BARRINGERI' CRATER, COCONINO COUNTY, Eugene Shoemaker CaliforniaInstitute of Technology David Roddy United States Geological Survey Carleton Moore Arizona State University Robert Dietz Arizona State University

A one-day field trip will visit Barringer Meteorite Crater. Partici­ pants will depart from Tempe in the evening and spend the night in Flagstaff. The field trip will depart from Flagstaff to the Crater and will return to Flagstaff and Phoenix on the same day. At the Crater participants may choose from a number of guided or self-guided op­ tions to visit this interesting geologic feature. In addition to the road guide included here participants will be provided with a ''Guidebook to the Geology of , Arizona" prepared for the 37th Annual Meeting of the Meteoritical Society. It has been reprinted and is available from the Center forMeteorite Studies, Arizona State Uni­ versity, Tempe, Arizona 85281. Access to the Crater is by the courtesy of the Barringer Crater Company and Meteor Crater Enterprises, Inc. Visitors must receive permission to enter nonpublic areas of the Crater.

SYNOPSIS OF THE The Supai Formation consists of interbedded red and GEOLOGY OF METEOR CRATER yellow fine-grained argillaceous sandstone and subordinate by Eugene M. Shoemaker siltstone. It is more than 300 meters (1,000 feet) thick in REGIONAL SETTING this region (Pierce, 1958, p. 84), but not more than 100 Meteor Crater lies in north-central Arizona in the Can­ meters or so (a few hundred feet) have been penetrated by yon Diablo region of the southern part of the Colorado drill holes at the crater. The Coconino Sandstone (McKee, Plateau (fig. 1). The climate is arid and the exposures are 1933) consists of about 210 to 240 meters (700 to 800 feet) exceptionally good. of fine-grained saccharoidal, white, cross-bedded sand­ In the vicinity of the crater, the surface of the Plateau has stone. Most of the Coconino sandstone is an unusually very low relief and is underlain by nearly flat-lying beds of clean quartz sandstone; in the lower part of the formation Permian and Triassic age. The crater lies near the anticlinal the quartzose sandstone is interbedded with red sandstone bend of a gentle monoclinal fold,a type of structurecharac­ beds of Supai lithology. Only the upper part of the teristic of this region. The strata are broken by wide-spaced Coconino is exposed at the crater. The Coconino is overlain northwest-trending normal faults, generally many kilome­ conformably by 2.7-meter- (9-foot-) thick unit of white to ters in length but with only a fewmeters to about 30 meters yellowish- or reddish-brown, calcareous, medium- to of displacement. Two mutually perpendicular sets of verti­ coarse-grained sandstone interbedded with dolomite re­ cal joints of uniformstrike occur in the region of the crater. ferredto the Toroweap Formation of Permian age (McKee, One set is subparallel to the normal faults and the other 1938). controls the trend of secondary stream courses (fig. 1). The Kaibab Formation of Permian age, which rests con­ Basaltic cinder cones and flows of late Tertiary and formably on the Toroweap Formation, includes 79.5 to 81 Quaternaryage lie at distances of about 11 to 29 km (7 to 18 meters (265 to 270 feet) of fossiliferous marine sandy miles) to the south, west, and northwest of the crater. dolomite, dolomitic limestone, and minor calcareous sandstone. Three members are recognized (McKee, 1938). PRE-QUA TERNARY STRATIGRAPHY The lower two members, the Gamma and Beta Members, Rocks exposed at Meteor Crater range from the Coco­ are chiefly massive dense dolomite; the upper or Alpha nino Sandstone of Permian age to the Moenkopi Formation Member is composed of well-bedded limestone and dolo­ of Triassic age. Drill holes in and around the crater have mite with several continuous thin sandstone interbeds. The intersected the upper part of the Supai Formation of Kaibab is exposed along the steep upper part of the wall of Pennsylvanian and Permian age, which conformably under­ the crater, and a large area west of Meteor Crater is a lies the Coconino. stripped surface on the Alpha Member (fig.1).

151 CANYON DIAILO REGION

Pk

C C C

....

111°15' 111° 00' 0 IOMil11 G1olo9y by Scale E. M. Shoemaker, 19S8 EXPLANATION ..,...,_.. Cliff Syncline 1howin9 trace of__,_ aaial plane �-Basalt }I� -­Contact Antlclinal bend ·. . .. lm . . . . • ·1 111 , . . . . . :! -i-- lon9er arrow indicates flatter dip Moenkopl formation � Normal fault } � dot on downthrown 1id1 Pk •• , Synclinal bend lon9er arrow Indicates flatter dip C KalbabI . lim11ton1 Anticline showln9 }I trace of a11ial plane Cinder�,,,�·� cone

Figure I. Sketch geologic map of Canyon Diablo region, Arizona.

