Genetic processes and environmental significance of Lower Devonian brachiopod shell concentrations in Longmenshan area, Sichuan, China

Journal of Asian Earth Sciences 115 (2016) 393–403

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Genetic processes and environmental significance of Lower Devonian brachiopod shell concentrations in Longmenshan area, Sichuan, China Fengjie Li ⇑, Xuelin Qu, Lingchun Du, Tingyong Dai, Yuchuan Yang, Junwu Li, Chengjin Yang Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China

a r t i c l e

i n f o

Article history: Received 27 May 2015 Received in revised form 22 August 2015 Accepted 26 October 2015 Available online 26 October 2015 Keywords: Lower Devonian Shell concentrations Taphonomy Genetic models Brachiopod Longmenshan area

a b s t r a c t The distinctive features of the Lower Devonian rocks of the Longmenshan area in southwestern China are brachiopod shell concentrations, especially in the Bailiuping, Ganxi and Xiejiawan Formations, where brachiopod shell concentrations occur widely throughout. Depending on the dominant skeletal elements, six types of shell concentrations can be distinguished: Protochonete, Acrospirifer, Howellella, Orientospirifer, polyspecific shell and polyspecific fragments concentrations. According to the shell features, taphonomic signature, host sediments and their relationships, four genetic models of the various shell concentrations are described in this paper. The genetic processes and distributions along an onshore–offshore area were clarified on the base of taphonomic analysis. Pavements of opportunistic species of Protochonetes are autochthonous assemblages living in quieter, deeper, more offshore waters near the maximum storm wave base. The pavements are the result of reduced sedimentation; the substrate was silty and water-saturated with variable turbidity soupy-mud. Transport by high-energy processes is interpreted as the final formation process of polyspecific fragments concentrations with most extensive scope from intertidal zone to the maximum storm wave base. The Acrospirifer, Howellella, and Orientospirifer concentrations have been stirred by storm wave action and quickly buried after short transport tempestite model. They are most easily preserved around the average storm wave-base. The polyspecific shell concentrations, which include large bivalves are autochthonous assemblages living in shallow and relatively quieter water near shore environments. Autochthonous assemblages of the opportunist Protochonetes bailiupingensis occurring in the Bailiuping Formation of the Longmenshan area not only record of storm events, but are also important features to identify and correlate the Bailiuping Formation in the field. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Shell beds or more precisely shell concentrations (e.g., Fürsich and Pandey, 1999) are an ubiquitous and highly conspicuous feature of the sedimentary stratigraphic record. As benthic organisms are highly sensitive to any environmental changes, they are a valuable tool for reconstructing the paleoenvironment (e.g., Kidwell, 1985; Brett and Baird, 1986; Davies et al., 1989; Chen, 1996). More and more studies demonstrate that shell concentrations carry the signature of the formational processes and are important indicators of palaeoenvironmental conditions (e.g., Kidwell, 1986; Kidwell et al., 1986; Banerjee and Kidwell, 1991; Fürsich and Oschmann, 1993; Fürsich, 1995; Simões and Kowalewski, 1998; Olszewski, 2004; Tomašovy´ch, 2006; Kolbe et al., 2011; Yamashita et al., 2011). Based on the main agents of the formation ⇑ Corresponding author. E-mail address: [email protected] (F. Li). http://dx.doi.org/10.1016/j.jseaes.2015.10.021 1367-9120/Ó 2015 Elsevier Ltd. All rights reserved.

of shell concentrations which include storm-induced waves and currents, sedimentation rate, settling behavior of taxa and productivity (Fürsich and Pandey, 1999), nine genetic types of skeletal concentrations are described along an onshore–offshore gradient, ranging from nearshore fair-weather wave-concentrations to offshore condensed concentrations (Fürsich and Oschmann, 1993). Genetic classifications pay attention to the taphonomic processes rather than to descriptive features of shell concentrations because the same features can be caused by several processes. It is useful to understand the processes of shell concentrations to recover the palaeoenvironmental and analysis the environmental significance. The concentrations processes include fair-weather waves, storms waves, proximal storm flows, distal storm flows, long term currents and the biological productivity (Fürsich and Oschmann, 1993). In fact, it is relatively difficult to reconstruct processes of shell concentrations because of the abundance of shells affected by the interplay of biogenic and sedimentological factors at the

