Showing papers in "Geological Society of America Bulletin in 1983"

Provenance of North American Phanerozoic sandstones in relation to tectonic setting

Abstract: Framework modes of terrigenous sandstones reflect derivation from various types of provenance terranes that depend upon plate-tectonic setting. Triangular QFL and QmFLt compositional diagrams for plotting point counts of sandstones can be subdivided into fields that are characteristic of sandstone suites derived from the different kinds of provenance terranes controlled by plate tectonics. Three main classes of provenance are termed “continental blocks,” “magmatic arcs,” and “recycled orogens.” Sandstone suites from each include three variants, of which the subfields lie within the larger subdivisions. Average modes for sandstone suites can be classified provisionally according to tectonic setting using the subdivided QFL and QmFLt plots. To test the validity of the classification, average modes for 233 Phanerozoic sandstone suites from North America were plotted on the triangular compositional diagrams and accompanying paleotectonic maps. Paired maps and ternary diagrams were prepared for eight different time slices, for each of which the tectonic setting of each major region within the continent remained relatively unchanged. Time slices are unequal in length but are controlled by the timing of major orogenic and rifting events that affected North America during the Phanerozoic. Comparison of the sandstone compositions with inferred tectonic setting through the Phanerozoic indicates that the proposed classification scheme is generally valid and yields satisfactory results when applied on a broad scale. Its application, together with other approaches, in regions of the world where over-all trends of geologic history are less well known could lead to important conclusions about the timing and nature of major tectonic events.

Andean tectonics related to geometry of subducted Nazca plate

Abstract: Seismological and geological data show that tectonic segmentation of the Andes coincides with segmentation of the subducted Nazca plate, which has nearly horizontal segments and 30° east-dipping segments. Andean tectonics above a flat-subducting segment between 28°S to 33°S are characterized by (from west to east): (1) a steady topographic rise from the coast to the crest of the Andes; (2) no significant Quaternary, and possibly Neogene, magmatism; (3) a narrow belt of eastward-migrating, apparently thin-skinned, Neogene to Quaternary shortening of the Andes; and (4) Plio-Pleistocene uplift of the crystalline basement on reverse faults in the Pampeanas Ranges. From about 15° to 24°S, over a 30°-dipping subducted plate, a west to east Andes cross section includes: (1) a longitudinal valley east of coastal mountains; (2) an active Neogene and Holocene andesitic volcanic axis; (3) the Altiplano-Puna high plateau; (4) a high Neogene but inactive thrust belt (Eastern Cordillera); and (5) an active eastward-migrating Subandean thin-skinned thrust belt. Tectonics above a steeply subducting segment south of 33°S are similar west of the volcanic axis, but quite different to the east. Early Cenozoic tectonics of western North America were quite similar to the Neogene Andes. However, duration of segmentation was longer and the width of deformation was greater in the western United States. Patterns of crustal seismicity are systematically related to Plio-Quaternary structural provinces, implying that current deformational processes have persisted since at least the Pliocene. Horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench. Above flat-subducting segments, crustal seismicity occurs over a broad region, whereas over steep segments, it is confined to the narrow thrust belt. Strain patterns in the forearc region are complex and perhaps extensional, and a broad region of the Altiplano-Puna and Eastern Cordillera appears to be aseismic.

An evaluation of criteria to deduce the sense of movement in sheared rocks

Abstract: Some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking. Apparently conflicting evidence from rotated pressure-shadow regions around porphyroclasts and porphyroblasts is clarified. The use of quartz-crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations.

Channel changes in badlands

Abstract: Stripping of the vegetation and soil from a 13-hectare site in Virginia underlain by coastal plain sediments created a rapidly evolving badland topography. Two types of channels developed: (1) sand-bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff, but they also generally eroded their beds slowly, and (2) steeper, bedrock-floored channels incised rapidly. In bedrock channels the erosion rate was proportional to the 4/9ths power of drainage area and the 2/3rds power of gradient. These exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows. Due to rapid mass wasting and reduced runoff, the alluvial channels became as much as 50% steeper during the winter than the summer, with an attendant yearly cycle of winter aggradation and summer entrenchment. The gradients, their seasonal variability, and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads. The hydraulic geometry of alluvial channels in the Virginia badlands were similar to that on the Morrison Formation in the western United States.

Entrainment of gravel from naturally sorted riverbed material

Abstract: The critical dimensionless shear stress, τ*ci, required to entrain a given size particle, di, was computed from extensive bedload-transport measurements in three self-formed rivers that have naturally sorted gravel and cobble bed material. The value of τ*ci was determined to be significantly affected by the size distribution of the riverbed material. For bed particles between 0.3 to 4.2 times the median diameter of the subsurface bed material, d50, the average critical dimensionless shear stress, τ*ci, is equal to τ*ci = 0.0834 (di/d50)-0.872 with a coefficient of determination of 0.980. For bed particles larger than 4.2 times d50, τ*ci appears to approach a constant value of approximately 0.020 in a noneroding channel. Thus, τ*ci varies from 0.25 to 0.020 for a given particle, depending on the ratio of di to d50. Previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0.25 to 0.020 for a given particle size. This analysis indicates that virtually all of the variation is due to differences in the subsurface bed-material size distribution. This analysis shows that τ*ci varies almost inversely proportional to the particle diameter for a nonuniform bed material; therefore, bed particles between 0.3 to 4.2 times d50 are entrained at nearly the same discharge. An investigation of the hydraulic characteristics and bed-material mobility of 24 self-formed gravel-bed rivers in Colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less. Particles as large as the 90th-percentile fraction were entrained by the bankfull discharge in nine of the rivers studied. Consequently, in these rivers, a significant fraction of the riverbed was entrained rather frequently over a period of years, although the transport rate was very small.

