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Triassic Stratigraphy and Chronology in New Mexico

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Triassic Stratigraphy and Chronology in New Mexico Triassicstratigraphy andchronology inNew Mexico bySpencer G. Lucas, New Mexico Museum of NaturalHistorv and Science. 1801Mountain Road NW, Albuquerque, NewMexico 87104 Abstract focused on particular regions of the state formably on Middle Permian (Guadalu- and thus did not produce a unified syn- pian) strata of the Artesia Group, Glorieta Tiiassic rocks in New Mexico are nonma- thesis of the Triassic stratigraphy and Sandstone, and San Andres Formation. rine strata of Middle and Late Tiiassic age. chronology in New Mexico. During the They Middle Triassic (Anisian) strata belong to are disconformably overlain by the Anton Chico Member of the Moenfooi past 12 years, I have studied the Triassic Upper Triassic Chinle Group strata or Formation, an unconformity-bounded tec- strata in New Mexico and their correla- younger rocks. Moenkopi strata are pre- tonosequence of fluvial red beds as thick as tives throughout the western United sent at the base of the Triassic across most 41 m Moenkopi strata are present across States. This work, done in collaboration of north and south-central New Mexico most of north and central New Mexico, and with several workers, notably O. Ander- (Lucas and Hunt,1987,1989b; Lucas and in southeast New Mexico thev crop out in son, S. Hayden, A. Heckert, P. Huber, A. Hayden, 1989a,b,1991; Lucas et aL,1990), Lincoln County. Hunt, and K. Kietzke, has produced a and in southeast New Mexico they extend Upper Triassic (late Carnian-Rhaetian) stratigraphic and chronological frame- to Bull Gap south of Carcizozo in Lincokr strata in New Mexico belong to the Chinle work for the Triassic strata in New Mex- (Lucas, Croup, an unconformity-bounded tectono- County 7991c). ico, sequence of fluvial, lacustrine, and rare which is reviewed here. Moenkopi strata in New Mexico are eolian red beds as thick as 650 m Chinle readily diJferentiated from underlying strata crop out across north, central, and Moenkopi tectonosequence Permian red beds (Guadalupian Artesia southeast New Mexico. Lithostratigraphy, The oldest Triassic strata in New Mexico Group) with which they were long con- and biostratigraphy based primarily on belong to the Moenkopi Formation and fused by lithologic characteristics of tex- tetrapod vertebrates, allow precise correla- are ture, mineralogy, bedform, and color as tion of Chinle of Middle Triassic (early Anisian) age. Group strata that comprise well as by fossil content (Fig. 2): three depositional sequences separated by Recognition of the Moenkopi Formation 1. Texture-Artesia Group sandstones basinwide unconformities. These deoosi- in New Mexico was long hindered by tional sequences resulted from base-level McKee (1,954), who concluded that no are very fine to fine grained, relatively changes driven by eustasy. Moenkopi strata are present in the state, well sorted, and rounded. Most of the and by a variety of geologists who resistant Artesia Group beds are either included siltstonesor silty sandstones.In contrast, Introduction Moenkopi strata in the Permian "Betnal" (=Artesia) Formation (Lucas and Moenkopi sandstones are fine to coarse Triassic rocks in New Mexico (Fig. 1) are Hayden, 1991). Some previous workers grained, poorly to moderately sorted, and nonmarine strata of Middle and Late Tri- (e.g., Cooley, 1957; Stewart et al., 1972b), subangular to subrounded. Intraforma- assic age. They belong to two tectonose- however, did recognize "Moenkopi(?) For- tional conglomerates of mudstone and quences with bounding pebbles unconformities mation" strata along the south edge of the siltstone are common in the that correspond to significant tectonic Colorado Plateau and as far east as the Moenkopi but virtually absent in the Arte- reorganizations of the depositional sys- Sevilleta Grant in Socorro County. sra. tem. Since their first recognition by Mir- Moenkopi strata in New Mexico belong 2. Mineralogy-Artesia sandstones are cou (1858), Triassic strata in New Mexico to the Anton Chico Member of Lucas and quartzarenites or slightly micaceous have received extensive study by stratig- Hunt (1987). They are fluvial red beds as quartzarenites and are frequently gypsif- raphers, sedimentologists, paleontolo- thick as 41 m dominated by trough-cross- erous. Moenkopi sandstones are mica- gists, geological field mappers, and eco- bedded micaceous litharenites and lithic ceous litharenites and lithic graywackes nomic geologists. Most of these studies graywackes. These strata rest discon- devoid of gypsum. 