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accumulation site (Kidwell, 1986). The original biogenic information can easily be erased by sedimentological reworking (Kidwell, 1986, 1991). Shell concentrations from the Lower Devonian of the Longmenshan area in southwestern China (Fig. 1) are described and interpreted in this paper. Based on identification of the species diversity of brachiopods which dominate the concentrations (Li and Chen, 1993; Chen, 1996), analysis of the geometries of skeletal accumulations (Kidwell et al., 1986) and observation of the taphonomic signatures including biotic and abiotic parameters (Brett and Baird, 1986; Miller et al., 1988; Speyer and Brett, 1988; Davies et al., 1989; Johnson, 1989; Kidwell, 1991; Kidwell and Bosence, 1991; Fürsich and Oschmann, 1993), we reconstruct the processes of the various shell concentrations and arrive at genetic models of their formation which contribute considerably to the environmental interpretation of shallow marine deposits of Lower Devonian in area. 2. Geological setting The shell concentrations described in the present paper are from the Lower Devonian which is well exposed in discontinuous outcrops in the Longmenshan region located on the western margin of the Yangtze Platform (Wang, 1985) (Fig. 1). The Ganxi section, a key Devonian section in China (Hou and Yang, 1941; Chen, 1988; Hou et al., 1988; Xian et al., 1995), crops out both banks of the Pingtonghe River, between Guxi and Shawozi village in Jiangyou city of Sichuan province (Fig. 2). Because of the abundant and highly diverse faunas of these rocks, geologists and palaeontologists studied the stratigraphy (Hou and Yang, 1941; Le, 1956; Chen, 1978; Hou et al., 1985; Xian et al., 1995), sedimentary environments and facies (Hou et al., 1988) and faunas (Chen, 1988; Li and Chen, 1993; Xian et al., 1995) since the early last century (Hou and Yang, 1941). The Lower Devonian is composed of siliciclastic rocks, mixed siliciclastic-carbonate rocks, and carbonate rocks in ascending order, which formed in marine environments ranging from nearshore to outer-shelf. The shell concentrations are mainly distributed across three formations (in ascending order), named Bailiuping, Ganxi, and Xiejiawan Formation (Le, 1956; Chen, 1978) (Fig. 3) and are dominated by brachiopods (Hou et al.,

1988; Chen, 1988; Chen et al., 1992; Li and Chen, 1993; Xian et al., 1995).

3. Methods Based on the methods described by Fürsich and Pandey (1999) in their study of upper Cretaceous shell concentrations from the Cauvery Basin, southern India, five transects in Guixi area were investigated in detail in the present study. The longest of these transects in the Shawan village (Fig. 3) is a composite transect that measures 16.5 m, while the other transects extend only for 4–10 m. For each shell concentration, taxonomic composition, biofabric, matrix, packing density, orientation patterns, taphonomic features of individual components (preservation quality; degree of disarticulation, breakage, abrasion, encrustation, and bioerosion), sedimentary structures and bedding plane features have been recorded. Altogether 18 shell concentrations (Fig. 4) have been investigated and some information on their main features is shown in Table 1.

4. Shell concentrations Based on the taxon or taxa that form the bulk of the skeletal material, seven shell concentrations are distinguished in the Lower Devonian of Longmenshan area. They are the Protochonetes, Acrospirifer, Howellella, Orientospirifer, polyspecific shell concentrations, and polyspecific fragments concentrations. The first five shell concentrations are main monotypic shell concentrations. They are different in thickness, species composition, diversity, orientation pattern and the taphonomic signatures of their components which include disarticulation, degree of fragmentation, abrasion, and boring.

