Showing papers in "Geological Society of America Bulletin in 1987"

The closing of Tethys and the tectonics of the Himalaya

Abstract: Recent geological and geophysical data from southern Tibet allow refinement of models for the closing of southern (Neo-) Tethys and formation of the Himalaya. Shelf sediments of the Indian passive continental margin which pass northward into deep-sea Tethyan sediments of the Indus-Tsangpo suture zone were deposited in the Late Cretaceous. An Andean-type margin with a 2,500-km-long Trans-Himalayan (Kohistan-Ladakh-Gangdese) granitoid batholith formed parallel to the southern margin of the Lhasa block, together with extensive andesites, rhyolites, and ignimbrites (Lingzizong Formation). The southern part of the Lhasa block was uplifted, deformed, and eroded between the Cenomanian and the Eocene. In the western Himalaya, the Kohistan island arc became accreted to the northern plate at this time. The northern part of the Lhasa block was affected by Jurassic metamorphism and plutonism associated with the mid-Jurassic closure of the Bangong-Nujiang suture zone to the north. The timing of collision between the two continental plates (ca. 50-40 Ma) marking the closing of Tethys is shown by (1) the change from marine (flysch-like) to continental (molasse-like) sedimentation in the Indus-Tsangpo suture zone, (2) the end of Gangdese I-type granitoid injection, (3) Eocene S-type anatectic granites and migmatites in the Lhasa block, and (4) the start of compressional tectonics in the Tibetan-Tethys and Indus-Tsangpo suture zone (south-facing folds, south-directed thrusts). After the Eocene closure of Tethys, deformation spread southward across the Tibetan-Tethys zone to the High Himalaya. Deep crustal thrusting, Barrovian metamorphism, migmatization, and generation of Oligocene-Miocene leucogranites were accompanied by south-verging recumbent nappes inverting metamorphic isograds and by south-directed intracontinental shear zones associated with the Main Central thrust. Continued convergence in the late Tertiary resulted in large-scale north-directed backthrusting along the Indus-Tsangpo suture zone. More than 500 km shortening is recorded in the foreland thrust zones of the Indian plate, south of the suture, and > 150 km shortening is recorded across the Indian shelf (Zanskar Range) and the Indus suture in Ladakh. There was also large-scale shortening of the Karakoram and Tibetan microplates north of the suture; as much as 1,000 km shortening occurred in Tibet. The more recent deformation, however, involved the spreading of this thickened crust and the lateral motion of the Tibetan block along major approximately east-west–trending strike-slip fault zones.

Late Paleozoic glacial episodes in Gondwanaland reflected in transgressive-regressive depositional sequences in Euramerica

Abstract: The Late Paleozoic glaciation of Gondwanaland comprised two short episodes, in the Famennian (I) and Visean (II) confined to Brazil and adjacent northwest Africa, and a long episode that started in the Namurian (IIIA) of eastern Australia and Bolivia/Argentina, expanded to cover much of Gondwanaland in the Stephanian/Asselian (IIIB), and collapsed in the early Sakmarian (IIIC). Dropstones in eastern Australia indicate that small ice centers lingered to the Kazanian. Across the belt of low latitudes north of Gondwanaland, short-ranging fossils in widespread shallow-marine and paralic deposits indicate synchronous deposition of transgressive-regressive sequences in different parts of Euramerica. These sequences correlate with glacial events in Gondwanaland at three levels: (a) four major regressions in Euramerica, in the Famennian (1), Visean (2), Namurian (3), Stephanian (4), and the Tastubian transgression that preceded the Sterlitamakian regression (5), also recorded in Gondwanaland, correlate with glacial episodes I, II, and IIIA, IIIB, and IIIC; (b) the time-interval of cyclothemic deposition in Euramerica (Brigantian or latest Visean to Sterlitamakian) correlates with that of glacial episode III; and (c) the dominant period of the Euramerican cyclothems, as estimated from the Middle and Late Pennsylvanian deposits of the mid-continent of North America, and of the thickest known Gondwanaland glacigenic sediment (the earliest Permian Lyons Group of Western Australia) is 0.4 Ma, equivalent in turn to the long orbital-eccentricity period of the Quaternary ice age, and the dominant period of fluctuation of the late Miocene Antarctic ice cap. The three levels of correlations confirm Wanless and Shepard9s (1936) hypothesis that the Late Paleozoic cyclothems are controlled largely by sea-level fluctuations related to the Gondwanaland glaciation.

Methane-derived authigenic carbonates formed by subduction-induced pore-water expulsion along the Oregon/Washington margin

Abstract: Authigenic magnesian calcite, dolomite, and aragonite are precipitated in the uppermost terrigenous sediments of the Washington/Oregon accretionary prism by subduction-induced dewatering. These distinctive carbonates are methane-derived and occur at sites of concentrated pore-water expulsion. Unique biologic communities that subsist, at least indirectly, on methane (Suess et al., 1985) are also found at some of these sites. The methane, which is dominantly biogenic, is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement. Carbonate precipitation occurs below the oxic layer, probably no deeper than several centimetres to a few metres below the seabed. Cementation may be induced by three factors: (1) increased carbonate alkalinity resulting from microbial sulfate reduction, (2) decreased σCO 2 solubility resulting from a pressure decrease when the pore water escapes the prism, and/or (3) the addition of Ca 2+ and Mg 2+ ions from sea water near the sediment/water interface. The convergent margin setting engenders precipitation of authigenic carbonates in several ways. Compressive stresses induce anomalously rapid compaction and dewatering rates, and they may cause overpressuring in migrating pore water, thereby delaying precipitation of carbonates until pressure is released near the sediment-water interface. Structural deformation of the accretionary prism creates pathways (such as fault zones), secondary fracture porosity, and dipping permeable layers (often exposed by mass movement) for efficient advection and expulsion of methane-enriched pore water. These characteristic conditions, which lead to the precipitation of methane-derived carbonates, may be found at other convergent margins.

Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping

Abstract: Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.

