Evidence of Internal-Wave and Internal-Tide Deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

中国科技论文在线 http://www.paper.edu.cn Geo-Mar Lett (2011) 31:509–523 DOI 10.1007/s00367-011-0253-z

ORIGINAL

Evidence of internal-wave and internal-tide deposits in the Middle Ordovician Xujiajuan Formation of the Xiangshan Group, Ningxia, China

You-Bin He & Jin-Xiong Luo & Xiang-Dong Li & Zhen-Zhong Gao & Zhan Wen

Received: 6 September 2010 /Accepted: 12 July 2011 /Published online: 19 August 2011 # Springer-Verlag 2011

Abstract The Xujiajuan Formation of the Lower Xiang- Introduction shan Group in Ningxia, China, is composed of grayish- green to yellowish-green, fine- to medium-grained sand- The study of internal waves in oceanography has a long stone, calcareous sandstone, siltstone, and shale. The upper history that can be traced back to the study of Stokes’ part is thin-bedded limestone. At the top of the second and interfacial wave theory of 1847 (Munk 1981). These are third members of the formation, a number of beds subaqueous waves that develop either between water layers intercalated between turbidites and deep-water shale show of different density or within layers with vertical density well-developed cross-bedding. These beds are composed gradients (LaFond 1966). They occur in all oceans and at mainly of thin- to medium-bedded calcareous siltstone, various depths (Munk 1981; Fang and Wang 1986; Du et al. fine-grained sandstone, fine-grained calcisiltitic limestone, 2001). Munk (1981) postulated that such internal gravity and silty shale. All bedding types reflect traction-current waves were as common as (or possibly even more prevalent action. The laminae of the bidirectional and unidirectional than) ocean surface waves. These gravity waves vary in cross-bedded units tend to dip either opposite to or at a amplitude, period, velocity, and depth (LaFond 1966; Munk large angle to the regional slope. The units vary in shape 1981; Gao et al. 1998). The longest periods correspond to and orientation in both upslope and downslope directions. those of semidiurnal or diurnal tides (Rattry 1960; LaFond A comprehensive evaluation of the sedimentary structures 1966; Munk 1981). Internal tides have frequently been and inferred paleocurrents suggests that the cross-bedded recorded in the oceans, and are more evident in deeper intervals were not formed by contour currents or turbidity water (depths exceeding 200–250 m; Shepard 1976). Both currents, but most probably represent internal-wave and internal tides and internal waves can generate large-scale internal-tide deposits. bidirectional flows at the seafloor, especially in submarine canyons and other types of submarine valleys (Shepard et al. 1979). Generally, the velocities of such currents range from 20 to 50 cm/s (Gao et al. 1998). Many aspects of internal waves are quite well understood because of Responsible guest editor: F.J. Hernández-Molina numerous observations and simulations. These include (1) Y.-B. He (*) : J.-X. Luo : X.-D. Li : Z.-Z. Gao : Z. Wen generation, superposition, propagation, and boundary-layer School of Geosciences, Yangtze University, phenomena (e.g., Nakamura and Awaji 2001; Tanaka et al. Nanhuan Road 1, 2003; Hibiya 2004; Lemckert et al. 2004; Nash and Moum Jingzhou 434023 Hubei, People’s Republic of China e-mail: [email protected] 2005; Rainville and Pinkel 2006), (2) breaking, reflection, diffraction, and attenuation when interacting with subma- Present Address: rine topography (e.g., Legg 2003; Small 2003; Troy and X.-D. Li Koseff 2005; Mercier et al. 2008), (3) swash–backwash Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Road 26, flows (e.g., Umeyama and Shintani 2004, 2006), (4) Beijing 100037, People’s Republic of China influence of various types of submarine topography (e.g.,

