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Triassic deposits of the - border area Edwin D. McKee, 1951, pp. 85-92 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.

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! TRIASSIC DEPOSITS OF THE ARIZONA-NEW considerablyin character from one part of theregion MEXICO BORDER AREA to another.In southeroNevada and southwestern Utahare the profiles of manychannels and valleys, I E. D. McKee carvedseveral hundred feet into the Kaibab limestone Universityof Arizona surfaceand subsequently filled with boulders, pebbles andother debris of initialMoenkopi deposition (Long- TheTriassic period is I representedin the Colo- well/ 1925).In easternUtah arerecorded(McKnight, radoPlateau area by twoprincipal groups of deposits: 1940)numerous illustrations of folding and bevelling (I)the Moenkopl formation of EarlyTriassic and of red Permianstrata accomplished prior to Moenkopi possiblyalso of MiddleTriassic age, in part,and development.In thisarea angular discordance between I (2) theShinarurnp conglomerate and Chinleformation Permianand Triassic beds and the truncation of anti- of LateTriassic age. A widespreadsurface of erosion clinescan be observed.In northeasternArizona on everywhereunderlies the Moenkopi; another separates theother hand I erosionvalleys are inconspicuous the Moenkopiformation from the Shlnarumpconglo- andstructure bevelling is notknown to exist;the I merate.An unconformityabove the Chinleformation unconformityis marked by a surfaceof slightrelief is lessconspicuous than those below1 but probably withIocally-derlved gravels filling shallow depressions i alsowidespread and significant, in thesurface of Permianrock. Strataof theMoenkopi formation contrast with Differentexplanations have been used for the thoseof theoverlying Upper Triassic formations in varioustypes of pre-Moenkoplerosion surfaces. Hill- numerousways. First, they include extensive marine and-valleytopography in the westernpart of the region I limestonesand calcareous shales, whereas the Shin- hascaused Longwell (1925) to suggestsubaerlal erosion arump-Chinledeposits are entirely continental, as theprocess responsible forthe surface of relief, Secondthey decrease in thicknessfrom many hundreds whereasthe smooth contact nofed in easternUtah is of feetin Nevadaand western. theresult of marineplana- I tonothingnear the explainedby Dane(1935) as Arizona-NewMexico line, whereas the UpperTri- tion..In northeasternArizona, where Continental assicdeposits extend as a blanketwith fairly uniform- depositsof theMoenkopi rest on theerosion surface, thicknessacross most of northernNew Mexico as well marineprocesses cannot be consideredresponsible so I as Arizona.Third, they contain casts of saltcrystals thelow relief probably is theresult of longcontinued andextensive gypsum beds, suggesting semi-arid subaerialerosion to nearbase level. climate,whereas the Shinarumpand Chinle contain noneof theevaporltes but an abundantand diversified The MoenkopiFormation I florasuggestive of morehumid conditions. Finally, theyinclude only fine- or medlum-graineddetrital GeneralDistribution: The Moenkopi formation is sediments,except for locally derived gravels at the composedof sedimentarydeposits of Triassicage that i base,even along margins of the basinwhereas the forma greatwedge extending across northern Arizona rUpperTriassic deposits contain many beds of large andsouthern Utah. Along its westernmargin in rounded.gravelsindicating the development of an southernNevada and southwesternUtah this wedge upliftedsource area to thesouth, attainsa thicknessof 2000feet. Farther east at the I typelocality near Cameron~ Arizona it is 400 feet Pre-MoenkoplUnconformity thickand in thevicinity of theUtah- border and the Arizona-NewMexico border, it thinsto the I Thepre-/Voenkopierosionsurface marks one of vanishingpoint. theimportant breaks in thestratigraphic record of the ColoradoPlateau. It is developedon rocksof Theeastern margin of thebasin in whichthe Permianage, including the Kaibab limestone in /Vbenkopiwas deposited forms a sinuousline from north I westernArizona and Utahand on variousmembers of to southas indicatedby outcropand well data. Isopach theCutler formation farther east. The time represented contoursshow that the basinincluded an eastward- by hiatusinvolves the upper half of thePermian period extendingembayment in the northand anotherin the (aboveLeonard time), plus the lowerpart (pre-Mee- southseparated by a westerly-extendingridge or i koceraszone) of theTriassic. peninsulain thearea of northeasternArizona.

