Hettangian Research Articles

Discussion on sea-level change and facies development across potential Triassic–Jurassic boundary horizons, SW Britain

Tony Hallam & Paul Wignall write: we welcome the paper by Hesselbo et al . (2004) for its thorough description and careful evaluation of the environmental significance of an important group of system boundary strata, but beg to take issue on a number of key points. (1) Most importantly, while we agree with the evidence of a regressive phase within the Cotham Member, we maintain in marked contrast to their opinion that there was also a sharp regression followed by a rapid transgression at the junction of the Langport Member (of the Lilstock Formation) and Blue Lias. It is pertinent before discussing the evidence in SW England further to take into account the broader European context (Hallam & Wignall 1999, with many relevant references cited therein). The evidence is especially clear in Germany where there was extensive shallowing in the latest Rhaetian marked by progradation of sandstone over shales. In northern Frankonia (Bavaria) fluvial sandstones infill channels incised into late Rhaetian marine strata and are overlain by marine Hettangian. The sea-level rise in the earliest Hettangian ( planorbis Zone) was evidently rapid, with the limit of marginal marine sandstones in the eastern part of southern Germany being pushed back at the expense of fully marine shales to the maximum extent achieved during the whole Hettangian Stage. Both in the north and south of Europe a similar pattern of successive sea-level fall and rise can be inferred. A clear end-Triassic regressive pulse can be recognized in the Danish Basin, while both in southern Sweden and NW Poland the upper Rhaetian is missing and there is an unconformity at the base of the Jurassic. In the Northern Calcaerous Alps of Austria, widespread emergence at the end of the Triassic is recognized, with the creation of karst surfaces on emergent reef complexes, while …

Sea-level change and facies development across potential Triassic–Jurassic boundary horizons, SW Britain

The Late Triassic to Early Jurassic aged succession of SW Britain (the Penarth and lower Lias Groups) comprises mudstone, sandstone and limestone strata deposited in a variety of marine to non-marine environments. Faunal and floral characteristics of these successions have led to the proposal that one location in SW England, St Audrie's Bay, should serve as the Global Stratotype Section and Point (GSSP) for the base of the Hettangian Stage and, thus, for the Triassic–Jurassic (Tr–J) boundary. The sections of SW Britain have also been used previously to infer sea-level change history and relate this to potential kill mechanisms associated with the Tr–J boundary mass extinction. Chemostratigraphic, biofacies and lithofacies data are used here to suggest alternative models of sea-level change in relation to possible Tr–J boundary horizons in the sections of SW Britain. A sea-level lowstand surface of erosion is inferred to occur within the Cotham Member of the Lilstock Formation, a unit deposited in an environment that was often subaerially exposed. In contrast to previous interpretations, the top surface of the overlying Langport Member (here inferred to be deposited on a carbonate ramp of depositional or tectonic origin) represents a drowning event of at least regional extent. All horizons regarded as plausible levels at which to place the Tr–J boundary based on fossil distributions lie within strata deposited during relative sea-level rise. However, it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and, additionally, there is no positive evidence that sea-level fall had any relation to the genesis of proposed Tr–J marker horizons. It is unlikely that sea-level fall played a significant role in the Tr–J boundary extinctions in either a local or a global context.

JURASSIC SEDIMENTARY AND TECTONIC PROCESSES AT MONTAGNA GRANDE (TRAPANESE DOMAIN, WESTERN SICILY, ITALY)

The Rosso Ammonitico of the Montagna Grande area is very interesting because of the great diversification in facies and thickness in three very closely situated sections. The Jurassic succession in this area is that typical of the Trapanese Domain. It starts with a thick pile of platform limestones (Inici Formation: Hettangian - Sinemurian) that are overlain by typically condensed and commonly nodular pelagic limestones (Rosso Ammonitico: Middle Jurassic - lowermost Cretaceous). The good exposure of this succession in active quarries allows observation of sedimentological and palaeontological details and to improve the understanding of the Jurassic tectono-sedimentary evolution. The Rocca chi Parra quarry shows a stepped surface, with a relief of some metres, incised in the Inici Formation. It is covered by lenticular or wedge-shaped lithosomes a few metres-thick of highly condensed Rosso Ammonitico. It is interpreted as a slide scar along which a thin but extensive block of lithified platform limestones was detached and slid downslope. The Poggio Roccione quarry shows neptunian sills and collapse structures in the middle part of the Rosso Ammonitico that on the whole is thicker (about 12 m) than at Rocca chi Parra. They indicate sea-floor instability probably due to seismic shocks with brittle to plastic behaviour of sediments depending on their coherence. The Montagna Grande outcrop shows an even thicker succession which includes a wedge of cherty limestones about 10 m thick intercalated between a lower and an upper Rosso Ammonitico calcareous unit. The features described in these three sections document a highly irregular sea-floor topography which in turn was controlled by several phases of extensional tectonics during Mid and Late Jurassic pelagic sedimentation. Structurally higher sectors closer to fault scarps were affected by a very condensed sedimentation, the opening of subvertical dykes and the triggering of large slides. Structurally lower sectors allowed preservation of thicker successions that were affected by gravitationally-induced soft to hard sediment deformation when tectonics resulted in an oversteepening of the slope.

