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Change from Rift to Retroarc Foreland Basin in Southwestern New Mexico

Change from Rift to Retroarc Foreland Basin in Southwestern New Mexico

Mid- (late Albian) change from to retroarc in southwestern

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 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, - cored rift shoulder that marked the northern boundary of the basin. Decreases in 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 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 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 (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 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 , 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

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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 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 , 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

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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). 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 . 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 Hill or Mural Formations in the Peloncillo Mountains and consists of a maximum of Cretaceous sedimentation in southwestern New Mexico occurred in 1,200 m of interbedded limestone, shale, and sandstone, with the relative two distinct stages. The first stage extended from Aptian through middle percentage of limestone increasing upsection (Gillerman, 1958; Zeller, Albian time and corresponds to the lower siliciclastic and middle 1965; Weise, 1982). The U-Bar Formation is late Aptian through middle carbonate-rich intervals. The second stage began in late Albian time and Albian in age in the Big Hatchet Mountains (Fig. 3; Zeller, 1965; Weise, continued into the Turonian and corresponds to the upper siliciclastic 1982). No index fossils have been found in the Hell-to-Finish Formation, interval. Each stage is characterized by a unique combination of subsi- but it must be Aptian in its upper part because of its conformable contact dence and sedimentation rate, sediment dispersal, and provenance. with the U-Bar Formation. The Hell-to-Finish and U-Bar Formations are present only south of the line labeled "rift shoulder" in Figure 2. informal lithologie Southwest Southeast Northwest Northeast intervals c o imiMUlULJ c o o Moreno Hill Moreno Hill Figure 3. Correlation chart of Cre- a. Atarque Atarque taceous rocks in southwestern New Mex- c ico, adapted from Mack and others o Boquillas Mancos Mancos upper o E (1986), Molenaar (1983), and W. A. Cob- a Bu da E a> Del Rio ban (1986, personal commun.). Southwest Q. o siliciclastic CL c region includes the Big Hatchet, Little a) Anapra Beartooth Z) U Hatchet, and Animas Mountains; south- Mojado Mesilla Sartén east region includes the East Potrillo c Muleros o Mountains, Eagle's Nest, and Cerro de £ Smeltertown V) Cristo Rey; northeast region includes the 3 Cooke's Range, Victorio Mountains, and O U-Bar middle CD carbonate- San Andres Mountains; northwest region O U-Bar O rich includes the Burro Mountains and Silver c o Hell-to- City area. The stratigraphy is divided in- O

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Figure 4. Subsidence curve for Lower Cretaceous rocks in the Big Hatchet Mountains, using the technique of Watts and Ryan (1976) and Steckler and Watts (1978). Thickness and depositional environ- ments are from Weise (1982) and Mack and others (1986), and the age of the rocks is from Zeller (1965) and Weise (1982). The Cretaceous time scale is that of Van Hinte (1976). Corrections for are based on the data of Perrier and Quiblier (1974); correction for sea- level change is based on the data of Vail and others: (1977).

