THE END OF THE PALEOZOIC AND THE EARLY MESOZOIC OF THE MIDDLE EAST

A.S. ALSHARHAN , A.E.M. NAIRN , in Sedimentary Basins and Petroleum Geology of the Middle East, 2003

Amanus Shale Formation (Early Triassic).

The Early Triassic Amanus Formation in the Dolaa well northeast of Palmyra was first described by Syrian geologists who recognized a Lower Amanus suite of sandstone (Late Permian) overlain by an Upper Amanus suite of shale (Early Triassic). The Amanus Shale in central Syria (about 150 m, or 492 ft) is composed of intercalated sandstone and argillite, grading upward into alternating limestone and carbonized argillites with rare carbonized plant fossils and ending in dolomitized limestone. In northeastern Syria (in wells Aoda-106 and 107), this sequence changes through the incoming of greater clastic influx, gray quartz sandstone and siltstone with an increase in the plant fossils found in the argillites. The lower unit in central Syria, as seen in wells Azar-1 and Habari-1, consists of interbedded red and green quartz sandstone and lenticular siltstone. To the northeast, in Markada-101, facies changes similar to those marking the lower unit occur as red and green sandstone and argillites with rare coalified plant fossils appear. However, to the southeast on the evidence from well Kari-1, the presence of red argillaceous limestone and of algal limestone suggests a continuation of the marine conditions found in central Syria. The age of the formation is Early Triassic (Scythian) based on the presence of foraminifera and ostracods.

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Carbonate systems

Luis Pomar , in Regional Geology and Tectonics (Second Edition), 2020

Triassic-Jurassic

Early Triassic marine biotas were seriously impoverished and recovery from the end-Permian extinctions was slow for many groups. The Middle Triassic marks the return of better conditions for the whole biosphere, the radiation of a number of key groups and the beginnings of the modern phytoplankton assemblage with the first clearly identifiable dinoflagellates (Falkowski et al., 2004; Katz et al., 2004). By the Late Triassic and Jurassic important planktonic organisms first appeared or experienced massive radiation: coccolithophorids, diatoms, dinoflagellates, planktonic foraminifera and ostracods. All of the new photosynthetic forms of the Jurassic belong to the 'red' plastid lineage, with chloroplasts of the red algae type, with chlorophyll c rather than the chlorophyll b of green plants and green algae (Falkowski et al., 2004). 'Red' plastids retained a much greater amount of its own DNA and thus had greater genetic independence from its host than 'green' plastids, which most genetic control relies on the hosts. Photosynthetic eukaryotes of the red plastid lineage also have higher nutritional quality (phytoplankton stoichiometry, the relative proportions of inorganic nutrients to carbon) than the green plastid lineage. But also important, 'red' and 'green' plastid lineages have different trace element requirements. The 'red' elements are cadmium, cobalt and manganese, while the 'green' elements are copper and iron. As marine iron concentrations are largely dependent on continental weathering, often drop from iron-bearing dust, the humid, equable conditions of the Mesozoic strongly reduced the availability of marine iron.

The replacement of the Paleozoic Assemblage (Sepkoski, 1981)—mostly extinct by the end-Permian extinction—by the Modern Assemblage involved the transition from sedentary epifaunal suspension feeders to mobile, energetic (high metabolism) infaunal suspension feeders, deposit feeders, and predators (Wagner et al., 2006; Leighton et al., 2013). Holland and Sclafani (2015) have suggested that an increase in primary productivity through time is the primary cause of Phanerozoic increases in fundamental biodiversity number, global richness, local richness, local evenness, abundance and body size.

Brachiopods and bivalves feed in similar ways and have occupied the same environments through geological time, but brachiopods were far more diverse and abundant in the Palaeozoic whereas bivalves dominate the post-Palaeozoic. Payne et al. (2014) concluded that this change in dominance is apparent, as metabolic activity of bivalves has increased by more than two orders of magnitude over this interval, whereas brachiopod metabolic activity has declined by more than 50%. Consequently, the increase in bivalve energy metabolism must have occurred via the acquisition of new food resources rather than through the displacement of brachiopods.

Modern-marine ecosystems are the result of numerous evolutionary innovations commonly referred to as the MMR (Buatois et al., 2016). This major evolutionary episode was responsible for the large-scale restructuring of shallow-marine benthic communities, including increases in the energy budgets of marine ecosystems and predation levels, the latter resulting in a number of coevolutionary developments (Fig. 12.24).