Thephysical record of theunc0nformity varies Theposition of thesouthern embayment is indicated

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

by thicknessesobtained at St.Johns, Arizona (8l Utah,but in generalthey correspond with the upper feet),at Cheehilgeethowell, New Mexico(50 feeti, partof theformation there. northwestof McGaffey,New Mexico(73 feet)and nearFort Wingate,New Mexico(30 feet).These The Wupatkiand Holbroakmembers are characterized depositsshow proximity to themargin of thebasin not by redbeds--sha[y si[tstones and structureless mudstones-- onlyby theirthinness, but also by thelarge percentage alternatingwith resistant, thick-bedded sandstones. of sandstoneand other coarse detrital materials. The Wlnslowmember between is composedlargely of light-coloredsiltstones and mudstones with considerable The westwardprojection of the Meenkopi beddedgypsum. It contrastsstrongly with the members boundaryin northeasternArizona is indicatedby aboveand below bath in colorand in slope-forming absenceof theformation at CarrlzoMountains (J. D. tendencies. Strobell,personal communication), Nazlini Canyon Canyonde Chellyand Lukachukai.At all of these Faunaand Flora:The invertebrate fauna from localitiesUpper Triassic deposits rest upon Permian themarine deposits of theMoenkopi in northwestern strata.Influence of the westwardprojection also Arizonaand southwesternUtah and in eastcentral is notedalong Echo Cliffs, midway across the state, Utahhas beenknown in a generalway for manyyears forsections thicken considerably both north and south (Reesideand Bassler, 1922). It consistslargely fromthe central part of thesecliffs near 1he Gap. mollusks,with pelecypods and gastropodsabundant and somecephalopods. Locally brachiopods and A northernembeyment of the Moenkopibasin crinoidsare common.This fauna has not been occursin southeasternUtah. Northeast of Moabthe studiedin detailand many of theforms are un- to,marionextends with moderate thickness into described. Coloradowhere it appearsto endabruptly against the ancientlandmass of Uncompahgre(Dane, 1935). In northeasternArizona where Moenkopl deposits areentirely continental, a moderately large and klthologicTypes: The Moenkoplformation is variedvertebrate fauna occurs. It includesreptiles, composedof bothcontinental and marinedeposits. It amphibiansand fish, with stegocephalians by far the consistsof sixprincipal lithologic types: (I) massive, mostcommon type (Camp, et al.¯ 1947).Skeletal cross-laminatedsandstone, (2) red shaly siltstone, remainshave been found in manylocalities in both (3)red brown structureless mudstone, (4) intraformational the Holbrookand Wupatkimembers. Trackways, (mudpellet and limestonepebble) conglomerate, (5) mostlyof reptiles,are alsocommon and have proved to gypsumand (6) limestone.Each of theserepresents be of somevalue in correlation(Peabody, 1948). distinctenvironment and occurs in varyingproportion acrossthe region. Theflora of theMoenkopi formation is very imperfectlyknown. Specimens are uncommonand The marinelimestones of theMoenkopi formation poorlypreserved. They include impressions of reeds arerestricted to the lower half and occur only in the andscouring rushes and a smallamount of petrified westernand central parts of the area.The massive wood. sandstonesare largely confined to theeastern borders of theregion, but the red siltstones and structureless Environmentof deposition:A wide variety of mudstoneswhich occur at variouslevels throughout the environmentsis represented in deposits of theMoenkopi. regionaccount for the characteristic redcolor. Themassive sandstones in the eastern part of thearea clearlyare the product of streamdeposition and form Stratigraphicsubdivisions: Three distinctive thinsheets over wide areas, developed during times membersare recognizedin the Moenkopiformation of regression.They contain abundant cusp-type throughoutthe Little Colorado River area of north- ripplemarks, both scour-and-fill anddelta-front cross- easternArizona (fig. I). These are from bottom laminationand the skeletons of amphibians. top,the (I)Wupatki, (2) Winslow and (3)Holbrook members.They do not correlateexactly with any of Redshaly siltstones were developed on mudflats - thesix membersof theformation that have been described eithertidal or floodplain.They are covered nearly by Gregory(1947) for the thick sections in southwestern everywhereby parallelcurrent ripple marks and locally ! !

