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Toarcian-Aalenian ammonite faunas from the western Middle Atlas (Morocco): controls of their distribution

El Hammichi F., Benshili K., Elmi S.

Volumina Jurassica

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2006

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Vol. 4, no. 1

169-169

EN

The SW part of the Moroccan Middle Atlas represents an area where the ammonite faunas are often abundant either in outer bioclastic and ferruginous beds or in outer distal platform marly limestones (Mibladéne Fm. and Al Yabés Fm.). Sedimentary discontinuities are frequently marked by condensed (bioclastic, ferruginous and encrusted limestones). The biostratigraphy is based on the distribution of 120 species, 46 genera and 7 families. The ammonite succession fits well with the West Tethyan standards. However, there are noticeable differences between the faunal communities of the diverse sectors (“subbasins”) of the studied region. This differentiation is largely due to a palaeostructural partitioning provoked by extensional tectonic movements that strengthen or lessen the eustatic variations. The faunas are largely cosmopolitan at the exception of those of the Gradata and Bonarellii zones having a strong South-Tethyan character with Collina, Collinites, Telodactylites and Furloceras to the lack of Haugia, Brodieia, Grammoceras, Esericeras and Hudlestonia. The main qualitative turnover occurs at the beginning of the Gradata Zone and it is followed by a near complete segregation of faunal assemblages. This well known scenario is complicated by local to regional conditions linked both to environments, tectonics and, perhaps, climates. For example, Phylloceratina are rare, but Lytoceratina can be abundant, although these two groups present great variations in proportion of their occurrence to the entire fauna (37% in the condensed sections; 22% in the expanded successions). These data must be compared with the percentages of Phylloceratina and Lytoceratina in other Tethyan localities: 20% on the neritic seamounts of Sicily and of the Southern Alps; 30% on their borders; 70% in small and strongly subsiding subbasins (umbilicus) as well as at some levels of the Djebel Nador ammonitico rosso (Western Algeria) Hammatoceratids of the Aalensis and Opalinum zones are strongly similar to those of Sicily and Southern Alps. Presence of involute and acute Pseudaptetoceras klimakomphalum (Vacek) and P. christianae (Elmi & Mouterde) documents the seaways between the Alpine Tethys and the peri-Atlantic basins (Portugal).

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Tectonically active Aalenian in the Northwestern Iberian Basin (Spain)

García-Frank A., Ureta S., Mas R.

Volumina Jurassica

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2006

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Vol. 4, no. 1

42-42

EN

The Alpine NW Iberian Range (N Spain), formed due to the tectonic inversion of the Iberian Basin. Four major episodes constituted the extensional development of the Iberian Basin during the Mesozoic: Permo-Triassic rifting; Jurassic post-rift subsidence; Upper Jurassic - Lower Cretaceous rifting; and Upper Cretaceous post-rift subsidence. Thus, the Aalenian in the Iberian Range was considered as a tectonically inactive period. However, the Aalenian in Northern Europe has been interpreted as a period of general regional regression, related to tectonic events. The Aalenian-Bajocian succession from the Northern Iberian Range is related with periods of low sedimentation rates and prolonged submarine exposure. The stratigraphic sequences defined in this area, highlight a progressive change in the sedimentary conditions of the basin. These sequences are established as precisely dated discontinuity-bounded sequences, defined by ammonoids. The complex depositional architecture of the basin, recognized on the basis of changes in isopach maps, is interpreted to be tectonically controlled, containing areas, ith low and high topographic platforms as a result of semigraben structures. Thus, two main paleogeographic settings are proposed for the studied region. The NW-SW-SE area is characterized by condensed sedimentation with five discontinuity-bounded sequences. The NE area is characterized by more expanded sedimentation without major discontinuity bounding sequences (except at the base of the succession) and with 10 coarsening-upwards sequences. The NE area corresponds to a different sedimentation setting, with more affinity with coeval rocks in the Cantabrian Range than those in the Iberian Range. In addition, the volcanism in nearby areas of the Iberian Basin within this period, is interpreted to be related to a crustal stretching phase. Tectonic instability during this period has been previously underestimated, although rarely documented in southern parts of this basin, and elsewhere in the Iberian Peninsula. Furthermore, to reinforce the synsedimentary tectonic activity, hydrothermalism related to exhalative events is recognized in the transitional beds of the Upper Toarcian - Lower Bajocian. All these data suggest that the Iberian Basin developed in a distensive context, and that the transition between Lower and Middle Jurassic was a period in which a certain tectonic instability is recognised.

