Cherry L. E. Lewis

Elevated K/T palaeotemperatures throughout Nortwest England: three kilometres of Tertiary erosion?

Abstract Apatite fission track analysis of samples from Northern and Central England demonstrates that most rocks presently at outcrop were subjected to palaeotemperatures greater than 90°C in the latest Cretaceous or early Tertiary. Available data indicate that Late Cretaceous/early Tertiary palaeogeothermal gradients were similar to present values, suggesting that heating was due to burial and that significant Tertiary erosion has removed approximately 3 km of overburden from the entire region. The Irish Sea and Cheshire Basins appear to have lost between 2.7 and 3.3 km of sediment but, more surprisingly, several kilometers of sedimentary cover may also have been removed from surrounding highs such as the Lake District and the Pennines. Further evidence for kilometre-scale erosion is found in fission track data from sediments in contact with the Tertiary Butterton dyke. While zircon fission track ages date the time of intrusion of the dyke at 61 ± 2 Ma, a reduced apatite age of 53 ± 1 Ma supports evidence that the present exposed levels experienced ambient temperatures of around 90°C at the time of intrusion. The cause of this widespread erosional event is considered to have been related to rifting in the North Atlantic and compression resulting from the Alpine orogeny. At 65 ± 5 Ma the timing and amount of cooling identified in this study fits well with a model of doming over a mantle hot-spot, immediately prior to rifting in the North Atlantic. However, associated with this rifting event was the Laramide pulse of the Alpine orogeny, which is known to have caused significant early Tertiary inversion of many basins in northwest Europe. The possibility that, by the end of the Cretaceous, up to 3 km of post-Triassic sediments lay across the Pennines, the Lake District and the Cheshire and Irish Sea basins demands considerable revision of ideas concerning the Late Cretaceous/early Tertiary geology of the region and leaves questions unanswered as to the tectonic and geomorphic processes which caused this regional event.

Elevated palaeotemperatures prior to Early Tertiary cooling throughout the UK region: implications for hydrocarbon generation

Elevated palaeotemperatures prior to Early Tertiary cooling, which affected wide areas of the UK region, have been revealed by Apatite Fission Track Analysis (AFTA™). All available evidence suggests that palaeogeothermal gradients were close to present values and that much of the observed heating was due to greater depth of burial, by 1 to 2 km or more of section that was subsequently removed by uplift and erosion. Uplift and erosion were not restricted to local inversion axes. The palaeotemperature data suggest a broad, regional warping, producing kilometre-scale Tertiary uplift and erosion across a wide area, within which recognized inversion axes represent local regions of maximum uplift and erosion. AFTA data show no thermal effects associated with Cimmerian unconformities, and any heating associated with Cimmerian events was of lesser magnitude than Late Cretaceous to Early Tertiary heating. Over much of the UK region, source rocks attained maximum temperatures and reached peak maturity during this later heating episode. The regional extent of heating at this time and its significance to hydrocarbon source rock maturation have not been fully recognized in the past. The timing of the events described here suggests a link to the development of the Atlantic margin, Laramide inversion tectonics and the onset of Alpine tectonism. However, definitive answers to such questions must await further research, particularly involving integration of AFTA and other thermal indicators with structural and geophysical data

Low-temperature effects of the Skye Tertiary intrusions on Mesozoic sediments in the Sea of Hebrides Basin

Abstract Fission track analysis of samples from the Sea of Hebrides Basin and surrounding regions demonstrates two very distinct age groups around 50 Ma and 300 Ma, regardless of the stratigraphic age. Apatite fission track results from sediments within 8 km of the Tertiary igneous complex on Skye were totally reset by temperatures >110°C during intrusion of the centres, but now yield ages younger than the granites. Zircon fission track results from the granites also demonstrate ages significantly younger than their hosts. Mean track lengths in apatites of <13.5 µm, and reduced apatite and zircon ages, suggest that temperatures remained elevated throughout the 6 Ma during which intrusive activity occurred, but were hotter within the granite bodies than the surrounding sediments. Prior to intrusion of the complex, temperatures in the basin sediments presently at outcrop were unlikely to have been higher than 50°C, and beyond the effects of the Tertiary intrusions, age and track length distributions from northern Skye, the Isle of Lewis and the west coast of Scotland illustrate that the area is unlikely to have been buried beneath more than 2 km of sediment at any one time since the end of the Devonian. North of the Highland Boundary Fault, Tertiary uplift and erosion in Scotland is considered to have been between 1–1.5 km. This is consistent with a regional pattern of Tertiary erosion identified across the British Isles, but considerably less than that recognised in Northern England.

