Journal of Palaeogeography 2013, 2(2): 127-137 DOI: 10.3724/SP.J.1261.2013.00022

Lithofacies palaeogeography and sedimentology

Genesis of an unusual clastic dike in an uncommon braided river deposit

Su Dechen1, 2, A. J. (Tom) van Loon3, *, Sun Aiping1, 2 1. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2. State Key Laboratory of Continental Tectonics and Dynamics, Beijing 100037, China 3. Geological Institute, Adam Mickiewicz University, Maków Polnych 16, 61-606 Poznan, Poland

Abstract A clastic dike containing unusually large clasts occurs in the Quaternary depos- its that unconformably cover the Mesoproterozoic sediments in the Fangshan District, Beijing area, China. The material into which the dike intruded is also uncommon because it consists mainly of loess-type silts that were deposited by braided rivers. The intrusion of the dike is explained as the result of the expulsion of pore water into the coarse, gravel-containing layers of a braided river system. The large size of the clasts in the dike is explained by an exception- ally strong upwards‑directed flow which owed its high energy to a high hydrostatic pressure that had been built up because pore water could not gradually seep through the impermeable silt-sized material during ongoing burial. This uncommon dike is compared with a second ex- ample, in similar Quaternary sediments covering the Mesozoic rocks in the Huairou District.

Key words soft-sediment deformation structures, clastic dike, braided river deposits, loess, Quaternary, Fangshan District, China

1 Introduction* Most commonly they are built of medium and fine sand (Lowe, 1975; Jolly and Lonergan, 2002; Obermeier et al., Clastic dikes, now also known as “injectites”; see, 2002); they are rarely coarser and truly coarse-grained among others Satur and Hurst (2007), Rodrigues et al. dikes (with boulder-sized clasts) are exceptional (Hubbard (2009), Scott et al. (2009), Kane (2010), Whitmore and et al., 2007). As a rule, they result from the sudden up- Strom (2010), Hurst et al. (2011) are soft-sediment defor- ward movement of a water/sediment mixture (Van Loon, mation structures (SSDS) that occur frequently in sedi- 2009), derived from a pore-water containing layer, through ments that have been deposited subaqueously, particularly the overlying sediments, into the direction of lowest pres- in successions with a high sedimentation rate. They tend sure, i.e. towards the sedimentary surface. At the surface, to be massive, without an internal structure (Peterson, the water/sediment mixture flows out and the water mixes 1968; Hiscott, 1979, although more or less developed with the water body in which the sediments are depos- flow structures may be present (Harms, 1965; Scott et al., ited. The expelled sedimentary particles either form mud 2009); some authors claim distinct banding of 1-10 mm or sand volcanoes at the sedimentary surface (Van Loon, (Diggs, 2007), due to differences in grain size (Archer, 2009, 2010; Van Loon and Maulik, 2011) or become re- 1984) or grain alignment (Hillier and Cosgrove, 2002). worked at the sedimentary surface by currents or waves. It is also possible (though much rarer), however, that