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Kunze et al. 2002; Pietrzak and Labeur 2004; Martin et al. al. 1997; He et al. 1998), Middle and Upper Ordovician 2006), (5) long and short periods (e.g., Marc et al. 1992; deposits of the central Tarim Basin (Gao et al. 1996, 2000; Anohin et al. 2006;D’Asaro et al. 2007), and (6) numerical He et al. 2003), Upper Paleozoic and Mesozoic deposits on simulations of sediment suspension (e.g., Bogucki et al. the western side of the Qinling Mountains (Jin et al. 2002; 1997; Venayagamoorthy and Fringer 2006). Wang et al. 2005), the Precambrian Anlelin and Xiushui Observations from underwater vehicles have revealed formations northwest of Jiangxi Province (Guo et al. 2003, that internal waves and internal tides can transport fine 2004), the Upper Ordovician at Linan, Zhejiang Province sand, and are able to generate ripples and dunes at water (Li et al. 2005a), the Precambrian Madiyi Formation at depths of several thousand meters (Mullins et al. 1982). Taojiang, Hunan Province (Li et al. 2005b), the Lower Lonsdale and Malfait (1974) suggested that the dune bed Cambrian Balang Formation at Shimen, Hunan Province forms observed in foraminiferal sands at a water depth of (He et al. 2005), the Middle Ordovician Pingliang Forma- 2,650 m on the northern flank of the Carnegie Ridge in the tion in the western Ordos Basin (He et al. 2007), and the eastern equatorial Pacific were generated by the spillover of Middle Ordovician Xiangshan Group (Li et al. 2009, 2010; dense, tide-driven water masses. Furthermore, using satel- He et al. 2010) in Ningxia Province. lite altimeter data from the Topex/Poseidon experiment, The investigation of internal-wave and internal-tide Egbert and Ray (2000) demonstrated that tidal dissipation deposits is still a young field in deep-water sedimentolog- in the open ocean also occurs at abyssal depths. Xu et al. ical research (He and Gao 1999;Shanmugam2008), (2002) reported the importance of tidal currents in the although some progress has been made these last years. Monterey Canyon, the largest submarine canyon along the One major obstacle is the lack of sufficient documentation. California coast. Shanmugam (2003) pointed out that tidal In the Xiangshan Group Xujiajuan Formation in the currents develop in deep-water canyons and channels. Xiangshan area, Ningxia, China, the recent discovery of Previous oceanographic investigations have shown that several depositional units considered to have been generat- internal waves and internal tides can also be important ed by internal tides and internal waves therefore offered a geological agents in deep waters, suggesting that such rare opportunity of studying such deposits in greater detail. sedimentary deposits should be preserved in the strati- The deposits comprise mostly mud and fine-grained sand graphic record. However, only a few sedimentologists have typically displaying sedimentary structures generated by considered the influence of internal tides in ancient deep- traction currents. Such structures, which include bidirec- water deposits. Thus, Laird (1972) reported tide-generated tional and unidirectional cross-bedding, flaser, wavy and bidirectional cross-laminations in pre-Devonian deep-water lenticular bedding, alternating parallel- and cross-bedded deposits of New Zealand. Klein (1975) identified flaser, laminae, double mud layers, climbing ripples, sigmoidal wavy and lenticular bedding in cores of Quaternary to cross-bedding, as well as reactivation surfaces, were used Cretaceous age recovered at water depths of 2,200–3,000 m here to reconstruct the mechanisms responsible for their on the Ontong-Java Plateau. McCave and Tucholke (1986) formation. proposed that mud waves could be produced by internal The main objective of this paper in this special issue is to waves. Karl et al. (1986) interpreted sand waves observed evaluate novel findings on sedimentary structures and at the Navarinsky Canyon head as internal-wave deposits, facies in terms of inferred internal-wave and internal-tide and Gao and Eriksson (1991) identified internal-tide action in the Middle Ordovician Xujiajuan Formation, deposits in the Ordovician of the central Appalachians, Xiangshan Group. Combined with existing regional knowl- USA. Gao et al. (1998) discussed the importance of internal edge, this should contribute to a better understanding of waves in the formation of large-scale sediment waves on these mechanisms in deep waters in general. modern seafloors, especially of sediment waves migrating upslope. Zhang et al. (1999) interpreted sediment waves in the northeastern Rockall Trough as internal-wave deposits. Regional and geological setting Finally, Shanmugam (2008) recently provided a comprehensive overview of the general characteristics of deep- The study area is located in the Xiangshan and Miboshan water tidal deposits. mountains near Zhongwei, Zhongning, and Tongxin Since the work of Gao and Eriksson (1991) on the (central–south Ningxia Hui Autonomous Region, North- Ordovician of the central Appalachians, sedimentologists in west China; Fig. 1), within the adjoining portions of the China have pursued such investigations (see overviews by ancient lands of Ordos and Alashan, as well as the Gao et al. 2006, 2010) and, indeed, ancient internal-tide Qinling–Qilian orogenic system (Fig. 2). Early Paleozoic deposits are now known from several regions. These tectonic activity is closely related to the evolution of the include the uppermost units of the Upper Ordovician Qinling–Qilian–Kunlun Ocean and the formation of the Yankou Formation in Tonglu, Zhejiang Province (Gao et North Qilian island arc (Zhang et al. 2004;Xuetal.2006). 中国科技论文在线 http://www.paper.edu.cn

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Fig. 1 Geological map of the study area showing outcrops of the Miboshan Formation, and the three formations of the Xiangshan Group. 1 Miboshan Formation, 2 Xujiajuan Forma- tion, 3 Langzuizi Formation, 4 Mopanjing Formation, 5 thrust fault, 6 transcurrent fault

Zhang et al. (2004) suggested that, during the Middle overlying Devonian Zhongning Formation is marked by an Ordovician, there was a certain degree of back-arc angular unconformity. extension and subsidence, although no oceanic crust The lower part of the Miboshan Formation consists of formation. The study area is on the continental side of gray to dark-gray, thin-bedded calcirudite, siliceous shale, the retro-arc foreland basin oftheNorthQilianislandarc. gravel-bearing shale, micritic limestone, and some fine- The dip of the regional slope was approx. southeast during grained feldspathic-quartzose sandstone. Casts and grooved the Middle Ordovician (Fig. 2). cast-like structures occur on the bottom surface of the The Xiangshan Group is located north of the Helan, calcirudites, whereas slump deformation structures are Xiangshan, and Miboshan mountains (Fig. 1). This group present in the limestone. The upper part of the Miboshan belongs to the Middle Ordovician (Zhang 1993;Li1997; Formation is composed of dark-gray, blocky micritic Wang and Zheng 1998; Zhang et al. 2004), and consists calcirudite, micritic limestone, gray-green shale, and mainly of celadonites, lightly metamorphic medium- to gravel-bearing shale. Slump deformation structures are fine-grained feldspathic-quartzose sandstones, shale, calcir- developed in the micritic limestone, whereas calcirudite udites, and siliceous rocks. Most of these rocks are lenses are intercalated in the shale. concentrated specifically in the upper part of the group, The Xujiajuan Formation consists mainly of gray-green the succession being conformably intercalated by a few thin and yellowish-green, medium- to fine-grained feldspathic- tholeiitic diabase sheets generated by submarine volcanic quartzose sandstone, calcareous sandstone, siltstone, and eruptions and hypabyssal intrusions. From bottom to top, shale. This formation has three members (Fig. 4). the Xiangshan Group can be subdivided into the Xujiajuan, The Langzuizi Formation can be subdivided into two parts. Langzuizi, and Mopanjing formations (Fig. 3). The outcrop The lower one is composed of gray-green as well as yellowish distribution of each formation is shown on the geological medium- to thick-bedded, fine-grained feldspathic-quartzose map of Fig. 1. The group conformably overlies the Middle sandstone and shale. The sandstone component decreases Ordovician Miboshan Formation, whereas the contact to the from bottom to top, whereas shale increases. The upper part is 中国科技论文在线 http://www.paper.edu.cn

