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Palaeogeography, Palaeoclimatology, Palaeoecology 252 (2007) 281 – 290 www.elsevier.com/locate/palaeo

Calcium carbonate seafloor precipitates from the outer shelf to slope facies of the Lower Triassic (Smithian-Spathian) Union Wash Formation, California, USA: Sedimentology and palaeobiologic significance Adam D. Woods a,⁎, David J. Bottjer b , Frank A. Corsetti b a

Department of Geological Sciences, California State University, Fullerton, Fullerton, CA 92834-0680, USA b Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA Accepted 30 November 2006

Abstract Formerly aragonite seafloor fans from the Lower Triassic (Smithian-Spathian) Union Wash Formation of east-central California were examined to determine the processes active during their growth. The seafloor fans were deposited in an outer shelf to slope environment along the western margin of Pangaea, and are contained within two stratigraphic intervals. The lower precipitatebearing interval is 130 m thick, comprised of micritic limestone, and contains precipitates that fall into 2 end-member morphologies: 1) laminated precipitate masses that contain distinct hemispheres or fans; and 2) precipitate masses that are composed of a complex interlocking mosaic of hemispheres or fans. Precipitates found in the bedded, lower half of the interval are typically 2–3 cm tall and 10–20 cm wide, and are commonly undercut, disturbed, and, in the case of the interlocking mosaics of hemispheres, show evidence of entrainment and transport by density currents. Precipitates are larger in the upper, massive portion of the lower precipitate-bearing interval and occur only as interlocking hemisphere/fan mosaics. Fans and hemispheres attain heights as tall as 30 cm, and precipitate bodies, made of interlocking mosaics of hemispheres and fans, attain thicknesses of up to 50 cm and widths of 1–2 m. Chaotic bedding, broken precipitates, and brecciation of precipitate bodies imply an unstable and mobile substrate. The upper precipitate-bearing interval is 30 m thick, comprised of bivalve-, ammonoid- and microgastropodbearing wackestone, and is separated from lower precipitate-bearing strata by 23 m of interbedded shale and calcareous siltstone. Much of the upper precipitate-bearing unit was deposited by subaqueous debris flows, as suggested by multiple breccias contained within the interval. Seafloor cements are found in undisturbed units and rarely within breccia clasts, suggesting that precipitates formed locally as well as higher on the shelf. Precipitate growth was likely the result of an increase in the saturation state of calcium carbonate in Early Triassic oceans, related to widespread oceanic anoxia. The presence of unusual carbonates during the period, and their correlation with depauperate faunas in the equivalent shallow facies of the Virgin Limestone, suggest that the remarkable oceanographic conditions associated with precipitate growth likely dampened the biotic recovery in the region until those conditions disappeared in the Middle Triassic. © 2007 Elsevier B.V. All rights reserved. Keywords: Early Triassic; Recovery; Precipitates; Carbonate sedimentology

⁎ Corresponding author. Tel.: +1 714 278 2921; fax: +1 714 278 7266. E-mail addresses: [email protected] (A.D. Woods), [email protected] (D.J. Bottjer), [email protected] (F.A. Corsetti). 0031-0182/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.11.053

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1. Introduction: anachronistic facies and the Early Triassic The Permian-Triassic mass extinction was the most devastating mass extinction in Earth history with up to 96% of all marine species eliminated (Raup, 1979). The effect of the Permian-Triassic crisis and associated environmental stresses is reflected in the unusual nature of Lower Triassic carbonates which appear to be more akin to the Proterozoic than the Phanerozoic (e.g.,

Grotzinger and Knoll, 1995; Schubert and Bottjer, 1995; Baud et al., 2005a). The Early Triassic period saw an expansion of microbialites in the form of crusts and mounds on carbonate platforms (e.g., Schubert and Bottjer, 1992; Sano and Nakashima, 1997; Baud et al., 1997; Lehrmann et al., 1998; Lehrmann, 1999; Ezaki et al., 2003; Pruss and Bottjer, 2004a; Baud et al., 2005b; Woods, 2005; Woods et al., 2005) and the concomitant appearance of microbial structures (wrinkle structures) in associated siliciclastic facies (Pruss et al., 2004). A

