Graptolites: Ordovician and Silurian biostratigraphy - an excursion - Earthwise
graptolite biostratigraphy, particularly in the Silurian, where biozones/subzones average under .. metric dates, they can chart the rates of evolution of. Sep 4, Use of trilobites in biostratigraphy and geological chronology & time scale. decay allowed scientists to date rocks via their isotope proportions, finally on graptolites (colonial marine invertebrates with distinctive skeletons). The succession can be regarded as one of the best Upper Darriwilian or Upper Middle Ordovician graptolitic successions of eastern North America, dated most.
About 3 km SE of Portpatrick at NX there is a track down to the south end of the bay; park by the roadside and walk 1. There is no room to park a bus.
Morroch Bay can also be reached on foot from Portpatrick by the cliff-top path, a pleasant walk of about 3 km in each direction, but involving an arduous scramble down steep overgrown slopes into the north end of the bay. The tide is not dangerous at this locality, but when it comes in, the bedrock geology is largely obscured.Episode 71: Graptolites
The intertidal zone exposes a thick sequence of mudstone dipping steeply and striking roughly SW, out to sea. There are several intrusive igneous dykes, more or less concordant with the bedding, and these can serve as useful landmarks when studying the mudstone succession.
The mud-stones generally become younger from south to north but the distribution of fossil zones indicates at least one major structural repetition, whilst the anomalous thickness of some units, together with the presence of minor folding and faulting, shows that the structure is complicated in detail. At the south end of the bay, black mud-stones 3aassociated with cherts and red and green mudstones have yielded a relatively diverse fauna of the N.
Conodonts have been recovered from red mudstones here. The succession to the north consists largely of red and green mudstones with beds of chert, but about m north of the cottage at the south end of the bay 3b and 3c you can collect faunas of the gracilis and possibly 'peltifer' biozones from black Glenkiln Shales.
The wilsoni Biozone is found in black shale 3d about m NW of the cottage, between chert beds and a large dyke near the middle of the bay. Climacograptus wilsoni is associated with Amplexograptus perexcavatus, Dicranograptus nicholsoni, and Glossograptus hincksii.
Close to the dyke is a bed 3e with Dicellograptus angulatus and other graptolites. Further north the mudstones are associated with some greywacke beds, and beside one of these 3fabout 50 m south of the northerly cottage, a fairly diverse D. The localities referred to so far are progressively younger northwards. However, a major strike fault to the north of here truncates the Hartfell Shale and reintroduces chert and Glenkiln Shale 3g with fossils of the gracilis and 'peltifer'biozones.
The northern end of the bay shows very clearly the interbedding of black mudstone with the basal greywackes of the Portpatrick Formation.
Many of the mudstone beds 3h and 3i are fossiliferous, and faunas of the clingani Biozone, including Climacograptus spiniferus, Dicellograptus morrisi, Dicranograptus ramosus, Neurograptus margaritatus and various Orthograptus spp.
This locality shows that the base of the Portpatrick Formation lies within the clingani Biozone, whilst the top of the formation exposed some 5 km to the NW in Killantringan Bay see Excursion 15 appears to lie within the overlying linearis Biozone Figure Approach on the A from Stranraer; 4 km south of Sandhead turn right, pass Ardwell House, and after 1.
From the farm a walk of about m takes you to Drumbreddan Bay. There are two principal exposures of Moffat Shale, separated by the greywacke beds that make up Grennan Point.
The northerly exposure is much the larger and displays a wider range of strata; the southerly exposure shows the relationship with the greywacke particularly well. Structural aspects of this locality are described in Excursion This specimen of graptolite, Dimorphograptus decussatus is possibly from the atavus Biozone of the Silurian and was found at Port Gower, Grennan Point. Dimorphograptus is a genus where the proximal part of the rhabdosome is uniserial due to loss of thecae while most of the rhabdosome is biserial.
During the Silurian the monograptid fauna developed from Lower Silurian simple thecal types of Monograptus and numerous biserial forms such as Orthograptus and Dimorphograptus. The Middle Silurian saw the acme of hooked types and dominance of cyrtograptids while the Upper Silurian saw a dominance of monograptids with simple thecal types. P The Moffat Shale Group is well exposed at low tide but largely covered when the tide is in.
The structure is broadly an anticline—syncline fold pair, though complicated in detail Figure They include thin black beds, and although these contain fragments of graptolites, it has yet to be shown whether they are referable to the complanatus, anceps or extraordinarius biozones.
Working north from the grey mud-stones, the first black beds encountered are basal Birkhill Shales 4b. Here you can collect fossils of the persculptus Biozone. Further to the north, 5 m from the faulted contact with the Gala Group greywackes 4cthe succeeding zone of Parakidograptus acuminatus is present.
The atavus Biozone is proved close to the greywackes, though the effects of strong faulting makes the collection of satisfactory material more difficult. Turning south from the pale barren mud-stones, the centre of Grennan Bay is occupied by a considerable thickness of Lower Birkhill Shale — massive black mudstones described by Lapworth as the vesiculosus Flags'.
