Michael H. Stephenson

A REVISION OF REDUVIASPORONITES WILSON 1962: DESCRIPTION, ILLUSTRATION, COMPARISON AND BIOLOGICAL AFFINITIES

Abstract Geochemical analyses of specimens of Reduviasporonites suggests that it is most likely of algal, rather than fungal origin. As a probable alga, Reduviasporonites is unlikely to be integral to the process of mass extinction occurring at or near the Permian‐Triassic boundary, as suggested by Visscher and other workers because it cannot have acted as a saprophytic metaboliser of dead vegetation resulting from that event. Moreover, it ranges outside the postulated time of mass extinction by at least 10 million years. Optical and electron microscopy of topotype material confirms that Reduviasporonites Wilson 1962 is the senior synonym of Chordecystia Foster 1979, and Tympanicysta Balme 1980. Moreover the type species of the last two genera, assigned in 1999 to Reduviasporonites by Elsik as R. chalastus (Foster) and R. stoschianus (Balme), are conspecific. The type species, R. catenulatus Wilson 1962, differs from R. chalastus in that its constituent cells are significantly smaller, more rounded, and h...

Tethyan oceanic currents and climate gradients 300 m.y. ago

We reconstruct the oceanic circulation pattern of the Tethys Ocean 300 m.y. ago by placing Late Carboniferous–Early Permian climate-sensitive biotic associations from Gondwana and Laurasia on a Pangea paleogeography constrained by selected paleomagnetic data. Warm-climate fossils and facies from Iran, located at that time along the Gondwanan margin of Arabia, are compatible with the existence in the Tethys Ocean of a warm subtropical surface current gyre, whereas cold surface currents swept the glaciated Gondwanan margin at higher southern latitudes, redistributing cold biota toward the tropics. This Tethyan surface current system and the associated narrow zonal barrier show similarities to recent glacial climate patterns. When placed on a large-scale paleogeographic reconstruction of Pangea of the B type, it neatly explains the otherwise problematic observation that the Carboniferous–Permian biota of Iran and northern Arabia is dominated by warm Euramerican and/or Russian taxa that are strikingly different from typical cold Gondwanan associations.

Crustaceans from bitumen clast in Carboniferous glacial diamictite extend fossil record of copepods

Copepod crustaceans are extremely abundant but, because of their small size and fragility, they fossilize poorly. Their fossil record consists of one Cretaceous (c. 115 Ma) parasite and a few Miocene (c. 14 Ma) fossils. In this paper, we describe abundant crustacean fragments, including copepods, from a single bitumen clast in a glacial diamictite of late Carboniferous age (c. 303 Ma) from eastern Oman. Geochemistry identifies the source of the bitumen as an oilfield some 100-300 km to the southwest, which is consistent with an ice flow direction from glacial striae. The bitumen likely originated as an oil seep into a subglacial lake. This find extends the fossil record of copepods by some 188 Ma, and of free-living forms by 289 Ma. The copepods include evidence of the extant family Canthocamptidae, believed to have colonized fresh water in Pangaea during Carboniferous times.

Pennsylvanian-Early Triassic stratigraphy in the Alborz Mountains (Iran)

