in a warm sea. The upper member marks a Morrowan at the widespread basal Pennsyl- return to a clastics-dominated system where a vanian unconformity and to northward trans- significant influx of silt- and clay-size material gression during Morrowan time. In spite of the acted to reduce carbonate production during Morrowan-Atokan boundary problem, the re- the final stages of Paradise deposition. While maining upper Atokan through Virgilian strata ilewMerico Geological Socieg the sources of the terrigeneous material in the make up the bulk of the section within the Paradise Formation are not well established, study area and are much better constrained The New Mexico GeologicalSociety an- the changes in the rate of sediment delivery to than the Morrowan and lower Atokan. Fusu- nual spring meeting was held at New the basin are consistent with increased tecton- linid biozones within these strata parallel the Mexico Institute of Mining and Technol- ism related to the onset of the Ouachita orog- lithoshatigraphic cydes and link sub-surfaceand ogy (Socorro)on April 16,1993.Following eny. surface stratigraphy. Findings are presented as are abstracts from sessionsgiven at that fence diagrams, isopach maps, correlation di- meeting. Abstractsfrom other sessionswill agrams, a composite type log for the study area, and a compilation of bio- appear in future issuesof NewMexico Ge- Sunsunracr srRATlcRApHy oF PENNSyLVANTAN stratigraphic data available thus far. A north- ology. of the fol- STRATABENEATN TNE JONNEOADEL MUERTO, south subsurface section consisting sourH-cENTRAL Nrw Mrxco, by A. M. West, lowing four wells, Gulf Oil Co. Sierra K State Stratigraphy, sedimentology, and T. F. Inwton, G. H. Mack, and R. E. Clemons, No. 1, Shell Oil Co. No. 1 Leeman Federal, session Department of Geological Sciences, New Exxon Corp. No. 1 Beard Federal, and Exxon Mexico State University, Las Cruces, New Corp. No. 1 Prisor Unit Federal, demonstrates Srnetrcnepny AND sEDTMENTATToNop rnE Pen- Mexico 88003 subsurface thickening and thinning trends aorsr FonvarroN (LarE within Pennsylvanian deposits. Fence dia- MrssrssrrnN) tN rnE Subsurface correlations tied to previous sur- Ppr-oNcrr-r-o grams and isopach maps linking previous sur- Moulralvs, Hroalco CouNry, face stratigraphic studies provide-an improved face work with subsurface sections show parallel Nnw Mrxco, by Daoid |. Siolls, New Mexico stratigraphic framework for the western por- Bureau of Mines strata thinning to the north and thickeniirg dra- and Mineral Resources, So- tion of the Orogrande Basin as well as the ad- corro, New Mexico matically basinward to the east. 87801 and Department of jacent northeastem portion of the Robledo shelf. Geoscience, New Mexico Institute of Mining Extensive work has been completed on out- and Technology, Socorro, New Mexico 87801; crops containing Pennsylvanian strata within and D. B. lohnson, Department of Geosci- this area located primarily within south-central ence, New Mexico Institute of Mining and New Mexico; however, previous work has done Typr Cunrrs AND SUMMERVTLLEFoRMATroNs, Technology, Socorro, New Mexico 87801 little to integrate existing surface knowledge sournresunN UrAH, by G. N. Pipiringos,U.S. The section of Paradise Formation exposed with the available surface database and pre- Geological Survey (retired), 1709 Brown Road, in the Peloncillo Mountains marks the north- viously unavailable fusulinid data. The focus Las Cruces, New Mexico 88005; and R. B. ernmost and westernmost exposure of Paradise of the presentation is an area located imme- O'Sullioan, U. S. Geological Survey, Box 25046, in New Mexico. The Paradis-eFormation is 60 diately north of Las Cruces and southeast of M!939, Federal Center, Denvet Colorado m in the central Peloncillo Mountains and is the San Mateo uplift, between the Caballo and 8022s-0046 composed of a series of yellowish-brown San Andres Mountains. The obiectives of the The , San Rafael Swell, weathering, slope-forming, mixed carbonates study are to determine the subsidence and dep- , is unfossiliferous but intertongues with and siliciclastics. Stratigraphically, the Paradise ositional history of the western part of the ba- and grades down into the Curtis Formation, lies conformably above t^heEscabrosa Group sin and adiacent eastern shelf as well as link which contains lithologies and identical and disconformably below the Pennsylvanian previous surface stratigraphic work with sub- to those in the Pine Butte Member of the Sund- Horquilla . Although the Paradise in surface findings. A combination of well-log ance Formation of late Middle age (Cal- the Peloncillos has undergone at least one rel- analysis, fusulinid biostratigraphy, cuttings lovian). The Pine Butte is unconformably atively mild metamorphic event, many of the descriptions, and previous geochemical anal- overlain by the Redwater Shale of original depositional textures in the carbonates yses and core descriptions creates a compre- age (Oxfordian). which in turn is unconform- remain intact, allowing for some interpretation hensive subsurface database that allows ably overlain by the Windy Hill Sandstone of of depositional environrnents. In the Peloncillo correlation of strata throughout the study area. Late Jurassic age (Kimmeridgian). The Sum- Mountains the Paradise Formation can be di- Pennsylvanian strata within the Orogrande Ba- merville and the Redwater Shale are uncon- vided into several informal members: a lower sin are markedly cyclic, dominantly shallow- formably overlain by the Tidwell Member of siliciclastic-dominated member, a middle car- marine deposits of Morrowan to Virgilian age. the Morrison. The Tidwell is the southwestern bonate-dominated member, and an upper, fine- The cyclic character of the well-log response extension of the Windy Hill Sandstone. These grained siliciclastic-dominated member. The within the subsurface sections permits delin- correlations are constrained bv the presence of lower siliciclastic member consists of a single eation of upward-shoaling parasequence sets the J-2, J-3 and J-5 unconformities in the San lithofacies composed of fine-grained, rounded consisting predominantly of shale and lime- Rafael Swell and by presence of tll.e JJ, J-4 to subrounded quartz arenites. The middle car- stone packages indicating periods of trans- and f-5 unconformities in southeastern Wyo- bonate member is composed of either a grain- gression and regression within the basin. The ming and in the Black Hills. Some or all of these rich or a grain-poor lithofacies. The grain-rich log response within the middle to upper Des- unconformities have long been overlooked, lithofacies contains abundant ooids, pelmato- moinesian clearly shows a marker unit believed looked for but not found, not looked for at all, zoan fragments, and rare brachiopods. The to be an unconformity truncating the stage. misunderstood, misplaced, or misidentified, grain-poor lithofacies is composed entirely of Consistent cvclicitv within these strata results most recentlv bv Anderson and Lucas (Nezp medium-grained, equigra nul-ar sparry caliite. in confident lithoitratigraphic correlation be- Mexico Geology, iggz, u. 74, no. 4, pp. 79-92) Very fine grained to silt-size quartz is found in tween wells, except with regard to the Mor- in the San Rafael Swell and vicinity. The re- trace amounts throughout the carbonates. rowan. Presence of the Morrowan within the sulting disagreements with prior work are due Overlying the carbonate member is a series of study area is subject to debate. While the fu- to the lack of effective means of communicating interbedded carbonates and shales. The car- sulinid biozone Eoschubertella indicates early stratigraphic field data. Some caveats and sug- bonates are similar to those in the middle mem- Atokan time (Clopine, 7992; tNilde, 1990), the gestions are offered to alleviate some of the ber. For the most part any textures in the shales fusulinid biozones Millerella and Eostaffelln are differences of opinion. were obliterated during metamorphism. The diagnostic of the Morrowan. Unforiunately, lower member is interpreted as representing Lower Pennsylvanian fusulinid samples avail- deposition in a shallow, offshore, siliciclasticl able in the subsurface sections are limited. Thick dominated system as sea level gradually fell Morrowan strata are present in surface out- MoRRISoN Forur.tertoN suBDIVIstoNs,NoRTHwEST during the Late . On a Q-F-R di- crops at Vinton Canyon (75 m thick) and Bishop Nnw Mexco: A REASsESSMENT,by Orin l. An- agram these sandstones plot exclusively on the Cap (44 m thick) located in the southwestem derson, New Mexico Bureau of Mines and Q pole indicating that these are rerycled sands portion of the basin. However, the Morrowan Mineral Resources, Socorro, New Mexico derived from a cratonic source. The middle car- thins substantially to the north (Clopine, 1992), 87807; Spencer G. Lucas, New Mexico Mu- bonate member marks a return to more stable, where it is only 1 m thick in the Derry Hills seum of Natural History and Science, 1801 clear-water carbonate conditions. The abun- section and is completelv absent in the Mud Mountain Road, NW, Albuquerque, New dant ooids and abraded grains suggest a lo- Springs section. Clirpine- (1992) attributes this Mexico 87lM; and Charles H. Maxwell, U.S. cation well within the zone of constant agitation substantial thinning to both the loss of the lower Geological Survey (retired) r=