152 In the vicinity of the crater, and to the east, beds of the on all the debris units, as well as on bedrock. The Pleis­ Moenkopi Formation(McKee, 1954) of Triassic age forma tocene alluvium forms a series of small, partly dissected thin patchy veneer resting disconformably on the Kaibab. pediments extending out fromthe crater rim and also occurs Two members of the Moenkopi are present in the vicinity of as isolated patches of pediment or terrace deposits on the the crater. A 3- to 6-meter (10- to 20-foot) bed of pale interstream divides. It is correlated on the basis of well­ reddish-brown, very fine-grained sandstone (McKee's developed pedocal paleosols with the Jeddito Formation of lower massive sandstone), which lies from0.3 to 1 meter (1 Hack (1942, pp. 48-54) of Pleistocene age (Leopold and to 3 feet) above the base, constitutes the Wupatki Member. Miller, 1954, pp. 57-60; Shoemaker, Roach, and Byers, Above the Wupatki are dark, reddish-brown fissile siltstone 1962) in the Hopi Buttes region, some 80 km (50 miles) to beds of the Moqui Member. About 9 to 15 meters (30 to the northeast. Holocene alluvium blankets about half the 50 feet) of Moenkopi strata are exposed in the wall of area within the first 0.8 km (half-mile) of the crater and the crater. extends along the floors of minor stream courses (fig. 2). It includes modern alluvium and correlativesof the Tsegi and Naha Formations of Hack (1942) of Holocene age QUATERNARY STRATIGRAPHY AND (Shoemaker, Roach, and Byers, 1962) in the Hopi Buttes STRUCTURE OF THE CRATER region. Meteor Crater is a bowl-shaped depression 180 meters Both the Pleistocene and the Holocene alluvium are (600 feet)deep, about 1.2 km(¾ mile) in diameter, encom­ composed of material derived from all formations rep­ passed by a ridge or rim that rises 30 to 60 meters (100 to resented in the debris and also contain meteorite fragments, 200 feet)above the surroundingplain. The rim is underlain (Merrill, 1907, 1908, pp. 472-478; Rogers, by a complex sequence of Quaternary debris and alluvium 1928, pp. 82-84, and 1930), other kinds of fused rock resting on disturbed Moenkopi and Kaibab strata (fig. 2). (Nininger, 1954, 1956, p. 117-134), and less strongly The debris consists of unsorted angular fragments rang­ shocked rocks with and (Chao, Shoe­ ing from splinters less than lµ, in size to great blocks more maker and Madsen, 1960; Chao and others, 1962). than 30 meters across. Because of the striking lithologic Oxidized meteoritic material and fragments of relatively contrast among the older formations fromwhich the debris strongly shocked Coconino Sandstone are locally abundant is derived, it is possible to distinguish and map units or in the Pleistocene alluvium where it occurs fairly high on layers in the debris by the lithic composition and strati­ the crater rim. Unoxidized meteoritic material occurs in two graphic source of the component fragments. principal forms: ( a) large crystalline fragments composed The stratigraphically lowest debris unit of the rim is mainly of two nickel-iron minerals, kamacite and taenite composed almost entirely of fragments derived from the (Merrill and Tassin, 1907), and (b) minute spherical parti­ Moenkopi Formation. Within the crater this unit rests on cles of nickel-iron (Tilghman, 1906, pp. 898-899, 907; the edge of upturned Moenkopi beds (fig. 3) or very locally Nininger, 1949, 1951a, b, pp. 80-81). The bulk of the grades into the Moenkopi Formation; away from the crater meteoritic material distributed about the crater is apparently wall the debris rests on the eroded surf ace of the Moenkopi. in the form of small particles. The total quantity of fine­ A unit composed of Kaibab debris rests on the Moenkopi grained meteoritic debris about the crater, which occurs not debris. The contact is sharp where exposed within the cra­ only in the Pleistocene and Holocene alluvium but also as ter, but, at distances of 0.8 km (half a mile) fromthe crater, lag and dispersed in colluvium, has been estimated by there is slight mixing of fragmentsat the contact. Patches of Rinehart(1958, p. 150) as about 12,000 tons. a third debris unit, composed of sandstone fragments from Beds ranging fromthe Coconino Sandstone to the Moen­ the Coconino and Toroweap Formations, rest with sharp kopi Formation are exposed in the crater walls. Low in the contact on the Kaibab debris. No fragments from the Supai crater the beds dip gently outward. The dips are generally Formation are represented in any of the debris. steeper close to the contact with the debris on the rim, and The bedrock stratigraphy is preserved, inverted, in the beds are overturned along various stretches totalling about debris units. Not only is the gross stratigraphy preserved, one-third the perimeter of the crater. Along the north and but even the relative position of fragments from different east walls of the crater, the Moenkopi can locally be seen to beds tends to be preserved. Thus most sandstone fragments be foldedback on itself, the upper limb of the fold consist­ from the basal sandstone bed of the Moenkopi Formation ing of a flap that has been rotated in places more than 180° occur near the top of the Moenkopi debris unit, fragments away fromthe crater (fig. 3). At one place in the southeast from the Alpha Member of the Kaibab limestone occur at corner of the crater, the flap grades outward into disaggre­ the base of the Kaibab debris unit, and brown sandstone gated debris, but in most places there is a distinct break fragments from the 3-meter-(9-foot-) thick Toroweap For­ between the debris and the coherent flap. mation occur just above the Kaibab debris unit. Rocks now represented by the debris of the rim have been Pleistocene and Holocene alluvium rests unconformably peeled back from the area of the crater somewhat like the

153 EXPLANATION � L___:__:_J \... Recent alluvium 0 '·" -Q.� ✓-�- � ilJi Pleistocene alluvium >­ a:: <( f1i�r Debris from Coconino z sandstone and a:: w .··'.>'l.t ,1.v Toroweap formation .... <( ../i ::, /1 0 Debris from Kaibab- limestone �,1 - YL Debris from .:�t Moenkopi formation ./ � u � (/) Moenkopi formation, Cl) ll <( ... _ 1 Wupatki member; a:: "Lower massive .... '" sandstone" of McKee ( 1954) unshaded rill Kaibab limestone, � / t Alp ha member a.. ·· .i of McKee 0938) . � ------/ � Contact; long dashes where approximately located, short}� ·:>. dashes where indefinite \1 Gtol09y by ��:��...:;;>_...-_j<]llllllffllllll E.M.1957-1 Shotm95ak8tr � 1 Small monoclinal fold, arrow 1 shows downthrown side 0 2 r3 Milts

Figure 2. Geologic map of area around Meteor Crater, Arizona. u n METEOR CRATER

Qr Qk Qp .

Qcl

5000' 5000'

4000•...______._ 4000' 0 1000 2000 feet Geol ogy byE. M. Shoemaker, 1957-1958 Qr, Recent alluvium Qd, mixed debris from Coconino, Toroweap, Kaibab, and Maenkopi lim, Moenkopi formation (Triassic) Ori, Recent playabeds formations; includes lechatelierile ond meteorillc material Pk, Koibob lime,tone ( Permion ) Qp, Pleistocene alluvium Qct, debri, from Coconino and Torawaap formations �I, Coconino ond Toroweop Qpl, Plaistocene lake bads Qk, debris from Kaibab limestone formations ( Perm Ion Qpt, Pleistocene la lus Qm, debris from Moenkopi formation PPs, S.ipa1 formation l Permian br, brecc,a ( includes lechalelierlle and mehlorltic malarial and Penn,y lvantan)

NUCLEAR EXPLOSION CRATERS

dd dd ° 4300' 4300

4250' Qp 4250' Qp

1 4200' 4200

--- - _ _ 4150 4 o'------_ _ _eo_l_ _ _ _ ._ - .- - - - k_ •- ,-9 _6_-1_95....1.8 ' 15 o 50 ,oo 5o, ,eel G ogy byE M Shoema �, 5

JANGLE U CRATER

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Center of nuclear device�

° 4250' 4250

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00•...______42 .14200' 0 l 8 .____ _, 50___ __,_ 100___ � 150 'feel Geo ogy byE.M Shoemaker, 195 dd, dump material moved by bulldour Ora, Recan t sond or power shovel Qrg, Recent grav�I ta, scree formed in crater Qpb, Plei,tocene paleo,ol, B horizon al, alluvium deposited in crater Qp, Pleistocene alluvium lo, fallout and throwout br,

Figure 3. Cross sections of Meteor Crater, Arizona, and nuclear explosion craters. petals of a flower. The axial plane of the fold in three comer of the crater a tom end of the overturned flap of the dimensions is a flat cone, with apex downward and concen­ east wall forms a projection suspended in debris. A few tric with the crater, that intersects the crater wall. If eroded normal faults, concentric with the crater wall, occur on the parts of the wall were restored, more overturnedbeds would southwest side. A number of small thrustfaults occur on the be exposed. north and west sides of the crater. Relative displacement of The upturnedand overturned strata are broken or tom by the lower plate is invariably away from the center of the a number of small, nearly vertical faults with scissors-type crater. Crushed rock is locally present along all types of displacement. A majority of these tears are parallel with the faults; it has been designated authigenic breccia, to distin­ northwesterly regional joint set, and a subordinate number guish it fromanother type of breccia under the crater floor. are parallel with the northeasterly set. Regional jointing has The floor of the crater is underlain by Quaternary strata, controlled the shape of the crater, which is somewhat debris, and breccia. Pleistocene talus mantles the lowest squarish in outline; the diagonals of the ''square'' coincide parts of the crater walls and grades into Pleistocene al­ with the trend of the two main sets of joints. The largest luvium along the floor. The Pleistocene alluvium, in tum, tears occur in the ''comers'' of the crater. In the northeast interfingers with a series of lake beds about 30 meters ( 100

