White Park Bay

The local Jurassic succession consists of marine mudstone with silty bands and calcareous nodules. Securely dated in-situ ammonites establish late Sinemurian Raricostatum Zone strata and the earliest Pliensbachian Taylori Subzone. The following five subzones are proved at the bay, but exposure is discontinuous and the available evidence does not justify drawing them as five equal-thickness beds. Nearly horizontal or gently north-dipping mudstones are interrupted by minor faults and basalt dykes; strong tilting at Oweynamuck is attributed to mass movement.

Densinodulum Subzone. In-situ mudstones in the centre of the bay and north of Oweynamuck contain Cheltonia howarthi, a species named from White Park Bay material. Central-bay nodules also yield Eoderoceras bispinigerum. The wider local Densinodulum assemblage includes Oxynoticeras lymense in a nodule with Gryphaea mccullochi, and small ammonites assigned to the Hemimicroceras subplanicosta group.

Raricostatoides Subzone. Echioceras raricostatoides and E. raricostatum are documented from local nodules, including an in-situ nodule in the centre of the bay and nodules at the eastern end. Their stratigraphic value depends on the fossil identification and provenance together. Loose nodules can preserve the same ammonites without revealing the precise location of their source bed.

Macdonnelli Subzone. The local fauna includes Leptechioceras macdonnelli, and pyritic Neomicroceras commune occurs in in-situ mudstone in the centre of the bay. Historical work also identified Macdonnelli-age fossils in the inland Lemnagh Burn. That stream occurrence should be distinguished from the shoreline exposures rather than transferred to whichever beach outcrop is visible today.

Aplanatum Subzone. This younger part of the late Sinemurian is among the subzones established from in-situ ammonites in the modern reassessment. Paltechioceras oosteri, represented by two specimens in one local nodule, provides an additional named Aplanatum indicator.

Taylori Subzone. The earliest Pliensbachian is demonstrated by Apoderoceras, including an in-situ A. aculeatum specimen at Oweynamuck. Local material also includes Phricodoceras and rare Tetraspidoceras associated with the interval around the Sinemurian–Pliensbachian transition, although these records do not all carry equally precise in-situ control.

Microfossil corroboration. Seven mapped mudstone samples provide a separate line of evidence. Samples WPB1–WPB5 were placed in the late Sinemurian JF8 microfossil zone; WPB6–WPB7 were tentatively assigned to earliest Pliensbachian JF9. The latter assignment remains provisional because the principal indicator, Vaginulinopsis denticulatacarinata, was absent. Foraminifera and ostracods therefore supplement the ammonite record without resolving every gap between exposures.

Historical sections beneath the cover. The original survey recorded 0.6 m of Jurassic shale with a few calcareous nodules below the basal conglomerate at the waterfall. At Oweynamuck, one inclined mudstone exposure was about 9 m thick, with a particularly dark, micaceous upper 1.3 m; a separate nearby landslipped exposure showed about 3 m beneath the conglomerate. These are measurements of different historical exposures, not components to add into a continuous modern log. Sand, landsliding and changing exposure prevent assuming that all remain visible.

Older and younger loose fossils. Ex-situ material extends beyond the securely dated mudstones: Asteroceras indicates the older Obtusum Zone, while Beaniceras, Androgynoceras and Oistoceras represent younger Lower Pliensbachian intervals. Loose calcareous sandstone contains Upper Pliensbachian Amaltheus and Pleuroceras. Some source beds may be local, but glacial transport, potentially from farther afield, remains a possibility for some material. These fossils are not evidence for a continuously exposed Obtusum-to-Spinatum succession. The historical Ibex-age interpretation of a 1956 Oweynamuck exposure has not been confirmed with the same modern in-situ control.

