sign in sign up
<<
Home , Igneous rock

New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/2

Tertiary and later igneous rocks of the San Juan Basin Eugene Callaghan, 1951, pp. 119-123 in: San Juan Basin (New Mexico and Arizona), Smith, C. T.; Silver, C.; [eds.], New Mexico Geological Society 2nd Annual Fall Field Conference Guidebook, 163 p.

This is one of many related papers that were included in the 1951 NMGS Fall Field Conference Guidebook.

Annual NMGS Fall Field Conference Guidebooks Since 1950, the New Mexico Geological Society has held an annual Fall Field Conference that visits some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed papers. These books have set the national standard for geologic guidebooks and are an important reference for anyone working in or around New Mexico.

Free Downloads

The New Mexico Geological Society has decided to make our peer-reviewed Fall Field Conference guidebook papers available for free download. Non-members will have access to guidebook papers, but not from the last two . Members will have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding in New Mexico. However, guidebook sales represent a significant proportion of the societies' operating budget. Therefore, only research papers will be made available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content will remain available only in the printed guidebooks. This will encourage researchers to purchase the printed guidebooks, which are essential references for geologic research in New Mexico and surrounding areas.

Copyright Information

Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from our website or printed and electronic publications may be reprinted or redistributed without our permission. us for permission to reprint portions of any of our publications. One printed copy of any materials from our website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires permission. This page is intentionally left blank to maintain order of facing pages. I NEW MEXICOGEOLOGICAL SOCIETY * SECONDFIELD CONFERENCE *. SAN JUANBASIN

. . . .:: ! TERTIARYAND LATERIGNEOus ROCKS OF THE As summarizedby Crossand Larsen..(1935, pp. SAN JUAN BASIN 50-54)the volcanicsequence of the San JuanMountains .. beganwith Lake Forkandesite of probablyearly Miocene ¯I EugeneCallaghan age.Any earlier volcanic center that supplied debris New MexicoBureau of Minesand MineralResources to the Animasand McDermottformations was destroyed by erosionduring early Tertiary time. The andeslte ! wasfollowed by thelatitic San juan - and by a Publishedwork on the SanJuan Basin indicates hostof rhyoliticI latiticand andesitic flows and tuff- thatthe central part of theBasin is nearlyfree of .These eruptions were followed by an bodiesof igneousrock. However, the outermargin intervalwhich produced the SanJuan peneplain. I is ringedby scatteredigneous masses. Most of these Volcanismlater than the peneplain supplied thin massesare erosionalremnants and many are landmarks accumulationsof and tuffand was followed : thathave a prominenceout of proportionto their areal in the RioGrande drainage area by basalticeruptions extent.The exactage of almostall of theigneous whichyielded rocks grouped as the Hinsdaleformation I rockbodies is unknown.The accumulation of great piles of Plioceneage. Some intrusive rocks in the Ouray of volcanlcsin theSan juan that extend regionare earlier than the erosion-surface at the base beyondthe margin of theBasin from the vicinity of of thevolcanic . Later, granular intrusive i DurangoI Coloradoto thatof Chama,New Mexico, rocks¯arealmostwhollyrestrictedtothe,lowerpart is believedto havebegun in Ollgocenetime and to of the volcanicsequence. Recent work in theSan