155 feetthick) toward the center of the crater r Up to 1.6 meters (Johnson, 1959, p. 10). Beds of alluvium exposed in the (6 feet) of Holocene alluvium and playa beds rest uncon­ rim are peeled back in an overturned syncline, just as the formably on the Pleistocene. Where exposed in shafts, the bedrock is peeled back at Meteor Crater. The upper limb of lowermost Pleistocene lake beds contain chunks of pumi­ the fold is overlain by and locally passes outward into debris ceous, frothylechatelierite. that roughly preserves, inverted, the original alluvial A layer of mixed debris underlies the Pleistocene talus stratigraphy. Shock-formed glass and other strongly and lake beds and rests on bedrock and on breccia. This shocked materials, some containing coesite, are present in layer is composed of fragments derived from all formations the uppermost part of the debris. A thin layer of debris intersected by the crater and includes much strongly formed by fallout or fall-back is also present in the crater. shocked rock and oxidized meteoritic material. The mate­ The floor and lower walls of the crater are underlain by a rial from all the different sources is thoroughly mixed. thick lens of breccia containing mixed fragments of al- Where intersected by a shaft in the crater floor, it is about 1 uvium and dispersed glass. Some of the fragments are 10.5 meters (35 feet) thick and almost perfectly massive, strongly sheared and compressed. The breccia was formed but exhibits a distinct grading, from coarse to fine, from by complex movement of material within the crater; locally, base to top. The average grain size, about 2 cm, is much the original bedding of the alluvium is roughly preserved in less than in the debris units of the rim or in the underlying the breccia. breccia; the coarsest fragments at the base rarely exceed O .3 A crater with the structure of Teapot Ess crater is formed meter ( I foot) in diameter. I believe that this unit was chiefly by two mechanisms, which operate in succession. formedby fallout of debris thrown to great height. It has not First, the alluvium, as it is engulfed by a compressional been recognized outside the crater, probably because it has shock caused by the explosion, is accelerated in all direc­ been entirely eroded away. Its constituents have been partly tions radially outward from the detonated device, and an redeposited in the Pleistocene and Holocene alluvium. expanding cavity is formed underground. When the shock Where exposed at the surface, the breccia underlying the reaches the free surface of the ground, it is reflected as a mixed debris is composed chieflyof large blocks of Kaibab, tensional wave. Momentum is trapped in the material above but the breccia exposed in shafts under the central crater the cavity, and it continues to move up and outward, indi­ floor is made up chiefly of shattered and twisted blocks of vidual fragments following ballistic trajectories. Kinetic Coconino. Extensive drilling conducted by Barringer (1905, energy is also imparted to the fragments from expanding 1910, 1914) and his associates (Tilghman, 1906) has shown gases in the cavity. Ejected fragments close to the crater that, at a depth of 100 to 200 meters (300to 650 feet), much maintain their approximate relative positions, inverted dur­ finely crushed sandstone and some fused and other strongly ing flight. Along ·1ow-velocity trajectories, their range is shocked rock and meteoritic material are present. Some largely a function of the angle at which they are thrown out, drill cuttings fromabout 180 meters (600 feet)depth contain which in turn varies continuously with their original posi­ fairlyabundant meteoritic material. In cuttings examined by tion relative to the nuclear device. The margin of the crater the writer the meteoritic material is chiefly in the form of is determined primarily by the radial distance, in plan, at fine spherules dispersed in glass, similar to the which the reflected tensional wave is just strong enough to described by Nininger (1954) and by Spencer (1933, pp. lift and separate the alluvium. The position at which this 394-399) from the Wabar and Henbury meteorite craters. occurs is just inside of and concentric with the hinge of the Cores of ordinary siltstone and sandstone of the Supai were overturned flap, i.e., the intersection of the axial plane of obtained at depths of 210 meters (700 feet) and deeper. The overturning with the surface of the ground. The reflected lateral dimensions of the breccia are not known because the tensional wave starts a fracture at t�e surface but does not drilling was concentrated in the center of the crater. Some appreciably change the momentum of most of the material of the meteorite-bearing or allogenic breccia was evidently thrown out. Beds are sheared off along a roughly conical encountered in a hole drilled from 1920 to 1922 on the south surface that starts at the surface slightly inside the hinge and rim of the crater (Barringer, 1924). (See log of hole in is propagated downward to join tangentially the lower part Hager, 1953, pp. 840-841.) of the hole expanded behind the shock front.Material brec­ ciated by very strong shock initially has the form of a MECHANISM OF CRATER FORMATION roughly spherical shell around the hole, but most of the Nearly all the major structural featuresof Meteor Crater, upper hemisphere is thrown out, and the lower hemisphere Arizona, are reproduced in a crater in the alluvium of Yucca is sheared out in the form of a concavo-convex lens. Over­ Flat, Nevada, formed by the underground explosion of a turning of the beds in the upper crater wall may be looked nuclear device. The Teapot Ess crater (fig. 3), about 90 on as drag along the conical shear zone. meters (300feet) across and originally about 30 meters (I 00 It is essential that the shock originate deeper than a cer­ feet) deep, was produced in 1955 by a 1.2 kiloton device tain minimum distance below the surface, relative to the detonated at a depth of 20 meters (67 feet)below the surface total energy released, in order to produce the overturned

156 Moe�i fonnotion Koibob hmestont I 1. Meteorite approaches ground at 15 km/sec. Coconino and Toroweop formations

L 2. Meteorite enters ground, compressing and fusing rocks ahead and flatten­ fused rock> ing by compression and by lateral flow. Shock into meteorite reaches back side of meteorite.

shock frontirustd rock 3. Rarefaction wave is reflected back through meteorite, and meteorite is decompressed, but still moves at about 5 km/sec into ground. Most of energy has been transferred to compressed fused rock ahead of meteorite. ,omJ------4. Compressed slug of fused rock and trailing meteorite are deflected later­ - ally along the path of penetration. Meteorite becomes liner of transient cavity. 5. Shock propagates away from cavity, cavity expands, and fused and strongly shocked rock and meteoritic material are shot out in the moving mass behind the shock front .

