Middle Toarcian Research Articles

Une plate-forme en distension, témoin de phases pré-accrétion téthysienne en Afrique du Nord pendant le Toarcien–Aalénien (synclinal Iguer Awragh–Afennourir, Moyen Atlas occidental, Maroc)

Abstract A stratigraphic study and mapping allow defining the particular features of this area of the southwestern Middle Atlas (Morocco) during the Toarcian and the Aalenian. Numerous spectacular variations in thickness and facies are documented by the discovery of coralline build-ups dated to the Late Toarcian. They occur along the palaeostructures. Development of the structures occurred in several stages: (1) initial differentiation during the Early and Middle Toarcian; (2) homogenisation during the Late Toarcian, after the construction of the reefs. The Aalenian history is similar: differentiation (Early Aalenian)–homogenisation (Middle to Late Aalenian). Two palaeostructural trends are evident: SW–NE and transverse SSW–NNE to south–north. These caused separation of several subbasins, which alternatively deepened and shallowed, probably due to the tilting of several blocks to the southwest. To cite this article: F. El Hammichi et al., C. R. Geoscience 334 (2002) 1003–1010.

Jurassic foraminifera from the Queen Charlotte Islands, British Columbia, Canada: biostratigraphy, paleoenvironments and paleogeographic implications

Lower and Middle Jurassic (Sinemurian to Callovian) foraminifera were examined from 66 localities of the Queen Charlotte Islands, British Columbia, Canada. Over 200 species of well preserved benthic calcareous and agglutinated taxa belonging to 50 genera were identified. Foraminifera from the Lower Jurassic are represented by an abundant and diverse, Tethyan-derived calcareous assemblage, with Boreally derived agglutinated taxa becoming gradually more dominant in the upper Lower Jurassic to lower Middle Jurassic. Sedimentary strata and their faunal components and position of hiatuses correlate approximately with the Jurassic eustatic sea-level curve. Sea-level changes have resulted in a variety of depositional environments ranging from deep-water euxinic shales with abundant tuff beds, characterized by small pyritized foraminifera, to a well oxygenated outer shelf setting, characterized by a major faunal increase, to shoreface sandstones and conglomerates with absent to rare benthic foraminifera. A gradual shallowing from a slope- to a shelf-dominated deposition, with a magmatic arc provenance, is suggested for the Lower Jurassic, changing in the Middle Jurassic to an inner shelf to upper shoreface setting dominated by sedimentary strata of volcaniclastic origin. A marked faunal turnover is observed in the Lower Jurassic Whiteaves Formation (middle Toarcian) with the extinction of several earlier species and the appearance of a new diverse assemblage of 225 taxa. This turnover is also observed in the ammonite and radiolarian assemblages of the Queen Charlotte Islands. In comparison to the dominantly Tethyan-derived taxa of pre-middle Toarcian age, many of the species from the middle Toarcian are found in northern latitudes such as on the northern slope of Alaska, Arctic Islands and the Northwest Territories. Hence, foraminifera, in conjunction with existing radiolarian and ammonite data, suggest a northward displacement of the Queen Charlotte Islands from low to mid-paleolatitudes by late Early Jurassic (Toarcian) to early Middle Jurassic (Bajocian) time.

Les series du Toarcien-Aalenien du SW du Moyen-Atlas (Maroc); precisions stratigraphiques et signification paleogeographique