Fig. 1. A) Early Triassic paleogeography and distribution of Lower Triassic outcrop in east-central California and southern Nevada (modified from Marzolf, 1993). D = Darwin; IM = Inyo Mountains; LCS = Lost Cabin Springs. B) Lower Triassic stratigraphy for the Union Wash Formation (Stone et al., 1991) and Moenkopi Formation (Larson, 1966). Asterisk denotes the approximate location of the strontium isotopic peak for the late Early Triassic (Corsetti, 2004). Ammonoid biozones (Tozer, 1994) and condont biozones (Orchard and Tozer, 1997) from western Canada. Abbreviations used for ammonoid genera: P = Proptychites; V = Vavilovites; Anawasatch = Anawasatchites; K = Keyserlingites; S = Silberlingites. Abbreviations used for conodont genera: N = Neospathodus; Ic. = Icriospathodus; Neogond = Neogondolella.

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facies are, by their nature, relatively rare, and likely require some driving mechanism that causes suppression or displacement of “normal” sedimentologic, geochemical, or palaeobiologic conditions. 2. Union Wash Formation The micritic limestones and calcareous shales of the Lower Triassic Union Wash Formation of east-central California were deposited along the outer edge of Pangaea from the early Smithian (on the basis of the conodont Parachirognathus ethingtoni Clark near the base of the lower member and the Meekoceras ammonoid bed at the base of the middle member) to the late Spathian or early Anisian (on the basis of the conodont Neogondolella timorensis Nogami and the Parapopanoceras ammonoid bed near the base of the upper member) (Silberling and Tozer, 1968; Stone et al., 1991). Previous work on the Union Wash Formation noted the occurrence of synsedimentary calcite precipitates within several stratigraphic intervals from the formation, and concentrated on documentation and description of these unusual features (Woods et al., 1999; Woods and Bottjer, 2000; Pruss et al., 2005; Woods et al., 2005). Precipitate-bearing intervals from the Union Wash Formation were examined near Darwin, California (Fig. 1) in order to determine how seafloor cement genesis and growth was related to, and affected by the sedimentologic and physical processes that were active during deposition. The precipitate-bearing rocks were

Fig. 2. Stratigraphic sections through the precipitate-bearing intervals at the Darwin and Darwin Hills localities.

sharp decline in bioturbation depth and intensity led to an Early Triassic resurgence of flat pebble conglomerates (Pruss et al., 2005), while the occurrence of large, synsedimentary inorganic calcite crusts and fans are indicative of an unusual ocean chemistry during the period (e.g., Baud et al., 1997; Woods et al., 1999; Heydari et al., 2003). These remarkable fabrics and facies are found in a variety of localities worldwide, both within the Tethyan and Panthalassic realms, occur across the span of Early Triassic time (Baud et al., 2005b), and represent an example of anachronistic facies, or sedimentary units that are more representative of earlier periods in Earth history (Sepkoski et al., 1991). Such

Fig. 3. Photomicrograph of several precipitate crystals along the upper boundary of a precipitate fan. Crystals that compose the seafloor cements are typically 0.25–0.5 mm across, up to 30 cm long, and have been replaced by a mosaic of equant, low-Mg calcite crystals in optical continuity with each other. Blunt crystal terminations, as shown here, coupled with a pseudohexagonal shape in cross-section, are indicative of an original aragonite mineralogy (Pruss et al., 2005). Crossed polarizers, scale bar = 1 mm.

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deposited during the latest Spathian to earliest Anisian and the Union Wash Formation in the Darwin area is interpreted to have been deposited in an outer shelf to slope environment based on the presence of soft sediment folds and brecciated beds at multiple stratigraphic horizons in the area (Stone et al., 1991; Woods, 1998). Synsedimentary seafloor precipitates are found within

two stratigraphic intervals in the lower portion of the upper member of the Union Wash Formation (Fig. 2): 1) a lower, 130-m-thick micritic limestone unit that contains abundant seafloor cements; and 2) an upper, 30-mthick wackestone, in which seafloor cements are less common. Precipitate-bearing rocks were examined for this study from two exposures, referred to as the Darwin

