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 subaqueous 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 m3; 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 3 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.

(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 experimental 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.

Table 1 Experimental conditions.

Sediment composition by volume (%) Duration (min) Water depth (cm)

3 1 3 1 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 contrast, the water flowed slowly and the load was low in the L-F.P. The stream became channelized and significantly eroded the unconsolidated sediments, with minor lateral deposition. During the L-F.P., almost no sediment was deposited on the delta plain (Figs 4E, F and 5E, F). Consequently, a new lobe formed in front of the channel, causing non-uniform progradation of the fan delta (Fig. 5G). Fig. 3 Sequence of the experiment, including water discharge (Qw), sediment discharge (Qs) and water depth over time in each period. Longitudinal section of the fan delta in different periods The resulting morphology is analo- channels repeatedly divided and The forced regression caused by the gous to a fan delta as defined by Ne- rejoined to form a braided stream decrease in water level in Stage 2 mec and Steel (1988). The fan delta pattern in the lower fan area (Video helped us to recognize the boundary developed differently in the different S1). In the L-F.P., however, there between the two stages (Fig. 6A). periods. During the M-F.P. and F.P., were 1 or 2 confined braided channels The thickness of the delta plain in the characteristics of the surface flow that widened downstream, leaving a Stage 1 is significantly greater than were similar to those reported by Ca- large part of the delta plain aban- that in Stage 2 but smaller in the zanacli et al. (2002) and Van Dijk doned at any given time. delta front (Fig. 6). This is because et al. (2009): avulsions and either During the M-F.P., flow made the 2-cm decrease in water level in channel bifurcation or sheet flow the greatest contribution to delta- Stage 2 led to less sediment accreting occurred primarily in the upper fan lobe growth (Figs 4A, B and 5A, on the delta plain, and more being region, lateral channel migration B), because the sediment supply transported to the delta front through bank erosion was more com- was greater than and the duration (Figs 5B and 6C). Additionally, the mon in the middle fan area, and the was twice as long as the L-F.P. thickness of the delta was different

Fan-delta Morphodynamics near the Shoreline At the channel mouth, turbulent eddies generated by momentum exchanges between the effluent and basin waters are responsible for the expansion, mixing and deceleration of the effluent water (Wright, 1977). Flow deceleration results in the (A) (B) unloading of sediments and formation of a subaqueous shoal (Fig. 7A, B). Continued basinward deposition of sediment makes the mouth bar grow until it ascends out of the water (Fig. 7). During mouth-bar evolution, the mouth bar first prograded, then accreted, before it backstepped and widened, with an increasing to decreasing depositional rate (Fig. 8). (C) (D) After the mouth bar emerged, it caused the flow to bifurcate, forming separate branches (Fig. 9A); each branch would likely bifurcate again following the formation of subsidiary mouth bars (Fig. 9B) if the experiment continued with the same conditions. Several bars may simultaneously develop basinward of the outlet if the channel is wide and shallow. In the experiment, such bars sepa- (E) (F) rated one channel into several subsidiary channels almost synchronously Fig. 4 Representative pictures illustrating fan delta development in each period. (A) (Fig. 9C, D). M-F.P. 8 min into Stage 2. (B) M-F.P. 32 min into Stage 2. (C) F.P. 8 min into Progressive deposition around the Stage 1. (D) F.P. 15 min into Stage 2. (E) L-F.P. 20 min into Stage 1. (F) L-F.P. 20 min into Stage 2. The uniform depressions are traces left by the measurements mouth bar increased the angle of of topography and have negligible effects on the experiment. The red lines hereafter bifurcation (Fig. 10). The resultant emphasize the contact between the sediment and the ponding water. flow widening and accretion led to headward migration of the diffluent point (Fig. 10A). Branch channels on in different periods because of and Hampson, 2009). Two classes of either side of the diffluent point cap- differences in sediment rates. The shoreline trajectories were identified tured different quantities of water thicknesses of both the delta plain in the experiment: an ascending and sediment (Fig. 10B–F). Conse- and the delta front were greater in regressive trajectory and an accre- quently, the main channel migrated the M-F.P. and F.P. than in the tionary descending regressive trajec- and the delta lobe developed trans- L-F.P., except for the M-F.P. in tory (Fig. 6A). The accretionary versely. The angle of the bifurcation Stage 2. In the L-F.P., however, descending regressive trajectory hap- grew over time, and the original deposition did not happen every- pened at the beginning of Stage 2 mouth environment evolved into the where in the delta front. Figure 6C due to the decline in water level. In top of the delta with delta plain shows that along x = 160 cm, no the ascending regressive process, the aggradation (Fig. 10B–F). sediment was transported to the delta angles were different in each period, It is worth mentioning that sedi- front during the L-F.P. of Stage 2, with the largest in the F.P. and the ment failures formed in front of tor- as also indicated by Fig. 5F. lowest in the L-F.P. (nearly flat; rential and highly loaded channels The topset–foreset rollover of the Fig. 6A). (Fig. 11). In the experiment, these depositional clinoforms is located Additionally, the gradient of the deposits mainly occurred during the close to the shoreline, so the shore- foreset slope varied according to the F.P. line trajectory can be observed in period due to different progradation the longitudinal profile (Fig. 6A). rates of the shoreline and foreset-toe. Shoreline trajectory can record strati- In the experiment, the average foreset Discussion graphic units representing transgres- slope angles in the F.P.s of the two Our experiment shows the surface sion, regression, aggradation and stages were 0.5430 and 0.4369 respec- morphology and stratigraphy of a self- degradation (Helland-Hansen and tively, shallower than those formed in organized fan delta under controlled Martinsen, 1996; Helland-Hansen the L-F.P. (0.8511) (Fig. 6B, C).