STRATIGRAPHY AND TECTONIC AND METAMORPHIC EVOLUTION OF THE PORTO AZZURRO UNIT IN THE MONTE CALAMITA PROMONTORY (SOUTHEASTERN ELBA ISLAND, TUSCANY)

The Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and of the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the SE part of the Island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piedmontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dikes. Moreover, in this part of the Island, the relationships between the uplift of the plutonic bodies and the final deformations of the tectonic stack are well exposed. To improve the geological knowledge of SE Elba, the authors carried out a 1:10.000 geological survey of the Calamita Promontory (mostly made up of the Porto Azzurro Unit) and performed petrographic and meso-/micro-structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, likely pre-Carboniferous basement (Mt. Calamita Fm.), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt.Calamita Fm., five main lithofacies were recognized and mapped. In particular, garnet-bearing, albite micaschist (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); Porphyroids-like rocks (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of the lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello Geothermal Field, whereas the other lithofacies can be probably correletable with the ?Ordovician formations of the Tuscan Metamorphic Units. The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (Dx), recognized in the Mt.Calamita Fm., which is defined by pre- Alpine schistosity and mineralogic relics (garnet); b) two Alpine tectono-metamorphic folding events (D1 and D2) in the Greenschists facies, which deformed also the Mesozoic covers; c) a following folding event (D3) which probably occurred during or immediately after the strong thermometamorphic imprint (including the magnetite-rich skarn bodies), due to the Neogene magmatic intrusions; d) Subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt.Calamita Fm. and the Mesozoic cover) and between the latter and the overlying tectonic Units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch). A final weak antiformal folding (D4) of the whole promontory took place before the development of NW-SE and N-S trending high –angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphic, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that defined for the Larderello geothermal region. Thus, the Mt.Calamita area can be considered as a little older, but similar geological model for all the future interpretations of the deep structure of southern Tuscany crossed by the Crop 18 profile.

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

JURASSIC PALAEOGEOGRAPHY OF THE TRANSDANUBIAN CENTRAL RANGE (HUNGARY)

The Transdanubian Central Range (TCR) is a flattened range of hills in northern Transdanubia (Hungary), formed mainly by Mesozoic carbonate rocks showing strong facies similarities with the Southern Alps and the Austroalpine domain. The Jurassic system is divided into several formations of predominantly pelagic limestones. Ammonoids are frequent and were collected bed-by-bed in numerous sections, providing an excellent biostratigraphic resolution. The thickness of the Jurassic system is usually small but changes along the strike of the TCR. It reaches a maximum thickness of 500 m in the western part; is very variable (10-400 m) in the central segment (Bakony Mts.) and rather low (less than 100 m) in the east (Gerecse). In the Bakony segment, the thickness variation reflects the strongly dissected topography of the Jurassic sea-floor. Synsedimentary tectonics is dominated by normal faults; tilted blocks and listric faults may be inferred only in the east. Five main steps were identified in the palaeogeographic evolution: 1) Late Hettangian: carbonate oolitic shoals prevail, except for a few sites where non-deposition or neritic sediments occur. 2) Sinemurian and Pliensbachian: tectonic disintegration resulted in an intricate pattern of submarine horsts and intervening basins, with condensed sedimentation or non-deposition on the horsts and thicker, continuous sedimentary sequences in the basins. The submarine topographic highs are surrounded by aprons of redeposited material (scarp breccias, brachiopod coquinas, crinoidal calcarenites, spiculitic cherty limestones), while pure or argillaceous limestones (Rosso Ammonitico) prevail in the distal areas. 3) Early Toarcian: the Tethys-wide anoxic event is superimposed on the previous submarine bottom topography; the resulting black shales and sedimentary Mn-ores are concentrated on the western sides of some horsts. 4) Dogger to Early Malm: radiolarites with heterochronous lower and upper boundaries (Aalenian to Kimmeridgian) prevail, except for the top of some submarine topographic highs. The absence of uppermost Bathonian to Lower Oxfordian carbonates suggests that the whole TCR sunk below the CCD in those times. 5) Latest Jurassic: the uniform deposition of Rosso Ammonitico and Biancone in the Late Kimmeridgian and Tithonian is interrupted only in the Early Tithonian by local intercalations of scarp breccias and coarse biodetrital limestones. This is interpreted as the last manifestation of synsedimentary tectonic movements along the faults bordering the submarine horsts. Based on palaeogeographic similarities and analogies in Jurassic times, the TCR is visualized as the northern foreground of the Trento platform/plateau (lying north of the later Insubric lineament), where the block-tectonic disintegration and differential subsidence started earlier and resulted in a bottom morphology more dissected than in the South Alpine part of this west Tethyan passive margin.

