Sedimentary characteristics and internal architecture of a river-dominated delta controlled by autogenic process: implications from a flume tank experiment
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Autogenic processes are widely found in various sedimentary systems and they play an important role in the depositional evolution and corresponding sedimentary architecture. However, autogenic processes are often affected by changing allogenic factors and are difficult to be identified and analyzed from modern and ancient records. Through the flume tank experiment under constant boundary conditions, the depositional process, evolution principles, and the sedimentary architecture of a river-dominated delta was presented, and a corresponding sedimentary architecture model was constructed. The evolution of river-dominated delta controlled only by autogenic process is obviously periodic, and each autogenic cycle can be divided into an initial progradational stage, a middle retrogratational stage, and a late aggradational–progradational stage. In the initial progradational stage, one feeder channel incised into the delta plain, mouth bar(s) was formed in front of the channel mouth, and small-scale crevasse splays were formed on the delta plain. In the middle retrogradational stage, the feeder channel was blocked by the mouth bar(s) which grew out of water at the end of the initial stage, and a set of large-scale distributary splay complexes were formed on the delta plain. These distributary splay complexes were retrogradationally overlapped due to the continuous migration of the bifurcation point of the feeder channel. In the late aggradational–progradational stage, the feeder channel branched into several radial distributary channels, overlapped distributary channels were formed on the delta plain, and terminal lobe complexes were formed at the end of distributary channels. The three sedimentary layers formed in the three stages constituted an autogenic succession. The experimental delta consisted of six autogenic depositional successions. Dynamic allocation of accommodation space and the following adaptive sediments filling were the two main driving factors of the autogenic evolution of deltas.
KeywordsRiver-dominated delta Autogenic process Depositional process Sedimentary architecture Flume tank experiment
There are a number of controlling factors that govern the depositional processes and the internal architecture of a sedimentary system including water and sediment supply, climatic changes, base-level variation, tectonic activities, and so on (Gong et al. 2016; Jia et al. 2018; Jia et al. 2016; Ventra et al. 2018; Zhao et al. 2018a; Zhao et al. 2018b). In order to clarify the controlling mechanism of each factor on the sedimentary system, Beerbower (1964) first classified the above factors into two types, including within-basin and extra-basin factors. The periodic and spontaneous evolution of geomorphology has been observed in most of the terrestrial sedimentary systems (such as alluvial fan, fluvial delta, braided river, and meandering river) when the extra-basin factors were maintained approximately constant (Carlson et al. 2018; Clarke et al. 2010; de Haas et al. 2016; Hajek and Straub 2017, 2017; Hamilton et al. 2013; Trampush et al. 2017; Van Dijk et al. 2009). As controlled by the within-basin factors only, the above processes are called autogenic processes (Miall et al. 2014). The widely existing fining upward sequence in the meandering river reservoir and the coarsening upward sequence in the delta are typical autogenic deposits (Hajek and Straub 2017; Miall et al. 2014).
Autogenic processes control the fundamental deposition of the sedimentary system, such as channel migration, avulsion, and abandonment (Clarke et al. 2010; Qi et al. 2018). It is these basic processes that give rise to the growth of sedimentary systems and to a large extent determine the internal sedimentary architecture. Even though an autogenic process only exists in a relatively short time compared to the allogenic process, the deposits formed in autogenic process get recorded in the stratigraphic framework created by the allogenic process (Hamilton et al. 2013; Miall et al. 2014). In other words, the autogenic deposits are the basic elements of the stratigraphy (Li et al. 2016; Wu et al. 2013). Therefore, it is not only helpful to guide the evaluation and development of oil and gas reservoirs scientifically, but also essential for sedimentology research to clarify the autogenic sedimentary process and its controlling effect on the internal architecture of sedimentary systems.
