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Lucas, S.G., DiMichele, W.A. and Krainer, K., eds., 2017, Carboniferous-Permian transition in Socorro County, New Mexico. New Mexico Museum of Natural History and Science Bulletin 77.

CARBONIFEROUS-PERMIAN TRANSITION IN SOCORRO COUNTY, NEW MEXICO, USA: AN OVERVIEW

SPENCER G. LUCAS1, KARL KRAINER2, BRUCE D. ALLEN3 and WILLIAM A. DIMICHELE4

1New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, New Mexico M 87104; email: spencer.lucas@state.nm.us; 2Institute of Geology, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria ;

3New Mexico Bureau of Geology & Mineral Resources, 801 Leroy Place, Socorro, New Mexico 87801; 4Department of Paleobiology, NMNH Smithsonian Institution, Washington, DC 20560

Abstract— This volume documents the results of 20+ years of field, laboratory and museum research on the Pennsylvanian-Permian rocks and fossils of Socorro County, New Mexico. The articles in this volume report studies of the Pennsylvanian-Permian strata east of Socorro (Joyita Hills-Cerros de Amado-Carthage area), in the Los Pinos Mountains (Sepultura Canyon area), in the Little San Pascual Mountains and at Bell Hill in the Southern San Mateo Mountains. Lithostratigraphy, sedimentary petrography, microfacies analysis and sedimentological interpretation as well as diverse paleontological studies (fossil plants, calcareous microfossils, conodonts, fossil insects, tetrapod footprints, coprolites and fossil fishes) are presented. The entire Pennsylvanian-Permian stratigraphic section in Socorro County is about 2 km thick. The Pennsylvanian strata are a complex succession of sedimentary rocks of marine and nonmarine origin deposited during the Middle-Late Pennsylvanian. These are synorogenic deposits of the ancestral Rocky Mountain (ARM) orogeny and associated marine carbonates of shallow seaways along the western periphery of equatorial Pangea. They are overlain by another complex succession of clastic and carbonate strata of Permian age, the oldest of which were deposited during the final phase of the ARM orogeny. Fossils (primarily the fusulinids and conodonts) provide the age control for the Pennsylvanian-Permian strata of Socorro County. They also are a significant part of the data used to interpret local and regional paleoenvironments and depositional systems. In addition, many of the fossils play a broader role in the understanding of late Paleozoic biotic and evolutionary events.

INTRODUCTIONSocorro County (Fig. 1) is located in central New Mexico and

encompasses an area of 6,649 square miles (17, 221 square kilometers). Most of the county is part of the Basin and Range physiographic province. Thus, the older sedimentary bedrock, of late Paleozoic age, is exposed in the rugged mountain ranges and other uplifts of Socorro County (Fig. 1). The oldest bedrock, which forms the cores of most of the mountain ranges, is Proterozoic basement, mostly metamorphic rocks and granite. Sedimentary rocks that rest on the basement are Pennsylvanian across Socorro County, except in the Sierra Ladrones and Magdalena Mountains, where a thin interval of Mississippian strata underlies the Pennsylvanian strata. The Pennsylvanian strata are a complex succession of sedimentary rocks of marine and nonmarine origin deposited during the Middle-Late Pennsylvanian. These are synorogenic deposits of the ancestral Rocky Mountain (ARM) orogeny and associated marine carbonates of shallow seaways along the western periphery of equatorial Pangea. They are overlain by another complex succession of clastic, carbonate and lesser evaporite strata of Permian age, the oldest of which were deposited during the final phase of the ARM orogeny. The entire upper Paleozoic stratigraphic section in Socorro County is about 1.5 km thick (Fig. 2).

The Pennsylvanian and Permian strata of Socorro County contain a diverse and extensive fossil record. Marine strata produce microfossils of cyanobacteria, algae, foraminiferans (including fusulinids) and conodonts as well as macrofossils, mostly of the shelly benthos—brachiopods, crinoids, bryozoans and molluscs. The nonmarine strata contain diverse paleofloras, fossils of arthropods (notably insects), trace fossils (mostly tetrapod footprints and coprolites) and tetrapod body fossils. These fossils (primarily the fusulinids and conodonts) provide the age control for the Pennsylvanian-Permian strata of Socorro County. They also are a significant part of the data used to interpret local and regional paleoenvironments and depositional systems. In addition, many of the fossils play a broader role in the understanding of late Paleozoic biotic and evolutionary events.

The articles in this volume are the outgrowth of the last two decades of field research on the upper Paleozoic strata of Socorro County. Some of this research has already been published in scientific journals (e.g., DiMichele et al., 2007; Falcon-Lang et al., 2011, 2016), in the 60th Guidebook of the New Mexico Geological Society (Lueth et al., 2009) and in Bulletin 59 of the New Mexico Museum of Natural History and Science (Lucas et al., 2013a). Even with the publication of

this volume, some data remain to be analyzed and written up, and these will be published elsewhere. The present article is intended to be an introduction to and overview of the contents of this volume.

SOME HISTORYLate Paleozoic strata and fossils have a long history of study in

Socorro County, extending back to the early works of Herrick (1900, 1904), Lee (1909) and Darton (1922, 1928), among others. Nevertheless, despite a long history of study, until recently relatively little was known of their paleontology, and their stratigraphy also was in need of more detailed study. A turning point in the study of these late Paleozoic rocks and fossils took place only a little over 20 years ago, in 1996.

In that year, Ed Frye and Mike O’Keefe, two engineering students at New Mexico Tech, showed one of us (SGL) some Pennsylvanian-Permian fossil sites they had discovered in the Cerros de Amado, east of Socorro. These were an accumulation of shark’s teeth and denticles in Middle Pennsylvanian strata, upright fossilized stumps and a black shale interval with conchostracans and the bivalve Dunbarella in the Upper Pennsylvanian rocks, and an extensive tetrapod footprint locality in the lower Permian red beds. The sites discovered by Frye and O’Keefe were an important indication of the highly fossiliferous nature of the Pennsylvanian and Permian strata in the Cerros de Amado. Clearly, the Pennsylvanian-Permian strata east of Socorro contained a largely unexplored and much understudied fossil record that awaited prospecting, collection and study. There had been some published studies of Pennsylvanian and Permian plants (Herrick, 1900; Hunt, 1983) and of Permian bones and footprints (e. g., Berman, 1993; Hunt et al., 1995) from this area, but much more, particularly in the Pennsylvanian strata, remained to be discovered.

In 2002, a major field campaign to collect fossil plants from these rocks was begun by Smithsonian paleontologists led by one of us (WAD). At that time, two of us (SGL and KK) were studying the Bursum Formation across New Mexico. This project expanded in Socorro County to look more closely at most of the upper Paleozoic section in order to produce more detailed lithostratigraphic, microfacies and biostratigraphic data. The effort in the Socorro area was joined by James Barrick of Texas Tech University, who extracted conodonts and used them for biostratigraphic interpretation of the ages of the Pennsylvanian strata in the Cerros de Amado and vicinity. The search for fossil plants also uncovered animal fossils, notably conchostracans, insects and fossil fishes.

In part driven by the planned publication of this volume, an

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effort was also made to study other Pennsylvanian-Permian outcrops in Socorro County. This involved fieldwork in the northern Oscura Mountains, at Abo Pass, in the Los Pinos and Little San Pascual mountains and at Bell Hill in the southern San Mateo Mountains (Fig. 1). Further work is underway and/or planned in the Oscura Mountains as well as in the Sierra Ladrones and Magdalena Mountains in order to decipher better the stratigraphy, depositional history and biostratigraphy of all the upper Paleozoic strata in Socorro County.

OVERVIEWLithostratigraphy and Biostratigraphy

East of SocorroThe area east of Socorro extends from the Joyita Hills on the north

and to Carthage on the south, with its center at and around the Cerros de Amado (Fig. 1). Because it does not have a single geographic name, we refer to this area as “east of Socorro.” The bulk of the research reported in this volume focuses on this area, which consists of rugged hills developed in faulted and folded upper Paleozoic strata exposed along the eastern flank of the Rio Grande rift. Much of the bedrock in the Cerros de Amado consists of Pennsylvanian sedimentary rocks, and, indeed, a complete, ~ 800 m thick Pennsylvanian section for this part of central New Mexico can be pieced together across different fault

blocks (Lucas et al., 2009a). Conodont biostratigraphy provides most of the published age control of this Pennsylvanian section (Lucas et al., 2009a; Barrick et al., 2013).