For trace fossils, the recovery from the end-Triassic mass extinction is characterized by a gradual increase in burrow size and the reappearance of deep-tier trace fossils, suggesting the return to 'normal' environmental conditions by the end of the Hettangian. In fact, an 8% increase in global ichnodiversity has been recorded in marine environments for the early Jurassic (Buatois et al., 2016).

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Triassic

J.G. Ogg , in The Geologic Time Scale, 2012

25.2.1.7 Marine Reptiles

Beginning in earliest Triassic, the seas became inhabited by large marine reptiles. Ichthyopterygia ("fish flippers", or Ichthyosaurs) with their dolphin-like morphology and Sauropterygia ("lizard flippers") that retained a more crocodile-like form rapidly diversified during the Early Triassic (e.g., Motani, 2010). The adaptation of the ichthyosaurs to the open seas, perhaps including a warm-blooded anatomy, enabled their expansion in the Middle and Late Triassic as coastal habitats for other marine reptiles declined (Motani, 2010). Most of the Sauropterygia clade vanished during the Late Triassic, although the long-necked Jurassic–Cretaceous plesiosaurs may represent an emergence from a distant branch.

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The Triassic Period

J.G. Ogg , ... H.S. Jiang , in Geologic Time Scale 2020, 2020

25.3.2 Early Triassic through Anisian age model

For the Early Triassic through early Anisian, the magnetic polarity pattern that was calibrated to short-eccentricity (100-kyr) changes in monsoon intensity recorded by clastic cycles in the Germanic Basin (e.g., Szurlies, 2007) has been duplicated in the cyclo-magnetostratigraphy of conodont-bearing carbonate deposits on the margins of the paleogeographic Yangtze Platform of South China (Li et al., 2016, 2018). This cycle-magnetic compilation of South China includes the GSSPs for the Induan (base-Triassic) at Meishan (Zhejiang province), the candidate for the Olenekian GSSP at West Pingdingshan near Chaohu (Anhui province), and a candidate for the Anisian GSSP candidate at Guandao (Guizhou province). The astronomical tuning is tied to the base-Triassic Induan GSSP at Meishan, which has an interpolated age of 251.902±0.024   Ma based on close-spaced U–Pb ID-TIMS dates (Burgess et al., 2014). [Note that an external uncertainty of c. 0.29   Myr should be included when comparing this version of EARTHTIME-standardized dates to previous versions, as explained in Burgess et al. (2014)].

The cyclo-bio-magnetostratigraphic reference section at West Pingdingshan near Chaohu in South China is a candidate for the Olenekian GSSP. According to the astronomical tuning, the Induan/Olenekian boundary level is 2.0   Myr later than the base-Triassic therefore has an age of 249.9   Ma (Li et al, 2016). This 2.0-Myr span is independently verified by the 1.9±0.1   Myr duration for the Induan stage derived from cyclo-biostratigraphy of conodont zones in the Montney Formation of British Columbia (Shen et al., 2017; Moslow et al., 2018; Henderson et al, 2018; Shen, 2018).

If the Griesbachian–Dienerian substage boundary is assigned as the base of the conodont S. kummeli Zone in these astronomically tuned sections of South China, then the base of Dienerian has a projected age of 250.5   Ma (Li et al., 2016). The 1.4-Myr duration of the Griesbachian Substage is also consistent with a U–Pb date of 251.50±0.6   Ma from the lower-middle Griesbachian at Meishan (Burgess et al., 2014). A reported date of 251.22±0.2   Ma on a volcanic ash from lower Smithian (Galfetti et al., 2007b) was based on pre-EARTHTIME standards, and therefore is anomalously old and should be reanalyzed (Burgess et al., 2014).

