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
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. sion breccia that varies in thickness from 0 soils of the rim ejecta. 2 and 4). Steepestobserved dips are at the to 150 ft, averaging 70 ft thick. The breccia edgesof the subsidedmass at the craterwall, consistsof angularblocks ofbasalt up to 5 ft Evidence for caldera-like collapse becoming flatter toward the centerof the cra- in diameterin a matrix of essentiallyunbed- Although most previous workers (except ter. Thus, available strike and dip data sug- ded, chaoticpyroclastic-fall deposits. The ba- Reicheand perhaps Shoemaker)visualized gest that the subsidedblock is saucer-shaped salt blocksare fragmentsof the Afton Basalt Kilbourne Hole crater being "excavated"by in three dimensions. There is no evidence disrupted by the eruption. Other accidental Surtseyan-typeexplosive blasts, there are two that stratal dips are the product of draping clastsin the brecciainclude mantle and lower lines of evidence that favor calderalike col- of pyroclastic debris across a pre-existing and upper crustal xenoliths up to 1.5 ft in lapseas the most important processin form- crater wall. Rather, faulting and tectonic ro- diameter.Much of the brecciamatrix appears ing the presentcrater. The first is the presence tation are indicated by the upbending and to be disaggregated sand and gravel from of largedownfaulted masses of tuff-ring ma- especially by the truncation of strata at the underlying bolsondeposits. Minor pyroclas- terial located at the base of the inner crater lower crater wall. tic-fall deposits with graded bedding have wall and beneaththe craterfloor. The second The ring fault that presumably borders the also been describedwithin or at the baseof is the small volume of pyroclastic material in subsided mass is nowhere visible becauseof the explosion breccia (Hoffea 7976; Stuart, the maar rim and on the La Mesa plains com- the thick colluvial apron of basaltblocks that 1981;Wohletz and Sheridan,1983). pared to the volume of the crater. mantle the lower slopesof the crater wall. In The upper part of the rim ejectaconsists Perhaps the most compelling evidencefor the northern part of the crater, however, the Iargelyof much finer grained,thinly bedded subsidenceof the tuff ring is the downfaulted fault can be confidently placed in the lower pyroclastic-surgeand fall deposits that are pyroclastic strata within the crater. These part of the crater wall beneath the Afton Ba- 100 ft thick or more on the eastrim. Reiche downfaulted stratawere also recognizedby salt (Figs.2 and 4). In this areait is clearthat (1940) interpretedthese strata to be fluvial in Reiche (1940)and constituted his basis for the fault is a simple fracture rather than a (1957), origin, but Shoemaker Hoffer (1976), emphasizing the subsidence origin for Kil- wide, complexzone. No fault, exceptthe ring (7979), (1979), Brenner Stuart and Brenner boume Hole. The downfaulted tuff-ring strata fracture, disrupts the continuity of either the (1981) and Stuart haveshown them to be the are exposedalong the northwestern,north- Afton Basalt or the downfaulted pyroclastic depositsof basesurges and air fall. The strata ern, and northeasterninner walls of the cra- beds. In fact, along the easternmargin of the exhibit beautiful sandwave structures and ter (Figs. 2 and 4), through an arc of nearly crater, nearly continuous outcrops of in- "repeatedaltemations in bedding from seams 180'. Elsewherethe subsidedmass presum- of coarseair-fall juvenile basalticlapilli" to finely bedded sandwavesto massivebeds with abundant accretionarylapilli (Wohletz and Sheridan, 1983).The abundanceof ju- venile lapilli increasesupward. Occasional basaltblocks or xenolith bombs are associ- ated with sag structuresin the section.The uppermost 10 ft of rim depositsare unstrat- ified, slightly palagonitized fine ash that De Hon (1965b)interpreted to be lahardeposits, but which Stuart (1981)recognized, at least at Hunt's Hole, asbioturbated, massive tuff. Accordingto