Palynofacies analysis of the Permian–Triassic transition in the Amb section (Salt Range, Pakistan): Implications for the anoxia on the South Tethyan Margin

Journal of Asian Earth Sciences 60 (2012) 225–234

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Palynofacies analysis of the Permian–Triassic transition in the Amb section (Salt Range, Pakistan): Implications for the anoxia on the South Tethyan Margin Elke Schneebeli-Hermann a,⇑, Wolfram M. Kürschner b, Peter A. Hochuli c, Hugo Bucher c, David Ware c, Nicolas Goudemand c, Ghazala Roohi d a

Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway Institute and Museum of Palaeontology, University of Zurich, Karl Schmid-Str. 4, CH-8006 Zurich, Switzerland d Pakistan Museum of Natural History, Garden Avenue, Islamabad 44000, Pakistan b c

a r t i c l e

i n f o

Article history: Received 20 January 2012 Received in revised form 31 August 2012 Accepted 4 September 2012 Available online 27 September 2012 Keywords: Permian–Triassic Palynofacies South Tethys Pakistan

a b s t r a c t The uppermost Chhidru Formation and the lower part of the Mianwali Formation were sampled in the Amb Valley, Salt Range, Pakistan for the study of the particulate organic matter (POM) content in order to evaluate the depositional environment during the Permian–Triassic transition. The POM content was assigned to four distinct palynofacies (palynofacies A–D). Palynofacies A recovered from siltstone within the white sandstone unit of the Upper Permian Chhidru Formation indicates a shallow marine oxic shelf setting. Recorded from the siltstone intercalations in the Kathwai Member of the basal Mianwali Formation, the Griesbachian palynofacies B is characterised by abundant acritarchs indicating a transgressive event. Palynofacies C recovered from the siltstone of the lowest Ceratite Marls (middle Dienerian) is dominated by terrestrial organic particles and indicates shallowing of the depositional environment, whereas 40 cm above, palynofacies D represents transgressive oxygen depleted conditions. The comparison with sections from the Australian Tethyan margin shows that oxygen depleted conditions occurred during the Griesbachian in the Perth Basin, while in the Bonaparte Basin oxygenated conditions prevailed. Hence, oxygen depleted facies do not correspond to a single, synchronous Permian–Triassic oceanic anoxic event but depend on local geography and bathymetry. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The research of the recent years has re-evaluated a plethora of hypotheses for the causes of the end-Permian mass extinction. It appears that the combined effects of a coherent chain of environmental changes linked to the Siberian Trap emplacement caused the mass extinction at the end of the Permian (e.g. Wignall, 2001). One of these environmental changes that were initially proposed as the unique cause for the mass-extinction is a widespread and long-lasting oceanic anoxic event (e.g. Wignall and Hallam, 1992; Wignall and Twitchett, 1996). During oceanic anoxic events degradation of sedimentary organic matter is reduced or even inhibited (e.g. Pilskaln, 1991). Therefore sediments deposited during these events are rich in organic matter and the composition of the sedimentary POM is characterised by enhanced preservation of amorphous organic matter of bacterial or algal origin (Batten, 1996; Hochuli et al., 1999; Tyson, 1993).

⇑ Corresponding author. Tel.: +31 30 253 26 47; fax: +31 30 253 50 96. E-mail address: [email protected] (E. Schneebeli-Hermann). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.09.005

In order to describe the specific composition of POM assemblages recovered from palynological preparation of sedimentary rocks Combaz (1964, 1980) coined the term palynofacies. In sedimentary successions distinct changes of palynofacies reflect changes in the depositional environment such as oxygenation, sea level changes, or distance from the shore (Combaz, 1964; Tyson, 1993, 1995). For example, high contents of amorphous organic matter (AOM) are typical for low oxygenation during deposition (e.g. Roncaglia, 2004). Therefore palynofacies studies are an excellent tool to evaluate the timing and distribution of the oceanic anoxic event during the end-Permian mass extinction and its aftermath. Previous studies indicate a distinct change of palynological assemblages across the Permian–Triassic boundary, i.e. an increase in acritarchs abundance towards the basal Triassic (Kathwai Member) of up to 80% (Balme, 1970; Sarjeant, 1970, 1973). Here we describe changes of the palynofacies of the Permian– Triassic transition from the sedimentary succession in the Amb valley in the Salt Range, Pakistan. The correlation of Permian–Triassic successions deposited in basins along the Australian Tethyan margin reveals a diachronous distribution of the oceanic anoxic conditions and challenges the idea of the occurrence of a single, synchronous oceanic anoxic event.