!'!"' """ ""'-- .....,.r�efltd_ed_ro-refo-ct.. ion-;f-.}!J fffe.-;·''..------6. Shell of breccia with mixed fragments and dispersed fused rock and meteoritic material is formed around cavity. Shock is reflected as rarefac­ ��.i.\:,;;,.... shock front ��j2··--··' tion wave from surface of ground and momentum is trapped in material above cavity. ,}, .:/-.,

7. Shock and reflected rarefaction reach limit at which beds will be overturned. Material behind rarefaction is thrown out along ballistic '$ Ri�Lt�;;�,,,I�---\ -� i;,,!t,-.w:ii.;;Ji)f (0}� / trajectories.

\,,�_ _,_..,,.. ,..,.,�.,,/

8. Fragments thrown out of crater maintain approximate relative positions except for material thrown to great height. Shell of breccia with mixed meteoritic material and fused rock is sheared out along walls of crater; upper part of mixed breccia is ejected.

9. Fragments thrown out along low trajectories land and become stacked in an order inverted from the order in which they were ejected. Mixed breccia along walls of crater slumps back toward center of crater. Fragments 0 1000 2000 FEET thrown to great height shower down to formlayer of mixed debris.

Figure 4. Diagrammatic sketches showing sequence of events in formation of Meteor Crater, Arizona. syncline in the rim. A crater formed near the Teapot Ess The structure of Meteor Crater would be produced by a crater by a nuclear device of the same yield, but detonated very strong shock originating about at the level of the pre­ at 5.4 meters (18 feet) depth rather than 20 meters (67 feet) sent crater floor ( 120 meters (400 feet) below the original (fig.3, Jangle U crater), has an anticline under the rim, not surface). A minimum scaled depth of penetration of the a syncline. An explosion at such a depth (shallow relative to meteorite or of a major part of the energy associated with the yield of the device) produces a crater more through the meteorite is indicated by the overturned synclinal struc­ radial expansion of the cavity behind the shock and less by ture of the rim. A closer estimate of the depth for the appar­ ejection of material than in the case of Teapot Ess crater. ent origin of the shock can be obtained fromthe base of the The greatest expansion of the hole occurs at the surface breccia under the crater floor by scaling from Teapot Ess because of upward relief of stressat the free surface, which crater. Some limits may also be set on the mechanics of manifests itself in concentricanticlinal buckling. impact by the factthat the shock was strong enough to fuse

