Air-Discharge Pits on the Yellow River Delta Plain

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Sedimentary Geology 170 (2004) 1–20 www.elsevier.com/locate/sedgeo

Air-discharge pits on the Yellow River delta plain

Zhong Jianhuaa,b,*, Wen Zhifenga, Wang Guanmina, Wang Xibina, Lu Hongboa, Shen Xiaohuac

a Institute of Earth Resources and Information, University of Petroleum Eastern China, Dongying, Shandong Province, 257061 PR China b Guangzhou Institute of Geochemistry, Chinese Academy of Science, Guangzhou 510640 PR China c Earth Science Department, Zhejiang University, Hangzhou, 310027 PR China Received 15 August 2002; received in revised form 13 August 2003; accepted 30 January 2004

Abstract

A large number of air-discharge pits, varying in size and shape, are developed on the Yellow River delta. Their planar cross- sections are circular, sub-circular, elongated and frequently irregular and their spatial configurations may be in form of a shallow dish to root like or other, extremely irregular, shapes. They range in diameter from less than 1 cm to over 60 cm and their depth varies from less than 1 mm to more than 20 cm. They most commonly occur individually or in groups on the point bar. Our investigation indicates that the occurrence of the air-discharge pits is closely associated with the unique hydrological, channel and hydrodynamic conditions caused by the intermittent flow interruptions of the Yellow River and the fine sediments typical of the river course. During a Yellow River flow interruption, an underground air-saturation zone in the dry layer of the riverbed may be formed due to the lowering of the ground water level. When flooding originates from the up-stream section of the Yellow River, the water quickly inundates the channel and rapidly submerges the extremely dry river bed and point bar. As a result, due to the blocking effect caused by the silty sand and mud, preferential percolation occurs along the outer edge of the point bar and levee, trapping air in the process. The resulting air pocket and pressure capsule are enclosed in the air saturation zone, providing the source material required for the subsequent formation of air-discharge pits. The trapped air may also form a high pressure capsule caused by the slow downward percolation of the river water. The differential, relatively lower pressure on the point bar permits the trapped air to escape. Provided the air is discharged in a shallow body of water ( < 10 cm deep), the force of the agitated water caused by the bubble explosion may liquify the surface of sedimentary deposits and thus facilitate the transportation of the sediment by the current flow which eventually results in the formation of an air-discharge pit. D 2004 Elsevier B.V. All rights reserved.

Keywords: Air-discharge pits; Flow interruption; Genesis; Delta plain and Yellow River

1. Introduction

The erosion pit (pot or pothole) is a common sedimentary structure (Quirke, 1930; Maxson, 1940; * Corresponding author. Institute of Earth Resources and Information, University of Petroleum Eastern China, Dongying, Higgins, 1957; Williamson, 1958; Allen, 1982; Mirria Shandong Province, 257061 PR China. and Petrovich, 1988; Myrow, 1992; Lopez, 2001; E-mail address: [email protected] (Z. Jianhua). Zhong et al., 2002a,b). It often occurs in ancient rock

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Fig. 1. (a) Map of the Yellow River valley and (b) location of the study area (hachured in Fig. 1a) (after Van Gelder et al.,1994). W: discharge; S: sediment yield; C: sediment load. ______中国科技论文在线 www.paper.edu.cn

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Fig. 2. Cross-section profile of the perched Yellow River (after Li and Finlayson, 1993). or recent sediment deposits. The main, causative channels formed by the currents small pits were factors determining the formation of erosion pits are produced on the surface of the sediment. Kindle as follows: (1996) used these results to explain the formation mechanism of the depressions (pits) in the Bay of (1) Water erosion (Allen, 1982; Myrow, 1992; Lopez, Fundy (Province of New Brunswick, Canada) 2001; Zhong et al., 2002a,b) and specific currents with diameters close to 30 cm and depths of 25 can lead to the formation of pits. Kindle (1996) cm. Cacchinone et. al., (1984) also used this type conducted an experiment and found that in a of interaction to explain the formation process of saline solution containing mud particles, currents the large scour pits (up to several hundred meters formed and moved upwards through the rapidly in diameter) off the coast of Califormia. Myrow settling mud, which, as soon as the currents (1992) suggested that pit (pot or pothole) ceased, precipitated inwards. Around the vertical formation was facilitated by unidirectional storm

Fig. 3. Grain size distributions of river sediments. ______中国科技论文在线 www.paper.edu.cn