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Fig. 2 Paleogeographic map of the Middle Ordovician showing the arrangement of lithofacies along the western margin of the Ordos Basin, China (modified from Gao et al. 1995). 1 Ancient land, 2 carbonate platform, 3 carbonate slope, 4 abyssal basin, 5 study area

composed of dark-gray medium- to thin-bedded chert, sections with two intersecting right angles were used to purplish-red gray-green shale, as well as grayish-white determine the dip directions of laminae. In the case of silica-bearing dolomicrite, with a diabase intrusion along a trough cross-bedding, the paleocurrent direction was mea- bedding plane. sured along its central axis. Only well-preserved flute casts The Mopanjing Formation consists of gray-green, and cross-bedded laminae were considered. purplish-gray and purplish-red thick- to massive-bedded, The in situ observations were corrected in the laboratory medium- to fine-grained feldspathic-quartzose sandstone using a Wulff net to reconstruct the approximate directions intercalated with shale and silty shale. The shale content of the original deposits. Rose diagrams of paleocurrent increases from bottom to top. The sandstone component in azimuths were drawn on the basis of corresponding this formation is markedly higher than that of the statistical analyses. In all, 36 flute casts and 365 cross- underlying Langzuizi and Xujiajuan formations. bedded sets were measured. At every suitable outcrop, more than ten grouped datasets were recorded, each as far as possible away from another. In the case of bidirectional Materials and methods cross-bedding, the directional sets were measured separate- ly, each set generally containing 5–8 data points (maximum As cross-bedding, current ripples, and flute casts are good 11). In a few cases the steep slopes of asymmetric ripples paleocurrent markers (see overview by Nichols 2009), these and groove casts were also measured. The rose diagrams of were used to estimate current directions in the field. The the paleocurrent azimuths of flute casts were used to orientations of flute casts on the bottom surfaces of reconstruct the flow directions of turbidity currents and turbidites were used to reconstruct the flow directions of the orientation of the regional continental slope in the turbidity currents and regional paleoslope orientations. Xujiajuan Formation, whereas those representing the When cross-bedding was documented in the field, the paleocurrent azimuths of cross-bedding were used to three-dimensional architecture of the cross-bedded units reconstruct the paleocurrent directions of other bottom was taken into consideration, making sure that vertical currents. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 3 Schematic stratigraphic column of the Xiangshan Group of the Middle Ordovician in the Xiangshan area of Ningxia, China. 1 Conglomerate, 2 breccia, 3 sandstone, 4 siltstone, 5 shale, 6 chert, 7 siliceous dolomicrite, 8 silty limestone, 9 muddy limestone, 10 limestone lens, 11 calcirudite lens, 12 diabase, 13 flute cast, 14 graded bedding, 15 cross-bedding, 16 slump deformation structure

Results

General description of sedimentary successions

The Xujiajuan Formation has three members (Fig. 4). The first (bottom) member consists of gray-green, medium- to thick-bedded, medium- to fine-grained feldspathic- quartzose sandstone intercalated with yellowish-green to gray-green shale and silty shale. Overall, the sandstone to shale ratio is approx. 2.46, whereas the ratio of individual sandstone layers to adjacent shale layers varies strongly from 0.44 to 8.69. The sandstone beds have lenticular shapes. Flute and groove casts are developed on the bottom surfaces of the sandstone beds. The beds are positively graded (Fig. 4), constituting sequences that become thinner and finer upward. Ta-e and Tb-e successions of the classical Bouma sequence can be observed. The second member can be subdivided into two parts. The lower one is composed of gray-green, medium- to thick-bedded, fine-grained feldspathic-quartzose sandstone intercalated with yellowish-green to gray-green shale and silty shale. A yellowish-green to gray-green shale and silty shale is also found in the overlying strata. Sandstone and shale successions form several cycles in this part. Overall, the sandstone to shale ratio is approx. 0.57, whereas the ratio of individual sandstone layers to adjacent shale layers varies strongly from 0.29 to 9.04. The bottom succession is 1 to 2 m thick, the sandstone thickness generally thinning upward. The other successions are 0.5 to 0.8 m thick, with the sandstone beds tending to thicken upward. They have lenticular or slightly lenticular shapes. Load casts are occasionally observed in the basal beds of the sandstones. The upper part of the second member is composed of similarly thick interbeds of yellowish-green to gray-green shale, silty shale, and gray-green medium- to thick-bedded, fine-grained feldspathic-quartzose sandstone. Gray-green medium- to thin-bedded calcareous siltstones and silty limestone are intercalated with these interbeds. Bidirection- al cross-bedding and small-scale hummocky cross-bedding are common. Ripple cross-laminations and parallel laminations are well developed in the calcareous siltstones. Overall, the sandstone to shale ratio is approx. 0.81, whereas the ratio of individual sandstone layers to adjacent shale layers varies from 0.59 to 1.26. The lithology of the third (top) member comprises gray- green shale that is interbedded with dark-gray micritic limestone. The limestone is intercalated with a few gray- 中国科技论文在线 http://www.paper.edu.cn

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recognizable spatial variation in lithology, and the distribution is fairly uniform. This member has therefore been used as a marker bed to distinguish the Xujiajuan from the Langzuizi formations.

