Jurassic to Palaeogene of the Southern Uplands

From Earthwise
Jump to navigation Jump to search
Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Introduction[edit]

Distribution of Palaeogene dyke swarms across the south of Scotland and surrounding regions. P912364.
Palaeogene dolerite dyke cutting altered serpentinite of the Ordovician Ballantrae Complex at Balcreuchan Port on the north side of Bennane Head. P005984.
Outcrop of the Cleveland Dyke across south-west Scotland, as determined from ground magnetic traverses. P912365.

The youngest strata preserved in the Carlisle Basin belong to the Lower Jurassic Lias Group, but their outcrop is restricted to an area west of Carlisle that does not extend north of the Scottish border. Almost 190 million years then elapsed between deposition of these Lias Group rocks and the onset of the Quaternary glacial episodes, yet the geological record across the south of Scotland provides little evidence of the original distribution and character of any deposits laid down during this time. It is widely agreed that Mesozoic rocks once covered at least some of the region, well beyond their present distributions in the Carlisle Basin and the offshore Solway Firth Basin. There are also extensive Palaeogene to Neogene successions in the offshore basins, but their onshore outcrop is very limited. None are preserved in southern Scotland, though at least 600 m of clay, sand and lignite form the Palaeogene Lough Neagh Clays Group in Northern Ireland.


Post-Triassic subsidence[edit]

It is likely that the Jurassic sedimentation pattern that had been established in the Carlisle and offshore basins continued into Early Cretaceous times. Comparison with regions to the south and east, where the Jurassic to Early Cretaceous stratigraphical record is more complete, suggests that the Carlisle and Solway Firth extensional basins would have continued to develop through normal faulting well into Early Cretaceous times. In west Cumbria, fault displacement demonstrably postdates the Sherwood Sandstone Group within the hanging-wall block of the Lake District Boundary Fault, whilst several faults in the Carlisle Basin displace Lias Group rocks. Supporting evidence for post-Triassic movements on the northern margin of that basin is provided by a radiometric (U-Pb) date of 185±20 Ma derived from uraninite veins within small north-west orientated tear faults in the footwall of the North Solway Fault at Sandyhills Bay [NX 890 546].

Early in Cretaceous times, areas such as the Lake District and Pennine blocks experienced another interval of erosion as the widespread, late Cimmerian unconformity developed; it seems highly probable that the Southern Uplands massif also experienced erosion at that time. Regional subsidence then dominated across the southern UK during Late Cretaceous times and probably extended to northern Britain, resulting in deposition of a relatively uniform Cretaceous sequence, dominantly of the Chalk Group. The Southern Uplands massif may possibly have formed an emergent landmass throughout this interval, but the presence of a Chalk Group sequence in Northern Ireland (Ulster White Limestone Formation) about 130 m thick, and of chalk clasts in volcanic vents in Arran, suggests that chalk deposition encroached across at least the lower lying parts of southern Scotland. Maximum post-Variscan burial of the region is likely to have been attained towards the end of the Cretaceous Period, prior to renewed uplift commencing in Late Cretaceous to Early Paleocene times.


Palaeogene magmatism[edit]

The south of Scotland was affected by distant events of epic proportions during the Cenozoic Era which triggered an episode of erosion that has continued, probably with little interruption, until the present day. The cause was thermal uplift along the north-west margin of Europe as a precursor to the formation of new oceanic crust and opening of the North Atlantic Ocean. Uplift was initiated by the the proto-Icelandic mantle plume acting on the base of the lithosphere in a pre-Atlantic region that included the west coast of Scotland, Northern Ireland and eastern Greenland. This area became the focus of intense magmatism from about 60 to 55 Ma, during which time immense volumes of basaltic magma were erupted from fissures and central volcanoes, with the accompanying intrusion of central-complexes, dyke swarms and sills.

Palaeogene mafic dykes cut across the Southern Uplands from north-west to south-east, as part of a regional swarm of high-level minor intrusions that occurs throughout western, central and southern Scotland and extends well into northern England (P912364). Abundant coeval dykes are found in Northern Ireland and the Isle of Man, whence they can be traced by their aeromagnetic anomalies into north-west England. The Scottish dykes have been generally thought to emanate from the Mull Centre and include a section of the 250 km long Cleveland Dyke, which has given radiometric ages (K-Ar) in the 56 to 59 Ma range. Other dykes in the central Southern Uplands appear to link the Blyth and Sunderland subswarms of Northern England with their putative origin at the Mull Centre. The more prominent Southern Uplands dykes are usually a few metres wide, with an exceptional 23 m recorded for part of the Cleveland Dyke. In northern England both the Cleveland and the Acklington dykes reach a width of 30 m. In the western part of the Southern Uplands and the Girvan area there are, in addition, many examples of thin Palaeogene dykes (<2 m across) cutting the Lower Palaeozoic rocks (P005984). These thin dykes appear to have no great length extent and may have originated as en echelon clusters of intrusions. Farther west, in the Firth of Clyde, thin Palaeogene dykes cut the microgranite of Ailsa Craig, itself dated at 61.5 ± 0.5 Ma (Rb-Sr whole-rock). This provides a maximum age for intrusion of the dyke swarm, which can be regarded as a Paleocene event.

