GEOLOGY AND COAL RESOURCESOFTHE LAKE QUADRANGLE, ATARQUE CIBOLA COUNTY, NEW MEXICO NEW MEXICO BUREAU O F MINES AND MINERALRESOURCES OPEN-FILEREPORT 167 bY O R I N J. ANDERSON J u n e , 1982 ( r e v i s e d 1, 9 8 3 ) Contents: (1) D i s c u s s i o n of G e o l o g y and C o a lR e s o u r c e s (attached) ( 2 ) G e o l o g i c map w i t h cross sesction (accompanying) T a b l e of C o n t e n t s GEOLOGY General P. S t u d y Area P. Structure P. Stratigraphy P. COAL RESOURCES p. 22 REFERENCES p. 23 Figures 1 - Measured s e c t i o n of Zuni and Dakota SS. p. 11 2 - M e a s u r e d s e c t i o n of A t a r q u e and Moreno H i l l Fms. p. 18 GEOLOGY General The Atarque Lake quadrangle l i e s i n t h e s o u t h w e s t e r n p a r t o f t h e Z u n i Basin, a b r o a d ,s h a l l o ws t r u c t u r a le l e m e n tt h a te x t e n d s southwestward from the Zuni Mountains of c e n t r a l Arizona. New Mexico i n t o e a s t - A s such it l i e sn e a rt h es o u t h e a s t e r nm a r g i n oftheColoradoPlateau. is The r e g i o n a ld i pi nt h es t u d ya r e a very gently northeastward toward the Gallup Sag which comprises thenortheasternpartoftheBasin.Thereare,however,broad, g e n t l e NW-SE t r e n d i n g f o l d s d i p s , and a t l e a s t which r e s u l t i n l o c a l southwestward up on t h e twoabruptmonoclinalflexures, n o r t h e a s ts i d e ,( o p p o s e dt or e g i o n a ld i p )t h a tc l e a r l yd e f i n et h e NW-SE structuralgrainofthearea,aswellas minor f a u l t i n g . These s t r u c t u r a l t r e n d s p a r a l l e l t h e a x i s o f t h e Zuni U p l i f t , b u t perhaps more i m p o r t a n t l y t h e y a p p e a r t o r e p r e s e n t the southeastward extension of the structural axes that around the southern andKiven (1975), wrap end of t h e D e f i a n c e U p l i f t , a s shown by Davis and t h e y a l s o a l i g n v e r y w e l l w i t h t h e northwest-trendingdikesystemintheFence quadrangle),Techado, Lake ( t h e Dyke Adams Diggings,andPietownareas. Of t h e t h r e e s t r u c t u r a l f e a t u r e s monoclines and f a u l t i n g - - broadfolds, monoclines are the most pronounced s i g n i f i c a n tn o to n l yl o c a l l y , Kelley (1955) s t a t e d ," I n mentioned b u t i n a much broadercontext. and As many r e s p e c t st h em o n o c l i n e sa r et h e Most of p r i n c i p a ls t r u c t u r a lf e a t u r e so ft h eC o l o r a d oP l a t e a u . thedeformationhasoccurredalong them." T h e i rs i g n i f i c a n c e is tied to the concept of northeast-southwest horizontal compression 1 -~ ~ of the Colorado Plateau during the Laramide. As a response to the regional compression, discrete, individual basement blocks were uplifted by reverse movements fracture zones (Davis, 1978). along segments of high-angle These reverse faults produced monoclinal folding in the overlying strata. Groupingsof these monoclines, some of which length, which have when been attempted related Precambrian may to basement the into be on traced the over 100 basis of similarity fracture a for miles of trends, zones to serve subdivide mosaic of crustal in the blocks. The area under consideration is underlain (1) by Cretaceous sandstone and shales that may exceed 500 ft in thickness where the Moreno Hill Formation (new name) is present, ( 2 ) highly cross bedded eolian sandstoneof the Jurassic that total about 140 ft in thickness, and ( 3 ) the Triassic shales, mudstones, siltstones, and sandstones that make up the Chinle Formation, which is estimated to be 800 to 1000 ft thick in this area. A 6 ft thick flat bedded top of the Chinle Fm crops out at one locality and represent present as silty sandstone the Rock Point a much thicker and sandy Member of the unit siltstone Wingate both to present the is thought to Sandstone the at north which and is southeast. In addition a1400 ft section of upper Paleozoic rocks capped by the San Andres locally as indicated by Atarque Lake quadrangle. an Limestone oil test 10 occurs in the subsurface miles to the north of the (Hole is in NE 1/4 sec. 5 , T 9N R 18W, drilled in 1963 by Cities Service Oil Co.; Maxwell and Nonini, 1977). This results in a cumulative sedimentary section of approximately 2900 feet overlying 2 the Precambrian basement in thoseareas where t h e Moreno H i l l Formation i s p r e s e n t . Major u n c o n f o r m i t i e s i n t h e s e c t i o n o c c u r a t t h e s y s t e m i c boundaries - t h eJ u r a s s i c / T r i a s s i c , and t h eC r e t a c e o u s / J u r a s s i c . i s aneolian The Upper J u r a s s i c ZuniSanndstone on t h e Rock P o i n t Member of fluvialunitdepositedunconformably t h e WingateFormation. and