Stratigraphic Description of the Tr-4 Unconformity in West-Central New
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Stratigraphicdescription of theTr-4 unconformity in west-centralNew Mexico and eastern Arizona by Andrew B. Heckert, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-1116; and SpencerG. Lucas, New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104 Abstract traverse the thickness of the Bluewater Second, proper identification of the Tr-4 Creek Formation remains constant at ap- erosional surface greatly facilitates recog- The Late Triassic Tr-4 unconformitv. proximately 50-60 m, demonstrating that nition of lithostratigraphic units in west- which approximates the Carnian-Norian the disappearance of the BIue Mesa Member central New Mexico, particularly in the is not due to intertonguing of the floodplain, stage boundary, occurs in west-central New structurally complex Lucero uplift. Third, Mexico and eastern Arizona as an overbank, and paleosol deposits that typify erosional this erosional surface marks the boundary surface developed at the base of the Sonsela that unit with the red-bed facies of the Blue- between two depositional systems tracts Member of the Petrified Forest Formation water Creek Formation Farther south, in (Chinle Group). Correlating measured the southeast part of the Lucero uplift, the in the Upper Tiiassic section and thus stratigraphic sections eastward from the San Pedro Arroyo Formation laterally re- allows us to make inferences about base- Petrified Forest National Park (Apache places the Bluewater Creek Formation and level changes in the Chinle depositional County, Arizona) through the Zunj Moun- is in tum overlain by the Sonsela. The Tr-4 basin during the Late Triassic. tains (McKinley County, New Mexico) to the unconformity developed as a response to a Lucero uplift in Cibola and Socorro drop in base level at or near the end of the Stratigraphy Counties, New Mexico, indicates that as late Carnian. much as 100 m of erosional relief character- The lower part of the Chinle Group in izes this unconformity. In eastern Arizona, Introduction eastern Arizona and western New Mexico the Sonsela rests disconformably on a thick (Fig. 1) consists of the following units (81t m) section of the Blue Mesa Member of The Tr-4 unconformity, as described by the Petrified Forest Formation. Farther (ascending): Shinarump Formation/ "mot- east, Lucas (1991a,1993), is particularly impor- measured sections in the Zuni Mountains tled strata," Bluewater Creek Formation, tant in correlating regional Triassic stratig- show that the Sonsela rests on Blue Mesa and Blue Mesa Member of the Petrified raphy for several reasons. First, tetrapod Member sections that average approximate- Forest Formation. In the Lucero uplift, the fossils (Lucas, 1993, in press; Lucas and ly 35 m thick. In the northern Lucero uplift, San Pedro Arroyo Formation iaterally Hunt, 1993) and pallnology (Litwin et al., the Blue Mesa has been entirelv removed bv reolaces the Bluewater Creek Formation pre-Sonsela erosion, and the'sonsela rests 1991) indicate that this unconformity south of the Rio Salado (Lucas and directly on red beds of the underlying Blue- approximates the Carnian-Norian bound- Heckert, 1994). West of St. Arizona, water Creek Formation. Throughout this aryl so it is of chronological significance. ]ohns, the basal Chinle unit is the Mesa Redondo Formation, which essentially overlies, yet exhibits a complex intertonguing relation- ship with, the Shinarump Formation (Cooley, 1957, 1958). Throughout the study area (Fig. 1) this sequence is trun- Alsoin Potentialenvironmental impact of the abandonedLa Bajada uraniummine on CochitiLake p.71 New MexicoGeological Society 1996and 1997Fall Field Conferences p.78 Service/News p.79 Upcomingmeetings p.79 Publicationsorder form o.80 Comingsoon PecosRiver vertebrates FIGURE 1-Map of New Mexico and eastern Arizona. Counties outlined are A, Apache County, Arrzona; M, McKinley County, New Mexico; C, Cibola county, New Mexico; s, Socorro County, New Fluviatdeposition in MimbresBasin Mexico. Numbers correspond to locations of measured sections in Fig. 2. Dashed line indicates tran- Oil and gas activitiesin 1995 sect line of restored cross section in Fig. 5 cated by an erosional surface associated Lucas, 1993) occurs as a thin, 4-20-m- Member of the Bluewater Creek Forma- with the Tr-4 unconformify, which is in thick, intermittent unit that crops out as a tion. The Mesa Redondo Formation has turn overlain by the Sonsela Member of prominent ledge of thinly bedded, ripple- not been explored with any great rigor the Petrified Forest Formation. laminated, micaceous sandstone with since the work of Cooley, so its interrela- minor intraformational limestone con- tionships with the overlying Petrified