Stratigraphicdescription of theTr-4 unconformity in west-centralNew Mexico and eastern Arizona

by Andrew B. Heckert, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-1116; and SpencerG. Lucas, New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104

Abstract traverse the thickness of the Bluewater Second, proper identification of the Tr-4 Creek Formation remains constant at ap- erosional surface greatly facilitates recog- The Late Triassic Tr-4 unconformitv. proximately 50-60 m, demonstrating that nition of lithostratigraphic units in west- which approximates the Carnian-Norian the disappearance of the BIue Mesa Member central New Mexico, particularly in the is not due to intertonguing of the floodplain, stage boundary, occurs in west-central New structurally complex Lucero uplift. Third, Mexico and eastern Arizona as an overbank, and paleosol deposits that typify erosional this erosional surface marks the boundary surface developed at the base of the Sonsela that unit with the red-bed facies of the Blue- between two depositional systems tracts Member of the Petrified Forest Formation water Creek Formation Farther south, in (Chinle Group). Correlating measured the southeast part of the Lucero uplift, the in the Upper Tiiassic section and thus stratigraphic sections eastward from the San Pedro Arroyo Formation laterally re- allows us to make inferences about base- Petrified Forest National Park (Apache places the Bluewater Creek Formation and level changes in the Chinle depositional County, Arizona) through the Zunj Moun- is in tum overlain by the Sonsela. The Tr-4 basin during the Late Triassic. tains (McKinley County, New Mexico) to the unconformity developed as a response to a Lucero uplift in Cibola and Socorro drop in base level at or near the end of the Stratigraphy Counties, New Mexico, indicates that as late Carnian. much as 100 m of erosional relief character- The lower part of the Chinle Group in izes this unconformity. In eastern Arizona, Introduction eastern Arizona and western New Mexico the Sonsela rests disconformably on a thick (Fig. 1) consists of the following units (81t m) section of the Blue Mesa Member of The Tr-4 unconformity, as described by the Petrified Forest Formation. Farther (ascending): Shinarump Formation/ "mot- east, Lucas (1991a,1993), is particularly impor- measured sections in the Zuni Mountains tled strata," Bluewater Creek Formation, tant in correlating regional Triassic stratig- show that the Sonsela rests on Blue Mesa and Blue Mesa Member of the Petrified raphy for several reasons. First, tetrapod Member sections that average approximate- Forest Formation. In the Lucero uplift, the fossils (Lucas, 1993, in press; Lucas and ly 35 m thick. In the northern Lucero uplift, San Pedro Arroyo Formation iaterally Hunt, 1993) and pallnology (Litwin et al., the Blue Mesa has been entirelv removed bv reolaces the Bluewater Creek Formation pre-Sonsela erosion, and the'sonsela rests 1991) indicate that this unconformity south of the Rio Salado (Lucas and directly on red beds of the underlying Blue- approximates the Carnian-Norian bound- Heckert, 1994). West of St. Arizona, water Creek Formation. Throughout this aryl so it is of chronological significance. ]ohns, the basal Chinle unit is the Mesa Redondo Formation, which essentially overlies, yet exhibits a complex intertonguing relation- ship with, the Shinarump Formation (Cooley, 1957, 1958). Throughout the study area (Fig. 1) this sequence is trun-