(A) (B)

(E) (F)

(G)

Fig. 5 Isothickness maps of total fan-delta deposits during the individual periods (A–F) and the boundaries of the total fan delta at the end of each period (A–G). (A) M-F.P. of Stage 1. (B) M-F.P. of Stage 2. (C) F.P. of Stage 1. (D) F.P. of Stage 2. (E) L-F.P. of Stage 1. (F) L-F.P. of Stage 2. (G) The total fan-delta boundaries at the end of each period show that the fan delta progrades uniformly in the M-F.P. and F.P., and new lobes form in the L-F.P. The black line (x = 160) indicates the location of the longitudinal profile analysed in Fig. 6.

(A)

(B)

(C)

Fig. 6 (A) The profile section (black line of x = 160 cm in Fig. 5G) of the laboratory-simulated fan delta. ①, M-F.P. in Stage 1; ②, F.P. in Stage 1; ③, L-F.P. in Stage 1; ④, M-F.P. in Stage 2; ⑤, F.P. in Stage 2. The yellow points identify topset–foreset rollovers, and the blue arrows, which connect the rollovers, indicate the shoreline trajectories. Sediments above Stage 2 were deposited under the same conditions as the M-F.P. of Stage 2 except for a shorter duration, which does not affect the results. (B, C) The profile sections (black line of x = 160 cm in Fig. 5G) of the simulated fan delta show the depositional characteristics of each period. The foreset slopes in the flood periods of Stages 1 and 2 reach 0.5430 and 0.4369 respectively; values that are lower than those in the low-flow periods. The dashed line that almost coincides with the nearby solid line in (C) is the surface of the delta after the L-F.P. of Stage 2, indicating that there was very little deposition along x = 160 cm in the L-F.P. of Stage 2. conditions. Such analogue experi- In a quiescent tectonic setting (i.e, inertia, turbulent bed friction and ments may reveal the sedimentary pro- no subsidence) with constant water outflow buoyancy. However, cesses that ultimately define the level, shoreline position is controlled Wright’s discussion did not empha- depositional architecture in deposi- by sediment supply (Kim et al., size the dynamic evolution of a tional systems to some extent (Peakall 2006). The angle of the shoreline tra- mouth bar that develops at the chan- et al., 1996; Postma et al., 2008; Van jectory is positively correlated with nel mouth of a deep water delta Dijk et al., 2009). sediment thickness (Cant, 1991; Hel- (Postma, 1990), the formation of land-Hansen and Hampson, 2009) which is mainly controlled first by (Stage 1 in Fig. 6A), and thus should outflow inertia and then by bed fric- Fan-delta evolution in different be correlated with sediment supply. tion. periods The variation in the delta thickness The transport capacity of the In the M-F.P., which was twice as long and the shoreline trajectory angles in effluent water diminished as water as and had higher sediment input than different periods also reveal that from the channel emptied into the the L-F.P., the fan delta grew both deposition was dominant in the F.P., lake, and sediments were deposited, progradationally and aggradationally. while erosional processes dominated causing the formation of a subaque- Similar phenomena took place in the the delta plain during the L-F.P. ous shoal (Fig. 7A, B). Due to F.P. Although the duration was (Fig. 6). their greater size and inertia, the halved, the high sediment concentra- coarser sands were deposited at dis- tion compensated for the lesser time, tal parts of the mouth. This is the Facies relationships so the fan delta also prograded and ag- inertia-dominated effluent diffusion graded in the F.P. In the L-F.P., how- model that produces a narrow Mouth-bar evolution and its relation- ever, erosion was dominant and the mouth bar (Wright, 1977). The con- ship with the subaqueous channel channel downcut the delta plain. Sedi- tinued basinward deposition of sediments from upstream were deposited Wright (1977) discussed river-mouth ment eventually decreases local in front of the confined channel, form- morphologies and depositional pat- water depth, which increases the ing a new smaller lobe (Fig. 5). terns under the influence of outflow relative influence of turbulent bed

