TABLE 2—Types of mineral deposits in New Mexico (North and McLemore, 1986; McLemore, in Abstracts press; Cox and Singer, 1986). NMBMMR classification USGS model number Placer gold 39a Placer tin none NMGS spring meeting Volcanic-epithermal vein 25b,c,d,e,g, 26b Rhyolite-hosted tin 25h Copper (± silver, uranium) vein deposits none The New Mexico Geological Society Great Plains margin 17, 22c, 18a,b, 18d, 39a annual spring meeting was held on April Rio Grande rift barite–fluorite–galena (formerly sedimentary–hydrothermal) none 9, 1999, at New Mexico Institute of Mining Carbonate-hosted Pb–Zn replacement 19a, 18c and Technology, Socorro. Following are Carbonate-hosted Ag–Mn replacement 19a,b Carbonate-hosted Pb–V–Mo replacement 19a the abstracts from all sessions given at that Laramide skarn 18b,c,a meeting. Laramide vein 22c Porphyry copper 17, 21a Sedimentary copper 30b Mississippi Valley-type (MVT) deposits 32a SESSION 1—ECONOMIC Replacement iron 18d GEOLOGY TODAY: DEPOSITS, Sedimentary iron 34f EXPLORATION, AND Proterozoic volcanogenic massive-sulfide deposits (VMS) 24a,b, 28a Veins and replacements in Proterozoic rocks 22c, 26b ENVIRONMENTAL SOLUTIONS Syenite/gabbro-hosted Cu–Ag–PGE none

significant Ag and/or Au as the primary The Jones Hill massive sulfide deposit is BASE AND PRECIOUS METALS AND product are placer, volcanic-epithermal, located in the Santa Fe mining district on the URANIUM DEPOSITS IN NEW MEXI- Great Plains margin, and carbonate-hosted border of Santa Fe and San Miguel Counties CO—AN OVERVIEW, by Virginia T. Ag–Mn replacement. Deposits that have pro- approximately 15 mi northeast of Santa Fe. McLemore, ginger@gis. nmt.edu, New duced significant base- and precious-metals The area is part of the Proterozoic Pecos Mexico Bureau of Mines and Mineral production include carbonate-hosted Pb–Zn greenstone belt in the southern Sangre de Resources, Socorro, NM 87801 replacement, Laramide vein, Laramide Cristo Mountains. The Jones Hill mine was Pedro de Abalos established the first min- skarn, porphyry-Cu, and Proterozoic mas- worked in the 1930s by three adits and one ing claim in the Fra Cristobal Mountains on sive-sulfide. Other deposits with minor met- shaft, but no production data exist from that March 26, 1685, although mining by the als production include Cu–Ag (±U) vein, Rio period. The massive sulfide deposit at the Indians and Spanish occurred before then. Grande rift, Mississippi Valley-type, sedi- Pecos mine 4 mi to the east produced over 2 Since then, millions of mining claims have mentary-Cu, and vein and replacement million tons of ore with average grades of been located, and numerous names given to deposits in Proterozoic rocks. Most of the 12.95% zinc, 4.00% lead, 0.078% copper, 3.4 the mining districts. Minerals have been and uranium in New Mexico was mined from ppm silver, and 0.011 ppm gold before shut- still are an important contribution to the sandstone deposits, with minor production ting down in 1939 due to bad ground at economy of New Mexico. More than $30 bil- from limestone deposits. depth. lion worth of minerals have been produced References The massive sulfide deposits are hosted by from New Mexico since the early 1800s Cox, D. P., and Singer D. A., editors, 1986, Mineral Proterozoic volcanic sequences of bimodal (Table 1). deposit models: U.S. Geological Survey, Bulletin mafic-felsic composition. The deposits are A mining district is a group of mines or 1693, 379 pp. generally lensoid and stratabound within a mineral deposits that occur in a geographi- McLemore, V. T., in press, Silver and gold in New host of metavolcanic and metaclastic rocks. cally defined area and are defined by geolog- Mexico: New Mexico Bureau of Mines & Mineral Deposition is from high temperature hydro- Resources, Resource Map 21, 59 pp., scale ic criteria. There are >230 mining districts in thermal solutions associated with volcanism. New Mexico. Districts producing >$20 mil- 1:1,000,000. North, R. M., and McLemore, V. T., 1986, Silver and The deposits are typically mined for lead and lion worth of cumulative production of met- zinc with precious metals as a byproduct. als include: Mogollon, Bayard, Burro gold occurrences in New Mexico: New Mexico Bureau of Mines & Mineral Resources, Resource The Jones Hill deposit is a Proterozoic vol- Mountains (Tyrone), Fierro–Hanover, Santa Map 15, 32 pp., scale 1:1,000,000. canogenic polymetallic deposit with copper- Rita (Chino), Pinos Altos, Lordsburg, Willow lead-zinc-silver-gold ore reserves projected Creek (Pecos), Chloride, and Magdalena (in to be 12 million tons in two blocks, an actual cumulative dollars at time of produc- GEOLOGY AND MINERALOGY OF THE upfaulted block projected at 5 million tons tion). Most of the uranium production has JONES HILL MASSIVE SULFIDE DE- and a lower block of 7 million tons of ore. come from Ambrosia Lake ($4 billion), POSIT, PECOS, NEW MEXICO, by Robert Upper block ore grades average 4.60% zinc, Laguna ($2 billion), Churchrock–Crown- C. Thompson, Department of Earth and 2.47% copper, and 0.072 ppm gold; the lower point ($400 million), and Smith Lake ($300 Environmental Science, New Mexico Insti- block ore grades average 3.81% zinc, and million) (Grants uranium district). tute of Mining and Technology, Socorro, 4.44% copper with an increase in the down- These deposits are found in 20 types rang- NM 87801; and Virginia T. McLemore, New dip direction. ing in age from Proterozoic through Holo- Mexico Bureau of Mines and Mineral The massive sulfide deposit occurs at the cene (Table 2). Deposits that have produced Resources, Socorro, NM 87801 contact between Proterozoic rhyolite domes

TABLE 1—Estimated total production of major commodities in New Mexico. Figures are subject to change as more data are obtained (these are minimum estimates). Commodity Years of Estimated Estimated Quantity Value Ranking production quantity of cumulative in 1997 in 1997 ($) in U.S. production value ($) in 1997 Copper 1804–1997 >9.21 million short tons >12 billion 286,825 short tons 598,071,508 3 Uranium 1948–1997 >171,000 tons >6.7 billion 216,393 lbs 2,185,557 6 Gold 1848–1997 >3.1 million troy ounces >363 million 28,709 troy ounces 10,474,832 10 Zinc 1903–1991 >1.51 million short tons >337 million none – – Silver 1848–1997 >116 million troy ounces >247 million 468,742 troy ounces 2,480,477 10 Molybdenum 1931–1997 >120 million pounds >148 million 8,343,729 lbs 107,851,657 6 Lead 1883–1991 >366,000 short tons >56.7 million none – –

30 May 1999 New Mexico Geology and overlying metamorphic rocks. The vol- Fluids in the aureole were thus both inter- tributed more or less randomly, or may show canic package was intruded by granite, min- nally and externally derived, and fracturing a preferred orientation within a small area or eralized, and folded and faulted. There is an events likely resulted initially from buildup single mine; this does not seem to uniquely angular unconformity between the Proter- of in situ pore fluid pressure followed by an favor any hypothesis. ozoic volcanics and the overlying Paleozoic increase in magmatic fluid pressure. Earthquakes occurring during sedimenta- sediments. The Jones Hill thrust fault then tion are increasingly recognized as a possible offset the deposit 2,000 ft. The primary ore cause of soft-sediment structures, sometimes minerals are sphalerite, galena, and chal- ORE-CONTROLLING FOLDS, TODILTO called "seismites," which can develop even copyrite. Bornite, argentite, molybdenite, URANIUM DEPOSITS, GRANTS DIS- on gentle slopes. Tectonic activity during proustite, native gold, and tetrahedrite are TRICT, NEW MEXICO, by William R. Todilto deposition is possible; active folding found in lesser amounts. The sulfide miner- Berglof, [email protected], University appears to have influenced the geometry of als in the gangue material include pyrite, of Maryland, Asian Division, Unit 5060 mineralized sandstones of the younger arsenopyrite, and pyrrhotite. The gangue Box 0100, APO AP 96328; and Virginia T. Morrison Formation. Some Todilto folds minerals are extensive but basically contain McLemore, New Mexico Bureau of Mines resemble ones occurring in the Pleistocene mafic minerals along with quartz, fluorite, and Mineral Resources, Socorro, NM Lisan Formation along the Dead Sea in Israel kaolinite, sericite, talc, and calcite. Alteration 87801 and Jordan, believed to be a sedimentologi- is stronger in the footwall near the ore zone. Several different types of folds and fold- cal analog of the Todilto in a different tecton- like structures, on different scales, affect the ic setting. The Lisan folds apparently formed Todilto Limestone (Luciano Mesa Member of at least in part from seismogenic processes QUARTZ VEINS AS PROXIES FOR the Todilto Formation, Lucas et al. 1995), or (Marco and Agnon, 1995). FLUID PRESSURE EVOLUTION IN are present within it, in the Grants uranium The Todilto uranium deposits are the THE CONTACT AUREOLE OF THE 1.4 district. These include: regional large-scale largest known deposits in limestone and GA SANDIA PLUTON, NEW MEXICO, folds affecting the Todilto and units above appear to reflect a rare combination of cir- by A. Cavosie, [email protected], L. and below it; large-scale intraformational cumstances. Intraformational folds in the Pletsch-Rivera, J. Selverstone, and Z. Sharp, folds with mappable axes; mounds or pil- Todilto, many of which localize uranium Department of Earth and Planetary low-like structures within the limestone, deposits, also appear to represent a some- Sciences, University of New Mexico, possibly stromatolites; and several types of what unlikely combination of multiple ori- Albuquerque, NM 87131; and B. Dutrow, small-scale intraformational folds. The latter gins; it is difficult to establish the relative Department of Geology and Geophysics, include sharp folding of varve-like thin bed- importance of each process. Louisiana State University, Baton Rouge, ding; within-layer folds resembling those References LA 70803-4101 described elsewhere as "enterolithic”; and Lucas et al., 1995, New Mexico Geological Society A fracture generation model proposed by micro-folding of thin layers, including the Guidebook 46, pp. 247–255. Dutrow and Norton (1995, JMG 13) predicts "crinkly" bedding common in the middle Marco and Agnon, 1995, Geology, v. 23, pp. 695–698. that the heating resulting from pluton intru- and upper portions of the Luciano Mesa sion into fluid-saturated, low-permeability Member of the Todilto. The large-scale host rock will repeatedly increase pore fluid intraformational folds have clearly influ- NEW MEXICO'S COAL INDUSTRY, by pressure and cause multiple fracturing enced the localization of uranium deposits in Gretchen K. Hoffman, [email protected]. events at individual localities. At conditions the Todilto, apparently by providing zones edu, New Mexico Bureau of Mines and of 3 kbar and 800°C, the model predicts the of permeability through which mineralizing Mineral Resources, Socorro, NM, 87801 greatest fracture density approximately 100 solutions moved. Such intraformational Coal production has played a significant m from the pluton contact. I used quartz folds are conspicuous only in or near known role in the economic development of New veins as proxies for fracture propagation in a uranium deposits. Not all of the folds are Mexico beginning in the 1850s and continu- natural contact aureole in order to test the mineralized, but all primary uranium ing to the present. One of the first document- assumptions and predictions made by this deposits in the Todilto are associated with ed mines began operating in 1861 at model. the folds. Many folds were exposed in mines Carthage by the U.S. Army, supplying coal to The contact aureole of the 1.4 Ga Sandia that are now inaccessible; others are exposed Fort Craig. With the westward expansion pluton records peak conditions of around 3 on rim outcrops. Small, isolated uranium and arrival of the railroads in the 1880s many kbar and 750°C. Field observations indicate deposits distant from Grants (Laguna, small coal areas, such as Monero and that quartz veins are most abundant Sanostee, and Box Canyon) were also associ- Madrid, were developed. The mines in these 400–1,000 m from the pluton contact and are ated with folds of this type. areas supplied coal for the steam engine rare close to the contact. XRF analyses indi- The origin of the ore-controlling folds has locomotives and coal and coke to the cate a 6 wt.% decrease in SiO2 and an remained controversial since they were first smelters in the Southwest. Yearly production increase in other elements in the host schist recognized during initial development and first exceeded 1 million short tons (st) in immediately adjacent the vein in one sample mining of Todilto uranium deposits shortly 1889. This first upward cycle of coal produc- location, whereas no depletion selvage has after 1950. An early suggestion that the ore- tion peaked in 1918, with over 4 million st of been determined in other sample locations. controlling intraformational folds relate to coal. Production dropped after World War I δ18O values of quartz at ~400 m from the con- Laramide tectonic activity seems unlikely, as and continued to decline with the economic tact are 13.3 per mil in the vein and 13.6 per isotopic dating of uraninite indicates that the depression of the 1920s and early 1930s. mil in the host rock. deposits formed in Jurassic time shortly after Conversion to diesel engines by the railroad The spatial distribution of quartz in veins, the host rocks were deposited. Evidence of and cheap natural gas also had an impact on SiO2 depletion in vein selvages, and host- soft-sediment deformation is abundant, con- the coal industry, and by 1958 New Mexico rock buffered δ18O values all support the sistent with early formation. coal production had dropped to 86,000 st. model prediction that sequential heating of One hypothesis is that the weight of The next upward cycle began in the early the country rock induced multiple fracturing encroaching dune sands of the overlying 1960s because of the growing population in events via in situ increase in pore fluid pres- Summerville Formation deformed soft lime the Southwest and California and the need sure. However, the absence of depletion sel- muds of the Todilto. Some folds resemble for economical energy. Several of the large vages at 3–4 m wide veins suggests far trav- tepees; along with small-scale enterolithic surface mines operating today (McKinley, eled, plutonic fluids. I prefer a model in folds, these could relate at least in part to Navajo, San Juan) in the San Juan Basin which initial heating increased the in situ forces of crystallization or hydration of cal- began operation in the 1960s or 1970s. pore fluid pressure initiating fracturing, and cite and gypsum. The extensive anhy- During this time, Kaiser Steel opened the produced quartz veins and a new aureole drite/gypsum beds of the overlying Tonque York Canyon underground mine (now Pitts- fluid network system; subsequently, pluton- Arroyo Member are absent where the lime- burg and Midway property) in the Raton derived fluids would have a conduit to stone is mineralized with uranium. Map- Basin, which operated until late 1995. The migrate outwards through the aureole. pable intraformational fold axes may be dis-

New Mexico Geology May 1999 31 Lee Ranch (Hanson Natural Resources) and important to the economy of the state. ca, garnet, sulfur, barite, fluorite, feldspar, La Plata mines (BHP/San Juan Coal) in the Potential for new economic IM deposits is nepheline syenite, pegmatite minerals, sul- San Juan Basin began production in the high for certain commodities, but regulation fur, and diatomite. Clay deposits are general- 1980s. Today, all of New Mexico's coal mines kept exploration and development very low ly sparse in New Mexico, but common clay is are surface operations. All of these are in the 1990s. used locally for pottery, brick, or adobe. Po- dragline operations except for La Plata, New Mexico leads the nation in produc- tential is low for borates and most other IMs. which is a truck and shovel operation. San tion of potash, perlite, and zeolite. It is 2nd in Juan mine started a pilot underground mine pumice and mica and 3rd in humate. In 1996, in 1997. New Mexico ranked 12th nationally for non- THE APPLICATIONS OF JAROSITE GEO- Most of the coal from New Mexico is used fuel mineral production at about $434 mil- CHRONOLOGY AND STABLE ISO- for electrical generation. Two-thirds of the lion or 45% of the estimated total value of TOPE GEOCHEMISTRY TO ORE state's coal production is consumed by the $963 million reported by the U.S. Geological DEPOSIT GENESIS AND WEATHER- three generating stations (San Juan, Four Survey. This contrasts with national totals ING—SOME EXAMPLES FROM THE 1 Corners, Escalante) located in northwestern where metals are about /3 and nonmetals RIO GRANDE RIFT, by Virgil W. Lueth, 2 New Mexico. Half of the electricity produced about /3 of total value. New Mexico differs New Mexico Bureau of Mines and Mineral here is sent over transmission lines to because population is low (small local mar- Resources, Socorro, NM 87801; Robert O. California and Arizona. The remaining coal ket) while the state is large in area (many Rye, U.S. Geological Survey, Denver produced in New Mexico is shipped by rail possible deposits) and remote (transport Federal Center, Denver, CO 80225; and to generating stations in Arizona from costs to distant markets dominate IM eco- Lisa Peters, New Mexico Bureau of Mines Pittsburg and Midway's (P&M) McKinley nomics). Additional IMs produced in New and Mineral Resources, Socorro, NM mine and Hanson Natural Resource's Lee Mexico are aggregate (sand and gravel; 87801

Ranch mine. P&M's Ancho surface mine near crushed stone), cement, gypsum, dimension Jarosite, KFe3(SO4)2(OH)6, and other K- Raton ships coal by rail to Wisconsin. stone (travertine and limestone), salt, scoria bearing minerals in the alunite group, can be New Mexico's 1997 coal production was and cinders, and clay (Table 1). dated by K/Ar and 40Ar/39Ar techniques and

