Geology Today最新文献

Large calcite-cemented sandstone concretions found in Middle Jurassic sandstones from the isles of Eigg and Skye are a striking feature of the Jurassic geology of the Inner Hebrides. Recent clumped isotope data unequivocally give concretionary calcite cementation temperatures above 50°C and up to 98°C. The high temperatures contrast with the mild thermal history of the enclosing less permeable Jurassic mudrocks. The clumped isotope data also enable calculation of the oxygen isotopic composition of the precipitating fluids: these included percolating rainwater that had interacted with Paleocene volcanics; then, later hot fluids of deeper, basinal origin, squeezed out of compacting mudrocks at depth as the Paleocene lava pile loaded the crust. There is thus direct connection between ‘Jurassic’ sedimentary features and the complex Paleocene volcanic history of the region.

The Bovey Basin in south Devon, UK, provides an instructive example of a Palaeogene pull-apart basin with a complex fluvial sedimentary fill. The basin, one of three that lie along the NW-SE-trending Sticklepath Fault Zone, is filled by about 1200 m of sediment, of which about the lower 700 m (mainly lower Bovey Formation) is concealed. The main Bovey Basin is separated from a smaller sub-basin (the Decoy Basin) south of Newton Abbot. Two main lithofacies are present in the Bovey Formation: lignite-clay; and clay-sand, with subordinate lithofacies of red-mottled clay and sand; and gravel and conglomerate. The upper 500 m or so of the sedimentary infill (middle and upper Bovey Formation) is divided into 10 members. Deposition in the basin was by a variety of fluvial processes, notably floodplain clays and sands deposited by meandering rivers; forest swamps with associated short-lived shallow lakes; and (mainly in the upper Bovey Formation) braided stream sands and gravels that formed mainly on alluvial fans. Lignites in the basin formed from plants that grew in place in forest swamps, a revision of the long-held view that the lignitic material was transported into the basin from forests of the conifer Sequoia couttsiae growing outside the basin on adjacent uplands. Newly discovered ichnofabrics occur in some lignitic sequences and probably represent root traces (rhizoliths).

Granites are coarse-grained igneous intrusive rocks, which, although present in most tectonic settings, are most characteristic of continental crust. While basalts are the most abundant volcanic rocks, granites are the most abundant intrusive rocks. On the scale of the Solar System, while basalts are ubiquitous, granites seem to be restricted to the Earth. The volcanic equivalent of granite is rhyolite, although rhyolites are of much lower abundance than granite, probably because such magmas are more viscous and less likely to reach the surface. In addition, many pyroclastic rocks have a granitic composition and are likely to have their origins in underlying granitic plutons. Granites may originate by differentiation of a basaltic magma, but the abundance of granites and the low abundance of intermediate rocks argues in general against this possibility and granite magmas are generally thought to have formed by melting of pre-existing rocks. Several distinct types of granite are recognized depending on their origin.

Following the end of the last ice age, decelerating global post-glacial sea level rises submerged the southern North Sea, and along this coast, 63 km of soft glacial deposits are still being rapidly washed away. The collapse of the massive ice sheets covering Britain, Ireland and Fennoscandia was a complex process, with warming after 14.7 ka. Limited glaciation then returned during the Loch Lomond Stadial at the end of the Pleistocene, Doggerland, connecting Britain with the continent, was slowly immersed, becoming a shrinking island which was rapidly submerged after 8 ka. This coincided with the Storegga tsunami which hit what remained of Doggerland’s shores and along the coasts of Scotland and Northumberland. Since then, the sea has risen by around 27.5 m. Now it is clear eustatic increases in global sea levels are starting to accelerate once again, we must start adjusting to this reality in the way we manage our coastal infrastructure and try to understand more about how this interacts with naturally dynamic shorelines.

The Waukesha Biota (Brandon Bridge Formation Lagerstätte) is a diverse assemblage of early Silurian (437 Ma) shallow marine fossils from Wisconsin, USA. Their exceptional preservation, in finely laminated dolomitic mudstones, was mediated by tidally-influenced conditions and/or microbial entombment. Shelly biomineralized animals are rare, but soft-bodied organisms are common. Arthropods dominate, including trilobites, phyllocarids, ostracodes, a thylacocephalan, a synziphosurine and various enigmatic groups. The fauna also includes evolutionary ‘holdovers’, such as palaeoscolecids and a lobopodian.

When a volcano erupts, it is often associated with destruction, particularly damage to infrastructure and loss of life. But these natural events also offer unexpected research opportunities, leading to serendipitous discoveries. This was the case for the volcanic events that made the headlines during 19 September to 25 December 2021, on the Canarian Island of La Palma. Rather than viewing the voluminous ash that erupted as a waste material needing to be removed as soon as possible, we saw the many possibilities that this remarkable material could offer science and engineering. Sustainability is a word that is commonly used in connection with geology these days. Here we present some possibilities of how the La Palma ash can be re-purposed for use on this planet but also help us to develop new ideas for the future living on the Moon.

The Vasquez Rocks of Agua Dulce and Santa Clarita, California, USA have become associated with other planets or dimensions since the 1960s, when the popular American television programme Star Trek used them as dramatic backdrops in three episodes, ‘Arena’, ‘The Alternative Factor’ and ‘Friday's Child’. Today they are a popular visitor attraction, but what is their geological background?

The Central Series of the Paleocene mafic to ultramafic Rum complex is the youngest of the three main cumulate series that make up the layered igneous complex of this famed Scottish island. The Central Series lies along the Long Loch Fault and provides insights into the feeder system to the Rum intrusion at an erosion level of about two kilometres below the former land surface. Much of the Central Series consists of a mélange of steep sided bodies of magmatic breccias that stretch along the Long Loch Fault (LLF) in a relatively narrow zone and is composed of blocks and clasts of all sizes derived largely from break-up of the former conduit walls. Repeated movements of the LLF are thought to have been responsible for opening and closing of the magma conduit, resulting in repeated replenishment events, each of which gave rise to new cumulate formation within the Central Series and the bordering Eastern and Western Layered Series, which crystallized under relatively tranquil conditions. The Central Series probably acted as the feeder zone supplying the neighbouring layered series. The more complete of these is the Eastern Layered Series in which 16 conformable units can be distinguished. Others are presumed either to lie unseen at depth or to have been stripped by erosion. The Central Series, although often neglected because of its relative inaccessibility and complexity, formed from successive magma replenishments alternating with large-volume side-wall collapses of previously deposited cumulate material. It could thus be thought of as representing the ‘pulsing heart’ to the Rum volcano and deserves to be regarded as a site of major volcanological and petrological importance. Here we present a summary of some 60 years of investigation into the cumulate rocks of the Central Series.