|Tircreven Burn (Mesozoic stratigraphy)||Londonderry|
|PHOTO TO BE ADDED Type locality for the Tircreven Sandstone Member of the Waterloo Mudstone Formation at Tircreven Burn, NE of Limavady, Co.Londonderry.|
|Site Type: ||Stream section|
|Site Status: ||PASSI|
|District: ||Limavady Borough Council|
|Grid Reference: ||C701328|
|Rock Age: ||Cretaceous, Jurassic (Cenomanian, Hettangian, Santonian, Sinemurian)|
|Rock Name: ||Hibernian Greensands Formation, Lias Undivided, Lower, Tircrevan Sandstone Member, Ulster White Limestone Formation, Waterloo Mudstone Formation|
|Rock Type: ||Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone|
|Fossil Groups: ||Ammonite, Annelid, Belemnite, Bivalve, Crinoid, Crustacea, Echinoderm, Fish, Foraminifera, Gastropod, Goniatite, Polyzoan, Sponge, Trace fossil|
|Other interest: ||No data, No Data|
Summary of site:
Around 60 million years ago as the North Atlantic was widening, great fractures opened along the line of the Irish Sea as Ireland threatened to detach from the rest of Europe. Along the northern stretch of this rift (from the north of Ireland up the west coast of Scotland) massive volcanic activity laid waste the warm temperate forests that clothed a dramatic karst landscape carved into the Ulster White Limestone. Intermittent bursts of activity lasted for about 3 million years heaping the stacks of lava flows whose remnants survive as the Antrim Plateau Basalts. This event, more than any other, is responsible for the preservation of the soft Mesozoic rocks of the area and the steep and deeply incised valley of Tircreven Burn cuts through the rock succession from the basalts on top, through the Ulster White Limestone and finally the Lower Jurassic (Liassic) mudstones before it becomes the Big Drain where the succession is lost beneath surface cover. The Tircreven stream section is the best exposure of these rocks in the north west of Northern Ireland.
The Liassic rocks of the locality all belong to the Waterloo Mudstone Formation and have a total thickness of 52 m. 15 m above the base of the section an important subdivision of the formation appears, the Tircreven Sandstone Member, 13 m thick. There are a further 24 m of mudstone above the sandstone.
The lowest 15 m of beds seen in the stream are predominantly grey mudstones and shales but there is a 25 cm thick limestone near the base and a very thin, biscuity limestone 1 to 4 cm thick towards the top. Fossils occur throughout, mainly bivalve molluscs such as the oyster Gryphaea, popularly known as the Devilís Toenail; the large, smooth, naturally polished Plagiostoma and many others, a few brachiopods and ostracods, and plant remains in various stages of preservation. The most important age indicator is a find of the ammonite genus Schlotheimia. Trace fossils are also abundant. The plant remains hint at the proximity of land.
The Tircreven Sandstone Member appears abruptly without any break in sedimentation. The first beds are thick grey-brown, silty, fine-grained sandstones with plant remains succeeded by a sequence of mostly white sandstones with few fossils. The rocks then darken to brown, even dark brown, silty sands and are succeeded by highly calcareous sandstones, verging on sandy limestones and fossils reappear in numbers, mostly large bivalve molluscs, one genus of sea snail and a spiral trace fossil. The sandstones persist in thin beds with minor limestones and beds of nodules until, in the top metre, more bivalve molluscs, including two oyster genera appear with a single cephalopod genus. The junction of the sandstone with the overlying mudstones was obscured when the site was logged. The mudstones and shales above the sandstone show all sign of slumping with slickensides (parallel striations on faults planes). Again they are blue-grey in colour and fossiliferous with many bivalve molluscs, a few brachiopods and some ostracods. In the top metre, immediately below the Cretaceous Basal Conglomerate, the rocks become richly fossiliferous and among the bivalves two important ammonites, Arnioceras semicostatum and a species of Euagassiceras provide vital dating evidence.
The time difference between the top bed of the Lower Lias and the base of the overlying Cretaceous conglomerate is around 100 million years, probably a time with more phases of deposition and subsequent erosive stripping but with a long period when the whole area of Northern Ireland emerged as land, at the height of the Age of the Dinosaurs.