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2. Geography, lithology and stratigraphy The Salt Range in Pakistan is one of the few areas where marine Permian–Triassic successions are accessible in numerous outcrops along valleys that intersect the sedimentary succession. During the Late Permian and Early Triassic times the Salt Range area was part of the southern Tethyan shelf of the Indian subcontinent (North Indian Margin) (Fig. 1a). Today, the Amb section is located ca. 20 km SE of Nammal and about 5 km S of the Sakesar mountain (N 32°290 48.100 ; E 071°560 20.600 ; Fig. 1b). The studied interval encompasses the uppermost part of the Chhidru Formation and the lowermost part of the Mianwali Formation. The uppermost part of the Chhidru Formation was informally named ‘‘white sandstone unit’’ by Kummel and Teichert (1970). Due to its deposition in shallow subtidal to intertidal environments, thickness and completeness of the unit varies throughout the Salt Range (Mertmann, 2003). At Amb, the white sandstone unit consists of a 9 m thick succession of alternating white to grey, medium grained sandstone and dark grey siltstone. Foraminifera indicate a Wuchiapingian age for the Chhidru Formation (Pakistan-Japanese Research Group, 1985). Conodont assemblages, however, suggest a correlation with the Upper Permian Changhsing Formation (China) (Wardlaw and Pogue, 1995). Mei and Henderson (2002) proposed the Wuchiapingian–Changhsingian boundary to coincide with the boundary between the Wargal and the Chhidru formations. Alternatively, it has been proposed to be located in the lower half of the Chhidru Formation based on the presence of the conodont species Clarkina longicuspidata that indicates late Wuchiapingian and early Changhsingian age (Shen et al., 2006; Mei et al., 2002; Wardlaw and Mei, 1999). The Changhsingian conodont fauna is marked by typical cool water elements such as Vjalovognathus and Merrillina (Mei et al., 2002). Thus the upper part of the Chhidru Formation, including the white sandstone unit is of Changhsingian age. The bulk organic d13C record from Amb section can be correlated with the chemostratigraphic scheme established for the expanded Permian–Triassic successions in Norway (Hermann et al., 2010). The bulk organic d13C data from Norway are characterised by a stepwise negative carbon-isotope shift in the latest Permian, which can be correlated to d13C data from globally distributed Permian–Triassic successions such as the

d13C data from the Bowen and Bonaparte Basins in Australia (Fig. 2). This stepwise negative shift of bulk organic d13C is also present the bulk organic d13C record at Amb, which is consistent with a latest Changhsingian age for the studied interval (Fig. 2). The contact between the Chhidru Formation and the overlying Mianwali Formation is an erosional unconformity with a large erosive relief that was interpreted to represent a sequence boundary (Baud et al., 1996; Mertmann, 2003; Hermann et al., 2011; Wignall and Hallam, 1993) and includes a sedimentary gap between the two formations (Fig. 2). The Mianwali Formation is subdivided into the Kathwai Member, the Mittiwali Member and the Narmia Member (Kummel, 1966). For this study only the basal part of the Mianwali Formation has been investigated, including the Kathwai Member, and the two basal lithological units of the Mittiwali Member, namely the Lower Ceratite Limestone, and the lowermost part of the Ceratite Marls (e.g. Waagen, 1895; Kummel and Teichert, 1970; Guex, 1978; Hermann et al., 2011). At Amb the 2.8 m thick Kathwai Member consists of dolomite and limestone both with terrigenous detritral contribution and several intercalated a few centimetre thick, dark grey siltstone layers. Glauconite is common in the Kathwai Member, reaching up to ca. 5% (Wignall and Hallam, 1993). Sedimentological data suggest that it was deposited in the transgressional phase after the sequence boundary at the base of the Kathwai Member (Mertmann, 2003). Based on lithology and fossil content, the Pakistani-Japanese Research Group (1985) subdivided the Kathwai Member into three subunits (see also Mertmann, 2003; Wignall and Hallam, 1993). At Nammal, these authors reported the conodont species Hindeodus parvus, the index fossil for the base of the Triassic (Yin et al., 2001) to occur in the middle unit of the Kathwai Member. Hence, the Permian–Triassic boundary was placed in the middle part of the Kathwai Member. At Nammal, the negative carbon isotope spike, which has been reported from Permian–Triassic boundary sections worldwide, occurs in the lowermost part of the Kathwai Member (Baud et al., 1996). In our collections from Amb, Hindeodus typicalis and H. praeparvus have been observed together with an ambiguous specimen of H. parvus in a sample from the lowermost part of the Kathwai Member. The work of Wardlaw and Mei (1999) documents the occurrence of the uppermost Changhsingian conodont C. meishanenis in the basal part of the Kathwai Member, followed by H. parvus

a

b

c

Fig. 1. Location of the study area. (a) Palaeogeographic position of the Salt Range together with the Perth Basin, Carnarvon Basin, and Bonaparte Basin (after Scotese (2001) and Gorter et al. (2009)). (b) Geographic position of the Salt Range in Pakistan. (c) Geographic position of the Amb and Nammal Valley in the Salt Range.

