous sedi- School of Energy Resources, China Univer- not been fully explored. ment with a density of approximately À3 sity of Geosciences, 29 Xueyuan Road, Beij- In this flume experiment, a fan 1400 kg m . The sediments were ing 100083, China. Tel.: +86 15810732717; delta was simulated under controlled dried and evenly mixed before being e-mail: [email protected] flood conditions. We describe the added to the flume to avoid clumping. 42 © 2014 John Wiley & Sons Ltd Terra Nova, Vol 27, No. 1, 42–53 J. Wang et al. • Surface morphology and stratigraphy of a simulated fan delta ............................................................................................................................................................ (A) (B) (C) Fig. 1 (A) Picture of the experimental facility. Measuring tapes are fixed around the flume to establish a three-dimensional coordinate system whose resolutions on X, Y and Z are 10 cm, 10 cm and 1 mm respectively. (B) Schematic of the experimen- tal facility. The flume is 6 m long by 3 m wide and 1 m deep. The initial ramp is 0.38 m high, 3 m long and 2.8 m wide, with a slope of 0.1260. The water volume was kept constant by a pump that delivered water into the drainage pond with equal inflow and outflow rates. The discharged water in the drainage pond was piped back to the input pond. (C) Overhead view of the experimental setup. The ramp range is from y = 50 cm to y = 350 cm. The original shoreline before sediments were sup- plied is almost coincident with the line y = 90 cm. The grain-size distributions of the and low-flow period (L-F.P.), con- A distance of 0.8 m between the three types of sediments are shown in ducted in the order M-F.P., F.P. and sediment feeder and the shoreline Fig. 2. L-F.P., with a time proportion of allowed part of the alluvial system to The experiment was performed in 2:1:1 (Fig. 3). The surface topography develop and excluded interactions two stages under controlled sediment was quantified by adding the initial between the feeder and standing composition, sediment discharge (Qs) slope topography to the sediment water. At the onset of the experi- and water discharge (Qw). Each stage thickness, which was measured by ment, there was an initial period of used the same parameters, except that inserting a ruler into the sediments. rapid adjustment as the slope of the the water level dropped by 2 cm in The bathymetry of the delta front was system gradually reached equilib- Stage 2 (Table 1), and each was measured directly with a ruler. Mea- rium, as recognized in other experi- divided into three periods to simulate surements were conducted at set time ments (e.g. Postma et al., 2008). The a flood hydrograph with a rising limb, intervals with a vertical resolution of experiment was conducted in a quies- crest and falling limb: a mid-flow 1 mm and a horizontal separation of cent tectonic setting in each stage, period (M-F.P.), flood period (F.P.) 10 cm. without tidal or wave interactions. In each stage, the water level rose slightly as sediment accumulated at the bottom. Results Landscapes in different periods As water and sediment were fed into the flume, a braided plain formed, Fig. 2 Grain-size distributions of the sorted sediments used in the experiment. extending into the standing water. © 2014 John Wiley & Sons Ltd 43 Surface morphology and stratigraphy of a simulated fan delta • J. Wang et al. Terra Nova, Vol 27, No. 1, 42–53 ............................................................................................................................................................. Table 1 Experimental conditions. Sediment composition by volume (%) Duration (min) Water depth (cm) 3 À1 3 À1 Periods Coarser sand Finer sand Silt & clayQs (cm s ) Qw (cm s ) Qs/Qw Stage 1 Stage 2 Stage 1 Stage 2 F.P. 33 50 17 110 1000 0.11 20 20 35 33 M-F.P. 17 66 17 38 500 0.08 40 40 35 33 L-F.P. 0 67 33 8 300 0.03 20 20 35 33 The sediment composition, sediment discharge (Qs) and water discharge (Qw) are modified from the design of Zhang et al. (2000a,b). The sediment and water volumes were measured by different types of measuring vessels. Sediments were delivered by a gauged barrel. The water discharge was controlled by a control valve. Additionally, the rivers that were abandoned in the L-F.P. would be activated again. During the F.P., the flow flooded existing channels, resulting in poorly channelized (sheet) flow that covered a large fraction of the sediment surface. The increased sediment transported by the unconfined flow caused aggradation and progradation of the fan delta. The fan delta thick- ened and expanded transversely and longitudinally. Generally, the delta developed a uniform planform fan- shape during the F.P. (Figs 4C, D and 5C, D, G), although channels were more likely to form along the medial line than on the flanks (French, 1992) (Video S1). In con- trast, the water flowed slowly and the load was low in the L-F.P.
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