Baseof the , Jurassic, olnorthwestern NewMexico and adjacent areas by)rin J.Anderson, 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 sand- stones and gypsiferous beds of the San Rafael Croup. Gilluly and Reeside recog- nized two members in their Morrison stri- ta. The lower was a light-gray to tan, coarse-grained, pebbly sandstone that exerted significant influence on topogra- phy 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 recog- nized 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 bound- ary, 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 forma- tion 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

August 1995 NewMerico Geology Anderson& Lucas (1995),this paper Gregory(1938) o= 9o 9X YO -st o 6L 2 ( C) .=o J s5o9 >= 69 =G LE E6 Es E= =o .9 G o lr J-5 unconformity- o o CL f o L o o o (J v, \. \:,,t\ \' V 0, o tr = E o ,!=o (g c L G :!o G' o tt :ao tr o l! .^ Ot ! .-c o o i= E ! @

FIGURE 4--Diagram_of stratigraphic sequence exposed at mouth of Recapture Creek, 8 km east of Bluff, UtalL contrasting the stratigraphic concepts of Gregory (1938) on right with concepts and nomenilature advocated in thii paper on left. *Crossbedded unit correlates wit"h Acoma f6ngue ol Zuni.

and exposed near Bluff, Utah (Fig. 1). At that encompassed a wide variety of litho- fluvial influence. We contend that the the type locality 5 mi (8 km) west of Bluff logies as well as a very significant, region- abrupt change in lithology and sedimenta- along Butler Wash, the unit is as thick as ally traceable scour surface (Fig. a). The ry features at the top of the "red sand- 126 ft (38 m) and consists of very fine to total reported thickness was 290 ft (88 m) stone" unit reflects a change from sabkha lower medium-grained, mostly ciossbed- and consistedof a basal40-55 ft (2-17 m\ conditions to fluvial braidplain deposition ded sandstone. It does not constitute a of red sandstone overlain by coarse- in the overlying unit. The contact is also facies or lithology that was previously grained, pebbly sandstone followed by marked by a regionally traceable scour considered to be present northward or a "shaIy" section. The significance of surface that represents a diastem or eastward in the Morrison Formation; nor Gregory's Recapture member is twofold. unconformity, albeit with a poorly con- does it occupy the same stratigraphic posi- First, the lower 40-55 ft (L2-17 m) of red strained time value, and as noted above, tion as the basal Salt Wash Member of sandstone are very fine grained, silty, par- a sequence boundary. We further contend Gilluly and Reeside's stratigraphy. More- allel bedded and slightly gypsiferous. this unconformity is the San Rafael over, Gregory did not offer an explanation These strata are very similar in lithology Group-Morrison Formation contact; as for the inclusion of this sandstone unit in and sedimentary features to the Summer- picked, this contact is consistent with the the Morrison. Goldman and Spencer ville Formation. The Summerville (which earlier lithologic descriptions of the San (1941) recognized the Bluff as a pre-Mor- underlies the Bluff Sandstone) was de- Rafael Group basal Morrison contact rison lithology (well-sorted, frosted, fine- posited in arid coastal plain and sabkha offered by Gifiuly and Reeside (1928), and to medium-grained, quartzose sandstones environments. Thus, it would appear that most importantly, the contact is readily of eolian origin) and correlated it with the the basal "Recapture Shale Member" of mappable. Thus, the basal 40-55 ft (12- Junction Creek Sandstone in southwestern Gregory was associated with the brief 17 m) of Gregory's "Recapture Shale" is Colorado. On a lithologic basis, Craig et al. return of sabkha conditions locally fol- reassigned to the San Rafael Group as (1955) removed the Bluff from the Mor- Iowing deposition of the eolian Bluff. the Recapture Member of the Bluff rison Formation and included it in the Gregory's basal "Recapture Shale" also Sandstone (F ig. 4, left side). Lithologically uppermost San Rafael Group. We concur intertonsues with the Bluff Sandstone (fine-grained, thinly bedded, gypsiferous with Craig e1 al. (iq55) anb regard the (Fig. 4),- demonstrating its genetic and sandstone and silty sandstone) and litho- Bluff Sandstone and the closely associated temporal association with that unit. This genetically (sabkha, arid coastal plain) Acoma Tongue of the Zuni Sandstone of intertonguing, though not so stated, these strata belong in the San Rafael west-central New Mexico (Anderson. undoubtedly was the reason for inclusion Grouo. 1993) as part of the upper San Rafael of the Bluff in the Morrison Formation. The second significant aspect of Greg- Croup throughout the Four Corners area In contrast, the sandstones above the ory's "Recapture Shale" is that the base of and southeastward into the San Juan basal 40-55 ft (12-17 m) of Gregory's the medium- to coarse-grained pebbly Basin. "Recapture Shale" are medium to coarse sandstone 40-55 ft (12-17 m) above the Gregory defined his overlying "Re- grained and conglomeratic with clay galls base at the type locality correlates with the capture Shale Member" from exposures and rip-up clasts and are fluvial in origin; unit that Lupton (1912) and Gilluly and near the mouth of RecaptureCreek 5 mi (8 the basal red sandstoneand interfingering Reeside (1928) recognized as the Salt Wash km) eastof Bluff. He descrjbed it as a unit Bluff Sandstone exhibit little evidence of

