Bedrock Geology UK North: Jurassic and Cretaceous
|This topic provides descriptions of the rock types appearing on the British Geological Survey 1:625 000 scale map of the UK North and gives a brief explanation of their origins.
Author: P Stone (BGS); Contributor: A A Jackson (BGS)
200 to 65 million years ago
The break-up of the Pangaea ‘supercontinent’ during the late Triassic brought an end to the prolonged period of mainly terrestrial conditions across the northern British region. Marine transgression extended across an ever-widening area until, by the early Jurassic, global sea levels were relatively high and most parts of the region were submerged. A period of uplift and erosion in the mid to late Jurassic is recorded by a widespread unconformity, with the maximum effect seen in the North Sea basin sequences. The commensurate fall in sea level continued during the early Cretaceous and an extensive unconformity developed across the surrounding land areas, until rising sea levels brought a marine transgression through the later part of the Cretaceous. Continental drift had carried Britain and Ireland to a latitude of about 35°N by the end of the Triassic and a slow northwards drift continued during the Jurassic and Cretaceous periods (Figure P785800f). The climate was strongly seasonal with warm, relatively dry summers and cool wet winters.
Although the deposition of Jurassic and Cretaceous strata was originally widespread, subsequent uplift and erosion have removed rocks of this age from much of northern Britain, leaving only scattered remnants. Much more extensive spreads of Jurassic and Cretaceous strata are preserved across southern Britain, from Devon to Yorkshire, and also underlie much of the North Sea. The lowermost Jurassic unit, the Lias Group (J1) and its correlatives, is characterised by grey, calcareous mudstone with thin interbeds of limestone, commonly nodular. Subordinate rock types present include sandstone and ironstone. The Lias Group is a marine deposit with a rich fossil fauna, from which the abundant bivalves and ammonites provide a biostratigraphical standard. Rocks of this age and character form the base of the sequence in Yorkshire and are also present, though poorly exposed, in the central part of the Solway Basin, west of Carlisle. They also occur in Northern Ireland, mostly appearing as a fringe to the protective cover of the Palaeogene Antrim lavas, and up to about 250 m of Lias Group have been proved in boreholes. Farther north, the Lias Group forms the base of the more extensive Jurassic sequences seen in the Hebrides and on the north-east coast of the Scottish mainland.
Jurassic rocks of the Hebrides
In the south of the Hebridean area, the lower part of the early Jurassic Lias Group consists mainly of the characteristic fissile calcareous mudstone with thin interbeds of limestone in a sequence about 30 m thick. Northwards, this lithological assemblage interdigitates with an increasing proportion of calcareous sandstone, and the sequence thickens to about 70 m. The calcareous sandstone comes to dominate the lower Lias Group sequences on Skye and Raasay where it is succeeded first by non-calcareous sandstone and mudstone and then by about 75 m of dark grey, fissile and micaceous mudstone. The top of the Hebridean Lias Group is marked by sandstone interbeds within a mainly mudstone sequence and by the appearance of an oil shale and 2 to 3 m of distinctive ooidal chamosite ironstone. The ironstone is cross-bedded in places and was deposited in shallow water.
A stratigraphical break and local unconformity separate the ironstone at the top of the Lower Jurassic, Lias Group, from a dominantly sandstone sequence of mid Jurassic age (J2–4) that is an approximate correlative of the Inferior Oolite Group seen in southern England. This sandstone sequence (Bearreraig Sandstone Formation), with sporadic mudstone and sandy limestone interbeds, ranges up to 550 m thick and includes the thickest sandstones known from the onshore British Jurassic (Plate P000885). The sandstones were deposited in shallow marine conditions with a strong tidal influence, contain abundant ammonites at certain horizons, and are succeeded by strata that accumulated as lagoonal deltas and mudflats encroached. These form part of the Great Estuarine Group and are mostly disposed in deltaic cycles of alternating sandstone and mudstone. The sandstone is highly variable: clean and white with large carbonate concretions (Plate P000654) to dark and muddy. The fissile mudstones may be calcareous or bituminous. The total thickness of the Great Estuarine Group in the Hebridean area is about 250 m. A marine transgression followed, and the Great Estuarine Group is succeeded by up to 50 m of marine sandstone and mudstone that extend the sequence up into the Upper Jurassic. In Skye the Middle Jurassic sandstones contain rare dinosaur remains — fossil bones and footprints.