* Corresponding author. Email: [email protected]; tom.van. dikes develop in a downward direction, for instance when [email protected]. faulting results in tension fissures that develop normal to Received: 2012-12-20 Accepted: 2013-01-10 the least direction of stress; such fissures may be filled by 128 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013 mobilized sand flows (Pratt, 1988; Schlische and Acker- ied layer (or layers) increases, until it is high enough to mann, 1995; Matsuda, 2000; Rowe et al., 2002; Silver and make a flow break through the overlying sedimentary seal. Pogue, 2002; Le Heron and Etienne, 2005). Large exam- This can occur when only a thin sedimentary cover has ples have been described by Netoff (2002) from the Jura- been built up, and this will result in small sediment/fluid- ssic in (USA), by Gozdzik and Van Loon (2007) from escape structures, which are commonly found in, for in- the Quaternary in Poland, by Parize et al. (2007) from the stance, shallow-marine sediments where sandy and clayey of France, by Whitmore and Strom (2010) layers alternate. Such structures are even more common in from the in Arizona, and by Lunina et al. (2012) lacustrine deposits (e.g. Moretti and Sabato, 2007; Tasgin, from the of Hungary. 2011; Lunina et al., 2012), particularly the Pleistocene gla- Many clastic dikes are related to seismic activity (Ober- ciolacustrine deposits (Brodzikowski et al., 1987; Chunga meier, 1996, 1998; Obermeier et al., 2002; Lunina et al., et al., 2007). 2012). This is because acoustic waves pass through soft Much rarer are escape structures-and consequently rock during an event; lasts some 0.8-2 s, much larger sedimentary dikes-in coarse-grained, poorly which is of the same order as the duration of the forma- sorted sediments such as those deposited by braided rivers. tion of clastic dykes, which propagate under turbulent flow This type of river is characteristic in areas where the river conditions with velocities of some 4-65 m/s (Levi et al., gradient is relatively high, a situation which tends to result 2011). The propagation of clastic earthquake-induced clas- in a high erosion rate; the consequence is a poorly sorted tic dikes thus couples two processes: fracture propagation sediment through which pore water from buried layers can (see Anderson, 1995) and the flow of clastics in a predomi- easily escape. nantly fluidized way. Escape structures and clastic dikes are even more rarely Many studies regarding clastic dikes pay attention to found in loesses: the silt-sized material tends to be insuffi- their geometry (Hurst et al., 2011). The dikes may be ciently permeable to allow seepage from buried sediment. straight and planar (e.g. Taylor, 1982), curved (Parize et Loess is principally deposited by wind, and pore water is al., 2007), and the dikes may be tapering (Rodriguez-Pas- therefore not necessarily present. Deposition tends to re- cua et al., 2000) or bifurcating (Hubbard et al., 2007) or sult in an undulating surface, however, so rain water ac- even show ptygmatic folding (Surlyk and Noe-Nygaard, cumulates in the lowermost part and may become covered 2001). It is remarkable that all these studies consider the by new loess layers; such new loess will become wet in its dikes as 2-D features, probably because, particularly in lower part, but if sufficient new loess is deposited imme- lithified sediments, 3-D exposures are scarce. It should be diately afterwards, the upper part will be dry. The conse- kept in mind, however, that dikes are, by definition, 3-D quence is that some moist loess may be included in overall bodies. In the example described here, we must conclude dry loess, but even in such a case escape structures are that the dike is not extended into two directions (i.e., it rare. This may also be due to the fact that much of the rain does not have the form of a sheet), but rather that it has a and run off water that may concentrate in loess depress- columnar geometry. ions will evaporate before new loess deposition prevents A prerequisite for the formation of upward‑directed further evaporation. clastic dikes is that a sufficiently high pressure (an “ab- As a result of the processes described above, clastic normal” pressure according to Montenat et al., 2007) is dikes in braided river deposits are rare, and clastic dikes built up to be able to make a flow break through the over- breaking through a loess body are also rare. The occurr- lying sediment (Jolly and Lonergan, 2002; Hurst et al., ence of such a feature is therefore remarkable. 2003; Chen et al., 2009). This happens as soon as the fluid pressure surpasses the lithostatic pressure exerted by the 2 The clastic dike weight of the overburden (Rodrigues et al., 2009). Such a process is not self-evident, as pore water tends, under The dike under study is not only remarkable because it common conditions, to seep gradually to the sedimenta- penetrates loess-like silty sediment (see Section 2.2), but it ry surface, through the pores of the overlying sediment. is also uncommon in that it contains large clasts (the larg- However, when the overlying sediment is not sufficiently est clast measured had a visible length of some 30 cm). The or not at all-permeable, thus forming some kind of seal, dike was found in August 2011 along a newly constructed such seepage cannot take place (or only extremely slowly), section of road G108 (near 75 km) in the Fangshan District so that the pore water pressure in the water-containing bur- of the Beijing area, China. Its position is 39°47’37”N and Vol. 2 No. 2 Su Dechen et al.: Genesis of an unusual clastic dike in an uncommon braided river deposit 129