New Mexico Geology August 1995 47 Member (Fig. a). Gregory's description of these strata at the type locality is very sim- ilar to the description that Lupton as well as Gilluly and Reeside gave for their basal Salt Wash Member. The two units occunv the same stratigraphic position atthough the Bluff Sandstone is not present to the north in the vicinity of Lupton's type Salt Wash, which is just southeast of Green River (Fig. 1). Distance between the two areas (southeast of Green River to Bluff) is approximately 100 mi (160 km); however, the stratigraphic sequence is similar. Not recognizing the significant lithologic break above the base of his "Recaoture ShaleMember" constituted a stratigraphic oversight by Gregory. Aside from the important correlation with the Salt Wash that was missed, he did not recognize a z maior change in lithology and deposition- o al environments at this break and thus L L included o o parts of the San Rafael Group -o -o E and the basal Morrison Formation in one E E member-his "Recapture Shale". o) o tr Gregory also noted the pebbly sand- o stone beds near Blanding, Utah, 20 mi (32 TL o o km) to the north in the same part of the (5 c z section as the "Salt Wash sandstones," but = (U o he hesitated to assert the correlation. It is = C) (6 L 2 of interest to note that Stokes L944\ did U) o tr recognize a Salt Wash unit low in (E tr Gregory's "Recapture Shale," but this E a o observation has been ignored for 50 years. o Gregory (1938) introduced another = member name for these pebblv sandstone beds near Blanding. Frbm the excellent outcrop near the confluence of Cotton- wood Wash and Westwater Canyon, iust south of Blanding, Gregory named a i75- ft-(53-m-)thick, sandstone section the "Westwater Canyon sandstone member." Significantly he did assign the name only "tentatively" because "its exact equivalen- cy to the typical Salt Wash sandstone has not been satisfactorily established." We recognize that the "Westwater Canyon TL sandstone member" is a synonym of Salt Tidwell Wash Member and that fhe LU litter name Member [*",, has priority. The type sections of the two units occupy the same stratigraphic posi- tion, are of essentially identical lithology, and are of similar rr thickness (Fig. 5). tIJ Correlation problems resulted not only from this duplicative nomenclature, but also because the type "Westwater Canyon matn Sandstone" was miscorrelated southward body f within a 20-mi (32-km) distance such that a at the type locality of the "Recapture Shale" the newly named "Westwater FICURE S-Type sections of Salt Wash and Westwater Canyon Members of the Morrison Formation. Measured sections are in Table 1. Canyon sandstone member" was includ- ed. The "Westwater" was erroneously cor- related with a locally prom inent, stratigraphic diagrams (Condon and Huff- the "Westwater Canyon sandstone" only fine-grained sandstone thit was present man, 1984; Peterson and Turner-Peterson, tentatively and that Stokes had earlier rec- 180 to 250 ft (54 to 76 m) above the'baseof 198f Peterson, 1988) that attempted to ognized a partial Salt Wash-"Recapture the pebbly sandstone at Recapture Creek, show the relationship among Gregory's Shale" correlation. Gregory invited fur- Further correlation problems resulting various units, namely the "Westwater Can- ther investigation with several of his state- from this duplicative nomenclature mani- yon sandstone," the "Recapture shale," ments. Of great significance was his fested thems-elves on many maps (Hintze and the earlier recosnized Salt Wash recognition of an unconformity at the base and Stokes, 1964; Hackman and Olson, Member. Aclear e"amp-le-6.4. of these efforts is of his type "Westwater Canyon Sandstone 1977; O'SulIivan and Beikman, 1963; included here as Fig. These attempts Member" and that this "sandstone may Moench, 1963, Thaden et aI., 7967), and did not acknowledge that Gregory named prove to mark the beginning of a cycle of