In this volume, Krainer et al. (2017a) detail the microfacies and petrography of the Pennsylvanian strata exposed in the Cerros de Amado area, east of Socorro. These strata, assigned to the (ascending) Sandia, Gray Mesa and Atrasado formations, encompass different kinds of limestone and intercalated siliciclastic sediments. In the Sandia Formation, most sandstones are quartzarenite. In the Garcia Member of the Gray Mesa Formation and in the clastic members of the Atrasado Formation, mixed siliciclastic-carbonate sandstone predominates. The most common limestone microfacies are bioclastic wackestone and floatstone. Other microfacies such as mudstone, packstone, grainstone and rudstone are rare, and boundstone is very rare. The most abundant fossils in the limestone facies of the Pennsylvanian strata are echinoderms (mostly crinoids), bryozoans and brachiopods, and the dominant grain association type is bryonoderm extended. The chloralgal grain association type is subordinate. Smaller and larger benthic foraminifers, calcareous algae (mostly phylloid algae), sponges (mostly spicules), corals, molluscs (bivalves and gastropods), and arthropods (ostracods and trilobites) are less common. Most encrusting organisms are cyanobacteria and Palaeonubecularia, and (subordinately) sessile foraminifers, algae, Tubiphytes and bryozoans.

FIGURE 1. Map of Socorro County, New Mexico, showing distribution of Pennsylvanian and Permian outcrops. Localities are: AP- Abo Pass, BC- Bruton Canyon (northern Oscura Mountains), BH- Bell Hill, CDA- Cerros de Amado, LPM- Los Pinos Mountains, LSPM- Little San Pascual Mountains, LU- southern Lucero uplift, S- town of Socorro, and SL- Sierra Ladrones.

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The Sandia Formation is characterized by distinct lateral and vertical variations in facies and thickness, and represents early synorogenic sediments deposited during the initial phase of the ARM orogeny. The Gray Mesa Formation, particularly the Whiskey Canyon Member, is mostly limestone that varies little in facies and thickness over a large area, indicating deposition on an extensive, deeper, open marine shelf that extended (at least) from Cedro Peak in the Manzanita Mountains of Bernalillo County south to the Caballo Mountains of Sierra County. The Atrasado Formation is a succession of alternating siliciclastic-dominated and limestone-dominated members. Viewed simply (climate fluctuation is also a factor, see below), siliciclastic-dominated members display strong lateral variations in thickness and facies, which are indicative of deposition during tectonically active periods, whereas the limestone-dominated members are much more consistent in facies and thickness, which, according to Krainer et al., (2017a), points to deposition during tectonically stable periods.

A particularly well known algal mound near Ojo de Amado east

of Socorro, briefly described decades ago by Hambleton (1962), is detailed by Krainer and Lucas (2017) in this volume. This phylloid algal mound complex is in the Tinajas Member of the Atrasado Formation, and the mound complex shows a distinct zonation into: (1) basal bioclastic micrite pile, (2) massive algal mound facies, (3) flank bed facies and (4) capping bed facies. Only the lateral part of the mound complex is exposed; the central part is cut out by a fault. The massive algal mound facies is composed of phylloid algal floatstone and formed below wave base in a low-energy environment where the algae grew in such profusion that they dominated the sea-bottom, forming “algal meadows,” and acted as efficient sediment producers and bafflers. The algae are not preserved in life position, so they do not form algal bafflestone. Rather, they were toppled in situ or were transported for very short distances, forming algal floatstone. Mound growth most likely was stopped when the algae were subjected to higher energy conditions, probably caused by a drop in sea-level.

The ARM orogeny in New Mexico is characterized by at least three pulses, one of which was at the very outset of the Permian and is reflected by deposition of the Bursum Formation. In this volume, Nelson et al. (2017) relate facies changes in the Bursum Formation east of Socorro to syndepositional tectonism. They identify three lithofacies of the Bursum Formation in the hills east of Socorro. The limestone-mudstone lithofacies comprises alternating layers of nodular to bedded marine limestone and non-fissile variegated claystone that represents paleosols. This facies is relatively thin and reflects deposition on stable, relatively high tectonic blocks where sea-level change was the prevalent factor driving sedimentation. The second facies is the laminated lithofacies of thinly layered gray to green shale, siltstone, and fine sandstone. These rocks occur in thick packages and contain fossil land plants and small invertebrates that indicate fresh to brackish water. The laminated facies accumulated on tectonic blocks that subsided rapidly and held standing water. The third facies of conglomerate and arkosic sandstone occurs widely at the base of the Bursum and intermittently in the middle and upper part of the formation. Representing deposits of shallow, low-sinuosity streams, these beds record distinct pulses of tectonic uplift in nearby ARM highlands.

In this volume, Elrick et al. (2017) analyze the depositional setting of fossil wood from the Tinajas Member east of Socorro. Harris and Lucas (2017) present a preliminary report on a large skull of an actinopterygian fish from the Tinajas Member. Lerner and Lucas (2017) document the unusual occurrence of the pseudofossil Astropolithon in the upper part of the Abo Formation. Voigt and Lucas (2017) review the tetrapod footprint record from the Abo and Arroyo de Alamillo formations across Socorro County. DiMichele et al. (2017b) document the extensive record of fossil plants from the Atrasado Formation east of Socorro, and DiMichele and Lucas (2017) report the first Annularia from the Abo Formation. Hunt and Lucas (2017) and Harris et al. (2017) describe extensive coprolite assemblages from the Pennsylvanian strata east of Socorro. These contributions are discussed further below.

Lucas and Krainer (2017) review the lithostratigraphy, microfacies, depositional history and biostratigraphy of the Permian strata east of Socorro. This Permian section is up to about 900 m thick and is assigned to the (ascending order): Bursum Formation, Abo Formation (Scholle and Cañon de Espinoso members), Yeso Group (Arroyo de Alamillo Formation and overlying Los Vallos Formation divided into Torres, Cañas and Joyita members), Glorieta Sandstone, San Andres Formation and Artesia Formation. The Bursum Formation is up to 120 m thick and consists of interbedded red-bed siliciclastics and marine limestones. The Abo Formation is up to 200 m thick and consists of siliciclastic red beds divided into the Scholle Member (~ 40 m of mudstone with channelized beds of crossbedded sandstone and conglomerate) overlain by the Cañon de Espinoso Member (~ 160 m of mudstone, siltstone and many thin beds of sandstone that display climbing ripple lamination). The lower formation of the Yeso Group, the Arroyo de Alamillo Formation, consists of ~ 107 m of red-bed siltstone, sandstone (mostly ripple laminar and laminar with some gypsiferous beds) and very minor dolomite. The overlying Torres Member of the Los Vallos Formation is ~ 156 m thick and is mostly gypsiferous siltstone, claystone, gypsum and a few prominent beds of dolomite and gypsiferous sandstone. The overlying Cañas Member is 66 m thick, consisting mostly of gypsum and a few beds of gypsiferous siltstone and dolomite. The Joyita Member is ~ 12 m thick and consists of red-bed sandstone that is crossbedded, ripple laminated and, in some beds, gypsiferous. The Glorieta Sandstone is up to 85 m thick and consists of crossbedded, laminar and ripple laminated quartzose sandstone.

FIGURE 2. Summary lithostratigraphic section of the Pennsylvanian and Permian strata of Socorro County, New Mexico.

4The San Andres Formation is up to 125 m of mostly limestone (lime mudstone). The Artesia Formation is up to 12 m thick and consists of red-bed siltstone and fine sandstone and minor beds of gypsum and dolomite. It is overlain by Middle Triassic strata of the Anton Chico Member of the Moenkopi Formation.

According to Lucas and Krainer, Bursum deposition took place in a mixture of nonmarine fluvial and shallow marine depositional environments in a tectonically active, mostly coastal setting. The Abo Formation preserves a series of extensive muddy floodplains traversed, early in Abo deposition, by incised rivers that later shallowed and gave way to extensive sheetflooding (also see Seager and Mack, 2003). Yeso sedimentation began with dominantly eolian deposition on an arid coastal plain (Arroyo de Alamillo Formation) followed by deposition of coastal sabkhas, dunes and restricted marine embayments (Torres and Cañas members of Los Vallos Formation). Yeso sedimentation ended with deposition of the Joyita Member of the Los Vallos Formation, which formed by eolian and fluvial processes during lowered sea level. The Glorieta Sandstone is mostly of eolian origin, and the San Andres Formation represents restricted to open, shallow marine deposits. The Artesia Formation represents a landward portion of the siliciclastic shelf that developed northwest of the reefs that fringed the Delaware basin in southeastern New Mexico-West Texas.

Fusulinids from the Bursum Formation east of Socorro indicate it is early Wolfcampian (Newwellian) in age. Local paleontological data coupled with regional correlations indicate that the Abo Formation is middle Wolfcampian-early Leonardian, the Yeso Group is Leonardian, the Glorieta and San Andres formations are late Leonardian, and the Artesia Formation is Guadalupian (Roadian-Wordian) in age. Lucero Uplift and Sierra Ladrones (Ladron Mountains)

Upper Paleozoic strata in the northwestern part of Socorro County are exposed in the southern part of the Lucero uplift and in a relatively narrow belt extending southward along the western flank of the Sierra Ladrones (Fig. 1). Structurally, the area lies along the western margin of the Rio Grande rift and the eastern margin of the Colorado Plateau. Vertical displacements along rift-bounding faults have brought Paleozoic and, in the Ladron Mountains, Proterozoic basement rocks to the surface. Exposures of upper Paleozoic strata in the Lucero uplift were mapped by Kelley and Wood (1946), introducing a stratigraphic nomenclature that provided the lithostratigraphic names Gray Mesa, Atrasado, and Los Vallos, which are presently used for upper Paleozoic formation-rank units across much of central New Mexico (Fig. 2).