Wohletzand Sheridan(1983), the stratigraphy suggeststhat "opening eruptions were dominantly low-energy strombolian ones that distributed fragments of the capping basaltflow and juvenile ma- terial toward the north and east.Subsequent eruptions becameSurtseyan and ejected dominantly reworked alluvial material from the underlying Santa Fe Group (Camp Rice and older formations) in highly inflated surges.These opening Surtseyaneruptions cleared the vent and excavatedthe large cra- ter. As eruptionsprogressed, the proportion FIGURE 3-Eastem part of crater wall and rim of Kilbourn Hole looking north. Qcr, Camp Rice of juvenile material in them increased.Final Formation; Qc, colluvium; Qb, Afton Basalt;Qe, basaltexplosion breccia; Qs, pyroclasticsurge deposits; eruptions were weakly Surtseyan,produc- Qal, alluvium
Nm Mexico Geology November 1987 ward-dipping,-but unbroken, tuff-ring strata blowing winds) and dispersedacross the La Afton Basalt (Qb), may have been eroded can be traced for 0.5 (Fig. , mi 4). Seeiringly, Mesa plains, then a total of 3.0 x 10 mi. of back from its original position. These pro- very large parts, if not all of the crater fldor. pyroclasticmaterial was ejectedduring the cesses of "normal backwasting," however, subsided as an unbroken, saucer- o, .on"- explosive phase of the volcano. Much of this have been minor in the formation of the cra- wasjuvenile materialfrom a deepsource and ter compared to caldera-like collapse. not a result of excavationof the crater.Con- Kilbourne Hole is not unique in exhibiting sequently, the volume of the ejecta, espe- subsidenceas a part of the evolution of i cially accidental ejecta, seemingly falls far phreatomagmaticeruption. Cratersin the Eifel short of filling the crater as it must if the district of Germany are thought to be true crater is to be excavatedentirely by phrea- maars produced by caldera-like subsidence tomagmaticexplosions. Subsidence must have because in most craters the volume of the played a maior role in the formation of the existing ejecta does not equal the volume of outcrop data in the northern part of the cra- crater. the crater (Frechen, 1951, 1962;Noll, 1962 ter, the extensive alluvial cover in the central Lorenz,1973).Maars of Seneze,France, also show evidenceof prolonged subsidenceby Summary and discussion their ring faults and thicli lake depositsen- The evidence favors the evolution of Kil- countered by the drill beneaththe craterfloor bourne Hole first as a typical phreatomag- (Lorenz,L973).In the Pinacatefield of north- matic tuff ring (Lorenz, 1973;Wohletz and westem Sonora,Mexico, Cerro Coloradoand A second, less certain line of evidence fa- Sheridan, 1983) and finally as a small cal- Kino Crater and particularly Crater Elegante voring subsidence at Kilbourne Hole is the dera-a maar (Lorenz, 1973).The Kilbourne are wonderful examplesof the role large-scale small volume of ejecta compared to the vol- Hole rim deposits,as well as pyroclasticshata subsidence plays in the formation of maars ume of the crater. If the crater were excavated in the subsided block, all exhibit character- (Jahns, 1959;Gutman,1976). Arcuate, con- by explosions, the volume of accidental ma- istics typical of a tuff ring rather than a tuff centricfaults and fault blocks,exposed along terial in the ejectashould approximatelv equal cone (Wohletz and Sheridan, 1983).Conse- the perimeter of Zuni Salt Lake-maar,Nevi the volume of the crater. Lorenz (1973) siug- quently, it seems likely that phreatomag- Mexico, indicate subsidence was also im- gests that because juvenile of the componeit matic explosions initially constructed a low, portant in the developmentof that structure of the ejecta apron, the'volume of material broad ring of ejecta.The size of the vent (Shoemaker, t962; Btadbury, t967; Cum- eru.pted from maars (before collapse) nor- associatedwith the tuff ring must have been mings, 1958;Wohletz and Sheridan, 1983). mally exceeds that of the crater. Calculations considerablysmaller than the modern crater, Hearn (1968)has shown that subsidenceof from Kilbourne Hole indicate that the vol- and presumably now lies buried beneath the 330 to 3,300 ft characterize the kimberlitic ume of the crater below the pyroclastic-fall centralcrater floor (Fig.2, crosssection). Cal- diatremes of Montana. Lorenz (1973) ob- deposits is on the order , of 6 x'70 mi3 and dera-like subsidenceloilowed all explosive served that "this clearly indicatesthat surface the volume of the pyroclastic rim deposits is expression(of the diatremes)must havebeen approximately 1.5 x 10 'mi3. If one issumes wide craters formed by subsidence. These craters must have appeared as maars or tuff rings modified by caldera-likecollapse (Fran- cis, 7970)." What causesfoundering of large-scale blocks during the formation of a maar?With-
subsidencewas repeatedand obviouslycon- current with a long interval of diatreme for- mation, the Woolsley et al. (1975)model applies nicely. The model also probably ap- plieswell to Kilbourne Hole and many irth-er maars, although in scarcelyeroded maarsthe causeof subsidenceis hardlv obvious.It mav involve evacuation of a magma chamber bv drainageor eruption as wel'[as by the prob- able predominant mechanismof mechinical enlargement of the eruption chamber by gas coring, attrition, and spallingat depth. Ac November 7987 Nan Mexico Geology Gile, L. H., 1987,A pedogenicchronology for Kilboume Hole, southem New Mexico-Il. Time of the explosions and soil events before the explosions: Soil Society of America Journal, v. 51,,pp.746-760. Gile, L., Hawley,J. W., and Grossman,R. 8., 1981,Soils and geomorphology in the Basin and Range area of southern New Mexico-guidebook to the Desert Proi ect: New Mexico Bureau of Mines and Mineral Re- sources,Memoir 39, 222PP. a\ Gutrnan,1.T.,1,976, Geology of CraterElegante, Sonora, Mexico: Geological Society of America Bulletin, v. 87, oo. 1778-7729. Hiwley, J. W., and Kottlowski, F. E.,1969, Quaternary J-','1\I\ geology of the south-central New Mexico border re- gion; in Kottlowski, F. 8., and LeMone, D. V (eds.), Border stratigraphy symposiun: New Mexico Bureau of Mines and Mineral Resources,Circular 104,pp. 89- ")?_ 115. Hearn, C. 8., Jr., 1968,Diatremes with Kirnberlitic affin- fi: ities in north-central Montana: Science,v. 159, pp.522- (L.-c .r, ?"| "nlt^--. 625. o Hoffer, J. M., L973, Kilboume Hole: El Paso Geological **;*filH " Fl*ll$m,.- Society,Guidebook 7, pp. 76-22. Hoffer, J. M., 1976,Geology of Potrillo basaltfield, south- central New Mexico: New Mexico Bureauof Mines and Mineral Resources,Circular 749,30 pp. Hoffer, J. M., 7985, Hunts Hole; in Hoffe9 f. M. (ed.), Geology of south-centralNew Mexico: El PasoGeolog- ical Society,Guidebook, pp. 9-10. Jahns, R. H., 1959,Collapse depressionsof the Pinacate volcanic field, Sonora, Mexico: Arizona GeologicalSo- ciety, Guidebook II, pp. 155-184. Lee, W. T., 1907,Afton aaters of New Mexico:Geological Society of America Bulletin, v. 18, pp. 211.-220. Lorenz, Y., 1973, On the forrnation of maars: Bulletin Volcanologique,v.37, pp. 183)M. NoIl, H. 1967,Maare und maar-dhnlicheExplosionskrater in Island. Ein Vergleichnit dem Maamlkanismus der Eifel: Sonderueroffentlichungdes GeologischesInstitut der Universitdt Kriln, n. 11, pp. 1.-7'1.7. Padovani, E. R., 1977, Granulite faciesxenoliths from Kil- boume Hole maar and their bearing on deep mstal evolution: Unpublished Ph.D. Dssertation, University of Texas(Dallas), Richardson,Texas, 772 pp. Padovani,E. R., and Carter,J. L., Dn, Aspectsof the deep cmstal evolution beneathsouth-central New Mex- ico;lz The earth's crust: American GeophysicalUnion, GeophysicalMonograph 20, pp. 79-55. Reeves,C. C., Jr., and De Hon, R. A., 1955,Geology of Potrillo maar, New Mexico and northem Chihuahua: American Journal of Science,v. 263, pp. 407-4W. Reiche,P, 1940,The origin of Kilbourne Hole, New Mex- ico: American Journal of Science,v.238, pp.212-225. Reid,J. B., 1976, Up per and lower limits to the geothermal 3 gradient beneath the southern Rio Grande rift (abs.): Geological Society of America, Abstracts