August 1995 NewMexico Geology AA TABLE 1-Type sectionof SaItWash Member of a ledge; unit is multistoried a cliff. Morrison Formation measuredin the NW%sec. channel bodies with Prominent 9. Sandstone; dark greenish- 79 T23S R18E, Grand County, Utah. Section scouted base. 7.3 yellow (10Y6/6) and startsat UTM 125906398,4295274Nand ends at Silty mudstone; same colors yellowish-gray (5Y7 / 2); IJTM 7258775A8, 4295482N. Strata are flat- and lithology as unit 8i lower very fine grained; quartzose iying. Stratigraphiccolumn is in Fig. 5. 1-2 m has ripple-laminated with some chlorite?; sandstones; forms a sloPe. 6.5 subangular; moderately Thickness sorted; not calcareous; Sandstone; pinkish-graY Unit Lithology (m) laminae; forms a cliff. 1.1 (5YR8/1); very fine grained; Brushy Basin Member: quartzose with a few Silty mudstone and muddY 11. Sandstone and conglom- feldspars; subrounded; siltstone; mudstone is eratic sandstone; sandstone well-sorted; calcareous; basal grayish yellow green (10YR8/2) is very pale orange conglomerates of (5GY7 /2); siltstone is with dark-gtay (N3) spots, reddish orange (10R6/6) to channei bodies are Pale- weathers pale yellowish olive (10Y6/2) mudstone pale reddish brown brown (10YR6/2); con- pebbles to 15 mm in (10R5/4); not calcareous; glomeratic sandstoneis diameter in pale yellowish- contains some nodules of (N8), weathers that very light gray bror,r.n (10YR6 /2) matrix; calcareous siltstone (5Y7 pale brown (5YR5/2); fine- trough-crossbedded; are yellowish gray /2); 7.1 to medium-grained; subar- unitls multistoried channel forms a slope. kosic; subangular; moder- bodies; forms a ledge. 9.3 7. Sandstone; yellowish-graY ately sorted; conglomerate (5Y8/1); very Silty mudstone; graYish- (5Y7/2) and clasts are mudstone pebbles red (10R4/2) and Iight fine to fine-grained; to 1 cm in diameter; trough- greenish-gray (5GY8/.1); quartzose; subrounded; crossbedded; forms a calcareous; has some teog)' well-sorted; not calcareous; ledge. Not measured beds of sandstone that are some medium-grained 10. Mudstone; greenish-gray very pale orange (10YR8/2), beds that are calcareous; a (sGY 6 / I) ; not calcareous; very fine grained, subarkosic, trough-crossbedded; forms upper 5 m very bentonitic; subrounded, well sorted and cliff; unit is multistoried 13.0 has ledgy lenses of sand- ripple laminated with much channel bodies. stone that are pinkish gray horizontal bioturbation on Sandstone; yellowish-graY (5YR8/1), very fine grained, bedding planes; forms a (5Y7 /2) and grayish-yellow subrounded, subarkosic, slope. 5.1 $Y8/a); very fine to fine- moderately sotted, and a 3. Sandstone; white (N9); verY grained; quartzose with calcareous; forms a slope. 73.6 fine grained; quartzose; few feldspars; subrounded; well-sorted; calcareous; Salt Wash Member: subr6unded; *eli-sorted; trough-crossbedded; f orms 9. Sandstone; very light graY calcareous; some conglomerate a chfJ; unit is multistoried (N8); fine- to medium- as in unit 5 at channel bases; channel bodies that laterallY grained; quartzose; sub- trough-crossbedded; f orms thins to give way mostlY to rounded; well-sorted; a ledge; multistoried sandy mudstone like unit 4- 4.7 calcareous; conglomeratic channel bodies. 4.0 very pale orange baseof unit is matrix- a. Silty mudstone; same colors Sandstone; supported pebbles to 1 cm and lithology as unit 4; (10YR8/2); fine- to medium- in diameter of light forms a slope. 1.5 grained; quartzosei subrounded; well-sorted; calcareous; greenish-gray (5GY8/ 1) 1. Sandstone; very light graY trough-crossbedded; base mudstone and chert and (N8); very fine to medium- of unit is matrix-suPPorted jasper pebbies to 0.5 cm in grained; subarkosic; conglomerate o{ chert, jasPer, diameter; trough-crossbedded; subangular; moderatelY and mudstone pebbles to forms a cliff; unit is sorted; calcareous;some 12 mm in diameter; unit multistoried channel bodies. 8.2 very coarse to coarse- forms base of big cliff. 0.8 8. Sandy siltstone; pale-red grained parts have red (5R6/2) with mottles of and black chert pebbles; 4. Sandy mudstone; pale-olive greenish gray (5GY6/1) trough-crossbedded; scour (10Y6/2) and light greenish- and pinkish gray (5YR8/1); base with flute casts and gr ay (5GB/ 1); calcareous; bentonitic; calcareous; tool marks; multistoried contains 0.3-0.6-m-thick are contains lenses of sandstone channel bodies that fine ledges of sandstone that very that are moderate brown upward to bioturbated toPs. 5.3 yeliowishgray (5Y8/1); (5YR8/1), well sorted, and fine grained, quartzose, calcareous; forms a slope. 8.3 subrounded, weil-sorted, calcateous, and massive to 7. Sandstone; very pale orange Type section of Westwater Canyon Member-of bioturbated. 6.7 (f0YR8/2); very fine to fine- the Morrison Formation measured in the grained; quartzose; subrounded; NW1/4 sec. 31 T37S R22E, San Juan County, Sandstone; yellowish-graY well-sorted; calcareous; Utah, near the mouth of Westwater Canyon (5Y7 /2); very fine to fine- trough-crossbedded; forms where it meets Cottonwood Canyon; section grained; quartzose; subrounded; besins at UT}r412628460E,4154718N and ends well-sorted; trough- atuTM 12628433E, 4154722N. strata are flat- crossbedded; forms a ledge. 0.9 lying. Stratigraphic column is in Fig' 5. 2. sandy mudstone; same sedimentation that continued until inter- colors and lithology as rupted by pre-Dakota ." We concur Thickness (m) unit 4. 1'.9 with this statement when the correct name Unit Lithology 1. Sandstone;grayish-yellow Salt Wash is substituted for "Westwater 11. Brushy Basin Member Not measured (5Y8/4) and very paie orange Canyon Member." He also stated that ulti- We stw at er Cnny on Member : (10YR8/2); same lithologY Morrison mately it may be found that the 10. Sandstone;yellowish-graY as unit 3; trough-crossbedded; pebbles; is composed of several formations. We (5Y7 /2); fine- to medium- some mudstone have read thoroughly the work of Gregory grained; quartzose; subrounded; unit is multistoried channel and remeasured most of his Morrison sec- well-sorted; slightlY calcareous; bodies with scour bases topset beds. 11.4 tions and have come to the conclusion that trough-crossbedded; forms and bioturbated !g

NewMexico GeologY August 1995 -=------_-_tu"hY WestwaterCanyon Member Morrison Fm.

