Fractured Basalt Research Articles

Direct Determination of Transport Parameters in Repository Materials

ABSTRACTA new flow technology has been developed that significantly decreases the time required to obtain transport data on saturated and unsaturated porous/fractured media. This technique is based on open-flow centrifugation and was developed to measure steady-state transport properties in most geologic materials within a matter of hours. Centripetal acceleration does not induce artificial effects in samples i.e., fracturing, collapse of interlayer structures, structural dewatering, compaction, chemical changes, etc., that occur with high-pressure methods. Using this technique, hydraulic conductivities (K) and diffusion coefficients (D) for compacted bentonite and four host rocks have been measured and re-interpreted. Based on these new data, K for compacted bentonite is less than 10−14 m/s, a factor of 1000 lower than previous pressure-gradient measurements, providing further assurance that radionuclide transport through bentonite backfill will be diffusion limited. Measured K for mudstone (1.8 × 10−12 m/s) indicates diffusion-limited far-field transport, while advective transport should occur for granite, basalt, and tuff, with expected matrix diffusion coefficients (correlated to measured D values) of 8.3 × 10−13 and 2.5 × 10−12 m2/s for fractured granite and basalt, respectively.

Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows

Research Article| May 01, 1992 Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows STEVEN W. ANDERSON; STEVEN W. ANDERSON 1Geology Department, Arizona State University, Tempe, Arizona 85287 Search for other works by this author on: GSW Google Scholar JONATHAN H. FINK JONATHAN H. FINK 1Geology Department, Arizona State University, Tempe, Arizona 85287 Search for other works by this author on: GSW Google Scholar Author and Article Information STEVEN W. ANDERSON 1Geology Department, Arizona State University, Tempe, Arizona 85287 JONATHAN H. FINK 1Geology Department, Arizona State University, Tempe, Arizona 85287 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1992) 104 (5): 615–625. https://doi.org/10.1130/0016-7606(1992)104<0615:CSIOER>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation STEVEN W. ANDERSON, JONATHAN H. FINK; Crease structures: Indicators of emplacement rates and surface stress regimes of lava flows. GSA Bulletin 1992;; 104 (5): 615–625. doi: https://doi.org/10.1130/0016-7606(1992)104<0615:CSIOER>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Crease structures are features commonly found on lava flow surfaces and consist of a fracture with curved walls that extend outward from a linear valley. These crease structures are found on flows of nearly all compositions and crystallinities. We have mapped the distributions of crease structures on many flows in the western United States and found that (1) axial length is not dependent upon composition and crystallinity; (2) adjacent crease structures are generally aligned in an en echelon pattern; (3) crease structures located adjacent to flow margins are generally perpendicular to these margins; and (4) at Mount St. Helens, Washington, large lobe-bisecting crease structures are found on lobes situated on slopes of less than 20 degrees. A primary surface feature found on many crease structures is striations. Striations are sets of long stripes on the walls of the crease structure that extend approximately parallel to the axis of the central valley, and they appear to be analogous to those found on the faces of cyclically fractured basalt columns.Observations of developing crease structures on 6 of the nearly 20 Mount St. Helens dome lobes show that they form throughout the extrusion of flows situated on slopes of less than 20 degrees, but only at the very beginning and/or end of extrusion of flows on steeper slopes. These observations imply that crease structures form when the lava flow is forced to spread laterally, either as the flow advances over a flat area, or as the down-slope movement stagnates near the end of extrusion (causing the rate of spreading to exceed the downslope rate of flow). This lateral spreading of lava results in the concentration of tensile stress along a line oriented perpendicular to the direction of spreading. The cooled crust of the extrusion is therefore torn apart about this line of tensile stress concentration, forming a central valley that exposes hot, ductile material from the flow interior to the atmosphere. The presence of striations on many crease-structure walls implies that the emplacement mechanism is similar to that suggested for columnar basalts, where each striation is produced by incremental fracturing.The stress responsible for crease structure formation is due to both cooling and lateral spreading of lava. Calculations show that stress due to spreading is three orders of magnitude higher than that due to cooling. Fracture advance times were calculated using a simple one-dimensional conductive cooling model adjusted for the special geometry of the crease structure. The model closely approximates measured crease-structure emplacement times at Mount St. Helens and gives reasonable estimates for crease structures found on older silicic lava flows. We then used this model to calculate emplacement times of entire domes that consist of a single crease structure. By dividing the dome volume by the formation time, we are able to calculate extrusion rates for these older domes. These values, which include the first flow-rate estimates reported for rhyolitic lavas (0.03-106 m3/s), similar to average flow rates of 0.7-40 m3/s calculated for Mount St. Helens dacite lobes from detailed topographic maps. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Scale dependence of continuum models for fractured basalts : Khaleel, R Water Resour ResV25, N8, Aug 1989, P1847–1855

