Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic Youfangzhuang Formation in Western Henan, Central China

Journal of Earth Science, Vol. 25, No. 4, p. 648–661, August 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0454-2

Min Wang, Yongan Qi*, Da Li, Mingyue Dai, Yuguang Chang Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000, China

ABSTRACT: Fluvial deposits from the Middle Triassic Youfangzhuang Formation in Henan Province, China contain a low-diversity trace fossils. Vertical dwelling burrows of opportunists dominate in active channel, whereas horizontal deposit-feeding, grazing, and vertical dwelling burrows occur in overbank deposits. Seven recurrent ichnofabrics were distingushed. The Palaeophycus tubularis ichnofabric pre- serves below scour interface between two fluvial cycles. The Skolithos linearis ichnofabric forms in middle to high-energy, soft substrates in the middle and upper parts of active channel deposits under semi-arid climate. The Skolithos linearis-Planolites montanus ichnofabric occurs in the sediments just a little above the Skolithos linearis ichnofabric and is formed in middle-energy, soft substrates in the upper part of active channel deposits. The Planolites montanus ichnofabric is mostly preserved in association with medium- to thick-bedded brown-red silty mudstones or siltstones in low-energy, perennial muddy bottom ponds in overbank settings. The Taenidium barretti ichnofabric is interpreted as periodically exposed ephemeral ponds or low-energy distal floodplain. The Skolithos isp. ichnofabric commonly occurs in middle to high-energy firm ground of proximal foodplain or crevasse-splay deposits. The root trace ichnofabric occurs in the crevasse splay deposits. The presence of thin palaeosols and the abundance of calcretization in mudstones indicate a seasonal climate ranging from humid to semi-arid, with high temperature, rainfall, and well-defined dry and wet seasons. KEY WORDS: trace fossil, ichnofabric, fluvial deposit, environmental interpretation, Middle Triassic, Youfangzhuang Formation.

0 INTRODUCTION 1993). The aim of this paper is to describe and analyse ichno- The study of the continental ichnofaunas has shown an fabrics from the fluvial deposits of the Middle Triassic You- explosive development during the last decade (e.g., Buatois and fangzhuang Formation in Henan Province, China, in respect to Mángano, 2007, 2004, 2002, 1998, 1995; Hasiotis et al., 2007; environmental interpretations, and comparison with other flu- Kim et al., 2005; Hu et al., 2002; Genise et al., 2000; Sarkar vial deposits. and Chaudhuri, 1992). Most of the studies focused on lacustrine settings, while fluvial settings received less attention, 1 GEOLOGICAL SETTING because thick successions of fluvial deposits containing trace The west-east trending Jiyuan Mesozoic Basin, Central fossils are commonly restricted to certain beds and depositional China, is filled with 3 km thick continuous fluvial and lacus- surfaces (Buatois and Mángano, 2007). The publications on trine sediments that overlie Upper Paleozoic marine sediments. fluvial ichnology are mainly focused on ichnoassemblages, It is bounded by the Songshan Mountains in the south, Wuzhi ichnocoenosis and continental ichnofacies (e.g., Buatois and Elevation in the east，and the Daiminzai Elevation in the west. Mángano, 2007, 2004, 1995; Hasiotis, 2002; Bromley, 1996; Its northern marginal part dips toward the Taihang Mountains Hasiotis et al., 1993; Sarkar and Chaudhuri, 1992; Woolfe, and is concealed beneath the Taihang overthrust. The basin fill 1990; Fitzgerald and Barrett, 1986; Bradshaw, 1981; Hobbs, contains meandering fluvial to lacustrine, and braided fluvial 1981; Bromley and Asgaad, 1979; Stanley and Fagerstrom, sediments (Qi et al., 2007; Zhang et al., 2005; Hu et al., 2002; 1974). Only a few studies involve in ichnofabric approach (e.g., Buatois et al., 1996; Wu, 1985). The meandering fluvial suc- Buatois and Mángano, 2007, 1998; Bromley, 1996; Smith, cessions, located in lower part of the basin and limited to its northern part, belong to the Lower and Middle Triassic Erma- *Corresponding author: [email protected] ying and Youfangzhuang formations. The lacustrine succes- © China University of Geosciences and Springer-Verlag Berlin sions are widely distributed in the basin, including the Upper Heidelberg 2014 Triassic Chunshuyao and Tanzhuang formations and the Lower–Middle Jurassic Anyao and Yangshuzhuang formations. Manuscript received April 12, 2013. Braided fluvial successions consist of Middle Jurassic Mawa Manuscript accepted November 28, 2013. Formation of yellow and white coarse sandstones in the north-