(A) (B)

(E)

Fig. 7 The mouth-bar development process. (A) The M-F.P. 8 min into Stage 2 shows the subaqueous channel and the mouth bar (white arrows). (B) The M-F.P. 35 min into Stage 2 shows deposits in the river mouth (white arrow). (C) The M-F.P. 36 min into Stage 2 shows an intensified unloading (white arrow). (D) The M-F.P. 37 min into Stage 2 shows the emergence of the mouth bar (white arrow). (E) The M-F.P. 38 min into Stage 2 shows the transverse extension of the mouth bar (white arrow). (B–E) are chronological and show the development of a single mouth bar. The black dotted bordered rectangle indicates the general location shown in Fig. 8. friction. This friction would, in sion of subaerial braided channels lower, and the channel bifurcated, turn, decrease Froude numbers and (Feng et al., 2014). Subaqueous with the diffluent point migrating the transport capacity of the efflu- channels fill as the mouth bar devel- upstream (Fig. 10A). Previous ent water, and the depositional rate ops, before younger subaqueous authors called the upstream sedi- would increase at the river mouth channels form around the mouth mentation a ‘morphological backwa- (Fig. 7C). In the shoaling process, bar. Accordingly, the subaqueous ter effect’ (e.g. Hoyal and Sheets, the increasing bed friction also channel can be regarded as the initia- 2009; Lamb et al., 2012). The back- caused upstream deposition of the tion of a mouth bar, which will even- water length is typically approxi- coarser materials. Then, the mouth tually be replaced by mouth-bar mated as 13.1 cm based on the bar rose to near the water surface deposits and then branching sub- formula given by Paola and Mohrig (Fig. 7D) and extended upstream. aqueous channels form. (1996). Compared to mouth-bar This observation may confirm the length (Fig. 8), the percentage of the friction-dominated effluent diffusion mouth bar that would be affected Relationship between mouth bars and model presented by Wright (1977), by the backwater length is approxi- channels which makes for a wider bar. We mately 11.5%. suggest that the mouth bar develops The velocity of the upstream flow After channel bifurcation, branch progradationally, aggradationally, gradually decreased near the mouth channels downstream of the fan delta retrogradationally and then trans- bar because the bar obstructed flow were also influenced by backwater versely from an inertia-dominated (Van Dijk et al., 2009). As a result, effects or upstream sedimentation model to a friction-dominated sediments were deposited headward, (Fig. 10C, D). Therefore, differences model (Figs 8 and 12). upstream of the bar. Consequently, in bed elevation or depth between At the channel outlet, subaqueous the central part of the channel the two branch channels due to dif- channels are the underwater exten- behind the mouth bar became shal- ferent upstream-sedimentation rates

(A) (B)

Fig. 8 Interpolation measurements of the mouth present in Fig. 7B–E. (A) Isothickness map of the mouth bar during the M-F.P. 30–35 min into Stage 2. (B) Isothickness map of the mouth bar during the M-F.P. 35–36 min into Stage 2. (C) Iso- thickness map of the mouth bar during the M-F.P. 36–39 min into Stage 2. The black dotted line (PP0) indicates the position of the profile shown in (D). The red dotted line indicates the shoreline. The blue arrows indicate flow direction. (A–C) are chronological and show that the mouth bar steps back as it widens. (D) The mouth bar’s evolutional trajectory (PP0) shows that it steps back during its progradation and aggradation. ①, ② and ③ are chronological. By comparing volume with duration, the rate of deposition is highest in (B or ② in D) and smallest in (C or ③ in D).