Scale dependence of continuum models for fractured basalts : Khaleel, R Water Resour ResV25, N8, Aug 1989, P1847–1855

RELIABILITY OF ROCKBOLT ANCHORAGE UNDER CYCLIC LOADING IN FRACTURED BASALT

ABSTRACT A preliminary feasibility study to determine the response of rockbolts to cyclic loading was conducted in highly fractured basalt at Gumboot Mountain, Washington. Nine ungrouted and six grouted rockbolts were tested. Yield strength and rockbolt movement were monitored while the rockbolts were subjected to between 200 and 400 loading cycles. The data from this study indicate that the ungrouted expansion shell rockbolts which were used are not suitable for cable logging system (CLS) anchorages in the highly fractured basalt found at Gumboot Mountain. The data for grouted rockbolts, although not conclusive, suggest that grouted rockbolts may provide suitable CLS anchorages in highly fractured rock.

The Linguado, Carapeba, Vermelho, and Marimba Giant Oil Fields, Campos Basin, Offshore Brazil: ABSTRACT

About 24 hydrocarbon accumulations have been discovered in the Campos Basin, including four giant fields in shallow to moderate water depths, in the period from 1978 to 1988. Linguado field is located in the extreme south of the producing area of Campos Basin, in water depths ranging from 95 to 110 m. The pool was discovered by the wildcat 1-RJS-49 in May 1978. The reservoir rocks, which occur between -1700 and -3000 m, are Hauterivian fractured basalts, Barremian pelecypod coquinas, Albian oolitic and oncolitic calcarenites, and, secondarily, Cretaceous turbidite sandstones. The main reservoir is formed by coquinas, which contain 80% of the total original recoverable oil volume, estimated at 130.15 million bbl. The field is located on a regional high, and the accumulation is strongly controlled by stratigraphic and diagenetic factors. High-quality oil (29 to 32° API) is produced through two floating production systems, and the cumulative oil productio amounted to 82.25 million bbl as of December 1990. Carapeba and Vermelho fields are situated at the northern limit of the Campos Basin producing area and, together with the smaller Pargo field, make up the so-called Northeastern Pole of the Campos Basin. Carapeba was discovered in February 1982 by the wildcat 1-RJS-193A and has an estimated recoverable oil volume of 183.81 End_Page 137------------------------ million bbl. Production comes mainly from two Late Cretaceous turbidite sandstone reservoirs. Vermelho field was discovered by the wildcat 1-RJS-241A in December 1982, and its main reservoir is formed by a massive Eocene turbidite sandstone. The estimated recoverable oil volume amounts to 121.55 million bbl. Both Carapeba and Vermelho fields are structural traps associated with the development of subtle anticlines caused by salt tectonics. The fields are gradually being put on stream through five fixed platforms installed in water depths ranging from 70 to 90 m. Marimba field, discovered in March 1984 by the wildcat 1-RJS-284 drilled in a water depth of 383 m, is considered the first deep water oil strike in Campos Basin. The field has an estimated recoverable volume of 174.18 million bbl of good-quality (28° API) oil in highly permeable Late Cretaceous turbidite sandstones, trapped under structural-stratigraphic conditions. The Marimba field has not yet been completely developed, but a floating production system is producing about 22,000 BOPD from four exploratory wells completed in water depths ranging from 383 to 485 m.

A magnetotelluric transect in the Oregon Coast Range

As part of EMSLAB, broadband magnetotelluric data have been collected and analyzed at 11 sites along an east‐west transect in the Oregon Coast Range. Using one‐ and two‐dimensional modelling, the Coast Range was found to consist of marine sediments about 4 km deep near the coast and volcanic rocks about 8 km deep farther inland. Willamette Valley consists of surficial conductive sediments to a depth of 1 km, which overlie an eastern extension of the rocks that compose the Coast Range. An east dipping conductor, at a depth of approximately 20 km, is associated with the upper surface of the subducting Juan de Fuca plate. Possible mechanisms for the high conductance are water‐saturated fractured basalts forming the surface of the plate, subducted sediments, and/or the dehydration of the subducting slab trapped by metamorphic layering within the lower crust. This conductor correlates with an east dipping reflector, 35–40 km below the eastern flank of the Coast Range, defined by COCORP profiling and thought to represent the surface of the subducting Juan de Fuca plate.

