Sacramento Mountains Research Articles

An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California

A 135‐km‐long, NW‐SE trending, seismic refraction/wide‐angle reflection profile provides a unique along‐strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two‐dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (< 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s−1) overlying a fairly homogeneous basement with velocities of 6.0 km s−1; (2) a localized, high‐velocity (6.4 km s−1) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s−1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s−1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a Pn velocity of 8.0±0.10 km s−1. The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s−1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle‐derived magmatism and lateral ductile inflow in the crust are proposed.

Permian trilobites from the San Andres Formation, New Mexico, and their relationship to species from the Kaibab Formation of Arizona

A new genus, Novoameura, is erected with the type species Anisopyge mckeei Cisne. Delaria macclintocki Cisne from the Kaibab Formation of Arizona is herein regarded as a junior synonym of Delaria sevilloidia (Chamberlain), which was originally described from the Phosphoria Formation of Wyoming. The Leonardian (Permian) San Andres Formation of the Sacramento Mountains, central New Mexico, contains trilobite species known both from the Kaibab Formation of Arizona and the Phosphoria Formation of Wyoming, but lacks trilobite species known from the nearby Permian of West Texas.

Aragonitic Pennsylvanian Phylloid Algae from New Mexico: The Missing Link

Remarkably well-preserved codiacean algae (Eugonophyllum and Anchicodium) retaining original aragonite are present in the Virgilian Holder Formation, Sacramento Mountains, south-central New Mexico. The algae are preserved in a 20-cm-thick packstone between two thick (> 5m) shale beds. Aragonite is preserved as a felt-like mesh of needles in the algal skeletons, in the shell fragments of molluscs, in the walls of sponges, and in botryoidal and isopachous marine cements. The aragonite is confirmed by X-ray diffraction, by visual inspection of pristine aragonite needles with SEM, and by a high content of Sr as revealed by microprobe analysis. The average Sr content of the algae (9,091 ppm, n = 21) is comparable to modern codiaceans. Preservation of internal structure in Eugonophyllum was previously unknown. The medullary (interior) region of the Eugonophyllum thallus is composed of an aragonite felt punctuated by small (20 {mu}m diameter), parallel utricles. As in modern codiaceans, the utricles in the cortical (exterior) region of the thallus increase in diameter and their bulbous tips coalesce to form the outer cortex of the plant. This occurrence provides a key piece of evidence in support of hypotheses concerning the nature and origin of phylloid algal bioherms. Because the internal structure ofmore » most fossil phylloid algae is replaced by sparry mosaic calcite, taxonomic classification has been difficult even at the fundamental level of division (phylum). The authors discovery confirms that at least some ancient phylloid algae resembled the modern green algae Halimeda or Udotea, and lends credibility to the suggestion that ancient phylloid algal mounds are analogous to modern Halimeda mounds of the South Pacific.« less

A reinterpretation of the timing, position, and significance of part of the Sacramento Mountains detachment fault, southeastern California

A contact previously considered to be part of the Sacramento Mountains detachment fault (SDF), exposed in the Sacramento Mountains metamorphic core complex, is reinterpreted as an unconformity between Tertiary rhyolite of Eagle Peak and cataclastically deformed crystalline lower-plate rocks. This reinterpretation is based on outcrop-scale topographic relief and the absence of deformation along the base of the rhyolite, even where underlying rocks are severely deformed and altered. The rhyolite is dated at about 14.3 Ma. Lower-plate rocks of the SDF comprise gneiss intruded by granodiorite, tonalite, and leucogranite. Mylonitic fabrics are variably developed in lower-plate rocks and are cut by brittle shear zones that contain ultracataclasite, and by high- and low-angle faults. Upper-plate rocks include gneiss; nonmylonitic granodiorite, tonalite, and granite; and middle Miocene sedimentary and volcanic rocks. On the south side of Eagle Peak, a thick ultracataclasite zone dips about 30° west, contains subhorizontal hematite-stained slickenlines, and is interpreted to be part of the SDF system. This ultracataclasite zone is unconformably overlain by more gently dipping, undeformed, 14.3 Ma rhyolite of Eagle Peak. The unconformity records unroofing of a domed SDF after a period of 8 to 9 m.y. of progressive unloading of the lower plate in an evolving extensional shear zone. Extension on part of the SDF was accommodated by a system of high-angle, oblique-slip faults.

