Problems of Stratigraphy of the Colorado Plateau and Adjoining Areas Clay T

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Problems of stratigraphy of the Colorado Plateau and adjoining areas Clay T. Smith, 1951, pp. 99-103 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.

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This page is intentionally left blank to maintain order of facing pages. i NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN I to arriveat a satisfactorycorrelation of the Jurassic FiguresI-4) which are used include little data other rocksof theNavajo country and a satisfactoryinter- than¯surface sections because of thegreat difficulty I pretationof theirenvironmental history, in separatingmany of thegroup sequences in well logs.In southwesternColorado and adjacentparts PROBLEMS OF JURASSIC STRATIGRAPHYOF THE of Utah,the Dolores formation contains equivalents COLORADO PLATEAU AND ADJOINING of boththe Chinleformation and the GlenCanyon I REGIONS. Groupthroughout much of its extent.Other units whichhave variously been included with both the ClayT. Smith* Morrisonformation or withparts of theSan Rafael I Grouphave been placed where most of the present Stratigraphicproblems in theJurassic section day workersin thearea believe they belong. of theColorado Plateau and surrounding regions can be groupedinto three closely related categories: McKee(1951, p. 488)has suggested that during I thefirst type of problemsare those which are con- Poleozoictime there were two: consistently postive cernedwith the areal distribution of the various rock areasin Arizona;one occupying roughly the position unitsand their correlations. A second type of problem of theDefiance uplift in northeasternArizona and I is paleogeographicin nature: the delineation of source the otherin southwesternArizona. The Uncompahgre areasfor thecomplex sedimentary series which have andFront Range positive elements and the Sierra alreadybeen defined and whose distribution is at GrandeArch were also well-deflned structural highs leastpartly known. A thirdtype of problemis more in Coloradoand New Mexico., By upperTriassic I detailed;it involvesthe recognition of facies changes timethese positive elements were greatly reduced,. : withinthe units and interpretation of sedimentary particularlythe Defianceuplift. The Chin le form- environments.Solutions for all theseproblems have ationwas a verywidespread formation accumulating becomecritical because of theextensive distribution materialfrom mult’iple sources (See figure I). I of uraniumore in the Jurassicsystem. Extensive and continuingrevision of correlationsand nomenclature A markedchange from earlier time in thedis- i tributionof positiveareas accompanied the transition Ihasoften contributed to confusion and misunderstanding evenamong those who are most: familiar with the area. to GlenCanyon time; these changes exerted continuing i controlsover all of Jurassicsedimentation. The De- fianceposHive element, was reduced to the extent ¯ Severalfacts regarding this rock sequence bear repeating:The section is composedalmost entirely thatit wasno longereffective in contr01~lingsediment- I ationin the FourCorners area. A new positive of clasticsediments with considerably more than half : theaccumulation being silt or coarsersized. Cross elementtrending slightly north of westbegan to rise beddingof all:typesis thegeneral rule and evenly or in centralArizona and New Mexico;it apparently parallelbedded sediments comprise a subordinatepart restrictedChinle deposition from extending southward. I of theassemblage. Fossils are so rareas to be essen- Thispositive element mlghtbe termed the "Navajo tiallynon-existent and age assignments are based Highland"because of itsmarked, and continuing uponeither over-extended lithologic correlations or effectson therocks exposed on theNavajo Reservation. I uponbroader relationships with overlying and under- Two thlngsare noteworthyabout the Navajo Highland; lyingsediments. Volcanic rocks or intrusiveigneous first,it didnot contribute materially to the accumu- masseswhich characterize the Jurassicrocks farther lationsof GlenCanyon time, exceptperhaps to the fluviatilefacies of theWingate formation described westare absent; a considerableproportion of someof I theclaystone sequences contain ash beds.Mineral- by Harshbargerwhich may haveder:ived some of its oglcallythe sedimentsare monotonoussand heavy materielfrom this area; second the gradualrise of mlneralswhlchmight be Of considerableassistance :in the Navajoelement caused a generalwestward tilt- correlationaresparse, ingof theGlen Canyon depositional basin so thatits I Kayentaand Navajoformations extend only a short Theclassic three-fold division of theJurassic distanceeastward into New Mexicoand the.deepest rockunits on the¯ColoradoPlateau is utilizedto partsof thedepositional basin lie several hundred I illustratethe broad features of thedistribution patterns milesto the westof the New Mexlco-Arlzonastate of theserocks. The isopach and faciesmaps (See line. I * New MexicoInstitute of Miningand Technology. ii 99 II

