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Shock Metamorphism of the Coconino Sandstone at Meteor Crater, Arizona

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Shock Metamorphism of the Coconino Sandstone at Meteor Crater, Arizona VOL. 76, NO. 23 JOURNAL OF GEOPHYSICAL RESEARCH AUGUST 10, 1971 ShockMetamorphism of the Coconino Sandstone at Meteor Crater, Arizona SUSAN WERNER KIEFFER 2 Division o] Geologicaland Planetary Sciences CaliIornia Institute o] Technology,Pasadena 9'1109 A study of the shockedCoconino sandstone from Meteor Crater, Arizona,was undertaken to examinethe role of porosity in the compressionof rocks and in the formation of high- pressurephases. A suite of shockedCoconino. specimens collected at the crater is divided into five classes,arranged in .orderof decreasingquartz content.The amountsof coesite, stishovite(measured by quantitativeX-ray diffraction),and glassvary systematicallywith decreasingquartz content. Coesitemay comprise% by weight of some rocks,whereas the stishovitecontent doesnot exceed1%. The five classesof rockshave distinct petrographic properties,correlated with the presenceof regionscontaining coesite, stishovite, or fused silica. Very few occurrencesof diaplecticglass are observed.In the lowest stagesof shock metamorphism(class 1), the quartz grains are fractured,and the voids in the rock are filled with myriads of small chips derived from neighboringgrains. The fracture patternsin the individualquartz grainsare controlledby the detailsof the initial morphologyof the collid- ing grains.In one weakly shockedrock, it was possibleto map the generaldirection of shock passageby recordingthe apparentdirection of collisionof individual grains.The principal mechanismof energydeposition by a shockwave in a porousmaterial is the reverberation of shock and rarefactionwaves through grains due to collisionswith other grains. A one- dimensionalmodel of the impactprocess can predictthe averagepressure, volume, and tem- perature of the rock if no phase changesoccur but cannot predict the observednonuni- formity of energy deposition.In all rocks shockedto higher pressurethan was necessaryto close the voids, high-pressureand/or high-temperaturephases are present.Locally high pressuresenduring for microsecondsand high temperaturesenduring for millisecondscon- trolled the phasesof SiO• that formed in the rock. Collapsingpore walls becamelocal hot spotsinto which initial depositionof energywas focused.Microcrystalline coesite in class2 rocks occursin symplekticregions on quartz grain boundariesthat were regionsof initial stress and energy concentration,or in sheared zones within the grains. The occurrenceand morphology of the coesite-richregions can be explained only if the transformationfrom quartz to coesiteproceeds slowly in the shockwave. In class3 rocks,microcrystalline coesite occursin opaque regionsthat surroundnearly isotropiccores of cryptocrystallinecoesite. The coresare interpretedto be the productsof the inversionof stishovite(or a glasswith Si in sixfoldcoordination) that initially formedin the shockfront in regionsof grainsshocked to pressuresnear 300 kb. Stishoviteis preservedonly in the opaqueregions, which are be- lieved to have been cooler than the cores.In class4 rocks vesicularglass occurs in core regionssurrounded by opaqueregions containing coesite. The relation of the glassto the coesite and quartz suggeststhat the glass was formed by inversion of stishovite formed above350 kb on releaseto lower pressure.Class 5 rocksare composedalmost entirely of glass,with vesiclesuniformly distributed in the glass.These vesicles were probablyformed by exsolutionof water that had been dissolvedin melted SiOn.during passage of the shock. A study of the shockedCoconino sandstone to investigatethe processof shock-wavepropa- from Meteor Crater, Arizona, was undertaken gation through a porous,granular medium and the effect of voids on the shock wave and the x Contribution 1959, Division of Geological and formationof high-pressurephases. The studyof Planetary Sciences, California Institute of Tech- nology, Pasadena 91109. shockprocesses in porousmedia is important • Now at the Institute of Geophysics,University becausenearly all known terrestriaI and sus- of California, Los Angeles 90024. pectedplanetary erosion processes act to frag- ment and disintegrateplanetary surfaces, and, Copyright ¸ 1971 by the American Geophysical Union. for this reason,a large proportionof impact 5449 5450 SUSAN WERNER KIEFFER 0 I000 ZOO0 3000 4000 FEET I I I I I GEOLOGIC MAP OF METEOR CRATER, ARIZONA Fig. 1. Geologicmap .of Meteor Crater, Arizona (courtesyof E. M. Shoemaker),showing structure of the crater, debris units containing shockedCoconino, and the location of sample- collecting areas (arrows). SI-IOCKMETAMORPttISM OF COCONINOSANDSTONE 5451 events produceshock waves that traverse por- ous,granular materials. Terrestrial craterssuch EXPLANATION