Metabasalts, axial hot springs, and the structure of hydrothermal systems at mid-ocean ridges

Abstract: Knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid-ocean ridge. Studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the Mid-Atlantic Ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy, mineral abundances, and mineral compositions, especially for chlorite. These relationships can be interpreted in terms of the distribution of alteration with respect to time, temperature, and water/rock ratio in young crust. Assemblages of chl-ab-ep-act, chl-ab-ep-act-qtz, chl-ab-qtz, and chl-qtz are produced at successively higher effective sea-water to rock ratios within the temperature range of the greenschist facies (−250 to 450 °C). Toward higher ratios, chlorites tend to become more Mg-rich. Pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending, rather than the ascending, limb of a convection system; this type of alteration, which reflects moderately high water/rock ratios, may characteristically occur above a still partially molten magma chamber. As the magma chamber solidifies and is then penetrated, more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water/rock ratios, producing solutions such as those recovered from springs on the Galapagos Rift and the East Pacific Rise at 21°N. Localized upwelling limbs of the convection system can produce veins filled with quartz, sulfides, and Fe-rich chlorite.

Paleohydraulic reconstruction of flash-flood peaks from boulder deposits in the Colorado Front Range

Abstract: Nine watersheds in the Colorado Front Range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits. The nine basins consist of eight small ungauged basins ranging in size from 1.6 to 29 km2 and the Big Thompson River at the mouth of the Big Thompson Canyon, draining 790 km2. Between 1923 and 1976, all nine basins had had one catastrophic flash flood, the magnitude of which has been estimated by the conventional slope-area method. In each basin, coarse boulder deposits of the large flash floods were identified, and three axes of the five largest boulders were measured, along with at least two profiles of the valley cross section. A simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape. Average depth was estimated as the arithmetic average of four values computed from the Manning equation, a regression equation for boulder size and unit stream power, a relative smoothness equation, and a modified Shields' relationship. The appropriate flood width for the estimated average depth was found by iteration, using the valley cross sections. The paleohydraulic discharges thus computed generally underestimate conventional slope-area discharge estimates on small streams by as much as 75%, although the average amount is only 28% too low, and the reconstructed discharge in one stream was 31% too large. The Big Thompson River flood of 1976 was overestimated by 76%. Reasons for discrepancy in reconstructed peaks could include (1) the possibility that floods may have been able to move boulders larger than those available to be moved; (2) overestimation of the slope-area discharge because high-water marks were set prior to erosion of the channel; (3) underestimation of original roughness coefficients; and (4) macroturbulent effects during fast, deep flows. The paleohydraulic technique is applied to two other streams in Colorado with sedimentological evidence of large flash floods, but no conventional indirect discharge estimates. A small tributary to the Big Thompson River draining 1.8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3/s from a flood in 1976. Using boulders excavated from a foundation site in Holocene alluvium along Boulder Creek in Boulder, Colorado, a paleohydraulic reconstructed flood peak of between 860 and 1,512 m3/s is calculated. This is 1.4 to 2.4 times the magnitude of the estimated 500-yr flood.

Experimental study of knickpoint and longitudinal profile evolution in cohesive, homogeneous material

Abstract: A series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal-profile evolution. Knickpoints are created by successive drops in base level, which simulate intermittent uplift. Channel morphology and flow characteristics vary systematically along a knickpoint reach. As the knickpoint lip is approached, width decreases, but depth, velocity, and bottom shear stress increase. As a result, knickpoints are rapidly destroyed through the process of (1) knickpoint replacement and (2) subsequent knickpoint inclination. Knickpoint replacement consists of two morphologically distinct, erosional reaches: the knickpoint face below the knickpoint lip, which decreases in slope, and an incising reach above the knickpoint lip, which increases in slope. Knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock. Within experimental limits, this study supports the notion that knickpoints do not undergo parallel head-ward retreat in homogeneous bedrock, given intermittent uplift, except in those possible cases of extensive and pervasive bedrock jointing. Only the erosional sediment-transport discontinuity above the knickpoint is developed in cohesive sediments. Below the knickpoint, a steep, bedrock channel slope, low sediment load, and rapid decrease in grain size of sediment load preclude development of the aggradational sediment-transport discontinuity. Knickpoint evolution can be characterized by three general models: parallel retreat, replacement, and inclination. These models are based on the following criteria within the knickpoint reach: (1) relationship between bottom shear stress, τ 0 , and critical shear stress needed to initiate motion, τ c ; (2) nature of the bed-load transport discontinuities; and (3) spatial variability of bedrock resistance to fluvial erosion.

Joint formation in granitic rock of the Sierra Nevada

Abstract: A single steeply dipping joint set in the Mount Givens Granodiorite, central Sierra Nevada, was studied to clarify the mechanics of fracture and joint formation in granitic rocks. The joints were filled with fluid during, or immediately following, formation; these fluids deposited epidote and chlorite within the joints. Examination of lithologic markers in outcrop and thin section demonstrates that relative displacements are normal to the joint surfaces. These observations rule out a shear origin for these joints. The measured extensional strain acommodated by joint dilation is on the order of 1 × 10 −4 to 5 × 10 −4 . A few joints in the area exhibit small strike-slip offsets. In these joints, the mineral fillings are sheared, indicating the strike-slip motion postdated the jointing. Individual joints consist of numerous subparallel, planar segments. The lengths of joints range from metres to nearly 100 metres. Shorter joints are more abundant than longer joints. The observed distribution of joint lengths is thought to result from the elastic interaction of adjacent joints. Shorter joints are prevented from further propagation by their long neighbors. Between mapped joints, small cracks that have lengths of several centimetres are found parallel to the longer joints. These cracks represent a growth stage between grain-scale microcracks and macroscopic joints. A method is developed for estimating the tensile stress responsible for initiating joint growth. The method requires knowledge of the extensional strain accommodated by joint dilation and the spatial density of joints, both of which can be determined by field observations. Calculations based on observations of joints in the Florence Lake area indicate relative tensile stresses (average remote stress plus internal fluid pressure) of approximately 1 MPa to 40 MPa. These values of stress and estimates of initial crack length are used to estimate the quasi-static fracture toughness of the granodiorite. The calculated fracture toughnesses range from 0.04 Mpa·m 1/2 to 4 Mpa·m 1/2 . The stress and fracture toughness estimates are compatible with existing data from laboratory fracture experiments.