Facies types

The sedimentary successions of the Xujiajuan Formation are dominated by turbidity-current and in situ deep-water deposits. There are a number of interbeds with well- developed cross-bedded units between these deposits. The cross-bedding includes planar, trough, small-scale hummocky and bidirectional sets, ripple cross-laminations, as well as climbing ripple cross-bedding and parallel bedding. All these bedding types reflect the action of traction currents. As both the underlying and overlying strata represent deep-water slope to abyssal plain depositional environments (Zhang et al. 2004; He et al. 2009), the cross- bedded units must also have formed in these deep-water settings. They are thus either the direct products of deep- water traction currents or the products of current-reworked older deposits.

Turbidite deposits

Abundant turbidity-current deposits occur in the first and second members of the Xujiajuan Formation. In vertical profile these constitute frequently alternating beds of sandstone and shale representing typical flysch deposits (Fig. 5a). The turbidites are composed mainly of fine- grained sandstone, siltstone, and shale. The thickness of individual layered sandstone beds generally varies from 20 to 400 cm, the minimum thickness being only 7 cm, the maximum thickness 800 cm. The average thickness is 130 cm. Sandstones with abrupt contacts to underlying shale have flute casts (Fig. 5b) and groove casts developed on their bottom surfaces. These structures are more Fig. 4 Lithologic columnar section of the Xujiajuan Formation at abundant in the northern part of the study area. The upper Kanglabai, Tongxin, Ningxia, China. 1 Fine-grained sandstone, 2 silty contacts are mostly gradational, but sudden changes in fine sandstone, 3 siltstone to fine-grained sandstone, 4 siltstone, 5 silty grain size are occasionally also observed. Generally, there shale, 6 shale, 7 limestone, 8 bidirectional cross-bedding, 9 unidirectional cross-bedding, 10 graded bedding, 11 flute cast, 12 limestone lens are no sedimentary structures visible within the sandstone beds. However, positively graded beds are quite common. Both complete and incomplete Bouma sequences have been green medium-bedded and fine-grained calcareous recorded. In the measured sections, the common feldspathic-quartzose sandstone beds. The limestone is thickening-up/thinning-up sequences of typical turbidites generally 3–8 cm thick, maximum and minimum thick- are observed. nesses being 15–18 and 1–2 cm, respectively. The bottom From the bottom to the top of the Xujiajuan Formation, the surfaces are uneven. The beds have lenticular shapes and number of sandstone layers progressively decreases. At the are laterally quite extensive. Herringbone cross-bedding same time the thickness of individual sandstone beds, and the and bidirectional cross-bedding are well developed. The ratio of sand to mud, decreases. In the measured section, thick- shale intercalated in the limestone is 1–3 cm thick, whereas bedded medium- to coarse-grained sandstones are characteristic the interbedded shale is 30–50 cm thick. The thickness of for the lower part. This indicates a mid-fan channel setting this member varies from 18.6 to 129.6 m. There is no within a larger submarine fan. The thickening-upward, 中国科技论文在线 http://www.paper.edu.cn

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Fig. 5 Turbidity-current deposits of the Xujiajuan Formation. a Rhythmic turbidite (flysch) interbeds in the middle part of the Xujiajuan Formation, at Daliush- ugou (actual length of line seg- ment is 2.64 m). b Flute cast at the bottom of massive fine- grained quartz sandstones in the lower part of the Xujiajuan For- mation, at Shangshipeng

thinning-upward sequences may represent the migration and interbedded with dark-gray micritic limestone (Fig. 6c). extinction of a submarine channel (Mutti 1977). The propor- The minimum thickness of the limestone is only 1–2 cm, tion of sandstone decreases, whereas that of shale increases in the maximum thickness reaching 15–18 cm (mostly 3– the upper part. This pattern indicates that the supply of coarser 8 cm). Limestone with uneven bottom surfaces occurs terrigenous material gradually decreased with time. The laterally in extensive lenticular beds (Fig. 6c). deposits gradually evolve upward into distal low-density Rare-earth elements in the shale samples have rather turbidity-current deposits, reflecting a gradual deepening of pronounced Ce and Eu anomalies (He et al. 2009)thatreflect the water. particular conditions of deep-water sedimentation. Chemical