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Stratigraphic distribution of brachiopods in the Lower Jurassic – Middle Jurassic transition of the Lusitanian Basin (Portugal) and a proposal of biozonation

Andrade B., Joral F., Henriques M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

143-143

EN

A detailed revision of the brachiopods of the Lower-Middle Jurassic transition in the Lusitanian Basin (Andrade 2006) has enabled the establishment of the stratigraphical distribution of this fauna. More than 2,000 specimens were collected at 11 sections throughout the basin, including the Bajocian GSSP in Murtinheira (Cabo Mondego). In all, 24 species, belonging to 14 genera, have been recognized along a stratigraphical interval that includes the Upper Toarcian, the Aalenian, and the Lower Bajocian. The Toarcian associations are characterized by species also recorded in neighbouring basins, such as Stroudithyris stephanoides, Sphaeroidothyris vari, Pseudogibbirhynchia bothenhamptonensis and Soaresirhynchia renzi; as well as species endemic to the Lusitanian Basin, such as Choffatirhynchia alcariensis, Nannirhynchia delgadoi, N. cotteri, Praemonticlarella conimbriguensis, Neozeilleria duartei and Pamirorhynchia(?) jorali. This mixed palaeobiogeographical character persists in the Aalenian, in which the associations include, together with widely distributed species such as Neozeilleria anglica, Pseudogibbirhynchia mutans or Lophrothyris withingtonensis, other species known in neighbouring basins, such as Sphaeroidothyris uretae and Neozeilleria sharpei, and other species recorded only in the basin, such as Soaresirhynchia minor, S. murtinheirensis and Sphaeroidothyris henriquesae. In the Lower Bajocian, excluding Loboidothyris perovalis, only endemic species are present (belonging mainly to endemic genera), such as Lusitanina bituminis, Stroudithyris choffati, Lusothyris atlantica and Mondegia limica. The interpretation of these distributions also enables to propose a brachiopod based biozonation for the studied interval. Three zones have been erected: 1. the Renzi Zone, for the Upper Toarcian, with two subzones: Renzi and Duartei; 2. the Anglica Zone, that ranges from the Aalensis Biochronozone of the Toarcian to the base of the Bajocian. It has been subdivided in 3 subzones: Nuskae, Anglica and Uretae; 3. the Choffati Zone, which comprises the main part of the Discites, Laeviuscula and Sauzei biochronozones, with two subzones: Bituminis and Limica; 4. this proposal of biozonation can be correlated with other established in neighbouring basins, such as the Iberian Range in Spain or the French Basins.

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Evolution of Graphoceratidae and the Aalenian/Bajocian boundary at Bearreraig, Isle of Skye, NW Scotland

Morton N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

196-196

EN

The GSSP for the base of the Bajocian Stage, and therefore the Aalenian/Bajocian boundary, has been defined at Cabo Mondego, western Portugal and ratified by IUGS. It was the first of the Jurassic Stages to be so defined. At the same time an Auxiliary Stratotype Point (ASP) at Bearreraig, Isle of Skye, NW Scotland was ratified by IUGS. The key marker event for the Aalenian/Bajocian boundary is evolution within the ammonite family Graphoceratidae. The original intention was to use as marker the first appearance of the genus Hyperlioceras (s.l. to include Toxolioceras and other mainly microconch “genera”) which evolved from the genus Graphoceras. However, in the light of detailed study of successions in various areas of western Europe and North Africa, this was modified. The earliest horizon of Hyperlioceras (H. incisum) proved to be too limited in distribution to be useful for correlation, so that the second Hyperlioceras horizon (H. mundum) was selected as the key marker for definition and correlation of the base of the Bajocian. The details of the evolution of Hyperlioceras from Graphoceras are best preserved and documented at Bearreraig, which is why this section was accepted as ASP. The succession is relatively thick (c. 24 m for the topmost Aalenian and lowermost Bajocian) and the ammonites are preserved mainly in a sequence of nodules. Each nodule contains an assemblage, dominated by juveniles, which approximates to a biological population. The dimorphic Graphoceras limitatum/carbatinum species remains virtually unchanged morphologically through some 15 m of strata (evolutionary equilibrium?) and overlap with the first two Hyperlioceras species. The main morphological change into the first Hyperlioceras species (dimorphic H. incisum/rotabilis) is in the shape of the venter (a punctuation event?). This is followed over 13 m of strata by gradational increases in size and involution through H. mundum/aspera and continuing into H. walkeri/contorta.

