Pedregosa Basin Research Articles

Mixed eolian–longshore sediment transport in the late Paleozoic Arizona shelf and Pedregosa basin, U.S.A.: A case study in grain-size analysis of detrital-zircon datasets

ABSTRACT Detrital-zircon U–Pb geochronology has revolutionized sediment provenance studies over the last two decades, and zircon has been successfully analyzed from nearly all sedimentary lithologies, depositional environments, and sediment grain sizes. However, despite the ubiquity of this method and the far-reaching interpretations supported by detrital-zircon data, few studies have investigated the potential role of zircon grain size on age spectra and provenance interpretation. In this study, we investigate the connections between sample grain size, zircon grain size, U–Pb age spectra, and interpreted provenance using 18 detrital-zircon samples (4999 individual grains) collected from Pennsylvanian–Permian strata in central and southern Arizona, USA. In these samples, there is no clear correlation between sample grain size and zircon grain size and no clear correlation between sample grain size and age spectra. However, when all grains are grouped by zircon minimum long-axis dimension, the abundance of some age groups is correlated to zircon grain size. In Pennsylvanian samples, < 400 Ma grains and 2500–3000 Ma zircons are more abundant in the finer fractions, and 1400–1900 Ma zircons are more abundant in coarser fractions of both Pennsylvanian and Permian samples. In Permian samples, 500–800 Ma zircons are most abundant in the finer fractions, and 2500–3000 Ma grains are concentrated in the coarser fractions. Based on changes in abundance and grain-size distribution of 500–800 Ma grains, we interpret a change in zircon provenance across the Pennsylvanian–Permian boundary that reflects regional climate and paleogeographic changes driven in part by the northward drift of Laurentia across the equator. Specifically, we interpret the concentration of 500–800 Ma zircons in Permian samples in central and southern Arizona to indicate that these grains were: 1) sourced from Gondwana, 2) deposited in, and subsequently eroded (recycled) from, Mississippian–Pennsylvanian strata in the Arkoma, Anadarko, and Fort Worth basins at the margins of Laurentia, and 3) finally transported into the Arizona study area as loess by easterly trade winds. This study serves as a case study in the value and interpretive power of basic grain-size characterization of detrital-geochronology datasets.

Provenance of Pennsylvanian–Permian sedimentary rocks associated with the Ancestral Rocky Mountains orogeny in southwestern Laurentia: Implications for continental-scale Laurentian sediment transport systems

AbstractThe Ancestral Rocky Mountains system consists of a series of basement-cored uplifts and associated sedimentary basins that formed in southwestern Laurentia during Early Pennsylvanian–middle Permian time. This system was originally recognized by aprons of coarse, arkosic sandstone and conglomerate within the Paradox, Eagle, and Denver Basins, which surround the Front Range and Uncompahgre basement uplifts. However, substantial portions of Ancestral Rocky Mountain–adjacent basins are filled with carbonate or fine-grained quartzose material that is distinct from proximal arkosic rocks, and detrital zircon data from basins adjacent to the Ancestral Rocky Mountains have been interpreted to indicate that a substantial proportion of their clastic sediment was sourced from the Appalachian and/or Arctic orogenic belts and transported over long distances across Laurentia into Ancestral Rocky Mountain basins. In this study, we present new U-Pb detrital zircon data from 72 samples from strata within the Denver Basin, Eagle Basin, Paradox Basin, northern Arizona shelf, Pedregosa Basin, and Keeler–Lone Pine Basin spanning ∼50 m.y. and compare these to published data from 241 samples from across Laurentia. Traditional visual comparison and inverse modeling methods map sediment transport pathways within the Ancestral Rocky Mountains system and indicate that proximal basins were filled with detritus eroded from nearby basement uplifts, whereas distal portions of these basins were filled with a mix of local sediment and sediment derived from marginal Laurentian sources including the Arctic Ellesmerian orogen and possibly the northern Appalachian orogen. This sediment was transported to southwestern Laurentia via a ca. 2,000-km-long longshore and aeolian system analogous to the modern Namibian coast. Deformation of the Ancestral Rocky Mountains slowed in Permian time, reducing basinal accommodation and allowing marginal clastic sources to overwhelm the system.

