Triassic nautiloid Sibyllonautilus from Gua Bama,Peninsular Malaysia and its regional stratigraphic implications

Coiled nautiloid Shells referred to Sibyllonautilus bamaensis Sone sp. nov. are reported from the top of the Gua Bama limestone hill in Pahang, Peninsular Malaysia. This is the first record of the genus in Southeast Asia; a pre-Ladinian, Triassic age is indicated for the occurrence. Based on the presence of Sibyllonautilus and previously reported Late Permian (Lopingian) foraminifers and algae, the Gua Bama limestones are interpreted to range from the Late Permian to the Triassic. It further seems plausible that some parts of Gua Bama are stratigraphically correlated to those of the nearby Gua Sei limestone hill, which has yielded basal Triassic conodonts, and that either or both the Gua Bama and Gua Sei hills may contain yet unconfirmed successions of the Permian-Triassic boundary.     

Generic demarcation of Permo-Triassic Claraia-like species and their biogeographic significance

Three types of byssal notch are present in Permo-Triassic Claraia-like species, i.e. Claraia-type, Pseudoclaraia-type and Claraioides-type. At present, there is no consensus in the use of the generic names. The preferred classification should be both useful in practice and compatible with the rules of taxonomy. For the genera in question, details of the byssal notch are important in assessing relationships because the morphology of this structure may closely reflect phylogenetic affinities. However, a particular feature of the notch may have great significance in one taxon but low significance in a related taxon. In order to seek the best taxonomic characters for classification of Permo-Triassic Claraia-like species, we should follow empirical evidence rather than preconceived opinions. Pseudoclaraia Zhang, 1980 should be considered a junior synonym of Claraia Bittner, 1901 Bittner, A. 1901. Über Pseudomonotis telleri und verwandte Arten der unteren Trias. Jahrbuch der geologischen Reichsanstalt Wien, 50: 559–592.  [Google Scholar] because the Pseudoclaraia-type and Claraia-type byssal notches form a continuum within populations. In contrast, all known Claraioides species possess the Claraioides-type byssal notch, which is their synapomorphy, indicating monophyly. The notch is characterized by ventral enlargement due to ventral dissolution and by its contracted anterodorsal outlet due to the accretion of shell material. This is a unique character in Palaeozoic bivalves. It implies that the Claraioides byssal notch must have had a different mode of development than the other two types of byssal notch. It is thus unreasonable to place Claraioides in synonymy with Claraia. A review of the biogeographic patterns of Permo-Triassic Claraia-like species indicates that trans-oceanic dispersal of Claraia from the Boreal Realm to the rest of the world may have occurred during the latest Permian. It is possible that volcanic-winter effects triggered the trans-oceanic dispersal and the tremendous change in Permian–Triassic global biogeographical patterns.     

A new temnospondyl amphibian from the Lower Triassic of South America

A new temnospondyl from the Early Triassic Buena Vista Formation (Paraná Basin) in Uruguay is described. The specimen is represented by a skull table fragment which includes the left orbit, and, although fairly incomplete, shows close affinities with the basal temnospondyl clade (Dvinosaurus + Tupilakosauridae). Previously, members of this clade were only known from Upper Permian and Lower Triassic deposits from the Northern Hemisphere. The new specimen described herein together with a recent record from the Lower Triassic of South Africa (Karoo Basin), support the hypothesis of an invasion of members of the basal (Dvinosaurus + Tupilakosauridae) clade into Gondwana as early as the Late Permian.     

Leaf galls on Dicroidium hughesii (Feistmantel) Lele from the Triassic of India—a new record

Disc-like galls are reported on the Triassic corystosperm leaf Dicroidium hughesii (Feistmantel) Lele from the Parsora Formation of South Rewa Gondwana Basin, central India. Although there have been numerous reports of arthropod–plant interactions from Permian and Lower Cretaceous successions, this is the first unequivocal report of arthropod–plant interactions from the Triassic succession of Peninsular India. The new record adds to global evidence that arthropod herbivory and gall formation, in particular, had rediversified by the Late Triassic in the wake of the end-Permian mass extinction.     