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locality and Darwin Hills locality, located 4 km and 5.5 km northeast of the town of Darwin, California, respectively (Fig. 1A). The Darwin locality offers a complete transect through both precipitate-bearing units, while the Darwin Hills locality only provides a partial transect, about 100 m thick, through the lower precipitate-bearing interval before the exposure is covered by alluvium and Tertiary volcanics. 3. Precipitate sedimentology 3.1. Precipitate description Seafloor cements within the Union Wash Formation at the Darwin Hills locality occur as fans and hemispheres composed of black, acicular to bladed calcite crystals, 0.25–0.5 mm in diameter and up to 30 cm long. In thin section view, the radiating crystals have been diagenetically replaced by a mosaic of low-Mg calcite equant crystals in optical continuity with one another (Fig. 3) (Woods et al., 1999). Blunt crystal terminations coupled with a pseudohexagonal shape in plan view indicate an original aragonite mineralogy (Pruss et al., 2005). Solitary fans and hemispheres are rare; instead “precipitate bodies” comprised of multiple interlocking fans and hemispheres are found throughout the two precipitate-bearing intervals. 3.2. Sedimentologic setting The precipitates typically occur as part of a threecomponent sedimentologic system comprised of: 1) laminated to massive micritic limestone that was deposited by a steady rain of lime mud into the region; 2) tan calcareous siltstones that are laminated or cross-laminated, and were likely swept into the region by occasional storms or density currents; and 3) seafloor cements (Fig. 4A, B). Precipitates likely grew during quiet periods of micrite sedimentation; growth ceased when buried by silt deposited from passing turbidity currents.

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4. Lower precipitate-bearing unit The lower precipitate-bearing unit is a 130-m-thick micritic limestone that is bedded on a 5-cm-scale within the lower 50 m and becomes massive in the upper 80 m of the unit. The bedded interval consists of couplets comprised of a thick micritic limestone bed (∼ 5 cm) and a thin (∼ 1 cm) tan, calcareous siltstone bed. The micritic limestone beds become progressively thicker upwards through the unit while the tan siltstone beds become progressively uncommon until they disappear approximately 80 m above the base, resulting in a massive, micritic limestone lithology for the remainder of the stratigraphic interval. 4.1. Lower 50 m of lower precipitate-bearing unit Precipitates first appear ∼ 3 m above the base of the lower precipitate-bearing interval. The seafloor cements comprise approximately 10–20% of the rock, and occur as black, discontinuous bodies, typically 2–3 cm in thickness and 10–20 cm across. The boundary between precipitate masses and surrounding rock is sharp, and the precipitates do not typically persist into the overlying bed. Precipitate bodies appear to have been frequently disturbed by turbidity or other currents as many show evidence of undercutting (Fig. 4C), scour (Fig. 4D) or are overturned (Fig. 4E). The presence of broken and damaged precipitate bodies atop and within siltstone layers and lenses suggests that precipitate bodies were occasionally entrained and transported by turbidity currents (Fig. 4F). Seafloor cements are present in this interval as 2 end-member morphologies: 1) laminated precipitate bodies (Fig. 4G); and 2) interlocking mosaics of well-developed hemispheres (Fig. 4H). The laminated masses consist of laminae of black calcite separated by thin (b1 mm thick), tan siltstone partings that appear to be laterally continuous into the dark gray micritic limestone that surrounds the masses (Fig. 4G). The siltstone partings were probably deposited

Fig. 4. Precipitate masses from the lower 50 m of the lower precipitate-bearing interval. A) The precipitate-bearing unit is comprised of: 1) micrite (M) which may be massive or laminated (as in this case); 2) laminated to cross-laminated tan calcareous siltstone (S); and 3) precipitates (P). The micrite is likely related to background sedimentation, while the calcareous siltstone beds are believed to represent the stochastic input of silts via turbidites. Note that the siltstone drapes the large precipitate bodies, implying synsedimentary growth. Scale bar = 1 cm. B) Edge-on view of a bedding plane showing infilling between precipitates by tan, calcareous siltstone, suggesting that the precipitates were a topographic high on the seafloor. Precipitate bodies were frequently disturbed by passing turbidity currents, as suggested by: C) undercutting of precipitate bodies by tan, calcareous siltstone; D) scour of precipitate bodies, followed by infilling of irregular scours by tan, calcareous siltstone; and E) overturn of precipitate bodies. F) Broken and damaged precipitate bodies within and on top of tan, calcareous siltstone lenses implies entrainment and transport of some precipitates by turbidity currents. Precipitates that occur within the lower 50 m of the lowermost precipitate-bearing interval fall into two end-member morphologies: G) laminated masses consisting of calcite laminae interlaminated with thin (b1 mm) siltstone partings (when the calcite laminae exceed ∼ 5 mm, crystals typically become oriented perpendicular to the seafloor and form undulating, mammillary crusts that are denoted with arrows in the photo); H) complex, interlocking mosaics of calcite fans and hemispheres. View is looking down on a bedding surface.