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Fig. 2. Correlation of the carbon isotope records from Amb in Pakistan with two Australian records. Chemostratigraphic intervals after Hermann et al. (2010). (A) Bulk organic carbon isotope record from Amb. (B) Bulk organic carbon isotope record Bowen Basin (Morante, 1996). (C) Bulk organic carbon isotope record Bonaparte Basin (Morante, 1996). LCL = Lower Ceratite Limestone, CM = Ceratite Marls.

in the Salt Range area. According to Mertmann (2003) the basal unit of the Kathwai Member is only locally preserved and is Permian in age. The middle unit of the Kathwai Member is mainly composed of yellowish to orange secondary dolomites and dedolomites with a lot of echinoderma debris (Mertmann, 2003), is basal Triassic in age with H. parvus and distributed throughout the area, being locally directly in contact with the upper Chhidru white sandstone unit. Consequently, the formational boundary is diachronous and H. parvus from the middle unit of the Kathwai Member can be found directly above the white sandstone. The diachronism between the two formations has already been proposed based on palynology (Hermann et al., 2011). Without further indications for the Amb section, we use the formational boundary between the Chhidru Formation and the Mianwali Formation as a proxy for the Permian–Triassic boundary. The 60 cm thick Lower Ceratite Limestone consists of an ammonoid rich grey coarse-grained limestone with abundant glauconite. Its ammonoid and conodont faunas are currently under further study to provide a detailed biostratigraphic scheme. Some preliminary results were shown during conferences (Ware et al., 2010, 2011). In Amb, the LCL contains the following index taxa: Gyronites dubium at the base (earliest Dienerian), Gyronites frequens in the penultimate bed (latest Early Dienerian) and Ambites atavus in the last bed (earliest Middle Dienerian). These species co-occur with the conodont Sweetospathodus kummeli. The overlying Ceratite Marls consist mainly of dark grey siltstones with intercalated limestone and sandstone lenses and beds. Ammonoids indicate a Middle Dienerian to Early Smithian age; the first 2 m studied herein being from the Middle Dienerian as indi-

cated by the presence of abundant ammonoids belonging to the genus Ambites. 3. Methods and material The Permian–Triassic Amb section in the Salt Range measures ca. 14 m. Thirty siltstone samples have been collected and prepared following standard palynological preparation techniques. Limestones and dolomites from the Kathwai Member have not been prepared due to the low preservation potential of these lithologies. The samples were cleaned, crushed and weighed (15–18 g) and subsequently treated with concentrated hydrochloric and hydrofluoric acid (Traverse, 2007). The residues were sieved over an 11 lm mesh screen and mounted for the analysis of POM. From the strew mounts a minimum of 300 particles per sample was counted. In order to determine the environmental gradients, which caused the largest variance in the POM dataset a multivariate statistical analysis using principle component analysis (PCA) has been performed. All analyses were conducted with a log10 transformation of the organic matter components data using C2 (Juggins, 2003). The results are illustrated as the species scores (=specific particle score) on the first and second axis in the PCA ordination diagram and sample scores for the same axis. 4. Results For the analysis of the POM the following categories have been identified: Translucent and opaque wood, cuticles, membranes and

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inertinite, pollen and spores as well as amorphous organic matter (AOM), acritarchs, leiospheres, and foraminiferal test linings. The following pollen groups have been distinguished: taeniate bisaccate pollen, non-taeniate bisaccate pollen, and other pollen. The group non-taeniate bisaccate pollen also includes undifferentiated bisaccate pollen, i.e. pollen of which a determination of taeniate or non-taeniate corpus morphology was impossible due to the poor preservation. The preservation of the spore and pollen is fair to excellent with a thermal alteration index of 2 (Batten, 1996). According to the relative abundance of the above mentioned categories four different palynofacies with distinct POM composition could be distinguished (Fig. 3, Pictures A–D): Palynofacies A is restricted to the Upper Permian white sandstone unit. It is marked by the dominance of terrestrial particles (>92%) with translucent and opaque phytoclasts occurring in approximately equal numbers of about 40%. Sporomorphs are represented by taeniate bisaccate pollen, non-taeniate pollen and spores of equal percentages (ca. 4%). Acritarchs are a minor component of the assemblage and reach percentages of <5% (Fig. 3). Bulk organic carbon isotope values in palynofacies A range from 27.5‰ (AMB 28) to 23.5‰ (AMB 29). Palynofacies B is restricted to the Griesbachian Kathwai Member. Acritarchs are represented in high numbers (53–63%) consisting of the genera Micrhystridium spp. and Veryhachium spp.

