David K. Loydell

Early Silurian sea-level changes

Global sea-level fluctuated markedly during the early Silurian, probably as a result of the waxing and waning of ice-sheets in the South American portion of Gondwana. The highest sea-levels of the Silurian are recorded by the Telychian upper crispus –lower griestoniensis and spiralis –lower lapworthi biozones. Other highstands occurred in the early Aeronian, during the convolutus Zone (mid Aeronian), guerichi Zone and late turriculatus Zone (early Telychian), and early Sheinwoodian. Low sea-levels characterized much of the argenteus and sedgwickii zones (Aeronian), the utilis Subzone (late guerichi –early turriculatus zones, early Telychian), the late Telychian (commencing in the mid lapworthi Zone) and, after a period of apparently only small amplitude sea-level fluctuations in the late Sheinwoodian and earliest Homerian, the mid–late Homerian, in particular the early nassa Zone. Facies (and faunal) changes in the Lower Silurian do not support the P and S model of Jeppsson and others, but are consistent with the sea-level changes proposed herein. Mid Telychian marine red beds appear to have been deposited during a minor sea-level fall immediately after a period of very high sea-levels, rather than during a transgressive episode as previously suggested. Comparison of the sea-level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early Silurian deep water (graptolitic) and shallow water (shelly) sequences. Improving the precision of this correlation should be a priority for future research.

Graptolites in British stratigraphy

697 taxa of planktonic graptolites are recorded, and their stratigraphical ranges are given, through 60 biozones and subzones in the Ordovician and Silurian strata of England, Wales and Scotland, in the first such stratigraphical compilation for Great Britain since the synthesis of Elles & Wood (1901–1918).

Integrated biostratigraphy of the lower Silurian of the Aizpute-41 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data is presented from the Rhuddanian through to lower Sheinwoodian of the Aizpute-41 core, Latvia. Correlation of the biozonation schemes based upon the three groups is achieved from the cyphus through to lowermost riccartonensis graptolite biozones, except for the upper Aeronian and lower Telychian, which lack both chitinozoans and graptolites, and upper lapworthi through to approximately base murchisoni graptolite Biozone, where there is interpreted to be an unconformity. Datum 2 of the Ireviken Event is correlated with a level at the base of or within the murchisoni Biozone. It is possible that the changes in conodont assemblages at Datum 2 on Gotland are the result of an unconformity here. Streptograptus? kaljoi sp. nov., from the lower spiralis graptolite Biozone, is described.

Discovery of Llandovery (Silurian) graptolites and probable Devonian corals in the Southalpine Metamorphic Basement of the Eastern Alps (Agordo, NE Italy)

We report the discovery of Aeronian (Middle Llandovery) graptolites, and corals of probable Devonian age, in boudins hosted by greenschists, within the Southalpine Metamorphic Basement. These discoveries provide key constraints to the depositional age range of the protoliths. This remarkable occurrence of almost undeformed graptolites and compound corals in boudins within a metamorphic shear zone indicates very marked local strain partitioning.

Graptolite biozone correlation charts

Charts are presented showing the correlation of graptolite biozonations for the Lower Ordovician to Lower Devonian, i.e. for the entire stratigraphical range of the planktonic graptoloid graptolites. Regions chosen are those for which the most detailed biostratigraphical studies have been undertaken. For Baltica, average graptolite zone (chron) duration appears to vary from c . 300000 years (Ludlow Epoch) to 2.4 Ma (Lochkovian Epoch).

Re-evaluation of the petroleum potential of the Kufra Basin (SE Libya, NE Chad) : does the source rock barrier fall ?

Abstract The Kufra Basin is a large, underexplored, Palaeozoic intracratonic sag basin in SE Libya and NE Chad with extensions into NW Sudan and SW Egypt. The basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to Cretaceous. Geologically, the basin is very similar to the Murzuq Basin in SW Libya which recently presented Libya with its largest oil discovery for over a decade. Most of the hydrocarbon play elements known from the Murzuq Basin also occur in the Kufra Basin: thick, porous Cambro-Ordovician sandstones are present and would form good reservoirs, lower Silurian shales may act as effective seals, and there are potential structural traps in seismically defined fault blocks. However, the source rock availability in the Kufra Basin is currently unclear. One of the two main source rock candidates in the basin is a lower Silurian shale unit (Tanezzuft Formation). The Tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins, but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by AGIP between 1978 and 1981. Hot shales developed at the base of this widespread Silurian shale unit form important source rocks in many areas of North Africa and Arabia. These hot shales are interpreted to have been deposited in palaeodepressions, such as incised valleys of the preceding lowstand, or intrashelf basins, during the initial transgression after the melting of the late Ordovician ice cap. The areal distribution of the organic-rich unit is, therefore, discontinuous. Fieldwork in the Kufra Basin has shown that the basal Tanezzuft horizon is not exposed on the northern and eastern margins of the basin. Deep infracambrian rift grabens have been interpreted on seismic lines from the Kufra Basin and, in analogy to Oman and Algeria, could contain organic-rich infracambrian deposits. The infracambrian succession in the Kufra Basin may contain a second major potential source rock and warrants further investigation.

