Lower Induan Research Articles

Strontium isotope stratigraphic insights on the end-Permian mass extinction and the Permian-Triassic boundary in the Dolomites (Italy)

The Dolomites (Southern Alps, Italy) is a most significant region to investigate the evolution of shallow-marine ecosystems during the end-Permian mass extinction (EPME). Shallow-marine ecosystems are complex places from an oceanographic viewpoint and combine high biological productivity and ecological diversity. Therefore, establishing the timing and correlation of globally recognisable events in this region are fundamental to interpreting the biological crisis that followed.We took advantage of the Strontium Isotope Stratigraphy (SIS) concept to correlate our stratigraphic succession, ranging from the upper Bellerophon (Bulla Member, upper Changhsingian) to the lower Werfen (Tesero Member, upper Changhsingian - lower Induan) Formations of the Dolomites, to the Meishan (Zhejiang Province, China), the Global Stratotype Section and Point (GSSP) for the Permian–Triassic boundary. We integrated new and previously published 87Sr/86Sr data from well preserved brachiopods and show that the topmost part of the Bellerophon Formation (the Bulla Member) correlates with beds 24 e-f of the GSSP section, while the Tesero Member, where the Permian-Triassic boundary is located, is characterised by more radiogenic Sr isotope ratios than expected. Differences in Sr isotope ratios may be due to small-scale variations in local paleoenvironmental and paleogeographic conditions/settings. Continental weathering of sedimentary or igneous rocks, distance from terrestrial environment and nearby occurrence of radiogenic rocks might also produce locally variable Sr isotope signatures especially in shallow water environments complicating the application of SIS principles.

Changes in size and growth rate of ‘Lilliput’ animals in the earliest Triassic

Marine invertebrate taxa that survived the late Permian (latest Changhsingian) mass extinction event are all much smaller than they were prior to the event, an example of the Lilliput effect. New taxa that first appeared in the immediate extinction aftermath are also small compared to their size in younger strata. The Lilliput effect is a temporary phenomenon, with most surviving taxa returning to pre-extinction size in the first two conodont zones of the Triassic. In this study, the growth rates, of some of the Early Triassic Lilliput animals, estimated by growth-line analysis, were compared from different stratigraphic levels to determine how this variable fluctuated as body size returned to pre-extinction values after the event. Specimens were collected from the Mazzin and Siusi members of the Werfen Formation in northern Italy, from facies deposited in a tropical, mixed siliciclastic-carbonate ramp setting. Measurements of the bivalves Unionites and Claraia, the gastropod Bellerophon and the brachiopod ‘ Lingula’ were made in the field, with subsequent growth-line analysis conducted on well-preserved specimens of ‘Lingula’. Specimens collected from the basal Triassic Mazzin Member were smaller than those from the younger Siusi Member. Growth-line analysis indicates that the smaller Mazzin Member specimens had a greater number of more closely spaced growth lines than the larger Siusi Member specimens. The small, ‘Lilliput’ animals living in the immediate aftermath of the Late Permian event therefore suffered more interruptions to growth and had overall slower growth rates than larger individuals living in later recovery times. Frequent interruptions to growth and an overall slower growth rate in specimens from the Hindeodus parvus and Isarcicella isarcica zones of the Mazzin Member (lower Induan) reflects suboptimal environmental conditions, such as the frequency or severity of low-oxygen episodes, temperature extremes, and/or disruptions to primary productivity.

DIVERSE ICHNOFOSSIL ASSEMBLAGES FOLLOWING THE P-T MASS EXTINCTION, LOWER TRIASSIC, ALBERTA AND BRITISH COLUMBIA, CANADA: EVIDENCE FOR SHALLOW MARINE REFUGIA ON THE NORTHWESTERN COAST OF PANGAEA

Abstract Diverse and locally abundant Lowermost Triassic (lower Induan, Griesbachian) trace-fossil assemblages are described and their significance for the location and characteristics of western Pangean environmental refugia are assessed. Trace fossils within the Montney Formation in northwestern Alberta and northeastern British Columbia record the activities of a wide variety of marine invertebrates. Many forms represent the dwelling and feeding traces of allochthonous storm-transported colonizers. Anachronistic forms—more typical of Paleozoic than Mesozoic successions—including Cruziana, Diplichnites, Monomorphichnus, and Trichophycus, are common. Notably these Paleozoic holdovers, as well as Rhizocorallium, Thalassinoides, and Spongeliomorpha, were likely constructed by marine arthropods. Trace fossils are rare in both shallow water (upper shoreface and foreshore) and offshore depositional settings, but are abundant in offshore transition to distal lower shoreface depositional settings. Low diversity ...