Compared to palynofacies A, woody particles are reduced to lower numbers (20%) and the relative abundance of sporomorphs is increased (18%) (Fig. 3). In palynofacies B bulk organic d13C values range from –30.1‰ (AMB 24) to 29.1‰ (AMb 120). Palynofacies C is restricted to the lowermost 20 cm of the Ceratite Marls (lowest middle Dienerian). In this assemblage the terrestrial fraction comprises on average 55% woody fragments and 27% sporomorphs. Spores alone account for 24% of the total particulate organic matter in assemblage C. The marine fraction consists of acritarchs (11%). The two samples representing palynofacies C show d13C values of 28.7‰ (AMB 26) and 28.6‰ (AMB 102), respectively. Palynofacies D is represented by a single sample in the middle Dienerian Ceratite Marls, about 40 cm above the base of the Ceratite Marls. It is characterised by high relative abundance of AOM (60%) the total marine fraction sums up to 74% of the total POM. Terrestrial woody plant fragments account for about 17% (Fig. 3). Sample AMB 103 shows a bulk organic d13C of 29.3‰. The principle component analysis (PCA) of the dataset is illustrated as species scores (=specific particle scores, Fig. 4a) and sample scores (Fig. 4b). The first two axis of the PCA explain together 72.5% of the variance within the dataset. On PCA axis 1, which explains 55.5% of the variance of the dataset, acritarchs and AOM have high positive scores, whereas phytoclasts (translucent and

D C

B

A

A

B

C

D

Fig. 3. Relative abundance of the particulate organic matter (POM) and subdivision into Palynofacies A–D, together with lithology, biostratigraphy, marine terrestrial ratios and PCA sample scores for axis 1 and 2. Pictures A–D correspond to Palynofacies A–D Scale bar: 50 lm. LCL = Lower Ceratite Limestone, CM = Ceratite Marls.

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opaque) score on the negative axis 1. PCA axis 2 explains 17.0% of the variance within the dataset. AOM plots on the positive axis 2, whereas palynomorphs plot on the negative side (Fig. 4a).

5. Discussion 5.1. Palaeoenvironmental interpretation of the palynofacies results

3.0

5.2. Comparison with Nammal One of the best studied Upper Permian–Lower Triassic section in the Salt Range is the Nammal section, about 20 km NW of Amb (e.g. Baud et al., 1996; Brühwiler et al., accepted for

2.0

a

Kathwai Mb

Palynofacies D

AMB 103

lowermost Ceratite Marls

AOM

Ceratite Marls

2.0

1.0

leiospheres translucent phytoclasts

0.0

foraminiferal test linings

opaque phytoclasts

1.0

AMB 34 AMB 30 AMB 29

AMB 36

AMB 35

AMB 31

Palynofacies C

AMB 44 AMB102 AMB 41 AMB 24

AMB 43 AMB 48 AMB 46

AMB 42

spores

AMB 26

AMB 101

0.0

-1.0

oxygen

b

Chiddru Fm

PCA axis 2, explains 17.0%

PCA axis 2, explains 17.0%

low oxygen

The relative abundances of the POM groups reflect not only the biological origin of the organic matter but also the depositional environment of the investigated sedimentary succession. Transgressive–regressive trends and trends in oxygenation, can be described (Combaz, 1964, 1980; Tyson, 1993, 1995). Additionally, palynofacies data combined with other geological data have been successfully used as a sequence stratigraphic interpretation tool (Batten, 1999; Batten and Stead, 2005; Tyson, 1995; Götz et al., 2008). Multivariate analysis methods such as PCA have been successfully used for the interpretation of palynofacies data and for analysing trends within a dataset (Kovach and Batten, 1994). The applied PCA allows evaluating the environmental gradients determining the POM assemblages. Since marine particles have high positive scores on PCA axis 1 while terrestrial phytoclasts show negative scores, PCA axis 1 (55.5% of the data variance) reflects the marine-terrestrial gradient within the data set. Therefore, we interpret PCA axis 1 as an indicator of relative sea-level changes. AOM comprises structureless organic matter derived from phytoplankton or bacteria (Lewan, 1986; Tyson, 1995). AOM with granular aspect (under light microscopy) has been shown to be a direct microbial product (Pacton et al., 2011). Dissolved organic matter and faecal pellets can also contribute to the formation of AOM (Tyson, 1995). Experimental studies show that dark anoxic conditions promote the production of AOM (Pacton et al., 2011). AOM is preferentially preserved under low oxygen conditions (Roncaglia, 2004; Tyson, 1995). It generally increases towards offshore basinal settings, where reduced water mixing and relatively low oxygen concentrations allows increased preservation of the less oxidation resistant organic matter (Leckie et al., 1990; Steffen and Gorin, 1993; Tyson, 1993, 1995; Wood and Gorin, 1998; Roncaglia, 2004). One of the possible processes that could lead to en-