Alsoin Potentialenvironmental impact of the abandonedLa Bajada uraniummine on CochitiLake p.71 New MexicoGeological Society 1996and 1997Fall Field Conferences p.78 Service/News p.79 Upcomingmeetings p.79 Publicationsorder form o.80 Comingsoon PecosRiver vertebrates FIGURE 1-Map of New Mexico and eastern Arizona. Counties outlined are A, Apache County, Arrzona; M, McKinley County, New Mexico; C, Cibola county, New Mexico; s, Socorro County, New Fluviatdeposition in MimbresBasin Mexico. Numbers correspond to locations of measured sections in Fig. 2. Dashed line indicates tran- Oil and gas activitiesin 1995 sect line of restored cross section in Fig. 5 cated by an erosional surface associated Lucas, 1993) occurs as a thin, 4-20-m- Member of the Bluewater Creek Forma- with the Tr-4 unconformify, which is in thick, intermittent unit that crops out as a tion. The Mesa Redondo Formation has turn overlain by the Sonsela Member of prominent ledge of thinly bedded, ripple- not been explored with any great rigor the Petrified Forest Formation. laminated, micaceous sandstone with since the work of Cooley, so its interrela- minor intraformational limestone con- tionships with the overlying Petrified Shinarump FormationL/"mottled strata" glomerates in the upper half of the Forest Formation are not clear. Although Bluewater Creek Formation. the Blue Mesa Member does overlie this Throughout the study area the base of Lithofacies are suites or packages of unit, we have not found a locality where a the Chinle Group is either the conglomer- specific lithology that occur together and comolete section of both units can be mea- ates and conglomeratic sandstones of the suggest similar depositional environ- sured together. Shinarump Formation or the pedogenical- ments. Identification of unique or specific ly modified siliciclastics of the "mottled lithofacies facilitates recognition of strati- Petrified Forest Formation slrata" (Stewart et al., 1972). Shinarump graphic units in the field. To this effect we strata are typicalty trough- to planar- recognize three principal lithofacies in the The Petrified Forest Formation consists crossbedded extrabasinal conglomerates Bluewater Creek Formation: (1) thinly of three members, the BIue Mesa, Sonsela, that fine upward into conglomeratic bedded, laminated to ripple-laminated and Painted Desert Members, in ascend- and/or quartzose sandstones and are as sandstones; (2) red beds of bentonitic ing order. Although all three crop out to thick as 2b m in this region (Coolev,7957; mudstone, rippleJaminated siltstone, and some extent in the study area, we are only Stewart eta1.,1972). Clasts are dominantly discontinuous, lenticular sandstones; and concerned with the lower two units, the extraformational pebbles and cobbles of (3) light-greenish-gray and light-gray ben- Blue Mesa and Sonsela Members. Paleozoic limestones, chert, and quartzite. tonitic mudstones and localized dark, car- Blue Mesa Member-At its type sec- These strata are channel-fill deposits on bonaceous shales. The sandstone lithofa- tion the Blue Mesa Member is more than the underlying Holbrook or Anton Chico cies is present at the base of the Bluewater 81 m of blue-gray, purple, and white ben- Members of the Moenkopi Formation. Creek Formation in its eastern outcrops, tonitic mudstones, siltstones, and thin "Mottled strata" are pedogenically such as the type section, the section at modified conglomerates, sandstones, silt- Maldonado Ranch, and in the Lucero stones, and mudstones that represent a uplift at Chicken Mountain Tank. This weathering profile deveioped prior to Iiihofacies also occurs higher in the section Chinle deposition. Locally, they may wherever the McGaffev Member is nre- underlie the Shinarump Formation (Lucas sent (Fig. 2). Most of the Bluewater Cieek New N4exreo and Hayden,7989; Fig. 2 section 8, Fort Formation is accounted for by the second Wingate 1). These strata can be quite thick, lithofacies. Mudstones are tJpically blue, GEOLOGY . as much as 20.3 m in the vicinity of Fort red, or reddish purple, slightly silty, and Scienceand Service Wingate (Lucas and Hayden, 1,989), and bentonitic. Occasional calcrete nodule rssN 0196-948X they can also occur as barely observable horizons indicate limited soil horizon Volume 18, No. 3, August 1996 horizons such as in the measured section development. Siltstones occur as thin, rip- Edllor Carol A. Hjellming at Rio Salado (Lucas and Heckert, 1,994; ple-laminated ledges that are predomi- EDITORIAL BOARD Fig. 2 section 17). nantly red with light-green flecks and Steve M. Cather, NMBMMR, Chairman The interrelationships of the Shinarump mottles. In the western oortion of the Thomas Giordmo, NMSU Laurel B. Goodwrn, NMIMT Formation and the mottled strata demon- study area the third, muddy-to-shaly Spencer G. Lucas, NMAzINHS strate how a complex incised topography lithofacies crops out in low badlands at Frank J. Pazzaglia, UNM developed during the interval between the base of tlie unit. We internret these Published quarterly by Moenkopi and Chinle deposition, an inter- New Mexico Bureau of Mines and deposits as representingu pooriy drained Mineral Resources val corresponding to the Tr-3 unconformi- Iowland area developed on the paleoto- a division of New Mexico Institute of ty (Lucas, 1993; Lrcas and Marzolf , 1993; pography generated during the Tr-3 Mining md Technology Lucas and Huber, 1994). We will later unconformity. These lowlands were only BOARD OF REGENTS show how a similar, but not as complex, Ex-Officio partially filled by mottled strata develop- Gary Johnsory Goaernorof New Mexico stratigraphic sequencedeveloped during ment and Shinarump deposition. Later, as Alan Morgm, Superintendentof Public lnstruction the latest Carnian and early Appointed Norian after base level continued to rise, aggradation Steve S. Torres, Pres.,1991-1997, Albuquerque the Tr-4 unconformity. of floodplain deposits covered these dark Delilah A. Yega, StudentMember, Secretaryfreasurer, shales with oxidized overbank deposits of 7995 1997, Socolro Diane D. Denish, 1992-7997,Albuquerque Bluewater Creek Formation the second red-bedslithofacies. J. Michael Kelly, 7992-7997,Roswell Charles A. Ztmnerly, 1991.-1997, Socorro Typical Bluewater Creek Formation sed- Mesa Redondo Formation New Mexico Institute of Mining and Technology iments are red beds that crop out as inter- President. ... DanielH. L6pez New Mexico Bureau of Mines and Mineral Resources bedded sandstones, siltstones, and mud- Cooley (1957) originally recognized the Director and StateGeologkt...... Charles E. Chapin stones throughout the study area. At its Bluewater Creek Formation as his "Red Subscriptions: Issued quarterly, February, May, type section (Lucas and Hayden, I989;Fig. Member" of the Chinle Formation, and August, November; subscription price $6.00,/calen- 2 section 13 and Fig. 4E), the Bluewater with it introduced the term Mesa Redondo dar year Editorial Matter: Articles submitted for publication Creek Formation consists of, in ascending Member for a correlative unit cropping should be in the editor's hands a minimum of five order, a basal sandstone, interbeddeJ out in the vicinity of St. |ohns, Arizona (5) months before date of publication (February, mudstones and siltstones, a thin, medial (Cooley, May, August, or November) md should be no 1958). In elevating the Chinle to longer than 20 typewritten, double-spaced pages- sandstone (the McGaffey Member of group status, Lucas (1993) concomitantly A11scientific papers willbe reviewed by at least two Anderson and Lucas, 1993), and addition- elevated the Mesa Redondo to a formation people in the appropriate field of study. Address inquiries to Carol A. Hjellming, Editor of Netr al mudstones and siltstones. This section rank unit. The Mesa Redondo superficial- Mexico Geology,New Mexico Bureau of Mines and is 53 m thick. The Bluewater Creek Forma- ly resembles the Moenkopi Formation, Mineral Resources, Socorro, New Mexico 87801- 4796. tion is present throughout most of the consisting primarily of reddish-brown silt- Publishedas public domain, thereforereproducible ulthout study area and is almost uniformly 50-60 stones and mudstones interrupted bv a permission.Source credit requestel. m thick. In the Bluewater Creek Formation conglomeraticand sandy unit that may or Circulation: 1,000 the McGaffey Member (Anderson and may not correlate with the McGaffey Printer: University of New Mexico Printing Services

