Texture Grade Research Articles

SOIL TAXONOMY AND EVALUATION OF SOME NEWLY RECLAIMED AREAS ADJACENT TO THE NILE DELTA RIMS, EGYPT

This study was carried out to identify the main soil characteristics, and thensoil classification and evaluation of some newly reclaimed areas adjacent to theeast, west and south Nile Delta rims, Egypt. Such work is considered as a scientificfundamental base for a successful agricultural development of such areas. Toachieve this target, the main soil physical and chemical characteristics as well asplant nutrients status of the different soil types developed on the studied areas weredetermined, with a carefully study through representative eleven soil profiles.The obtained analytical data indicate that soil texture is dominated withmedium (i.e., sandy clay loam) and relatively coarse grades (i.e., sandy, loamy sandand sandy loam), except some soils of El-Hussania and Tal El-Yahudia areas thatare characterized by fine texture grade of clayey and sandy clay, respectively. Thatis true, since these soils are developed on different origins, i.e., the Nile alluviumand desert formations calcareous or siliceous in nature as well as under variousenvironmental conditions. Also, the soils under investigation are mostly non-saline,except of some ones are suffering from slight to high saline conditions at ElHussania, El-Nubaria and Abu-Zaabal areas, which are located at the east, west andsouth Nile Delta rims, respectively. In addition, the available plant nutrients statusin the studied soils was more attributed with the additions of organic manures andagro-management practices as well as the nature of soil sediments and locallyprevailing environmental conditions.Soil taxa of the studied soils could be categorized into three orders, i.e.,Vertisols, Entisols and Aridisols, and six families, as follows:i. Vertisols include one family of Typic Haplotorrerts, fine clayey, smectitic,thermic (i.e., soils developed on El-Hussania area).ii. Entisols: include four families of Typic Torrifluvents, sandy, mixed, thermic (i.e.,soils developed on El-Qurein and El-Salhia areas); Typic Torrifluvents, clayey,smectitic, thermic (i.e., soils developed on Tal El-Yahudia village); TypicTorriorthents, sandy, mixed, thermic (i.e., soils developed on El-Alikat ElBahariya village) and Typic Torripsamments, siliceous, thermic (i.e., soilsdeveloped on El-Sadat and Abu-Zaabal areas).iii. Aridisols: include one family of Typic Haplocalcids, fine loamy, mixed, thermic(i.e., soils developed on El-Beheira and El-Nubaria areas).The data of the current suitability of the studied soils for irrigated agricultureindicate the existing of three suitability classes, i.e., marginally (S3, soils of ElSadat, Alikat and Abu-Zaabal areas), moderately (S2, soils of El-Qurein, El-Salhia,El-Beheira, El-Nubaria and Tal El-Yahudia areas) and highly suitable (S1, somesoils of El-Beheira area). Soils of El-Nubaria can be shifted from the currentsuitability class of S2 to the potential suitability one of S1, due to their soillimitations are able to correct, i.e., wetness and salinity/alkalinity conditions.It is noteworthy to mention that the similarity of suitability classesrecognized in the studied areas for both current and potential conditions are mainlyattributed to most of the identified soil limitations can not able to correct. So, it could be recommended that the severity of soil texture (coarser in nature) can becorrected by application of organic and inorganic soil amendments as well asapplying drip and sprinkler irrigation systems to sustain a soil moisture content at afavourable condition for grown plants and biological activity in the soil.

The Pimenta Bueno kimberlite field, Rondônia, Brazil: Tuffisitic kimberlite and transitional textures