Cretaceous -=='=td

Morrison - -- =- ;;;;;: Fm. o_-- | fJm{ --' - correlativewith AcomaTongue SaltWash Member of ZuniSs. Jzal Jbr. - )za \------/ __<:---: _ __t__-_:_ _ _:___s=::I__ - J-5/ -- Tidwell Member of / BluffSandstone eluffSs. Summerville Formation SummervilleFormation Summerville B (mainbody) Fm. (not to scale)

FIGURE 6-A, Stratigraphy and correlation of Middle and Upper Jurassic rocks in Four Corners region as envisioned by Peterson and Turner-Peterson (1987). Jmru, upper part of Recapture Member of Morrison Formation; Jmru=, I upper Recapture equivalent. Line of section in inset at left. B, Stratigraphy and conelation of Middle and shown I Upper Jurassic rocks, same area as A above, as proposed in this paper. Major contrasts betr,veen this and A 1ie in the recognition of regionally traceable scour surface at base of Salt Wash Member as shown in B, and in the fact that A I canr-rotbe applied by the field mapper. Jmj, Jackpile Member of Morrison recognized in southeastern San Juan Basin; I Jmf, Fiftymile Member of Morrison, a unit recognized by Peterson and Turner-Peterson (1987); Jza, Acoma Tongue of Zuni Sandstone; and Recapture Member of Bluff Sandstone. Both A and B are diagrammatic. i1 Jbr,

O 1oomi\ |_ o 200km F, I {uu, i his intuition was good and his statement recognized from Montana and South Morrison Formation, was prophetic-his lower "Recapture Dakota southward to Arizona and New northwestern New Mexico shale" most surelv is part of the SanRafael Mexico, as well as eastward into Oklaho- Early attempts (1950s) to correlate and Group. Our coirelitions (Fig. 68) are ma (Fig. 1). Gregory, however, named it recognize a four-member Morrison For- based on original definitions and lithos- for exposuresapproximately 9 mi (14 km) mation in northwestern New Mexico were tratigraphy, plus recognition of sequence northwest of Blanding, Utah, in a topo- related to uranium exploration efforts. boundaries, and contrast sharply with graphic feature called Brushy Basin. At Some of this work was classifiedinforma- those of Peterson and Turner-P-eterson this locality the Brushy Basin Member is tion, other efforts were completed as in- (1987; Fig. 6.4), which we contend are approximately 450 ft (136 m) thick with house reports and hence are not generally unapplicable in the field. only 42 ft (13 m) described as sandstone. available to the scientific community (e.g., Our reconnaissance,however, indicates Rappaport et al., 1952), and some of the Brushy Basin Member that the base of this member is not well work was in the form of graduate student To complete the discussionof Gregory's exposedat the type locality. Elsewhereit is theses (e.g., Chenoweth, 1953). A11,how- four-part Morrison Formation we must well exposed, and commonly we see an ever, left some of the aspects of the correla- consider the upper, most widely recog- interfingering relationship with the un- tions unclear; for example, no diagrams nized, and most extensiveunit. It is a clay- derlying Salt Wash Member. Thus, the satisfactorilv explained the Salt Wash- stone-dominated (largely smectitic), upper member-the Brushy Basin-of the Recapture ielationship (reportedly inter- slope-forming unit with lesseramounts of Morrison is generally a noncontroversial fingered) in the Four Corners area and sandstone and nodular limestone. The unit and the only one of the four proposed southward, nor how the Westwater Can- typical grayish-green cast, but common by Gregory (1938) that has any strati- yon Sandstone could be extended south- color banding, and the abundant free- graphic utility with respect to the Mor- ward from Recapture Creek into the swelling clays are the characteristicsthat rison Formation (Bluff and Recaptureare southern San Juan-Basin (C. T. Smith, oral stratigraphers most associatewith Mor- retained but properly belong with San comm.; Smith, 1954). Most of what was rison Formation strata. This unit may be RafaelGroup strata). called "Recapture Shale" in New Mexico