Fan-deltas and braid deltas: Varieties of coarse-grained deltas

Abstract: Two types of coarse-grained deltas are recognized: fan-deltas and braid deltas. Fan-deltas are gravel-rich deltas formed where an alluvial fan is deposited directly into a standing body of water from an adjacent highland. They occupy a space between the highland (usually a fault-bounded margin) and the standing body of water. In contrast, braid deltas (here introduced) are gravel-rich deltas that form where a braided fluvial system progrades into a standing body of water. Braid deltas have no necessary relationship with alluvial fans, as exemplified by fluvioglacial braid deltas. Braid deltas have previously been classified as fan-deltas even though the geomorphic and sedimentologic settings of the two systems can be vastly different. Braid deltas are a common present-day geomorphic feature and are abundant in the geological record. Fan-deltas and braid deltas can be distinguished in the rock record by distinctive subaerial components of these depositional systems; the shoreline and subaqueous components of both are similar. Fan-delta sequences have a subaerial component that is an alluvial-fan facies comprising interbedded sheetflood, debris-flow, and braided-channel deposits. Fan-deltas produce small (a few tens of square kilometres), wedge-shaped bodies of sediment, commonly displaying high variability in paleocurrent patterns and abrupt changes in facies. The deposits are generally very coarse grained (with large out-sized clasts), very poorly sorted, matrix-rich, polymictic, heterolithic, partially cemented by penecontemporaneous carbonate, and have low porosity and permeability. Braid-deltas, in contrast, have a subaerial component consisting entirely of braided-river or braidplain facies. Their deposits display better sorting, roundness, and clast orientation than do fan-delta sediments; they lack a muddy matrix; they display size grading and bar migration; they commonly have a sheet geometry with high lateral continuity (tens to hundreds of square kilometres); and they exhibit moderate to high porosity and permeability. Valuable paleogeographic and tectonic information concerning the proximity of highlands and major fault zones may be misinterpreted or lost if these two coarse-grained deltaic systems are not differentiated.

Field, chemical, and physical constraints on mafic-felsic magma interaction in the Lamarck Granodiorite, Sierra Nevada, California

Abstract: Evidence of magma interactions resulting in both hybridization and mingling are preserved in a diverse suite of gabbroic to intermediate rocks associated with the compositionally zoned hornblende-biotite Lamarck Granodiorite of the eastern Sierra Nevada, California. Ellipsoidal mafic enclaves were formed by quenching of small amounts of high-alumina basaltic magma upon injection into and dispersal through granodiorite magma early in its crystallization. Synplutonic intrusions of hornblende gabbro through hybridized mafic granodiorite represent injection of mafic magma at a later stage of crystallization of the granodiorite, as they crosscut regional trends in foliation and compositional zoning in the host pluton. Where compositional contrasts between intrusion and host granodiorite are large, contacts are sharp and abundant enclaves derived from the mafic intrusion are present in the granodiorite. Where the host is relatively mafic or where the local-scale proportion of mafic magma is large, contacts are zones of extensive hybridization that contain both enclaves and hybrid schlieren. Uncontaminated mafic intrusions have high-alumina basaltic compositions, whereas hybridized intrusions have silica contents as high as 63.5%. Mafic intrusions locally contain coarse-grained cumulus gabbro inclusions. Mafic schlieren in granodiorite far from mafic intrusions represent localized accumulations of hornblende, Fe-Ti oxides, and biotite from the granodiorite. Intrusion of late mafic dikes mobilized and entrained granitic residue from the granodiorite and formed composite dikes of aplite and pillowed diorite. Whether interacting magmas mix or mingle is a function of the heat contents and mass fractions of the end members. Calculations that account for compositions, heats of fusion, heat capacities of liquids and crystals, and a range of initial temperatures, crystallinities, crystal sizes, and magma water contents indicate that in most circumstances the basalt end member quenches; the resulting large viscosity contrast between the end members prevents hybridization. Homogenization is likely only if the compositional difference between host and injected mafic magma is less than 10% SiO2 or if the mass fraction of mafic magma is greater than 0.5. Resulting mixtures have the composition of tonalite or mafic granodiorite; thus, the more silicic rocks of the granodiorite pluton must represent differentiation products rather than direct hybrids of mafic or intermediate magma and felsic magma.

Conodont color and textural alteration: an index to regional metamorphism, contact metamorphism, and hydrothermal alteration.

Abstract: Experimental and field data are used to extend the utility of conodonts as semi-quantitative thermal indices into the regimes of regional and contact metamorphism, as well as hydrothermal alteration. An Arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color-alteration indices (CAI) above 300 °C, that is, for CAI values of 5½ through 8. Such CAI values occur in very low- to medium-grade, regionally metamorphosed, contact-metamorphosed, and hydrothermally altered rocks. The uniformity or variability of CAI values within a sample, together with conodont texture, can help to distinguish grades and environments of metamorphism, particularly in metacarbonate sequences. Induced CAI by pyrolysis in a water-methane mixture at ½ kbar results in retardation of CAI and in a disparate mixture of both low and high CAI values within each experimental sample. In this system, color-alteration processes, above a CAI of 2 to 3, seem to change from predominantly carbonization to predominantly loss of organic matter, presumably by oxidation and volatilization of oxides. These experiments approximate the type of CAI mixture characteristically found in conodonts recovered from hydrothermally altered rocks. These data indicate that CAI values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization.

Rainfall, ground-water flow, and seasonal movement at Minor Creek landslide, northwestern California: Physical interpretation of empirical relations

Abstract: Simple ground-water flow analyses can clarify complex empirical relations between rainfall and landslide motion. Here we present detailed data on rainfall, ground-water flow, and repetitive seasonal motion that occurred from 1982 to 1985 at Minor Creek landslide in northwestern California, and we interpret these data in the context of physically based theories. We find that landslide motion is closely regulated by the direction and magnitude of near-surface hydraulic gradients and by waves of pore pressure caused by intermittent rainfall. Diffusive propagation of pore-pressure waves accompanies downward ground-water flow along nearly vertical hydraulic gradients that exist in most of the landslide. Field data combined with a pore-pressure diffusion analysis show that single rainstorms typically produce short-period waves that attenuate before reaching the landslide base. In contrast, seasonal rainfall cycles produce long-period waves that modify basal pore pressures, but only after time lags that range from weeks to months. Such tune lags can depend on antecedent moisture storage and can explain variable delays between the onset of the wet season and seasonal landslide motion. Limit-equilibrium analysis shows that when seasonal pressure waves reach the landslide base, they establish a critical distribution of effective stress that delicately triggers landslide motion. The critical effective-stress balance is extremely sensitive to the direction and magnitude of hydraulic gradients. Although pervasively downward gradients instigate seasonal motion, we infer from theory and limited data that ground water also may circulate locally in near-surface cells. The circulation can further reduce the landslide's frictional strength, particularly in areas of nearly horizontal ground-water flow that occur beneath steep faces of hummocks. Hummocky topography that results from slope instability may therefore cause ground-water flow that perpetuates instability.