26.77 million st, up 8% from the previous Potential is high for potash and salt, per- contain OH and SO4 groups that can provide 18 year's production. New Mexico ranks 13th in lite, zeolite, pumice, and humate. The four stable isotope analyses: δD, δ OSO4, 18 34 coal production for the nation. Wyoming is potash- and salt-producing area in south- δ OOH, and δ S. Age dating and analysis of first with ten times the production of New eastern New Mexico is a small part of the these isotopic parameters can provide infor- Mexico. Coal is one of the four mineral fuels surrounding lower-grade potash resource. mation on the genesis and natural destruc- produced in New Mexico, ranking third in Deeper potash may be developed through tion of the deposits by weathering. This value behind natural gas (including coalbed technology. Perlite and zeolite potential is information in turn can provide insight into methane) and crude oil. Total revenues from high in the volcanic terrane of southwestern the climatic, geomorphic, and tectonic evolu- coal were $34.3 million in 1997. The majority New Mexico where several deposits have tion of an area, as well as the hydrologic con- of these revenues from coal go into funds been drilled or had minor production. trols on present day water quality. A number that support public schools and education. Humate is abundant in the San Juan Basin in of jarosite occurrences in the Rio Grande rift The coal industry is also a major employer in northwestern New Mexico at weathered coal have been studied, and preliminary results San Juan and McKinley Counties. outcrops and in some carbonaceous shale. are reported here. Coal resources underlie 20% (15 million High-calcium limestone, limestone, and gyp- Geochronologic and stable-isotope studies 2 acres) of the state's total area. Surface sum are abundant in the southern /3 of the of supergene jarosite are underway at the minable coal reserves for the San Juan Basin state, but these sources are often distant from copper porphyry deposits in the Organ and have recently been estimated by NMBMMR consumption centers. Sand and gravel are Jarilla Mountains on the southeast margin of at 6.35 billion st. The Department of Energy abundant along the Rio Grande but are local- the rift. These studies are patterned after the estimates an additional 0.75 billion st of sur- ly restricted near urban areas, which raises work at Creede, Colorado, by Rye et al. face minable coal in the Raton Basin. These delivered cost; other areas use crushed stone (1993). Jarosite and alunite can be used to reserves are low-sulfur coals of subbitumi- or caliche, both of which have moderate to define the hydrogeochemical environment of nous to bituminous rank that have potential high potential. supergene jarosite and alunite mineraliza- for development. Potential is moderate for alunite (possible tion, record the age of paleowater tables, New Mexico coal costs are high; average potash/aluminum co-products), stone, sili- reconstruct climate, geomorphic, and tecton- price per ton is $22.64 for 1997, compared to $6/ton for Wyoming coal. Cost factors are Table 1—Estimated industrial mineral volume and value for 1996 by New Mexico producers. ratio of seam thickness to amount of over- Industrial mineral Volume Value Price POTENTIAL and comments burden, health and safety regulations, and commodity '000 mt '000 US$ US$/mt (data may be rounded) reclamation costs. New Mexico coal can not easily compete with Wyoming coal because Cement (est.) 455 29,575 65.00 HIGH for limestone, cement supply adequate Clay, common 32 165 5.16 LOW for large deposits, locally common of the tremendous difference in seam thick- Dimension stone <2 210 128.79 HIGH, local demand is low ness and the lack of an integrated rail net- Gypsum (est.) 600 4,260 7.10 HIGH, mainly in south-central NM work, particularly in the San Juan Basin. Humate 16 960 60.00 HIGH, large resource, mainly northwest NM Cost of mining and transportation are signif- Mica 20 4,400 220.00 MODERATE, mainly north-central NM icant economic factors, particularly because Perlite (est.) 600 17,520 29.20 HIGH, large resource, mainly southwest NM Potash 2,430 225,000 92.59 HIGH, large resource, deep, southeast NM of the deregulation of the power industry. Salt (est.) 315 15,750 50.00 HIGH, mainly southeast NM These changes are requiring the New Mexico Pumice and pumicite 102 527 5.17 HIGH, large resource, north-central NM coal mining industry to be as competitive as Sand and gravel 9,880 48,500 4.91 HIGH, large resource, urban resource lower possible. Concrete & concrete products 1,580 9,450 5.97 Highway and building construction Asphaltic concrete 500 3,200 6.40 Highway construction Road base & coverings 885 4,150 4.69 Highway construction Fill 320 627 1.96 Highway and building construction INDUSTRIAL MINERAL PRODUCTION Other 6,595 31,073 4.71 Highway, buildings, and landscaping AND POTENTIAL IN NEW MEXICO, Stone, crushed 3,480 18,850 5.42 HIGH, mainly in eastern NM 2 by James M. Barker and George S. Austin, Limestone 1,350 6,090 4.50 Abundant in southern /3 of NM New Mexico Bureau of Mines and Mineral "Granite" 1,490 9,240 6.20 Abundant in northern NM Resources, Socorro, NM 87801 Cinder, scoria 283 2,170 7.66 Hundreds of volcanic cones western NM Other stone 357 1,382 3.87 Basalt (traprock), quartzite, caliche and others Industrial minerals, rocks, and materials Zeolite (est.) 15 1,125 75.00 HIGH, mainly southwest NM are nonmetallic commodities (IM hereafter). TOTAL 31,307 434,224 13.87 Average price (unweighted) Some produced in New Mexico lead the Sources: US Geological Survey, NM Bureau of Mines & Mineral Resources, NM Energy & Natural Resources nation in several categories, and all are Department, unpublished files, company data and estimates (est.). st = mt x 1.1023114; $st = $mt/1.1023114

32 May 1999 New Mexico Geology ic history of the area. Sulfur isotopes trace hypogene copper sulfides, and of former heterogeneities, the waste rock piles contain sulfur to precursor sulfide sources. Hydro- enrichment zones as a result of in situ oxida- variable grain sizes. There are 100,000s of gen and oxygen isotopes can be used to infer tion of "enrichment blankets." The formation waste rock piles in New Mexico of varying climate changes during the course of super- of copper oxide minerals can occur in any cli- size and composition. One of the controver- gene oxidation. The ages and distribution of mate, but extremely arid conditions engen- sial problems facing scientists today is how jarosite and alunite in time and space define der the formation of exceptionally deep oxi- to sample these waste rock piles quickly and periods of uplift related to tectonic events. dized zones as a result of sparse flushing economically in order to adequately assess An extensive geochronological and stable- with surface water and very deep phreatic and prioritize environmental hazards. isotope study of jarosite at the Copiapo mine, zones. Oxide zones of laterally continuous A sampling strategy was developed to Doña Ana County, New Mexico, has defined copper oxide development in extremely arid geochemically characterize four mine waste a new mode of occurrence—hydrothermal environments have been noted as deep as rock piles in the Hillsboro mining district in jarosite related to sour gas from sedimentary 500 m below the present topographic sur- Sierra County, New Mexico. The piles repre- basins. Mineralization is hosted by a fault face. sented four different mineral deposits and cutting Pennsylvanian-age limestone and The depth of the local phreatic zone, in include: a placer gold waste rock pile (Site consists of a replacement body with sequen- general, controls the depth of oxidation. A), a Laramide polymetallic vein waste rock tial deposition of halloysite, fluorite, proso- Oxidizing conditions persist into the phreat- pile (Site B), a carbonate-hosted Pb–Zn waste pite, gypsum, hematite, jarosite and natro- ic zone, although the oxygenated water at rock pile (Site C), and a carbonate-hosted jarosite. 40Ar/39Ar age dates on hydrothermal depth is very reduced compared to the near- Ag–Mn waste rock pile (Site D). The waste jarosite and natrojarosite display a narrow surface water. Reducing conditions persist rock piles have highly variable grain sizes. range of apparent ages from 5.0 ± 0.12 to 4.5 into the oxidized volume as well, resulting Chemical analyses of six size fractions indi- ± 0.16 Ma with the natrojarosite consistently from capillary action moving reduced water cated the less than 0.25 mm mesh typically younger. Low sulfur isotope values for the along fractures. As a result of the channel- contained the highest metal concentrations. jarosites (δ34S = –16 to –24) indicate a basin- ized fracture flow and capillary action a In addition to each of the grain size fractions, derived source of H2S with subsequent oxi- vadose zone is formed in which a combina- chemical heterogeneity is found within an dation during mineralization. Hydrogen tion of sulfide and oxide minerals are stable. entire waste rock pile. As an example, 45

(δDsmow = –64 to –96) and oxygen isotope Acid conditions are created as oxygenated samples were analyzed by XRF for each of 18 (δ Osmow = –4 to –10) values of parent waters ground water migrates, especially along the sample sites within a grid overlying the indicate an exchanged meteoric water origin fractures, and oxidizes sulfide minerals. The Laramide polymetallic vein waste rock pile typical of basin derived brines similar to the result of this is the formation of oxide and (Site B). Copper values range from 460 to deep saline ground waters found in the sulfate minerals that are dissolved and trans- 2700 ppm, lead from 73 to 730 ppm, zinc basins today. Calculated temperature of for- ported by periodic flushing into reduced from 120 to 1400 ppm, and arsenic from 4 to mation, from the fractionation of 18O between rock volumes where supergene sulfide min- 76 ppm. the SO4 and OH sites in many of the jarosites, erals form. If the local phreatic zone is low- Each of the four waste rock piles was sam- is approximately 130º C. ered and if the sulfide minerals experience in pled using unique grids that conformed to Analysis of jarosite from the Hansonburg situ oxidation, the result will be a geochemi- their respective dimensions. To ensure equal deposits, Socorro County, New Mexico, cally thick copper oxide zone. treatment of the samples, sampling was per- reveals a range of apparent ages from 7.9 ± The copper oxide minerals that form are a formed using a less than 0.25 mm stainless 0.85 Ma to 1.63 ± 0.06 Ma. Each specific age function of local pH conditions within the steel sieve. Each of the waste rock piles was of jarosite mineralization contains a unique deposit. Chalcanthite and kröhnkite, along sampled with densities of 15, 30, and 45 sam- mineral paragenesis and distinct stable-iso- with a series of iron oxides, occur in very ples within each of the respective grids. This tope signatures. Both hydrothermal and acid conditions; these minerals are very study indicates that heterogeneous mine supergene jarosites are present. Hansonburg, water-soluble and tend to be redissolved, waste rock piles can be adequately sampled Copiapo, and many other jarosite-bearing surrendering copper to copper oxide miner- and chemically characterized by homogeniz- deposits along the rift contain abundant als such as antlerite and brochantite. In mod- ing samples collected from a grid containing hydrothermal fluorite, suggesting that they erate pH conditions minerals such as 15–30 sample cells. have fundamentally similar origins con- neotocite, copper pitch, and cuprite are sta- trolled by tectonic events, climate, and the ble. Conditions favorable for malachite and evolution of brines and sour gas in sedimen- azurite exist when there is a sufficient MINE WASTE ROCK PILE MINERALO- tary basins. amount of reactive carbonate material in the GY AND GEOCHEMISTRY IN SELECT Reference wall rock or sufficient carbonate in solution. AREAS OF THE HILLSBORO MINING Rye, R. O., Bethke, P. M., Lanphere, M. A., and Chrysocolla will form if there is adequate sil- DISTRICT, SIERRA COUNTY, NEW Steven, T. A., 1993, Age and stable isotope sys- ica in solution. MEXICO: INSIGHTS INTO METAL tematics of supergene alunite and jarosite from MOBILITY, by Erik A. Munroe, Depart- the Creede mining district, Colorado: implica- tions for supergene processes and Neogene geo- ment of Earth and Environmental Science, morphic evolution and climate of the southern SAMPLING PROTOCOL FOR MINE New Mexico Institute of Mining and Rocky Mountains: Geological Society of America WASTE ROCK PILE CHARACTERIZA- Technology, Socorro, NM 87801; Virginia T. Abstracts and Programs, v. 25, p. A-274. TION IN THE HILLSBORO MINING McLemore, and Nelia Dunbar, New Mexico DISTRICT, SIERRA COUNTY, NEW Bureau of Mines and Mineral Resources, MEXICO, by Erik A. Munroe, Department Socorro, NM 87801 COPPER OXIDE DEVELOPMENT IN of Earth and Environmental Science, New In New Mexico, there are at least 100,000 ARID ENVIRONMENTS, by Chadwick J. Mexico Institute of Mining and Techno- abandoned mine waste rock piles with wide- Spencer, [email protected], and William logy, Socorro, NM 87801; Virginia T. McLe- ly ranging mineralogical and geochemical X. Chávez, Jr., [email protected], De- more, New Mexico Bureau of Mines and compositions. To better understand the envi- partment of Mineral Engineering, New Mineral Resources, Socorro, NM 87801; ronmental implications of metal mobility in Mexico Institute of Mining and Tech- and Philip Kyle, Department of Earth and regions of minimal precipitation (< 250 nology, Socorro, NM 87801 Environmental Science, New Mexico Insti- mm/year) a mineralogical and geochemical Copper oxides are presently a topic of tute of Mining and Technology, Socorro, study was implemented for five mine waste great interest in the mining and metallurgical NM 87801 rock piles, some of which contained sulfides, industry because of the ease with which they Mine waste rock piles are anthropogeni- as well as the drainage systems from these can be leached and the resulting low cost of cally created landforms in mining areas that areas in the Hillsboro mining district. copper extraction. Copper oxide minerals are derived from open pit and underground Furthermore, this study will give insight into occur in the oxidized zone of porphyry cop- mining methods. These waste rock piles the mineralogical mechanisms governing per deposits as a result of the oxidation of exhibit a diverse mineralogy and geochem- metal mobility in arid environments. istry. In addition to the mineral and chemical The average metal content of a Laramide

New Mexico Geology May 1999 33 vein waste rock pile is 1,200 ppm copper, 230 localities within the Precambrian Grand Electron microprobe analyses suggest that ppm lead, 550 ppm zinc, and 26 ppm arsenic, Canyon Supergroup yield a complex array of the muscovites from the Dox samples have but a stream sediment sample directly below crystal ages. The Grand Canyon Supergroup been recrystallized. Additionally, the micro- the area contains 190 ppm copper, 52 ppm is exposed exclusively in eastern Grand probe reveals that the sandstone matrix is lead, 150 ppm zinc, and 8 ppm arsenic. The Canyon and rests with angular unconformi- rich in K and Fe, possibly reflecting a signif- mineralogical assemblage of the same ty on top of the Granite Gorge Metamorphic icant fluid influx event. This event is proba- Laramide vein waste rock pile consists of (in Suite. The Supergroup is grossly sub-divided bly ca. 1,100 Ma and may be associated with order of abundance) quartz, albite, micro- into the Unkar Group, which is uncon- the igneous activity that occurred at this cline, chlorite, illite, hornblende, muscovite, formably overlain by the Nankoweap time. Combined, the geochronological pyrite, chalcopyrite, hematite, minor bornite, Formation, which in turn is unconformably results indicate that the Dox Formation is chalcanthite, and actinolite. The stream sedi- overlain by the Chuar Group. Age con- older than 1,100 Ma and that the young ment contains igneous rock fragments, straints on these important Precambrian sed- (~650 Ma) ages from the Sixtymile Formation quartz, albite, chlorite, hematite, minor iments are limited; however, the top of the may be the result of argon loss and therefore pyrite, calcite, gypsum, and cuprite. Unkar Group includes the ca. 1.1 Ga can not be used to argue that the Sixtymile Mineralogical characteristics of mineral Cardenas Basalts, thereby placing a mini- Formation is younger than 650 Ma. grains and their weathering rind products mum age on the Unkar Group. The Chuar were examined with the electron microprobe Group is constrained to be younger than the to examine the chemical breakdown of min- Cardenas Basalts and older than Middle QUARTZ–KYANITE PODS IN PROTERO- erals that release metals to the environment. Cambrian (Tapeats Sandstone). ZOIC ROCKS IN NORTHERN NEW Several minerals have weathering rinds of Muscovite from four separate samples of MEXICO: SHEAR ZONE FORMATION different mineralogical and chemical compo- the Dox Formation within the Unkar Group ALONG AN OLDER HYDROTHER- sitions than their cores. Pyrite and chalcopy- were analyzed. Single crystal laser fusion MAL ALTERATION HORIZON, by Mary rite appear to be the most reactive to surficial (SCLF) analyses were conducted on 110 crys- C. Simmons, Department of Earth and Pla- weathering and tend to show the thickest tals, and additionally 19 individual crystals netary Sciences, University of New Mex- weathering rinds. Chemical composition of a were incrementally heated for age spectrum ico, Albuquerque, NM 87131 pyrite core/rind transect showed iron con- analysis. Forty-seven crystals from the Large quartz–kyanite schist pods of unu- centration increased from 46% to 66% while Tapeats Sandstone were dated with SCLF sual bulk composition enclosed by shells of sulfur decreased from 54% to 2%. Chemical methods, and 7 grains were step heated. Fif- sericite schist occur within the 1.7 Ga Vadito composition of a chalcopyrite core/rind teen muscovite crystals were also analyzed Group metarhyolite in northern New Mex- transect showed copper concentration from the Sixtymile Formation, which uncon- ico. These pods are discontinuous, lenticular, decreased from 33% to 3%, iron increased formably caps the Chuar Group. The majori- symmetrically zoned, and are stratiform from 30% to 58%, and sulfur decreased from ty of the SCLF results from the Dox For- within a map-scale sericitic horizon. Pre- 34% to 0.3%. Sulfur lost from the system will mation yield ages between 1,050 and 1,200 vious studies have not resolved whether the be removed by the formation of sulfuric acid Ma with a strong peak at ca. 1,100 Ma. The high-Al bulk compositions of the quartz– after oxidation occurs. Clays, iron crystals from the Sixtymile Formation are kyanite pods were the result of weathering, oxide/hydroxide, iron sulfate, and primary typically between 1,200 and 1,400 Ma, but hydrothermal alteration, or shearing. Geo- texture rinds exist in the waste rock piles one grain is ~650 Ma and one ~1,000 Ma. Tra- chemical differences between the quartz– analyzed. ditionally, the youngest detrital age is inter- kyanite/sericite schist pods and the sericite- Metal ratios of mine waste rock preted to be a maximum estimate for the sed- rich layer that connects them suggest more pile/stream sediment sample are copper 6.3, imentary age thereby suggesting that the than one alteration process. This study uses lead 4.4, zinc 3.6, and arsenic 3.2. Metal Dox Formation is younger than ~1,100 Ma geochemical, structural, and metamorphic mobility, therefore, can be described in the and that the Sixtymile Formation is poten- data to evaluate the origin and tectonic evo- following manner: As > Zn > Pb > Cu. Super- tially as young as 650 Ma. Considering that lution of the quartz–kyanite rocks. gene oxidation of galena created a cerrusite the Supergroup sediments overlie Precam- Geochemical data from sampling travers- rind, which effectively shields the galena brian crystalline rocks with muscovite cool- es, mineral textures, and map patterns indi- grain from further weathering and release of ing ages between 1,400 to 1,600 Ma, it is cate that the quartz–kyanite pods obtained lead and sulfur. This may decrease the level intriguing that very few detrital grains yield their unusual (high Al, low K, Na, Ca, Fe) of lead mobility depending on the overall these ages. Moreover, the grouping of ages at compositions through hydrothermal alter- acid producing capability of the waste rock ca. 1,100 Ma is conspicuously near the age of ation associated with volcanism. Geochem- pile. Metals present in sulfide mineral the Cardenas Basalts and other 1,100 Ma ical profiles are symmetrical with depletion weathering rinds are in higher concentra- intrusive rocks. These observations suggest of Ca, Na, K, Fe and enrichment of Si toward tions than those in rinds surrounding oxides that argon loss from the detrital muscovites the center of the alteration zone. Higher K or silicates. Oxides and silicates need more may have occurred since deposition, and and Fe compositions in the sericite-rich layer time and water to break down and showed therefore traditional interpretations to con- that connects the pods suggest a different at most a minimal clay rind. This indicates strain sedimentary ages may be inappropri- alteration process. Truncation of stratigraph- metal mobility is higher in the sulfide miner- ate. ic map units, grain-size reduction, S–C fab- als, which therefore enhances metal avail- The age spectrum analyses of the single rics, and asymmetric porphyroblasts suggest ability to the environment. grains provide additional insight towards that this second alteration process was relat- the meaning of the overall young SCLF ed to a top-to-the-south shearing episode

results. One sample of the Dox Formation (D1) along a bedding-subparallel zone before

shows copper mineralization, and initial D2 (N-vergent) deformation produced map- SESSION 2—PRECAMBRIAN heating steps of the age spectra yield ages scale folds. GEOLOGY AND VOLCANOLOGY between 200 to 500 Ma, clearly indicating Microstructural studies show that kyanite post-deposition argon loss. However, age is an early (S1) metamorphic mineral pro- spectra for other samples of the Dox duced prior to shearing of the previously WHERE HAVE ALL THE OLD MICAS Formation do not yield ages below ca. 1,100 altered volcanic rock, shown by alignment GONE?, by M. T. Heizler, New Mexico Ma. Combined, these results are interpreted and grain-size reduction of kyanite within

Bureau of Mines and Mineral Resources, to record two alteration events, one at ca. the earliest fabric (S1). Subsequent metamor- Socorro, NM 87801; K. E. Karlstrom and M. 1,100 Ma and one in the Phanerozoic (?Lara- phism and shearing may have enhanced the J. Timmons, Department of Earth and mide?). concentration of silica and aluminum in this Planetary Sciences, University of New The geochronologic results demonstrate zone and linked the pods of altered rock into