4.1. Protochonetes concentrations Protochonetes concentrations (A2, A3, B1, B2, B3; Table 1) occur as thin pavements 1–2 valves thick of Protochonetes distributed on the surface of plane. Some Protochonetes are articulated, and single valves are nearly always oriented in a convex-up position (Fig. 5a–c). The shells all belong to Protochonetes bailiupingensis (Chen, 1996). They are small (around 5–13 mm in size), thin and concavo-convex articulated or single valves. The state of preservation is very good and the shells are rarely encrusted, bored, or fragmented. The matrix is a gray-black and sallow silty mudstone. Despite the thin nature of the pavements, their lateral distribution is quite large and may extend for a few kilometers. The pavements are so rich that 3–4 pavements occurred within one centimeter in some strata. The density of shells in every pavement is very high (110–130 articulated or single valves per one square decimeter). Although marine benthic communities are often strongly dominated by one or two species, such overwhelming dominance by a single species may indicate that an opportunist continually invaded an unoccupied habitat (Levinton, 1970).

4.2. Acrospirifer concentrations

Fig. 1. Outline map of China showing position of Sichuan Province (modified from Qi et al., 2008); the star represents the study area of the Ganxi section in Jiangyou City which is shown in detail in Fig. 2.

Acrospirifer concentrations usually consist of complete but disarticulated shells which are oriented overwhelmingly convexupward in thin pavements (Y1, Y2) (Fig. 5d). The density of shells is high and shells may rest on top of each other. The matrix is gray-black silty mudstone. Apart from Acrospirifer, the brachiopods of Luanquella Kwangsiensis may occur in the pavement.

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Fig. 2. Locality of the study area.

4.3. Howellella concentrations Howellella concentrations are the most abundant and occur as thin pavements or loosely packed shells lenses. Although the shells are always complete but disarticulated, they differ in orientation. The single valves are oriented overwhelmingly convex-upward in the pavements (Fig. 5e) and mainly convex-upward with few convex-downward or vertical valves in the lenses (Fig. 5f). Small or large lenses (up to 5–20 cm) (B4, B5) have a more regular geometry with tapered lateral terminations and a sharp, erosional base. The matrix is a gray-black silty mudstone in the pavements and a gray packstone in the lenses.

4.4. Orientospirifer concentrations Orientospirifer concentrations are also abundant and two subtypes are recognized according to the taphonomic signatures: (1) Well sorted thin pavements (1–2 valves), the shells are usually matrix-supported, disarticulated and convex-up oriented (Fig. 5g). (2) Moderately sorted packstones with an irregular and erosional base ranging from 20 to 80 cm in thickness, with stacked and nested, randomly orientated bioclasts of Orientospirifer. At the Anle village section, in spite of the dense packing and rarity of articulated shells, the proportion of fragmented valves is below 10%. The abundance of Orientospirifer decreases and that of crinoid

fragments increases from bottom to top in the packstone beds (A1, S2, S3, S4) (Fig. 5h and i). These changes correspond to normal graded bedding (A1) (Fig. 6a). Packstones in the upper parts of graded bedding contain mostly dispersed or loosely packed, poorly sorted and randomly oriented fragments of Orientospirifer which constitute less than 10%.

4.5. Polyspecific shell concentrations In polyspecific shell concentrations several taxa dominate. They are composed of brachiopods including Luanguella and Howellella and of bivalves such as Praecardium, Ganxiella, and Ptychopteria in black mudstone of the Ganxi Formation in Shawan section (S4), (Fig. 6b). The brachiopod contains single valves of Euryspirifer and individuals or single valves of Dicoelostrophia in the Xiejiawan Formation of the Ganxi section (G1, G2) (Fig. 6c) where shell preservation is better. Single valves are convex-up oriented and articulated flat lie on the surface of beds. The density of shells varies and the thicknesses of interbeds between the pavements are very inconsistent. Occasionally the pavement is very thin so that it is difficult to separate the upper from the lower of the two shell layers. Complete bivalve individuals commonly cut off two or more pavement beds. It is hard to distinguish stacked pavements and shell beds with muddy interbeds.

