Quantoxhead Fossil Hunting

Early Jurassic ammonite faunas – Quantoxhead became important for very early Sinemurian ammonites
The East Quantoxhead coast is one of the most important Lower Jurassic ammonite localities in Britain because it preserves one of the earliest known Sinemurian ammonite assemblages. Fossils from this section, especially arietitid ammonites, became crucial for recognising the base of the Sinemurian Stage.

1995 – East Quantoxhead proposed as a candidate GSSP for the base of the Sinemurian
Kevin Page proposed the East Quantoxhead section as a candidate Global Stratotype Section and Point for the base of the Sinemurian Stage. This was a major scientific milestone, as it meant the cliffs here were being considered as the international standard for a boundary in the geological timescale.

2000 – East Quantoxhead selected as the global reference section for the base of the Sinemurian
The East Quantoxhead section was selected as the GSSP for the base of the Sinemurian Stage. The boundary is defined by the first appearance of the ammonite genera Vermiceras and Metophioceras, making the locality internationally important in Jurassic stratigraphy.

2002 – the East Quantoxhead GSSP was fully formalised in print
The formal published account of the Sinemurian GSSP confirmed East Quantoxhead as the world reference point for this stage boundary. This cemented Quantoxhead’s place as one of the most scientifically important Lower Jurassic coastal sections in Britain.

Modern collecting – Quantoxhead remains important for ammonites and marine reptile remains
As well as its stratigraphic importance, Quantoxhead is also well known as a productive fossil locality. The foreshore and fallen blocks can yield ammonites, bivalves, brachiopods and occasional marine reptile material, including ichthyosaur bones, showing that the site remains important both scientifically and for fossil collecting.

LIAS GROUP

Blue Lias Formation (Hettangian To Lower Sinemurian)

The Quantoxhead coast has to be read as part of the wider west Somerset composite Lower Lias section, not as one isolated vertical cliff log. The internationally important Limekiln Steps exposure lies in the upper Hettangian to lowermost Sinemurian part of the succession, while lower and higher numbered beds are correlated along the same coastal tract; the bed numbers used below are the formal published west Somerset numbers of Whittaker & Green. In current BGS lithostratigraphy the Blue Lias extends from the base of the fissile mudstone above the Sun Bed to the top of Bed 238, even though the published numbered succession continues upward into the basal Charmouth Mudstone Formation.

Beds 1–7 — Basal Pre-Ammonite Blue Lias (c. 5 m)

Grey calcareous mudstones and rather nodular dark blue-grey limestones form the basal Lias above the Sun Bed. About 5 m of strata are present before ammonites become established. The fauna is chiefly bivalved, with Liostrea hisingeri especially characteristic at some levels, together with other marine shells. These beds record the earliest fully marine Blue Lias conditions on the west Somerset coast, but they remain pre-Psiloceras in ammonite terms and are therefore important for understanding the transition from the latest Penarth facies into the earliest Jurassic sea.

Beds 8–12 — Earliest Psiloceratid Beds

The first psiloceratid ammonites appear as low as Bed 8. Weakly ribbed Psiloceras, including P. erugatum, together with Neophyllites, occur below the more typical smooth Psiloceras forms of the classic planorbis interval. Lithologically these beds continue the familiar Blue Lias rhythm of dark calcareous mudstone, marl and thin limestone, with marine faunas still comparatively sparse. They are significant because they show that the earliest Jurassic ammonite radiation at Quantoxhead begins below the abundance phase of P. planorbis.

Beds 13–19 — Psiloceras planorbis Beds

Unequivocal Psiloceras planorbis is abundant in these beds, making them the classic lower Planorbis Subzone in the west Somerset sequence. The succession is composed of interbedded dark grey mudstones and thin nodular limestones, with fossils commonly flattened in the mudstones but more three-dimensional in the limestones. The Planorbis Subzone extends upward to the base of Bed 25 and is about 4.5 m thick in the standard west Somerset log. These beds are among the key British reference levels for the earliest ammonite-defined Jurassic.

Beds 20–23 — Upper Planorbis Subzone Below The Laminated Mudstones

These beds continue the upper part of the Planorbis Subzone below the famous laminated Bed 24. Bivalves remain common, and the broader Planorbis interval in west Somerset has yielded Liostrea, Camptonectes, Protocardia, Pteromya, echinoids such as Diademopsis and Eodiadema bechei, and occasional vertebrate material. Deposition was still strongly cyclic, but the increasing development of laminated, organic-rich mudstones foreshadows the more restricted conditions of Bed 24.