August 1996 New Mexico Ceology sandstones (Lucas, 1993). Calcrete-nodule- formity, the Tr-4, is marked by a variety of depositional slope. These observed rela- rich soil horizons are common in the mud- both lithologic and nonlithologic indica- tionships are not consistent with the con- stone intervals. Generallv. the contact tors. Lithologic indicators include trough cept of a depositional pinchout of the BIue between the Blue Mesa and underlying scours, rip-up clasts, and stratigraphic Mesa Member, and instead represent Bluewater Creek Formation is fairly relief, whereas regionally apparent scouring and beveling of paleotopogra- abrupt, with very thin (< 1 m) sheet sand- chronological indicators constraining the phy associated with the Tr-4 unconformity stones of the BIue Mesa Member overlying timing of this unconformity include tetra- and onset of Sonsela deposition. red-bed deoosits of the Bluewater Creek pod biochronology, megafossil plant, and Formation. We interpret the BIue Mesa palynofloral evidence (Lucas, 1993, in Biochronology Member to represent more distal fluvial press; Fig. 3). The Tr-5 unconformity is deposits than the Bluewater Creek found below the Rock Point Formation Lucas and Hunt (1993) recently recog- Formation-the stacked mudstones and and its equivalents, often at the top of the nized four land-vertebrate faunachrons calcrete horizons typifying floodplain Owl Rock Formation, and is not pertinent (lvf) that they used to subdivide the deposits and soil development, respec- to discussion here. Chinle. Two of these, the Adamanian and tively. The unit thins progressively, but the Revueltian, are pertinent to discussion somewhat irregularly, to the east, averag- Lithologic evidence here. The Adamanian lvf is defined as late ing approximately 35 m throughout the for the Tr-4 unconformity Carnian in age on the basis of the aetosaur Zuni Mountains before disappearing alto- Stagonolepis and the phytosaur Rutiodon, gether in the Lucero uplift. A variety of lithologic and lithostrati- palynomorphs (Litwin et aI., 1991), and Sonsela Member-We recognize the graphic indicators demonstrate that chan- magnetostratigraphy (Molina-Garza et Sonsela Member of the Petrified Forest nel-fill deposits of the Sonsela Member a1.,1993; Fig. 3). Similar lines of evidence, Formation as a tripartite package consist- rest disconformably on underlying units. including the phytosaur Pseudopalatusand ing of a lower conglomeratic and sandy These include the presence of intraforma- the aetosaur Typothorax, indicate that the channel-fill surface, a middle, muddy tional clasts in the Sonsela, channel scours Revueltian is Norian in age. interval, and an upper conglomeratic and at the BIue Mesa-Sonsela contact, and Strata of the Bluewater Creek Formation sandy package. The conglomerates and changing stratigraphic thickness of units contain fossils of the Adamanian lvf, conglomeratic sandstones of the Sonsela underlying the Sonsela, indicating the including the aetosaur Stagonolepisand the Member are coarse-grained, high-energy development of as much as 100 m of strati- phytosaur Rutiodon (Fig. 2). Originally fluvial deposits that locally scoured as graphic relief. based on fragmentary fossils in the study much as 7 m into the underlying Blue Even in structurally complex areas such area (Hunt et a1.,1989;Lucas and Hayden, Mesa Member (Deacon, 1990). Sonsela as the Lucero uplift, the Sonsela Member 1989), this age assignment has been clasts typically consist of intraformational is readily distinguished from grossly simi- immeasurably strengthened by the real- mudstone rip-ups and calcrete nodules. lar units such as the Shinarump Formation ization that the Placerias quarry near St. Petrified logs are very common, with lens- by the abundance of intraTormational Johns, Arizona is in the lower illuewater es of chert pebbles and other extrabasinal clasts relative to extraformational clasts. Creek Formation (Lucas et al., 1995; our debris less common. Some outcrops Chert and quartzlte pebbles, while some- observations, Fig. 2). The overlying Blue include a O.5-m-thick coouina of unionid times present, are a very minor comPo- Mesa Member also produces abundant bivalves. Mudstone rip-ups and calcrete nent of Sonsela sandstones and conglom- Adamanian fossils, especially in the Pet- nodules were probably derived by canni- erates in this study area. "Limestone" rified Forest National Park (Long and balizatton of Blue Mesa sediments and clasts in the Sonsela are almost uniformly Murry, 1995). Thus these units are well their equivalents by avulsive processes calcrete nodules, whereas those of the constrained to the late Camian by verte- during Sonsela channel-cutting and depo- Shinarump are Paleozoic extrabasinal brate fossils. sition. Trough- and planar-crossbedded clasts, some of which contain fossils The Sonsela Member contains a limited conglomerates and conglomeratic sand- (Stewart et a1.,1972).Sonsela Member con- fauna typical of the Revueltian lvf. stones, grading upward into ripple-lami- glomerate clasts are typically mudstone Throughout the Petrified Forest National nar and laminar sandstones, are the domi- rip-ups and calcrete nodules that we inter- Park the Sonsela and overlying Painted nant bedforms and lithology, commonly pret to represent deposits of material Desert Members have numerous fossil with lateral accretion sets and minor s1'n- scoured and eroded from underlying occurrences of Typothorax and Pseudopala- depositional slumping (Cooley, 1,957; Chinle Group units. tus (Long and Murry, 1995), in addition to Stewart et al., 1972; Deacon, 1990). The stratigraphic relief introduced many other vertebrates, indicating that Paleocurrents indicate a northerly to above and extensively documented below these strata are, at the oldest, early Norian northeasterly pattern of drainage in cannot be explained bv basinal tectonics in age (Fig. 2). Sonsela river channels (Deacon, 1990). The because of itivariability. The fact that the contact between the Sonsela and the over- Blue Mesa Member is irregularly beveled Study area transect lying Painted Desert Member is grada- off indicates that the unit was cannibal- tional but not well exposed and is often ized by channel incision associated with The outcrop pattern and internal varia- ambiguous because the Sonsela usually deposition of the Sonsela rather than sys- tion of the stratigraphy detailed above is crops out as either a mesa top (Petrified tematically thinned by changing basin well exposed in four distinct areas. From Forest National Park) or as the dipslope oarameters. Paleocurrent measurements west to-east these are eastern Arizona, Fort on a hogback (Zuni Mountains and Lucero in the Sonsela (Deacon, 1990) indicate a Wingate, the eastern Zuni Mountains, and uplift). paleoslope descending from south-south- the Lucero uplift. west to north-northeast in the study area. The Tr-4 unconformity Therefore, the stratigraphic thinning of the Eastern Arizona Blue Mesa Member from west to east Lucas (1991a,1993) proposed that two across the study area cannot be accounted In the south part of the Petrified Forest regional unconformities are present in the for by depositional thinning coincident National Park the Blue Mesa Member is Chinle Group. in addition to the Tr-3 and with the depositional dipslope. lnstead, as the dominant lithologic unit. Although its ]-0 unconformities that bound the unit documented in Fig. 2, the Blue Mesa base is not exposed at its type section, the below and above (Pipiringos and O'Sulli- Member thins irregularly from west to exposed portion of the unit measures 81* van,1978). The lowest intragroup uncon- east more or less perpendicular to the m (Fig. 2). Most of the unit is dominated

Nf?a Mexico Ceology August lgq6 WNW

120km 30 km

Fossillocalities: M = metoposaurid P = phytosaur A = aetosaur D = dinosaur S = sYnaPsid MV= microvertebrates c = conchostracans Pt = plant fossils F = fossil logs

Generic names used as applicable

Generalized column "DyingGrounds" - Stagonolepis, *. Buettneria. " Rutiodon, 4 SonselaMember Placerias Petrified Blue Hills Blue | "r"*rouo",. I Sandstone" Mesa Forest 5 Member FortWingate Formation GravelPit

BluewaterCreek

I- Formation PM ' 7 Type McGaffey

Downs'euarryreve PlaOefi?S quafry S, P, M, A, D, MV....._ "mottledstrata" Shinarump Formation

FIGURE 2-{orrelated measuredsections of lower Chinle Group rocks in eastemArizona and west-centralNew Mexico.