According to the turbidite lithofacies model of Mutti analyses show that the mean TiO2/MnO and Al2O3/MnO (1977), the first member of the Xujiajuan Formation is ratios are 10.72 and 192.77, respectively (Table 1). In the interpreted to represent mid-fan channel deposits of a larger underlying Miboshan Formation, by contrast, the turbidite fan. The lower part of the second member is corresponding ratios are 17.7 and 344.3, respectively (Li dominated by channel-mouth turbidity-current deposits in a 1999). This indicates that the influence of terrigenous more distal fan setting. The sandstone layers intercalated material was relatively small at the time when the Xujiajuan with shale, together with the overlying shale, constitute a Formation was deposited. The water depth must at that time complete channel-mouth depositional sequence of a turbi- have been deeper than that of the shelf-margin to slope dite fan. setting of the Miboshan Formation (Li 1999). The mean Sr/ Ba ratio of the dark-gray, thinly bedded micritic limestone of In situ deep-water deposits the third member of the Xujiajuan Formation is 24.3, the maximum value reaching 52.5, the minimum one 10.6 In situ deep-water deposits refer to sedimentary deposits (Table 2); these are characteristic of marine conditions. formed by vertical accretion in deep-water sedimentary Moreover, the overlying Langzuizi Formation composed environments. The sedimentary types are therefore pelagic of “varicolored” shale and siliceous rock (Fig. 6d)isa or hemipelagic in origin (Stow et al. 2001). Deeper-water product of in situ deep-sea deposition. A diabase intrusion continental shelf deposits can also be included here. having geochemical characteristics similar to that of mid- In situ deep-water deposits of the Xiangshan Group are ocean ridge (MORB) tholeiites was found in this formation developed mainly in the middle to upper parts of the (Deng et al. 2007). The upper part of the underlying Xujiajuan Formation. They can be subdivided into two Miboshan Formation comprises dark-gray shale and thinly types. One is composed of gray-green shale and silty shale, bedded micritic limestone in which slump deformation varying in thicknesses from several to tens of meters structures are well developed (Fig. 7). (Fig. 6a). Sometimes a few 30–50 cm thick gray-green In summary, combined with the lithologic features of the (yellow-brown after weathering) calcareous siltstones to overlying Langzuizi Formation and of the underlying fine-grained sandstones are intercalated. Gray-green Miboshan Formation, the lithologic features of the Xujia- medium- to thick-bedded, fine-grained feldspathic- juan Formation suggest that the sediments were deposited quartzose sandstones and gray-green silty shale between in a deep-water environment (probably bathyal to abyssal). thicker shale beds occur in a number of cycles (as shown by the arrows in Fig. 6b). The individual sandstone beds are Sedimentary structures 20–70 cm thick, whereas the individual silty shale beds are 10–40 cm thick, the cycles being approx. 50–90 cm thick The sedimentary structures of the Xujiajuan Formation (rarely more than 100 cm). The cumulative thickness of the comprise mainly graded bedding, cross-bedding, ripples, cycles is less than 10 m. The other type is gray-green shale flute casts, and groove casts. The first member, and the 中国科技论文在线 http://www.paper.edu.cn

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Fig. 6 Shale, limestone, and siliceous rocks of the Xiang- shan Group. a Grayish-green shale and silty shale interbed, second member of Xujiajuan Formation, northwest of Kan- glabai. b Grayish-green shale intercalated with medium- to thin-bedded calcareous siltstone, upper part of Xujiajuan Forma- tion, northwest of Kanglabai. White arrow Sandstone/shale cycle in a larger set of shale. c Gray thin-bedded micritic limestone alternating with grayish- green shale, third member of Xujiajuan Formation, northwest of Kanglabai. d Purplish-red thin-bedded siliceous rocks and gray shale, top of Langzuizi Formation, Mopanjing

lower part of the second member, is dominated by graded bedded types. Climbing ripples, parallel-bedded sets, and bedding and flute casts with subordinate cross-bedding and ripple cross-laminations (or combined-flow ripple lamina- ripples. The upper parts of the second and third members tions) also exist. The cross-bedded laminae show both are composed mainly of cross-bedded units. unidirectional and bidirectional arrangements. The cross-bedding shown in Fig. 8a is developed in Cross-bedding medium- to thin-bedded calcareous siltstones in the third member of the Xujiajuan Formation. Sets A and B are Cross-bedding in the Xujiajuan Formation is developed approx. 3–4 cm thick. The laminae of these sets intersect at mainly in the medium- to thin-bedded calcareous siltstone acute angles with flow-reversal tendencies. The laminae of and fine-grained sandstone, fine calcisiltitic micritic lime- set A are relatively straight, whereas those of set B are stone, and silty shale. Cross-bedding is overall relatively slightly curved and convergent downward. When corrected rare, but can be abundant in individual layers. Generally, by means of a Wulff net, the dip directions of set A vary cross-bedding occurs in two or more sets that vary in shape, from 0 to 29°, and the dip angles from 7.5 to 22.5°. The dip and consist of both parallel and curved inclined laminae directions of set B are 138–190°, and the dip angles 7.5–17°. (Fig. 8). The sets are 1.2–6.7 cm thick (generally 2–4 cm) The cross-bedding illustrated in Fig. 8c occurs in thin- and include planar, small-scale hummocky and trough- bedded calcareous siltstones in the upper part of the second

Table 1 Average contents of mineral oxides in shale of the Xiangshan Group

Strata Lithology Sample number Al2O3 TiO2 MnO Al2O3/MnO TiO2/MnO

Langzuizi formation Shale M-2 13.19 0.740 0.070 188.43 10.57 Shale P7-1 13.30 0.828 0.104 127.88 7.96 Xujiajuan formation Shale P6-1 17.29 0.900 0.066 262.00 13.64 Mean value 192.77 10.72 Miboshan formation (Li 1999) Shale Omb4 13.94 0.60 0.10 139 6 Siliceous shale Omb2 6.39 0.30 0.02 320 15 Shale Omb1 8.61 0.48 0.015 574 32 Mean value 344.3 17.7 中国科技论文在线 http://www.paper.edu.cn