August 1995 NewMexico Geology with the Recapture Member of the Bluff was ore-Morrison-facies interdunal or minor flu- Zuni Sandstone and discussed in terms of vial that interfingered with the regional significance, event stratigraphy, Sandstone (Fig.7). We also recognize that upper part of the "Cow Springs Sand- and simplification of nomenclature by at this geographic locality, remote from stone"; Cow Springs was a name that Anderson (1993), the Acoma Tongue is the tJpe area of the Acoma Tongue, it may Harshbarger et al. (1951) suggested as a now recognized over a much broader area/ be appropriate to assign a local name to junior synonym for the Zuni Sandstone, a based partly on observations by Condon the crossbedded facies that herein corre- distinctive crossbedded eolianite in the (1985) in the Bluff, Utah area. Condon late with the Acoma Tongue. Perhaps it as a local member of southwestern San |uan Basin and associat- noted a distinctively crossbedded facies should be regarded ed Gallup-Zuni Basin (Anderson, 1993). with eastward-dipping foresets very simi- the Bluff Sandstone given the nature and Thus was born the erroneous concept that lar to the Acoma Tongue of the Zuni. subtlety of the bedform contrast between Morrison strata, specifically "Recapture While a revelation in terms of regional cor- the units. The base of the overlying Mor- Shale," intertongued with or graded relation of eolian facies in uppermost San rison by comparison is readily picked by southward into the eolian Zuni Sandstone Rafael Group rocks, the significance went the geologic maPper at the abrupt change fluvial (or to complete the confusion-into "Cow unrealized when Condon assigned the in grain size and the presence of at a sur- Springs Sandstone"). distinctively crossbedded facies to the characteristics encountered scour Formal adoption of Gregory's four- Morrison Formation. This left him with face cut and backfilled by the Salt Wash member Morrison Formation in New the unenviable task of having to identify a Member. This regionally traceable surface of the Mexico came with the work of Craig et al. group-rank boundary (top of San Rafael is the lower bounding surface (1955).As noted above, Craig did not rec- Group) in a continuous eolian sandstone Morrison fluvial sequence. and into the ognize the Bluff Sandstone as part of his sequence, based only on a change in bed- The area northwest of Bluff (Fig. outside Morrison Formation, advocating instead form (the crossbedded facies). This San Rafael Swell 1), although in this that the Bluff be included with the San approach ignores the lithologic basis_of the main area of consideration the pre-Mor- Rafael Group. He did not note, however, eiilier descriptions of the Morrison and is report, is of interest because different. that the reddish, fine-grained facies at the inconsistent with the North American rison stratigraphy is somewhat present in that top of the Bluff was, depending upon loca- Code of Stratigraphic Nomenclature, The Bluff Sandstone is not part of the tion, either interdunal or sabkha-related, which recommends that boundaries of area, and in addition the upper distinc- and preferred to accept the erroneous rock-stratigraphic units be placed at posi- Summerville Formation contains beds and interpretation that this fine-grained facies tions of lithologic change. Condon and tive grayish-green mudstone Peterson (1988) was Dart of the Salt Wash Member. Thus, Peterson (1986), in an effort to defend their bedded gypsum, which of the Craig made the observation or conclusion, placement of the basal Morrison contact, referred to as the Tidwell Member litho- largely from the literature, that in the Four ieferred to it as "an unrecognizable time Morrison Formation. Because these main Corners area the Salt Wash and Bluff inter- boundary," presumably because there was logies are common to the underlying and bedding tongue, detracting from the value of reas- no lithologic basis for their decision. One portion of the Summerville thinly bedded signing the Bluff to the San Rafael Group. must ask the question: if it is unrecogniz- iharacteristics (parallel, are similar, we see little utility in In the San Juan Basin the interbedded able to them, of what utility is it to the geo- strata) in the Morri- reddish facies at the top of, or overlying, logic mapper? We contend that the placing the Tidwell Member the the Bluff Sandstone is locally thicker than distinctly crossbedded facies records the ion. The local coarser grained facies at by Peterson its counterpart in Utah (herein named change in wind pattern as this part of the base of the Tidwell, noted Recapture Member of the Bluff) and also North American continent drifted north- (1988), we interpret as wadi deposition during exhibits much more fluvial influence. At ward into the zone of prevailing wester- (shallow desert arroyos active only latter part of some localities such as at Thoreau, New lies. As such it is not a lithostratigraphic heavy rainstorms) during the the Mexico, this fluvial influence is present at unit but rather a widely recognized facies, Summerville time. At the type area of intervals throughout 1,20ft (36 m) of sec- on the basis of bedform, with chrono- Tidwell Member in sec. 24 T22S R13E, we tion above the Bluff Sandstone, and thus stratigraphic significance. Thus the time noted minor wadi-type channel deposits part the the eolian Acoma Tongue of the Zuni is boundary need no longer remain "unrec- scattered throughout the upper of locally displaced by these unnamed water- ognizable," as it may now be related to Tidwell in otherwise typically Summer- laid and interdunal deposits. All of the flu- that most basic of all tectonic processes/ ville lithologies. Conversely, the scoured vial facies described here lie below the continental drift, and be regarded as an base of the immediately overlying Salt regional scour surface that marks the base example of event stratigraPhY. Wash Member defines the horizon above of the overlying Morrison Formation. We reject the concept of major eolianites which no Summerville t)?e strata are pre- Thus, the initial correlation of these flu- in the basal Morrison Formation for the sent. Accordingly, we include Tidwell stra- vially influenced strata with Cregory's following reasons: (1) the Morrison type ta in the Summerville Formation (Fig. 68). "Recapture Shale" (Craig et a1., 1955 and section in the Colorado Front Range con- The base of the Morrison in the San Juan subsequent USGS workers) was in error tains no eolianites; (2) the original defini- Basin is similarly placed at the scoured because most of the type "Recapture tion of the "western Morrison" by Gilluly base of a prominent, cliff-forming sand- (1955) Shale" Iies above the regional scour or and Reeside (1928) did not include any stone that since Craig et al. has been secruence boundarv. In contrast all the eolian strata; (3) the base of the Morrison called "Westwater Canyon Sandstone (Fig. 7). This scour surface also strita called "Recapture" in the San Juan as described by Gilluly and Reeside (1928) Member" Basin by the USGS (Harshbarger et a1., can be projected into the San Juan Basin of marks an abrupt change in grain size and 1957; Condon and Huffman, 1984; Con- New Mexico and is clearlY above the sedimentary characteristics. Most diag- don and Huffman, 1988) demonstrably crossbedded eolian strata recognized by nostic is the appearance of pebbly sand- fluvial underlie the regional scour-sequence Condon (1985) and by Anderson and stones, clay clasts, rip-up clasts, boundary (Fig. 6) that marks the basal Lucas (1992);and (4) from a lithologic and crossbedding, and smectitic clay in the Morrison. a paleoclimatic perspective, the eolianites interbedded mudstone (overbank) de- New perspective on the correlation of are clearly related to San Rafael Group posits. As such the scour surface is every- rocks associated with the San Rafael arid depositional svstems rather than to where recognizable, commonly has a thin, Group-Morrison contact has come from Morrison Formation fluvial systems. Thus irregular pedogenic carbonate unit, and observations regarding the distinctively we recognize the distinctive crossbedded can be correlated with the base of the dis- crossbedded eolianite at the top of the strata of the Acoma Tongue of the Zuni tinctive Salt Wash Member into the Four Bluff Sandstone. Noted by Anderson and Sandstone as far northwestward as Bluff, Corners area. ]ust south of Beclabito, New Lucas (1992) as the Acoma Tongue of the Utah, where it is coeval and intertongues Mexico (Fig. 1), near the Four Corners, the