p a r t l y The o v e r l y i n g DakotaSandstoneformsthe basal Cretaceous rocks which consist of the nonmarine, marginal marine, and marinesandstones front of and s h a l e s t h a t were deposited in and immediately behind the shoreline of the advancing Western I n t e r i o r Seaway. I nt h i sa r e at h e unconformably on t h e ZuniSandstone, progressivelyolderrocks;inthe Dakota r e s t s b u t southward it r e s t s on Zuni S a l t Lake a r e a t h e Dakota r e s t s on theChinleFormation. Scattered remnants of the areapresentthroughoutthearea Creek on thesouthtoPinehaven Upper Tertiary Bidahochi Formation from t h e n o r t h s i d e o f C a r r i z o on thenorth. apron of alluvial material deposited along fluvial axis in the I t r e p r e s e n t s an a northwest trending upperreachesoftheancestral ColoradoRiverdrainagebasin.IntheAtarque formation consists of sandstones Little Lake a r e a t h e and coarse conglomerate largely of volcanic material derived for the D a t i l , Mangas, and GalloMountainssouth composed most p a r t from t h e and e a s t o f Quemado, a s w e l l a s some reworking of older coarse grained fluvial deposits on t h e Zuni P l a t e a u and SantaRita Mesa. Northofthevillageof Zuni and i n t h e P i n e h a v e n a r e a t h e p r e s e n c e o f g r a n i t i c d e b r i s i n theFormationindicates Mountains(Repenning, a sourceareainthegranitecored e t .a l . , 1958). 3 Zuni Study Area Structure The narrow, l i n e a r zone of deformation that trends northwestward through the southwestern corner of the Atarque Lake q u a d r a n g l e c a n b e t r a c e d f o r a d i s t a n c e o f a b o u t 1 5 m i l e s a n di s t h e mostprominent and s i g n i f i c a n tl o c a ls t r u c t u r a lf e a t u r e . h a sb e e ni n t e r p r e t e da s It a f a u l t by v a r i o u s i n v e s t i g a t o r s ( s e e Sears, 1925, p. 23) and i s shown a s a f a u l t on t h e Geologic Map of New Mexico (Dane and Bachman, 1965),upthrown on t h e e a s t bringingTriassicChinleagainstCretaceousrocks. investigationhas The p r e s e n t i s more than 100 shown, however, t h a t t h e r e f e e t of Jurassic sandstone cropping out i n a hogbacksequence t h a t d i p s f r o m 140 t o 460 t o t h e s o u t h w e s t a n d d e f i n e s t h e z o n e of maximum d e f o r m a t i o n : t h e d i p s d e c r e a s e r a p i d l y t o t h e SW. A t t h eb a s eo ft h e Chinle Formation - a f a u l tc o n t a c t . i s i n t e r p r e t e d as anabruptmonoclinal h e r e i n named the Atarque Monocline, related to anglereversefaultalong i nt h e a high a northwesttrendingfracturepattern Precambrianbasement t h es o u t h e r n the The c o n t a c t w i t h t h eo v e r l y i n gJ u r a s s i c i s a d e p o s i t i o n a lo n e ,n o t T h e r e f o r e ,t h es t r u c t u r e flexure theupperpartof and t h e Rock Point Member of the Wingate Formationareexposed. ZuniSandstone hogbacksequence NE and (see c r o s ss e c t i o n and westernpartofsec.10, A-A' on map). In T. 6N, R 19W, anabrupt change in strikeofthemonocline fromN 750WtoNN 200Wtakes place in a very short distance:, one mile t o the southeast of this s t r i k e change t h e monocline i s buried under doesnotappearagainas a distinctfeature. 4 a basalt flow and The southwest-facingAtarqueMonocline A s t h eu p p e r( a n t i c l i n a l ) r e g i o n a ld i p . limb hasbeenerodedoff a x i s of t h e f e a t u r e , a s l i m b i s concealed,the and thelower i s opposed t o t h e shown on t h e accompanying map, i s a r b i t r a r i l y p l a c e d a t o r n e a r Z u n i Sandstone. t h eb a s eo ft h e convergeswith The axisofthemonocline a s y n c l i n a la x i st ot h es o u t h e a s t .T h i s may i s dying o u t i n t h a t d i r e c t i o n , indicatethestructure the dips are not decreasing b a s a l t flow. McLelland, however, where t h e monoclinepassesunderthe Haschke, and Robinson, (1981) have indicated extensive faulting within and s o u t h o f t h i s b a s a l t f l o w on t h e Rincon Hondo quadrangle. Othernotableaspectsofthemonoclineare t h ep r e s e n c eo fc r o s sf o l d s . more o r l e s s The a x e so ft h e s ec r o s sf o l d sa r e normal t o the monoclinal axis across the monocline, on theVenadito i t s s i n u o s i t y and and appear t o extend b u t a r e much more i n e v i d e n c e t o t h e west Camp quadrangle(Anderson,1982a).Thesecross folds probably represent deformation along basement fractures of a d i v e r g e n tt r e n d .I nd i s c u s s i n gt h ec r o s sf o l d i n g characteristic of the ColoradoPlateau stated that the inherent nature of and s i n u o s i t y monoc1ines;Davis (1978) basement f r a c t u r e zones envisionedforthePlateau,coupledwiththerecurringnatureof movements w i t h i n t h e s y s t e m would t e n d t o f a v o r t h e crossfoldinginthevicinityoffracture Folds created by movement along one fracture t r a n s f o r m e di n t o phenomenon of zone i n t e r s e c t i o n s . zone may be domes, saddles,ordoublyplungingfoldsby superposedorcontemporaneous movement along a f r a c t u r e zone of d i f f e r e n t strike. 