Late Silurian and Early Devonian biostratigraphy in the Hill End Trough and the Limekilns area,New South Wales

Precordilleran Late Palaeozoic brachiopod genera considered for this palaeobiogeographical analysis belong to four faunal associations: the low diversity Early Carboniferous (Tournaisian) Protocanites scalabrinii-Azurduya chavelensis Zone; the Late Carboniferous (Bashkirian-Moscovian) Levipustula fauna, which appears in sequences associated with glacial intervals; the latest Carboniferous-earliest Permian (Gzhelian-Asselian) Rhipidomella-Micraphelia and Tuberculatella-Aseptella associations, and the Early Permian (mid to late Asselian) Tivertonia jachalensis-Streptorhynchus inaequiornatus Biozone. The brachiopod affinities of the Precordillera are compared with the biogeographical Late Palaeozoic regions previously suggested by other authors using cluster analysis. During the Bashkirian-Moscovian, the Precordilleran faunas show a high affinity with the Austral Realm because of the presence of the Levipustula faunal elements in Eastern Australia. At the same time the affinity with the central and North American Regions is very low. The Early Permian Precordilleran faunas demonstrate highest affinity with the Gondwanan Realm with several genera also linked to the Tethyan and Boreal Realms. After the Late Carboniferous glacial episodes that affected the southwestern Gondwanan margin, the Precordilleran region underwent climatic amelioration. Subsequently, the Early Permian Precordilleran brachiopod assemblages are characterized by typical cool to cold water genera widely developed in the core Gondwanan Realm, with fewer genera (such as Kochiproductus, Rhynchopora and Neochonetes) suggesting a warmer water influence. The Precordilleran faunas demonstrate an increase in brachiopod diversity from the Carboniferous to the Permian, related to water temperature changes and to the palaeogeographical evolution of the southwestern Gondwanan margin and the movement of Gondwana across the South Pole.     

Chonetoidea (Brachiopoda) from the Lopingian (Late Permian) of South China

Ten species of the superfamily Chonetoidea from the Lopingian (Late Permian) of South China are described or revised. A review of all recorded Chonetoidea species from the Lopingian (Late Permian) of South China indicates that some 22 species of five genera can be recognised. Species of Tethyochonetes and Neochonetes are characteristic in the lithofacies dominated by mudstone, siltstone or siliceous rocks in the Lopingian and some argillaceous limestone and clay rock facies near the Permian-Triassic boundary. New taxa are Neochoneles (Zhongyingia) subgen. nov., Neochonetes (Huangichonetes) subgen. nov. and Tethyochonetes flatus sp. nov.     

Late Triassic (Carnian) corals from Timor-Leste (East Timor): their identity,setting, and biogeography

Four scleractinian coral taxa are described from limestones within a sandstone-shale séquence correlated with the Late Triassic Babulu Formation, Manatuto township, on the northern coast of Timor-Leste (East Timor). The coral fauna consists of three phaceloid taxa, Paravolzeia tìmorìca gen. et sp. nov., Craspedophyll ramosa sp. nov., Margarosmilia confluens (Münster), and a generically indeterminate solitary taxon attributed to the family Margarophylliidae. Ali four corals are related at various taxonomie levels to Carnian faunas from the Dolomites of northern Italy. Previously, only Norian coral faunas were known from the Triassic of Timor. The fauna exhibits both similarities to and differences from Carnian faunas of the Dolomites and helps confirm palaeogeographic affinities with the western Tethys, although during Late Triassic time Timor lay in the distant southeastern portai of the Tethys. Despite isolation from the western Tethys, the presence of two species foundalso in the Dolomites indicates that larvai dispersai occurred between the two areas.     