| NEW .MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN n

Figure2 illustratesthe distribution of the Glen Canyon (seefigure 3) combinesmost of thecorrelations and Groupof sedimentsin termsof ithecombined thick- equivalentssuggested in theprevious papers. n nessesof all units.The Wingate sandstoneand its ¯ equivalentsare the only representatives of the group Theextreme facieschanges in theSan Rafael in most of NewMexico.The Navajosandstone is groupand a lackof. consistently recognizable form- theonly representative in far western Arizona and ationalbreaks suggest.that thegroup should be re- ¯ fartherwest. The depositionalbasin of GlenCanyon ducedto formationaistatus or at leastshould comprise timeseems, in manyways, a continuationof the a two-folddivision rather than the present four-fold upperTriassic deposifional area although more or .. sequence;furthermore, some of thefacies changes lesscontinuously restricted toward the end of Glen are so Well-markedthat new formatlonal names are ¯ Canyontime. Some authorities have argued that this justified. coincidenceof distribution pattern and the lack of a well-definedhiatus at thetop of theChinle formation The facieschanges in theCarmel formation ¯ in someareas is strongevidence for including all Of describedin theprevious papers probably reflect the GlenCanyon group in theTriassic period, the upliftand westward tilting of theNavajo high- | : landin lateGlen Canyon time. Durlng.Entrada The problemsposed by the GlenCanyon group timesome rejuvenation.of theearlier positive masses ¯ aresummarized as follows: musthave taken place to resultin thethinning of I. Is ithecommonly accepted Jurassic (?) age Entradasandstone across the Sierra Grande arch in .thegroup correct? .. northeasternNew Mexico and distortion of its mm 2. Whatwere the sourceareas for these rocks? isopachsaround the Four Corners area above the old ¯ The Uncompahgreand FrontRange highlands Defiance¯highland ¯. The Uncompahgrehighland may n weretoo worndown to furnishmuch detritus, havehad localrejuvenation but depositionwas not althoughthey served as. effective blocks to completelyblocked across it as hadbeen the case ¯ deposition, earlierin.Mesozoic time. The extreme easterly 3. Whatsedimentary environmentresults in the extentof themassive sandstane..facies of the Entrada thickaccumulation of cross-bedded sandstone fo’rmation("Slick Rim" of someauthors) suggests that suchas the Navajosandstone in ZionCanyon the.effects of theWestward tilting brought about by g and to thewest? therise of theNavajo highland were relatively 4. Why doesthe Navajosandstone show little nullifiedby erosionduring Carmel time. facieschange over large areas in a region n wherechanges are abundantand abruptin DuringEntrada time additional uplift took place g superand subjacent strata ? on the Navajohighland; the maximumelevations on 5. Doesthe developmentand growthof the Navajo thisrenewal of activitywere probably somewhat highlandfurnish an adequateexplanation for fartherwest than those which caused the tilting in late ¯ thewestward shift in depositionduring Glen- GlenCanyon time. For the firsttime since the devel- Canyontime’?, opmentof the Navajohighland detritus wascontrlbuted to theJurassic basin. Nearly all the material was TheSan Rafael group was originallydefined from derivedfrom the erosion of sedimentaryrocks; large I exposuresin theSan Rafael Swell of south-central amountsof sandwere contributed to the southern part Utah.By accretionand piracyfrom adjacent form- of thedepositional basin almost continuously to the ationsthe grouphas grown from the originalfour endof Morrisontime. ¯ members¯ to overa dozencorrelated or relatedmembers. A goodmany of thecorrelations are reliable,part- The Curtisformation marks a secondmarine icularlythose with the Sundance sequence to thenorth; invasionfrom the north in Utahand Colorado; the theSan Rafael group contains the only marine faunas Todiltoformation of New Mexicomay be a correlative of anyconsequence in the entire Jurassic section on of theCurtis and it indicatesperhaps marine or lacus- II the ColoradoPlateau. Other correlations have varying trineconditions encroaching from the east. Slight degreesOf certainty,generally ;varying inversely as renewalof uplifton theold Defiance highland pre- n theirdistancefromthetypelocality. The isopachmap ventedthe Curtis and Todilto seaways from joining g ! !