val. Substantial lateral thickness changes over distances, of a few kilometres or less suggest that the basal conglomerate was deposited in a region of irregular topography (Mack and others, 1986). Furthermore, the composi- tion of clasts in the basal conglomerate can usually be correlated with formations directly underlying the conglomerate, implying a local provenance. In southeastern Arizona and northern Chihuahua, Mex- ico, similar thickness variations and local provenance of the Glance Con- glomerate are associated with syndepositional movement on west-north- west-trending, high-angle or normal faults (Bilodeau, 1982; Bilodeau and Lindberg, 1983; Brown and Dyer, 1986). Early Cretaceous faults have not been positively identified in southwestern New Mexico but may be re- vealed by detailed mapping. of the upper part of the lower siliciclastic and middle carbonate-rich intervals do not display small-scale lateral thickness and facies variations characteristic of the basal conglomerate, suggesting that most of the local relief was buried. Siliciclastic sediment dispersal at this time was southward. Evidence for this interpretation, which is docu- mented by Mack and others (1986), includes (1) southerly change from Subsidence and Sedimentation Rates braided to meandering fluvial fades in the lower siliciclastic interval, (2) southerly paleocurrents in the fluvial facies of the lower siliciclastic A subsidence curve was constructed, using the technique of Watts interval, and (3) a southward decrease in maximum detrital clast size in and Ryan (1976) and Steckler and Watts (1978), from data from the Big both the lower siliciclastic and middle carbonate-rich intervals. These data Hatchet Mountains, which have the thickest and best age-dated Lower show that sediment dispersal was perpendicular to the trend of the basin Cretaceous section in southwestern New Mexico (Fig. 4). The lower curve and suggest that the source area was to the north of the basin. of Figure 4 represents total subsidence, that is, cumulative sediment thick- By middle Albian time, during deposition of the upper part of the ness and average water depth of deposition. Because the average water middle carbonate-rich interval, the rate of siliciclastic influx into the basin depth of deposition was shallow and represents a small component of total was very low, and carbonate deposition dominated (Zeller, 1965; Weise, subsidence, the slope of the lower curve can also be used as a measure of 1982; Mack and others, 1986). The decrease in siliciclastic influx probably sedimentation rate. The upper curve is the tectonic component of subsi- reflects a substantial reduction in relief in the northern source and dence, which is calculated by subtracting the effects of sediment and water marks the end of the first stage of sedimentation. loading from the total. The second stage of sediment distribution and dispersal began in late Southwestern New Mexico shows two distinct stages of subsidence Albian time and corresponds to deposition of the upper siliciclastic inter- (Fig. 4). The initial stage is characterized by relatively low rates that val. The shallow-marine carbonate environment that occupied the entire decrease with time. By middle Albian time, when the rates of tectonic southern portion of the study area in middle Albian time was replaced by subsidence and sedimentation reached their lowest points, carbonate depo- as much as 1,500 m of upper Albian and lower Cenomanian shallow- sition of the upper U-Bar Formation was widespread and siliciclastic influx marine and fluvial siliciclastic sediment (Mack and others, 1986). More- was virtually zero (Fig. 4; Mack and others, 1986). over, for the first time in the Cretaceous, -300 m of siliciclastic sediment Subsidence and sedimentation rates increase significantly in late Al- was deposited north of the rift shoulder line in Figure 2. This sediment bian time, corresponding to deposition of the upper siliciclastic interval onlapped Paleozoic sedimentary and Precambrian crystalline rocks (El- (Mojado Formation in Fig. 4). It is important to note that the increase in ston, 1958; Chafetz, 1982; Mack and others, 1986). basin subsidence was not merely due to sediment and water loading, but Dispersal of the upper siliciclastic interval was (Eastward, southeast- rather had a significant tectonic component (Fig. 4). Unfortunately, the ward, and northeastward. The Mojado Formation and coeval rocks at subsidence curve for the Big Hatchet Mountains cannot be extended Cerro de Cristo Rey display an eastward decrease in grain size. Trough beyond Early Cretaceous because Upper Cretaceous rocks are not exposed crossbed and parting paleocurrent data frcm a thick (800 m) in the southweste rn portion of the study area. fluvial interval of the Mojado Formation in the Big Hatchet and Animas Mountains indicate eastward transport (Mack and others, 1986). Sim- Sediment Dispersal ilarly, planar crossbed paleocurrent data from a thin (50 m) fluvial interval of the Sarten Formation in the Cooke's Range indicate southeastward Initial uplift in the region along and south of the line labeled "rift transport (Mack and others, 1986). In Late Cretaceous, sediment dispersal shoulder" in Figure 2 resulted in deposition of the lower siliciclastic inter- in central and northern New Mexico appears to have shifted northeast-

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TABLE 1. DETRITAL MODES OF

MXQ PXQ KSP PLG CHT PRF CRF MRF VRF

Upper silicidastic x 91.2 2.1 0.8 1.2 2.1 0.8 0.0 0.0 1.8 interval (n = 93) s 11.7 2.5 1.6 2.2 2.0 1.7 0.0 0.0 7.8

Lower silicidastic Upper part x 56.6 7.5 12.7 4.5 7.0 1.0 10.2 tr 0.5 and middle (n = 32) s 14.3 7.6 7.5 5.5 5.4 2.3 10.6 0.0 2.0 carbonate-rich intervals Basal 100 m x 37.2 7.7 0.0 0.0 29.4 0.5 25.2 0.0 0.0 or less (n = 9) s 20.4 4.7 0.0 0.0 17.2 1.3 16.1 0.0 4.1