Pennsylvanian strata exposed in northwestern Socorro County include the (ascending) Sandia, Gray Mesa, and Atrasado formations. The Pennsylvanian rocks rest unconformably on Proterozic basement or, in the southern Sierra Ladrones, a relatively thin (up to about 27 m) section of Mississippian siliciclastic and carbonate strata assigned by Armstrong (1958) to the Caloso and overlying Kelly formations. The Pennsylvanian section is overlain by Pennsylvanian-Permian strata of the Bursum Formation. Permian strata overlying the Bursum Formation include (ascending) the Abo Formation, Yeso Group, Glorieta Sandstone and San Andres Formation. The San Andres Formation is overlain unconformably by Triassic strata assigned to the Moenkopi Formation. The Permian Artesia Formation is not present above the San Andres Formation in the northwestern part of Socorro County.

Kelley and Wood (1946) measured several sections in the vicinity of the Lucero uplift, including a section in the Navajo Gap area (their graphic section 1) that lies within Socorro County, just northwest of the Sierra Ladrones. The thickness of the Sandia, Gray Mesa and Atrasado formations in their measured section 1 amounts to approximately 628 m. A short distance to the north, still within Socorro County, Kelley and Wood’s (1946) section 2 (Monte de Belen) shows 490 m of Sandia, Gray Mesa, and Atrasado, although the Sandia Formation portion of the section is incomplete. Kottlowski (1960) compiled thickness estimates for Pennsylvanian strata in the Lucero uplift/Sierra Ladrones area, including a reported value of about 823 m in the southern Ladron Mountains a few km north of the Rio Salado (also see Siemers, 1978, 1983). Based on relatively high thickness estimates for the Pennsylvanian System in the Lucero uplift, Kottlowski (1960) followed previous workers (e.g., Wengerd, 1959) by delineating a regional depocenter termed the Lucero basin on his Pennsylvanian isopach maps.

Since publication of the Kelley and Wood (1946) geologic map and the Kottlowski (1960) compilation, reports that include lithostratigraphic descriptions of the Pennsylvanian strata in the Lucero uplift/Sierra Ladrones area include Hammond’s (1987) M.S. thesis on

the structural geology of the Navajo Gap area, description of the type sections of the Gray Mesa and Atrasado formations by Krainer and Lucas (2004a), and Scott and Elrick’s (2004, Elrick and Scott, 2010) cycle-stratigraphic investigations of the Gray Mesa Formation on Mesa Sarca (Monte de Belen) in the southern Lucero uplift.

Hammond (1987) utilized a high-resolution stratigraphic nomenclature for the Pennsylvanian System in this area in order to delineate details of the structural geology. Thus, he mapped Pennsylvanian rocks using the stratigraphic nomenclature created by Thompson (1942), as modified by Rejas (1965) for the area east of Socorro. This is the same nomenclature, with some modifications (e.g., many of Thompson’s [1942] formations are now considered member-rank units), that is presently being used for Pennsylvanian strata over a large portion of central New Mexico (Fig. 2). Hammond’s (1987) reported thicknesses for the Sandia, Gray Mesa and Atrasado formations in the Navajo Gap area yield a total thickness of 573 m.

In terms of its overall stratigraphic architecture, the Pennsylvanian section in the Lucero uplift/Sierra Ladrones area is similar to other areas in Socorro County to the east. Thus, the Sandia Formation is characterized by a dominance of siliciclastic deposits, the Gray Mesa by a preponderance of marine carbonate, and the Atrasado Formation by a succession of alternating slope-forming intervals containing a relative abundance of siliciclastics and ledge-forming intervals dominated by marine carbonate. Wengerd (1959, fig. 6) showed the stratigraphic distribution of some Desmoinesian and Missourian fusulinid genera in his stratigraphic section for the Monte de Belen area, but comparatively little detailed biostratigraphy is available for the Pennsylvanian of northwestern Socorro County.

A substantial literature regarding the overlying Bursum Formation has been produced from the Carrizo Arroyo area in the Lucero uplift, 22 km north of the Socorro County line. There, the Bursum Formation is about 100 m thick and consists of variegated nonmarine shale, mudstone and siltstone, with some beds of marine shale and carbonate. At Carrizo Arroyo, the Bursum Formation (Red Tanks Member) contains extensive fossil assemblages that include palynomorphs, calcareous algae, charophytes, plant megafossils, non-fusulinid foraminifers, fusulinids, bryozoans, brachiopods, gastropods, bivalves, nautiloids, eurypterids, ostracods, syncarid crustaceans, conchostracans, insects and some other arthropods, echinoids, crinoids, conodonts, fish ichthyoliths and bones of amphibians and reptiles (see articles in Lucas and Zeigler, 2004a). Deposition of the Bursum Formation at Carrizo Arroyo began during the latest Pennsylvanian and continued into the early Permian (e.g., Lucas et al., 2013b).

Information regarding the Bursum Formation is sparse for the Socorro County portion of the Lucero uplift and the Ladron Mountains. Kelley and Wood (1946) depicted approximately 63 and 96 m of Bursum Formation (their Red Tanks member) in their graphic sections 1 and 2. Hammond (1987) reported a partial thickness of the Bursum Formation (incomplete due to faulting) of about 30 m in the Navajo Gap area. Krainer and Lucas (2004b) documented a poorly exposed Bursum section at Coyote Draw on the western flank of the Sierra Ladrones that is about 120 m thick. During a recent visit to the western side of the Sierra Ladrones, we identified an area a few km south of Hammond’s sections that looks promising in terms of piecing together a reasonably complete and well exposed section of the Bursum Formation, including a good exposure of its upper contact with the overlying Abo Formation.

As with the Bursum Formation, information regarding younger Permian stratigraphic units in northwestern Socorro County is sparse. A complete Permian section was described by Lucas and Zeigler (2004b) in the northern Lucero uplift in the vicinity of Carrizo Arroyo. There, the Abo Formation is about 150 m thick, consisting of red-bed mudstone, arkosic sandstone and siltstone. The Yeso Group overlies the Abo Formation and is approximately 348 m thick. It can be divided into the DeChelly (70 m thick) and overlying Los Vallos (278 m thick) formations. The DeChelly Formation is dominantly beds of quartzose sandstone. The Los Vallos Formation is very fine-grained sandstone, siltstone, and gypsum, with some relatively thin but persistent marine carbonate beds. The Glorieta Sandstone overlies the Yeso Group, is approximately 53 m thick, and consists of quartzose sandstone and a single, 7 m thick bed of gypsum. It is conformably overlain by the San Andres Formation, which is about 109 m thick and mostly gypsum and limestone, with lesser amounts of sandstone and shale. In the Lucero uplift, the Middle Triassic Moenkopi Formation rests disconformably on the San Andres Formation.

Permian rocks in the Monte de Belen measured section of Kelley and Wood (1946) were pieced together just north of the Socorro County

5line. The section traverses large tracts mapped as Quaternary deposits covering the Abo and Yeso portions of the section, and it appears that the upper part of the section extends outside the map area to the west. The graphic section depicts 268 m of Abo Formation, 32 m of DeChelly Formation (their Meseta Blanca member) and 273 m of Los Vallos Formation (total Yeso Group thickness of 305 m), together with 66 m of Glorieta Sandstone and 137 m of San Andres Formation. The San Andres Formation in this traverse contains significant amounts of gypsum.Magdalena Mountains

Pennsylvanian strata in the northern Magdalena Mountains are the stratotype of Gordon’s (1907) Magdalena Group (Fig. 3), a term still used by some to refer to the entire Pennsylvanian section in southern New Mexico (though see Thompson [1942] and Kues [2001] for compelling arguments to abandon the term Magdalena Group). These strata are much faulted and otherwise altered by intrusives, so thickness estimates and some of the actual stratigraphic succession are not definitely determined. Loughlin and Koschmann (1942) presented the most detailed data and mapping of the upper Paleozoic section in the Magdalena Mountains, and more recent summaries are provided by Kottlowski (1960) and Siemers (1978, 1983).

The Pennsylvanian section has been assigned to the Sandia and Madera formations by these later workers, the latter divided into the gray limestone and overlying arkosic limestone members. We have only examined these strata in a reconnaissance fashion, and assign them to the Sandia, Gray Mesa and Atrasado formations. According to Siemers (1978, 1983), the Sandia Formation is 167 m thick, the Gray Mesa 172 m thick, and the Atrasado Formation is only 61 m thick in the Magdalena Mountains. This is a total thickness of about 400 m, but Siemers (1983) states that well data suggest an actual thickness of the Pennsylvanian section of 500-550 m (also see Krewedl, 1974; Siemers,

FIGURE 3. Gordon’s (1907) stratigraphy of the Magdalena Group in the Magdalena Mountains.