(A) (B)

Fig. 9 The 2 types of mouth-bar development near the water surface. (A) M-F.P. 20 min into Stage 2. (B) L-F.P. 6 min into Stage 1. (A) and (B) show multi-stage bifurcations of channels by sequentially formed mouth bars. a, main channel; b, c, branch channels; ①, main mouth bar; ② and ③, subsidiary mouth bars. (C) M-F.P. 18 min into Stage 2. (D) L-F.P. 20 min into Stage 2. (C and D) show simultaneous bifurcations of a channel by multiple mouth bars.

cated sequentially by mouth bars that form in multiple stages (Fig. 9A, B). Consequently, the delta expands transversely during progradation. However, our experiment shows that a channel can also be bifurcated by multiple, simultaneously formed mouth bars. Such bars may form and separate one channel into several (A) (B) subsidiary channels almost synchronously if the channel is wide and shallow (Fig. 9C, D). Previous obser- vations have also indicated that a series of small-scale mouth bars might form during a single diffluence (Damuth et al., 1983). Studies by Zhang et al. (2010) and Yin et al. (C) (D) (2012), using data from a maturely prospected area of an oil field and Google Maps images of the South Dongting and South Poyang Lakes in China, found that multiple arrays of mouth bars developed at the river mouth. While these studies were conducted in topset-dominated deltas (Edmonds et al., 2011), similar phenomena could occur in foreset-domi- (E) (F) nated deltas (Edmonds et al., 2011), according to our experimental obser- vations. Fig. 10 The relationship between mouth-bar and fan-delta-lobe formation. (A) The M-F.P. 8 min into Stage 2 shows the diffluent point moving upstream (black arrow). (B) M-F.P. 15 min into Stage 2. (C) M-F.P. 18 min into Stage 2. (D) Relationship between floods and M-F.P. 22 min into Stage 2. (E) M-F.P. 30 min into Stage 2. (F) M-F.P. 35 min slide-slump deposits into Stage 2. (B–F) are chronological. (B, C and E) show heavier water flow to the right, while (D and F) show heavier water flow to the left (blue arrows). Together, Sediment loading on the delta slope (A–F) show the formation of a new lobe. (A, C, D, F) show increasing bifurcation can generate sand failures (Jiang angles and upstream migration of the diffluent point. et al., 2008; Chen et al., 2009). In the delta front, the uncompacted and less dense argillaceous sediment was overlain by rapidly accumulated, coarser, and relatively denser sands (Fig. 11). Additionally, the higher depositional rates in the river mouth result in significant oversteepening of the delta front (e.g. Postma, 1990; Girardclos et al., 2007). During the flood period in the experimental basin, many channels (A) (B) flow into the lake and the interdis- tributary bays form narrow delves that transport sediments farther into Fig. 11 (A) F.P. 15 min into Stage 2. The arrows show slide-slump deposits. (B) The morphology of the model fan delta shows that the narrow delves and sand the basin (Fig. 11B). Floods brought bodies are superimposed onto muddy facies. a mass of sediments into the river mouth, leading to an abrupt increase in the weight of the sediments, which increased the load on the sub- may cause an uneven distribution of quently, the delta lobe develops by strate, leading to slope failure and the flow. The flow may avulse into multiple avulsions (Fig. 10E, F). the production of subaqueous slides. one of its bifurcations while the Thus, delta formation is a continu- In addition, the supercritical flow other closes off, and this uneven ous process of channel bifurcation destabilized the deposits near the flow distribution would alternate and progradation related to mouth- mouth. Therefore, sediment failure in the branches (Fig. 10B–F). Conse- bar formation. A channel is bifur- occurred more frequently along the

(A)

(C)

(D) (B)