A new Late Permian brachiopod fauna from Tanggula,Qinghai-Tibet Plateau and its palaeogeographical implications

Weihong He, Jianjun Bu, Zhijun Niu & Yang Zhang, June, 2009. A new Late Permian brachiopod fauna from Tanggula, Qinghai-Tibet Plateau and its palaeogeographical implications. Alcheringa 33, 113–132. ISSN 0311-5518.

A brachiopod fauna described from the Late Permian of the Gongri and Lizai villages, northwest of the Qoima Co Lake, Tanggula area, southern Qinghai, northwest China, includes ten species in nine genera. This fauna from the (Qiangtang Block) has a strong affinity to coeval faunas of South China, sharing 74% of its species. In addition, the Late Permian Tanggula brachiopod assemblages also demonstrate a clear link with the Middle Permian brachiopod faunas of neighbouring microcontinents including the Indochina block (Malaysia and Cambodia) and the Shan-Thai block (Thailand), as suggested by the presence of Caricula sp. cf. C. salebrosa, Transennatia termierorum and Strophalosiina. This phenomenon indicates that there were relatively narrow seaways between these microcontinents to enable ready interchange of brachiopods during the Permian, and that brachiopods tended to retreat towards the palaeoequatorial region throughout the period.     

Late Triassic megaspores from the Amery Group,Prince Charles Mountains,East Antarctica

Cantrill, D. J., & Drinnan, A. N., 1994:03:28. Late Triassic megaspores from the Amery Group, Prince Charles Mountains, East Antarctica. Alcheringa 18, 71–78. ISSN 0311-5518.

Megaspores referable to the genera Cabochonicus Batten & Ferguson 1987 and Minerisporites Potonié 1956 are a common component of the palaeoflora recovered from the Jetty Member within the Flagstone Bench Formation of the Amery Group. The known ranges of these two genera, in conjunction with the macrofloral remains, suggest a Late Triassic age. Two new species, Cabochonicus sinuosus and Minerisporites triangulatus, are described. Scanning electron microscopic examination of the spores indicates that standard palynological treatment of megaspores can result in sculptural degradation, possibly leading to incorrect generic assignment.     

A new Changhsingian (Late Permian) Rugosochonetidae (Brachiopoda) fauna from the Zhongzhai section,southwestern Guizhou Province,South China

Zhang, Y., He, W.-H., Shi, G.R. & Zhang, K.-X., 2013. A new Changhsingian (Late Permian) Rugosochonetidae (Brachiopoda) fauna from the Zhongzhai section, southwestern Guizhou Province, South China. Alcheringa 37, 221–245. ISSN 0311-5518.

This paper describes 20 species (including three undetermined species) of Rugosochonetidae (Brachiopoda) in an upper offshore fauna from the Permian–Triassic Boundary Zhongzhai section, southwestern Guizhou Province, South China. New taxa are Tethyochonetes sheni, Tethyochonetes cheni, Neochonetes (Huangichonetes) archboldi, Neochonetes (Sommeriella) waterhousei, Neochonetes (Sommeriella) rectangularis and Neochonetes semicircularis.

Yang Zhang [zyan@deakin.edu.au] and G.R. Shi [guang.shi@deakin.edu.au] (corresponding author), School of Life and Environmental Sciences, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia; Weihong He [whzhang@cug.edu.cn] (corresponding author) and Kexin Zhang [kx_zhang@cug.edu.cn], State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Hongshan, Wuhan 430074, PR China. Received 8.6.2012; revised 19.9.2012; accepted 7.10.2012.     

New Late Permian rugose corals from Pahang,peninsular Malaysia

Aye Ko Aung, Ng Tham Fatt, Kyaw Kyaw Nyein & Myo Htut Zin, 2013. New Late Permian rugose corals from Pahang, peninsular Malaysia. Alcheringa 37, 422–434. ISSN 0311-5518.