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Aalenian carbon-isotope stratigraphy: a tool for the ammonite radiolarian and nannofossil zonation calibrations in the Western Tethys

Sandoval J., Aguado R., Bartolini A., Bill M., Bruchez S., O'Dogherty L., Perilli N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

212-213

EN

Here we report a detailed carbon-isotope stratigraphy for the Aalenian (Middle Jurassic) pelagic carbonates in the Betic Cordillera (southern Spain), which represents an ideal region to directly tie the stable carbon-isotope curves to ammonite zones and subzones, and thereby for the first time achieve an accurate chronostratigraphic calibration. For this purpose we have selected two sections (Agua Larga in the province of Granada and Cerro de Mahoma in the province of Murcia) which represent basinal deposits of the southern Iberian palaeomargin. In these sections ammonite are common and relatively wellpreserved. Biostratigraphic analyses enable the recognition of the uppermost Toarcian (Aalensis Zone), the complete Aalenian (Opalinum, Murchisonae, Bradfordensis and Concavum zones) and the lowermost Bajocian (Discites Zone). Calcareous nannofossils and radiolarians (at discrete beds of the Upper Aalenian - Lower Bajocian) are also common in these Median Subbetic hemipelagic sections. The Subbetic Aalenian is characterized by a monotonous and rhythmical alternance of marlstones and marls in continuous sedimentation throughout the analysed interval. We present a ?13C curve very detailed (bed by bed) for the uppermost Toarcian - lowermost Bajocian interval. The curve shows a relative minimum (around 1‰) in the Upper Toarcian, a weak positive shift (around 2‰) in Lower Aalenian (Comptum Subzone), a decreasing values (newly around 1‰) in the Middle Aalenian (Bradfordensis Zone), a positive peak of 2.7‰ in the Upper Aalenian (Concavum Zone and Subzone) and a new relative minimum (1.5-1.7‰) at Aalenian/Bajocian boundary. A good correlation among the isotope curve and the different biostratigraphic zonations is accurately presented. We explore the powerfulness of this well-calibrated ?13C curve as a tool for stratigraphic correlation. In this sense, the biochronostratigraphic position of the radiolarian UAzones 1-2 (mainly based on Italian sections with scarce record of ammonites, Baumgartner et al. 1995) are here redefined by means of the isotope reference curve of the Subbetic. We present the results of a biostratigraphic study of nannofossils assemblages throughout the studied interval. In addition, on the base of a semiquantitative study, we have selected the most abundant taxa (>10%) from the whole assemblage to perform a multivariate analysis (principal components): Biscutum dubium, B. intermedium, Crepidolithus crassus, Carinolithus superbus, Discorhabdus striatus, Schizosphaerella spp., Watznaueria contracta and the genus Lotharingius. This multivariate analysis reveals a strong correlation between the abundances of oligotrophic (C. crassus and Schizosphaerella spp.) and eutrophic (B. dubium and B. intermedium) taxa and the fluctuation of the C degree curve.

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First record of Jurassic (Toarcian - Bajocian) ammonites from the northern Lut Block, east-central Iran

Seyed-Emami K., Fürsich F. T., Wilmsen M., Schairer G., Majidifard M. R.

Acta Geologica Polonica

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2004

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Vol. 54, no. 1

77--94

EN

A very thick and lithologically rather unusual marine sedimentary succession is described from the Kuh-e-Shisui area, northwestern Lut Block (east-central Iran). It contains a low diversity ammonite fauna comprising the families Dactylioceratidae, Hildoceratidae, Graphoceratidae, Hammatoceratidae, and Sonniniidae, which are concetrated in several levels, indicating the Lower-Lower Middle Toarcian, Upper Toarcian, Aalenian, and Lower Bajocian. The ammonite fauna, consisting of 21 taxa, descibed for the first time from the Lut Block, corresponds to that of the Badamu Formation of the Kerman-Ravar region (southern Tabas Block, to the west of the Lut Block), but is far less diverse. An exception is the occurrence of Lower Toarcian Harpoceratinae and Hildoceratinae, which hitherto have not been recorded from east-central Iran. The ammonite fauna is closely related to that of northwestern Europe.