Palaeoclimatic inferences from upper Palaeozoic siltstone of the Earp Formation and equivalents, Arizona‐New Mexico (USA)

AbstractThe Late Palaeozoic configuration of Pangaea contributed to a palaeoclimatic extreme that was characterized by both icehouse and monsoonal conditions. This study uses sedimentological, geochemical, and provenance data from silty facies of the Earp and equivalent Supai Formations (Arizona, New Mexico) to shed light on atmospheric circulation and glacial–interglacial climate change in westernmost equatorial Pangaea. Five silt‐rich facies comprise both loessite and marine and fluvially reworked loessite. An initial aeolian origin for the silt is indicated by the remarkably invariant grain size and the laterally continuous, sheet‐like geometry of beds. The silt‐rich facies occur in repetitive facies associations (1–20 m scale) that form mixed continental‐marine (loess, marine‐reworked loess), shallow‐marine, and continental (loess, palaeosol) ‘sequences’. Facies repetitions of both mixed continental‐marine and shallow‐marine sequences reflect a linked glacioeustatic–glacioclimatic control, whereas the continental (loess–palaeosol) couplets reflect a primary glacial–interglacial climatic cyclicity linked to glacioeustasy. Stratigraphic interpretations suggest that aeolian silt flux maximized during glacial to incipient interglacial stages (lowstand to early transgression), and decreased significantly or ceased during interglacials (highstand to early falling stage). Detrital‐zircon geochronological data indicate a transition from dominantly north‐easterly winds during the Middle Pennsylvanian to north‐westerly and south‐easterly winds by the Early Permian, which trend is inferred to reflect the onset of monsoonal circulation in western Pangaea. Relative grain‐size data support the detrital‐zircon data, and exhibit a significant decrease from the Sedona arch/Central Arizona shelf (north) to the Pedregosa basin (south) sections. Whole‐rock geochemical data suggest a relatively unweathered source for the silt in the north, and detrital‐zircon data indicate significant silt was derived from the local basement. These large piles of silt(stone) preserve valuable information for reconstructing both long‐term evolution in atmospheric circulation and short‐term fluctuations in glacial–interglacial climate. Many such indicators for long have been applied to ‘recent’ (Plio‐Pleistocene) loess, but are equally applicable to ‘deep‐time’ strata.

Discovery of Latest Devonian/Earliest Mississippian microfossils in San Salvador Patlanoaya (Puebla, Mexico); biogeographic and geodynamic consequences

Abstract Carbonate microfossils Petschoria (?), Kettnerammina, Kamaena, and Tolypammina (?) allow to biostratigraphically date the base of the sedimentary cover in San Salvador Patlanoaya (Puebla State, Mexico). They are indicative of the former Tournaisian, i.e. the Latest Devonian (Strunian) and the Earliest Mississippian Kinderhookian (= Hastarian). Deposits of this period are very poorly known in Mexico, and appear to be confined to a rift affecting only Patlanoaya, some areas of the Sonora and Chihuahua states (Mexico) and the Pedregosa Basin (New Mexico, USA). This rift is connected with the Rheic Ocean that opens, whereas the Antler Ocean is probably entirely closed in this period, as confirmed by the microfossil migration ways. To cite this article: D. Vachard, A. Flores de Dios, C. R. Geoscience 334 (2002) 1095–1101.

Walther's law, climate change, and upper Paleozoic cyclostratigraphy in the ancestral Rocky Mountains

Geologists routinely apply Walther9s Law in interpreting paleoenvironments and paleogeographic scenarios from stratigraphic successions. The application of Walther9s Law, however, may be problematic in strata that have been influenced by significant and geologically rapid climate change. Such climatic fluctuation can force depositional environments to change fundamentally rather than migrate laterally, resulting in a stratigraphic succession wherein depositional facies vertically superposed rarely or never coexisted laterally. Upper Pennsylvanian carbonate-siliciclastic cycles deposited in basins of the southern Ancestral Rocky Mountains display stratigraphic relations that cannot be explained by autogenic and/or glacioeustatic mechanisms alone; rather, intracyclic (glacial-interglacial) climate change was a significant influence. Cycles of the northern Pedregosa basin, for example, comprise eolian-marine siltstone in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing suggests a climatic control on silt influx. Cycles of the western Orogrande basin comprise deltaic siliciclastic strata in sharp contact with subtidal-peritidal carbonate facies; absence of facies mixing, truncated siliciclastic progradation, and carbonate facies with emergence features similarly support a climatic influence on the siliciclastic component. In both systems, intracyclic (glacial-interglacial) climatic shift reconfigured depositional environments. In cases such as these, application of Walther9s Law could produce erroneous conclusions. Severe glacial-interglacial climate change helps explain the pervasive association of mixed carbonate-siliciclastic cyclicity that typifies upper Paleozoic strata in many parts of the world.