hanced deposition of AOM in shallow depositional settings could be the upward migration of the oxygen minimum zone, thus providing the low oxygen environment for the preservation of AOM. Consequently, we interpret the second PCA axis as the gradient from oxygenated to dysoxic environment. Strikingly, spores plot on the marine side of the PCA. The spore fraction consists predominantly of cavate lycopod spores. Their outer exine might function as a saccus-like structure that increases the spores’ buoyancy. Selective transport of terrestrial palynomorphs in the marine environment is a well known phenomenon and has been described as the Neves effect (e.g. Chaloner and Muir, 1968; Traverse, 2007). This may explain why in the present study spores show an increase in abundance in a distal setting, similar to bisaccate pollen. In this case, it also means that the spore/pollen ratios are not governed primarily by sea-level changes but by the vegetation of the catchment area. Therefore, spores plotting on the marine side of the PCA might reflect the vegetation of a low lying, low energy catchment area. Trajectories of the sample scores on PCA axis 1 and 2 (Fig. 4b) indicate a trend from shallow marine, well oxygenated conditions of the Chiddru Formation (palynofacies A) to basinal oxygen depleted conditions (palynofacies D). Palynofacies A reflects shallow, oxygenated conditions corresponding to the lithological interpretation of Mertmann (2003). The relative abundance of acritarchs in palynofacies B of the Kathwai Member indicates an increased marine influence and deepening of the depositional environment remaining under well oxygenated marine regime (positive excursion of the PCA axis 1 in Fig. 3). The lowermost samples of the Ceratite Marls (palynofacies C) plot between the marine and the terrestrial samples showing a shallowing trend in a well oxygenated milieu. Basinal and oxygen poor conditions are reflected in palynofacies D. The rise in sea-level together with increasing oxygen depletion is indicated by the positive excursions of PCA axis 1 and 2 for the single samples from the basal Ceratite Marls (Fig. 3).

acritarchs

AMB 120

Palynofacies A

taeniate bisaccate non-taeniate undiff

Palynofacies B -1.0

-2.0 -2.0

terrestrial

-1.0

0.0

1.0

PCA axis 1, explains 55.5%

2.0

3.0

marine

-1.0

0.0

1.0

2.0

PCA axis 1, explains 55.5%

Fig. 4. Principal components analysis (PCA) ordination diagrams of particulate organic matter (POM). (a) Particles score. (b) Samples score; grey arrow represents the general trajectory of the samples.