Wang, M., Qi, Y. A., Li, D., et al., 2014. Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic Youfangzhuang Formation in Western Henan, Central China. Journal of Earth Science, 25(4): 648–661. doi: 10.1007/s12583-014-0454-2 Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 649 ern part of the basin. The outcrops are exposed at the western retention and conservation. Most of the burrows, irrespective of part of Jiyuan City, Henan Province, Central China (Fig. 1). orientation, always occur in or are connected with brown-red sediments which are regarded as to be formed under semi-arid 112º15' 112º25' 112º35'E climate. They are seldom preserved in yellow-green sediments N and gradually disappear in the yellow-green sediments at the Z Z top of the formation. Z Q2 Pt Є T3 E Є K P Z 2 ICHNOFABRICS IN ACTIVE CHANNEL DEPOSITS O2 35º10'N Ar Ar Three recurrent ichnofabrics are distinguished in the active T1-2 Jiyuan channel deposits of the Youfangzhuang Formation (Fig. 3). C 35º05' P Environmental interpretations are based on sedimentology, T3 Study area facies analysis, trace fossil composition and diversity, their E cross-cutting relationships, and degree of bioturbation (Taylor O2 J Q2 and Goldring, 1993). 35º00' T1-2 O2 P C T3 T1-2 Є C J 2.1 Palaeophycus tubularis Ichnofabric C Q2 P This ichnofabric has the lowest ichnodiversity and is characterized by the exclusive trace fossil Palaeophycus tubu- Figure 1. Geologic map of the study area. laris (Figs. 4a and 4b). The bioturbation index (BI), which expresses the degree of bioturbation in relation to primary bed- The Youfangzhuang Formation, 415 m thick, comprises ding (Taylor and Goldring, 1993), is BI 2. This ichnofabric seventeen upward-fining fluvial cycles (Fig. 2). Every cycle occurs at intervals of in 0.1–0.3 m thick, with which up to four begins with an undulate scouring surface, 5–10 m thick active horizons containing Palaeophycus tubularis are present. The channel deposits in the lower part and 5–20 m thick overbank abundance of trace fossils gradually decreases from bottom to deposits in the upper part. Purple muddy conglomerates are top (Fig. 8a). This trace fossil is a simple, unbranched, straight locally preserved at the bottom of the lowest two fluvial cycles. to curved, unlined, parallel to inclined, shallow tubular burrow, They are relatively coarse grained (maximum size up to 12 cm), passively filled by the overlying yellow medium sandstone. It poorly-sorted, imbricated, and clast-supported. They occur in has a mean diameter of 0.5 cm and a mean length of 2.3 cm erosionally based, lenticular bodies that usually incised in (Fig. 8a). The maximum observed length of the burrows is purple-red mudstone or silty mudstone of underlying floodplain about 10 cm, being limted by the size of the outcrop. The deposits. The active channels consist of thickly bedded fine to ichnofabric generally abruptly overlies the interface between coarse-grained sandstones and display a variety of internal two fluvial cycles. The parallel laminated, very thin mudstone physical sedimentary structures, including wedge and planar beds below the interface are variably disrupted by shallow, 1–5 cross laminations, parallel beddings, and common wave ripples. cm-thick Palaeophycus tubularis traces crosscutting the bed- The wedge cross beddings and parallel beddings are prevalent. ding in a subvertical to inclined pattern. Above the interface, Trough and planar cross-beddings occur only in one or two of the yellow medium sandstone displays a flow-ravinement sur- seventeen cycles. The depth of individual active channel fills face, massive or thick-bedded lamination near the interface, and always exceeds 7 m in the lower cycles and gradually decreases develops small brown-red muddy conglomerate and parallel upwards. Overbank deposits are dominated by cyclically interbedding upward 20 cm above the interface. It is the production laminated to interbedded thin horizontally laminated, occasion- of rapid deposition of the active channel that sharply contacts ally massive or thick-bedded, brown-red calcareous siltstones, with the underlying layer. mudstones of floodplains, natural levees and lenticular, yellow The mudstone beds above mentioned deposited in flood- to pale brown very fine- to fine-grained sandstones of crevasse plain environment of meandering river under semi-arid climate splays. Irregular calcareous concretions occur in mudstone of condition. After their deposition, a new fluvial cycle begans floodplain and become much richer upwards. High sand/mud with the channel migration and the current scour to underlying ratio (1.3 : 1) in the lower strata gradually deceases upwards, mudstone. The trace makers of Palaeophycus tubularis were up to 0.5 : 1 in the upper strata. Trace fossils occur, usually taken by high speed channel water and burrowed the active abundantly, in siltstones, silty mudstones and thinly bedded channel sandstones and passed through scour interface down parallel laminated fine-grained sandstones. into the brown-red muddy substrate. Because of high sedimen- The climate condition at the time of deposition of the tary rate of sandstone beds, the trace maker had to escape from Youfangzhuang Formation within Jiyuan Mesozoic Basin is the burrow after the short burrowing, leaved the opened burrow generally regarded as semi-arid in the oxidizing environments. which was passively and rapidly filled by the sediments after The thin palaeosols and the common calcretization of the mud- then. Therefore, Palaeophycus tubularis ichnofabric is pre- stones indicate a seasonal climate ranging from humid to served in the sediments near the interface between two fluvial semi-arid, with high temperature and rainfall, and well-defined cycles and represents the beginning of a new fluvial cycle. This dry and wet seasons. This seasonality and overall climate re- ichnofabric corresponds to the Skolithos ichnofacies. gime would have clearly influenced the organisms’ behavior, food resource, reproduction intervals, and especially water

