Galdera-likecollapse atKilbourne Hole maar, New Mexico byWilliam R. Seager,New Mexico State University, Las Cruces, NM 88003 Introduction Afton craters to similar craters in Germanv, subsidence in the formation of Kilbourne Kilbourne Hole, one of a group of Pleis- India, Mexico, and at Zuni Salt Lake, New Hole. Shoemaker(1957) recognized evidence tocenevolcanic craterscollectively known as Mexico. In 1935,Dunham (1935)described for explosive volcanic activity in the ejecta the Afton craters, is located about 20 mi Kilboume Hole and alsoseemed satisfied with blanket that forms the crater rim, and, in a southwestof las Crucesin south-centralNew the explosion hypothesis for the origin of the later paper (Shoemaker,1962), he noted "the Mexico (Fig. 1). Perhapsbest known for its crater. In the most detailed study up to 1940, Afton craters . exhibit downfaulting or wonderful array of mantle and lower crustal Reiche (1940)rejected the explosion hypoth- collapse along the crater wall," an apparent xenoliths, Kilbourne Hole has vielded con- esis, suggestinginstead that the craterswere referenceto the evidencethatled Reiche(19110) siderable information about the mantle and formed by subsidence.However, Reiche to his subsidencehypothesis. De Hon (I965a, deep crust of southem New Mexico (e.g., (1940)believed that subsidencegreatly post- b) studied Hunt's Hole and Kilbourne Hole Carter, 1965,1970; Padovani, 1977; Padovani dated explosivevolcanic activity and was only and found evidence of subsidence lacking. 'Acollapse and Carter, I977;Reid,1976;Reid and Woods, vaguely related to it. In fact, Reiche (1940) origin may be dismissed. (be- r97q. suggestedthat subsidencewas causedeither cause)inward dips are absent at the holes as Geologic studies focusing on the origin of by subsurface solution of limestone pro- are ring dikes; (the) raised rims are not ex- the ejecta blanket as well as on the crater moted by acidified volcanic waters, or by a plainable by a collapsemechanism" (De Hon, began in 1,907.ln that year Lee (1907)inter- vague "volcanic subsidence"that occurred at I965a). De Hon (I965a, b) and Reevesand preted both Hunt's Hole and Kilbourne Hole the site of an older volcanic vent but long De Hon (1955) believed that phreatomag- as explosion craterscaused by ground water after the vent had becomeinactive and bur- matic explosions were responsible for the flashing to steam as the ground water was ied by hundreds of feet of "fluvial" deposits. craters, although they did recognizethe role invadedby basalticdikes. Darton (1916)sup- SinceReiche's (1940) paper was published, of "down faulting or slumping, accompanied ported Lee's hypothesisand comparedthe little more has been said about the role of by normal backwasting of the wall" ur pt: lo70 lo5" Alsoin this issue Sedimentologyof the Cutotf Formation p.74 Pictographsin Arroyodel Tajo p. 80 Oil andgas discovery wells drilledin 1986 p.82 Extractiveminerals industries,198ut-1986 p.87 lO 15mi Service/News p. 88 Taxeson naturalresource production p. 89 Indexfor volume 9 p. 91 Staffnotes p.92 Gomingsoon Castileand Salado Formations. DelawareBasin N LowerCretaceous strata under southernHigh Plains Mineralparagenesis and structure at the U.S.Treasury mine MineralSymposium abstracts FIGURE l-Location map of the Kilboume Hole maar and vicinity cessesthat "enlargethe diameterand filled the result of "some form of coring or rasping ring. Suchcaldera-like subsidence along ring the craters"(De Hon, 1965b).Hotrer (7973, by jets of sediment-laden stream (which) fracturesis an important part of the evolu- 1976,7986) summarized the resultsof his and gradually formed the crater" (Stuart, 7981,,p. tion of maarsand diatremesin Meico, France, previous workers'studiesof KilbourneHole 71). Germany, Montana, and elsewhere (e.g., and Hunt's Hole. Like others before him, Evidence presented in this paper supports Jahns,1959; Lorcnz, 1973;Gutman,7975). exceptReiche and probablyShoemaker, Hof- the interpretation that major subsidence was fer apparentlyaccepted the cratersas prod- an important process in the formation of Kil- Generalfeatures ucts of phreatomagmaticexplosions, with bourne Hole, and that the crater is therefore Kilbourne Hole is a roughly elliptical crater "normal backwastingand slumpageenlarg- a maar in the sense of Lorenz (1973). The approximately1.7 mi long along the major ing the craterto its presentshape and size" subsidence immediately or closely followed axis and a little more than 1 mi wide (Fig. (Hoffea 7976).By evaluatingDe Hon's (1965a) Phreatomagmatic explosive activity and in- 2). It is surrounded on all sides exceptthe alternativehypotheses for formation of Hunt's volved caldera-like collapse, along a ring south by a prominent rim of ejectathat rises (1981) Hole, Stuart also concludedthat vol- fracture, of the tuff ring that had accumu- as much as 150ft abovethe surrounding La canic explosion was most likely ("blasting lated above and around the vent. The pres- Mesaplains and nearly450 ft abovethe playa during eruption could havepulverized rock, ent dimensions of Kilbourne Hole are not so at the centerof