Sedimentologic and geomorphic variations in storm-generated alluvial fans, Howgill Fells, northwest England

Abstract: In June 1982, a storm with a return period greater than 100 yr, but lasting less than 2.5 hr, destabilized hillslopes and produced a suite of geomorphologically and sedimentologically diverse alluvial fans. Thirteen major fans were deposited at the tributary junctions between small ( 2 ) catchments and two north-flowing; headwater streams of the River Lune, northwest England. Storm-generated fans spread over or became inset into older stable fans and produced localized vertical accretion of as much as 3 m and lateral accretion of as much as 100 m. Sedimentary processes operating during deposition involved debris flow, transitional flow, and streamflow. Six facies types are recognized on the basis of depositional topography, sedimentary texture and fabric, and matrix content: viscous debris flow (D1), dilute debris flow (D2), transitional flow (T1), fluvial bars and lobes (S1, S2), and fluvial sheet gravels (S3). Regionally, streamflow deposition prevails over debris-flow deposition, and type S3 facies has the greatest areal extent. Temporal and spatial variations in facies deposition during the storm, however, resulted from water:sediment ratio variations. Fan deposition involved an early phase of debris flow to transitional flow due to large inputs of sediment from hillslope failures. This was followed by a systematic change to more dilute conditions, resulting in streamflow deposition and eventually in channel incision. A significant amount of geomorphic work and complex variations in sedimentary processes during the storm resulted, in part, from extensive overland flow and hillslope destabilization. Discriminant analyses indicate that catchment size, channel gradient, and percentage of area eroded during the storm controlled whether debris-flow or streamflow facies dominated a fan sequence. Smaller, steeper catchments had a greater percentage of the area yielding sediment and are dominanted by debris flows, whereas larger catchments produced more runoff resulting in dilution and streamflow. The results indicate that the facies sequences and fan entrenchment in the Howgill Fells, which are typically considered products of longer term climatic change or tectonics in other localities, are here primarily affected by thresholds related to catchment geomorphology, by the type of sediment available, and by the position within the storm cell.

Surface morphology of columnar joints and its significance to mechanics and direction of joint growth

Abstract: Columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes. New observations show that each band contains a single plumose structure and thus represents an individual crack, or joint segment, formed during a discrete growth event. Analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing, and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies. Columnar joints form by nucleation and growth of new cracks on the edges of older cracks. Each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face, where thermal stress is concentrated. Inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle-ductile transition of lava; outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava. Cracks often diverge slightly from the planes of previous cracks, probably because of spatial and temporal changes in directions of local principal stresses. Mechanical interaction causes a diverging crack to overlap, curve toward, and usually intersect the previous crack behind its edge, leaving a blind tip that points in the overall growth direction of the columnar joints. This and other directional criteria are applied to determine joint-growth patterns in several lava flows of the western United States. In two-tiered and multi-tiered flows, downward-growing columnar joints usually meet upward-growing joints well below the middle of the flows, which indicates very rapid cooling of upper portions relative to lower portions. This supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows, whereas conduction is probably the dominant cooling mechanism at the bases.

Overview of the COCORP 40°N Transect, western United States: The fabric of an orogenic belt

Abstract: The COCORP 40°N Transect of the Cordillera of the western United States crosses tectonic features ranging in age from Proterozoic to Recent and provides an acoustic cross-section of a complex orogen affected by extension, compression, magmatism, and terrane accretion. The key features of the transect, centered on the Basin and Range Province, include (1) asymmetric seismic fabrics in the Basin and Range, including west-dipping reflections in the eastern part of the province and predominantly subhorizontal ones in the west; (2) a pronounced reflection Moho at 30 ± 2 km and locally as deep as 34 km in the Basin and Range with no clear sub-Mono reflections; and (3) complex-dipping reflections and diffractions locally as deep as 48 km in the Colorado Plateau and Sierra Nevada. The eastern part of the transect, shot above known and inferred Precambrian crystalline basement, probably records features related to the entire history of the orogen, locally perhaps as old as 1800 Ma. In this region, major paleotectonic features probably controlled subsequent structural development. In title western half of the transect, however, most reflectors are probably no older than Mesozoic. Within the Basin and Range Province, there appears to be a strong Cenozoic overprint that is characterized by asymmetric half-grabens, low-angle normal faults, and a pervasive subhorizontal system of reflections in the lower crust; no one model of intracontinental extension is universally applicable. Processes that produce or are accompanied by thermal anomalies (magmatism, enhanced ductility, and extension) appear to be essential in developing a highly layered lower crust.

The Mortagne granite pluton (France) emplaced by pull-apart along a shear zone: Structural and gravimetric arguments and regional implication

Abstract: The emplacement mechanism and kinematics and the 3-D geometry of the Variscan granite pluton of Mortagne are interpreted, using systematic measurements of the granite structure allied with micro-structural observations, from a detailed gravimetric study. It is proposed that magma infilled a pull-apart structure which formed along a jog of a northwest-southeast–trending sinistral zone of shear. A second episode of shearing, under a dextral regime and independent of emplacement, was responsible for perigranitic tectonites. These successive transcurrent opposite movements, already documented for Variscan times in western France, are discussed concerning the age of the Mortagne pluton (313 ± 15 m.y.).