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Fig. 3. Stratigraphic table of Lower Devonian strata in the Longmenshan area, Sichuan Province, southwestern China. Based on Chen and Wang (1996).

4.6. Polyspecific fragments concentrations Polyspecific fragments concentrations mainly consist of fragments of brachiopods, bivalves, crinoids, and trilobites. The preservation quality of shells is poor and the fragments vary in size. Even abundant shells are difficult to identify. Apart from a few bigger, complete single valves of brachiopods and bivalves or head or tail of trilobites, debris of crinoids are the dominant bioclasts fragments the concentrations (S5, G3) (Fig. 6d–f). Polyspecific fragments concentrations are matrix-supported and usually exhibit graded beddings. The enclosing rocks which are rare or rich in fossils are usually siltstones, muddy siltstones, mudstones, silty mudstones and shales. 5. Discussion 5.1. Genetic models of shell concentrations According to the shell features, taphonomic signatures and the appearance in outcrop, the various shell concentrations have been classified in four genetic models which also trace their genetic histories. The main features of each genetic model are shown in Table 2. 5.1.1. Opportunistic species explosion model Opportunistic species are organisms which can enter a new ecologic zone very quickly where the species can settle, survive, breed,

and develop. They may also create conditions which makes it easier for other organisms to subsequently enter into the new biochore (Levinton, 1970; Chen, 1988). Opportunistic species are usually small, thin-shelled and flat which finally form a community (Alexander, 1977). Opportunistic species explosion refers to the rapid increase in individual followed by extermination (Levinton, 1970). The frequency of pavements in the strata distribution varying from high to low reflects an unstable sedimentary environment. The thick mudstone interlayer suggests that the pavement formation was due to sediment starvation. The substrate of the pavements was silty and muddy. There was so high water content that the substrate became paste. These properties required invasive opportunistic species to have light and flat bodies, otherwise they would sink into and become buried by mud. At the same time, they are a highly adaptive to turbulent and turbid water environments. Based on the features and distributions of Protochonetes bailiupingensis and interlayers between pavements in the Lower Devonian Bailiuping Formation (A2, A3, B1, B2, B3) (Fig. 5a–c) of the Longmenshan area, Protochonetes bailiupingensis displays the conditions of an opportunistic species (Chen, 1988). The mechanism of pavement formation is sediment starvation. The genetic processes of opportunistic species explosion can be summarized as follows five stages (Fig. 7). (1) In environments of high physiological stress (Levinton, 1970) such as very turbulent, turbid water and the high influx of terrigenous material, most benthic organisms are not adapted to survive for long time. A few infaunal organisms may exist (Fig. 7a). (2) In environments of low physiological stress (Levinton, 1970) such as the quiet and clear water with very low to input of terrigenous material, a few larvae of opportunistic species first tentatively invade and may settle, breed, and develop (Fig. 7b). (3) As Protochonetes bailiupingensis appears to an c-selective, opportunistic species with a strong ability to adapt to the environment, it can not only survive, but also rapidly breed and develop in the new ecologic zone where predators and competitors are initially absent. The high fecundity birth rates, short life and strong dispersal promote a rapid expansion of the communities. With time, the communities continuously extended their ecologic zone and occupied large areas of the sea floor (Fig. 7c). (4) As the living conditions of Protochonetes bailiupingensis fluctuated, the brachiopods often became buried by mud stirred up by waves and flows. The individuals of Protochonetes bailiupingensis are small, thin-shells and have a flat shape, so that a one-centimeter-thick mud layer may bury all of them. When compacted the storm layer is only 2–3 mm thick, so that 3–4 pavements can be accommodated in one-centimetermudstone. At times, bigger storm waves might have stirred up muddy sediments and brought it into suspension thus increasing the water turbidity. As a result many Protochonetes bailiupingensis died of suffocation. In this case, the mudstone interlayers may have been thinner or even a just mud film covering the surface of the shells. At the same time, due to their morphology, individuals could easily vibrate and become in the water column. Dead shells became concentrated and became disarticulated, and were rearranged convex-up on the sea floor (Fig. 7d). (5) The stirred-up mud would bury the shells and blanket when a storm ceased stop or the turbulent underflow slowed down. When the shells were no longer destroyed (final burial), they could have become preserved as pavements (Fig. 7e). 5.1.2. Tempestite model The Acrospirifer, Howellella, Orientospirifer concentrations are almost monospecific shell pavements or thin shell beds. In some cases, they are lenticular, apparently filling a scour and exhibit graded bedding. All shells are disarticulated and more than 90% of them are oriented convex-up and distribute on the surface of beddings (Y1, Y2, B4, B5, S2, S3, S4) (Fig. 5d–h). Some shells are