115°46’31”E at an altitude of 280 m above sea level (Fig. loess has been eroded away. 1). The base of the dike is, unfortunately, no longer ex- The dike remained practically unchanged after it had posed, because a wall has been constructed (Fig. 2). been found in August 2011 until a heavy flood occurred on July 21, 2012 (Fig. 2 A, 2B). It appeared in September 2.1 Description of the dike 2012 that the upper part of the dike had, probably as a The dike, which is now partly hidden behind a wall that consequence of the flood, been largely washed away (Fig. hides its lower part (Fig. 2) extended before the wall was 2C), leaving only the already mentioned vertical string of constructed to the level of the road and must therefore be stones (Fig. 4). at least 8.5 m height; it is about 60 cm wide at its base 2.2 Description of the host sediment and some 40 cm in its top part. The boundaries between the dike and the surrounding loess-like material (for the The dike breaks through a unit that consists largely of sake of simplicity called “loess” in the following text) and well sorted silt-sized material that shows all the characteris- gravel are very clear and almost vertical. tics (including the formation of vertical cliffs) of true loess From bottom (Fig. 3) to top (Fig. 4), the dike gradu- deposits. It can not, however, be a primary, purely aeolian, ally contains less large clasts, until only a discontinuous loess, as strings (Fig. 5), horizons and discontinuous layers “vertical string” of clasts remains; it cannot be excluded, of pebble-to cobble-sized clasts occur, as well as isolated however, that this apparent upward diminishing amount clasts. The clasts were eroded from the tilted Mesoprotero- of clasts should be ascribed to the columnar character of zoic, Wumishan Formation, which underlies the loess-like the dike, which seems not entirely straight but a bit ir- host sediment. regular. The size of the gravel clasts ranges from a few The clast-rich layers in the silty host sediment may be millimeters to 30 centimeters; the clasts do not show considered as breccias or conglomerates, as some of the any sorting and their shapes range from very angular clasts are rounded to subrounded; whereas most clasts are to subrounded. The gravel in the dike consists entirely subangular to (mainly) very angular. For the sake of sim- of dolomite, indicating that the clasts are derived from plicity, we will call them “conglomerates”. The conglom- the Mesoproterozoic Wumishan Formation, which is the erates are poorly sorted, i.e. they consist of clasts of all only rock unit under the loess, and is exposed where the sizes (up to a maximum of 30 cm), but they contain rela-

Fig. 1 Satellite and geological map of the Fangshan area with the clastic dike. 1-Mesoproterozoic (mainly dolomites of the Wum- ishan Formation); 2-Lower Paleoproterozoic limestones; 3-Upper Paleoproterozoic; 4-Mesozoic volcaniclastics; 5-Pleistocene; 6-Holocene; 7-Mesozoic intrusion; 8-Fault. 130 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

Fig. 2 Overview of the clastic dike. A- The dike in its geological context, as present on June 12, 2012. B- Closer view on August 19, 2011; C- Closer view on July 3, 2012; D- Closer view on September 6, 2012. tively little sand-sized material; the matrix consists mainly occurs within the fine-grained parts of the conglomerates of the same loess that the dike intruded, and in which the (Fig. 6), and extremely rarely in the silty material. It can, conglomerate layers and lenses are embedded. however, not be excluded that the search for such struc- The lack of rounding of the clasts indicates that they tures in the “loess” has biased the researchers. One might were not transported over long distances. This is supported therefore describe the fine-grained material most properly by the anomalously low amount of sand in the conglom- as massive. erate layers: there was apparently insufficient transport to abrade the clasts to a considerable degree. 3 Interpretation The uniform grain-size of the silty material obscures any original that may be present. A sound interpretation of the genesis of the clastic dike Some faint indications of lamination are thought to oc- requires insight into the conditions under which it was cur locally, commonly in the form of parallel lamination, formed. For this reason, the age and the origin of the host exceptionally as cross-bedding; this almost exclusively sediment is interpreted first. Vol. 2 No. 2 Su Dechen et al.: Genesis of an unusual clastic dike in an uncommon braided river deposit 131

Fig. 3 The lower part of the dike, as far as still visible on Sep- Fig. 4 The top part of the dike on September 6, 2012, consisting tember 6, 2012, with very densely packed clasts of different almost exclusively of a vertical string of clasts that are still of deci- (mostly large) sizes, showing no preferred orientation. All clasts meter size, but distinctly smaller than those in the bottom part. All are dolomites of the Wumishan Formation. clasts are the Wumishan dolomites.

tons and as mass-flow deposits. Neither interpretation ap- 3.1 Age of the host sediment and the clastic dike plies here. The loess was deposited during the Malan interval, A genesis as a glacially deposited diamicton (till) can which belongs to the Late Pleistocene (Q3 in Fig. 1B) (Lü be excluded for several reasons. The most important is Jinbo and Li Wei, 2000). The thermoluminescence age of that diamictons do not show alternating layers of gravel 33-35.6 ka and the carbon age (of the calcium carbonate) and silt, but true mixtures of the various particle sizes. An- of 23 ka (Lu Yanchou et al., 1987) of the Malan loess at other argument is that the clasts do not show glacial striae. the Zhaitang section indicate that the host material is very Moreover, a land-ice cover tends to bring material from young. Obviously, the dike must be even younger, but no more or less remote areas, but all large clasts are of local precise dating is possible. nature. Finally, no other morphological or sedimentologi- cal signs of a glaciation are present, nor has this type of 3.2 Genesis of the host sediment sediment (here or elsewhere in the Fangshan District) been The joint, common occurrence of clasts of several deci- described as glacial. meters large in layers of only centimeters to a few decime- A genesis as a mass-flow deposit can also be exclud- ters thick, alternating with decimeter to meter thick layers ed. Low-viscosity flows such as turbidites and mudflows (Fig. 7) of silt requires specific depositional conditions. destroy primary sedimentary structures (and sometimes Most similar occurrences are interpreted as glacial diamic- produce new structures) because of complete mixing of 132 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

Fig. 5 Thin strings of pebbles are common in the silty host rock, excluding an exclusively aeolian depositional environment.