Lithologic mapping in arid regions with Landsat thematic mapper data: Meatiq dome, Egypt

Abstract: Digital Landsat thematic mapper (TM) data were evaluated for lithologic mapping capabilities over the Meatiq dome area in the hyper-arid Eastern Desert of Egypt Bi-directional spectral reflectance data (04–25 µm) for powders of the major rock types exposed in the dome and published spectral reflectance data were used as guides in selecting TM band reflectance ratios that maximize discrimination of individual rock types on the basis of their respective mineralogies Comparison of TM data with field and petrographic observations shows (1) increasing amounts of magnetite and other opaque minerals, with low, flat spectral reflectances, decrease the ratio of TM band 5 (155 to 175) to band 1 (045 to 052 µm); (2) increasing amounts of hydroxyl-bearing minerals, with hydroxyl ion vibrational absorptions in TM band-7 wavelength region (208 to 235 µm), increase the ratio of TM band 5 to band 7; (3) increasing amounts of Fe-bearing aluminosilicates that absorb in the band-4 wavelength region (076 to 09 µm) increase the product of the following two TM ratios: band 5 to band 4 and band 3 (063 to 069 µm) to band 4; and (4) thin (≤5 µm), desert varnish that covers many outcrops modulates, but does not obscure, the spectral reflectance signatures of the Meatiq rocks The varnish consists of amorphous to poorly crystalline dioctahedral smectite, iron oxides, and/or oxyhydroxides Serpentinite, mafic mylonite, massive amphibolite, quartzofeldspathic mylonite, biotite schist, and quartz phyllonite were mapped on the basis of their unique values in one or more of the three ratio images, whereas coarse- and fine-grained granites, granite gneiss, and tonalite, with similar mineralogies and TM band ratios, were mapped as a group Finer subdivisions were made where field traverses provided local verification Results demonstrate that appropriate processing and presentation of Landsat TM data can significantly augment field observations for lithologic mapping of large areas in arid regions

Temporal variations in column height and magma discharge rate during the 79 A.D. eruption of Vesuvius

Abstract: The 79 A.D. plinian eruption of Vesuvius ejected ∼4 km3 (ORE) of phonolitic magma over a period of ∼19 hr. A change in magma composition during the eruption is marked by a sharp transition from white, evolved phonolitic pumice to denser, overlying gray pumice, at mid-level within the fall deposit. Deposition of the upper, gray pumice fall was interrupted six times by the emplacement of pyroclastic surges and flows. Reverse size grading is conspicuous in the fall deposit. Measurements of maximum pumice and lithic diameters have been used to construct isopleths for eight chronostratigraphic levels within the fall deposit. The temporal evolution of eruption column height and magma discharge rate have been evaluated from these isopleths, using a theoretical model of pyroclast dispersal from explosive eruptions. During ejection of the white pumice, the column height rose from 14 to 26 km, as the magma discharge rate increased to 7.7 × 107 kg/s. Shortly after onset of the gray pumice fall, the column reached its maximum altitude of 32 km, with a discharge rate of 1.5 × 108 kg/s. Subsequent generation of surges and pyroclastic flows was associated with fluctuations in column height, supporting an origin by column collapse. At the white-gray boundary in the fall deposit, pumice density increases abruptly from 0.60 g/cm3 in the white pumice to 1.10 g/cm3 at the base of the gray pumice. Higher in the gray fall, the density decreases continuously to 0.60 g/cm3. The variation in pumice density is attributed primarily to differences in volatile content of two magmas which were tapped and mixed in varying proportions during ascent and eruption.

Quantitative characteristics of sinuous distributary channels on the Amazon Deep-Sea Fan

Abstract: Morphometric analysis of meandering channels on the middle and lower Amazon Deep-Sea Fan demonstrates that these channels have definite similarities with meandering subaerial rivers. The relationships between meander wavelength and both channel width and radius of meander curvature for fan channels are similar to those observed for large rivers; however, channel width, depth, and cross-sectional area decrease down a fan channel. Channel slope or gradient, measured along the channel axis, decreases smoothly down fan even though the fan slope (valley slope) which the channel traverses decreases irregularly down fan. Channel sinuosities range from about 1.05 to 2.6 on the fan, and sinuosity along a single channel, especially on the middle fan, appears to increase or decrease locally to compensate for varying fan surface slope (valley slope) to maintain a smoothly decreasing channel slope. This dynamic relationship between valley slope and channel sinuosity suggests that the sinuosities of the Amazon Fan channels have changed (that is, the channels have meandered) to obtain the optimum channel slopes, and the optimum channel slope decreases down fan. It is not possible, however, to determine whether that meandering occurred early in the development of the channel/levee system or throughout its evolution. Down-channel changes in fan and channel slope and maximum flow thickness (combined with variations in flow density) may produce systematic changes in flow characteristics and channel facies down fan.

Playa-lake basins on the Southern High Plains of Texas and New Mexico: Part I. Hydrologic, geomorphic, and geologic evidence for their development

Abstract: Playa-lake basins of the Southern High Plains, Texas and New Mexico, may originate wherever water periodically can collect in a surficial depression. They expand, however, by hydrologic and geomorphic processes including (1) dissolution of lithologic carbonates by infiltrating water; (2) transport downward of fine-grained clastic and organic material by the infiltrating ground water, leading to continuing processes of oxidation and carbonate dissolution in the subsurface; and (3) eolian removal of clastic material from the floor of playa lakes, which at some sites appears to have deepened playa depressions. Evidence for largely hydrologic processes of playa-basin development on the Southern High Plains includes (1) a geographic occurrence restricted to relatively flat areas of the High Plains surface that have poorly developed fluvial drainage and which are underlain by generally unsaturated clastic and calcrete beds; (2) a tendency to occur where water collects and infiltrates, as along ephemeral streams and lineations suggestive of fracture systems; and (3) hydrologic, geochemical, petrographic, and bore-hole data, which suggest that recharge to the High Plains aquifer is principally from playa lakes, that various geochemical changes including carbonate dissolution and enhancement of secondary porosity occur as water moves downward through the unsaturated zone beneath playa lakes, and that calcrete beds often are missing or significantly dissolved beneath playa floors.

