No description available.

No description available.

The primary factors that influence stratigraphy are tectonic subsidence, eustasy, and sediment supply. Change in any of these factors potentially produces a similar response in the form of a change in accommodation space. Accordingly, distinguishing the origin of a stratigraphic response is difficult, but theoretically possible by analysis of temporal and spatial extents of the accommodation signal. Correlation is critical for distinguishing eustasy from tectonism. Upper Pennsylvanian strata of the Pedregosa and Orogrande basins (southern Ancestral Rocky Mountains) were deposited during a time of continental collision and extensive continental glaciation, and contain a composite record of changing tectonism, eustasy, climate, and sediment supply. High-frequency stratigraphic cyclicity expressed as repetitive stacks of lithofacies at the scale of 10¹ m pervades all sections and displays features that collectively imply a primary glacioeustatic origin, notably: (1) abrupt juxtaposition of dissimilar lithofacies, signaling a rapid rate of baselevel change, (2) apparent intrabasinal, interbasinal and, provisionally, interregional correlation of high-frequency cycles across and between contrasting tectonic environments, and (3) cycle frequencies that approach the 413 ka periodicity of orbital eccentricity, the probable forcing mechanism for Pennsylvanian glaciations. Glacial-interglacial climate change expressed as precipitation and circulation fluctuations in the equatorial Pedregosa and Orogrande basins accompanied Pennsylvanian glacioeustasy. Intensified aridity and wind strength during peak glacials led to decreased fluvio-deltaic sedimentation and increased eolian activity where siliciclastics were available. Conversely, increased precipitation during interglacials reactivated and/or intensified fluvio-deltaic sediment yield. Eustasy dictated fluvial aggradation versus degradation and coastal sediment trapping versus bypassing. Coupled glacioeustatic-glacioclimatic change was sufficiently severe to reconfigure environments between climatic extremes, which implies that Pennsylvanian stratigraphic cycles should be viewed in at least partially non-Waltherian terms. Each cycle potentially recorded contrasting facies mosaics that were to some degree temporally exclusive. Multiple-cycle trends in facies and/or thickness also occur to define low-frequency stratigraphic patterns at the scale of 10² m. Qualitative analysis of these trends implicates distinct eustatic and tectonic processes as contributing influences. The eustatic component may derive from low-frequency glacioeustasy as well as tectonoeustasy related to evolving continental paleogeography. The tectonic component probably reflects late Paleozoic Marathon-Ouachita collisional orogenesis.

The Chama area straddles the physiographic and structural boundary between the San Juan Basin and the San Juan Mountains. The Mesozoic formations, the Upper Jurassic Entrada and Morrison and the Upper Cretaceous Dakota and Mancos, are separated by a marked angular unconformity from the Oligocene (?) Blanco Basin Formation and the upper Miocene Conejos Quartz Latite and Treasure Mountain Rhyolite. Late Cenozoic eastward tilting and high-angle normal faulting have affected all the bedded rocks. The present topography has been profoundly modified by extensive mass movements and Pleistocene glaciation during the Cerro, Durango, and Wisconsin stages with related outwash terrace deposits. / text

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No description available.

A strip in Taos County, New Mexico, eight miles long and three miles wide along New Mexico Highway 3 from U.S. Hill to Talpa, Fort Burgwin Ridge, has outcrops of Precambrian metaquartzite (1800 my bp) and cataclastic granite (1760 my bp) overlain by Mississippian limestone and Pennsylvanian fan delta sediments of the western Taos Trough. Twelve cross sections are used to demonstrate (1) Pennsylvanian syndepositional vertical movements, (2) Laramide thrusting that folded the Mississippian and Pennsylvanian rocks along splay faults to the east of the Pecos-Picuris Fault (see Fig. 40), and (3) Late Miocene to present-day normal and strike slip faulting, which is related to Rio Grande Graben rifting and appears to have reactivated earlier fault zones. The north-trending structures appear to be subsidiary to the Pecos-Picuris Fault, a major geofracture that parallels the map area to the west. / text

The middle sandstone member of the Upper Triassic Chinle Formation in northeastern New Mexico consists of ephemeral-stream deposits adjacent to the Ancestral Rocky Mountain uplift and perennial braided-stream deposits on the plains to the east. The two fluvial systems were contemporaneous with the ephemeral rivers existing as transverse tributaries to a perennial trunk system flowing from north to south. Proximal ephemeral-stream deposits are characterized by 60-80 m thick sequences of coarse-grained sediments deposited principally by overbank sheetflooding processes. Facies associations are dominated by horizontally-stratified, medium-grained sandstone with subordinate sets of tabular-tangential and trough cross-stratified medium-grained sandstone. Streamflooding events are indicated by basal conglomeratic channelized deposits and single-story massive and trough cross-stratified channel sandstones. Uppermost portions of proximal ephemeral-stream sequences contain 0. 5 to 1. 0 m-thick units of trough cross-laminated, fine-grained sandstone and siltstone. Distal ephemeral-stream deposits are distinguished by their lenticular geometries, thicknesses of 20 to 30 m and sedimentary structures indicative of lower flow-regime streamflooding processes. Channelized conglomerates and trough cross-stratified and parallel-stratified sands~ones are capped by tabular and wedge cross-stratified, trough cross-laminated and subordinate horizontally-stratified sandstones. Facies associations in the deposits of the trunk system are comparable to those of the Platte and South Saskatchewan braided rivers (Miall, 1977). Sets and cosets of tabular-tangential or trough cross-stratified medium-grained sandstone are separated by variable thicknesses of horizontally-stratified medium-grained sandstone; these dominate 10 to 30 m-thick, sheet-like sandstones which are traceable laterally for tens of kilometers. The sheet-like sandstones are separated by equivalent or lesser thicknesses of red mudstones. Effects of the allocyclic controls of tectonics and climate are recognizable in sedimentary architectures of individual members or of the whole formation. In contrast, the effects of autocyclic controls can be recognized when internal geometries of individual members are focussed upon. The concept of base level is helpful in realizing the time dependency of controls on sedimentation because allocyclic controls determine the position of base level and base level determines which autocyclic controls operate. / M.S.

No description available.

No description available.