5 In the adjacent Venadito associated with the monocline Camp quadrangle the cross folding and i t s a s s o c i a t e d d e p r e s s i o n on t h e downwarped side has produced a series of structural basins and saddles.IntheAtarque Lake q u a d r a n g l et h el a r g e s t topographicfeatureproduced by t h e c r o s s f o l d i n g i s the west f a c i n g mesa i n t h e S 1 / 2 s e c . 5 and t h e S E 1 / 4 s e c . 19W. This mesa o f f e r st h eo n l ye x p o s u r e so ft h e Tongue o f t h e 6, T 6N R RioSalado Mancos Shale on the quadrangle. Mesa Colorado i n t h e n o r t h e a s t c o r n e r o f t h e q u a d r a n g l e d o e s t o crossfolding;rather notappeartoberelated partofthesouthwest predominant NW-SE it r e p r e s e n t s a limb of a s y n c l i n e t h a t p a r a l l e l s t h e s t r u c t u r a lg r a i no ft h ea r e a . The morphology and r e l i e f on t h e mesa i t s e l f i s p a r t l y t h e r e s u l t o f l o c a l v a r i a t i o ni nt h i c k n e s so ft h e offoldaxes ZuniSandstone. The sinuousnature is evidentinthisnortheastarea;theaxisofthe synclinenortheastof Mesa Colorado and t h e m o n o c l i n a l a x i s t o t h es o u t h w e s ti ns e c s .1 3 b u t a r e of and 14arebothcurved, d i v e r g e n tt r e n d ,i n d i c a t i n gt h e ya r es e p a r a t es t r u c t u r e s . F a u l t i n g on the quadrangle is limited to the minornorth- south trending normal fault at the southwest corner of the Colorado.Here thewestblockhas movedup30 Mesa t o 40 f t r e l a t i v e t o t h e e a s and t the middle shale unit of the Dakota Sandstone has been juxtaposed against the upper marine sand unit of the east The senseof side. movement a l o n g t h e f a u l t i s easilyconfused from a d i s t a n c e . In addition minor faults are likely in the highly fractured area in the E 1/2 s e c 10, T 6N, R 19W where t h e s t r i k e o f t h e 6 AtarqueMonoclinechangesabruptly. A 3 f tt h i c kl i g h tg r e e n i s h graybentonitjr: clay found atone locality j u s t west ofthe road i n t h e SE 1 / 4 SW 1 / 4 s e c l 0 p r e s e n t s a s t r a t i g r a p h i c p r o b l e m t h a t i s included i s p r o b a b l yr e l a t e dt of a u l t i n g .A l t h o u g ht h ec l a y i n an area of Atarque Sandstone outcrops because of scale l i m i t a t i o n s it i s u n l i k e l y t h a t Outcrops a r e p o o r it belongs in the Atarque. and s t r u c t u r a l r e l a t i o n s h i p s remain somewhat uncertain at this locality. Stratigraphy Although the Chinle Formation s t u d i e di nd e t a i l . maroon,and was mapped, t h i s u n i t was not The upper p a r tc o n s i s t s ofpurplishred, graysiltstone,mudstone,claystone, and t h i n sandstonesthatareassignedtothePetrifiedForest Stewart,Poole, o ft h eC h i n l ea t S t . Johns Ariz. andWilson ( 1 9 7 2 ) described and measured s e c t i o n s t w o n e a r b yl o c a l i t i e s . A t one l o c a l i t y n o r t h o f between 34042'N and 34046'N, t h e upper p a r to f the Petrified Forest Chinlesequencethat Member i s 591 f e e t t h i c k w i t h i n i s 827 f e e tt h i c k . The thicknessoftheChinle a total A t t h eo t h e rl o c a l i t y , near Zuni, New Mexico, 178 f e e t o f P e t r i f i e d F o r e s t present. Member. on theAtarque quadrangle w a s n o t d e t e r m i n e d i n t h e p r e s e n t e s t i m a t e d t o be 800 t o 1000 f t thick, based Member i s Lake work, b u t i s on breadth of outcrop and i n f e r r e d d i p s on t h e u p p e r l i m b o f t h e m o n o c l i n e , a n d t h e absenceof San AndresLimestoneoutcrops.O'Sullivan (1977) showed t h e 400 m ( 1 3 2 0 f t ) i s o p a c h o f t h e C h i n l e p a s s i n a g few milestothenorthoftheAtarquearea;theChinle thesouthinthisarea. 