Fig. 6 Vague parallel lamination in a relatively coarse part of the fine-grained host material. the original sediment, but cannot form new gravel lay- have been transported and deposited by water. The pres- ers in fine-grained sediment. High-viscosity flows such ence of large clasts indicates that this must have been run- as slumps can preserve some of the original layering, but ning water. Running water capable of transporting large results-due to the processes involved-in bent, commonly clasts may be shallow marine (e.g. deposition by long- broken up layers, if any layering is preserved at all. shore current in the neighborhood of a hard-rock cliff). The succession must therefore be explained in another Marine transport may also have taken place by waves, way. The normal stratification of the gravelly layers and forming a pebble beach in a bay surrounded by hard-rock lenses indicates that “normal” transport media (water, ice, units. However, a marine origin must be excluded because wind) must have been involved. The size of the clasts ex- of the position of the study site 280 m above level, since cludes wind, and the absence of a glacial context and of this would imply that the area would have been invaded by glacial characteristics excludes ice. The material thus must the sea before or during the Quaternary, and was uplifted Vol. 2 No. 2 Su Dechen et al.: Genesis of an unusual clastic dike in an uncommon braided river deposit 133

Fig. 7 Characteristic facies of the host rock: pebble strings and pebble layers a few centimeters thick to a few decimeters thick, inter- calated with decimeter to meter-thick silty layers. at least 280 m during the past 35,000 years. No evidence is fluvial deposits and shifting of meanders may also result in available for such a dramatic event. Moreover, the angular vertical successions with alternating channel and floodplain shape of the gravel clasts is inconsistent with a transgres- deposits. However, in the sediments under study, the chan- sion conglomerate or a pebble beach. The conclusion must nels are far too small and shallow to be ascribed to a mean- therefore be that the sediments have a continental origin. dering river. Moreover, the small channels at the study site Continental environments where large clasts are trans- are incised in the silts that should represent the floodplain ported by water currents, while also transporting large deposits, but in this case they should represent crevasse- amounts of silt-sized material, are scarce. They include splay deposits. They are not, however, as crevasse-splay fans, rivers and deltas. Fans are characterized by numer- deposits are continuous and show a clear gradual decrease ous channels that may be incised deeply, by a dominance in thickness away from the main channel. The deposits thus of mass-transported sediments, and by a distinctly inclined cannot be explained as deposited by a meandering river, surface. The sediments under study do not fulfill any of which leads inevitably to the conclusion that they must rep- these requirements, so such a fan origin must be ruled out. resent a braided river system. More or less the same holds for a (lacustrine) delta. This If the silts at the study site were more poorly sorted leaves a river as the only possibility. sediments, the overall impression from the sedimentary Meandering rivers are, as a rule, characterized by deep succession would be that of a braided river deposit, indeed channels (with a gravel lag) and floodplains with fine- (Fig. 7). This leaves the question of how such an uncom- grained overbank deposits. These subenvironments are mon braided river deposit can have been formed. The situated beside each other, but ongoing accumulation of answer cannot be found in the transport or depositional 134 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013 conditions, as these are similar for braided rivers all over decimeter; see Fig. 8) in the middle part of the dike, and the world. The solution must therefore be the lithological even smaller in the upper part, which seems to narrow up- nature of the sediments in the source area, and this source wards, where the boundaries with the host rock also be- area cannot have been far away, as indicated by the angular comes less clear. clasts. The trigger for the sudden upward escape of the over- As mentioned above, the clasts consist of local Me- pressurized sediment/water mixture cannot be reconstruct- soproterozoic material, mainly dolomites. These do, ed with any certainty. Possibly shock waves released dur- obviously, not provide quartz silt (even though some of ing an earthquake were responsible, numerous researchers the dolomites may contain minor amounts of fine silici- have not found that dikes are one type of soft-sediment clastic particles). There is, however, one specific source deformation that commonly originates as a result of earth- that covers older rocks over extensive areas: true loess. quakes. It should be noted in this context that 67 earth- It must thus be concluded that such loess deposits, which quakes have been documented for the Beijing area during occur frequently in the neighborhood and which tend to recorded human history with magnitudes that are now in- be many meters thick, constitute the source of the silt- terpreted to have exceeded a magnitude of 7.0. sized material in the Quaternary deposits under study, although it cannot be excluded that the loess was deposit- 4 Discussion of the sedimentary envi- ed, entirely or partly, at or nearby the study site by wind. ronment As sandstones or quartzites do not occur in the neigh- borhood, it is logical that quartz sand is not present in We interpret the sedimentary environment as a braided the deposits. The bimodal grain-size distribution of these river system (see Section 3.2). This system could develop uncommon Quaternary braided river deposits can thus be because rain water ran periodically off the hills and moun- well explained. tains that surround the study site, and formed sheet floods in the more level parts of the area. These sheet floods carried 3.3 Genesis of the clastic dike clasts that were eroded from the Mesoproterozoic rocks, The uncommon grain-size distribution of the host rock as well as loess that was encountered along the way. If forms the clue for the genesis of the clastic dike. As men- this is the case, a characteristic braided river environment tioned above, braided river deposits are rarely intruded by existed (Fig. 7). clastic dikes, because pore water from buried layers can It is also possible that the area was actually a more or seep through the commonly poorly sorted and fairly per- less level place where aeolian deposition of loess pre- meable overlying braided river deposits to the sedimentary vailed. In that case, it might be considered as an aeolian surface. environment occasionally reached by sheet floods result- In the present case, this was, however, impossible be- ing from drainage of rain water that fell in the surround- cause a pore-water containing gravel layer was apparently ing hills and mountains. These sheet floods reworked the rapidly covered by an impermeable loess layer. Thus, pres- previously deposited loess. This only underlines that na- sure could gradually build up. The hydrostatic pressure be- ture does not draw distinct boundaries between different came consequently so high that a sediment/water mixture environments, so it seems a matter of semantics is terms of was able to find a way upward through what may have been how the environment is classified. Whatever choice may a zone of weakness. Because overpressure in the buried be made in this respect, it also underlines that only a more gravel was exceptionally high, large clasts could not only be complete palaeogeographic picture might indicate whether transported laterally to the place where the break-through the aeolian or the fluvial character prevailed most of the took place, but also upwards through the tunnel just formed time. through the silt-sized material. It is interesting in this con- text that the dike also cuts through a few gravelly layers, 5 Conclusions which may also have supplied both clasts and water, thus intensifying the process. Analysis of the environmental conditions of a succes- When the sediment/water mixture moved upwards, sion built of loess-sized material with intercalations of the hydrostatic pressure gradually decreased, resulting in gravel layers and horizons indicates that it must have acc- a diminishing transport capacity. This is reflected by the umulated in a braided river environment where aeolian smaller size of the clasts (although still up to over one activity was strong and may or may not have prevailed. A Vol. 2 No. 2 Su Dechen et al.: Genesis of an unusual clastic dike in an uncommon braided river deposit 135