The Sistan suture zone of eastern Iran

Abstract: A deformed accretionary prism and a flanking forearc basin extending from Birjand southeast to Zahedan, Iran, record the destruction of an arm of the Neo-Tethys during Senonian-Paleocene time and consequent collision of the Afghan and Lut eratonic blocks. The accretionary prism at 32 °N is subdivided into two northwest-trending en echelon belts termed the “Ratuk” and “Neh” complexes, respectively. On the east, the Ratuk complex is characterized by ophiolitic block-against-block or serpentinite-matrix melange and large fault slivers of epidote blueschist tectonite. The Ratuk complex was built prior to Maastrichtian time. The Neh complex to the southwest is Senonian to Eocene in age and includes, in addition to ophiolitic melange, weakly metamorphosed marine sedimentary rock exposed in extensive belts bounded by steeply dipping faults. The Sefidabeh forearc basin deposits onlap both the Neh and Ratuk complexes and the southwest margin of the Afghan block. They make up as much as 8 km of Cenomanian to Eocene terrigenous elastics and carbonates that display a complex but coherent stratigraphy. Facies relations demonstrate the uplift and subaerial exposure of the Ratuk structural high, followed by its subsidence contemporaneous with construction of the Neh complex and calc-alkalic volcanism on the northeast (inner) side of the basin. The accretionary prism-forearc basin polarity, the structural vergence and general younging of the accretionary prism to the southwest, as well as the position of the (relatively) high P T metamorphic rock on the inner side of the prism are consistent with northeast-dipping subduction. Widespread emergence of the entire belt and the initiation of folding of the Sefidabeh basin deposits during middle Eocene are interpreted to be consequences of the entry of the Lut block into the subduction zone. Continued convergence of the continental blocks is expressed by a regional system of folds and transcurrent faults corresponding to east-northeast compression. These structures are buried by mildly deformed Miocene volcanic rocks. Extensive post-Miocene right-slip faulting is inferred to be an effect of Miocene “terminal” collision of Arabia and Eurasia.

Paleoceanographic implications of Miocene deep-sea hiatuses

Abstract: Miocene paleoceanographic evolution exhibits major changes resulting from the opening and closing of passages, the subsequent changes in oceanic circulation, and development of major Antarctic glaciation. The consequences and timing of these events can be observed in variations in the distribution of deep-sea hiatuses, sedimentation patterns, and biogeographic distribution of planktic organisms. The opening of the Drake Passage in the latest Oligocene to early Miocene (25–20 Ma) resulted in the establishment of the deep circumpolar current, which led to thermal isolation of Antarctica and increased global cooling. This development was associated with a major turnover in planktic organisms, resulting in the evolution of Neogene assemblages and the eventual extinction of Paleogene assemblages. The erosive patterns of two widespread hiatuses (PH, 23.0–22.5 Ma; and NH 1, 20–18 Ma) indicate that a deep circumequatorial circulation existed at this time, characterized by a broad band of carbonate-ooze deposition. Siliceous sedimentation was restricted to the North Atlantic and a narrow band around Antarctica. A major reorganization in deep-sea sedimentation and hiatus distribution patterns occurred near the early/middle Miocene boundary, apparently resulting from changes in oceanic circulation. Beginning at this time, deep-sea erosion occurred throughout the Caribbean (hiatus NH 2, 16–15 Ma), suggesting disruption of the deep circumequatorial circulation and northward deflection of deep currents, and/or intensification of the Gulf Stream. Sediment distribution patterns changed dramatically with the sudden appearance of siliceous-ooze deposition in the marginal and east equatorial North Pacific by 16.0 to 15.5 Ma, coincident with the decline of siliceous sedimentation in the North Atlantic. This silica switch may have been caused by the introduction of Norwegian Overflow Water into the North Atlantic acting as a barrier to outcropping of silica-rich Antarctic Bottom Water. The main aspects of the present oceanic circulation system and sediment distribution pattern were established by 13.5 to 12.5 Ma (hiatus NH 3), coincident with the establishment of a major East Antarctic ice cap. Antarctic glaciation resulted in a broadening belt of siliceous-ooze deposition around Antarctica, increased siliceous sedimentation in the marginal and east equatorial North Pacific and Indian Oceans, and further northward restriction of siliceous sediments in the North Atlantic. Periodic cool climatic events were accompanied by lower eustatic sea levels and widespread deep-sea erosion at 12 to 11 Ma (NH 4), 10 to 9 Ma (NH 5), 7.5 to 6.2 Ma (NH 6), and 5.2 to 4.7 Ma (NH 7).

The evolution of Middle America and the Gulf of Mexico–Caribbean Sea region during Mesozoic time