August 1996 NewMexico Geology ESE

60 km 60 km 35 km

Localionof sections: 1) BlueMesa Membertype section: sec.21-23, T18N, R24E, Apache County, AZ 2) Placeriasquarry:SW1/4 NW1/4 sec. 14 T12N,R27E, Apache County, AZ 3) St. Johnssouth: NW1/4SW1/4 sec. 8, T12N,R28E, ApacheCounty,AZ 4) Blue Hills: SW1/4SE 1/4sec. 24 T13N R28E,Apache County, AZ 5j FortWingate gravel pit: SE1/4NW1/4 sec. 13 (unsurveyeo;,fi+ru, R17W,McKinley County, NM..- . o) fort Wing?tesiootin! range:NE1/4 SW1/4 sec. 32 (unslrveyed),T15N R16W,McKinley County, NM zi rype HrtdGaffey:NEil+ NE1/4sec. 17 (unsurveyed),T14N R16W,McKinley County,-NM 8) F6rtWingatell: W1/2 NE 1/4sec g (unsurveyed),T14N, Rl6W, McKinleyCounty, N\{- 9) FortWingate l: SE1/4NW 1/4sec. I (unsurveyed),T14N, R16W, McKinley Cou$V, fM 1()) Six MileCanyon ll: NW1/4NW 1/4sec. 13 (unsurveyed),T14N R16W,,McKinley County, NM 11) Six MileCaniron l: SW1/4SE1/4 sec. 13 (unsurveyed),T14N R16WMcKinley County, NM 12i Sii Mi6 Spriig Roao:wtpsetn sec 25; T14N R'toWanO E1l2 NW 1/4sec.'30(Cnigtpyqg.), T14NR1.5W, McKinley Countv, NM 13j BluewaterCrelkFormation type section:NW1/4 SE1/4 and W1/2NE1/4 sec. 36, T13N,R12W, Cibola County, NM 14) MitchellDraw: sec.4, T12N,R11W, Cibola County NM 15) ChickenMountain Tank: UTM 3838396N,132863'59E to 3830751N, 13285105E (Zone 13), NM 16) CaffadaBonita: UTM 3830910N, 13280990E to 38330897N,1 3281 040E (Zone 1 3), NM 15 16 Chicken Cafrada Bonita 10 Mountain SixMile Canyonll 13 14 BluewaterCreek Mitchell Draw Six MileCanyon | 12 Tvoe Section

SanPedro Arroyo Formation

Paratypothorax

"mottledstrata" LEGEND

f-F-tr Conglomerate tTL-il Gypsiferoussiltstone f,;I;I;..] Sandyconglomerate f.'.-.1-.i Conglomeraticsandstone EE! Limestone(pedogenic)

tri-r=-rl Muddysiltstone/silty mudstons G Gypsumnodules f- Mudstone/shale .14/ Crossbedding ffi Mudstonewirh calcrete nodutes SCALE: I = 5 metefS

New Merico Geology August 1995 55 or, if the latter is absent, the Bluewater Lithologic Other Palyno- LVF Phytosaurs Aetosaurs Magneto- Base Creek Formation. Cover and structural unil tetraoods stratioraohvstratioraDhv Level complexities have thus far prevented us, or any other worker, from measuring Rock either a complete Bluewater Creek Forma- Point Apachean tion section or an intact Blue Mesa section Formation resting on the Mesa Redondo Formation in this area/ but we can draw numerous I r-5 stratigraphic conclusions regarding these rnconformit\ units here. upper Owl Petrilied The basal Bluewater Creek Formation is Rock Forest best exposed south of St. Johns in the z Formation fE o palynofloral vicinity of Ihe Placeriasquarry. Here, basal = zone o Bluewater Formation s Creek strata consist tr Revueltian s ,* of dark shales and gypsum-bearing ben- tonitic mudstones that overlie c o t color-mot- Painted o z 6 tled, gypsum-nodule-bearing mudstones E Desert \ Member and siltstone of the mottled strata. These L strata are identical lithologically and stratigraphically to Bluewater Creek q Formation strata of the "Lake Ciniza" o Sonsela locality of Ash (1978)near Fort Wingate, o fr-4 New Mexico (Fig. 2) and representdepos- its of our third lithofacies of the Bluewater Blue Creek Formation. Farther south, the Blue- Mesa tower water Creek Formation is poorly exposed Member 'bo Petrified Adamanian C on a stripped surfacedeveloped in the top Bluewater .e Foresl palynofloral of the Shinarump Formation (Fig. 2 sec- z Creek Formation zone tion 4, Blue Hills, and Fig. 4B). Here, the -T- base of the Bluewater Creek Formation z consistsof typical red beds of the second TE lithofacies. Temple North of St. extensive badlands Shinarump Johns, Mountain o Formation exposuresof the Bluewater Creek Forma- palynofloral tion crop out east of A2-61 in the Blue zone Hills. Here, the base of the Bluewater Creek Formation is nowhere exposed,but the upper half of that unit and its contact FlcuRE.3-Biochronological and magnetostratigraphiccontrols on the timing of the Tr-4 unconJor, with the Blue Mesa Member are preserved mity and inferred baselevel changes.Palynology is after Litwin et al. (1991),letrapodbiochronolo- where a thin but complete section of the gy follows Lucas and (1993), Hunt and magnetostratigraphyis after Molina-Garzi et al. (7993). Blue Mesa Member rests conformably on red beds of the Bluewater Creek Forma- by mudstones and calcrete-nodule-bear- ately yellowish brown, whereas those of tion (Fig. 2 section4, Blue Hills). This sec- ing mudstones (63%), although there are the Bluewater Creek Formation are darker tion also marks the westernrnost known significant sandstone units such as the and often grayish red. Blue Mesa Member extensionof the McGaffev Member. which Newspaper Sandstone (Billingsley, 1985) sandstones are typically " ashy" and much cropsout as 5.3 m of red-ripple-laminated totalling 37% of the section (Figs. 2, aA). more poorly sorted with a significant sandstonesthat are identical in lithology With no base exposed, it is important to component of powdery bentonitic mud, and bedform to the type McGaffey Mem- demonstrate that this relativelv sandv whereas Bluewater Creek Formation ber near Fort Wingate (Anderson and Blue Mesa Member section is in fact the sandstones are generally better sorted and Lucas,1.993). Blue Mesa and does not overlap either the cemented euartz- and litharenites. Mesa Redondo or the Bluewater Creek On the blsis of these lines of evidence, Fort Wingate Formations. Several lines of reasoning the Blue Mesa Member is clearlv the onlv involvhg lithologic differences demon-- unit cropping out in the southern Petrified The village of Fort Wingate, New strate that the Blue Mesa Member is the Forest National Park beneath the Sonsela, Mexico, and the nearby, now-defunct only unit present in the southern Petrified and it is here that the unit is its thickest. To army depot from which it gets its name Forest National Park. Nowhere does the the south and east the first outcrops of are in the midst of excellent outcrops of Blue Mesa Member resemble the Moen- units lower than the Blue Mesa Member the lower Chinle Group. We have mea- kopi-like Mesa Redondo Formation, are found along the Little Colorado River sured composite complete sectionson the therefore the latter unit is clearly not where Cooley measured up to 51 m of the grounds of the Fort Wingate Army Depot included in the Wpe Blue Mesa section. Mesa Redondo Formation (Cooley, 1957). and east of Fort Wingate in Six Mile Blue Mesa Memb^er mudstones appear The Mesa Redondo Formation appears to Canyon. Additional sections were mea- more bentonitic ("waxy") in hand speci- grade into the Bluewater Creek Formation sured in the immediate vicinity of Fort men than their Bluewater Creek Forma- in the vicinity of St. Johns, as the latter Wingate by Lucas and Hayden (1989)and tion counterparts and produce a more unit rests on either the Shinarumo Anderson and Lucas (1993),and these are "popcorn"-l ike wea thering surface typical Formation or "mottled strata" south of Si. also included in our discussionhere (Fig. of very bentonitic strata. Sandstones of the Johns at the Placerias quarry and along 2). Theseexposures represent the western- Blue Mesa Member are lightly colored, U$180 (Fig.2). most Triassic outcrops in the Zuni dominantly white and pale yellow with In the St. johns area, "mottled strata" Mountains. darker sandstones being'shades of moder- underlie either the Shinarumo Formation The Fort Wingate area exhibits some of