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flow, proposing a model which emphasizes the action of water droplets produced from melting importance of a flow along a vertical shear surface ice, can form pits in fine sediment, especially on in generating deep hole vortices. However, the surface of loose fine silt and clay that is according to Alexander (1932) the current does unconsolidated (Zhong et al., 2002a,b). In the not directly produce a pit (pot) resulting from lower reaches of the Yellow River and the Yellow erosion. River delta large numbers of such pits are (2) Underdrainage (Williamson, 1958). Rising ground generated during the spring thaw every year. water currents and the associated water level differences can lead to the formation of pits. But Prior to the year 2000 many pits were observed on generally speaking they can only develop on the the Yellow River delta. we hypothesize that some of outside of a levee. In Dismal River, Nebraska, them were produced by air discharge, and not by other circular pits (conduits) have been observed (Guh- gases or formation mechanisms. They consequently man and Pederson, 1992), ranging in size from a represent a new type of pit. pencil like diameter of unknown depth to at least 10 min diameter and 44 min depth. (3) Gas erosion (Quirke, 1930; Maxson, 1940). The 2. Study area and upstream survey erosive action produced in the vicinity of rising gas bubbles of methane originating from the As the second largest river in China and one of decomposition of organic debris can lead to the the ten largest rivers in the world, the Yellow River formation of gas pits. The gas pits produced by has a drainage basin covering f 752,443 km2 (Fig. this process in Lake Mead, USA are up to six feet 1a), a length of f 5464 km and a mean discharge of in diameter and four feet in depth (Maxson,1940). f 44.8 Â 103 m/year. It is especially known for its (4) Erosion resulting from melting ice. The erosion tremendous sediment load of f 10.1 Â 109 m3/year effects caused by the dripping and spattering which consists mainly of silt and clay derived from

Table 1 Data for the flow interruptions of the Yellow River from Lijin to its estuary Year Flow interruption dates Flow interruption Flow interruption record Total lengths of flow First day Last day occurrences Flow interruption Flow Total interruption reach days days days Yellow River 1972 4.23 6.29 3 15 4 19 310 1974 5.14 7.11 2 18 2 20 316 1975 5.31 6.27 2 11 2 13 278 1976 5.18 5.25 1 6 2 8 166 1978 6.03 6.27 4 < 1 5 5 104 1979 5.27 7.09 2 19 2 21 278 1980 5.14 8.24 3 4 4 8 104 1981 5.17 6.29 5 26 10 36 662 1982 6.08 6.17 1 8 2 10 278 1983 6.26 6.30 1 3 2 5 104 1987 10.01 10.17 2 14 3 17 216 1988 6.27 7.1 2 3 2 5 150 1989 4.04 7.14 3 19 5 24 277 1991 5.15 6.01 2 13 3 16 131 1992 3.16 8.01 5 73 10 83 303 1993 2.13 10.12 5 49 11 60 278 1994 4.03 10.16 4 66 8 74 308 1995 3.04 7.23 3 117 5 122 683 1996 2.14 12.18 6 123 13 136 579 1997 2.07 12.13 13 202 24 226 700 ______中国科技论文在线 www.paper.edu.cn

Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 5 the loess plateau (Cheng, 1991; Van Gelder et al., and shallow, often being less than 10 m wide and 2 1994). A quarter of the sediment load is deposited in m deep, facilitating the overflow of flood water on to the channel of the lower river drainage basin, about the point bars and channel bars. half in the estuary region and the remaind are in the The sediment load in the river is extremely fine shallow waters of the Bohai Sea (Li and Finlayson, grained. Recordings taken at the Lijin Hydrometric 1993). As a result the riverbed of the lower river Station in 1987 show that the sediments consisted of drainage areas is continuously aggrading, leading to less than 10% fine sand, 50–70% silt and 30–40% the formation of a perched river with its riverbed clay (Cheng, 1991). For our granulometric analysis, elevated 3–10 m above the neighboring plains (Fig. we collected five samples in the study area in which 2) (Li and Finlayson, 1993). From the village Ning- the air-discharge pits are developed (Fig. 3) located on hai, Lijin county, to the estuary, the river course a bed material consisting mostly of fine silt with a eventually evolves into the modern Yellow River grain size distribution between 0.032–0.128 mm. delta with its channels located 0.5–2.0 m above Because granular materials of this size are easily the surrounding plain (Gao et al., 1989). As a result, liquefied, suspended and transported, they constitute the water level both of the ground water and the an important component in the formation process of surface water in the channel is always higher than air-discharge pits. the one found in the adjacent regions of land In 1972 the Yellow River experienced its first flow bordering on the levee. Additionally, because of the interruption and as a result some parts of the river heavy silt deposits, the main channel is both narrow course completely dried up. Subsequently this event