7 is thinning to As previously mentioned, 6a ft thick bed of reddish brown, flat bedded, silty sandstone is exposed at the base of south the end of Mesa Colorado in sec 18 T 7N, R18W, resting on Chinle Formation mudstones and shales with no apparent break in deposition. It is considered to be the equivalentof the Rock Point Member of the Wingate Sandstone described by Harshbarger, Repenning, and Irwin, (1957,p. 8) as a pale reddish brown silty sandstone and sandy siltstone at the type locality near Rock Point, Arizona. They further described it as a quiet water deposit that - Point Lagoon accumulated in a southward plunging - basin Rock fed by streams entering from the northeast, east, of the lagoon and southeast. An embayment at the eastern margin is shown extending across the New Mexico state line in the Zuni- Atarque area. Southeastward from the Mesa Colorado exposure the Rock Point Member thickens before pinching out. An area of excellent exposures de Yeso called and 30 ft of the Los Fence flat Pilares Lake bedded at the quadrangales silty boundary is sandstone between described as comformably the Mesita having nearl overlying the Chinle and overlain by the largely eolian Zuni Sandstone (Anderson, 1982b). The white, fine-grained, highly cross-bedded sandstone comprising the uppermost Jurassic rocks and outcropping in monocline is considered to be the Zuni Sandstone. The Zuni Sandstone, as redefined by Anderson (1983), consists of the undivided equivalents of the Entrada andCow Springs Sandstones. et al, (1951), The name Cow Springs was proposed by Harshbarger 8 the Fig. 1 Measured section Zuni Sandstoneand Dakota Sandstone in E+ NWk sec.18, T 7N R 18w scale (ft.) : . L Pale 'yellowish brn to pale red. brn, fn to med. gr. ss, with thin pebble cyl. zones; burrowed bioturb. in low. part; 10 ft thick white& purple well indurated s s in upper part overlain by erosional surface with pebble cgl. I- Dakota Sandstone - re. ~ u v ~ a ~ ~ Lt gray & It. gray. brn, paludal shales, arenaceous shales and siltstones, and carbonaceous clay; interbedded fn sandy siltst and silty clay in lower part overlain'by carbonaceous zone;' 2 ft thick burrowed and root penetrated sandstone near ' 50 I up ~ " ~ _I 2 I- I & paludal Lt. brn to paleyell. brn up fn- low mediumgr s s ; :trough cross bedding and channel scours; clay galls in lower part. 75- \ - " . ... I loo-. aeolian 125- 150- .Zuni Sandstone "" . .... .:...: _I ,. :: " 1.75- I I Fluvial and aeolian .. . Pinkish gray and grayish orange pink lower .._.. fn grs s ; planar and .trough cross bedded ....... ..... J:,:;::!,?.: except for ext&eme basal portion; thin I con'glomeratic zones in fining upward 'i""'"'"'" channel fill sequences in middle and lower ";..---4--,I Durrowea bioturbated -root tube -tj . *.i.. A r i p p l e marks , ., ..: ., C a r b o n a c e o u s zon - p.,y. ,.:-, ... . . I .. 'Roc% Point Member .reddish brown silty ss; flat bedded. . . for the greenish gray to light yellowish gray, fine grained, cross bedded sandstone that weathers to grayish white rounded cliffs. At the type section 4 miles east ofCow Springs, Arizona, it is 342 ft thick and is unconformably overlain by the Dakota Sandstone. Additional work by Harshbarger, et a1 (1957), extended theCow Springs into the Zuni Reservation of 449 a ft thick section at Black Rock was New Mexico and designated terminology asCow Springs Sandstone, unconformably overlain by the Dakota Sandstone. On the Zuni Reservation and southward it becomes difficult underlying to Entrada distinguish snadstone and the Cow thus name the Springs Zuni from the Sandstone (first used by Dutton, 1885) has been applied to this interval by Anderson (1983). notch, Basin The Zuni sandstone, whichcommonly has a medial maybe traced quadrangle in to outcrop Mesa southward Colorado on the through Atarque although it thins considerably two at localities 19W and sec 31T 8N R 18W. southward the contact - the Lake Plumasano quadrangle, sec. 35 T 9N R From the Lupton, Arizona area between Cow the Springs and the underlying Entrada was reported by HarshbaFger, eta1 (1951) as "unidentifiable." Hackman and Olson (1977) used the term Cow Springs-Entrada (JCSe) for this interval in the Zuni area. Northwest Mesa the serve to of the Summerville divide study and area the the Zuni.into its toward Bluff Fort Formation component Defiance are - and present parts Entrada Black and and Cow Springs. Northward and northeastward near Todilto Park, the upper part of the Zuni,i.e. Cow Springs, appears to intertongue with the westwater Canyon Member 10 of the Morrison Formation is and considered tobe an eolian deposit contemporaneous with the fluvial Morrison (Harshbarger, et.al.,1957, p. 50). In the Atarque Lake area the "medial notch", which is thought the to represent the break between Cow Springs equivalent, is not as the Entrada equivalent and well defined as it is at Zuni Pueblo. It can, however, be identified at the north endof NW1/4 sec 6, T 7N R 18W, and to a lesser Mesa Colorado in the extent at the south end in sec 18, 7N RT 18W. At this southern location bedded the ledge medial break is represented 1 by ft athick, cross forming sandstone, somewhat