The transition from a passive margin to an Upper Cretaceous foreland basin related to ophiolite emplacement in the Oman Mountains

Abstract: The Upper Cretaceous Muti Formation in the Oman Mountains exemplifies the transition from a passive continental margin to a foreland basin related to thrusting and ophiolite emplacement. Field data largely substantiate theoretical models that depict a flexural forebulge moving continentward of an advancing thrust-load, followed by flexural downwarping and collapse as nappes override a continental margin. Five units of the Upper Cretaceous (? late Cenomanian–late Campanian) Muti Formation depositionally, or structurally, overlie Mesozoic passive-margin paleoenvironments ranging from abyssal plain to continental rise, slope, and carbonate platform. The Arabian continental margin and Tethys ocean rifted in Permian and Triassic time, giving rise to a mature passive margin in the Jurassic and Lower Cretaceous. At the end of Lower Cretaceous time, northeastward subduction apparently began, and the Semail ophiolite formed, probably in a marginal basin setting. As subduction continued, the trench migrated toward the Arabian continent preceded by a flexural fore-bulge that soon reached the adjacent Arabian continent (Cenomanian-Turonian; ca. 90−88.5 Ma), halting deposition regionally (“Wasia-Aruma break”). Flexural uplift was greatest near the shelf edge, with removal of as much as 600 m of the platform edge succession, and redeposition onto the base of slope as slump sheets, debris flows, and lithoclastic turbidites. As the advancing thrust-load impinged on the edge of the Tethys ocean, the crust was downflexed (late Coniacian–Campanian time; 88.5−73 Ma), creating a foredeep that partly filled with terrigenous clastics, olistostromes, detached blocks of rift-related carbonate build-ups, seamount volcanics and continental basement rocks. The basin depocenter migrated inboard with time, collapsing, in turn, Mesozoic abyssal plain, base of slope, and slope settings to establish a foreland basin on the foundered shelf. Terrigenous sediments accumulated in deep water below the carbonate compensation depth (CCD), together with lithoclastic limestones and detached blocks shed from submarine fault scarps. Successive foreland basins were overridden by the advancing nappes. During the later stages of thrusting onto the continent, Mesozoic sediment decollement nappes were bulldozed ahead of the Semail nappe, ploughing into the existing foreland basin. In conjunction with the final emplacement of the Semail ophiolite, the foreland basin migrated to its most continentward location and filled with clastics derived by subaerial erosion of the Semail nappe and other units. The nappes were submerged again in late Maastrichtian time, possibly in response to flexural relaxation, followed by re-establishment of a carbonate platform. Further compression and thrusting in the Paleocene-Eocene gave rise to a successor foreland basin adjacent to the northern Oman Mountains front.

Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence

Abstract: The Bonneville Flood resulted from catastrophic outflow from Pleistocene Lake Bonneville about 15,000 yr ago, when the lake overtopped its rim at Red Rock Pass in southeastern Idaho and discharged a vast volume of water down the Snake River. This paper provides revised estimates of the paleodischarge, volume, and duration of the Bonneville Flood, based on new evidence of its height and on current understanding of the amount of lowering of Lake Bonneville. Evidence for the revised height of the flood is derived from the altitude of erosional features and flood deposits at the head of a constricted reach of the Snake River Canyon at the mouth of Sinker Creek and from the altitudes of flood deposits at several places about 53 km upstream. Using the step-backwater method, we estimate that peak discharge for the Bonneville Flood through the constricted reach was from 793,000 to 1,020,000 m 3 /s and most likely was 935,000 m 3 /s. This discharge is 2.2 times the discharge previously reported and is the second largest flood known to have occurred in the world. At this rate of discharge, the shear stress for the flood would have been 2,500 N/m 2 , and the unit stream power would have been 75,000 N/m/s, as compared with values of 6 to 10 N/m 2 and 12 N/m/s for recent floods on the Mississippi and the Amazon. Other recent studies of the history of Lake Bonneville show that the volume of water released was 4,700 km 3 , or about 3 times greater than the volume previously inferred. Although this volume indicates a flood duration of 8 weeks at constant peak discharge, an accurate estimate of the duration would require dam-break modeling at Red Rock Pass. From a dam-break model, flood hydrographs at Red Rock Pass and the hydraulics of the flood wave along the Snake River could be computed.

Structural style and kinematics of an underplated slate belt, Kodiak and adjacent islands, Alaska

Abstract: The Kodiak Formation, composed of coherent Maastrichtian turbidites, is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest Cretaceous. It consists of about 80% coherent landward-dipping thrust packets; zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder. About half of these disrupted sandstone zones are related to pre-accretion deformation, and the rest formed along late-stage, strike-slip faults that postdate development of slaty cleavage. The formation is divided into three structural belts. The landward and seaward belts include steeply dipping structures, and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain. The central belt probably acted as a low-angle, southeast-verging, floor thrust zone beneath the landward belt. The structural history of the Kodiak Formation includes (1) early soft-sediment disruption; (2) tectonic stratal disruption; (3) thrust faulting, slaty cleavage (S 1 ) development, and folding (F 1 ); (4) intrusion of granodioritic plutons, dikes, and sills and associated normal faulting; (5) development of crenulations (F 2 ) and crenulation cleavage (S 2 ); (6) thrust faulting; and (7) development of right-lateral, strike-slip faults. Event 3 produced the dominant northwest-dipping structural grain and caused the greatest amount of shortening; the timing of event 7 relative to events 5 and 6 is not certain. The dominant structures of the Kodiak Formation are interpreted as developing during underthrusting, underplating, and intra-wedge shortening during latest Cretaceous time within an accretionary wedge. The maximum time from deposition to emplacement was about 12 m.y., and the timing is constrained by the maximum depositional age (74 m.y.) and the age of plutonic rocks (62 m.y.) that crosscut the dominant fabric. Pre-cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting. Slaty cleavage, thrusts, and F 1 folds developed during underplating that also resulted in the formation of duplexes. Crenulations are probably related to post-underplating subhorizontal shortening of sediments within the accretionary wedge. Syn-deformation dynamic recrystallization of quartz, minimum syn-accretion metamorphic temperatures between 205 and 250 °C, and pressures of at least 2.65 kb recorded during the earliest stages of deformation suggest underplating at >10 km.