157 Coconino Sandstone, at a depth of some 90 meters (300 Hack, J. T., 1942, The changing physical environment of the Hopi feet) or more below the surface, and that part of the meteor­ Indians: Peabody Museum Nat. HistoryPapers, v. 35,p. 3-85. Hager, Dorsey, l 953, Crater mound (Meteor Crater), Arizona, a ite itself was fused by shock on impact, as shown by the geologic feature: Am. Assoc.Petroleum Geologists Bull., v. 37, dispersal of spherical particles of meteoritic material in p. 821-857. some of the fused rock. Johnson, G. W., 1959, Mineral resource development by the use of nuclear explosives: Calif. Univ.,Lawrence Radiation Lab. Rept. Not all meteorite craters exhibit overturned synclines in 5458,18 p. the rim, as at Meteor Crater, which indicates that the scaled Kennedy, G. C.,Wasserburg, G. J., Heard, H. C., and Newton, R. depth of penetration on impact is not always so great. The C.,1962, The upper Millphase region in the system SiO2-H2O, Am. Jour. Sci.,v. 260,p. 501. rim of the main meteorite crater at Odessa, , for Kieffer, S. W., 1970, 1. of the Coconino example, is underlain by an anticline similar to that of the Sandstone at Meteor Crater, Arizona; 2. The specific heat of Jangle U crater (compare with Hardy, 1953; Sellards and solids of geophysical interest: Ph.D. thesis,California Institute of Technology,Pasadena. Evans, 1941). Kieffer, S. W., 1971, Shock metamorphism of the Coconino The reconstructed sequence of events in the formation of Sandstone at Meteor Crater, Arizona: Jour. Geophys. Res., v. Meteor Crater, based on theoretical analysis (Shoemaker, 76,p. 5449. Kieffer, S. W.,Phakey, P. P., Christie, J. M., Shock processes in 1960), is illustrated in figure 4. The crater was formed by porous quartz (in prep). impact of meteoritic iron bolide about 30 meters (100 feet) Kluth, C. F.,and Kluth, M. J.,1974, Geology of the Elden Mountain across . On the basis of scaling from the Sedan Crater in area,in Geology of Northern Arizona for the Geological Society of America,Rocky Mt. Section Meeting, Flagstaff,Arizona, pt. Nevada, the energy released by impact was 4 to 5 megatons II,p. 521. TNT equivalent. This event occurred a few tens of Leopold,L. B.,and Miller,J. P.,1954:_A post-glacial chronology for thousands of years ago, as shown by the mid-Wisconsin age some alluvial valleys in Wyoming: U.S. Geol. Survey Water­ SupplyPaper 1261,90 p. of the oldest sedimentary deposits on the rim and in the McKee, E. D., 1933, The Coconino Sandstone-its history and ori­ interior of the crater. gin,in Contributions toPaleontology: Carnegie Inst. Washington Pub. 440,p. 78-115. McKee, E. D., 1938, The environment and history of the Toroweap and Kaibab Formations of northern Arizona and southern Utah: REFERENCES CITED Carnegie Inst.Washington Pub. 492,221 p. McKee, E. D., 1951, Triassic deposits of the Arizona-New Mexico Baker, A. A., and Reeside, Jr., J. B., 1929, Correlation of the Per­ border area: in New Mexico Geo!. Soc. Guidebook, 2nd Field mian of Southern Utah, northern Arizona, northwestern New Conf.,San Juan Basin,p. 85-92. Mexico, and southwestern Colorado: Bull. Amer. Assoc. Petrol. McKee, E. D., 1954, Stratigraphy and history of the Moenkopi For­ Geo!.,v. XIII, p. 1413-1448. mation of Triassic age: Geo!. Soc. America Mem. 61,133 p. Barringer, D. M., 1905, Coon Mountain and its crater: Acad. Nat. Merrill,G. P., 1907, On a peculiar form of metamorphism in siliceous Sciences,Philadelphia Proc., v. 57,p. 861-886. sandstone: U.S. Nat. Mus.Proc., v. 32,p. 547-550. Barringer, D. M., 1910, Meteor Crater (formerly called Coon Moun­ Merrill, 1908, The Meteor Crater of Canyon Diablo, Arizona: its tain or Coon Butte) in northern central Arizona: Published by the history, origin, and associated meteoritic irons: Smithsonian Inst. author,24 p. Misc. Coll.,v. 50, p. 461-498. Barringer, D. M., 1914, Further notes on Meteor Crater, Arizona: Merrill, GeorgePerkins, and Tassin,Wirt, 1907, Contributions to the Acad. Nat. SciencesPhiladelphia Proc., v. 66,p. 556-565. study of Canyon Diablo : Smiths. Misc. Col. 50, p. Barringer, D. M., 1924, Further notes on Meteor Crater in northern 203-215. central Arizona (No. 2): Acad. Nat. Sciences PhiladelphiaProc., Moore,R. B., Wolfe, E. W.,and Ulrich,G. E.,1974, Geology of the V. 76,p. 275-278. eastern and northern parts of the San Francisco , Chao, E.C. T., 1967, Shock effects in certain rock-forming minerals: Arizona,in Geology of northern Arizona for the Geological Soci­ Science,v. 156, p. 192-202. ety of America, Rocky Mt. Section Meeting,Flagstaff, Arizona, Chao, E.C.T., Fahey, J. J., Littler, Janet, and Milton, D. J., 1962, pt. II,p. 465. Stishovite, SiO2 A very high pressure new mineral from Meteor Nininger, H. H., 1949, Oxidation studies at Barringer crater. Metal­ Crater,Arizona: Jour. Geophys. Res.,v. 67, p. 419-421. center pellets and oxide droplets: Am Philos. Soc. Yearbook, p. Chao, E.C.T., Shoemaker, E. M., and Madsen,B. M., 1960, First 126-130. natural occurrence of coesite: Science,v. 132, p. 220. Nininger, H. H., 1951a, Condensation globules at Meteor Crater: Colton,H. S. (revised 1967),The basaltic cinder cones and lava flows Science, v. 113, p. 755-756. of the San Francisco Mountain volcanic field: Mus. of Northern Nininger, H. H., 1951b, A resume of researches at the Arizona ArizonaBull. No. 10,Northland Press, Flagstaff, Ariz., 58 p. meteorite crater: Sci. Monthly,v. 72, p. 75-86. Damon,P. E., 1965,Correlation and chronology of ore deposits and Nininger, H. H., 1954, Impactite slag at Barringer crater: Am. Jour. volcanic rocks: Ann. Prog. Rept., COO-689-50 Contract AT Sci.,v. 252,p. 277-290. (l l-1)-689, U.S. Atomic Energy Comm.,p. 36-43. Nininger, H. H., 1956, Arizona's meteorite crater: Denver, Colo., Damon,P. E., Shafiqullah, M., and Leventhal, J. S., 1974, K-Ar WorldPress, Inc., 232 p. chronology for the San Francisco volcanic field and rate of ero­ Noble, L. F., 1914, The Shinumo Quadrangle: U.S. Geo!. Survey sion of the Little Colorado, in Geology of Northern Arizona for Bull. 549,p. 1-100. L the Geological Society of America, Rocky Mt. Section Meeting, Opdyke, N. D., and Runcorn, S. K., 1960,Wind direction in the Flagstaff,Arizona, pt. I,p. 221. western United States in the late Paleozoic: Geol. Soc. America Darton, N. H., 19 I 0,A reconnaissance of parts of northwestern New Bull. v. 71, p. 959-971. L Mexico and northern Arizona: U.S. Geo!. Survey Bull. 435, Peirce, H. W., 1958,Permian sedimentary rocks of the Black Mesa p. 21. basin area: New Mexico Geo!. Soc. 9th Field Conference, Fairchild, H. L., 1907, Origin of Meteor Crater (Coon Butte), Guidebook of the Black Mesa Basin, Northeastern Ariz., p. Arizona: Bull. Geo!. Soc. America,v. 18, p. 493-504. 82-87. .. 158 Peirce, H. W., and Scurlock, J. R., I 972, Arizona well information: lower Wupatki Member of the Moenkopi Ariz. Bur. Mines Bull. 185, 195 p. Formation of Early Triassic age (McKee, Read, C. B., 1950, Stratigraphy of the outcropping Permian rocks around the San Juan Basin, in New Mexico Geol. Soc. 1951). Guidebook of the San Juan Basin, New Mexico and Colorado, p. San Francisco Mountain, including 62-66. , and Elden Mountain are Reiche, Parry, An analysis of cross-lamination the Coconino Sandstone: TheJour. of Geology, v. XLVI, no. 7, p. 905-932. visible at 10 and 11 o'clock, respectively, Rinehart, J. S., 1958, Distribution of meteoritic debris about the on the left. San Francisco Mountain, a Arizona meteorite crater: Smithsonian Inst. Contrib. Astrophys., largely andesitic strato-, probably V. 2, p. 145-160. Rogers, A. F., 1928, Natural history of the silica minerals: Am. attained a total eruptive elevation of more Mineralogist, v. 13, p. 73-92. than -4,500 m (15,000 feet).The volcano Rogers, A. F., 1930, A unique occurrence of lechatelierite or silica may have been modified by either summit glass: Am. Jour. Sci., 5th Ser., v. 19, p. 195-202. Schuchert, C., 1918, On the carboniferous of the Grand Canyon of collapse or explosive eruption. More re­ Arizona: Amer. Jour. Sci., v. XLV (4th Ser.), p. 347-361. cent fluvial and glacial erosion have also Shoemaker, E. M., 1960, Penetration mechanics of high velocity modified the form of the volcano. Hum­ meteorites, illustrated by Meteor Crater, Arizona: lnternat. Geo!. Con., 21st Session, pt. 18, Copenhagen, p. 418-434. phreys Peak, elevation 3,851 m (12,633 Shoemaker, E. M., Roach, C. H., and Byers, F. M., Jr., 1962, feet), is the highest mountain in Arizona. Diatremes and uranium deposits in the Hopi Buttes, Arizona, in Elden Mountain (2,835 m; 9,299 feet) is a Petrologic Studies, a volume to honor A .F. Buddington: Geo!. Soc. America, p. 327-355. dome, complex in detail because Smiley, T. L., 1958, The geology and dating of , part of it (north) appears to be laccolithic, Flagstaff, Arizona, in Guidebook of the Black Mesa Basin, tilting and exposing pre-Kaibab Paleozoic northeastern Arizona, New Mexico Gepl. Soc. 9th Field Conf., p. 186-190. rocks as old as Devonian, while another Spencer, L. J., 1933, Meteoric iron and silica-glass from the meteorite part (the south end, visible from the road) craters of Henbury (central Australia) and Wabar (Arabia): was extrusive (Kluth and Kluth, 1974). Mineralog. Mag., v. 23, p. 387-404. Tilghman, B. C., 1906, Coon Butte, Arizona: Acad. Nat. Sciences Large viscous flow lobes with marked Philadelphia Proc., v. 57, 887-914. jointing can be seen from the highway on von Engelhardt, W., and Stoffler, D., 1967, Stages of shock the south and east sides of the mountain. metamorphism in crystalline rocks of the Rees Basin, Germany, in French, B., and Short, N., eds., Shock Metamorphism of 0.6 0.6 The road cut exposes the top of the Alpha Natural Minerals, Baltimore, Md. Member of the Kaibab Formation of Per­ mian age (McKee, 1938). The top of the ROAD GUIDE FROM Kaibab is an ancient karst surface. The FLAGSTAFF TO METEOR CRATER upper 20 to 30 meters (65 to 100 feet) of by the Kaibab Formation appear to have been Eugene M. Shoemaker, modified by solution. Susan W. Kieffer, and Robert L. Sutton 0.4 1.0 The road crosses a southeast-trending es- MILES carpment bordering a small valley Interval Total ("park") which is probably a graben. 0.0 0.0 Butler Street interchange, U.S. Interstate 1.9 2.9 Route 89 north toward Page and Grand Highway 40 (I-40), heading east toward Canyon exists here. Continue west on Meteor Crater and Winslow. Just north of I-40. U.S. Route 66 in Flagstaff, at 9:00 below 0.4 3.3 Proceed slowly! About 100 meters ahead the horizon, is Switzer Mesa, which is of you, a fault-line scarp is visible. The capped by Pliocene basalt flows dated at downthrown block is on the west and 5.80 ± 0.34 m.y. (Damon and others, forms the small depression which you are 1974). The relatively straight margins of traversing. On the left at 9:00 o'clock is Switzer Mesa suggest that basaltic lava Sheep Hill, a cinder cone of the Tappan once flowed down a graben valley and that age-group (Moore and others, 1974). As subsequent erosion produced the present you approach the fault-line scarp', a quick ''reverse topography.'' This filled graben glance to the rear at about 5:00 o'clock is one of many straight-walled valleys and will reveal a basalt flow in the depression escarpments formed in the pre Plio­ below I-40. This flow emanated from the Pleistocene deposits by pervasive normal base of Sheep Hill. The east margin of this faulting. Cropping out below the protec­ depression is sometimes occupied by an tive lava cap of Switzer Mesa is part of the ephemeral lake, Bill Fill Lake.