They yield fossils of the acuminatus Biozone in the centre of the bay 4d and several species of the atavus Biozone including species of Atavograptus, Coronograptus, Dimorphograptus and Normalograptus within 10 m of the greywackes of Grennan Point 4e. Closer still to the greywackes 40, you can collect diverse faunas of the convolutus Zone, including species of Coronograptus, Glyptograptus, Petalolithus, Rhaphidograptus and several Monograptus species Margutus, M. Deformation associated with strong nearby faulting hampers collecting but excellent specimens were collected last century and the source of those remains to be rediscovered.
The pale greenish or whitish beds of soft claystone interbedded in the Birkhill Shale are metabentonites altered volcanic ash. These indicate frequent and large-scale eruptions, but the position of the volcanic source is unknown. On the south side of Grennan Point a relatively narrow exposure of the Birkhill Shale 4g extends along the base of the greywacke cliffs.
Despite the presence of a strike fault, some of the graptolites found here are well preserved. Examples representing the cyphus Biozone?
The Birkhill Shale passes up into a few metres of thinly bedded grey shale and laminated siltstone 4h followed conformably by massive greywackes.
The thinly bedded unit contains thin black mudstone seams and with patience you can collect rare but well-preserved graptolites of the convolutus Biozone which serve to date the onset of greywacke deposition. The northern exposure youngs to the north, as is generally the case in Moffat Shale inners, but the southern exposure is unusual in that it seems to young towards the south. This was formerly taken as evidence for a major anticlinal structure centred on Clanyard Bay, but more recently the complex thrust-related structure has become apparent.
This is described in some detail as part of Excursion Grennan Quarries, nearby, afford an opportunity to collect graptolites from interbeds in the greywacke succession and can be visited at times when the tide covers exposures in Clanyard Bay.
The exposures can only be studied in detail at low tide. The northern outcrop is the larger. They include black mudstone interbeds that have yielded Dicellograptus.
Working towards the greywackes at the northern edge of the bay, you find Birkhill Shale, and from various beds in a general south-to-north sequence can collect graptolites of the atavus, acinaces, triangulatus?
In addition, an old collection contains well-preserved fossils indicating a low level in the turriculatus Biozone; the exact locality is unknown but it is possible that, if careful collecting were undertaken, the torriculatus locality might be rediscovered and evidence obtained for the presence of other zones.
On the south side of the bay, the Birkhill Shale is exposed with associated beds of bentonite. Graptolites of the cyphus Biozone have been collected about 20 m from the base of the cliffs.
Farther south, about 10 m from the cliff, the presence of the convolutus Biozone provides evidence for southward younging here. Grennan Quarries These quarries are situated just west of the A, 3 km north of Drummore. One on the west of the track to Grennan NX displays greywackes of Gala Group 8 or Port Logan Formationwhich here includes shaly interbeds.
From these you can collect well-preserved graptolites of the crispus Biozone, including Monograptus crispus, M. Figure 66 Locality map for Gillespie Burn Locality 6. From Glenluce take the A south towards Port William.
A car may be parked near the track-junction NX and Gillespie Burn approached over land farmed from Culroy, where prior permission for access and parking must be sought. From Culroy walk due east to the tributary stream which flows down to join the Gillespie Burn NX where it flows through a small ravine. Silurian About 40 m upstream of the place where a stone wall crosses this tributary, the left NE bank displays greywacke beds with shale interbeds 6a.
Graptolites: Ordovician and Silurian biostratigraphy - an excursion
Graptolites from these interbeds include climacograptids Normalograptus spp. Several hundred barrels of fluid were lost in some of these porous intervals during drilling, indicating high permeability. The sandstone intervals are porous and permeable and are texturally and compositionally mature.
The permeable sandstone and porous dolomite intervals are laterally extensive and could serve as important DEFF Research Database Denmark Jakobsen, Kristian Grube A profound transformation of the marine biosphere took place during the Great Ordovician Biodiversification Event GOBErecognized as the longest interval of sustained biodiversification in the history of life.
The bivalves are the most dominant and diverse group in the central Australian fauna, whereas the brachiopods are the most dominant and diverse group in the Tasmanian fauna.
Biostratigraphy - Wikipedia
Thirteen trilobite, five brachiopod, four conodont and 22 mollusc taxa have been documented Although highly endemic at species level, the investigated brachiopod, trilobite and mollusc faunas display a high degree of overall similarity at generic level with Middle Ordovician faunas from North China. The upper boundry of the Gurpi Formation is more noticeable and there are different overlying formations. In Khozstan province,the Gurpi Formation is overlain by purplr shale of pabdeh Formation with signs of unconformity.
The Gurpi Formation is not always concurrent. This research study aims at describing the biostratigraphy of Gurpi Formation.