Mid-Cretaceous (late Albian) change from rift to retroarc foreland basin in southwestern New Mexico GREG H. MACK Department of Earth Sciences, New Mexico State University, Las Cruces, New Mexico 88003 ABSTRACT Cretaceous (Aptian-Turonian) sedimentary rocks in southwest- ern New Mexico were deposited in two stages that were controlled primarily by tectonism. During the first stage, from Aptian through middle Albian time, a maximum of 1,900 m of siliciclastic and carbon- ate sediment accumulated in a west-northwest-trending rift basin. Lithic and arkosic sediment was derived initially from intrabasin up- lifts and subsequently from a west-northwest-trending, basement- cored rift shoulder that marked the northern boundary of the basin. Decreases in subsidence and siliciclastic sedimentation rate in mid- Albian time were a response to wearing down of the rift shoulder and marked tbe end of the first stage of sedimentation. Beginning in late Albian time, what is now southwestern New Mexico experienced increases in tectonic subsidence and siliciclastic sedimentation rate. As much as 1,500 m of quartzarenite and shale was deposited in the southwestern part of the study area, and -300 m of upper Albian, Cenomanian, and Turanian sediment onlapped the former rift shoulder. Paleocurrent and facies data indicate that sedi- ment dispersal was eastward, southeastward, and northeastward. The second stage of sedimentation took place in a retroarc foreland basin that was complementary to a compressional orogenic belt in what is now southeastern Nevada, southeastern California, western Arizona, and/or Sonora, Mexico. INTRODUCTION Cretaceous tectonism in the southwestern United States and north- western Mexico was the result of eastward subduction of the Farallon plate beneath North America (Dickinson, 1981). In response to subduc- tion, a magmatic arc formed in what is now western Nevada, western Arizona, California, Baja California, and Sonora, Mexico (Fig. 1; Gastil, 1975; Silver and others, 1975; Coney, 1978; Rangin, 1978; Dickinson, Figure 1. Paleotectonic maps illustrating two different tectonic models for mid-Cretaceous time. a. The Chihuahua trough is a rift basin that is separated from the retroarc foreland basin by a rift shoulder and lowlands. Adapted from McGookey and others (1972), Dickinson (1981), Bilodeau and Lindberg (1983), and Mack and oth- ers (1986). b. A retroarc foreland basin extends across Utah, Colo- rado, Arizona, and New Mexico and is complementary to the Sevier thrust belt and/or an orogenic belt in southeastern California, western Arizona, and Sonora, Mexico. Adapted from McGookey and others (1972), Dickinson (1981), Molenaar (1983), and Mack and others (1986). The solid line extending from southwestern New Mexico to southeastern Utah is the position of the late Cenomanian shoreline, inferred by Molenaar (1983). Geological Society of America Bulletin, v. 98, p. 507-514, 5 figs., 1 table, May 1987. 507 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/5/507/3445177/i0016-7606-98-5-507.pdf by guest on 02 October 2021 508 G. H. MACK Figure 2. Index map of the locations of measured sections (solid circles) of Cretaceous rocks. Contour lines are isopachs in metres. The line labeled "rift sho ulder" separates thick (>500 m) from thin Cretaceous rocks and is the northern limit of Aptian through middle Albian rocks. 1981). The Sevier thrust belt developed east of the magmatic arc and can Dickinson and others (1986a) suggested that the trough resulted from be traced southward with some degree of confidence to the vicinity of Las either back-arc rifting or was a fa iled arm of the Gulf of Mexico rift. In this Vegas, Nevada (Fig. 1; Armstrong, 1968; Dickinson, 1981). South of Las extensional model, the northern, limit of the basin is a west-northwest- Vegas, in southeiistern California, western Arizona, and Sonora, Mexico, trending rift shoulder that acted not only as a sediment source but also as a the tectonic histoid is less well constrained, but scattered evidence indicates barrier separating the Chihuahua trough from the relroarc foreland basin middle and Late Cretaceous compressional deformation (Hamilton, 1982; to the north (Fig. la; Dickinson, 1981; Bilodeau and Lindberg, 1983). The Burchfiel and Davis, 1971, 1972, 1975, 1977; Drewes, 1978; Rangin, rift shoulder is not clearly defined in southwestern New Mexico but may 1978; Reynolds and others, 1980). A retroarc foreland basin, which correspond to a line that separates thick (>500 ni) from thin Lower formed east of ths Sevier thrust belt, subsided in response to thrust loading Cretaceous rocks (Fig. 2). and received detiital sediment from the thrust belt (Fig. 1; McGookey and The rift model for the Chihuahua trough is not