1975). Both Kottlowski (1960) and Siemers (1983) note that much of the Atrasado Formation has been removed by faulting (tectonically thinned) in the Magdalena Mountains.

Krainer and Lucas (2009) were convinced that the Abo Formation rests directly on the Atrasado Formation in the area around the Kelly Mine, so they show a zero thickness for the Bursum Formation in the Magdalena Mountains. Abo Formation strata are the youngest Paleozoic rocks exposed in the Magdalena Mountains and are much faulted and intruded. Loughlin and Koschmann (1942) estimated that a maximum (but incomplete) Abo thickness of about 53 m is preserved locally. Loughlin and Koschmann (1942) also listed records of both fusulinids and fossil plants from the Pennsylvanian and Permian strata in the Magdalena Mountains, but these have not been examined by us.Bell Hill (Southern San Mateo Mountains)

The southern San Mateo Mountains of Socorro County (Fig. 1) are a large volcanic edifice composed primarily of Eocene-Oligocene igneous rocks at the eastern edge of the Mogollon-Datil volcanic field (e.g., McLemore, 2012). Along the southeastern edge of the range, relatively small fault blocks expose Paleozoic sedimentary rocks. One of these exposures, at Bell Hill (Fig. 1), has long been important to the regional Pennsylvanian stratigraphy as the supposed thickest (~ 800 m thick) Pennsylvanian section in south-central New Mexico and the focal point of a late Paleozoic San Mateo depositional basin (e.g., Kottlowski, 1960; Furlow, 1965; Kelley and Furlow, 1965; Siemers, 1978, 1983).

In this volume, Lucas et al. (2017a) present the results of a restudy of the Pennsylvanian section at Bell Hill, and determine that it is thinner than previously reported, only 495 m thick. At Bell Hill, the Pennsylvanian strata overlie a thin (~ 70 m thick) lower Paleozoic section that consists of the Cambro-Ordovician Bliss Formation and the Ordovician El Paso Group (Sierrite Member of the Hitt Canyon Formation) and Montoya Formation (Cable Canyon and Upham members). The Middle Pennsylvanian Red House Formation unconformably overlies the Montoya Formation, and is at least 31 m thick (structural complications prevent a certain estimate). It is overlain by the Gray Mesa Formation, which is 158 m thick and divisible into the Elephant Butte (at least 18 m thick), Whiskey Canyon (53 m thick) and Garcia (87 m thick) members. The overlying Bar B Formation is 306 m thick, substantially thicker than to the southeast, and accounts for most of the relatively great thickness of the Bell Hill Pennsylvanian section. The Bar B Formation is overlain by the Bursum Formation, ~ 26 m thick, which is overlain by the Abo Formation.

Fusulinid and conodont biostratigraphy indicate that at Bell Hill the Red House Formation is Atokan, the Gray Mesa Formation is Desmoinesian and the Bar B Formation is Desmoinesian-Virgilian. The new estimate of the thickness of the Pennsylvanian section at Bell Hill (495 m) requires redrawing the late Paleozoic San Mateo basin as a platform–an area underlain by a relatively thick succession of Pennsylvanian strata, between the even thicker Pennsylvanian fill of the Lucero and Orogrande basins to the north and south (Fig. 4).Little San Pascual Mountains

The Little San Pascual Mountains (LSPM) are a small uplift on the eastern side of the Rio Grande river, about 40 km south of Socorro (Fig. 1). Most of the sedimentary rocks exposed across the uplift are of Pennsylvanian age, overlain to the south-southeast by Permian strata (Kottlowski, 1960; Geddes, 1963; Siemers, 1978, 1983).

In this volume, Lucas et al. (2017b) present new lithostratigraphic, petrographic and biostratigraphic data on the Pennsylvanian section in the LSPM. The Pennsylvanian section is at least 637 m thick and assigned to the (ascending order) Red House (200 m +), Gray Mesa (168 m) and Bar B (269 m) formations. The base of the Red House Formation is covered/faulted, so its thickness is a minimum. The Red House Formation in the LSPM consists of a lower interval of shale and limestone, a middle interval of shale and an upper interval of alternating shale and limestone. The overlying Gray Mesa Formation can be divided into Elephant Butte (45 m), Whiskey Canyon (37 m) and Garcia (87 m) members. The Elephant Butte Member is interbedded limestone and shale, the Whiskey Canyon Member is mostly cliff-forming, cherty limestone, and the Garcia Member is alternating limestone and covered intervals with a distinctive crossbedded sandstone at its base. The Bar B Formation is mostly slope-forming intervals that are covered/shale alternating with limestone and dolomite. The Bursum Formation overlies the Bar B Formation and is 50-60 m of interbedded red-bed mudstone, sandstone and limestone, overlain by siliciclastic red beds

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of the Abo Formation. Conodont biostratigraphy indicates that in the LSPM, the Red

House Formation is of Atokan age, the Gray Mesa Formation is of Atokan-Desmoinesian age and the Bar B Formation is of Desmoinesian-Virgilian age. The Pennsylvanian section in the LSPM includes the northernmost outcrops of the Red House and Bar B formations. The Bar B has a stratigraphic architecture that indicates it is transitional to Atrasado Formation sections to the north.

At 637+ m thick, the Pennsylvanian section in the LSPM is thicker than the Pennsylvanian section at Bell Hill. As already noted, these sections do not define a San Mateo basin of Pennsylvanian deposition, but instead identify a platform, between the Lucero and Orogrande basins (Fig. 4). Northern Oscura Mountains

At the northern end of the Oscura Mountains in southeastern Socorro County (Fig. 1), a complete Pennsylvanian stratigraphic section is overlain by an Abo Formation section that dips under the Yeso Group strata that form the southwestern edge of Chupadera Mesa (Wilpolt and Wanek, 1951). The upper part of the Pennsylvanian section here is key to the regional Pennsylvanian lithostratigraphy because it contains the type sections of units named by Thompson (1942) that are now recognized as members of the Atrasado Formation (Figs. 2, 5-6) across much of Socorro County and as far north as the Sandia Mountains of Bernalillo and Sandoval counties.

Lucas and Krainer (2009) provided the most recent description of the Pennsylvanian section in the northern Oscura Mountains, assigning it to the Sandia, Gray Mesa and Atrasado formations. Here, a thin Sandia Formation about 13 m thick rests nonconformably on Proterozoic granite, and is overlain by the limestone-dominated Gray Mesa Formation, which is about 150 m thick. The overlying Atrasado Formation is ~ 121 m thick and is divided into the (ascending) Adobe, Council Spring, Burrego, Story, Del Cuerto and Moya members, all of which were units of formation rank in Thompson’s (1942) stratigraphy (Fig. 5). Currently, three of us (SGL, KK and BDA) are restudying the Atrasado Formation section in the northern Oscura Mountains to improve both lithostratigraphic and biostratigraphic resolution.

At the northern end of the Oscura Mountains, the Bursum Formation is about 80 m thick and includes the strata that were the type section of Thompson’s (1942) Bruton Formation. About 15 km to the southwest, in the Hansonburg Hills, is the type section of the Bursum Formation of Wilpolt et al. (1946), redescribed by Lucas et al. (2000, 2002). Although Bruton Formation had priority over and refers to

almost the same stratigraphic interval as Bursum Formation, the latter has been generally used. Lucas and Krainer (2004) revived the name Bruton as a member of the Bursum Formation at outcrops where most of its thickness is marine limestone and shale.

On the northern end of the Oscura Mountains, a complete section of the Abo Formation is exposed, dipping under the Yeso Group section that forms most of the flanks and lowlands of and around Chupadera Mesa (Wilpolt et al., 1946; Wilpolt and Wanek, 1951; DiMichele et al., 2007). Here, the Abo Formation is ~ 150 m thick and almost evenly divided into the Scholle and Cañon de Espinoso members. Fossil plants and tetrapod footprints are found at several levels in the Cañon de Espinoso Member (DiMichele et al., 2007; Lucas and Spielmann, 2009a; Voigt and Lucas, 2017). The overlying Arroyo de Alamillo Formation of the Yeso Group is about 100-110 m thick but poorly exposed in the lowlands southwest of Chupadera Mesa. It yields tetrapod tracks from sites in the lower 10 m of the formation (Voigt and Lucas, 2017). As is the case east of Socorro, this section is one that documents the boundary between the Dromopus and Erpetopus biochrons, which is close to the Abo-Yeso boundary (Voigt and Lucas, 2017).