Mexico, Albuquerque, NM 87131 argon loss since deposition but can not dis- a map-scale sericite-rich (S1) shear zone. Single crystal 40Ar/39Ar age determina- cern between thermal resetting versus loss Other minerals that formed early in the tions of detrital muscovites from several associated with alteration or diagenesis. deformational history of these rocks include

34 May 1999 New Mexico Geology staurolite, chloritoid, paragonite, and albite, other structures that Hamblin identified as striking contrast in silica content. This might indicating peak P–T conditions of up to basalt pillows, some of which are 10–20 m also be explained by a high water content for

~550°C, ~5 kbar for S1 fabrics. across, and this association led him to the felsic melt. To test this idea we are cur- attribute the ellipsoidal structures, as well as rently calculating the viscosity of the two the giant pillows, to interaction with water, melts for different water contents. These ORIGIN OF SPHEROIDAL AND ELLIP- which is supported by the position of the water contents will be compared to experi- SOIDAL FLOW(?) STRUCTURES IN Toroweap flows relative to the Colorado mental haplogranite water solubility data to THE BEARWALLOW MOUNTAIN AN- River, past and present. Where the similar estimate a depth of melting. This depth will DESITE, MOGOLLON–DATIL VOL- structures in the Bearwallow Mountain be compared to the inferred stratigraphic CANIC FIELD, NEW MEXICO, by James Andesite are found, there are no obvious section beneath the volcanoes for considera- C. Ratté, U.S. Geological Survey, P.O. Box relationships with major pre-existing water tion of potential protolith horizons. 25046, MS-905, Denver, CO 80225 courses, but further consideration of this Enigmatic structures in the Bearwallow possibility may be warranted. Mountain Andesite, or Bearwallow Moun- I conclude that the spheroidal and ellip- EFFECTS OF SHALLOW BASALTIC tain Formation, are here interpreted to be soidal structures in the Bearwallow INTRUSION INTO SILICIC TEPHRA, flow features. The Bearwallow Mountain Mountain Andesite are probably flow struc- GRANTS RIDGE, NEW MEXICO,by Andesite is a voluminous assemblage of tures because of the encircling, stretched Giday WoldeGabriel, [email protected], Gor- andesitic to basaltic lava flows that covers vesicles, and because they appear to grade don N. Keating, and Greg A. Valentine, Earth large areas of the Mogollon–Datil volcanic into more common flow structures such as and Environmental Sciences Division, Los field of southwestern New Mexico. Flows ramped flow layers. In keeping with the Alamos National Laboratory, Los Alamos, were erupted from numerous polygenetic, interpretation as flow features, I offer the NM 87545 and perhaps some monogenetic, centers best perhaps outrageous hypothesis that they A shallow basaltic intrusion into chemical- described as shield volcanoes. Many litera- might be akin to standing waves in water, ly homogeneous silicic tephra at depths of ture references to them as stratovolcanoes but because of the much greater viscosity of <200 m was investigated as part of a natural seem inappropriate because of their compo- andesitic and basaltic lava, they are essen- analog study on the effects of subsurface dis- sition, low, domal profiles, and lack of appre- tially standing waves frozen in space. posal of high-level radioactive waste. This is ciable interlayered pyroclastic material. The Finally, I hope others will find an opportu- an attempt to understand the effects of intru- Bearwallow volcanoes erupted about 25 mil- nity to see these interesting flow(?) struc- sions into unsaturated zeolitic tuffs that will lion years ago, and many are aligned along tures in the Bearwallow Mountain Andesite potentially host high-level radioactive waste. predominant northeast–southwest and west- and look for new evidence to confirm their Moreover, the extent of man-made hydro- northwest–east-southeast trends believed to origin. thermal system that could develop from reflect early crustal extension that preceded radioactive decaying in a shallow repository Basin and Range deformation in this area. is evaluated. The spheroidal and ellipsoidal structures CHARACTERISTICS AND ORIGIN OF A localized aureole up to 10 m wide devel- are somewhat uncommon but have been MELTED XENOLITHS AT VULCAN oped around a 150-m wide, 2.6 Ma basaltic seen at widely separated localities, in lavas CONE AT THE ALBUQUERQUE VOL- plug at Grants Ridge, New Mexico. The plug from different eruptive centers. They may be CANOES, by Kathleen E. McLeroy and intruded into nonwelded, pumice-rich com- most easily observed in road cuts along the Gary Smith, Department of Earth and positionally homogeneous tuff and volcani- National Forest road leading south and east Planetary Science, University of New clastic sediments of similar age (3.3 Ma). from Reserve to Negrito Creek and in cliffs Mexico, Albuquerque, NM 87131 Color variation (pinkish to orange), strong along the Tularosa River about 2 mi above its Melted xenoliths are present at Vulcan local contact welding, brecciation, and stop- confluence with the San Francisco River, in cone, highest of the middle Pleistocene ing characterize the host rock within the con- Catron County. Albuquerque volcanoes. The inclusions at tact zone. Despite the high-temperature The structures are most conspicuous Vulcan cone are light colored, are more than basaltic intrusion, there is no indication for where they are spheroidal or elliptical in 90% glass, and, in many cases, are extremely extensive convective heat transfer and per- form, but at some places, they appear to vesicular. Petrographic examinations show vasive hydrothermal circulation and alter- grade into ramp structures, as commonly that the inclusions are about 15–30% round- ation. The proportion of volcanic glass, loss found at flow fronts or where flows traverse ed quartz grains and glass (locally hydrated on ignition (LOI), fluorine, iron, and some uneven topography. Where spheroidal in to unidentified zeolites) and vesicles. trace- and rare-earth-element contents in the form, they are commonly 1–5 m in diameter, XRF analyses indicate a SiO2 content of host rocks are depleted near the intrusion. but the long axes of more ellipsoidal struc- 84%. This composition is unreasonable for an Conversely, the degree of devitrification and tures may be 10 m long. igneous protolith but, along with the other the potassium content are higher along the Initially, these spheroidal and elliptical oxides, is consistent with an arkosic sand- contact. Vapor-phase expulsion of elemental features might bring to mind filled lava stone (70% quartz, 13% K-feldspar, 17% pla- species as fluoride, chloride, hydroxide, sul- tubes, but the presence of circumferential, gioclase [An 28]). fide, and carbon oxide complexing may have stretched vesicles would seem to rule out The basalt is not quenched where it is in been responsible for the depletion of the ele- that idea. Alternative explanations, gleaned contact with the exotic melt, leading to the ments during the devitrification of silicic from a summary search of the literature, conclusion that the magma had reached ther- glass at near- solidus temperature related to include accretionary lava balls, lava channel mal equilibrium. In places the melts have the basaltic intrusion. fillings, lava squeeze-ups, lava coils, and thoroughly mingled, and a hybrid liquid The results of finite-difference numerical structures formed by interaction of lava and appears to have formed. We have found no modeling of the intrusion as a dry, conduc- water. However, none of these seem to quite reference to similar extremely melted sedi- tion-dominated system agree well with geo- fit the ellipsoidal and spheroidal structures mentary xenoliths, suggesting unusual con- chemical and mineralogical data. Contact in the Bearwallow Mountain Andesite. Then, ditions for their formation. welding of the host rocks apparently occur- I came across Kenneth Hamblin’s excellent Our hypothesis is that extreme melting red at temperatures >700°C under a density- descriptions of lava flows and lava dams in was a consequence of wet-melting at suffi- driven lateral load of approximately 1 MPa, the Grand Canyon in GSA Memoir 183. Dr. cient depth that the pore water was not corresponding to the observed depth below Hamblin describes ellipsoidal structures in expelled but participated in the melting reac- the former ground surface of <200 m. Other Flows E and F of the Toroweap lava dam, tions and was incorporated into the newly physical changes in the first 10 m of host and his sketch of these features, in his figure formed melt. The extreme vesicularity of the rock, represented by partial devitrification 29, closely resembles those in the Bear- ejecta affirms a high dissolved volatile con- and color changes, apparently occurred at wallow Mountain Andesite at the Tularosa tent for the melted xenoliths. Mingling of the temperatures of 500–600°C, which probably River locality. However, the features in the two melts also suggests similar viscosity at persisted for up to 55 yrs after the emplace- Toroweap basalt flows are associated with some point in their combined history, despite ment of the basaltic plug. Devitrification is

New Mexico Geology May 1999 35 generally enhanced by the presence of aque- K–Ar biotite, McLemore, 1983, NMGS v. 27: 4, pp. 945–965). Thus, preliminary data ous fluids; however, the abundance of vol- Guidebook, 34:235–240) in central New Mex- support a magmatic origin for the syenites. canic glass within a short distance (~ 10 m) ico; and a volcanic rock from a depth of from the plug is consistent with our infer- about 2,800 m in an oil test well near Tula- ence that the limited water-rock interaction rosa (541 ± 21, K–Ar whole rock; Loring and ARC ABANDONMENT AS A CAUSE FOR in the nonwelded ignimbrite and fallout was Armstrong, 1980, Geology, 8:344–348). The PASSIVE CONTINENTAL RIFTING: caused by intrusion into unsaturated envi- chemistry of these rocks and in some cases COMPARISON OF THE JURASSIC ronment. Our study appears to suggest that the short time interval between intrusion MEXICAN BORDERLAND RIFT AND basaltic intrusion into nonwelded unsaturat- and deposition of the overlying Ordovician THE CENOZOIC RIO GRANDE RIFT, ed tuffs has localized contact metamorphic sedimentary rocks suggest that they were all by Timothy F. Lawton and Nancy J. effects and insignificant hydrothermal alter- generated during continental rifting. Thus, McMillan, Department of Geological ation. we propose that an aulacogen, similar to the Sciences, New Mexico State University, Southern Oklahoma aulacogen, existed in Las Cruces, NM 88003 New Mexico during Cambrian and Early Two rift systems, the late Mesozoic Ordovician time. Borderland rift that includes the Bisbee and SESSION 3—RIFT-RELATED McCoy Basins and the Cenozoic Rio Grande MAGMATISM, SEDIMENTATION, rift, in the southern Cordillera of North AND TECTONICS ORIGIN OF THE SYENITIC ROCKS OF America formed along the inner flanks of THE RED HILLS, SOUTHERN CABAL- former continental-margin arcs. Both rift sys- LO MOUNTAINS, NEW MEXICO, IN tems were initiated when arc magmatism THE LOBO HILL ALKALIC COMPLEX, THE LIGHT OF FIELD, PETROGRAPH- abandoned its former inboard extent as a TORRANCE COUNTY, NM: CAMBRI- IC, AND GEOCHEMICAL STUDIES result of retrograde motion of the subducted AN MAGMATISM IN THE NEW MEXI- ACCOMPANIED BY THE COMPARI- slab. Similarities in the tectonic and geo- CO AULACOGEN, by N. J. McMillan, SON WITH TWO PETROGENETIC chemical stratigraphy preceding and during [email protected], Department of MODELS, by S. Pandurang, spandura crustal extension of each rift system suggest Geological Sciences, New Mexico State @nmsu. edu, N. J. McMillan, Department a three-phase magmatic-depositional model University, Las Cruces, NM 88003; and V. of Geological Sciences, New Mexico State for the formation of passive continental rifts T. McLemore, New Mexico Bureau of University, Las Cruces, NM 88003; and V. above a foundering subducted slab. Phase 1: Mines and Mineral Resources, Socorro, T. McLemore, New Mexico Bureau of Continental margin arc magmatism during NM 87801 Mines and Mineral Resources, Socorro, normal subduction weakens the continental New analyses of alkalic igneous rocks and NM 87801 crust. This phase is represented in the carbonatites from the Lobo Hill alkalic com- The brick-red, silica-depleted, microcline Borderland rift system by the Mount plex near Moriarty, NM, provide new evi- microperthite syenites of the Red Hills at the Wrightson Formation and related calc-alka- dence for a widespread, early Paleozoic, Caballo Mountains occur within the Pre- line volcanic successions, and in the south- alkalic igneous event in New Mexico. The cambrian granites in the form of elongated ern Rio Grande rift by the Rubio Peak and complex consists of brick-red, fine-grained bodies. The syenites are studied with the aim Palm Park Formations. Phase 2: Incipient ret- syenitic and carbonatitic dikes that intrude of determining the petrogenesis with the aid rograde motion of the slab, or "slab founder- the Precambrian basement and clearly cross- of field studies, petrography, and geochem- ing," initiates mantle return into the wedge- cut the Precambrian foliations. Previous istry. Field studies include sample collection shaped volume between slab and overlying attempts at radiogenic isotope dating of the across a zone of metasomatism and from continental lithosphere, causing crustal Lobo Hill complex were unsatisfactory; how- individual syenite bodies (some containing extension, lithospheric melting, and deposi- ever, a new 40Ar/39Ar age determination on granite xenoliths). The syenites are being tion of conglomerate in nascent rift basins biotite from a syenite yields an age of 518 ± analyzed for major elements and trace ele- stratigraphically above and adjacent to the 5.7 Ma. Syenites have aphanitic porphyritic ments. extinguished arc. Caldera-related silicic vol- textures, with altered plagioclase pheno- Field observations, petrographic analysis, canism and coeval extrusion of lithosphere- crysts set in felty to intergranular ground- and literature survey indicate that the petro- derived basalt and basaltic andesite define a masses; the rocks are almost devoid of ferro- genesis of the syenites can be tested by two bimodal "ignimbrite flare-up." This phase is magnesian minerals. Textures formed during models. In Model 1 the syenites are magmat- represented in the Borderland rift system by rapid cooling indicate that the 40Ar/39Ar date ic with accompanying metasomatism and an assemblage of outflow and intracauldron can be interpreted as the age of emplacement formed in a failed rift during Cambrian– tuffs informally known as the "quartz por- and cooling at shallow depths. Na2O and Ordovician time (Ervin, S. D., and McMillan, phyries" (for example, the Cobre Ridge Tuff

K2O concentrations are highly variable. N. J., 1997, Ann. G.S.A. Meeting, Abs. with and Canelo Hills Volcanics) and in the south-

K2O/Na2O ratios vary from 0.34 to 2.96; two Prog., p. 231; McLemore, 1986, 37th N.M.G.S. ern Rio Grande rift by the Bell Top For- samples from the top of the complex have Field Conf. Guidebook, pp. 151–159). Model mation. Mafic volcanism, represented in

K2O/Na2O of ca. 130, have late-stage fluorite 2 suggests that the syenites are late-stage southern New Mexico by the Uvas basaltic and high concentrations of Rb, Pb, Th, and hydrothermal alteration products of the andesite, block faulting, and extensional sed- Zn, and record metasomatism at the fringes granite accompanied by quartz dissolution imentary-basin formation continue after the of the system. Incompatible trace elements Y, and replacement (metasomatism) by secon- end of silicic volcanism. Phase 3: Decom-

Zr, and Nb increase with increasing SiO2 in dary K-feldspars. Unfortunately, the syenites pression-induced partial melting of convect- the suite, while Ba and Sr decrease, suggest- are devoid of datable minerals, and all the ing asthenosphere in the mantle wedge cre- ing that fractional crystallization of feldspar rocks are significantly altered, precluding ates basalts with ocean-island chemical was the dominant differentiation process. age determinations. affinities intercalated with alluvial or marine The Lobo Hill alkalic complex is one of Modal analysis across two zones of meta- sedimentary rocks in extensional basins. This several similar intrusions of Cambro–Ordo- somatism (29 samples) indicates an abrupt phase is represented in southern Arizona by vician age in New Mexico and Colorado. change in composition across the the Glance Conglomerate, Crystal Cave Although many of these complexes lack syenite/granite contact accompanied by a Formation, and interbedded volcanic rocks, modern age determinations, the well-dated decrease in quartz from 40–0 % and increase and in southwestern New Mexico by newly suites include the Florida Mountains alkali in microcline from 1–94%. Petrographic discovered marine and volcanic strata feldspar granite in southern New Mexico studies of the Red Hills syenite have shown beneath the Lower Cretaceous Hell-to-Finish (503 ± 10 Ma, U–Pb zircon, Evans and Cle- that the alteration of the syenites can be dis- Formation. In the Rio Grande rift system, mons, 1988, AJS, 288:735–752; 491 ± 5 Ma, tinguished from that of the granites, based phase 3 is recorded by the Santa Fe Group, 40Ar/39Ar hornblende, Ervin, 1998, MS thesis, on the changes in primary mineralogy and including the Hayner Ranch, Rincon Valley, NMSU); the Lemitar carbonatites (449 ± 16, the type of secondary vug filling and Camp Rice Formations. (Cathelineau, M., 1986, Journal of Petrology,