haveextended well into Pliocene time. The central Juanregion has indicated that the beginning of :..’:. partof MountTaylor may be as old as lateMiocene volcanicaccumulation may wellhave been in J butmost of the associatedvolcanics are probably much Oligocenetime. younger.Large parts of theJemez volcanic sequer~ce eastof theBasin are probablyof Pleistoceneage. Many The LaPlataMountains northwest of Durango I of thevolcanic necks and associated flows on thewest Colorado,contain a variety of porphyriticrocks interJ- sideof theBasin are probably of Plioceneage. The mediatebetween and monzonit%and even- valleyflows along U. S. Highway66 areprobably of granularsyenite, , and dioriteintruded latePleistocene age andthe McCartysflow may be intosedimentary rocks as youngas theMancos shale. I lessthan 1,200 years old. Evidence of volcanic No extrusiverocks occur in thisarea. Eckel, (!9491 activityearlier than the mld-Tertlary, hasbeen found p. 32)i statesthat these intrusive rocks may be late at thenorth side of the Basin.Those who took the Cretaceousor earlyTertiary. An earlymid-Tertlary fieldtrip of theSociety in 1950will remember the ageis suggestedby analogywith other eruptive areas I andesiticvolcanic debris in thesedimentary Anlmas in New Mexicoand Colorado. and McDermottformations of uppermostCretaceous and possiblyPaleocene age. Ute Mounta!nsouthwest of Cortez,Colorado, is I a steep-sidedmass which ¯rises 3,000 feet above the : Asidefrom the Son Juanand Jemezvolcanics, surroundingsurface. It is brieflydescribed by (Coffin. whichare considered as being.outsidetheBasin, the . 19211p. 121)as a stockand the was interpreted flowrocks and volcanic necks are dominantly of basaltic forCoffin as an andeslteporphyry. J or basaltic-andesitecomposition and appearance.Those at thewest side of the.Basin are abnormal in havingthe Themany small bodies of igneousrock that occur. lowsilica content of basalticrocks but a highcontent of alongthe western side of the.Basin between the Four I potash-~ which is suggestedmineralogically in their con Cornersand Galluphave been studied by Howel tentof biotiteand, in somelocalities, of leucite. Williams(19361. pp~ 111-172). Aside from the diorite ... Theserocks are therefore classified as trachybasolt porphyrylaccollth of CarrizoMountain;. the rocks are: andminette~. Rhyolite, ; , and andeslte alkalineand arecharacterized by a hlghcontent of i occurin the coreof MountTaylor and makeup the potasheven though the content of silicais low.ThoUgh greaterproportion of therocks of theJemez and San identicalmineraiogically I theflaw rocks are called JuanMountains. The larger intrusive bodies are of trachybasalt,andthe rocks are called minette, I dioriticor monZonlticcomposition, whichis a basaltic-appearingrackwith abundant i ! i 119 II NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN II ! flakes.Most of thevolcanic necks contain large rockfragments from the pre-Cambrlanbasement. This amountsof fragmentsof pre-Cambrianbasement rocks andother igneous masses are the source of thepyrope andsome consist largely of sedimentaryrock debris, garnetsand the or perldotcrystals that are g Fromdata in theHopl area in Arizona,it is foundin thisregion. Zilditloi has a judgedthat the age of mostif notall of theseigneous cap250 feetthick composed of trachybasalt.Fluted bodiesis Pliocene.Notes on the variousbodlesltaken Rockis a smalllaccolith of minette. fromWilliams’ account follow. :. Thevolcanic necks in theChuska Valley along The Carrizolaccolith which cove~s an areaof the highwaybetween Gallup and Shlprockare perhaps 100square miles is theonly large intrusive body in themost striking