Fig. 12 Simple evolutional model of a mouth bar according to our experiment. (A) This plan view shows the chronological development patterns of a mouth bar observed during the experiment, showing the growth of a mouth bar as illustrated by Wright (1977). ① is the first step in the formation of the mouth bar, and the process continues into ② and ③. The solid-col- our triangles correspond to the depositional sites of the coarsest particles at the mouth bar in each step. As the mouth bar develops, it steps backward and widens, resulting in widening and bifurcation of the channel as new bars form. Different col- ours hereafter correspond to different steps. AA0 and BB0 are the transverse and longitudinal sections, which are shown in (C and D) respectively. (B) The chronological development patterns of the mouth bar according to the experiment. The mouth bar first develops progradationally, then aggradationally, retrogradationally and transversely, until new bars form. (C) The transverse section shows the mouth bar widening upward. (D) The longitudinal section shows the mouth bar stepping backward. delves during flood periods. Slope cation by sequentially formed References failures seldom occurred on the stee- mouth bars and simultaneous Burns, B.A., Heller, P.L., Marzo, M. and per slopes that formed in the L-F.P. bifurcation by arrays of mouth Paola, C., 1997. Fluvial response in a (Fig. 6B). This was probably bars. The bifurcation makes the sequence stratigraphic framework: because the lower depositional rate delta grow radially with the difflu- example from the Montserrat fan delta, and less torrential channel flow ent point moving upstream. Spain. J. Sed. Res., 67, 311–321. could not induce failure through 4 Slide-slump deposits are more eas- Cant, D.J., 1991. Geometric modelling of local loading near the delta front. ily formed in flood periods. facies migration: theoretical We suggest that the high sediment development of facies successions and local unconformities. Basin Res., 3,51– load and torrential flows detach sed- Acknowledgements 62. iment and move it down slope Cazanacli, D., Paola, C. and Parker, G., through mass failure. This study was supported by the 2002. Experimental steep, braided flow: China National Science and Technology application to flooding risk on fans. J. Major Project (2011ZX05009-002), the Hydraul. Eng., 128, 322–330. Conclusions National Natural Science Foundation of Chen, D.X., Pang, X.Q., Jiang, Z.X., China (41102089) and the Basic Scientific 1 Fan deltas aggrade and prograde Zeng, J.H., Qiu, N.S. and Li, M.W., Research Foundation (2010ZY11). We 2009. Reservoir characteristics and their uniformly in flood and mid-flow thank Weiling Li, Chao Liang, Jianguo effects on hydrocarbon accumulation in periods, while they prograde non- Zhang, Quanda Jiang, Qiwei Wang, and lacustrine turbidites in the Jiyang uniformly in low-flow periods. Shaofeng Bu and Boyu Zhao for their Super-depression, Bohai Bay Basin, 2 The subaqueous channel is eventu- creative group effort to make the experi- China. Mar. Pet. Geol., 26, 149–162. ally replaced by a mouth bar. The ment possible. In particular, we thank Connell, S.D., Kim, W., Paola, C. and mouth bar develops first prograda- Hui Liu, Li’an Liu, Jie Xu and Wei Du Smith, G.A., 2012a. Fluvial tionally, then aggradationally, ret- for their useful comments for improving morphology and sediment-flux steering the experiment. Pinghua Yang, Yaru Cao rogradationally and transversely of axial-transverse boundaries in an and Yanpeng Sun polished the manu- 82 from the initial formation of the experimental basin. J. Sed. Res., , script for publication. Reviews by Baren- 310–325. subaqueous channel. dra Purkait, four anonymous reviewers Connell, S.D., Kim, W., Smith, G.A. and 3 There are two modes of channel and the Associate Editor greatly Paola, C., 2012b. Stratigraphic bifurcation, i.e. multi-stage bifur- improved the manuscript.