New Merico Geology August 1993 As originally defined in the Colorado Front Natural History and Science,1801 Mountain Iargely red-bed strata of the Wanakah Forma- Range, the consists of var- Road, NW, Albuquerque, New Mexico 87104; tion of northwest New Mexico correlate with iegated shale or claystone, largely smectite (al- and O. /. Anderson,New Mexico Bureau of the Curtis Formation and largely red-bed strata tered volcanic ash), with lesser sandy shale and Mines and Mineral Resources,Socorro, New of the overlying Summerville Formation in the thin sandstones. As currently used on the Col- Mexico 87801 San Rafael Swell, Utah. Part of their position orado Plateau it includes diverse lithologic units During the past 25 yrs, the U.S. Geological is a statement (A&L, p. 88) erroneously attrib- that cannot be traced eastward into the type Survey has reinterpreted Middle and Late Ju- uted to Peterson (1988) that the type Cow area. Subdivision of the Morrison on the pla- rassic stratigraphy and sedimentation on the Springs Member of the Entrada Sandstonein teau dates from Cilluly and Reeside (1928)who Colorado Plateauby: (1) using and creating re- northeast Arizona is a "pre-Todilto unit." No used Lupton's (1914) informal name, Salt Wash dundant and parochial stratigraphicnames; (2) such statement was made in that report, and sandstone member, to designate a basal Mor- purportedly tracing marker beds, which can- the regional relationships by O'Sullivan (1978), rison conglomeratic sandstone unit. The over- not, however, be replicatedin the field; (3) pos- Peterson (1988), and Condon (1989)strongly lying shale-dominated beds were left as tulating regional unconformities for which no suggest the opposite-that the type Cow Springs undivided Morrison Formation. The correla- stratigraphic or geochronologic evidence ex- is stratigraphically higher, and younger, than tion from the Front Range was based on sim- ists; and (4) creating sedimentologicmodels to the Todilto. The samevertical seouenceof units ilarity of fauna and lithology, although equal which stratigraphic units are retrofitted re- alsooccurs in the southernSan Juan Basin and emphasis was given to a possible correlation gardless of actual stratigraphic relationships. is why we continue to apply the name Cow with the Lower Cloverly Forma- Examplesof thesepractices include: (1) coining Springs there. The reports cited above show tion. Ten years later Gregory subdivided Mor- the redundant name Horse Mesa for the unit that the vertical and agesequence of units from rison strata in southeastern Utah into the long called Bluff and using and extending the northwest New Mexico to Cow Springs, Ari- Recapture Shale (a lateral facies of the Salt Wash), preoccupied name Wanakah in place of Sum- zonal to the San RafaelSwell, Utah, is Todilto the Westwater Canyon Sandstone Member, and merville; (2) claiming that a marker bed in the (oldest), type Cow Springs, and Curtis (young- the overlying Brushy Basin Shale. The West- "Wanakah Formation" in southwest Colorado est), which makes it extremely difficult to cor- water was described as a ouartzose sandstone can be traced into the Curtis Formation of relate Todilto with Curtis. A&L apparently with chert and quartzite-conglomeratic string- southeast Utah, although the "marker bed" eschew stratigraphicmarkers whereasour ers, occupying a stratigraphic position close to cannot be tracedacross gaps in the outcrop belt studies are based largely on identifying and that of the Salt Wash, somewhat lower in the and, at different outcrops, is of differing thick- tracing out such markers to establishthe lateral section than has generally been recognized in ness, lithology, and stratigraphic position; (3) and time relationships becauseso many of the regional correlations. workers Later have rec- positing a regional J-5 unconformity for which beds are undatable by other means. Using these ognized volcanic pebbles in the Westwater but no evidence exists; (4) creating a sedimento- markers, O'Sullivan (unpubl. data 1980a,1980b, not specifically in the type area. We thus con- logical model whereby sandstonesof the Salt 1991)and Condon (1989)can now show that tend that in the western type area all members Wash Member of the Morrison Formation must the Curtis correlateswith the Moab Tongue of except the Brushy Basin are largely prevolcanic have fed a lake and then creating the lake by the Entrada and that most of the Wanakah of and likely to be all or in part older than the co-opting upper Summerville strata and re- northwest New Medco correlates with the type Morrison Formation. Moreover the naming them the Tidwell Member of the Mor- Moab/Curtis and underlying upper part of the quartzose character of the lower sandstones rison Formation, even though these strata are Entrada; their work also shows that onlv up- suggests affinities to the , al- nowhere laterallv eouivalent to the Salt Wash. permost Wanakah beds may correlate wiih though genetically there is a contrast. A di- Particularly significint here is the correlation lowermost Summerville beds. For this reason, lemma exists, however, because neither of Wanakah and Curtis, becausethis suggests but mainly becausered beds in the Wanakah lithology nor genesis have been applied con- that the Todilto Formation of northern New contain other lithologies than typical Summer- sistentlv to placement of the basal Morrison Mexico is older than the Curtis (contrarv to all ville red beds, the name Wanakah is more ap- contact. Recint USGS work by Condon and availabledata, as summarizedby Harshbarger propriate than Summerville for upper San Rafael Peterson (1986) has advocated an "unrecog- et al.,1957;Imlay, 1980;Kocurek and Dott, 1983; Group strata in the eastern Colorado Plateau, nized time boundary" as the base of the Moi- Blakey et al., 7988;and Ridgley, 1989;among including northwest New Mexico. A&L failed rison Formation. Adhering to the definition of others) and that the New Mexico Entrada is to consider the implications of the Redwater a formation is more appropriate (mappability), older than the Utah Entrada. We concludethat Shale Member of the , and hence we would place the contact on the the recent U.S. Geological Survey interpreta- which only occurs as far south as northeast basis of lithic characberistics. This, unfortu- tions of Middle and Late Jurassicstratigraphy Utah and northwest Colorado, and of the J-4 nately, is not an easy task because of great lat- on the Colorado Plateau have totally and er- and J-5 unconformities at its base and top. eral variability of these strata and the failure of roneously confused a relatively straightfor- Contrary to A&L (p. 89),Pipiringos and O'Sul- rnany previous workers to recognize this se- ward stratigraphy well understood by earlier livan (1978)did not say or indicate that the J- quence as transitional between San Rafael Group workers. We prefer to base a stratigraphy of 4 unconformity "is present in the northern San and Morrison Formation. Recent work on Hay- these rocks on sound stratigraphic