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Fig. 4. Section through the Lower Devonian in Ganxi area. Left column: Litholog of the Bailingping Formation in Anle Village (A) and Bailiuping Village (B). Middle column: Litholog of the Ganxi Formation in Shawan Village (S). Right column: Litholog of the Bailingping Formation in Yinzuiyan Section (Y) and the Xiajiawan Formation in Ganxi Village (G). c = clay; s = silt; fs = fine stone; ms = medium sand; cs = coarse sand; g = gravel; w = wackestone; p = packstone; g = grainstone.

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Table 1 Charateristics of shell concentrations in Lower Devonian strata of the Longmenshan area, Sichuan, southwestern China. Serial number

Formation

Thickness (cm)

Lithology

Type of supported

Types of species

Geometry

Base boundary

Shell orientation

Sorting

A2

Bailiuping

30

Matrix-supported

Protochonetes

Pavements

Well

Bailiuping

60

Matrix-supported

Protochonetes

Pavements

Convex-up

Well

B1

Bailiuping

30

Matrix-supported

Protochonetes

Pavements

Convex-up

Well

B2

Bailiuping

60

Matrix-supported

Protochonetes

Pavements

Convex-up

Well

B3

Bailiuping

90

Clast-supported

Protochonetes

Pavements

Convex-up

Well

Y1

Bailiuping

15–20

Mudstone

Acrospirifer

Bed, lens

Convex-up

Moderate

Y2

Bailiuping

10–15

Mudstone

Acrospirifer

Bed, lens

Sharp

Convex-up

Moderate

A1 B4 B5 S1

Bailiuping Xiejiawan Xiejiawan Ganxi

100 28 60 20–40

Packstone Packstone Packstone Packstone

Clast-to matrixsupported Clast-to matrixsupported Clast-supported Clast-supported Clast-supported Clast-supported

Planar, indistinct Planar, indistinct Planar, indistinct Planar, indistinct Planar, indistinct Sharp

Convex-up

A3

Silty mudstone– mudstone Silty mudstone– mudstone Silty mudstone– mudstone Silty mudstone– mudstone Mudstone

Orientospirifer Howellella Howellella Orientospirifer

Bed Lens Bed Bed, lens

Sharp Sharp Sharp Sharp

Moderate Moderate Moderate Moderate

S2

Ganxi

30

Packstone

Clast-supported

Orientospirifer

Bed

Sharp

S3

Ganxi

20–40

Packstone

Clast-supported

Orientospirifer

Bed, lens

Sharp

G1

Xiejiawan

15–20

Mudstone

Matrix-supported

Lens

Sharp

G2

Xiejiawan

10

Mudstone

Matrix-supported

Lens

Sharp

S4

Ganxi

5–20

Mudstone

Matrix-supported

Bed

Indistinct

G3 S5

Xiejiawan Ganxi

36 5–10

Packstone Packstone

Clast-supported Matrix-supported

Euryspirifer, Dicoelostrophia Euryspirifer, Dicoelostrophia Luanguella, Howellella Crinoids, Euryspirifer Crinoid fragments

Random Random Random Mainly convex-up Mainly convex-up Mainly convex-up Mainly convex-up Mainly convex-up Mainly convex-up