architecture of an experimental basin Jiang, Z.X., Cao, Y.C., Yang, W.L., Postma, G., 1990. An analysis of the with interacting drainages. J. Sed. Res., Wang, T. and Zhang, L., 2008. variation in delta architecture. Terra 82, 326–344. Accommodation space transformation Nova, 2, 124–130. Dabrio, C.J., 1990. Fan-delta facies system in faulted basin. Open Geol. J., Postma, G., Kleinhans, M.G., Meijer, associations in late Neogene and 2,10–17. P.Th. and Eggenhuisen, J.T., 2008. Quaternary basins of southeastern Kim, W. and Paola, C., 2007. Long- Sediment transport in analogue flume Spain. In: Coarse-Grained Deltas (A. period cyclic sedimentation with models compared with real-world Colella and D.B. Prior, eds). Spec. constant tectonic forcing in an sedimentary systems: a new look at Publ. Int. Ass. Sediment., 10,91–111. experimental relay ramp. Geology, 35, scaling evolution of sedimentary Damuth, J.E., Kolla, V., Flood, R.D., 331–334. systems in a flume. Sedimentology, 55, Kowsmann, R.O., Monteiro, M.C., Kim, W., Paola, C., Voller, V.R. and 1541–1557. Gorini, M.A., Palma, J.J.C. and Swenson, J.B., 2006. Experimental Reitz, M.D. and Jerolmack, D.J., 2012. Belderson, R.H., 1983. Distributary measurement of the relative importance Experimental alluvial fan evolution: channel meandering and bifurcation of controls on shoreline migration. J. channel dynamics, slope controls, and patterns on the Amazon deep-sea fan Sed. Res., 76, 270–283. shoreline growth. J. Geophys. Res., 117, as revealed by long-range side-scan Kim, W., Sheets, B.A. and Paola, C., F02021. doi:10.1029/2011JF002261. sonar (GLORIA). Geology, 11,94–98. 2010. Steering of experimental channels Sheets, B.A., Hickson, T.A. and Paola, Edmonds, D.A., Shaw, J.B. and Mohrig, by lateral basin tilting. Basin Res., 22, C., 2002. Assembling the stratigraphic D., 2011. Topset-dominated deltas: A 286–301. record: depositional patterns and time- new model for river delta stratigraphy. Lamb, M.P., Nittrouer, J.A., Mohrig, D. scales in an experimental alluvial basin. Geology, 39, 1175–1178. and Shaw, J., 2012. Backwater and Basin Res., 14, 287–301. Ethridge, F.G. and Wescott, W.A., 1984. river plume controls on scour upstream Van Dijk, M., Postma, G. and Tectonic setting, recognition, and of river mouths: implications for fluvio- Kleinhans, M.G., 2009. Autocyclic hydrocarbon reservoir potential of fan- deltaic morphodynamics. J. Geophys. behaviour of fan deltas: an analogue delta deposits. In: Sedimentology of Res., 117, F01002. doi:10.1029/ experimental study. Sedimentology, 56, Gravels and Conglomerates (E.H. 2011JF002079. 1569–1589. Koster and R.J. Steel, eds). Can. Soc. Martin, J., Paola, C., Abreu, V., Neal, J. Van Dijk, M., Kleinhans, M.G., Postma, Pet. Geol. Memoir, 10, 217–235. and Sheets, B., 2009. Sequence G. and Kraal, E., 2012. Contrasting Feng, D., Deng, H.W., Zhou, Z., Gao, stratigraphy of experimental strata morphodynamics in alluvial fans and X.P. and Cui, L.T., 2014. under known conditions of differential fan deltas: effect of the downstream Paleotopographic controls on facies subsidence and variable base level. boundary. Sedimentology, 59, 2125– development in various types of braid- AAPG Bull., 93, 503–533. 2145. delta depositional systems in lacustrine McPherson, J.G., Shanmugam, G. and Van Heijst, M.W.I.M. and Postma, G., basins in China. Geosci. Front.,In Moiola, R.J., 1987. Fan-deltas and 2001. Fluvial response to sea-level press. doi: 10.1016/j.gsf.2014.03.007. braid deltas: varieties of coarse-grained changes: a quantitative analogue, French, R.H., 1992. Preferred directions deltas. Geol. Soc. Am. Bull., 99, 331– experimental approach. Basin Res., 13, of flow on alluvial fans. J. Hydraul. 340. 