principles Rafael Swell where it represents a break be- stack Mountain has indicated that a two-phase and practicesthat eschewthe use of redundant tween the upper and lower Curtis or locally transition exists. The sandstone facies in the nom-enclature,are basedon demonstrablefield between the Summerville and the Redwater"- lower part are generally well-sorted high-sphe- relationships, recognize rock-stratigraphic units indeed, the J-4 and the Redwater are not even ricity quartzose sandstone very similar to San by their lithology and mappability not their age, present there. Pipiringos and O'Sullivan also Rafael Group lithologies but contrasting in lith- avoid correlationbased on imaginary uncon- showed that the Redwateris cut out southward ogenesis. The sandstone facies in the upper formities, and develop sound regional rock by the J-5 unconformity at the baseof the Mor- part consist of a mixture of the quartzose facies stratigraphy as the basis of sedimentological rison. A&L (p. 87) criticized our lack of plus a more angular. poorly sorted lithic frac- interpretation, not the reverse. control and isotopic dates, yet they present no tion containing chert, quartzite. and volcanic such data either. Enough such data are now fragments. Each of these might be recognized available, however, to shed light on an impor- as members of a pre-Morrison unit in the San tant aspect of this problem. An {Ar/3eAr date Juan Basin. The base of the Morrison thus be- Connele.ttoNcoNTRovERSrEs-MtDDLE AND Up- of 154.9+ 1.5Ma from a bentonitebed 8 ft above comes the base of the local Westwater Canyon PERJURASSIC STRATICRAPHY OFTHE COLORADO the baseof the Tidwell Member of the Morrison Member, which may not be the same unit as Plergeu AS RELATEDTo NoRTHWEsTERNNEw Formation near Notom, Utah, determined by type Westwater Canyon in southeast Utah. If MEXco, by Fred Peterson,R. B. O'Sullitan, f . D. Obradovich (in Peterson, 1992)indicates further detailed work establishes these as two and S. M. Condon,U.S. GeologicalSurvey, an age essentially on the Oxfordian-Kimmer- separate units, consideration will have to be Box25046,M!939, Federal Center, Denvet idgian boundary for lowermost Tidwell strata Biven to applying Smith's (1954)name, Prewitt Colorado 80225 (time scaleof Harland et al., 1990).Supporting Sandstone Member, to strata previously called Anderson and Lucas (New Mexico Geology, this are charophytes from the Tidwell near '1.4, Westwater Canyon Member. 7992, v. no. 4, pp. 79-92) (A&L) recently Grand Junction (Peck,1952 Ott, 1958)that sug- criticized the present state of stratal relation- gest an age no older than Kimmeridgian ac- ships on the Colorado Plateau that we have cording to biostratigraphic studies by Schudack been working on for the past 25 yrs or so. Here (1990). A&L include Tidwell in their Summer- Mrools-Upprn Junassrc srRATrcRApHyAND sED- we resDond to the most salient points of their ville, which would make its top considerably IMENTATIONON THE COLORADOPLATEAU. bV argumints. A major contentionof A&L is that younger than most workers currently accept. Spencer G. Lucas, New Mexico Museum of the Todilto Limestone Member and overlying More importantly, by including Tidwell in

August 1993 New MexicoGeology Summerville, A&L gloss over significant ero- and ash fallout from the caldera-forming erup- zation. Results are given in the table below. sional and depositional events (Redwater and tion, up to 721 m of coarse-grained debris-flow Eleven of 13 samplesmeet the requirementsof bounding unconformities) that occurred in the strata and rockfall debris were deposited along a plateau age. Uncertainty of each plateau age post-Summerville pre-Tidwell time interval. with interstratified basinward ephemeral-lake is calculated by quadratically combining the Working with marker beds and surfacesis dif- deposits. Subsequently, up to i3 m of lacus- standarddeviation of plateaugas fractions'ages ficult, frustrating, and time consuming, but in trine margin strata were deposited, followed with the measured uncertainty in irradiation our opinion it is a better methodology than by up to 550 m of turbiditic and laminated la- parameter J and uncertainties in isotopes pro- lumping superficially similar lithologies to- custrine strata with numerous l-l5-m-thick ash- duced by interfering reactions during irradia- gether and neglecting the detailed interrela- fallout tuffs. The fallout tuffs are of broadly tion. Samplesrange in agefrom 4.36J.40Ma. tionships of the strata. similar compositions but tend to progress from Errors (1.o)determined by the above method dacitic to rhyolitic compositions through time. range from 0.02-0.05 Ma. A total of 109 ori- The hrffs are thought to have been derived from ented samples were collected from 13 sites in dacitic to rhyolitic ring-fracture lava domes that the RPV. Principal component analysis (PCA) NActMtENToFonuerloN STRATIGRApHyAND interfinger with sedirnentary rocks and ash-flow was performed on all samplesto determine po- BIOSTRATICRAPHY: IMPLICATIONS FOR PALEO. and ash-fallout tuffs in the southern part of the larity and the direction of characteristicrem- CENE TECTONISM AND EVOLUTION OF THE SAN caldera. Back-filled alluvial valleys in the walls anent magnetization. Site-meandirections and JunN Besw, New Mexrco, bv ThomasE. Wil- of the caldera suggest that the caldera filled cones of 95% confidence (i.e. ce5)were deter- liamson,Department of Earfh and Planetary with approximately 1025 m of sedimentary and mined using Fisher statistics.The analytical ac- Sciences,University of New Mexico, Albu- volcanic strata. Intracaldera fill of the Platoro curacy of oAr/3eAr dates is determined by querque/New Mexico87131 and Uncompahgre calderas also contains thick comparing each site's magnetic polarity and The has been divided deep-water lacustrine strata but are dominated age to the geomagnetic-polaritytime scale.In- into three membersbased largely on exposures by intermediate to felsic lavas and outflow ash- consistenciesarise when RPV samples don't of the unit at Mesa de Cuba and Torreon and flow tuffs from later calderas. Intracaldera fill correlate polarity direction of the time scale. EscavadaWashes. These members are, in as- of the Bursum and Emory (south) cauldrons is For example, there are two sites that have re- cending order, the Arroyo Chijuillita Member, dominated by ring-fracture Iava domes and ash- verse polarity but fall within a normal chron the Oio Encino Member, and the Escavada flow tuffs with intercalated alluvial sedimen- on the time scale.These inconsistencieseither Member. Study of the Nacimiento Formation tary rocks draped against the topography of indicate the geomagnetic-polaritytime scaleis on outcrop and in the subsurfaceto the north the domes. Lacustrine strata are of minor sig- wrong or, more likely, the 1o error resolved by oAr/3eAr as far as the San fuan River indicates that the nificance and represent only-fill shallow-water fi- error-analysis methods is wrong. When EscavadaMember is present,but the lower two cies. Similar intracaldera seouences are 2o errors are applied to theseproblematic sam- membersbecome incieasingly indistinct north- described for the Muir, Geronimo Trail, and ples polarity reversals agree, within error, to ward. A possible new member consistins of a Turkey Creek (AZ) cauldrons. Broad similari- the geomagnetic-polarity time scale.The true thick, sh6et sandstoneunderlies the Escivada ties exist between all intracaldera sedimentary error may lie somewhere between 1o and 2o. Member in the Kutz Canyon area. Correlation fill sequences examined, however, the thick li- Paleomagnetic analysis is a well-established, to the time scale using magnetostratigraphy custrine sequences that are prominent in some inexpensive method for correlation of radio- and biostratigraphybaJed on fossil malmirats San Juan Mountain calderas are absent in those metric dates. As technology advances,isotope- indicates that accumulation rates of the Naci- of southwestern New Mexico. This discrep- detection limits increase,and analytical errors miento Formation decreasedthrough the Pa- ancy may be partially explained by climatic dif- become smaller, geomagnetic-po[arity corre- leocene,concurrent with a generalincrease