Fig. 4. Field photographs of a Yellow River flow interruption. (a) During 1996, the point bar was completely desiccated and mud cracks were developed on the riverbed. (b) Wind action resulted in many wind-eroded structures, one of them a mushroom-like shape up to 30 cm high. (c) An aeolian sand ridge about 50 cm high, 10 m meters wide and more than 100m long formed in the main channel. (d) The point bar was mostly covered by aeolian sand. ______中国科技论文在线 www.paper.edu.cn

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Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 9 was repeated almost every year. But in the 1990s the with the exception of a very few authors, research on flow interruption occurred more frequently. The river the effects of river flow interruption on the geology course downstream of the Lijin hydrological station and sedimentology has been inadequate until very experienced a flow interruption lasting 136 days in recently. During a flow interruption, the regions 1996 and 236 days in 1997. The flow interruption data affected become extremely dry (Fig. 4) and are often from 1972 to 1997 are summarized in Table 1. Flow covered with deposits of aeolian sand (Fig. 4c,d).Asa interruption cause many problems with regard to result, the pores of the sedimentary deposits of point industry and agriculture, but at the same time create and channel bars, and even the riverbed itself, become a series of interesting earth science phenomena. Many increasingly filled with air. geologists have studied the Yellow River delta (Cheng During flow interruption periods, distinct hydro- et al., 1986; Gao et al., 1989; Jongerius et al., 1989; logical and ecological changes took place. In addition, Hu and Xu, 1989; Cheng, 1991; Wright et al., 1998; the sedimentary features of the Yellow River are Zhong et al., 1997, 1999; 2002a,b; Zhong and Wang, characterized by the formation of many new geolog- 1999; Zhong and Li, 2000), but to our knowledge, ical phenomena, such as unique face sequences and

Fig. 5. (a–h) Field photographs documenting various air-discharge pit configuration characteristics. (a) Small size, quite circular and shallow, with a small air-discharge hole at the center and some small residual air bubbles around the hole (Shengli No. 1 point bar). (b) Small size, extremely shallow with air-discharge holes at the center and black material deposited around the border; clearly visible ring formations around the pit surface (Shengli No. 1 point bar). (c) Small size, distinctly circular and shallow, showing ring formations on the pit wall. Several clearly visible fissures which have been formed by air-heave mechanisms are located on the lower right section partially covered by the ruler (Shengli No.1 point bar). (d) Small size, circular, funnel-like shaped, with a well developed air-discharge hole or pipe in the center. Some ring structures, which are actually bedding, have developed mainly on the pit wall. Small, carbonized plant detritus deposit is visible in the lower part of the pit wall (Shengli No. 2 point bar). (e) Small size, loudspeaker shape, showing a well developed air-discharge pipe (conduit) over 20 cm in depth. (f) Small size, elliptically shaped with minute current rill marks along the slope of the right wall section. Air-discharge hole located on the left (Shengli No. 1 point bar). (g) Small size, well developed air-discharge hole with a diameter of about 4.2 cm, becoming smaller towards the bottom, with a depth of about 8.6 cm (Shengli No. 1 point bar). (h) Two small air-discharge pits occurring almost entirely in the form of air- discharge pipes with underdeveloped pit walls (Shengli No. 1 point bar). (i–p) (i) Large size, quasi-circular, showing relatively precipitous walls causing a small-scale collapse, and some fecal pellets. Additionally, small rill marks and ring structures, formed by bedding, are seen on the right of the pit wall. In the right bottom section are five small air-discharge holes. The ruler is 30 cm long (Shengli No. 1 point bar). (j) Large size, quasi-circular, very steep wall with three compound air-discharge holes at the center. Located on the pit wall ring structures formed by horizontal bedding associated with numerous, distinct rill marks and a weak lip rising on the right margin. Diameter is ca. 50 cm and depth ca. 40 cm (Shengli No. 1 point bar). (k) Large size, extremely irregular spatial structure. The pit wall is remarkably uneven throughout, with air discharge pits and rill marks very well developed. Bedding is distinct on the pit wall. The film box is 5.8 cm long. (l) Large size, extremely irregular spatial structure. The pit wall is remarkably uneven throughout. Some mud originating from outside has been deposited within the pit. The air discharge hole (pipe or conduit) and rill marks very well developed. The film box is 5.8 cm long. (m) Large size, extremely irregular structure. The pit wall is remarkably uneven throughout. Three air-discharge holes (pipe of conduit) are very well developed as are rill marks. The film box is 5.8 cm long. (n) Compound form, extremely irregular, formed by the agglomeration of about 6 single air-discharge pits. The pit wall is uneven, parts of it vertical and even overhanging. On most of the bottom part mud and silt have been deposited at a later point in time preventing the observation of any primary structures, such as air-discharge holes. Around the rim a wide and flat lip or spine, not easily seen, has been developed. (o) Compound form, resembling a honeycomb and formed by the combination of more than ten air-discharge pits. A thin mud layer with numerous wrinkles has covered most of individual pits (Xihekou point bar). (p) Extremely complex, compound shape, consisting of about 30 single, large pits, mostly covered by fine sediments (Shengli No. 1 point bar). q–x (q) Compound shape. Rill marks and small-scale ripples have developed on the wall. Three very small, circular secondary pits have developed on the bottom of the lower part. The film box is 5.8 cm long (Xihekou point bar). (r) Long, compound shape like a lotus root, formed by several, single air-discharge pits arranged in formation and around the rim a distinct lip or spine has been developed. The film box is 5.8 cm long (Xihekou point bar). (s) Extremely irregular shape, difficult to analyze because of its unique structure and a covering mud layer. The film box is 5.8 cm long. (Shengli No.1 point bar). (t) A special kind of an air- discharge pit. It is extraordinarily similar to a mud volcano with a well developed lip or bulge. The air-discharge hole is concealed due to the blocking effect created by a thin, pulp-like layer of mud (Shengli No. 2 point bar). (u) Mid-size. The pit wall slopes gradually in the direction of the current flow but is sharp inclined in the up-flow direction. Almost all sediments originating from the pit are deposited just around the pit, resulting in the disappearance of the ripples forming a flat petticoat-like structure. Diameter is about 20 cm (Shengli No. 2 point bar). (v) Mid- size. A thin muddy layer with concentric wrinkles has covered the two pit, obscuring the air discharge hole. The film box is 5.8 cm long. (Dongzhang point bar). (w) Large discharge pits occurring in groups, developed in a shallow point bar channel in the vicinity of which many air- heave structures have developed and covered by a thin layer of mud. The bottle is 30 cm high (Shengli No. 2 point bar). (x) Mid-size. The whole pit is covered by a thin layer of mud and accordingly cannot be differentiated (Shengli No. 2 point bar). ______中国科技论文在线 www.paper.edu.cn

