Geoff Clayton

The Carboniferous System. Use of the new official names for the subsystems, series, and stages

As a result of votes by the Subcommission on Carboniferous Stratigraphy [SCCS] that were ratified by the International Commission on Stratigraphy [ICS] and the International Union of Geological Sciences [IUGS] over the period 1999-2004, the official subdivision of the Carboniferous System has been substantially modified. For subsystems, the terms Mississippian and Pennsylvanian should be used in all regions of the world to replace the more ambiguous and more awkward terms Lower and Upper Carboniferous. Regional geographic names for series and stages may continue to be used in those regions in which they developed, specifically in Western Europe, the USA, and China. However, their global equivalents should be denoted equally, particularly as they become better correlated, in order to facilitate global correlation in future work. The SCCS also voted to standardize the scale of all regional units termed stages at rough equivalency with the global stages now recognized in the Carboniferous (which are similar in scale to those in the adjacent Devonian and Permian Systems). Therefore, the up to 26 subdivisions of the Tournaisian, Visean, Namurian, Westphalian and Stephanian of the regional western European classification should now be ranked and termed only as substages.

The post-Carboniferous evolution of Ireland: evidence from Thermal History Reconstruction

Integration of Apatite Fission Track Analysis (AFTA ® ) and vitrinite reflectance (VR) data from onshore Ireland reveals a complex thermal history, characterized by multiple cooling episodes of late Carboniferous, Jurassic, early Cretaceous, early Tertiary and late Tertiary age. Peak palaeotemperatures in each episode decrease through time to produce an overall long-term cooling trend since the late Carboniferous. Thermal history styles across the region are very similar, though the magnitude of peak palaeotemperatures in individual episodes shows some variation. Similar thermal histories are also identified in the surrounding offshore regions. The regional nature of all these palaeo-thermal episodes, and their correlation with regionally significant unconformities, suggests that heating was due primarily to greater depth of burial, with subsequent cooling representing the progressive unroofing of the present onshore region since late Carboniferous times. In Northern Ireland, explanations of early Cretaceous and early Tertiary palaeotemperatures in terms of greater depth of burial are more difficult to reconcile with geological evidence, and heating due to hot fluid movement appears more likely. This applies particularly to early Tertiary effects, for which the Tertiary Igneous Province provides a ready explanation. Over the entire onshore region, maximum maturity levels in Carboniferous and older units were reached at the end of the Carboniferous, and preservation of hydrocarbons to the present day, through several tectono-thermal episodes, appears unlikely.

Palynomorph Darkness Index (PDI) – a new technique for assessing thermal maturity

Palynomorph Darkness Index (PDI) is proposed as a new thermal maturity indicator. PDI is calculated from measurement of the red, green and blue (RGB) intensities of light transmitted through palynomorphs, using standard palynological microscopes and digital cameras. PDIs determined by different microscope and camera combinations show excellent correlation, suggesting that the method is largely platform-independent, though calibration is required, preferably using photographic filters as standards. Investigation of PDI from experimentally heated Tasmanites reveals a progressive increase with increasing temperature, suggesting that the technique is applicable through a broad temperature range encompassing the whole of the oil window and at least part of the zone of dry gas generation. Potential applications of this inexpensive method include the estimation of thermal maturity of rocks deficient in vitrinite, such as the Lower Palaeozoic rocks of Arabia and the Upper Devonian black shales of North America.

New evidence concerning the thermal history of Devonian and Carboniferous rocks in the South Portuguese Zone

The Late Palaeozoic rocks of southern Portugal have a complex thermal history. Vitrinite reflectance determinations from 90 samples confirm that the organic maturity of Late Palaeozoic rocks in SW Portugal is very high, mainly corresponding to meta-anthracite coal rank. The optic fabric of vitrinite in oriented coal samples from the Brejeira Formation suggests maturation under simple, non-tectonic, burial conditions with peak temperatures being attained prior to Variscan deformation. The lack of any increase in vitrinite reflectance with depth through c. 1u2009km of section in borehole AC-1 is not consistent with conductive heat transfer and is interpreted as the result of late synorogenic to post-orogenic advective heating. This heating episode generated temperatures sufficiently high to produce levels of maturity corresponding to meta-anthracite coal rank through much of the terrane now exposed, but not high enough to result in overprinting of the pre-deformation optic fabric of vitrinite in the thin coals. The slightly lower maturity of older, platform-facies rocks in the Aljezur–Bordeira area may reflect either deposition on a basement high followed by less burial than adjacent parts of the basin, or the area being the site of a sink for descending relatively cool fluids.

The Dispersed Spore Retusotriletes loboziakii sp. nov., Affiliated with the Enigmatic Late Devonian Alga Protosalvinia Dawson 1884

The miospore Retusotriletes loboziakii sp. nov. is described from rocks containing the problematic algae Protosalvinia spp. from two localities in eastern Kentucky, USA. Both sections are from the Upper Devonian (Famennian) Ohio Shale Formation. Single, dispersed specimens of Retusotriletes loboziakii sp. nov. are morphologically identical to spores within dispersed tetrads. Moreover the dispersed tetrads are identical to in situ tetrads within Protosalvinia thalli, and Retusotriletes loboziakii sp. nov. is therefore considered to be the dispersed miospore of Protosalvinia spp. Other taxa in the dispersed miospore assemblage suggest that the Protosalvinia-bearing samples investigated should be assigned to the Famennian VCo Miospore Biozone.

The Palynology of the Hannibal Shale (Mississippian) of Northeastern Missouri, U.S.A. and Correlation with Western Europe

Abstract Mississippian palynomorphs have been recovered from two successions of the Hannibal Shale in Missouri, U.S.A., which is the lowermost Carboniferous unit in the Mississippi Valley. The assemblages are dominated by acanthomorph acritarchs, with Micrhystridium stellatum being the principal species. The miospore assemblages are well-preserved and of low taxonomic diversity. They are dominated by long-ranging taxa that survived the latest Devonian mass extinction event. This assemblage closely resembles assemblages assigned to the Vallatisporites verrucosus-Retusotriletes incohatus (VI) Biozone of Western Europe. However the assemblage from the Mississippi Valley is relatively impoverished in comparison with European assemblages, and many key European zonal taxa are absent. Independent biostratigraphic correlation suggests that the Hannibal Shale is equivalent to the VI Biozone, the Cristatisporites hibernicus-Umbonatisporites distinctus (HD) Biozone, and possibly the lowermost part of the Spelaeotriletes balteatus-Rugospora polyptycha (BP) Biozone in Europe. This indicates the relatively condensed nature of the succession. Palynofacies analysis suggests a shelf to basin depositional environment for the Hannibal Shale; this is consistent with the paleogeographic location of this area during the Early Carboniferous.