There is ID-TIMS radiometric dating of the volcanic-ash beds in the Olenekian–Anisian boundary interval at the GSSP candidate at Wantou (Guangxi province) section (Ovtcharova et al., 2015) and at the Guandao section (Guizhou province) (Lehrmann et al., 2015). These dates, when coupled with the composite cyclostratigraphy for Early Triassic through Anisian (Li et al., 2016), indicate that the lowest occurrence of the conodont Ch. timorensis s.str. as the preferred marker for the base of Anisian at approximately 246.7   Ma (Chen et al., 2020). U–Pb-dated volcanic-ash beds bracketing this base-Anisian marker at Wantou by Ovtcharova et al. (2015) had been interpreted by them to imply a slightly older age (c. 247.3   Ma) for a slightly different conodont marker for the base-Anisian, but they caution that there are inconsistencies in the progression of interpreted dates from the zircon populations in successive layers and the statistical derivation of a mean date from each ash bed was influenced by the decisions of which individual zircons within the distribution of ID-TIMS dates should be included. Therefore the cyclostratigraphic age of 246.7   Ma relative to the base-Triassic control date is used here.

If the Smithian–Spathian substage boundary is placed at the lowest occurrence of conodont Novispathodus pingdingshanensis in these sections, then the base of the Spathian has a projected age from the astronomical tuning of 248.1   Ma (Li et al., 2016).

The magnetostratigraphy and interpreted 405-kyr eccentricity-driven cycles at the conodont-rich Guandao section span the Anisian through lowest Ladinian (Lehrmann et al., 2015; Li et al., 2018). This cyclo-magnetostratigraphy age model was applied to estimate the ages for the bases of Anisian substages according to their Tethyan ammonoid-based correlations to magnetic polarity zones (e.g., Hounslow and Muttoni, 2010). The corresponding cyclo-magneto-ammonoid age model projects the base of Bithynian at 245.0   Ma (base of Kocaella ammonoid zone), the base of Pelsonian at 244.2   Ma (base of B. balatonicus ammonoid zone), and the base of Illyrian at 243.3   Ma (base of Paraceratities trinodosus ammonoid zone).

In some cases, these ammonoid-defined substages are different from their conodont-based placements relative to polarity zones in South China (e.g., Lehrmann et al., 2015), for example, the basal substage of Aegean includes 5 ammonoid zones (Jenks et al., 2015), but its conodont-based duration at Guandao would have been only 0.3   Myr, compared to a c. 1.7-Myr duration based on ammonoid correlation to the reference magnetic polarity pattern elsewhere (Hounslow and Muttoni, 2010). Therefore because the bases of Anisian aubstages have been traditionally assigned with ammonoid zones, these versions for the substages are dashed in the GTS2020 charts.

The ages for all other biozones, sea-level sequences, geochemical trends, and other events are derived from their published calibrations to the astronomical-tuned age model for Early Triassic through Anisian conodont zones of South China or to their calibration to the magnetic polarity zones. For example, the age model for ammonoid zones of the Arctic is according to magnetostratigraphy studies (e.g., Ogg and Steiner, 1991; Hounslow et al., 2008a, 2008b; Hounslow and Muttoni, 2010) as correlated to the Chaohu and Guandao cyclo-magnetostratigraphic scales.

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Evolution and Biostratigraphy

Coordinated byF.M. Gradstein , ... S. Esmeray-Senlet , in Geologic Time Scale 2020, 2020

3J.3 Mesozoic

Worldwide, earliest Triassic macrofloras are extremely rare and associations show a very low diversity. It should be noted, however, that the number of plant-bearing localities is limited and nearly all are in the Paleotethyan region. All over Eurasia, the lowermost Triassic is dominated by Pleuromeia, an up to 1.2   m high lycophyte that could form dense stands along shores and streams, growing associated with a few horsetails and ferns (Grauvogel-Stamm and Ash, 2005). The dominance of Pleuromeia is also apparent from the palynological record, that is, its spore Densoisporites nejburgii. Younger, early Anisian floras from France and Germany are more diverse and dominated by herbaceous conifers (Aethophyllum) and several genera of voltzialean conifer trees; other common elements are sphenopsids, ferns, and ginkgophytes. Pteridosperms and cycadophytes are present but rather rare. A late Anisian flora from the Alps is similar in composition, but ferns and cycadophytes are far more common. This gradual increase in diversity continued and Ladinian floras from the Alps and the Germanic Basin are characterized by a fairly wide variety of sphenopsids, lycophytes, ferns, cycadophytes, pteridosperms and ginkgophytes, and conifers. The increasing diversity is ascribed to a change of climate that became less arid. During the Middle Triassic, several new groups appeared, including matoniaceous and dipteridaceous ferns and probably caytonialean seed ferns. These groups spread rapidly and soon became typical for Northern Hemisphere Mesozoic floras together with podocarpalean conifers and bennettitales; the latter group was recently traced back to the late Permian (Blomenkemper et al., 2018). The bennettitaleans were a group of plants with cycad-like leaves—in fact bennettitalean and cycadalean leaves are often hardly distinguishable without cuticle; they partly also had a similar growth habit. However, the complex flower-like reproductive organs, which are bisexual in several species, clearly separate them from cycads.