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by passing turbidity currents, and appear to have interrupted precipitate growth. Where the calcite laminae are thin (b 5 mm), they are comprised of small (b0.1 mm) crystals with random orientations. Where laminae thickness exceeds about 5 mm, crystal growth appears to become more ordered, with larger (0.25–0.5-mm-thick) crystals oriented perpendicular to the seafloor, forming undulose, mammillary crusts that radiate upwards from the laminated body (crusts typically b 1 cm thick) (Fig. 4G). Evidence of scour or undercutting of laminated masses is common (e.g., Fig. 4C, D), however, the laminated bodies do not appear to have been transported by passing turbidity currents. The low profile of the masses (radiating crusts are typically only 5–10 mm high) likely allowed turbidity currents to pass overhead and made entrainment and transport unlikely. The second precipitate body end-member is comprised of clumps of well-developed hemispheres that commonly form a complex, interlocking mosaic (Fig. 4H). Many of these bodies have irregular shapes that imply transport and breakage along their edges (refer to Fig. 4F). Bedding plane observations of these bodies demonstrate that they formed a topographic high above the seafloor (refer to Fig. 4B which shows tan siltstone filling in topographic lows around the bodies) and would have likely been easier to entrain and transport than the low profile laminated masses. Complex patterns of interlocking hemispheres and fans are likely the result of lateral and upside-down growth of precipitates into cavities created between adjacent hemispheres; no evidence exists for precipitate growth downwards into the sediment (e.g., displacement of mud by crystal growth). 4.2. Upper 80 m of lower precipitate-bearing unit The lower precipitate-bearing unit becomes massive around 50 m above the base. Seafloor cements comprise a much larger portion of the rock (up to 75%) and large (up to 30 cm in radius), well-developed hemispheres and fans become common. An increase in hemisphere size may imply faster growth rates or may be the result of less frequent smothering of precipitate growth by silt. Hemispheres and fans typically occur in large, irregular clusters that may extend 1–2 m laterally and up to 50 cm vertically (Fig. 5A). The precipitate masses exhibit multiple phases of growth, suggesting that new growth preferentially occurred on older precipitate substrates. The seafloor appears to have become unstable during deposition of the upper portion of the lower precipitatebearing unit. Precipitates are commonly broken, and chaotic bedding comprised of multiple broken and damaged precipitate masses becomes common (Fig. 5B).

Pockets of breccia occur within and along the edges of a number of large precipitate bodies, suggesting that those bodies were moving downslope (Fig. 5C). Breccias commonly exhibit internal infilling of cavities with newer generations of precipitates (Fig. 5D). Most gravity-related movement of the substrate appears to have occurred at the surface; only a single example of a deep-seated glide plane, in the form of a highly brecciated bed, 1 m-thick, has been noted. The occurrence of an intervening shale unit between the two precipitate-bearing intervals is interpreted to represent an increase in the flux of terrestrial silt to the area, leading to conditions that stifled precipitate genesis and growth. The change to shale deposition is sudden based on the sharp upper boundary of the lower precipitate-bearing unit. Precipitate growth occurred up until the end of deposition of the lower precipitate-bearing unit; no seafloor cements have been found in the intervening shale unit. 5. Upper precipitate-bearing unit Much of the upper precipitate-bearing unit exhibits evidence of mass movement; the interval contains multiple 1–2 m-thick brecciated beds as well as thinner breccia beds and lenses throughout. This unit also differs from the lower precipitate-bearing unit in that it is fossiliferous, containing abundant bivalves and less abundant microgastropods and ammonoids. Seafloor cements are found within the brecciated beds as well as within undisturbed micritic limestone or wackestone beds. Undisturbed precipitate bodies commonly occur intermittently along stratigraphic horizons (implying episodic growth), are 30–50 cm across and 5–10 cm high, and consist of interlocking hemispheres. In some cases the seafloor cements appear to have been growing on top of breccia layers (Fig. 5E), while other precipitates appear to have grown in cavities created in the breccia rubble (Fig. 5F). Precipitates in breccia beds occur as small bodies contained within larger micritic limestone clasts or as irregular cement masses that were damaged by transport. 6. Precipitate growth mechanism and Early Triassic palaeoceanography Growth of seafloor precipitates during the Early Triassic has been attributed to the buildup and degassing of CO2 in a chemically-stratified ocean with a welldeveloped chemocline between oxic surface waters and anoxic (perhaps sulfidic) deeper waters (Woods et al., 1999). A variety of studies have suggested that a