coarser grained than the adjacent units (Fig.1). The lower portion ofthe Zuni(be1owthe medial notchor break) contains scattered conglomeratic zones at the of base thin channel fill sequences (Fig. 1). chert and quartzite pebble It also contains a well rounded conglomerate that is well exposed in the NE1/4 sec6, T 7N R 18W. The conglomerates represent atypical facies for Entrada equivalent rocks, but fluvial facies are not unknown in the Entrada. These conglomerates are probably related to streams traversed this northward course medial silty which originated area of largely toward member of eolian the the in the Mogollon sedimentation on embayment or lagoon Entrada had been Highland and a in which the deposited (Harshbarger and others,1957, p. 44-46). Cretaceous rocks in the area consist in ascending of order the main body Sandstone-Mancos of the Shale Dakota sequence, Sandstone, the Rio an intertongued Salado Dakota Tongue of the Mancos (new name), the Atarque Sandstone (new name), and the coal-bearing Moreno Hill Formation (new name). The rocks called 11 t h e mainbody of the Dakota Sandstone are the marine, marginal marine, andnon-marinerockstha Cretaceoussequence makeup i nw e s t - c e n t r a l t h e lowest p a r t of t h e New Mexico (Hook,Cobban, Dakota i s comprisedof Landis,1980).Inthestudyareathe basalcross-bedded,fluvialsandstoneofvaryingthickness, paludalshalesequencecontainingcarbonaceousshale thin coal beds, and marginalmarine form theupper 2 5 f t of t h eu n i t( f i g . locally in the and a a and very and m a r i n e s a n d s t o n e s t h a t 1). F o s s i l sc o l l e c t e d upper marine sands include the bivalves Pycnodonte c f . P. k e l l u m i andExogyra l e v i s , and T u r i t e l l a s p, a n dv a r i o u s " othergastropods. The Dakota i s o v e r l a i n by t h el o w e rp a r to f t h e Mancos Shale,an18-foot-thickarenaceousshalethat is exposed only in the west-central portion of the quadrangle: elsewhere it i s coveredorunrecognizable. This shaletongue probably eqivalent to the Clay is Mesa Tongue which h a s a t y p e s e c t i o n d e s i g n a t e d i n t h e Laguna, New M e x i c o a r e a ( a s d o m o s t o f t h ei n t e r t o n g u e d Cobban, 1973). Dakota-Mancos units)(Landis, Dane and However, becauseofthepinchoutofanunderlying sandstonetongue(theCubero) a few milessouthwestof Laguna, t h e t e r m C l a y Mesa c a n n o t b e e x t e n d e d i n t o t h e s t u d y a r e aand Mancos Shale'' i s used hence the informal term "lower part of the h e r e (Hook, Cobban, and Landis, 1980). Overlying t h i s s h a l e Sandstone; it c o n s i s t so f i s t h e Paquate Tongue o f t h e a 20-foot-thick,massive,cross-bedded, coarsening upward s a n d s t o n e u n i t t h a t zone a t t h e t o p Dakota commonly h a s a f o s s i l h a s h composed a l m o s t e x c l u s i v e l y o f Pynodonte c f . P. kellumi. Large Exogyra l e v i s and brown f e r r u g i n o u s / c a l c a r e o u s concretions may also be present in the upper part. The tongue characteristically color, but near weathersto a the Atarque light tan or pale Monocline it grayish may be orange reddish brown. Topographically, the Paguate is generally a small cliff former, but outcrops are very restricted on this quadrangle. Northward the unit merges with the main body of the Dakota, as the lower part of the Mancos pinches out. Near the village Two of Wells, 14 miles due northof Zuni, at the type localities of the Twowells Tongue of the Dakota and the Whitewater Arroyo Tongue of the Mancos, the Paguate is not present as a lithologic unit distinct from the main body of the Dakota (Hook, Cobban, and Landis, 1980 ) The . Whitewater Arroyo Tongue of the Mancos Shale overlies the Paguate Tongue throughout the Zuni Basin. The name was proposed by Owen (1966) who applied it to a "well defined, persistent tongue of marine shale separating Twowells the (Tongue) from the rest of the Dakota Sandstone in the southwestern area). part of the San Juan Basin" (and in the Gallup Sa A type section was designated in Whitewater Arroyo in section17 o f T 12N, R 1 9 W near the village of Two Wells, where it is 80 feet thick and described as a gray to olive-gray, silty, oyster-bearing shale. It does not crop out in the present study area but it consists of an estimated 55 feet of medium-to dark- gray fissile shale based on outcrops in adjacent areas, and contains the relatively large oyster Exogyra trigeri (Coqband) in its middle portion; also very near the middle is a distinctive white to orange-weathering 15-inch thick bentonite bed. Good exposures of the Whitewater Arroyo Tongue and the bentonite 13 ~ bed occur on the adjacent