The autogenic depositional processes and the associated sedimentary mechanism have received extensive attention and have been thoroughly researched. As described by Hajek and Straub (2017), the typical autogenic processes include dune migration, upstream migrating cyclic steps, channel migration/meandering, channel bifurcation, avulsion, and regrading of depositional surface. All these processes are very common in modern sedimentary systems and stratigraphy records (de Haas et al. 2016; Lin et al. 2018a, b). For a sedimentary system, when the allogenic boundary condition is constant or approximately stable, its topography and sedimentary architecture is completely dominated by the autogenic processes (Clarke et al. 2010; Hajek and Straub 2017; Van Dijk et al. 2009).
In general, autogenic behavior is usually observed during allogenic variations in field, which makes it difficult to study its characteristics and significance (Van Dijk et al. 2009). In order to clarify the autogenic depositional process and associated internal architecture of sedimentary system, flume tank experiments have been performed by researchers on various sedimentary systems in recent years (Carlson et al. 2018; Esposito et al. 2018; Trampush et al. 2017; Wang et al. 2015; Yu et al. 2017; Zhang et al. 2016). To date, extensively experimental studies have been performed to investigate the autogenic behavior of alluvial fans, fan deltas, meandering rivers, and so on (Clarke et al. 2010; Clarke 2015, 2014; Crosato et al. 2011; van Dijk et al. 2013; Egozi and Ashmore 2009; Ganti et al. 2016; Kim et al. 2013; Nicholas et al. 2009; Paola et al. 2009; Reitz and Jerolmack 2012; Van Dijk et al. 2009). As pointed out in a review by Paola et al. (2009), flume tank experiments produce spatial structure and kinematics that, although imperfect, compare well with natural systems despite differences of spatial scale, time scale, material properties, and number of active processes (Hajek and Straub 2017). Therefore, the experimental sedimentary systems can be regarded as a substitution of the field scaled ones. In recent years, flume experiments were used for investigating the autogenic process of fan delta (Van Dijk et al. 2009), fluvial delta (Foreman and Straub 2017; Carlson et al. 2018) using non-cohesive sediment and constant sea level (not constant relative base level). These types of sedimentary systems usually developed in arid and semiarid climate. In humid climate, the sediment in various types of sedimentary systems was cohesive. However, the experimental studies on autogenic process-controlled river-dominated deltas using cohesive sediment and constant relative base level are very limited (Straub et al. 2015). For river-dominated delta, even though its extreme complex sedimentary architecture was investigated by numerous studies using outcrops, modern deltas, and numerical models (Wright 1977; Edmonds and Slingerland 2007; Ahmed et al. 2014), but the autogenic successions included in the sedimentary records were affected by changing boundary conditions (Wellner et al. 2004). The formation process and mechanism of the mouth bar in the delta was investigated by numerous scholars by numerical simulation and field work (Wright 1977; Edmonds and Slingerland 2007; Ahmed et al. 2014), but these studies only focused on mouth bar or a part of river-dominated deltas. Thus, the complete and detailed sedimentary architecture of an entire delta was not clear enough yet. Therefore, the autogenic evolution of depositional process and associated internal architecture of cohesive sediment-dominated deltas in an unconfined catchment still need to be investigated.
In this paper, we investigated the spontaneous sedimentation and evolution in the autogenic processes of cohesive sediment-dominated deltas based on the experiment performed at Tulane University by Straub et al. (2015). The entire process during the formation of the experimental delta had been recorded with a digital camera and a 3D laser scanner. The digital elevation models (DEMs) had been constructed with scanned topography data and were utilized for topography and sedimentary evolution analysis with the assistance of a newly developed program. Based on the analysis of the autogenic process and the internal architecture of the experimental delta, six autogenic cycles have been identified.
2 Experimental design
Boundary condition of the experiment (modified from Straub et al. 2015)
Experiment run time, h
g polymer per kg sediment
Water level rise rate, mm/h
TDB-13 (weakly cohesive)
1.72 × 10−4
3.91 × 10−4
To understand the evolution of the depositional process and the resulting sedimentary architecture of the experimental delta, we developed a program with MATLAB for the analysis of the DEMs. With the help of the program, four sections across the flow direction and one section along the radial flow direction were figured (Fig. 2).