Abstract Toarcian and Aalenian deposits from south west of the Middle Atlas named "Couches du Seloum", were classically considered as a "border facies" of the Middle Atlas trough, which was extensively developed in the northern part of the folded Middle Atlas. Both studies realized on the border of the Bekrit-Timahdite syncline and the Iguer Awragh-Afennourir syncline, allow to specify the stratigraphy, the spatial distribution of deposits and to reconsider the paleogeographic organization of this "border" in the integral context of the northern Middle Atlas trough during the Toarcian and the lower Bajocian interval. A lithostratigraphic succession of the "Couches du Selloum" raised in the western part of Bekrit-Timahdite syncline shows the superposition of three units. Unit 1 is made of clay, reddish and gray marl, locally gypseous, assigned to the lower Toarcian. Unit 2 is essentially composed of a subnodular calcareous series, which consists of discontinuous, wavy and upward-thickening beds. The upper offshore is the most frequent environment. It is mainly made of crinoidal calcarenites, yielding some ammonites and brachiopods rich levels. The Middle Toarcian (Bifrons zone and Gradata zone) was identified in the lower part of this unit. Most of the upper Toarcian zones were distinguished (Speciosum zone, Meneghinii zone and Aalensis zone), they are represented in the upper part by brachiopods (Stroudithyris) and ammonites (Hammatoceras, Dumortieria and Catulloceras) rich set. On several lateral sections, the uppermost part of these pseudo nodular limestones is dated from Opalinum zone (levels with Leioceras and Tmetoceras). Unit 3 shows several facies: bioclastic calcarenites arranged in decimetrics beds, showing cross stratifications; microsparitic limestones in thick beds with silexites. The macrofauna is scarce, often represented by some crinoidal and mollusks fragments; some Zoophycos appear in the top bed of the set. The following deposits illustrate the evolution from the shoreface to the upper offshore. This unit is overlain by a sedimentary discontinuity, materialized by a boring surface, some pockets of accumulations with belemnites and a cover of ferruginous nodules. Ammonites drowned on the discontinuity surface or in the basis of the first interbedded marls which overlies this surface sign the Aalenian-Bajocian transition (top of the Concavum zone and the basis of Discites zone). Immediately above we found some index fauna of the Laeviuscula zone. The series overlaying the "Couches du Selloum" comprises thinning up alternations of limestone/marl, passing progressively to the "Marnes de Boulmane" Formation. Lateral variations and interpretations; SW to NE variations: The thickness and facies variations observed in the Bekrit-Timahdite syncline (C1 to C9), denote the existence of marine intervals generally more dilated, with some marly deposits developed towards the SW; on the contrary towards the NE, series are exclusively calcareous, reduced in thickness, and often condensed in their upper part. The same evolution is observed between the deposits of the Iguer Awragh syncline to the SW and those of Afennourir in the NE [Benshili et al., 1999]. SE to NW variations: Some observations realized in a Toarcian-Aalenian outcrop recently discovered in the sector of Outghalline (C11), shows that an uplifted area separates the marly basin of Bekrit from the Iguer Awragh one. Such variations denote a partition of the Toarcian Selloum basin, located in south west of the Middle Atlas trough, into several entities which are regrouped under the denomination of "Blocks of Selloum"; two NE-SW trending notably subsiding gutters are recognized, the Iguer Awragh-Afennourir and the Bekrit-Timahdite, separated by a shoal (ridge of Ijadrane). Paleogeographic framework of the SW border of the Middle Atlas trough: A synthesis of the geometry of the "Blocks of Selloum" and some paleogeographic and structural features of the NW border of the Middle Atlas trough, as they appear in a recent synthesis [Benshili, 1989; Elmi et al., 1989; Benshili et Elmi, 1994; Charriere, 1990; Fedan, 1993; Laadila, 1996; Sabaoui, 1998; El Arabi et al., 1999], is shown in 3 episodes from Toarcian to Aalenian. The southwestern part of the Middle Atlas studied here, does not appear in any case as a coastal area surrounding the Middle Atlas trough, but like a gulf occupied by marine deposits which is isolated from the Middle Atlas trough by the shoal areas called the "Haut-fond de Boulmane". In the Middle Atlas trough, the sequences prograde in a NE direction and show blocks tilting generally towards the same direction. On the other hand, in the "Blocks of Selloum" the tilting acts toward the SW, against the direction of the global Tethyan polarity.

Bioconstructions a spongiaires siliceux dans le Lias terminal du Bassin lusitanien (Portugal); stratigraphie, sedimentologie et signification paleogeographique