August 1996 NezoMexico Geology FIGURE 4-Photographs of selected outcrops of lower Chinle Group strata in east-central Arizona and west-central New Mexico. A, Camp Mesa, sec. 23 T18N R24E, Petrified Forest National Park, Apache County, Arizona. B, Measured section in Blue Hills, SW1/+SE1/qsec. 24 T13N R288, Apache County, Ari- zona- C, Fort Wingate Shooting Range, NE%SW1/s sec. 13 (unsurveyed) T15N R16W McKinley County, New Mexico. D, Six Mile Canyon I section, SW% SE% sec. 13 (unsurveyed) T14N R16W. E, Typ" Bluewater Creek Formation, NW%SE% and WZzNET+sec. 36 T13N R12W, Cibola County, New Mexico. F, Cafrada Bonita, UTM 3830910N, 13280990E, Socorro County, New Mexico. BC, Bluewater Creek Formation; BM, Blue Mesa Member, Petrified Forest Formation; S, Sonsela Member, Petrified Forest Formation. Line segments indicate the Bluewater Creek Formation-Blue Mesa Member contact.

Neu Mexico Ceology August 1996 67 ESE -=h --r I -t- l.-l .-,;-- l-----;- + Petllhd Fod 51. Johnq rorlfi*",", Eibrn anl Lu€ro Uplifr ildlml Park Arbna Mw il.ii6 llounbin3

FIGURE 5-Restored cross section from Blue Mesa, Petrified Forest National Park in Apache County, Arizona, to Rio Salado, Socorro County, New Mexico, with inferred base-level changes on left. The cross section is based on the measured sections in Fig. 2. the thickest-known sections of mottled pinches and swells considerably in this much as 1.5 m of relief visible locally. strata,with 20.3m measuredby Lucas and region. Outcrops in the vicinity of Fort TWenty kilometers to the south and east, Hayden (1989)southeast of Fort Wingate Wingate demonstratethe variability of the in Six Mile Canyon, a similar situation (Fig. 2). Here is also one of the few places McGaffey Member in the Bluewater Creek exists, with approximately 44 m of Blue where Shinarump Formation deposits Formation. At its type section and else- Mesa resting on red beds of the Bluewater unambiguouslyoverlie mottled strata,as where in the immediate vicinity of Fort Creek Formation and overlain by at least in the Fort Wingate 1 section. Elsewhere, Wingate the unit is approximately 10 m 18 m of Sonsela. the Shinarump rests directly on the thick and holds up numerous small hog- Severalfeatures of theseand other near- Moenkopi or is absent. Mottled strata in backs and ridges (seemap by O. j. Ander- by measured sections are worth dis- this area are typified by two primary son in Anderson and Lucas,1993,p. G23). cussing here. At or near both locations facies,a blue and white, heavily silicified, To the west, the unit thins to a single, both the Bluewater Creek Formation and resistant "porcellanite" and very muddy, lenticular calcrete-nodule conglomerate the Blue Mesa-Sonselacontacts are trace- grayish- to bluish-green-mottled,crumbly 1.5m thick at our Fort Wingate Gravel Pit able at the surface for many kilometers siliciclasticswith large orange mottles and section (Fig. 2). Eastward, the McGaffey (see map by O. I. Anderson in Anderson nodules and veins of gypsum. Member is strikingly persistent through and Lucas, 7993,p. G23). We have taken The basal Bluewater Creek Formation the head of Six Mile Canyon and along the advantage of this to (1.)correlate both of around Fort Wingate is typified by bluish- road connecting that canyon to Four Mile these sections with complete sections of gray and green mudstones and occasional Canyon. In the northem reaches of Six the Bluewater Creek Formation and (2) black shalesof our third Bluewater Creek Mile Canyon, however, the McGaffey demonstrate that the changing thickness Formation lithofacies. This package is at pinches out about 100-200m south of our of the Blue Mesa Member cannot be most 10-15 m thick, probably present in Six Mile Canyon section (Fig. 3, and note accountedfor by interfingering of the Blue our covered interval near the Fort Wingate the absenceof the McGaffey in Fig. aD). Mesa Member with either the Bluewater gravel pit, and includes Ash's (1978)"Lake Whether or not the McGaffey Member is Creek Formation or the SonselaMember. Ciniza" locality and a fossiliferoushorizon present, the upper Bluewater Creek On the grounds of the Fort Wingate near Six Mile Spring Road (4 Mile Canyon Formation consistsof additional red beds Armv Depot the Blue Mesa-Bluewater of Hunt and Lucas, 1993) that has pro- siimilar to the lower units (Fig. 4D). The Creek con^tactis laterally traceablefor 12 duced bones of at least one theropod ind Bluewater Creek in this area is consistent- kilometers to the southwest, where it ties numerous other microvertebrates(Heckert ly a total of 60 m thick. into the completeBluewater CreekForma- et al., 1994). A thin sandstone in this unit We have measured numerous strati- tion section near the Fort Wingate gravel has produced footprints of an ornithischi- graphic sections in the vicinity of Fort pit. Here, the Blue Mesa Member overlies an? dinosaur (Hasiotis et al., 1994).These Wingate. TWo of these seitions, the Fort approximately 60 m of the Bluewater prints represent the oldest dinosaur Wingate Shooting Range and the Six Mile Creek Formation. The "missing" base of remains from New Mexico and are among Canvon II sections.are oarticularlv infor- the Bluewater Creek Formation here is a the oldest known dinosaurs from North mative regarding the Tr-4 uncon?ormity covered interval that almost certainlv reo- America. (Fig. 2). At the Fort Wingate Shooting resentsthe basal fine-grained lithology bf Above these strata, classic red beds of Range section, measured approximately the third Bluewater Creek Formation interbedded red and blue mudstones and 150 m WSW of a small-arms firing range, lithofacies. red siltstonespersist up to the base of the 34.5 m of Blue Mesa Member sediments In anv case,the mottled strata are well McGaffey Member. The McGaffey Mem- rest on top of the Bluewater Creek Forma- exposedhere in a roadbed stream cut and ber, defined by Anderson and Lucas (1993) tion, which is only locally exposed (Fig. constrain the maximum thickness of the as 6-12 m of rippleJaminated sandstone 4C).Above the Blue Mesa the 12.3-m-thick Bluewater Creek Formation. A complete that form a prominent bench in the upper tripartite Sonsela described earlier has Blue Mesa section 44 m thick rests on the half of the Bluewater Creek Formation. channeled an irregular surface with as Bluewater Creek here, although it is not