159 0.1 3.4 In the escarpment directly east of Big Fill blanket of variable thickness covers the land Lake and in the road cut, the jumbled beds surface. The cinders probably came from of the karst horizon of the Alpha Member fourcinder cones just south of the highway. of the Kaibab Formation are again ex­ 0.4 12.4 Winona exit. The basaltic cinder cone on the posed. A distinctive red bed sequence may right has a rootless lava flow on its north­ be seen below the top of the Kaibab; this western flank. This cone is probably the unit thins toward the east and is not present source of the lava flow at mile 12.0. A last at Meteor Crater. The entire Kaibab For­ view of Sunset Crater silhouetted in front of m a ti on pinches out 30 miles beyond O'Leary Peak may be obtained by looking Meteor Crater. back to the left ( about 8 o'clock?). 0.1 3.5 Many quarries are dug into the cinder 1.2 13.6 The road drops back to the stripped surface cones, such as Sheep Hill (at 9:00 on the Kaibab, thinly strewn with cinders. o'clock), Wildcat Hill (11:00 o'clock), To the left, just beyond the Santa Fe railroad and Turkey Hill. The cinders are used in tracks, is the margin of a basaltic lava flow. road metal and in cinder blocks. There are 1.0 14.6 Looking ahead, the road rises over a gentle more than 400 cinder cones in the San anticline, reflected as a swell on the stripped Francisco volcanic field. surface on top of the Kaibab Formation. 2.5 6.0 Turn off to Walnut Canyon National 0.4 15.0 Three miles to the north at 9:30 o'clock, Monument. Piper Crater, a basaltic cinder cone, is 0.2 6.2 The red beds in the Alpha Kaibab Forma­ superposed on the south side of Rattlesnake tion are again exposed in the road cut north Crater, an older maar-type vent. Merrill Cra­ of eastbound 1-40. ter is east of Piper Crater at 10 o'clock. 2.2 8.4 Cosnino road exit. 2.1 17.1 Merriam Crater is located in the distance at 0.6 9.0 Normal fault, downthrown to the west. 9:00 o'clock. 0.7 9.7 From this section of the road, the view to 1.2 18.3 The small cinder cone at 3:00 o'clock is the the south is of a stripped surface developed nearest Quaternary volcanic feature to on top of the Kaibab Formation. Topog­ Meteor Crater. raphy in this area accurately reflects the 0.7 19.0 The eastward gradient of the highway bedrock structure, except where it is mod­ steepens noticeably as it comes over the crest erately dissected by gullies. of the anticline mentioned at mile 14.6. 0.3 ,10.0 At about 8 o'clock, below the horizon, Sun­ From here, the road descends into Padre set Crater is visible as a bright-rimmed cin­ Canyon. The Hopi Buttes volcanic field can der cone, devoid of vegetation. It lies just to be seen in the distance ( about 80 km; 50 mi) the right of, and below, O'Leary Peak. The at 10 to 11 o'clock, across the Little Col­ most recent eruption in the volcanic fieldoc­ orado River valley). On the skyline to the curred in 1064 A.D. ·at Sunset Crater. The right is Anderson Mesa, capped by basalt date is based on dendrochronology (Smiley, flows dated at 6.2 ± 1.2 m.y. (Damon, 1958). 1965). 1.2 11.2 Along the highway good exposures of thin 0.9 19.9 Padre Canyon· and Twin Arrows Trading grey beds within the Alpha Member of the Post. Kaibab Formation are visible beneath 1 to 3 1.3 21.2 As one looks out across the broad stripped m of the jumbled rocks in the karst horizon. surface on the Kaibab, it should be kept in The Kaibab here is mostly silty dolomite and mind that in this region of the Colorado dolomitic sandstone with some chert. Plateau, the topography reflects the struc­ Marine fossils (brachiopods such as Derbya; tures beneath. A broad anticline is visible in productids; cephalopods; invertebrates; a the distance ahead. Normal faults which few fishteeth) are abundant in many beds. have been crossed are controlled by a re­ 0.1 11.3 At 10:30 o'clock is a large cinder quarry (the gional pattern of jointing. Two orthogonal Darling pit) at Cinder Mountain, owned by sets of joints which trend predominantly the Santa Fe Railroad. northeast and northwest are parallel to im­ 0.4 11.7 Walnut Canyon portant regional structures in this part of the 0.3 12.0 A lava flow of the Tappan age-group Colorado Plateau (Colton, 1936). (Moore and others, 1974), is visible in the 1.7 22.9 The long ridge between about 2 and 3 road cut. For the next 2.5 miles a cinder o'clock is an irregular basalt-capped ridge