Much of the upper Paleozoic section at the southern end of the Oscura Mountains, at Mockingbird Gap on the White Sands Missile Range, is in Socorro County, and it was described and mapped by Bachman (1968). We have mainly studied the Bursum, Abo and Yeso strata in this area, publishing thus far only on the fossil footprints in the lower Yeso Group (Lucas and Spielmann, 2009b).Chupadera Mesa

Chupadera Mesa is a large tableland that extends from near Mountainair in Torrance County on the north to near Carrizozo in Lincoln County on the south, a distance of about 86 km, most of it in Socorro County (Fig. 1). The mesa is developed in early Permian rocks.

Lucas et al. (2016a) recently re-evaluated the Pennsylvanian section to the west of Chupadera Mesa, in the southern Manzano Mountains, most of it in Torrance County. This includes the type sections of units named by Myers (1973) and long applied to Pennsylvanian strata throughout the Manzano and Manzanita Mountains. Detailed restudy indicates these strata are very similar to the Pennsylvanian section in the Cerros de Amado, ~60 km to the SW, so the stratigraphic nomenclature introduced by Thompson (1942) and elaborated by Rejas (1965) and Lucas et al. (2009a) can be applied to the Pennsylvanian section in the Manzano and Manzanita Mountains (also see Lucas et al., 2011a, 2014).

FIGURE 4. Kottlowski’s (1960) isopach map of Pennsylvanian strata in south-central New Mexico identified a small San Mateo basin centered on Bell Hill (left) in Socorro and Sierra counties. New thickness data on the Pennsylvanian sections at Bell Hill and in the Little San Pascual Mountains redraw the thickness patterns so that the San Mateo basin is no longer evident; instead, there is a trough located on the platform separating the Lucero and Orogrande basins (right) (from Lucas et al., 2017a).

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Relatively recent regrading and excavation of the BNSF railroad tracks through Abo Pass has cut fresh outcrops through most of the Pennsylvanian section. Here, in the Burrego Member of the Atrasado Formation, an extensive paleoflora was collected and is detailed by DiMichele et al. (2017b) in this volume. This is the only substantial paleoflora of Virgilian age known from Socorro County.

Lucas et al. (2016b) presented a comprehensive study of the Permian stratigraphic section at Abo Pass at the southern tip of the Manzano Mountains and onto the western flank of Chupadera Mesa. Here, the Permian section is ~ 800 m thick and is assigned to the (ascending order): Bursum Formation (Red Tanks Member), Abo Formation (Scholle and Cañon de Espinoso members), Yeso Group (Arroyo de Alamillo Formation and overlying Los Vallos Formation divided into Torres, Cañas and Joyita members), Glorieta Sandstone and San Andres Formation. The Bursum Formation is ~ 35-40 m thick and consists of interbedded red-bed siliciclastics (mudstone, sandstone and conglomerate) and marine limestones. The Abo Formation is ~ 310 m thick and consists of siliciclastic red beds divided into the Scholle Member (~ 140 m of mudstone with channelized beds of crossbedded sandstone and conglomerate) overlain by the Cañon de Espinoso Member (~ 170 m of mudstone, siltstone and many thin beds of sandstone that display climbing ripple lamination). The lower formation of the Yeso Group, the Arroyo de Alamillo Formation, consists of ~ 80 m of red-bed sandstone (mostly ripple laminar and laminar with some gypsiferous beds) and very minor dolomite. The overlying Torres Member of the Los Vallos Formation is ~ 180 m thick and mostly consists of gypsiferous siltstone, claystone, gypsum

and a few prominent beds of dolomite and gypsiferous sandstone. The overlying Cañas Member is 16-52 m thick, consisting mostly of gypsum and includes a few beds of gypsiferous siltstone and dolomite. The Joyita Member is ~ 21 m thick and consists of red-bed sandstone that is crossbedded, ripple laminated and, in some beds, gypsiferous. The Glorieta Sandstone is ~ 78 m thick and consists of crossbedded, laminar and ripple laminar, quartzose sandstone. In the Abo Pass area, the upper part of the San Andres Formation has been eroded, leaving up to 91 m of mostly limestone (lime mudstone). It is overlain by Triassic strata east of the Abo Pass area. Important tetrapod footprint localities in the Abo Formation (Voigt and Lucas, 2017) as well as vertebrate bone localities in the Bursum and Abo formations (Berman et al., 2015) are just to the east of Abo Pass, in Torrance County (Lucas et al., 2016b). Fossil plants of Pennsylvanian and Permian age are also known from the Abo Pass area (DiMichele et al., 2017a), though most of these localities are also in Torrance County.Los Pinos Mountains

The Los Pinos Mountains are part of a north-northeast chain of mountains that border the east side of the Rio Grande Valley (Fig. 1). Geologically, the range is a basement-cored uplift that formed along the eastern margin of the Rio Grande rift during the late Cenozoic. In this uplift, above a thick and complex basement core, sedimentary rocks of late Paleozoic (Pennsylvanian-Permian) age are exposed. The complete Pennsylvanian section was described in a generalized way in the older literature of New Mexico geology (Darton, 1922, 1928; Stark and Dapples, 1946). More recently, these rocks have been mapped at a

FIGURE 5. Thompson’s (1942) type sections of units he named for the upper part of the Pennsylvanian succession in the northern Oscura Mountains.

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scale of 1:24,000 (Myers et al., 1986; Allen et al., 2013a, 2014).In this volume, Krainer et al. (2017b) present a new study of

the Pennsylvanian strata in the Los Pinos Mountains. These strata nonconformably overlie Proterozoic metamorphic and granitic basement and are disconformably overlain by the lower Permian Bursum Formation. Krainer et al. assign the approximately 424 m thick Pennsylvanian section in the Los Pinos Mountains to the (ascending order) Sandia (172 m thick), Gray Mesa (45 m thick) and Atrasado (206 m thick) formations. This section of the Sandia Formation is unusually thick, and fusulinids and conodonts demonstrate that its upper 80 m are of Desmoinesian age. Thus, at Sepultura Canyon, about 80 m of Desmoinesian strata contain substantial beds of sandstone and conglomerate. This distinguishes them from Desmoinesian strata to the north and south, which contain few or no beds of sandstone. This sandy interval thus indicates a local facies of Desmoinesian age not seen elsewhere in central New Mexico. This facies may be explained most easily by tectonic activity and proximity to the ARM Joyita uplift, over which the Gray Mesa Formation pinches out (Kottlowki and Stewart, 1970; Nelson et al., 2013b, fig. 9).

Limestone of the unusually thin Gray Mesa Formation is dominated by wackestone and floatstone with a diverse fossil assemblage indicating deposition in a low-energy, open, normal marine shelf environment. The Atrasado Formation is composed of alternating siliciclastic-dominated and limestone-dominated intervals that can be arranged in eight lithostratigraphic members that are recognizable over a large part of central New Mexico. The Atrasado Formation is overlain by siliciclastic redbeds at the base of the Bursum Formation.

In the southern Los Pinos Mountains, Stark and Dapples (1946) named the Aqua Torres Formation (Aqua Torres is a corruption of Agua [de] Torres, the name of a local well) for a stratigraphic interval about 61 m thick between the Atrasado and Abo formations that is now assigned to the Bursum Formation (Krainer and Lucas, 2009). Allen et al. (2013b) described a Bursum section just south of the Los Pinos Mountains, where it is about 40 m thick and contains an early Wolfcampian (Newwellian) assemblage of schwagerinid fusulinids. Immediately south and east of the main part of the Los Pinos Mountains, Abo Formation and Yeso Group strata crop out but have not been studied by us.

SEDIMENTATION: ANCESTRAL ROCKY MOUNTAIN TECTONICS, GLACIO-EUSTASY AND CLIMATE

Most students of late Paleozoic sedimentation in New Mexico posit glacio-eustasy and/or ARM tectonism as the principal drivers of sedimentation. Indeed, where discussed in this volume (see especially Krainer et al., 2017a, b; Nelson et al., 2017), the influence of ARM tectonism on Pennsylvanian-Permian sedimentation is heavily emphasized. Little discussed, or considered, are the possible effects of local or regional climate fluctuations on sedimentation.

Potentially, sedimentation during the Pennsylvanian and the early part of the early Permian in central New Mexico was influenced by tectonic movements related to the ARM system, by glacio-eustatic sea-level fluctuations related to the Gondwana glaciations and/or by climate fluctuations in what was then western equatorial Pangea, some of which were far field effects of the late Paleozoic ice ages. Local sedimentary processes were also influenced by local/regional climate and topographic relief. These varied allocyclic and autocyclic drivers of sedimentation all played a role in late Paleozoic sedimentation in central New Mexico, but the work necessary to tease apart the different drivers and their effects has only just begun.

In New Mexico, ARM tectonism created a series of intracratonic uplifts of Pennsylvanian to Permian age, bounded by a number of sedimentary basins. The timing of uplift and basin subsidence varies from basin to basin. In general, basins in northeastern to eastern New Mexico subsided earlier (latest Mississippian to Early Pennsylvanian) than basins in the southern and western part of New Mexico (climax during the early Permian) (see summary in Kues and Giles, 2004).