Scale dependence of continuum models for fractured basalts

Numerical simulations are carried out to determine the scale above which an equivalent porous medium (EPM) model can be used to represent flow through the dense interiors of fractured basalts. Steady laminar flow is assumed to occur through the interconnected network of conduits formed by the intersecting column‐defining fracture patterns of the basalt dense interiors. The basalt rock matrix is considered to be impermeable. A porous medium equivalence is established in terms of fluid (Darcian) fluxes. A polar plot for the equivalent hydraulic conductivity is used to test whether a given fractured basalt system can be approximated as an EPM. For interconnected colonnade network models of filled or unfilled fractures with uniform apertures and a column diameter of 1 m, EPM models are appropriate at a length scale of approximately 6 times the column diameter. For network models of filled or unfilled fractures with a lognormal aperture (b, millimeters) distribution (mean In b=−1.945 and σln b=0.896), EPM models can be justified at length scales of approximately 22 and 27 times the column diameter, respectively. The computed equivalent K values for clay‐filled fractures compare favorably with the field‐measured data. This suggests that a model comprised of an interconnected network of filled fractures may not be an unlikely representation of flow through the dense interiors at the small scales considered in this study. Several limitations of the study are noted.

The statistical structure and filter characteristics of tritium fluctuations in fractured basalt

The statistical structure and spectral‐covariance characteristics of tritium fluctuations are examined from injection of low‐level radioactive waste in a fractured basalt aquifer. The tritium fluctuations are the result of a time‐varying injection well concentration. The integral scale of the fluctuations increases with distance from the source and the observation well spectra are characteristic of a dispersive‐type filter. On the basis of these qualitative attenuation characteristics, a theoretical filter is constructed and transport parameters are estimated. The analysis indicates that environmental tracers may provide a useful extension to controlled tests over large length scales (&gt; 100 m). Although current research has concentrated on the importance of intrinsic variability such as porous media heterogeneities, the present study addresses the control exerted by extrinsic variability as exhibited by source strength fluctuations.

The determination of resistivity and porosity of the sediment and fractured basalt layers near the Juan de Fuca Ridge

Summary. In the summer of 1984 an electrical survey using magnetometric off-shore electrical sounding (MOSES) was conducted at two sites in Middle Valley, part of the northern Juan de Fuca Ridge complex. MOSES has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer. Site 1, at 48°32N, 128°42W, is in the central part of the turbidite-filled basin. Using a two-layer model of conductive sediments overlying a fractured basalt basement, the sediment resistivity and thickness were found to be 0.82 ± 0.06 Ωm and 1800 ± 300 m, respectively. The basement resistivity, although not well constrained by the data is consistent with the results obtained at site 2. Site 2, located at 48°10N, 128°50W, has a thinner sediment layer, which appears to vary with position. The sediment conductivity—thickness product is the parameter determined by the data. If the sediment resistivity were the same as at site 1, the sediment thickness would be 140 ± 30 m to the SE of site 2, and 240 ± 55 m to the NW. The fractured basalt basement has a resistivity of 8.5 ± 3.4 Ωm and is at least 1000 m thick. Using temperature-corrected pore fluid resistivity, the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement. These values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment-filled basin and determined for layer 2A/B basalts in DSDP hole 504B, respectively.

