Lower Permian, Wolfcampian), Robledo Mountains, New Mexico
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Sequence stratigraphy of the lower-middle Hueco transition interval (lower Permian, Wolfcampian), Robledo Mountains, New Mexico Greg H. Mack, Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003; Katherine A. Giles, Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968; Corey W. Durr, Bureau of Land Management, 1800 Marquess Street, Las Cruces, New Mexico, 88005 Abstract and siliciclastic strata that were deposited in Frontrangia environments ranging from offshore marine A 96-m-thick, mixed siliciclastic-carbonate to supratidal. Decimeter- to dekameter-scale Uncompahgre interval spanning the boundary between interbedding of these rocks provides a the lower and middle members of the lower Permian (Wolfcampian) Hueco Formation in high-resolution record of Permian sea-level the southeastern Robledo Mountains, New fluctuations at a variety of temporal scales. Mexico, contains depth-sensitive lithofacies Zuni-Defiance Sierra In addition, several of the facies display that allow delineation of sea-level cycles on the Grande many of the features described on modern Robledo Shelf. Carbonate lithofacies include sabkhas of the Trucial Coast of the Persian Amarillo (1) fenestral dolomudstone (supratidal to high Florida Pedernal Gulf, providing an excellent field laboratory intertidal), (2) peloidal dolomudstone (low Central Basin for students. The goals of this study are (1) Platform intertidal to lagoon), (3) foram packstone and to interpret the depositional environments Diablo grainstone (restricted marine), (4) intraclast, of the strata, (2) to use the stacking patterns fossiliferous grainstone (tidal channel), and (5) fossiliferous packstone (open marine), and the nature of the contacts to interpret whereas (6) shale (offshore marine) constitutes sea-level changes in the context of sequence the only siliciclastic lithofacies. The presence of stratigraphy, and (3) to consider the roles of both siliciclastic and carbonate lithofacies may Marathon Mountains paleoclimate and tectonics on sedimentary be related to sea-level change but may also facies and stacking patterns. have been influenced by paleoclimate, with Pedernal uplift carbonate sediment deposited during more Tectonic setting and stratigraphy arid periods and siliciclastic sediment depos- Sacramento Shelf ited during more humid periods. Compared paleoequator Pennsylvanian and lower Permian strata in to the remainder of the Hueco Formation, the Orogrande Basin Robledo Shelf experienced more restricted New Mexico were deposited in intermon- marine conditions, was shallower, and was tane basins that were complementary to not traversed by rivers during deposition of modern uplifts of the Ancestral Rocky Mountains Caballo the lower-middle Hueco transition. Mountains (Fig. 1). Located west of the Pedernal uplift Several scales of sea-level cyclicity are pres- was the Orogrande Basin, which contains ent within the lower-middle Hueco transition, Robledo Shelf approximately 1,600 m of Pennsylvanian including decimeter-to meter-scale upward- modern and lower Permian sedimentary rocks shallowing parasequences, decimeter- to Robledo 50 km (Kottlowski et al. 1956; Kottlowski 1963). meter-scale sequences primarily represented Mountains by interbeds of carbonate and shale, and The eastern margin of the Orogrande Basin meter- to dekameter-scale composite mega- (Sacramento Shelf) was tectonically active sequences composed of deeper-water sets of during Pennsylvanian and early Permian sequences overlain by shallower-water sets time (Pray 1961), and lower Permian FIGURE 1—The Permian–Pennsylvanian of sequences. A rough estimate of the average Ancestral Rocky Mountains in the American conglomerates exposed in the Caballo duration of the sequences is about 87 k.y., Southwest. Inset shows the location of the mod- Mountains suggest localized uplift along which is consistent with a glacial-eustatic ern Robledo and Caballo Mountains with respect the western margin of the basin as well origin. The average duration of the composite to the Permian Robledo Shelf and Orogrande (Lawton et al. 2002; Seager and Mack 2003). megasequences (about 600 k.y.), however, is Basin, adapted from Kues and Giles (2004). The Orogrande Basin was bordered on the longer than the longest glacial-eustatic cycle (about 400 k.y.), suggesting the possible role west by the Robledo Shelf, named for expo- of non glacial eustacy and/or tectonic subsid- Hueco Formation, (2) Laramide (latest sures in the Robledo Mountains (Kottlowski ence as controlling factors. Cretaceous–Eocene) faults, paleocanyons 1963). Lower Permian strata display an and their conglomeratic fill, and regionally abrupt facies change from carbonate-rich