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Tertiary and Later Igneous Rocks of the San Juan Basin Eugene Callaghan, 1951, Pp

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Tertiary and Later Igneous Rocks of the San Juan Basin Eugene Callaghan, 1951, Pp 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 years. Members will have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology 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. Contact 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 -tuff and by a Publishedwork on the SanJuan Basin indicates hostof rhyoliticI latiticand andesitic flows and tuff- thatthe central part of theBasin is nearlyfree of breccias.These eruptions were followed by an erosion 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 rhyolite 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 Mountains that extend regionare earlier than the erosion-surface at the base beyondthe margin of theBasin from the vicinity of of thevolcanic succession. 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 diorite and monzonit%and even- valleyflows along U. S. Highway66 areprobably of granularsyenite, monzonite, 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 rock 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, trachyte; latite, 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 dike rocks are called minette, I dioriticor monZonlticcomposition, whichis a basaltic-appearingrackwith abundant biotite 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 olivine or perldotcrystals that are g Fromdata in theHopl Buttes area in Arizona,it is foundin thisregion. Zilditloi Mountain has a lava 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.Shiprock, 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 hornblende and andeslne and smaller of thetuff-breccia andits dip toward the center suggest quartzandbiotiteina microgranularintergrowthof thatthere were probably no lavaflows from this neck. g II quartzand orthoclase. 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 sandstone 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 Butte 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
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