36 May 1999 New Mexico Geology DEVELOPMENT OF A MAJOR RIFT nal warps, hot springs, ground-water flow, ern San Luis Basin and northern Española TRANSFER ZONE (EMBUDO FAULT): and asymmetric drainages. Basin, has purported that rift subsidence did INSIGHTS INTO THE EVOLUTION OF not begin until middle or late Miocene THE NORTHERN RIO GRANDE RIFT (Baldridge et al., 1994, GSA Bull. 105, p. 1538; IN NEW MEXICO, by Paul Bauer, New MOUNTAIN-FRONT GEOLOGY OF THE Ingersoll et al., 1990, GSA Bull. 102, p. 1280). Mexico Bureau of Mines and Mineral TESUQUE QUADRANGLE, SANTA FE We have found evidence for earlier onset of Resources, Socorro, NM 87801; Keith I. COUNTY, NEW MEXICO, by Claudia I. rift subsidence in the Oligocene–early Kelson, William Lettis and Assoc., 1777 Borchert, [email protected], and Gary Miocene Abiquiu Formation, which strad- Botelho, Walnut Creek, CA 94596; and Smith, Department of Earth and Planetary dles the margin of the Rio Grande rift and Peggy Johnson, New Mexico Bureau of Sciences, University of New Mexico, Albu- Colorado Plateau near Abiquiu, New Mines and Mineral Resources, Socorro, querque, NM, 87131 Mexico. Structural and stratigraphic rela- NM 87801 Our research in the Tesuque quadrangle, tionships between Mesozoic strata of the Three major fault systems intersect in the Santa Fe County, New Mexico, has produced plateau and the upper member of the Taos area of northern NM: 1) the repeatedly insights to controversial stratigraphic and Abiquiu Formation provide evidence for reactivated, N-striking, 5-km-wide Picuris– structural relationships along the Sangre de syndepositional subsidence of the Abiquiu Pecos fault system (PPF); 2) the Holocene, Cristo Mountain front. embayment, a shallow structural bench of eastern, rift-bounding Sangre de Cristo fault Volcaniclastic strata of the Bishop's Lodge the rift adjacent to the deeper San Luis and zone (SdCF); and 3) the Embudo fault zone Member have been variously placed within Española Basins. The data presented here (EF), which is the transfer zone between the or unconformably below the Tesuque indicate riftward stratigraphic thickening in San Luis and Española rift basins. The moun- Formation. Our mapping clearly substanti- the upper Abiquiu Formation across rift- tain front zone between Pilar and Cañon pro- ates early workers' choice of including these bounding faults. vides preeminent exposures of the termina- strata within the Tesuque. The volcaniclastic Stratigraphic thickness between intervals, tion of a major rift transfer fault. New, layers are interbedded with and overlie char- defined by vertical changes in lithofacies and detailed (1:12,000 and 1:6,000 scale) mapping acteristic arkosic, conglomeratic sandstone clast composition, increases from the plateau of bedrock, rift fill, surficial deposits, and the of the Nambé Member of the Tesuque. to the rift. Compositional changes, evident faults that cut them all, provides important Within the older, arkosic strata of the Nambé petrographically and in mesoscale, reflect controls on the geometry and kinematics of Member lie several olivine basalt flows, like- the introduction of newly erupted material Laramide to Holocene tectonism. ly similar in age to a nearby 24.9 ± 0.6 Ma from the Latir volcanic field (~60 km to the The southern end of the SdCF (Cañon and basalt (Baldridge et al.; 1980, EPSL v. 51, pp. northeast in the Taos area) to the deposition- Hondo sections of the SdCF) is a 20-km-long, 309–21). We submit that this relationship is al system. Such Latir-derived volcanic clasts arc-shaped fault zone that defines the Taos evidence that subsidence of the Española as tuff, sanidine, and quartz-bearing pumice embayment. The SdCF continues north Basin was underway prior to 25 Ma and that each make their first appearance progres- (Questa section) into Colorado along a rela- the Bishop's Lodge Member correlates sively higher in stratigraphic sections toward tively linear range front. The transition regionally to the Oligocene–lower Miocene the rift, as do lithofacies transitions within between the NE-striking, left-oblique normal Abiquiu and Picuris Formations. Previous the upper Abiquiu. These signs point to EF and the N-striking, normal Cañon section restriction of the Nambé Member to the mid- deposition of the upper Abiquiu Formation is a smooth curve that cuts the PPF in a struc- dle Miocene overlooked the presence of having taken place while the basin was sub- turally complex zone near Talpa. South of diagnostic fossils only in the upper part of siding along rift-bounding faults. the EF/SdCF, the volcaniclastic Picuris the member. Formation (34–18 Ma) of the Miranda graben The contact between the Tesuque For- is cut by strike- and oblique-slip faults of the mation and the mostly Proterozoic rocks of TECTONICS AND VOLCANISM OF THE PPF. The PPF projects northward across the the Sangre de Cristo Mountains is both fault- LATE MIOCENE BEARHEAD MAG- Taos Valley to align with the Questa section. ed and depositional. Field and geophysical MATIC EPISODE IN THE SOUTH- The Taos graben, identified primarily by data do not support a single, range-bound- EASTERN JEMEZ MOUNTAINS, NEW geophysics and drillholes, is a buried, N- ing fault with possible Quaternary move- MEXICO, by Gary A. Smith, Department trending, 13-km-wide, 5,000-m-deep graben. ment, an interpretation proposed by some of Earth and Planetary Sciences, Uni- The Taos graben is the major rift feature in workers for the eastern margin of the Espa- versity of New Mexico, Albuquerque, NM the southern San Luis Basin. The eastern ñola Basin. Near Nambé Lake, the Tesuque 87131 edge of the graben (Town Yard fault) lacks Formation is clearly in depositional contact Relationships between Neogene volcan- Quaternary expression but is aligned with upon the Paleozoic and Proterozoic rocks. ism and faulting in the Jemez Mountains the PPF to the south and the Questa section Farther south, a fault along the contact juxta- have been only poorly defined. Recent map- of the SdCF to the north. poses granite and Tesuque Formation along ping indicates, however, that upper Miocene A preliminary conceptual geologic model one segment, but offsets different beds of the Bearhead Rhyolite magmatism was coeval of the Taos Valley is as follows. The PPF and Nambé Member along another, demonstrat- with, and spatially related to substantial sub- SdCF are reactivated pre-Laramide faults. ing that the mountain-front faults are of com- sidence along NNW intra-rift faults and NE- The Miranda and Taos grabens were origi- parable magnitude to the myriad, minor striking reactivated basement structures nally parts of an oblique-slip, Oligocene-to- faults found farther west into the basin. Most along the Jemez lineament. Extrusion of Miocene basin. The PPF and Questa section Pliocene and Quaternary stream deposits Bearhead Rhyolite flows/domes and coge- of the SdCF represent the exposed eastern close to the mountain front have not been netic Peralta Tuff Member pyroclastic edge of the graben. The Town Yard fault is offset, precluding significant Quaternary deposits occurred mostly between 7.0–6.7 the buried intermediate section of the fault movement. Ma. Although eroded in many places and graben. Sometime after 18 Ma, rift kinemat- buried to the south beneath younger fill of ics changed, and the EZ-Cañon/Hondo sec- the Santo Domingo Basin, Bearhead–Peralta tion severed the PPF, leading to its extinc- THE OLIGOCENE–EARLY MIOCENE eruptive products comprise a minimum vol- tion. As the rift widened and extension ABIQUIU FORMATION, NORTHERN ume of 35 km3 and may have exceeded 100 slowed, the Taos graben was abandoned, NEW MEXICO: EVIDENCE FOR RIO km3. Several workers (most notably L. Justet, and faulting migrated eastward to form the GRANDE RIFT INITIATION SYN- UNLV M.S. thesis, 1999) have suggested that Taos embayment. CHRONOUS WITH DEPOSITION, by the Bearhead Rhyolite may represent erup- This model explains a variety of geologic, Jessica D. Moore, [email protected], and Gary tions from a single magma chamber. Most hydrogeologic, and physiographic features A. Smith, Department of Earth and Plan- vents are concentrated in an ~165 km2, NE- of the Taos Plateau, including the Rio Gran- etary Sciences, University of New Mexico, elongated elliptical region parallel to, and de gorge, intrabasinal faults such as Los Albuquerque, NM 87131 just north of, a discontinuous zone of NE- Cordovas faults, Pliocene basalts, broad basi- Previous work on the western margin of striking faults, at least 30 km long, that coin- the Rio Grande rift, in the area of the south- cides with the volcanic front of the southern

New Mexico Geology May 1999 37 Jemez Mountains and a regional geophysical Taos Range and flow west onto the Taos can be used to support either scenario, we lineament. Northeast-striking faults are com- Plateau. Previous workers have determined believe that geomorphic evidence found in a mon north of, but are rare south of, this zone. approximate rates of vertical incision that major river system on the high plateaus of This zone of faults and vents may more allow for a field-based test of the stream the western Great Plains argues for late appropriately represent the southern margin power law. Field measurements of discharge Cenozoic mantle-driven, epeirogenic rock of a broadly defined Jemez lineament, rather for the two streams show that discharge gen- uplift. than the narrow line through the Valles erally increases with drainage area for these The Jemez lineament is one of many dis- caldera that is typically depicted on maps. two basins, but discharge does decrease in tinct zones of high heat flow and prolific late Most Bearhead Rhyolite magmatism was the more alluvial stretches (developed in Cenozoic volcanism identified as “fingers” focused along the margins of the Bearhead basin fill) downstream of the mountain front. of low-velocity mantle that extend beneath Basin, a 6.5 km-wide asymmetric, west-tilted Discharge then increases once the streams parts of the western United States. Regional- graben that accommodated at least 700 m of enter lower gorges incised through basalts. scale warping of the Tertiary Ogallala For- volcaniclastic strata between about 7.0 and We suggest that the stream loses water to the mation implies Neogene–Quaternary defor- 6.2 Ma. Erosional remnants of relatively thin relatively permeable basin fill sections, while mation across the lineament. The Canadian Peralta Tuff pyroclastic aprons are found it gains water once it intersects a regional River canyon, northeastern New Mexico, near rhyolite vents outside of the basin, but a ground-water system in the basalts of the straddles this crust-penetrating structure thick sequence of pyroclastic deposits and lower gorges. Stream power maps using and cuts through the warped Ogallala cap tuffaceous sedimentary strata accumulated field-derived data indicate that unit stream rock. A broad convexity in the longitudinal within the basin, demonstrating subsidence power (m = 0.5, n = 1) and another variant profile of the Canadian where it traverses the contemporaneous with late Miocene volcan- (m = n = 1) best predict the rates of incision Jemez lineament provides further evidence ism. The restricted extent of hypabyssal for both the Red River and Rio Hondo. for rock uplift. No such broad convexities rocks of the Cochiti mining district can now Furthermore, field-based stream power exist in the profiles of other major river sys- be explained by a footwall uplift along the maps match incision rates better than their tems (e.g. Arkansas River, South Platte River, NW margin of the Bearhead Basin. Pre- map-based counterparts. Similarities and the Pecos River) that drain the eastern Miocene(?) roof pendants in mineralized between the map-based and field-based unit flank of the southern Rockies. Fluvial stratig- intrusions suggest >1 km of uplift. Alteration stream power distributions suggest that this raphy of the Canadian River canyon is tem- in the mining district, and broadly through may be the best form for predicting rates of porally constrained by 40Ar/39Ar dating of a the southern Jemez Mountains, has been vertical incision using just maps. We suggest basalt flow within the canyon and by radio- dated at 5–7 Ma (WoldeGabriel and Goff, that the similar, generally parallel profile carbon dating of organic material found in 1989, Geology, 11:986). shapes of both the modern valleys and ter- the terrace deposits. Numerical age dates All of these observations suggest that a race treads for both rivers (indicating con- yield uniform incision rates over the past 1.5 large, late Miocene magma chamber devel- stant rates of incision spatially) suggest that Ma. These uniform rates coupled with near- oped along the crustal-scale boundary repre- these streams are at grade. If so, the stream ly parallel terrace profiles suggest that inci- sented by the Jemez lineament. This magma power law parameterized for unit stream sion is controlled by relatively uniform, chamber may have induced hydrothermal power (m = 0.5, n = 1) provides a potential mantle-driven rock uplift along the Jemez mineralization/alteration of a large region of simple test for grade of river systems in areas lineament and not by hydrologic shifts the southern Jemez Mountains. Movement where base level fall or regional tectonics linked to climate change. These combined along intra-rift faults intersected this cham- cause net fluvial incision. data provide strong evidence for late ber and tapped it incrementally prior to suf- Cenozoic epeirogenic rock uplift associated ficient evolution of a gas-rich cap that might with localized mantle anomalies. Rock uplift have led to a single, voluminous pyroclastic EVIDENCE FOR LATE CENOZOIC ROCK in the western United States, therefore, may eruption. Subsidence along these faults UPLIFT IN THE WESTERN UNITED be localized to regions where the crust is enhanced accumulation and preservation of STATES FROM THE GEOMORPHIC underlain by buoyant mantle, but not neces- volcaniclastic deposits recording this mag- EVOLUTION OF THE CANADIAN sarily associated with areas of high elevation matic episode. RIVER, NORTHEASTERN NEW MEXI- and high relief. CO, by Paul A. Wisniewski, pawis@unm. edu, Department of Earth and Planetary RESULTS OF A FIELD-BASED STREAM Sciences, University of New Mexico, RIO GRANDE INCISION HISTORY— POWER LAW TEST ON THE RED Albuquerque, NM 87131 PRELIMINARY RESULTS, SOCORRO RIVER AND RIO HONDO, NORTH- Post-Laramide rock uplift in western BASIN, CENTRAL NEW MEXICO, by ERN NEW MEXICO, by David K. Mitchell, United States has been the subject of a long- Harland L. Goldstein and Bruce Harrison, Department of Earth and Planetary standing debate. Central to the debate are Department of Earth and Environmental Sciences, University of New Mexico, the following questions: has the West experi- Science, New Mexico Institute of Mining Albuquerque, NM 87131 enced rock uplift, where have rocks come up, and Technology, Socorro, NM 87801 While it has been proposed that the rate of and what is the nature of the uplift mecha- Constrained within a classic continental vertical fluvial incision of a stream is propor- nism? Workers speculate that the rugged, rift zone, the Rio Grande is a major through- tional to stream power, no field-based tests high topography of the Rocky Mountains flowing drainage system within the south- have been conducted to validate this pro- reflects late Cenozoic rock uplift, generated western United States. Although rifting posed relationship. The general form of this perhaps in part by active mantle processes. began in the Oligocene, the Rio Grande was relationship is dz/dt = kQmSn, where dz/dt is Recent teleseismic data suggest that plumes not an established axial river system until the vertical incision rate, k is a basin-specific of low-velocity, dynamic mantle currently early Pliocene. Furthermore, the onset of constant, Q is discharge, S is the energy buoy an almost uniformly over-thickened river incision did not occur until around the slope, and m and n are coefficients that scale crust. Apatite fission-track cooling histories middle Pleistocene. The history of Rio the relative influence of discharge and slope. of various mountain chains seem to corrobo- Grande downcutting is an important compo- At least 3 different combinations of m and n rate this story. Paleobotanical data, however, nent of the evolution of continental rift values have been suggested as most appro- indicate that the Laramide Rocky Mountains zones. Although the rift zone, as well as the priate for predicting rates of vertical fluvial stood at equal or greater elevations than the Rio Grande itself, has received a lot of atten- incision, and all previous stream power law modern Rockies. These high elevations have tion, detailed incision histories have had lim- tests have substituted drainage basin area for been attributed to up-basin hydrological ited attention. discharge. Using a field-based approach, we shifts such as those that accompany global This study assesses the timing of Rio determined discharge–drainage area rela- climate change and glacial-interglacial Grande incision within and around the tionships and stream power values for the cycles. While the presence of deeply Socorro Basin in central New Mexico. The Red River and Rio Hondo in northern New entrenched river systems of the Colorado approach of this study is to use tributary ter- Mexico. These two streams both head in the Plateau, Great Plains, and Basin and Range races to reconstruct paleo-Rio Grande eleva-

38 May 1999 New Mexico Geology tions within the rift zone. Tributary terraces bers to the southeast in the southern Big COSMOGENIC NUCLIDE: PRELIMI- are the main focus of this study because axial Burro Mountains. This initial stage was fol- NARY RESULTS FROM THE SOCOR- Rio Grande terraces are not well preserved lowed by uplift and tilting of the strata of RO CANYON FAULT, by John P. Ayarbe, and are often complex in that they are these two members. The second stage was Fred M. Phillips, J. B. J. Harrison, Depart- interfingered with alluvial fan deposits. It is the deposition of the Pliocene to Pleisto- ment of Earth and Environmental Science, assumed that the tributary terraces form in cene(?) Pearson Mesa Member, which con- New Mexico Institute of Mining and response to Rio Grande base level change. sists of 75 m of alluvial-flat and distal to mid Technology, Socorro, NM 87801; and David Thus, they provide a record of Rio Grande alluvial-fan lithofacies in a northwest-trend- Elmore, and Pankaj Sharma, PRIME Lab, elevations. Terrace profiling will provide a ing, northeast-tilted, internally drained half Purdue University, West Lafayette, IN means to correlate the tributary terraces to graben. Clast composition and paleocurrent 47907-1396 remnant Rio Grande terraces, as well as directions indicate a provenance for this Determining fault-scarp chronologies is reconstruct paleo-Rio Grande profiles. Four member to the north in the Rileys Peak area. important in the assessment of earthquake tributary terrace sequences within the There is no depositional record of a major hazards and to paleoseismology. In arid Socorro Basin will be used in this study. Soil through-flowing, axial-fluvial system during environments the low abundance of organic development will be used to correlate terrace the time of deposition of the Gila Conglom- carbon often prevents 14C dating of displace- surfaces between tributaries and to establish erate in the study area that would be analo- ments; therefore, in these regions fault-scarp relative ages. One numeric age has been gous to the modern Gila River. diffusion modeling has been applied to date determined for a terrace surface in Socorro scarps. A drawback to this technique, how- Canyon. The numeric age and the carbonate ever, is that the geomorphic diffusivity is profile mass of this surface will be used to LATE QUATERNARY PALEOSEISMICITY often unknown and must be estimated. calibrate carbonate accumulation rates in the OF THE ALAMOGORDO FAULT Because the diffusivity of unconsolidated study area and will provide a basis for rela- ADJACENT TO THE SACRAMENTO material can vary by three orders of magni- tive age determination. MOUNTAINS, by D. J. Koning and F. J. tude a large amount of uncertainty is intro- Although the timing of downcutting is a Pazzaglia, Department of Earth and Pla- duced into the calculated age of a scarp. major focus of this study, the ultimate goal is netary Sciences, University of New Mex- Better estimates of a scarp age can be to provide an explanation of the external ico, Albuquerque, NM 87131 obtained by constraining the value of the dif- causes of incision. That is, is downcutting of The Alamogordo fault is a major Rio fusivity using a cosmogenic nuclide. the Rio Grande a result of climatic condi- Grande rift structure in southern New Samples, collected along two vertical tran- tions, tectonic conditions, or both? Mexico along which the Tularosa Basin has sects near the fault plane of the Socorro been down-dropped relative to the Sacra- Canyon fault (central New Mexico), were mento Mountains. This fault has displaced analyzed for 36Cl. The analyses provided ver- DEPOSITIONAL ENVIRONMENTS AND surfaces developed on Quaternary deposits tical 36Cl profiles that were used to calibrate a PROVENANCE OF THE CENOZOIC by as much as 10 m. The timing of late model that couples the accumulation of 36Cl GILA CONGLOMERATE OF THE DUN- Quaternary rupture events along the Alamo- to fault-scarp morphology. The calibration of CAN AND CANADOR PEAK QUAD- gordo fault adjacent to the Sacramento the model allowed the diffusivity to be con- RANGLES, SOUTHWESTERN NEW Mountains was investigated by: (1) mapping strained and the timing of ruptures to be MEXICO, by Shane V. Smith and Greg H. proximal piedmont deposits along the west- assessed. Preliminary results suggest the sur- Mack, Department of Geological Sciences, ern foot of the Sacramento Mountains, (2) face was displaced around 120 ka and 44 ka. New Mexico State University, Las Cruces, describing exposures of offset Quaternary The geomorphic diffusivity is approximately NM 88003 sediment and collecting 11 samples of data- 0.0004 m2/yr. Lithofacies distribution, paleocurrent, and ble material, and (3) measuring fault scarp provenance data are used to define the evo- profiles at 40 localities. lution of the Gila Conglomerate in the Basin The ages of four late Quaternary surface HYDROGEOLOGIC CHARACTERIZA- and Range tectonic province of the Duncan rupture events are constrained by strati- TION OF A LARGE DISPLACEMENT and Canador Peak quadrangles, southwest- graphic relationships and C-14 dates. South NORMAL FAULT IN POORLY LITHI- ern New Mexico. Crustal extension in this of the city of Alamogordo, two large rupture FIED SEDIMENT, by G. C. Rawling and L. part of the Basin and Range resulted in fault- events probably occurred within a time span B. Goodwin, Department of Earth and block mountains and complementary basins of a few thousand years. A limiting upper Environmental Science, New Mexico filled with up to 250 m of conglomerate, numeric age indicates that both happened Institute of Mining and Technology, sandstone, siltstone, and mudstone of the shortly before 12.6 ka (radiocarbon years). Socorro, NM 87801 Gila Conglomerate. The Gila Conglomerate The estimated average displacement associ- An understanding of the impact of fault is divided into upper and lower stratigraph- ated with each of these two events is approx- zones on subsurface fluid flow is of impor- ic units that are separated by an angular imately 3–4 m. North of the city of Alamo- tance in the construction of realistic ground- unconformity. The lower unit consists of gordo, the youngest interpreted surface rup- water flow models. While conceptual mod- strongly consolidated conglomerate, sand- ture event is constrained by C-14 dates to els and quantitative hydrogeologic data exist stone, and mudstone, and the upper unit has have occurred between 10.4 and 11.4 ka for fault zones in lithified sedimentary rock, unconsolidated to poorly consolidated silt- (radiocarbon years). South of the city of very little work has addressed faults in poor- stone, mudstone, and sandstone with Alamogordo, the youngest surface rupture ly lithified sediments. We are developing a uncommon conglomerate. Three mappable very likely occurred in the early Holocene hydrogeologic model of fault zones in poor- members were identified in these two units during a period of major alluvial fan aggra- ly lithified sediments based on the Sand Hill including the Wilson Mine and Nichols dation. This event probably had an average fault, by field mapping at a variety of scales, Canyon Members of the lower Gila displacement of ~1 m. The alluvial fan in situ and laboratory permeability measure- Conglomerate and the Pearson Mesa stratigraphy suggests that these four rup- ments, structural analysis, and statistical Member of the upper Gila Conglomerate. tures belong to a temporal clustering phe- treatment of the data. The Gila Conglomerate of the Duncan and nomena that occurred over a period of ~8 ka The Sand Hill fault is a large-displacement Canador Peak quadrangles shows a two- during the latest Pleistocene and early (up to 600 m), basin-bounding growth fault stage evolution. The initial stage was the Holocene. These four ruptures probably pro- in the Rio Grande rift west of Albuquerque, deposition of the late Oligocene(?) to early duced seismic moment magnitudes within NM. The fault juxtaposes synrift sediments Miocene Wilson Mine and Nichols Canyon the range of 6.8–7.3. of the Oligocene–Miocene middle and Members that consist of 175 m of sediment Pliocene–Pleistocene upper Santa Fe Group. deposited on the distal edge of an alluvial Previous work has shown that the width, fan and alluvial flat and in a lacustrine playa. EARTHQUAKE CHRONOLOGY ESTAB- structural complexity of fault zone architec- Clast composition and paleocurrent direc- LISHED BY CALIBRATING A FAULT- ture, and extent of fault zone cementation are tions indicate a provenance for both mem- SCARP DIFFUSION MODEL WITH A qualitatively predictable based on the local

