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1

Ammonite biostratigraphy of Central High Atlas. a reference section for Tethyan Domain

100%

El Hariri K. , Dommergues J.-L. , Chafiki D. , Meister C.

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tom Vol. 4, no. 1

170-170

EN

The Sinemurian times of the Moroccan Central High Atlas saw important palaeogeographic changes which were of primary importance in the geodynamic evolution of the Atlas Basin. Sedimentation in the subsiding basin was dominated by the hemipelagic, thin bedded sediments of the Aberdouz and Ouchbis formations. The ammonite fauna gives the basis for precise dating of the deposits and recognition of changes of facies along a proximal/distal direction which corresponded to the main breakup of the platform at the turn of Early and Late Sinemurian. This study is concentrated on Sinemurian ammonites of Central High Atlas (Morocco). It refers to four sections namely Bou-Hamid, n’Zala, Tillicht, Foum Zaabel in the High-Atlas of Midelt-Rich and two sections namely Tamadoute, Taguendouf in the High-Atlas of Beni-Mellal. A careful systematic description of 70 taxa belonging to the Phylloceratidae, Juraphyllitidae, Lytoceratidae, Schlotheimiidae, Arietitidae, Echioceratidae, Acanthopleuroceratidae, Oxynoticeratidae, Eoderoceratidae, led to the construction of a detailed biostratigraphical framework for the Sinemurian. Ten horizons were recognized in the Lower Sinemurian (Obtusum Zone) – Upper Sinemurian (Raricostatum Zone) interval up to Lower Carixien (Jamesoni Zone). The bulk of the species show Tethyan paleogeographical affinities and can be correlated with those of the northwestern European standard zonation.

2

The Upper Sinemurian ammonite succession in the Sierra Madre Oriental (Mexico)

75%

Meister C. , Blau J. , Dommergues J.-L. , Schlatter R. , Schmidt-Effing R.

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tom Vol. 7, no. 1

31-36

EN

In the Sierra Madre Oriental (Mexico), the biostratigraphic subdivision of the Upper Sinemurian can be refined. It has been possible to differentiate 11 horizons from the upper Obtusum Zone to the top of the Raricostatum Zone which are correlated with the NW European standard zonation. The index species of each horizon is figured.

3

Sinemurian ammonite succession from the Sierra Madre Oriental (Mexico)

75%

Blau J. , Meister C. , Dommergues J.-L. , Schlatter R. , Schmidt-Effing R.

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tom Vol. 4, no. 1

147-148

EN

In a series of studies (Blau et al. 2001, 2002, 2003; Meister et al. 2002, 2005) dedicated to Liassic (Sinemurian) ammonites of the Sierra Madre Oriental (Mexico), we have described 42 taxa, amongst them one new genus and five new species. After a critical analysis of Erben’s collection and with our own collections, we propose a synthetic succession of 13 correlable units “biohorizons”. Even though the Sinemurian sediments of the Huayacocotla Basin are very thick only two periods are well represented: the Bucklandi and Semicostatum zones for the Lower Sinemurian and the upper Obtusum and Raricostatum zones for the Upper Sinemurian. These two time intervals can be well correlated with South and North American biostratigraphic schemes. The Lower Sinemurian is characterized by the acme of Arnioceras a genus well distributed on the eastern Panthalassian rim. The Upper Sinemurian allows a much more refined biostratigraphic subdivision and more acute correlations. In the upper Obtusum Zone Euerbenites is an index fossil and can be correlated from Sonora to Peru. In the Raricostatum Zone several horizons allow acute correlation with South America (O. incaguasiense and P. tardecrescens horizons) and with North America (P. harbledownense and P. rothpletzi horizons). Mainly in the Upper Sinemurian there is quite strong endemism of the ammonite fauna in the Huayacocotla Basin (nearly 40%) which may be due to a quite isolated palaeogeographical position of the basin. On the other hand the affinities to Tethyan faunas are obvious. There is little palaeontologic evidence for a Sinemurian connection between Eastern Pacific (Panthalassa) and Western Tethys through the Hispanic Corridor. First palaeontological evidence indicates a possible connection not before the Pliensbachian. If this connection was not established in the Sinemurian, we must imagine other migration ways to explain the Tethyan affinities. There are at least four possibilities or combinations of migration routes: southern peri-Pangean, peri-Asiatic, latudinal trans-Panthalassian, and Boreal (Viking Corridor, Arctic seas). All these migration routes remain hypothetical and the establishment of the Hispanic Corridor already in the Sinemurian cannot be ruled out completely.

4

High resolution ammonite stratigraphy of the Charmouth Mudstone Formation (Lower Jurassic: Sinemurian-Lower Pliensbachian) in south-west England, UK

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Page K.