Relationship of Laramide and Basin and Range Structures in the Pedregosa Basin, Southwestern New Mexico: ABSTRACT

The Pedregosa basin of southwestern New Mexico has long been recognized as a frontier hydrocarbon exploration target. The Paleozoic sedimentary rocks contain petroleum source and reservoir units that correlate with important producing zones in the Permian basin of west Texas and southeastern New Mexico. Factors that are commonly considered to limit the petroleum potential of the region include multiple episodes of deformation, heating due to local igneous intrusion and volcanism, and fresh-water flushing. Laramide (late Cretaceous-early Tertiary) fold-and-thrust traps are structural targets. Subsequent fracturing by Basin and Range (late Tertiary) normal faulting may have partially destroyed the Laramide traps. Seismic reflection and gravity data from the Playas and Hatchita valleys in southwestern New Mexico constrain the structural styles of Laramide shortening and later Basin and Range extensional deformation. These data show that Precambrian basement was involved in Laramide thrusting. This observation refutes the interpretation of some previous workers who interpreted the Precambrian basement surface as a regional decollement for Laramide thrusting. An east-west seismic profile that crosses the southeast end of the Little Hatchet Mountains images a range-bounding listric normal fault and associated antithetic faults beneath the Hatchita Valley fill. Near the range, [open quotes]antithetic[close quotes] faults show a normalmore » sense of motion, but farther east, faults are reverse. The latter may be high-angle Laramide structures or unusual features of Basin and Range deformation. The major range-bounding fault is steeply dipping Near the surface, but becomes a low-angle listric fault at a depth of about 10 km.« less

Preservation and paleoclimatic significance of eolian dust in the Ancestral Rocky Mountains province

Upper Paleozoic (Pennsylvanian-Permian) silty strata of the northern Pedregosa basin (southern Ancestral Rocky Mountains province) exhibit distinctive features that suggest origin of the silt as eolian dust prior to deposition in marine environments. Studies of mechanisms controlling modern eolian dust accumulation imply that combined marginal-marine-epeiric-sea environments constitute an ideal sink for eolian dust. Accordingly, Pedregosa siltstones may reflect the prevalent mode of eolian dust preservation downwind from coeval ergs, and significant volumes of eolian dust may be preserved within analogous epeiric strata. Because eolian dust reflects conditions of aridity and windiness, it bears on paleoclimatic and paleogeographic reconstructions. Recent studies within several basins of the Ancestral Rocky Mountains suggest an eolian derivation for many siltstones and thus corroborate inferences of increasing aridity and concomitant monsoonal conditions for western Pangea. Further regional analysis may provide valuable insights for refining concepts of the zonal to monsoonal transition that accompanied Pangean assembly.

Interbasinal cyclostratigraphic correlation of Milankovitch band transgressive-regressive cycles: Correlation of Desmoinesian-Missourian strata between southeastern Arizona and the midcontinent of North America

The cyclic pattern of deposition in the Horquilla Limestone of southeastern Arizona resulted from glacio-eustatic sea-level fluctuations with periodicities within the Milankovitch band. The depositional surface fluctuated from above sea level to below the depth of autochthonous carbonate production. By means of characteristics of the cycles, correlations can be established not only between sections in the Pedregosa basin of southeastern Arizona, but also to the midcontinent of the United States. These correlations exceed the resolution of biostratigraphy and distinguish the eustatic component of relative sea-level fluctuations in widely spaced basins with different depositional settings.