The oldest coal-forming Triassic vegetations are known from the Ladinian but it was not until the Carnian that large amounts of plant biomass were deposited on a wide scale; the Carnian is often associated with a pluvial event. Carnian coal-forming floras are typically dominated by bennettitaleans and cycads, commonly associated with sphenopsids and ferns. Northern Hemisphere Rhaetian floras are the most diverse floras of the Triassic and remarkably similar over large distances; regional differences are mainly of quantitative nature. Prominent constituents were the peltaspermalean seed fern Lepidopteris and the Caytoniales, a group of seed ferns with characteristic quadripartite leaves possessing a cuticle with simple stomata, similar to those of angiosperms, and very small seeds deeply hidden in a semienclosed cup-shaped structure, and simple pollen sacs containing small bisaccate pollen. Also common were sphenopsids, various ferns, a wide variety of cycads and bennettitaleans, and various types of conifers, including several new families such as Voltziaceae, Cheirolepidiaceae, Araucariaceae, Podocarpaceae, and Pinaceae; the latter three survive until today. Triassic Gondwana floras also show an increasing diversity throughout the Triassic. They are often strongly dominated by the seed fern Dicroidium, a species-rich genus that during the Triassic spread over the entire Southern Hemisphere. Peltasperms did not arrive in the Southern Hemisphere until the Triassic. Other typical elements are conifers, including podocarpaceous conifers and Heidiphyllum, a conifer that was widespread and common throughout Gondwana. Sphenopsids, ferns, cycads, bennettitaleans, and ginkgophytes were also represented. Within the Southern Hemisphere, different subprovinces related to the paleoclimatic and paleogeographical position can be distinguished, varying from relatively dry and warm to more temperate (Césari and Colombi, 2013). Dicroidium species from northern and central Gondwana often had relatively large leaves, whereas trees from the Antarctic were deciduous and had small leaves, being an adaptation to the long period of darkness during the polar winter (Bomfleur and Kerp, 2010).

Like the macroflora, palynological associations show a strong increase in diversity during the Triassic. Earliest Triassic microfloras are dominated by lycophyte and bryophyte spores and two species of acritarchs. Striate pollen is a typical feature of Triassic palynomorph assemblages, disappearing near the end of the Triassic. A characteristic and widespread Triassic genus is Aratrisporites, a double-layered, monolete lycophyte spore. In the marine realm, dinoflagellate cysts first appeared in the Upper Triassic (see, e.g., Mangerud et al., 2019). From then on, these organic-walled phytoplankton cysts, now commonly found in marine, brackish and freshwater environments, play a prominent role in biostratigraphy (e.g., Stover et al., 1996).

Although the Triassic–Jurassic boundary marks one of the big mass extinctions, only one plant group, the common and widespread genus Lepidopteris (peltasperms) was affected severely and became extinct. In some regions in central Europe, the Triassic–Jurassic boundary is not clearly expressed in the terrestrial macrofossil record, whereas in other regions such as East Greenland highly diverse Triassic communities of Podozamites, a broad-leafed conifer, and bennettitaleans were replaced by lower diversity forests, dominated by taxa that had been relatively minor components of Late Triassic forests such as the Czekanowskiales, a group of plants with narrow, strongly dissected leaves and seed-bearing structures consisting of two valves, ginkgophytes (Sphenobaiera) and osmundaceous ferns (McElwain et al., 2007).

Cycads and bennettitaleans were important components of vegetation during the Jurassic and Early Cretaceous and ginkgophytes reached their maximum diversity. Pteridosperms were still present (Pachypteris, Caytonia), but as a whole, the group shows a gradual and steady decline. New groups of modern conifers evolved such as the Cephalotaxaceae, Taxaceae, and Cupressaceae; the latter have small, scale-like leaves. In general, the Triassic and Jurassic were periods of major radiation within the ferns and the conifers.