I00 I NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN

! anddirect tracing of theequivalence of these two landas a new regionalsource area? formationsis, hence,not possible. The Todilto 3. Whatconditions of accumulationand sedimen- limestoneand associated gypsum beds: intertongue tatloncan give rise to suchextensive distribu- I withthe upper part of therocks correlated with the tionof cross-beddedsandstone as is illustrated Entradasandstone. The unconformable relationships by thedevelopment of the "Slick Rim ’i faciesof reportedby Gilluly(1929) at the topof the Entrada theEntrada sandstone. I sandstoneinthetypelocality are not found. The Entradasandstone may represent combined transgressive TheMorrison formation too hasaroused conten- andregressive units which are not now differentlable, tionamong stratigraphers. Parts of manyof theunits Similarrelationships maybe presentin theMoab describedin thisand the previous papers have at I Tongue-Summervillesequence described by Craigand varioustimes been included in it as wellas unitsin Holmes(1951). sequencesreferred to the lowerCretaceous. The Morrisonformation can be dividedinto two major I Themarine deposits of theCurtis and Todilto facies;a lower sandstone (principally fluvlatile), formationsgrade upward into siltstones and sandstones andan upperclaystone: (principally quiet water assignedto theSummerville formation. In theupper deposition)with minor sandstone and limestone. The partof theSummerviJle there are several units whose claystonehas much the greater extent of thetwo i stratigraphicpositions have been in considerable whereasthe distribution of the sandstone was controlled doubt.Various authors have placed them as lowas at leastslightly, by thewestern part of theNavajo theNavajo or Entradaformations while others have positivearea. Over most of the ColoradoPlateau consideredthem Morrlson equivalents; a third alter- the faciesare co-extenslve and the formation averages I nativehas been to givesuch beds formational rank’, from800 to 1000feet thilck. Toward the southwestern Someof theseunits, such as TheJunction Creek sand- marginof depositionthe sand unit thickens and occupies stoneor theCow Springs sandstone, contain undlff- allof theinterval assigned to theMorrison f~rmation. i ¯ erentlatedbeds which are equivalentto boththe Alongthe southwesternmargin of outcropthe SanRafael group and theMorrlson formation, formationthickness, measurements are of doubtful accuracybecause separation of Morrisonsand facies Themost significant feature of sedimentationin (CowSprings sandstone) from similar sand facies in the I theSan Rafael Group is theintroduction of small SanRafael group (Cow Springs sandstone) is difficult. localsource areas giving rise to suchunits as the In northwesternColorado and in theDinosaur National JunctionCreek sandstone, the Bluffsandstone, the Monumentof Utahthe sandstonedisappears and clay- Wanakahformation and thellke; where these units stonerepresents Morrison time. The claystone thickens I thinand die outtheymay show tongues or affinities considerablyto the west from this area and is somewhat to bothSan Rafael and Morrisonsedimentation and thickerto thenorth. Eastward the claystone thins thustheir position is in considerabledoubt. Another acrossthe old Uncompahgrehlghland:and then thickens i significantfeature of SanRafael sedimentation is againalong the eastern slope of theFront Range high- the continuedcontribution of sand from the Navajo land.Figure 4 illustratesthe general relations of the highlandwhich resulted in the markedfacies changes differentfacies as wellas someof themore restricted I of allthe San Rafael beds to thesouth and southwest, membersof theMorrlson formation. Questionsraised ¯ by therelations in theSan Rafael Theclaystone of theMorrlson formation¯ is the g~oupare summarized as follows: firstunit in theJurassic sequence to blanketall the old I I. Isa four-foldformationaldivision of thegroup positiveareas. Morrlsonconditions thus set the stage warrantedby thepresent information, or would for thegreat Cretaceous inundations which were to understandingof these rocks be facilitatedby follow. reductionto formationaland member rank.’? I 2. How canthe facieschanges introduced by local In northernUtah and northeasternNew Mexico sourceareas be reconciledwith and-integrated alternatingsandstone and shale beds referred to lower intothe broader facies changes resulting from Cretaceouson thebasis of marinefaunas conformably I malineand non-marlneenvironments ofdepos- overliethe Morrison formation. As theseunits are itionand the introduction of the Navajo high- tracedtoward the Colorado Plateau region their ! ! I IOI NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN I I identitybecomes more or lessinextricably merged assignedto partsof bothJurassic and Cretaceous withthe uppermost units of theclaystone facies of periods?The tendencyin recentyears has the Morrlsonformation. Such gradations serve to beento removebeds formerly assigned to the castdoubt upon the upper contacts drawn in some formation. I partsof theColorado Plateau region. Muchof theconfusion concerning the Jurassic stratigraphyof the south and west sides of theSan Theproblems related to theMorrlson formation I aresummarized as follows: JuanBasin in partlcular,and the Colorado Plateau I. Manymembers described within the Morrison in general,will be resolvedwhen we havesolved formationhave ¯distributionswhich overlap somebasic geologic problems. For example:¯what areasof majorfacies change. How should sedimentaryenvironments result in thedistribution of I theresulting variations be interpreted and coarseclastic deposits over large areas? What is the evaluated? effectof localfacies changes as a resultof local upliftwhen these are superimposedupon broader 2. Whatare the source areas for most of the I claystonefacies and some parts of thesand- regionalfacies changes? How can marinefacies stonefacies? be separatedfrom non-marine facies in areaswhere 3. Whatmarks the upperContact of theMorri’son thereis littlelithologic change and fossil evidence formationand should the formation be is limited? I SelectedReferences

