Scotland : Hebridean Terrane

The Cambrian succession in the Hebridean Terrane extends for about 200 km from near Durness on the northern coast of the Scottish mainland SSW to Skye (Fig. 14). The classic account of the whole region is the memoir by Peach et al. (1907), which work had a great influence on British geological studies in the Lower Palaeozoic during the earlier part of the 20th century. The geology of the region was described by Park et al. (2002) and some critical localities were reviewed by Prigmore & Rushton (1999). Sedimentological studies by Swett and his collaborators (Swett & Smit 1972) and by McKie (1990a-c, 1993) and new litho- and biostratigraphical work, including the study of microfossil faunas and floras, have led to a re-appraisal of the stratigraphical succession and its correlation

Welsh Basin

The Cambrian rocks of Wales mostly lie within the Avalon composite terrane, apart from a small area of Cambrian rocks of the Monian composite terrane that is discussed in Chapter 9. The Cambrian rocks of the Welsh basin form the greater part of the Dyfed Supergroup of Woodcock (1990), the base of which overlies a widespread early Cambrian unconformity and the top of which extends to the late Tremadocian (Ordovician). The Dyfed Supergroup extends onto the Midland microcraton in attenuated form, with substantial gaps in the successions locally. The correlation of the basal parts of the Dyfed Supergroup is uncertain because of the lack of suitable evidence, but Woodcock was able to interpret the supergroup as a megasequence composed of five sequences, labelled in ascending order Ia to Ie, each separated by eustatic, tectonic or volcanic events, or a combination of these (Woodcock 1990, fig. 6). Sequence Ib includes strata assigned to the later Terreneuvian and all of Series 2; the strata of Sequence Ic are those of Stage 5 and the Drumian. The bases of sequences Id and Ie are particularly strongly marked, mainly by eustatic regressions, the former in the Guzhangian and the latter near the top of the Furongian, so that Ie is essentially composed of Tremadocian strata. Based principally on his extensive work in the eastern North American sector of Avalonia, Landing (1996) divided the Cambrian to Tremadocian successions in Avalonia into ten epeirogenic sequences (Landing 1996, figs. 2, 5). He recognized equivalents of some of these sequences in selected Welsh and English successions (Landing 1996, p. 51, fig. 7), for example the base of his Sequence 6 (which corresponds to Woodcock’s Ic) and Sequence 9 (part of Woodcock’s Id). Although some correlations are doubtful, the presence in Britain of a hiatus at the level of his Sequence 5, as suggested by Landing’s analysis (1996, fig. 7), is a possibility that merits further investigation. On the other hand, the downward extension of the Arvon ‘Slate Belt’ succession to the base of the Terreneuvian appears speculative

The stratigraphy, correlation, provenance and palaeogeography of the Skiddaw Group (Ordovician) in the English Lake District

A new lithostratigraphy is presented for the Skiddaw Group (lower Ordovician) of the English Lake District. Two stratigraphical belts are described. Five formations are defined in the Northern Fells Belt, ranging in age from Tremadoc to early Llanvirn. They are all mudstone or sandstone dominated, of turbidite origin; in ascending order they are named the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirk Stile formations. Two formations are defined in the Central Fells Belt, ranging in age from late Arenig to Llanvirn. These are the Buttermere Formation - a major olistostrome deposit - overlain by the Tarn Moor Formation, consisting of turbidite mudstones with volcaniclastic turbidite sandstone beds. A revised graptolite and new acritarch biostratigraphy for the Skiddaw Group is presented with eight graptolite biozones and thirteen acritarch assemblages and sub-assemblages. The provenance of the group is assessed from detailed petrographical and geochemical work. This suggests derivation, in the early Ordovician, largely from an old inactive continental arc terrane lying to the south-east, with the appearance of juvenile volcanic material in the Llanvirn. Comparisons and correlations of the Skiddaw Group are made with the Isle of Man and eastern Ireland

Biostratigraphical divisions

Since the 19th century geologists working on Cambrian rocks world-wide have relied largely on trilobites as biostratigraphical guides, and these remain important, especially in Series 3 and the Furongian Series, where they enable refined correlations. In Series 2, especially in its lower part where trilobite biostratigraphy becomes increasingly difficult, other criteria are employed, for example the distribution of small shelly fossils, bradoriid arthropods and, where available, archaeocyaths. In Avalonia there are no archaeocyaths, but the bradoriids have been revised and exploited stratigraphically (Siveter & Williams 1997, Williams & Siveter 1998). The recent development of acritarch biostratigraphy, which has made a vital contribution, is discussed below. Towards the base of the Cambrian, body fossils may be very scarce and trace fossils have been used biostratigraphically (Narbonne & Myrow 1988; Bland & Goldring 1995; McIlroy & Horák 2006), though they may be difficult to work with. All the zones referred to are biozones (Rawson et al. 2002), most of those in the Terreneuvian and Series 2 and 3 being assemblage zones, whilst those in the Furongian are local range-zones of selected species