Vennadito Camp quadrangle NE in1/4 the sec 30, T 7N, R 19W, where it is protected by a cover of Twowells Sandstone. The bentonite bed has also been recognized 18 miles to the southwest on the Twentytwo Spring quadrangle, and 75 mi. to the east on D-Cross Mountain where it mayonly be 8 in. thick (Hook, Cobban and Landis1980). The Twowells Tongue of the Dakota Sandstone does not crop out in the study area. It also may be seen on the adjacent Venadito Camp quadrangle where it is about 30 feet thick and consists of (1) a lower very fine-grained, flat-bedded sandstone with a few burrows; ( 2 ) a middle very fine to fine-grained, intensely burrowed and bioturbated sandstone, locally quite friable; in at least one locality it has a flat-bedded, undisturbed, well-cemented, ledge-forming sandstone near the top of the bioturbated beds which in turn is overlain by a 10 ft covered interval probably composed of arenaceous shale or interbedded very fine sandstone and siltstone; (3) the upper part is an upper fine- to medium-grained, planar cross-bedded unit, generally 6 to 8 feet thick, containing clay clasts and, locally, moderate concentrations of burrows including Ophiomorpha. Both the Twowells represent and minor the Paguate progradational Tongues events are in here the considered to Dakota-Mancos transgressive sequence. The Rio SaladoTongueofthe Mancos Shale (new represents a return to open marine conditions or name) an interruption in sediment supply following deposition of the Twowells. It consists of up to 240 feet of medium-gray I 14 and grayish-brown shale, calcareous shale, and thin calcarenites, with an interbedded shale and very fine-grained sandstone sequence at the top where name) it grades into the overlying Atarque Sandstone (new . The name Rio and Cobban (1983). between Salado Tongue is proposed in Hook, Molenaar, It is defined as the shale tongue lying the Twowells Tongue of the Dakota Sandstone and the Atarque Sandstone (the latter unit is also being defined by the same authors) and is coextensive with these two units. The thin calcarenite and calcareous shale beds that occur 30 to 40 feet above the base of the Rio Salado in thi-s area represent the equivalent of the Bridge Creek - Limestone Member of the Greenhorn Formation (Hook, Cobban, and Landis, 1980). The beds may oftenbe recognized atadistance yellow-weathering calcarenites stand inoutcropbecause out in contrast to the the typically gray Mancos Shale. They appear at only one locality on the map-the NE 1/4 sec 31, T 7N, R 19W. Deposition of these beds is related to the very late Cenomanian (Greenhorn) transgressive maximum, an event that wasmarked by the deposition of limestone beds throughout most of the Western Innterior Seaway. These beds form important marker horizons and also contain the guide fossil Pycnodonte newberryi (Stanton) in abundance or at just below this interval (Hook and Cobban, 1977). P. newberryi was collected in - the NE 1/4 sec 31 and NW 1/4 sec 32 T 7N, R 19W and in the SW 1/4 sec 11 T 6N, R 19W. In the subsurface the Bridge Creek beds may. be recognized by the distinctive resistivity the e-log, which is helpful in correlations. kick they produce on L About 100 feet above the Bridge Creek equivalent, limestone concretions appear in the section; examples may be found in the SE 114 sec 6 T 6N, R 19W. Associated with the concretions are ammonites Mammites depressus, M. nodosoides, the Proplacenticeras cummingsi, and Neoptychites cephalotus. Also found in association with the concretions are Ostrea sp., Veniella Baculitessp., Turritella sp. and mortoni, other gastropods. interbedded shale and sandstone sequence at the oftop Rio The Salado Tongue can be seen at the same 6 sec locality. The Atarque Sandstone (new name) is the redefined term proposed in Hook, Molenaar, and Cobban (1983) for the regressive coastal barrier sandstone unit Tongue of the Mancos Shale and regression of the seaway that overlies the Rio Salado as such marks the first major following the Dakota-Mancos transgression. As the shoreline had assumed a general NW-SE trend in this area the Atarque prograded northeastward into the Mancos Seaway. It is a diachronous unit that becomes younger to the northeast. Throughout the Zuni Basin it is a cli,ff-forming unit and consistsof a lower flat-bedded sandstone, which appears in most outcrops as the first massive unit overlying the transitional zone at the base, and coarsens upward from very fine to fine grained; and an upper cross-bedded unit, generally 15 to 20 feet thick that is lower to upper fine grained (see Fig. 2). Only the upper cross bedded unit is well'exposed on the Atarque Lake quadrangle. Thesetwo units are similarto