The chemistry and hydrology of thermal springs on efate, vanuatu, sw pacific

Two zones of geothermal spring activity on the island of Efate are described. The hydrothermal systems are associated with a deep-seated graben through Pleistocene volcanics and fringing reef limestones. The Teuma group of springs have temperatures up to 59°C and discharge around 1 MW of heat, although total discharge into rivers is estimated from conductivity changes to be 10 MW. The hydrochemistry and stable isotope hydrology of these springs indicate that there are two thermal components: residual sea-water with estimated base temperature no more than 114°C and a less saline thermal water which has a much smaller surface discharge but is indicated by cation and SiO 2, geothermometry to have a base temperature up to 218°C. The Takara group of springs discharge in the coastal reef limestone where the meteoric water table intersects the ground surface; the proximity of this thermal water table also gives rise to an expanse of “hot ground” inland of the spring line. The excess heat flow from this system is around 20 MW, and varies according to the seasonality of fresh-water recharge and flux. A heat source at depth to the landward side of the thermal area is indicated. The hydrochemistry of the Takara system shows that it comprises sea-water, possibly mixed with meteoric water prior to circulation in the thermal reservoir. Mg 2+ and SO 2− 4 are strongly depleted in the thermal water relative to unreacted sea-water; however there is no detectable δ 18O shift which suggests a high water:rock ratio in the thermal reservoir. Quartz geothermometry indicates a base temperature up to 174°C which is similar to that indicated by cation geothermometry although the unusual absence of K + enrichment casts some doubt on use of the Na—K—Ca method. The interpretation emphasizes the need to examine and understand alteration mineralogy in order to place more confidence in geothermometry and to obtain more information on evolution of the geothermal systems. No drilling has yet taken place and much of the reservoir geology and description is speculation. The Teuma system probably comprises circulation in fractured basalt dykes associated with graben activity, and flow may therefore be very limited at depth. The Takara system has some similarities with other marine systems, mostly at oceanridge locations, where sea-water circulates convectively in basalts with a regionally-high geothermal gradient.

Geology and Exploration in Takutu Basin, Guyana: ABSTRACT

The Takutu basin is an intracratonic graben 280 km (174 mi) long and 40 km (25 mi) wide in northern Brazil and adjoining Guyana, lying entirely within the center of the early Precambrian Guyana shield. Acidic metavolcanic rocks and thick Proterozoic quartzite lie to the north of the basin. Granulite, gneiss, and granite border the graben to the south and east. High mountains arise along the south-bounding fault whereas more subdued topography flanks the north side of the basin. Triassic basalt forms a wide band of outcrop along the southern and eastern margins of the rift. There are very poor and sparse outcrops of the basin fill. The graben is filled with up to 4,000 m (13,123 ft) of Cretaceous and Jurassic sedimentary rock underlain by 1,500 m (4,921 ft) of mafic volcan cs of Triassic age and possibly older (i.e., Proterozoic). The geologic history of the Takutu graben is interpreted to extend back into Precambrian time because it occupies an ancient suture zone in the Guyana shield. Renewed rifting and major subsidence occurred in Mesozoic time resulting in the deposition of thick nonmarine clastics, evaporites, and carbonates. A basal(?) Jurassic clastic-carbonate sequence overlies the eroded basalt. It is overlain by thick Cretaceous Aptian salt and interbedded shales that were deposited over most of the basin and contain good oil source rocks. The only indication of a marine environment is found within the subsurface post-salt clastics in Brazil. Lacustrine an deltaic depositional processes were dominant as indicated from well and seismic data. Two main structural styles, namely pre-salt and post-salt, occur in the basin. The former is characterized by block faulting and horst and graben development. Non-piercement halokinetic forms swells and ridges in the post-evaporite Takutu Formation. Wrenching and salt solution are interpreted on seismic records. A large, cross-basin arch is present in Guyana where at least six undrilled prospects have been mapped. Three widely spaced exploratory wells have been drilled down to the mafic volcanics. The wells are located on structures near the rift margin or in areas of thinner basin fill. Two of the tests were dry and abandoned while Home Karanambo #1 was classed as a noncommercial oil discovery in fractured basalt. Several clastic depocenters have been interpreted and delineated from the seismic and drilling results. They lie near the southern and eastern unexplored basin margins, distant from the wells drilled to date. End_of_Article - Last_Page 444------------