Sedimentation in the Chile Trench: Depositional morphologies, lithofacies, and stratigraphy

Abstract: The depositional bodies of the Chile Trench (trench fans, the axial channel, sheeted basins, ponded basins, and axial sediment lobes) control the spatial distribution of modem lithofacies in the basin. Sheeted basins (south of 41°S) are presumably fed by closely spaced submarine gullies that approximate a line of source of sediment supply along the base of the slope. Trench fans (41°S−33°S) are built at the mouths of major submarine canyon systems which act as point sources of sediment supply. The axial channel follows the northward gravitational gradient, draining the distributary networks of trench fans into the longitudinal transport system. Down-gradient (northern) fan lobes are severely dissected by erosional processes; evidently, periods of proximal deposition alternate with periods of massive sediment remobilization and progradation of the axial dispersal system into more distal environments. Channelized basins in the canyon-mouth areas yield to sheeted basins (depositional surface maintains an axial gradient) or ponded basins (depositional surface is strictly flat) in inter-canyon areas. Tectonic disruption of the oceanic basement can locally augment the axial gradient and stimulate flow channelization, or reverse the gradient and induce sediment ponding. A large, margin-parallel sediment lobe is built at the base of a high axial escarpment near 33°S, where the axial channel crosses a transverse discontinuity at the convergent plate boundary. Five lithofacies are defined by Q-mode factor analysis of sediment textures and hydro-dynamic structures in 27 cores from the Chile Trench. The Channel facies (thick, amalgamated sand, massive to laminated or cross-bedded) is deposited by high-energy processes within the coarse-grained bedload of turbidity currents; it forms in distributary and axial channels. The Levee facies (rhythmically bedded, internally structureless, graded sand and graded silt) is deposited from concentrated sediment suspensions that quickly lose momentum during channel spillover; it forms on channel flanks, although constructional levees are not always present. The Basin-1 facies (more complete Bouma sequences, both upper and lower flow regime structures) forms in ponded basins where flows are confined by a high-relief, seaward trench wall. The Basin-2 facies (graded and laminated silt, lower flow regime structures) forms in low-energy environments, such as interchannel areas, distal basins, trench walls, and elevated topographic features. The Contourite facies (silt and sand laminations winnowed from hemipelagic muds and distal turbidites) is best developed in sediment-starved basins where geostrophic currents are constricted and accelerated between the steep inner and outer trench walls. The trench wedge records a coarsening-upward sequence as the oceanic plate migrates toward and into the trench during plate convergence, and becomes more proximal to sediment sources along the base of the continental margin. Near canyon mouths, prograding trench fans drive the axial channel seaward into the trench wedge, and the coarsening-upward sequence is truncated by a time-transgressive erosional unconformity. Abandoned axial channel deposits are carried landward beneath prograding fans to record a fining-upward sequence above the basal unconformity. Channel migration and lobe aggradation may produce fining- and coarsening-upward sequences on depositional fan lobes, but sequences on the erosional lobes are fragmented by numerous truncation surfaces.

A depositional model for outwash, sediment sources, and hydrologic characteristics, Malaspina Glacier, Alaska: A modern analog of the southeastern margin of the Laurentide Ice Sheet

Abstract: The Malaspina Glacier on the southern coast of Alaska is a partial analog of the late Wisconsinan Laurentide Ice Sheet that occupied New England and adjacent areas. Ice lobes of the Malaspina are similar in size to end moraine lobes in southern New England and Long Island. Estimated ablation rates, surface slopes, and meltwater discharge per unit of surface area for the Laurentide Ice Sheet are comparable to measured ablation rates, surface slopes, and meltwater discharge rates for the Malaspina Glacier. Meltwater moves from the surface of the Malaspina down-glacier and toward the bed of the glacier along intercrystalline pathways and through a series of tunnels. Regolith beneath the glacier, which is eroded and transported to the margin of the glacier by subglacial and englacial streams, is the source of essentially all fluvial and lacustrine deposits on the Malaspina Foreland. By analogy, a similar hydrologic system existed at the southeastern margin of the Laurentide Ice Sheet. Subglacial regolith, which was eroded from beneath the ice sheet by meltwater, was the source of most stratified sediment deposited in New England and adjacent areas during the late Wisconsinan. Similarly, Wisconsinan ice-contact landforms in New England were built by the same processes that are constructing landforms composed of stratified sediments in contact with the Malaspina Glacier. For the Malaspina Glacier and the Laurentide Ice Sheet, therefore, we reject the concept of the “dirt machine” by which debris near the base of the glacier is carried to the surface of the glacier along shear planes and then washed off the surface to form ice-contact stratified deposits.

Nature and distribution of deformation across the Banda Arc–Australian collision zone at Timor

Abstract: Recently acquired seismic-reflection and SeaMARC II (side-scan and swath bathymetry) profiles near Timor show that the Banda Arc–Australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system. Deformation is occurring, at present, most intensely at the foot of the inner slope of the Timor Trough. This deformation front is discontinuously advancing southward as new thrust slices develop within the subducted Australian margin strata. In contrast, present deformation is apparently negligible in the Savu Basin, the complex fore-arc basin north of Timor. A possible significant exception is a postulated right-lateral, northeast-trending fault zone offsetting the outer-arc high between Savu and Roti. Although back-arc thrusting has been documented north of the volcanic arc, this component of convergence is minor compared with the scale of ongoing deformation in the Timor Trough. The detailed nature of these surveys has also led to the recognition of along-strike variations in deformation in the Timor Trough and in the Savu Basin. These variations may be related to the variable degree of involvement of the Australian continental margin along the arc.

Hydrothermal sulfide and oxide deposits on seamounts near 21°N, East Pacific Rise

Abstract: Hydrothermal deposits were sampled by submersible from the calderas and summit benches of two seamounts 20 and 30 km west of the East Pacific Rise at 21°N. The deposits document a wide variety of off-axis hydrothermal activity, including samples from the interiors of seafloor sulfide deposits. In contrast to most mid-ocean ridge deposits, sulfide deposits on Green Seamount are rich in Si, Fe, and Cu, are poor in Zn, and contain abundant quartz. Sulfide chimneys grew by quenching of hydrothermal fluids and deposition of walls of colloform pyrite, marcasite, Zn sulfide, and minor galena, whereas coarse-grained pyrite and chalcopyrite were deposited at higher temperatures (>250 °C) in chimney interiors. As chimney walls grew outward into sea water, increased temperatures in the inner walls resulted in recrystallization to pyrite and deposition of pyrite and chalcopyrite in pore space. Cooling in the outer portions of chimneys caused deposition of opal at temperatures of about 80–170 °C. As chimneys coalesced and opal-rich material was heated in the interiors of sulfide edifices, opal was recrystallized to chalcedony and quartz. Faulting has exposed the interiors of sulfide deposits, where further recrystallization and deposition of chalcopyrite and quartz occurred at temperatures of 230–320 °C. Clogging of chimneys, changes in flow paths, and cooling of the deposits led to the deposition of late mixed-layer illite-smectite, iron sulfides, Fe-poor sphalerite, opal, and barite. The sulfide deposits formed over a time span of at least 140,000 to 70,000 yr ago, probably coincident with caldera and pit crater formation. The ages of the sulfide deposits provide constraints on the last volcanic and tectonic activity, and they also suggest that it took a maximum of ∼260,000 yr for the seamount to reach its present height. Red Seamount is presently hydrothermally active, and deposits consisting predominantly of amorphous Fe-oxyhydroxide are forming on small pillow cones in the caldera at temperatures of 10–15 °C. Hydrothermal nontronite deposits capped by Mn-oxide crusts cover much of the southern summit bench of Red Seamount and formed at temperatures of around 30 °C. Iron-rich talc and sulfide grains in nontronite provide evidence of former high-temperature activity and suggest that a sulfide deposit is present nearby.