Two hypotheses have been developed to explain the tectonic processes of the ARM (Ye et al., 1996; Kluth, 1998; Dickinson and Lawton, 2003). One explains the deformation by the collision of Laurentia and Gondwana, resulting in the formation of the Ouachita orogen and the basins and uplifts of the ARM. The other hypothesis relates ARM deformation to northeasterly directed subduction along the southwestern margin of Laurentia evidenced by a volcanic arc in east-central Mexico. The interpretation of ARM deformation on a more regional scale is presented by Armstrong (1962), Armstrong and Mamet (1988), Peterson (1980), Kluth and Coney (1981), Kluth (1986) and Woodward et al. (1999).

According to Kluth and Coney (1981), the late Paleozoic ARM uplifts and basins of Colorado, New Mexico and adjacent areas formed by NW-SE crustal shortening caused by the collision of North America with South America/Africa that produced the Ouachita-Marathon orogeny. This model is based on the assumption that north-striking faults of late Paleozoic age underwent left-lateral slip, which is consistent with northwest-southeast crustal shortening. In contrast, Woodward et al. (1999) proposed that most of the 125 km of pre-Laramide right-lateral offset in northern New Mexico occurred along north-trending major faults during the late Paleozoic. Late Paleozoic deformation therefore was more likely caused by northeast-southwest crustal shortening related to a northeast-directed subduction zone along the southwest margin of North America (Ye et al., 1996).

Glacio-eustatic sea-level fluctuations related to the Gondwana glaciation influenced sedimentation on tectonically stable shelfs such as the American Midcontinent, producing well developed cyclic successions. Cyclic sedimentation resulting from glacio-eustatic sea-level fluctuations has been reported from New Mexico, particularly from the Gray Mesa Formation (e.g., Soreghan 1994a, b; Scott and Elrick

FIGURE 6. Bruton Canyon in the northern Oscura Mountains, looking west. This is the area Thompson (1942) used to define his stratigraphic nomenclature for Upper Pennsylvanian strata in New Mexico. A few of the cliff-forming members of the Atrasado Formation (originally designated as formations by Thompson) are labeled.

92004; Wiberg 1993; Wiberg and Smith 1994) (also see discussion in Nelson et al., 2013a). Rygel et al. (2008) distinguished distinct phases of glacioeustatic sea-level fluctuations during the late Paleozoic, some of which likely influenced sedimentation in central New Mexico.

The Sandia Formation encompasses the lower part of the Pennsylvanian sedimentary succession across much of northern and central New Mexico, and represents synorogenic deposits related to the onset of the ARM orogeny. Krainer and Lucas (2013) pointed out that the Sandia Formation is characterized by marked lateral facies changes and variation in thickness. The Sandia Formation is generally thin and dominantly siliciclastic in the northern part of New Mexico at sections close to the ARM uplifts, and becomes thicker towards the south (Krainer and Lucas, 2013).

Tectonic activity during sedimentation of the Gray Mesa Formation was comparatively low. The Whiskey Canyon Member in particular is free of coarse siliciclastic deposits and almost free of fine-grained siliciclastic sediment, indicating that deposition took place during what may have been a relatively stable period tectonically. However, it may also reflect a period of relatively dry climate, when clastic runoff from nearby land surfaces was diminished.

According to Krainer et al. (2017a) in this volume, the limestone-dominated members of the Atrasado Formation reflect periods of relative tectonic quiescence, whereas the clastic-dominated members represent tectonically active periods during which rapid uplift of basement blocks occurred. Based on these inferences, four tectonically active phases may have occurred during deposition of the Atrasado Formation. The Tinajas Member marks the strongest, and the Del Cuerto Member, which contains only small amounts of siliciclastic sediment, represents the weakest tectonic phase.

East of Socorro, the Bursum Formation is characterized by considerable lateral variations in facies and thickness, indicating that regional synsedimentary tectonic movements of the ARM orogeny strongly influenced Bursum sedimentation (Krainer and Lucas, 2009). A tectonic pulse responsible for the unconformity at the base of the Bursum Formation caused a sharp facies change from dominantly shallow marine carbonate sedimentation of the uppermost Atrasado Formation to mixed siliciclastic-carbonate sedimentation of alternating nonmarine and marine sediments of the Bursum Formation. The unconformity on top of the Bursum Formation was caused by a major tectonic pulse of the ARM deformation, resulting in a significant rejuvenation of basement uplifts and increased siliciclastic influx and deposition of nonmarine red beds of the Abo Formation.

There is also evidence for tectonic activity during sedimentation of the Bursum Formation. The widespread Bursum Formation (Virgilian-early Wolfcampian) thus is the result of significant synsedimentary tectonic activity related to the ARM orogeny. Glacioeustatic sea-level changes related to the Permo-Carboniferous glaciation of the Gondwana supercontinent seem to have been of minor importance during sedimentation of the Bursum Formation and were overprinted by tectonic movements (Krainer and Lucas, 2009; Lucas et al., 2013b; also see Nelson et al., 2017).

In summary, following Krainer et al. (2017a), among others, several phases of the ARM orogeny can be distinguished in central New Mexico. The main source for siliciclastic sediment was the Pedernal uplift, subordinately the Joyita uplift (Fig. 7). The biostratigraphy of the Pennsylvanian succession in central New Mexico (e. g., Krainer and Lucas, 2013, Nelson et al., 2013a, b; Lucas et al., 2016a, b, c) indicates the following timing of ARM tectonic phases:

1. The Sandia Formation represents the first tectonic phase of the ARM orogeny during the Atokan, starting locally during the latest Morrowan.

2. The Gray Mesa Formation was deposited during a period of little tectonic activity, although local intercalation of sandstone and conglomerate in the lower (Elephant Butte) and upper (Garcia) members suggest some tectonic activity. This period of relative tectonic quiescence lasted for much of the Desmoinesian in most areas. It should be noted, though, that the Desmoinesian appears to be the time of the highest sea level of the Middle Pennsylvanian with the lowest magnitude of sea-level fluctuations until the very end. Thus, the Gray Mesa could be seen as being deposited during a time of globally high sea levels, and the overlying Bartolo Member of the Atrasado Formation could be correlative with the extreme sea-level fluctuations that appeared during the late Desmoinesian throughout the entire equatorial area of Euramerica. This possibility, of course, diminishes the importance of ARM tectonism in the onset of Atrasado Formation deposition.

3. Four tectonic phases may be recorded by the clastic-dominated

members of the Atrasado Formation. These tectonic phases indicate rapid uplift of basement blocks and are separated by periods of tectonic quiescence during which limestone was deposited. The Bartolo Member represents the first tectonic phase during the latest Desmoinesian/earliest Missourian. The second phase during which the Tinajas Member was deposited was the strongest and took place during the Missourian. The third tectonic phase is represented by the Burrego Member and occurred during the early Virgilian. The last tectonic phase occurred during the late Virgilian. During this phase, which was the weakest, the Del Cuerto Member was deposited.

4. Tectonic activity continued during deposition of the Bursum Formation (lower Wolfcampian), which contains substantial amounts of siliciclastic sediments, including nonmarine conglomerate and arkosic sandstone.

5. The onset of deposition of the overlying nonmarine red beds of the Abo Formation (and Cutler Group in northwestern New Mexico) marks the last and strongest tectonic phase, causing uplift and deposition of large amounts of siliciclastic sediments during the middle and upper Wolfcampian, over large areas of New Mexico.

PALEONTOLOGY AND CORRELATIONFossil Plants

Plant fossils are abundant in strata of the Sandia, Atrasado, Bursum, and Abo formations in Socorro County, both in the area east of Socorro and in other areas of the county where strata have been investigated (e.g., Hunt, 1983; DiMichele et al., 2007, 2017a; Lucas et al., 2009b; Falcon-Lang et al., 2011, 2016). In this volume, DiMichele et al. (2017b) describe numerous fossil floras from the Atrasado Formation east of Socorro, encompassing more than 50 species ranging in age from late Desmoinesian to early Virgilian. The floral succession of the Atrasado Formation is important as a baseline study of vegetation from the western Pangean equatorial region during the extensive time interval sampled.

The floras are primarily allochthonous, with two notable exceptions. One autochthonous flora has been described in detail by Falcon-Lang et al. (2011, 2016). Of Missourian age, the plant fossils consist of in situ silicified coniferopsid tree stumps and fallen logs of the same. These fossils are rooted in a micritic carbonate, and buried in carbonate-rich, subaqueous to eolian-deposited gypsiferous dunes.

Elrick et al. (2017), in this volume, examine the sedimentary features of this deposit and conclude that it formed in a sabkha setting, but that the trees grew during a brief return to fresh-water conditions on an algal flat. The other autochthonous flora is of late Desmoinesian age, and is dominated by stems and leaves of the pteridosperm Neuropteris ovata, with only a few specimens of other species. Excavated in three locations, separated by up to 2.5 km, this deposit was likely a large, coastal swamp, the closest thing in the section to what, in the central portion of Pangea, would have been a coal bed.