Shinarump FormationL/"mottled strata" glomerates in the upper half of the Forest Formation are not clear. Although Bluewater Creek Formation. the Blue Mesa Member does overlie this Throughout the study area the base of Lithofacies are suites or packages of unit, we have not found a locality where a the Chinle Group is either the conglomer- specific lithology that occur together and comolete section of both units can be mea- ates and conglomeratic sandstones of the suggest similar depositional environ- sured together. Shinarump Formation or the pedogenical- ments. Identification of unique or specific ly modified siliciclastics of the "mottled lithofacies facilitates recognition of strati- Petrified Forest Formation slrata" (Stewart et al., 1972). Shinarump graphic units in the field. To this effect we strata are typicalty trough- to planar- recognize three principal lithofacies in the The Petrified Forest Formation consists crossbedded extrabasinal conglomerates Bluewater Creek Formation: (1) thinly of three members, the BIue Mesa, Sonsela, that fine upward into conglomeratic bedded, laminated to ripple-laminated and Painted Desert Members, in ascend- and/or quartzose sandstones and are as sandstones; (2) red beds of bentonitic ing order. Although all three crop out to thick as 2b m in this region (Coolev,7957; mudstone, rippleJaminated siltstone, and some extent in the study area, we are only Stewart eta1.,1972). Clasts are dominantly discontinuous, lenticular sandstones; and concerned with the lower two units, the extraformational pebbles and cobbles of (3) light-greenish-gray and light-gray ben- Blue Mesa and Sonsela Members. Paleozoic limestones, chert, and quartzite. tonitic mudstones and localized dark, car- Blue Mesa Member-At its type sec- These strata are channel-fill deposits on bonaceous shales. The sandstone lithofa- tion the Blue Mesa Member is more than the underlying Holbrook or Anton Chico cies is present at the base of the Bluewater 81 m of blue-gray, purple, and white ben- Members of the Moenkopi Formation. Creek Formation in its eastern outcrops, tonitic mudstones, siltstones, and thin "Mottled strata" are pedogenically such as the type section, the section at modified conglomerates, sandstones, silt- Maldonado Ranch, and in the Lucero stones, and mudstones that represent a uplift at Chicken Mountain Tank. This weathering profile deveioped prior to Iiihofacies also occurs higher in the section Chinle deposition. Locally, they may wherever the McGaffev Member is nre- underlie the Shinarump Formation (Lucas sent (Fig. 2). Most of the Bluewater Cieek New N4exreo and Hayden,7989; Fig. 2 section 8, Fort Formation is accounted for by the second Wingate 1). These strata can be quite thick, lithofacies. Mudstones are tJpically blue, GEOLOGY . as much as 20.3 m in the vicinity of Fort red, or reddish purple, slightly silty, and Scienceand Service Wingate (Lucas and Hayden, 1,989), and bentonitic. Occasional calcrete nodule rssN 0196-948X they can also occur as barely observable horizons indicate limited soil horizon Volume 18, No. 3, August 1996 horizons such as in the measured section development. Siltstones occur as thin, rip- Edllor Carol A. Hjellming at Rio Salado (Lucas and Heckert, 1,994; ple-laminated ledges that are predomi- EDITORIAL BOARD Fig. 2 section 17). nantly red with light-green flecks and Steve M. Cather, NMBMMR, Chairman The interrelationships of the Shinarump mottles. In the western oortion of the Thomas Giordmo, NMSU Laurel B. Goodwrn, NMIMT Formation and the mottled strata demon- study area the third, muddy-to-shaly Spencer G. Lucas, NMAzINHS strate how a complex incised topography lithofacies crops out in low badlands at Frank J. Pazzaglia, UNM developed during the interval between the base of tlie unit. We internret these Published quarterly by Moenkopi and Chinle deposition, an inter- New Mexico Bureau of Mines and deposits as representingu pooriy drained Mineral Resources val corresponding to the Tr-3 unconformi- Iowland area developed on the paleoto- a division of New Mexico Institute of ty (Lucas, 1993; Lrcas and Marzolf , 1993; pography generated during the Tr-3 Mining md Technology Lucas and Huber, 1994). We will later unconformity. These lowlands were only BOARD OF REGENTS show how a similar, but not as complex, Ex-Officio partially filled by mottled strata develop- Gary Johnsory Goaernorof New Mexico stratigraphic sequencedeveloped during ment and Shinarump deposition. Later, as Alan Morgm, Superintendentof Public lnstruction the latest Carnian and early Appointed Norian after base level continued to rise, aggradation Steve S. Torres, Pres.,1991-1997, Albuquerque the Tr-4 unconformity. of floodplain deposits covered these dark Delilah A. Yega, StudentMember, Secretaryfreasurer, shales with oxidized