Although the basal conglomerate of the Cretaceous is no more than 8 cm thick, it is important and poses a number of problems discussed in record 120 (also Tircreven Burn). It is a dark, greenish rock crowded with Liassic fossils derived from the beds below but with small, fossilised fish or amphibian droppings, a few bones, possibly of amphibians or reptiles and a preponderance of small fish teeth. It is succeeded by a soft, fawn sandstone 50 cm thick with polished grains and pebbles. The following thick sandstone has prominent broken fragments of a thick, prismatic-shelled bivalve of the genus Inoceramus. There follow around 3 m of green, glauconitic (glauconite is a green mineral normally in small grains, indicating a marine environment) sands with a revealing fauna of belemnites, incorporating Actinocamax verus, sea urchins, particularly Micraster rogalae, bivalve molluscs including the spiny oyster Costagyra lacineata and trace fossils in the form of burrow-fills called Thalassinoides. Taken as a whole, this fauna indicates a time stage called the Santonian that started around 85 million years ago. This 4 m section constitutes the total thickness of the Hibernian Greensand Formation in this area.
Similarly the Ulster White Limestone that follows is represented by no more than 8 m of rock, mostly white limestone, with a pale green tinge near its base due to glauconite. There are 3 beds; a greenish, massive limestone with phosphatic pebbles starts the sequence. It contains a rich fauna including sea urchins of the genera Micraster and Echinocorys, belemnites including Gonioteuthis, a tiny sponge and oyster species. It is succeeded by another massive bed of pale green, glauconitic limestone which is topped by a reddish-yellow stromatolite which is a finely laminated structure, created by a combination of cyanobacteria and algae, no more than a centimetre thick. The final bed is 5.4 m thick with pink flint nodules, poorly fossiliferous but with the belemnite Belemnitella. It was poorly exposed when the section was logged.
Compared to other Cretaceous sequences in Northern Ireland the Tircreven succession is much attenuated and this requires explanation. The sea bed in this area was not of even depth while the Cretaceous rocks were being deposited. It consisted of a series of basins separated by ridges or axes and fringed by stable shelves. The Tircreven section was on the Londonderry Shelf which was never submerged to any great depth.
The Jurassic (Liassic) and Cretaceous rocks of Northern Ireland were formed on shallow sea beds just before and during the opening of the North Atlantic, on the southern fringe of the supercontinent of Laurasia (incorporating modern North America, Greenland, Europe, Siberia and parts of China). These sea beds were a few degrees north of 30 degrees latitude, about the latitude of the northern margins of the Sahara desert at the present time. The waters swarmed with a wide variety of marine life, many animals feeding on or near the sediment but some, like the ammonites, swimming and drifting freely so they had a widespread distribution. They also evolved quickly so most species spanned very brief periods, geologically speaking, around half a million years on average. This makes ammonites ideal tools for dating and at Tircreven they provide clear and dependable time zones. The ammonites in the Lias confirm the presence of the Arnioceras semicostatum Zone, fixing a date about 200 million years ago.
The sea urchins and belemnites provide the time frame for the Cretaceous, aided to a lesser degree by ammonites and sponges. With the exception of the enigmatic basal conglomerate, the rocks appear to be no older than the Santonian stage, between 86 and 83 million years old. The conglomerate could be significantly older, perhaps more than 90 million years.
When it comes to more detailed correlation with the Cretaceous rocks of the north and east coasts and the succession north of the Lagan Valley and Belfast Lough, the fossils of Tircreven present major problems. The Tircreven fossils are very different and there is much debate about how they relate to the rest. A further complication is the mixing of warm water, southern forms with species considered to be to be from colder, northern latitudes and a number of scenarios are possible. The rocks of Tircreven have much information yet to yield and there is enormous scope for future research.
By any standards Tircreven is a site of national importance. It presents a picture of the very different sea beds and their inhabitants in the early Jurassic when compared with the east and south margins of the Antrim Plateau and contains the type section of the Tircreven Sandstone Member of the Waterloo Mudstone Formation, the only thick sandstone sequence in the Irish Jurassic. The enigmatic Cretaceous Basal Conglomerate takes a unique form in the stream and with the rich and puzzling Cretaceous faunas, offers immense research potential that should be protected from threat of development. The site is unlikely to be at risk due to its extreme difficulty of access and its fundamental instability.