Fig. 8 Clasts in the middle part of the dike. dominance of loess-like silty sediments in a braided riv- Xu, and Mr. Guo Rongtao for valuable suggestions and er system is uncommon. This silty material must at least for their help in the field. The second author is indebted have been partly derived from elsewhere, but may partly to Prof. Feng Zengzhao and Prof. Bao Zhidong, who pro- also have been deposited at or close to the study locations. vided the financial means to come to China to carry out This implies an uncommon sedimentary environment that the fieldwork. might be characterized as fluvio-aeolian. The alternation of poorly sorted and highly permeable References conglomerates and impermeable loess-like material led to increasing hydrostatic pressure in water-saturated sheet Anderson, T. L., 1995. Fracture mechanics (2nd ed.). Boca Raton; flood deposits, eventually resulting in the upward escape CRC Press, 688. of pore water (with clasts), thus forming in the Fangshan Archer, J. B., 1984. Clastic intrusions in deep-sea fan deposits of the District a large dike with exceptionally large clasts, which Rosroe formation, lower , western Ireland. Journal of Sedimentary Petrology, 54: 1197-1205. must also be considered as an uncommon phenomenon for Brodzikowski, K., Gotowala, R., Haluszczak, A., Krzyszkowski, a braided river succession. D., Van Loon, A. J., 1987. Soft-sediment deformations from glaciodeltaic, glaciolacustrine and fluviolacustrine sediments in Acknowledgements the Kleszczów Graben (central Poland). In: Jones, M. E., and Preston, R. M. F. (eds). Deformation of sediments and sedimen- The authors are indebted to Prof. Lü Jinbo, Mr. Duan tary rocks. Geological Society, London, Special Publication, 29: 136 JOURNAL OF PALAEOGEOGRAPHY Apr. 2013

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