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publication, Hermann et al., 2011; Kummel and Teichert, 1970; Mertmann, 2003, Pakistani–Japanese Research Group, 1985; Wignall and Hallam, 1993). A combined approach of sedimentological and palynofacies data has been used to describe the depositional environment and to establish a sequence stratigraphic framework of the Lower Triassic succession at Nammal (Hermann et al., 2011). Sedimentological and palynofacies data of the Amb and Nammal sections vary partially, which leads to different interpretations of the depositional environments of the two sections. As mentioned before, the distribution of siltstone and sandstone as well as the thickness of the white sandstone unit varies laterally. While we observed several siltstone intervals in the 9 m thick white sandstone unit at Amb, at Nammal it consists of ca. 1 m of sandstone. Kummel and Teichert (1970) described a bed of siltstone between the Chhidru Formation and the Kathwai Member close to the river, which could not be found during the recent field campaigns (2007–2010) due to debris coverage. Higher up in the valley the Kathwai Member rests directly on white sandstone. Because of the lack of appropriate samples for palynological analysis there are no palynofacies data for the Chhidru Formation at Nammal. At Nammal the Kathwai Member consists of dolomites and limestone with terrigenous detritral contribution and sandstone and does not include similar siltstone beds as the Kathwai Member of the Amb section. Even the most promising sample at the Nammal section from the sandstones near the top of the Kathwai Member has yielded only opaque phytoclasts (Hermann et al., 2011). The poor preservation of organic matter in the Kathwai Member at Nammal is likely the result of its shallower marine setting compared to slightly deeper shelf setting of the Amb section (Fig. 5). In accordance with the sequence stratigraphic interpretation of the Kathwai Member of Mertmann (2003) and Baud et al. (1996) the middle and upper unit of the Kathwai Member represents a transgressional phase rather than a lowstand system tract, which is supported by the new data from Amb. At Nammal the preservation of the organic matter in the Lower Ceratite Limestone is as poor as in the Kathwai Member and palynofacies C has not been observed. However, the assemblages from the base of the Ceratite Marls correspond to palynofacies D as described in this study (Hermann et al., 2011 and unpublished data, see Fig. 5). 5.3. Comparison with Western Australian basins The sedimentary successions of the Perth, Carnarvon, and Bonaparte Basins in Western Australia include marine sedimentary archives of Late Permian to Early Triassic age, which were located some 1000 km SE of the Salt Range on the Australian shelf during the Early Triassic (Fig. 1). Biostratigraphic dating of the sedimentary sequences of these basins is based on conodont assemblages as well as bivalves, ammonoids and palynology (Dolby and Balme, 1976; McTavish, 1973; McTavish and Dickens, 1974; Nicoll, 2002; Nicoll and Foster, 1998; Metcalfe et al., 2008) (Fig. 6). The position of the Permian–Triassic boundary is controversially discussed. In the Perth Basin, the lower part of the Kockatea Shale comprises the lower Inertinitic Interval, with the Dulhuntyispora parvithola and the Protohaploxypinus microcorpus Zone, and the upper Sapropelic Interval with palynological assemblages assigned to the Kraeuselisporites saeptatus Zone (Thomas et al., 2004). Gorter et al. (2009) proposed the Permian–Triassic boundary to correspond to the base of the P. microcorpus Zone, therefore to be located within the Inertinitic Interval (Fig. 6); an interval dominated by opaque charcoal and wood fragments. In contrast, Thomas et al. (2004) located this boundary at the base of the K. saeptatus Zone corresponding to the lower boundary of the Sapropelic Interval, assigning a Griesbachian–Dienerian age to the lower parts of the K. saeptatus Zone based on brachiopods, bivalves, ammonoids, and palynomorphs (Fig. 6). Metcalfe et al. (2008) recorded the

Changhsingian conodont species Clarkina jolfensis in the basal part of the Sapropelic Interval and therefore proposed a position of the Permian–Triassic boundary within the lower part of the Sapropelic Interval. The Sapropelic Interval with up to 5% TOC was deposited under anoxic conditions and is a highly productive oil and gas source rock (Thomas et al., 2004). Due to the importance of the hydrocarbon accumulation in the Lower Triassic of the Perth Basin, exploration focussed on the lateral extent of the source rock and the reasons for its distribution in the basins in Western Australia (Gorter et al., 2009). Based on palynological correlation, the Sapropelic Interval could be recognised as far north as the northernmost well in the Perth Basin (Livet-1), but is absent in the Bonaparte Basin (Gorter et al., 2009) (Fig. 6). In the Perth Basin, the deposition of the Sapropelic Interval in the Kockatea Shale was probably caused by the palaeogeographic disposition. During the late Changhsingian– Griesbachian the Sapropelic Interval was deposited in a transgressive shallow-marine setting with restricted circulation leading to the preservation of organic matter under oxygen deficient conditions (Gorter et al., 2009, 2010). A tectonic or volcanic event during the Late Permian or Early Triassic had an additional impact on the sedimentation pattern. This event uplifted the region north of the Perth Basin and led to a shallowing of the depositional environment and probably erosion of latest Permian sediments (Gorter et al., 2009). Griesbachian deposits have not been reported from the Carnarvon Basin (Dolby and Balme, 1976). For comparison between the Western Australian and the Salt Range successions, ranges of selected shared palynomorphs taxa are illustrated for the Amb section and their possible correlation with the Australian palynological zones is indicated (Fig. 6). In contrast to the Perth Basin, Griesbachian sediments were deposited on a shelf with oxic open marine circulation in the Salt Range and Bonaparte Basin. In the Salt Range these oxygenated conditions prevailed during the Late Permian and the Griesbachian. Similar to the Bonaparte Basin, palynofacies data from the white sandstone unit at Amb indicate shallow marine depositional environment in the late Permian. The sequence boundary at the base of the Mianwali Formation includes a hiatus representing a sedimentary gap (Mertmann, 2003). We conclude that the depositional environment in the late Permian and during the Permian–Triassic transition at Amb was too shallow to develop anoxic facies (Tyson, 1995). The situation only changed during the middle Dienerian and oxygen deficient conditions prevailed as indicated by the high AOM content of palynofacies D and TOC values of up to 3% in the lower part of the Ceratite Marls at Nammal (Hermann et al., 2011). In the Perth Basin the deposition of organic rich sediments persisted until the late Dienerian (Metcalfe et al., 2008, 2012), while the records of the Bonaparte Basin showed still oxygenated conditions (Fig. 7b) (Gorter et al., 2009). At Nammal, a middle Dienerian transgression is marked by the lithological change from the limestone deposits of the Lower Ceratite Limestone to the finegrained siliciclastics with intercalated limestone and sandstone lenses of the Ceratite Marls. It correlates with a change of the marine/terrestrial ratios in the palynofacies data. Therefore, the onset of accumulation of amorphous organic matter in the middle and upper Dienerian of the Salt Range was induced by sea-level change. Based on pyrite framboids dysoxic conditions have been suggested for the upper Permian–basal Triassic succession of Kashmir (Wignall et al., 2005). At Selong in South Tibet the presence of pyrite together with low Th/U ratios has been used as indices for oxygen depleted conditions in the Dienerian (Wignall and Newton, 2003). Thus the varying distribution of organic rich sediments on the southern Tethyan coast was determined by the regional palaeogeography (embayment) and by the differential tectonically induced sea-level changes on the passive margin of Gondwana.