The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of 40Ar/39Ar thermochronology to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The 40Ar/39Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The 40Ar/39Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1°–5°C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman‐Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500°C which reset the K–Ar systems of minerals from Proterozoic rocks. High‐grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman‐Piute batholith at 73±1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were ∼100°C/m.y. between 73 and 70 Ma and 5°–10°C/m.y. from 70 to ∼30 Ma. Between 65 and 25 Ma the entire eastern Mojave Desert underwent a period of cooling at a rate of 2°–10°C/m.y. By 30 Ma, rocks exposed in the Old Woman‐Piute, Marble, Ship, Clipper, and Turtle Mountains were below ∼100°C. The 40Ar/39Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23±1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at ∼180°C and ∼350°C, respectively. This suggests that the exposed part of the Chemehuevi detachment fault initiated at a dip of 5°–30° or as a series of higher‐angle faults that cut to a depth of 10–12 km and were later rotated to their present dip. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10°–50°C/m.y. between 22 and 15 Ma.

Late Pennsylvanian Storm-Dominated Shelf Sand Ridges, Sacramento Mountains, New Mexico

ABSTRACT Unlike previous researchers who concentrated on carbonate strata in the Sacramento Mountains, New Mexico, we conducted facies analysis on sandstone units within the Upper Pennsylvanian Holder Formation (Virgilian). Detailed work on several exposures in Laborcita Canyon revealed that the sandstone bodies have flat bases and upward-coarsening grain size sequences and are composed of three main subfacies. The sandstone subfacies in ascending vertical order are 1) thin-bedded, fine-grained sandstone with parallel-laminae and hummocky stratification with graded tempestite sequences; 2) fine-grained sandstone similar to subfacies 1, but occurring within medium to thick-bedded, large-scale foresets up to 5 m high; and 3) medium-grained sandstone with multidirectional cross-stratification, ipples and abundant marine trace fossils. The sandstones were deposited in a storm-dominated, inner shelf environment, and the preserved depositional topography, lenticularity, distribution of sedimentary structures and paleocurrent relationships suggest that the Laborcita Canyon sandstones represent sand ridges. The sand ridges migrated and accreted seaward during storms that generated large waves (estimated wave heights = 1.25-6 m) and downwelling pressure gradient currents, which combined to transport sand efficiently offshore. Storm-flow paleocurrent indicators trend roughly normal to the regional paleoshoreline, suggesting that the ridges were deposited in a friction-dominated or transitional shelf dynamic zone rather than in a fully-developed geostrophic zone. Sand ridges on the U.S. Atlantic inner shelf are modern proc ss analogues to the Laborcita Canyon sandstones because of the similarities in scale, internal bounding surfaces, grain-size sequences and storm processes that control deposition.

Late Quaternary Fossils of Poa fendleriana (Muttongrass): Holocene Expansions of Apomicts

Late Quaternary Fossils of Poa fendleriana (Muttongrass): Holocene Expansions of Apomicts