Late Permian rugose corals are described from a limestone unit of the Gua Musang Formation at Selborne Estate, Padang Tengku area, Pahang, peninsular Malaysia. These include one genus, Iranophyllum, which is reported for the first time from Malaysia, with two new species Iranophyllum aequabilis and I. pahangense belonging to Waagenophyllidae. A Late Permian age is confirmed by a Paleofusulina–Colaniella–Reichelina foraminiferal fauna co-preserved with the corals.

Aye Ko Aung [akaung.mm@gmail.com], Ng Tham Fatt [thamfatt@gmail.com], Kyaw Kyaw Nyein [konyein@gmail.com], Department of Geology, University of Malaya, 50603, Kuala Lumpur, Malaysia and Myo Htut Zin [myohtutgreat@googlemail.com], Lab. Services, Pte. Co. Ltd., Singapore. Received 16.10.2012; revised 5.1.2013; accepted 17.1.2012.     

Late Triassic palynoflora from the Pranhita–Godavari Valley,India: evidence from vertebrate coprolites

Vijaya, Prasad, G.V.R. & Singh, K., June, 2009. Late Triassic palynoflora from the Pranhita–Godavari Valley, India: evidence from vertebrate coprolites. Alcheringa 33, 91–111. ISSN 0311-5518.

The Upper Triassic Maleri Formation, represented by red clays and sandstones, has to date not produced any plant macrofossils or palynomorphs. Many spiral and non-spiral coprolites collected during this study from the Maleri Formation of the Pranhita-Godavari Valley were analysed for palynomorphs. Based on shape, nature of coiling and size, the Maleri coprolites are classified into seven groups. Of these, only Group-I, Type 5 (non-spiral) and amphipolar (spiral) types yielded diverse gymnospermous and pteridophytic spores, pollen, other plant debris and sparse fungal spores and algal remains. Occurrences of Antulsporites varigranulatus, Aratrisporites spp., Cadargasporites baculatus, Dubrajisporites isolatus, Enzonalasporites vigens, Foraminisporis coelatus, Grandispora spinosa, Kraeuselisporites saeptatus, Polycingulatisporites reduncus, Staurosaccites spp., Tethysispora unica and Tikisporites balmei confirm a Late Triassic age for the coprolite-bearing red clays. Records of Classopollis classoides and Callialasporites turbatus/dampieri in these assemblages more precisely suggest a Norian to Rhaetian age. The non-spiral coprolites were possibly produced by aquatic piscivorous animals whereas the spiral coprolites may have been produced by an as yet unidentified fish taxon. The coprolite-producing animals (spiral and non-spiral groups) possibly ingested gymnospermous and pteridophytic plant remains passively along with water or their herbivorous prey.     

Umkomasia (megasporophyll): part 1 of a reassessment of Gondwana Triassic plant genera and a reclassification of some previously attributed

The genus Umkomasia, a megasporophyll, belonging to the pteridosperms (seed ferns) in the family Umkomasiaceae (Corystospermaceae), is reassessed comprehensively worldwide. All previous records are analysed. Certain fertile structures previously attributed are reclassified. Umkomasia is shown to be restricted to the Triassic of Gondwana where it is associated with the genus Pteruchus, a microsporophyll, and the genus Dicroidium, a vegetative leaf. It is well represented from Argentina, Australia and southern Africa where the Molteno Formation is by far the most comprehensively sampled with eight species described. Two specimens from the upper Permian of India attributed to Umkomasia are reclassified as cf. Arberiopsis sp. A whorled fertile structure from Antarctica, previously assigned to Umkomasia, is reclassified in a new genus as Axsmithia uniramia. Another compression fossil and the permineralized Umkomasia resinosa remain as valid records from Antarctica. The material described as Umkomasia from the Triassic of China is reclassified as Stenorachis asiatica. The Lower Jurassic record from Germany is placed in a new genus as Kirchmuellia franconica. The records of Umkomasia sp. from the Rhaetic of Germany are reclassified as cf. Kirchmuellia sp. and the single specimen from the Jurassic of Libya as genus et sp. indet. The Lower Cretaceous record from Mongolia has been reclassified by other researchers as Doylea mongolica. A pictorial key to Umkomasia species is provided, geographic and stratigraphic distributions are tabulated.     