Sedimentology and tectonic significance of Wolfcampian (Lower Permian) conglomerates in the Pedregosa basin: Southeastern Arizona, southwestern New Mexico, and northern Mexico

A single interval of red chert-clast conglomerate and associated strata (RCC/CRCC interval) occurs within the Earp Formation (Pennsylvanian-Lower Permian) at many localities in southeastern Arizona, southwestern New Mexico, and northern Mexico; this interval records a middle Wolfcampian erosional event in the northern Pedregosa basin and adjacent shelf. The RCC and CRCC intervals are probably proximal and distal braided-stream deposits, respectively, consisting of conglomeratic longitudinal and transverse bar deposits; fluvial-channel sandstone; and interfluvial fine-grained sandstone, siltstone, and shale, and they are interstratified within a much thicker sequence of shallow-marine deposits. The RCC/CRCC interval is uniformly thin (average 5 m thick), yet it is characterized by sheet-like lateral continuity, indicating that deposition occurred on an extensive, low-lying surface on which fluvial channel migration and channel switching were important processes. Paleoflow directions were generally southeastward. Biostratigraphic evidence suggests that middle and lower Wolfcampian strata below the RCC/CRCC interval were beveled northward. Much clastic material of the RCC/CRCC interval may be composed of reworked residuum of the beveled strata, with additional contributions possibly from regions north and northwest of the Pedregosa shelf. Middle Wolfcampian emergence of the northern Pedregosa basin and shelf and accompanying deposition of the RCC/CRCC interval were probably caused by tectonics. Other less important factors, such as eustatic sea-level fluctuations, may have amplified the regression. Evidence favoring a tectonic cause of regression are synchronous timing of tectonism and synorogenic sedimentation throughout a wide area in the Ouachita foreland. The RCC/CRCC interval was probably deposited on a forebulge peripheral to the Pedregosa foreland basin. Stratigraphic evidence indicates that the southern Pedregosa basin evolved rapidly from a slowly subsiding platform to a deep foreland basin during early or middle Wolfcampian time. Subsidence of the Pedregosa foreland basin was presumably caused by downflexure under northward-verging thrust sheets in the Chihuahua region during the Ouachita orogeny. Downflexure of the Pedregosa foreland basin evidently caused flexural forebulging and subaerial exposure of large areas cratonward. Flexural models predicting deflection of lithosphere under isostatic thrust and sediment loads agree satisfactorily with the forebulge concept for the origin of the RCC/CRCC interval.

Foraminiferal characterization of the Morrowan-Atokan (lower Middle Pennsylvanian) boundary

Designation of a lower Atokan (lower Middle Pennsylvanian) boundary stratotype is necessary because the traditional type Atokan Series is inadequate as a chronostratigraphic standard of reference. Foraminiferally, the boundary between the Atokan and the underlying Morrowan Series (Lower Pennsylvanian) is best defined by the appearances of the primitive fusulinaceans Eoschubertella and Pseudostaffella . Recrystallized staffellids ( Staffella and Nankinella ) appear at or near this level at some localities. The appearance of the fusiform fusulinid Profusulinella is considerably higher, and its range defines an informal middle Atokan interval. Upper Atokan rocks are characterized by the range of Fusulinella below the appearance of Beedeina and/or Wedekindellina . Foraminiferal successions across the Morrowan-Atokan boundary have been documented at many localities in the south-central and western United States and in the Arctic. The merits of these sites as potential boundary stratotypes are evaluated herein. The Arctic is unsuitable as a reference area for the Atokan Series because taxon ranges there are seemingly atypical of ranges elsewhere in North America. Of the remaining sites where successions have been reported, those in southern New Mexico (type Derryan area), southeastern Arizona (Pedregosa Basin), southern Nevada (Las Vegas area), and east-central Idaho (Lost River and Lemhi Ranges) exhibit the best foraminiferal faunas in apparently continuously deposited strata.

Paleozoic Paleotectonics and Sedimentation in Southern Rocky Mountain Region: ABSTRACT

During the Paleozoic, the southern Rocky Mountain region included most of New Mexico and Arizona and at least the northern parts of adjacent Chihuahua and Sonora. It was particularly stable part of the North American craton during the Cambrian through Middle Devonian. Slow End_Page 863------------------------------ deposition of shelf clastics and dolomitic carbonates was interrupted by several long erosional hiatuses. Major recognizable tectonism first appeared in the Devonian with at least one depositional basin formed west of the Defiance-Zuni uplift. Thin Early Mississippian shelf carbonates and evaporites covered nearly the entire region. The most significant tectonic activities started in the late Chesterian and extended with increasing magnitude until the end of Wolfcampian time. Local basins and uplifts date from this interval and occurred in two belts. One belt was about 80 mi (130 km) wide along the western sides of the Hueco and Pedernal uplifts and along both sides of the Uncompahgre uplift. Another belt extended northwest from the Pedregosa basin into southeastern Arizona. Major tectonic events initiated the Morrowan, Atokan, and Missourian Epochs and occurred twice within the Wolfcampian Epoch. Leonardian, Guadalupian, and Ochoan Epochs were times of tectonic stability. During the Leonardian, sediments from the Uncompaghre uplift gradually covered all the other uplifts. The timing of these Paleozoic tectonic events suggests a cause-effect relationship with plate-tectonic histories that brought North American and northern Europe together in the Late Devonian (Acadian orogeny) and Euramerica and northwestern Gondwana together in the Late Mississippian through Early Permian (Appalachian orogeny). End_of_Article - Last_Page 864------------