10 Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 sedimentary structures (for example the air-discharge 6c), cylindrical pipe (Figs. 5g,h and 6d), pot (Figs. 5i pits). and 6e) or lotus-root (Figs. 5r and 6f), but others are Field investigations conducted in the Yellow River much more complex (Figs. 5j–q,s,v and 6g). region during the past 5 years show that a large number of air-discharge pits have developed along a 3.2. Structure and texture 30-km long reach of the river channel from Dongz- hang Village, Kenli county, to the Gudao Hydrometric The air-discharge pits on the Yellow River delta are Station (Fig. 1b). In previous publications dealing essentially composed of four parts: rim, wall, bottom with the Yellow River delta the existence of air- and pipe (conduit or hole). The rim is the fringe discharge pits has never been adequately addressed. around the surface of the pit. Due to the different hydrodynamic and material conditions in the channel, the rims are of many types: (1) Smooth (Fig. 5a–f); 3. Characteristic of air discharge pits (2) uneven type (Fig. 5g–s); (3) lip or spine shape (Fig. 5n,t). In addition several other structures may be 3.1. Shape and size observed such as ripple marks (Fig. 5w) or scour channels depending upon the subsequent action of Air-discharge pits on the Yellow River vary in waves or running water. shape with their size ranging from less 10 cm to over A lip or spine shaped bulge may develop on just 60 cm in diameter and a depth of a few millimeters to one or simultaneously on two sides of the rim and over 20 cm. Their horizontal cross-sections may be even around the pit surface (Fig. 5n,t). The lip bulge circular (Figs. 5a–e and 6a–e), slightly irregular (Fig. measures several mm to 1 cm in height and several 5f, g) or extremely irregular (Fig. 5h–s). Their forms mm to several cm in width., with steeply sloped inner often resemble a simple dish (Figs. 5a–c and 6a), sides and gradually sloping outer sides. In some cases funnel (Figs. 5d and 6b), loudspeaker (Figs. 5e and it may be superimposed by a concentric structure, the

Fig. 6. Plane and cross-section of typical air-discharge pits. ______中国科技论文在线 www.paper.edu.cn

Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 11 water-level mark. The air-discharge pits with well bulge are indicative of the existing specific air dis- developed lip bulges around their border closely charge and running water characteristics. A well- resemble mud volcanoes in shape (Fig. 5t). (Note: developed lip bulge is formed by large amounts of their origin, however, is quite different—the former is air escaping into quiet water with great intensity and produced by the action of discharged air, but the latter for a prolonged period of time. A poorly developed lip is formed by escaping water). According to our obser- bulge originates from the action of an intensive vations, some of the mud-volcanoes on the Yellow current that transports the sediment, which is tirred River delta may occasionally be formed by discharged by discharged air, away from the pit. Generally air rather than by escaping water. In Xinjiang, China, speaking, the development of a lip bulge necessitates mud-volcanoes formed by methane gas discharge may unidirectional flow during the formation of an air- also be observed. Occasionally the sediments coming discharge pit, whereas the development of a lip rising from the pit are deposited around the pit surface on two or more sides suggests a bi-or multidirectional forming a small-scale plate (Fig. 5u), whereas some flow pattern. The analysis of other sedimentary struc- relatively large sized materials such as fecal pellets, tures developed around the air-discharge pits corre- are only deposited within the pit (Fig. 5i).These sponds with that of the lip bulge. For example, ripple materials were originally produced from pit walls that marks and scour channels are absent around air- collapsed as the result of air-discharge action mecha- discharge pits with a well-developed lip bulge. In nism. In Fig. 5n, the upper wall edge is upright or comparison, abundant ripple marks or scour grooves over-hanging, and as a result it is unstable and may with a large ripple symmetry index (RSI = 4–8) are collapse at any time. developed around those pits that lack a well-devel- Based on the descriptions above we propose that oped lip bulge. Undirectional current ripple marks are some similar structures in ancient formations may developed around those pits with a lip bulge on only have been formed by air-discharge rather than by one side whereas the complex, super-imposed current water escape mechanisms. Indeed, pit (and hill) struc- ripple marks as well as reformed ripple marks, are tures may also involve into small, blister shaped hills formed around pits with a lip bulge on two or more with cones situated at their center. These are quite sides. similar in structure to air-discharge pits with lip bulges The lip bulge may be absent around most air- developed around the border. Their origin is specifi- discharge pits. However, a flat, marginal rim, subse- cally related to air-escape action mechanisms (upward quently referred to as the marginal apron, is often movement of bubbles). developed around the pit, yielding a sharp contrast to A lip bulge is the result as air discharge or the the ambient ripple marks (Fig. 5u). The sediments in interaction between air discharge and running water. the air-discharge pit account for the formation of Accordingly, the development and structure of the lip marginal aprons where the sediment is spread around

Fig. 7. Shapes of typical air-discharge pipes. (a) Tube-like with a constant diameter. (b) Elliptical. (c) Conical with a large top and a small bottom. (d) Figure ‘‘8’’ shape, formed by the fusion of two discharge pipes. (e) Complex shape resulting from the fusion of two elliptically shaped discharge pipes. ______中国科技论文在线 www.paper.edu.cn

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Fig. 8. Field photographs of air-heave structures. (a) Mid-size, circular, with small, narrow and upright seam-like air-discharge pits developing near the center. Diameter c 1 m. (b) Mid-size. Two compound, small air-discharge pits are formed at the center surrounded by many, small ice- induced wrinkles. The film box is 5.8 cm long. (c) Mid size. About 1m in diameter and 10 cm high. Numerous radiating fissures have developed around the center. The structure is surrounded by a ring of green algae marking the water level. The ruler is 30 cm long. (d) Mid-size. Approximately 1 m in diameter. Formed in a riverbed that is covered by still water and erupting air when it was photographed. The part above water surface is about 40 cm in diameter. (e) Large-size. Small, erupting fissures near both the center and the edge and some additional collapsed fissures. Some fragments were formed by explosions around the erupting fissures. Diameter>2 m. (f) Six air-heave structures arranged in a line in the direction of the river flow, accompanied by air-discharge pits. ______中国科技论文在线 www.paper.edu.cn

Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 13 the pit by the combined action of gas, running water vary considerably ranging in diameter from several and wave motions. The wall that reflects the forma- mm (Fig. 5a–c) to 2–3 cm (Fig. 5e–m). The cross- tion process of the pit, constitutes the principal sectional areas may be circular (Figs. 5a–g and 7a,c), structural component of the air-discharge pit and sub-circular to elliptical (Figs. 5h and 7b) or even essentially determines its shape. According to the elongated like a seam (Fig. 8a). When two pipe-like slope angle, the wall can be divided into three major pits are combined the resulting structure resembles a categories such as gentle slope (Fig. 5a–c), moderate figure ‘‘8’’ (Figs. 7o and 8b) or the form like Fig. 7E. (Fig. 5d) and steep (Fig. 5j–n,r). In addition, the This kind of pit consists entirely of a pipe (conduit), a wall structure can be divided into two types, even phenomenon caused by the rapid fall of flood levels or (Fig. 5a–f,t,u) and uneven (Fig. 5j–m). In the case an insufficient discharge of air. The following five of compound air-discharge pits, formed by the ag- basic structural configurations may be differentiated glomeration of several pits, the wall is often poorly (Fig. 7). developed and evolves into the complex forms (Fig. Air-discharge pits are often accompanied by air- 5o–q). Furthermore, air-discharge pits may be cate- heave structures (Fig. 8a–f) which implies that a gorized as symmetrical (Fig. 5a–e) are asymmetrical potential, morphogenetical relationship exists between (Fig. 5h,j–m) based on the degree of wall symmetry. the two. Individually (Fig. 5a–m), in groups (Fig. 5w) Commonly, air-discharge pits have a distinct wall, or in combined form (Fig. 5n–s,v) air-discharge pits however, it may be absent in some cases. For in the Yellow River delta are generally developed on example, the air-discharge pits shown on Fig. 5g,h point bars (Fig. 9). In many instances, however, a pipe lack a wall. cannot be observed at all due to subsequent particulate Small air volume discharge holes most common in material deposition and hydrodynamic processes the Yellow River delta. The bottom is the base of the caused by running water (Fig. 5n–p,s–x). Pipes or air-discharge pit but only in specific types, such as sand holes produced by air discharge may often be pot-shaped pits with a steeply dipping wall, can a observed on modem beaches. As a result of tidal distinct bottom be observed. The bottom tends to action mechanisms air is driven out of the sand merge with the wall and is even absent in some escaping through the upper, saturated layers to form instances such as is the case in the loudspeaker shaped pipes in the swash zone. Normally the pipes consist of air-discharge pit (Figs. 5e and 6c). On the other hand, upright, cylindrical or irregular shaped tubes measur- in air-discharge pits with a gently dipping wall, the ing several mm across that reach to the surface of the bottom is still vaguely recognizable (Fig. 5a–c). sand releasing streams of bubbles into any covering The pipe (conduit or hole) is an additional impor- water (Allen, 1982). In Dismal River, cylindrical tant structurally visible component of air-discharge conduits produced by boiling sand springs can be as pits (Figs. 5a–m,q,r and 7), unless covered by subse- large as 10 m in diameter and 44 m in depth (Guhman quent deposits such as mud layers (Fig. 5n–p, s–x). and Pederson, 1992). Nevertheless, in sedimentary Generally an air-discharge pit has a single pipe (Figs. rocks pipe (conduit) structures may be formed by 5a–h and 7a–c), but some (Figs. 5i–k,m and 7d,e) other processes as well. For example, there are pipe have several pipes. The shape and size of the popes structures formed by fluidization in Uratanna forma-

Fig. 9. Sketch to illustrate the location of the air-discharge pits. ______中国科技论文在线 www.paper.edu.cn

14 Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 tions (Lower Cambriam) in South Australia measur- 5. Nearly all of the pits lack surrounding drainage ing 1–6 cm in diameter and from 20 to over 50 cm in gullies (Fig. 5), which would have to form around length (Mount, 1993) which are slightly larger than pits produced by ascending water springs, as is the ones found in the Yellow River delta. Massari shown by the spring pits in Dismal River which (2001) also reported some giant pipes produced by each display a small gully that drains the flow to escaping water. the outside (Guhman and Pederson, 1992). Fur- thermore, if the pits are formed by ascending water springs they should develop during flow interrup- 4. Analysis of origin tions when the ground water level is lower and the water head difference outside the riverbed is higher The origin of the air-discharge pits on the Yellow than that in the riverbed. This phenomenon has River delta is unique mainly as a result of the existing never been observed. hydrological, channel and hydrodynamic conditions 6. Pit formation in the Yellow River delta is not the as well as the riverbed structure generated by the result of gas discharge Although we were able Yellow River flow interruption and the fine-grained occasionally to locate some gas domes, they cannot sediment load. be considered as the principal pit formation We propose that the formation of some pits devel- mechanism. The gas domes observed were com- oped on the Yellow River delta is related to air paratively small in size, generally measuring less discharge mechanisms but not to springs or other than a few centimeters in diameter and about one potential causative processes. The main arguments cm in height. In addition, insufficient quantities of in support of our hypothesis are as follows: gas are available for the formation of large pits 40– 50 cm in diameter and ca. 30 cm deep. Our field 1. The vast majority of pits are associated with air- observations show that organic matter in the heave structures (Fig. 8);somehaveformed sediments is quite sparse, so that biodegradation directly on the m (Fig. 8a,b). processes would not produce the amounts of gas 2. Occasionally gas which was not flammable was required to form the gas pits. We therefore observed bursting from several pits. In Fig. 5a, hypothesize that the pits found in the Yellow River small residual air bubbles surround the pit. delta reflect a specific air-discharge morphogene- Elsewhere extensive and violent air bubble explo- sis, whose underlying, detailed formation mecha- sions were observed at the water surface. nisms and processes are quite complex. 3. The pits only develop following a prolonged flow interruption, generally 1–2 days after the subse- We propose the following underlying mechanisms quent flooding. During flow interruptions we never and sequential processes for the formation of air- observed pits being formed by spring-type pro- discharge pits. cesses. During 2001 and 2002, the water manage- ment regulations ensured a continuous water 4.1. Air entrapment supply in the lower reaches of the Yellow River and flow interruptions did not occur. As a result we Just before a flow interruption, the river flow never observed any air-discharge pits during our velocity is slow (Fig. 10a), allowing the deposition regular field trips. of fine sediments (mainly silt and clay) (Fig. 10b).A 4. Because the Yellow River course is higher than the temporary ground water funnel is formed, sloping surrounding land (Fig. 2) during both flow from the natural levee into the channel, as a result interruptions and flooding, the water head in the of the flow interruption, together with an underground river is always higher than in the surrounding land. air-saturation zone beneath the dry riverbed, due to the As a result, upward movement of ground water lowering of the underground water surface. Although occurs only on the outside of the levee. This the ground water head in the levee is higher than that prevents the formation of spring-generated pits in the riverbed at that period of time, no springs or within the river course. spring-type pits are, developed in the river course. The ______中国科技论文在线 www.paper.edu.cn