Flood geomorphology of the Katherine Gorge, Northern Territory, Australia

Abstract: The Katherine Gorge is a narrow, deep canyon system developed in resistant sandstone. The tropical monsoonal climate results in rare flood events of very large magnitude. These floods flush sand-sized sediment as washload through the gorge, but local accumulations are preserved as slack-water deposits at tributary mouths. Paleoflood hydrologic analysis of the deposits, employing step-backwater flow modeling, allows quantitative estimates to be made of geomorphically significant flows. For an annual exceedence probability of 10 −2 , the gorge may experience discharges of 6,000 m 3 /s, mean velocities of 7.5 m/s, flow depths of 15 to 45 m, stream powers per unit area as great as 1 × 10 4 watts/m 2 , and bed shear stresses of 1.5 × 10 3 N/m 2 . Such flows are required to transport boulders 3 m or more in intermediate diameter that occur at large-scale riffles. At maximum stage, pools reach depths of 45 m and are preferentially developed at vertical joint intersections in the rock, probably through intense hydraulic action. Pool-and-riffle development thus reflects the channel boundary characteristics for extreme flow conditions. Additional indicators of especially intense flow phenomena include potholes, flutes, abraded facets on rock bar surfaces, and scabland development on upland bedrock surfaces.

Crustal structure of eastern Nevada from COCORP deep seismic reflection data

Abstract: The western Nevada segments of the COCORP 40°N deep seismic-reflection survey of the North American Cordillera reveal the geometry of structures of Cenozoic and possibly earlier ages to travel-times of > 10 s, corresponding to depths of >30 km. The most striking feature of the data is a band of prominent reflections, typically at traveltimes of 9.5 to 10.5 s, that are present discontinuously across the entire data set. Few reflections are observed from beneath the base of this reflective zone, which is interpreted as the crust-mantle transition. This “reflection Mono” is inferred to be continuous across the survey area, varying gradually in depth but without resolvable offsets. It appears to have taken its present form or position during basin-range crustal extension. The middle to lower crust in much of the survey area is characterized by discontinuous reflections that are typically subhorizontal and locally dip gently west. These reflections may represent intrusions or shear zones related to basin-range or pre-basin-range extension, but some are likely to be inherited from earlier compressional deformation. Reflections from the upper crust are interpreted as images of basin-fill strata, basin-range normal faults, and Mesozoic and Paleozoic thrusts related to back-arc thrusting and accretion of oceanic and arc-related rocks.

Thermal genesis of dissolution caves in the Black Hills, South Dakota

Abstract: Jewel Cave (118 km of mapped passages beneath an area of 2.7 km 2 ) and Wind Cave (70 km beneath 1.8 km 2 ) are, respectively, the fourth and tenth longest known cave systems and the world9s foremost examples of three-dimensional, rectilinear networks of solutional passages. Other caves in the Black Hills are similar. They occur in 90–140 m of well-bedded Mississippian limestone and dolomite. Walls throughout Jewel Cave are lined with euhedral calcite spar as much as 15 cm thick. Wind Cave displays lesser encrustations and remarkable calcite boxwork. Since 1938, opinion has favored cave excavation by slowly circulating meteoric waters in artesian confinement similar to that surrounding the Black Hills. We believe that the caves were developed by regional thermal waters focusing on paleospring outlets in outlying sandstones. Four sets of criteria are evaluated: (1) morphological—the three-dimensional, one-phase maze form having convectional features is similar to known and supposed thermal caves in Europe; (2) petrographic and mineralogical study of the chief precipitates shows a record of carbonate solution → calcite precipitation consonant with a model of cooling, then degassing, waters; (3) a thermal anomaly at regional hot springs is shown to extend beneath Wind Cave, where basal lake-water samples show chemical and isotopic affinities with the thermal waters; and (4) δ 13 C and δ 18 O measurements place all suspected paleothermal water precipitates in the domain of thermal calcites reported by others and being deposited at the modern hot springs. Finally, U-series dates show that the Wind Cave deposits are Quaternary and that the cave is still draining. Jewel Cave is truly relict and divorced from the modern thermal ground-water system; its great calcite spar sheets are probably older than 1.25–1.50 Ma.

Tectonic significance of an Early Proterozoic two-province boundary in central Arizona

Abstract: A compilation of U-Pb zircon dates for lower Proterozoic rocks in central Arizona shows that, although rocks tend to be older in the northwest (1800−1696 m.y.) than the southeast (1738−1630 m.y.), there is no single boundary separating distinct geochronologic provinces in Arizona. Instead, the distribution of isotopic ages reflects the presence of two major tectonic provinces separated by a regionally subhorizontal boundary or boundaries. The northwestern part of central Arizona contains the Yavapai Series (1800−1755 m.y.) and calc-alkaline batholiths (1750−1696 m.y.), both believed to represent oceanic island-arc materials. The southeastern part of central Arizona is dominated by the Alder, Red Rock, and Mazatzal Groups and related hypabyssal intrusions (1710−1692 m.y.), with voluminous rhyolitic ash-flow tuffs and quartz arenite believed to record a relatively stable continental tectonic setting. Two working hypotheses emerge to explain the juxtaposition of representatives of these two tectonic provinces over a 100-km-wide zone in central Arizona. One interpretation (model 1) suggests that rocks of the southeast province were deposited with angular unconformity on newly accreted continental crust composed of northwest province rocks. A second interpretation (model 2) suggests that the two areas represent allochthonous terranes that evolved separately and were juxtaposed by large subhorizontal movements on thrusts and strike-slip faults. An important new constraint is that the 1699-m.y.-old strongly peraluminous Crazy Basin Quartz Monzonite was emplaced in the northwest province during ductile deformation at depths greater than 8 km at the same time that rhyolitic ash-flow tuffs and quartz arenite were being deposited in the southeast province. For model 1, this implies a rapid change of tectonic regimes about 1700 Ma, from convergence to uplift, erosion, sedimentation, and possibly extension. For model 2, the differences in crustal level, structural style, and petrologic affinity between ∼1700-m.y.-old rocks in both provinces are believed to result from juxtaposition of different crustal blocks after 1700 Ma.