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Fig. 5. Precipitate masses from the upper 80 m of the lower precipitate-bearing interval and the upper precipitate-bearing interval. A) Precipitates become quite large in the upper 80 m of the lower precipitate-bearing interval. Precipitate bodies are comprised of interlocking hemispheres and may be up to 50 cm thick and 1–2 m across. Precipitate growth appears to have occurred preferentially on older cement substrates, based on the irregular shapes that many of the large precipitate bodies have in this interval. Width of coin = 2.5 mm. B) Chaotic bedding implies an unstable substrate and debris flows during deposition of seafloor cements; C) breccias present within and along the edges of large precipitate bodies are indicative of movement of the large precipitate bodies. D) Precipitate growth (2) from a previous fan (1) into a cavity created within a brecciated pocket. Upper precipitate-bearing interval. E) Precipitate body that is growing on top of a brecciated layer (BM = brecciated micritic limestone). F) Growth of precipitates into a cavity between breccia clasts.

stratified, anoxic ocean existed during the Early Triassic (e.g., Wignall and Hallam, 1993; Hallam, 1994; Isozaki, 1997; Wignall and Twitchett, 2002), and degradation of organic matter by sulfur-reducing bacteria below the chemocline is thought to have led to an increase in the

amount of bicarbonate and carbonate in the deep ocean (Woods et al., 1999). Upon mixing of deep, alkaline, anoxic waters with oxygenated surface waters, perhaps in regions of upwelling, CO2-degassing is believed to have taken place, resulting in supersaturation of the

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remaining waters with respect to calcium carbonate and leading to initiation and growth of the precipitate bodies (Kempe, 1990; Grotzinger and Knoll, 1995; Woods et al., 1999). Strontium isotope stratigraphy demonstrates that the precipitate-bearing intervals within the Union Wash Formation correlate with subtidal stromatolite occurrences within the onshore equivalent to the Union Wash Formation, the Virgin Limestone (Fig. 1B) (Moenkopi Formation) (Corsetti, 2004). Stromatolite growth during the Early Triassic may have been enhanced by the oceanographic phenomena that led to deposition of the inorganic seafloor precipitates, as the formation of both structures are ultimately linked to calcium carbonate supersaturation (Fig. 6). Therefore, widespread stromatolite occurrences may have been fostered by persistent stressful environmental conditions that suppressed metazoan grazers and burrowers (Pruss and Bottjer, 2004b) and further augmented by conditions favorable to widespread calcium carbonate precipitation. 7. Conclusions and palaeobiological significance The occurrence of inorganic calcium carbonate precipitates may signify an increase in the levels of CO2 in surface waters and the impingement of anoxic waters onto the continental shelf during the Early Triassic (Woods et al., 1999). Such conditions would have been stressful to marine biota, and may have acted to suppress

biotic recovery from the Permian-Triassic mass extinction (sensu Hallam, 1991). Wignall et al. (1998) noted that the recovery from the Permian-Triassic crisis appears to commence earlier at high latitudes, and this observation suggests that the timing of the onset of the recovery may have been directly related to the retreat of stressful conditions; in those areas where environmental stresses persisted, the recovery was put on hold until those conditions dissipated. Indeed, evidence from Lower Triassic (Griesbachian) rocks from the Wadi Wasit Block in Oman supports this hypothesis: in the absence of marine anoxia the recovery began almost immediately and proceeded rapidly (Krystyn et al., 2003; Twitchett et al., 2004). Data from western North America further supports the contention that persistent harsh conditions hindered recovery. Studies of palaeocommunities from the shallow-water, lateral equivalent of the Union Wash Formation, the Virgin Limestone, show that the fauna in the region remained in a postextinction “mode” (i.e., low diversity, low complexity, dominated by generalists) for much of the Early Triassic interval (Schubert and Bottjer, 1995; Pruss and Bottjer, 2004a). Meanwhile, studies of palaeocommunities from higher latitudes where seafloor cements have not been documented demonstrate that the recovery began shortly after the extinction event (Wignall et al., 1998; MacNaughton and Zonneveld, 2003; Beatty et al., 2005). Therefore, it appears that lateral and temporal variability in stressful environmental conditions played a strong role