Bed Lens

Erosional Erosional

vertical (S1, Fig. 5i). The matrix is a mud or wackestone. The shells are rarely broken and well preserved. These features indicate deposition by high energy currents of short duration, where dead shells are reworked, winnowed and transported for short distances (Fürsich and Pandey, 1999). Thus, the Acrospirifer, Howellella, and Orientospirifer concentrations are interpreted as products of proximal tempestites (Aigner, 1985; Fürsich and Oschmann, 1993; Fürsich and Pandey, 1999; Dattilo et al., 2012) or distal tempestites (Fürsich and Oschmann, 1993). Distal tempestites differ from proximal ones in the beds tending to be thinner, better sorted and the size of the components being smaller than those of the proximal tempestites (Fürsich and Oschmann, 1993; Li et al., 2014). The genetic processes of the tempestite model can be summarized as follows (Fig. 8). (1) As environmental conditions are favorable, rich epifaunal become established on the seafloor and abundant shells were buried in the sediment (Fig. 8a). (2) Multiple storm waves eroded and scoured these areas. As a result live and dead shells became winnowed and scoured by storm waves. This might have led disarticulation or even partial shell fragmentation of most of shells (Fig. 8b). (3) Intense storm action could have cleared off the sedimentary layer on top of shells, sometimes concentrating single valves and fragments together. They easily floated and were carried offshore where they were deposited convex-up (Fig. 8c). (4) With decreasing flow velocity, the shells will be buried under mud stirred up by the intense storm (Fig. 8d).

5.1.3. Hydrodynamic transport model The polyspecific fragments concentrations are characterized by a variety of biodebris, different sized components, a distinctly irregular erosional base and by poor preservation (S5, G3) (Fig. 6d–f). The polyspecific fragments imply transport and physical breakage of skeletal elements by hydrodynamic processes

Moderate Moderate Moderate Moderate Poor Poor Poor

(Fürsich and Oschmann, 1993). The concentrations most likely reflect long-lasting transport of the shells across the shell. (1) A rich and diverse epifauna such as brachiopods, bivalves, crinoids, and trilobites lived on the seafloor. The hydrodynamic force of the repeated storms stirred up live and dead epifaunal organisms and transported them to shallow-water nearshore areas. While in transport they may have been become fragmented layer shells are dragged and rolled by currents across the sea floor, which easily could have fragmented them (Fig. 9a). (2) When the storm subsided strong offshore-directed currents transported sediment, including shells debris, toward the open shelf, where the debris would sink and concentrate together. They would be overlaid by storm sediment quickly. These were polyspecific fragments concentrations if the concentrations could be preserved permanently. This type of shell concentrations occurs from the intertidal zone to the maximum storm wave base (Fig. 9b). 5.1.4. Biogenic and diagenetic compaction model Polyspecific shell concentrations exhibit a high species diversity, lack sorting and shells are randomly oriented. Preservation of shell concentrations is better in black mud. The concentrations lack sharp bases and consist mainly of complete individuals or single valves (G1, G2) (Fig. 6c), especially the well preserved large valves of the bivalve Ptychopteria (S4) (Fig. 6b). The features suggest that polyspecific shell concentrations are mixed assemblages and largely autochthonous community relics (Fürsich and Pandey, 1999). However, the completely convex-up oriented shells show that they have been winnowed by gentle waves or currents. The polyspecific shell concentrations thus can be interpreted as primary biogenic concentrations (Fürsich and Oschmann, 1993; Fürsich and Pandey, 1999). (1) During medium to high rates of sedimentation, several epifaunal taxa lived and breed in different depth on the sea floor

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Fig. 5. Shell concentrations in the Lower Devonian. (a–c) Protochonetes concentrations, Bailiuping Formation, Anle section, bedding plane view; (d) Acrospirifer concentration, Bailiuping Formation, Yingzuiyan section, bedding plane view; (e and f) Howellella concentrations, Xiejiawan Formation, Bailiuping section, bedding plane view; (g–i) Orientospirifer concentrations, Ganxi Formation, Shawan section, (g) bedding plane view, (h and i) cross-section.