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

theycontain rain pits, shrinkage cracks and casts of suchplaces Chinle deposits rest directly upon those saltcrystals. Red brown structureless mudstones of the Moenkopi.Such observations show plainly probablyare the result of clayand silt settling in thatthe unconformity is the result of a longperiod pondsor lakes.Gypsum beds doubtless indicate of subaerialerosion and not a localfeature. lagoonsor playasback from the margins of thesea andthe limestonesand calcareousshales with marine Theamount of timerepresented by the hiatus faunasclearly indicate shallow sea deposits, involvedin the Moenkopi-Shinarumpunconformity is difficultto determine.The age of thetopmost beds In brief,the depositional environment of the in the Moenkopiformation is not known;they may Moenkopiconsists of wide,flat plains extending belonglate in theLower Triassic as suggestedby froma topographicallylowsource area on theeast stratigraphicposition high above the Meekoceras- to a geosyncllnalsea on thewest. Streams introducedl bearingbeds, or theymay be of MiddleTriassic age sandand silt from the eastern margins, shallow ponds as suggestedby some of thevertebrate" fossils (Welles, andlagoons were widespread over the nearly flat surfaceand the sea alternatelyadvanced and re- 1947).Further-more, the timewhen Shinarump treatedfrom the west.Evaporation was of considerable depositionbegan is opento question,as discussed magnitudeindicating that the climatewas semiarid underthe ageof thatformation. In anyevent, the orarid. timeof hiatusmust have been considerable, even in a geologicsense, and during that time, apparently, Ageof formation:The lowerpart of the Moenkopi regionalUplift to thesouth took place (McKee, 1951) as representedby the Tlmpoweapmember in southwesternandrenewed upwarping along anticlinal structures in Utahand theSinbad limestone in San RafaelSwell, thenortheastern part of thebasin occurred (McKnight, Utah,is datedby thepresence of theammonite 1940). Meekoceras.This marks the middleof the Lower ShinarumpConglomerate and ChinleFormation Triassicaccording to theclassification of zones by Smith(1932). The age of the youngerfauna in the Generaldistribution: The con- Virginlimestone has notyet been determined, slstsof a seriesof depositsthat extend with relatively uniformthickness across most of northernNew Mexico In theupper part of theMoenkopi most of the andArizona into southern Nevada, also northerly into vertebrateremains indicate a Lower Triassic age centralUtah and southwestern Colorado. It is considered accordingto Welles(1947), though one of the common a correlativeof theDolores formation of Coloradoand forms(Cyclotosaurus) belongs in theMiddle Triassic thelower units may be equivalentto the Dockumof of Europe.Likewise the reptilian tracks mostly are ,in part(Camp, 1950). In the ChinleValley comparableto LowerTriassic forms in theOld World area,Arizona which is thetype ~locality, it has a butsome are similar to speciesfrom the Middle Tri- thicknessof 900to 1000feet. Farther east near Fort assic.Thus the Moenkopiformation definitely began WingateNew Mexicoit is about1400 feet thick in EarlyTriassic time but may have continued into (Harshbarger,personal communication). MiddleTriassic. TheShinarump conglomerate which is considered Moenkopi- ShinarumpUnconformity by manygeologists (Camp, et al.¯ 1947)as a basal memberof theChinle formation occurs as a blanket The contactbetween the Moenkopiformation and depositof sandand gravel across most of horthern the Shinarumpconglomerate is irregular and wavy even Arizonaand southern Utah, eastward at leastas far withinshort lateral distances. In someplaces, channels as FortWingate, New Mexico(Harshbarger, personal and pocketsexcavated in the upperMoenkopi surface communication).Its thickness varies considerably extenddownward many feet and are filledwith sand fromplace to placelargely as a resultof irregularities and gravelof the Shinarump.Elsewhere the Moenkopi on the underlyingMoenkopi surface, but itsmaximum stoodas hillsor ridges,around and over which sediments thicknessprobably does not greatly exceed 160 feet as of the Shinarumpaccumulated. In a few areasresidual foundon thenorth end of theDefiance Uplift. In Moenkoplmasses were sufficiently high to standabove numerousareas includlng that east of Holbrookalong the generalsurface of Shinarumpaccumulation and in EchoCliffs and near Ganado, Arizona it is absent