Abstract The Upper Liassic series in the western border of Iberia (Lusitanian Basin, Portugal), show an important lutitic sedimentation, characterized generally by a monotonous marl/limestone alternation. Small scale siliceous sponge mudmounds occur in these deposits from Middle Toarcian to Lower Aalenian age. The scope of this work is to pinpoint the stratigraphical and sedimentological context and to characterize controlling factors of the spongioliths. Stratigraphic and facies analysis. Relevant sections were observed and investigated in different locations of the Lusitanian Basin (e.g., Alvaiazere, Porto de Mos, Rabacal, Coimbra and Cantanhede) (fig. 1). The siliceous sponge facies correspond to the upper part of the S. Giao Unit and to the lower part of the Povoa da Lomba Unit (fig. 2). Considering the sequential scheme of Duarte [1997], the sediments correspond to groups of third-order depositional sequences MST3 and MST4 (mainly in the upper part of this sequence: MST4B). The sedimentary evolution of these units shows a stacking pattern composed of shallowing upward sequences deposited in an outer homoclinal ramp setting, dipping northwestwards. Both units increase in thickness from south to north (fig. 3) and their vertical facies associations correspond to a very bioturbated (Chondrites, Zoophycos, Planolites and Thalassinoides) marl/limestone succession (figs. 4 and 5). MST3 is demonstrably more marly than MST4B. The base of MST4 [MST4A in Duarte, 1997] corresponds to a marl/marly limestone alternation, very poor in siliceous sponge mudmounds. The first unit (MST3) which includes sponge mudmounds is dated as uppermost Bifrons zone through the base of the Bonarellii zone. The majority of the siliceous sponge mudmounds occur within this time slice. These mounds are characterized by a great diversity of accompanying fauna mainly composed of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves. The initial growth of the sponge build-ups can be correlated basin-wide to the intra Bifrons regional flooding surface (MST2/MST3 boundary). The second unit (MST4), particularly its upper part (MST4B), corresponds to the top of the Meneghinni-Opalinum interval and is related to a carbonate progradational phase. In the eastern part of the basin, the calcareous facies of MST4B are more bioclastic. Siliceous sponge mudmounds. The Toarcian mudmounds of the Lusitanian Basin are usually only a few decimetres thick and most display irregular knob-like to flat lenticular morphologies. Some build-ups are round and can reach 1,5 metres in thickness and ten metres in diametre. Also worth mentioning is a siliceous sponge biostrome developed at the base of MST3 in the Porto de Mos section (figs. 3 and 6). The upper mound surface is normally rough and uneven. In both sequences they are always related laterally with carbonate beds, which corresponds to the top of fourth order sequences. The mudmounds consist of mostly brownish iron-rich calcified siliceous sponges and a greyish, sometimes peloidal allochthonous micritic matrix. In general, the sponges themselves consist of dense leiolitic microbolites [automicrites sensu Reitner and Neuweiler, 1993]. The sponge spicules are diagenetically transformed into calcite. The great majority of the sponge specimens belong to the Hexactinosa (Class Hexactinellida) and are unknown and undescribed to date. "Lithistides" (polyphyletic desma-bearing demosponges) are very rare and only occur as forms encrusting Hexactinosan sponges. The benthic macrofauna is abundant and consists of monospecific crinoids, rhynchonellids, terebratulids and bivalves (mainly pectinids and ostreids). Encrusting organisms are serpulids, bryozoans and foraminifera, as well as "Lithistids" mentioned above. They are entirely restricted to the stratonomical surfaces of the siliceous sponges. The sponge bioherms consist of several microfacies types (wackestones, packstones, floatstones and boundstones). All of them are micrite dominated and represent low energy environments. They differ mainly in the amount of siliceous sponges, micrite, microbialites and the accompanying fauna. Palaeoenvironmental significance. The amount of microbial induced carbonate clearly mirrors the importance of microbial activity in respect of the reef building potential. Furthermore, three other controlling factors played an important role in the initiation of the siliceous sponge mudmounds of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation rate. The role of the first two factors is evident because the siliceous sponge mudmounds are particularly important (abundance and volumetric expression) in the eastern part of the basin (Rabacal-Alvaiazere region). They are practically absent towards the west (essentially in MST4B) where the series show hemipelagic sedimentologic features (figs. 7 and 8). Reduced sedimentation rate is a precondition for the settlement of siliceous sponges and Hexactinosa in particular. Compared to all other Toarcian sequential units, MST3 and MST4B are the thinnest and therefore reflect the lowest sedimentation rates (fig. 8).

Calibration of early–middle Toarcian nannofossil events based on high-resolution ammonite biostratigraphy in two expanded sections from the Iberian Range (East Spain)

