Base of the Morrison Formation, Jurassic of Northwestern New

Baseof the Morrison Formation, Jurassic, olnorthwestern NewMexico and adjacent areas by)rin J.Anderson, New Mexico Bureau of Mines and Mineral Resources, Campus Station, Socorro, NM 87801; andSpencer G. Lucas, New l\4exico Museum of NaturalHistory and Science, 1801 l\4ountain Road NW, Albuquerque, Nl\487104 Abstract Introduction face that is of great stratigraphic signifi- The Morrison Formation-San Rafael Grouo Considerable uncertainty and differ- cance and mapping applicability. contact is herein resolved and olaced at a ence of opinion exist with regard to the Extending this four-member subdivision regionally traceable scow surface and sequence base of the Morrison Formation (Upper into the San Juan Basin in the 1950s dur- boundary at the base of the Salt Wash Member Jurassic) in the San Juan Basin of north- ing the height of uranium exploration of the Morrison Formation. [r additiory the western New Mexico and adjacent areas activities resulted in uncertain correla- intemal skatigraphy of the Morrison is simpli- (Fig. 1). reflects tions and a Morrison basal contact that fied with only two members, the Salt Wash and Much of this confusion the Brushy Basiry recognized for regional corre- the fact that in this area the base of the was not based on lithologic contrast. This lation purposes. Thus, the confusing, duplica- Morrison was not determined by correla- trivialization of the formation concept tive, and overlapping members, which includ- tion with the lithologies at the formation further resulted in the inclusion of Sin ed Bluff Sandstone, Recapture Shale, and type section near Morrison, Colorado Rafael Group strata in the overlying Westwater Canyon Sandstone, are not l€cog- (Waldschmidt and LeRoy, 1.944), but Morrison. We critically review the perti- nized, and the names are either abandoned or rather by correlation to a western nent literature and present our own are reassigned to the San Rafael Group. This Morrison or the Morrison of Utah. We observations based primarily on field- newly established Morrison concept and inter- have found that the longstanding subdi- work and restudy of many of the strati- nal stratigraphy recognizes and follows useful vision this western Morrison into four graphic sections reported in the older mapping units that aid in regional correlations. of The sequence boundarv at the base of the or more members lacks lithostratigraphic literature. Laboratory studies, primarily Morrison has climatic implications related to significance, contains overlapping units, grain size and cementing agents in the the northward drift of the NorthAmerican con- employs duplicative nomenclature, and sandstones, provided further information tinent through Jurassic time. ignores a regionally traceable scour sur- for our interpretations and conclusions. 110' tuo 104" Gaeen River Ugrb a Monlro9e Jl Oqrsy 0 10 20 30 40 50mi \ t!Lr---Jl \ \\ Recaptute a..^ creek \ ?n"r Jl) uu ---+-/ I Becl ab ito { Horae Meea M** l ,\ll Fomerovitte Haoan l-S"iin aAlbuquerque 10' 108' 106" FIGURE 1-lndex map of northwestern New Mexico and adjacent areas. |, outcrops of Jurassic rocks. August 1995 NewMexico Geology Review and discussion of previous work The first formal use of the name Mor- rison as a stratigraphic unit in Utah was that of Gilluly and Reeside (1928), who designated the strata overlying their San Rafael Group as Mor.rison Fornration. The basic lithologic distinction was that varie- 25' gated shale, medium- to coarse-grained sandstones, and mudstones of the Mor- rison tended to be slope formers above the brightly colored, thick, chff-forming sandstones and gypsiferous beds of the San Rafael Croup. Gilluly and Reeside recognized two members in their Morrison stri- ta. The lower was a light-gray to tan, coarse-grained, pebbly sandstone that exerted significant influence on topography near the San Rafael Swell and-Green 20' River and northwest of Moab (Fig. 1). This lower unit was called the S;lt Wash Sandstone Member, a name that Lupton (1912) had introduced for these beds in Salt Wash, 15 mi (24 km) southwest of Creen River. (Lupton described this unit as a member of the McElmo Formation, a name subsequently abandoned as essen- tially a synonlrn of the Morrison Forma- tion.) In retrospect, Gilluly and Reeside's basal contact of the Morrison was a well- justified stratigraphic decision. It provided a Morrison base that is an easiiy recognized lithologic break. The pebbly, trough- crossbedded SaIt Wash Member contains rip-up clasts, clay galls, and other sedi- A mentary features that readily identify it as a fluvially deposited sandsione very dif- ferent from underlying fine-grained San Rafael Group strata. Tie chanlnel scour at their Morrisbn base is a regionally trace, able surface,which again is1 major