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Fig. 10. Sketch to illustrate the formation of a high pressure air-capsule. ______中国科技论文在线 www.paper.edu.cn

16 Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 main reasons for this hydrodynamic anomaly may be structures and air-discharge pits on the point bar as follows: (Fig. 10d). Previous researchers found that under special con- (1) The sediments in the river course consist of ditions dried sediments can entrap air to form a extremely fine, particulate matter (silt and clay), unified air bag or pressure capsule as shown in this preventing the groundwater within the levee from paper, in the process producing some unusual struc- seeping into the river course. tures such as cavernous formations, sand domes and (2) The ground water head difference in the levee is sand holes (Allen, 1982). Such air-entrapment struc- too small to form springs or spring-type pits. tures are widely developed on the upper parts of (3) Because of the differential resistance to water beaches and on the crests of intertidal beach bars, flow, ground water within the levee will seep into where the normally fine or very fine grained sand the cultivated land beyond the levee. drains well at low tide. Some researchers have showed experimentally that air entrapment and cavernous After flooding and prior to the occurrence of a structures develop after repeated flooding and drying flow interruption, the Yellow River has a slow flow of sand. velocity and often becomes stagnant. As a result, the surface of the channel may become veneered by a 4.2. Air discharge deposit of silty sand or mud (Fig. 10b), which dries out during the flow interruption. Moderate flooding The detailed process of the air-discharge pit for- (exceeding 500 m3/s) originating upstream will rap- mation may be described as follow. The air trapped in idly inundate the point bars. Field observations show the pressure capsule moves upward because of the that the channel and point bars become rapidly existing density difference from its surroundings, and submerged, within about 1 h. As a result, preferential discharges into the water through various types of percolation is confused to the outer edge of the point pipe-like channels. The discharged air may lead to an bar adjacent to levee (Fig. 10c), as the silty sand and explosion or the air–water interface if the pressure mud layer forms an aquiclude. Thus the water cannot capsule is located in shallow body of water (less than seep quickly into the air-saturated zone beneath the 10 cm deep). The explosive force agitates the surface dried riverbed. This zone is sealed off, trapping the water triggering vibrations which cause the liquefac- air in the porous sediment, to form an air bag. Owing tion of the saturated sediments around the discharge to the relatively deep water and the concomitant high hole, by lowering the coefficient of friction between water pressure, the river water tends to percolate the sedimentary particles. downward into the channel. This downward percola- We conducted a simple experiment to determine tion in conjunction with the lateral percolation along whether the saturated sediments in the Yellow River the point bar entraps the air in the point bar forming a bed could become liquefied under the action of air- relatively high pressure air capsule (Fig. 10c). Be- discharge. We inserted a fine pipe into saturated sedi- cause of slow water percolation below the point bar, ments (mainly fine silt) in the Yellow River bed the air in the high pressure capsule is continuously covered by water less than 10 cm deep and connected forced upward by water percolation, beneath the the opposite and of the pipe to an air cylinder. When we channel process generating a series of air-heave pumped air into the pipe the sediment was rapidly