This work deals with refined nannofossil biostratigraphy in two well-documented lower–middle Toarcian sections, in the Iberian Range. The sampled shallow carbonate ramp deposits belong to the Turmiel Fm. and consist of an alternation of marls and limestones. Both lithologies yield common and well-preserved ammonite assemblages, which provide a refined ammonite biostratigraphic framework. A significant calcareous nannofossil composition change has been recognized within the Serpentinus Zone, and a useful succession of nannofossil events has been calibrated to the ammonite biostratigraphy. This succession includes: FO of Lotharingius sigillatus (Mirabile Subzone); LO of Mitrolithus jansae (Semicelatum Subzone); FO of Carinolithus cantaluppii (Strangewaysi Subzone); LOs of Biscutum grande and Biscutum finchii (lower part of the Serpentinus Zone); FO of Carinolithus superbus (Strangewaysi–Falciferum boundary); FO of Lotharingius velatus (Falciferum Subzone); FOs of Discorhabdus striatus and Discorhabdus ignotus (Serpentinus–Bifrons boundary) and “acme end” of Lotharingius hauffii (Variabilis–Illustris boundary). In the studied sections, the composition changes, the biohorizons recognized and their succession are closely comparable with other early–middle Toarcian Boreal sections.

New U–Pb zircon ages integrated with ammonite biochronology from the Jurassic of the Canadian Cordillera

The accuracy of published Jurassic time scales is compromised by a paucity of sufficiently accurate and stratigraphically well-constrained isotopic dates. U–Pb dating of volcaniclastic rocks within fossiliferous marine successions is a method suitable of providing additional useful calibration points. Stikinia and Wrangellia, two of the major accreted terranes of the Canadian Cordillera, comprise volcanosedimentary assemblages of magmatic arc origin. Within them we selected sections from which we report 14 new U–Pb zircon ages that are integrated with ammonite biochronology at the zonal level. Poor zircon recovery and complex U–Pb systematics made it difficult to obtain precise ages for many of the samples. The following ages were determined (with 2σ errors): 194.0+9.1–1.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian), 191.5 ± 0.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian) to the Whiteavesi Zone (lower Pliensbachian), 185.6+6.1–0.6 Ma from the Freboldi Zone (lower Pliensbachian), 184.7 ± 0.9 Ma from the Kunae Zone (upper Pliensbachian), 180.4+8.0–0.4 Ma from the Kanense to Planulata zones (lower to middle Toarcian), 179.8 ± 6.3 Ma from the Yakounensis Zone (upper Toarcian) to the lower Bajocian, 167.2+10.5–0.4 Ma from the Rotundum Zone (upper Bajocian), 158.2+1.9–0.4 Ma from the upper Bathonian, and 162.6+2.9–7.0 from the upper Bathonian to lower Callovian. Five other dates are inferior in precision, but still provide some constraints for time-scale calibration. The U–Pb dates reported here together with a wealth of other recently obtained Cordilleran isotopic dates will be used for a significant revision of the Jurassic time scale.

New U–Pb zircon ages integrated with ammonite biochronology from the Jurassic of the Canadian Cordillera

The accuracy of published Jurassic time scales is compromised by a paucity of sufficiently accurate and stratigraphically well-constrained isotopic dates. U–Pb dating of volcaniclastic rocks within fossiliferous marine successions is a method suitable of providing additional useful calibration points. Stikinia and Wrangellia, two of the major accreted terranes of the Canadian Cordillera, comprise volcanosedimentary assemblages of magmatic arc origin. Within them we selected sections from which we report 14 new U–Pb zircon ages that are integrated with ammonite biochronology at the zonal level. Poor zircon recovery and complex U–Pb systematics made it difficult to obtain precise ages for many of the samples. The following ages were determined (with 2σ errors): 194.0+9.1–1.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian), 191.5 ± 0.8 Ma from the Plesechioceras? harbledownense Assemblage (upper Sinemurian) to the Whiteavesi Zone (lower Pliensbachian), 185.6+6.1–0.6 Ma from the Freboldi Zone (lower Pliensbachian), 184.7 ± 0.9 Ma from the Kunae Zone (upper Pliensbachian), 180.4+8.0–0.4 Ma from the Kanense to Planulata zones (lower to middle Toarcian), 179.8 ± 6.3 Ma from the Yakounensis Zone (upper Toarcian) to the lower Bajocian, 167.2+10.5–0.4 Ma from the Rotundum Zone (upper Bajocian), 158.2+1.9–0.4 Ma from the upper Bathonian, and 162.6+2.9–7.0 from the upper Bathonian to lower Callovian. Five other dates are inferior in precision, but still provide some constraints for time-scale calibration. The U–Pb dates reported here together with a wealth of other recently obtained Cordilleran isotopic dates will be used for a significant revision of the Jurassic time scale.