Some new fossil records and notabilia from the Falkland Islands

During recent geological investigations in the Falkland Islands new fossil material, including some previously unrecorded species, was collected at several stratigraphical levels. The sedimentary rock sequence preserved in the islands (Figure 1) comprises marine but near-shore, clastic strata in the lower part, the West Falkland Group of Siluro-Devonian age, succeeded by marine to lacustrine clastic lithologies of the Lafonia Group, of Carboniferous to Permian age (Aldiss and Edwards 1999). At the base of the Lafonia Group is a glaciogenic unit, the Fitzroy Tillite Formation. The sequence is broadly comparable to those present in the originally adjoining parts of the Gondwana supercontinent now fragmented into South America, Africa and Antarctica. Particularly close stratigraphical similarities are seen between the Falklands and the Cape Fold Belt and Karoo Basin of southern Africa

An Early Cambrian archaeocyath–trilobite fauna in limestone erratics from the Upper Carboniferous Fitzroy Tillite Formation, Falkland Islands

Rare clasts of limestone contained in the uppermost Carboniferous Fitzroy Tillite Formation of the Falkland Islands contain a rich Cambrian fauna of archaeocyaths together with a radiocyath and a few trilobites. Neither Cambrian strata nor limestone are present in the indigenous rock succession and the clasts are regarded as exotic erratics, introduced during the Permo-Carboniferous glaciation of southern Gondwana, prior to its Mesozoic break-up. Nineteen archaeocyath taxa have been identified, with seven (plus a radiocyath) occurring in a single clast. Trilobite identifications are less definitive, but they are compared to Yorkella and the Siberian genera Edelsteinaspis, Namanoia and Chondrinouyina. The archaeocyath fauna has an Australo–Antarctic character and the Transantarctic Mountains seem the most likely source for these unusual erratics. Most recent reconstructions of Gondwana rotate a Falklands microplate into a position between South Africa and East Antarctica. There, it is in proximity with the Eastern Cape Province, where tillites within the Permo-Carboniferous Dwyka Group are correlatives of the Fitzroy Tillite Formation, and the ‘Atlantic’ end of the Transantarctic Mountains. The Dwyka Group tillites also contain rare clasts of archaeocyathan limestone and the rotational reconstruction produces a continuity of the apparent ice-flow directions in South Africa and the Falkland Islands

Charles Darwin, Bartholomew Sulivan and the geology of the Falkland Islands: unfinished business from an asymmetric partnership

When in 1846 Charles Darwin published the first account of the geology of the Falkland Islands he drew on his experiences in 1833 and 1834 during the voyage of HMS Beagle, and on collections made at that time by the ship’s Assistant Surgeon, William Kent. Aboard HMS Beagle Darwin struck up a particular friendship with Lieutenant Bartholomew Sulivan who subsequently revisited the Falklands between 1838 and 1845 in command of HMS Arrow and HMS Philomel. The surviving letters that Sulivan wrote to Darwin during those voyages contain a wealth of additional geological observations. Sulivan also dispatched additional geological specimens to Darwin, one of which can be identified in the extant Beagle collection. Darwin utilised some of Sulivan’s structural geology sketches in his 1846 paper, but otherwise the detailed and accurate observations were only partly and ambiguously recorded there in a footnote. Sulivan described fold and cleavage relationships, made the first record of intrusive dolerite dykes in the Falkland Islands, and recorded landforms and lithologies that could have led to the early recognition of both Quaternary and Late Palaeozoic glaciation in the Falklands. Glacial phenomena were not generally understood at the time and Darwin at first misinterpreted some of Sulivan’s prescient observations. The acknowledgements in Darwin’s 1846 paper do scant justice to Sulivan’s contributions which, as a result, have remained generally unappreciated. This paper seeks to rectify that situation whilst reconciling Darwin’s account with the modern geological interpretation and reviewing his specimen collections in current terms

Biostratigraphical control of thrust models for the Southern Uplands of Scotland

Graptolite biostratigraphy affords a robust and relatively accurate means of correlating Ordovician and Silurian hemipelagite and turbidite sequences and has been used to establish the structural development of the regional thrust belt in the Southern Uplands of Scotland. The overall structural pattern has long been recognised: greywackes within individual thrust slices, deposited within a relatively short time-interval, become sequentially younger southwards; each overlies the basal Moffat Shale Group which was deposited over a longer time. However, recent refinement of the graptolite biozonal scheme has allowed the better assessment of along-strike variations within the thrust belt which are here illustrated by two transects; one, based on work in the Rhins of Galloway and the Kirkcudbright areas (SW Southern Uplands), and the other in the Peebles-Hawick area (NE Southern Uplands). The SW transect most closely approximates to the regular pattern wherein a southward-propagating thrust-front incorporated sequentially younger greywacke units. The uniform geometry is interrupted only locally, towards the southern margin of the thrust belt, by a system of back-thrusts producing structural pop-ups. The NE transect departs from this regular model: a northern sector shows the orderly initiation of the thrust belt, but towards the SE a more irregular distribution of the thrust-slice agescan be best explained by outof- sequence movement. This transect also shows more repetitive imbrication of the same biostratigraphic interval than is apparent farther SW. In both transects the fundamental changes in thrustbelt geometry took place from mid-Llandovery times onwards, with a reversion to forward-breaking, in-sequence thrusting at the beginning of the Wenlock. The cause is a matter for speculation, but may be linked withthe accommodation of an obstacle to forward-thrust propagation. However it is recognised that such variationsin thrust geometry are a fundamental feature of most thrust belts and do not require a single regionally significant cause