Paleosols of Late Pennsylvanian cyclic strata, New Mexico*

ABSTRACTCyclically deposited siliciclastic and carbonate rocks of the Late Pennsylvanian (Virgilian, Holder Formation) of south‐central New Mexico were deposited on a narrow shelf between the Pedernal uplift and the Orogrande basin. In the Dry Canyon area, Sacramento Mountains, shelf‐to‐basin palaeotopography is indicated by onlapping beds and primary dips. Calcareous alteration zones are developed on the tops of major carbonate units. These detailed studies confirm earlier interpretations that alteration zones represent subaerial exposure events and provide new evidence for and further documentation of a paleosol environment. The paleosols contain: (1) rhizoliths; (2) tangential needle fibres of calcite; (3) five types of alveolar texture; (4) upward oriented ‘ribbon spar’; (5) brownish and clear isopachous spar; (6) blackened grains; (7) random needle fibres of calcite; (8) coated grains; (9) micritized grains; (10) glaebules; (11) desiccation cracks; (12) possible Microcodium; and (13) well‐developed laminated crusts. Isopachous cements, upward‐oriented cements (which mark the former tops of capillary fringes), dark colour and shallow depth of penetration of rhizoliths (less than 1 m) indicate significant paleosol formation in waterlogged (hydromorphic) and drier conditions. Pennsylvanian palaeohydrogeology of the system is reflected in the type of paleosol alteration.Paleosols developed on subtidal strata and paleosols which drape tens of metres of palaeotopography indicate cyclic sedimentation was not controlled by delta lobe switching. Local tectonic up and down movement would be an unlikely mechanism to explain the repeated tens of metres of relative sea‐level fluctuation. Eustatic sea‐level fluctuations, induced by Late Pennsylvanian Gondwana glaciation, probably were of the correct magnitude and frequency to have caused the Holder cycles.

Cement Stratigraphy of Pennsylvanian Holder Formation, Sacramento Mountains, New Mexico

Cyclic strata of the Holder Formation (Virgilian, New Mexico) were deposited across a Pennsylvanian shelf-to-basin transition during a time when sea level fluctuated over tens of meters. Cement-stratigraphy studies indicate abundant calcite precipitation from low-temperature fresh water within shelf, shelf-crest, and shelf-edge marine limestones. Freshwater cementation occurred during 15 periods of intraformational subaerial exposure. These early cements are least abundant in basin and lagoon limestones. The distribution of the early cement zones suggests that cementation was controlled by paleotopography, stratigraphic position below subaerial exposure surfaces, lithology, and configuration of the paleoaquifer system. Distribution of early calcite cements provides new data for interpretation of cycles, diagenetic systems, and porosity evolution in petroleum reservoirs. Trace-element analyses support a low-temperature, freshwater origin for the early cements. Cement-stratigraphy studies, fluid-inclusion analyses, and trace-element analyses indicate a later cement that occluded remaining limestone porosity precipitated from a sodium- and calcium-rich brine at a temperature of about 100°C. Stratigraphic reconstruction dates this cementation as Cretaceous or later. Fluorescent, oil-filled fluid inclusions were trapped along fractures in the late cement, indicating oil migration during or after late-stage cementation.

A Spectrum of Late Paleozoic and Cretaceous Shelf Bars, Western United States: ABSTRACT

Shelf-bar sandstones of the Cretaceous Western interior seaway of North America form excellent stratigraphic traps for hydrocarbons. Modern shelf sands occur in a variety of settings and are affected by such processes as storms and tidal, longshore, and shelf currents. Case studies of several Cretaceous and late Paleozoic sandstones document a spectrum of ancient shelf-bar types analogous with modern shelf systems. Several shelf bars deposited in nearshore environments (8-10 mi offshore) were associated with deltaic axes. Sands were reworked from these axes by currents moving along the shoreline that were diverted by a headland. Such shelf-plume sandstones were subsequently reworked by storms and other shelf processes. The relative effects of storm and shelf or coastal currents greatly influenced the geometry and characteristics of the nearshore shelf bars, of which several variations can be documented. Tidal currents are an important factor in forming and modifying modern shelf bars in the North Sea. Examination of primary structures and directional features suggests tidal currents were relatively unimportant in nearshore shelf bars of the Cretaceous Interior seaway. Examples of Pennsylvanian shelf-bar sandstones from the Sacramento Mountains of New Mexico have features characteristic of both storm and tidal processes. Detailed examinations of primary sedimentary structures and facies relationships are useful in recognizing the various shelf-bar types and predicting the occurrence and distribution of similar potential reservoirs. End_of_Article - Last_Page 1442------------