Jurassic–Cretaceous unconformity and basal conglomerate. At Oweynamuck and a separate waterfall exposure, a glauconitic, phosphatic conglomerate with a chalk matrix rests on Jurassic mudstone. One studied exposure measured 0.5 m, but this is not a bay-wide thickness. Jurassic fossils were eroded and incorporated alongside Cretaceous echinoids and bivalves, producing a mixed-age assemblage. Historical records include derived ammonites at Oweynamuck and Uptonia at the waterfall. These do not date the enclosing matrix by themselves. Accounts differ over the layer’s relationship to the Hibernian Greensands and Cloghfin Sponge beds; no separately proved Cloghfin formation or complete greensand succession is inserted here.

The lowest securely identified chalk formation is Santonian Galboly Chalk, beneath the Cloghastucan Chalk. At White Park Bay it is flintless, an important local departure from regional descriptions that include flints elsewhere. It forms the bottom of the condensed three-formation Oweynamuck succession above the basal Cretaceous deposit.

This Santonian formation succeeds Galboly Chalk and includes the characteristic Marsupites-bearing interval. Its regional lithology is gritty chalk with chalk-pebble horizons and flints, but the succession diminishes and loses flint westwards between Oweynamuck and Portbraddan. Marsupites testudinarius-bearing chalk is also documented in a tilted block at the western end of White Park Bay. Such displaced exposures are valuable geological records but should not be mistaken for an undisturbed vertical section.

Oweynamuck Flint Band. Within the Cloghastucan Chalk at the eastern headland, the flint band is semicontinuous and locally reaches 17 cm in thickness. The original account places it normally about 2 m above the formation’s base and records Marsupites only as high as 25 cm below it. These are observations from the local reference section, not universal dimensions or evidence that the fossils occur inside the flint. Westward loss of flint means the marker cannot be expected to retain the same appearance across the bay.

Soft-sediment slumping below the Oweynamuck Flint Band. The basal Cloghastucan Chalk in the middle slipped Oweynamuck section contains minor slumps formed while the sediment was still incompletely consolidated. Similar deformation was recorded in the eastern Oweynamuck section, near the base of the topmost metre below the flint band. This movement during chalk deposition is distinct from the much later landsliding that displaced parts of the coastal succession.

Campanian Creggan Chalk overlies the Cloghastucan Chalk and completes the lower Oweynamuck succession. Abundant inoceramid bivalve debris is a defining feature of this interval. Across the lower chalk, flint development decreases westwards, and the pre-Boheeshane chalk at Portbraddan is flintless.

Bendoo Pebble Bed. At the top of the Creggan Chalk in the western bay, glauconitised chalk pebbles and an erosional surface mark interruption and reworking before the overlying Boheeshane Chalk. The bed provides a useful boundary marker above the lower inoceramid-rich chalk.

The Boheeshane Chalk is Campanian and overlies the Creggan Chalk above its upper pebble-bed boundary. The historical working subdivisions A, B and C, with numbered units, are useful for locating the western-bay and Portbraddan fossil horizons below. They are subdivisions of this formation, not three additional formations or invented members. Regional descriptions emphasise finer chalk, flint bands and local pebble horizons; the details below are restricted to explicitly documented White Park Bay and Portbraddan occurrences.

Lower Boheeshane A1: Offaster below erosion surfaces. Small Offaster were recorded below the surfaces labelled a and b in fallen-block sections east of Portbraddan and at the White Park Bay waterfall, corresponding to sections F and D in the original correlation diagram. These records belong to the lower, largely flintless post-Creggan interval; they are separate from the much higher Offaster occurrence in the Altachuile Breccia.

Belemnitella praecursor Bed, A2. The Portbraddan shore platform exposes this named belemnite-rich horizon. Large guards assigned historically to Belemnitella ex gr. praecursor dominate the assemblage described there.

Orbirhynchia Band, B1. At Portbraddan, a roughly 1.5 m interval contains Orbirhynchia bella, with the strongest concentration in its upper 15 cm. A tabular flint marks the top near the cliff base.