Abstract: A plate-tectonic model for the evolution of Middle America and the Gulf of Mexico-Caribbean Sea region is presented. The model, which is based upon the existence of the Mojave-Sonora megashear, incorporates into the Triassic Pangea reconstruction three microplates between North and South America, thus avoiding the overlap of the Bullard fit. These plates are the Yaqui, bounded on the north by the Mojave-Sonora megashear; the east and west Maya plates, bounded on the north by the Mexican volcanic zone and on the south by a predecessor of the Motagua fault zone; and the Chortis plate (parts of Guatemala and Honduras). During Late Jurassic time, as North America split away from Europe, Africa, and South America, shear, with left-lateral sense of displacement, occurred along the transform faults that bounded the micro-plates. If ∼800 km of left-lateral displacement along the Mojave-Sonora megashear, ∼300 km along the Mexican volcanic belt, and ∼1,300 km along a proto-Motagua megashear are restored, and if Yucatan and Cuba are rotated to fit against northern South America, then (1) a curvilinear belt of late Paleozoic rocks that show lithologic as well as paleontologic similarities extends across the reconstruction and links outcrops in Texas, eastern Mexico, nuclear Central America, and Colombia; (2) a Mediterranean-like sea is delineated that was a precursor of most of the present Gulf of Mexico; (3) correlation is implied between the distinctive quartzose San Cayetano Formation of Cuba and the Caracas and Juan Griego Groups of Venezuela. Geometric constraints suggest that probably shear initially occurred along the Mexican volcanic zone near the end of the Middle Jurassic. Subsequently, probably about 160 m.y. ago, displacements that total ∼800 km began along the Mojave-Sonora megashear. Contemporaneously, Yucatan and fragments of pre-Cretaceous rocks that compose parts of central and western Cuba migrated northward toward their present positions. Rotation of Yucatan was facilitated by considerable displacement along the proto-Motagua zone and along a zone that is probably coincident with the modern Salina Cruz fault. Accumulation of widespread major salt units of Late Jurassic (Callovian to early Oxfordian) age in the Gulf Basin probably occurred contemporaneously with the arrival of these blocks at their present positions. Clastic units that interfinger with some of the youngest salt units and rim the Gulf of Mexico have not recorded major recognized translations since their accumulation. Clockwise rotation of South America and the Chortis plate occurred during Early Cretaceous time. This movement, which was manifested by subduction of Jurassic ocean floor against the previously rifted precursor of the island of Cuba and under parts of Hispaniola and Puerto Rico, is recorded by circum-Caribbean orogeny. Abrupt changes in the relative motions between North and South America during Late Cretaceous time may have resulted in extension and outpourings of basalt upon the Jurassic rocks of the ocean floor of the Venezuelan Basin. West of Beata Ridge, sea-floor spreading formed the Colombian Basin. Related subduction occurred as the Chortis plate (including part of Central America, the Nicaraguan Rise, and southeastern Cuba) was sutured against the Maya East plate along the present Motagua fault and Cayman Trench. Our model is constrained by published geologic data, the relative positions of North and South America from Atlantic sea-floor magnetic anomalies, and the requirement that the major transform faults be compatible with the poles of rotation for the appropriate relative motions between North and South America. Paleomagnetic data from Middle America are sparse but do not conflict with the predicted motions of some of the microplates, especially Chortis.

Origin of the Torlesse terrane and coeval rocks, South Island, New Zealand

Abstract: The Carboniferous to Lower Cretaceous Torlesse terrane and Haast Schist derivatives constitute the major part of the complexly deformed facies of the Eastern Province of New Zealand. Strata consist mainly of quartzofeldspathic graywacke and mudstone, intercalated with minor but widely distributed conglomerate, and volcanics with associated chert and limestone. Clastic rocks were deposited largely by sediment gravity-flow mechanisms in a deep-marine environment. Also present are a few highly fossiliferous shallow-marine and terrestrial deposits of limited areal extent, which rest unconformably on or in fault contact with Torlesse flysch. Several periods of deformation are recognized and melange is present on both local and regional scales. Metamorphism ranges from zeolite to greenschist facies. The bulk of the rocks fall into five areally extensive and mutually exclusive fossil zones of the following ages: Permian ( Atomodesma ), Middle Triassic, early Late Triassic (?) ( Torlessia ), Late Triassic ( Monotis ), and Late Jurassic-Early Cretaceous. Contacts between major fossil zones are mainly tectonic. Petrographic analysis permits subdivision of Torlesse sandstones into five major petrofacies that correspond in age to the five major fossil zones. The sandstone petrofacies (mainly arkosic), together with the composition of conglomerate clasts (mainly indurated Torlesse rocks), indicate that the source terrane was a continental volcano-plutonic arc, probably part of Gondwanaland, coupled with autocannibalistic reworking of older uplifted Torlesse rocks. In contrast to the quartzofeldspathic nature of the Torlesse, coeval sedimentary rocks of the Eastern Province are volcanogenic. They are thought to represent related forearc-basin (Maitai-Murihiku terranes) and trench-complex (Caples terrane) deposits derived from a volcanic island arc (Brook Street terrane). Three petrofacies are established for Maitai-Murihiku and Caples sandstones. The petrofacies indicate a common, evolving, immature to submature volcanic island arc source for these terranes. A reconstruction of New Zealand's Eastern and Western Provinces is proposed. In Permian and Triassic times, the Torlesse was deposited in trench, slope, or borderland basins along a trench-transform margin fronting a continental volcano-plutonic arc source (Western Province-Gondwanaland). Deposition was spasmodic but voluminous and was accompanied by concurrent deformation and accretion resulting in parallel belts of Torlesse rock younging outward from the Gondwanaland margin. At the same time, the Brook Street terrane volcanic arc and associated terranes were forming to the west of the Torlesse site, separated from Gondwanaland by a marginal sea. In latest Triassic or Early Jurassic times, the Torlesse was rafted into the volcanic arc system via transform faulting approximately parallel to the Gondwana margin. The collision event resulted in tectonic thickening of Torlesse and Caples rocks at the plate interface and metamorphism to Haast Schist. The source was then dominated by older, partly metamorphosed Torlesse terrane, newly uplifted along the collision front. Closing of the marginal sea behind the Brook Street terrane in Late Jurassic-Early Cretaceous times resulted in juxtapositioning with the Western Province (Gondwanaland) along the Median Tectonic Line.