New Mexico Geology May 1999 39 stratigraphy. These parameters generally ence permeability. Calcic soils are document- coëvally with basin and range tectonic activ- increase with increasing grain size of the ed as a map overlay and incorporated into ities. As a result, the basin fill consists of allu- host sediment. the measured sections. vium interlayered with basalt flows. Based This study focuses on determining the The lithologic units are divided into five on drilling records from wells in the valley, relationships between mappable fault zone mappable units within two major deposi- some of the basalt layers are highly fractured structural units and fault zone hydrogeolog- tional environments. In the upper Santa Fe and probably act as aquifers, while others ic units identified by statistical and geostatis- Group we document basin-scale fluvial sedi- appear intact and probably function as tical measures of the permeability data. To ments present at the time of fault propaga- aquitards. place the permeability data in geologic con- tion and fault-related syn-tectonic deposi- Over 40 water samples from the area have text, structural study of the fault zone will tional wedge (STDW) deposits that occur as been analyzed for major ion content, δD and emphasize identification of active deforma- a result of unstable, fault-related, surface δ18O; approximately 15 are from springs and tion mechanisms and development of struc- morphology due to fault propagation. The flowing artesian wells in the center of the tures with displacement. Hydrologic, petro- upper Santa Fe Group fluvial deposits are basin, and the remainder are from springs, physical, and structural data will be com- divided into three units: gravel- and con- seeps, and creeks in the Chiricahua Moun- bined with classification and regression glomerate-dominated, sand-dominated, and tains (the bounding range on the northwest analysis to identify the primary petrophysi- silt- and mud-dominated units. The STDW of the valley; it is assumed to be the primary cal factors and/or tectonic processes control- consists of a poorly sorted colluvial wedge recharge area). ling permeability. unit, an eolian unit, and a well-sorted, fault- Based on their chemical signatures, waters Preliminary data suggest that highly parallel fluvial unit that contains reworked from the center of the basin form two distinct deformed fault zone structural units have colluvial wedge and upper Santa Fe Group groups—well waters are dominated by – + distinct hydrologic signatures, with low fluvial sediments. For mapping purposes the HCO3 and Na , while most spring waters – means and variances of permeability. These eolian and fault-parallel fluvial sediments have HCO3 /mixed cation compositions but units are characterized by penetrative planar within the STDW are grouped together as show greatly elevated levels of Mg2+ com- and linear fabrics and grain-scale tectonic both represent well-sorted sands. pared to the well waters. Both well and 2– mixing. We are employing a petrographic The STDW sediments are commonly asso- spring waters typically have TDS and SO4 image analysis system to quantify fabric ciated with soil-bounded stratigraphic inter- of roughly 400 and 10 mg/L, respectively. development and petrophysical factors relat- vals, within the upper Santa Fe Group, rep- Data from stable isotope analyses of the well ed to permeability. resenting episodic movement along the SHF. and spring waters suggest that they share a The fluvial facies acts as background sedi- common source area. mentation within the basin. The calcic soils Water samples from springs near the crest HYDROGEOLOGY OF THE UPPER represent periods of relative tectonic and of the Chiricahuas are dilute (TDS <50 SANTA FE GROUP ADJACENT TO landscape stability between rupture events mg/L), with mixed-ion composition. The THE SAND HILL FAULT, ALBUQUER- along the SHF. To test the viability of these waters from lower elevations on the eastern QUE BASIN, NM, by D. G. Smyth, lithologic units as hydrostratigraphic units, side of the range (that bounding the San Department of Earth and Environmental permeability measurements are being under- Bernardino Valley) are predominately 2+ 2– Science, New Mexico Institute of Mining taken using a variety of methods: air mini- Ca /SO4 waters with elevated TDS and Technology, Socorro, NM 87801, and permeameter, grain-size analysis, and (460–1,200 mg/L). S. Connell, New Mexico Bureau of Mines falling-head permeability. Stable isotopic data suggest that the and Mineral Resources, Albuquerque, NM Chiricahuas are the primary source area for 87106 waters in the basin center. The cause of 2– Little is known about fault controls on decreasing TDS and SO4 levels from the fluid flow through unconsolidated sedi- SESSION 4—HYDROLOGY, SOILS, basin margin to the basin center is not under- ments, although many important aquifers in AND PALEONTOLOGY stood but may be revealed by examining the United States are found in such basins. chemical data from water sources in inter- The Sand Hill fault (SHF) is a major north- mediate areas of the basin. south trending, high-angle normal fault PRELIMINARY RESULTS OF HYDRO- associated with the Rio Grande rift. This GEOCHEMICAL AND ISOTOPIC IN- growth fault locally separates the synrift VESTIGATION OF GROUNDWATER SEASONAL GEOCHEMICAL RESPONSE deposits of the upper and lower Santa Fe FLOW IN THE SAN BERNARDINO OF A SHALLOW ALLUVIAL AQUIFER Group. Our project is part of a collaborative VALLEY, ARIZONA AND SONORA, by ASSOCIATED WITH A FIRST ORDER effort trying to characterize fluid flow S. Earman, F. M. Phillips, and B. J. O. L. MONTANE STREAM IN NORTHERN through unconsolidated sediments in fault- McPherson, Department of Earth and Envi- NEW MEXICO, by Richard M. Ortiz, ed basins, for the purpose of more accurate ronmental Science, New Mexico Institute Armand Groffman, and Laura J. Crossey, De- fluid flow modeling of these systems. The of Mining and Technology, Socorro, NM partment of Earth and Planetary Sciences, focus of our field study is the hydrogeology 87801 University of New Mexico, Albuquerque, of the hanging-wall sediments associated An ongoing study is focused on under- NM 87131 with the SHF in the Pliocene to Pleistocene standing the hydrogeology of the San Rio Calaveras, located in the Jemez upper Santa Fe Group. Bernardino Valley, located in southwestern Mountains of northern New Mexico, is a first Physical characteristics of sediments, such Arizona, USA, and northeastern Sonora, order montane stream with a shallow allu- as grain size and sorting, act as indicators of Mexico. Ground water from artesian wells in vial aquifer system. The hydrogeology of the possible pathways and barriers to fluid flow. the center of the basin is used to provide system has been well documented during Calcic soil horizons, which are common in habitat for several species of endangered and the past 6 years allowing for an understand- the Albuquerque Basin, can change the threatened fish, and the long-term viability ing of the seasonal dynamics. The aquifer is hydrologic properties of a sedimentary unit of this supply is uncertain if water-use pat- ideal for the examination of the seasonal by depositing calcium carbonate around and terns in the valley are altered. response of redox processes in a dynamic between grains, closing off pathways for The San Bernardino Valley is located in the near-surface environment because 1) the fluid flow. Our research utilizes 1:8,000 litho- Basin and Range physiographic province. It chemistry shows a seasonal response, 2) facies mapping to document lateral changes trends north-south and is approximately 15 vadose-zone and aquifer sediments are rela- and several measured sections to document km wide and 35 km long. The geology and tively high in organic carbon (an important vertical/temporal changes. The measured hydrogeology of the basin have not been consideration in biogeochemically mediated sections document lithofacies changes in widely investigated and are poorly under- systems), 3) a large biogeochemical and greater detail than the mapping, allowing a stood. hydrological data base has been assembled, better idea of lithologic variations within Basaltic volcanism was active in the area and 4) it is a pristine environment. individual mappable units that may influ- In addition to surface water sampling at

40 May 1999 New Mexico Geology the site, there are also floodplain and transect with changing vegetation. Typically, the bet- AVAILABLE MOISTURE CONTROLS ON wells. These three main sampling localities ter-developed soils are observed in the THE DISTRIBUTION OF CALCIUM add up to a combined total of 54 sampling forested areas. Laboratory analyses revealed CARBONATE WITHIN SOILS OF A locations. Measurements performed in the textural and vegetation controls on both FIRST ORDER DRAINAGE BASIN, field and in the lab include pH, oxidation- organic matter and carbonate content. SEVILLETA WILDLIFE REFUGE, reduction potential (ORP), dissolved oxygen, Organic carbon weight percentages are high- SOCORRO, NEW MEXICO, by D. Mc- and major anion and cation analysis (for er and penetration of organic matter is deep- Mahon, Desert Research Institute, Reno, major and minor elements including iron er in riparian forest soils than in desert Nevada; J. B. J. Harrison, and J. M. H. Hen- and manganese). patchland soils. Although weight percent drickx, Department of Earth and Environ- This study compares hydrologic data calcium carbonate is less than 0.5% in both mental Science, New Mexico Institute of (stream discharge, water-table fluctuations) the desert patchland and riparian forest, car- Mining and Technology, Socorro, NM with geochemical parameters (anion and bonate penetration is deeper and concentra- 87801 cation concentrations, pH, ORP, and dis- tions are lower in the riparian forest soils. A A small first-order drainage basin devel- solved oxygen content). The water table fluc- positive relationship also exists between oped in early Pleistocene fan gravels shows a tuates up to 0.7 m during an annual cycle finer-grained sediments and organic content marked contrast in vegetation between with a maximum reached during spring in both soils. Our findings suggest that: (1) north- and south-facing slopes. The south- snow melt and minimum during winter short-term pedogenesis is surprisingly sig- facing slope is characterized by a creosote- baseflow conditions. During periods of high nificant on the middle Rio Grande floodplain black gramma grassland, whereas the north- discharge (spring snow melt), dissolved oxy- when contrasted with soils of deposits of facing slope has black gramma-juniper gen in the system is at a maximum (2–6 similar age elsewhere in the region; and (2) woodland vegetation. There is a concomitant mg/L). Elements such as iron and man- vegetation exerts some control over the rate change in the depth and amount of calcium ganese are predominantly present in their of incipient stages of soil development. In carbonate in the soils on the opposing slopes. oxidized states and remain stationary within other words, one must consider that the Depth to calcic horizon and the average pro- the aquifer system. During periods of base- apparently accelerated soil formation is file mass of calcium carbonate in south-fac- flow conditions, dissolved oxygen within the largely attributable to the construction of ing slope soils is 5 cm and 10g/cm2 respec- system is at a minimum, and iron and man- Cochiti Dam. tively, compared to 30 cms and 15g/cm2 for ganese are reduced. Iron and manganese soils on the north-facing slope. Catenary become mobile in the system with concentra- relations are not strongly developed for the tions of iron and manganese up to 15 mg/L CARBON DIOXIDE SOIL GAS STUDIES soils on either slope. However, a very strong and 2 mg/L respectively. Understanding the IN THE SEVILLETA, by L. J. Wardell and catenary relationship exists in the soils of the dominant hydrologic pulse (snowmelt) and B. Harrison, Department of Earth and east-facing headslope of the drainage basin. how it affects the geochemistry of the system Environmental Science, New Mexico A decrease in profile mass of calcium car- allows for the application of this knowledge Institute of Mining and Technology, bonate and an increase in depth to calcic to perturbed systems. Socorro, NM 87801 horizon occurs with lower slope soils. Previous work on two opposing slopes at The initial driving force for these contrasts a Sevilleta field site inferred different paleo- is the variation in solar radiation, with the THE SPATIAL VARIABILITY OF ENTI- climate conditions and vegetative patterns north-facing slope receiving less winter sun- SOLS ON A RIPARIAN FOREST from measuring stable carbon isotope con- light than the south-facing slope; however, FLOODPLAIN AS A FUNCTION OF centrations of pedogenic carbonate layers. the greatest differences in PET occur in the VEGETATION AND SEDIMENT TEX- However, calcium carbonate layers form spring and fall. These differences in PET TURE: IMPLICATIONS FOR SHORT- over several thousands of years and thus determine the nature of the vegetation com- TERM PEDOGENESIS, by Nicole M. represent an integrated climate signal over munities on the opposing slopes, which in Bailey, [email protected], and Laura this time period. To better understand the turn influences the available soil moisture. Hagan, Department of Earth and Planetary short term variability we are conducting soil North-facing soils also have greater amounts Sciences, University of New Mexico, Albu- gas measurements periodically over at least of organic carbon and silt than south-facing querque, NM 87131 four consecutive seasons. soils, and these features increase the avail-

In the past, active floodplain soils were By combining the seasonal trends of CO2 able moisture for vegetation on the north- considered much too young to contain any flux and carbon stable isotope composition facing slopes. CO2 measurements through- significant amount of soil development, such in the soil gas, we will be able to relate CO2 out the year indicate that north-facing soils that few soil studies have been conducted on production with the degree of vegetation res- have a greater CO2, flux which may be the floodplains to determine rates of soil forma- piration and carbonate deposition. Yearly main factor controlling the difference in pro- tion in time scales of years or decades. This is variations will provide understanding to the file mass of calcium carbonate between the particularly so in arid regions, where rates of degree of variability that can be expected in two slopes. soil development are generally slower than the more general determination of a paleocli- those in more humid regions. The middle mate condition. Differences in CO2 produc- Rio Grande floodplain at the Rio Grande tion rates and concentrations within the soil SEDIMENTOLOGY OF ESTANCIA BA- Nature Center in Albuquerque provides an on the two different slopes in the study pro- SIN GROUND-WATER DISCHARGE opportunity to study a unique floodplain vide information on their different vegeta- PLAYAS, CENTRAL NEW MEXICO, by environment, where the surface has been tive growth patterns. Measurements started T. A. Loveland, Department of Earth and artificially stabilized by the Cochiti Dam in spring of 1998 and are planned to contin- Planetary Sciences, University of New construction in 1942. Soil stratigraphy in this ue through 1999. Mexico, Albuquerque, NM 87131 region is characterized by a complex Results show that these two slopes with The Estancia Basin playa complex, located sequence of overbank flooding deposits and different vegetation patterns show different about 40 km east of the Manzano Mountains lateral accretionary deposits that have been patterns of seasonal behavior with respect to in central New Mexico, consists of ~87 subjected to pedogenesis for the past 57 yrs. surface CO2 flux and subsurface CO2 concen- ephemeral salt lakes (playas) and associated Eight soil pits were excavated along north- trations. Little variation is observed in the lunette dunes, which together encompass an 2 south and east-west transects of the Rio isotopic composition of the subsurface CO2. area of approximately 400 km . The playas Grande in order to characterize the soil spa- Isotopic carbon values for both slopes favor occupy ~12% of this area (50 km2). The playa tial variability. Soil stratigraphy, vegetation C4 vegetation. This is inconsistent with the basins are mid-Holocene eolian deflation species, and vegetation density are variable pedogenic carbonate that indicated one of features cut into the latest Pleistocene lake along both transects. Pedogenic features the two slopes having a near equal distribu- beds, which were deposited on the floor of such as O and A development, root diame- tion of C3 and C4 vegetation. pluvial Lake Estancia in the center of the ters and densities, and degree of sedimenta- Estancia Basin. The lunette dunes, adjacent ry structure preservation vary noticeably to and downwind from the playas, are com-