Note: detrital modes, on the basis of 300 points per sample, exclusive of cement and matrix, n = total number of samples; x = mean; s = standard deviation; tr = trace amounts. The grain types are those of Basu (1976). Sample distribution is Hell-to-Finish Formation = 31; U-Bar Formation = 10; Mojado Formation = 68; Sarten Formation = 20; Beartooth Quartzite = 5. MXQ = monocrystalline quartz; PXQ = polycrystalline quartz; KSP = K-feldspar; PLG = plagioclase feldspar; CHT = chert; PRF = pelitic rock fragments; CRF = carbonate rock fragments; MRF = metamorphic rock fragments; VRF = fragments.

ward. This interpretation is supported by northwest-trending shorelines carbonate-rich intervals (Fig. 5; Table 1). Sandstones in the basal 100 m of that faced offshore to the northeast (Molenaar, 1983), by northeastward the lower silicidastic interval consist primarily of monocrystalline quartz, progradation of nonmarine facies (Devine, 1980; Flores and Erpenbeck, chert, and carbonate rock fragments. Some of the monocrystalline quartz 1981), and by northeasterly directed paleocurrents in fluvial and deltaic grains are anomalously well rounded and have abraded overgrowths, at- sandstones (Molenaar, 1973). testing to their polycyclic history. The carbonate grains possess many of These data suggest that the source area for silicidastic sediment dur- the features determined by Zuffa (1985) to indicate a detrital origin. Many ing the second stage of basin history was to the west, northwest, and/or detrital chert grains contain inclusions of carbonate minerals or relict southwest of what is now southwestern New Mexico. This represents a outlines of shallow-marine fossils, indicating that they were derived from significant departure from the southward dispersal pattern during stage one chert nodules in carbonate rocks. The suite of detrital grains suggests that sedimentation. Indeed, the area north of and parallel to the rift shoulder the source terrane was initially composed of cherty carbonates and sand- line in Figure 2, which was the predominant source terrane during stage stones. This interpretation is consistent with the distribution of Paleozoic one, was not an important source of sediment during stage two. that unconformably underlie Cretaceous rocks in southwestern New Mexico (Kottlowski, 1965; Greenwood and others, 1977). Provenance In addition to the grain types described above, detrital feldspar ap- pears in the upper part of the lower silicidastic interval and generally The type and relative abundance of rocks that were exposed in the increases in abundance upsection (Fig. 5; Table 1). The percentage of total northern source terrane during the first stage of sedimentation are inferred feldspar averages 17% but in some samples, is as high as 40% (Table 1). from detrital modes of 41 sandstones of the lower silicidastic and middle K-feldspar is represented by microcline, orthoclase, and perthite and is generally more abundant than is plagioclase (Table 1). The type and abundance of detrital feldspar indicate granitic and/or gneissic source 0 rocks. A Precambrian source for the detrital feldspar is supported by the fact that in the Burro Mountains, the Cretaceous Beartooth Quartzite directly overlies Precambrian crystalline basement (Elston, 1958; Hewitt, 1959). The upsection increase in feldspar is probably the result of unroof- ing of basement rocks. An estimate of the magnitude of uplift and erosion in the northern source terrane is possible by extrapolating Paleozoic iso- pachs into the area of the Burro Mountains (Greenwood and others, 1977). This analysis indicates that 1.2 km of Paleozoic sedimentary rocks were stripped from the basement prior to deposition of the Beartooth Formation. The provenance of sandstones changed dramatically during the sec- ond stage of sedimentation. The majority of sandstones of the upper silicidastic interval (87%) are quartzarenites composed of monocrystalline quartz and small amounts of polycrystalline quartz and chert (Fig. 5; Table 1). The predominance of monocrystalline quartz suggests derivation from sedimentary source rocks (Suttner and others, 1981; Mack, 1981). It might be argued that the quartzarenites resulted from extreme chemical weathering of crystalline rocks. Such first-cycle quartzarenites require for their development a combination of low erosion rate and tropical climate (Potter, 1978; Franzinelli and Potter, 1983). These conditions do not apply to the upper silicidastic interval because southwestern New Mexico was quartz, excluding chert; F = total feldspar; L = total rock fragments not situated in tropical paleolatitudes during Cretaceous time (Habicht, (lithics), including chert and carbonate rock fragments. Squares rep- 1979) and because the high sedimentation rate of the Mojado Formation resent samples from the basal 100 m of the lower silicidastic interval. (Fig. 4) suggests short residence time of the detritus in the soil profile. Open circles represent samples from the upper part of the lower Transportational modification of detrital composition can also be ruled out silicidastic interval and from the middle carbonate-rich interval. as the origin of the quartzarenites. Many studies have shown that fluvial Closed circles represent samples from the upper silicidastic interval. transport is incapable of producing quartzarenites from arkosic or lithic

