However vertical leakage between the two aquifer units may occur via fault systems. The Great Oolite and Inferior Oolite are generally regarded as two distinct aquifers with different water tables separated by the low permeability Fullers Earth Clay. Oolite is a type of sedimentary rock, usually limestone, made up of ooids cemented together. Spring lines are well-developed at the boundary of geological contacts and provide significant baseflow to rivers. Spring discharge in the Cotswolds is considerable and is estimated to exceed artificial abstraction. Massive-bedded oolitic limestone is the characteristic lithology of the Burrington Oolite, which is up to c. Being heavily fractured and faulted transmissivity values are high and the resultant combination of low storativity and high transmissivity gives rise to a highly responsive aquifer with large seasonal variations. Burrington Oolite Subgroup and Vallis Vale Limestone Formation. The control of faulting on aquifer drainage routes is well observed in the Cotswolds with rivers following local fault systems. Consequently matrix porosities are low and primary aquifer storage is limited.Īquifer storage is further constrained where the saturated thickness of the unconfined limestones in the higher Cotswolds is dissected by heavily incised river valleys and extensive faulting. The Oolitic limestones are well-cemented and have a low intergranular permeability. The shallow marine depositional environment of the Great and Inferior Oolite Group gives rise to an alternating sequence of limestones and clays the limestone beds themselves being thinly bedded and laterally non-persistent. The principle Jurassic aquifers are the Inferior Oolite and Great Oolite limestones, which overlie the Lias Group and have an outcrop area of approximately 600 km 2 in the Upper Cotswolds. Haines limestone quarry on the west side of the A34 looking south the Great Oolite White Limestone. The Crocodile Spring at Compton Abdale in the Cotswold Hills. Plainly, then, what we are looking at is ooids cemented together to form oolite.The distribution of Limestone outcrop in the Thames Basin. Not only are the grains the right size and shape to be oolites, but in cross-section one can see the nuclei around which they formed and the growth patterns typical of oolites. This resemblance might be fortuitous, but any such conjecture is laid to rest by looking through a microscope at a cross-section of oolite, as shown in the lower picture to the right. It was deposited during the Middle Jurassic. Photomicrograph of the grains composing oolite, in cross-section. The Inferior Oolite is a sequence of Jurassic age sedimentary rocks in Europe. (The limestone in this photograph and the one below is from the Carmel Formation in southern Utah.) Magnified, it looks like it is made of ooids, as shown in the top picture to the right. It is very easy to distinguish limestone formed from ooids. The term oolith may also be used as a term either for the rock or for an individual ooid. Rock formed from Calcium carbonate ooids is, by definition, a form of limestone, and is known as oolitic limestone or oolite. Like other small grains, ooids can be cemented together to form a kind of rock. Today ooids are to be found in a number of locations with warm shallow water, including the Bahamas, Shark Bay in Australia, and the Persian Gulf, all of which are marine sites but they are also sometimes found in inland waters such as the Great Salt Lake in Utah. Such ooids are typically formed in water rich in calcium carbonate (for obvious reasons) and for preference warm shallow water agitated by waves. These grains are typically between 0.25mm and 2mm in diameter in fact, some authors use a different term for ooids of different sizes, but in this article we shall use the word "ooid" as a catch-all term.Īlthough there are a number of minerals which can form ooids, in this article we are interested in ooids formed from calcium carbonate, and from now on we shall confine our discussion to them. In this article we will discuss the formation of ooids, and how to recognize oolitic limestone in the geological record.Īn ooid consists of a nucleus (a fragment of shell, a grain of sand, or whatever) around which layers of minerals are deposited to form roughly spherical grains. All varieties in the Calcite-Dolomite horizontal column are possible here. Ooids from a beach on Joulter's Cay, Bahamas. Siliceous Oolite: > 50 oolitic Oolitic Chert: < 50 oolitic Diatomite: diatom tests Radiolarite: radiolarian tests: Siliceous Limestone: disseminated silica Cherty Limestone: containing chert nodules Etc.
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