Correlation of upper Cenozoic tephra layers between sediments of the western United States and eastern Pacific Ocean and comparison with biostratigraphic and magnetostratigraphic age data

Abstract: Five widespread upper Cenozoic tephra layers that are found within continental sediments of the western United States have been correlated with tephra layers in marine sediments in the Humboldt and Ventura basins of coastal California by similarities in major-and trace-element abundances; four of these layers have also been identified in deep-ocean sediments at DSDP sites 34, 36, 173, and 470 in the northeastern Pacific Ocean. These layers, erupted from vents in the Yellowstone National Park area of Wyoming and Idaho (Y), the Cascade Range of the Pacific Northwest (C), and the Long Valley area, California (L), are the Huckleberry Ridge ash bed (2.0 Ma, Y), Rio Dell ash bed (ca. 1.5 Ma, C), Bishop ash bed (0.74 Ma, L), Lava Creek B ash bed (0.62 Ma, Y), and Loleta ash bed (ca. 0.4 Ma, C). The isochronous nature of these beds allows direct comparison of chronologic and climatic data in a variety of depositional environments. For example, the widespread Bishop ash bed is correlated from proximal localities near Bishop in east-central California, where it is interbedded with volcanic and glacial deposits, to lacustrine beds near Tecopa, southeastern California, to deformed on-shore marine strata near Ventura, southwestern California, to deep-ocean sediments at site 470 in the eastern Pacific Ocean west of northern Mexico. The correlations allow us to compare isotopic ages determined for the tephra layers with ages of continental and marine biostratigraphic zones determined by magnetostratigraphy and other numerical age control and also provide iterative checks for available age control. Relative age variations of as much as 0.5 m.y. exist between marine biostratigraphic datums [for example, highest occurrence level of Discoaster brouweri and Calcidiscus tropicus (= C. macintyrei )], as determined from sedimentation rate curves derived from other age control available at each of several sites. These discrepancies may be due to several factors, among which are (1) diachronism of the lowest and highest occurrence levels of marine faunal and floral species with latitude because of ecologic thresholds, (2) upward reworking of older forms in hemipelagic sections adjacent to the tectonically active coast of the western United States and other similar analytical problems in identification of biostratigraphic and magnetostratigraphic datums, (3) dissolution of microfossils or selective diagenesis of some taxa, (4) lack of precision in isotopic age calibration of these datums, (5) errors in isotopic ages of tephra beds, and (6) large variations in sedimentation rates or hiatuses in stratigraphic sections that result in age errors of interpolated datums. Correlation of tephra layers between on-land marine and deep-ocean deposits indicates that some biostratigraphic datums (diatom and calcareous nannofossil) may be truly time transgressive because at some sites, they are found above and, at other sites, below the same tephra layers.

Timing of structural events in the Himalayan foothills of northwestern Pakistan

Abstract: The Attock-Cherat Range forms the southern boundary of the Peshawar basin and includes rocks transitional between metasediments of the Lesser Himalaya and foreland-basin strata to the south. The Attock-Cherat Range comprises three fault-bounded structural blocks which are, from north to south, (1) Precambrian metaclastic strata overlain by unfossiliferous limestone which is itself apparently overlain by Paleozoic strata with the contact not exposed; (2) unfossiliferous flysch of Precambrian(?) age overlain by Cretaceous and Paleogene marine strata and Murree red beds at least in part of early Miocene age; and (3) unfossiliferous limestone, argillite, and quartzite correlated in part to Paleozoic strata in the Peshawar basin, overlain by a Tertiary sequence generally similar to that in block 2. Farther south, in the Kala Chitta Range, strata of Triassic to Eocene age occur in south-verging folds and thin thrust sheets. The similarity of the Tertiary sequences in the Kala Chitta Range and in blocks 2 and 3 demonstrates that the pre-Tertiary sequences were juxtaposed by faults prior to deposition of the Paleocene Lockhart Limestone. This may coincide with initial contact of the west-northwest–facing passive margin of India with Eurasia or nearby microplates. Major late Tertiary imbricate thrusting and folding took place prior to uplift of the Attock-Cherat Range and to deposition of Peshawar intermontane basin fill of Pliocene-Pleistocene age. The Peshawar basin formed as the Kala Chitta Range was faulted south on the Main Boundary thrust (MBT), forcing Siwalik foreland basins still farther south. Late Quaternary deformation in the southern Peshawar basin occurred along a seismically active zone of en echelon , stepped-left faulted pressure ridges that may reflect a subsurface ramp on the older MBT.

Rates and processes of soil development on Quaternary terraces in Cajon Pass, California

Abstract: Field and laboratory analyses of soils on 11 well-dated fluvial terraces spanning the past 0.5 m.y. demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils. Correlations with respect to time among iron species, soil morphology, and soil silt and clay demonstrate that the chronosequence at Cajon Pass reflects primarily an evolutionary, largely time-dependent trend and does not reflect differences in external factors such as climate. Most of the soil development on Holocene terraces of the Cajon Pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels. This process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering. Latest Pleistocene and older Pleistocene soils have developed clay and authigenic iron oxide-rich B horizons at the expense of organic-matter-rich A horizons and color B horizons as the extent of chemical weathering has increased. This conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time (< 4,000 yr) and is herein defined as a type of pedologic threshold. In the Cajon Pass area, the attainment of the threshold and subsequent development of the argillic B horizon of soils on latest Pleistocene terraces occurred during the Holocene; thus, the absence of argillic horizons in soils on Holocene terraces is attributable to simply their younger age rather than to the Pleistocene-to-Holocene climatic change. The threshold is a function of several variables, including influx rate of eolian dust and initial soil permeability; therefore, the time required to attain the threshold will vary in chronose-quences characterized by geomorphic or geographic settings that are different from conditions found in Cajon Pass.