of all., which rises over Ii thegroup on thewest side Of theBasin. It consists 1400feet above its base is composedof tuff-breccla II entirelyof dioriteporphyry in whichthere are large traversedby a few dikesof minette.The crudebedding phen0crystsof and andeslne and smaller of thetuff- andits dip toward the center suggest quartzandbiotiteina microgranularintergrowthof thatthere were probably no lavaflows from this neck. g II quartzand . Theradiating en echelondikes of minetteare also a sourceof greatinterest. Bennett Peak and Ford Buttes Thedikes and necksin theRedrock Volley area, aremade up of fragmentsof sedimentaryrocks rarely / includingMitten Rock and ThumbRock, are composed morethan an inchin diameterand cutby a few small mainlyof minettelava rather than breccia, though dikesof mlnette. somesuch as ThumbRock contain many foreign in- clusions. The TwinCones near Gallup consist of 90 percent of co¯minutedCretaceous and onlya small II Igneousbodies in theChuska Mountains are both fractionof admixedminette. A few largeblocks of necksand remnants of flo~s.Black Pinnacle is a alaskiteand Jurassic sandstone as wellas other m neckof minette.Sandstone in contactwith it was sedimentaryrocks occur in thesenecks. I changedto quartziteor to buchite,a silica glass formedby meltingof thesandstone. Tubby is Remnantsof basalticflows occur at PowellMoun - a neckcomposed of tuff-brecciacontaining large rainand LookoutMountain on eitherside of U.S. ¯ blocksof sandstoneand. shale. It is cutby a dikeof Highway66 betweenGallup and Grants. II platyminette~ Lava caps occur on Palisadesand SonselaButtes. In Washington Passflows are either On thesouth side of theBasin the most prominent coarse-gralnedwithlarge of olivine,auglte, featureis theMount Taylor volcanic field, ably I biotite,and sanidlne or fine-gralnedwith sporadically describedby C. B. Hunt (1938,pp. 51-80).Mount I distributedphenocrysts. FIowsof trachybasalt also occur Tayloris a lowcone set on a ,the Mesa in the LukachukalMountains. Chivato,which is surroundedby steepslopes leading I downto thevalley of theRio Puerco and to thatof II Of theSeveral igneous structures near Ft. I Defiance, theRio San Josewhose course is followedby Uo S. themost interesting is Buell Park. This circular de- Highway66. The platformis an olderosion surface presslonin sandstoneis regardedby Williamsas having cut on Cretaceousrocks r whichreach upward to a beenformed by cauldronsubsidence. A cylindrical pointabout 2,000 feet below the crest of themountain. II blockabout 2½ milesin diameterdropped at least Volcanicnecks occur on all.sides of theMount Taylor 1000feet, probably after the eruptlons of lavaand areabut are especially numerous ~n thevalley of the ¯ greenlapilll tufts. Outlet neck and: The Beast are RioPuerco. Hunt shows that the centralcore of Mount II necksdiffering somewhat in internatstructure. Outlet Taylorconsists of rhyolitetuffr rhyolite trachyte, and neckhas a coreof blockyminette surrounded by a latlte.The rhyolitic vltric tuff is theearliest and most sheathof brecciaof theenclosing sedimentary rocks, widespreadof the eruptivematerials. Hunt regards ~Jj Someof thesedimentary material is vitrified.The thismaterial as of probablelate Miocene age. The !1 Beastis mainlytuff-breccla with dikes that show tran- sillclcgroup of rockswas almost covered by sitlonfrom minette to monchiquite.The GreenKnobs andesiteas possiblythe latest phase of theMount Taylor areremnantsof a neckwhich is madeup of a minette eruptivesequence, in the southeasternmargin of the g thatweathers readily and contai:ns an abundanceof mountain,pumiceous tuff containing fragments of - ! !