August 1996 New Mexico Geology included in Figs. 2 and 5. This thickness is Bluewater Creek Formation. As Fig. 2 Blue Mesa Member in the eastern Zuni greater than, but consistent with that shows, this section is slightly thinner than Mountains. The possibility exists that the observed at the Fort Wingate firing range. the type Bluewater Creek Formation, Blue Mesa Member has graded laterally A similar situation exists in Six Mile although if one allows 5 m at the base of into an expanded Bluewater Creek Forma- Canyon where the best Blue Mesa the Mitchell Draw section to account for tion sectiorywhich in turn has graded into Member section is located well down the the sandstone that is not exposed, then the 90-m-thick San Pedro Arroyo Forma- canyon in an area of relatively modest this accommodation amounts to only an tion. We choosenot to acceptthis hypoth- relief. Here, as well. 44 m of the Blue Mesa extra 1-3 m at the tlpe Bluewater Creek esis on the grounds that it requires more rest on only a few meters of exposed Formation. rapid lateral variation of the lithostratigta- Bluewater Creek Formation sediments, phy in the vicinity of the Lucero uplift but the contact is laterally traceable for Central New Mexico than occurs throughout the rest of the several kilometers up canyon (south) to study area. This hypothesis therefore another measured section of the Blue- The easternmost outcrop belt that requires more assumptions regarding the water Creek Formation (Fig. 2 section 11, encompassesthese units is the structurally lack of continuity of facies than our Six Mile Canyon I, and Fig. 4D), where a complex Lucero uplift in Cibola, Valencia, hypothesis that the Blue Mesa is beveled minimum of 52 m of Bluewater Creek and Socorro Counties, New Mexico. As we awayby erosiveaction associatedwith the Formation sediments are exposed. reported earlier (Lucas and Heckert, 1994), Tr-4 unconformity and the Sonsela rests the Blue Mesa Member is entirely absent directlv- uDon the Bluewater Creek or San Eastern Zuni Mountains in the Lucero uplift. Instead, throughout Pedro Arroyo Formation. The latter is most of the uplift, the Sonsela rests direct- unusually thick, perhaps as a reflection of Lucas and Hayden (1989) named the ly on red bedi of the Bluewater Creek For- paleotopography associated with the Bluewater Creek Formation for basal mation as in our Cafrada Bonita section underlying Ti-3 unconformity. Chinle outcrops along that drainage south (Fig. 2 section 16 and Fig. aF) and in nu- The Sonselain the Lucero uplift is typi- of Prewitt, New Mexico (Fig. 4E). Since merous other locations, such as Cerro fied by a significantly higher percentageof then most regional stratigraphic work on Pelon, Paint Tank, and Cafr.on del Alamito mudstone rip-up clasts. This increase the Chinle has been focused on more west- (Lucas and Heckert, 1994). As a result of comes at the expense not only of extra- ern outcrops (Lucas, 1993; Anderson and extensive faulting, we have been unable to basinal clasts,but also of calcretenodules. Lucas, 1993) or in the Lucero uplift (Lucas locate a comolete Bluewater Creek Forma- More detailed, quantified petrographic and Heckert 1994). Here we briefly s1m- tion section in the Lucero uplift. Frag- work is necessary,but we h)?othesize that thesize this new information and an addi- mentary sections in the vicinity of Chicken this change in clast composition may tional measured section with the strati- Mountain Tank indicate that the Blue- reflect the entrenchment of Sonselachan- graphic data published by Lucas and water Creek Formation may exceed 70 m nels in lower Chinle Group floodplains Hayden (1989). in the Lucero uplift (Fig. 2),but this may after generating enough relief (up to 100 Nowhere in the eastern Zuni Mountains include some fault repetition, particularly m) to completely remove the Blue Mesa is the contact between the Bluewater in the red beds. Member. Thereforethe Tr-4 unconformity Creek Formation and the BIue Mesa Mem- South of the Rio Salado, in the south- is readily apparent in the Lucero uplift, ber quite as distinct as it is farther west. ernmost portion of the Triassic outcrop where the Sonsela exhibits dramatically Here the two units are not divided bv a belt in the Lucero uplift, the Bluewater different conglomerateclast lithology than thin tuffaceous sandstone as they a.e ,rLu. Creek Formation grades into the San the stratigraphically earlier and lower Fort Wingate, but the Blue Mesa is still Pedro Arroyo Formation, another red- Shinarump Formation. readily differentiated lithologically from bed-dominated unit that is characterized the Bluewater Creek Formation. Factors by extensive fluvial mudstones with Conclusions used here to distinguish the Blue Mesa nulnerous very thin (0.5-2.0 m), laterally from the Bluewater Creek include the persistent, ripplelaminated to laminar- The Tr-4 unconformity is readily corre- presence in the Blue Mesa Member of a sheet sandstones. We reported a complete lated across numerous lithostratigraphic high degree of bentonitic alteration in the measured section of this unit previously units in easternArizona and west-central mudstones, often expressed as slopes (Lucas and Heckert, 1994),noting that it is New Mexico. Differential erosion on this exhibiting "popcorn" weathering sur- capped by the Sonsela. Accordingly, this surface has resulted in the removal of up faces, and the presence of multiple, Iater- modifies the description of the San Pedro to 100m of sedimentarysection, principai- persistent ally calcrete nodule horizons. Arroyo Formation fiven by Lucas (1991b), ly from the Blue Mesa Member of the BIue Mesa sediments are dominantlv verv restricting the San Pedro Arroyo Forma- Petrified ForestFormation, with the great- light green or shades of purple, witn tn'e tion to his units 2-9 (Lucas and Heckert, est erosion occurring in the eastern (land- red hues typical of the Bluewater Creek 1994). The San Pedro Arroyo Formation ward) portion of the study area. This is Formation much less common. rests directly on the mottled strata in the clearly demonstrated by examination of Lucas and Hayden measured 21.0 m of southern Lucero uplift, where it is as our simplified restored cross section (Fig. Blue Mesa Member (their "lower part of much as 90 m thick. Very few fossils are 5). Petrified Forest Formation") sediments known from the unit, so it is not certain In Figs. 2 and 5 we use the Bluewater above the tvpe Bluewater Creek Forma- that the base of this unit correlates with Creek Formation-Petrified Forest Forma- tion. Farthei east, near Mitchell Draw, we the base of the Bluewater Creek Formation tion contact to hang our measured sec- measured another section at the eastern and it may in fact be older. It is cleat how- tions on because it is the only contact terminus of Chinle outcrops in the Zuni ever, that the base of the Sonsela here is between lithostratigraphic units in this Mountains, where we fiound 22 m of Blue also an unconformity. interval for which there is no evidence of Mesa Member (Fig.2 section 14). Here we note that it is difficult to ascer- an unconformity. Therefore, it is most Although the base of the Mitchell Draw tain properly the relationships of the informative to consider changes of Blue section is not the Bluewater Creek Forma- Sonsela Member with the San Pedro Mesa Member thickness independent of tion-Shinarump Formation/mottled stra- Arroyo Formation. In our single complete changes in Bluewater Creek Formation ta contact, the lowest sandstone, exposed section of the San Pedro Arroyo Formation thickness. It is possible that there may be a in a valley floor on the south point of the that unit is approximately 90 m thick (Fig. minor depositional hiatus or paraconfor- measured section, is lithologically identi- 2), a thickness comparable to the com- mity at the end of Bluewater Creek cal to the basal sandstones at the tvpe bined Bluewater Creek Formation and Formation deposition, as evidenced by