in ferences but may also be due to differing lation will continue to be a reliable, independ- averagegrain size upsection. Changes in thick- physiographic expressions of calderas in the ent check on accuracyof radiometric dates. ness of the Nacimiento Formation, which var- two volcanic fields. ies from about 260 m to 470 m in the study area, can be attributed primarily to intrafoi- Structural geology and volcanology session mational thinning toward the basin margin. Much of the rapid variations in thicknesi of ANALYZING ANALYTICALACCURACY oF RADIoME- Pnorrnozorc oRoGENrcHrsroRy tN NEw Mrxco; the EscavadaMember in the Mesade Cuba area TRICDATES: CASE STUDY FROM THE R-IOPUNNCO A MERGING OF CONFLICTING MODELS, bY K. E. can be explained by erosional scouring at the NECKS/WESr-CENTRAL NEw MExrco, by R. B. Karlstromand I. A. Grambling,Department of base of the San JoseFormation on local struc- Hallett, Department of Geoscience. New Earth and Planetary Sciences,University of tural highs rather than a more regional angular Mexico Institute of Mining and Technology, New Mexico, Albuquerque, New Mexico unconformity at the basin margin. The hiatus Socorro, New Mexico 87801 87737 separating €Ar/3eAr the Nacimiento Formation and the Comparison of age determinations A reconciliation of conflicting models sug- basalSan fose Formationin the southernSan and geomagnetic polarity with published ge- geststhat Proterozoicrocks in New Mexico rec- Juan Basin is significant and estimated to be oma6netic-polarity time scales is used as an ord a polyphase tectonic history involving between3and6m.y. independent check of geochronologic accu- orogenicactivity at 7.75-7.65, 1.4, and 1.3-1.0 rary specifically in refining analytical error. Ba- Ga. Early deformation and metamorphism took saltic plugs from 13 volcanic necks located in placeduring severalpulses of Early the Rio Puerco vallev (RPV) east of Mt. Tavlor convergence.These events accompanied Covpantsol oF sEDIMENTARyCALDERA-FILL 1.75- have been dated by'the 4Ar/1"Ar method ind 1.65 Ga plutonism juvenile SEQUENCESIN THE SAN MOUNTAINS, and assembly of JUAU analyzed by alternating-field (AF) demagneti- Colonaoo ANDsoLnHwEsrERN NEw Mrxco. by DanielLarsen, Department of Earth and €ArfeAr Paleo4e€nelilm_ PlanetarySciences, University of New Mex- ico, Albuquerque, New Mexico 82131 Location Sample age (Ma) error (Ma) Vo polarity nlnr k o* Tuffaceous and alluvial sedimentary rocks, Cerro de Guadalupe 2.40(i) +0.30 t2.2 R 8/0 92.8 5.8 pyroclastic rocks, and a variety of intermediate Plug Cerro de la Celosa plug 2.54 + 0.03 r.18 N 8/0 316.8 3.1 to felsic lavas are commonly observed to fill GonzolasRanch dike 2-Ol + 0.03 1.15 R 70t4 250.6 3.1 moat depressions between the topographic Cerro de Nuestra Sefrora 2.60(' +0.90 34.L R 5/3 28.2 L4.6 margin and resurgent dome of modern and Plug Cerro Chafo flow 2.64 + 0.06* r.77 N u4 56.2 12.4 ancient resu4gentcalderas. The sedirnentary fill Cerro Cuate 2.66 + 0.02 0.75 N 414 21 4 20.3 of the Creedecaldera in the centralSan iuan Plug Cabezon Peak 2.66 +0.06* 7.20 N 6t3 11.3 20.8 Mountains in southwest Colorado has 6een Plug Cerro Chato 2.90 + 0.02 0.51 N 5/3 73.3 8.9 studied in detail in outcrop and scientific drill Plug Cerro Parido flow 2.97 + 0.03 0.98 N 7t7 782.4 4.5 core.Similar sequences have been examined in Hilt 6793 3.08 + 0.03 1.00 R 612 73.9 7.9 other calderasin the San Mountains and Plug Juan Cerro Negro 3.39 + 0.02 0.62 N 517 79.6 8.6 in the Bursum and Emory cauldrons in New Plug Cerro de facobo 3.59 + 0.05 1.51 N 7t7 207.4 4.3 Mexico. The Creede Formation is the Plug Picacho Peak 4.36 +0.14* 3.27 R 6t2 53.1 9.3 sedimentary fill of the Creede caldera in the Plug Tertiary * San fuan volcanic field in southern : 2o error, all others are 1o; (i) : isschron age, all others are plateau ages;rVnr: number used/ Colorado. Following emplacementof ash flows number reiected.

Nao Mexico Geo/ogy August 1993 arcs (Indonesian model) in Arizona and Col- PonpHvRoeLesr-MATRIX RELATIoNSHIpSusED To ton can therefore be separated into Pre-, orado; similar events probably affected New RECOGNIZEPOLYPHASE GEOLOGIC HISTORY IN syn-, and post-emplacementSenerations, but Mexico. A regional northeast-shiking, steeply MTDDLEPnorrnozorc RocKs oF CENTRALNEw iI remains unclear whether theserepresent dis- dipping (S) shortening fabric predates 1.7 Ga Mrxco, by Amy G. Thompson and Karl E. creteevents or Parts of a contrnuum. (Arizona, Needle Mts.), 1.65 Ga (Magdalena Karlstrom, Department of Earth and Plane- Mts.), and 1.45Ga plutons (PicurisMts., Man- tarv Sciences. Universitv of New Mexico, zano Mts.) and thus in part recordsEarly Prot- Al6uquerque, New Mexico 87131 pLUToN erozoic continental assemblv.Earlv Proterozoic Porphyroblast-matrix textural relationships Syr.rcrnoruous EMPLACEMENTAND DEFoR- metamorphism was perhaps dominantly are often ambiguous and, used in isolation, can MArroN:rP'E 1,.42 Ge SeNpn GneNrrE,by E. greenschist faciesbut reached amphibolite fa- lead to conflicting interpretations of the geo- KirW, K. E. Karlstrom,and.C. Andronicos,De- cies where plutons were closely spacedin time logic history of an area because deformation partment of Earth and Planetary Sciences, and space(e.9. northwest Arizona). In central and metamorphism are generally heterogene- University of New Mexico, Albuquerque, New Mexico, peak metamolphic conditions vary ous in both sDace and time. This is true in New Mexico 87131 from greenschist to upper amphibolite facies, Proterozoic rbcks in southwestern North The Sandia Granite is one of a series of 1.4 and metamorphic field gradients pass close to America where the debate about a tectonic ver- Ga granitoid plutons exposedin rangesbound- the AlrSiOr triple point. The age of this triple sus anorogenic Middle Proterozoic geologic ing the Rio Grande rift. The pluton cores the point metamorphism remainscontroversial. Is- history continues. Some geologists describe the Sandia Mountains and croDsout over 300km2. ograds crosscut contractional structures, sug- Middle Proterozoic as a time of tectonic qui- Recentstructural work suggeststhat the pluton gesting that peak metamorphism postdated escence whereas others suggest that the Prot- was emplaced syntectonical$ with respect to regional contraction. Elevated geotherms im- erozoic crust was reactivated by possibly several a transtensionalshear zone on its southeastern ply either pluton-enhanced metamorphism (at tectonic episodes. Studies of the emplacement side. The shear zone is exposed just north of 1.65 and/or 1.45 Ga) or conductivedecay re- of the 1.43 Ga Priest quartz monzonite in cen- Tiieras Canyon; it is I-2 km wide, strikes 030', sulting from crustal thickening. Metamorphic tral New Mexico help elucidate timing of tec- and dips 45'northwest under the pluton. We P-T paths suggest substantial amounts of de- tonic events. This pluton is interpreted: L) to thus infer it to rePresent a deformed base or compression on the retrograde path, but the have postdated moit of the regionil (Dr) short- lower side of the pluton. A northwest-south- timing and characterof the prograde path re- ening deformation and some amphibolite-fa- east bansect acrossthe zone consistsof 1) un- mains poorly constrained. In New Mexico Early cies metamorphism; 2) to have been deformed Sandia pluton-megacrystic Proterozoic deformation and metamorphism synchronous with continued or renewed monzogranite with a locally developed mag- were variably overprinted by Middle Protero- northwest-southeast shortening and devel- matic foliation; 2) sheared Sandia pluton-a zoic tectonic events. One model suggeststhat opment of a 1-2-km-wide metamorphic aure- mylonitic augen orthogneiss whose matrix is regional deformation and metamorphism were ole; and 3) to have been overprinted by low- depleted in quartz and K-feldspar and en- the product of a complex interplay of contrac- temperature deformation after cooling. Appar- riched in biotite relative to the undeformed tion and extension (Himalayan model) broadly ently conflicting porphyroblast-matrix assem- pluton; 3) Cibola granite-leucocratic, fine- to synchronous with emplacementof 1..4Ga plu- blages and disequilibrium mineral assemblages coarse-grained, equigranular granite that in- tons (Grambling