The Pimenta Bueno kimberlite field of Rondônia State, Brazil is located near the edge of the Amazon Craton and contains more than thirty bodies. A late to mid-Triassic emplacement age has been obtained for two kimberlites. The Pimenta Bueno kimberlite field includes pipes of up to 15 ha and a few apparently flat-lying hypabyssal kimberlite sills. In the three pipes and one sill examined, three textural end member types of kimberlite are recognized: hypabyssal kimberlite (HK), tuffisitic kimberlite (TK), and resedimented volcaniclastic kimberlite (RVK). There are gradations in textures between the three types. The overall gradation with depth is VKB, VK, TK, TKt, HKt to HK (where “t” denotes transitional varieties), and the textural gradations are apparent at all scales, megascopically to microscopically. Importantly, these gradations appear to occur within single phases of kimberlite. TK and the associated transitional varieties observed in the Pimenta Bueno field have striking similarities to those of many kimberlites of southern Africa and Canada. The Pimenta Bueno kimberlite field thus represents a newly recognized example of southern-African-style kimberlite pipes occurring on a third continent. The TK-bearing pipes of Pimenta Bueno, southern Africa, and Canada are markedly different from those found in the Canadian Prairies and Lac de Gras, Northwest Territories, which indicates that pipes containing TK and related textures are repeated in space and time but only form in certain circumstances. There is an apparent correlation between kimberlite texture and pipe size. In the larger pipes (> 5 ha), the TK is overlain by VKB. In the small pipes (< 3 ha) there is a textural gradation with depth from TK to HK and VKB is not present. One TKt–HKt transition appears to be ~ 60 m thick. The nature of the TK–TKt–HKt–HK textural transition suggests that TK developed by the textural modification of magmatic kimberlite within the pipe. The textures may reflect increased degrees of magma disruption associated with volatile exsolution.

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

AbstractNaturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha–1, 6.2 t ha–1, and 12.4 t ha–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils.While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha–1 and 12.4 t ha–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments.We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.

Turbidite emplacement on the southern Balearic Abyssal Plain (western Mediterranean Sea) during Marine Isotope Stages 1–3: an application of ITRAX XRF scanning of sediment cores to lithostratigraphic analysis

Abstract The upper part (0–20 m) of a long piston core from the SE Balearic Abyssal Plain — spanning the past 50 ka — has been studied using the ITRAX micro-XRF core scanner to obtain downcore elemental profiles. The Ca/Fe ratio was found to be an effective parameter to distinguish between turbidites and pelagites, because turbidites generally have higher Fe contents and lower Ca contents compared with pelagic intervals. Beds that were obscure when visually logged could be identified as turbidites or pelagites on their geochemical characteristics, allowing more complete subdivision of the sequence into genetic units. The ITRAX XRF data also provide useful information on textural grading, bioturbative mixing, identification of geochemically distinctive marker beds, indications of differences in provenance, and confirm or query the presence of early arrivals during turbidite emplacement. A chronostratigraphic framework for the core based on accelerator mass spectrometry (AMS) radiocarbon dating and correlation with oxygen isotope stages of pelagic intervals in other cores (using calcium carbonate stratigraphy) was also established. This shows that turbidite emplacement on this part of the Balearic Abyssal Plain has been modulated strongly by climate and sea-level change, with turbidite emplacement most frequent during the early Holocene when the rate of post-glacial sea-level rise was greatest. Deposition of the coarsest (i.e. sand and silt-based) turbidites at the core site was restricted to the full and Late Glacial (11–25 ka). Turbidite emplacement during Oxygen Isotope Stage 3 was rare. Most of the turbidites at the site are distal, but some coarse-grained-based turbidites are characterized by higher Sr/Ca ratios (possibly indicating a higher aragonite content), higher Ca and lower Fe contents compared to other turbidites, and are interpreted as having a more proximal shelf source. Such turbidites are generally rare, however, and restricted to full Glacial and Younger Dryas time. There is little evidence for large-scale seismogenic turbidites (expected to be seen as randomly timed emplacement, seemingly independent of eustatic control) at the core site, despite proximity to the seismically active Algerian margin 100 km to the south. This suggests that seismogenic turbidites must largely bypass this part of the plain. Although the ITRAX core scanner provides a rapid and non-destructive means of characterizing downcore geochemical distributions in great detail, interpretation of the data requires caution and assessment from an informed standpoint. Analytical artefacts such as those caused by water or organic content, degree of compaction, grain-size and mineral effects, unevenness of the cut core surface and poor discrimination of closely spaced element XRF peaks need identification and elimination.