In general aspect, the allochthonous floras of the Atrasado Formation are of a “mixed” character. They consist of species typical of wetland habitats that were important elements of central Pangean coal basins of North America and Europe, intimately intermixed with species generally considered xeromorphic and tolerant of seasonally dry settings. Included in this latter group are conifers, cordaitaleans, taeniopterids, noeggerathialeans, and some pteridosperms not usually found in wetlands. Such mixed floras indicate that the background climate was likely seasonally dry, with the wetland elements living in local swamps and riverine corridors.

DiMichele and Lucas (2017), in this volume, describe the first occurrence of the calamitalean sphenopsid foliage Annularia from the early Permian Abo Formation red beds east of Socorro. Although heavily sampled throughout its outcrop area from northern to south-central New Mexico, no calamitaleans have previously been reported from the red beds of the Abo Formation. In light of the discovery of this specimen, the authors discuss the matter of rarity in the living flora, and contrast it with rarity in fossil assemblages. They conclude that although rarity in the standing vegetation may be expected to translate into rarity in the fossil record, the latter may also result from a variety of taphonomic factors that may mask even abundant species in the parent vegetation.

Tetrapod FootprintsSiliciclastic red beds of the Abo Formation and its homotaxial

equivalents in New Mexico contain one of the World’s great records of early Permian nonmarine trace fossils (e. g., Lucas and Heckert,

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FIGURE 7. Isopach map of Pennsylvanian-Permian Bursum Formation and tectonic features of the ancestral Rocky Mountain orogeny in central New Mexico.

111995; Lucas et al., 2011a). East of Socorro, at Abo Pass and in the northern Oscura Mountains, the Abo red beds (and, locally, the Arroyo de Alamillo Formation of the Yeso Group) contain a substantial number of ichnofossil assemblages dominated by tetrapod footprints.

In this volume, Voigt and Lucas (2017) present the first comprehensive review of these tetrapod ichnofaunas based on almost 500 track specimens. The fossil footprints are assigned to the ichnogenera Amphisauropus Batrachichnus, Dimetropus, Dromopus, Erpetopus, Hyloidichnus, Ichniotherium, Limnopus, Tambachichnium and Varanopus, and represent temnospondyl, reptiliomorph anamniote, pelycosaurian-grade synapsid, eureptilian, and probable parareptilian trackmakers. Some rare and questionable tracks resemble Robledopus, Notalacerta, and Merifontichnus, and are traces of supposed early non-diapsid eureptilian and therapsid origin.

The early Permian red beds of central New Mexico record the turnover from the Dromopus biochron to the Erpetopus biochron (Voigt and Lucas, 2017). The boundary of these two biochrons almost coincides with the lithostratigraphic boundary between the Abo Formation and the overlying Arroyo de Alamillo Formation, and this boundary is, according to the global Permian tetrapod footprint record, not older than late Artinskian in age. Moreover, the early Permian ichnofauna of the Abo Formation in Socorro County statistically confirms previous hypotheses (e. g., Hunt and Lucas, 2006) that reptiliomorph anamniote tracks (Amphisauropus, Ichniotherium) are more common in inland deposits, and small temnospondyl (Batrachichnus) and pelycosaurian-grade synapsid (Dimetropus) tracks dominate coastal plain deposits.

CoprolitesEast of Socorro, the Tinajas Member of the Atrasado Formation

yields prolific assemblages of coprolites, first discussed by Hunt et al. (2012). In this volume, both Hunt and Lucas (2017) and Harris et al. (2017) present analysis of extensive coprolite assemblages from the Tinajas Member.

Hunt and Lucas (2017) document a new coprolite locality (named the Erickson site) in the Tinajas Member that contains a very large sample of coprolites. Most specimens are found as a weathered lag that eroded from a shale, although a small number of specimens occur in situ in thin limestone beds. Most of the coprolites are spiral in morphology (heteropolar, amphipolar, scroll). Six ichnotaxa are heteropolar spiral in morphology and include a new ichnotaxon, Crassocoprus mcallesteri, Kalocoprus oteroensis, Heteropolacopros texaniensis and Speirocoprus isp. There are numerous specimens of Crassocoprus mcallesteri, and amphipolar coprolites are represented by Hyronocopros amphipola. There are several specimens of the scroll coprolite Bibliocoprus beemanensis, which is identified outside of its type locality for the first time.

Hunt and Lucas (2017) note that there are four bromalite assemblages of Missourian age in New Mexico that represent an ecological transect from lacustrine to basinal marine: (1) Tinajas Lagerstätte – lacustrine; (2) Kinney Brick Quarry Lagerstätte – lagoonal; (3) Erickson site – nearshore marine; and (4) Sacramento Mountains – offshore marine. There are clear trends through these ichnoassemblages (Tinajas-Kinney-Erickson-Sacramentos): (1) flattened preservation in matrix to isolated three dimensional; (2) diverse bromalites to only coprolites; and (3) increasing proportion of spiral coprolites. The Erickson locality is transitional in all these aspects between Tinajas and the Sacramentos. The ichnoassemblage of the Beeman Formation is representative of the shark surplus paradox, in which there is an apparent disproportionate diversity and abundance of spiral (probably chondrichthyan) coprolites relative to the fish fauna (preserved or inferred). The Kinney ichnoassemblage is typical of lagoonal/estuarine ichnofaunas of the Paleozoic and Mesozoic.

Harris et al. (2017) document a rare, laterally extensive nonmarine coprofauna preserved in lacustrine sediments of the Tinajas Member of the Atrasado Formation east of Socorro. Here, abundant coprolites from five recently discovered localities in the Tinajas black shale interval are distinguished from those of the stratigraphically equivalent Tinajas Lagerstätte by their large size and three-dimensional preservation. Together, the Tinajas black shale coprolites represent a remarkable record of morphological and preservational variation within the context of a single paleolake system. Three coprolite taphotypes are recognized: (1) enclosure within a goethite concretionary layer; (2) cementation within a quartz-rich mantle; and (3) non-concretionary hydroxylapatite permineralization. X-ray diffraction analysis (XRD) was used to determine the mineralogical composition of a representative sample of coprolites from each taphotype. Early diagenetic (pre-compaction)

nucleation and mineralization of coprolites at the sediment-water interface was a function of possible microbial activity and prevailing geochemical conditions such as iron and silica activity, pH, and oxygen fugacity of lake bottom water and sediment at the time of deposition. Infilling of barite within the cracks of some coprolites represents either microbial sulfate reduction within the microenvironment of the original fecal material or fault-related secondary mineralization that occurred long after deposition.

Nonmarine ArthropodsIn Socorro County, body fossils of nonmarine arthropods (notably

conchostracans and insects) have been a byproduct of the search for fossil plants in the Pennsylvanian strata east of Socorro. Notable, and reported on earlier in a preliminary way (Martens and Lucas, 2005; Lerner et al., 2009), is the black shale interval of the Tinajas Member of the Atrasado Formation in the Cerros de Amado, which contains a prolific assemblage of conchostracans and much smaller numbers of other arthropods, notably blattoids (cockroaches).

In this volume, Schneider et al. (2017) describe fossil insects from the Bartolo and the Tinajas members of the Atrasado Formation east of Socorro. Particularly interesting is a relatively small collection of fossil insects from lacustrine black shales of the Tinajas Member. Most of them are blattoids, but they include one blattinopsid. The blattoids belong to the genus Sysciophlebia of the family Spiloblattinidae, the genera Phyloblatta and Anthracoblattina of the family Phyloblattidae, as well as to the genera Dictiomylacris, Opsiomylacris and Neorthroblattina of the family Mylacridae. The insect-bearing horizons in the Atrasado Formation are bracketed by marine limestones that contain conodonts, which allow for correlation of the nonmarine-insect-bearing strata to the Standard Global Chronostratigraphic Scale. The occurrence of insect biozone species in the Tinajas Member indicates that the range of the Sysciophlebia rubida-Syscioblatta lawrenceana insect zone is middle Kasimovian to early Gzhelian in age. Based on this, Schneider et al. hypothesize that the gypsum-bearing evaporitic deposits in the Atrasado Formation may partly correspond to a phase of drier climate with widespread development of red beds containing vertic and calcic aridisol horizons in the middle Kasimovian to middle Gzhelian of Europe in eastern Laurasia. It is worth noting that similar indicators of more moisture-limited conditions are present in the Appalachian Basin during the Missourian, at about this same time period, resulting in the appearance of numerous floras dominated by coniferophytes and other xeromorphic taxa (e.g., Bassler, 1916; McComas, 1988; Lyons and Darrah, 1989; Martino, 2017)

What is currently known of the Pennsylvanian record of insects east of Socorro is very limited. No doubt field efforts to specifically collect fossil insects would add substantially to what is now known of the Pennsylvanian insects in the area east of Socorro.