aid to geologic mapping. In modern terminolo- wYo gy, the base of the Morrison, as picked bv Gilluly and Reeside,is a sequenie boundary, one that reflects climjtic and base- level changes and that records the onset of widespread braidplain deposition of Mor- rison strata during Late Jurassic time. The upper part of the Morrison Forma- tion in southeastern Utah consists largely of claystone and was not given a membe. Sall Wash name by Gilluly and Reeside (1928). This earliesl upper unit is the only part of the formation that can be correlated lithologically with confidence to the tvpe Morrison Foi- mation near Morrison, Colorado (Fig. l ). ^.,/; \.j slreams FIGURE 2-A, Middle Jurassic paleogeography of southern margin of Sundance Seiwiy ind associatedtidal flats,arid coastalplain. sibkha depositional systems (horizontal dashes). Erg development representedby Moab longue (oi Entrada Sandstone) and the Zuni Sandstone. Evaporites represented by the Todilto. Modified from Anderson and Lucas, 1994. B, Late Jurassic paleogeography of area south of sundance Seaway. illuslrating onset of flurial deposition across San Rafael Group strata bv earliest "Morrison" streams. Mod-ified from Anderson and Lucas, 1994. 20' NewMexico Geology August 1995 Perhaps the most significant attribute of Ma this early-recognized Morrison-San Rafael 550 500 450 400 350 300 250 200 150 100 50 Group contact is the lithogenetic contrast it represents. The uppermost San Rafael Group strata were assigned by Gilluly and Reeside to the Summerville Formation. The Summerville consists of thinlv bed- Temoeratezone of ded, parallel-bedded, very fine grained gypsiferous sandstone or siltstone, with westerlies lesser mudstone and bedded gypsum or Yn** 'e '/,*e,, carbonate near the top. Summerville strata Subtrooicalbelt e"J}.,9,Y and related San Rafael Group eolianites form a sequencedeposited in a marginal 320 Phanerozoic marine, tidal flat, arid coastal plain, or .= paleolatitude sabkha setting with persistent eig devel- (U Northeasttrade winds 10 track of Four Corners opment (Entrada and Zuni Sandstones) around the southern margins of the Sun- o dance (Fig. E seaway 2A). This paleogeogra- LO Eouatorial belt of Cordilleran -rt- phy was also recognized by Imlay (1954), :\r continental oo7.o Kocurek and Dott (1983), Harshbarger et margin al. (1957), Brenner (1983), Anderson and Lucas (1992, 1994), and Ridgley (1989), Southeast trade winds Laramide among others. With this paleogeographic \ episode and paleodepositional basin concept, the post-San Rafael Group sequence takes on added significance. The pebbly, coarse- grained, trough-crossbedded sandstones New Circum-Pacific arc magmatism Mexico Paleozoic platform of the Salt Wash Member stratal of the Morrison successrons Deltaicand terrestrial Formation represent fluvial (braidplain) assem0tages Carbonatesand red beds deposition and higher energy systems, which in turn indicate higher gradients FIGURE 3-Phanerozoic paleolatitude track (stippled area) of the Four Corners area, illustrating the and an increase in coarse siliciclastic sup- northward ddft of that area into the zone of prevailing westerlies in Middle to Late Jurassic time. Modified from Dickinson, 1989. ply during Late Jurassictime (Fig. 2B). Paleoflow direction was generally east- ward (Craig et a1.,1955; Harshbarger et a1., 1957; Saucier, 1967; Turner-Peterson, son, 1989). Basal Morrison beds rest dis- tongues with the upper claystone facies, 1986). This delineates a source area to the conformably on these foresets near Gallup, providing further reason to regard the Salt west in an area of known Middle to Late at Church Rock, near Laguna, and at Mesa Wash as part of the Morrison. The sharp Jurassic back-arc volcanism (Fig. 2B). Gigante (Fig. 1); however, at the later lithologic contrast at the base of the Salt Uplift preceding and associated with the localitv. horizontallv bedded red siltstone Wash has already been discussed. Herein volcanism provided the source area for the and fine-grained sindstone intervene to lie the reasons for the differences between basal Moriison pebbly sandstone beds, form the top of the San Rafael Group. the tvoe Morrison Formation and that of which backfilled a verv moderatelv in- Comr:rarison of the above described Utah<ir the "western Morrison"; the west- cised topography. Continued volcanism, "westein Morrison" with the tvoe Mor- ern Morrison represents a more proximal dominated by latite and rhyolitic ash rison of the Front Range may-iaise the Iac1es. (Kowallis et al., 7991), provided much of legitimate question of why the lower These regional aspects of Morrison the source rock for the claystone-dominat- sandstone-dominated interval (Salt Wash stratigraphy and the basal contact with the ed (largely smectite)

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