Integrated biostratigraphy of the lower Silurian of the Kolka-54 core, Latvia

Integrated graptolite, conodont and chitinozoan biostratigraphical data are presented from the Llandovery and Wenlock of the Kolka-54 core, Latvia. Correlations between graptolite and chitinozoan biozones are consistent with those published from other East Baltic sections and the Welsh Basin. While most correlations between graptolite and conodont biozones agree with those presented in previous studies, there are important exceptions. Significantly, we report here the discovery of Distomodus staurognathoides Biozone conodonts in the lowest Aeronian Demirastrites triangulatus graptolite Biozone. The base of the D. staurognathoides Biozone was previously considered to lie much higher in the Aeronian. Also it is shown that Walliserodus survived the late Wenlock Mulde Event, during which it was considered previously to have become extinct.

Integrated Silurian Chitinozoan and Graptolite Biostratigraphy of the Banwy River Section, Wales

The succession of 38 upper Llandovery–lower Wenlock chitinozoan taxa from graptolitic horizons in the Banwy River section (Powys, Wales) is described. Five new species are named: Bursachitina nestoraeConochitina leviscapulaeConochitina mathrafalensisBelonechitina caveiBelonechitina meifodensis. A further ten taxa are described under open nomenclature. Seven chitinozoan biozones are recognized in the Banwy River section, three of which (Cingulochitina bouniensisConochitina acuminataSalopochitina bella) are new. The base of each biozone is correlated with the graptolite biostratigraphical scheme as follows: Angochitina longicollis Biozone — upper spiralis Biozone; Conochitina acuminata Biozone — lowermost lapworthi Biozone; Margachitina banwyensis Biozone — upper lapworthi Biozone; Margachitina margaritana Biozone — lowermost insectus Biozone; Cingulochitina bouniensis Biozone — upper murchisoni Biozone; Salopochitina bella Biozone — upper firmus Biozone. The succession of chitinozoan biozones in the Banwy River section is compared with that in other sections which have graptolite biostratigraphical control. This has highlighted the following: (1) the correlation of the base of the dolioliformis Biozone with the graptolite biozonation is imprecise; (2) E. dolioliformis is recorded only from levels after the first A. longicollis in Sweden (although this may reflect previously unrecognized synonymies); (3) the longicollis Biozone may be diachronous, its base correlating with levels low in the Telychian in Sweden, Norway and Estonia and with the upper Telychian spiralis Biozone in Wales and the Prague Basin; (4) data herein and from the Prague Basin indicate that the base of the margaritana Biozone correlates with a level low in the insectus Biozone.

Integrated graptolite and chitinozoan biostratigraphy of the upper Telychian (Llandovery, Silurian) of the Ventspils D-3 core, Latvia

Integrated graptolite and chitinozoan biostratigraphical data are presented from the upper Telychian (Oktavites spiralis and Cyrtograptus lapworthi graptolite biozones) of the Ventspils D-3 core, Latvia. The base of the Angochitina longicollis chitinozoan Biozone is approximately coincident with that of the spiralis graptolite Biozone, as it is elsewhere in the East Baltic, although in Wales it lies within the upper spiralis graptolite Biozone. Conochitina proboscifera appears in the upper spiralis graptolite Biozone in the Ventspils D-3 core, but at lower and higher horizons elsewhere, presumably reflecting its patchy distribution during the lower part of its stratigraphical range. Ramochitina ruhnuensis appears to be a stratigraphically useful, although geographically restricted, species, appearing at a level close to the base of the lapworthi graptolite Biozone. The most remarkable feature of the Ventspils D-3 chitinozoan record is the very early occurrence, in the upper spiralis graptolite Biozone, of two chitinozoan biozonal index species: Margachitina banwyensis and M. margaritana. Previously, these two taxa were considered unequivocal indicators of the uppermost Telychian to Sheinwoodian or Homerian, respectively.

Carbon isotope stratigraphy of the upper Telychian and lower Sheinwoodian (Llandovery-Wenlock, Silurian) of the Banwy River section, Wales

δ 13 Corg and TOC data are presented from the upper spiralis Biozone (Telychian, Llandovery, Silurian) through to the upper Sheinwoodian (Wenlock, Silurian) of the Banwy River section, Wales. In laminated hemipelagites from the Telychian, δ 13 Corg values rise through the upper lapworthi Biozone to a maximum in the lower insectus Biozone after which they decline slightly. The most conspicuous feature of the δ 13 Corg curve is the prolonged positive excursion in the Sheinwoodian, commencing in the upper murchisoni Biozone and ending in strata yielding Monograptus flexilis. This Sheinwoodian positive δ 13 C excursion in the Banwy River section correlates precisely with that recognized in the East Baltic. The interval with the highest δ 13 Corg values also records the highest TOC values, suggesting that for the Sheinwoodian at least, burial of carbon may have contributed to the positive δ 13 C excursion. Bioturbated strata yield very low TOC values; whether the δ 13 Corg values from these beds reflect a primary signal or the result of biostratinomic or diagenetic modification is uncertain.