et al., 1989).This model lumps can be reconciled with this polyphase history. trudes laterally continuous screensof quartz- all the higher grade metamorphism and defor- Evidence that a major component of the north- ite. Contacts between the lithologies are irreguLar mation into a single Middle Proterozoicevent. west-southeast shortening predated pluton and, in many places, diffuse. The shear zone It may overestimatethe magnitudeof 1.4 Ga emplacement is: 1) the pluton crosscuts the re- is truncated to the southeastby the Tijeras fault, contraction, but it highlights the importance of gional, subvertical Sz fabric and isodinal Fz folds; a Phanerozoicbrittle fault that juxtaposes the 1.4 Ga tectonism and the uncertainties in dat- 2) the pluton is generally unfoliated compared Cibola granite with the unsheared amphibo- ing deformational and metamorphic fabrics. to the intensely deformed country rocks; and lites, pelitic schists, and quartzites of the Ti- Some areas show the direct juxtaposition of 3) most contact-metamorphic minerals (75V") jeras greenstone. Kinematic indicators triple point rocks tightly infolded with but gen- epitarjally overgrow a differentiated 52 cleav- (asymmehic porphyroclasts,S-C fabrics)in the erally abovehigher grade gneisses(750'C, 8 kb; age. Prepluton metamorphism may be repre- shear zone suggest top-to-the-north, exten- Cimarron, Taos, and Rincon Mts.). This ge- sented by kyanite, rare staurolites that are sional movement with inferred slip parallel to ometry is interpreted to representa mid-crustal stretching lineation plunging 40"- €Ar/3eAr included in contact-metamorphic garnets, and an average extensional shear zone. hornblende cloudy, anhedral gamets that are locally over- 330'). Small-scalesynthetic shear bands often and U-Pb monazite ages of 1.4 Ga from the grown by contact-metamorphic gamets. Tec- transpose foliation into C{' relationships, while higher grade rocks indicate regional heating in tonism also accompanied plutonism. Poles to antithetic shear bands appear to have accom- plutons areaswhere no 1.4 Ga have been iden- tabular and locally folded dikes define a great modated back rotation of discreteblocks. Field tified. Extensional deformation and accomDa- circle (northeast strike, subvertical dip), the pole and microstructural relationships suggestthat nyin8 metamorphismmay have takenplacd at to which is interpreted to record subhorizontal, pluton crystallization was synchronous with 1.4 Ga. A secondmodel suggeststhat 1.4 Ga northwest-southeast shortening. Some (257o) deformation in the shear zone. Evidencein- plutons produced locally enhanced deforma- of the contact-metamorphic garnets and stau- cludes: 1) fine-grained melt injections of leu- tion and metamorphism in their aureoles. Ei- rolites, especially in the area northwest of the cocratic Cibola granite fill S and C planes and ther model is compatible with broad contact pluton, have shaight inclusion trails that are shear bands in mylonitic orthogneiss;2) a gen- aureoles and syn-pluton ductile deformation subperpendicular to the subvertical "Sr" matrix eral correlation between degreeof deformation around 1.4 Ga granites, both extensional(San- fabric, suggesting early-synlSz" Browth. Al- and intrusion sequence-the deformed host dia) and contractional (Priest), and by de- though these might represent earlier meta- Sandia pluton is crosscutby progressivelyless compressionalP-T paths for metamorphism in morphic minerals, we prefer an interpretation deformed Cibola granite, aplites, and Pegma- several areas. It is clear that 1.4 Ga was not a that F, folds were tightened and "S2" reacti- tites; 3) undeformed pegmatite dikes that time of "anorogeny," but of regional heating, vated and intensified on the northwest, high- crosscutearly shear fabrics. Shear-zonemove- uplift, and deformation. The extent of 1.4 Ga strain side of the pluton during emplacement. ment was clearV synchronouswith pluton em- metamorphism (regional heating versus plu- Post-emplacement deformation is recorded by placement and produced impressive ton-enhanced metamorphism) and the kine- discrete mylonite zones in the granite. Tem- deformation in the margin of the Pluton. In matic regime for 1.4 Gi ductile deformation peratures of less than 350'C are suggested by light of this, we suggestthat the Sandiapluton (regional contraction and extensionversus plu- core and mantle strucfures, undulose extinc- is not "anorogenic" in the strictest senseof the ton-enhanced local contraction and extension tion and deformation bands in quartz, unre- word and that other 1.4 Ga plutons should be accompaniedby reactivation of earlier fabrics) covered kinked muscovite, and cataclastic examined for similar deformation. Further work remain to be established. New evidence indi- deformation in feldspar grains. This low-tem- is required, however, to constrain whether de- cates that post-l.4 Ga northwest-verging con- perature deformation may be associated with formation was related to pluton emplacement tractional deformation affected several areas retrogression of contact minerals and, possibly, or to regional deformation. (Priest, Sandia,Picuris, El Oro, Cimarron, Taos with movement on the Monte Largo Canyon Mts.). Microfabrics associatedwith this defor- shear zone (4 km to the north). oAr/3eAr mus- mation indicate low-temperature deformation €Ar/3eAr covite ages of about 1350 Ma suggest cooling (<350"C). Regional resetting of mus- through 350'C at this time and may give a max- TRANsTENsIoNALDEFoRMATIoN oF THE 1.4 GA covite ages at 1.35-1.0Ga suggeststhat this imum age for this low-temperature deforma- Servon GnarvrrE, A REcoRDoF MIDDLEPRo- deformation was at least in part an inboard tion. Complex polyphase deformation and rERozolcTECTONISM, by C. L. Andronicos,K. expression of Grenville convergence. metamorphism around the 1.43 Ga Priest plu- E. Karlstrom,andE. KrbV, DePartment of Earth

August 1993 NrzoMexico Geology and Planetary Sciences,University of New margin of the Yavapai province and records sedimentary fragments. Explosive eruntive ac- Mexico, Albuquerque, New Mexico 87131 continental collision of a younger Proterozoic tivity ended with the deposition of a 2-m-thick The 1.4 Ga Sandia Granite is exposedin the province (1.65 Ga) in central New Mexico (Ma- TF. Extrusion of a dome is inferred to have Sandia Mountains of central New Mexico. The zatzal province). Proterozoic rocks in New occurred following the explosive activity from southern margrn of the pluton is bounded by Mexico can be placed into the \avapai (7.74- each vent. Near the intersection of Peralta and a transtensional ductile shear zone, here termed L.72Ga\or Mazitzal (1.55Ga) provincesLased Colle Canyons the TWM and TLPC are inter- the Seven Springs shear zone (SSSZ).The plu- upon age and isotopiccharacteristics. A broad calated. The absence of inter-eruptive sedi- ton's northern margin is an intrusive contact boundary between provinces is formed where ments, erosional surfaces, or paleosols indicates with supracrustal rocks. Both margins of the the volcinogenic bisement (1.74-7.72 Ga\ of that these two eruptions were simultaneous. pluton show evidence of synchronous defor- northern New Mexico and southern Colorado Despite the close proxirnity of the two vents mation, magmatism,and metamorphism.Su- (Yavapaiprovince) and overlying sedimentary and simultaneity of the eruptions, the differing pracmstal rocks of the pluton's northem margin cover (Hondo Group, Uncompahgre Forma- substrates are thought to have influenced the record a complex structural and metamorphic tion) were deformed together during the 1.65 amount of ground water available to each history involving at least three generations of Ga Mazatzal orogeny. The brittle character of magma. The greater phreatomagmatic com- deformation. The D, deformation is recorded the Picuris-Pecos, Borrego, Nacimiento, and ponent of the TLPC eruption is apparently re- by compositional layering, which strikes east- other faults suggestsposi- move- lated to the mixing of magma with the west to northwest. D, fabrics are overprinted ment. Although some Pennsylvaniandisplace- underlying basin-fill sediments at a topo- by Dt deformation and contactmetamorphism. ment is likely, the right