Thermal history of the Kunlun Batholith, N. Tibet, and implications for uplift of the Tibetan plateau

Abstract At an average elevation of 5 km, the Tibetan plateau is the highest in the world. The timing of uplift and the mechanism by which the crust was thickened to 70 km have long been a subject for debate. Preliminary Rb-Sr and 40 Ar/ 39 Ar biotite ages combined with zircon and apatite fission track dates from the Kunlun batholith in northern Tibet allow for comparison with the previously established uplift history of southern Tibet, and inferences to be made about uplift of the plateau as a whole. A major divide in the Kunlun Terrane is identified at the Golmud Fault, across which apatite ages decrease from 100 Ma in the north to 20 Ma in the south. This is believed to be as a result of a thrusting event at 120 Ma which uplifted the Northern block and overthrust the Southern block to the south. Subsequent to this event, surface approach rates (SAR) of less than 70 m Ma -1 from biotite closure at 120 Ma to apatite cooling at 20 Ma in the Southern block, indicate a long uninterrupted period of thermal equilibration in an essentially static continental block. Mean confined track lengths of 12.5–13.5 μm in apatites suggest that samples exposed at the surface today resided in the fission track partial annealing zone prior to the onset of uplift which must have occured subsequent to 20 Ma. This indicated that at least 3–4 km have been uplifted since this time and that the onset of this phase could have been as recent as 8 Ma.

Doctoring geology: the medical origins of the Geological Society

Abstract Four of the Geological Societys 13 founders were medical men: William Babington, James Parkinson, James Franck and James Laird, the Societys first Secretary. All were physicians and mineralogists except Parkinson, an apothecary surgeon and fossilist. At least 20 percent of the Societys early members were also medical practitioners whose prime interest was mineralogy. The subject was taught as part of medical training, required as it was in the fabrication of medicines, thus medical men were drawn into mineralogy and on into geology. In 1805 a number of medical practitioners broke away from the constraints of their parent body, the Medical Society of London, to form the Medical and Chirurgical Society, which became a role model for the young Geological Society when challenged by its parent body, the Royal Society. Driven by wealthy mineral collectors and patrons of science like Charles Greville, one reason – perhaps the reason – for founding the Society was to map the mineralogical history of Britain. Towards this endeavour, Babingtons expertise in mineralogy brought people together, Laird organized them and Parkinson was invited because he was not a mineralogist. Franck was unable to participate significantly, being away at war for much of the time. The contribution made to the founding of the Geological Society by each of the medical founders is examined, and a biographical sketch of each man reveals the close relationship between medicine and the emergence of this new science of geology.

Arthur Holmes’ vision of a geological timescale

Abstract Arthur Holmes (1890–1965) was a British geoscientist who devoted much of his academic life to trying to further the understanding of geology by developing a radiometric timescale. From an early age he held in his mind a clear vision of how such a timescale would correlate and unify all geological events and processes. He pioneered the uranium-lead dating technique before the discovery of isotopes; he developed the principle of ‘initial ratios’ thirty years before it became recognized as the key to petrogenesis, and he wrote the most widely read and influential geology book of the twentieth century. But despite all this, much of his contribution to geology has gone unrecognized in the historical literature. This paper attempts to redress this omission, to dispel some of the myths about Holmes’ life, and to trace his contribution to the development of the geological timescale.