650 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang

180 MF SF Trough (m) Ff Fp cross bedding Sk Cs Lf Pm Tabular 345 MFSF MFSF Pf Fp Sk Tf Fp cross bedding 87 (m) Rh Cs Lf Cs Lf Wedge (m) cross bedding Rh Pb Sc Tf Fp Tb Tf Fp Wave cross bedding 80 170 338 Tb Tf Fp MF SF Parallel bedding SP 258 Sl Pb Sc Pm Pf Fp Sl Sk Cs Lf (m) Rh Horizontal bedding Tf Cs Lf Pb Sc Pm Pf Sk Fp 415 MFSF Ff Fp Tf (m) Tb Pt Tf Rh Ff Tf Fp Rh Cs Lf Cs Lf Pm Pf Fp Tf Fp Fp Pb Sc Tf Sl Pf Cs Lf Pf 60 Tf Fp 150 240 318 Tf Fp 405 Sl Tf Fp Pb Sc Tf Sk Cs Lf SP Tf Nl Lf Sl Fp Ro Pb Sc Cf Pm Fp Cf Cs Lf Rh Tf Fp Cs Lf SP Pb Sc Sk Cs Lf Pb Sc Sk Cf Tf Fp Cf Rh Tf Sl Sk Fp Pf Tf Fp Cs Lf Sk Tf Pm 40 130 SP 220 298 Sl 385 Pt Tf Sl Pf Pb Sc Pm Pb Sc Fp Fp Pf Fp Fp Tf Tb Pf Tf Ff Tf SP Pf Fp Pb Sc Sl Sc Tf Pb Pf Tf Pm Fp SP SP Tf Fp Sl Sl Pb Sc Rh Pf Cs Lf Sk Cs Lf Cs Lf Pb Sc Pf Fp Tf Fp Pb Sc Cf 20 110 200 Fp 278 365 Pb Tf Sk Sc Cs Lf Pf Fp Sl Tf Tf Fp Pb Sc Tf Sk Pf Cs Lf Pt Fp Sk Cs Lf Fp Tb Sk Tb Pm Ff Ff Pb Sc Fp Sl Sl Tb Tf Tf Tf Fp Pb Sc SP Pt Pf Sl Fp Cf Pm Tf Pb Sc Pb Sc Sl Pb Sc 0 Sl Pb Sc A B C DE

Figure 2. Detailed sedimentologic log of the Youfangzhuang Formation including sedimentary lithofacies and ichnofabric distribution. Pt. Palaeophycus tubularis ichnofabric; Sl. Skolithos linearis ichnofabric; SP. Skolithos linearis-Planolites montanus ichnofabric; Pm. Planolites montanus ichnofabric; Tb. Taenidium barretti ichnofabric; Sk. Skolithos isp. ichnofabric; Rh. root trace ichnofabric. Exposed profile from bottom to top: A, B, C, D, E. SF. Subfacies; MF. microfacies; Sc. active channel subfacies; Lf. levee subfacies; Fp. floodplain subfacies; Sl. active channel lag microfacies; Pb. point bar microfacies; Nl. natural levee microfacies; Cs. crevasse splay microfacies; Pf. perennial pond microfacies; Ff. distal floodplain microfacies; Cf. proximal floodplain microfacies; Tf. ephemeral pond microfacies.

2.2 Skolithos linearis Ichnofabric contrast in color with the burrow fills and host rock. The bur- This ichnofabric consists of the monospecific burrow Sko- row fills are yellow-green fine sandstone from the overlying lithos linearis, that vary in diameter from 0.2 to 0.4 cm and 1 to bed, indicating passive fills of the burrows. These burrows are 10 cm in length (Figs. 4c–4h, 8b). The maximum ichnoabun- abundant in parallel laminated sandstones which have thin dance and burrow length occur in the middle part of the forma- yellow-green beds interbedded with very thin brown-red beds. tion (Fig. 8b), representing a steady sedimentation and a con- They always penetrate the very thin brown-red beds down to tinuous colonization window for trace makers. The burrows are the thin yellow-green beds. But they entirely disappear where usually lined with brown-red fine sandstone, which is a clear very thin brown-red beds are absent. The overall bioturbation

Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 651 index of this ichnofabric is BI 2. In the most localities, this beds, but not by the lithology of host beds. In conclusion, Sko- ichnofabric succeeds unbioturbated thick-bedded sandstones lithos linearis ichnofabric was formed in middle energy, soft with wedge or planar cross-beddings. substrate in the middle and upper part of the fluvial channel This ichnofabric is observed only in thinly laminated par- deposits under semi-arid climate. This ichnofabric corresponds allel bedding fine sandstone. Other beddings, for example, to the Skolithos ichnofacies. thick bedded parallel bedding, trough cross bedding, tabular cross bedding, and wedge cross bedding, resulting from high 2.3 Skolithos linearis-Planolites montanus Ichnofabric rates of sedimentation, were in the absence of bioturbation. The strata, which preserved this ichnofabric, can up to 5 m Shifting granular sands seemingly are not the most favorable in thickness. It occurs in the sediments just a little above the sites for the development of Skolithos linearis ichnofabric, as in Skolithos linearis ichnofabric and is formed right after the latter shallow marine environments. The vertical burrows in the pre- during a continuous deposition process. It is characterized by a sent study were constructed in a low rate sedimentation period combination of vertical to subvertical Skolithos linearis and of very thin brown-red bed or on subaerial omission surfaces horizontal Planolites montanus, P. beverleyensis, Palaeophycus under semi-arid climate. The burrows were preserved as open striatus, Gordia isp. (Fig. 5). The bioturbation index is BI 3–4. structures that acted as sediment traps and were buried and Skolithos linearis is 2–10 mm in diameter, and its maximal filled due to high rate of sedimentation of thick yellow-green length is more than 3 cm (Fig. 8c). They were passively filled bed under wet climate. That may explain why the burrows are with the yellow-green fine sandstone but their lining was always lined with brown-red lining, while the burrow filling is brown-red in color. The horizontal trace fossils (e.g., Planolites yellow-green in colour. The occurrence of burrow lining in montanus, Planolites beveleryensis, Gordia isp.) are completely Skolithos linearis indicates that sediment was loose, so the restricted to brown-red laminar layers, cross-cutting each other. trace-makers had to secrete mucus and build lining to prevent Planolites montanus is about 2 mm wide and 1–2 cm long, the burrows from collapse. In the scenario of a discontinuity filled with yellow-green fine sandstone. Planolites berveyesis between emplacement of the substrate and burrow filling, the shows very thin lining and continued parallel ornamentation. Its lithology of the filling is dictated by the lithology of overlying diameter ranges from 4 to 10 mm with an average of 5 mm, and