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detritus (Russell, 1937; Hayes, 1962; Mack, 1978). Furthermore, shallow- basin is the complementary compressional orogenic belt (Dickinson, 1976, marine destruction of labile grains is only effective under conditions of 1981). If an Early Cretaceous orogenic belt existed in southwestern United extremely low sedimentation rate, which does not apply to the upper States and northwestern Mexico, it was hundreds of kilometres to the west siliciclastic interval (Mack, 1978; Suttner and others, 1981). There is also or southwest of southwestern New Mexico (Fig. la). The fact that the no significant difference in composition between nonmarine and marine principal source terrane was to the north of the basin during the first stage sandstones of the upper siliciclastic interval. The quartzarenites were thus of sedimentation in southwestern New Mexico would appear to preclude a most likely derived from sedimentary source rocks. retroarc foreland basin model, unless the northern source terrane was the A small percentage of lithofeldspathic sandstones (13%) in the upper peripheral bulge of the retroarc foreland basin (Cohen, 1982). A periph- siliciclastic interval contain plagioclase and volcanic rock fragments eral bulge is a region of uplift on the distal margin of a foreland basin that (Fig. 5; Table 1). The small amount of these grains indicates that volcanic results from flexural loading of the crust (Cohen, 1982). The bulge mi- rocks were uncommon in the source terrane, although two samples are grates cratonward with time in response to the cratonward propagation of composed almost exclusively of plagioclase and volcanic rock fragments the thrusting. The peripheral bulge is commonly represented in the rock (Fig. 5). record by an erosional unconformity (Cohen, 1982). The northern source terrane during stage one in southwestern New Mexico probably was not a TECTONIC IMPLICATIONS peripheral bulge because uplift arid dissection of the northern source ter- rane are an order of magnitude greater than is associated with a peripheral Cretaceous sedimentation in southwestern New Mexico was primar- bulge, and there is no evidence to indicate cratonward migration of the ily a response to tectonism, although eustatic sea-level changes and northern source terrane. paleoclimate played subordinate roles (Mack, 1986; Mack and others, Intrabasin uplifts are also uncharacteristic of retrcarc foreland basins 1986). The characteristics of the two stages of sedimentation can be used (Dickinson, 1976,1981). An exception to this rule may have occurred in to evaluate the rift-basin model of Bilodeau (1978, 1982), Dickinson the Early Cretaceous foreland basin in southwestern (Schwartz, (1981), Bilodeau and Lindberg (1983), and Dickinson and others (1986a) 1982; Schwartz and others, 1983; Suttner and others, 1985; DeCelles, and the retroarc foreland basin model of Hayes (1970) and Molenaar 1986). The degree of uplift and dissection in Montana, however, is much (1983). less than that of the northern source terrane (rift shoulder) in southwestern The first stage of sedimentation in what is now southwestern New New Mexico and southeastern Arizona (Bilodeau, 1982; Bilodeau and Mexico and adjacent areas compares favorably to standard models for the Lindberg, 1983; Mack and others, 1986). development of rift basins (Miall, 1984, p. 371-384; Mitchell and Read- The second stage of sedimentation in southwestern New Mexico ing, 1986, p. 478-490). During initial extension, brittle failure of the upper began in late Albian and continued into Turanian time and corresponds to produces a series of horsts and . This phase of the upper siliciclastic interval. That this stage represents a period of re- rifting is represented by the basal conglomerate of the lower siliciclastic newed tectonism is suggested by increases in tectonic subsidence and. interval in southwestern New Mexico and by the Glance Conglomerate in siliciclastic sedimentation rates (Fig. 4). Nontectonic models for this stage, southeastern Arizona and northern Chihuahua, Mexico (Bilodeau, 1982; such as expansion of the drainage basin beyond the rift shoulder or climat- Bilodeau and Lindberg, 1983; Brown and Dyer, 1986). The standard rift ically induced increase in erosion rates, are not consistent with the increase basin is bordered by a rift shoulder, which is analogous to the northern in tectonic subsidence. source terrane in southwestern New Mexico (Figs, la, 2). Bilodeau and Four lines of evidence suggest that the increase in subsidence and Lindberg (1983) suggested that uplift of the Early Cretaceous rift shoulder sedimentation rate was not the result of reactivation of the northern rift in southeastern Arizona and southwestern New Mexico was thermotecton- shoulder. (1) The Precambrian core of the rift shoulder was eroded to sea ically rather than fa.ult controlled. There is no evidence to support or reject level and onlapped by shallow-mairine sediment, (2) quartzarenites of the this hypothesis in southwestern New Mexico because of lack of exposures upper siliciclastic interval were no t derived from crystalline basement and of the margin of the rift shoulder. Data presented herein, however, indicate Paleozoic carbonate rocks of the rift shoulder, (3) fades of the upper that the rift shoulder was a major tectonic feature that underwent kilome- siliciclastic interval are too distal in character to have been deposited tres of uplift and erosion. within a few tens of kilometre;» of a reactivated rift shoulder, and In the final phase of standard rift models, the irregular topography (4) sediment dispersal is easterly, southeasterly, and northeasterly rather produced by initial extension is reduced in elevation by erosion, and the than southerly. entire basin subsides thermotectonically. The predominant source of sedi- The second stage of sedimen tation conforms best to a retroarc fore- ment at this time is the rift shoulder. This phase of evolution of the rift land basin model. The sudden increase in tectonic subsidence was a re- basin corresponds to deposition of the upper part of the lower siliciclastic sponse to tectonic loading. Erosion of the orogen produced a great volume and middle carbonate-rich intervals. If rifting continues, the sea invades the of siliciclastic sediment that spread eastward, southeastward, and north- basin, and new is created along the rift axis. Marine trans- eastward across the former rift basin and onlapped the former rift shoulder gression into what is now southwestern New Mexico is well documented (Fig. lb). Sedimentary rocks and, to a lesser extent, volcanic rocks were (Hayes, 1970; Greenwood and others, 1977; Bilodeau and Lindberg, exposed in the orogenic belt and provided quartzarenites and minor vol- 1983; Mack, 1986; Mack and others, 1986), but there is no evidence of canic arenites to the basin. This provenance is similar to that of other Cretaceous oceanic crust. Cretaceous retroarc foreland basin sandstones in the Western Interior In contrast to the rift-basin model, the first stage of sedimentation in (Dickinson and others, 1986b). southwestern New Mexico does not compare favorably with a retroarc Critical to the argument of a late Albian through Turonian retroarc foreland basin model. The principal source area of a retroarc foreland foreland basin in southwestern New Mexico is evidence of mid-Cretaceous