A UPb age from the Toarcian (Lower Jurassic) and its use for time scale calibration through error analysis of biochronologic dating

The Early Jurassic is the most poorly constrained part of the geochronologic scale within the Mesozoic, owing to the scarcity of calibration points. We report a new UPb zircon age of 181.4 ± 1.2 Ma (2σ) from an ash layer within the type section of Crassicosta Zone, a North American regional standard ammonite zone in the Middle Toarcian. For time scale calibration, the isotopically dated level needs to be tied to the Jurassic chronostratigraphic framework established in northwest European successions. Three major contributing factors (difference between local observed and true ranges, uncertainties of correlation, and taxonomic noise) are assessed to determine the maximum biochronologic dating error. The local ammonite distribution data are robust in that 95% confidence level extensions of observed ranges of common ammonite taxa would not significantly modify the zonal assignment. The computer-assisted unitary association (UA) method is used for world-wide biochronologic correlation. Representative North and South American, western Tethyan, and northwest European sections were selected to establish the global maximum ranges for 103 Middle Toarcian ammonite taxa and a sequence of 40 UA. Maximum permissible correlatives of the Crassicosta Zone lie between the northwest European standard Semipolitum Subzone (late Bifrons Zone) and Bingmanni Subzone (early Thouarsense Zone). In particular, the dated tuff layer cannot be older than the Semipolitum Subzone or younger than the Illustris Subzone (Variabilis Zone). Taxonomic inconsistencies are present in the source data but their effect in corrupting UA-based correlation is shown to be negligible. The precise UPb age (2 σ error < ± 1%) that is constrained at the (sub)zonal level furnishes a useful new time scale calibration point. It warrants adjustments to previous Toarcian stage boundary age estimates and allows a significant reduction of their associated uncertainties.

The Early Toarcian Anoxic Event in the Westernmost Tethys (Subbetic): Paleogeographic and Paleobiogeographic Significance

A positive excursion reveals an oceanic anoxic event in the Subbetic, corresponding to the westernmost end of the Tethys during the Early Jurassic. This positive excursion exists in five stratigraphic sections consisting of Lower and Middle Toarcian pelagic sediments. The positive excursion in the Subbetic is located in the upper part of the Early Toarcian, serpentinus ( falciferum) biozone, while in other Tethyan regions (Alps, Apennines, and Dinarides) it has been found in the lower part of the same biozone. The most significant positive excursion detected in the Subbetic is therefore about half a million years younger, although other relative peaks had been detected. This isotopic anomaly can be correlated with a maximum transgression that occurred earlier in the regions closer to the open sea of the Tethys ocean. Other features of the anoxic event (faunal crises and black-shale deposits), appear at different ages, related to the activity of the transgressive phenomenon that occurred through the Early Toarcian.

Equatorial origin for Lower Jurassic radiolarian chert in the Franciscan Complex, San Rafael Mountains, southern California

Lower Jurassic radiolarian chert sampled at two localities in the San Rafael Mountains of southern California (∼20 km north of Santa Barbara) contains four components of remanent magnetization. Components A, B′, and B are inferred to represent uplift, Miocene volcanism, and subduction/accretion overprint magnetizations, respectively. The fourth component (C), isolated between 580° and 680°C, shows a magnetic polarity stratigraphy and is interpreted as a primary magnetization acquired by the chert during, or soon after, deposition. Both sequences are late Pliensbachian to middle Toarcian in age, and an average paleolatitude calculated from all tilt‐corrected C components is 1° ± 3° north or south. This result is consistent with deposition of the cherts beneath the equatorial zone of high biologic productivity and is similar to initial paleolatitudes determined for chert blocks in northern California and Mexico. This result supports our model, in which deep‐water Franciscan‐type cherts were deposited on the Farallon plate as it moved eastward beneath the equatorial productivity high, were accreted to the continental margin at low paleolatitudes, and were subsequently distributed northward by strike‐slip faulting associated with movements of the Kula, Farallon, and Pacific plates. Upper Cretaceous turbidites of the Cachuma Formation were sampled at Agua Caliente Canyon to determine a constraining paleolatitude for accretion of the Jurassic chert sequences. These apparently unaltered rocks, however, were found to be completely overprinted by the A component of magnetization. Similar in situ directions and demagnetization behaviors observed in samples of other Upper Cretaceous turbidite sequences in southern and Baja California imply that these rocks might also give unreliable results.

