Konkresi (geologi)

Konkresi adalah massa kompak yang keras, terbentuk dari presipitasi semen mineral di dalam rongga-rongga partikel, dan ditemukan di batuan sedimen atautanah. Konkresi berbentuk seperti telur ataupun bulat, namun bentuk-bentuk iregular juga sering terjadi. Istilah 'konkresi' berasal dari bahasa latin Con yang artinya 'bersama' dan crescere yang artinya 'tumbuh'. Konkresi-konkresi terbentuk di perlapisan strata sedimen yang telah terendapkan terlebih dahulu. Mereka biasanya terbentuk lebih awal ketika terjadi penumpukkan sedimen, sebelum kemudian sedimen tersebut mengeras menjadi batuan. Semen konkresi ini sering menyebabkan konkresi lebih keras dan lebih tahan terhadap pelapukan dibanding perlapisan induknya.

Konkresi-Konkresi bongkah yang terlapukkan dari Batugamping berumur kenozoikum, california, Amerika Serikat.

Konkresi di bagian barat Kazakhstan

Ada perbedaan penting antara konkresi dan nodula. Konkresi terbentuk dari presipitasi mineral di sekitar sejenis nukleus sedangkan nodula adalah tubuh yang tergantikan.

Deskripsi yang telah dilakukan semenjak abad ke-18 membuktikan bahwa konkresi telah menarik perhatian ilmuan sejak masa lalu. Hal itu disebabkan adanya variasi bentuk yang tidak biasa, ukuran dan komposisi, yang membuatnya sempat dikira telur dinosaurus, hewan, dan fosil tumbuhan, hingga puing antariksa atau artefak manusia.

Galeri

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  Concretions of various shapes
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  Cannonball concretion in Jameson Land, East Greenland.
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  Concretion in the Cretaceous of western South Dakota.
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  Encrusted hiatus concretion from the Kope Formation (Upper Ordovician) of Covington, Kentucky. Encrusters include bryozoans and crinoids.
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  'Kettle' concretion at Kettle Point, Ontario, defaced by graffiti.

Referensi

• Al-Agha, M.R., S.D. Burley, C.D. Curtis, and J. Esson, 1995, Complex cementation textures and authigenic mineral assemblages in Recent concretions from the Lincolnshire Wash (east coast, UK) driven by Fe(0) Fe(II) oxidation: Journal of the Geological Society, London, v. 152, pp. 157–171.
• Boles, J.R., C.A. Landis, and P. Dale, 1985, The Moeraki Boulders; anatomy of some septarian concretions:, Journal of Sedimentary Petrology. v. 55, n. 3, pp. 398–406.
• Chan, M.A. and W.T. Parry, 2002, 'Mysteries of Sandstone Colors and Concretions in Colorado Plateau Canyon Country PDF version, 468 KB : Utah Geological Survey Public Information Series. n. 77, pp. 1–19.
• Chan, M.A., B.B. Beitler, W.T. Parry, J. Ormo, and G. Komatsu, 2005. Red Rock and Red Planet Diagenesis: Comparison of Earth and Mars Concretions PDF version, 3.4 MB : GSA Today, v. 15, n. 8, pp. 4–10.
• Davis, J.M., 1999, Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow: Water Resources Research, v. 35, p. 1705-1712.
• Hattin, D.E., 1982, Stratigraphy and depositional environment of the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas: Kansas Geological Survey Bulletin 225:1-108.
• Hobbs, D., and J. Hafnaer, 1999, Magnetism and magneto-structural effects in transition-metal sulphides: Journal of Physics: Condensed Matter, v. 11, pp. 8197–8222.
• Hoffmann, V., H. Stanjek, and E. Murad, 1993, Mineralogical, magnetic and mössbauer data of symthite (Fe9S11) : Studia Geophysica et Geodaetica, v. 37, pp. 366–381.
• Johnson, M.R., 1989, Paleogeographic significance of oriented calcareous concretions in the Triassic Katberg Formation, South Africa: Journal of Sedimentary Petrology, v. 59, p. 1008-1010.
• Loope D.B., Kettler R.M., Weber K.A., 2011, Morphologic Clues to the origin of Iron Oxide-Cemented Sphereoids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A, The Journal of Geology, Vol. 119, No. 5 (September 2011), pp. 505–520
• Loope D.B., Kettler R.M., Weber K.A., 2011, Follow the water: Connecting a CO2 reservoir and bleached sandstone to iron-rich concretions in the Navajo Sandstone of south-central Utah, USA, GEOLOGY FORUM, November 2011, Geological Society of America doi:10.1130/G32550Y.1
• McBride, E.F., M.D. Picard, and R.L. Folk, 1994, Oriented concretions, Ionian Coast, Italy: evidence of groundwater flow direction: Journal of Sedimentary Research, v. 64, p. 535-540.
• McBride, E.F., M.D. Picard, and K.L. Milliken, 2003, Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A.: Journal of Sedimentary Research. v. 73, n. 3, p. 462-483.
• Mozley, P.S., 1996, The internal structure of carbonate concretions: A critical evaluation of the concentric model of concretion growth: Sedimentary Geology: v. 103, p. 85-91.
• Mozley, P.S., and Goodwin, L., 1995, Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations: Geology: v. 23, p 539-542.
• Mozley, P.S., and Burns, S.J., 1993, Oxygen and carbon isotopic composition of marine carbonate concretions: an overview: Journal of Sedimentary Petrology, v. 63, p. 73-83.
• Mozley, P.S., and Davis, J.M., 2005, Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale microbially induced, chemical heterogeneity in groundwater: Geological Society of America Bulletin, v. 117, 1400-1412.
• Pratt, B.R., 2001, "Septarian concretions: internal cracking caused by synsedimentary earthquakes": Sedimentology, v. 48, p. 189-213.
• Raiswell, R., and Q.J. Fisher, 2000, Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition: Journal of Geological Society of London. v. 157, p. 239-251
• Scotchman, I.C., 1991, The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England: Sedimentology. v. 38, pp. 79-106.
• Thyne, G.D., and J.R. Boles, 1989, Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand: Journal of Sedimentary Petrology. v. 59, n. 2, pp. 272-279.
• Voigt, E., 1968, Uber-Hiatus-Konkretion (dargestellt an Beispielen aus dem Lias): Geologische Rundschau. v. 58, pp. 281–296.
• Wilson, M.A., 1985, Disturbance and ecologic succession in an Upper Ordovician cobble-dwelling hardground fauna: Science. v. 228, pp. 575-577.
• Wilson, M.A., and Taylor, P.D., 2001, Palaeoecology of hard substrate faunas from the Cretaceous Qahlah Formation of the Oman Mountains: Palaeontology. v. 44, pp. 21-41.
• Zaton, M., 2010, Hiatus concretions: Geology Today. v. 26, pp. 186–189.