NewMexico Geology August 1996 soil formation at the top of the Bluewater (HST) of stacked floodplain and paleosol Litwiru R. J., Traverse, A., and Ash, S. R., 1991, Pre- Creek Formation in, for example, the Six deposits of the Blue Mesa Member (Fig. 3). liminary palymological zonation of the Chinle For- and its to Mile Canyon I section. mation, southwestern U.S.A., correlation However, the pri- At some point after deposition of the Blue the Newark Supergroup (eastem U.S.A.): Review o{ mary variable influencing Blue Mesa Mesa Member, base level dropped precip- Palaeobotmy and Pallmology, v 68, pp. 259-287. Member thickness throughout our tran- itously and erosion was initiated. The Long, R. A., and Murry, P. A., 1995, Late tiassic sect is erosion and channeling associated Sonsela therefore represents the onset of (Camian and Norian) tetrapods from the southwest- Mexico Museum Natural with the Tr-4 unconformity em United States: New of and initial the next TSf. when base level rose again History and Science, Bulletin 4,254 pp. deposition of the Sonsela Member (Fig. 5). and re-initiated deposition on the paleoto- Lucas, S. G.,1991a, Correlation of Triassic strata of the The same cannot be said for the Blue- pography we have documented here. Colorado Plateau and southem High Plains, New water Creek Formation. Bluewater Creek AcrNowuocMENTs-Numerous peo- Mexico: New Mexico Bureau of Mines and Mineral . 47-56 . Formation thickness Resources, Bulletin 1.37, pp is instead con- ple, including Orin Anderson, Mike Lucas, S. G., 1991b, Triassic stratigraphy, paleontology, strained by multiple variables related not Grubensky, Steve Hayden, Adrian Hunt, and correlation, south-central New Mexico; ln Bar- to the Tr-4 unconformity, but rather to the and Harold Rowe, assisted in measuring ker, J. M., Kues, B. S., Austin, G. S., and Lucas, S. G. Ti-3 unconformity. Principal among these sections. Brad Petersen was instrumental (eds.), Geology of the Sierra Blanca, Sacramento and New Mexico: New Mexico are the extent ofpaleotopography Capitan Ranges, generat- to our work in east-central Arizona. Geological Society, Guidebook, 42, pp. 243-259. ed by, and filled during, the Tr-3 uncon- Various personnel at the Fort Wingate Lucas, S. G.,1993, The Chinle Group: revised stratig- formity. Although the contact of the Military Depot and the University of New raphy and biochronology of Upper tiassic nonma- Bluewater Creek Formation with the BIue Mexico Office of Contract Archaeology rine skata in the western United States: Museum of Bulletin 59, pp.27-50. Mesa Member does Northern Arizona, not demonstrate any facilitated access to the Fort Wingate Lucas, S. G., in press, The Upper Triassic Chinle topographic irregularity, even where the Army Depot. Orin Anderson and Clay Group, westem United States, nonmarine standard latter consists of basal, low-angle cross- Smith reviewed an earlier version of this for Late tiassic time; ln Dickins, J. M., Yin, H., & bedded sandstones, the basal Bluewater paper. The Maldonado family and Lucas, S. G. (eds.), Permetiassic of the circum- Jerry Pacific: Cambridge. Creek Formation is everywhere either Gillette generously allowed us access to Lucas, S. G., and Hayden, S. N., 1989, Triassic stratig- lithologically or topographically grada- their land. The Dinosaur Societv. a Vice- raphy of west-central New Mexico; ln Anderson, O. tional. Where the Shinarump Formation is President's Research and Grint Fund J., Lucas, S. G., Love, D. W, and Cather, S. M. (eds.), present, the Bluewater Creek Formation grant, a Student Research and Allocations Southeastern Colorado Plateau: New Mexico Geological Society, Guidebook 40, pp. 191-211. contact is interpreted as the first flaggy to Committee grant, and the New Mexico Lucas, S. G., and Heckert, 4.8.,1994, Triassic stratig- sheety, ripple-laminated sandstone. Else- Museum of Natural History and Science raphy in the Lucero uplift, Cibola, Valencia, and where the contact is picked simply at the all supported this work. Socorro Counties, central New Mexico; in Love, D. top of the mottled strata. This results in a W, Hawley, J. W, Kues, B. S., Adams, J. W., Austin, G. S., and Barker, J. M. (eds.), Carlsbad region: New highly irregular base to the Bluewater References Mexico Geological Society, Guidebook 44, pp. Creek Formation as mottled strata differ- 241.