lateral strike-slip graphic position >300 m lower than the TWM Texturesthat support the interpretati6nthat displacementis probably Laramideage. ThG vent, which is underlain by relatively dry, older contact metamorphism is synchronous with interpretation is supported by the association volcanic rocks that could not supply enough deformation include: 1) contact-metamorphic of faults with en echelon folds ofJurassic-Cre- ground water to sustain a PM eruption. andalusite that overgrows S, foliation and is taceous rocks in the Nacimiento Mountains and aligned in S, foliation; 2) sillimanite that is li- the regional balance of Laramide foreland neated parallel to L, extension lineations, which shortening in the Wyoming province and ex- are oriented 45-030; 3) shear bands and S-C tension in Arizona. Right lateral displacement INvoLvEveNr oF THECoMANCHE/SANTA FE FAULT fabrics that are filled with pegmatite and aplite^S-C was related to decoupling and northward ZONE WITH THE RIO GNAIOT RIFT,TIMING AND dikes. Orientations of sheai bands and movement of the Colorado Plateaurelative to KINEMATICS,FROM CARRIZO ARROYO NORTH- fabrics are consistentwith top-to-northeastex- the midcontinent. This movement was driven WARDALONCTAT LUCINO UPLIFT,bV S\CACNN. tension. Deformation in the SSSZis interpreted by body forces associatedwith gravitational Hayden, Department of Earth and Planetary to have promoted segregationof melt fractions collapseof the overthickenedArizona segment Sciences, University of New Mexico, Albu- during crystallization of the Sandia Granite as of the Cordilleran orogen. These crustal-scale querque/ New Mexico 87131 indicated faults localized by the distribution of diverse com- later Tertiary extension result- Along the Lucero uplift, consistent transten- positional ing in units. Granitic rocks can be dirrided the opening of the Rio Granderift. sional fault geometries, steeply raking linea- on the basis of mineralogy and textures in the tions on fault planes, and rare ductile shear field. Coarse-grainedbiotite-rich granites (de- indicators, such as S-C fabrics and shear bands pleted Sandia Granite) are concenlratedin the in bedded gypsum of the Perrnian Yeso and northwest margin of the SSSZ, while fine- Enurnvp HIsroRyFoR Two volcANtc vENTSAs San Andres Formations, demonstrate the dex- granites Brained (interpreted to be segregated RECORDEDrN THE PsnAlre Turr: Jevez tral transtensional nature of the deformation melts) are concentrated and southeast of the Moururaws, Npw Msxrco, by KyleR. Gay and along the Comanche-Able Hill-Santa Fe fault SSSZ. Granites in the central portion of the Gary A. Smith, Department Earth and Plan- zone for severa.l miles from Able Hill, south of SSSZshow complex intrusive relationships be- etary Sciences,University of New Mexico, Carrizo Arroyo, to the Tiieras accommodation tween fine-grained leucogranitesand coarse- Albuquerque,New Mexiio 87131 zone to the north. Mafic hypabyssal rocks in- grainedbiotite granites.Leucogranites crosscut The Peralta Tuff Member of the Bearhead trude this fault zone locally and have been dated coarse-grainedbiotite granites as dikes, shear Rhyolite is superbly exposed in the Peralta with K-Ar whole-rock isotopic methods to be bands and C planes in S-C fabrics. These field Canyon/Tent Rocks area in the southeastern about27.7 Ma. Dikes of this rock intruding the relations suggest that the Cibola Gneiss (here JemezMountains. These tuffs record episodic, Comanche fault display lineations on fault named the Cibola Granite) is in fact comag- rhyolitic, volcanic activity during the late Mi- planes that rake 60'-75" south on planes that matic with the Sandia Granite. This interpre- ocene and are composed of primary and re- strike north-south and dip 75'-85'east. These tation is supportedtentatively by geochemical worked pyroclastic-flow (PF), tephra-fall (TF), show horizontal projection directions from 315" data. Maior elements show a linear fractiona- and pyroclastic-surge(PS) deposits, along with to 330", showing motions to have a larger dex- tion trend between the Sandia Granite and the intercalatedbraided-stream deposits. The tuffs tral- than dip-slip component to last movement Cibola Granite. of West Mesa (TWM) and thi tuffs of lower on the fault. Along the Able Hill fault, between Peralta Canyon (TLPC) are informally named the Comanche and Santa Fe faults, which in- eruptive units within the PeraltaTuff. In most volves evaporitic sediments of the San places, the TWM consists of 20 m of PF and Andres Formation, bedded gypsum has RrsronanroNor Leneruroe(?)nrcm LATERALsrRrKE un- minor PS depositserupted from a vent in the dergone ductile deformation that locally dis- SLIP IN NORTHERN NEW MTXICO: TECTONIC IM- BearheadPeak dome complex.The TLPCcon- plays both normal and reverse dextral-shear PLTCATIONS FROM THE PNOTTNOZOTC TO THE sist of 50 m of PF,PS, and TF and were erupted textures. In the area north of Carrjzo Arrovo. CENozorc,by C. G. Daniel and K. E. Knrts- from a vent locatedin lower PeraltaCanvon, the Santa Fe fault, long thought to be the;ift trom,Departrnent of Earth and PlanetarySci- 7 km southeastof BearheadPeak. Granulo- margin, exclusive of the Comanche fault to ences, University of New Meiico, the metric, vesiculation, petrographic, and strati- west, has a steep (>70") westward Albuquerque, New Mexico 87131 dip, making graphic data were collected to determine the it a reverse fault locally. The Comanche fauft Regional piercing points in Early Proterozoic interaction between the rhyolitic magmas and zone in this area is less than 0.5 km wide and rocks, defined by the intersections of subhor- the ground water during the TWM and TLPC dips steeply east, indicating that they are part izontal isobaric surfaceswith steeply dipping eruptions. The TWM eruption began with a of the same system at a shallow structural level. stratigraphic markersand structures,allow resl phreatomagmatic(PM) phaseresufting in the The Santa Fe fault here shows very weak, steep toration of 50-150 km of right lateral strike slip tormation of a tuff ring at least 70 m thick. dextral lineations where brecciated mudstones along a network of northlsouth-striking Lai- Subsequentmagmatic PF traveled>6 km from of the are faulted against the amidg(?)faults that formed the precursor-ofthe the vent leaving a >20 m deposit. The TLPC Upper Cretaceous Mancos Formation, Middle Rio Grande rift in northern New Mexico. The eruption began with several PM eruptive cy- , or Upper Tri- restoration of slip provides new insight into cles, each of which began with wet, lsh-rich, assic . Coarse sediments that the structure of Proterozoic rocks and the re- low-angle crossbedded15, progressedthrough cap the Santa Fe Group contain clasts of the gional conelation of Proterozoic provinces. Re- a massive, poorly sorted PF, and ended with a <4 Ma Carr2o Mesa basalt and have been lo- alignment of Proterozoic rocks ln the Tusas, drier, pumiceous, crossbeddedPS. Abundant cally deformed and uplifted by motion on the Picuris, Truchas and Rio Mora uplifts shows clasts of underlying basin-fill sediments de- Santa Fe fault. This would indicate that dextral an originally east-west- to northwest-hending crease_upwardin the cycles.Approximately 30 motion has occurred at least locally since the ductile thrust belt that may extend westward m of the vent's tuff ring was deposited in this late . These features are gebmetrically to the Needle Mts. of soutliem Colorado.This manner. These cyclesgave way to 20 m of pu- identical to what seismologists have referred thrust belt probably formed near the southem miceous, crossbeddedPS containing only rare to as an extensional, or reverse, flower strut-