Process and rate of dedolomitization: Mass transfer and 14C dating in a regional carbonate aquifer

Abstract: Regional dedolomitization is the major process that controls the chemical character of water in the Mississippian Pahasapa Limestone (Madison equivalent) surrounding the Black Hills, South Dakota and Wyoming. The process of dedolomitization consists of dolomite dissolution and concurrent precipitation of calcite; it is driven by dissolution of gypsum. Deuterium and oxygen isotopic data from the ground water, coupled with regional potentiometric maps, show that recharge occurs on the western slope of the Black Hills and that the water flows northward and westward toward the Powder River Basin. A significant part flows around the southern end of the Black Hills to replenish the aquifer to the east of the Hills. Depth of flow was inferred from interpretation of the silica geothermometer based on the temperature-dependent solubilities of quartz and chalcedony in water. Chemical effects of warm water in the Pahasapa Limestone include changes in the solubility products of minerals, conversion of gypsum to anhydrite, solution and precipitation of minerals, and increases in the tendency for outgassing of carbon dioxide. Where sulfate reduction is not important, sulfur isotope data show that (1) in the Mississippian aquifer, most of the sulfate is from dissolution of gypsum and (2) some wells and springs have a hydrologic connection with overlying Permian and Pennsylvanian evaporites. Sulfate ion concentration, a progress variable, shows a strong correlation with pH as a result of the combined effects of the dedolomitization reactions. Mass-balance and mass-transfer calculations were used to adjust 14C values to determine a range of ground-water flow velocities between 2 and 20 m/yr. These velocities are characteristic of carbonate aquifers. The average rates of dolomite and gypsum dissolution are 1.7 × 10−4 and 3.4 × 10−4 mmol/kg of H2O/yr, respectively. The precipitation of calcite is occurring at the rate of 3.4 × 10−4 mmol/kg of H2O/yr. The close agreement among the model results demonstrates that dedolomitization is controlling water-rock interactions in this regional carbonate aquifer system.

An 18O/16O and D/H study of Tertiary hydrothermal systems in the southern half of the Idaho batholith

Abstract: During Eocene time, 37 to 49 m.y. ago, a series of large hydrothermal systems was developed around a group of epizonal granite plutons in the Idaho batholith. These systems involved deep and extensive circulation of fluids derived from low-δ 18 O (∼−16) and low-δD (∼−120) meteoric waters. Water-rock interactions occurred at temperatures of 150 to 400 °C, lowering the 18 O/ 16 O and D/H ratios in the surrounding Mesozoic rocks (tonalite, granodiorite, and granite), such that the feldspar δ 18 O and biotite δD values became as low as −8.2 and −176, respectively. These values contrast markedly with the primary isotopic compositions of +9.3 ± 1.5 and −70 ± 5, respectively. Widespread propylitization of the Mesozoic plutonic rocks accompanied these isotopic exchange effects. Systematic mapping shows that anomalous δD and δ 18 O values occur over more than 15,000 km 2 , indicating the extensive lateral dimensions of the ancient circulating systems. The former zones of intense hydrothermal activity are marked by low- 18 O zones, which were mapped in the vicinity of the margins of several Eocene plutons (for example, at Rocky Bar) and also within a giant (5- to 20-km wide, 60- to 40-km diam) ring zone that surrounds the Sawtooth Mountains. The latter anomaly is coincident with the high-permeability ring fracture zone of an Eocene caldera system. Most of the ore deposits in the southern half of the Idaho batholith are epithermal and mesothermal Au-Ag veins that are located near the periphery of the low- 18 O zones (that is, near the outermost δ 18 O = 8 isopleth). This association links these deposits with the Tertiary hydrothermal activity and has great potential as an exploration tool in the heavily forested region. Evidence is presented that the Eocene ground-water circulation pattern was affected over large lateral distances (25 to 50 km) and great depths (5 to 7 km). These conclusions, together with the indications that large amounts of water (> 7,000 km 3 ) were involved in some systems and that the circulation patterns probably are related to caldera ring structures, may be of particular importance in geothermal exploration and exploitation of analogous modern systems. For example, the “fossil” hydrothermal activity mapped in the Idaho batholith may be characteristic of deep-level fluid circulation in geothermal systems such as Yellowstone National Park, Wyoming. In such regions, the major zones of hydrothermal activity seem to be principally associated with either (1) the caldera ring zones or (2) the central plutons (resurgent domes).

Structure and emplacement of a rhyolitic obsidian flow: Little Glass Mountain, Medicine Lake Highland, northern California

Abstract: Many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows. The stratigraphy of the Holocene Little Glass Mountain rhyolitic obsidian flow consists of (from bottom to top): air-fall tephra deposits, basal breccia, coarsely vesicular pumice, obsidian, finely vesicular pumice, and surface breccia. Slightly crystalline rhyolite occurs near the vent areas. A model of obsidian-flow emplacement based on these textural relations is presented. This stratigraphy may reflect the distribution of volatiles within the magma source region, with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body. The distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows. This complex structure, as mapped on the Little Glass Mountain flow, reflects both the initial flow stratigraphy and subsequent deformational processes. During emplacement of the lava, three processes disrupt the initial flow configuration: the rise of coarse pumice diapirs from the base of the flow, the inward propagation of fractures in areas of extension, and surface folding in sites of flow-parallel compression. Subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava; most of the observed upper surface of a dome originates as a fracture plane. The structure of domes that form over vent areas may reflect the orientation of dike-like conduits as well as the local state of stress during extrusion.

Eolian deposition in the northeast Pacific Ocean: Cenozoic history of atmospheric circulation

Abstract: The mass accumulation rate and grain size distribution of the total eolian component of North Pacific pelagic clays have been used to evaluate changes in eolian sedimentation and in the intensity of atmospheric circulation that have occurred during the past 70 my Eolian deposition was relatively low, approximately 12 mg/cm2/1,000 yr, in the Late Cretaceous and Paleocene, reflecting an increasingly humid environment A peak in accumulation rates across the Paleocene-Eocene boundary may be the result of episodic volcanism in Mexico A sharp decrease in grain size across the Paleocene-Eocene boundary is not readily interpreted Fine eolian grain size and the low accumulation rates, averaging 8 mg/cm2/1,000 yr, in the Eocene and early Oligocene are in agreement with low early Tertiary thermal gradients and less vigorous atmospheric circulation Large increases in grain size between 37 and 30 my ago, 20 and 10 my ago, and 5 my ago to the present suggest that atmospheric circulation intensified by a factor of 15, 18, and 14, respectively, at these times Eolian accumulation rates increased from 8 mg/cm2/1,000 yr at the Eocene-Oligocene boundary to approximately 25 mg/cm2/1,000 yr at the end of the Miocene, with significant changes at 25 and 13 my ago Increased rates about 25 my ago probably reflect movement of the core site into the influence of the westerlies, and the increase 13 my ago appears to be a response to steepening thermal gradients and intensifying atmospheric circulation resulting from major expansion of Antarctic ice in the mid-Miocene along with a general increase in global aridity in the late Cenozoic An order of magnitude increase in eolian accumulation rates (25 to 226 mg/cm2/1,000 yr) in Pliocene-Pleistocene time coincides with the onset of Northern Hemisphere glaciation and consequent climatic deterioration The large increase in accumulation rates in Pliocene-Pleistocene time coincides with a relatively smal! increase in the intensity of atmospheric circulation compared to that in Oligocene and Miocene time, suggesting that the increased deposition rates were the result of an increase in global aridity in glacial times, rather than a further increase in the vigor of the global wind systems