Fig. 11. Sketch to depict the formation of an air-discharge pit (Horizontal cross-section of the central core). (a) Normal deposits in river bed (Ia) and still water deposits during the first flow interruption (Ib). (b) Normal deposits in river bed (Ia) underlying a layer of fine deposits formed during the first flow interruption (Ib). Layer IIa was formed during the discharging, resulting in leakage and lateral seep age making the water table rise and compressing the trapped air. (c) With standing rise of the river, layer IIb was deposited, which leads to a higher pressure and the leakage and extruding of trapped air. Layer Ib, formed by fine deposits during the earlier flow interruption will be broken through by trapped air with a high enough pressure. The leakage will result in the air-discharging channel in layer II. The leaked air will explode when reaching water surface forming vibrating waves and liquifying the recently deposited sand layer. (d) Liquified silts are carried away by flow, resulting in the formation of an air discharge pit. ______中国科技论文在线 www.paper.edu.cn

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18 Z. Jianhua et al. / Sedimentary Geology 170 (2004) 1–20 liquefied and started to flow easily. Nichols (1994) 5. Conclusions experimentally reproduced the formation processes of fluid escape structures, such as dish structure, pillar- (1) Numerous air-discharge pits (and air-heave type structures and mud volcanoes. Owen (1996) structures) are developed on the Yellow River experimentally showed that when saturated sand delta plain, mainly as a result of the unique body is shaken it is easy to liquefy. Other investi- hydrological, channel and hydrodynamic condi- gators have also studied liquefaction mechanisms of tions caused by the Yellow River flow interrup- unconsolidated sediments (Dlrymple, 1979; Ishihara, tions as well as by the fine grained sediments 1993; Nichols, 1995). Some think that fluidization found in its delta. and liquefaction process is mainly caused by earth- (2) The air-discharge pit is a new type of structure, quakes (Sead and Idriss, 1967; Lin, 1997; Obermeier, which has not yet been reported previously in the 1995). Neither of these researchers considered the sedimentological literature, or described in detail. possibility that air discharge into shallow water can (3) The paper explains the origin and formation cause unconsolidated sediment to become liquefied. mechanisms of air-discharge pits. The ascending air bubbles can make the water move (4) The characteristic features and formation pro- upwards thus giving rise to a circular movement cesses of air-discharge pits are indicative of the pattern in the water body near the air-discharge hole water depth and the associated, hydrodynamic or air-discharge pipe (conduit). As a result the sedi- conditions. ments in the pit bottom can be stirred up and (5) Some air-discharge pits on the Yellow River delta continuously carried away by the slowly running have many similarities with potholes (Aigner and water, forming air-discharge pits (Fig. 11). This kind Fetterer, 1978; Allen, 1982; Myrow, 1992) in of structure, termed as air-discharge pit, was not several respects, such as shape, texture and addressed in several important sedimentological pub- structure. Our knowledge will contribute signif- lications (Shrock, 1948; Reineck and Sengor, 1979; icantly to a better understanding of pothole Allen, 1982; Reading, 1985). This leads us to believe formation mechanisms. that the air-discharge pit is a novel sedimentary (6) The detailed study of air-discharge pits on the structure. Yellow River delta provides us with more insight Neither environments with deep water nor envi- into the development of the pits. The research ronments without water favor the formation of air- results seem to indicate that the formation discharge pits. Very deep water can prevent the processes are much more complex than expected. liquefaction process resulting from stirred up sedi- Their degree of complexity is correlated with the ments, the critical condition in the formation process occurrence and combined interaction of the of air-discharge pits. The complete absence of water unusual hydrological, channel and hydrodynamic cannot lead to the formation of a pressure capsule and conditions caused by the Yellow River flow consequently an air-discharge pit. It is equally im- interruption. portant that the sediments be fine to very fine grained. There is some similarity in the formation mecha- Acknowledgements nisms of the air heaven structure and the air discharge pit. The difference is that the air in the high-pressure We are indebted to the National Science Founda- capsule continually migrates upwards, driven by the tion of China, University Doctoral Foundation of the bottom percolating water and, when it meets the thin Education Administration of P.R.C. and the Natural fine soft sediment layer, it accumulates continually, Science Foundation of Shandong Province for giving rise in the process to some air-heave structures their financial support. We would like to acknowledge (Fig. 10d) in the relatively lower places of the point the students of 1992–1995 grades, Department of bar where the sediments contain more abundant Resources, University of Petroleum, for their active muddy material which is more suitable for sealing participation in our field investigations. We are the air. particularly grateful to Dr. Keith A. W. Crook and ______中国科技论文在线 www.paper.edu.cn

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