New Mexico Geology August 1993 ture where steep faults dipping toward one later deformation being partitioned into the nofaunas in the Chinle Group of the westem another converge to a master fault or zone at megacrystic gneiss. The nature of the struc- United States.Several ichnotixa are biochron- depth. In this casethe east-dipping Comanche tures (e.9., very strong mineral lineation, ologically significant. For example, Pseudotetra- fault is the best candidate for the master fault. asymmetric tails around K-feldspar porphy- europus is restricted to Rhaetic shata (Redonda, roclasts in the megacrystic gneiss) indicates a Sloan Canyon Formations of New Mexico; Rock non-coaxial strain historv, with kinematic in- Point Formation of Utah). The TucumcariBasin dicators suggestinga souiheastover northwest also contains the best sequenceof superposed Poster session movement sense (in their present orienta- body-fossil fauna in the Chinle Group. All for- tions). If the effects of Phanerozoic deforma- mations are dominated by specimensof phy- Srnucn;nal DEVELopMENToF THEPRorERozotc tions are removed, foliations become nearly tosaurs; aetosaursand metoposaursconstitute RocKsrN THEloyrrA Htt-r-s,Soconno CouNry, vertical, with a nearly vertical stretching line- the majority of other fossils. The body-fossil New Mrxco, by Stnen Ralser,Department ation, with the south side up relative to the and trace-fossilrecords of the Tucumcari Basin of Geoscience,New Mexico Institute of Min- north side. The distribution of structures in- exhibit poor correspondance:(1) limulid body ing and Technology, Socorro, New Mexico dicates that in the foyita Hills there is a tran- fossils are unknown; (2) footprints of phyto- 87801 sition between dominantly L-tectonites in the saurs are absent; (3) Brachychirotheriummay Proterozoicrocks are well exposedin a fault- south and L>S tectonites in the north. Both represent an aetosaur,but it is common in only bounded block within the core of the Iovita the nature and distribution of the structures one formation; (4) saurischianbones and teeth Hills (approximately 35 km northeast of So- suggestthe presenceof a wide shearzone, with are rare, but their tracks are common in the corro). This block is largely composed of two a minimum thickness of 2-3 km. The zone of Redonda Formation; and (5) Pseuilotetrasauro- granitic gneisses: a massive, coarse-grained L-tectonites, which shows evidence of the most pus may represent a prosauropod, but body quartz-feldspar-biotite+ hornblende gneiss, intense deformation, has a thicknessof at least fossils of these dinosaurs are absent.Most for- consisting predominantly of large crystalsof 1.5 km. The regional significance of such a shear mations of the Tucumcari Basin are of Tvpe 4b K-feldspar(up to 30mm in sizeand comprising zone is currently unknown. (bonesmore abundant than footprintsaird of greater than 50% of the rock) in a fine-grained different animals) of Lockley (1989).The Re- matrix of quartz, K-feldspar, plagioclase,bio- donda is of Type 3b (bones subequal in num- tite, and hornblende; and a megacrysticgneiss, bers with tracks and of different animals). consisting of megacrysts of K-feldspar (up to Venrrnnarr AND TNVERTEBRATETRAcKs AND 20 mm in size) in a matrix of quartz-feldspar- TRACKWAYS FROM UPPER TruASSIC STRATA OF THE biotite. The megacrystic gneiss is associatedwith Tuclr"rcaru Besw, easr-crvrnal NEw MEXCo, strongly deformed aplite dikes, and both show by Adrian P. Hunt and.Martin G. Lockley,De- Wger oo pRosAURopoDTRACKS LooK LIKE? intrusive relations into the massive gneiss. Pods partment of Geology,University of Colorado Posstst-pANswERs FRoM THE LATE Truesstc rnacr and dikes of amphibolites crosscutboth gneisses at Denver, CampusBoxI72, P.O.Box173364, RECORDSOFNEw MrxrCOaNO Ur ^H,by Martin and commonly have a northeast--easttrend. Denver, Colorado 80217J364;and SpencerG. G. Lockleu,Adrian P. Hunt, and Kellu L. Con- Pegmatites (consisting of quartz*K-feld- Lucas and Philip Birchet'f, New Mexico Mu- rad, Department of Geology, University of spar+biotite) occur throughout the area, are seum of Natural History and Science,1801 Colorado at Denver, Campus Box 772, P.O. commonly from 5 cm to 50 cm in width, and Mountain Road, NW, Albuquerque, New Box 173364,Denver, Colorado 80217-3364 locally can be traced for up to a few hundred Meyico 87104 Viewed from a global perspective, prosau- meters.Pegmatites show ihree distinct orien- Vertebrateand invertebratetrackways are lo- ropods are a significant component of Late Tri- tations. In the southern half of the area they cally abundant in Upper Triassic strata of the assic archosaur-dominatedfaunal assemblages. are either shallowly dipping (30') to the north Tucumcari Basin in east-centralNew Mexico. Howevet to date they are virtually unknown or northwest or are vertical and northwest The oldest trackways are from the Los Esteros from skeletal remains, from this epoch, in the trending. Most pegmatitesin the northern do- Member of the Santa Rosa Formation (latest western United States(Chinle Group sensuLu- main are approximately vertical and approxi- Carnian) in Santa Fe County and pertain to cas 1993).Little is known of the Lite Triassic mately north trending. A strong stretching limulids (horseshoe crabs) and represent the track record of prosauropods except in south- lineation, plunging 50' towards 150', is the genus Koupichnium.The Bull Canyon Forma- ern Africa, where two ichnogenera Tetrnsauro- dominant structural feature of the Proterozoic tion (early-middle Norian) contains many in- pus and Pseudotetrasauropuswere defined by rocks of the foyita Hills. Foliations are locally vertebrate trackways in Quay County. All Ellenberger (1972). Ellenberger included two developed in the south of the areabut are more trackways represent arthropods; trackway ichnospecies in the former ichnogenus (both pervasively developed in the northern half of widths are less than 2 cm. Such trackways are quadrupedal) and eight ichnospeciesin the lat- the area. Foliations show a range in orienta- locally abundant in the basal Bull Canyon near ter, of which only two were describedas quad- tions, from northeast-to southeast-trendingand Logan and the middle Bull Canyon at Bull Can- rupedal. Thus the ichnogenus Pseudotetra- generallyeast dipping. However, poles to all yon and BarrancaCreek. The Barrancaarea also sauropusis predominantly (7570)composed of foliations lie on a single great circle, normal to includes several vertebrate tracks and track- bipedal ichnospecies.Both ichnogeneramainly the stretching lineation. In the southem part ways. Two isolated tracks represent "swim- represent large animals (pes lengths 45-60 cm), of the area foliations show the whole range of ming traces" and consist of parallel scratch though three ichnospeciesof Pseudotetrasauro- orientationsand arebest developedwhere there marks. One slab contains severaltrackways of pus arc smaller (pes lengths 18.5-30cm). Re- is a strong rheological contrast between gneiss a small (pes length : 6 cm), undescribed rep- cent work in the of the western and someplanar structure(e.g., veins and ap- tile with a tetradactyl manus and pes. The Re- United States has revealed that a small ich- lite dikes). The northern part of the area is donda Formation (Rhaetian) is rich in nospeciesof Pseudotetrasauropusis present at at characterized by west-dipping, north-north- invertebrate tracefossils, and vertebrate tracks least two localities in New Mexico and Utah. east-trending foliations, which are Iocally de- and trackways are common at three localities This is the first record of the ichnogenus in veloped in the massive gneiss. K-feldspar in Quay County: (1) Mesa Redonda, (2) Red North America (Lockley et al., 1992).A reex- porphyroclasts in the megacrysticgneiss com- Peak,and (3)Apache Canyon. Thesefootprints amination of trackways from the SloanCanyon monly have well developedasymmetric tails, occur in epirelief on the undersides of laterallv Formation in the Cimarron Valley alsosuggests indicatinga southeastover northwestsense of continuods calcarenitesthat representcyclic la- that the ichnogenus Tbtrasauropus(or another shear. The relative timing of emplacement of custrine-shorelineenvironments. At Mesa Re- larger species of Pseudotetrasauropus)rs also the granitic protoliths to the two gneisseswith donda these cyclesrepresent the highest strata present. Conrad et al. (1987)noted that large respect to deformation can be determined. The in the Redonda and contain numerous tracks tracks from the Cimarron Valley area assigned massive gneiss is the oldest rock type within of a tridactvl theropod. One slab contains a to Chirotheriumsp. "resemble those of small the foyita Hills and was intruded by the me- well-preserved trackway of Pseudotetrasauro- sauropods," and prosauropodswere sug- gacrystic gneiss/aplitedikes. Both gneissesand pus. Two isolated tracks pertain to Brachychi- gested "as possible trackmakers." However, the deformed aplite dikes exhibit stretchinglin- rotherium.Most tracks at Red Peak represent a some differencesbetween these forms and ?