Abstract New fieldwork was carried out in the central and eastern Alborz, addressing the sedimentary succession from the Pennsylvanian to the Early Triassic. A regional synthesis is proposed, based on sedimentary analysis and a wide collection of new palaeontological data. The Moscovian Qezelqaleh Formation, deposited in a mixed coastal marine and alluvial setting, is present in a restricted area of the eastern Alborz, transgressing on the Lower Carboniferous Mobarak and Dozdehband formations. The late Gzhelian–early Sakmarian Dorud Group is instead distributed over most of the studied area, being absent only in a narrow belt to the SE. The Dorud Group is typically tripartite, with a terrigenous unit in the lower part (Toyeh Formation), a carbonate intermediate part (Emarat and Ghosnavi formations, the former particularly rich in fusulinids), and a terrigenous upper unit (Shah Zeid Formation), which however seems to be confined to the central Alborz. A major gap in sedimentation occurred before the deposition of the overlying Ruteh Limestone, a thick package of packstone–wackestone interpreted as a carbonate ramp of Middle Permian age (Wordian–Capitanian). The Ruteh Limestone is absent in the eastern part of the range, and everywhere ends with an emersion surface, that may be karstified or covered by a lateritic soil. The Late Permian transgression was directed southwards in the central Alborz, where marine facies (Nesen Formation) are more common. Time-equivalent alluvial fans with marsh intercalations and lateritic soils (Qeshlaq Formation) are present in the east. Towards the end of the Permian most of the Alborz emerged, the marine facies being restricted to a small area on the Caspian side of the central Alborz. There, the Permo-Triassic boundary interval is somewhat similar to the Abadeh–Shahreza belt in central Iran, and contains oolites, flat microbialites and domal stromatolites, forming the base of the Elikah Formation. The P–T boundary is established on the basis of conodonts, small foraminifera and stable isotope data. The development of the lower and middle part of the Elikah Formation, still Early Triassic in age, contains vermicular bioturbated mudstone/wackestone, and anachronostic-facies-like gastropod oolites and flat pebble conglomerates. Three major factors control the sedimentary evolution. The succession is in phase with global sea-level curve in the Moscovian and from the Middle Permian upwards. It is out of phase around the Carboniferous–Permian boundary, when the Dorud Group was deposited during a global lowstand of sealevel. When the global deglaciation started in the Sakmarian, sedimentation stopped in the Alborz and the area emerged. Therefore, there is a consistent geodynamic control. From the Middle Permian upwards, passive margin conditions control the sedimentary evolution of the basin, which had its depocentre(s) to the north. Climate also had a significant role, as the Alborz drifted quickly northwards with other central Iran blocks towards the Turan active margin. It passed from a southern latitude through the aridity belt in the Middle Permian, across the equatorial humid belt in the Late Permian and reached the northern arid tropical belt in the Triassic.

A revised correlation of Carboniferous rocks in the British Isles

The report revises and expands upon the 1976 and 1978 publications for the Dinantian and Silesian, respectively, combining them into a single account of British and Irish Carboniferous stratigraphy. The need to update the two Special Reports reflects the considerable advances in Carboniferous geology over the last 30 years. The report covers developments in international chronostratigraphy and incorporates wholesale reassessments of British lithostratigraphy. A huge volume of biostratigraphical information has been published over recent decades and the report summarizes the key information. Carboniferous rocks have long been of economic importance, but it is the search for hydrocarbons, in its infancy at the time of the previous reports, which has greatly increased our understanding of Carboniferous successions offshore and at depth, particularly in southern and eastern England.

How cold were the Early Permian glacial tropics? Testing sea-surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Abstract: Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater palaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting δ18O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The δ18O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 °C (for δ18Oseawater = 0‰ V-SMOW) or from +30.3 to +35.4 °C (for δ18Oseawater = +1.0‰ V-SMOW) for the early Asselian. Results for δ18O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 °C (for δ18Oseawater = 0‰) or +29.2 to +32.8 °C (for δ18Oseawater = +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 °C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.

Palaeoecological and possible evolutionary effects of early Namurian (Serpukhovian, Carboniferous) glacioeustatic cyclicity

Early Namurian (Serpukhovian, Carboniferous), sedimentary cycles in the Throckley and Rowlands Gill boreholes, near Newcastle upon Tyne, UK, consist of fossiliferous limestones overlain by (usually unfossiliferous) black mudstone, followed by sandstones and often by thin coal seams. Sedimentological and regional geological evidence suggests that the largest are high-amplitude cycles, probably of glacioeustatic origin. δ13C (bulk organic matter) delineates marine and non-marine conditions because of the large difference between terrestrial and marine δ13C, and indicates that full marine salinity was only intermittent and resulted from glacioeustatic marine transgression superimposed on a background of inundation by freshwater from large rivers, which killed off the marine biota. Palynology suggests that plant groups, including ferns and putative pteridosperms, were affected by changing sea level, and that there is a theoretical possibility of connection between cyclicity and the first appearance of walchiacean conifer-like monosaccate pollen such as Potonieisporites. Long-term terrestrial and marine increasing δ13C (organic) may reflect the onset of major glaciation in Gondwana, as there is evidence to suggest that the two are coeval, but no specific mechanism can be suggested to link the trends.