Paleotopography and Substrate Lithology as Controls on Initiation of Waulsortian Reef Growth: Examples from Sacramento Mountains, New Mexico: ABSTRACT

Frameless mud mounds, usually called Waulsortian reefs, are common in Osagean and Kinderhookian rocks of North America and western Europe. Spectacular Waulsortian reefs crop out in the Sacramento Mountains of New Mexico along 10 mi of continuous exposure, where detailed petrographic studies and field measurements of the strata between the top of the Devonian and the base of the reefs reveal: (1) down-to-the-southwest paleoslope on an uneven, gently dipping ramp; (2) widespread deposition of skeletal packstones and siliciclastics to the northeast; (3) patchy, local thicks of skeletal packstones surrounded by shaly wackestones to the southwest; (4) relict highs on the Devonian surface beneath the skeletal packstone pods; (5) clusters of sheetlike reefs weakly associated with paleotopography in the northern outcrops; and (6) large, dome-like individual reefs strongly associated with depositional topography and skeletal packstone/grainstone substrates in the southern outcrops. The pre-reef strata do not exhibit abrupt changes in thickness or lithology to indicate a break in regional slope, and the reefs are not aligned with patterns in thickness of facies that would distinguish shelf-edge environments from lagoonal environments. Like their European counterparts, the Osagean reefs in the Sacramento Mountains grew on a ramp where the nonreef facies were grainy updipmore » and muddy downdip, and reef anatomy varied from sheetlike updip to dome-like downdip. The association between paleotopography, substrate lithology, and the initiation of Waulsortian reef growth provides new information about regional depositional patterns in the Early Mississippian.« less

New Taxa in Oenothera (Onagraceae)

A new species, three new sections, and one new subsection of Oenothera are described and a new combination in 0. flava is made here so that the names will be available. Oenothera coryi (sect. Megapterium), which ranges from north-central Texas to the southern part of the panhandle, is described. The combination 0. flava subsp. taraxacoides (sect. Lavauxia) is made for large-flowered, modally outcrossing populations of 0. flava from three disjunct areas: 1) Mogollon Plateau, Arizona to Catron Co., New Mexico; 2) Sacramento Mountains and Sierra Blanca in southeastern New Mexico; and 3) Sierra Madre Occidental from Chihuahua to Durango, Mexico. As part of a genus-wide reevaluation of the infrageneric classification, three new sections, sect. Ravenia, sect. Eremia, and sect. Contortae, are proposed for species formerly included in subg. Pachylophus, which is no longer recognized, and subsect. Australis is described for the two white-flowered South American species of sect. Lavauxia.

Abo Formation Alluvial Facies and Associated Basin Fill, Sacramento Mountains, New Mexico: ABSTRACT

Outcrops of the Abo Formation (Wolfcampian to early Leonardian age) in the Sacramento Mountains of south-central New Mexico record the evolution of a dry alluvial fan system as it was deposited off the pedernal uplift into the Orogrande basin. The location and orientation of present-day outcrops allow us to observe an inferred east-to-west transverse facies tract consisting of: (1) proximal alluvial fans (lower Abo), which are contiguous in places with underlying Laborcita Formation fan-deltaic sediments; (2) medial anastomosed streams (middle Abo); and (3) distal low-gradient mud-dominated flood basins characterized by either distributary streams (upper Abo) or clastic tidal flats (Lee Ranch Tongue of the Abo) with associated marine carbonates (Pendejo Tongue of the Hueco Formation). Tectonism in the Pedernal highlands, which climaxed during the Late Pennsylvanian, apparently continued well into the Wolfcampian in this region, as evidenced by a major basal Abo unconformity and distinct stacked megasequences of lower Abo alluvial fan lithofacies. However, by the middle Abo, tectonic activity had quiesced and the uplift began eroding and retreating to the north and east. By the late Abo, a pediment surface had formed that was subsequently onlapped by upper Abo and eventually Yeso Formation sediments.