New occurrences ofLeukadiellaandParoniceras(Ammonoidea) from the Toarcian (Lower Jurassic) of the Canadian Cordillera

Previous studies of the Toarcian of the North American Cordillera have mentioned the rare occurrence ofParonicerasin the Queen Charlotte Islands. Recent work has identified the presence ofLeukadiellain the Middle Toarcian of the Queen Charlotte Islands, the Spatsizi area, and the Hazelton area. They occur withRarenodia planulata, Peronoceras pacificum, Peronoceras verticosum, andPhymatocerascf.P. pseudoerbaense.TheLeukadiellaspecimens are well preserved and generally larger than those found in the Mediterranean region. Taxa present in North America includeParoniceras sternale, Leukadiella ionica, Leukadiella amuratica, Leukadiellaaff.L. helenae, andLeukadiellaaff.L. ionica.MorphologicallyLeukadiellais closely related to such genera asHildaitesandHildocerasand is more suitably placed within the subfamily Hildoceratinae rather than the Bouleiceratinae. The distribution ofLeukadiellaandParonicerasindicates the influence of the Hispanic Corridor linking western Tethys and the eastern Pacific during the Middle Toarcian.

Hummocky cross-stratifications and trace fossils in the Middle Toarcian of some sequences of Umbria-Marche Apennines

Some Toarcian sequences of the Umbria-Marche Apennines show variable thicknesses of marly deposits rich inorganic material (MS, Marne del M. Serrone Formation, Late Domerian — Middle Toarcian) and well-oxygenated, red and nodular marly limestones (RAUM, Rosso Ammonitico Umbro-Marchigiano, Middle/Late Toarcian). In the MS, gravity flow deposits and turbidites are locally abundant. The MS/RAUM transition, instead, exhibit sharpbased hummocky cross-stratified (HCS) deposits, that, in the Middle Toarcian, range in thickness from several centimeters to 60 cm, and show wavelengths mostly from 70 to 140 cm. A complete sequence of HCS deposits consists of three divisions: I) a lower, massive or slightly graded bivalve and echinoderm lag, (0.5 to 40 cm thick) that is generally erosive, coarse-grained and discontinuous and probably represents proximal materials, deposited during the high-energy phase of a storm event (basal lag); II) a middle, well-sorted and matrix-poor calcisiltitic division (10 to 60 cm thick), that exhibits a well preserved hummocky cross-stratification; III) an upper calcilutitic deposit (from 0.5 to 20 cm), with pervasive bioturbation in the upper part of the bed. Division III (often absent) probably reflects the waning-energy phase of depositional event. A vertical transition from turbidites at the Domerian/Toarcian transition to sharp-based HCS deposits in the upper part of the sequences (Middle Toarcian), seems to indicate a regressive trend in the investigated Umbria-Marche sections. In fact, according to literature, HCS beds were deposited generally about or above a storm wave base. At this depth the storm waves suppressed the Bouma divisions and formed HCS under a combined (oscillatory/unidirectional) flow regime. Highly packed concentrations of concave bivalve shells, showing shelter-porosity (winnowed beds) and abraded tests of benthic foraminifers seem to indicate a dominant near-bottom oscillatory flow regime below the fairweather wave base level. A shallowing-upward trend in the investigated sections from the Early Toarcian to the Middle Toarcian, as testified also by microfaunal content, seems to be due to tectonics and/or sea-level fall and ranges from an upper bathyalouter shelf in the Early Toarcian to a middle shelf in the Late Toarcian. At this last time trace fossil assemblages increase in abundance, diversity and penetration depth at the top of HCS beds and in normal basinal sediments, in relation to increased oxygen availability and to favourable substrate conditions.

Palaeoenvironment of the Jurassicsystem in the Western and Central Tethys (Toarcian, Callovian, Kimmeridgian, Tithonian): An overview

Palaeoenvironment maps of Western and Central Tethys (Central America to India) have been established for Middle Toarcian, Middle Callovian, Early Kimmeridgian and Late Tithonian times. They will be extended to Australia and Indonesia. Using these maps an attempt is made to recognize the main environmental parameters. Four have been evidenced (1) latitude positioning which mainly controls temperature and illuminations (2) morphology of seas and oceans, (3) hydrodynamics of surface and deep marine currents, (4) presence of new organisms rock-forming species. The feasibility of modelling is evaluated.