Sources and composition of fluids associated with fluorite deposits of Asturias (N Spain)

The fluorite deposits of Asturias (northern Iberian Peninsula) are hosted by rocks of Permo‐Triassic and Palaeozoic age. Fluid inclusions in ore and gangue minerals show homogenization temperatures from 80 to 170°C and the presence of two types of fluids: an H₂O–NaCl low‐salinity fluid (<8 eq. wt% NaCl) and an H₂O–NaCl–CaCl₂ fluid (7–13 wt% NaCl and 11–14 wt% CaCl₂). The low salinity and the Cl/Br and Na/Br ratios (Cl/Br_molar 100–700 and Na/Br_molar 20–700) are consistent with an evaporated sea water origin of this fluid. The other end‐member of the mixture was highly saline brine with high Cl/Br and Na/Br ratios (Cl/Br_molar 700–13 000 and Na/Br_molar 700–11 000) generated after dissolution of Triassic age evaporites. LA‐ICP‐MS analyses of fluid inclusions in fluorite reveal higher Zn, Pb and Ba contents in the high‐salinity fluids (160–500, 90–170, 320–480 p.p.m. respectively) than in the low‐salinity fluid (75–230, 25–150 and 100–300 p.p.m. respectively). The metal content of the fluids appears to decrease from E to W, from Berbes to La Collada and to Villabona. The source of F is probably related to leaching of volcanic rocks of Permian age. Brines circulated along faults into the Palaeozoic basement. Evaporated sea water was present in permeable rocks and faults along or above the unconformity between the Permo‐Triassic sediments and the Palaeozoic basement. Mineralization formed when the deep brines mixed with the surficial fluids in carbonates, breccias and fractures resulting in the formation of veins and stratabound bodies of fluorite, barite, calcite, dolomite and quartz and minor amounts of sulphides. Fluid movement and mineralization occurred between Late Triassic and Late Jurassic times, probably associated with rifting events related to the opening of the Atlantic Ocean. This model is also consistent with the geodynamic setting of other fluorite‐rich districts in Europe.     

Elchaxylon,a new corystosperm based on permineralized stems from the Late Triassic of Argentina

Artabe, A.E. & Zamuner, A.B., March 2007. Elchaxylon, a new corystosperm based on permineralized stems from the Late Triassic of Argentina. Alcheringa 31, 85‐96. ISSN 0311-5518.

Anatomically preserved stems of Late Triassic corystosperms from the Río Blanco Formation of Argentina are described and assigned to Elchaxylon zavattieriae gen. et sp. nov. The polyxylic gymnospermous axes have two discontinuous cambial rings, which develop centrifugal secondary xylem and centripetal secondary xylem around the mesarch primary xylem bundles. The centrifugal undivided pycnoxylic secondary xylem incorporates uniseriate rays, uni- to biseriate radial pitting either alternate or opposite, and one or two simple pits in each cross-field. Atypical secondary growth is restricted to localized development of centripetal secondary xylem (inverted xylem). Elchaxylon could represent a basal member of the rhexoxyloid lineage. This basal clade is characterized by limited centripetal secondary xylem, lack of perimedullar bundles, and modest development of centrifugal xylem.

Analia E. Artabe [aeartabe@museo.fcnym.unlp.edu.ar] and Alba B. Zamuner [azamuer@museo.fcnym.unlp.edu.ar]. División Paleobotánica, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque SN, 1900 La Plata, Argentina and Consejo Nacional de Investigaciones Científicas y Técnicas; received 20.5.2005, revised and accepted 14.9.2005.     