The Triassic–Jurassic boundary is much clearer expressed in the palynological record than in the macrofossil record. In the Northern Hemisphere, up to 90% of the pollen and spore species became locally or regionally extinct. Highly diverse assemblages of monosulcates and monosaccates were replaced by low diversity assemblages dominated by Classopollis, the pollen of cheirolepidiaceous conifers, which first appeared in the Rhaetian and persisted until the earliest Paleogene. Cheirolepidiaceae were conifers with small scale–like leaves and extremely, up to 100   µm, thick cuticles. They were widespread at low latitudes and adapted to arid and (hyper)saline environments. They formed monospecific forest in coastal and lagoonal settings but were also part of mixed forests in fluvial environments.

Until the end of the Early Cretaceous, gymnosperms ruled the world. The first angiosperms flowers, carpels, and stamens appeared in the late Barremian–early Aptian (Friis et al., 2011). Angiosperms differ from gymnosperms in a number of features, for example, in having true flowers, pollen with a much more complex wall structure, and in leaf morphology (leaf shape, venation, and cuticle). Although angiosperms are a highly diverse and extremely successful group, their origin is still enigmatic, even though in recent years many fossils of early flowering plants have been found. Various gymnosperm groups have been discussed as potential ancestors. Pollen showing remarkable similarities to angiosperm pollen have been reported from the Middle and Upper Triassic (Cornet, 1989; Hochuli and Feist-Burkhardt, 2013), but there are no convincing records of coeval angiosperm macrofossils. The earliest pollen grains unequivocally assigned to angiosperms are small (<10   µm); they are very rare in the Hauterivian. Angiosperms originated in paleoequatorial regions being small herbs growing in humid environments, maybe even partly submersed. Initially, they played a minor role, but they diversified and spread rapidly and during the Late Cretaceous they were already dominant, except for the high latitudes where the vegetation was still mainly composed of conifers and ferns. During the Cretaceous, groups that had been most successful in the Jurassic were outcompeted and several became extinct, for example, the bennettitaleans and pteridosperms, but also cycads and ginkgophytes show a drastic decline. A part of the success of angiosperms is probably related to the efficiency of insect pollination; angiosperms and insects show a remarkable coevolution.

The rapid radiation and spreading of angiosperms are well documented by the microfossil record; palynological associations from the successive stages show a fast diversification and increasing abundances. Aquilapollenites is a typical Cretaceous pollen type that has been recorded throughout the Northern Hemisphere but is also found in South America (Vajda and Bercovici, 2014); it reached its maximum distribution during the Maastrichtian. In the tropical areas, palm pollen became characteristic elements of the assemblages. By the end of the Cretaceous, provincialism reached its peak and several floral provinces had developed (Herngreen et al., 1996), mostly along latitudinal lines.

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Volume 5

Charlotte S. Miller , Viktoria Baranyi , in Encyclopedia of Geology (Second Edition), 2021

Early-Triassic Climate

The c. 5 Ma long Early Triassic was an interval of high environmental variability characterized by high sea surface temperatures and episodes of ocean anoxia. Unlike other hyperthermal events, for example the Paleocene–Eocene Thermal Maximum (in which recovery after the extinction event occurred within c. 200 kyr), the P-T event was extreme and warmth continued for c. 5   Ma after the extinction interval, spanning the Early Triassic. It has been suggested that the large quantity and the high rate of CO2 emissions during the P-T interval exceeded the climate regulating capacity of the chemical weathering of silicate rocks, a process which consumes atmospheric CO2. Low uplift rates and high continentality may have further contributed to the climate regulation failure and the long duration of hot-house climatic conditions into the Early Triassic.

The hot-house Early Triassic climate was punctuated by large-scale temperature variations with subsequent environmental perturbations which were likely responsible for the delayed recovery of life after the P-T extinction. As well as environmental stress, reduced post-extinction competition may have enabled species to exist across the full range of their niches, resulting in the occurrence of generalized taxa in the fauna and flora.