An ammonite zonation for the Toarcian (Lower Jurassic) of the North American Cordillera

This is the second in a series of papers intended to establish a Lower Jurassic ammonite zonation that takes into account the biostratigraphic and biogeographic peculiarities of the North American succession. In North America the lower boundary of the Toarcian is drawn at the first appearance of Dactylioceras above the last occurrence of Amaltheus and Fanninoceras. The lower Toarcian is represented by the Kanense Zone; the middle Toarcian by the Planulata and Crassicosta zones; and the upper Toarcian by the Hillebrandti and Yakounensis zones. Section 5 on the Yakoun River in the Queen Charlotte Islands is designated the stratotype for the Planulata, Crassicosta, and Hillebrandti zones; section 3 on the Yakoun River is designated the stratotype for the Yakounensis Zone; an ideal stratotype for the Kanense Zone is not presently known. Reference sections further illustrating the faunal associations that characterize the zones are designated in eastern Oregon (Snowshoe Formation) and northern British Columbia (Spatsizi Group). The Dactylioceratidae, Harpoceratinae, and Hildoceratinae provide the most important zonal indicators for the lower Toarcian; Dactylioceratidae, Phymatoceratinae, and Bouleiceratinae for the middle Toarcian; and Phymatoceratinae, Grammoceratinae, and Hammatoceratinae for the upper Toarcian. Phymatoceras hillebrandti is described as a new species.

First Data on Clay Mineral Assemblages and Geochemical Characteristics of Toarcian Sedimentation in the umbriamarche basin (central italy)

AbstractA moderate to high rate of clay sedimentation is characteristic of the Toarcian sequences in the Umbria-Marche basin. The clay mineral assemblages and geochemical characteristics indicate that the Marne di Monte Serrone Formation and the Rosso Ammonitico Umbro-Marchigiano Unit were deposited in a shallow marine environment. In this general palaeogeographic scheme, a reducing subenvironment must have exsisted in which black shale-type facies were deposited resulting in geochemical anomalies in As, Sb, Zn, Co, Cu, Pb, V, Cr, and Ba, among other elements. The high values of both the detrital index and the Ce/Ce* ratio reveal the influence of proximal emerged reliefs. It is suggested that the palaeosoils that developed on the Liassic carbonate Laziale-Abuzzese platform were the source area of these sediments, following a palaeogeographic scheme analogous to that proposed for the Betic Cordilleras (Spain) during the Middle Domerian to Middle Toarcian. The positive and negative Eu anomalies are due to the decisive influence of the weathering process in which sediments with heterogeneous Eu anomaly size were mixed. The final distribution pattern of REE is the result of the different environments to which the clay minerals were subjected and the differences in intensity of weathering.

Croissance et miniaturisation de quelques Hildoceras (Cephalopoda) en liaison avec des environnements contraignants de la Téthys toarcienne

The stratigraphy of the Bifrons Zone (Middle Toarcian) has been well established on the NW margin of the Tethys as well as on the southern rim, especially in Italy and North Africa. In these areas, the zonal succession is based on the evolution of the Hildoceras lineage. The general evolutionary trend concerns the appearance and the position of the lateral groove, but some specimens present ornemental and dimensional particularities which are obviously linked to the environmental conditions of the locality where they have been collected. It is especially the case with Italian and Algerian localities which are situated on the bottom of small basins (ombilics) resulting from the splitting of the South Tethyan platform by tilted-blocks tectonic. In these localities, Hildoceras, present a remarkably reduced size compared with the size of their, NW-tethyan equivalents. Ontogenesis has been evaluated through septal spacing of the shells. This criterion seems to have recorded growth rate variations, between several individuals or during the life of one individual, more accurately than classic dimensional criteria (such as diameter). At least two models of growth have been recognized depending on the stratigraphic age. Moreover, they have evidenced that miniaturized ammonites coming from ombilic bottoms showed the same typical growth than the other representants of the species. In fact, their growth was affected by chronological alteration of the development, such as paedomorphosis, resulting in an abridged life cycle which is probably a better strategy to such unfavourable, unstable, isolated environments. So, it would appear to be a consequence of the constraining conditions prevailing in the ombilics which would have influenced some particularly malleable parameters of the shell, such as septal spacing.