Sedimentology and ichnology of the Lower Triassic Montney Formation in the Pedigree-Ring/Border-Kahntah River area, northwestern Alberta and northeastern British Columbia

Abstract Very fine-grained sandstone beds in the lowermost Triassic (lower Induan = Griesbachian) lower Montney Formation are the primary reservoir lithology in the Pedigree, Ring-Border and Kahntah River gas fields of northwestern Alberta and northeastern British Columbia. Hydrocarbon production from Pedigree and Ring-Border fields occurs primarily from well-sorted, low-angle to hummocky and trough cross-stratified sandstone beds. Hydrocarbon production from Kahntah River field occurs primarily from highly bioturbated silty/muddy sandstone with hummocky and trough cross-stratified sandstone interbeds. The three gas fields included in this study occur within shallow marine successions in strata that partially coincide stratigraphically, but are successively younger, from east to west. Thus the cored interval in Pedigree Field overlaps with, but is overall slightly older than, the cored interval in the Ring/Border Field, which overlaps with, but is overall slightly older than, the cored interval in the Kahntah River Field. Six lithofacies have been identified in the study interval: L-I (dark gray laminated shale); L-II (interbedded very fine-grained sandstone, siltstone and silty shale); L-III (low-angle to hummocky cross-stratified, very fine-grained sandstone); L-IV (convolute-bedded shale, siltstone and sandstone); L-V (bioturbated muddy/silty sandstone); and L-VI (trough cross-stratified, very fine-grained to fine-grained sandstone). These lithofacies comprise an overall shallowing upwards proximal offshore through shoreface succession. Strata interpreted as proximal offshore contain few trace fossils, primarily isolated Planolites. Trace fossils in strata interpreted as offshore transition include Aulichnites, Conichnus, Cruziana, Diplichnites, Fustiglyphus, Halopoa, Helminthopis, Lockeia, Monomorphichnus, Palaeophycus, Planolites, Protovirgularia, Rhizocorallium, Rusophycus, Spongeliomorpha, Thalassinoides and Treptichnus. Although trace fossil diversity is very high, abundances are low and few of these taxa represent indigenous infauna. Most forms represent the dwelling and feeding traces of storm-transported, opportunistic colonizers. Strata deposited in the lower shoreface contain a diverse trace fossil assemblage consisting of Asterosoma, Cylindrichnus, Cruziana, Diplocraterion, Palaeophycus, Phycosiphon, Phycodes, Planolites, Rhizocorallium, Rosselia, Skolithos, Spongeliomorpha, Teichichnus and Thalassinoides. Upper shoreface sandstone beds are apparently barren. Several lines of evidence, including low diversity of indigenous infauna, support the hypothesis of dysoxic depositional conditions in the lower Montney offshore transition during the Griesbachian. Decreased thickness and preservation of storm beds suggest that the intensity and frequency of storms in the study area appear to have decreased through the Griesbachian, resulting in longer interstorm intervals and deeper, more pervasive bioturbation. Periodic influx of turbid fresh/brackish water by local ephemeral deltas may have also had a dampening effect on local biodiversity.

Lower Triassic conodont stratigraphy

A conodont stratigraphic subdivision of the Lower Triassic is made, and considerations about the Permian/Triassic boundary are developed. Data received during the last few years are generalized with emphasis laid on results obtained on conodonts found by the authors in their samples from the Himalayas and different parts of Europe. The conventional Permian/Triassic boundary is situated in the stratigraphic range of Clarkina subcarinata-Anchignathodus typicalis C.-R.-Z. although as an alternative possibility (based on conodont, ammonite and other faunas) the base of Kashmirella kummeli R.-Z. is indicated. The conodont subdivision of the Lower Triassic is based upon 14 conodont zones as follows: Clarkina subcarinata – Anchignathodus typicalis C.-R.-Z., Anchignathodus parvus L-Z., Isarcicella isarcica R.-Z., Clarkina carinata A.-Z. – for the Griesbachian (Lower Induan); Kashmirella kummeli R.-Z., Neospathodus dieneri – Neospathodus cristagalli C.-R.-Z., and parts of Kashmirella nepalensis R.-Z. – for the Dienerian (Upper Induan); middle and upper parts of Kashmirella nepalensis R.-Z., Neospathodus waageni A.-Z., Scythogondolella milleri R.-Z. and Parachirognathus – Furnischius Beds – for the Smithian (Lower Olenekian), and Neospathodus triangularis - Neospathodus hommeri A.-Z., Platyvillosus – Foliella Beds and Kashmirella timorensis R.-Z., – for the Spathian (Upper Olenekian). The boundary between the Lower and Upper Triassic is set with proving of the Kasmirella gondolelloides R.-Z.