Storm-controlled oblique dunes of the Oregon coast

Abstract: The large (mean height 25 m, spacing 300 m), relatively straight-crested dunes of the central Oregon coast migrate an average of 3.8 m/yr toward an azimuth of 26°. The dunes are transverse to the strong, south-southwesterly winter storm winds that are responsible for their basic form, orientation, and migration. The dry, moderate, north-northwesterly summer winds modify the dune form but not the dune trend. Comparison of the sand transport calculated from wind data and the transport measured from dune migration indicates that the actual transport by the wet southerly winds is only one-third of the amount calculated assuming dry conditions. The resultant (vector-mean) transport rate, as recalculated by comparison of the measured and initially calculated rates, is 34 m 3 /m·yr toward an azimuth of 45°. The dunes are thus oblique by our definition of an oblique dune (angle between dune trend and resultant transport direction between 15° and 75°). The internal structures of the dunes confirm northward migration during wet conditions. Evidence for deposition during wet conditions includes slipface deposits deformed mostly by sliding and various structures formed by the adhesion of sand grains to wet surfaces. Most summer deposits are not preserved, but those on the basal apron (the gentle north slope at the base of the winter slipface) have a high preservation potential. A depositional model based on dune climbing predicts that the preserved record of oblique dunes formed by an obtuse-bimodal wind regime would consist of tabular sets of crossbeds in which the dip angles increase upward from the base of each set.

Sedimentological and structural evolution of the Arabian continental margin in the Musandam Mountains and Dibba zone, United Arab Emirates

Abstract: The northernmost extremity of the Oman Mountains, the Musandam Peninsula, is composed of an allochthonous sequence of Permian to middle Cretaceous shelf carbonates. These are separated from ophiolitic rocks to the south by a northeast-southwest-trending belt known as the Dibba zone. This structurally complex belt is composed of allochthonous slope- and basin-facies sediments, Haybi volcanic rocks, “Oman Exotic” limestones, sub-ophiolitic metamorphic rocks, and ultra-mafic slices. Lithofacies correlation confirms that fragmentation and rifting of a vast east-facing carbonate platform occurred in Middle to Late Triassic time, resulting in the establishment of a shelf edge and small ocean basin. This margin was typified from northwest to southeast by ooid-skeletal lime sand shoals or small bioherms on the shelf edge, a bypass foreslope of well-laminated periplatform ooze, and basin-margin accumulations of carbonate turbidites or debris flows. The shelf edge now coincides with the northern boundary of the Dibba zone and appears to have remained stationary from Middle Triassic to Late Jurassic time. Periods of arrested carbonate sedimentation on the platform either because of exposure (Late Triassic and middle Cretaceous) or because of drowning (Late Jurassic) are represented in the basin by starved sedimentation and deposition of radiolarian cherts. A spectacular platform margin collapse occurred in the middle and Late Cretaceous, as represented by massive conglomerates, with clasts as young as Albian, above an unconformity that progressively removed all of the Lower Cretaceous. The Tethyan basinal and ophiolitic rocks of the Dibba zone were emplaced from the east-southeast during the Turonian to lower Maastrichtian. After emplacement of these allochthonous units, compressional deformation in the mid-Tertiary resulted in large-scale, open, “whaleback” folds with wavelengths as much as 15 km, generally with north-south axes. In places, these folds, which affect the complete shelf and allochthonous sequences, are overturned toward the west, and thrusting has caused previously lower tectonic units of the Late Cretaceous stacking order to be thrust over previously higher tectonic units, thus reversing the Late Cretaceous tectonostratigraphy. The maximum amount of translation on the later thrusts is in excess of 5 km on the Hagab thrust, where the complete shelf carbonate sequence of the Musandam Mountains has been thrust west-northwest over the previously higher Hawasina and Haybi thrust sheets. The Tertiary folding and thrusting can be correlated in time and space with the Zagros fold belt of southwestern Iran.

Channels and valleys on Mars

Abstract: Tentative conclusions about the origins of channels and valleys on Mars based on the consensus of investigators who have studied the problem are presented. The morphology of outflow channels is described in detail, and the morphology, distribution, and genesis of Martian valleys are addressed. Secondary modification of channels and valleys by mass-wasting phenomena, eolian processes, cratering, and mantling by lava flows is discussed. The physics of the flows needed to account for the immense volumes of Martian outflow channels is considered in detail, including the possible influence of debris flows and mudflows, glaciers, and ice sheets. It is concluded that Mars once probably possessed an atmosphere with higher temperatures and pressures than at present which played an essential role in an active hydrological cycle.