Rocks of the Late Triassic and Early Jurassic age in the Carlisle Basin, Cumbria (north-west England)

SUMMARY The thickness and nature of the rocks of Late Triassic (Penarth and Lias groups) and Early Jurassic (Lias Group) age in a poorly exposed outlier west of Carlisle, Cumbria, have been proved in two recent boreholes. A third borehole proved that the Mercia Mudstone Group (Triassic) crops out, beneath superficial deposits, within an area previously mapped as Lias. Cores from the lower part of the Lias Group have yielded biostratigraphical evidence for the position of the base of the Psiloceras planorbis Zone and, consequently, of the Hettangian Stage and the Jurassic System. Almost 70 m of Lias Group rocks were penetrated above 13–14 m of rocks of the Penarth Group which is proved for the first time in this area. The borehole data, combined with seismic reflection data, suggest that the outlier is both more extensive and is located further west than was previously envisaged, and that it is fault bounded to the west, to the south, and to the north and north-east. However, to the east, the Penarth Group-Lias Group succession may crop out beneath superficial deposits.

The base of the Jurassic System: new data on the first appearance of Psiloceras planorbis in southwest Britain

AbstractRecent collecting in the Lower Lias of localities at St Audrie's Bay, north Somerset, Sedbury Cliff, Gloucester and Lavernock Point, South Glamorgan has yielded Psiloceras planorbis (J. de C. Sowerby) in beds considered by some to be of Rhaetian, Triassic age. These are the earliest records of this ammonite to date in southwest Britain. Evidence is provided of a major facies change at the top of the pre-planorbis beds followed by the diachronous first occurrence of Psiloceras planorbis. These new data have implications for the use of earliest occurrence of Psiloceras planorbis as a chronostratigraphic marker in defining the base of the Jurassic System, and for the definition of the base of the Hettangian Stage in the recently proposed candidate Global Stratotype Section at St Audrie's Bay.

St Audrie's Bay, Somerset, England: a candidate Global Stratotype Section and Point for the base of the Jurassic System

AbstractIn 1967 the Somerset coastline near Watchet was proposed as the type area of the basal (Planorbis) chronozone of the Hettangian Stage and thus of the Jurassic System. Neither at that time nor subsequently, however, has a type locality and section been nominated from those available in the area. There is urgent need to select a Global Stratotype Section and Point (GSSP) for the base of the Hettangian, and of the Jurassic System. The cliff forming the headland at the west side of St Audrie's Bay, three kilometres east of Watchet, Somerset, is here proposed as the type locality and section, with the base of the Hettangian Stage, at the base of the Planorbis Chronozone, being placed at the horizon currently recognized as that at which ammonites of the genusPsilocerasappear. In this section the base of the Planorbis Chronozone corresponds with the base of thePsiloceras planorbisBiozone. The proposal of this section is conditioned by the availability of comprehensive litho- and biostratigraphic information, and the ability of the section to fulfil International Commission on Stratigraphy (ICS) requirements for a candidate GSSP.

THE LATE TRIASSIC-HETTANGIAN BIVALVES TURNOVER IN LOMBARDY (SOUTHERN ALPS)