Permian brachiopods from the Tupe Formation,San Juan Province,Precordillera, Argentina

The brachiopod fauna from the Tupe Formation at La Herradura Creek, located on the west flank of Perico Hill, San Juan Province, Argentina, palaeogeographically belongs to the western sector of the Paganzo basin (‘Guandacol embayment’). The stratigraphical section of the Tupe Formation at La Herradura Creek is the stratotype of the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone, was previously regarded as being of Late Carboniferous age but here is assigned to the earliest Permian (Asselian). We describe and review the biozone assemblage, which consists of Streptorhynchus inaequiornatus, Tivertonia jachalensis, Kochiproductus sp., Costatumulus sp., Coronalosia argentinensis, Tupelosia paganzoensis, Trigonotreta pericoensis, Septosyringothyris sp. aff. Septosyringothyris jaguelensis and Crurithyris? sp. This brachiopod assemblage is related to Indian and Australian Early Permian faunas and its presence in the La Herradura Creek section provides new evidence in support of an Asselian (Early Permian) age for the Tivertonia jachalensis-Streptorhynchus inaequiornatus biozone. This assemblage is also important for intra- and inter-basinal correlation because several of its characteristic species have been identified from other sections of the Paganzo basin and the Río Blanco basin. The proposed age for this biozone is consistent with the age of palynological data from slightly above the marine faunas from the stratotype locality.     

Two middle Cambrian trilobite genera,Cyclolorenzella Kobayashi, 1960 and Jiulongshania gen. nov., from Korea and China

As is widely recognised, fossil florules are difficult to correlate because they appear to have been more controlled by environmental and ecological factors than uniform changes which reflect the passage of geological time. Here I present a worked example of a generally applicable approach to this problem in dating and mapping fossil plant associations.

Floral associations are best described and named by the classical methods of phytosociology. The three dimensional shape of an association in rocks and its palaeoecology can then be critically assessed. For example, during Middle Triassic time the following plant associations would have been encountered on a traverse from the coast to several hundred kilometres inland from the Pacific margin of Gondwanaland: Pachydermophylletum (mangrove scrub), Linguifolietum (coastal swamp woodland), Dicroidietum odontopteroidium (floodplain forest), Phoenicopsetum (levee bank scrub), Dicroidietum odontopteroidium xylopterosum (xerophytic woodland) and Johnstonietum (mallee-like woodland).

The fourth dimension of fossil plant associations (time) is best assessed from the evolution of a prominent group of plants. In conjunction with the ranges of other plant megafossils, evolutionary changes can be used to define Oppel-zones. For the Late Permian to Early Jurassic of eastern Australasia, I have used the evolution of Dicroidium and leaves of related pteridosperm plants together with other species of restricted stratigraphic range or prominent in the definition of the fossil plant associations. From this evidence four Oppel-zones can be recognised. ‘Thinnfeldia’ callipteroides Oppel-zone (Chhidruan to Smithian, 250–230 × 10⁶ years), Dicroidium zuberi Oppel-zone (Smithian to Middle Anisian, 230–220 × 10⁶ years), D. odontopteroides Oppel-zone (Late Anisian to Ladinian, 220–210 × 10⁶ years) and Yabeiella Oppel-zone (Carnian to Rhaetian, 210–200 × 10⁶ years.

In a complete revision of the pteridosperm form-genera Dicroidium, Johnstonia and Xylopteris, a large number of new combinations and the following new taxa are proposed; Dicroidium gouldii sp. nov., D. odontopteroides var. argenteum var. nov., D. odontopteroides var. moltenense var. nov., D. radiatum sp. nov., D. townrovii sp. nov. and Johnstonia coriacea var. obesa var. nov.     