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D C

B

A

Fig. 5. Correlation of the Amb and Nammal sections. The colour code corresponds to the colours used for the different categories in Fig. 3. Palynofacies of Nammal after Hermann et al. (2011) with additional data (sample 092204-3P not included in Hermann et al. (2011)). LCL = Lower Ceratite Limestone, CM = Ceratite Marls.

5.4. Bearings of high acritarchs abundance for the role of bioproduction in the end-Permian crisis It has been hypothesised that primary productivity was low in the aftermath of the end-Permian extinction. The low diversity of skeletozoans in the Griesbachian and the overall reduced biomass in the Early Triassic have been put forward as evidence for a low productivity (e.g. Fraiser, 2011; Twitchett, 2001). This idea is in accord with the ‘‘phytoplankton blackout’’ hypothesis that suggests low diversity and low primary productivity in a time span from the Devonian to the Late Triassic (Riegel, 2008) that would have been caused by predominant physical weathering on continents and therefore insufficient nutrient supply to the oceans. The disruption of primary production has been regarded to represent a possible bottom up perturbation of ancient food webs that would explain ecological crises during mass extinctions (e.g. Martin, 1996), because it triggers a cascade of secondary extinctions of primary and higher trophic level consumers (e.g. Roopnarine, 2010). However, there are problems with this hypothesis. First, evolutionary models suggest that a significant lag of recovery of higher trophic levels occurs only if more than 75% of primary producers

are removed. Secondly, production recovers quickly after the perturbation primary whereas consumers show a protracted recovery (Solé et al., 2002). For the earliest Triassic this could imply that the lack of consumers led to the abundantly preserved acritarchs as observed in numerous sections (e.g. Balme, 1979; Balme and Foster, 1996; Grebe, 1970; Hochuli et al., 2010; Jansonius, 1962; Thomas et al., 2004; Ouyang and Utting, 1990; Utting, 1994) However, highly diverse benthic invertebrate communities of Griesbachian age have recently been described, which represent primary consumers and indicate that marine recovery was not suppressed during this time (Hautmann et al., 2011; Hofmann et al., 2011; Krystyn et al., 2003, 2010; Twitchett et al., 2004). Additionally, it has been suggested that increased marine productivity contributed to the development of dysoxic/anoxic facies during the Early Triassic (Metcalfe et al., 2012 and references therein). Based on the observation that diversity of primary consumers remained high during the Late Palaeozoic ‘‘phytoplankton blackout’’, an alternative view explaining the reduced diversity of acritarchs during the so called ‘‘phytoplankton blackout’’ including the end-Permian mass extinction interval, proposed that parts of the primary production was provided by non-encysting algae

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B

A

Fig. 6. Correlation of the transgression near the Permian–Triassic boundary in the section of Amb, Bonaparte Basin (after Gorter et al., 2009, units of the palynofacies data are not given), and Perth Basin (after Thomas et al. (2004)). Note the different use of the Permian–Triassic boundary in the two Western Australian sections. Three different position for the Permian–Triassic boundary have been proposed: (a) base of the P. microcorpus Zone after Gorter et al. (2009) (b) base of K. saeptatus Zone after Thomas et al. (2004) (d) within the lower part of the Sapropelic Interval after Metcalfe et al. (2008). A preliminary correlation of palynomorph assemblages based on key taxa is given (hatched areas). See Fig. 5 for colour code. LCL = Lower Ceratite Limestone, CM = Ceratite Marls.

Griesbachian

middle Dienerian

open oxic circulation? restricted circulation?

