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Lite Geology 14 Winter 1995 L I T E NewMexico Bureau ~.~, .~,..,.~ "~ ~,,~..~,~.~.,..~,~ of .............. ¯ Mines and Mineral .’..:. .i .,.."".. Resources (NMBM&MR) A quarterly publication for educators arid the public- contemporary geological topics~ issues and events EadhBriefs "Concretions, Bombs, and Ground Water Peter S. Mozley Departmentof Earth and EnVironmental Science, NewMexico Tech Concretions are hard masses of sedimentary and, more rarely, volcanic rock that form by the preferential precipitation of minerals (cementation) in localized portions of the rock. They are commonlysubspherical, but frequently form a variety of other shapes, including disks, grape-like aggregates, and complex shapes that defy description (Figs. 1, 2, and 3). Concretions are usually very noticeable features, because they have a strikingly. different color and/or hardness than the rest of the rock. In someareas this is unfortunate, as the concretions have attracted the unwanted attention of local graffiti artists. Commonly, when you break open concretions you will find that they have formed around a nucleus, such as a fossil fragment or piece of organic matter. For a variety of reasons, this nucleus created a more favorable site for cement precipitation than other sites in therock. ~erhapsthe mostunusual concretion "[ spent too much time in thesame place in the back swamp .nucleiarefound in a modemcoastal’ and the clanged concretion went and nucleated on me." saltmarsh in England.Siderite (FeCOa) concretionsin the marshformed aroundWorld-War-II era military shells,bombs, and associated shrapnel, ThisIssue: includingsome large unexploded shells(AI-Agha et al.,1995). A British Earth Briefs--how does Nature conceal Reptiles’onthe Rocks--someunique geologiststudying these concretions bombs and record ancient water-flow photosof homedlizards in New realizedthis only after striking a large pathways? Mexico unexplodedshell repeatedly with his rockhammer (yes, he livedto tellabout Have you ever wondered... Howhas our Trips and worksl~ops for Earth science it)!The concretions formed climate varied in the past? teachers (see page 10 and 12) preferentiallyaround the military debrisbecause it providedan abundant sourceof ironfor the siderite. In shales,concr&ions often preserve New Mexico Bureau of Mines and Mineral Resources 1 Lite Geology,Winter 1995 Figure 2--Complexelongete concretions in the Zia Formation (Miocene), NM.I.ens cap for scale. Figure l~"Cannonball" concretions in the Zia Formatum (Miocene), NM. Photo courtesy of Dave I ,ove. Figure 3--1 luge elongate concretions in the Zia Formation (Miocene), NM. Figure 4---~,ptarian calcite concretion from the Mancos Shale, NM.Top concretior~ has been cut on a rock saw to reveal septarian fractures filled with several generations of coarsely crystalline calcite cement. Top concretion is 10 cm in diameter. Concretions courtesy of NMBM&MR Mineral Muset,m. Winter 19q5, I.ite Geolo£,tt 2 New Mexico Bureau of Mines and Mineral Resources features of the original sediment--such (Raiswell, 1971; Astin, 1986). Mexico’s most important aquifer as burrows, fossils, and sedimentary Alternatively, some authors have (Mozley and Davis, 1996). Elongate layering--that cannot be seen in the rest suggested that they originate as tensile concretions have even been found in of the rock. Preservation of primary fractures produced in response to faults cutting the Santa Fe Group, where features occurs because concretions burial and compaction (Astin, 1986). they record the past flow orientation of usually form relatively early, before The name septarian originates from the ground water in the fault zone (Mozley sediment compaction and other Latin word saeptum (enclosure or wall), and Goodwin, 1995). processes disrupt the original sediment. referring to the raised cracks on the Thus, cementation of the concretions outside of some septarian concretions. Acknowledgements "freezes" the early sediment structure, If you are interested in hunting for Thanksto Stuart Burley for relating his hair raising encounter with’the Linconshire forming a rigid mass that resists later septarian concretions in New Mexico, Washconcretions. This article benefitted alterations. Examining the concretions many are found in the Mancos Shale, a from the commentsand suggestions of Virgil is the only way of understanding these marine Cretaceous unit found in Lueth. early features. northwest New Mexico. Septarian concretions are the most Recently hydrologists have become References commontype of concretion found in interested in elongate concretions. AI-Agha,M. R., Burley, S. D., Curtis, C. D., rock and mineral shops. In these These concretions range in size from and Esson, J., 1995, Complexcementation concretions the fine-grained concretion pencil- and cigar-like bodies to those textures and authigenic mineral body (usually composed of calcite, that resemble large fallen logs (Figs. assemblages in Recent concretions from CaCO~)is cut by a radiating network of and 3). They are thought to form from the Linconshire Wash(east coast, UK) fractures filled with coarsely crystalline flowing ground water, with the long driven by Fe(0) Fe(lI) oxidation: Journal of the Geological Society, London,v. 152, calcite and other minerals (Figs. 4 and axis of the concretion oriented parallel pp. 157-171. 