New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/2 Cretaceous stratigraphy of the San Juan Basin Caswell Silver, 1951, pp. 104-118 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. 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Sandstonepredominates to J thesouthwest, shale to thenortheast; marine deposits make ¯ mm CaswellSilver ~ up theentire section to thenortheast whereas deposits ConsultingGeologist to thesouthwest are of intertonguingmarine and contlnentaifacies. I Possiblyfew systems of rocksin westernUnited Sears,Hunt, and Hendricks(1941 , p. 102)reviewed Stateshave been written about more than the Cre- theconcept, in a paperwhich has becomea classic, thatthe deposits under discussion were laid down in a g taceousrocks of theSan JuanBasin. The publications gm listedat theend of thispaper include many papers broadshallow trough that extended across the area in describingthese rocks in everypart of theBasin and lateCretaceous time. The general orientation of the adjoiningareas with a wealthof detailnot possible shorelinewas northwest to southeastand fluctuations [] here.Thispaper will not re-describe all the strati- in theposition of thisshoreline were generally in a II graphicunits. The subject of thispaper is thesequence northeastto southwestdirection. Further, they elaborated of bedsof differinglithology, the lateral variations theconcept first stated by Lee(1915, p. 28)that both the in lithologyof these beds and their relations to the transgressiveandregressive deposits were laid down economicdeposits of coal,oil and gasthat have been duringcontinued sinking of thetrough and thatboth II foundand may possiblybe foundin theSan JuanBasin. transgressiveor regressive deposits were a reflection Theworks of previousauthors are drawn upon freely of therate of sinkingin relationto thesupply of HI forthis purpose. If afterreading this paper newcomers material.Figure 2 fromSears, Hunt, and Hendricks havean understandingof the thickness and position of (1941,p. 105)shows the theoretical progression. This | theCretaceous layered rocks and an understandingof wasborne out by the "widespreadoccurrence of trans- thepossible distribution of the contained fluids, the itionzones between shale and overlying regressive ¯ author’spurpose will have been served, sandstoneand by theextent of thicknessand uniformity [] of suchsandstones. With continued subsidence there TheCretaceous sediments of thecentral part of wouldalso be Opportunityfor the development of thick i theSan Juan Basin can be Iogically:divldedinto coal-bearingcoastal-swamp deposits behind and en- ¯ twomajor groups: the older and lowergroup which croachingupon the beachsands as the sandsgrew up- I includesall the sediments deposited while marine conditions ward and seaward. " (Ibid, p. 101).As regression enduredand theupper and younger transitional group continuedflood-plain deposits encroached upon the [] whichbridged the transition from marine to wholly near-shareand coastal deposits. At suchrime as the continentalconditions of deposition.If one considers rateof sinkingin thetrough exceeded the supply of theduration of marineCretaceous sedimentation in materialregression ceased and the sea again trans- thecentral part of thebasin as extendingfrom the gressedthe land to the southwest.The upwardsuccession n baseof theDakota sandstone to thetop of thePictured of sedimentswas the reverse of thatduring regression U Cliffssandstone, the valuable stratigraphic reference coastaldeposits building upward to thesouthwest over intervalof theolder group is established(See Figure i.) flood-plaindeposits, which were in turnfollowed by R Thisinterval contains most of thecommercial deposits of near-shoresands and marine sands. oil andgas that have been found in Cretaceousrocks of thebasin at thistime, as wellas mostof theyet un- Age of theFormations. discovereddeposits. This interval lies across theSan Earlyworkers in theBasin were perplexed by the ¯ JuanBasin as a sedimentaryblanket of almostuniform conflictingpaleontologic data relating to theage of [] thickness.Over much of thebasin this interval is made thevarious lithalogic units, It wasnot until the up entirelyof clasticsand is constantin thicknesswith- formationswere mapped on a strictlylithologic basis in a fewhundred feet of 4,700.However, there is a disregardingageconsiderations that the present clear ¯ remarkableuniformity in theVariation in composition stratigraphlcrelationships became evident. As early Ill of thisblanket. In allbut a fewplaces the bottom and as 1915Lee (p. 36-37) pointed out that it wasthe tophorizons consist of sandstonea few hundred feet customthen to assigna sandstoneto theDakota if it [] thick.Between these containing sandstone horizons occurredimmediately below shale known from fossil n ! ! 104 I NEW MEXICO GEOLOGICAL SOCIETY * SECOND FIELD CONFERENCE * SAN JUAN IBASIN ! evidenceto be of Bentonage and thatthe Dakota DakotaSandstone. of onelocality was not the exact time equivalent "TheDakota sandstone," in thewords of Gregoryl I of theDakota of otherlocalities. The transgressive (1917,p. 72)"is highly variable in structure,texture; natureof the’Dakotashown by Lee(1915) and the and composition.It is characterizedmore by a transgressiveandregressive Mesaverde sandstones persistentcombination of features than by theper- I describedby Sears,Hunt,andHenrlcks,(1941)and sistenceof any givenbed. The baseis commonlybut ¯ Pike(1947) have established clearly the fact that by no meansuniversally marked by conglomerateand theseformations cut across time lines and cannot be the topis in manyplaces a coarsebrownor gray sand- definedsolely in termsof age.Figure 3 is a time stonebed but may be a groupof interbeddedsandstones i correlationchart which shows the time relation of andshales or whollysandy shale of yellowor gray thevarious units in theBasin referred to thestandard tones.Coal lenses occur prevailingly in the middle ¯ timescale of theGreat Plains. This figure is in- of theDakota but are found in allpositions from top I herentlyinaccurate as the foregoing discussion has to bottom.The formationis everywherelenticular; shown.:The age of theseformations can onlybe lensesand wedgesof sandstoneof conglomerate, discussedin termsof a specificlocality, of shale,and of coaltens of feetor a fewinches thickoverlap, appear, and disappear along the strike i LowerCretaceous Rocks (?) andvertically in a mostcapricious manner. " The Rocksof earlyCretaceous age havebeen abovegeneral description in the opinion of thisauthor tentativelyidentified by Reeslde (1944) only in the is thebest heretofor written on theDakota sandstone northernpart of theBasin near the town of Dolores of theSan Juan Basin and contains within it cluesto I in MontezumaCounty, Colorado, where they have theerratic production history of thisunit. Other a thicknessof approximately100feet.: They consist morecasual observers in theBasin have attempted to of white,medium to fine-grainedsandstone and green systematizethe Dakota.Prevalent are suchexpressions andgray shale. Similar rocks of likethickness appear I as "the secondDakota,