(Fig. 10a). (2) Due to the low rate of sedimentation, shells of dead epifaunal organisms remained on the sea floor for a long time. They experienced winnowing by distal storm waves, distal storm flows, or weak currents, which disarticulated the shells and oriented them convex-up (Fig. 10b). (3) Once the rate of sedimentation increased or storm mud was rapidly deposited, the shells became buried (Fig. 10c). (4) Diagenetic compaction increased the close-packing of the single valves and articulated individuals to form the shell concentrations (Fig. 10d). It is necessary to emphasize that all the genetic models presented above are simplified and any given shell concentrations may be not easily fit. The main features of each model are provided in Table 2.

5.2. Environmental significance of shell concentrations 5.2.1. Frequency of storm events The rocks enclosing Protochonetes bailiupingensis (Chen, 1996) concentrations are gray black and sallow silty mudstones or mudstones with well-preserved shells. These features suggest that the brachiopod lived below the normal storm wave-base. Only rare, extremely intense storms touched the bottoms in this environment, and fragment the small, thin and flat shape Protochonetes bailiupingensis shells. The brachiopods suffocated by the deposition of mud related to severe storms. The frequency of Protochonetes bailiupingensis pavements represents the frequency of the most severe storms events. Johnson (1989) estimated 12–15 years to be the

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Fig. 6. Shell concentrations in the Lower Devonian. (a) Orientospirifer concentration, Bailiuping Formation, Anle section, normal graded bedding, cross-section; (b) Protochonetes, Howellella, and Ptychopteria, Ganxi Formation, Shawan section, bedding plane view; (c) Euryspirifer and Dicoelostrophia, Xiejiawan Formation, Ganxi section, bedding plane view; (d) concentrations of Luanquella, Howellella, Ganxi Formation, Ganxi section, cross-section; (e) crinoid concentration, normal graded bedding, Ganxi Formation, Shawan section, cross-section; (f) Luanquella and crinoids, normal graded bedding, Ganxi Formation, Ganxi section, cross-section.

frequency of storms affecting the sea floor below the normal stormwave base in the Rytteraker Formation, Llandovery in southern Norway, based on the population age of the brachiopod Pentamerus and the sedimentary cycles between storm to non-storm layers. In view of the above frequency, we estimate the time of Protochonetes recovery and its thriving to be no more than two decades. Besides, this estimate can be used to calculate the average rate of sedimentation of Protochonetes pavements in the Bailiuping Formation. 5.2.2. Environmental implications The different genetic models of shell concentrations reflect different hydrodynamic conditions that represent various

sedimentary environments. It is possible to differentiate the various types of shell concentrations with respect to their positions along an onshore–offshore gradient. The opportunistic Protochonetes bailiupingensis lived in quieter, deeper, more offshore waters below the average storm wave-base, most likely near the maximum storm wave-base. The dominance of shell fragments implies that transport and physical breakage of shells occurred under the influence of high-energy processes including waves and currents. The distributions of the polyspecific fragments concentrations have the most extensive distributions from intertidal zone to the maximum storm wave-base (Fürsich and Oschmann, 1993). In comparison, Acrospirifer, Howellella and Orientospirifer

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F. Li et al. / Journal of Asian Earth Sciences 115 (2016) 393–403 Table 2 Main features of genetic models of shell concentrations in the Lower Devonian of the Longmenshan area, Sichuan, southwestern China. Genetic model

Taxonomic composition

Features of individuals

Preservation

Orientation

Density

Lateral distribution

Concentration

Source of shells

Time of formation (years)

Opportunistic species explosion model Tempestite model

Monotypic

Concavo-convex or flat individuals and single valves

Good

Convex-up

High

Wide

Biologic (Ecologic)

