GSSP for Tremadocian Stage
Definition:
The base of the Ordovician System and base of the lowest Ordovician stage (Tremadocian) is exposed in the coastal platform at Green Point, western Newfoundland, at 101.8m level, within Bed 23, in the measured section (Lower Broom Point Member, Green Point Formation), coinciding with the first appearance of the conodont Iapetognathus fluctivagus, and 4.8m below the earliest planktonic graptolites.Definition for the auxiliary (ASSP) section:
The auxiliary Xiaoyangqiao Section of the Ordovician System and Tremadocian Stage is exposed near Dayangcha Village, Jilin Province, North China. The ASSP level (19.9m (± 0.2m)) in the Xiaoyangqiao Section lies within the upper Cordylodus intermedius conodont zone. The FAD of graptolite Rhabdinopora proparabola at 20.9 m is a secondary marker. See "additional comments" below for more detail.
Location:The Green Point Section is located within the Gros Morne National Park, about 70km from Deer Lake Airport and about 10km north of the village of Rocky Harbour, western Newfoundland. |
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Sedimentology:The Green Point Section is comprised of alternating black, grey and green shales, micritic nodular and ribbon limestone and grainstone. Limestone conglomerate is largely confined to one bed (Bed 19) and a lenticular bed within Bed 25. |
Primary Markers:
First appearance of conodont species Iapetognathus fluctivagus within Bed 23.Secondary Markers:
Graptolites:Planktonic graptolites appear 4.8m above the boundary in the Green Point Section. The earliest planktonic graptolite species , Staurograptus dichotomus and Rhabdinopora praeparabola, are both present in the Green Point Section.
Carbon isotopes:
The boundary level coincides closely with the peak of a positive cycle in 13C, representing the highest value reached in the boundary interval.
Correlation Events:
Conodont FAD Iapetognathus fluctivagusOther Locations around the World:
The Cambrian-Ordovician Boundary lies in the lower Ibex Series of North America, at the base of the Warendian Stage of north Australia, in the lower Tremadoc as used in Kazakhstan, and in the late Fengshanian of north China.Auxiliary Xiaoyangqiao Section is exposed near Dayangcha Village, Jilin Province, North China. See additional comments for more detail.
Additional Comments:
Definition:The auxiliary Xiaoyangqiao Section of the Ordovician System and Tremadocian Stage is exposed near Dayangcha Village, Jilin Province, North China. The ASSP level (19.9m (± 0.2m)) in the Xiaoyangqiao Section lies within the upper Cordylodus intermedius conodont zone. The FAD of graptolite Rhabdinopora proparabola at 20.9 m is a secondary marker.
Location:
The auxiliary Xiaoyangqiao Section is exposed near Dayangcha Village, Jilin Province, North China at a latitude of 42°3’24"N (42.056° N), and a longitude of 126°42’21"E (126.705°E).
Sedimentology:
The ASSP for the Ordovician lies within the upper part of the 40 m thick Dayangcha Beds. The Dayangcha Beds can be subdivided into 4 units. From bottom to top:
Unit I is a succession of bedded shale and limestone with thin layers of carbonate lenticles.
Unit II starts with a stromatolite horizon followed by shallow to deep-water carbonate deposits.
Unit III is composed of shale, siltstone interbedded with nodular limestone beds. The ASSP is placed at 19.9 m (½ 0.2 m) in the section.
Unit IV is composed of green to black shale and grey siltstones with fine-grained sandstone beds.
Primary Markers:
The ASSP level (19.9m (½ 0.2m)) in the Xiaoyangqiao Section lies within the upper Cordylodus intermedius conodont zone. The FAD of graptolite Rhabdinopora proparabola at 20.9 m is a secondary marker.
Secondary Markers:
Markers across the Cambrian - Ordovician Boundary (primary and auxiliary markers):
In stratigraphic order from oldest to youngest:
| Meters | Markers | Eoconodontus notchpeakensis and Cordylodus primitivus conodont association | Start of the Cordylodus evolutionary lineage |
|---|---|
| FAD of Cordylodus proavus | Cordylodus proavus is cosmopolitan and widely distributed in Midcontinent and North Atlantic realms |
| Maximum values of the positive excursion in δ 13Ccarb at ca. 0.6 m | Global event that marks the basal part of the Cordylodus proavus conodont zone |
| FAD of Cordylodus caboti | The species is cosmopolitan in all biogeographic realms |
| positive spike at 17.4 m HSS | Excursion from high negative to high positive values is a global event |
| FAD of Cordylodus intermedius | The species is cosmopolitan at middle shelf and greater depth with occurrences in all biogeographic realms |
| Minimum C isotope peak 2 at 19.6 m | This lies immediately below the GSSP horizon |
| Auxiliary GSSP at 19.9m (½ 0.2m) | |
| FAD of Cordylodus lindstromi | The species is cosmopolitan. It is a close proxy for the FAD of planktonic graptolites |
| FAD of conodont Iapetognathus preaengensis | The species is the first of the Iapetognathus evolutionary lineage |
| FAD of Iapetognathus jilinensiss | The species is an early member of the Iapetognathus evolutionary lineage |
| FAD of graptolite Rhabdinopora proparabola at 20.9m | This species is the first planktonic graptolite and marks the lower Ordovician |
| Maximum peak (no. 3) of the positive excursion in δ 13Ccarb at ca. +21.4 m | This is a global event |
| FAD of Rhabdinopora parabola | This species is well represented and has worldwide distribution |
| FAD of graptolite Anisograptus matanensis | Primary marker, base of Chron C33r, base of Campanian Stage |