Bed 24 — Laminated Psiloceras Mudstone (2 m Or More)

Bed 24 is one of the classic west Somerset Blue Lias horizons, comprising 2 m or more of indurated laminated mudstone. Crushed iridescent ammonites of the Psiloceras planorbis group are abundant, and small pyritized colonies of Heterastraea have been recorded from laminated mudstones at this level. Fish remains and ichthyosaur material are known from the Planorbis Zone in the wider GCR coastal section, including an embryo within a large ichthyosaur skeleton from the lower part of the succession. The finely laminated, bituminous lithology indicates deposition under very low-energy, locally dysoxic to intermittently anoxic bottom-water conditions.

Beds 25–42 — Johnstoni Subzone (c. 3.4 m)

Bed 25 is a limestone in which the first appearance of Caloceras marks the base of the Johnstoni Subzone. The subzone is about 3.4 m thick and consists of limestones and mudstones, with hard laminated mudstones in Bed 36 yielding Caloceras johnstoni and Bed 37 yielding C. intermedium. These beds are a classic low Hettangian reference interval in which ammonite biostratigraphy becomes much more precise than in the Planorbis Subzone below.

Beds 43–66 — Lower Liasicus Zone / Portlocki Subzone

The base of the Liasicus Zone is taken at the appearance of Waehneroceras sensu lato in Bed 43. The full Liasicus Zone is about 28 m thick, and its lower part, up to about Bed 69, is notably mudstone-dominated with only subordinate limestone bands. The macrofauna becomes more diverse, including Camptonectes, Gervillia, Lucina, Liostrea, Modiolus, Plagiostoma and Pseudolimea, together with the lowest Gryphaea cf. obliquata. These beds represent a comparatively complete offshore marine record of the early Hettangian shelf in the Bristol Channel–Central Somerset basin system.

Beds 67–79 — Upper Liasicus Zone / Laqueus Subzone

Laqueoceras appears around Bed 67 and marks the boundary between the Portlocki and Laqueus subzones. The lithology remains mainly dark grey calcareous mudstone with persistent Blue Lias limestone couplets, but the succession is less ammonite-poor than the lower Liasicus interval. Quantoxhead is especially valuable here because the expanded mudstone thickness suggests little erosion and a comparatively complete basinward record.

Beds 80–94 — Lower Angulata Zone Below The Complanata Subzone

Bed 80 yields Schlotheimia cf. amblygonia, taken as marking the base of the Angulata Zone. The Angulata succession at west Somerset is about 40 m thick in total and extends upward to Bed 145 through grey mudstones and nodular limestones. Macrofossils are not especially abundant at first, but Gryphaea arcuata becomes increasingly common and the brachiopod Calcirhynchia calcaria becomes an important guide fossil in this part of the section.

Beds 95–133 — Complanata Subzone

The base of the Complanata Subzone is placed at Bed 95. Bed 95 has yielded Schlotheimia complanata and S. complanata polita; Bed 99 produced S. oxygonia together with members of the angulosa–phobetica group; and Bed 133 yielded Schlotheimia ex gr. striatissima. These beds are part of the richly resolved upper Hettangian ammonite sequence for which Quantoxhead and adjacent west Somerset sections are famous, even though preservation is commonly flattened where ammonites occur in mudstone rather than limestone.

Beds 134–144 — Depressa Subzone

The Depressa Subzone begins at Bed 134. The upper surface of Bed 134 has yielded Schlotheimia cf. princeps, probably close to the horizon of S. depressa, and the interval above is characterised by a marked reduction in species diversity. Schlotheimia pseudomoreana is present virtually throughout the subzone and becomes the dominant latest Hettangian guide fossil. Lithologically these beds remain classic Blue Lias paper shales, marls and nodular limestones, but biostratigraphically they approach the most important stage boundary in the whole section.

Bed 145 Lower 0.90 m — Latest Hettangian Paper Shale

Bed 145 is a bituminous paper-shale unit within the normal Blue Lias cycle of bituminous shale – marl – limestone – marl. The lower 0.90 m still belongs to the latest Hettangian and carries the dwindling Schlotheimia fauna, especially S. pseudomoreana. Thin concentrations of fish scales and bones help to identify the bed in the field. The boundary is not at the base of a new cycle, and there is no recognisable sedimentary break at this level.

Bed 145 At 0.90 m Above Base — Hettangian–Sinemurian Boundary GSSP

The internationally agreed base of the Sinemurian Stage lies 0.90 m above the base of Bed 145, immediately below and on either side of the Limekiln Steps section. It is defined by the first appearance of Vermiceras quantoxense, V. palmeri and Metophioceras. This is one of the most important biostratigraphic levels in the entire British Jurassic because the earliest Sinemurian ammonite assemblage is more complete here than anywhere else in Europe, and because the boundary shows no obvious lithological break or major hiatus.