Investigating the record of Permian climate change from argillaceous sedimentary rocks, Oman

A standard method for the analysis of carbon isotope composition of bulk organic matter (δ13Cbulk) in palaeoenvironmental and stratigraphic studies of sedimentary basins associated with hydrocarbons is suggested. This method includes the removal of interstitial hydrocarbon within the sediments, which is shown to have a negative effect on δ13C. Using the method, a δ13Cbulk trend (c. −21 to −24‰) was found within the Lower Permian Al Khlata and lower Gharif formations of Thuleilat-16 and -42 well sections, south Oman. Palaeontological and sedimentological evidence indicates considerable palaeoenvironmental change, from a cold climate lowland fern flora and upland primitive conifer flora low in the sequence, to a lowland cycad-like and upland glossopterid or other gymnospermous flora higher in the sequence. The lithologies range from glacial diamictite at the base to calcrete horizons and redbeds at the top. It is therefore likely that the δ13Cbulk trend is related to palaeoenvironmental change.

THE AGE OF THE CARBONIFEROUS–PERMIAN CONVERRUCOSISPORITES CONFLUENS OPPEL BIOZONE: NEW DATA FROM THE GANIGOBIS SHALE MEMBER (DWYKA GROUP) OF NAMIBIA

Abstract The establishment of the Converrucosisporites confluens Oppel Zone in the Canning Basin of Australia in cored intervals from the Calytrix No. 1 Borehole was considered to be an advance in Gondwana Carboniferous—Permian palynostratigraphy. This was because the zone is associated with a marine fauna that suggests a correlation with the standard Russian Early Permian stages. Moreover the index species has a wide occurrence in Gondwana outside Australia, for example Antarctica, Argentina, Brazil, India, Oman, Saudi Arabia, and Uruguay. The zone was originally considered middle to late Asselian in age, but this was later revised to latest Asselian to Early Sakmarian. Converrucosisporites confluens is reported here from the Ganigobis Shale Member of Namibia in a well-preserved and diverse assemblage including four of the fourteen specified accessory taxa for the Converrucosisporites confluens Oppel Zone. Ash layer IIb of the Ganigobis Shale Member is radiometrically dated as 302.0 ±3.0 Ma (i.e. Pennsylvanian; Gzhelian or Kasimovian) thus the Converrucosisporites confluens Oppel Zone may range earlier than previously thought. Preliminary study of the range top of Converrucosisporites confluens in Argentina and Uruguay suggests that it ranges younger there than in Western Australia with the possibility that the Converrucosisporites confluens Oppel Zone may also be younger than previously thought.

Biostratigraphy and palaeoenvironments of the Ballagan Formation (lower Carboniferous) in Ayrshire

Synopsis A local palynostratigraphical subdivision for the Ballagan Formation in Ayrshire is established, based on the Heads of Ayr coastal section. The lower part of the formation is characterized by a low-diversity CM Biozone Colatisporites-dominated assemblage, while the upper part is characterized by high-diversity assemblages containing Acanthotriletes cf. macrogaleatus and Radiizonates mirabilis, and assemblages dominated by Leiosphaeridia algal palynomorphs. Sedimentological analysis of the lower part of the formation suggests deposition in an arid coastal floodplain environment. Up sequence, there is a gradual reduction in sandstones, suggesting reduced fluvial influence. The Ballagan Formation yields ostracodes indicative of marginal marine and brackish environments, which supports the sedimentological interpretation. The lower part of the formation contains species that are typical of the Tournaisian Beyrichiopsis glyptopleuroides-Eriella Biozone, providing a stratigraphical tie with the CM Biozone assemblage. The upper part of the formation yields ostracodes that elsewhere have been reported from the Viséan, but Lycospora pusilla, the index taxon for the Viséan Pu palynomorph Biozone, was not recovered. Two age interpretations can be offered for this part of the section: either late Tournaisian or early Viséan. A late Tournaisian–early Viséan age for the Ballagan Formation would imply that L. pusilla is so rare in the lower parts of its range that dense sampling failed to recover it. By contrast, a Tournaisian age for the entire Ballagan Formation at the coast implies recalibration of some ostracode ranges. Above the Ballagan Formation, mid-Viséan palynomorph assemblages from the Lawmuir Formation allow the Heads of Ayr volcanic vent to be dated in the range late Tournaisian-mid-Viséan, implying correlation with the Clyde Plateau Volcanic Formation.