PHYSICAL, CHEMICAL AND MACRO-MICROMORPHOLOGICAL CHARACTERISTICS OF SOME ALLUVIAL SOILS IRRIGATED WITH DIFFERENT WATER RESOURCES الخصائص الطبيعية والکيميائية والمورفولوجية و الميکروموفولوجية لبعض الأراضى الرسوبية النهرية المروية بمصادر مائية مختلفة فى محافظة أسيوط-مصر

In addition to the Nile River, which is the main resource of water, there are alternative sources, i.e. groundwater, agricultural drainage water and sewage water are used for irrigation in many parts of Egypt. The aim of this work was to study the morphological and micromorphological features as well as some physical and chemical properties of the alluvial soils irrigated with these waters at Assiut city. Four sites were chosen to represent the soils irrigated with different water sources, i.e. the fresh Nile water, artesian water, Nile water polluted with incompletely treated sewage water and Nile water polluted with agricultural drainage water. One profile was chosen and dug to represent the soils of each site. The profiles were morphologically described and some physical and chemical characteristics of their layers were measured. Thin sections were prepared from undisturbed surface and subsurface soil samples, then micromorphologically examined. The obtained results revealed that the studied soils are generally flat to almost flat, deep, of water table deeper than 150 cm and are moderately well drained (the sites irrigated with artesian and agric. drainage polluted Nile waters) to well drained (the sites irrigated with Nile and polluted sewage waters). Although, the surface and subsurface layers of soils irrigated with Nile, artesian and sewage polluted Nile waters have the same texture grade (sandy loam), some properties such as, matrix colour, structure and consistency, differ in the surface layer irrigated with sewage polluted Nile water. Soil irrigated with sewage polluted Nile water has the highest contents of organic matter, pH, available Zn, Cu and Pb, whereas, irrigated with artesian water has the highest amounts of calcium carbonate, EC, ESP, soluble Ca, Mg and K, and extractable Fe, B and Ni. Moreover, the soil irrigated with agric. drainage polluted Nile water contains higher values of soluble Na, CEC and available Mn than the others. Micromorphological features of the studied soil samples indicate that they have apedal soil materials, with mainly primary structure. Porphyroskelic and agglomeroplasmic fabrics characterize the soil surfaces representing soils irrigated with fresh Nile, artesian and sewage polluted Nile waters, while samples irrigated with agric. Drainage polluted Nile water show silty-sized grains embedded in dense soil plasma. The argillasepic plasmic fabric predominates over the other in the examined samples of the studied soils. Plasma separations are noticed as vo-in-sepic in all studied samples. The dominant void type is vugh, that is mainly interconnected, irregular, ortho and/or metavugh. Vesicles are noticed in all-surface samples. Channels and chambers are seen in the subsurface samples irrigated

Geology of the Gahcho Kué kimberlite pipes, NWT, Canada: root to diatreme magmatic transition zones

The Cambrian Gahcho Kué kimberlite cluster includes four main pipes that have been emplaced into the Archaean basement granitoids of the Slave Craton. Each of the steep-sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite (HK) to diatreme-facies tuffisitic kimberlite breccia (TKB). The TKB displays many diagnostic features including abundant unaltered country rock xenoliths, pelletal lapilli, serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate. The HK contains common fresh olivine set in a groundmass composed of monticellite, phlogopite, perovskite, serpentine and carbonate. A number of separate phases of kimberlite display a magmatic textural gradation from TKB to HK, which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity, proportion of fresh olivine and the degree of xenolith digestion.The pipe shapes and infills of the Gahcho Kué kimberlites are similar to those of the classic South African pipes, particularly those of the Kimberley area. Similar intrusive magmatic emplacement processes are proposed in which the diatreme-zone results from the degassing, after breakthrough, of the intruding magma column. The transition zones represent ‘frozen’ degassing fronts. The style of emplacement of the Gahcho Kué kimberlites is very different from that of many other pipes in Canada such as at Lac de Gras, Fort à la Corne or Attawapiskat.

Strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation: an example from high-grade rocks of the Norwegian Caledonides