Classification of sedimentary furrows and a model for furrow initiation and evolution

Abstract: Sedimentary furrows are longitudinal bed forms which form in fine-grained, cohesive sediments of the deep-sea floor as well as in similar sediments in estuaries and in large lakes. Deep-sea furrows have been observed where bottom currents often flow in one direction at 5 to 20+ cm/sec. Currents are much stronger (greater than 50 cm/sec) in furrowed estuarine environments, but sediment accumulation rates are higher. A review of furrows studied in a variety of locations and environments suggests that these bed forms fall into two broad morphological classes. Those developed in muddy, cohesive sediments tend to have steep walls and flat floors, while those developed in carbonate-rich cohesive sediments are broad and have gently sloping walls. At abyssal depths, furrows can often be detected by their distinctive echo patterns on 3.5-kHz echo sounders—a series of similar, evenly spaced hyperbolic echoes which often have their apices tangent to the sea floor. However, different furrow cross sections may generate distinctive hyperbolic echo patterns. A model for furrow initiation and development in muddy cohesive sediments suggests that furrows develop in depositional environments swept by recurring, directionally stable, and episodically strong currents. Furrows are initiated when secondary circulations in the boundary layer align coarse, generally light debris in sand ribbon-like bed forms. As the coarse, light sediments move along the sea floor, they abrade small longitudinal troughs into the muds. The grooves may fill in during intervals between strong current events if sedimentation rates are too high. Once a furrow is established on the sea floor, further development may be controlled by the balance between sediment deposition and erosion, where deposition occurs continuously, and erosion occurs only episodically during brief periods of increased current strength. “Narrow” furrows (width 1/5 to 1/15 of spacing) appear to develop where deposition exceeds erosion, whereas “wide” furrows (width 1/2 of spacing) develop where erosion exceeds or equals deposition. Dissolution may play an important role in the formation and evolution of furrows in carbonate-rich cohesive sediments. Secondary circulation patterns over established furrows may be strongly controlled by bed-form topography.

Sources and dispersal patterns of clay minerals in surface sediments from the continental-shelf areas off Alaska

Abstract: The Three mineral suites were recognized in the Gulf of Alaska. The glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates, micas, and chlorites, derived locally from argillite, graywacke, and metavolcanics under conditions of mild chemical weathering. More glycol-expandable clay minerals (GECM) in the north Gulf substantiate clay supply to that region primarily from the Yakataga and Poul Creek Formations, and to a lesser extent from the Copper River. The northern Gulf clay-mineral suite is transported to central Prince William Sound and northeast Kodiak Shelf by the Alaska coastal current. In the western Gulf, clay minerals adjacent to the Alaska Peninsula are derived principally from the Susitna River via Cook Inlet, whereas those of the outer shelf presumably have their provenance in Kodiak Island and the Katmai Ash submarine deposit. In the southeast Bering Sea, a progressive northwestward decrease in GECM, from 65% to 11%, is caused by a gradually decreased sediment export from the Alaska Peninsula and the Aleutians, with a possible minor increase in illite from the Kuskokwin River. The Yukon River is the predominant clay source for the north Bering Sea, as signified by an abrupt increase in that area of GECM (20% to 35%) and of kaolinite/chlorite ratios. Clay mineralogy indicates that the central Chukchi Sea is the major repository of the Yukon clays, suggesting that clay-mineral stratigraphic studies have implications for interpretation of the Quaternary paleogeography of “Beringia.” In the Beaufort Sea nearshore, influence of different fluvial inputs and their dispersals by currents are sharply defined. The lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and/or reworking and redistribution of sediments resulting from ice gouging of the sea substrate. Movement of sediment to the west in the Alaskan portion of the inner shelf of Beaufort Sea is evident. An apparent seaward increase in illite/GECM ratio in the Colville Delta is tentatively ascribed to regeneration of degraded illite-micas by K + fixation in saline waters. An isolated area of sediment rich in expandable clay minerals in the central Beaufort Shelf presumably is relict.

Large-scale bed configurations (macroforms), Platte River Basin, Colorado and Nebraska: Primary structures and formative processes

Abstract: Large-scale, current bed forms, defined here as bed configurations having dimensions commensurate with the size of the turbulent boundary layer, are considered by many workers to be hydrodynamically equivalent to the regime bed forms. These bed forms were investigated at formative flows (bankfull flows) on the South Platte and Platte Rivers in Colorado and Nebraska to determine structures and formative processes. Results of this study suggest that large-scale bed forms, here termed macroforms, are not hydrodynamically equivalent to the regime bed forms but constitute a unique hierarchical class of bed configurations produced by turbulent vortices that involve the entire boundary layer. Macroforms are oriented obliquely to the mean flow direction and are attached to one or both channel banks. Three members of a continuum of geometries are recognized in the channels of the Platte River Basin. The internal stratification for each of the three macroform types recognized in the basin is similar and in its simplest form consists of the coarsening-upward sequence apron laminae-foreset laminae-topset laminae. Although it may not be possible to differentiate the three macroform types solely on the basis of internal stratification, the presence of a distinctive coarsening-upward sequence in coarse-grained fluvial systems offers a potentially powerful tool for identifying these environments in the geologic record.

Modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the New England Fold Belt (east Australia): A Paleozoic forearc terrane

Abstract: The New England Fold Belt of southeast Australia is divided by the Peel Fault System into two time-equivalent zones showing contrasting depositional, structural, and metamorphic histories. These are inferred to represent a western magmatic arc–frontal arc succession and an eastern subduction complex assemblage. The composition of the early to middle Paleozoic clastic rocks from both east and west of the Peel Fault System indicates derivation from an undissected volcanic arc. Detrital quartz is absent or present only in minor amounts and always as monocrystalline grains. Lithic fragments are almost exclusively volcanic, and normally dominant over feldspar which is invariably plagioclase (now largely albitized). To the west of the fault, the distribution of lathwork, microlitic, felsitic, and vitric rock fragments, as well as variation in the amount of detrital quartz, indicates changes in provenance through time, and two cycles of basaltic to silicic volcanic activity have been identified. The composition of detrital calcic clinopyroxene grains, particularly their low Ti and Al contents, and their Ti:Mn:Na ratios, indicates that they crystallized mainly from subalkaline, probably calc-alkaline, volcanic arc magmas. The dominantly volcanogenic provenance of sandstone from east of the fault and the low Ti content of most detrital clinopyroxene grains indicate that the volcanic chain that lay to the west of the Peel Fault was the most likely source for the major portion of the clastic debris in the eastern rocks. However, the presence of Ti-rich detrital clinopyroxenes, identical in composition to those in the metabasalts of the subduction complex, indicates that uplifted segments of the complex provided an additional sediment source.