New Mexico Geology May 1999 41 posed of material deflated during playa locality yields fossils that represent a diverse mations of the Bisbee Group. A lack of basin formation, mostly gypsum and clay. vertebrate and invertebrate fauna. The inver- regional stratigraphic studies (most have These dunes are up to 1 km wide, with ele- tebrate fauna consists primarily of conchos- focused on a single mountain range), the rel- vations of up to 50 m above the adjacent tracans, although a possible decapod crus- ative rarity of biostratigraphically useful fos- playa surface. Depth of post deflation playa- tacean was also discovered at the site. The sils, and the apparently rapid lateral facies fill sediment in different playas ranges from vertebrate fauna consists of both fish and changes in parts of the Lower Cretaceous 0.4–2.4 m. Radiocarbon dates from gastro- abundant tetrapods. A partial, articulated section have hindered the development of pod shells, taken from the base of one of the fish skeleton represents a semionotid, and convincing regional correlations. However, a dunes, and from cysts of the brine shrimp numerous scales have also been recovered lithologically and biostratigraphically dis- Artemia salina, taken from the bottom-most from the site. The tetrapod fauna is dominat- tinctive interval of the U-Bar Formation, the sediment of one of the playas, both indicate ed by archosaurs, including indeterminate Carbonate Hill Member, is present in all out- ~7,000 yrs BP as the time of playa basin phytosaurs, the aetosaur Desmatosuchus hap- crop areas of the U-Bar Formation in south- deflation. locerus, a coelophysoid theropod dinosaur, western New Mexico. It thus provides a The playas are zones of ground-water dis- and another, larger theropod; a distal sound basis for the correlation of U-Bar sec- charge for the hydrologically closed Estancia humerus of a possible cynodont? was also tions throughout the region. Basin (approximately 5,000 km2). The Estan- recovered. Plants are represented by non- Zeller (1965, NMBMMR Memoir 16) intro- cia Basin (central valley floor, town of diagnostic woody debris. duced the term "oyster-limestone member" Estancia) receives 30 cm (12 inches) of pre- Stratigraphically, the site is high in the of the U-Bar Formation in the Big Hatchet cipitation and experiences 152 cm (60 inches) Petrified Forest Formation, approximately 60 Mountains for the same lithostratigraphic of potential evaporation in an average year. m below the Entrada Sandstone. Tetrapod unit that Gillerman (1958, NMBMMR Ground water discharging into the playas is biochronology, based on the presence of the Bulletin 57) had already named the rich in the dissolved ions Na+, Ca++, Mg++, aetosaur Typothorax and the phytosaur Carbonate Hill Limestone. The Carbonate 2– – – SO4 , Cl , and HCO3 . Evaporative concentra- Pseudopalatus, indicates a Revueltian (early- Hill Member of the U-Bar Formation is 60– tion of these ions results in precipitation of mid Norian, approximately 210–218 Ma) age 610 m thick and consists mostly of dark-gray gypsum, calcite, halite, dolomite, and to a for the Petrified Forest Formation in this to black, bioclastic limestone (packstone) lesser extent bloedite, in and on the playa area. characterized by numerous bivalve or gas- sediment. Playa sediment consists of 30–50% Tetrapod bones from locality 3845 are tropod shells and interbedded with cal- gypsum, 2–10% carbonate (calcite and dolo- extremely well preserved and occur primari- carous shale, calcarenite, and minor lime- mite), and the remainder silt and clay. Efflor- ly in a bonebed of greenish-gray intraforma- stone-pebble conglomerate. It yields late escent crusts on playa surfaces are mainly tional conglomerate that fines upward into Aptian ammonites (e.g., Kazanskyella spathi, halite and thenardite. Crystal morphology of sandy mudstone of similar colors. Con- Acanthoplites berkeyi, Hypacanthoplites immu- gypsum crystals, which grow in the sedi- chostracans are preserved in close associa- nitis) in the Peloncillo and Big Hatchet ment at a depth of 30–40 cm, is controlled by tion with tetrapod bones. The fish skeleton Mountains. At all outcrops, a diverse assem- ground-water flow (delivery of ions), tem- was found approximately 1.5 m above this blage of bivalves from the Carbonate Hill perature, salinity, and concentration of dis- horizon in laminated mudstone above a sim- Member includes species of Quadratotrigonia solved organic carbon. Growth of these ilar, fining-upward sequence. The decapod? that indicate a late Aptian age. At some out- sand-sized gypsum crystals results in was found in a spoil pile at the site but was crops, the large, late Aptian–early Albian increased hydraulic conductivity of this probably originally close (<1 m) to the bone- foraminiferan Orbitolina texana is present in zone. bearing horizon. the Carbonate Hill Member. A few of the wettest playas are colonized Coelophysoid theropod material recov- The Carbonate Hill Member of the U-Bar at their surface by microbial communities. At ered thus far includes elements of the pelvic Formation crops out in the Big Hatchet, Little the top of the sediment column, just below girdle and the hind limbs as well as a partial Hatchet, Animas, Peloncillo, Victorio, and the halite crust, is an algal mat composed of tooth. The presence of two proximal left tibi- East Potrillo Mountains and at Eagle Nest in filamentous cyanobacteria (blue-green ae and fibulae indicates the presence of at the West Potrillo Mountains. It thus provides algae). This mat excludes oxygen from the least two individuals. An aberrant tetrapod the basis for a first order correlation of U-Bar zone below where Desulfovibrium, an anaero- specimen consists of a fused tibia-fibula- Formation outcrops throughout southwest- bic sulfate reducer, performs dissimilatory astragalus-calcaneum that is also fused prox- ern New Mexico. sulfate reduction (the reduction takes place imally. This probably represents a large (>3 outside the cell), producing hydrogen sulfide m body length) theropod with an as-yet- gas. Some of the reduced sulfur combines undetermined pathology. A NEW TYRANNOSAURID (DINO- with aqueous ferrous ions and results in pre- Particularly important aspects of this site SAURIA: THEROPODA) PARTIAL cipitation of botryoidal pyrite both free and are: (1) the abundant and highly diverse fau- SKELETON FROM THE UPPER CRETA- on the surfaces of growing gypsum crystals. nal elements, including aquatic inverte- CEOUS KIRTLAND FORMATION, brates, fish, and both semi-aquatic and ter- SAN JUAN BASIN, NEW MEXICO, by restrial tetrapods; (2) the exceptional preser- Thomas E. Williamson, New Mexico Muse- A DIVERSE NEW TRIASSIC FOSSIL vation that characterizes all taxa found at the um of Natural History and Science, 1801 ASSEMBLAGE FROM THE PETRIFIED site; (3) the 10+ m extent of the fossiliferous Mountain Rd. NW, Albuquerque, NM FOREST FORMATION (REVUELTIAN: horizons; and (4) the relatively rare taxa, 87104; and Thomas D. Carr, Royal Ontario EARLY-MID NORIAN) NEAR ABI- including decapods?, dinosaurs, and cyn- Museum, 100 Queen's Park, Toronto, QUIU, NEW MEXICO, by A. B. Heckert, odonts? found at the site. Ontario M5S 2C6 Canada Department of Earth and Planetary Sci- A partial skeleton of a large tyrannosaurid ences, University of New Mexico, Albu- (NMMNH P-27469) from the Upper Cre- querque, NM 87131; L. F Rinehart, S. G. CARBONATE HILL MEMBER OF THE U- taceous (upper Campanian) Hunter Wash Lucas, New Mexico Museum of Natural BAR FORMATION: KEY TO CORRELA- Member, Kirtland Formation represents only History and Science, 1801 Mountain Rd. TION OF THE LOWER CRETACEOUS the third relatively complete tyrannosaurid NW, 87104 (NMMNH); A. Downs, Ruth SECTION IN SOUTHWESTERN NEW to be collected from New Mexico. Previously Hall Museum of Paleontology, Ghost MEXICO, by Spencer G. Lucas and John W. reported partial skeletons of tyrannosaurids Ranch Conference Center, Abiquiu, NM Estep, New Mexico Museum of Natural from Upper Cretaceous strata of New 87510; J. W. Estep, J. D. Harris, P. K Reser, History and Science, 1801 Mountain Rd. Mexico include OMNH 10131, a skull and and M. Snyder, NMMNH NW, Albuquerque, NM 87104 skeleton referred to Aublysodon cf. A. miran- Recently, one of us (MS) discovered a pro- Lower Cretaceous strata exposed in south- dus from the Fruitland or lower Kirtland lific new locality (NMMNH locality 3845) in western New Mexico (Hidalgo, Luna, Grant, Formation, and NMMNH P-25049, a partial the badlands of the Petrified Forest For- Sierra, and Doña Ana Counties) are assigned skeleton of an immature tyrannosaurid from mation near Ghost Ranch, New Mexico. This to the Hell-to-Finish, U-Bar and Mojado For- the Farmington Member, Kirtland Formation

42 May 1999 New Mexico Geology that has been referred to Albertosaurus but the early Irvingtonian. The overlapping due to its fragility, preparation is proceeding probably represents a new species of range zone for these three species is late slowly. The specimen measures 48 mm long Daspletosaurus. The new specimen is tenta- Blancan. The three other new sites from the by 19 mm wide exclusive of legs and anten- tively identified as an Albertosaurus based on Albuquerque Basin, two in the vicinity of nae. No clear indication of eyes is seen in the the relatively uninflated ectopterygoid. A Los Lunas in Valencia County and one near present state of preparation; however, two more precise and certain identification must Veguita in northern Socorro County, each pairs of antennae are present; one pair is await further preparation of the specimen. consist of only one species of mammal. A apparently very long. Of the five pairs of NMMNH P-27469 is estimated to be from tooth of the typical Blancan camel Hemi- legs, the first pair are slender and apparently 40% to 60% complete. It is largely or com- auchenia blancoensis occurs in one of the Los bear small chelae. Limb pairs two, three, and pletely disarticulated and includes portions Lunas sites, and several postcranial elements four are robust uniramous pereiopods, and of the skull including portions of both den- of either the large camel Camelops or a giant the legs of pair five terminate in small pad- taries, a maxilla, and an ectopterygoid, and camelid (Blancocamelus or Gigantocamelus) dle-like structures. The wide, flat body, postcranial elements including numerous were identified from the second Los Lunas reduced abdomen, and lack of a caudal fan vertebrae and ribs, a partial femur, and most site. The Veguita site has a maxilla with a indicate the animal is probably of the of the left pelvic girdle. It also shows evi- complete dentition of the large horse Equus brachyurous (short-tailed) type, which dence of pathologies including a possible scotti, a species typical of middle Blancan includes the crabs. The type and sequence of puncture wound of a palatal bone through early Irvingtonian faunas. thoracic limbs is typical of swimming crabs. (ectopterygoid) with related infection of the The presence in many of the Albuquerque However, only the ventral surface is bone and nearby periosteal tissue and a rib Basin faunas of mammals that occur in mid- exposed, so the carapace, whose morpholo- that shows a healed fracture. dle Blancan and younger faunas excludes an gy is particularly diagnostic of the Mala- The new tyrannosaurid specimen was col- early Blancan (3.7–4.5 Ma) age for most of costraca, is not visible. However, based pri- lected from the Bisti/De-na-zin Wilderness these sites. Several of these faunas appear to marily on the presence of five pairs of tho- Area under an excavation permit issued by be late Blancan in age (1.8–2.5 Ma) based on racic limbs, we tentatively assign it to the the Bureau of Land Management (BLM)— the few age-diagnostic species present, Decopoda. the first permit of its kind to be issued on any although the absence of Neotropical immi- Decapod crustaceans first appear in the federally designated Wilderness Area. grant mammals (e.g., ground sloths, glyp- Late Devonian. Ten families of decapods are todonts, capybaras, and porcupines) from all known from the Triassic; four of these are Albuquerque Basin Blancan faunas may be found in North America, and two are known TWO NEW PLIOCENE (BLANCAN) VER- significant. The arrival of these immigrants from the Chinle Group. Therefore, this spec- TEBRATE FAUNAS FROM THE ALBU- from South America at about 2.5 Ma charac- imen is particularly important because it QUERQUE BASIN, NORTH-CENTRAL terizes most late Blancan faunas from the sheds light on a rarely preserved aspect of NEW MEXICO, by G. S. Morgan and S. G. southwestern United States, suggesting that the Chinle fauna and thus provides insight Lucas, New Mexico Museum of Natural some Albuquerque Basin Blancan faunas into the fresh-water ecosystems of the Upper History and Science, 1801 Mountain Rd. may be middle Blancan in age (2.5–3.7 Ma). Triassic. NW, Albuquerque, NM 87104 Ten sites from the Albuquerque Basin in north-central New Mexico have produced GEOLOGY AND TAPHONOMY OF THE vertebrate fossils of Blancan (Pliocene) age. POSTER SESSION PETERSON SITE, NEW MEXICO'S Five of these are unpublished sites discov- MOST EXTENSIVE LATE JURASSIC ered within the past 5 yrs, and five of the DINOSAUR QUARRY, by Rodney Peter- sites were known previously. Two of the new A PROBABLE DECAPOD CRUSTACEAN son, Ronald Peterson, N. V. D'Andrea, sites, the Loma Colorado de Abajo fauna FROM THE UPPER TRIASSIC PETRI- Spencer G. Lucas, and Andrew B. Heckert, from Rio Rancho in Sandoval County and FIED FOREST FORMATION OF THE New Mexico Museum of Natural History the Belen fauna from near Belen in Valencia CHINLE GROUP, NORTH-CENTRAL and Science, 1801 Mountain Rd. NW, County, contain age-diagnostic species of NEW MEXICO, by L. F. Rinehart New Albuquerque, NM 87104 mammals. The Loma Colorado de Abajo Mexico Museum of Natural History and The Peterson site, NMMNH locality 3282, fauna is composed of three species. A small Science, 1801 Mountain Rd. NW, Albu- is the most extensive Jurassic dinosaur local- land tortoise (cf. Hesperotestudo) is identified querque, NM 87104; A. B. Heckert, Depart- ity yet discovered in New Mexico. Located in from a shell fragment; the ground squirrel ment of Earth and Planetary Sciences, Bernalillo County near Cañoncito, it is a Spermophilus sp. is known from a partial University of New Mexico, Albuquerque, mass burial assemblage of dinosaur bones in skull; and the primitive pocket gopher NM 87131; and S. G. Lucas, New Mexico the Brushy Basin Member of the Morrison Geomys (Nerterogeomys) is represented by one Museum of Natural History and Science, Formation. The bone-bearing horizon at the complete and one partial skull and a 1801 Mountain Rd. NW, Albuquerque, site is a 1.1- to 3-m-thick, trough-crossbed- mandible fragment. The Loma Colorado NM 87104 ded sandstone approximately 25 m below Geomys is indicative of a Blancan age but New Mexio Museum of Natural History the base of the Jackpile Member of the does not permit a more precise placement of and Science (NMMNH) locality L-3845 in the Morrison Formation. Discovered in 1963, the this fauna within the Blancan. The Belen Petrified Forest Formation of the Chinle site has been regularly excavated since 1989 fauna includes six species: the partial skele- Group (Rio Arriba County, NM) yields fos- and has yielded more than 100 dinosaur ton of a colubrid snake; a partial mandible of sils representing a rich fauna of vertebrates bones, but it is far from completely excavat- the talpid Scalopus (Hesperoscalops) sp.; a (fish and reptiles) and invertebrates (mostly ed. Dinosaur taxa from the site include a mandible of the geomyid Geomys (Nertero- conchostracans) of early-mid Norian (~218– large allosaurid theropod, a diplodocid geomys) paenebursarius; a metatarsal of the 210 Ma) age. We discuss here a probable sauropod and other(?) sauropods. horse Equus calobatus; a partial metapodial of decapod crustacean (NMMNH P-29041) The dinosaur bones at the Peterson site are a small antilocaprid; and a complete set of found in the spoil pile at the site. The speci- mostly disarticulated and partially articulat- mandibles with right and left m2-m3 of the men is in matrix of slightly sandy mudstone ed limb bones, vertebrae, and ribs with a gomphotheriid proboscidean Stegomastodon that contains poorly sorted, texturally general NW–SE alignment that parallels the mirificus. The Belen mole mandible repre- mature sand- to gravel-size clasts of very fine paleocurrent as indicated by SE-dipping sents the first record of the family Talpidae grained mudstone. Location of the specimen trough crossbed axes. This provides strong from New Mexico in either the fossil or mod- and rock color indicate that it probably orig- prima facie evidence of fluvial transport and ern fauna and the westernmost occurrence of inated in one of several fining-upward hydraulic concentration of the bones. How- the genus. G. (N.) paenebursarius is restricted sequences between 0.3 and 1 m above the ever, clay balls associated with the bones, the to late Blancan faunas, E. calobatus occurs in main bone-bearing layer. large size of the bones, and their evident lack the late Blancan and Irvingtonian, and S. mir- The decapod specimen is dorsoventrally of abrasion suggest that transport distances ificus is found in the middle Blancan through compressed, preservation is only fair, and, were relatively short. Some bones at the site

New Mexico Geology May 1999 43 are tilted on bedding planes with a dip of FROM THE MIOCENE OF THE tus from Permian to Middle Triassic time (at about 8° down to the NW, suggesting they ESPAÑOLA BASIN, NEW MEXICO, by least 30 million years) and is overlain by may have been mired on a bar or levee mar- Spencer G. Lucas, Andrew B. Heckert, and Moenkopi fluvial deposits. Tr-3 is the uncon- gin. Gary S. Morgan, New Mexico Museum of formity between early Middle Triassic The Peterson site thus is a fluvially trans- Natural History and Science, 1801 Moun- (Moenkopi) and early Late Triassic (Chinle) ported and winnowed lag deposit of large tain Rd. NW, Albuquerque, NM 87104 rocks (about a 10 million year hiatus), and dinosaur bones, mostly of sauropods. It A nearly complete skull of the primitive this surface is overlain by initial fluvial thereby resembles taphonomically most of proboscidean Gomphotherium, New Mexico deposits of the Chinle Group. J-2 is the ero- the large dinosaur bone quarries known Museum of Natural History (NMMNH) P- sional surface between latest Triassic and from the Morrison Formation in the western 28972, was collected just east of Española, Middle Jurassic strata (about a 45 million United States. The Peterson site also sup- Rio Arriba County, in the area Galusha and year hiatus) overlain by Entrada eolianites. ports recognition that deposition of the Blick (1971, American Museum of Natural 2. There is much less regional stratigraph- Brushy Basin Member took place on riverine History Bulletin 144) referred to as "First ic relief on the Tr-4 unconformity. This is con- floodplains, not in a large, shallow lake. Wash." This skull is from the Miocene (late sistent with the fact that the Tr-4 unconfor- Barstovian–early Clarendonian) Pojoaque mity is an intrabasinal unconformity that Member of the Tesuque Formation. This is represents a relatively short temporal hiatus A PARTIAL SKULL OF A PACHYCEPH- also the type horizon of Mastodon productus close to the Carnian–Norian boundary ALOSAURIAN DINOSAUR FROM Cope, 1875, now referred to as Gompho- 3. The nature of the Tr-5 (= J-0) unconfor- THE UPPER CRETACEOUS KIRTLAND therium productum and supposedly the only mity is difficult to evaluate using these data FORMATION, SAN JUAN BASIN, Miocene proboscidean taxon from New because in most of the sections the youngest NORTHWESTERN NEW MEXICO, by Mexico. Triassic strata pre-date the Tr-5 unconformi- Thomas E. Williamson, New Mexico Muse- NMMNH P-28972 has complete tusks and ty. Nonetheless, the varying thicknesses of um of Natural History and Science, 1801 M2-3. The right tusk is worn, but the left tusk the Painted Desert Member of the Petrified Mountain Rd. NW, Albuquerque, NM is unworn, indicating "right handedness" in Forest Formation and its correlatives may in 87104 this Miocene proboscidean. The M2's are part be due to erosion beneath the Tr-5 A partial skull (NMMNH P-27403) from worn, but the M3's are little worn and not unconformity. the Upper Cretaceous (upper Campanian) completely erupted. Assignment of NMM- De-na-zin Member, Kirtland Formation, San NH P-28972 to Gomphotherium is justified by Juan Basin, represents a pachycephalosaur its relatively bunodont upper cheek teeth, THE LOWER CRETACEOUS SECTION AT (Dinosauria: Pachycephalosauria). Previous which have single trefoils, and its M3 with EAGLE NEST, LUNA COUNTY, NEW identifications of Pachycephalosauria from four lophs. Size of the M3 (length = 135 mm, MEXICO, by John W. Estep and Spencer G. the San Juan Basin have been questionable or width = 65 mm) is relatively small for Lucas, New Mexico Museum of Natural erroneous and were based only on isolated Gomphotherium but within the size range of a History and Science, 1801 Mountain Rd. teeth. Therefore, this represents the first cer- very polymorphic species, G. productum, as NW, Albuquerque, NM 87104 tain report of Pachycephalosauria in New conceived by Tobien. We, however, are skep- Lower Cretaceous strata exposed at Eagle Mexico. tical of the validity of so variable a species Nest in Luna County (sec. 34 T27S R5W) The skull of P-27403 includes most of the and note that the newly collected Gompho- overlie Permian limestone and are over- fronto-parietal dome as well as most of the therium skull falls into the size range of a turned to the southeast. The Lower Cre- right supraorbital II, the right postorbital small species of Santa Fe Group gom- taceous section here is at least 400 m thick and postfrontal, the right squamosal, and photheres that should be called G. produc- and is assigned to the Hell-to-Finish and U- much of the basicranium. The new specimen tum. A large size group of Santa Fe Group Bar Formations of the Bisbee Group. The differs from all previously described pachy- Gomphotherium, however, may better be Hell-to-Finish Formation disconformably cephalosaur taxa in North America. The termed a separate species for which the overlies Permian strata (probably Colina skull lacks a distinct squamosal shelf as is name G. pojoaquensis (Frick) is available. Limestone) and is 175 m thick. The lower seen in Stegoceras and Stygimoloch and third of the Hell-to-Finish Formation (about exhibits derived "pachycephalosaurine" fea- 50 m thick) consists of interbedded cobble tures such as closed supratemporal fenes- CONTINUITY OF TRIASSIC STRATA conglomerates (clasts are Paleozoic lime- trae, a high dome, and marked anteroventral AND UNCONFORMITIES ACROSS stone and chert up to 15 cm in diameter) and rotation of the occiput that unite it with THE RIO GRANDE RIFT, NORTH-CEN- red-bed mudstones. The upper two-thirds Prenocephale and Pachycephalosaurus. The TRAL NEW MEXICO, by Spencer G. are interbedded red-bed mudstone, calcare- new specimen further resembles Prenocephale Lucas, Andrew B. Heckert, and John W. ous shale, calcarenite, nodular (pedogenic?) prenes from the Nemegt Formation, Estep, New Mexico Museum of Natural limestone, limestone-cobble conglomerate, Mongolia, in its possession of a single row of History and Science, 1801 Mountain Rd. and sandstone. low conical nodes along the posterior and NW, Albuquerque, NM 87104 We place the contact of the Hell-to-Finish lateral margins of the squamosal. Middle Triassic strata of the Moenkopi and U-Bar Formations at the base of a 5-m- The new pachycephalosaur specimen is a Formation and Upper Triassic strata of the thick ledge of bivalve packstone. Above this new member of the Willow Wash local fauna Chinle Group are readily correlated from the bed, U-Bar strata are interbedded bivalve that also includes the dinosaurs Parasaur- Colorado Plateau eastward across the north- and gastropod packstones, calcareous shale, olophus tubicen, Naashoibitosaurus ostromi, ern Rio Grande rift to the southern High and calcarenites. The exposed U-Bar For- Pentaceratops sternbergi, and a new genus and Plains of eastern New Mexico. Litho- mation at Eagle Nest is about 230 m thick, if species of Ankylosauridae. The new pachy- stratigraphic and biostratigraphic data thus a 75-m-thick covered interval about 25 m cephalosaur specimen increases the distinc- provide the basis for a detailed correlation of above its base is included, but no top of the tiveness of the Willow Wash local fauna com- Triassic strata from Fort Wingate on the west U-Bar Formation is exposed here. About 100 pared to near contemporaneous faunas of to Lamy on the east. This correlation demon- m below the top of the U-Bar Formation at western North America. This marked faunal strates the continuity of Middle and Late Eagle Nest, at NMMNH locality 3755 (UTM difference supports the argument for rela- Triassic deposition across northern New 3533302N, 279207E, zone 13, NAD 27), fos- tively strong provinciality among terrestrial Mexico and that: sils of Quadratotrigonia and other bivalves faunas during the Late Cretaceous of west- 1. There is substantial (tens of meters of) indicate a late Aptian age. Lithology and ern North America. stratigraphic relief across three pervasive biostratigraphy thus support assignment of unconformities: Tr-1, Tr-3, and J-2. These are the exposed U-Bar section at Eagle Nest to tectonosequence boundaries that correspond the Carbonate Hill Member. The upper NEWLY DISCOVERED SKULL OF THE to significant tectonic reorganizations of the Aptian Carbonate Hill Member of the U-Bar PROBOSCIDEAN GOMPHOTHERIUM Triassic–Jurassic depositional basins. Thus, Formation is present at all U-Bar sections in the Tr-1 erosional surface represents the hia- southwestern New Mexico and thus pro-

