Wyniki wyszukiwania - BazTech

1

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

Page K.

Volumina Jurassica

|

2009

|

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.

2

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

Page K.

Volumina Jurassica

|

2006

|

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.

3

Event stratigraphy of the Badenian selenite evaporites (Middle Miocene) of the northern Carpathian Foredeep

Bąbel M.

Acta Geologica Polonica

|

2005

|

Vol. 55, no. 1

9-29

EN

The Middle Miocene (Badenian) evaporites of the northern Carpathian Foredeep were deposited in a salina-type basin. Calcium sulphate sediments were deposited mainly on the broad northern margins of the basin, on vast evaporite shoals (mainly as fine-grained microbialite gypsum) and in large shallow saline pans (as coarse-crystalline selenites). 125 sections of these primary deposits, exposed from Moldova, Ukraine, Poland to the Czech Republic, were subjected to stratigraphic analysis based principally on the methodology of event and high-resolution stratigraphy. Due to an extremely gentle relief and a predominantly aggradational type of deposition, typical of a salina basin, the environmental changes or events were recorded nearly instantaneously in the whole area. Some events, such as water-level or brine-level fluctuations (emersions and floods, which can be very rapid in a salina basin),fluctuations in the average pycnocline level, aeolian dust or ash falls, accretion of specific gypsum microbialites, produced sets of marker beds which are perfectly correlated over distances of tens to hundreds of kilometres. Some thin grass-like selenite beds, representing deposits of shallow flat-bottomed saline pans, were correlated precisely over such great distances and are interpreted as isochronous or near-isochronous. Each bed was presumably deposited during the average pycnocline level highstands in the saline pan. The thick-bedded selenite units do not show long-distance bed-by-bed correlation, presumably because they were deposited in deeper pans in which the pycnocline fluctuations were recorded by bedding planes (i.e. by intercalations of fine-grained gypsum or dissolution surfaces) only on the shallow slopes or swells. However some apparent growth zones in the gypsum crystals from such selenite units were correlated throughout the basin, proving that the selenite growth However some apparent growth zones in the gypsum crystals from such selenite units were correlated throughout the basin, proving that the selenite growth was isochronous.