One of the major mass extinctions of the Phanerozoic took place at the Rhaetian-Hettangian boundary. According to some researchers, it might have been preceded by a minor event at the end of the Carnian. The faunal association present in the Late Trixsic formations of Lombardy and their changes related with the lithofacies changes are analysed here. In the Carnian Gorno Formation and Val Sabbia Sandstone, five bivalve assemblages have been distinguished. From the trophic analysis of these molluscs, we observe the infaunal suspension feeders predominance in the 1st and 2nd assemblage. They are gradually replaced by epibyssate species in the following associations, closely related with the lithofacies changes. In the overlying Norian Dolomia Principale, most bivalves disappear. They are replaced by species of Neomegaladon , Isognomon and gastropods. It is pointed out that this decline is not due to mass extinction, but to the settlement of different conditions in the basin which fitted better the euryhaline bivalve assemblages. In the overlying Riva di Solto Argillite, Zu Limestone and Conchodon Dolomite, 4 bivalve assemblage zones have been distinguished. Both, the new genera appearance and the number of species increase are closely linked to the substitution of calcareous facies of peritidal platform with argillaceous and arenaceous sequences of inner basin. In the Hettangian Sedrina Limestone, the fauna treated by Gaetani (1970), rich in bivalves, is examined. The diversity and the phyletic relationship of this species with those of the Rhaetian zones are pointed out. The diversity, the origination and extinction rate are calculated for the bivalve species of Lombardy and for the Megalodontidae and Dicerocardiidae revised by Vegh-Neubrandt (1982). Eventually, some morphologic changes in the phyletic lineage of megalodontids have been pointed out too. Concluding, faunal crisis like mass extinction is not recorded by the bivalve assemblages of the Late Triassic of Lombardy and Southern Alps in general. They reflect pseudoextinctions, sudden and severe turnover, certainly depending on the changing environmental conditions. The affinity among the late Rhaetic bivalves and those of the Hettangian demostates that the big faunal crisis at the Rhaetian-Hettangian boundary in Lombardy is traceable back to a rapid faunal turnover consequent to the rifting and sinking of the carbonate platform.

Stratigraphic correlation of presumed Milankovitch cycles in the Blue Lias (Hettangian to earliest Sinemurian), England

ABSTRACTSimple digital filtering methods allow the correlation of patterns of metre‐scale cycles between outcrop sections and well logs through the Hettangian to earliest Sinemurian part of the Lower Lias in southern England. The matching of cyclic patterns is sufficiently confident to allow the identification of a significant break in the Blue Lias section at Lyme Regis (Dorset). The standard ammonite zones of the Hettangian Stage are shown to be of quantifiably unequal duration.

Palynology and the triassic/jurassic boundary

Characteristic latest Triassic palynofloras are first developed in the Norian with the association of Camarozonosporites laevigatus, Chasmatosporites apertus, C. magnolioides, Cingulizonates rhaeticus, Densosporites fissus, Granuloperculatipollis rudis, Kyrtomisporis laevigatus, K. speciosus, Limbosporites lundbladii, Ovalipollis ovalis, Perinosporites thuringiacus, Rhaetipollis germanicus, Ricciisporites tuberculatus, Selagosporis mesozoicus, Semiretisporis gothae, Triancoraesporites ancorae, Zebrasporites interscriptus and Z. laevigatus. These forms persist into the Rhaetian and, in some instances, into the basal part of the overlying Jurassic. Palynofloral resolution of the Triassic/Jurassic boundary is largely based on the disappearance of typically late Triassic morphotypes; undoubtedly post-Triassic species, e.g. Cerebropollenites mesozoicus and C. thiergartii, however, are rarely conspicuous elements of the basal Jurassic palynofloras and may not become dominant elements until the late Hettangian or Sinemurian. A palynological definition of the Triassic/Jurassic boundary is further complicated by the lack of agreement on the legitimacy of the Rhaetian Stage, the location of the type section of the basal zone of the Hettangian Stage, and the uncertainty over the precise definition of the top Triassic/basal Jurassic within the stratotype sections proposed. In this context, the unsatisfactory nature of a palynologically defined Triassic/Jurassic boundary in northwest Europe is discussed.

Hettangian (Lower Jurassic) rocks and faunas, northern Yukon Territory

The Lower Jurassic Hettangian Stage is documented with certainty for the first time in the Canadian Arctic. It is represented by a basal Jurassic sandstone unit in the Bonnet Lake area of northern Yukon Territory. The two subzones of the Early Hettangian Planorbis Zone, i.e., the Planorbis Subzone and the Johnstoni Subzone, are indicated by poorly preserved Psiloceras sp. indet. and Psiloceras (Caloceras) cf. P. (C.) johnstoni (J. de C. Sowerby), respectively. The varied but poorly preserved bivalve fauna associated with P. (C.) cf. P. (C.) johnstoni includes Prosogyrotrigonia (?) sp. cf. P. inouyei (Yehara), Cardinia sp. cf. C. hybrida (J. Sowerby), C. sp. aff. C. concinna (J. Sowerby), Pleuromya (?) sp., Meleagrinella (?) sp., Oxytoma (Oxytoma) sp., and Parallelodon (?) sp. The bivalves closely resemble approximately coeval forms described from Japan. The above-mentioned faunas are figured as is a specimen of Psiloceras cf. P. erugatum (Phillips), which was previously described from the Hettangian of southern Yukon. Other occurrences of the Hettangian in Canada and Alaska are reviewed.