44 May 1999 New Mexico Geology vides an important basis for regional correla- formably overlies some of the jasperoids, al., 1994). These flows are massive and gen- tion of the Lower Cretaceous. indicating jasperoid formation prior to for- erally faulted into large blocks. Many of the mation of the Emory caldera. flows have a lower medium dark-gray base The Copper Flat porphyry-copper deposit that is heavily fractured into thin (~5 cm) GEOCHEMISTRY OF HOST ROCKS, consists of Cu, Au, Mo, and Ag dissemina- lensoids, possibly a texture resulting from VEINS, REPLACEMENTS, AND JAS- tions and quartz veins in a breccia-pipe in cooling and/or shear stress. In other loca- PEROIDS IN THE HILLSBORO DIS- the quartz monzonite stock. It is predomi- tions, the lava bases exhibit thick (up to 4 m) TRICT, SIERRA COUNTY, NEW MEXI- nantly a low-grade hypogene deposit that is flow breccias. Underlying this flow to the CO, by Virginia T. McLemore, ginger@gis. concentrated within a breccia pipe contain- south is a pyroclastic breccia and lapillistone nmt.edu, New Mexico Bureau of Mines ing pyrite, chalcocite, chalcopyrite, azurite, and an underlying, coeruptive flow. Both and Mineral Resources, Socorro, NM malachite, and cuprite. Copper Flat has flows are tilted and faulted along with 87801; M. T. Heizler and Erik A. Munroe, reported reserves (Dec. 31, 1997) of underlying sediments and are, in places, Department of Earth and Environmental 56,549,000 tons of ore grading 0.432% Cu, overlain by eolian and fluvial sand and grav- Science, New Mexico Institute of Mining 0.14 ppm Au, 2.19 ppm Ag, and 0.014% Mo. el lithofacies. A black dacitic ash, up to 2 m and Technology, Socorro, NM 87801 Many workers in the district have recog- thick, which is overridden and deformed by New geochemical, geochronological, and nized district zoning. The Copper Flat por- the extensive upper flow, is exposed within a geological data, combined with earlier stud- phyry-Cu deposit forms the center. prominent NW–SE canyon separating the ies, have provided a refinement of the evolu- Propagating outward radially from the old and young centers. Several dacitic intru- tion of the mineralization in the Hillsboro Copper Flat porphyry are Laramide sives occur as isolated hills to the south and (Las Animas) district in central New Mexico. Au–Ag–Cu veins hosted by many of the southeast of the volcano and are believed to Laramide (polymetallic) vein, placer Au, car- latite dikes. Chemical analyses range from be age-correlative with the older center. bonate-hosted Ag–Mn, Laramide skarn, and 8–64,600 ppb Au, <0.2–590 ppm Ag, 40– The younger volcano of the Los Lunas porphyry-Cu deposits are found in this dis- 57,337 ppm Cu, <1–475 ppm Mo, 57–8,906 complex forms the central edifice with two trict. Past production has been predominant- ppm Pb, and 138–17,026 ppm Zn. Carbonate- prominent lava lobes that extend to the E ly from the Laramide veins, although minor hosted replacement deposits (Ag, Pb, Mn, V, and SE from the summit. The volcano con- production has occurred from the carbonate- Mo, Zn) are found in the southern and north- sists of at least five eruptive units (four lavas hosted deposits. During 1877–1982 an esti- ern parts of the district, distal from the cen- and one pyroclastic deposit) and has been mated 270,000 troy oz Au (lode and placer), ter. Chemical analyses range from <5–99 ppb faulted and uplifted 150 m above the sur- 78,000 troy oz Ag, 24 million lbs Cu, and Au, 1–<50 ppm Ag, 131–173 ppm Cu, 2–140 rounding topographic top of basin fill. All of 153,387 lbs Pb were produced. The Copper ppm Mo, 30–>10,000 ppm Pb, and 123– the units are trachyandesite in composition, Flat porphyry-Cu deposit in the central part >20,000 ppm Zn. Collectively, the evidence and one lava has been dated at 1.22 ± 0.01 of the district was discovered in 1975. suggests that the deposits found in the Ma (Love et al., 1994). The oldest unit is a Quintana Minerals Corp. produced approxi- Hillsboro district were formed by large, con- massive chocolate-brown flow that forms a mately 7 million lbs of Cu in March–June vective hydrothermal systems related to the laterally continuous (but faulted) lava and 1982, prior to closure of the open-pit mine. Copper Flat volcano and subsequent intru- eruptive edifice along the top of the pre- Alta Gold Co. is applying for mining permits sion of the quartz monzonite and latite dikes. eruptive surface on the west, north, and to reopen the Copper Flat mine. northeast sides of the young volcano. The The geology of the Hillsboro district is flow is generally less than 8 m thick but is dominated by Cretaceous andesite flows A VOLCANO REVISITED: A PRELIMI- ponded to the north (18 m) and on the SW (75.4 ± 3.5 Ma, 40Ar/39Ar), breccias, and vol- NARY REPORT OF THE GEOLOGY OF edge (20 m) of the volcano. Crustal canoclastic rocks that were erupted from an LOS LUNAS VOLCANO, CENTRAL xenocrysts are common within the choco- andesite volcano. The Copper Flat quartz NEW MEXICO, by Kurt Panter, kpanter late-brown flow. It is overlain by a light gray- monzonite porphyry (74.93 ± 0.66 Ma, @western.edu, Western State College, ish-red flow on the east side. The next 40Ar/39Ar) intruded the vent of the volcano. Gunnison, CO 81231; Bruce Hallett, bhal- youngest lava forms a small isolated outcrop The unmineralized Warm Springs quartz [email protected], Golder Associates Inc., to the west of the main summit. A 2–5-m- monzonite (74.4 ± 2.6 Ma, 40Ar/39Ar) is south Edgewood, NM 87015; Dave Love, dave@ thick, red, scoriaceous pyroclastic fall, con- of the Copper Flat porphyry. A third altered, gis.nmt.edu; Chris McKee, [email protected]. sisting of individual bombs and lapilli agglu- unmineralized quartz diorite crops out in the edu, New Mexico Bureau of Mines and tinate, mantles both chocolate-brown and northern part of the district. These two intru- Mineral Resources, Socorro, NM 87801; high western lava. All three units are cut by sions most likely represent small, satellite Rebecca Thompson, [email protected], a prominent N–S-trending fault exposed stocks that intruded along fracture zones on Western State College north of the summit and which displaces the the flanks of the volcano. Latite and quartz Preliminary results show Los Lunas vol- units as much as 30 m down to the east. The latite dikes intruded the andesite and cano is a polygenetic center composed pri- youngest unit is a light-gray to brown Copper Flat porphyry and radiate outwards marily of intermediate lavas with subordi- flow(s) originating from a central vent near from the Copper Flat porphyry. A 40Ar/39Ar nate pyroclastic lithologies. Los Lunas is one the summit. This unit forms extensive flow plateau age of 70.21 ± 0.20 Ma of a latite dike of six principal Pliocene–Pleistocene vol- lobes to the E and SE of the summit. is distinctly younger than the andesite and canic centers erupted within the Albu- Preliminary results from major and trace quartz monzonite and could represent the querque Basin—an extensionally formed elements (XRF analysis) reveal some impor- true cooling age or was reset by younger depression within the Rio Grande rift sys- tant differences between the two volcanoes. hydrothermal activity. The igneous rocks are tem. Los Lunas volcano is currently being Although the dacitic compositions of the part of a differentiated comagmatic suite. mapped under the U.S. Geological Survey's older volcano appear more evolved (12–18% The andesites are metaluminous and alka- National Cooperative Geologic Mapping Q-norm) relative to the younger trachyan- line; the quartz monzonites and latites are Program. Approximately 70 samples have desite volcano (3–8% Q-norm), the range in peraluminous and alkaline to subalkaline. been collected for geochemistry and Mg#'s (100*Mg/Mg+Fe) are roughly equiva- 40 39 The linear variation in Nb/Zr, Zr/TiO2, Ar/ Ar geochronology. lent (44– 51). In addition, highly incompati-

V/TiO2, and various major elements sug- Based on volcanic stratigraphy, landscape ble trace elements such as Ba, Th, and Nb gests that the igneous rocks are comagmatic. morphology, and two previous 40Ar/39Ar have significantly lower concentrations in Alteration of the igneous rocks consists of dates, Los Lunas volcano is considered to the dacites relative to trachyandesites—all of locally intense silicification and K-metaso- have at least two main periods of eruptive which suggest a fundamental petrogenetic matism. Large jasperoid bodies have activity. The older center, a low-lying jum- difference between the two volcanoes. replaced limestones belonging to the El Paso bled mass SW of the 5,955-ft central summit, Reference Formation, Fusselman Dolomite, and Lake consists of four volcanic units. The youngest Love, D.W., Reynolds, C.B., Hallett, B., Lozinsky, Valley Limestone in the southern part of the of these units is an extensive flow complex of R.P., and Niemyjski, T., 1994, Sedimentation, district. The Sugarlump Tuff (35 Ma) uncon- dacitic lavas dated at 3.88 ± 0.01 Ma (Love et deformation, and erosion related to Los Lunas

New Mexico Geology May 1999 45 volcanoes, central New Mexico (abs.): New PRELIMINARY STUDIES OF THE occur rapidly in many environments (Birke- Mexico Geology, v. 16, no. 3, p. 57. RECENT SOIL-GEOMORPHOLOGIC land, 1984), and therefore rates of soil devel- HISTORY OF A FLUVIAL SYSTEM, opment on the surfaces are reflected in time- RIO CALAVERAS, JEMEZ MOUN- dependent soil features. On such a short MIDDLE PROTEROZOIC METAMOR- TAINS, NEW MEXICO, by Ivan Erchak time-scale, spatial dependence is also very PHISM AND DEFORMATION IN THE and Tim Gere, Department of Earth and important to consider. For example, the pres- PECOS COMPLEX, NEW MEXICO, Planetary Sciences, University of New ence of a single tree has greatly influenced U.S.A., by Erwin A. Melis, eamelis@nmt. Mexico, Albuquerque, NM 87131 local soil development. Studying and inter- edu, Laurel B. Goodwin, Department of The disciplines of pedology and geomor- preting the history and evolution of late Earth and Environmental Science, New phology have been integrated with great Holocene fluvial systems such as this one Mexico Institute of Mining and Tech- success, helping to illuminate processes of requires an interdisciplinary approach nology, Socorro, NM 87801; and Matt landform development during the Qua- involving geomorphology, sedimentology, Heizler, New Mexico Bureau of Mines and ternary. In this research we show that the pedology, and ecology. This research is the Mineral Resources, Socorro, NM 87801 recent history of the fluvial system in a small first stage of a larger project to be conducted The regional significance of ca. 1,400 Ma drainage basin can be understood by cou- as a part of Master's thesis at the University deformation of Proterozoic continental crust pling the detailed descriptions of soils with of New Mexico. This study should help bet- in the southwestern United States is contro- geomorphic and other field criteria. While ter define the geomorphic framework in versial. Some workers have proposed that many soil-geomorphologic studies have which the hydrologic system is operating, metamorphism and deformation were been conducted on the Pajarito Plateau, thereby providing valuable input to ongoing restricted to the margins of ca. 1,400 Ma plu- located on the east flank of the Jemez hydrogeoecologic studies of nutrient cycling tons. Others suggest that ca. 1,400 Ma defor- Mountains of northern New Mexico, rela- related to a seasonally fluctuating water mation and metamorphism were regional in tively few have been conducted on the Jemez table. extent. Our work supports the latter inter- Plateau, located on the west flank. The study pretation but indicates that strain was dis- area is part of the Rio Calaveras drainage, tributed heterogeneously in New Mexico which is cut into the Bandelier Tuff and A DOUBLE-POROSITY MODEL OF during Middle Proterozoic deformation. located approximately 3 km west-northwest GROUND-WATER FLOW IN THE The Pecos complex of northern New of the Valles caldera. Fluvial landforms that MADERA FORMATION BASED ON Mexico provides an ideal site to study dominate the inner valley at the site are SPRING HYDROGRAPHS AND AQUI- Proterozoic deformation and metamor- shown to have developed within the past 700 FER TEST ANALYSES FROM PLACI- phism, as it includes dated granitoid plutons yrs. Two distinct, laterally continuous geo- TAS, NEW MEXICO, by Peggy Johnson, of three distinct ages: ca. 1,718 ± 5 Ma morphic surfaces are observed. The upper New Mexico Bureau of Mines and Mineral (Yavapai orogeny), 1,650 Ma (Mazatzal surface is a fluvial terrace, located between 2 Resources, Socorro, NM 87801 orogeny), and ca. 1,480 Ma. All three plutons and 3 m above the current stream level, and The Madera Formation forms a carbonate exhibit a generally easterly striking, south- the lower surface is the current floodplain, aquifer of regional significance for ground- dipping, solid-state foliation; where present, located about 0.5 m above the active channel water development in the Sandia Mountains a stretching lineation plunges shallowly. bed. Each of these surfaces exhibits different east of Albuquerque, New Mexico. Charact- Locally developed kinematic indicators in degrees and types of soil development, erization of carbonate aquifers is problemat- both the oldest and youngest intrusions which we conclude are both time and space ic due to the localized nature of ground- record dextral strike-slip shear. The foliation dependent. The ages of the soils and associ- water flow in fractured limestone. Spring is variably developed in all three plutons; ated surfaces were derived in part by estab- hydrographs and aquifer test data indicate much of the youngest pluton is not foliated. lishing ages through preliminary den- that ground water in the Madera aquifer Definitive evidence of older structures has drochronological studies of the trees on the moves as combined diffuse flow and fracture not been found; however, intrafolial folds in surfaces. The initial stages of pedogenesis flow. Aquifer test drawdown data fit a dou- one locality and variations in orientation of the foliation in the older plutons suggest that an older foliation may have been overprint- ed. Deformation was accompanied by amphibolite-facies metamorphism. Local ret- rogression of hornblende to actinolite appears to have largely postdated deforma- tion. 40Ar/39Ar dates on hornblende fall within the range of 1,372 ±1 0 Ma. Cooling through the closure temperature of hornblende (~500°C) therefore postdates intrusion of the ca. 1,480 Ma granitoid. The timing of dextral strike-slip shear, which occurred under amphibolite-facies conditions, therefore is bracketed by dates on intrusion and the cool- ing of hornblende. We thus have evidence of a regionally extensive, ca. 1,400 Ma fabric in the Pecos complex. It is significant that this fabric formed at least in part through strike- slip shear, since previous models of this deformation event emphasized either regional shortening or extension. This work suggests that deformation was regional in extent, that strain was distributed heteroge- neously, and that current models are inade- quate to explain the kinematics of the struc- tures we observe in the field.