New dinosaur remains from the Late Triassic of southern Brazil (Candelária Sequence,Hyperodapedon Assemblage Zone)

Pretto, F.A., Schultz, C.L., Langer, M.C. 4.02.2015. New dinosaur remains from the Late Triassic of southern Brazil (Candelária Sequence, Hyperodapedon Assemblage Zone). Alcheringa 39, xxx–xxx. ISSN 0311-5518

The oldest unequivocal dinosaurian records come from Upper Triassic strata, mostly from Argentina and Brazil. Recent fieldwork in the Brazilian deposits of Ischigualastian age (late Carnian–earliest Norian) have resulted in the collection of a new specimen (UFRGS-PV-1240-T) identified as dinosaurian by the presence of a concave emargination on the ventral surface of the femoral head and a well-developed deltopectoral crest on the humerus, which extends for ca 40% of its estimated proximodistal length. Although fragmentary, the material increases the dinosaurian record from the Late Triassic of Brazil, which is still modest when compared with coeval Argentine strata. Additionally, UFRGS-PV-1240-T augments the dinosaurian diversity at the ‘Sítio Janner’ site, which was until now restricted to the sauropodomorph Pampadromaeus barberenai. The new specimen is larger than most early dinosaurians (except for herrerasaurids), implying that members of the group were already occupying niches otherwise explored by other larger-bodied taxa in the ‘Sítio Janner’ palaeofauna, and Late Triassic palaeoenvironments in general.

Flávio A. Pretto [flavio_pretto@yahoo.com.br] and Cesar L. Schultz [cesar.schultz@ufrgs.br] Universidade Federal do Rio Grande do Sul, Instituto de Geociências, Setor de Paleovertebrados, Av. Bento Gonçalves, 9500—Bloco J, Prédio 43127, Campus do Vale, Agronomia?Porto Alegre, Rio Grande do Sul, Brazil, CEP 91540-000; Max C. Langer [mclanger@ffclrp.usp.br] Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Departamento de Biologia, Av. Bandeirantes, 3900, CEP 14040-901, Ribeirão Preto, São Paulo, Brazil.     

Organic–inorganic rock–fluid interactions in stylolitic micro‐environments of carbonate rocks: a FIB‐TEM study combined with a hydrogeochemical modelling approach

Stylolites and the interfaces to the host limestone have been investigated by means of a multidisciplinary analytical approach (thin section microscopy, FIB‐TEM, organic geochemistry and petrography). Carbonate dissolution assuming different boundary conditions was simulated by applying a generic hydrogeochemical modelling approach. It is the conceptual approach to characterize and quantify traceable organic–inorganic interactions in stylolites dependent on organic matter type and its thermal maturity, and to follow stylolite formation in carbonates as result of organic matter reactivity rather than pressure solution as a main control. The investigated stylolite samples are of Upper Permian (Lopingian, Zechstein), Middle Triassic (Muschelkalk) and Late Cretaceous (Maastrichtian) age and always contain marine organic matter. The thermal maturity of the organic matter ranges from the pre‐oil generation zone (0.4–0.5% R_r) to the stage of dry gas generation (>1.3% R_r). The results of the generic hydrogeochemical modelling indicate a sharp increase of calcite dissolution and the beginning of stylolite formation at approximately 40°C, which is equivalent to a depth of less than 800 m under hydrostatic conditions considering a geothermal gradient of 30°C and a surface mean temperature of 20°C. This temperature corresponds to the pre‐oil window when kerogens release an aqueous fluid enriched in carbon dioxide and organic acids. This aqueous fluid may change the existing pore water pH or alkalinity and causes dissolution of carbonate, feldspar and quartz, and clay mineral precipitation along the stylolite. Dissolution of limestone and dolostone leads to reprecipitation of calcite or dolomite opposite of the dissolution side, which indicates only localized mass redistribution. All these integrated hydrogeochemical processes are coupled to the generation of water during organic matter maturation. In all of the calculated hydrogeochemical scenarios, H₂O is a reaction product and its formation supports the suggested hypothesis.