NewMexico Geology August 1995 FIGURE 7-Scour surface at base of Salt Wash Member of Morrison Formation (A) near Church Rock, New Mexico, and (B) in hogback south of Gallup, New Mexico.

so-called three-part Salt Wash Member in the San ]uan Basin. We adopt this The Salt Wash Member thins gradually (William Chenoweth, oral comm , 1993) nomenclature but do not ignore the fact though nonuniformly southwestward to a apparently includes a relatively fine grain- that the basal Morrison in the San Juan wedge edge. South of Gallup, along the ed basal unit up to 30 ft (9 m) rhick and Basin has a different source area than the hogback near latitude 35"22', the Salt showing oxidized (red) colors. This unit Salt Wash Member. Source area for the Salt Wash is beveled off by pre-Dakota erosion, likely correlates with the reddish, parallel- Wash was inferred to be to the west in and from this latitude southward the bedded unit that formed the base of south-central Utah (Craig et al., 1955). Dakota rests on Zuni sandstone or older Gregory's "Recapture Shale Member" and Source area for the "Westwater Canvon". Mesozoic rocks. Eastward from the is here reassigned to the San Rafael Group. considered to be an "alluviating disiribu- Thoreau-Crants area the Salt Wash also Thus, (crossbedded, the middle part peb- tary system of braided channels" (Craig et thins, though not uniformly, and at Mesa bly sandstone)of the so-called three-part al., 1955) was inferred to lie to the west Cigante, 30 mi (48 km) west of Albuquer- Salt Wash is the actual base of the Salt and southwest in adiacent Arizona. These que at longitude 107"15', the Salt Wash is Wash Member, in the corrected stratigra- observations are supported in a general sporadically present. The significant aspect phy presented here, and it is that horizon way by paleoflow interpretations of cross- of the Morrison Formation in this east- or that we correlate with the base bedded strata and geometry of the sand- ward direction is that the fine-grained of what has been called "Westwater Can- bodies. upper member, the Brushy Basin, persists yon Sandstone" juan in the San Basin. In addition to the different source area is all the way out to the southern High Although the name "Westwater Canyon the fact that the Salt Wash has a somewhat Plains.Thus the extent of the Brushy Basin Sandstone" was recognizedas duplicaiive different weathering profile than the basal Member approaches500 mi (800km; in an earlier in this paper and consequently rec- Morrison of the San Juan Basin. This is pri- east-west direction, greater than has ommended for abandonment, we none- marily due to a lower ratio of channel- heretofore been acknowledged in the liter- theless point out that the unit referred to deposit thickness to floodplain-deposit ature. This is significant to any undertak- as "Westwater Canyon" in the San Juan thickness in the type Salt Wash; in other ing of a basin analysis study of the upper Basin is coarser grained and does not cor- words, the basal Morrison sandstone in Morrison. relate directly with the "Westwater Can- the southern San Juan Basin characteristi- In the few areas of the southeastern San yon" Member designated in Gregory's cally has thicker channel sandstone de- Juan Basin where a basal Salt Wash Mem- reierence section at the mouth of Re- posits than the Salt Wash. ber is not present, the mapping contact capture Creek. The two may be correlative For these reasons, a case could be made may be based on the lithologic contrast in part, but the base of the San Juan Basin to recognize the basal Morrison sandstone between the greenish, variegated smec- unit is stratigraphically lower and hence of the San juan Basin as a separate mem- titic claystones of the Morrison and the perhaps older than the base of the "West- ber as Smith (1954)advocated, and corre- underlying reddish, silty sandstones of water Canyon" designated at the Recap--is late it with the Salt Wash Member. Smith. the San Rafael Group. In addition, ture Creek section by Gregory. This dinosaur bone fragments are relatively significant; most workers use the Recap- abundant in the Morrison of this area,but ture Creek reference section as the basis none have been reported locally in the for correlations southward into the San San Rafael Group. Juan Basin primarily because (1) the type "Westwater Canyon Sandstone" did not Conclusions have an exposed base and was more diffi- lithologies are not dissimilar to those of We recognize a two-part Morrison For- cult to access, and (2) exposures are not so the Salt Wash Member. We, accordingly, mation throughout northwestern New good or extensiveas they are at the mouth have chosen to simplify the nomenclatuie Mexico and southeastern Utah. Previous of Recapture Creek. and here term the basal Morrison sand- attempts to recognize and correlate Greg- Having said this, the logical . stratigraph- stones of the San Juan Basin the Salt Wash ory's (1938) four divisions of the Morrison ic procedure would be to apply thsname Member pending further study of their in Utah and New Mexico were met with Salt Wash to the basal Morrison sandstone internal stratigraphy and lithology. failure for the following reasons: (1) his