Beds 145 Upper Part–146 — Quantoxense Biohorizon

The upper part of Bed 145 and Bed 146 belong to the quantoxense Biohorizon. These beds preserve the unique west Somerset overlap between surviving Schlotheimia and the first true arietitids, a relationship not known elsewhere in Britain or north-west Europe. The lithology remains finely laminated and organic-rich, emphasizing that the faunal revolution took place within continuing offshore basin sedimentation rather than at a sudden facies change.

Bed 147 — Metophioceras sp. B Biohorizon

Bed 147 yields the Metophioceras sp. B Biohorizon, one of the lowest Sinemurian horizons distinguished in west Somerset. It is a very narrow but highly important interval because it demonstrates the rapid stepwise replacement of the latest Hettangian fauna by earliest Sinemurian arietitids and keeps the Quantoxhead sequence far more finely resolved than most correlative sections elsewhere.

Beds 148–149 — Conybearoides Biohorizon Interval

Beds 148–149 carry the conybearoides interval, with Bed 149 especially important for Metophioceras brevidorsale and Vermiceras rougemonti. This is effectively the third recognised Sinemurian biohorizon in the local succession and, at many other north-west European localities, would be the lowest Sinemurian level preserved at all. Its presence at Quantoxhead is therefore strong evidence that deposition across the stage boundary was unusually complete.

Beds 150–154 — Rotaries, Rouvillei And Rotator Biohorizons

The upper part of Bed 152 marks the rotaries Biohorizon, Bed 153 the rouvillei Biohorizon, and the upper part of Bed 154 the rotator Biohorizon. The rapid faunal turnover across these beds is one of the reasons Quantoxhead became the world reference section for the base of the Sinemurian. Sedimentologically the rocks remain thinly cyclical, but biostratigraphically they record an exceptionally fine-grained arietitid succession.

Beds 155–164 — Upper Conybeari Subzone: Elegans, Conybeari And Silvestrei Horizons

The interval from Beds 145 upper to 164 constitutes the Conybeari Subzone, about 14.2 m thick in Page’s interpretation, and the higher part of it carries several further recognised biohorizons. The upper part of Bed 160 marks the elegans Biohorizon, Bed 161 the conybeari Biohorizon, and Bed 163 the silvestrei Biohorizon. Microfossils reinforce the boundary definition in this part of the section: the latest Hettangian is characterised by Lingulina tenera plex. substriata, whereas Planularia inaequistriata and the Frondicularia terquemi plexus group appear consistently above the stage boundary.

Beds 165–167 — Defneri Biohorizon And Base Of The Rotiforme Subzone

The top of Bed 166 to the base of Bed 167 marks the defneri Biohorizon at the base of the Rotiforme Subzone. These beds show the transition from the tightly packed Conybeari succession below into the thicker lower Sinemurian mudstone-dominated interval above. The faunas remain strongly arietitid, but lithologically the section is still dominated by dark calcareous mudstones with limestone interbeds.

Beds 168–202 — Main Rotiforme Subzone (within a c. 32.6 m interval for Beds 165–202)

The remainder of the Rotiforme Subzone, through Bed 202, is a thick lower Sinemurian offshore succession dominated by mudstone with recurring laminated, nodular and more persistent limestones. Although the higher biohorizons are less often quoted bed by bed in general summaries, this interval remains important because it preserves substantially more thickness than many equivalent southern British sections and therefore a more complete record of early Sinemurian basin sedimentation.

Beds 203–238 — Upper Bucklandi Interval And Top Of The Blue Lias Formation

Beds 203–238 represent the upper part of the Bucklandi interval and the formal top of the Blue Lias Formation in modern BGS lithostratigraphy. Mudstones remain predominant, in part fissile and bituminous, with limestones, some laminated, at frequent intervals. Macrofossils become locally more diverse upward, with epifaunal and shallow-infaunal bivalves, rhynchonellid and terebratulid brachiopods, gastropods, serpulids and vertebrate remains. The top of Bed 238 is the lithostratigraphic point at which persistent Blue Lias limestone rhythms give way to the more mudstone-dominated Charmouth Mudstone Formation.

Charmouth Mudstone Formation (Lower Sinemurian)

In older west Somerset literature the numbered beds continue seamlessly above the Blue Lias, and those published bed numbers are retained here because they remain the practical field scheme for placing Quantoxhead within the wider coastal succession. Under current BGS lithostratigraphy, however, Beds 239 upward belong to the basal Charmouth Mudstone Formation rather than the Blue Lias.

Beds 239–244 — Basal Charmouth Mudstone Still Within The Bucklandi Zone

The lowest Charmouth Mudstone beds are still within the Bucklandi Zone in Page’s interpretation, showing that the ammonite zonation does not coincide exactly with the modern formation boundary. The lithology becomes more mudstone-dominated and the limestone beds less regular and less laterally persistent than in the Blue Lias below, but deposition remained fully marine and offshore. This interval is transitional in field appearance: still recognisably lias in style, yet no longer the classic evenly rhythmic Blue Lias.