Introduction extensive andesitic volcaniclastic rocks, (3) sediment on the Robledo Shelf to more middle Cenozoic rhyolite sills, and (4) late shale- and sandstone-rich sediment along In March 2009 the Congress of the United Cenozoic faults, conglomerates, and basalts the western margin of the Orogrande Basin States designated the southeastern part of related to the Rio Grande rift (Seager et al. (Seager et al. 1976). In addition to tectonism, the Robledo Mountains near Las Cruces 2008; Fig. 4). These rocks and structures sedimentation in the Orogrande Basin and as the Prehistoric Trackways National provide important insight into the geologic on the Robledo Shelf was influenced by Monument to protect and enhance research history of southern New Mexico and, like eustatic sea-level fluctuations (Stoklosa et on the remaining vertebrate tracks in the Permian vertebrate trackways, are wor- al. 1998; Mack et al. 2003; Mack 2007) and the Permian Abo Tongue of the Hueco thy of preservation and study. changes in paleoclimate (Mack 2003, 2007). Formation (Figs. 1, 2, 3; Lucas and Heckert This study focuses on a unique 96-m-thick Lower Permian stratigraphic terminology 1995). Exposed within and adjacent to the stratigraphic interval that straddles the con- in this study conforms to the geologic map borders of the national monument are other tact between the lower and middle members of the Robledo Mountains by Seager et al. geologically important features, including of the Permian Hueco Formation. This (2008), who applied the terminology of (1) three other members of the Permian interval, which is widely exposed within the Kottlowski (1963). This approach follows national monument, consists of carbonate May 2013, Volume 35, Number 2 NEW MEXICO GEOLOGY 27 precedence and allows future geologists to International North relative locate the members discussed here on the American sea level stages stage Robledo Mountains geologic map. In the Robledo Mountains, 279.3 Ma low high the Hueco Formation is divided into four mappable members, which in ascend- upper ing order are the lower Hueco, middle Hueco Hueco, Abo Tongue, and upper Hueco member (Fig. 2). Fusulinids in the lower part of the lower Hueco Member indicate an early Artinskian ? Wolfcampian (Nealian) age (Wahlman and King 2002), but fusulinids are absent in the remainder of the formation in the Robledo Abo Mountains. Conodonts and a marine Tongue 290.1 Ma invertebrate fauna from the Abo Tongue have been interpreted as late Wolfcampian ? (Lenoxian; Kues 1995; Kozur and LeMone upper 1995). Biostratigraphic data are cur- middle interval rently insufficient to determine whether Hueco the Wolfcampian–Leonardian boundary member lower Sakmarian Wolfcampian exists within the upper Hueco member. The interval ? interval Hueco Formation in the Robledo Mountains of contains three stratal packages deposited lower upper study during relatively high sea level, separated interval 100 by two intervals deposited during lower sea Hueco level (Fig. 2). Each of these large-scale inter- 295.5 Ma member vals, which correspond to third-order cycles lower 50 of Van Wagoner et al. (1990), is composed Asselian interval of smaller-scale sea-level cycles that are the meters focus of this study. 298.9 Ma 0 The lower and middle members of the Hueco Formation are separated by a 1.5-m-thick, ledge-forming marker bed composed of tan dolomudstone contain- Gray, ledge-forming limestone Interbedded gray limestone ing abundant dark-gray and brown chert and shale and brown sandstone nodules. However, the strata for 61 m below Interbedded tan dolomudstone, “Marker bed” of tan, cherty and 33.5 m above the marker bed, which are gray limestone, and gray shale dolomudstone at lower-middle the subject of this study, are nearly identical, Hueco contact consisting of interbedded tan dolomud- Interbedded red siltstone, gray stones, gray limestones, and shales (Fig. limestone, gray shale, and tan Pebbly sandstone dolomudstone 2). The base of the study interval is placed above a brown, pebbly sandstone in the FIGURE 2—Generalized stratigraphy of the lower Permian Hueco Formation in the Robledo lower interval of the lower Hueco member, Mountains, adapted from Seager et al. (2008). International stages are from the International whereas the upper contact is beneath thick- Chronostratigraphic Chart (2012). Currently, the exact correlation between the members of the Hueco bedded, ledge- and cliff-forming, dark-gray Formation in the Robledo Mountains and the international stages is unknown. limestones of the upper interval of the middle Hueco member (Fig. 2) dolomudstone, (2) peloidal dolomudstone, a few bubble-shaped vugs are also present. (3) foram packstone and grainstone, (4) Also common are millimeter-scale wavy Methods intraclast, fossiliferous