120 I

NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN

stonerests on thinflows of basalticrock and is over- andscoria overlying both the Santa Fe andlater lainby water-laidmaterial which is succeededby the terracesands and gravels as in theAlbuquerque area. porphyriticandeslte. The remainderof theplatform is coveredby basaltflows that had many local sources. Of particularinterest to thetraveler along U. S. Whereerosion has destroyed the platformand removed Highway66 betweenMesita and Bluewaterare the theflows the numerous vents or volcanicnecks stand out basalticvalley flows. The flow at Bluewaterextends in reliefabove the dissected sedimentary rocks. Erosion froma localsource north of thehighway to thecarrot hascut as muchas 2000feet below the level of the fieldsnorthwest of Grants.Another flow floors the basaltflows which Hunt regards as of probablyPliocene valleysouth of therailroad at Grantsfrom the overpass age. westof Grantseastward toward McCartys. This flow hadits source many miles to thesouth in whatis almost SinceHunt’s work was publishedthe mine of the a shieldarea of basalticflows. A morerecent basaltic PumiceCorporation of Americawas openedin the flowcalled the McCartys by Nichols(1946, pp. 1049-1086) pumiceoustuff near the east end of GrantsRidge. is superimposedover the olderflow. The McCartys flow A geologicsection of oneof thebasaltic volcanoes coversa largearea to thesouth but approaches the San is wellexposed at themine. Just east of themine JoseValley in a verythin stream and reaches as faras thepumiceous tuff gives way to rhyoliteand to McCartys.Nichols regards this flow as probablyless massesof perliteor volcanicglass that are intimately than1,200 years old. All the features of flowsOf this associatedwith the rhyolite on thenorth side of the type,such as spatter,cones, ropy surface, ridge:.Cursory examination suggests that the rhyolite ridgesand treetrunk molds, are verywell shown. An andperllte represent one or moredome eruptions at olderflow is exposedalmost continuously from the thissite and are not connec ted with the Mount Taylor vicinityof Lagunato thatof Mesita. center.Exploratory work has been done on theperllte witha viewto commercialutilization. The JemezMountains in Sandovaland RioArrlba countieseast and southeast of Cubaare east of the ¯ Manyof the basalticnecks in theMount Taylor Naclmientouplift and are thus not in theBasin. The regionare composedof columnarmasses of basaltbut "entireJemez mass is beingCarefully mapped by C. S. others¯ arecomposed of basalticbreccia whose bedding Rossand RobertSmith, of the U. S. GeologicalSurvey. tendsto dipinward toward the center in a manner Sufficeto saythat the Jemez area has been a volcanic similarto thatof thetuff breccias of thenecks in the centerfor a considerableperiod. Most of thefeatures westernpart of theBasin. An exampleis theneck of the mountainsare of post-Plioceneage. The most nearCubero. The most impressive of the volcanic prominentfeature is thegreat of theValle ¯ necksis CabezonPeak which rises 2000 feet above Grandewhich has been in partfilled by rhyolitic the RioPuerco. The exposedbasaltic core is 800feet domes.Visitors to LosAlamos and Frljoles Canyon highand 1500feet in diameter, in BandelierNational Monument are familiarwith ¯ thegreat cliffs of tancolored welded tuff which issued Basalticrocks occur at severalerosional levels in as a kindof flowthrough breaks in thewall of the thearea to thesouth of theMount Taylor region. caldera.The Jemez region is doubtlessthe best at threelevels are distinguished in the Lucero uplift exampleof thevarious of therhyolitic eruptive (Kelleyand Wood,1946). One capsthe highestpart processin theUnited States and the report of Ross of theMesa Lucero. Another occurs at a lowerlevel andSmith is eagerlyawaited. as theprominent cap on the MesaRedonda near High- way 6, fourmiles southwest of Correoand thethird theCerro Colorado flow which came from the south intothe valley of theRio San Jose at Correoand followedthe valley for 12 milessoutheastward. This latestflow is considerablyeroded and is wellabove thepresent stream |evel below Correo. In theRio Grandetrough region outside the SanJuan Basin the¯SantaFe formationcontains basaltic flows (Wr:ight,1946, pp. 412-413) and there are flows

122 I I NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN B~SIN - i

I SelectedReferences Coffin,R- C. ~ (1921)Radium, , and vanadium deposits of southwesternColorado: Colo. Geol. Survey i Bull.16 Cross,Charles W. ¯ andLarsen I EsperS., (1935)A briefreview of thegeology of the SanJuan region southwesternColorado: U. S. Geol.Survey Bull. 843 I EckellE. B. andothers (1949). Geology and depositsof the LaPlatodistrict, Colorado: U.S. Geol. Survey Prof.Paper 219, p. 179.

HuntC. B. (1938)Igneous geology and structureof the MountTaylor volcanic field: U. S. Geol.Survey I Prof.Paper 189-B, pp. 51-80.

Kelley,V. C., and Wood,G. H., (1946)Lucero uplift~ Valencia, Socorro, and BernalilloCounties, New I Mexico:U. S. Geol.Survey Oil and Invs.Prelim. Map no. 47.

Nichols~R. L. (1946)McCartys basalt flow~ Valencia County~ New Mexico:Geol. Soc. Am., Bull.,vol. I pp. 1049-1086. Williams~Howel (1936) Pliocene volcanoes af the Navajo-Hopicountry: Geol. Soc. Am. Bull.,vol~ 47, pp. i 111-172. Wright,H. E. Jr.,(1946) Tertiary and Quaternarygeology of the LowerRio Puertoarea, New Mexico:Geol. Soc.Am., Bull., vol. 57, pp. 383-456. I I I I I I I I I I 123