Baker,A. A.,Dane, C. H.,and Reeside, J. B.,(1936) Correlation of the Jurassic formations of partsof Utah, I Arizona,New Mexicoand Colorado:U.S. Geol. Survey Prof. Paper 183. i Baker,A. A., Dane,C. H.,and Reeside, J. B.,(1947) Revised correlation of Jurassic formations of ports Utah,Arizona, New Mexicoand Colorado:Am. Assoc.Petroleum Geologists Bull., vol. 31, no. 9. I

Craig,L. C., and Holmes,C. N., (195i)Jurassic Stratigraphy of Utahand Colorado~New MexicoGeol. SocietyGuidebook of SecondField Conference, San Juan Basin,New Mexicoand Arizona. I Dutton,C. E., (1885)Mount Taylor and ZunlPlateau: U. S. Geol.Survey 6th Ann.Rept.

Gilluly,James (1929) Geology and Oil andGas Prospectsof Partof the San RafaelSwell, Utah, U. S. Geol. I Survey,Bull. 806, pp. 69-130.

Gregory,H. E., (1917)Geology of the Navajocountry: U. S. Geol.Survey Prof. Paper 93. I Gregory,H. E., (1938)The San Juan¯ country, Utah: U. S. Geol.Survey, Prof. Paper 188. Hoover¯,W. B., (1950)Jurassic formations of partsof Utah,Colorado, Arizona and New Mexico:New Mexico I Geol.Society Guidebook of the San Juan Basin,New Mexicoand Colorado.