On the mechanisms and kinetics of oxygen isotope exchange in quartz and feldspars at elevated temperatures and pressures

Abstract: Stereological and scanning electron microscope (SEM) analyses of quartz-water and plagioclase feldspar–water interactions have been combined with 18O/17O and 17O/16O measurements of the associated isotope exchange, in an attempt to develop an understanding of mineral-water interactions at elevated temperatures and pressures. The quartz-water exchange (T = 250 to 600 °C; Ph2o = 1 to 22 kbar) is dominated by an “Ostwald ripening” recrystallization mechanism, in which reprecipitated quartz develops as faceted overgrowths on original grains that have been rounded by initial solution processes. Correspondingly, there is a linear decrease in specific surface area (S) as a function of fractional extent of exchange (f). The unrecrystallized quartz cores exchange through diffusional mechanisms that also become dominant in later stages when recrystallization becomes retarded and increasingly ineffective as a mechanism of isotopic exchange. This transition from solution-precipitation to diffusional mechanisms accounts for a “plateau” pressure effect on exchange rate, observed in later stages of exchange. Substantial recrystallization does not occur in feldspar-water exchange at 500 and 600 °C, Ph2o = 2 to 15 kbar; the feldspar grains rapidly degrade into smaller fragments and become incoherently coated by a small amount of fine precipitated crystals. Subsequently, isotopic exchange occurs without further textural change and at constant S. These observations, the preservation of Al/Si order in albite throughout exchange, the high calculated values of oxygen-diffusion coefficients and possible dependence of these coefficients on (Ph2o)½ indicate inhibition of feldspar recrystallization and are consistent with diffusional mechanisms for penetration of water into the feldspar and exchange with shared oxygen atoms in alumino-silica tetrahedra. Equilibrium 18O/16O fractionation factors for both quartz-water and albite-water agree well with those measured previously in this laboratory.

The role of woody vegetation in stabilizing slopes in the Cincinnati area, Ohio

Abstract: Tree roots markedly stabilize colluvium on steep hillsides in the Cincinnati area, Ohio, according to detailed analysis of a landslide complex on the south side of Rapid Run Creek, a tributary of the Ohio River. The shear surface of the slide developed near the interface between bouldery, silty clay colluvium and underlying calcareous Ordovician shale, and it broke woody roots that penetrated through the colluvium and into the underlying bedrock. Measurements of positions, sizes, and tensile strengths of the roots indicate that the average shear strength contributed by the roots was about 5.7 kN/m 2 of the shear surface. The average strength contributed by residual friction of the soil was about 0.7 kN/m 2 . The tree roots increased the factor of safety against sliding nine-fold. Root strength allows forested, colluvium-mantled hillslopes in the Cincinnati area to resist sliding at slope angles as high as 35°, whereas similar slopes devoid of trees are subject to sliding at slope angles of 12° to 14°. The contribution of tree roots to slope stability should be evaluated before removal of trees for development of hillsides.

Porosity reduction by pressure solution: A theoretical model for quartz arenites

Abstract: Pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones. This paper presents a one-dimensional model for a well-sorted quartz arenite sequence undergoing diagenesis. The sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts. Quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains. This solution-and-precipitation process reduces porosity and forces the migration of pore fluid. Permeability decreases with porosity, hindering fluid migration, and excess fluid pressures are generated. Distributions of porosity, pore-pressure, and fluid-flow rates are given for several sedimentation rates and thermal gradients. Complete reduction of the primary, intergranular porosity was accomplished, in all cases examined, by a depth of 3.5 km.

Stratigraphy and depositional setting of the Proterozoic Snowy Pass Supergroup, southeastern Wyoming: Record of an early Proterozoic Atlantic-type cratonic margin

Abstract: Metasedimentary rocks in the Medicine Bow Mountains and Sierra Madre are divided into four groups. The > 3-km-thick Phantom Lake Metamorphic Suite contains strongly deformed metavolcanic and metasedimentary rocks that are crosscut by late Archean granites. The > 2.5-km-thick Deep Lake Group unconformably overlies the Phantom Lake Suite and late Archean granites and contains fluvial sediments, including radioactive quartz-pebble conglomerates, and glaciomarine deposits. Both successions are intruded by large sills of tholeiitic gabbro. The 4.5-km-thick lower Libby Creek Group is inferred to be in thrust-fault contact with older units and contains sediments recording transgressions and regressions across a macrotidal delta. This succession is intruded by the 2,000-m.y.-old Gaps Intrusion and comagmatic tholeiitic to weakly alkalic dikes. The 3-km-thick upper Libby Creek Group is bounded by a thrust fault below and by the Cheyenne Belt above and contains carbonates and marine slates. The early Proterozoic Deep Lake, lower Libby Creek, and upper Libby Creek Groups collectively are named the Snowy Pass Supergroup. Lithologies and stratification sequence in the well-preserved Medicine Bow Mountain section suggest transgressive, miogeoclinal sedimentation during the early Proterozoic. Paleocurrent data indicate that fluvial, then deltaic, sedimentation of the Deep Lake and lower Libby Creek Groups took place on a southwest-dipping paleoslope, parallel to the inferred south cratonic boundary of the Wyoming Province. This and a few west-directed paleocurrents suggest a continental or microcontinental block to the south, bounding sedimentation in a northeast-elongated basin. A rift setting for deposition of these units explains the transgressional character of the sediments, the deltaic sedimentation with paleocurrents parallel to the cratonic boundary, and the 120° bend in the Cheyenne Belt between the Medicine Bow Mountains and the Sierra Madre. The upper Libby Creek Group is interpreted to represent open marine conditions following separation of the two continental blocks. Tholeiitic sills in the Deep Lake Group and tholeiitic to weakly alkalic dikes in the Libby Creek Group are thought to be related to basaltic igneous activity associated with compound early Proterozoic rifting between 2,300 and 2,000 m.y. ago.