160 ,. which extends toward Meteor Crater and knobby or biscuit-like outcrops just above will be seen end on from road to Meteor the Kaibab limestone. Crater. 2.8 34.2 Meteor Crater exit from1-40. For the next 2 miles the highway traverses an 2.4 25.3 1. anticline where an oil test well was drilled to 0.6 34.8 STOP The first stop is at Indian Country Precambrian strata in 1962, about half a mile Road intersection. To the east may be seen north of the highway (NE¼ NW¼ sec. 12, an abandoned stone building located just T. 20N., R. l lE.). The well was converted north of the old alignment of U.S. Highway H. H. for watering stock. Following are subsurface 66. This building once housed thicknesses of Paleozoic formations pene­ Nininger's meteorite collection. It was oper­ trated by this well (Peirce and Scurlock, ated as a museum, open to the public at a 1972, p. 159): fee, where visitors heard lectures by Nininger and could study the collection and Feet Meters purchase specimens. The operation of this Kaibab (Permian) ...... 283 86.3 museum was the source of Nininger' s liveli­ Coconino (Permian) ...... 890 271.3 Supai (Permo- hood for many years. Visitors could climb Pennsylvanian) ...... 1370 417.7 the stone tower and look off to the north to Naco (Pennsylvanian) ...... 51 5 157.0 get a view of Meteor Crater without deviat­ Molas (Pennsylvanian) ...... 12 3.7 Redwall-Zones A, B, C ing fromtheir route on Highway 66. (Mississippian) ...... 110 33.5 The museum is built just above a low es­ Martin (Devonian) ...... 380 115.8 Precambrian, depth ...... 3560 1085.4 carpment which trends northwest-southeast Precambrian, elevation ...... 2175 663.1 and can be traced by eye for a distance of Total depth of well ...... 3630 1106.7 about two miles. The escarpment is along a The Toroweap Formation, a marine unit small normal fault which offsets the lower between the Kaibab and the Coconino For­ beds of the Moenkopi at this place. The mations in the Grand Canyon, was not rec­ throw on the faultis about 9 meters (30 feet). ognized in this well as it is readily confused On the upthrown side may be seen outcrops with the Coconino (McKee, 1938). Approx­ of the lower Massive Sandstone unit of the imately the lower 73 m (240 ft) of Coconino Wupatki Member of the Moenkopi Forma­ in this well are correlated with the DeChelly tion, which are the lowermost beds of the Sandstone, which thickens toward the De­ Moenkopi at this place. Erosional remnants fiancePlateau (Pierce and Scurlock, 1972). of the Lower Massive Sandstone form a line 0.2 25.5 The escarpment crossed by the road is a of small knobs trending offto the southeast. fault-linescarp on the flank of the anticline. About 3 meters (10 feet) of siltstone and The bedrock is downthrown to the east and very finesandstone of the Moqui Member of exhibits reverse drag. the Moenkopi Formation are preserved loc­ 0.2 25.7 Crest of anticline. ally above the Lower Massive Sandstone. 1.5 27 .2 A good view is obtained here of the rim of Nininger's old museum is built of pieces of Meteor Crater in the distance at 1 :00 Moenkopi sandstone from both the Wupatki o'clock. The Hopi Buttes volcanic field has and Moqui Members. Some of these pieces become more prominent on the left horizon are beautifully ripplemarked. at about 11 o'clock. A water well, 100meters (300 feet)away, 1.9 29.1 The road rises over the crest of another anti­ at 11 o'clock, brings water up fromthe mid­ cline. East and West Sunset Buttes are visi­ dle of the Coconino Sandstone at a depth of ble in the distance to the right of Meteor 200 meters (600 feet). The Coconino Crater. Sandstone is the principal aquifer in this part 0.5 29.6 A fault-line scatp bounding the drainage at of northern Arizona. Here the water table 1:30 to 3:00 o'clock at a distance of about slopes gently to the north, in the same direc­ one half mile is along a normal fault tion as the average regional dip of the beds. downthrown to the west. Meteor Crater is located on the southern 1.1 30.7 Canyon Diab lo. flank of the Black Mesa basin; the beds dip 0.7 31.4 Weathered outcrops of massive sandstone generally about one degree to the northeast. beds in the Wupatki Member of the Moen­ 0.5 35.3 The road drops into a shallow valley where kopi sandstone (Triassic) form characteristic Wupatki sandstone is well exposed.

161 lnterleaf. Upturned edges of stratified rock on west side of interior of Meteor Crater, Arizona; in distance. (Photograph No. PK 14,092 by Troy L. Pewe, January 2, 1970.) n n n rr n n o a It J] an_ n_ an a a a a an� H n "n" H n nun " 0.9 36.2 The road drops to the level of the which is Mt. Humphreys. At 3,851 meters Moenkopi-Kaibab contact. A few scattered (12,633 feet), Mt. Humphreys is the highest blocks of Kaibab Limestone may be seen to point in Arizona. The San Francisco the side of the road. is composed mainly of ande­ site lava flows and ashes. Rhyolites and da­ 0.9 37.1 STOP 2. Here the road comes over the crest cites occur as domes perches on the flank of of a rise, and the visitor obtains a first good the older stratovolcano. The largest of these overall view of the crater rim. At this point, domes forms Mount Elden, which is the the road is on the Moqui Member of the prominent toe extending to the left of the Moenkopi, about 5 meters (15 feet) above main mountains. Domes which have been the base of the Moqui Member. dated range in age from2.71 ± 0.13 m.y. to To the left and behind the rim of Meteor 0.212 ± 0.011 m.y. (Damon and others, Crater may be seen the lava-capped mesas 1974). known as East Sunset Butte and West Sunset To the right of the San Francisco peaks is Butte. Both buttes are capped by basalt, an extensive field of Pleistocene basaltic which rests on the Chinle Formation of cinder cones. This is the main locus of basal­ Upper Triassic age. The flanksof the buttes tic activity, at the present time, in the San visible from the vantage point along the road Francisco volcanic field. The youngest cin­ are mantled with landslide debris which sup­ der cone in this field, not visible from this ports a pygmy forest of pinyon and juniper. vantage point, is Sunset Crater, which has In places along the 11anks of the buttes, not been dated by dendrochronological methods visible fromthis point, a complete section of at A.D. 1064 (Smiley, 1958). The nearest the Moenkopi Formation and a partial sec­ prominent reddish colored cinder cone is tion of the Chinle Formation are exposed. Merrill Butte, which lies about 27 km (17 Three cinder cones are visible on the buttes miles) airline distance. At the extreme right which mark the vents from which the basal­ of the volcanic field is a large symmetrical tic lavas were erupted. cinder cone called Merriam Crater. The cin­ To the right of Meteor Crater, fromabout ders on the side road, extending to the west, 12 o'clock to about 3 o'clock, the skyline is and also used as aggregate in the asphalt formed by basalt-capped mesas of the Mor­ capping of the Meteor Crater road, have mon Mountain volcanic field. The basalts in been excavated frombasaltic cinder cones. this field are Miocene and Pliocene in age. One of the basalts capping Anderson Mesa, 1.9 39.0 STOP 3. AT JUNCTION OF BAR-T-BAR RANCH at 3 o'clock, is dated at 6.2 ± 1.2 m.y. ROAD. Low buttes on each side of the road (Damon, 1965). The two highest points vis­ are formedby beds of the Moqui Member of ible in the volcanic field are Hutch Moun­ the Moenkopi Formation. An excavation tain, at about 1:30, and Mormon Mountain, around the flank of the butte, on the right at about 2 o'clock. side of the road, is a fossil quarry excavated The high point between Hutch Mountain by the University of California. Materials and Mormon Mountain is the end of a long, recovered from this quarry include an exten­ northeast-trending ridge that extends directly sive collection of footprints and fossils of toward the viewer fromthe more distant part ganoid fishes and amphibians. On the basis of the volcanic field. The ridge is capped by of regional studies of the stratigraphy of the a single lava flow, most of which has now Moenkopi Formation and on fossils obtained been destroyed by landsliding on both sides from this and other nearby quarries, the beds of the ridge. Red beds of the Chinle Forma­ at this horizon can be placed near the base of tion and Moenkopi Formation may be seen the Middle Triassic. The fossils were found exposed in parts of the landslide blocks on in freshwater dolomite and siltstone beds in the near end of the ridge. the floor of the quarry. These beds are no Between 3 o'clock and 5 o'clock lie the longer exposed. volcanoes of the San Francisco volcanic field. The largest volcano is San Francisco 0.7 39.7 STOP 4. This stop is just beyond a broad Mountain. A circle of peaks surround the curve in the road at a point where road heads eroded core of this volcano, the highest of nearly due east. To the north of the road, at 9