Vertebrate FossilsVertebrate fossils are found throughout much of the upper Paleozoic

section east of Socorro, but are only concentrated and abundant in the Abo Formation. In this volume, Harris and Lucas (2017) describe a large skull of an actinopterygian fish from Pennsylvanian strata east of Socorro. Hodnett and Lucas (2017) review the record of fossil fishes from the Sandia, Atrasado, Bursum and Abo formations east of Socorro. These are mostly of selachians from marine facies (e.g., Zidek and Kietzke, 1993; Lucas and Estep, 2000; Ivanov et al., 2009; Itano and Lucas, 2015; Hodnett and Lucas, 2015). However, both the Bursum and Abo formations contain some nonmarine fish fossils.

According to Hodnett and Lucas (2017), the upper Paleozoic strata of Socorro County preserve a series of four distinct environmental phases and their corresponding fish assemblages from the Middle Pennsylvanian through the early Permian: (1) a Middle Pennsylvanian marine phase, (2) a Late Pennsylvanian short marine regression/coastal lacustrine phase, (3) a latest Pennsylvanian/earliest Permian short marine transgression, and (4) an early Permian freshwater fluvial phase. Many of the late Paleozoic Socorro fish assemblages are dominated by isolated shark teeth. Rigorous field collection of macro specimens and screen washing programs could greatly expand the taxa and data of the Socorro fish assemblages.

Northeast of Socorro, vertebrate body fossils (and coprolites) from the Abo Formation are best known at the Gallina Well locality, stratigraphically low in the Scholle Member (Berman and Reisz, 1986; Berman, 1993; Harris et al., 2005; Spielmann et al., 2009; Cantrell et al., 2011, 2012, 2013; Berman et al., 2015). The collective vertebrate-fossil assemblage from this locality consists of palaeonisciform fishes;

12the temnospondyl amphibians Eryops, Trimerorhachis, Zatrachys and Platyhystrix; captorhinimorphs; the lepospondyl Diplocaulus; and the eupelycosaurs Ophiacodon, Sphenacodon and Dimetrodon. This is a tetrapod-dominated assemblage of the Coyotean land-vertebrate faunachron, which is of latest Virgilian-middle Wolcampian age (Lucas, 2006, 2017).

Marine MicrofossilsMarine microfossils from the Sandia, Gray Mesa, Atrasado and

Bursum formations in Socorro County are primarily cyanobacteria, algae, foraminiferans (including fusulinids) and conodonts. The conodonts have already been documented by Lucas et al. (2009a) and Barrick et al. (2013) for the area east of Socorro. In this volume, Lucas et al. (2017b) document conodonts from Bell Hill, and Krainer et al. (2017a) document them from the Los Pinos Mountains. These conodont records provide some precise correlations of the marine strata, notably matching parts of the Atrasado Formation east of Socorro to mid-continent cyclothems (Fig. 8), but much more remains to be done in terms of sampling and studying Pennsylvanian-Permian conodonts from Socorro County.

In the Los Pinos Mountains and Cerros de Amado region, cyanobacteria occur as encrusting organisms on various skeletal grains, locally forming coated grains and oncoids in many limestones of the Sandia, Gray Mesa and Atrasado formations. Thin microbial mats (stromatolites) are present, and, locally, cyanobacteria and other encrusting organisms form small patches of boundstone in the Tinajas Member (Krainer et al., 2017a, b). Cyanobacteria are also present in many limestones of the Red House, Gray Mesa and Bar B formations of the Little San Pascual Mountains (Lucas et al., 2017b) and in the Bar B Formation at Bell Hill (Lucas et al., 2017a). Cyanobacteria are also present in the Torres Member of the Los Vallos Formation (Yeso Group) east of Socorro, locally forming cm-sized oncoids (Lucas and Krainer, 2017). In the San Andres Formation, cyanobacteria are common, forming stromatolites (Lucas and Krainer 2017).

Calcareous (mostly phylloid) algae are common in many limestone beds of the Gray Mesa and Atrasado formations, in part forming phylloid algal wackestone and floatstone. Other algae are subordinate, such as Eugonophyllum, Archaeolithophyllum missouriense, Anthracoporella (Garcia Member), Epimastopora (Amado Member), Efluegelia (Whiskey Canyon, Garcia members) and Claracrusta (Bartolo Member) (Krainer et al., 2017a, b). In the Little San Pascual Mountains, phylloid algae are common in limestones of the Red House, Gray Mesa and Bar B formations. Archaeolithophyllum missouriense is rarely found in the Red House Formation, and Komia is rare in the Gray Mesa Formation (Lucas et al., 2017b). At Bell Hill, phylloid algae are common in the Gray Mesa and Bar B formations, Archaeolithophyllum missouriense occurs in the Gray Mesa Formation and Komia is rarely present in the Gray Mesa and Bar B formations (Lucas et al., 2017a). Phylloid algae are also present in some limestone beds of the Bursum Formation, but less common than in the underlying Pennsylvanian limestones. In the San Andres Formation, some limestone beds contain abundant recrystallized calcareous algae (algal wackestones) (Lucas and Krainer, 2017).

Smaller benthic foraminifers are present in almost all limestones of the Sandia, Gray Mesa and Atrasado formations in the Los Pinos Mountains and the Cerros de Amado region. They are present in all microfacies, except in peloidal wackestone, packstone, grainstone and mudstone. The diversity is lower in the Sandia Formation than in the Gray Mesa and Atrasado formations (Krainer et al., 2017a, b). The most common smaller foraminifers are species of Eotuberitina, Tuberitina, Earlandia, Endothyra, Planoendothyra, Bradyina, Climacammina, Deckerella, Spireitlina, Tetrataxis, Polytaxis, Globivalvulina, Calcivertella, Ammovertella, Palaeonubecularia, Hemigordiellina, Syzrania, and Syzranella, which are partly documented in Krainer et al. (2017b). Smaller foraminifers are also present in limestones of the Red House, Gray Mesa and Bar B formations of the Little San Pascual Mountains (Lucas et al., 2017b) and at Bell Hill (Lucas et al., 2017a). The diversity of smaller foraminifers in the Bursum Formation is less than in the Pennsylvanian limestones. Smaller benthic foraminifers are rare in the Yeso Group (Torres Member of the Los Vallos Formation). Limestones of the San Andres Formation east of Socorro contain a low diversity association of smaller foraminifers (Lucas and Krainer, 2017).

Fusulinids are present in some limestones of the Sandia Formation (Fusulinella), Gray Mesa Formation (Beedeina, Wedekindellina, Plectofusulina, rare Eoschubertella and Millerella) and Atrasado Formation (various species of Triticites) in the Los Pinos Mountains

FIGURE 8. Correlation of the Atrasado Formation east of Socorro based on conodont biostratigraphy (modified from Barrick et al., 2013).

13and Cerros de Amado (Krainer et al., 2017a, b). Fusulinids are present in some limestones of the Red House, Gray Mesa and Bar B formations of the Little San Pascual Mountains, but were not studied in detail (Lucas et al., 2017a). At Bell Hill, fusulinids occur in the Red House Formation (including Millerella), the Gray Mesa Formation (species of Beedeina and Wedekindellina) and Bar B Formation (various species of Triticites) (documented in Lucas et al., 2017b). Some limestones of the Bursum Formation in the Little San Pascual Mountains contain large Triticites (Lucas et al., 2017a).

Marine MacrofossilsMarine macrofossil faunas of the Sandia, Gray Mesa, Atrasado,

Bursum and San Andres formations fossils are dominated by the characteristic shelly benthos of the late Paleozoic, namely brachiopods, crinoids and bryozoans. Molluscs (mostly bivalves and gastropods) are much less common. Little effort has been made to collect and publish on these fossils, one of the few exceptions being Kues (2002). These fossils have been too little studied in the New Mexico upper Paleozoic section to provide a detailed biostratigraphy. They provide some paleoecological data, but need much more collecting and study.

PROSPECTUSThis volume documents extensive and diverse research on the

Pennsylvanian-Permian rocks and fossils in Socorro County. As detailed as some of the data and analyses presented here are, all of the upper Paleozoic outcrops in Socorro County merit further research, particularly to refine further their biostratigraphy and correlation. The two decades of research reported here thus can be considered a strong start to a much needed detailed study of the stratigraphy, sedimentology and paleontology of the upper Paleozoic rocks and fossils of New Mexico.

ACKNOWLEDGMENTSThe work reported in this paper was assisted by too many people

to name here. Most notable are Scott Elrick and John Nelson for diverse collaboration on geological issues; Dan Chaney for collecting plant fossils; Jim Barrick for conodont biostratigraphy; Daniel Vachard for his work on calcareous microfossils and their biostratigraphy; Sebastian Voigt, Adrian Hunt, Susan Harris and Allen Lerner for their work on trace fossils; and Joerg Schneider for his studies of fossil insects. Staff and volunteers of the New Mexico Museum of Natural History provided diverse field assistance, notably Amanda Cantrell, Sebastian Dalman, Alan Erickson, Larry Rinehart, Justin Spielmann and Tom Suazo.

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