The disappearance of the “Ammonitico Rosso”

Results of the research for the reconstruction of the Tethys paleoenvironments show that the Ammonitico Rosso disappeared in the late Berriasian. No typical analogues have been recorded so far in younger levels. The Ammonitico Rosso basically developed during the rifting of the continental margins pointing up the phases of the Tethys opening. It is found on tilted-block tops and slopes as well as in the related basins. The Ammonitico Rosso developed especially in areas characterized by a complex paleogeography such as Apulia or southern Iberia. Few occurrences are reported from the central Atlantic margins, although the information is scarce and only available by Deep Sea Drilling cores. The maps 1:20,000,000 of middle Toarcian, early Kimmeridgian and late Tithonian paleoenvironments of the Tethys illustrate the gradual disappearance of this characteristic facies. This disappearance in the Berriasian is correlated with: (1) the increase of biogenic (e.g., the bloom of nannoplankton) and clastic sediment supply (development of flysch facies); (2) the establishment of an oceanic circulation, which was not suitable for the AR formation, from the Himalayan branch of the Tethys to the Caribbean; (3) the end of the rifting of the western Tethys continental margins, around Late Jurassic-Early Cretaceous times, which began to sink because of thermal subsidence; (4) the evolution of oceanic chemistry. However, these factors were controlled by the global paleogeographic evolution. The Tethys ocean occupied a triangle-shaped area bounded by huge continental masses situated in roughly tropical latitudes. The gradual fragmentation of Pangaea and Cretaceous-Tertiary continental collisions definitely modified the global paleogeography never again producing this kind of ocean.

Hummocky cross-stratified deposits and turbidites in some sequences of the Umbria-Marche area (central Italy) during the Toarcian

Some sections of the Umbria-Marche basin (central Italy) were characterized during the Toarcian by alternately thicker and thinner organic-rich marls and well-oxygenated, red and nodular marly limestones. These sequences exhibit sharp-based hummocky cross-stratified (HCS) deposits and turbidites, ranging in thickness from several centimetres to 200 cm. Palaeocurrent measurements of HCS beds and turbidites indicate the same SE-NW direction. HCS deposits, that show wavelengths mostly from 70 to 140 cm, are grainstones and matrix-poor packstones. A complete sequence consists of three divisions. (a) A lower, coarse-grained bivalve and echinoderm lag, from 0.5 to 40 cm thick. The lag is generally erosive and discontinuous and probably represents a proximal material deposited during a high-energy phase of a storm event. (b) A middle, well-sorted calcarenite/calcisiltite division (from 20 to 100 cm thick), that exhibits well preserved hummocky cross-stratification. Bioclastic calcareous sands transported by offshore currents from storm surges were organized into HCS beds under an oscillatory/unidirectional flow regime near the bottom. (c) An upper, oscillatory ripple-bedded calcisiltite division from 0.5 to 25 cm thickness. Ripples were probably formed during a dominant oscillatory regime (wave action?) after the sand sedimentation had stopped. Locally a pervasive bioturbation is common in the upper part of this bed. Turbidites (10–200 cm thick) are packstones and rudstones with abundant muddy matrix and generally show the classical Bouma BC sequence. The lower and middle interval of the studied sequences, from the late Domerian to the early Toarcian, is dominated by fining-upward turbidites. On the contrary, the upper part of the sequence, middle/late Toarcian in age, is dominated by sharp-based HCS beds. The vertical transition from turbidites to sharp-based HCS deposits is indicative of a general regressive trend. The causes of the sea-bottom depth changes can be related respectively to a sea-level rise documented for the early Toarcian and to the sea-level fall of the middle Toarcian. Lateral and vertical distribution, frequency, grain size, structures and composition of detritic beds and microfaunal assemblages confirm the differentiated topography of the Umbria-Marche basin during Toarcian time.

Le premier squelette de Plesiosaurede France sur le Causse du Larzac (Toarcien, Jurassique inférieur)

Features observed in the skull, girdles and limbs of the Plesiosaur recently discovered at Tournemire (Aveyron) allow its assignment to the genus Plesiosaurus Conybeare, 1821 with the systematic content of Persson. The specimen exhibits a novel association of anatomical features and is so regarded as the type of a new species: Plesiosaurus tournemirensis nov. sp. Discovered in Upper Toarcian Beds, the species could be a descendant of P. brachypterygius V. Huene from the Middle Toarcian of Holzmaden (Western Germany).

Lenticulina [Marginulinopsis] pennensis foraminifère nouveau du Toarcien de l'Europe occidentale

A new species of free Foraminifera (Nodosariidae)Lenticulina [Marginulinopsis] pennensis occurs in many middle Toarcian exposures of the west European area. This species is distinguished by the thickness of the middle part of the sides and the compression of the marginal edges.