Lithology and structure of the volcanic sequence in eastern Iceland

The Reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99.7%. Subaerial lava flows comprise 54% of the core, dikes 41%, and clastic rocks about 5%. In addition, a 1.3‐km‐thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled. The lava flows are chiefly low‐magnesia basalts and ferrobasalts with lesser amounts of icelandite, basaltic andesite, and olivine tholeiite. In the drill core, complete flows range from 1.2 to 19.7 m in vertical thickness and average 7.1 m; in the exposed section, flow thicknesses range from 1 to 30 m and average 5.2 m. Most of the flows are characterized by an upper scoriaceous zone 1–3 meters thick, grading downward through a zone of fractured basalt into a massive flow interior. Some flows also have a thin basal breccia. Olivine tholeiites and some low‐magnesia basalts are commonly mottled, whereas ferrobasalts and icelandites have well developed flow banding. Most of the flows are fine‐ to medium‐grained, aphyric to sparsely phyric, and moderately vesicular. A few are moderately plagioclase phyric; others have scattered phenocrysts of clinopyroxene and olivine. Over 100 intrusive units are recognized in the core, and they span approximately the same compositional range as the flows. They are characterized by having steeply inclined chilled contacts, a uniform massive aspect, and an absence of vesicles or brecciation. Most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures. Many of the dikes are also aphyric, but some contain notable plagioclase or olivine phenocrysts. Groundmass textures range from fine‐grained, intersertal to coarse‐grained, ophitic. Clastic units occur typically as thin, varicolored layers between lava flows, rarely exceeding 0.5 m in thickness. These are chiefly fine‐ to coarse‐grained, poorly‐ to well‐bedded lithic tuffs showing varying degrees of reworking. Most are silicic to intermediate in composition, but basaltic units are also present. Two rhyolitic ash flow tuffs occur in the section, one of which is 30 m thick. We interpret the lavas to be the flanking portions of broad low‐angled central volcanoes centered on silicic complexes. Some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms. Most of the dikes were probably intruded laterally into the lava pile; thus they do not represent direct vertical feeders from deep magma bodies.

Solubility equilibria in basalt aquifers: The Columbia Plateau, eastern Washington, U.S.A.

A speciation-solubility geochemical model, WATEQ 2, was used to analyse geographically-diverse groundwater samples from the aquifers of the Columbia Plateau basalts in eastern Washington. The groundwater samples compute to be at equilibrium with calcite which provides both a solubility control for dissolved Ca and a pH buffer. Amorphic ferric hydroxide, Fe(OH) 3 (a), is at saturation or modestly oversaturated in the few water samples with measured redox potentials. Most of the groundwater samples compute to be at equilibrium with amorphic silica (glass) and wairakite, a zeolite, and are saturated to oversaturated with respect to allophane, an amorphic aluminosilicate. The water samples are saturated to undersaturated with halloysite, a clay, and are variably oversaturated with regard to other secondary clay minerals. Equilibrium between the groundwater and amorphic silica presumably results from the dissolution of the glassy matrix of the basalt. The oversaturation with respect to the clay minerals other than halloysite indicates that their rate of formation lags the dissolution rate of the basaltic glass. The modeling results indicate that metastable amorphic solids limit the concentration of dissolved Si and suggest the same possibility for Al and Fe. The results also suggest that the processes of dissolution of basaltic glass and formation of metastable secondary minerals are continuing even though the basalts are of Miocene age and presumably have undergone a long history of contact with groundwaters. The computed solubility relations are found to agree with the known assemblages of alteration minerals in the basalt fractures and vesicles, if account is taken of both the difficulty of identifying amorphic phases and the slow rate of formation of clay minerals at low temperatures. Because the chemical reactivity of the bedrock will influence the transport of solutes in groundwater, the observed solubility equilibria are important factors with regard to chemical-retention processes associated with the possible migration of nuclear waste stored in the earth's crust. Specifically, the occurrence of secondary minerals will enhance the sorption of dissolved radionuclides. Speciation-solubility geochemical models provide an important means of determining these solubility-equilibria relationships.

The magnetic properties of some DSDP basalts from the North Pacific and inferences for Pacific plate tectonics

One hundred seven basalt samples from seven DSDP sites in the North Pacific studied for magnetic properties were found to have a mean magnetization of 0.005 emu/cm3, a mean susceptibility of 0.001, and a mean Koenigsberger ratio of 10. The mean remanence coercivity as measured by the mean demagnetizing field is 90 Oe. The titanomagnetite in most of these old (13–115 m.y.) and highly fractured basalts has been largely oxidized to titanomaghemite during sea floor weathering. The titanomaghemitization is manifest in an increase in Curie temperature from about 160°C to 300°–350°C and a threefold to fourfold decrease in the saturation magnetization. Although a 50% decrease in remanence intensity and susceptibility can be attributed to oxidation at one site, the relation between degree of oxidation and these two properties is obscured at most sites by magnetic grain size differences. A decrease in remanence intensity no doubt occurs but was not observed because the most highly oxidized basalt, that in thin flows and in the margins of thicker flows, is also the finest grained and therefore had a higher initial remanence intensity. Remanence coercivity, while mainly dependent on magnetic grain size, increases about 50% with oxidation. The remanence direction does not appear to be affected by oxidation. Although the remanance inclinations differ considerably from the present geomagnetic field inclination at most sites, the expected range of secular variation is such that the remanence, with the exception ofthat in Meiji Seamount, could have been acquired by the basalts at the present site latitudes. But the site paleolatitudes calculated from the remanence inclinations correspond more closely to the paleolatitudes predicted by assuming that the Pacific plate has moved northward with the motions deduced by Morgan and others from linear volcanic chains. In particular, the coincidence of the magnetic paleolatitude, 19°, measured in Meiji Seamount at the northern end of the Emperor Seamount chain, with the latitude of the island of Hawaii, supports the fundamental assumption of the Morgan model of Pacific plate motion, wherein the volcanic locus or ‘hot spot’ forming the Emperor‐Hawaiian chain has remained essentially stationary with respect to the earth's axis.