3.1 The periodicity and general characteristics of the autogenic depositional process
The autogenic processes were constrained under the constant boundary condition during the experiment. The water and sediment supply, the total accommodation space, and relative base level remain stable (Hajek and Straub 2017). Therefore, the variation of depositional process and the complex internal sedimentary architecture were spontaneously formed. In the experiment, the spontaneous periodic evolution of the delta has been observed.
3.1.1 Periodically vertical transition of sedimentary architecture elements
3.1.2 Periodic evolution of delta geomorphological characteristics
The fraction of the delta top covered by active flow (fW) is an indicator of the development degree of the channels on the top of the experimental delta. Meanwhile, the roughness of shoreline (RSL) quantified the fluctuations of the shoreline along the delta plain (Straub et al. 2015). In an autogenic process, fW experienced several cycles of rising first and then falling. On the contrary, RSL experienced several cycles of falling first and then rising. It was worth noting that the trends of fW and RSL were exactly the opposite. Both curves of fW and RSL suggested that the geomorphological characteristics (including development degree of channels and the roughness of the shoreline) in the delta had obvious periodic changes under the control of autogenic process (Fig. 4b–c). The spontaneous periodic evolution in distributary channel development degree and shoreline roughness demonstrated that the autogenic processes were very complex rather than simple and stable.
3.1.3 Geomorphology dynamics in the autogenic processes
It was observed that there were six sedimentary cycles driven by the autogenic process in the experiment. According to the evolution of fW and RSL of the experimental delta, deposition on the delta experienced a sequential variation in each cycle, and each cycle was similar (Fig. 4b, c). The whole delta was constituted by the deposition of six such cycles. Therefore, investigation on the depositional process and associated sedimentary evolution in each cycle is essential to understand the nature of geomorphology dynamics of the delta driven by autogenic processes.
At the initial stage, a feeder channel incised into the delta plain and continuously brought sediments to the delta front. Under continuous sediment feeding, a lobe-shaped mouth bar was formed and grew continuously (Fig. 5a, b). The middle stage of an autogenic cycle was characterized by retrogradational distributary splays (Fig. 5c, e). During the late stage, the feeder channel branched into several small-scale distributary channels, and each of these distributary channels fed a terminal lobe at the distal part of the delta plain (Fig. 5f).
3.2 Geomorphology dynamics and associated internal architecture during different stages
3.2.1 The initial progradational stage
According to the observation on digital images and the DEMs, the incised channel was relatively stable; meanwhile, deposition on the delta plain was characterized by crevasse channels and associated crevasse splays (Fig. 8). At the beginning of this stage, only one or a limited number of isolated crevasse splays developed on the delta plain (Fig. 8a, b), and the development degree of levee was too low to be identified (Fig. 8a, b). With the development of the mouth bar in the delta front, the area, quantity, and sediment thickness increment in the crevasse splays on delta plain increased gradually (Figs. 6 and 8). At the end of the initial stage, the crevasse splays were laterally overlapped rather than isolated (Fig. 8d, e). Overlapped map of the areas that has an increment more than 3 mm in A-E demonstrated that most of the sedimentation on the delta plain during the initial stage were deposited at the later hours. Meanwhile, there were nearly no sediments deposited in a large fraction of the delta plain (Figs. 8f and 10). In the latter 21 h during this stage, levees tended to develop well by usually distributing along the channel bank continuously (Fig. 8c, e) and were about 0.25 m wide (Fig. 9).
It was worth noting that the feeder channel was laterally migrating during this stage (Fig. 10). The maximum migration distance was about 0.06 m. The crevasse splays were distributed on the side that was eroded by the migrating feeder channel (Fig. 10a–c).
3.2.2 The middle retrogratational stage
To clarify the precise sediment distribution during the middle stage, the sediment thickness increment per 5 h is calculated and shown in Fig. 12. In each of the increment plot of sediment thickness, the distributary splays were symmetrically distributed on both sides of the feeder channel (Fig. 12). The shape of distributary splay complexes was often short elliptical or irregularly lobate, and several smaller and nearly paralleled small channels developed in the complexes (Figs. 11 and 12b). Compared to the crevasse splays formed during the initial stage, the distributary splays were obviously larger in area and thickness. The distributary splay complexes formed in 5 h were as large as 0.35 m2 in average, and the maximum thickness of the splays in 5 h was thicker than 10 mm (Fig. 12).