Initiation and Reactivation of Proterozoic Aulacogen, Northern Mexico: ABSTRACT

Geochemical and petrologic affinities of late Proterozoic (~ 1Ga) bimodal igneous rocks of the Franklin Mountains, west Texas, suggest a rift origin. Scattered occurrences of similar rocks southward into the state of Chihuahua, Mexico, indicate a southerly trend for the feature. The feature is bounded by stable blocks: the stable craton of west Texas to the east and northeast, and the Sierra del Nido block to the west and southwest. Separation of the Sierra del Nido block from the craton occurred about 1 Ga. Gravity gradients mark the boundaries of the blocks, and a northwest-trending Bouger gravity high may mark the axis of the aulacogen. The aulacogen and the Sierra del Nido block are truncated to the south by the Mesozoic Mojave-Sonora discontinuity. The aulacogen was reactivated, at least in part, in the late Paleozoic as the Pedregosa basin and in the Mesozoic as the Chihuahua trough. These reactivations were apparently not full-fledged rifting events, but did result in basin development. The Sierra del Nido block was a paleographic high throughout the Paleozoic, and the Aldama platform developed on this block during the Cretaceous. The most recent reactivation of the aulacogen is as the southern extension of the Rio Grande rift, as evidenced by trends of high heat flow, recent mafic magmatism, and regional extensional faulting. End_of_Article - Last_Page 258------------

Conodont-Based Thermal Maturation of Paleozoic Rocks in Arizona

Color changes in conodonts are established thermal indices that have been related to the thermal window for hydrocarbon generation and preservation and that also have value for targeting mineralization potential. Conodont color-alteration values from 141 localities in Arizona appear to characterize the thermal maturation of the major Paleozoic outcrop areas in the state. Paleozoic rocks in the Colorado Plateau, in part of the belt of faulted plateau-like rocks bordering the plateau, and in a small area (from 31°20^prime to 31°44^primeN and from 110°00^prime to 110°40^primeW) in southeast Arizona have thermal potential for oil. Paleozoic rocks in the same areas as well as in the Pedregosa basin (southeasternmost Arizona) have thermal potential for gas. f these areas, southeast Arizona appears to have the best hydrocarbon potential because of the association of possible source beds and favorable thermal-maturation levels.

Overview of the Central North American Basins and Their Relation to Deep Crustal Structure: ABSTRACT

As our knowledge of deep structure of major central North American basins has increased, it has become clear that they have experienced long and complicated tectonic histories. A knowledge of these histories is especially important to efforts to formulate exploration strategies for deeper horizons and frontier areas. Regional geophysical and geologic studies of these basins indicate that Precambrian features have often exerted considerable control on basinal development (e.g., Anadarko basin, Rome trough, Rough Creek graben, Pedregosa basin). A particularly important tectonic event was the Eocambrian continental breakup which extensively rifted the southern margin of North America. Although this rifting event is manifested in various ways, its extent can be estimated by m pping the deep-seated crustal anomalies which probably formed at this time. Although age relations are uncertain in most cases, deep-seated anomalies are associated with the Arkoma basin, Anadarko basin, Illinois basin, Mississippi embayment, and Permian basin. There are many similarities in the development of these basins, but they all can be shown to have unique tectonic histories. End_of_Article - Last_Page 495------------