the lower and upper shoreface units of Molenaar (1973). shoreface unit is thought to have 16 been deposited The lower offhsore beyond the zone where wave action or longshore currents effected sedimentation. Deposition of the upper shoreface unit probably took place in the zone where longshore currents were active. Burrows, including Ophiomorpha, are cammon in bedded unit, the lower and are often present in the extreme flat- lower andupper parts of the cross-bedded unit. The Atarque Sandstone, present usage, corresponds to the Lower Gallup,or the Atarque Member of of Molenaar Gallup, the (1973); he also, however, included the overlying nonmarine carbonaceous shales, fluvial channel sandstones, and thin coal beds in this member. The faunal evidence presented in Hook, Molenaar, and Cobban (1983) points to a significant age difference between the Atarque a,nd the Gallup. They recognize the Atarque cycle than Gallup by as having been the Gallup Sandstonne Pescado Tongue of the the deposited during and as Mancos an being earlier separated regressive from the Shale. The Pescado Tongue is not present in the study area. Its southwesternmost occurrence is some 10 miles to the east on the Shoemaker Canyon Southeast quadrangle. Where present, the Pescado Tongue separates the Gallup Sandstone from the (older) (Hook, Molenaar, and Cobban, redefined Tres Hermanos Formation 1983). The base of their Tres Hermanos is the regressive marine sandstone for which they propose name the Atarque Sandstone Member of the Tres Hermanos Formation. In their regional stratigraphic framework southwestward or landward landward from the a nomenclature pinchout pinchout the of the Atarque change exists Pescado is the Tongue; raised member to formation. Likewise the overlying nonmarine, 17 at in rank from ' 75- xncreasinq grain sire 18 carbonaceous shales, fluvial channel sanstones, and thin coals that Molenaar (1973) included in the Atarque Member of the Lower Gallup, are currently recognized as a separate stratigraphic unit that must also undergo the nomenclature change at the landward pinchout of the Pescado Tongue. The term proposed for that nonmarine section in'Hook, Molenaar, and Cobban (1983) is the Carthage the Member Pescado of Tongue the is Tres Hermanos present, and the Formation Moreno in Hill the area Formation wh in the area landward of the Pescado pinchout (McLelland, et. al., 1983). The Moreno Hill Formation, as defined comprises the nonmarine carbonaceous shales, fluvial channel sandstones and minor thin coals that overlie the Atarque Sandstone and that locally represent the youngest Cretaceous rocks. The geographical (the is area Atarque roughly Mesaverde in the Sandstone indicated Group on Zuni and by the Basin Moreno the area in Hill which two these new Formation) northwest of Quemado Geologic ofMapNew Mexico (Dane names may be mapped applied as and Bachman, 1965). Most ofthe MorenoHill Formationhasbeen erodedoff inth Atarque Lake quadrangle. Exposures of the remaining section are limited to two areas in sec 5, T 6N, R 19W. The basal 25 ft consists of a cross bedded fluvial channel sandstone that rests on the Atarque Sandstone (see Fig.2). An overlying paludal shale at this locality contains very thin coaly beds, 2 to 4 inches thick; this is the only coal occurrence on the quadrangle. 19 The light-gray and pinkish-ggay fluvial sandstones and conglomerates that occur largely as isolated patches and remnants unconformably overlying the Cretaceous, Jurassic, and Triassic rocks are considered to be part of the Bidahochi Formation. The name Bidahochi was first proposed by Reagan (1924) for the conglomerate that overlies Cretaceous rocks in the Ganado, Arizona area just eastof the Hopi Buttes volcanic field. Later work by McCann (1938), Re.iche (1941) and Hack (1942) established a correlation between the Bidahochi and the Tertiary sediments of the upper Little Colorado River drainage in the Zuni Basin. Repenning and Irwin (1954) established a reference section for the Bidahochi15 Arizona and of the east miles recognized three settlement members, a of Bidahochi, lower largely lacustrine member, a middle volcanic member, and an upper member. The Zuni Basin deposits probably correlate In the present study exposed but for most unit is gentle the sandy area the part slopes with the at the upper formation is topographic places member. locally well expression of the strewn with cobbles or boulders. The coarser cobble-and conglomerates boulder-size in the unit fragments of vesicular are composed basalts, of basaltic andesites, minor rhyolite and other volcanic rock, some with quartzite and chert and are well exposed in the vicinity of the Atarque