5). The origin of the fractures is poorly to the ground-water flow direction understood, but they may result from (McBride et al., 1994). In formations Astin, T. R., 1986, Septarian crack formation internal shrinkage of the concretion where such concretions are common, in carbonate concretions from shales and mudstones: Clay Minerals, v. 21, pp’. 617- body (like the cracks found in seasoned measuring concretion orientation can L 632. firewood). This shrinkage may be provide a direct measurement of the related to dehydration or past ground-water flow orientation over McBride, E. E, Picard, M. D., and Folk, R. transformation of a gel-like mineral a large area. Such concretions are L., 1994, Oriented concretions, Ionian Coast, Italy--Evidence of groundwater precursor in the concretion interior common in the Santa Fe Group, New flow direction: Journal of Sedimentary Research, v. A64, pp. 535-540. Mozley,P. S., and Davis, J. M., 1996, Relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer, Sierra Ladrones Formation, New Mexico: Journal of Sedimentary Research, v. 66, pp. 11-16. Mozley, P. S., and Goodwin,L., )995, Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations: Geology,v. 23, pp. 539-542. Raiswell, R., 1971, The growth of Cambrian and Liassic concretions: Sedimentology,v. 17, pp. 147-171. Figure 5--Moeraki boulders, Moeraki Formation, Paleocene, South lslanct, NewZealand. These huge septarian calcite concretions were exposed when the shale host rock was eroded by wave action. Photo courtesy of James Boles. New Mexico Bureau of Mines and Mineral Resources 3 Life Geology, Winter 1995 Have you ever wondered... ... Howour climate has research is leading and point out some weather-shaping, upper-level winds examples pertinent to NewMexico. called jet streams to alter their course varied in thepast? (Fig. 2). The effects are often droughts Part 1: The El Nifio effect Oceans and atmospheric currents in Australia, India, and Africa; floods in First-order effects on climate are the South Americaand California; and col,d, CharlesE. Chapin circuJation patterns in the oceans and wet winters in NewMexico. For ’ Director and State Geologist, NMBM&MR atmosphere. The two are strongly example, the Albuquerque Journal on interrelated as has been graphically De~:ember17, 1992 (an El Nifio year) Manyof us have listened .to illustrated by the El Niflo effects in reported that Novemberwas the grandparents and elderly neighbors recent years. The El Nifio or ENSO(El coldest in Albuquerque since 1938 tell of how when they were young Nifio--southern Oscillation) is (average,temperature 39.7 °F versus the climate was different. Small warmingof surface waters in the eastern normal average of 44 °F) and the streams were bigger then and had Pacific near the Equator(Fig. I). It snowiest since 1946 (5.9 inches versus more fish in them, or winters were caused by a weakening of the trade 0.38 inches). Of(icial snowfall more severe, etc. Such stories are winds that normally blow westward Albuquerque up to December 17th was usually received with polite across the equatorial south Pacific. As 19.6 inches, making1992 the fifth interest and considerable they weaken, warmPacific surface snowiest year of the past 100 years. skepticism. But howvariable is our waters that usuallybuild up near Asia Whata contrast with 1995! climate.., and how can we fiod begin flowing eastward.t0ward Central The El Nii’lo-Southern Oscillation is out? and South America (Fig. 1A) bringing actually a cycle with both warmand Widely publicized worries about unusually warmcurrents to the region. cold extremes. The warm extreme (El the buildup of carbon dioxide in Whythe trade winds weaken is not Nifio) is driven by an eastward the Earth’s atmospl~ere, as a result known. propagating, broad wave of of burning fossil fuels, and the The build up of warmsurface waters, downwelling warm surface water that possibility of global warming, with thousands of miles across, in the eastern depresses the therm0cline (the thin the melting of ice caps and rise of Pacific causes the globe-circling, layer of steep temperature gradient sea level, etc., have elevated research
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