Planolites montanus ichnofabric Taenidium barretti ichnofabric

Skolithos isp. ichnofabric

Flood plain Root traces ichnofabric Perennial pood

Alluvial fan

Channel deposit Point bar

ichnofabric A B Skolithos linearis-Planolites montanus ichnofabric Palaeophycus tubularis ichnofabric Skolithos linearis

Figure 3. Seven ichnofabric models in the Youfangzhuang Formation. The lower three preserve in the active channel deposits, and the upper four belong to overbank deposits of the Youfangzhuang Formation.

652 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang

Figure 4. Trace fossils from active channel deposits. (a), (b) Palaeophycus tubularis, a negative epirelief; (c) Skolithos linearis; (d) S. linearis, a positive hyporelief; (e), (f), (g), (h) S. linearis, wall lining different from the host rock. the maximum length is larger than 36 cm. The filling of the from their lining in colour. Where cross-cutting relations were burrows is the same with the host rock, which is yellow-green observed, vertical burrows consistently cross-cut the horizon- fine sandstones. tally oriented burrows, and thus they post-date the horizontal This ichnofabric contains two phases of bioturbution: the burrows. The water at the sediment/water interface was fully first is represented by mainly horizontal feeding trails, which oxygenated. Planolites is interpreted as a paschichnia, made by are limited to brown-red bedding planes; the second is chiefly sediment grazers colonizing a softground substrate (Bromley, vertical dwelling tubes, which were produced at the deposition 1996). The shallow tier Planolites-Gordia makers, probably period of brown-red sandstone bed and were buried by the worms, exploited nutrients in the upper part of the sediment, deposition of overlying yellow-green sandstone bed. This may while the deep tier Skolithos producer, suspension feeders, explain why the filling of vertical dwelling tubes is different lived in a deeper zone, probably 2–10 cm below the sediment/

Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 653 water interface, exploited food from above the burrow. The formation. ichnofabric is low to moderate diversity, but at a high abun- This ichnofabric is only preserved in parallel laminated dance. The increase in ichnodiversity and ichnoabundance fine sandstone and formed in low-middle energy, soft substrate compared with Skolithos linearis ichnofabric indicates a de- in the upper part of fluvial channel deposits under semi-arid crease in energy and an increase in colonization window. climate. The thinly laminated parallel laminated bed is 1–3 cm However, Skolithos linearis-Planolites montanus ichnofabric thick and is dominated by interbedding of thin yellow-green has a narrow distribution and disappears in the upper part of the beds with very thin brown-red beds. No matter vertical or

Figure 5. Trace fossils and bioturbational structures from active channel deposits. (a) abundant bioturbational structures positive hyporelief; (b) horizontal Planolites montanus occasionally cut by vertical Skolithos linearis, preserved as positive hyporelief; (c) horizontal Gordia isp. can occasionally be cut by vertical Skolithos linearis; (d) horizontal Planolites montanus with many crossing; (e), (f) abundant Palaeophycus striatus occasionally cut by vertical Skolithos linearis; (g) horizontal Planolites montanus occasionally cut by vertical Skolithos linearis.

654 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang

Figure 6. Trace fossils from overbank deposits. (a), (b) abundant Planolites montanus in positive hyporelief; (c) steeply-inclined Palaeophycus tubularis; (d) vertical Skolithos linearis and horizontal Planolites montanus in same deposit; (e) horizontal Taenidium barretti showing many crossings; (f) the mottled soft-ground Planolites montanus can be reworked by sharp-walled Skolithos isp.; (g), (h) Taenidium barretti. horizontal the trace fossils are, their preservation always has ing floodplains, nature levees, crevasse splay, and so on. They close relationship with brown-red sandstone beds, which usually host the most diverse and abundant trace fossil assem- formed under low rate of sedimentation and semiarid climate. blages in fluvial environments. The trace fossils are mainly This ichnofabric corresponds to the Skolithos ichnofacies. horizontal grazing trails and feeding trails, but the vertical dwelling structures are also present. Four recurrent ichnofabrics 3 ICHNOFABRICS IN OVERBANK DEPOSITS were identified in the overbank deposits (Fig. 3, the upper four The overbank deposits are relative steady settings, includ- ichnofabric models).

Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 655

3.1 Planolites montanus Ichnofabric retti. The supraposition of soft-ground controlled burrows and This ichnofabric comprises four ichnotaxa, namely, and in stiff-ground controlled burrows is also common. The decreasing order of abundance, Planolites montanus, Palaeo- soft-ground controlled burrows include Planolites montanus phycus tubularis, Skolithos isp., and Planolites beverleyensis and Palaeophycus tubularis, which have indistinct boundaries (Figs. 6a–6d). It has a BI 3–5 and generally overlies the layer between burrows and host rock. They are usually reworked by containing Skolithos linearis-Planolites montanus ichnofabric. Taenidium barretti and Skolithos isp., whose burrows have Numerically, Planolites montanus overwhelmingly dominates clear outline or sharp-walled. The coalified plant stem frag- the ichnofabric that commonly mixed but may be monospecific. ments can be seen occasionally. Locomotion traces, vertebrate It is subcylindrical, straight or slightly curved burrow with a traces, and exposure marks (e.g., mud-cracks, raindrops) are diameter of 0.2–0.6 cm and a length up to 6 cm. The filling of absent. burrows shows darker colors a little than that of the host rock. The ichnofabric was developed in interbedded horizontal The outline of traces is not very clear. This ichnofabric essen- and ripple-laminated, pale brown or gray-green siltstone or silty tially reflects simple, horizontal, actively filled (e.g., Planolites mudstone. Occupation of the pond by producers of Planolites montanus, P. beverleyensis) or passively filled (Palaeophycus montanus and Palaeophycus tubularis probably occurred im- tabularis) burrow systems of, respectively, deposit-feeding or mediate after the original flooding event. The sediments at that predaceous organisms, most likely annelids (Pemberton and time were soft with burrows having indistinctly defined walls. Frey, 1982). It has a moderate ichnodiversity and ichnoabun- They gradually changed from soft to stiff with burrows pos- dance and disappears in the upper part of the formation (Fig. sessing clearly defined walls as the partial desiccation of the 9d). Bioturbation is locally too intensive to identify the ichno- pond. The supraposition of sharp-walled burrows to taxa. vague-walled burrows indicates a change of the substrate from This ichnofabric is mostly preserved in association with soft to firm. The absence of locomotion traces, vertebrate traces, middle- to thick-bedded, brown or brown-red silty mudstone or and exposure marks also suggests that the exposed duration siltstone. The complicated overlapping relationship indicates was not so long. The absence of burrows with scratches and that the colonization window of bioturbation was long, so the brush like marks, resting and locomotion traces and vertebrate organisms had enough time to disturb the substrate. The indis- footprints and mudcracks also indicates that complete desicca- tinct boundary between burrows and host rock suggests these tion did not occur. Sediments taken by the following flooding burrows were formed in a softground, subaqueous environment. rapidly buried the firm substrate and related trace fossils. As a Physical structures indicating subaerial exposure are absent, conclusion, the sedimentary environment of this ichnofabric is reflecting overbank vertical accretion with little associated interpreted as periodically exposed ephemeral ponds or erosion commonly allows good preservation of the ichnofauna low-energy distal floodplain. This ichnofabric corresponds to and preludes desiccation of the water bodies. Morphologic the Scoyenia ichnofacies. details of the trace fossils are very poorly preserved, suggesting that they were formed in a water-saturated substrate. The oc- 3.3 Skolithos isp. Ichnofabric currence of argillaceous sediments and horizontal beddings This ichnofabric shows a thickness of up to 1 m and is indicates that the low-energy depositional settings promote dominated by Skolithos isp.. The burrows are vertical to sub- generally stable ecosystems that were in equilibrium. Such vertical, straight to slightly curved, sharp-walled, and unlined settings allowed for the accumulation of organic detritus and (Figs. 9f, 7a–7h). The width of most burrows is uniform promote the success of deposit-feeding organism. In conclusion, throughout their length. Some burrows become inclined to this ichnofabric is interpreted as low-energy, perennial muddy subhorizontal toward their lower end and appear as J-, or bottom ponds in overfilled overbank. It corresponds to the S-shaped structures. The burrows are generally 4.5 mm in di- Mermia ichnofacies. ameter and 10 mm in length. They are passively filled with brown-red silty mudstone and occasionally with small pebbles 3.2 Taenidium barretti Ichnofabric that came from the overlying sediments. Arenicolites isp. is the This ichnofabric is characterized by the backfilled burrows subordinate component which burrows are also filled with the Taenidium barretti and has a BI 3–5 (Figs. 6e–6h). It has a above brown-red siltly mudstone (Fig. 7g). This ichnofabric has moderate ichnodiversity and moderate to high ichnoabundance a low to moderate ichnoabundance and a BI 2–4 (Fig. 9f). It and is widely distributed in the middle and top parts of the generally overlies Planolites montanus ichnofabric in the lower formation (Fig. 9e). The single Taenidium barretti is straight to part of the formation or Taenidium barretti ichnofabric in the slightly curved, horizontal to subhorizontal, sylindrical, unlined, upper part of the formation and goes up into the overlying meniscate burrows, with a filling slightly darker than the host Planolites montanus ichnofabric or Taenidium barretti ichno- rock. Menisci are tightly ranged, and merge laterally at the fabric. Some Skolithos isp. cuts Planolites montanus or Tae- burrow boundary, forming pseudo-wall (Figs. 6g, 6h). The nidium barretti ichnofabrics. diameter is 2–6 mm, with a modal value at 3–4 mm and a The traces were constructed as dwelling burrows. Many maximum length of 14 cm. Taenidium barretti occurs more other domiciles have discretely lined walls (Frey and commonly in patches. Branching was not observed, although Pemberton, 1985). Absence of a wall lining, without any inter- interference and crossovers between burrows are common. nal structure, and having sharp contact between burrows and Interference structures may give the impression of branching. host rock in the present dwelling structures apparently point to The sediments were intensively bioturbated by Taenidium bar- a high consistency of the host substrate into which the burrows

656 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang were dug. The burrows remained open after produced by and emplaced from waning currents of sheet flows associated trace-makers, allowing passive filling by brown-red mud from with the episodic overbank flooding of the larger channels. The subsequent depositional events. The ichnofabric commonly ichnofabric also occurs in wedge cross-laminated beds and occurs in thin sheets of light brown or light gray-green fine deformed cross-laminated beds of light brown or light sandstone or siltstone with parallel stratifications to current gray-green fine sandstone or siltstone lenses deposited in ripple cross-laminations. These thin sheets are inferred to be crevasse-splays as sediment from active channel spilt overbank. middle to high-energy flood deposits which spilt over the bank In conclusion, this ichnofabric was formed in middle to

Figure 7. Trace fossils from overbank deposits. (a)–(f), (h) vertical, high-inclined, J-, or S-shaped Skolithos isp., which displays sharp-walled, unlined, passively filled with silty mudstone from overlying bed. (g) Arenicolites isp., passively filled with silty mudstone from overlying bed. (i)–(j), Root traces, some of them have not been completely oxidized and preserved a distinct carbonized core.

Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 657 high-energy firm ground of proximal foodplain or crevasse- nospecies, with intensive bioturbation on bedding planes and a splay deposits. This ichnofabric corresponds to the Skolithos- complex cross-cut relationships between vertical and horizontal Glossifungites ichnofacies. burrows. The Skolithos linearis-Planolites montanus ichnofabric displays a long colonization window that provided the trace 3.4 Root Trace Ichnofabric makers with enough time to colonize the sand substrate. These The root traces display blurry outlines and a desultory dis- conditions allow preservation of not only vertical burrows, but tribution. They generally have a diameter of 0.4–0.5 cm and a also horizontal intensively bioturbated traces, The active chan- length from 1 to 6 cm (Fig. 9g). The overall Bioturbation Index nels are characterized by relatively high energy, rapid fluctua- is BI 1–2. Units of the ichnofabric are 0.3–0.5 m in thickness. tion in rates of sedimentation and erosion, and larger grain sizes The root traces are dark brown, apparently darker than host than those typically deposited in adjacent environments (Bua- rock. Some of the root traces have not been completely oxi- tois and Mángano, 2007). The overbank deposits in the lower dized and preserved distinct carbonized cores. They were de- part of the formation consist of rhythmic thin alternating beds veloped in light gray-green or light brown heavy-bedded fine of mudstone or thick-beded mudstone. Because of overfilled sandstone or siltstone (Figs. 7i, 7j). The desultory distribution and rapid sedimentation, trace fossils are rare, only the Plano- of the root traces indicates that the roots are allochthonous bur- lites montanus ichnofabric can be found in oriented parallel to ial after a distant transportation. Occasionally, the root traces the bedding plane, indicating very shallow tier emplacement. occur in light brown, parallel bedding fine sandstone or The ephemeral exposed Taenidium barretti ichnofabric is ab- heavy-bedded siltstone, which may gradually pinch out laterally. sent (Fig. 9). Above all, this ichnofabric is preserved in the deposits of cre- All of the seven ichnofabrics widely occur in active fluvial vasse splay. channel and overbank deposits of the middle part of the You- fangzhuang Formation (Figs. 2b, 2c, 8, 9). The mudstone con- 4 EVOLUTION OF ICHNOFABRICS glomerate of fluvial lag deposits are absent at this stage. The Ichnology of the lower part of the Youfangzhuang Forma- Palaeophycus tubularis ichnofabric located in the bottom of tion is dominated by the active fluvial channel ichnofabrics active fluvial channels gradually disappears. The lithology of such as the Palaeophycus tubularis ichnofabric, the Skolithos active fluvial channels is similar as that of the lower part of the linearis ichnofabric and the Skolithos linearis-Planolites mon- formation, but the types of cross-bedding are more diversified, tanus ichnofabric (Figs. 2a, 8). including trough, wedge and planar cross-bedding, parallel There is a gradually reducing trend on hydrodynamic en- stratification or current ripple cross-lamination.The Skolithos ergy and burrowing depth, as well as the scale of cross-bedding linearis ichnofabric and the Skolithos linearis-Planolites mon- and sizes of sand grain form the Palaeophycus tabularis ichno- tanus ichnofabric still occur in parallel stratified to current fabric in fluvial lag deposits, via the Skolithos linearis ichno- ripple cross-laminated fine sandstone and show no clear fabric, to the Skolithos linearis-Planolites montanus ichnofab- changes from the lower part of the formation in burrow diame- ric in fluvial channel deposits. Generally, these ichnofabrics ter, burrowing depth and ichnodiverdity, but has a higher ich- contain only single ichnotaxa, such as Palaeophycus tubularis noabundance. ichnofabric or Skolithos linearis ichnofabric. However, in the The overbank deposits in the middle part of the formation upper part of active fluvial channels, Skolithos linearis- are much more complicated than that in the lower part of the Planolites montanus ichnofabric that formed in middle energy formation and consist of rhythmicly alternating thin beds of and steady sedimentary environments, has more than four ich- mudstone and thin-middle bedded siltstone and fine sandstone,

400 400 400 (m) (m) (m)

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0 0 0 01050120 360 50 100 0105 0 5 10 0 360 100 200 010501 10 0360 100 200 Diameter Length Diversity Abundance Diameter Length Diversity Abundance Diameter Length Diversity Abundance 2 (mm) (cm) (species) ()n/m2 (mm) (cm) (species) ()n/m2 (mm) (cm) (species) (n/m ) A. Palaeophycus tubularis B. Skolithos linearis C. Skolithos linearis-Planolites montanus ichnofabric ichnofabric ichnofabric

Figure 8. The main parameters of the three ichnofabrics in active channel deposits of the Youfangzhuang Formation.

