Hibernian Greensands Formation, Cretaceous, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
W I Mitchell
Hibernian Greensands Formation
The main outcrop of this formation is in southeast Co. Antrim and all the sediments were deposited prior to the Uintacrinus socialis Biozone (late Santonian) when chalk deposition commenced . The four members (P947943) are about 30 m thick with a maximum of 14 m recorded for the lowest three members at the section in Collin Glen [J 269 722], 350-412 m upstream from Glen Bridge (P947855).
The transgressive base of the fossiliferous Belfast Marls Member, formerly known as the Glauconitic Sands , rests on the Waterloo Mudstone Formation, Penarth Group and the Mercia Mudstone Group. The fauna is dominated by bivalves, especially large specimens of the oyster Amphidonte obliquata (P948080) Fossil 1, but also contains brachiopods, cephalopods, gastropods and serpulids. The earliest Cenomanian (Mantelliceras mantelli Biozone) age of the lower part of the member is based on the presence of the ammonites Schloenbachia and Mantelliceras (P948080) Fossil 2. At Cloghfin [J 484 942] (P947855), the belemnite Actinocamax primus (P948080) Fossil 3 confirms a mid-Cenomanian age for the upper part of this unit.
At the base of the Island Magee Siltstones Member is a transitional zone lithologically resembling the top of the Belfast Marls Member but consisting of 0.23 m of glauconite-depleted, grey silty marl with a fauna of smooth-shelled bivalves lacking Amphidonte. The previous name of this member, ‘Yellow Sandstones and Grey Marls’ emphasised the pale bluish grey colour of the rocks when fresh and calcareous, and their pale yellow hue when weathered and decalcified. The mid-Cenomanian age is indicated by the ammonite Cunningtoniceras cunningtoni (P948080) Fossil 4 which is typical of the Acanthoceras rhotomagense Biozone, and the bivalve ‘Inoceramus’ hamiltoni, which is characteristic of the late mid-Cenomanian in southern England. In Collin Glen, the member is at least 9 m thick, but the fauna is restricted to rare brachiopods and serpulids. At Cloghfin Port (J484 942), the member forms foreshore reefs with fossils including the corals Micrabacia cf. coronula and ‘Stephanophyllia’ cf. bowerbankii, serpulids, inoceramid bivalves and Gryphaeostrea canaliculata. The echinoid ‘Epiaster’ is typical of warm water southern Tethyan faunas. The member is overlain unconformably by the Collinwell Sands Member.
The upper surface of the Island Magee Siltstones Member is commonly bored, with pipes of younger sediment from the Collinwell Sands Member extending down for up to 0.6 m. The outcrop of the two informal units of this member is located southwest of Belfast with exposures of the upper unit restricted to small pits west of Collin Glen . Although abundant fossils occur in the lower unit (P947943), the absence of zonal index taxa makes dating problematical and the attributed late Cenomanian to mid-Turonian age is based on the occurrence of large specimens of the oyster Rhynchostreon suborbiculatum.
The basal conglomerate of the Kilcoan Sands Member rests unconformably on the three older members (P947943) and its sand component was largely derived by erosion of the Collinwell Sands Member. The thickest section of the member (7.3 m) is at Kilcoan [J 461 986] on Island Magee (P947855), but elsewhere the succession thins beneath the unconformity at the base of the Ulster White Limestone Formation. The succession is divided into three parts based on the presence of three, closely spaced, shell bands packed with Inoceramus fragments. The pre-Inoceramus bands strata are early Coniacian based on the association of the echinoid Conulus raulini and rhynchonellid brachiopods including ‘Cretirhynchia’ robusta. The Inoceramus bands contain the Coniacian zonal index fossil Volviceramus involutus (P948080) Fossil 5 that occurs near the base of the Micraster coranguinum Biozone. They also have numerous oysters including Gryphaeostrea canaliculata and Hyotissa semiplana and terebratulid brachiopods such as Concinnithyris and Gibbithyris. Brachiopods, including large specimens of the flat terebratulid Gibbithyris hibernica, are also common in the post-Inoceramus bands succession, but the Santonian age is based on the presence of the small and medium-sized echinoids Conulus, common Echinocorys and rare Micraster.
At Tircrevan Burn [C 701 326], the Kilcoan Sands Member consists of 3m of chert-free and cherty greensand and is divided into two parts by an unconformity . In the lower part, the sandstones only contain long-ranging fossils indicative of the Cenomanian to early Coniacian. However, the basal sandstones of the upper part contain the belemnite Actinocamax verus and the spinose oyster Costagyra laciniata, which typify the upper part of the Micraster coranguinum Biozone and are Santonian (P947943). Poorly fossiliferous strata at the top of the upper sandstones are bioturbated by chert-filled Thalassinoides burrows and contain large specimens of the flat echinoid Micraster rogalae, indicating the upper Micraster coranguinum Biozone and a Santonian age. In southeast Co. Antrim this is coeval with either the top of the Kilcoan Sands Member or the Cloghfin Sponge Beds Member of the Ulster White Limestone Formation.
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