Autochthonous

1–10

Monotypic and polytypic

Good to very good

Variable

Biologic sedimentologic

Parautochthonous

10–100

Polytypic

Poor

Convex-up few convexdown or perpendicular Variable

Variable

Hydrodynamic transport model Biogenic and diagenetic compaction model

Single valves of biconvex individuals and flat, concavoconvex individuals Fragments and bioclast

High

Wide

Sedimentologic

Allochthonous

2–3

Single valves and individuals

Varying Mostly very good

Variable mostly convex-up

Variable

Restricted

Biologic and diagenetic

Auto to parautochothonous

10–100

Polytypic

Fig. 7. Genetic processes for the shell concentrations dominated by explosion of opportunistic species (see text for explanation, based on Chen (1996) and edited).

Fig. 8. Genetic processes for the shell concentrations dominated by tempestite (see text for explanation).

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Fig. 9. Genetic processes for the shell concentrations dominated hydrodynamic transportation (see text for explanation).

Fig. 10. Genetic processes for the shell concentrations dominated biogenic and diagenetic compaction (see text for explanation).

living in shallow water close to the fair-weather wave-base or slightly beneath the base (BA2 to BA3 (Chen and Wang, 1999)) where the shells have been stirred by storm waves and became quickly buried after short transport. The optimal preservation potential of the Acrospirifer, Howellella and Orientospirifer concentrations are deep water environments, probably around the average storm wave-base. As the polyspecific shell concentrations usually include a few bivalves, it can be deduced that the water depth at which these concentrations formatted was not very deep and probably belongs to the BA2 ecology zone (Chen and Wang, 1999). Based on the analysis of storm influence, they reflect relatively quiet environment. In some cases, the dominating concentration mechanism is biological rather than physical concentration.

5.2.3. Sign of stratigraphic correlation The Protochonetes pavements formed within a short time (12–15 years) and may extend laterally for a few kilometers. They can be regarded as isochronous stratigraphic bodies that can be used for stratigraphic correlation, and helps to increase the accuracy of such correlations. The pavements of monotypic Protochonetes bailiupingensis are important features for the

identification and correlation of the Bailiuping Formation in the field.

6. Conclusions (1) Shell concentrations are a distinctive feature of the Lower Devonian in the Longmenshan area, southwestern China. They are dominated by brachiopods. Depending on the dominant skeletal elements, six types of shell concentrations can be distinguished. (2) Based on the shell features, taphonomic signatures and appearance in outcrops, four genetic models to explain the various shell concentrations are described in this paper. (3) The genetic processes and distribution along an onshore– offshore gradient are reconstructed based on taphonomic analysis. Opportunistic species of Protochonetes pavements are autochthonous assemblages living in quieter, deeper, offshore waters, near the maximum storm wave-base. Transport by high-energy hydrodynamic processes is interpreted as the final formation process of polyspecific fragments concentrations which have the most extensive distribution from the intertidal zone to the maximum storm wave-base. In the

F. Li et al. / Journal of Asian Earth Sciences 115 (2016) 393–403

tempestite model of the Acrospirifer, Howellella and Orientospirifer concentrations shells have been stirred up by storm waves and became quickly buried after short transport. The optimal preservation potential was around the average storm wave-base. The polyspecific shell concentrations, in particularly with large bivalves, are autochthonous assemblages living in shallow near-shore water, but in relatively quiet water environments. (4) Autochthonous assemblages of the opportunistic species Protochonetes bailiupingensis occurring in the Bailiuping Formation of Longmenshan area not only indicate the presence of storm events, but are also important features for identifying and correlating the Bailiuping Formation in the field.

Acknowledgements This study has been supported by the National Natural Science Foundation of China (No. 41172100), the Key Project of Natural Science in Sichuan Education Department (No. 09ZA007, No. 12ZA012). The reviewer of Professor Franz T. Fürsich and another anonymous reviewer are acknowledged for their critical reviews, which led to much improvement of this work. I sincerely appreciate Professor Franz T. Fürsich for his hard work on the manuscript trying to improve the English.

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