Paleomagnetic Constraints on Timing of Reddening in Ferruginous Draping Beds, Sacramento Mountains, New Mexico: ABSTRACT

Paleomagnetic Constraints on Timing of Reddening in Ferruginous Draping Beds, Sacramento Mountains, New Mexico: ABSTRACT

Depositional Environments of Laborcita Formation (Wolfcampian), Northern Sacramento Mountains, New Mexico: ABSTRACT

Depositional environments that change dramatically over short lateral distances are represented by exposures of the Laborcita Formation. A siliclastic source area lay to the east and southeast, in the Pedernal Mountains. To the west, a shallow marine sea filled the orogrande basin. Alternating cycles of marine and nonmarine sedimentation resulted from fan-delta lobe shifting and eustatic sea level movements. In clear-water areas not affected by fan-deltaic sedimentation, deposits become increasingly calcareous. Various carbonate facies resulted from organisms adapting to changing environmental conditions. Mud-cracked algal mats, digitate algal stromatolites, and small phylloid red algal mounds and rhodoliths indicate deposition in shallow-water subtidal to supratidal settings. Large buildups (20 m thick) of phylloid green algae associated with abundant submarine cement occurred in a position near the edge of the narrow shelf. Widespread skeletal detritus beds overlie and extend hundreds of meters away from the massive buildups. Influx of terrigenous mud and silt in advance of a prograding fan-delta system terminated growth of the buildups. The next transgression is represented by a carbonate grainstone exhibiting characteristics of shallow-water marine, storm-dominated shelf bars. The shelf bars migrated in a northwest-southeast direction.

Influence of Canopy Characteristics of One-Seed Juniper on Understory Grasses

Studies were conducted in the Sacramento Mountains, New Mexico, to determine the intluence of juniper (Juniperus monosperma [Engelm.] Sarg.) canopy on understory vegetation. The basal area of grass species was estimated at 6 locations beneath the canopies of 50 one-seed junipers. Other parameters measured were litter depth, canopy height, canopy cover, canopy closure, tree height, trunk diameter, north-south crown diameter, and east-west crown diameter. Locations adjacent to the trunk had the greatest juniper canopy cover and litter depths, and the lowest height to canopy. Locations at the end of the canopy had the least crown cover and litter depths, and the greatest height to canopy. All but one of the grass species had greater basal areas at the edge locations and the least at the interior locations beneath juniper canopies. Pinyon ricegrass(Piptochuetiumfimbriatum [H.B.K.] Hitch.) was the exception; it was never found at the exterior locations. Regression models indicated that shading influenced the basal areas of most grass species. Litter depth was negatively correlated with grass basal cover in only 4 models and positively correlated in 1. Basal area of pinyon ricegrass was positively correlated with trunk diameter, a reflection of tree age, indicating that the grass requires time to become established. Also, basal area of pinyon ricegrass was positively correlated with canopy cover, indicating that this species requires the modified microenvironment afforded by shading.

Early Diagenesis of a Phylloid Algal-Mound Complex, Laborcita Formation, Southeastern New Mexico: ABSTRACT