Beds 245–257 — Higher Lower Sinemurian Mudstones, Probably Lyra Subzone In Part

Above the Bucklandi interval, about 50 m of largely mudstone-dominated succession continues upward to Bed 257, with only a few mostly nodular limestones. Page assigned this higher part of the succession to the Lyra Subzone and recognised distinct ammonite horizons in correlative west Somerset sections, but the higher lower Sinemurian subzonal sequence at Quantoxhead is much less securely proved than the boundary interval below. These beds are therefore best treated conservatively as basal Charmouth Mudstone of higher lower Sinemurian age rather than as a fully settled bed-by-bed zonal sequence.

Structural Style

The Quantoxhead coast is not a single flawless vertical reference face. Across the wider Blue Anchor–Lilstock coast the succession is interrupted by both normal and reverse faults, and local repetition or omission of shale intervals is real, especially eastward toward Lilstock. Even so, the boundary interval north of East Quantoxhead is exposed over a long stretch of cliff and foreshore and individual limestone markers can be traced well enough for a robust composite log. This is why the section works so well as an international stratotype even though the wider coast is structurally complex.

Stratigraphic Significance

Within about 27 m of strata, from the upper Complanata Subzone to the lower Rotiforme Subzone, fifteen distinct ammonite biohorizons have been recognised. That level of resolution, together with the unique overlap of Schlotheimia and the earliest arietitids in Bed 145 and the supporting foraminiferal changes above the boundary, is why Quantoxhead functions as the world standard for the base of the Sinemurian Stage.

Depositional Environment

The Quantoxhead succession was laid down in a fully marine shelf basin within the Bristol Channel–Central Somerset basin system. Repeated Blue Lias cycles of bituminous shale, marl, limestone and marl reflect alternation between quieter, more organic-rich mud accumulation and more oxygenated carbonate-rich phases. Laminated paper shales indicate reduced bottom-water oxygenation, locally reaching dysoxic or anoxic conditions, whereas the limestones and bioturbated mudstones record intervals of better benthic activity. Rapid subsidence explains both the unusual thickness of mudstone at Quantoxhead and the exceptional completeness of the ammonite record.

Preservation And Typical Fossils

Collector and research value at Quantoxhead lies not just in the ammonites but in the style of preservation. Ammonites from laminated organic-rich mudstones can retain iridescent aragonite, whereas shells in limestones are commonly more three-dimensional. Typical fossils through the succession include Psiloceras, Caloceras, Waehneroceras, Schlotheimia, Vermiceras and Metophioceras, together with oysters such as Liostrea hisingeri, gryphaeids, pectinids, brachiopods, serpulids, echinoids, fish remains and occasional ichthyosaur material.

Total Thickness Covered Here

The formal Blue Lias Formation at Quantoxhead extends from the base of the Lias to the top of Bed 238. Older west Somerset measured successions continue the published numbered bed scheme upward to Bed 257, now classed as basal Charmouth Mudstone Formation. Quoted total thicknesses for the numbered lower Lias succession vary because of faulting, correlation differences and later lithostratigraphic revision: Palmer recorded about 178.9 m, whereas Whittaker & Green recorded about 203 m. For website use, it is best to treat Quantoxhead as a thick composite Hettangian–lower Sinemurian coast section rather than to force a single spurious total for one small cliff face.

References

Palmer, C.P. (1972) on the Lower Lias succession between Watchet and Lilstock, west Somerset.
Whittaker, A. & Green, G.W. (1983). Geology of the Country around Weston-super-Mare.
Warrington, G. & Ivimey-Cook, H.C. (1995) on the Penarth Group and basal Lias of coastal west Somerset.
Hodges, P. (1994) on the lowest Jurassic ammonites in west Somerset.
Page, K.N. & Bloos, G. (1998) on the basal Hettangian ammonite succession of west Somerset.
Bloos, G. & Page, K.N. (2000, 2002) on the Hettangian–Sinemurian boundary succession and the Global Stratotype Section and Point at East Quantoxhead.
Page, K.N. (1992, 1994, 2001, 2002) on Sinemurian ammonite biohorizons and the Quantoxhead boundary interval.
Page, K.N. and co-authors (2000) on the East Quantoxhead Sinemurian GSSP proposal.
Hylton, M.D. (1998) on foraminiferal changes across the Hettangian–Sinemurian boundary at East Quantoxhead.
British Geological Survey Lexicon: Blue Lias Formation and Charmouth Mudstone Formation.
JNCC Geological Conservation Review account: Blue Anchor–Watchet–Lilstock Coast.
International Commission on Stratigraphy GSSP register: base of the Sinemurian Stage, East Quantoxhead.