the craterfloor. Although rim forming the crater"). Both Stuart (1981)and much a result of explosive excavation, ejectaslopes moderately to gently outward De Hon (1955a)visualized Hunt's Hole (and slumping, Iandslides, and erosion as they are away from the crater,the inner walls of the the other craters)as developing"passively," a product of large-scalesubsidence of the tuff crater are generallysteep or even vertical. Figure 3 shows the stratigraphyexposed in the inner walls of the crater.Pre-explosion rocks form the lower half of the crater walls and include the Camp RiceFormation (Qcr) of early to middle Pleistoceneage (Gile et al., 1981)overlain by Afton Basalt(Qb). Exposed Camp Rice strata consistof basin-floorbol- son sedimentsand minor fluvial gravelcon- taining multiple paleosols,including conspicuouscaliche. The upper surface of the Camp Ricestrata is the constructionalLa Mesa surface upon which the paleosols evolved since the surfacestabilized approx- imately0.5 Ma. (Gileet al., 1981).The Afton Basalt,as much as 15 ft thick, locallyburied )\ the La Mesa surface and Camp liice For- ,l (l )F) !l J.r New AAexnc@ I GEOLOGY . Scienceand Seruice \k\ Volume 9, No.4, November 1987 Edifor Deborah A Shaw Drnfter:Mo te M Brown Publishedquarterly by )) New Mexico Bureau of Mines and Mineral Resources )t a division of New Mexi@ Institute of Mining & T<hnology I )l BOARD OF REGENTS ) ,N Ex Officio Garry Carruthers, Couernorol Nru Mexico Alan Morgan, SupetintefldenLof Publiclnsfruction ADpointed GilbertL Cano,Pi; ,1985-1989,Albuquerque LentonMalry, SeclTreos ,1985-1991. Albuquerque l- Y3:. Robert O Anderson, 1987-7993,Roswell Donald W Morris, 1983-1989,Ins Alamos -:::j57 Steve Torres, 1967-1991,Socono ta ?- New Mexico Institute o( Mining & Technology ' -,' President Laurence H Lattman o o.5 lmi New Mexico Bureau of Mines & Mineral Resources DirTtot Frank E Kottlowski ff A O O.5 lkm A. fr DeputyDirector Ceorge S Austin 4,400 4,400 Subscriptions:Issued quarterly, February, Mat August, November; subscription price $6 O0/calendaryear 4,OOO 4,OOO Editoilqlmatter: Articles submitted fot publicationshould 3,600 be in the editor's hands a minimum of five (5) months 3,600 before date of publication (February, May, August, or 3,200 3.200 November) and should be no longer than 20 typewrit- ten, double-spacedpages All scientific PaPerswill be reviewed by at least two people in the aPProPriatefield of studv Address inoui.ies to Deborah A Shaw, Editor ffi Corp RiceFormof ion [@ bosoltexplosion breccio Topogrophic of Nru'MexicoGeoLog11, New Mexico Bureau of Mines & -a flm Mineral Resources,Socorro, NM 87801 @ attonBosotl [iTl pyroclosticsurge deposits f Londslide Publishedas publicdomain, therelore reproduable without Per- blocks mission Sourcecredit reqrcsted laqr-loltuvium Circulalion:1,6N ffilcolluvium Printer:Universit'J of New Mexico Printing Plant FIGURE2-Geologic map and crosssection of Kilbourne Hole. All units are Quaternary November 1987 Neu Mexico Geology mation before formation of Kilbourne Hole. ing wet, massive beds of ash that moved ably underlies the crater floor but is hidden The basalt has yielded radiometric dates downslope as lahars." by alluvium. rangingfrom 0.5m.y. to about0.1 m.y. (Seager The age of Kilbourne Hole is late Pleisto- The subsided beds are mostly correlative et al., l9U; Hoffer, 1976;Hawley and Kott- cene. It is clearly younger than the Afton with the thinly bedded surge and fall de- lowski, 1969).Based on soil development, Basalt,which has yielded conflictingage es- posits that composethe upper part of the Gile (1987)believes the basalt is approxi- timatesbetween 0.5 and 0.1Ma. If Kilbourne rim ejecta, although basal explosion breccia matelv0.1 m.v. old. Hole is approtmately the same age as Po- constitutes part of the subsided mass at the Tufi-ring ejectaoverlies the Afton Basalt trillo maar, a very similar structure located northern edge of the crater. Relative to the and forms the upper half of the crater wall, on the Mexico-USA border 14 mi south- samestrata in the maar rim, the downfaulted as well as the rim and back slopes.De Hon southwest of Kilbourne Hole, then an age of pyroclastic deposits have subsided a mini- (195b),Hoffer (1976),Brenner (i979), Stuart 180.000vears is indicated.This date is based mum of 350ft. This has resulted in the crater and Brenner (1979),and Stuart (1981),give on a radiometric date from a late-stageba- floor being 200 to 300 ft lower than the sur- detaileddesciptions of the stratigraphy,and salticflow on the floor of Potrillo maar (Seager rounding La Mesa plains, clear evidence for Wohletz and Sheridan(1983) provide an ex- et al., 1984).On the other hand, Gile (1987) subsidence (Cas and Wrigh;7987). cellent summary. In general,the rim ejecta estimates the age of Kilbourne Hole to be Observeddips of stratain the sunken block canbe divided into two units. The basalunit, approximately24,000 years, based on the ex- range from 8 to 35', and all dips are centro- first describedby Reiche(1940), is an explo- tent of pedogenic carbonatedevelopment in clinal, toward the center of the crater (Figs.