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Table 2 Trace element contents of micritic limestone from the upper With dip directions of 132.5–143°, the paleocurrent part of the Xujiajuan Formation directions in sets B and C are basically identical. The dip Sample number Sr (10−6) Ba (10−6) Sr/Ba angles of the laminae in set B are gentle, whereas in set C they are steep. The paleocurrent directions measured in sets K29-1B1 1,120 58.7 19.1 A, D, and E are opposite to those of sets B and C (Fig. 9, K31-B1 730 13.9 52.5 diagram h). K33-B1 643 34.9 18.4 The unidirectional cross-bedding shown in Fig. 8e K35-B1 1,280 121 10.6 occurs in medium-bedded, calcareous fine-grained sand- P1-B3 848 41.2 20.6 stone in the upper part of the second member of the Mean value 24.2 Xujiajuan Formation. Three sets can be distinguished from bottom to top, i.e., A, B, and C. The sets are approx. 2– 4.5 cm thick and are characterized by distinctly erosional member of the Xujiajuan Formation. Five sets, A to E, were interfaces at their bases. The laminae are concave-curved in identified from bottom to top. The sets are approx. 1–2cm shape, with steep upper parts and gentle lower parts that thick. The laminae of sets A and B are slightly curved and converge downward. In all, 45 paleocurrent measurements show flow-reversal tendencies. The set boundaries are with azimuths between 241° and 304° were obtained. wave-like in appearance. Set C shows parallel bedding, Figure 8f exhibits wave-generated ripple cross-bedding whereas the right-hand part of set D is characterized by or tidal-bundle sequences. The laminae in the middle and mild climbing-ripple bedding, its direction opposite to that lower parts of the cross-beds (short arrow) are convex-up. of set E. The rose diagrams of paleocurrent azimuths The tops, which form clear erosional boundaries, are obtained from sets A and B are shown in Fig. 9, diagrams f incised by the troughs of the overlying wave-generated and g. ripple cross-beds (long arrow). The crests of the overlying The cross-bedding in Fig. 8d is developed in thin-bedded cross-beds are smooth, the wave ripples apparently being calcareous siltstone to fine-grained sandstone that are asymmetrical. The laminae thicken downward to the ripple intercalated with very thin shale beds. Five sets, A to E, troughs, and thin upward toward the ripple crests. These were identified from bottom to top. These sets are approx. features suggest that the sedimentary structures may 1–3 cm thick. Due to the poor outcrop situation, it was not represent combined-flow ripple laminations (Harms 1969; possible to determine whether the bottom surface of set A Nøttvedt and Kreisa 1987; Arnott and Southard 1990; was erosional. However, erosion surfaces are apparent at Lamb et al. 2008). the set boundaries of B, D, and E, although not at the bottom of set C. Overall, set C is obscure, but faint laminae Paleocurrent analysis were observed at the location indicated by the arrow in Fig. 8d, their inclination being opposite to that observed in Oriented sedimentary structures in the Xujiajuan Formation set D. The laminae recorded in each set were corrected by are composed mainly of cross-bedding and flute casts. Flute means of a Wulff net. The dip directions in set A are 351.5– casts are found in the western part of the study area 34°, and the dip angles 7–10.5°; in set B, the direction is (Figs. 5b and 9), whereas cross-bedded deposits can be 138°, and the angle 11°; in set C the directions are 132.5– found in the entire study area. The corrected paleocurrent 143°, and the angles 17–21°; in set D the directions are azimuths of the directional measurements of the 36 flute 329–336.5°, and the angles 10–19°; in set E the dip casts and 365 cross-bedded sets are illustrated in the rose directions are 283.5–294.5°, and the dip angles 12.5–26°. diagrams of Fig. 10a and b, respectively. These data reveal

Fig. 7 Slump deformation structure in thin-bedded micritic limestone of the Miboshan For- mation, to the south of the Miboshan Mountains. a Top of Miboshan Formation. b Upper part of Miboshan Formation 中国科技论文在线 http://www.paper.edu.cn

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Fig. 8 Cross-bedding in outcrops of the Xujiajuan Forma- tion. a Bidirectional cross- bedding in medium- to thin- bedded calcareous siltstone, third member of Xujiajuan For- mation, Langzuizi Section; note that the dip direction of the laminae in set A is opposite to that in set B (length of scale is 5 cm). b Calcareous sandstone interbedded with shale, upper part of Xujiajuan Formation, northern Mopanjing Section. Cross-laminations are developed in calcareous sandstone. Sets A, B, and C are intercalated with shale. The sets are approx. 1– 2 cm thick with relatively planar laminae. c Argillaceous siltstone, upper part of second member of Xujianjuan Forma- tion, Langzuizi Section. d Cal- careous siltstone to fine-grained sandstone, middle part of third member of Xujianjuan Forma- tion, Kanglabai Section. e Cal- careous fine-grained sandstone, upper part of second member of Xujianjuan Formation, Kangla- bai Section. A to E represent different sets. f Wave-generated ripple cross-bedding in calcareous siltstone, second member of Xujiajuan Formation, northern Kanglabai Section, with asymmetric crests (long arrow); coin diameter is approx. 2 cm (for profile position, see Fig. 9)

that the paleocurrents of the flute casts basically point in the paleocurrent directions of the bidirectional cross-beds are same general SSW direction. This represents the direction mainly up- and downslope; (2) the angles of the individual of the turbidity currents and the orientation of the directions are tightly confined, irrespective of whether they paleoslope (Li et al. 2009). The directions of the cross- flow up- or downslope (e.g., NW in Fig. 9, rose diagram f, beds, by contrast, scatter widely, although three dominant and SE in Fig. 9, rose diagram g); and (3) alternating directions are evident: NNE, SSW, and WNW. Of these, the current directions display a transitional character between SSW trend almost coincides with the regional slope the main directions (Fig. 9, rose diagram h). In combination direction, whereas the NNE trend practically points in the with the lithologic characteristics of the Xujiajuan Forma- opposite direction. Overall, the data on paleocurrents reveal tion, the alternating up- and downslope current character- that these were multidirectional at the time of deposition. istics are indicative of an unchannelized, gently sloping The cross-bedding in the Xujiajuan Formation is depositional environment. developed mainly in the medium- to thin-bedded calcareous siltstone, the fine-grained sandstone, the silty limestone, and the silty shale. These cross-beds are generally com- Discussion posed of two or more parallel and curved laminae sets. In Fig. 9, the rose diagrams e–h represent the paleocurrent Recognition of internal-wave and internal-tide deposits azimuths of the four groups of bidirectional cross-bedded sets. The paleocurrent directions indicate clear reversals. The Xujiajuan Formation is characterized by turbidity- Furthermore, the angles between the dominant paleocurrent current and in situ deep-water deposits. However, in the azimuths are rather large, and the values reveal that (1) the upper part of the second and the third members, there are 中国科技论文在线 http://www.paper.edu.cn