Fig. 6. Generalized model of the distribution of Lower Triassic anachronistic facies across the western margin of Pangaea. Intertidal to shallow subtidal facies were comprised of ooid shoals and stratiform stromatolites (Mary and Woods, 2005). Small stromatolitic and thrombolitic bioherms, 1–2 m high and comprised of smaller, cabbage-head stromatolites and thrombolites, occur in inner to middle shelf facies (Pruss and Bottjer, 2004b). Outermost shelf and slope facies contain sedimentary seafloor precipitates that grew as hemispheres and are commonly brecciated and broken due to the unstable substrate of the continental slope. In situ hemispheres that grew in cavities between talus boulders are common here. Basinal facies typically consist of interbedded siltstone turbidites and seafloor precipitate crusts (Woods et al., 2005). Extensional faulting and erosion has led to the subsequent loss of facies between the middle and outermost shelf.

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in determining when the biotic recovery began and the direction of the recovery from the Permian-Triassic mass extinction. Acknowledgments This work greatly benefited from discussions with Maria Mutti, Alfred Fischer, Sara Pruss, and Pedro Marenco. The senior author wishes to acknowledge Mike van Ry and Daniel Sturmer for valuable field assistance and discussions. This research was supported by funds supplied to ADW by the Department of Geological Sciences at California State University, Fullerton. FC and DB were supported by a grant from the National Science Foundation (EAR0447019). Aymon Baud, Yukio Isozaki, and Paul Stone provided helpful and insightful comments that greatly strengthened the manuscript. References Baud, A., Cirilli, S., Marcoux, J., 1997. Biotic response to mass extinction: the lowermost Triassic microbialites. Facies 36, 238–242. Baud, A., Richoz, S., Pruss, S., 2005a. The Lower Triassic anachronistic carbonate facies in space and time. Albertiana 33, 17–19. Baud, A., Richoz, S., Marcoux, J., 2005b. Calcimicrobial cap rocks from the basal Triassic units: western Taurus occurrences (SW Turkey). Comptes Rendus Palevol 4, 501–514. Beatty, T.W., Zonneveld, J.-P., Henderson, C.M., MacNaughton, R.B., 2005. Lower Triassic shallow marine ichnofossil assemblages from the northwest margin of Pangea: insight into biotic recovery after the Permian-Triassic mass extinction. AAPG Annual Convention Abstracts, vol. 14, p. A13. Corsetti, F.A., 2004. Did enhanced lithification play a role in the Proliferation of Early Triassic stromatolites? A case study from the Virgin Limestone Member, Moenkopi Formation, western Nevada. Annual Meeting of the Geological Society of America, Abstracts with Programs, vol. 36, p. 182. Ezaki, Y., Jianbo, L., Natsuko, A., 2003. Earliest Triassic microbialite micro- to megastructures in the Huaying area of Sichuan Province, south China; implications for the nature of oceanic conditions after the end-Permian extinction. Palaios 18, 388–402. Grotzinger, J.P., Knoll, A.H., 1995. Anomalous carbonate precipitates: is the Precambrian the key to the Permian? Palaios 10, 578–596. Hallam, A., 1991. Why was there a delayed radiation after the endPaleozoic extinctions? Historical Biology 5, 257–262. Hallam, A., 1994. The earliest Triassic as an anoxic event, and its relationship to the end-Paleozoic mass extinction. In: Embry, A.F., Beauchamp, B., Glass, D.J. (Eds.), Pangea: Global Environments and Resources. Canadian Society of Petroleum Geologists, Calgary, AB, Canada, pp. 797–804. Heydari, E., Hassanzadeh, J., Wade, W.J., Ghazi, A.M., 2003. PermianTriassic boundary interval in the Abadeh section of Iran with implications for mass extinction; part 1, Sedimentology. Palaeogeography, Palaeoclimatology, Palaeoecology 193, 405–423. Isozaki, Y., 1997. Permo-Triassic boundary superanoxia and stratified superocean: records from the lost deep sea. Science 276, 235–238. Kempe, S., 1990. Alkalinity: the link between anaerobic basins and shallow water carbonates? Naturwissenschaften 77, 426–427.

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