Sulphur isotope compositions of sulphides and sulphates, DSDP Leg37

Sulphur isotope abundances were determined for 15 sedimentary and 9 basalt specimens from DSDP Leg 37 cores. Sedimentary sulphides had δ34S values as low as −54‰, which is consistent with isotopic selectivity during bacterial sulphate reduction. The δ34S values for soluble sedimentary sulphates were generally lower than ocean water sulphate. This is probably due to contributions of oxidized sedimentary sulphides. Pyrite in basalt fractures had δ34S values ranging from −33‰ to −5‰. An unique interpretation of these data is not possible at this time.

Mathematical simulation of subsurface flow contributions to snowmelt runoff, Reynolds Creek Watershed, Idaho

A mathematical model of subsurface flow is used to complement a field study of snowmelt runoff in a small upstream source area in the Reynolds Creek Experimental Watershed near Boise, Idaho. Field measurements from an instrumented cross section of this small watershed show that the mechanism of streamflow generation is subsurface delivery of meltwater over limited distances through shallow high‐permeability low‐porosity formations of altered and fractured basalt. The mathematical model provides a two‐dimensional transient saturated‐unsaturated analysis of the subsurface flow at the field site. It proved to be a valuable aid to a unified interpretation of the field measurements. For mathematical models that consist of boundary value problems with boundary conditions that are time and space dependent, the boundary condition sensitivity can thwart the rational calibration‐validation procedure. This, together with the more serious limitations of data availability, funds for data acquisition, and computer capacity, precludes the imminent use of fully deterministic hydrologic response models on a regional scale.

Microbiological Quality of Subsurface Drainage Water from Irrigated Agricultural Land

AbstractIrrigation and subsurface drainage waters sampled from an 82,150‐hectare (203,000‐acre) irrigation district in southern Idaho were evaluated for bacteriological quality. The soils in the district are wind deposited over fractured basalt, calcareous, and have a pH near 7.8. Drainage, where needed, is provided by horizontally mined tunnels or by tile drains connecting shallow relief wells that flow the year around. For the 12 month ending September 30, 1969, a 2‐meter (6.5‐foot) depth of water for the entire irrigation tract was diverted, and 50% of the water passed through the soil becoming subsurface drainage. The irrigation water and seven subsurface drains were sampled at 2‐week intervals during the summer of 1969. Coliform, fecal streptococci, starch hydrolyzers, and bacteria able to grow at temperatures from 0 to 55C were counted. The diverted irrigation water contained from 140 to 3,300 coliform per 100 ml, but 86% of the subsurface drainage samples contained 5 or fewer coliforms per 100 ml. Numbers of other microorganisms were also low in the drainage waters. The outflow samples were oxygen saturated and the temperatures were 13.0 ± 1.1C for all samples. Percolation through the soil improved the water quality almost to domestic water standards.

Magmatic Differentiation in Mount Girnar

The Girnar laccolith was intruded into a thick covering of basalt of late Cretaceous or early Eocene age in Kathiawar, India, which was lifted up in the form of a dome. Slow cooling of the magma gave rise to olivine-gabbro, diorite, monzonite, syenite, nepheline-syenite, and granophyre. This paper deals principally with the mechanism of intrusion and differentiation. Three phases of movement are represented by olivine-gabbro, diorite-monzonite, and granophyre, respectively. Diorite and monzonite occur in the center and are surrounded by olivine-gabbro. Granophyre is intruded in the marginal hills of basalt in the form of large dikes. This structural feature is explained by a circular system of fractures in basalt due to arching. The differentiation is shown to be the result of fractional crystallization; sediment-assimilation or immiscibility in the liquid state is regarded as improbable. The physical conditions which prevailed indicate quiet crystallization and absence of convection currents and crystal settling. Field evidence has been noticed supporting the origin of nephelite-syenite by the dissociation of the polysilicate feldspar.