88 i . . NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN I locallybecause of non-depositionwhere hills of Moenkopi The Shlnarumpconglomerate consists of con- stoodabove the generalsurface of accumulation, glomeratesandstone, and minoramounts of mottled mudstone.The "D" memberof the Chinleinto which i Lithologlctypes: The Shlnarump-Chinledeposits it gradesis composedof alternatingbeds of sandstone, includefive principal litholagic types. These are mudstoneand claystone.Like the Shinarump,it is conglomerates,sandstones, mudstones, claystones characterizedby much channel-fill cross-stratification i andlimestones. All are of continentalorigin, but andit containslenses of conglo,menate so it is sometimes theyrepresent a variety of deposltionaleenvironments,confusedwith the Shinarump. In somelocalities this Theconglomerates, sandstones and limestonesare memberis not recognizableor identifiable. The "C" resistantand form ledges or cliffs;the mustones and memberor middleunit makes up morethan one half i claystonesare exceptionally weak and Weather to of thetotal thickness of theChinle. It is composed badlandtopography, almostentirely of claystoneand mudstone in thetype area,although toward the southand southeastinter- n Conglomerateis abundantin theShinarump,and beddedsandstones are progresslvely I thickerand more alsooccurs as lensesand lenticular beds at various numerous(Harshbarger, personal communication). horizonswithin the overlying Chlnle. Gravels in the theFort Wingate area a conglomeraticsandstone that conglomerateare mostly of durablerock types such as . has beencalled, erroneously r Shinarump,actually i quartzite,jasper and chert and, in mostplaces along occursnear the middle of the member(Bates, 1933). theLittle Colorado valley, range from i/4 inch up to The "B"member of. Gregory,or top-mostunit of the 2 I/2 inchesin diameter.In the Shinarumpconglom- Chinlein. this clas,~--’~ication, consistsof limestone cratethey average progressively smaller in successi.ve and interbeddedmu~stone and claystonebeds. I northerlyoutcrops across Arizona; in theChinle they do notextend far north of areasalong the southern Fauna.an~.Flora: The flora .of the Upper Triassic marginof thedepositional basin. (McKee, 1936). of Arizonaand Hew Mexicois famousbecause of the i abundanceof petrifiedtreesthat it includes.Most Clayst0neand mudstone,which together make up nu~nerousof theseare the conifers Araucarioxylon o majorpart of the Chlnleformation, commonly have ¯ and Woodworthia,though others are present.. Altogether beenreferred to as "marls",though many are non- 35 generaand 38 speciesof plantsincluding many I calcareous.They are of variouscolors including cycadsand fernshave been recognized (Daugherty, grayishred, purple, greenish gray and variegated 1941).Fossil plants occur in boththe Shinarump combinationsof these. The claystonesinclude ben- conglomerateand the Chinleformation, but well i tonitebeds. Brownish gray,thin-beddedlimestones preservedforms have been found only in theChinle. of lentlcularcharacter are numerous in theupper part of the formationnorth of WinslowArizona but are A considerablevertebrate fauna, including representedby only two beds eastward near Fort amphibians~fish and reptiles of severalmajor groups, i Wingateand are absentin the BlackRock area of occursin the Chinleformation. Most numerous in New Mexico. numberand varietyare thephytosaurs, described by Camp(1930), and stegocephalianamphibians. Small I Stratlgraphlcsubdivisions: At the type locality dinosaursfrom the Chinleat GhostRanch, New Mexico, of theChinle formation in Chinle:Valley,Arizona haverecently been discovered by Colbert.In addition theformation was dividedby Gregory(1917) into four to thevertebrate remains, large concentrations of fresh memberswhich, in descendingorder, were designated waterclams are found at numerouslocalities and fresh i as "A","B~.’~ "C" and "D"(fig. 2). The underlying watergastropods have been described from the limes- Shinarumpconglomerate which grades upward into the stonebeds. "D" membermay be consideredan additionalmember of the Chinleformation. On the otherhand essentially Environmentof Deposition:"The Upper Triassic i all of Gregory’s"A" memberappears logically to belong of Arizonawas characterized by subtropical to tropical to theGlen Canyon group rather than to theChinle temperatures,as indicated by thefern element" formation,on the basis of lithologiccharacter and accordingto Daugherty(1941). He disagreeswith many i stratigraphicpositionabovea wlde-spreadgranule earlierwriters who considered the climate to havebeen conglomerate.(Callahan, 1951). aridor semiaridand concludes on thebasis of plant | ! 89 |

II NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN ! lifethat the climate was like that of thesavannas in Triassicage hasbeen suggested by Stokes(1950) who thetropics of today.He envisionsdense forests concludesthat it was formedin themanner of a pediment, I borderingpermanent streams and swampyareas, with attainingits distributionandgrowthduring thegeneral moreopen forests in theuplands, periodOf erosionrepresented by theMoenkopi-Chinle unconformity.The presenceof comparablegravel Thereappears to be generalagreement that the depositswithin the Chinle seems to thepresent writer I Shinarumpand Chinleof the areadiscussed were to disprovethis idea. depositedon thesurface of a wide,low floodplain. In theearly stages of sedimentation,streams trans- Post-ChinleUnconformity I portedabundant gravels and sands from a recently raisedarea to thesouth and gradually spread them Whatis believedto be an irregularerosion northwardas a thinblanket (Shinarump), the result surface,overlain by a granule-pebblecongjomerate of haselevelremaining nearly constant. At later occursnear the contactof Gregory’s"A" and "B" I stagesin Shinarump-Chlnlehistory, new butlsimilar membersof the Chinlein manyplaces. This uncon- blanketsof gravelalso advanced northward across formablesurface is considered(Callahan, 1951) theflood plain for short distances. In general, the representthe close of Chinledeposition and to mark trendin sedimentationwith the passing of timewas the boundarybetween the Chinle and strataof the I towarddeposition of finer and finer materials, overlyingGlen Canyon group. The markedlithologic similaritybetween the strata immediately above the Sedimentsof theChlnle illustrate a transition unconformityand otherunits of the GlenCanyon I fromdominantly fluvlatile conditions, shown by cross- group,especially the Kayenta formation, support stratificationchannelling and c:onglomerate lenses thisinterpretation. Whether the break is alsoa in "D" membersandstones, to an environmentof boundarybetween Triassic and Jurassic deposits is nearlycontinuous , quiet-waterdeposition represented notknown and doesnot necessarilyfollow. i by the mudsand claysof the "C" member.Scattered lentlcularbeds of conglomerateand sandstone,similar Somelocalities where evidence of the post- to thosefound in "D" memberand in theShinarump, Chinleunconformity are well exposed are (I) Tuba i indicateintermittent fluvlatile deposition. This ~/as Cityarea, Arizona,where a maximumrelief of 50 fallowedby limestoneprecipitation in.bodles of feetand a granuleconglomerate occur (Callahan, 1951) freshwater("B"member) andfinally by a complete (2)north of Winslow,Arizona, where a reliefof cessationof sedimentationover much of theregion, feetand a sharpllthologlc break are present; (3) I BlackRock area,New Mexico,where a one-to-two- Age of Formation:The Chinlehas commonlybeen footthick pebble conglomerate occurs and limestone assignedto theUpper Triassic on thebasis of similaritiesbedsof theChinle are absent, (4) near Fort Wingate, betweenits vertebratefauna and that of theEuropean New Mexico,where cross-lamlnated, normal Wingate I Keuper.Although some paleontologists have suggested sandstone,rests upon the Chinle claystone surface thatthe lower part of theChinle may representMiddle and(5) northof Lupton,Arizona, where nearly Triassictime, considerable faunal evidence obtained feetof pebbleconglomerate containing numerous frag- i by Camp(1930) has indicated that the entire formation mentsof Chlnleoverlies the erosion surface. The above belongsto theUpper Triassic and is youngerthan the data,except that from Tuba City, were furnished by J. PopoAgle of Wyomingand alsoprobably, except for Harshbarger(personal communication) who statesalso thelowest part younger than the Dockumof Texas. thatthe unconformity is slmilarily exposed at various i Fossilplants likewise indicate a Late Triassic age for localitiesin thenorthern part of theNavajo reservation. theChinle according to Daugherty(1941). i Theageof the Shinarumpconglomerate hasnot beendetermined on the basis of vertebratefossils, but petrifiedwood within it is consideredUpper Triassic by Daugherty(1941). Furthermore, its stratlgraphic I positionconformably below the Upper Triassic Chinle supportsthis contention. On theother hand, a middle ! ! 91 | NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN BASIN I ! SelectedReferences .... i Bates,Robert L. (1933)The oil and gas resourcesof New Mexico,New Mex.Sch. Mines, Bull. 18, 320 pp.