Monocline. Quartzite, chert, and jasper comprise a much larger proportion of the pebble conglomerates thando they of the coarser deposits: all have a calcareous cement. The sandstones associated with the conglomerates 20 are white to very light gray, medium to coarse grained, poorly sorted, rich in lithic fragments including chert, and jasper, and are generally very friable. Low angle cross beddingof highly variable directions is common a feature. The clast lithologies suggest a source area to the southeast in the Datil, Gallo, Mangas Mountains area and deposition was along A finer a persistent grained, light northwest brown to trending very light fluvial grayish axis. brown, highly friable sandstone occurs in the north and north central parts of the quadrangle. These sandstones are not asociated with to be conglomerates, however, and tend more quartz rich than those that are. They rest on the more gentle topography developed on exposed. may Chinle Fm., but the base is nowhere well It is for the most part a fluvial deposit, but there be aeolian facies near theoftop the In the SE 1/4 SW 1/4 sec. 2 T locally. white the altered tuff was found resting remaining section 7N 19W R a 2 5 ft thick on sandstone of the Bidahochi Fm. An x-ray analysis ofthe tuff at the New Mexico Bureau of Minesand Mineral Resources by Robert M. North identified feldspar and kaolinite as the principal constituents. In their discussion of the lithology and depositional features of the Upper Member of the Bidahochi Fm. Repenninget. al, (1958) stated the Another unit containned distinctive a few of beds white facies forms the resistant tops on the of the rhyolitic Formation is ash. present and mesas in the E 1/2 sec 19 and the NE 1/4 sec 30 T7N R19W. It is a dense, calcareous sandstone, matrix and cement supported. A fresh surface is yellowish gray, but it weathers somewhat darker; thickness varies but may approach 2 0 ft in places; small root casts were noted at several 21 ~~ horizons and localities. The,deposit maybe related to a shallow impondment monocline formed or it behind may be in the part COAL The Atarque Lake hogback created pedogenic-a by caliche the truncated caprock. RESOURCES quadrangle encompasses a small portion of the Salt Lake coal field. The field is essentially defined as that area lying for the most part south of the North Plains basalt flow andwest underlain by the of Upper the continental Cretaceous divide, Moreno Hill that is Formation of McLelland, et. al, (1982) (in preparation). In effect it is delineated on the State Geologic Map (Dane and Bachman, 1965) as a Mesaverde Group outcrop area. A small portion of the field, however, lies northof the basalt flow: most of this "outlier" is on the Venadito Camp quadrangle the west trending Monocline miles but across The coal a the flow very and on the the small southwest resources in northwest portion corner of the the triangular trending extends bounded are Lake by Atarque eastwardfewfor Atarque quadrangle area a quadrangle. limited to the Moreno Hill Fm. outcrops in this southwest corner and are essentially nil. At the outcrop in the SE 1/4 NE 1/4 sec 5 , T 6N R 19W several 4 inch thick coaly beds are present in a shale and carbonaceous shale section resting on the fluvial sandstones. The coal bearing sequence as noted and described on the Venadito Camp quadrangle (Anderson, 1982a) is higher in the section and has been eroded 22 off here. adjacent REFERENCES Anderson, Orin J.. 1983, Preliminary reporton redefinition of Zuni Sandstone, west-centralhiew Mexico: New Mexico Geology, vol. 5, no. 3 . , pp 56-59. , 1982a, Geology and Coal Resources of the Venadito Camp Quadrangle, Cibola County, New Mexico: New Mexico Bureau , of Mines and Mineral Resources Open-file report 163. 198213, Geology and Coal Resources of the Mesita de Yeso Quadrangle, Cibola County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Open-file 171 (in preparation). Dane, Carl H., and Bachman, GeorgeO., 1965, Geologic Map of New Mexico: U. S. Geol. Surv. 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H., 1954, Bidahochi Formation of Arizona and New Mexico: American Assoc.of Petroleum Geologists Bull., vol. 38, p. 1821. Repenning, C. A., Lance, J. F., and Irwin, J. H., 1958, Tertiary Stratigraphy of the Navajo Country: in New Mexico Geological Society9th Field Conference Guidebook,p. 128. Sears, J. D., 1925, Geology and Coal Resourcesof the Gallup-Zuni Basin, New Mexico: U. S. Geol. Surv. Bull 767, p. 23. Stewart, J. H., Poole, P. G., and Wilson, R. F., 1972, Stratigraphy and Orgin of the Chinle Formation and Triassic strata in the Colorado Plateau region: U. Surv. prof. paper690. 26 related S. Geol.