Lee,W. T. (1902)The MorrisonShales¯ of southernColorado and northernNew Mexico.Jour. Geol. vol. 10, pp. 45-46. I

McKee,E. D.,(!948) Classification andInterpretation of Crass-laminatlon (abstract): Geol. Soc. America Bull., vol.59, p. 1378. I McKee,E. D., (1951)Sedimentary Basins of Arizonaand adjoiningareas. Geol. Soc. America Bull., vol. 62, no. 5, pp. 481-506. I Geol.Soc of AmericaMemoir 33 pp. Stark,J. T. et al (1949)Geology and Origin of SouthPark i Colorado. 47-48. I Weir,G. W., (1951)Cross-lamination in the SaltWash sandstone member of the Morrisonformation (abstract): Geol.Soc. America Cordilleran Section, 47th. ann. meeting, official program, p. 22. I

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

MEASURED STRATIGRAPHICSECTION FOUR MILES EAST OF COW SPRINGS., ARIZONA I Measuredby J. W. Harshbarger,1947.

I CRETACEOUS: FEET

DAKOTA SANDSTONE: 10 Sandstone:yellowish orange, fine-grained, thin-bedded (½ to 5 inches),cross-laminated; I calcareous-ferruglnauscement; abundant limonite streaks; forms rough cliff ...... 15.0

U NCO NFORMITY: I Erosionalrelief,4 inchesto I.5 feet;markedby coarse-grainedsandstone and granulecon- glomerate;sharp contact due to colorchange, I JURASSI C: COW SPRINGS SANDSTONE: 9 Sandstone:light gray, fine-grained, thln-bedded (I to 4 inches),partially flat-bedded andpartially cross-bedded; firm calcareous-ferruginous cement; contains lenticular I manganesebeds (2 toil2inches, 50 to 100feet long) near base; iron-manganese con- orations(6 inchesto 3 feetcommon); forms rough pitted, rounded cliff ...... 104.0

8 Sandstone:dusky-yellow, fine-grained, massive, fiat-bedded~ weak cement; contains I manganesestringer near top; abundant black iron specks; forms smooth rounded cliff ...... 28.0

7 Sandstone:llke No. 4; containsal~undant !imonite nodules (½to 3/4 inch)...... 51.0 I ... 6 Siltstone:greenish-gray; silty, thin-bedded (i/8 to I/2 inch),flat-bedded; weak ca|~are~uscement; forms recess in mainrounded cliff...... °..~...... 2.0 .., .. ..- . ....’ _ i 5 Sandstone:greenish-gray , veryfine-grained, thin-bedded (I/I.6 to 4 inches)cross-bedded " largescale; firm calcareous cement; contains green, hard calcareous pods: (I x 6 inches);forms 94.0 I smooth.. : round,. slick cliff ...... ~ ...... 4 Sandstone:pinkish gray, very fine-grained, thick-bedded (6 to 14 inches);cross-bedded largescale; firm cement; contains several green, weakly cemented, silty stringers (I to I inches);forms smooth, round, slick cliff ...... 40.0 3 Sandstoneand siltstone: reddlsh-gray, very flne-gralned, flat and cross-bedded, very weak cement;gradational unit between Entrada sandstone and Cow Springssandstone; forms poorlyexposed slope ...... 23.0.. I 342.0

E NTRADA SANDSTONE: : i 2 Sandstone: light brown, fine-grained,thln-bedded (I/8 to 3/4cross-laminated; inch) very weakcalcareous cement; alternates with light gray sandstone beds; contains several reddishbrown shale partings (I/4 to I inch);forms poorly exposed, color-banded smooth slope.. 67.0

Sandstone:light-brown, Very fine-grained, thin-bedded, cross-bedded on large scale; firmcalcareous cement; contains numerous criss-cross light-gray streaks or bands;forms smooth,round, slick, steep slope ...... 134.0 j TotalEntrada sandstone (incomplete) 2-0~.0 Valley:covered by alluviumand vegetation. I i IO3