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Fig. 9 Rose diagrams of paleocurrent azimuths in outcrops of the flute casts, 4 flow directions of turbidity currents inferred from Xujiajuan Formation. 1 Rose diagram of flute cast azimuths, 2 rose synthetical analyses. Sections: a Shangshipeng, b Xiashipeng, c diagram of paleocurrent azimuths of bidirectional cross-bedded sets, 3 Wangjiagou, d Daliushugou, e Ganliushu, f Langzuizi, g, h Kanglabai; flow direction of turbidity currents deduced from the alignment of n number of measurements

abundant cross-bedded strata intercalated between the current-generated sedimentary structures perched between turbidity-current and in situ deep-water sedimentary the turbidite and the in situ deep-water deposits are most sequences. The rock types in these strata consist of fine- probably the products of either deep-water traction currents grained sandstones, siltstones, calcareous siltstones, and or deep-water tidal currents (Shanmugam 2003, 2008), sandy limestone. These bedding types are mostly the certainly not of turbidity currents. The additional possibility products of traction currents, not of sediment gravity flows. of deep-water combined flows needs further investigations. Moreover, the analysis of the depositional setting indicates The bidirectional cross-bedded units are found mainly in that the sedimentary setting of the Xujiajuan Formation medium- to thin-bedded calcareous siltstones and fine- comprises deep-water slope to abyssal plain depositional grained sandstones, argillaceous siltstones, and fine-grained environments. Therefore, the rocks with the characteristic calcisiltitic micritic limestone of the Xujiajuan Formation. The laminations are well preserved and have clear directional characteristics (Fig. 8). The rose diagrams of paleocurrent azimuths indicate either directly opposing (Fig. 9, rose diagrams f and h) or almost opposing paleocurrent directions (Fig. 9, rose diagrams e and g). Cross-bedding that developed up- and downslope cannot be generated by either contour currents or turbidity currents, because the turbidity-current directions are uniformly downslope. Contour currents, by contrast, flow parallel to Fig. 10 Rose diagrams of paleocurrent azimuths of flute casts (a)and the regional slope (Faugères and Stow 1993; Gao et al. laminations (b) in the Xujiajuan Formation; n number of measurements 1998). Therefore, only alternating up- and downslope 中国科技论文在线 http://www.paper.edu.cn

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Fig. 11 Idealized sedimentary currents induced by internal waves and internal tides can sequence model of the Xujia- have formed the bidirectional cross-beds in the deep-water juan Formation. Layers I slope environments investigated in this study (cf. Gao et al. Turbidity-current deposit, II 1998; He et al. 2008). traction-flow deposit, IIa combined-flow action, IIb The dip directions of the laminae of the unidirectional reworking by internal waves and cross-beds are opposite to the regional slope, being oriented internal tides, III hemipelagic NNE in Fig. 10b. These structures cannot have been formed deposit; 1 massive or normally by turbidity or contour currents, but only by dominant graded sandstone, 2 combined- flow ripple lamination, 3 cross- upslope currents. Unidirectional flows may be the result of bedding, 4 shale long-period internal waves superimposed on a weaker internal tide (e.g., Gao et al. 1998). Rocks displaying this type of unidirectional cross-bedding can most plausibly be attributed to the action of internal waves and internal tides. The unidirectional cross-bedding oriented at large angles relative to the regional slope (Fig. 9, rose diagram e) could have been formed by contour currents. In the case of contour currents, however, bioturbation structures are well developed (Stow and Lovell 1979; Faugères and Stow 1993; Stow et al. 1998), whereas the Xujiajuan Formation and, as a matter of fact, the whole Xiangshan Group is contacts can be abrupt or gradational. This unit can be known for its scarcity in fossils and bioturbation. As a further subdivided into two types, namely IIa and IIb. The consequence, this kind of unidirectional cross-bedding in a former (layer IIa) is characterized by parallel bedding, deep-water setting is more likely to have been formed by ripple cross-laminations (or combined-flow ripple lamina- internal waves and internal tides. tions), and occasional small-scale hummocky cross- The ripple cross-bedding with asymmetrical smooth bedding. It probably represents the product of combined crests, as well as the convex or curved laminae may flows generated by the interaction of short-period internal represent combined-flow ripple laminations (Lamb et al. waves with turbidity currents. Layer IIb is characterized by 2008). This type of ripple can be the result of weak unidirectional and bidirectional cross-bedding, and includes oscillatory flow superimposed on a unidirectional current occasional parallel-bedded sets. Combined-flow ripple (Yokokawa et al. 1995), as is typical for deposition in low- lamination does not occur in layer IIb, which may thus energy combined flow (Lamb et al. 2008). Such combined- have been formed by internal waves superimposed on flow deposits in deep water may thus have been formed by internal tides. the interaction of short-period internal waves with marginal The hemipelagic sedimentary unit (layer III) consists of turbidity-current flows (Li et al. 2010). shale, or micritic limestone thinly interbedded with shale. This layer is dominated by deep-water hemipelagic depo- Idealized vertical sedimentary sequence of the Xujiajuan sition formed by particles settling from the water column. Formation A statistical analysis of the 75 sedimentary sequences recorded in the Xujiajuan Formation has revealed that 30% In the idealized lithological column representing the of the complete sequence is composed of layers I, II, and Xujiajuan Formation in Fig. 11, a basal deposit generated III. Incomplete sequences composed of layers I and II, by density currents (I) is overlain by traction-current layers I and III, as well as layers II and III contribute 20%, deposits (II), followed at the top by a hemipelagic 36%, and 12%, respectively. The former two sequences are sedimentary unit (III). The turbidity-current unit (I) is well developed over the entire study area, whereas the latter composed of 20–400 cm thick layers of fine-grained sequence is observed only in the southern part of the study sandstone and silty sandstone. The contact to the overlying area, which is quite remote from any sedimentary source. unit II is mostly gradational, but abrupt grain-size changes The present study on internal-wave and internal-tide can occasionally occur. Massive bedding is well developed, deposits has shown that directional sedimentary structures and occasional positively graded beds can be found. The are one key characteristic of internal-wave and internal-tide deposits may have been formed by depletive flows that are deposits in deep-water sedimentary environments (bidirec- spatially heterogenetic (Kneller and Branney 1995; Mulder tional cross-lamination, cross-lamination dipping upslope, and Alexander 2001). multidirectional cross-lamination, and other traction-current The traction-current sedimentary layer (II) is composed sedimentary structures such as flaser, wavy and lenticular of 3–60 cm thick siltstone. Both the lower and upper bedding, double mud layers, and reactivation surfaces). 中国科技论文在线 http://www.paper.edu.cn