‘Isotope stratigraphy’ has been applied to an undated, high-grade carbonate sequence from the Norwegian Caledonides in order to constrain the depositional age. Sokumfjell Group marbles of the Beiarn Nappe Complex, in the Uppermost Allochthon of the Nordland Caledonides, were subjected to several phases of deformation culminating in amphibolite-facies metamorphism. The ca. 1000 m thick marble sequence studied is composed of five different lithofacies. Lithofacies 1–4 are calcite marbles and Lithofacies 5 is dolomite marble. All the calcite marble lithofacies exhibit, throughout, rhythmic bedding and textural grading on different scales, suggesting that carbonate material was deposited in a pelagic, turbidite system. 123 samples were analysed for major and trace elements, and acid-soluble Mg and Ca, of which 42 samples of calcite marbles and four samples of dolomite marbles were selected for δ 13C, δ 18O and 87Sr/ 86Sr analyses. All selected calcite marbles have high Sr contents, averaging 2000 ppm. They are characterised by 87Sr/ 86Sr ratios ranging between 0.70642 and 0.70999, and δ 13C values ranging between +1.3 and +8.4‰ (V-PDB). δ 18O values are highly variable, 14.1–24.8‰ (V-SMOW). The ‘best preserved’ samples selected on the basis of Mn, Sr and Rb concentration, and Mn/Sr and Mg/Ca ratios yield average 87Sr/ 86Sr values of 0.70669, 0.70697, 0.70727, 0.70749 and 0.70737 for Lithofacies 1, 2, 3, 4 and 5, respectively. Post-sedimentary alterations including amphibolite-facies metamorphism resulted in only a slight increase of 87Sr/ 86Sr values in the marbles studied apparently due to aragonite precursor, high Sr concentration, and a very early and fast diagenetic alteration caused by fluids isotopically comparable with that of seawater. Projection of these values along with δ 13C measurements onto 87Sr/ 86Sr and δ 13C calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 635 and 600 Ma.

Structure and neotectonics on the Main Divide, upper Wilberforce valley, mid Canterbury, New Zealand

The upper Wilberforce valley drains the Main Divide of the northern portion of the actively rising Southern Alps. The Wilberforce area lies in a transition zone between oblique convergence to the south and a predominantly strike‐slip segment to the north. The structure of the area is dominated by the 2 km wide, currently active, Main Divide Fault Zone (MDFZ), which strikes northeast and consists of two main strands. The western strand crosses the Main Divide at Browning Pass and dies out northwards. The eastern strand runs parallel to the Main Divide immediately east of the divide in the upper Wilberforce valley and then crosses the divide farther north. A broad region of relatively low relief and topography at Browning Pass has developed between the two fault strands. The MDFZ was initially dominated by southeast‐directed oblique reverse faulting. Neotectonic features in the Browning Pass area, between the two fault strands, show dextral strike‐slip motion with as much as 50 m horizontal displacement affecting recently deglaciated surfaces but <10 m of vertical displacement. South of the MDFZ, the structure of the area is dominated by north‐striking faults which intersect the MDFZ immediately east of the divide. These faults are subparallel to bedding in the host Torlesse Terrane sediments, and faults and bedding control topography. Bedding becomes rotated subparallel to the MDFZ along the Main Divide. A spaced cleavage in metasediments on the hanging wall of the MDFZ may indicate that the hanging‐wall rocks are of higher textural grade than the textural zone I footwall rocks.

Age and isotopic characterisation of geological terranes in Marlborough Schist, Nelson/Marlborough, New Zealand

K‐Ar and Rb‐Sr ages of Marlborough Schist and Pelorus Group metasediments show coherent regional metamorphic/cooling age patterns, irrespective of their terrane protolith, in which ages are primarily inversely related to metamorphic grade and, more locally, with distance from the Picton Fault Zone. The oldest ages, from the lowest grade (prehnite‐pumpellyite facies, textural grade 1'IIA) Pelorus Group are mainly in the range 175–200 Ma, indicating regional metamorphism occurred at least in earliest Jurassic or latest Triassic times. Progressive cooling of Marlborough Schist resulted in a spectrum of cooling ages, mainly 110–180 Ma, which reflects slow regional uplift during the Jurassic and Early Cretaceous. Some disruption of this pattern occurred at the Picton Fault Zone where post‐Early Cretaceous movement has juxtaposed high and low structural levels. Initial 87Sr/86Sr isotopic ratios (i) at the time of metamorphism (t), derived from Rb‐Sr whole‐rock isochron data, further characterise terrane protoliths identified by field, petrographic, and geochemical evidence. The (t)‐(i) data largely confirm these prior regional groupings but do not entirely confirm the terrane identifications. A less radiogenic group, (i) < 0.7055, termed Western Marlborough Schist, comprises the Pelorus Group, and schist equivalents, west of a postulated suture/terrane boundary in the schists, and is correlated with Caples Terrane rocks in Otago. “Torlesse"‐type schists are more radiogenic, (i) > 0.7062, but do not match Torlesse (Rakaia) Terrane (t)‐(i) data from Wellington or Otago. They correlate better with Kaimanawa Schist of the central North Island. An intermediate group, (i) = 0.7042–0.7062, is separate from the “Torlesse” group, but partly overlaps the Caples Terrane group. This correlates well with Waipapa Terrane of the central North Island and Haast Schist of uncertain terrane affinity in Otago. The combined radiogenic and intermediate groups, termed Eastern Marlborough Schist, define a lineage which thus includes Kaimanawa Schist, Pahau Subterrane rocks, and both Waipapa‐type and Torlesse‐type schists in Marlborough. It is possible that the Moonlight Fault Zone of Otago has a continuation in Eastern Marlborough Schist as the Picton Fault Zone.