August 1995 New Mexico Geology Petroleum basal Bluff Sandstone Member is basically phv alluded to above relates to the con- Colorado: American Association of Geologists, Bulletin, v 25, pp. 1745-1767 a fine-grained eolianite with affinities to ieft ttrat this part of the North American Gregorv H. E., 1938, The San Juan Country: U.S. the San Rafael Group, and it demonstrably cohtinent drifted into the zone of prevail- Giological Survey, Professlonal Paper 188, 123 pp. lies below a regional unconformity and/ ing westerlies at the close of San Rafael Hackman, R. ]., and Olson, A. B , 1'977,Geology, strtc- or paleosol that signals the onset of fluvial Group deposition (late Middle jurassic) ture, and urmium deposits of the Gallup 1' x 2' deoosition under a more mesic climatic and eolianites of that age reflect this in the quadrangle, New Mexico and Arizona: U S. Geological Survey, Map I-981, 1:250,000. regime; (2) his type "Recapture Shale prominent east-dipping foresets. Harshbarger, W, Reprenning, C.A, and Irwin, ]. H., AcrNowlnpcMENrs-The manuscript J. Member" included sabkha deposits at the 1957 , Stratigraphy of the uppermost Tiiassic and the top of the San Rafael Group, the unconfor- was reviewed and improved by reviews Jurassic rocks of the Navajo country: U.S. Geological mity at the base of the Salt Wash Member, from William Chenoweth, Adrian Hunt, Survey, Professional Paper 291,71 pp. and the entire Salt Wash Member; (3) his and Frank E. Kottlowski. Lyrrne Hemen- Harshbarger, J. W, Repeming, C. A., and Jackson, R. 1951, stratigraphy of the Navajo country; type "Westwater Canyon sandstone mem- way and Terry Telles processed the manu- L., Jurassic ir Smith, C. T., and Silver, C. (eds.), San Juan Basin: ber" is demonstrably correlative with the script, and Rebecca Titus drafted the New Mexico Geological Society, Guidebook 2, pp. Salt Wash Member, so the name "West- illustrations. We are grateful to all these 95-99. water Canyon Member" is superfluous; people for their assistance. The New Hintze,L. F., and Stokes, W. L., 1954, Geologic map of and (4) most of the type "Recapture," by Mexico Bureau of Mines and Mineral southeastern Utah: University of Utah, 1:250,000. Sumary of history in the original definition, lies above the regional Resources and the New Mexico Museum lmlay, R., 1954, Jurassic western interior of the US.: Billings Geological unconformity, whereas the Recapture unit of Natural History and Science supported Society, Third Amual Field Conference, pp.79-85. that was correlated into the San Juan Basin this research. Kocurek, G., and Dott, R. H., Jr., 1983, Jurassic paleo- lies demonstrably and entirely below this geography and paleoclimate of the central and regional unconformity and hence is part of References iouthem Rocky Mountains; ln Re1mo1ds,M. W, and Dolly, E. D. (eds.), Mesozoic paleogeography of 1993, Zuni Sandstone and Acoma the San Rafael Grouo. Anderson, O.1., west-central United States, Rocky Mountain Section, Mexico Geology, voI. 15, no. 2, Recapture and Westwater Canyon are Tongue defined: New Society of Economic Paleontologists and Mineralo- thus superfluous names as members of oo.38-39. gists, pp. 101-116. Anderson, O. J., and Lucas, S. G., 1992, The Middle Kowallis, B. I., Christiansen, E. H, and Deino, A. L., the Moirison Formation in northwestern Summerville Formation, northern New Jurassic 1,991, Age of the Brushy Basin Member of the New Mexico. The valid name Salt Wash Mexico: New Mexico Geology, v 1'4,no. 4, pp. 79-92 Morrison Formation, , western Anderson, O. J., and Lucas, S. G., 1994, Middle Membcr pertains to the prominent pebbly USA: Research, t 12, pp 483493 Jurassic stratigraphy sedimentation and paleogeog- sandstone at the base of the Morrison T, 1912, Oil and gas near Green River, raphy in the Colorado Plateau and Southern High Lupton, C. Grand County, Utah: U.S. Geological Survey, Formation throughout its area of western Pliins; ln Caputo, M. V, Peterson, J. A., and distribution, and it is overlain by and Franczyk, K. (eds.), Mesozoic Systems of the Rocky Bulletin 541. intertongues with the distinctive, clay- Momtain region, U.S.A.: Rocky Mountain Section, Moench, R. H., 1963, GeoloSlc map of the Lagma quadrangle, New Mexico: U.S. Geological Suruey, stone-dominated (smectitic) Brushy Basin SEPM (Society for Sedimentary Geology), pp. 299-3L4. Map GQ-208. Member. In the more distal areas such as Brenner, R. L.,1983, Late Jurassic tectonic setting and O'Sullivan, R. B., and Beikman, H M , 1'963,Geology, at Mesa Gigante in the southeastern San paleogeography of Western lnterior, North America; structure, and uranium deposits of the Shiprock Juan Basin, the Salt Wash Member is not ln Re;molds, M. W, and Dolly, E. D. (eds.), Mesozoic Quadrangle, New Mexico and Arizona: U.S. present, and the Brushy Basin Member paleogeography of west-central U.S: Rocky Geological Suruey, Map I-345, 1:250,000. Mountain Section, Society of Economic Paleon- 1988, Stratigraphy and nomenclature of rests on a more profound unconformity Peterson, F-, tologists