- eations with similar orientations. Locallv. a tridactyl theropod, but Pseudotetrasauropusalso trasauropusfrom Africa causedthese authors to crosscuttingrelation is seen between the li- occurs here. At Apache Canyon, where the dismiss an affinity to that ichnogenus. Instead, neated massivegneiss and both the contact with, track-bearing cycles are overlain by terrestrial it was inferred that the tracks were attributable and foliation within, dikes of the megacrystic strata, Pxudotetrasauropushacks dominate, and to phytosaurs, another large animal known to gneiss. This relation may indicate that intru- Brachychirotheriumand the tridactyl theropod have been abundant at that time. It now ap- sion of the granitic protolith to the megacrystic tracks are less common. The Tucumcari Basin pears that this dismissal of the prosauropod gneiss occurred during the deformation, with contains the best sequenceof superposedich- interpretation may have been premature and

August 1993 Nats Mexico Ceology that we must seriously consider that the tracks may be allied to Tetrasauropus.If this interpre- NewMexico Geological Society tation is corect then this is also the first report 1994Fall Field Conlerence for this ichnogenus in North America. It ii ac- Callfor papers cepted, at least by some ichnologists, that the Lower Jurassicichnogenus Otoioum is attrib- The 1994 NMGS Fall Field Conference for the conferenceguidebook. NMGS utable to a prosauropod and that it is similar will tour the Mogollon Slope of west-cen- guidebooks typically cover a wide range to Pseudotetrasauropus and Tetrasauropus.Gwen tral New Mexico from September 28 to of topics in geology, that Otozoumand geophysics, geo- another prosauropod track October l, 7994. The Mogollon Slope is Naoahopusare both known in the Lo*er chemistry, economic geology, hydrology, Juras- generally defined southerly sic of the western United Statesand thai Late by dipping archeology, engineering, and regional Triassicskeletal remains of at least one prosau- Tertiary volcanic and volcaniclastic strata history. If you plan to submit an article or ropod are known, it is reasonable to infer that that overlap the southern structural mar- minipaper for the guidebook, pleasesend prosauropod tracks should occur at this time. gin of the Colorado Plateau between So- a tentative title, estimate of manuscript The ichnogenera Tetrasauropusand Pseudotetra- corro, New Mexico and Springerville, length, and information concerning fig- suropus wafiant further study to determine their Arizona. The conferencewill focus on the ures and tables to: NMGS-1994FFC, distribution in western Norlh America and to Cenozoic stratigraphic, structural, and Richard M. Chamberlin, New Mexico Bu- establish_ifthey are ofprosauropod afrinity,as topographic evolution of the Plateaumar- reau of Mines and Mineral Resources, proposed by Ellenberger. So- gin. Additional topics will be recent wild- corro, NM 87801,prior tolantary7,1994. cat oil-testwells in the MangasMountains Also, be sure to request"Instructions to (Continues in next issue) area, ongoing development of an open- Authors" before you start writing. All ar- pit coal mine near Quemado by the Salt ticles and road log contributions for the River Project, aspectsof regional hydro- Mogollon Slope guidebook must be sub- logic studies, and seismicreflection pro- mitted by February 15, 1994. Managing files for the San Agustin Plains region. A editor for the guidebook will be BarryrKues, four-wheel-drive caravan is planned to and technical editors will be Richard permit accessto the scenic back country Chamberlin, Jim Barker, and Bill McIn- north and south of U.S. 60. tosh. For additional information, contact Scientific papers, technical review ar- Richard Chamberlin (505-835-5310/5420; ticles, and minipapers are being solicited fax (505-835-6333).

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Bauer et al. (Continuedfrom p. 55) Condie,K. C., and BuddingA. 1979, precambrian , 1., Geologyand geochemistry of Torrance Counties, New Mexico: U.S. Geological Survey, Geologic Quadrangle rocks, central and south-centralNew Mexico: New Mexico Bureau of Mines and Map GQ-1412,scale 1:24,000. Mineral Resources,Memoir 35, 58 pp. Myers, D. A., and McKay, E. J.,7972, Geologicmap of the Capilla Peakquadrangle, Daniel,C..G- Thompson, A. C., and Grambling,I. A.,lDz, Decompressionmet- Torranceand ValenciaCounties, New Mexico: U.S. GeologicalSuruey, Geologic amorphic P-T paths from kyanite-sillimanite-andalusite bearinq rocks in north- Quadrangle Map GQ-f008, scale1:24,000. central New Mexico (abs.): Geological Society pro_ of America, Abitracts with Myers, D. A., and McKay, E. 1.,7974,Geologic map of the southwestquarter of grams/ v. 24, no.7, p. A264 the Torreon 1S-ninute quadrangle,Tonance and ValenciaCounties, New Mexico: Goodwin, L. B., Ralser,S., Bauer, P. W., and Karlstrom, K. E., 1992,Mvlonitization U.S. Geological Suruey, Miscellaneous Investigations Series, Map I-820, scale events bracketing a granitoid intrusion-a Proterozoicexample from [he Manzano l:24,000. Mountains, New Mexico (abs.): Geological Society of America, Abstracts with Paterson,S. R., and Tobisch, O. T.,1988, Using pluton ages to date regional de- Programs, v. 24, no. 7, p. A746. formations-problems with commonly used criteria: Geology, v. 16, pp. 1108- Grambling;J.A.,lgsz,PrecambrianstructuresinCaiondelTrigo,ManzannMoun- "1111. tains, central New Mexico: New Mexico Geological Sociew,-Guidebookto 33rd Shastri, L. L.,1993, Proterozoicgeology of the Los Pinos Mountains, central New Field Conference,pp. 217-220. Mexico-timing of plutonism, deformation, and metamorphism:Unpublished MS Grambling, A., and'Dallmeyer, proterozoic J. R. D., 1990, tectonicevolution of the thesis, New Mexico Institute of Mining and Technology,Socono, New Mexico, Cimarron Mountains, north-central New Mexico:New Mexico GeologicalSociety, 82 Guidebook to 41st Field PP Conference, pp.16t-770. Shastri,L. L., and Bowring, S. A.,1992, Timing of Proterozoicdeformation, plu- Grambling, A., and Dallmeyer, proterozoic J. R. D.; m press, Tectonicevolution of tonism, and metamorphism in the Los Pinos Mountains, central New Mexico rocks in the Cimanon Mountains, northein New Mexico, U.S.A.: Joumal of Met- (abs.):Geological Society of America,Abstracts with Programs,v.24, no.7, p. amorphic Geoloqy. A177. Grambling, A., proterozoic J ihompson, A. G., and Dallmeyer,R. D., 1992,Middle Simpson, C., 1985,Deformation of granitic rocks acrossthe bdttltsductile transition: thrusting in central New Mexico: GeologicalSociety of America, Abstracts wrth Journal of Structural Geology, v. 7, no. 5, pp. 503-511. Programs, v. 24, no. 7, p. A92. Stark, J. T., 1956, Geology of the south Manzano Mountains, New Mexico: New Grambling, A., J. William-, M. L., Smith, R. F., and Mawer, C. K., L989,The role Mexico Bureau of Mines and Mineral Resources,Bulletin 34, 45 pp. of crustal extension in the metamorphism of Proterozoicrocks in New Mexico; in Thompson, A. G., Grambling, f. A., and Dallrneyer,R. D., 1991,Proterozoic tectonic Grambling, A., and Tewksbury,g. (eas. proterozoic J. J. ), geologyof the southern history of the Manzano Mountains, central New Mexico: New Mexico Bureau of Rglky Mountains: papei .- Gmlogical Societyof America, Special Z3S,pp. 87_170. Mines and Mineral Resources,Bulletin 137, pp.71,-77. Karlstrom, K. E., pioterozoii and Bowring, S. A., 1988, Early ussembi!.of tecto_ Thompson,A. G., and Karlstrom,K. E.,193, Porphyroblast-mabrixrelationships nostratigraphic terranes in southwestern North America: fournal of Geology, v. used to recognizepolyphase geologic history in Middle Proterozoicrocks of cential 96, pp. 561-576. New Mexico: New Mexico GeologicalSociety, Spring Meeting, Proceedings, Karlstrom, So- K. E., and Bowring, S. A., 1991,,Styles and timing of Early proterozoic cono, New Mexico, p. 18. defomation in Arizona

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