The Tertiary Period (65 – 2.6 Ma)

The Tertiary geological period covers the time span 65-2.6 Ma – from the end of the mass extinction of the dinosaurs to the Quaternary ‘Ice Age’. It was a period of immense contrasts, ranging from the violent volcanic activity that gave birth to the Atlantic Ocean to the gentler, slower weathering processes that shaped Britain’s landscape before the arrival of the glaciers. Today, many geologists use the terms Palaeogene and Neogene to describe this time span, with the Palaeogene comprising, from oldest to youngest, the Palaeocene, Eocene and Oligocene epochs and the Neogene consisting of the Miocene and Pliocene. The Palaeogene, Neogene and Quaternary collectively make up the Cenozoic Era. When exactly the Pliocene ended and the Quaternary began, however, still remains a hotly debated geological topic. In general, the Tertiary was a period of climatic cooling. At the start, mammals replaced reptiles as the dominant vertebrates. Hominoids, the earliest relatives of humans, appeared towards the end of the period.

The Atlantic opens …..

The breakup of Pangaea began about 170 million years ago, in Jurassic times, and in the following 100 million years, Laurentia separated from the amalgamated Baltica, Siberia and Africa (and the rest of the microcontinents that make up Western Europe), whilst the Gondwana split later in the Cretaceous Period and both North and South America drifted westwards. Incidentally, most of old Avalonia remained with Western Europe, excepting the area of the modern North American continent between New England and Newfoundland, which consists of Avalonian rocks. A new ocean – the Atlantic – was developing and widening, with much volcanic activity, including an intense episode of volcanism and associated igneous intrusions, focused along the western seaboard of Scotland, early in the Cenozoic Era. By this time, Anglesey lay between 40 –50 degrees north of the equator.

Tertiary Olivine Dolerite Dykes

Despite a small outcrop area, Anglesey’s Tertiary rocks have been disproportionately important to our understanding of the geological evolution of Great Britain and to the development of geological science. First, a swarm of Palaeogene dykes bears witness to the plate tectonic processes that split the ancient continent of Pangaea and saw the birth of the Atlantic Ocean around 65 Ma. These dykes were born from the same volcanic activity that formed the spectacular basalt columns of the Giant’s Causeway in Northern Ireland.

The dykes are rich in iron-bearing silicate minerals, and are rather prone to deep spheroidal weathering, so that their outcrops often display the classic “onion-skin” texture. Mapping of the dykes, by aeromagnetic survey, has revealed a series of apparent offsets that have been interpreted as evidence that significant amounts of sinistral faulting have occurred in North Wales in the last 55 million years. However, because some of the dykes are exposed at outcrop, these apparent offsets can be examined in detail. The outcrops show that the intrusions ‘skipped’ from following one plane of weakness to another, and so the offsets appear not to be due to later faulting. The uncertainties require that the outcrops are preserved and subjected to future investigation.

tertiary dolerite dyke in Porth Dafarch
tertiary dolerite dyke in Porth Dafarch ( J Conway)
Above: an early Cenozoic dyke cutting older rocks on the coast at Porth Namarch, near Holyhead. Basic in composition, with a lot of iron-bearing silicates, the dyke-rock shows the distinctive, spheroidal “onion-skin” weathering pattern [J Conway]

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This conference poster by BGS staff includes magnetic scans which very clearly show the swarm of Tertiary dykes crossing Anglesey

Beamish, D.; Schofield, D.I.. 2010. HiRes airborne geophysical survey of Anglesey: a key dataset for unravelling complex geology and establishing an environmental baseline. [Poster] In: Geological Society of America Joint Meeting, Baltimore, USA, 14-16 March 2010.

Miocene

The recent discovery of a tiny pocket of Miocene deposits in northern Anglesey could indicate that a substantial Miocene cover once existed, but was largely eroded following Plio-Pleistocene uplift. It provides a rare glimpse of the climates and processes that shaped Britain’s landscape and heralded the onset of the Quaternary ‘ice ages’, some 2.6 Ma. It seems likely that the Menaian surface (and the adjacent Snowdonian massif) were well-established landscape features before the end of the Miocene. The few areas of upstanding relief may represent inselbergs exhumed from a deep saprolitic cover.

Possible Miocene sediments in a solution pit in limestone near Cemaes [ J Conway]

The Cenozoic Era also brought into existence a mineral that is named after Anglesey. The lead sulphate, anglesite, was first described by Beudant in 1832 at Parys Mountain, although the mineral, previously known as “vitriol de plomb”, had been known there for many years. It formed during the deep, prolonged subtropical weathering that the climate permitted at the time, during which the sulphide orebodies were deeply oxidised into a thick “gossan”, much of which survived later erosional processes and was instead removed by the miners in their search for copper ore.

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