with Pro- grams,v.8, no.5, p.621. FIGURE S-Diagrammatic evolution of Kilbourne Hole. 1) Initial perforation of Camp Rice Formation Reid,J. B., and Woods,C. A.,1979, Oceanicmantle be- and Afton Basalt and formation of basalt explosion breccia; 2) formation of tuff-ring through accu- neath the southern Rio Granderift: Earth and Planetary mulation of pyroclastic-surge deposits during Surtseyan-type eruptions; 3) caldera-[kJcoilapse of part ScienceLetters, v. 47, pp.303-315. of tuff ring to produce the maar structure. The modern crater has been widened somewhat by slurnping Seager,W. R., Shafiqullah, M., Hawley, J. W, and Mar- and erosion, and the crater floor has been buried to a great extent by alluvium. vin, R., 1984, New K-Ar dates from basalts and the evolution of the southern Rio Grande rift: Geological Society of America Bulletin, v. 95, pp. 87-99. Shoemaker,E. M., 7957, Primary structuresof maar rims References and their bearing on the origin of Kilbourne Hole and Bradbury, I. P., 7967, Origin, paleolimnology, and lim- Daton, N. H.,7916, Explosioncraters: Scientific MonthJy, Zuni Salt Lake, New Mexico (abs.):Geological Society nology of Zuni Salt Lake maar, west-centralNew Mex- v.3, pp. 477-430. of America Bulletin, v. 58, p. 1.864. ico: Unpublished Ph.D. dissertation,University of New De Hon, R. A., 1965a,A maar origin for Hunt's Hole, Shoemaker,E- N{-, 1962,Interpretation of lunar craters; Mexico, Albuquerque, 247 pp. Dofla Ana County, New Mexico:Unpublished M.S. the- in Zdenck Kopal (ed.), Physics and astronomy of the Brenner, M. G., 7979,Sedimentary feahrresin basesurge sis, T€xasTech University, Lubbock, Texas,70 pp. moon: Academic Press,New York, 538 pp. tuffs, Kilbourne Hole, Potrillo Mountains, Dofra Ana De Hon, R. A., 1955b,Maare of La Mesa: New Mexico Stuart, C. J., 1981,Hunt's Hole maar volcano, Dofra Ana County, New Mexico (abs.): Geological Society of Geological Society, Guidebook to 15th Field Confer- County, south-centralNew Mexico; in Geology of the America, Abstractswith Programs,v. 11, no. 6, p-267. ence,pp. 2M-ZW. border, southern New Mexico-northern Chihuahua: El Carter,J. L., 1965,The origin of olivine bombsand related Dunham, K. C., 1935,The geology of the Organ Moun- PasoGeological SocietyGuidebook, pp. 64-72. inclusions in basalts:Unpublished Ph.D. dissertation, tains with an accountof Dofla Ana County: New Mex- Stuart, C. J., and Brenner, M. G., 1979, "Low regime" Rice University, Houston, Texas,264 pp. ico Bureau of Mines and Mineral Resources,Bulletin base surge dune-an example from Kilbourne and Cfiter, J. L., 190, Mineralogy and chemishy of the eilth's 7r,272 pp. Hunls Holes, south-central New Mexico (abs.): Geo- upper mantle based on the partial-fusion partial-crys- Francis, E. H.,7970, Bedding in Scottish (Fifeshire)tuff logical Society of America, Abstracts with Programs, v. tallization model: Geological Society of America Bul- pipes and its relevanceto maars and calderas:Bulletin 1.1,no.7, p. 525. letin, v. 81, pp.2O2l)0M. Volcanologique,v.U, pp. 597-712. Wohletz, K. H., and Sheridan,M. F., 1983,Hydrovolcanic Cas, R. A. F., and Wright, J. V., 7987,Volcanic succes- Frechen,J., 1951,Die Vulkaneder Westeifel;in Hopmann, explosions-Il; in Evolution of basaltic tuff rings and sions, modem and ancient:Allen and Unwin. Boston. M., Frechen, J., and Knetsch, G. (eds.), De Vulkan- tuff cones:American Journal of Science,v. 283,pp. 385- pp.376-382. ische Eifel: Georg Fisher Verlag, Wittlich, pp.50-n. 473. Cummings, D., 196f, Geologicmap of the Zuni Salt Lake Frechen, J., 1962, Fiihrer zur Vulkanologisch-petrogra- Woolsley, T. S., McCallum, M. 8., and Schumm, S, A., volcanic crater,Catron County, New Mexico: U.S. Geo- phischen Excursionen im Siebengebirgeam Rhein, 1975, Modelling of diaheme emplacement by fluidi- logiol Swey, Mscellaneous Investigations Map I-5M, laacher Vulcanarbeit, und Maargebiet der Westeifel: zation: Physics and Chemistry of the Earth, v.9, pp. scale1:6000. Schwartzbart, Stuttgart, 151pp. 29-42. trr NetaMexico Geology November 1987