Salt Range

Bonaparte Basin Carnarvon Basin Perth Basin

Salt Range

Bonaparte Basin Carnarvon Basin Perth Basin

Fig. 7. Distribution of sediments deposited under oxygen depleted conditions. (a) Griesbachian. (b) Middle Dienerian. Palaeogeography after Scotese (2001) and Gorter et al. (2009).

(Strother, 1996). This view is plausible since only a minority (13– 16% of living dinoflagellate species are known to produce cysts (Head, 1996). Moreover, acmes of Prasinophyceae and the Micrhystridium/ Veryhachium acritarchs complex, which have been interpreted as algal blooms, occur abundantly in some late Palaeozoic and early Mesozoic sediments, especially in association with extinction events (Riegel, 2008; van de Schootbrugge et al., 2007; Kürschner et al., 2007; Bonis et al., 2010). In recent studies increased chemical

weathering rates enhancing primary productivity have been proposed to occur during the Early Triassic (e.g. Algeo and Twitchett, 2010). Some authors even suggest that this high primary productivity suppressed the faunal recovery in the aftermath of the end-Permian extinction event (Meyer et al., 2011). In addition to possible phytoplankton production by nonencysting algae, the present POM data from the Amb section show that acritarchs flourished in the Griesbachian, a fact that has previously been reported from other Lower Triassic sections (e.g. Balme,

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1979; Balme and Foster, 1996; Grebe, 1970; Hochuli et al., 2010; Jansonius, 1962; Thomas et al., 2004; Ouyang and Utting, 1990; Utting, 1994). Biomarker data from the Late Permian (abundance of Pristane, Phytane, and C33-n-ACH (IV)) from Spitsbergen also indicate a phytoplankton bloom after the marine ecosystem collapse (Nabbefeld et al., 2010). Finally, in the light of the discovery of diverse benthic invertebrate communities of Griesbachian age (Hautmann et al., 2011; Hofmann et al., 2011; Krystyn et al., 2003, 2010; Twitchett et al., 2004) representing primary consumers and indicating considerable bioproduction the hypothesis of reduced primary production as a cause of the end-Permian mass extinction and an allegedly delayed recovery is not conclusive.

6. Conclusions The POM content of the Amb Valley section provides the archive to describe the depositional environment in this region during the Permian–Triassic transition. Four distinct palynofacies have been distinguished.  Palynofacies A in the Upper Permian Chhidru Formation is characterised by the dominance of terrestrial phytoclasts.  Palynofacies B of the Kathwai Member (Mianwali Formation, Griesbachian) shows high abundances of acritarchs.  In palynofacies C of the basal Ceratite Marls (Mianwali Formation, earliest middle Dienerian) terrestrial components, especially spores are dominant.  Palynofacies D of the Ceratite Marls (Mianwali Formation, middle Dienerian) is characterised by the dominance of AOM. The succession of the four distinct palynofacies types indicates the development from a shallow-marine, oxygenated near shore setting in the Upper Permian to oxygenated marine conditions in the Griesbachian. A shallowing event is documented at the base of the middle Dienerian and oxygen depleted fully marine conditions prevailed in the middle Dienerian. There is no evidence that a long-lasting Permian–Triassic oceanic anoxic event. Anoxic conditions did not establish before the middle Dienerian in Pakistan. Comparison with the Western Australian basins demonstrates that the timing and lateral extent of the Permian–Triassic anoxic conditions was determined by local palaeogeography and paleobathymetry. Acknowledgements We acknowledge financial support from the Swiss National Science Foundation PBZHP2-135955 (to E.S.-H.) and 200020-127716 (to H.B.). The Pakistan Museum of Natural History is thanked for support during field work in the Salt Range. We are grateful to Michael Hautmann for discussions on the manuscript. We thank three anonymous reviewers for helpful comments, which improved an earlier version of this paper. References Algeo, T.J., Twitchett, R.J., 2010. Anomalous Early Triassic sediment fluxes due to elevated weathering rates and their biological consequences. Geology 38 (11), 1023–1026. Balme, B.E., 1970. Palynology of Permian and Triassic strata in the Salt range and Surghar range, West Pakistan. In: Kummel, B., Teichert, C. (Eds.), Stratigraphic Boundary Problems: Permian and Triassic of West Pakistan. The University Press of Kansas, Lawrance, pp. 305–453. Balme, B.E., 1979. Palynology of Permian–Triassic boundary beds at Kap Stosch, east Greenland. Meddelelser om Grønland 200 (6), 1–37. Balme, B.E., Foster, C.B., 1996. Triassic (Chart 7). In: Young, G.C., Laurie, J.R. (Eds.), An Australian Phanerozoic Timescale. Oxford University Press, Melbourne, pp. 136–147.

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