Carbon isotope records from the Tethys indicate that the C isotope values did not return to Permian levels until the Middle Triassic. Instead large and short-lived perturbations in the C cycle persisted into the Early Triassic. The largest, globally significant C isotope excursion following the P-T extinction occurred at the Smithian-Spathian boundary (SSB; Fig. 1), coinciding with a major synchronous change in terrestrial and marine ecosystems (Fig. 4). The C isotopic perturbation at the SBB interval coincided with abrupt cooling and major marine extinctions (Fig. 4). Conodonts, ammonoids and foraminifera suffered severe biodiversity losses at the SSB boundary (Fig. 4B and C). The SSB also coincided with large changes in marine redox conditions, with temporal correlation between anoxia and low marine biodiversity levels. Warm sea surface temperatures during the middle/late Smithian coincided with widespread anoxia. Cooler sea surface temperatures may have been responsible for the reduction of anoxia across the SSB. A major change from spore-dominated to gymnosperm-dominated assemblages is evident at the SSB in the Boreal realm (Fig. 4E), likely as a result of severe climatic perturbation from stable humid to more arid climatic conditions. Cool intervals during the Early Triassic were associated with diversification pulses in both primary and secondary consumers and nekton-pelagic and benthic communities, whereas pulses of intense warming were associated with high floral and faunal turnover. Increased atmospheric CO2 possibly from a short eruptive event of the Siberian Traps may have led to ocean acidification, anoxia, changes in redox conditions and high sea surface temperatures evident through the SSB event.

Fig. 4

Fig. 4. Climatic variations in the Early Triassic over the Smithian-Spathian boundary (SSB). (A) δ18O data from conodonts and the estimated temperature curve across the SSB. (B) Number of conodont species and (C) ammonoid genera. (D) Boreal stable C isotope record of bulk organic matter. (E) Ratio between xerophytes/hygrophytes plant types as derived from palynology.

 Data from: (A) Sun, Y., Joachimski, M. M., Wignall, P. B., Yan, C., Chen, Y., Jiang, H., Wang, L., and Lai, X. (2012). Lethally hot temperatures during the Early Triassic Greenhouse. Science 338, 366–370. (B and C): Stanley, S. M. (2009). Evidence from ammonoids and conodonts for multiple Early Triassic mass extinctions. Proceedings of the National Academy of Sciences 106, 15264–15267. (D and E) Galfetti, T., Hochuli, P.A., Brayard, A., Bucher, H., Weissert, H., and Vigran, J.O. (2007). Smithian-Spathian boundary event: Evidence for global climatic change in the wake of the end-Permian biotic crisis. Geology 35, 291–294.

Early Triassic "red beds," common across central Europe, were traditionally viewed as indicating hot and dry climates. However, more recently the Early Triassic red beds, particularly paleosols, have been interpreted as indicative of seasonally dry conditions. In northern Europe eolian and fluvial sandstones, lacustrine-playa mudstones and halite and gypsum deposited in continental basins, provide evidence for a dry climate with braided river systems and high seasonal changes in discharge. Nevertheless, the widespread distribution of eolian deposits during the Early Triassic suggests that the Pangaean interior experienced arid to semi-arid conditions. The existence of warm-climate floras at high paleolatitudes indicates that the Early Triassic was characterized by low meridional temperature gradients. High-paleolatitude paleosols from the Sydney Basin (65–85°S) show substantial evidence for chemical and textural weathering that are more characteristic of soils formed in lower latitudes (40°–58°), providing evidence for warm polar regions. Modeling studies based on ammonoid paleobiogeography indicate that the Smithian was characterized by lower meridional temperature gradients, with a steeper temperature gradient modeled for the Spathian. Climate simulations for the Early Triassic indicate globally warm and dry conditions, with a mean surface temperature of 17.3   °C, c. 4   °C more than the present-day. Vegetation globally was broadly similar across Pangaea, with low diversity assemblages dominated by lycopsids and conifers persisting until the end of the Early Triassic. Succulence, small leaves and needle leaves indicate plant adaptation to highly seasonal climates and low water availability. Extensive peat swamps in Australia, South Africa, China and Siberia disappeared in the Early Triassic, forming a "coal gap." This was possibly the result of the extinction of peat-forming woody plants.