Retrogradational distributary splays formed during this stage nearly covered the whole delta plain (Fig. 12f), and the average sediment thickness increment during this stage was about 20 mm on delta plain (Fig. 13). Therefore, at this stage, sedimentation mainly occurred on the delta plain, and the fraction of the delta top covered by active flow (fW) was larger than the initial stage (Fig. 3b). The distributary splays were fed by multiple small channels, each of these small channels dominated a single small splay. Consequently, the edge of the large splays was very rough. As the delta plain consisted of a set of retrogradational distributary splays, it had a larger RSL than that in the initial stage (Fig. 3c).
3.2.3 The late aggradational–progradational stage
3.3 Sedimentary architecture of autogenic process dominated delta
Combined with the sedimentary characteristics and evolution in different stages of the experiment, the sedimentary architectural model of autogenic process forced river-dominated delta in the three stages was established according to the observations made via the delta deposition process and quantitative analysis.
3.3.1 Sedimentary architecture in the initial stage
3.3.2 Sedimentary architecture in the middle stage
Distributary splay and feeder channel filling were the main sedimentary architecture elements developed during the middle stage. Retrogradational bifurcation was the typical behavior of the feeder channel. Near-symmetric large distributary splay complexes developed on both sides of the feeder channel and the feeder channel was also filled with sediment (Figs. 14 and 19b). Due to the retrogradational bifurcation of the feeder channel, distributary splay complexes and channel filling was manifested as retrogradational patterns (Figs. 12 and 13). The continuous bifurcation of the feeder channel and the sediment deposition made the surface of the delta smoother, and the surface elevation gradually decreased from the feeder channel to both sides. Compared to the crevasse splays developing in the initial stage, the distributary splays developing during this stage were significantly larger and thicker (Fig. 19a, b).
3.3.3 Sedimentary architecture in the late stage
In this stage, the experimental delta was characterized by multi-branched radial distributary channels and terminal lobes. Originating from the source, these distributary channels were obviously smaller than the feeder channels developing in the former two stages (Fig. 11). These distributary channels often transport sediment to the area close to the shoreline and form terminal lobes (Fig. 19c). The lobes overlap each other, leading to a significantly smooth shoreline (Figs. 11c and 19c).
The main sedimentary architecture elements were terminal lobes that were deposited near the shoreline. For the most time of this stage, radial distributary channels just act as a shallow sediment transport pathway and thus hardly form channel filling deposits (Figs. 16 and 19). In addition, since the distributary channels were very shallow, overflow developed on both sides of the distributary channels. As a result, a small fraction of the sediments was deposited on the delta plain (Figs. 17 and 19).
4 Discussion: driven mechanism of the autogenic evolution in the experimental delta
The periodic evolution of the delta autogenic deposition process is completely spontaneous in the absence of external environmental forces like sediment supply, water flow, tectonic activity, etc. (Straub et al. 2015). Scholars have also discovered autogenic cycles in sedimentary systems such as meandering rivers (Miall et al. 2014), alluvial fans (Clarke et al. 2010), and deltas (de Haas et al. 2016; Trampush et al. 2017; Van Dijk et al. 2009; Yu et al. 2017). A similar experiment presented by Van Dijk et al. (2009) investigated the sedimentary process of a fan delta using non-cohesive sediment and a constant sea level. Cycles of alternating sheet and channelized flow demonstrated that the autogenic process was significantly periodic. In the natural environment, a large number of similar fan delta could be identified along the shoreline of basins in arid or semiarid climate (Van Dijk et al. 2009; Jia et al. 2016, 2018). In contrast, the river-dominated delta presented in this paper was simulated using cohesive sediment and constant relative base level. In modern environment and ancient stratigraphy records, this type of delta mainly deposits along the shoreline of basins in humid climate (Li et al. 2015; Ielpi et al. 2018).