269–292. Eng., 118, 1002–1013. Muto, T. and Steel, R.J., 2001. Wang, J.H., Jiang, Z.X., Zhang, Y.F., Girardclos, S., Schmidt, O.T., Sturm, M., Autostepping during the transgressive Gao, L.M., Wei, X.J. and Zhang, W.Z., Ariztegui, D., Pugin, A. and growth of deltas: results from flume 2013. Physical simulation of deltaic Anselmetti, F.S., 2007. The 1996 AD experiments. Geology, 29, 771–774. deposits. Oil Gas Geol., 34, 758–764. (in delta collapse and large turbidite in Nemec, W. and Steel, R.J., (Eds.), 1988. Chinese with English abstract). Lake Brienz. Mar. Geol., 241, 137–154. Fan Deltas: Sedimentology and Tectonic Wright, L.D., 1977. Sediment transport Helland-Hansen, W. and Hampson, Setting. Blackie and Son, London, vii– and deposition at river mouths: a G.J., 2009. Trajectory analysis: ix, 444 pp. synthesis. Geol. Soc. Am. Bull., 88, 857– concepts and applications. Basin Res., Paola, C. and Mohrig, D., 1996. 868. 21, 454–483. Palaeohydraulics revisited: palaeoslope Xia, C.H., Zhang, C.S., Zhou, H.B., Shi, Helland-Hansen, W. and Martinsen, O.J., estimation in coarse-grained braided B.Q., Liu, Z.B. and Li, L., 2002. Study 1996. Shoreline trajectories and rivers. Basin Res., 8, 243–254. on sedimentary simulation of fan delta sequences: description of variable Paola, C., Mullin, J., Ellis, C., Mohrig, in Lower Sub-member Es3 of Baimiao depositional-dip scenarios. J. Sed. Res., D.C., Swenson, J.B., Parker, G., Gas Field in Puyang Depression. Oil 66, 670–688. Hickson, T., Heller, P.L., Pratson, L., Gas Geol., 23, 218–223. (in Chinese Hickson, T.A., Sheets, B.A., Paola, C. Syvitski, J., Sheets, B. and Strong, N., with English abstract). and Kelberer, M., 2005. Experimental 2001. Experimental stratigraphy. GSA Yan, J.H., Chen, S.Y., Song, G.Q., Jiang, test of tectonic controls on three- Today, 117,4–9. Z.X. and Qiu, G.Q., 2004. Preliminary dimensional alluvial facies architecture. Paola, C., Straub, K., Mohrig, D. and study on the formation of J. Sed. Res., 75, 710–722. Reinhardt, L., 2009. The “unreasonable fluxoturbidite in front of delta. Acta Hoy, R.G. and Ridgway, K.D., 2003. effectiveness” of stratigraphic and Sedimentol. Sinica, 22, 573–578. (in Sedimentology and sequence geomorphic experiments. Earth-Sci. Chinese with English abstract). stratigraphy of fan-delta and river-delta Rev., 97,1–43. Yin, T.J., Li, X.Y., Zhang, C.M., Zhu, deposystems, Pennsylvanian Minturn Peakall, J., Ashworth, P.J. and Best, Y.J. and Gong, F.H., 2012. Sand body Formation, Colorado. AAPG Bull., 87, J.L., 1996. Physical modelling in fluvial shape of modern shallow lake basin 1169–1191. geomorphology: principles, applications delta sediments: by taking Dongting Hoyal, D.C.J.D. and Sheets, B.A., 2009. and unresolved issues. In: The Lake and Poyang Lake for example. J. Morphodynamic evolution of Scientific Nature of Geomorphology Oil Gas Technol., 34,1–7. (in Chinese experimental cohesive deltas. J. (B.L. Rhoads and C.E. Thorn, eds), with English abstract). Geophys. Res., 114, F02009. doi:10. pp. 221–253. John Wiley & Sons Ltd., Zhang, C.S., Liu, Z.B., Shi, D., Cheng, 1029/2007JF000882. Chichester. Q.G., Zhang, R.B. and Ma, C.Y.,

2000a. Physical simulation of 2006. Simulation of ﬂuxoturbidite in Supporting Information formation process in distributary front of delta. Acta Sedimentol. Sinica, channels and debouch bars in delta. 24,50–55. (in Chinese with English Additional Supporting Information Earth Sci. Front. (China Univ. Geosci., abstract). may be found in the online version Beijing), 7, 168–176 (in Chinese with Zhang, C.M., Yin, T.J., Zhu, Y.J. and of this article: English abstract). Ke, L.M., 2010. Shallow-water deltas Video S1. Characteristics of the Zhang, C.S., Liu, Z.B., Shi, D. and Jia, and models. Acta Sedimentol. Sinica, surface ﬂow during F.P. The video A.L., 2000b. Formed proceeding and 28, 933–944. (in Chinese with English records F.P. 11 min to 11.5 min into evolution disciplinarian of fan delta. abstract). Stage 1. Acta Sedimentol. Sinica, 18, 521–526. (in Chinese with English abstract). Received 11 April 2014; revised version Zhang, G.L., Chen, S.Y., Yan, J.H., accepted 7 November 2014 Jiang, Z.X., Song, G.Q. and Qiu, G.Q.,