universally accepted, others, 1972; Dickinson, 1976, 1981; Jordan, 1981). however. Drewes and Hayes (1983) challenged the field interpretations of During Early Cretaceous time in southeastern Arizona, southwestern Bilodeau (1982) upon which the rift model is based. Alternatively, Hayes New Mexico, west Texas, and northern Mexico, a maximum of 3.2 km of (1970) proposed that Lower Cretaceous detrital sedimentary rocks in siliciclastic and carbonate sedimentary rocks accumulated along the southeastern Arizona and southwestern New Mexico were derived pri- northwestern edge of the Chihuahua trough (Fig. la; Kottlowski, 1965; marily from a source to the west or southwest of the basin. Similar disper- Zeller, 1965; Hayes, 1970; Greenwood and others, 1977; Bilodeau and sal models have been proposed for Upper Cretaceous sedimentary rocks in Lindberg, 1983; Mack and others, 1986). Two different tectonic models northern and central New Mexico (Sabins, 1964; Fassett and Hinds, 1971; have been proposed for the origin of the Chihuahua trough. Bilodeau Molenaar, 1973,1983; Cumella, 1983). A westerly sediment source raises (1978, 1982), Dickinson (1981), Bilodeau and Lindberg (1983), and the possibility that the Chihuahua trough was a retroarc foreland basin that Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/5/507/3445177/i0016-7606-98-5-507.pdf by guest on 02 October 2021 MID-CRETACEOUS RETROARC FORELAND BASIN, NEW MEXICO 509 was complementary to a compressional orogenic belt that may have ex- A variety of stratigraphic names have been applied to interbedded tended south and southeast of the Sevier thrust belt (Fig. lb). sandstone and shale of the upper siliciclastic interval (Fig. 3). In the Big The purpose of this paper is to present evidence to suggest that Hatchet, Little Hatchet, and Animas Mountains, the Mojado Formation is Cretaceous sedimentation in southwestern New Mexico occurred in two ~ 1,500 m thick and is late Albian to earliest Cenomanian in age (Zeller, different stages that reflect different tectonic settings. The sedimentology 1965, 1970; Zeller and Alper, 1965). Rocks in the Peloncillo Mountains and petrology of Aptian through middle Albian rocks are consistent with that are lithologically equivalent to the Mojado Formation were mapped the rift model of Bilodeau (1978, 1982), Dickinson (1981), Bilodeau and as the Still Ridge and Johnny Bull Formations by Gillerman (1958) and as Lindberg (1983), and Dickinson and others (1986a). In late Albian time, the Cintura Formation by Drewes and Thorman (1980a, 1980b). A late however, southwestern New Mexico experienced dramatic increases in Albian to early Cenomanian age has also been established for the Sarten tectonic subsidence and siliciclastic sedimentation rate, as well as changes Formation in the Cooke's Range (Clemons, 1982; W. A. Cobban, 1986, in sediment dispersal and provenance. These changes mark the beginning personal commun.), and upper Albian and Cenomanian rocks are also of a retroarc foreland basin that persisted into the Late Cretaceous present at Cerro de Cristo Rey, New Mexico and Mexico (Figs. 2, 3; (Fig. lb). Lovejoy, 1976). The Beartooth Quartzite, exposed near Silver City and in the Burro Mountains, does not have index fossils but is assumed to be late STRATIGRAPHY Albian and/or early Cenomanian in age because of its similarity to the Sarten Formation and its conformable position beneath the Colorado In southwestern New Mexico, Cretaceous sedimentary rocks can be Formation (Figs. 2, 3; Mack and others, 1986). The Colorado Formation divided into lower and upper siliciclastic intervals and a middle carbonate- ranges in age from middle to late Cenomanian to middle Turonian (Mole- rich interval (Fig. 3). The lower siliciclastic interval consists of a basal naar, 1983; W. A. Cobban, 1986, personal commun.) and consists of a conglomerate (=s42 m thick) that is overlain by a maximum of 700 m of lower shale member and an upper sandstone member (Hewitt, 1959; conglomerate, sandstone, siltstone, and shale (Mack and others, 1986). Jones and others, 1967). Molenaar (1983) recommended abandonment of Throughout most of the region, the lower siliciclastic interval is called the the name Colorado Formation in favor of the names Mancos, Atarque, Hell-to-Finish Formation (Fig. 3; Zeller, 1965, 1970; Zeller and Alper, and Moreno Hill Formations, which are used in central New Mexico. The 1965), but in the Peloncillo Mountains, the names McGhee Peak (Giller- top of the Colorado Formation is an erosional unconformity. man, 1958) or Glance and Morita Formations (Drewes and Thorman, 1980a, 1980b) were applied. The middle carbonate-rich interval is repre- EVIDENCE OF TWO STAGES OF SEDIMENTATION sented by the U-Bar Formation and the Carbonate