The dykes are typically composed of microgabbro or basalt to basaltic andesite, with phenocrysts of plagioclase, clinopyroxene and orthopyroxene contained within an altered glassy groundmass. Margins to the thicker dykes are chilled whereas the central portions are composed of coarser-grained microgabbro, consisting of randomly orientated to weakly aligned plagioclase laths with intergranular ophitic to subophitic clinopyroxene and interstitial quartz. Olivine, where present, is typically replaced by chlorite and serpentine. Some of the larger intrusions, such as the Cleveland Dyke, show more compositional variation and are andesitic in places. The contact metamorphic aureoles adjacent to the dykes, even the largest of them, are typically very narrow.

The larger dykes, such as Cleveland and Acklington, represent huge volumes of magma. For the Cleveland Dyke alone this has been estimated to be at least 85 km3. Until recently, the preferred mechanism for their intrusion involved lateral emplacement from one of the major volcanic centres. Modelling of the Cleveland Dyke had suggested that it represents a single pulse of magma that spread laterally from a reservoir beneath Mull at a velocity of up to 18 km per hour, reaching its farthest extent in less than 5 days. Only the clusters of small, en echelon dykes were thought to have been intruded vertically, perhaps as offshoots from deeper, laterally emplaced bodies. However, detailed work on the Cleveland Dyke in Scotland is now taken to favour more general vertical intrusion.

It has now been established that the Cleveland Dyke, at least in its Scottish outcrop, shows unusual and considerable compositional heterogeneity along its length, being andesitic to dacitic in places. It is also a more complex intrusion than previously thought; at some localities it appears as a single feature and at others as two or more subparallel dykes that may differ in composition. Individual dyke segments sometimes overlap to produce an en echelon arrangement with, in some cases, offsets of more than 1 km between segments and considerable variation in trend between them. The major dyke offsets generally occur at the intersections with the north-east to south-west, Caledonian tract boundary faults (P912365). Curiously, a comparison of the Cleveland Dyke’s aeromagnetic signature with that derived from ground traverses shows that the two anomalies are not everywhere symmetrical, or even parallel, and that the subsurface body is broader than the dyke outcrop. The dyke outcrop would seem to be produced by relatively thin ‘blades’ rising from a larger body at depth. All of this information is more readily reconciled with vertical intrusion of the dykes rather than with lateral emplacement. It now seems most likely that the Palaeogene dykes of the south of Scotland were sourced from small, high-level magma chambers fed by a regional reservoir. The latter most probably arose through magmatic underplating of the lithosphere by the proto-Icelandic mantle plume.


Cenozoic uplift and erosion[edit]

During Neogene (probably Miocene) times, a further major episode of uplift affected the Solway Firth and Carlisle sedimentary basins, probably as a distant effect of the Alpine Orogeny. This arose from the collision, away to the south, of the African and European plates. The basins’ pre-existing, extensional boundary faults were reactivated with a reverse sense of movement, which also resulted in the folding of adjacent basin strata. As a result of the two major tectonic uplift events during the Cenozoic, and the subsequent erosion that they initiated, it is estimated that between 700 and 2500 m of strata have been stripped from parts of northern England, including the entire cover of Upper Jurassic and Cretaceous rocks.

At least the lower-lying parts of southern Scotland, peripheral to the Southern Uplands massif, would be expected to have experienced a similar history, albeit that the possible Mesozoic cover might have been thinner. However, it is worth noting that the minimum figure of 700 m of erosion derived from northern England came from the Scafell area of the central Lake District, where the current surface altitude (ca 950 m) exceeds that of the Southern Uplands. The Scafell data were obtained by apatite fission track studies, a technique that measures the radiation damage suffered by individual crystals and the degree to which it has been annealed by the rising temperature experienced during burial. The greater thickness, 2500 m, was calculated to have been removed from the Carlisle Basin sequence, with the Lias Group alone accounting for as much as 1500 m of the loss.

No Cenozoic sediments are known to have survived in the south of Scotland, though the products of pervasive weathering during this time may have been preserved locally. Possible examples occur in the Cheviot Hills, where both the Devonian volcanic rocks and the granite are intensely altered and disintegrated in places to depths of between 2 and 50 m; the residual deposit of sand and clay is referred to as saprolite. The preservation in the saprolite of original igneous and volcanic textures shows that transformation occurred in situ. The formation of such deeply weathered rock profiles would have been aided by the warm, humid conditions that prevailed for much of Cenozoic time, most notably during the Eocene Epoch (about 34–56 Ma).


Bibliography[edit]

Chadwick R A, Kirby, G A, and Baily, H E. 1994. The post-Triassic structural evolution of north-west England and adjacent parts of the Irish Sea. Proceedings of the Yorkshire Geological Society, Vol. 50, 91–102.

Dagley, P, Skelhorn, R R, Mussett, A E, James, S, and Walsh, J N. 2008. The Cleveland Dyke in southern Scotland. Scottish Journal of Geology, Vol. 44, 123–138.

Green, P F. 2002. Early Tertiary palaeothermal effects in northern England: reconciling results from apatite fission track analysis with geological evidence. Tectonophysics, Vol. 349, 131–144.

Holliday, D W. 1999. Palaeotemperatures, thermal modelling and depth of burial studies in northern and eastern England. Proceedings of the Yorkshire Geological Society, Vol. 52, 337–352.

Jolly, R J H, and Sanderson, D J. 1995. Variations in the form and distribution of dykes of the Mull swarm, Scotland. Journal of Structural Geology, Vol. 17, 1543–1557.

MacDonald, R, Wilson, L, Thorpe, R S, and Martin, A. 1988. Emplacement of the Cleveland Dyke: evidence from geochemistry, mineralogy and physical modelling. Journal of Petrology, Vol. 29, 559–583.