Jurassic rocks of north-east Scotland
Small outcrops of Lias Group strata (J1) occur in the Lossiemouth area, on the south side of the Moray Firth, whilst on the north-west shore of the Firth a more extensive, but still restricted coastal sliver of mainly Jurassic strata, the Sutherland Group (J1–6), is the onshore representative of a much more extensive offshore sequence. Marine transgression did not extend into this region until well into the Jurassic Period, and the Lower Jurassic sequence (J1), about 120 m thick, begins with alluvial fan conglomerates that are partly derived from the underlying, terrestrial Triassic sandstone. Above the conglomerate, a deltaic mudstone and sandstone succession contains thin coals and seatearths, and an interbedded quartzite unit deposited in an estuarine channel. This contains much washed-in plant debris; lush vegetation clearly covered adjacent land areas. Towards the end of the early Jurassic, marine conditions finally prevailed and micaceous mudstone with a fossil fauna of ammonites and brachiopods is preserved.
A stratigraphical break follows the Lower Jurassic marine mudstone, above which the Middle Jurassic sequence (J2–4) shows a return to nonmarine lithologies: fluvial sandstone and alluvial plain mudstone are succeeded by lagoonal, carbonaceous and bituminous mudstone and a 1 m-thick coal seam. Above the coal, the Upper Jurassic sequence (J4-6) is dominantly marine, with a variety of interbedded sandstone and mudstone lithologies, and with muddy limestone appearing towards the top. Occupying the highest part of the Upper Jurassic sequence is a spectacular boulder conglomerate that may exceed 500 m in thickness (Plate P002200). It is a chaotic rock-fall breccia containing huge foundered blocks and was derived from Old Red Sandstone strata that were exposed in a major, synsedimentary, submarine fault scarp. A shallow water fauna of reef coral and large brachiopods was derived from the top of the scarp and incorporated into the boulder beds, which accumulated in relatively deep water and locally alternate with dark grey, marine mudstone containing ammonites and other fossils. The reef corals are the most northerly examples of their kind known from the Jurassic.
Jurassic rocks of the Cleveland Basin, north-east England
Calcareous mudstone and nodular limestone with sporadic sandstone interbeds form the lower part of the Lias Group (J1) in the Cleveland Basin, where it is about 450 m thick. In the middle part of the group the Staithes Sandstone and Cleveland Ironstone dominate locally before a return to mainly mudstone in the upper part. The latter is locally bituminous, with washed-in plant material sometimes converted to jet. The Lias Group was deposited in a shallow-water, near-shore, marine environment. The succeeding Middle Jurassic contains several stratigraphical hiatuses. At the base up to 12 m of sideritic sandstone form the Dogger Formation. Above this, the Ravenscar Group (J2–3; 125 thick) together with some succeeding beds, is an intricate interfingering of mudstone, sandstone, conglomerate, ironstone and limestone (ooidal in places) that accumulated in fluviodeltaic spreads, in lagoons and in brackish creeks. Plant remains are common and there is a profusion of shelly fossils in some beds; more rarely, insect remains and reptilian footprints are preserved. Some of the more calcareous units, such as the Scarborough Formation, have a wide distribution and record marine transgressions. The Cornbrash Formation, which immediately overlies the top of the Ravenscar Group, was also deposited during a major marine transgression and marks a substantial rise in sea level.