658 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang

400 400 (m) (m)

300 300

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0 0 01051050 0360 100 200 01051050 0 100 200 Diameter Length Abundance Diameter Length Diversity Abundance 2 (mm) (cm) (species) ()n/m2 (mm) (cm) ()n/m F.Skolithos isp. ichnofabric G.Root trace ichnofabric

400 400 (m) (m)

300 300

200 200

100 100

0 0 01051050 0360 100 200 01051050 0360 100 200 Diameter Length Diversity Abundance Diameter Length Diversity Abundance (mm) (cm) (species) ()n/m2 (mm) (cm) (species) ()n/m2 D.Planolites montanus ichnofabric E.Taenidium barretti i chnofabric

Figure 9. The main parameters of the four ichnofabrics in overbank deposits of the Youfangzhuang Formation. but the thick-bedded mudstone is absent. Trace fossils are served in perennial or ephemeral exposed ponds represent a widely distributed in these deposits and have a moderate ichno- shallow tier and have been produced under thin films of water diversity and high ichnoabundance. They are dominated by or in emerged sediments. However, the traces in the Skolithos horizontal or low-angle inclined simple or meniscate feeding isp. ichnofabric represents a deep tier because their producers burrows and vertical or high-inclined burrows, but the burrows had to excavate deep burrows to obtain water and foods when with scratches and brush-like marks, resting and locomotion the substrate was exposed to air. There is a complex crosscut- traces, and vertebrate footprints are absent. Four ichnofabrics ting relationships between different types of traces. Poorly pre- occur in overbank settings from low-energy, softground, peren- served traces may be crosscut by better defined softground nial ponds or mixed with soft- and firm-ground ephemeral ex- trace fossils, reflecting improving taphnomic conditions due to posed ponds, to high-energy, firm ground of proximal flood- the increasing firmness of the substrate. Some general relation- plain, and natural levee or crevasse-splay deposits. Traces pre- ships regarding the sequence of colonization of the ponds can

Ichnofabrics and Their Environmental Interpretation from the Fluvial Deposits of the Middle Triassic 659 be recognized. The emplacement of different ichnofabrics and Mángano, 2007). The ichnofabrics reported in this paper mostly happened subaqueously in water bodies, which filled by show a slightly higher diversity, including more than three ich- overbank vertical accretion without clear desiccation. nogenera that are dominated by simple vertical or steeply in- The deposits in the upper part of the Youfangzhuang For- clined burrows, and have a changing abundance from low to mation (Figs. 2d, 2e) are a little different from that of the mid- very high. They occur in scour surface between active channel dle part. Wedge cross-bedding and parallel stratification or sandstone and underlying overbank mudstone and parallel current ripple cross-lamination prevail in active fluvial chan- stratified or current ripple cross-laminated fine-grained sand- nels, but trough and planar cross-beddings are absent. The stone, and are absent in wedge and planar cross-bedding sand- thickness of active fluvial channels is smaller than those in the stones. They are a little different from the Skolithos ichnofabric middle part of the formation, while that of the overbank set- proposed by Buatois and Mángano (2007, Fig. 17.5D therein) tings is larger. The ichnodiversity and ichnoabundance are which was mainly preserved in trough cross-bedding sandstone. lower than those in the middle part of the formation and be- The trace fossils in abandoned channel and overbank set- come lower and lower upwards. The Planolites montanus tings are widely distributed in deposits from Silurian to Pleis- ichnofabric formed in low-energy, softground, perennial ponds tocene all over the world and include 48 ichnogenera and 81 is absent, but the ephemeral exposed Taenidium barretti ichno- ichnospecies. Paleozoic trace fossils are characterized by fabric is well developed in overbank deposits. All traces finally monospecific assemblages of meniscate traces that have been disappear as purple sediments wholly gave place to yellow- variably assigned to Beaconites and Taenidium. Variants of the green sediments at the top of the formation. Beaconites-Taenidium ichnofabric seem to be common in abandoned channel and overbank deposits and represent proba- 5 COMPARISON OF THE ICHNOFABRICS IN FLU- bly the earliest continental ichnofabrics. Since Triassic, menis- VIAL SETTINGS cate ichnofabrics are commonly associated with desiccated The trace fossils in active channels are not as abundant as substrates and can be divided into two kinds of ichnofabrics, those in abandoned channels and associated overbanks, as well including the softground (Beaconites-Taenidium) and firm- as in nearshore high-energy settings. Some adverse conditions, ground (Scoyenia) suites. Ichnofabric emplacement often took such as high to relatively high-energy, rapid fluctuations in place in a response to substrate consistency changes (from rates of sedimentation and erosion, and coarser grain sizes ob- softground to firmground) due to progressive desiccation of struct the burrowing of trace makers and preservation of trace water body. This resulted in the crosscutting between different fossils. The oldest burrows in active channels are Skolithos ichnofabrics and formed Beaconites-Taenidium+Scoyenia linearis and Beaconites antarcticus in the Devonian Taylor composite ichnofabric (Buatois and Mángano, 2007). However, Group of the southern Victoria Land, Antarctica, which occur the Beaconites-Taenidium ichnofabric does not always repre- in planar bedded sandstone and decimeter scale trough sent softground suite. A detailed study by Morrissey and cross-bedded sandstone and gritstone (Woolfe, 1990; Bradshaw, Braddy (2004) suggests that the Beaconites-Taenidium ichno- 1981). Other Palaeozoic trace fossil records in active channels fabric recorded colonization of subaerially exposed sediment in are S. linearis in fine to coarse sandstone and fine conglomerate response to seasonal desiccation with animals excavating into from the Permian Feather Conglomerate of southern Victoria the substrate to aestivate or mould at water table level. The Land, Antarctica and the Takrouna Formation of northern Vic- Taenidium barretti ichnofabric in this paper also recorded a toria Land, Antarctica (Fitzgerald and Barrett, 1986). Mesozoic trace suite in periodically exposed ephemeral ponds or trace fossils in active channels are Skolithos isp. and escape low-energy distal floodplain. Softground ichnofabric in this traces from the Triassic Bhimaram Sandstone and the Maleri paper is the Planolites montanus ichnofabric, which was Formation of India and formed mainly in scour-fill crossbeds formed in low-energy, perennial muddy bottom ponds in over- and parallel-laminated units (Sarkar and Chaudhuri, 1992). filled overbank. Some authors reported the Camborygma Other trace fossil records include Taenidium isp. in sand bar ichnofabric in abandoned channel, overbank and lake margin and overbank of the Antenor Navarro Formation of NE Brazil deposits, but it is not widely distributed. The Camborygma (Fernandes and Carvalho, 2001), Skolithos isp., Taenidium isp. ichnofabric is dominated by the crayfish burrow Camborygma and root traces in channel and floodplain of the Cretaceous whose morphology is related to the position of the water table. North Horn Formation of Utah, USA (Bracken and Picard, This trace fossil has have complex architectures with many 1984), Beaconites coronus, ?Skolitnos isp. in sand bar and branches and chambers constructed by many burrowers in areas abandoned channel of Miocene Gering and Monroe Formations, of high water table. But their deep simple burrows are domi- Nebraska, USA (Stanley and Fagerstrom, 1974), and Palaeo- nant in areas with low and/or highly fluctuating water table and phycus tubularis in sand bar of Miocene Ridge Route Forma- have considerable disruption of the sedimentary fabric (Ha- tion of California, USA (Smith et al., 1982). Buatois and Mán- siotis, 2002; Hasiotis et al., 1993). gano (2007, Fig. 17.5D therein) proposed the Skolithos ichnofabric for representing the animal colonization in active fluvial 6 CONCLUSION channels after the summary on trace fossil records all over the Abundant trace fossils occur in fluvial deposits of the world. This ichnofabric is dominated by vertical burrows and Middle Triassic Youfangzhuang Formation in western Henan commonly contains only one ichnotaxon. It reflects the em- Province, China. They are abundant in low-energy overbank placement of moderately deep to deep burrows and most likely and active channels deposits. The climate conditions at the time records opportunistic colonization of emplaced sands (Buatois of deposition is generally regarded as semi-arid in an oxidizing