Structural and textural development in Singhbhum shear zone, eastern India

The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation, folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa during mylonitization.

Effects of the physical properties ofsphagnumpeat on the nursery growth of containerizedpicea marianaandpicea glaucaseedlings

Containerized black spruce (Picea mariana [Mill.] B.S.P.) and white spruce (Picea glauca [Moench] Voss) seedlings were grown from seed in Sphagnum media at two different degrees of humification (light and moderate) and at two artificially created textural grades (with or without particles < 1.3 mm). During the 26‐week growing period, the water content in the media was maintained at 60% of container capacity. At the end of the growing period, white spruce seedlings grown in the peat media enriched in fine particles (<1.3 mm) had a greater height, diameter, and root and shoot dry mass than those grown in peats deprived of fine particles. The degree of humification per se had little influence on growth. Black spruce seedlings, on the other hand, showed only a small increase in height with the addition of fine particles. Significant (p <0.05) linear relationships were found between physical properties of the media and final morphological measurements of white spruce: between easily available water and both he...

Effects of the physical properties ofsphagnumpeat on water stress in containerizedpicea marianaseedlings under simulated field conditions

Containerized black spruce seedlings, grown in Sphagnum peat at two degrees of humification (light and moderate) and two artificially created textural grades (with or without particles <1.3 mm), were subjected to controlled degrees of water stress using polyethylene glycol (PEG) at 0, 20, 75 and 175 g PEG kg−1 H2O, with and without dialysis membranes. After a 4‐day exposure, shoot water potential and water potential at the soil‐root interface were strongly affected by the humification of the peat at the highest PEG concentration. Differences in fine particle content of the peats had little effect on the physiological variables measured. Needle conductance was affected greatly by the presence of the dialysis membrane itself. The results suggest that high aeration, one of the most desirable physical properties of a growing medium for nursery production, may become undesirable once the seedlings are outplanted.

Growth and carcass characteristics of Angus and American Wagyu steers

Ten purebred Angus and ten crossbred (3/4–7/8) American Wagyu steers were fed a corn and barley-based diet for 552 days and slaughtered. All USDA and Japanese yield and quality grade factors were evaluated and the amount of extractable lipid and moisture in totally trimmed muscle was determined. Angus steers gained 0·9 kg/head/day and American Wagyus gained 0·7 kg/head/day. This difference in growth rate resulted in Angus steers having a heavier final weight ( P < 0·05). Angus steers required less feed per unit of gain than did American Wagyu steers. Adjusted fat thickness over M. longissimus dorsi opposite the 12th rib was approximately 3·5 cm and was not statistically different between the two breeds ( P > 0·05). Average ribeye area and kidney, pelvic and heart fat were similar for the two breeds. The calculated USDA yield grade for both breeds exceeded 6. Average USDA marbling score was nearly a degree of marbling higher for American Wagyu than for Angus, but variation within breed groups was high. Average USDA quality grade was well into USDA Prime for all carcasses and did not differ by breed ( P > 0·05). Japanese yield grade factors were similar for both breeds except for cold left side weight and the yield estimation which includes an adjustment factor that favors American Wagyu. American Wagyu steers merited a higher Japanese marbling score than did Angus ( P < 0·05). Beef color score, firmness, texture and firmness and texture grade were also different between the breeds ( P < 0·05). Fat colour, luster and quality were not different ( P > 0·05). These data clearly show that some American Wagyu steers have the genetic ability to deposit as much marbling as Japanese Black cattle raised in Japan.