163 o'clock, the skyline is formed by a ridge of Crater is stratified and can be divided into siltstone and sandstone of the Moqui three distinct mappable units: Member of the Moenkopi. The south slope 1) a unit composed of fragments of the of the ridge is mantled with late Pleistocene Moenkopi Formation at the base, colluvium containing abundant fragments of 2) a unit composed of fragmental debris sandy dolomite from the Kaibab Formation. fromthe Kaibab Formation, and These fragments have been derived by re­ 3) a unit composed of fragments of the working of a blanket of ejecta that surrounds Coconino Sandstone at the top. the crater. At one time this Exposures of the Kaibab debris unit can extended beyond the ridge of Moenkopi, but be seen between 10 o'clock and 3 o'clock. the more distant parts of that blanket have At 3 o'clock the Kaibab debris rises high on now been eroded away during the time the crater rim. Low mounds with large, elapsed since the crater was formed.The col­ light-colored blocks, lying 100 to 300 meters luvium extends to the south into a shallow (300 to 1,000 feet) from this vantage point, drainage where it is overlain by a thin de­ between 10 o'clock and about 1 o'clock, are posit of Holocene alluvium. The outer exposures of the Kaibab debris unit that pro­ eroded edge of the ejecta blanket lies essen­ trude through the surrounding Pleistocene al­ tially at this position, and is concealed be­ luvium. This part of the crater rim is essen­ neath late Pleistocene and Holocene alluvial tially a stripped surface on the top of the deposits. Kaibab debris unit. The gently hummocky The difference in elevation between the topography of the rim reflects the original crest of the Moenkopi ridge and the lowest configuration of the contact between the outcrop in the ejecta blanket, near the drain­ Kaibab debris unit and the overlying age, provides a minimum estimate of the re­ Coconino debris unit, which has been lief on the precrater surf ace. It is clear that largely stripped away. the ridge of Moenkopi is an erosional feature At 2 o'clock, on the horizon, is the that predated the crater and had a precrater chimney of a building now destroyed by fire. relief of at least 15 meters (50 feet). This building was the headquarters for the On the near side of the drainage is a sur­ exploration activity of the old Standard Iron face that climbs gradually toward the crater Company and the latter-day Barringer Crater rim and which is underlain by late Pleis­ Company. During the 1930's and 1940's, tocene alluvium. The road bed is built on after exploration had ceased, the building this alluvial unit. The age of this and other also served as a visitor center and a small alluvial deposits at Meteor Crater has been museum for the public visiting the crater. determined by correlation of pedocal This building is constructed on a high-level paleosols developed on them. A well­ pediment surface that is cut into the rim units defined sequence of Pleistocene and of the crater. The pediment is capped by Holocene deposits has been studied in the relatively coarse alluvial and colluvial debris Hopi Buttes region, about 80 km (50 miles) derived from still higher parts of the rim. to the northeast. (The nearest of the Hopi The pediment can be traced as a cut surface Buttes are just visible on the northeast hori­ between about 1:30 and 3 o'clock. The zon.) There the deposits have been dated by easternmost edge of the pediment lies just to means of vertebrate fossils, C-14 analysis, the left of a powder magazine, which has and on the basis of artifacts contained in the been dug in a brilliant white deposit beneath youngest alluvial units. A well-developed the pediment gravels. This white deposit is a paleosol, corresponding to the youngest remnant of the Coconino Sandstone debris Pleistocene soil in the Hopi Buttes, is ex­ unit, which is preserved in a pocket beneath posed in a shallow borrowpit in the alluvium the pediment gravels. 50 meters (170 feet)to the south of the road. The difference in elevation between the This Pleistocene alluvium rests directly on highest recognizable pediment on the crater the preserved part of the ejecta blanket. rim and the upper edges of the latest Pleis­ tocene alluvial deposits at this locality pro­ The ejecta blanket surrounding Meteor vide a minimum estimate for the amount of

164 1,1 \I ,n, fJ �I �

�

Interleaf. Upturnededges of stratified rock on southwest side of interior of Meteor Crater, Arizona. (Photograph No. PK 14,093 by Troy L. Pewe, January 2, 1970.) dissection of the rim by erosion that has oc­ ter rim and for the residences of the museum curred since the crater has formed. On the staff. northeast flank of the crater rim this differ­ ence is about 12 meters (40 feet). The total amount of degradation of the rim is, of 0.2 40.2 STOP 6. IN LOWER PARKING LOT. Expo­ course, substantially greater than this. A sures in cuts at the southeast corner of park­ conservative but realistic estimate of the total ing lot provide an excellent introduction into reduction in rim elevation by erosion would the nature of the rim debris units. A part of be 15 to 20 m (50 to 75 feet). In places the the Kaibab debris unit is exposed in this cut. rim crest may have been eroded down as The debris consists of angular to subangular much as 30 m ( 100 feet). In most places the fragments ranging in size from 1 micron or Coconino debris unit has been entirely strip­ less up to blocks more than a meter across. ped away. The size frequency distribution of this mate­ rial follows a rather well-known fragmenta­ tion law. The cumulative mass of the debris is a simple power function of the particle 0.2 40.0 STOP 5. BY TRAILER INSTRUCTION SIGN size. The exponent of this power function is AND JUST BEYOND ROAD TO WATER WELL. such that about 50 percent of the total mass At approximately 1 o'clock may be seen a fallsin the largest three phi intervals. As one tongue-shaped mass of light-colored rocks, examines the debris in the cut, about 50 per­ extending to the crest of the crater rim. This cent of the volume of the material consists of mass is a remnant of the Coconino easily recognized fairly coarse blocks set in a Sandstone debris layer preserved in a depres­ matrix of finer crushed material. In general, sion on the top of the Kaibab debris layer. t,he finesare only recognizable in freshexpo­ Kaibab debris underlies the grassy slopes sures such as this. The effect of weathering surroundingthe Coconino debris remnant. A is to wash away the fines, leaving the surface water well at 7:30 extends slightly below the mantled with the coarser debris. When seen water table at a depth of about 200 meters on a natural outcrop, the debris unit appears (600 feet). This well provides the water for to be composed almost entirely of the coarser the museum and the visitor center on the era- blocks.

166