Pranala luar

• Dietrich, R.V., 2002, Carbonate Concretions--A Bibliography, The Wayback Machine. and PDF file of Carbonate Concretions--A Bibliography, CMU Online Digital Object Repository, Central Michigan University, Mount Pleasant, Michigan.
• Biek, B., 2002, Concretions and Nodules in North Dakota North Dakota Geological Survey, Bismarck, North Dakota.
• Epoch Times Staff, 2007, Mysterious Huge Stone Eggs Discovered in Hunan Province Epoch Times International. Photographs of large cannonball concretions recently found in Hunan Province, China.
• Everhart, M., 2004, A Field Guide to Fossils of the Smoky Hill ChalkPart 5: Coprolites, Pearls, Fossilized Wood and other Remains Part of the Oceans of Kansas web site.
• Hansen, M.C., 1994, Ohio Shale Concretions PDF version, 270 KB Ohio Division of Geological Survey GeoFacts n. 4, pp. 1–2.
• Hanson, W.D., and J.M. Howard, 2005, Spherical Boulders in North-Central Arkansas PDF version, 2.8 MB Arkansas Geological Commission Miscellaneous Publication n. 22, pp. 1–23.
• Heinrich, P.V., 2007, The Giant Concretions of Rock City Kansas PDF version, 836 KB BackBender's Gazette. vol. 38, no. 8, pp. 6–12.
• Hokianga Tourism Association, nd, Koutu Boulders ANY ONE FOR A GAME OF BOWLS? and Koutu Boulders, Hokianga Harbour, Northland, New Zealand High-quality pictures of cannonball concretions.
• Irna, 2006, All that nature can never do, part IV : stone spheres
• Irna, 2007a, Stone balls : in France too!
• Irna, 2007b, Stone balls in Slovakia, Czech Republic and Poland
• Katz, B., 1998, Concretions Digital West Media, Inc.
• Kuban, Glen J., 2006-2008. Nevada Shoe Prints?
• McCollum, A., nd, Sand Concretions from Imperial Valley, a collection of articles maintained by an American artist.
• Mozley, P.S., Concretions, bombs, and groundwater, on-line version of an overview paper originally published by the New Mexico Bureau of Geology and Mineral Resources.
• United States Geological Survey, nd, Cannonball concretion
• University of Utah, 2004, Earth Has 'Blueberries' Like Mars 'Moqui Marbles' Formed in Groundwater in Utah's National Parks press release about iron oxide and Martian concretions