3. Bedforms-Artesia Group sandstones typically are laminated, rippie laminated, or massive and form laterally persistent, v repetitive (cyclic) beds. Moenkopi sand- ,l stones are typically trough crossbedded and have lenticular beds. tr:3 4. Color-Artesia Group red beds are characteristically moderate reddish brown (I0 R 4/6), commonly called "brick red" or "orange red," with some grayish- Sreen/yellowish-gray reduction mottles or thin bands. Moenkopi strata, in con- ( trast, are characteristically grayish red (5 R 4/2 or I0 R 4/2). 5. Fossil content-Artesia Group strata Iack body fossils, whereas Moenkopi strata (especially conglomerates) have bones and bone fragments, ostracods, ?OOKM charophytes, and tetrapod footprints. NEWMEXICO These Moenkopi fossils from New Mex- ico (Lucas and Morales, 1985; Kiet- ,$/ rnrnssrcRocKS 2ke,1.989a,b; Lucas and Hayden, 1989a; Hunt and Lucas, 1993c) indicate correla- tion of the Anton Chico Member with the FIGURE l-Outcrop distribution of Tiiassic strata in New Mexico (after Dane and Bachman, 1965) Holbrook Member of the Moenkopi For- and inset showing the two tectonosequences of riiassic strata present in the state. mation in north Arizona and assignment February 1995 NewMexico Ceology of an early Anisian age to the Anton Chico rocks are mostly red beds of fluvial, lacus- formity above older rocks (Tr-3 unconfor- Member (Morales,1987). Magnetostratig- trine, and rare eolian origin deposited in a mity of Pipiringos and O'Sullivan, 1978), raphy supports this correlation (Steiner single vast basin that extended at least and the top of the Chinle Group also is an and Lucas, 1992;Steiner et aL.,1993). from Wyoming to Texas and from east unconformity of basinwide scope (J-0 Nevada to west Oklahoma (Lrcas, 1993; unconformity of Pipiringos and O'Sulli- Chinle tectonosequence Lucas and Marzolf , 1993; Marzolf , 7993). van,1978). Lucas(1993) assigned all Upper Triassic All Upper Triassic strata in New Mexico nonmarine strata in the west-central belong to the Chinle Croup. The base of Chinle Group chronology United Statesto the Chinle Group. These the Chinle Group is everywhere an uncon- Chinle Group biostratigraphy and bio- chronology is based on palynomorphs, megafossil plants, ostracods, nonmarine molluscs (unionid bivalves and gas- SANTAROSA FORMATION tropods), vertebrate coprolites, and tetra- TecolotitoMember pod vertebrates (Fig. 3). Articles in Lucas and Hunt (1989a) and Lucas and Morales unconformity (1993) present much of this biostratigra- phy and biochronology, and Lucas (1995) mottledstrata provides unconformity an extensive review Hunt and Lucas (1993a) summarized tetrapod bios- tratigraphy and biochronology of the Upper Triassic strata of New Mexico. No syndepositional radiometric ages are available for Chinle Group strata in the state, but magnetostratigraphy has been published for several New Mexico sections (reviewed by Molina-Garza et al., 7993). A few salient points are: MOENKOPI 1. The base of the Chinle Group is FORMATION everywhere of late Carnian (Tuvalian) age (Hunt and Lucas, 1991a). Chinle strata below the medial sandstone-conglomerate Anton Chico complex termed Sonsela, Poleo, or Trujillo Member are also of late Carnian age; the Carn- ian-Norian boundary is approximated by the base of the medial sandstone-con- + '"t"r. glomerate complex. Strata of this complex | and the overlying mudrock-dominated units (Painted Desert Member of Petrified Forest Formation and Owl Rock Forma- tion and their correlatives) are of earlv- middle Norian age. The youngest Chinie Groun strata (Rock Point Formation and correiatives) are here regarded as of Rhaetian age, though they could be rSrAssrc slightly older, of late Norian age. No unconformity Chinle Group strata are of age. PERMIAN ]urassic 2. Using the Harland et al. (1990) numerical timescale, the base of the Chinle Group is about 225 Ma, and the top is 208 c onglomerate Ma or slightly older (Fig.3). Chinle Group deposition thus took place during about ss - trough crossbedded 17 million years. 3. Tetrapod biochronology of the Chinle "BERNAL ss - planar crossbedded Group provides the most precise correla- tions because tetrapod fossils allow four ss - laminar intervals of time to be characterized bios- FORMATION' tratigraphically. Palynomorphs and ss - massive megafossil plants allow only three time intervals to be discriminated, whereas siltston€ other fossil groups allow recognition of only two intervals (Fig. 3). Nevertheless, limes t one/dolomite correlations based on different kinds of Chinle Group fossils are consistent with gypsum eacn otner (i19.rnl J,. 4. Correlation of the Chinle Group to the SCCS (standard global chronbstrati- graphic scale) for the Late Triassic (Carn- unconformity ian, Norian, and Rhaetian stages and their SAN ANDRESFORMATION subdivisions: Tozer, 1984, 1994; Visscher, 1992) is based on palynomorphs and FIGURE 2-Measured stratigraphic section at Bemal Butte (from Lucas and Hayden, 1991)to show contrast between "Bernal" (=Artesia) Formation and Moenkopi Formation rs- New Mexico Ceology February 1995
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