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Chemostratigraphy of the Permian–Triassic Strata of the Offshore Persian Gulf, Iran

Vahid Tavakoli , in Chemostratigraphy, 2015

14.1 Introduction

The Late Permian–Early Triassic transition is generally referred to as a global greenhouse interval (e.g., Fluteau et al., 2001; Woods, 2005; Schneider et al., 2006). It is characterized by the widespread deposition of carbonates and evaporites in the Middle East and negative carbon stable-isotope excursions linked to the end-Permian mass extinction. Several hypotheses have been suggested to explain this catastrophic event. The most commonly accepted theories include oceanic anoxia, changes in seawater chemistry, volcanism, asteroid/comet impact, methane clathrate dissociation events, and extreme climate change (Stanley, 1988; Ellis and Schramm, 1995; Renneet al., 1995; Knoll et al., 1996; Isozaki, 1997; Bowring et al., 1998; Retallack et al., 1998; Becker et al., 2001; Erwin et al., 2002; Wignall and Twitchett, 2002; Wit et al., 2002; Heydari et al., 2003; Kidder and Worsley, 2004; Kump et al., 2005; Retallak and Jahren, 2008; Tavakoli and Rahimpour-Bonab, 2012).

Regardless of any of the hypotheses, the ocean chemistry has been changed during that time. These changes were reflected in the geochemistry of the sediments. Geochemical data may point to the nature of original seawater and paleoclimate in various carbonate environments (e.g., Walgenwitz et al., 1992; Emmanuel et al., 1999). So, Permian–Triassic boundary (PTB) sections with carbonate lithology are unique for investigating the end-Permian mass extinction because of their potential for recording changes in seawater geochemistry (Grotzinger and Knoll, 1995; Lehrmann et al., 2003; Ehrenberg et al., 2008). Chemostratigraphy of these strata can reveal the paleoceanic conditions at the time of deposition and may aid in high-resolution correlation using the results of isotopic and elemental analyses. Anomalous diagenetic overprints and possible siliciclastic influx must be differentiated from such evidences (Ehrenberg et al., 2008; Rahimpour-Bonab et al., 2009; Tavakoli et al., 2011). Most of the chemostratigraphic researches have been focused on the nature of changes in isotopic or elemental concentrations or the cause of extinction (e.g., Heydari et al., 2008; Liu et al., 2013; Heydari and Hassanzadeh, 2003; Wang et al., 2007; Heydari et al., 2000; Twichett et al., 2001). The results of the previous studies (op cit.) show that the end-Permian extinction is associated with distinct changes in the C and Sr isotopic ratios of seawater. Although fewer studies considered the elemental concentrations of sediments at the Permian–Triassic transitions (e.g., Dolenec et al., 2001; Kaiho et al., 2006; Algeo et al., 2007), they focused only on the redox conditions at the time of deposition as the cause of extinction. Thus, chemostratigraphic application in documenting the paleoceanic conditions at the time of deposition is yet to be attempted.

The PTB sections studied here are located in the offshore Persian Gulf, Iran (Fig. 1). The Persian Gulf PTB sections are significant in terms of the occurrence of organosedimentary thrombolites that have been reported from the other PTB sections around the world (Sano and Nakashima., 1997; Lehrmann, 1999; Kershaw et al., 1999, 2002; Wignall and Twitchett, 2002; Ezaki et al., 2003; Hips and Haas, 2006; Pruss et al., 2006). These thrombolites have been generally described as microbialite buildups that were formed by a result of a benthic microbial community (Burne and Moore, 1987). Microbialites exhibit unique clotted accretionary textures that are rarely found in post-Cambrian strata, and may have resulted from the distinct ecological and geochemical conditions of the post-PTB mass extinction. This marker bed could be used as a good indicator for Early Triassic time correlation through the area.

Figure 1. Study area at the time of deposition (left) and at the present time (right). The study area is highlighted in both pictures (the Permian–Triassic global map from Stampfli and Borel, 2002).

In this paper, high-resolution carbon, oxygen, and strontium isotope ratios as well as elemental analysis (Fe, Mg, Sr, Na, and Th/U ratios) of the three wells in the Persian Gulf are presented, which provide new insights to calibrate the PTB from geochemical aspects. Elemental concentrations are introduced here for the first time in this area. These new data are important to understand the Late Permian and Early Triassic major paleoceanographic conditions in this region. Also, this study provides a new approach for calibration and correlation of the Upper Permian to Lower Triassic strata if the section does not have enough biostratigraphic data for absolute age dating. These data fill the gap in Permian–Triassic ocean geochemistry in this part of the world. Based on the chemostratigraphic data as well as sedimentological and biological characteristics, the exact horizon of the PTB is located, and the sequence is compared to other PTB sections around the world. Discussion will be extended to find the significance of the geochemical data that provides an insight into paleoceanographic conditions during the greatest crisis in the history of life.