660 Min Wang, Yongan Qi, Da Li, Mingyue Dai and Yuguang Chang environment. Most of the trace fossils, irrespective of their nology in Facies and Squence Stratigraphic Analysis of orientation, always occur in or are related to brown-red sedi- Fluvio-Lacustrine Successions. In: McIlroy, D., ed., The ments formed under semi-arid climate. Application of Ichnology to Palaeoenvironmental and Seven ichnofabrics were recognized in fluvial deposits of Stratigraphic Analysis. Geological Society, Special Publi- the Youfangzhuang Formation. Three of them are in active cation, London, 228: 311–333 channels, including the Palaeophycus tubularis ichnofabric in Buatois, L. A., Mángano, M. G., 2007. Invertebrate Ichnology the sediments near the scour interface between two fluvial cy- of Continental Freshwater Environments. In: Miller, W. III, cles, the Skolithos linearis ichnofabric in middle- to high- ed., Trace Fossils: Concepts, Problems, Prospects. Elsevier. energy, soft substrate in middle and upper part of fluvial chan- 285–323 nel deposits, and the Skolithos linearis-Planolites montanus Buatois, L. A., Mángano, M. G., Wu, X. T., et al., 1996. Trace ichnofabric in middle-energy, soft substrates in upper part of Fossils from Jurassic Lacustrine Turbidites of the Anyao fluvial channel deposits. The ichnofabrics preserved in over- Formation (Central China) and Their Environmental and bank deposits comprise the Planolites montanus ichnofabric in Evolutionary Significance. Ichnos, 4: 287–303 low-energy, perennial muddy bottom ponds, the Taenidium Fernandes, A. C. S., Carvalho, I. S., 2001. Ichnofosseis de barretti ichnofabric in periodically exposed ephemeral ponds or Intertebrados da Bacia de Sousa (Estado da Pariba, Brasil): low-energy distal floodplain deposits, the Skolithos isp. ichno- A Localidade de Serrote do Letrero. Colecao Chapada do fabric in middle- to high-energy, firmground of proximal Araripe, 1: 147–155 floodplains or crevasse-splay deposits, and the root trace Fitzgerald, P. G., Barrett, P. J., 1986. Skolithos in a Permian ichnofabric in the crevasse splay deposits. Braided River Deposit, Southern Victoria Land, Antarctic. Trace fossils in fluvial deposits have a low ichnodiversity Palaeogeography, Palaeoclimatology, Palaeoecology, 52: and a low to middle, occasionally high-abundance. The ichno- 237–247 fabric approach can also be used in trace fossil study and envi- Frey, R. W., Pemberton, S. G., 1985. Biogenic Structures in ronmental interpretation of fluvial deposits as well as those in Outcroups and Cores. I. Approaches to Ichnology. Bulletin marine settings. of Canadian Petroleum Geology, 33: 72–115 Genise, J. F., Mángano, M. G., Buatois, L. A., et al., 2000. 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