‘Our favourite science’: Lord Bute and James Parkinson searching for a Theory of the Earth

Abstract John Stuart, the third earl of Bute and the British Prime Minister from 1762 to 1763, and the apothecary surgeon James Parkinson both amassed large and important geological collections; both believed in the biblical Deluge; both admired the work of Jean André de Luc; and both were fascinated by the study of geology. Each sought a theory that would explain the geological phenomena they observed but which also allowed them to maintain their religious integrity. They were men of their time, struggling to come to terms with a new science that challenged their strongly held religious beliefs. Butes Observations on the Natural History of the Earth, never published, provides us with a snapshot of his thinking about prevailing theories of the Earth. He dismissed all except those that fitted the geological facts as understood at the time, but was nevertheless unable to progress from a rigid belief in the biblical Flood having been a miracle. Parkinsons Organic Remains of a Former World reveals a man fully conversant with contemporary geological ideas being propounded elsewhere in Europe. Also highly religious, Parkinson oscillated between his deeply held beliefs and the contradictory evidence provided by the fossils he held in his hand.

Arthur Holmes’ unifying theory: from radioactivity to continental drift

Abstract Only ten years after the discovery of radium in 1897, Arthur Holmes (1890–1965) began his studies at the Royal College of Science in London where he completed the very first U/Pb age determination designed specifically for that purpose. His continued interest in radioactivity and its effect on the thermal history of the Earth led to his early recognition that the age of the Earth should be measured in thousands, not hundreds, of millions of years, a subject he pursued for the rest of his career, despite considerable opposition from traditional geologists. Following a short period in Burma, he returned in 1922 to find that not only had attitudes to the age of the Earth changed, but that geologists were embroiled in a new controversy over continental drift. Evidence is put forward that suggests Holmes may have been aware of Wegener’s theories virtually from the time they were proposed, and that by 1924 he was already searching for his own theory which would explain all geological processes. His profound understanding of the effects of radioactivity on the internal processes of the Earth, and his advanced knowledge of petrology, placed him in a unique position to develop a mechanism for driving continental plates around the globe. The progression of his ideas for this mechanism — convection currents in the mantle — and the unifying theory that that led to, is traced through his papers and letters to colleagues.

Celebrating the age of the Earth

Abstract The age of the Earth has been a subject of intellectual interest for many centuries, even millennia. Of the early estimates, Archbishop Ussher’s famous calculation of 4004 bc for the date of Creation represents one of the shortest time periods ever assigned to the Earth’s age, but by the seventeenth century many naturalists were sceptical of such chronologies. In the eighteenth century it was Nature that provided the record for Hutton and others. But not all observers of geology enquired about time. Many, like William Smith, simply earned a living from their practical knowledge of it, although his nephew, John Phillips, was one of the first geologists to attempt a numerical age for the Earth from the depositional rates of sediments. For more than fifty years variations of that method prevailed as geology’s main tool for dating the Earth, while the physicists constrained requirements for a long timescale with ever more rigorous, and declining, estimates of a cooling Sun and Earth. In 1896 the advent of radioactivity provided the means by which the Earth’s age would at last be accurately documented, although it took another sixty years. Since that time ever more sophisticated chronological techniques have contributed to a search for the oldest rocks, the start of life, and human evolution. In the attempt to identify those landmarks, and others, we have greatly progressed our understanding about the processes that shape our planet and the Universe, although in doing so we discover that the now-accepted age of the Earth is but a ‘geochemical accident’ which remains a contentious issue.

The age of the Earth in the United States (1891–1931): from the geological viewpoint

Abstract In North America, prior to the Second World War, discussions on the age of the Earth were a minuscule part of the geological literature, as demonstrated by the small number of papers indexed to the subject in bibliographies. Indeed, during the first quarter of the twentieth century, there were few general papers on this topic circulating among those geologists who dealt with sedimentary rocks and fossils; nevertheless, evidence is provided that many geologists were aware of the ‘debate’ going on in Britain. As the methodology for determining the length of geological time dramatically changed during the four decades represented here, so too did the evolution of ideas about the age of the Earth. These can conveniently be divided into three time periods: before, during and after the discovery that radioactivity could be applied to the dating of rocks. The first section reviews the attitudes of geologists in America to the age of the Earth in the 1890s. It is followed by their reactions to the discovery of radioactivity. The third part discusses two major publications on the age of the Earth which reflect the ultimate acceptance, by geologists, of the long timescale revealed by radioactivity. Because much of the early work on radioactivity was being done in Europe, American geologists were marginally later than their British counterparts in accepting the concept of radiometric dating, but by the end of the period under consideration they led the field in geochronology.