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Southern Cordillera☆

A.W. Snoke , in Reference Module in Earth Systems and Environmental Sciences, 2013

Early Mesozoic Continental to Oceanic Magmatic Arc

The Middle Pennsylvanian–Middle Early Triassic oblique truncation of the continental margin of the Southern Cordillera was subsequently overprinted by the development of a northwest–southeast-trending magmatic arc. It can be traced from southern Arizona where it is built on continental (sialic) crust to the eastern Klamath Mountains where it is built on oceanic (ultramafic to mafic) crust. The transition from continental to oceanic arc is inferred to occur at about 39º N latitude in the Sierra Nevada. This Early Mesozoic magmatic arc is thought to have been west-facing with an eastward-dipping subduction zone. Late Triassic (~  220   Ma) blueschist-facies metamorphic rocks in the northern Sierra Nevada, Klamath Mountains, and near Mitchell, Oregon (inlier of the Blue Mountains Province) are interpreted as the innermost subducted rocks of an accretionary complex that developed seaward of the magmatic arc. Its development initiated a long-lived convergent plate boundary zone along the Southern Cordillera. This west-facing magmatic arc served as the 'backstop' for the accretion of numerous tectonostratigraphic terranes that were added to the western North American continental margin from mid-Jurassic through Early Tertiary time.

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NORTH AMERICA | Southern Cordillera

A.W. Snoke , in Encyclopedia of Geology, 2005

Early Mesozoic Continental to Oceanic Magmatic Arc

The Middle Pennsylvanian–Middle Early Triassic oblique truncation of the continental margin of the Southern Cordillera was subsequently overprinted by the development of a north-west/south-east-trending magmatic arc. It can be traced from southern Arizona, where it is built on continental (sialic) crust, to the eastern Klamath Mountains, where it is built on oceanic (ultramafic to mafic) crust. The transition from continental to oceanic arc is inferred to occur at about 39° N latitude in the Sierra Nevada. This Early Mesozoic magmatic arc is thought to have been west-facing with an eastward-dipping subduction zone. Late Triassic (∼220 Ma) blueschist-facies metamorphic rocks in the northern Sierra Nevada, Klamath Mountains, and near Mitchell, Oregon (inlier of the Blue Mountains Province) are interpreted as the innermost subducted rocks of an accretionary complex that developed seaward of the magmatic arc. Its development initiated a long-lived convergent plate boundary zone along the Southern Cordillera. This west-facing magmatic arc served as the 'backstop' for the accretion of numerous tectonostratigraphic terranes that were added to the western North American continental margin from mid-Jurassic through Early Tertiary time.

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Volume 4

Arthur W. Snoke , James B. Chapman , in Encyclopedia of Geology (Second Edition), 2021

Late Paleozoic to Early Mesozoic Continental to Oceanic Magmatic Arc

The Middle Pennsylvanian-middle Early Triassic oblique truncation of the continental margin of the Central Cordillera was subsequently overprinted by the development of a northwest-southeast-trending magmatic arc. It can be traced from southern Arizona where it is built on continental (sialic) crust to the eastern Klamath Mountains where it is built on oceanic (ultramafic to mafic) crust. The transition from continental to oceanic arc is inferred to occur at about 39° N latitude in the Sierra Nevada. This early Mesozoic magmatic arc is thought to have been west-facing with an eastward-dipping subduction zone ( Barth et al., 2011). Late Triassic (~   220   Ma) blueschist-facies metamorphic rocks in the northern Sierra Nevada, Klamath Mountains, and near Mitchell, Oregon (inlier of the Blue Mountains Province) are interpreted as the innermost subducted rocks of an accretionary complex that developed seaward of the magmatic arc. Its development initiated a long-lived convergent plate-boundary zone along the Central Cordillera. This west-facing magmatic arc served as the "backstop" for the accretion of numerous tectonostratigraphic terranes that were added to the western North American continental margin from mid-Jurassic through early Paleogene time.

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https://www.sciencedirect.com/science/article/pii/B9780124095489121244