More uniform conditions prevailed after the ‘Cornbrash transgression’ with deposition of sand — the Kellaways Formation — and, later on, mud. This trend continued into the late Jurassic when mud accumulated under quiet marine conditions; up to 45 m of the grey-green mudstone, the Oxford Clay Formation and the Osgodby Formation (J4), is now preserved in the Cleveland Basin. The mudstone is richly fossiliferous, containing ammonites, belemnites and a variety of bivalves. The succeeding Corallian Group (J5) (Plate P006779) is about 100 m thick, and comprises calcareous sandstone and limestone. It formed in a shallow-marine environment and includes coral reefs that were progressively reworked into bioclastic limestone; other limestones are variously sandy or ooidal. The highest part of the Jurassic sequence records a return to mainly mud deposition (J6). The Ampthill Clay Formation is up to 90 m thick and contains siderite nodules, and the Kimmeridge Clay Formation is about 150 m thick, highly bituminous and contains limestone nodules. These clay-dominated formations were deposited when relatively uniform, shallow-marine conditions prevailed over a wide area of southern and eastern England and the North Sea. Conditions were ideal for the preservation of organic matter and the Kimmeridge Clay Formation is now the principal source rock for oil throughout the region.
In the Cleveland Basin a non-sequence separates Cretaceous marine strata from the underlying Kimmeridge Clay Formation. About 100 m of grey clay with sporadic calcareous and phosphatic nodules makes up the Speeton Clay Formation (K2–3), which spans the Lower Cretaceous as the lateral equivalent of the Wealden and Lower Greensand groups of southern England; it contains several additional non-sequences. The Chalk Group, which follows, lies at the northern extremity of a broad outcrop extending south to The Wash and thence across south-east England. It was deposited at the acme of the late Cretaceous marine transgression in a warm sea with an estimated depth range of 50 to 300 m. Chalk is a micritic limestone that incorporates the debris of planktonic algae and foraminifera, together with coarser fragments of marine invertebrates. There was very little terrigenous input and intense bioturbation would have created a thick sea-floor carbonate sludge. The widespread siliceous flint nodules were nucleated on the remains of animals such as sponges or sea urchins or, in some cases, formed as burrow fills. Just below the Chalk in the Cleveland Basin, several metres of red chalk form a condensed sequence that accumulated very slowly; the colour was imparted by red mud washed in from a low-lying, contemporary land area. The red chalk (Hunstanton Formation: K4) is equivalent to the Gault and Upper Greensand formations of southern England. Above it the Chalk Group is represented by more than 300 m of grey and white chalk with flint layers in some formations (K5 & K6). It forms the spectacular sea cliffs at Flamborough Head (Plate P006782).
A number of small, isolated outcrops of Late Cretaceous strata are scattered across the Hebrides, wherein fairly restricted and localised sequences rarely exceed 20 m in thickness. Their deposition followed a period of uplift and erosion, when renewed marine transgression established a low-energy, shallow-marine depositional environment. The most common rocks are fine-grained sandstones (either quartzose or green and glauconitic) and chalk; conglomerate also occurs, but rarely.
In Northern Ireland, only Upper Cretaceous rocks are present (K6) and include the youngest chalk strata seen in Britain. They are exposed in the spectacular cliff sections along the coastline of Antrim, preserved there and elsewhere beneath a protective cover of Palaeogene lavas (Plate P225312 and P225483). At the base of the sequence about 30 m of glauconitic sandstone with thin siltstone and mudstone interbeds form the Hibernian Greensands Formation, although in places the chalk rests directly on Dalradian rocks. There are several minor unconformities within the Hibernian Greensands sequence, culminating in the unconformable contact with the succeeding Ulster White Limestone Formation, which consists of about 130 m of white chalk with flint nodules. Although this chalk has the same composition as the chalk of south-east England it is very much harder due to secondary calcite cement. Consequently the Irish chalk is commonly referred to as limestone.