as Meteor Crater (Arizona), Campo del Cielo (Argentina), GossesBluff (Northern Territory, Alluvium Playa beds Australia), Henbury (Northern Territory), and UNCON FORM I T Y Wabar (Arabia) contain the products of shock metamorphism of granular materials. To in- Alluvium terpret the record of impact on a planetary sur- 9•"••. Lakebeds face, it is necessaryto be able to relate the Telus UNCON FORM I T Y observedshock-produced changes to the initial state of the surface material and to the pres- Mixed debris from Coconine, sure, temperature,and duration of the shocks Toroweep, Kinbob, end to which it has been subjected. Moenkopi formations:, includes lechoteller•te and meteor•hc The Coconinosandstone from Meteor Crater, material Arizona, was selectedfor this study because Debris from Coconine send- naturally occurringshocked samples are readily stone and Toroweap formation available at the crater. These samples,which [ I have been subjected to a range of shock pres- Debris from Kalbab limestone sure and temperature histories, represent a broad range of final states that result from a Debris from Moenkopi meteorite impact. The Coconino sandstoneis formebon UNCONFORMITY composedalmost entirely of quartz, the only mineral for which su•cient data exist to allow Moenkop• formation an interpretation of the complexbehavior that UNCONFORMITY results from shock loading. I METEOR CRATER AND TI-IE COCONINO Kaybob I,mestone, dotted I•ne •s SANDSTONE sandstone bed Meteor Crater (Figure 1) lies in the Canyon Toroweap formation and Cocoreno sand- Diablo region of the southern part of the stone Colorado Plateau. The crater was formed in Contact sedimentary rocks (the Coconino sandstone, Faults, nearly vertical Toroweap, Kaibab, and Moenkopi formations) or normal ranging from Permian to Triassic age. The rim Thrust fault, teeth are of Meteor Crater is blanketed by an over- on side of upper plate turned flap (with inverted stratigraphy) ejected Authigemc breccia; from the crater; this, in turn, is overlain by fragments not mixed, occurs mostly along faults Pleistocene and Recent alluvium (Figure 1; Shoemaker[1960, 1963]). The lower walls and AIIogenlc breccio floor of the crater are covered by Pleistocene fragmentsmixed, •ncl'udes lechatelierlte and meteorltlc and Recent deposits. Talus along the crater material walls gradesinto alluvium on the floor of the Shaft •::--- crater, which,in turn, interfingerswith a series of lake beds about 30 meters thick. These de- P•t positsoverlie a layerof thoroughlymixed debris (the fallout) composedof fragmentsof Coco- Dump nino sandstone,Toroweap, Kaibab, and Moen- Drill hole kopi formations,and meteoriticfragments. The Explanation of Figure 1 mixed debris unit (attributed by Shoemaker to fallout of debris thrown to great height) is exposedin the crater walls and has been pene- trated by severalshafts in the bottom of the 5452 SUSAN WERNER KIEFFER crater. The mixed debris on the crater floor is goethite occur as secondaryminerals McKee underlainby a brecciacomposed of fragmentsof [1934] reportedzircon, green tourmaline, blue Coconino sandstone. tourmaline,leucoxene, monazite, rutile, mag- The Coconino sandstone underlies 8.3 X 10' netite, and anatase in total amount less than km• of the ColoradoPlateau provincein north- 0.2% in the Coconino in northern Arizona. ern Arizona. At Meteor Crater, outcropsof Recrystallizedquartz is the primary cementin Coconino are best seen on the southeast wall, the Coconino a.t Meteor Crater. but small outcrops also occur on the north, The detrital grains of the Coconinoare well east, and southwalls. The nearestexposures out- rounded (about 0.7-0.8 on the scaleproposed side the crater are 24 km to the south. The by W. C. Krumbein in 1941 for estimating upper part of the Coconino as exposed at roundness);see Figure 2. Including the sec- Meteor Crater is a pale buff, white, or pink ondary overgrowths,the grains are generally cross-bedded sandstone. The coarseness of the slightly ellipticalin shapeand may have angu- crossbedding causesa massiveappearance in lar boundaries. The boundaries of the detrital outcrop.It is convenientto recognizetwo types grainsagainst secondary overgrowths of quartz of sandstonein hand specimenand thin sec- are revealedby layersof dustand vesicles(Fig- tion: (1) massive sandstone,in which lamina- ure 2). The grainshave reentrantsonly in those tions, if they exist, are several centimeters places where quartz has been dissolvedfrom thick; and (2) laminatedsandstone, containing the boundaries.In this study, the shapeof the parallellaminations that are 0.5 to 2 mm thick. grains was characterizedby the ratio of two The widest variations of mineralogy, porosity, perpendiculardiameters, one of which was the and grain size are associatedwith theselamina- longest grain dimension.Measurements of this tions. ratio, dm•/d•, obtainedin traversesof
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