and Mineralogisis, pp. 119-132 Middle Jurassic rocks, western Colorado Plateau, marked by a well-developed paleosol. Chenoweth, W. L., 1953, The Variegated Member of Utah, and Arizona: U.S. Geological Suruey, Bulletin The name Recaoture Member is retained the Morrison Formation in the southeastern part oi 1633-8,pp.17-56. New Mexico: as a subdivision bf the Bluff Sandstone in the San ]uan Basin, Valencia County, Peterson, F, and Tumer-Peterson, C. E., 1987, The Unpublished M.S. thesis, University of New Mexico, southeastern Utah to define a 40-55-ft- Monison Formation of the Colorado Plateau; recent Albuquerque,86 pp. advances in sedimentology, stratigraphy, and paleo- -m-)thick, of the (12-17 fine-grained, parallel- Condon, S. M., 1985, Lithologic descriptions tectonics: Hmteria, v.2, no. 1, 18 pp. selected intervals of Jurassic rocks in southeastern bedded silty sandstone. This sandstone Rappaport, I., Hadfield, J. P, and Olson, R. H., 1952, Utah, northeastern Arizona, and northwest New represents local sabkha conditions at the rocks of the Zmi uplift: U.S. Atomic Energy Mexico: U.S. Geological Survey, Open-file Report Jurassic Commission, Rept. RMO-642,55 PP. close of San Rafael Group deposition. As 85-223,100 pp. Ridgley, L.,1989, Trace fossils and mollusks from the so defined the Recapture Member repre- Condon, S. M., and Huffman, A. C., Jr', 1'984, J. upper member of the Wanakah Formation, Chama sents nothing more than an upper unit of Stratigraphy and depositional environment of Bliin, New Mexico-evidence for a lacustrine ori- Jurassic rocks, San ]uan Basin, New Mexico, with the Summerville Formation. However, gin: U.S. Geological Suruey, Bulletin 1808-C, pp. emphasis on the south and west sides: Rocky c11L6. because these strata formed the base of Mountain Section of Geological Society of America, in the Gregory's type Recapture Member, we Field Trip Guidebook, 37th Annual Meeting, Saucier, A. 8., 1967, The Morrison Formation Zuni, have chosen to perpetuate the name. Durango, Colorado, pp. 93-148. Gallup region; ln Trauger, F. D. (ed.), Defiance, 'l'988, Taylor Region: New Mexico Geological Society, Condon, S. M., and Huffman, A. C., Jr., Revision Mt. The distinctively crossbedded upper Guidebook 18, pp. 138-144. in nomenclature of the Middle Jurassic Wanakah oart of the Bluff Sandstone contains thick Formation, northwestem New Mexico and north- Smith, C. T., 1954, Geology of the Thoreau quadrangle, sets of east-dipping foresets and has an eastern Arizona: U.S. Geological Suruey, Bulletin McKinley and Valencia Comties, New Mexico: New intertonguing relationship with the 1.633-4,1-2pp. Mexico Bureau of Mines and Mineral Resources, Recapture Member. The crossbedded unit Condon, S. M., and Peterson, F, 1986, Stratigraphy of Bulletin 31,36 pp. rocks of the San L., 1944, Morrison md related deposits in is reiognized in the San Middle and Upper /urassic ]uan Stokes, W southern Juan Basin; historical perspectives, current ideas and an adjacent to the Colorado Plateau: Geological Basin in similar stratigraphic position, remaining problems: American Association of Society of America, Bulletin, w 55,pp.951-992. where it is designated as the Acoma Petroleum Geologists, Studies in Geology no.22,pp. Thaden, R. E., Santos, E. S., and Ostling, E. J.,'1967, Tongue of the Zuni Sandstone. This unit is 7)6. Geologic map of the Dos Lomas quadrangle, New Craig, L. C., et al., 1955, Stratigraphy of the Morrison Mexico: U.S. Geoiogical Suruey, Map GQ-680 more significant in terms of event stratig- regiory a and related formations, Colorado Plateau Turner-Petersory C. E., 1986, Fluvial sedimentology of raphy than as strictly a lithostratigraphic Bulletin preliminary report: U.S. Geological Suruey, a major uranlum-bearing sandstone-a study of the unit, and thus we will leave the matter of 1009-E, pp. 125-168. Westwater Canyon Member of the Morrison setting of Arizona nomenclature for this unit in Utah to local Dickinson, W. R., 1989, Tectonic Formation, San Juan Basin, New Mexico: American Reynolds, S. mappers or investigators. We cannot ac- through time; in lenney, J. P, and J. Association of Petroleum Geologists, Studies in (eds.), Geologic evolution of Arizona; Arizona Geology no.22, p. 60. cept the proposal by previous workers Geological Society Digest, no. 17, pp. 1.-16. Waldschmidt, W. A. and LeRoy, L. W, 1944, Recon- (Condon the gra- Gilluly, and Reeside, 8., 1928, Sedimentary and Peterson, 1986) that J., 1. Jr., sideration of the Morrison Formation in the tlpe rocks of the San Rafael Swell and some adiacent dational base of this crossbedded sand- area, Jefferson County, Colorado: Geological Society areas in eastern Utah: U.S. Geological Survey, 'l'097-1114. stone represents an unrecognized time of America Bulletin, v 55, pp. tr Professional Paper 150, pp. 61-84. boundary that defines the base of the Goldman, M. I., and Spencer, A. C., 1941', Correlation Morrison Formation. The event stratigra- of Cross' La Plata Sandstone, southwestern

New MexicoGeology August 1995