SCIENTIFIC REPORTS

 

Modzalevskaya, Tatialana L.

All-Russian Geological Research Institute (VSEGEI), St.Petersburg, Russia

 

The research on correlation of the Silurian sequence of Timan-Northern Ural Region with Baltic sections and with the International Standard (Antoshkina et al., 2000) defines more precisely the position of Ordovician-Silurian boundary in the Urals on brachiopods and conodonts. The abrupt replacement of the middle Ashgillian brachiopod fauna with Holorhynchus giganteus (Yaptikshor Regional Stage) by the Rhuddanian-Aeronian brachiopod fauna with Virgiana barrandei (Yareney Regional Stage) take place within the lithologically quite homogenous secondary dolostone succession. In the majority of studied sections between the strata with Ordivician fauna below and Silurian fauna above lies interval without any identifiable fossils. This interval much more likely corresponds to Hirnantian so as the youngest Ordovician fauna, characterised by Holorhynchus gifanteus in this region, is known to have become extinct before Hirnantian time (Brenchley et al., 1997).

Based on the latest ones and conodont data, the Ordovician-Silurian boundary (=the boundary between the Yaptikshor and Yarenej Regional Stages) in the north of the Urals lies in an interval coinciding with a regional regressive event (Antoshkina, 1996).

 

 

Wang, Xiaofeng

Yichang Institute of Geology and Mineral Resources, China

 

The Huanghuachang section, 20km from Yichang is well-exposed and plenty of conodonts, brachiopods, trilobites, chitinozoans and some graptolites-bearing interbeds from deflexus/ protobifidus Zone to Exigraptus Zone. A complete conodont sequence from communis Zone to originalis Zone can be recognized. Their relationship with other fossils is shown in Fig. 1.

 

 

conodont

 grapt.

chitinozoa

trilobite

cephal.

brachio.

 

 

 

Darriwanian

parva

sinoden-tatus

 

pirum

Hanchogo-

litus

 

 

 

originalis

navis

trianglaris

 

 

 

 

brevis

 

Protocyclo.

 

 

Uuorthisia

 

Bathmo.
 
 
 
Leptella

 

   

 

     Doubao-

     wanian

evae

 

suecicus

Pseudocaly-

mene

 

 

communis

deflexus

baculata

 

 

 

 

 Xinchangian

 

diversus/

proteus

 

symmetrica

 

Manchuroces-Koreanoceras
Nanorthis-

Eupuctata

 

 

 

Fig. 1.  Showing the relation of conodont Zone with other fossils zone between the Late Tremadocian to “Arenigian” at Huanghuachang section (Modified from Wang et al. 1987, 1994, 1992, 1996, Chen et al. 1996).