Fine analysis of the periodic autogenic evolution process on the experimental delta shows that the main mechanism was the dynamic distribution of accommodation space in the sedimentary system and the following adaptive compensation of sediments (Trampush et al. 2017).
With gradual progradation of the mouth bars, the accommodation space in the area in front of the feeder channel mouth gradually reduced (Hajek and Straub 2017). Once the channel mouth area ran out of accommodation space, the feeder channel bifurcation formed a large-area distributary splay. Hydrodynamics of the bifurcated flow greatly reduced, leading to the rapid formation of the distributary splays. At this time, the accommodation space on the delta plain was significantly larger than that in the delta front. As the growing distributary splays continuously blocked the flows, bifurcation point was forced to retreat upstream. The delta plain was gradually filled from the mouth of the feeder channel to the source (Foreman and Straub 2017), eventually forming multi-period retrogradational and overlapping distributary splay complexes (Figs. 19 and 20). In the middle stage, few sediments were carried to the delta front, thus continuously increasing its accommodation space. As the consumption of accommodation space on the delta plain, the middle stage ended and the late stage began (Fig. 20).
In the late stage, because the accommodation space in the delta plain has been very limited, particularly the little space on both sides of the feeder channel, several smaller distributary channels radiating from the source point were formed (Fig. 20). These small distributary channels extended through a locally lower area to the delta front, and terminal lobes developed at the area close to the shoreline (Fig. 19). In general, in the late stage, the sediments mainly concentrated near the shoreline of the experimental delta (Clarke et al. 2010; Trampush et al. 2017). On one hand, the delta plain was dominated by aggradational deposits, and on the other hand, the delta front was dominated by progradational deposits.
Overall, the dynamic change of accommodation space between the delta plain and the delta front during the autogenic deposition process and the adaptive compensation of the sediments led to the migration of the sedimentary center in the delta (Fig. 20). Changes in the morphology, scale, and combined style of the sedimentary architecture elements ultimately led to complex configurations within the delta (Carlson et al. 2018; Foreman and Straub 2017; Hamilton et al. 2013; Miall et al. 2014).
Controlled by only autogenic process, the river-dominated experimental delta experienced several autogenic cycles. Each autogenic cycle generally consisted of three stages: the initial progradational stage, the middle retrogradational stage, and the late aggradational–retrogradational stage.
In the initial stage, one channel developed from the source point to the delta front. Consequently, a large-scale mouth bar was formed at the end of the feeder channel. The feeder channel incised into the delta plain and remained unfilled until the end of this stage. Several small-scale crevasse splays and levees were deposited on both sides of the channel (s).
In the middle stage, the feeder channel was blocked by the mouth bar which grew out of water at the end of the initial stage and resulted in the bifurcation of the feeder channel and the development of large-scale distributary splays. Due to reactivated accommodation space, continuous feeder channel bifurcation and associated distributary splays gradually retreated toward a sediment feeder, and the feeder channel was filled with sediments.
In the late stage, the feeder channel branched into a number of small-scale, radial channels. Each of these distributary channels fed a terminal lobe in the area close to the shoreline. With rapid migration of the distributary channels, a set of distributary channel-terminal lobe complexes were formed and stacked together.
The distribution of accommodation space and deposition on the river-dominated delta were not balanced everywhere. Active deposition can usually control a fraction of the area. When deposition happens at this place, cumulation of the accommodation space happens in the remaining area. Allocation of the accommodation space and the following adaptive compensation of sediments were the main control factors driving the autogenic evolution of the delta.
The authors wish to express sincere gratitude to Professor Straub and Dr. Qi Li from Tulane University for their open experimental data and patiently providing suggestions to the users, which has greatly assisted our research. This study was supported by a National Natural Science Foundation of China (No. 41802123), a China Postdoctoral Science Foundation funded project (No. 2018M630843) and an Open Fund of Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. K2017-31). We highly appreciate the anonymous reviewers for the constructive comments and suggestions, which helped us to improve the paper.
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