Marine carbonate cementation was the initial stage in the paragenesis of phylloid algal mounds in the Laborcita Formation (Wolfcampian), Sacramento Mountains, New Mexico, and the cements are almost identical to those in Holocene coral reefs of Belize. These cements include relicts of botryoids and crusts of needle crystals, in part defined by inclusion patterns and luminescent ghosts in mosaic calcite. Individual needle crystals are pseudohexagonal in cross section and range from less than 1 to 30 µm wide. These nonluminescent early cements line cavity walls, coat phylloid-algal blades and stromatolites, and are interlayered with marine sediment. Early cements also include bladed, fibrous, and rare radiaxial fibrous calcites, which are microdolomite-rich. They have a proximal nonluminescent zone, a central bright-luminescent zone, and a distal blotchy, moderate-luminescent zone. The bright zone may be time equivalent to bright-luminescent micritic coatings on botryoids and grains. Botryoids are encrusted by isopachous bladed cement, some of which has prismatic overgrowths containing an early inclusion-rich zone. This initial cementation was followed closely by: (1) dissolution of algal blades and mollusks, (2) in-situ brecciation, and (3) cementation by blocky calcite. Botryoidal and acicular cements are interpreted as originally marine aragonite precipitates, based on morphology, occurrence, susceptibility to diagenesis, and similarity to Holocene reef cements. The same criteria, plus the microdolomite inclusions, indicate that the bladed, fibrous, and radiaxial cements had a marine Mg-calcite precursor. This assemblage followed by prismatic overgrowths, dissolution, and blocky-calcite cementation indicates an evolution from marine to freshwater diagenesis. This early diagenesis was followed by, in order: (1) cementation by multiple stages of blocky calcite (interrupted by episodic fracturing), (2) local dolomitization, (3) stylolitization, (4) dedolomitization and blocky-calcite cementation, and (5) minor chertification. End_of_Article - Last_Page 315------------

Depositional Trends In Upper Paleozoic Terrigenous Clastic Rocks, Sacramento Mountains, New Mexico

Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis­ play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm­ dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.

Ecofacies Transect of Lake Valley Shelf, San Andres Mountains, New Mexico, and Its Relation to Early Mississippian Orogrande Basin: ABSTRACT

South-central New Mexico has been well known for its excellent exposures of Mississippian strata. The Sacramento Mountains have become famous for their fortuitous exposures of the Lake Valley shelf margin. A transect of measured sections there exposes the transition from shallow carbonate shelf with beautifully developed reefs to a starved basin with isolated pinnacle reefs to the south. A new transect, to the west in the San Andres Mountains, reveals a complete shelf-to-basin transect, from shoreline to shelf-margin to starved basin. There, however, no reefs are present in time-equivalent strata. Studies there, integrating physical stratigraphy, depositional systems, microfacies analysis, and conodont biostratigraphy and biofacies, allow formulation of a leeward shelf ec facies depositional model for the Lake Valley formation. This is combined with the Sacramento Mountains transect to delineate a model of an Early Mississippian Orogrande basin in the same position as the Pennsylvanian-Permian Orogrande basin and the present Tularosa basin. A series of such basins may have existed along the southern flank of the ancient Transcontinental arch in the southwestern United States. End_of_Article - Last_Page 143------------

Biogeographic implications of a packrat midden sequence from the Sacramento Mountains, south-central New Mexico

Thirteen packrat ( Neotoma spp.) and two porcupine ( Erethizon dorsatum) middens from 1555 to 1690 m elevation from the Sacramento Mountains, New Mexico, provide an 18,000-yr vegetation record in the northern Chiuahuan Desert. The vegetation sequence is a mesic, Wisconsin fullglacial (18,000–16,000 yr B.P.) pinyon-juniper-oak woodland; a xeric, early Holocene (ca. 11,000–8000 yr B.P.) juniper-oak woodland; a middle Holocene (ca. 8000-4000 yr B.P.) desert-grassland; and a late Holocene (ca. 4000 yr B.P. to present) Chihuahuan desertscrub. The frequency of spring freezes and summer droughts in the late Wisconsin probably set the northern limits of Pinus edulis and Juniperus monosperma at about 34°N, or 6° south of today's limit. Rising summer tempratures in the early Holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope. At this time pinyon-juniper woodland and pine forest dominated by Pinus ponderosa probably began their spectacular Holocene expansions to the north. Continued warming in the middle Holocene led to very warm summers with strong monsoons, relatively dry, cold winters, and widespread desert-grasslands. Desertscrub communities in the northern Chihuahuan Desert did not develop until the late Holocene when the biseasonal rainfall shifted slightly back toward the winter, catastrophic winter freezes decreased, and droughts in all seasons increased. The creosote bush desertscrub corridor across the Continental Divide between the Chihuahuan and Sonoran deserts was probably connected for the first time since the last interglaciation.