Depressed Echinocorys Bed. An echinoid-bearing horizon is recorded in the White Park Bay amphitheatre. Its correlation with the B2 bed recognised at Larry Bane is tentative: the White Park occurrence may lie slightly higher. The local record is retained without turning that uncertain lateral comparison into a fixed numbered bed position.

Whitehead Flint Band and the C1–C2 division. In the regional Boheeshane C scheme, the higher of two massive central flint courses is the Whitehead Flint Band. It separates C1 from C2 and was used to correlate the upper limit of Gonioteuthis. This provides context for the C2 beds documented at Portbraddan, rather than a new claim that the full marker succession has been measured there. The historical Lower–Upper Campanian boundary drawn at this level was already recognised as lacking secure international correlation.

C2 ammonite occurrence. A small indeterminate pachydiscid ammonite is recorded specifically from Portbraddan within the C2 fauna. The available identification supports a chalk ammonite occurrence, but not a named species.

Upper C2 Echinocorys-rich bed. At the northern end of the Portbraddan section, an Echinocorys-rich horizon occurs near the top of the Boheeshane Chalk, below the Larry Bane Chalk. This is a separate local echinoid concentration from the tentatively correlated depressed Echinocorys horizon.

Larry Bane Chalk succeeds Boheeshane Chalk and underlies Ballintoy Chalk in the documented Portbraddan succession. It is Campanian. The formal regional description identifies flinty, shell-debris-rich chalk divided by persistent erosion surfaces, useful context for recognising interruptions in chalk sedimentation.

The Campanian Ballintoy Chalk follows Larry Bane Chalk and is documented at Portbraddan. The formation is a stratigraphic name: its occurrence here does not make fossil records from Ballintoy Harbour part of White Park Bay. Regional descriptions distinguish small scattered flints below from small discontinuous tabular flints above. The strongest specifically local upper marker is the Altachuile Breccia, which terminates the formation beneath Glenarm Chalk.

Altachuile Breccia. At Portbraddan, the upper Ballintoy Chalk contains disturbed, pellety chalk with angular flint fragments. Intact Offaster within this deposit was interpreted as evidence of animals recolonising the sea floor after disturbance. A storm origin was proposed for the brecciation, but the deposit does not independently prove a particular hurricane.

Glenarm Chalk follows the Altachuile Breccia in the higher Campanian succession represented towards Portbraddan. Regionally, the formation contains changing nodular and tabular flint development and records interruptions in deposition. At White Park Bay, the upper formations belong in the context of the separate Portbraddan fault-block section, rather than a continuous extension of the lower-chalk log at Oweynamuck.

The Campanian Garron Chalk lies above Glenarm Chalk and below Portrush Chalk in the upper succession represented by the Portbraddan fault-block composite. Its formal regional description emphasises conspicuously shelly chalk and very large flints. These characteristics distinguish it within the wider chalk classification, although no separate local thickness or complete three-bed log is established here.

Portrush Chalk succeeds Garron Chalk beneath Ballymagarry Chalk in the Campanian upper fault-block succession. Regional descriptions emphasise tabular flint bands and inoceramid shell debris in its upper part. Its inclusion records the documented local stratigraphic range; the South Antrim hardgrounds described elsewhere are not demonstrated in this section.

The separately mapped Portbraddan fault block reaches into the Campanian Ballymagarry Chalk, the highest chalk formation supported for this composite. The regional unit is characterised by exceptionally large, flat-lying flints that can approach continuous bands. The local range does not establish that the whole formation or its upper boundary is exposed.

Later structure and coastal cover. The Portbraddan Fault downthrows northwards, helping explain why different chalk levels occur close together. Basalt dykes cut the older rocks, and landsliding has displaced both Jurassic and chalk exposures. Beach sand and other cover further interrupt what can be seen. Superficial material and loose transported fossils are therefore kept separate from the bedrock hierarchy: an exposed block, a historical stream section and an undisturbed bed are different kinds of geological evidence.