-254. entially infilled incised paleotopography Anderson, O. J., and Lucas, S. G., 1993, McGaffey Lucas, S. G., and Huber, P' 1994, Sequence strati- Member of Upper Triassic Bluewater Creek and do not represent a uniform planar graphic correlation of Upper Triassic marine and Formation, west-cenkal New Mexico: New Mexico nonmarine strata, western United States and surface. This is best illustrated by the Museum of Natural History and Science, Bulletin 3, Euope; in Pmgea: Global Environments and Re- noticeable change in thickness between pp. G23, G30-C31. sources: Canadian Society of Petroleum Geologists, the Bluewater Creek Formation sections at Ash, S. R. (editor), 1978, Geology, paleontology, md Memoir 17, pp. 241--254. paleoecology of a Late tiassic 1ake, western New Six Mile Canyon I and Six Mile Spring Lucas, S. G., and Hunt, A. P, 1993, Tetrapod biochron- Mexico: Brigham Young University, Geology ology of the Chinle Group (Upper tiassic), western Road, where the latter is approximately 10 Studies, v. 25, 100 pp. United States: New Mexico Museum of Natural m thicker iust 2 km south-southeast of the Billingsley, G. H., 1985, General stratigraphy of the History and Science, Bulletm 3, pp. 327129. Petrified Forest Six Mile Canyon I section. A detailed look National Park, Arizona: Museum of Lucas, S. G. and Marzolf, J., 1993, Stratigraphy and Northern Arizona, Bulletin 54, pp. 3-8. at the lithostratigraphy of these units sequence stratigraphic interpretation of Upper Cooley, M. 8., 7957, Geology of the Chinle Formation Triassic strata in Nevada; in Dunn, G. and McDou- shows that this is in large part due to infill- in the upper Little Colorado drainage area, Arizona gall, K. (eds.), Mesozoic paleogeography of the west- ing of paleotopographic lows in the vicin- and New Mexico: Unpublished MS thesis, Univer- ern United States-Il: Pacific section SEPM, Book 71, ity of the latter section, represented by a sity of Arizona, 317 pp. pp.375-378. Cooley, M. E., 1958, The Mesa Redondo Member of the substantially thicker package of basal Lucas, S. G., Heckert, A. 8., and Hunt, A. P,1995,Stra- Chinle Formation, Apache and Navajo Counties, tigraphy and biochronoiogy of the Placerias qtarry, mudstones and shales. Indeed, the mot- Arizona: Plateau, v 31, no. 1, pp. 7-15. Apache County, Arizona: Geological Society of tled strata exposed at Six Mile Canyon I Deacon, M. W., 1990, Depositional malysis of the America, Rocky Mountain Section, Abstracts with are themselves undulose in nature and Sonsela Smdstone Bed, Chinle Formation, northeast Programs, w 27, pp. 4445. Arizona and northwest New Mexico: Unoublished thus only locally exposed in a valley floor Marzol| J,, 1993, Sequence-stratigraphic relationships MS thesis,Northern Arizona University, i7 pp. across the palinspastically reconstructed Cordilleran that is otherwise composed entirely of Hasiotis, S. T., Dubiel, R. F., Conrad, K. I., and Lockley, Triassic cratonal margin: New Mexico Museum of Bluewater Creek Formation sediments. M. G., 1.994,Footprint evidence of North America's Natural History md Science, Bulletin3, pp. 331-344. We believe that the Tr-4 unconformitv. earljest dinosaur, Upper Triassic Chinle Formalion, Molina-Carza, R., Geissman, J. W., and Lucas, S. G., Fort Wingate, New Mexico: Geological Society of 7993, Late Carnian-Early Norian magnetostratigra- as documented here, represents erosive America, Rocky Momtain Section, Abstracts widr ^17. phy from nonmarine strata, Chinle Group, New activity associated with a regional fall in Programs, w 26, p. Mexico: Contributions to the Triassic magnetlc base level marking the end of a deposi- Heckert, A. B., Lucas, S. G., and Hmt, A. P., 1994, A polaritv time scale and the correlation of nonmarine tional sequence as argued by Marzolf Late Camian theropod from New Mexico; implica- ind mirine Triassic faunas: New Mexico Museum ol tions for the early evolution of Theropoda: of (1993; Fig.5). This sequence consists of a Joumal Natural History and Science, Bullet\n3, pp.344-352. Vertebrate Paleontology, Supplement to v 13, no. 3, Pipiringos, C. N., and O'Sullivan, R. B., 1978, Principal transgression systems tract (TST) deposit- oo.28-29. unconformities in Triassic and furassic Rocks, girnt, ed on and in a paleotopography that was A. P, and Lucas, S. G.,1993, Triassic vertebrate Western Interior United States-a preliminary sur- represented in places by mottled strata paleontology and biochronology of New Mexico: vey: U.S. Geological Survey, Professional Paper New Mexico Museum of Natural History and that developed during regional lowstand. 1035-A,29 pp. Science, Bulletin 2, pp. 49-60. Stewart, l. H., Poole, F. G., and Wilson, R. F., 1972, This TST includes the channel-fill deposits Hmt, A. P, Lucas, S. G., Martini, K., and Martini, T., Stratigraphy and origin of the Chinle Formation and of the Shinarump Formation and the flu- 1989, Tiiassic stratigraphy and paleontology, Mesa related Uooer Tiiassic strata in the Colorado Plateau vially dominated Bluewater Creek Forma- del Oro, Valencia County, New Mexico; ln Anderson, region: U-.S. Geological Survey, Professional Paper O. Lucas, S. G., Love, D. W, and Cather, S. M. J., 6q0. 336 Pp. tion and its equivalents. These units fine (eds.), Southeastem Colorado Plateau: New Mexico upward into the high-stand systems tract Geological Society, Guidebook 40, p. 8.

August 1996 New MexicoGeology