Callahan,J. T. (1951)Geology of theGlen Canyon group along Echo Cliffs, Ariz., Plateau, vol. 23, i no. 4, pp.49-57. I

Camp,Charles (1930) A studyof the Phytosaurswith description of new materialfrom western North America, Univ.Calif. Mere., vol. 10, 160 pages, i Camp,C. L.,Colbert, E. H., McKee,E. D. ~ and Welles,S. P. (1947)A guideto the continentalTriassic of northernArlzona~ Plateau, vol. 20 no. I, 10 pages. i Dane,C. H. (1935)Geology of the SaltValley Anticline and adjacentareas, Grand County, Utah, U.S:. i Ge01.Survey, bull. 863, 184 pages. ..~¯ . dig Daugherty,L- H. ¯(1941)The UpperTriassic Flora of ArizonaCarnegie Inst~ Washlngton~ Publ. 526, 108 pages. I Gregory~H. E. (1917)Geology of the Navajocountry, U. S. Geol.Survey, Prof. Paper 93, 161 pages.

Gregory,H. E. and Williams,¯N. C. (1947)ZionNational Monument, Utah, Geol. Sac. Am. Bull.~Vol. i p. 211-244

.. Longwell,C. R. (1925)The pre-Trlassic unconformity in southern Nevada~ Am. Jour. Sci. ~ fifthser., vol. 10~ ¯p. 93-106. I

McKee,Edwin D. (1936)Triassic pebbles in northernArizona containing invertebrate fossils, Am. Jour.Scl., ¯ fifthser., vol. 33, p. 260-263.

McKee,Edwin D. (1951)Sedlmentarybasins of Arizonaandadjolning areas, Geol. Sac. Am., Bull., vol,62, p. 481-506. ~ i i FMcKnight, E. T. (1940) Geology of area between Green and CoJorado rivers, Grand and San Juan counties Utah, U.S. Geol. Survey, Bull.908, 147 pages. ¯ . .. . [] Peabody, Frank (1948)RepfiJe and amphibian trackways from the Lower Triassic Moenkopl formation of Arizona and Utah;Univ. CaJlf. Publ.:, Bull. Dept. Geol. S0c., vol. 27., No. 8, pp. 295-468.

Reeside, JohnB., Jr., and Bassler, Ha~ey (1922) Stratlgraphic sections in southwesternUtah and northwestern Utahand northwesternArizona, U.S. Ge01. Survey, Prof. Paper 129-D p. 52-77.

Smith,J. P. (1932)LowerTriassic ammonoids c~f NorthAmeri~:a, U.S. Geol. Survey, Prof. Paper !67, 199 pp. i Stokes,W. L. (1950)Pediment concept applied to Shinarumpand similarconglomerates, Geol. Sac. Am., Bull., vol.61 pp. 91-68 [] Welles,S. P.,(1947) Vertebrates from the upperMoenkopi formation of NorthernArizona, Univ. Calif. Publ., Bull.Dept. Geol. Sci.~ vol. 27 no.;7~pp. 241-294. ! ! !

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