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tom Vol. 7, no. 1

19-29

EN

The "Lower Lias" mudrocks of the Charmouth Mudstone Formation inWest Dorset coast are world famous for their ammonite faunas, which range from mid Lower Sinemurian (Semicostatum Chronozone) to Lower Pliensbachian (topmost Davoei Chronozone) in age. The succession includes significant non-sequences, however, and as certain other intervals yield only crushed and relatively poorly preserved material, much of the sequence of ammonite faunas of this interval in south-west England has remained poorly understood. Inland, however, although it has been realised for many years that some of the missing horizons reappear, the Formation is very poorly exposed and as a consequence little has been known about its detailed stratigraphy and palaeontology. The systematic recording over 40 years by Mr H.C. Prudden (Montacute) of temporary excavations in East Somerset (around 20 km north of the Dorset coast), combined with material collected by others from similar exposures has now, however, revealed a virtually complete sequence of ammonite faunas through the interval represented by the Formation including from many of the which are missing on the Dorset coast. In particular, only one subchronozone remains to be conclusively proven in the region, the terminal Sinemurian, Aplanatum Subchronozone (Raricostatum Chronozone). This faunal succession is correlated with that on the coast to provide a detailed synthesis of the sequence of ammonite biohorizons in the region, which is correlated with a contemporary Standard Zonation and high-resolution biohorizonal/ zonule scheme for interval in North-West Europe. The significance for regional and international correlations of the Lower Lias is also discussed.

5

Sinemurian to lowermost Toarcian ammonites of the Brescian Alps (Southern Alps, Italy): biostratigraphical framework and correlations

63%

Meister C. , Schirolli P. , Dommergues J.-L.

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tom Vol. 4, no. 1

184-185

EN

A set of 28 ammonite biohorizons or faunal assemblages can be proposed for the Sinemurian, the Pliensbachian and the lowermost Toarcian in the Brescian Alps, in part based on the data from Dommergues et al. (1997) and partly on new results, deriving both from recent field investigations and from the study of the historical collection of the Lower Jurassic ammonites preserved in the Museum of Natural Sciences of Brescia (Northern Italy), placed into their stratigraphical framework (Fig. 1). The biohorizons are referred to the Liassic carbonate succession of the Brescian Alps, outcropping between the eastern surroundings of Brescia (Botticino), to the East, and the Lake Iseo, to the West. Since the Hettangian the region was subjected to the paroxismal phase of the Jurassic rifting and the area of study was located in the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin of the Tethys. At the beginning of Jurassic, an articulated fault-system, composed of N-S master faults (W-dipping) and W-E transfer faults, located from Brescia to the North, separated a western growing basinal area from the eastern Botticino structural high. Since Hettangian to the onset of Toarcian, almost 1000 m of well-stratified cherty marly limestones of the Medolo Group (Gardone Val Trompia Limestone and Domaro Limestone formations) represent the synrift deposition of the Sebino Basin, following the drowning of the Raethian-Hettangian Corna Platform. In contrast, after the Early Sinemurian a coeval reduced sequence (50 m thick), composed by the Rezzato Encrinite (Lower Sinemurian to Upper Carixian) and the overlying thinly-bedded and nodular ammonitic marly limestones of the Botticino Corso Rosso (Upper Carixian and Domerian), covered the Corna Platform in the Botticino High. The recognized ammonite biohorizons and assemblages are quite well integrated and correlable with either the NW European standard zonation or the different zonations given for the Tethyan realm (Appennines, Subbeticas and Austrian and Hungarian Upper Austroalpine). Most of these stratigraphical units are based on Tethyan taxa. Only two horizons (U. cf. jamesoni horizon and P. solare horizon) show NW European affinites. Moreover the E. quenstedti horizon is only known from the northern margin of the Tethys, mainly from the Upper Austroalpine units.

6

High resolution ammonite stratigraphy of the Charmouth Mudstone Formation (Lower Jurassic: Sinemurian – Lower Pliensbachian) in South West England (UK)

63%

Page K.

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tom Vol. 4, no. 1

200-200

EN

The “Lower Lias” mudrocks of the Charmouth Mudstone Formation in West Dorset coast are world famous for their ammonite faunas, which range from mid Early Sinemurian (Semicostatum Chronozone) to Early Pliensbachian (topmost Davoei Chronozone) in age. The succession includes significant non-sequences, however, and as certain other intervals yield only crushed and relatively poorly preserved material, much of the sequence of ammonite faunas of this interval in South West England has remained poorly understood. Inland, however, although it has been realized for many years that some of the missing horizons reappear, the Formation is very poorly exposed and as a consequence little has been known about its detailed stratigraphy and palaeontology. The systematic recording over 40 years by Mr H. C. Prudden (Somerset Geology Group) of temporary excavations in East Somerset (around 20 km north of the Dorset coast) has now, however, revealed a virtually complete sequence of ammonite faunas through the interval represented by the Formation. In particular, many levels have now been identified which are missing in the major non-sequences on the Dorset coast, thereby revealing a much more complete stratigraphical sequence in the region than previously realized. In particular, only one subchronozone remains to be conclusively proven – the Aplanatum Subchronozone of the Raricostatum Chronozone – although this could still be due to collection failure as nodular facies do not appear to be present at this level and near-surface clay exposures are often too degraded to yield determinable specimens. This faunal succession is correlated with that on the coast to provide a detailed synthesis of the sequence of ammonite biohorizons in the region, which is correlated with a contemporary Standard Zonation and high-resolution biohorizonal/zonule scheme for interval in North West Europe. The significance for regional and international correlations of the Lower Lias is also discussed.