46 May 1999 New Mexico Geology ble-porosity model and show that ground plate tectonics, and geology of New Mexico. reports, maps, and all NMBMMR open-file water is primarily transmitted through large Illustrations showing how to make a tetra- reports. Recently a coal library has been fractures, but the majority of aquifer storage hedron and hexagonal illustrations that can established to store geophysical logs, chemi- is attributable to the limestone matrix. be printed and mounted on the sides of cal analyses, maps, and reports from coal- Fracture transmissivity ranges from 170 to hexaflexagons are available at the New exploration projects in New Mexico. The 200 m2/d. Total storativity, for both fractures Mexico Bureau of Mines and Mineral NMBMMR also houses information on the and matrix, is 0.20–0.25. The fractures trans- Resources earthquake education Web site state's water resources, including informa- mitting the bulk of spring discharge are asso- (http://tremor.nmt.edu/). The files are cur- tion for use in developing and protecting ciated with faults in the Madera Formation. rently available as downloadable and print- water supplies, evaluating water-quality and Spring hydrographs from fault-controlled able .gif files and may be available as higher pollution problems, operating and reclaim- springs near the village of Placitas may pro- resolution .pdf files in the near future. ing mines, and utilizing geothermal vide a potentially valuable source of data on resources. Some National Uranium Resource Madera aquifer hydraulic properties, includ- Evaluation (NURE) data are available at the ing effective porosity, transmissivity, storage, DRILL CORE, CUTTINGS, AND GEO- NMBMMR, along with various geologic water budgets, and recharge. Hydrograph LOGICAL RESOURCES AVAILABLE maps, reports, and logs. separation and recession curve analysis yield AT THE NEW MEXICO BUREAU OF Professional staff are available for consul- preliminary estimates of dynamic storage MINES AND MINERAL RESOURCES, tation on the different resources provided at and recharge for three Placitas village by E. R. Fleming, G. K. Hoffman, and A. S. the NMBMMR. For a complete overview of springs. Further work is required before the Read, New Mexico Bureau of Mines and the NMBMMR's resources visit our web site full potential of spring hydrograph data can Mineral Resources, Socorro, NM 87801 at http://geoinfo.nmt.edu. be utilized as a regional aquifer characteriza- The core and cutting library archives at the tion tool in the Madera Formation. New Mexico Bureau of Mines and Mineral Resources (NMBMMR) make available rep- resentative samples from rock formations in Two abstracts from the poster session (nos. ORIGAMI LEADS TO OROGENY: USE different parts of New Mexico. Drill cuttings 6 and 7) were the catalyst for a letter to New OF THREE-DIMENSIONAL PAPER from 15,000 wells and more than 1,000 dif- Mexico Geology. Abstracts 6 and 7 are printed MODELS FOR GEOSCIENCE EDUCA- ferent cores are stored at the NMBMMR. The below, followed by the letter and a response TION FROM MINERALOGY TO core library consists of five warehouses, a from the abstracts’ authors. EARTHQUAKES, by David W. Love, Jan processing area in each building, equipment Thomas, and William C. Haneberg, New for slabbing core, and microscopes for the Mexico Bureau of Mines and Mineral examination of cuttings. The library is an JURASSIC STRATIGRAPHY IN THE Resources, Socorro, NM 87801 important source of geological and engineer- TIJERAS SYNCLINE, BERNALILLO Basic geometric forms such as cubes, octa- ing information regarding exploration for, COUNTY, NEW MEXICO, by Spencer G. hedra, rhombohedra, and tetrahedra may be and development and preservation of, oil Lucas, John W. Estep, and Orin J. Anderson, made by folding flat paper or envelopes, and natural gas, water, coal, uranium, metal- New Mexico Museum of Natural History both reasonably priced starting materials. lic and industrial minerals, and carbon diox- and Science, 1801 Mountain Rd. NW, These may be used to help visualize solid ide in New Mexico. An excerpt from the Albuquerque, NM 87104 forms in crystallography and mineralogy, database of archived core is shown in Table Jurassic strata are intermittently exposed and numbers of them may be used to create 1. This data will be available on the NMB- along the western, northern, and northeast- unit cells. Tetrahedra in particular are easy to MMR web site at http://geoinfo.nmt.edu. ern limbs of the Tijeras syncline (Cedar make by folding and cutting small enve- Additional resources at the NMBMMR Crest–Sandia Park areas, Bernalillo County). lopes. Tetrahedra may be linked to build sil- include a number of different types of infor- The singlemost complete and well-exposed icate and carbonate crystal models. mation related to the core and cuttings in its section crops out in the NW⁄SW⁄ sec. 31 T11N A ring of six tetrahedra may be linked repository. The Subsurface Library contains R5E and well represents the outcrop belt. along two sides of each to form a hexa- well records, drillers’ logs, electric and other Here, the Jurassic strata are 169 m thick and flexagon. Spillhaus and the GeoLearning geophysical logs, sample logs, and descrip- are (ascending): (1) Dewey Bridge Member Corporation (Sheridan, Wyoming) have pro- tions. Source-rock analyses, biostratigraphic of Entrada Sandstone, 11.5 m of mostly red- duced hexaflexagons with illustrated geo- data, petroleum-exploration maps, geologic dish brown, ripple-laminated, fine sandstone graphic, geologic, and astronomic themes on maps, production data, petroleum-related that disconformably overlies red-bed mud- each side printed on stiff paper (Spillhaus publications, field and pool data and maps, stones of the Upper Triassic Petrified Forest copyright, 1985). These models are also rea- and a number of core analyses and drill-stem Formation of the Chinle Group; (2) Slick sonably priced and require assembly. As an test records are also available. The Geologic Rock Member of Entrada Sandstone, yellow, alternative, we have combined the rapid pro- Information Center (GIC) is a specialized coarse, trough-crossbedded sandstone about duction of tetrahedra from envelopes with library and archive focusing on geologic data 10.5 m thick; (3) Luciano Mesa Member of various geological themes printed in color relating to New Mexico's mining, milling, Todilto Formation, 2.2 m of thinly laminated, on hexagons to produce customized hexa- petroleum industries, and water resources. kerogenic limestone; (4) Tonque Arroyo flexagons covering the topics of earthquakes, The GIC also contains unpublished mine Member of Todilto Formation, 20.5 m of mas- sive, white gypsum; (5) Summerville Formation, 31 m of red, ripple-laminated sandstone, massive gypsiferous sandstone, Table 1—Excerpt from District Core Archive Table and cyclically bedded, variegated siltstone, District/Field/ Company name Twp Rng Sec Type Total wells shale, and nodular limestone; (6) Salt Wash Basin name Member of Morrison Formation, 21 m of fine Willow Creek Perry, Knox 18N 12E 27 Lead/Zinc 6 to coarse, trough-crossbedded feldspathic & Kaufman sandstone with lenses of clay-pebble con- District Total Number of wells 6 glomerate and red-bed mudstone; (7) Brushy Basin Member of Morrison Formation, 32 m Zuni Mountains Conoco Metallics 11N 12W 6 Uranium 2 of mostly covered, smectitic, green claystone District Total Number of wells 2 with some thin lenses of nodular limestone and fine-grained sandstone; (8) Jackpile Albuquerque Basin Shell Oil 8N 2E 16 Oil & Gas 1 Member of Morrison Formation, 32 m of District Total Number of wells 1 trough-crossbedded kaolinitic sandstone overlain by the Oak Canyon Member of the Cretaceous Dakota Formation. The Jurassic

New Mexico Geology May 1999 47 section in the Tijeras syncline thus is very (1999a,b) reported significant findings Formation occur within Mancos Shale of similar to that exposed nearby in the Hagan regarding the Mesozoic stratigraphy of cen- central New Mexico, with the Twowells Basin and at Galisteo Dam, though thinner. tral New Mexico. The findings where pre- Tongue being the youngest and the Cubero The unit mapped in the Tijeras syncline by sented in two abstracts focused on Cre- Member being the oldest. All of the Dakota Ferguson et al. (1996, NMBMMR Open-file taceous and Jurassic stratigraphy. Since two Formation sandstones underlie the Green- Digital Map OF-DM-1) as the Bluff of the abstracts deal directly with areas horn Formation (which includes the Myti- Sandstone is actually the Salt Wash Member recently mapped and described in detail by loides mytiloides-bearing black micrite bed), of the Morrison Formation. New Mexico's STATEMAP program, I feel and all consist chiefly of bioturbated, marine, compelled to respond to some of the state- quartz-rich sandstone, although the Two- ments that I believe are misleading and wells is generally coarser grained (Owen and THE INTERTONGUED DAKOTA–MAN- incorrect. Siemers, 1977). It seems that the only evi- COS (CRETACEOUS) SECTION IN THE dence Lucas et al. (1999a) have for suggest- TIJERAS SYNCLINE, BERNALILLO Cretaceous ing this reinterpretation is the relative thick- COUNTY, NEW MEXICO, by Spencer G. Lucas et al. (1999a) report the discovery of nesses of the intervening shales, a criteria Lucas, Orin J. Anderson, and John W. Estep, the critical outcrop for reinterpreting the that most geologists would find equivocal New Mexico Museum of Natural History stratigraphy of the intertongued Dakota– considering that the next available outcrops and Science, 1801 Mountain Rd. NW, Mancos section in the Tijeras syncline. The that include this sequence are nearly 30 km Albuquerque, NM 87104 outcrop they describe is merely another to the north. Ferguson et al. (1996) mapped The oldest Cretaceous strata exposed in exposure of the Mytiloides mytiloides-bearing the sandstone in question as upper Dakota the Tijeras syncline (Cedar Crest–Sandia black micrite bed that we identified and Formation and merely suggested that it Park area, Bernalillo County) belong to the described at two other nearby localities might correlate with the Twowells Tongue. intertongued Dakota Formation and Mancos (Ferguson et al. 1996, p. 4). An excerpt from The sandstone may indeed be the Cubero Shale of Cenomanian (early Late Cretaceous) our report is included. Member, but until definitive biostratigraphic age. Previous workers interpreted this strati- The most continuous exposure of evidence is presented, I suggest that this thin graphic interval as either: (1) undifferentiat- Mancos Shale is near the head of Arroyo and relatively minor sandstone remain as we ed Dakota Formation directly overlain by San Antonio, along the eastern half of the mapped it, an informal unit. undifferentiated Mancos Shale (Kelley and boundary between secs. 25 and 36 T11N Northrop, 1975, NMBMMR Memoir 29) or R5E. In this gully, the oldest exposed rock Jurassic (2) lower Dakota Formation overlain by a is a dark-gray shale with a 20–60-cm-thick Lucas et al. (1999b) report the discovery of covered interval followed by a thin sand- black micrite bed containing the bivalve the most complete and well-exposed section stone interval probably correlative with the Mytiloides mytiloides. This limestone is of Jurassic rocks in the Tijeras syncline at the overlain by at least 100 m of black, noncal- Twowells Tongue of the Dakota Formation south end of a north-striking hogback near careous shale with abundant septarian (Molenaar, C. M., 1983 in Mesozoic paleo- the Cedar Crest Post Office (NW⁄ SW⁄ sec 31 nodules as much as 1 m in diameter. geography of the west-central US: RMS- Above the septarian nodule shale is a T11N R5E), an area mapped in detail by SEPM; Ferguson et al., 1996, NMBMMR shale sequence with several thin- to medi- Ferguson et al. (1996). I agree that the section Open-file Digital Map OF-DM-1). However, um-bedded calcareous sandstone beds. is well exposed, but the outcrops are cut by detailed stratigraphy of this interval and its These sandstone beds, which also are pre- numerous faults, and they lie in an excep- correlation to nearby, better exposed sections served in the next gully to the north, con- tionally structurally complex area adjacent to in the Hagan Basin and at Galisteo Dam sup- tain abundant bivalve fragments and the the Tijeras fault. There may be structural rep- port a different interpretation of the Dakota– ammonite Prionocyclus novimexicanus etitions or omissions of the section depend- Mancos section in the Tijeras syncline. (sample F-95-54). ing on where it is measured, and as a struc- The critical outcrop is along Gutierrez The Mancos shale is also exposed along tural geologist I would (and have) cautioned Canyon in the SE⁄NW⁄ sec. 29 T11N R6E. the headwaters of Gutierrez Canyon in the against using any section of strata along this Here, the section is: (1) about 10 m of trough- east limb of the Tijeras syncline. hogback for determining thicknesses of crossbedded quartzarenitic sandstone, with Incomplete exposures here suggest a simi- units. a bioturbated uppermost 0.6 m, of basal lar stratigraphic sequence to that along The section is also not representative of Dakota Formation disconformably overlying Arroyo San Antonio. A Mytiloides myti- the Jurassic sequence in Tijeras syncline as is Upper Jurassic Jackpile Member of Morrison loides-bearing black micrite (sample F-95- suggested by Lucas et al. (1999b). In particu- Formation; (2) a 31-m-thick interval of gray 49) is present just above the uppermost lar, the sandstone body that directly overlies shale that forms a mostly covered slope; (3) a Dakota Formation exposure. Farther the Summerville Formation is atypical. This downstream a medium-bedded calcareous 7-m-thick bioturbated sandstone cuesta; (4) a sandstone, which Lucas et al. (1999b) identi- sandstone contains fragments of the mostly covered 63-m-thick interval of dark- fied as the Brushy Basin Member of the ammonites Prionocyclus novimexicanus and gray shale with numerous thin bentonite Scaphites whitfieldi and the bivalve Inocer- Morrison Formation, thickens to the north beds; (5) the Bridge Creek Member of the amus rotundatus (sample F-95-52). The Sca- and changes character into a medium- to Greenhorn Formation, a 3.7-m-thick lime- phites whitfieldi indicates a late Turonian thick-bedded, planar-laminated and cross- stone with the index bivalves Pycnodonte biostratigraphic age for this interval, and, stratified, moderately to well-sorted, felds- newberryi and Mytiloides mytiloides at based on precise laser-fusion radiometric pathic sandstone (Ferguson et al., 1996). I NMMNH locality 4105 (UTM 379273E, ages from sanidine phenocrysts in ben- agree that the sandstone identified as Brushy 3890240N, zone 13, NAD27). By correlation tonites elsewhere in the western interior Basin Member by Lucas et al. (1999b) is sed- to the Hagan and Galisteo sections, units 1–2 basin, an absolute age of between 90.21 ± imentologically and compositionally more = Oak Canyon Member of Dakota, unit 3 = 0.72 Ma and 88.34 ± 0.60 Ma (Obradovich, akin to the Morrison Formation and that the Cubero Member of Dakota and unit 4 = 1993). Kauffman et al. (1993) reported an map ought to be revised in this small area. I Graneros Shale. Stratigraphic position, age of 89.4 Ma for the Scaphites whitfieldi think, however, that issuing the statement, lithology, relative thicknesses, and position (T12) biozone. "The unit mapped in the Tijeras syncline by relative to the Greenhorn confirm that in the Based on the "discovery" of this limestone, Ferguson et al. (1996) as the Bluff Sandstone Tijeras syncline the upper Dakota sandstone Lucas et al. (1999a) reinterpret a thin and dis- is actually the Salt Wash Member of the Mor- interval is Cubero, not Twowells. continuous, bioturbated sandstone approxi- rison Formation," is wrong and misleading. mately 63 m below the limestone as the The sandstone mapped by Ferguson et al. Cubero Member of the Dakota Formation, (1996) as Bluff Sandstone in most areas of the April 30, 1999 and not the Twowells Tongue. They cite stra- Tijeras syncline was distinguished from the Dear NMG, tigraphic position relative to the black overlying Morrison Formation by its "higher This letter is in response to a discussion micrite bed, lithology, and relative thickness degree of sorting, its lack of mud-chip clasts, with Spencer Lucas at the 1999 NMGS spring of the intervening shales as evidence for the and the less-altered (pinkish-colored) nature meeting at a poster session where Lucas et al. correlation. At least three tongues of Dakota of its feldspar sand grains." Although very

48 May 1999 New Mexico Geology poorly exposed, the sandstone also displays May 3, 1999 Park quadrangle, so interested parties will abundant eolian sedimentological features Ferguson et al.’s (1996) geologic map of have a chance to evaluate the Jurassic stratig- such as large-scale, planar-tabular to wedge- the Sandia Park quadrangle presents a raphy firsthand. planar cross-stratification, and planar lami- Jurassic-–Cretaceous section strikingly dif- Spencer G. Lucas and Orin J. Anderson nations reminiscent of sub-critically climbing ferent from the much better exposed New Mexico Museum of Natural History translatent ripple stratification. An eolian Jurassic–Cretaceous sections in the nearby and Science body in this part of the section would corre- Hagan Basin (20–25 km to the north) and at 1801 Mountain Rd. NW late with the Bluff Sandstone, an important Galisteo Dam (40–45 km to the north-north- Albuquerque, NM 87104 eolian unit of the interior Colorado Plateau, east). However, our recent field studies indi- and is significant because it would represent cate that the Jurassic–Cretaceous section References an eastern outlier of this lithofacies on paleo- exposed on the Sandia Park quadrangle is Ferguson, C. A., Timmons, J. M., Pazzaglia, F. J., geographic maps. strikingly similar and thus readily correlated Karlstrom, K. E., Osburn, G. R., and Bauer, P. W., by lithostratigraphy and, where available, by 1996, Geology of Sandia Park 7.5' quadrangle, A suggestion biostratigraphy to the sections in the Hagan Bernalillo and Sandoval Counties, New Mexico: The advancement of science depends to a Basin and at Galisteo Dam. New Mexico Bureau of Mines and Mineral Thus, the upper tongue of the Dakota on Resources, Open-file Report DM-1, 1:12,000 scale large degree on accumulated knowledge and map, cross sections, text. information and on the ability of scientists to the Sandia Park quadrangle is the Cubero Lee, W. T., 1912, The Tijeras coal field, Bernalillo tap into these resources. A scientist who Tongue (Lucas et al., 1998, 1999a). And, the County, New Mexico: U.S. Geological Survey, starts from the beginning on every project sandstone above the Summerville Formation Bulletin 471, pp. 575–578. would never get very far. Although it is is the Salt Wash Member of the Morrison Lee, W. T., 1917, Geology of the Raton Mesa and sometimes difficult to acquire obscure maps Formation, not the Bluff Sandstone (Lucas et other regions in Colorado and New Mexico: U. S. and reports about an area, it is always in the al., 1999b). As in the Hagan Basin and at Geological Survey, Professional Paper 101, pp. best interest of the field scientist to be aware Galisteo Dam, this sandstone in the Sandia 9–221. of what has already been done. I discovered Park quadrangle has fluvial bedforms and Lucas, S. G., Anderson, O. J., and Estep, J. W., 1998, Stratigraphy and correlation of middle during my discussion with Dr. Lucas that he pebbly rip-ups; it lacks eolian bedforms. Cretaceous rocks (Albian–Cenomanian) from the had not read our map or report on the Sandia Ferguson et al.’s map of the Sandia Park Colorado Plateau to the southern High Plains, Park quadrangle (Ferguson et al., 1996) quadrangle contains numerous other mis- north-central New Mexico: New Mexico before his field investigations. It was also takes with regard to the Jurassic–Cretaceous Museum of Natural History and Science, clear that at the time of our discussion, he section, which include: (1) locally, Dakota Bulletin 14, pp. 57–66. still had not read our report. Although I strata are mapped as Morrison; (2) the map Lucas, S. G., Anderson, O. J., and Estep, J. W., applaud his effort and his important discov- fails to identify several readily mappable 1999a, The intertongued Dakota–Mancos ery of another index fossil in the Mancos units of the Mancos Shale interval—the (Cretaceous) section in the Tijeras syncline, Greenhorn, Carlile, Juana Lopez, D-Cross, Bernalillo County, New Mexico, in Proceedings Shale, I find that his methods leave much to volume, 1999 Annual Spring Meeting: New be desired. Our mapping program was Tocito, and El Vado; (3) locally, slump blocks Mexico Geological Society, p. 54. directed principally at the structural compli- of Mesaverde sandstone are mapped as a Lucas, S. G., Estep, J. W., and Anderson, O. J., cations of the Sandia Mountains, but we also Turonian marine sandstone; and (4) the 1999b, Jurassic stratigraphy in the Tijeras syn- made a serious effort to identify important Mesaverde Group is mapped as units that cline, Bernalillo County, New Mexico, in biostratigraphic features, and we invested a lack regional significance; Lee (1912, 1917) Proceedings volume, 1999 Annual Spring great deal of time integrating this informa- presents a more stratigraphically useful Meeting: New Mexico Geological Society, p. 53. tion into our maps and reports. interpretation of the Mesaverde Group here. Owen, D. E., and Siemers, C. T., 1977, Lithologic Sincerely, These aspects of the mapping of Ferguson correlation of the Dakota Sandstone and adjacent units along the eastern flank of the San Juan Charles A. Ferguson et al. (1996) will be detailed and corrected by Basin, New Mexico, in Fassett, J. E., and James, Arizona Geological Survey us in an upcoming publication. Furthermore, H. L. (eds.), San Juan Basin III, northwestern 416 W. Congress, #100 the Second Day fieldtrip of the 50th NMGS New Mexico: New Mexico Geological Society, Tucson, Arizona 85701 Fall Field Conference this September will Guidebook 28, pp. 179–183. REPLY TO FERGUSON examine the Jurassic section on the Sandia

New Mexico Geology May 1999 49