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Conclusions Egbert GD, Ray RD (2000) Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data. Nature 405:775–778 The deep-water internal-wave and internal-tide deposits Fang XH, Wang JM (1986) A review on oceanic internal waves (in described in this paper have only recently been discovered. Chinese with English abstract). Adv Mech 16(3):319–330 The sedimentary structures characterized by cross-bedding Faugères J-C, Stow DAV (1993) Bottom-current-controlled sedimen- intercalated with turbidity-current deposits in the upper part tation: a synthesis of the contourite problem. Sediment Geol 82:287–297 of the Xujiajuan Formation, Ningxia, China, indicate that Gao Z-Z, Eriksson KA (1991) Internal-tide deposits in an Ordovician internal waves and internal tides are the most probable submarine channel: previously unrecognized facies? Geology 19 mechanisms responsible for their formation. These findings (7):734–737 add to the stratigraphic record of such deposits in general Gao Z-Z, Luo S-S, He Y-B, Zhang J-S (1995) Ordovician submarine fan system in west margin of Ordos (in Chinese with English and, more importantly, have practical implications for the abstract). J Oil Gas Geol 16(2):119–125 interpretation of the sedimentary settings and paleocurrent Gao Z-Z, Peng D-T, Liu X-F, He Y-B, Fu X, Li D-S, Zhong G-F features associated with the deposition of the Xujiajuan (1996) Ordovician internal-tide deposits in TZ30 well, Tarim Formation, as well as the entire Xiangshan Group. Basin (in Chinese with English abstract). J Jianghan Petrol Inst 18(4):9–14 The study of internal-wave and internal-tide deposits is a Gao Z-Z, He Y-B, Li J-M, Li W-F, Luo S-S, Wang Z-Z (1997) Internal- new field in deep-water research that has developed over wave deposits are found in China. Chin Sci Bull 42(13):1113– the last 20 years. Until recently, well-documented examples 1117 were rare. The discovery of internal-wave and internal-tide Gao Z-Z, Eriksson KA, He Y-B, Luo S-S, Guo J-H (1998) Deep-water traction current deposits: a study of internal tides, internal waves, deposits of the Xiangshan Group Xujiajuan Formation in contour currents and their deposits. Science, Beijing Ningxia has provided new insight that may contribute to the Gao Z-Z, He Y-B, Zhang X-Y, Zai Y-H, Hu Y-Y, Yang H-J, Li X-S, Li identification of such deposits elsewhere. Y (2000) Internal-wave and internal-tide deposits of the Middle- Upper Ordovician in the Central Tarim Basin (in Chinese with English abstract). Acta Sedimentol Sin 18(3):400–407 Acknowledgements This research was supported by the China Gao Z-Z, He Y-B, Liu C-X, Xing F-C, Wang C-Y (2006) History, National Natural Science Foundation (Nos. 40672071 and status and prospect of study on deep-water traction current 41072086) and the Research Fund for the Doctoral Program of deposits (in Chinese with English abstract). J Palaeogeogr 8 Higher Education in China (No. 20104220110002). We express our (3):331–338 sincerest gratitude to Prof. Shunshe Luo, as well as the postgraduate Gao Z-Z, He Y-B, Li X-D (2010) Study of internal-wave and internal- students Dan Wang, Hui Xu, Jianke Dai, and Hongwei Wang of the tide deposits in the ancient stratigraphical record from China. Yangtze University for their active participation. Dr. Qingchun Wang Geotemas 11:47–48 of the Shijiazhuang University of Economics is also acknowledged. Guo J-Q, Zhang X-H, Zhang Z-J (2003) Internal-wave and internal- We are grateful to Senior Engineers Zhaochang Zheng and Cheng tide deposits of Xiushui Formation of Shuangqiaoshan Group in Wang of the Exploration and Development Bureau of Geology and Middle Proterozoic in Xiushui area, Jiangxi Province (in Chinese Mineral Resources of Ningxia for their help during the field work. with English abstract). 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