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deformation to the west, northwest, or southwest of the basin. The age of probably took place in a retroarc foreland basin that was complementary syntectonic sedimentary rocks indicates that the Sevier thrust belt was to a compressional orogenic belt in what is now southeastern Nevada, active in Utah and southeastern Nevada in mid-Cretaceous time (Carr, southeastern California, western Arizona, and Sonora, Mexico. The retro- 1980; Bohannon, 1983; Lawton, 1985). In southeastern California, west- arc foreland basin in southwestern New Mexico was disrupted by orogenic ern Arizona, and Sonora, Mexico, scattered evidence also indicates mid- activity in latest Cretaceous or early Tertiary time. Cretaceous compressional deformation. Burchfiel and Davis (1971,1972, 1975,1977) suggested that thrust faults and folds of the Sevier thrust belt ACKNOWLEDGMENTS in southeastern Nevada can be traced southward into the Clark and New York Mountains of southeastern California. Farther south, Hamilton Discussions with William R. Seager, Russell E. Clemons, Joe A. (1982) described an Early or mid-Cretaceous compressional and meta- Galemore, and Warren B. Kolins were very helpful in formulating the morphic event in the Big Maria Mountains, California, and Reynolds and ideas presented herein. Cornelius M. Molenaar and William L. Bilodeau others (1980) delineated a folding event that was bracketed between Late read an earlier version of this paper and offered many suggestions for its and mid-Cretaceous in the western Harquahala Mountains of improvement. This research was funded, in part, by a grant from the U.S. western Arizona. In northern and central Sonora, Mexico, northeast- Geological Survey, Denver, Colorado. vergent folds and thrust faults of mid-Cretaceous age involve sedimentary rocks and, to a lesser extent, arc volcanic and plutonic rocks (Rangin, 1978). 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