Grainstone

Grainstone

Sedimentary rocks

Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017

W.S. MacKenzie, A.E. Adams, K.H. Brodie

176 is a very porous limestone from the Arab D Formation in Saudi Arabia. This unit forms the world’s biggest oil reservoir. The limestone is a grainstone with a mixture of bioclasts, intraclasts and peloids. There is a high intergranular porosity between the grains and some intragranular porosity within Foraminifera (chambered microfossils seen, for example, on left hand side of photograph). Some of the pore-spaces between grains look quite large and it is possible that a component of the rock has dissolved out to increase overall porosity.

Global geoheritage significance of Ordovician stratigraphy and sedimentology in the Cliefden Caves area, central western New South Wales

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Journal Information

Published in Australian Journal of Earth Sciences, 2019

M. Brocx, V. Semeniuk, I. G. Percival

Grain types comprising the grainstones are skeletal (generally crinoid ossicles, fragments of bryozoa, calcareous algae, brachiopods, gastropods, pelecypods, and bryozoa), lithoclasts (comprising muddy limestone fragments and some sparry-calcite-cemented grainstone fragments), and calcrete ooids, pellets, and intraclasts (of muddy limestone). Hence, these limestones range from skeletal grainstone, pellet grainstones, intraclast grainstone, lithoclast grainstone and calcrete-ooid grainstone. Grainstones vary from structureless, to laminated, to cross-bedded at the decimetre scale. The range of fossils in skeletal grainstones includes brachiopods, gastropods, bryozoa, trilobites, ostracodes, crinoid ossicles and calcareous algae.

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Sedimentary rocks

Global geoheritage significance of Ordovician stratigraphy and sedimentology in the Cliefden Caves area, central western New South Wales