Oil and Gas Exploration and Development in Arizona: ABSTRACT

Recent oil and gas exploration activity has been widespread throughout Arizona. Development drilling has continued in the Dineh-bi-keyah and Teec-nos-Pos fields in the northeastern corner, and exploratory drilling continues to test potential Paleozoic reservoirs elsewhere on the plateau. Several shallow wells north of the Grand Canyon encountered shows and limited recoveries of oil from Permian and Triassic rocks. The greatest activity has occurred along the trend from northwestern to southeastern Arizona. Several million acres were leased and eight exploratory wells drilled along this trend. None were discoveries, but the presence of a Laramide thrust fault in the vicinity of Tombstone was established. The other tests have neither proved nor disproved the concept of the Overthrust belt in southern Arizona. Recent discoveries in the nonmarine Tertiary and marine Paleozoic of southern Nevada have stimulated interest in the oil potential of similar rocks and structures in the Basin and Range province of Arizona, which are coincident with the Overthrust trend. Reported gas discoveries by Pemex in Miocene marine sediments of the Gulf of California have stimulated leasing in the Yuma area, where one uncompleted well is reported to be a potential producer. The Pedregosa basin of extreme southeastern Arizona remains an area of great interest to explorationists because of the presence of a 25,000-ft (7,600-m) sequence of Paleozoic marine sediments similar to those of the Permian basin, and Cretaceous marine rocks, including coral-rudist reefs, similar to those that produce in Texas and Mexico. End_of_Article - Last_Page 944------------

Petroleum Exploration in Pedregosa Basin, Southwestern New Mexico: ABSTRACT

In southern Hidalgo and Grant Counties, New Mexico, the northwestern end of the Pedregosa basin has a high petroleum potential. Paleozoic rocks, dominantly shallow-marine carbonates, are over 11,000 ft (3,350 m) thick. Of the 11 formations, ranging in age from Cambrian to Permian, 7 contain favorable oil- or gas-source units. Mesozoic rocks, generally shallow-marine limestones and deltaic sandstones-mudstones, are nearly 10,000 ft (3,050 m) thick. Two of the three Lower Cretaceous formations contain favorable source units. Gas-prone kerogens are more abundant than the oil-prone types. At normal depths of burial, organic matter in the source units has reached thermal maturity, and some in the older formations is overmature. In the lower plate of a major Laramide thrust fau t, the Lower Cretaceous units are overmature and the older Paleozoic units may be thermally metamorphosed. Best reservoir objectives are the porous dolostones, totaling 484 ft (148 m) in thickness, in the upper part of the Horquilla Formation (Pennsylvanian-Permian). They are located at the shelf margin of a deep-marine basin. Other favorable reservoir units are indicated in surface exposures. In this frontier area, only 11 exploration wells have been drilled to Precambrian, Paleozoic, or Mesozoic rocks. Shows of oil or gas were reported in 6 of the wells. None of the wells have tested the better reservoir objectives in the deeper parts of the graben valleys, where commercial accumulations of petroleum are likely to be preserved. Tertiary-intrusive and volcanic-cauldron complexes have thermally metamorphosed older sedimentary units only locally. Basin and Range faulting has disrupted subsurface fluid systems in many parts of the area. Despite the challenging risks, the high potential encourages further exploratory drilling on selected petroleum prospects. End_of_Article - Last_Page 248------------

Oil and Gas Exploration Wells in Pedregosa Basin: ABSTRACT

Oil and Gas Exploration Wells in Pedregosa Basin: ABSTRACT

Dolostone Reservoirs in Horquilla Formation (Pennsylvanian-Permian), Big Hatchet Peak Section, Hidalgo County, New Mexico: ABSTRACT

Dolostone reservoirs with net thickness of 484 ft (148 m) in upper Horquilla, are exposed in the Big Hatchet Peak section of southwestern New Mexico and constitute the best petroleum objective demonstrated to date in the Pedregosa basin. This shallow-marine carbonate section includes uppermost Paradise Formation (Chesterian) and 3,230 ft (985 m) of Horquilla (Morrowan-Wolfcampian). The lower member of the Horquilla consists of limestone and chert; a disconformity at the top formed during the Desmoinesian. The upper member is at least 1,867 ft (569 m) thick; limestones contain phylloid-algal biostromes and ancient solution cavities; alternating laterally extensive dolostones are 54 to 148 ft (16.5 to 45 m) thick and exhibit much vuggy porosity. Petrographic evidence demonstrates that the limestones were stabilized within freshwater diagenetic environments, that they contained much primary or secondary porosity, and that practically all of it was occluded by marine or freshwater cements. Dolostones also contained much secondary intercrystalline and moldic porosity that was partly occluded by epitaxial cements and coarse recrystallization of initial neomorphic rhombs. Anhydrite porphyroblasts were emplaced by hypersaline waters and were dissolved later by fresh ground waters to form molds with distinctive stairstep outlines; this tertiary (third stage) porosity in dolostones was partly filled by gravitational cements, and some microstalactite tips were dolomitized paramorphically. Dark Horquilla limestones are rated as fair petroleum-source units. Land-derived kerogens indicate sources of gas; alteration indices of 3 to 3+ indicate moderate thermal history. Dolostones in the upper member are rated as fair to good reservoir units. Matrix porosity and permeability are poor but should improve farther south along the shelf edge and on the slope into Alamo Hueco basin. End_of_Article - Last_Page 571------------

Oil and Gas Potential of Southern Arizona: ABSTRACT

End_Page 1015------------------------------Two highly prospective areas in southern Arizona are the Pedregosa basin in Cochise County and central Yuma and Maricopa Counties. The best source rocks were deposited during the Paleozoic before the inception of thrust faulting. They include dark mudstones and dark-gray cherty limestones. In addition, the Jurassic evaporitic environment may have contained prolific growth of phytoplankton which was preserved and matured into a rich source rock. Good reservoir rocks are found in Paleozoic marine sediments (primarily Ordovician, Silurian, and Permian) and in Cretaceous marine sediments. Early Cretaceous reefs may be especially good reservoirs. Two major types of tectonism provide models for structural traps: (1) overthrust faulting, and (2) basin-and-range block faulting. Sediments resting on a glide plane of Jurassic evaporites may have been thrust over reservoir rocks. This trap would be analogous to that of the Pineview field in northern Utah with a carbonate source rock, sand reservoir, and a salt seal. The primary exploration tool is reflection seismology. Basin-and-range faulting would create the fracture systems necessary for petroleum genesis through hydrothermal convection, hydrocarbon migration, and the upward travel of Jurassic salt. Salt diapirism may have caused additional fracturing in flanking reservoir rocks, thus increasing porosity and permeability. Reconnaissance prospecting for basin-and-range traps shou d include gravity surveys to locate salt and infrared air photo surveys to detect heat flow. End_of_Article - Last_Page 1016------------

Tertiary Volcanic Centers as Constraints on Oil and Gas Potential of Basin and Range Province: New Mexico Segment of Pedregosa Basin: ABSTRACT

Stratigraphic similarities with the Permian basin and structural similarities with the Overthrust belt make the New Mexico segment of the Pedregosa basin an attractive target for petroleum exploration. Middle Tertiary volcanic cover and postvolcanic faults introduce constraints, as elsewhere in the Basin and Range province. Geologic mapping in southwesternmost New Mexico (Hidalgo County) has traced Oligocene ash-flow tuff sheets to at least nine major cauldrons, including Valles-type resurgent cauldrons 10 to 40 km wide (Apache Hills: Apache cauldron; southern Pyramid Mountains: Muir cauldron; southern Animas Mountains: Tullous, Juniper, Animas Peak, Cowboy Rim, San Luis cauldrons; southern Peloncillo Mountains: Rodeo and Geronimo Trail cauldrons). Faulted-off cauldron segments continue beneath the Playas, Animas-San Luis, and San Simon-San Bernadino Valleys. Composite batholiths probably underlie cauldron clusters, as in the San Juan and Mogollon-Datil volcanic fields. Exposed monzonitic plutons (Apache Hills, Pyramid Mountains, Animas Mountains) are apophyses, emplaced during resurgence. Other plutons, apparently unrelated to cauldrons, are present in the northern Pyramid and Little Hatchet Mountains. Distribution of cauldrons and plutons suggests that much of the Pyramid, Peloncillo, Little Hatchet, and Animas Mountains, Apache Hills, and adjacent valleys are unfavorable for petroleum accumulations. However, destructive thermal effects may be confined to aureoles extending only about 1,000 m from intrusive contacts, as in the KCM 1 Forest Federal wildcat. Cauldrons and thermal effects deleterious to petroleum potential are unknown in so theastern Hidalgo County (Dog, Alamo Hueco, Big Hatchet Mountains, southern Sierra Rica). End_of_Article - Last_Page 759------------