Environment of Deposition of Downdip Lower Wilcox Sandstones, Provident City Field, Lavaca County, Texas: ABSTRACT

ABSTRACT The Lower Wilcox section at Provident City Field produces dry gas from thin-bedded, silty sandstones, at depths of 12,500 to 14,100 ft (3810 to 4298 m). The sandstones lie downdip of the Colorado and Guadalupe deltas of the Rockdale Delta System, within the Wilcox fault zone. Regional stratigraphy and structural trends indicate that the sandstones were deposited in a deep marine environment. Cores show that sandstone cosets range 0.1 to 2.7 ft (0.03 to 0.82 m) and average 0.58 ft (0.18 m) in thickness. Sedimentary structures within the cosets range upward from a massive unit (A) to a planar-laminated unit (B) to a ripple-laminated unit (C). The cosets have an average composition of lithic arkose and show textural grading indicative of deposition from turbidity flows. A growth fault, having approximately 1000 ft (3048 m) of throw at a depth of 12,300 ft (3750 m), bounds the field to the northwest and largely controls the distribution of lithofacies. Stacked, AB-type, turbidite cosets indicate channel facies. The M Sandstone was deposited as a constructional channel, with abrupt lateral grading to overbank facies, where turbidites of the BC- and C-type are dominant. The S Sandstone was deposited as a series of thin, constructional channels, mostly with turbidites of the AB- and ABC-type, that are generally stacked, causing superimposed, dip-trending lobes on an otherwise strike-trending sandstone.

Latest Cretaceous and early Tertiary orogenesis in south-central Arizona: Thrust faulting, regional metamorphism, and granitic plutonism

Rocks in a number of mountain ranges in southernmost central Arizona are juxtaposed by thrust faults, regionally metamorphosed, and intruded by garnet–two-mica granites. Field relations and K-Ar and U-Pb isotopic geochronology indicate that thrust faulting, metamorphism, and granitic plutonism were closely related aspects of a latest Cretaceous and early Tertiary orogenic episode. The regionally metamorphosed rocks, chiefly greenschist-facies quartzofeldspathic schists, were derived from Jurassic and Cretaceous sedimentary, volcanic, and plutonic rocks. In five ranges, these metamorphic rocks are overlain, along synmetamorphic thrust faults, by Precambrian gneiss or Late Jurassic or Late Cretaceous plutonic rocks. In three additional ranges, upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range. Seven of these eight thrust faults have single lower plates, but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides. Areal structural relations and the regional arrangement of distinctive Jurassic igneous rock units strongly suggest that one large range, the Baboquivari Mountains, and several smaller ranges are fensters in a thrust-fault system of regional extent, and that several adjacent ranges are allochthonous. The regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early Tertiary leucocratic granites characterized by various combinations of accessory biotite, muscovite, and garnet. At least two of these granites contain inherited Precambrian zircon indicative of generation by crustal anatexis. K-Ar and(or) U-Pb isotopic ages of premetamorphic granites, metamorphic rocks, thrust-zone mylonites, synmetamorphic granites, and metamorphogenic fissure veins indicate that the orogenic episode commenced in Late Cretaceous time and culminated in early Tertiary time, 58 to 60 m.y. ago. The observed and inferred close spatial and temporal relations between thrust faulting, regional metamorphism, and granitic plutonism lead to the following hypothesis for orogenesis in south-central Arizona: crustal compression caused overthrusting of crystalline rocks, resulting in crustal thickening; and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place.

Lead-210 sediment geochronology in a changing coastal environment

Sediment accumulation rate studies utilizing excess 210Pb and 137Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm −2 · yr −1 or approximately 8.4 to 11.8 cm · yr −1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the 210Pb profile in order to establish a geochronology. Grain size 210Pb distribution measurements at three depths reveal that the specific activity (dpm · g −1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess 210Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events. Sixteen 137Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm −2 · yr −1.

Contrasting Depositional Processes of Sub-Clarksville and Woodbine Reservoir Sandstones: ABSTRACT

ABSTRACT The Sub-Clarksville and Woodbine sandstones are newly discovered gas reservoirs in Grimes County. Although the sandstones accumulated in a dominantly shallow marine environment, depositional processes and transport mechanisms were different for each sand body. The Sub-Clarksville represents rapid deposition by storm-generated bottom currents. Sand accumulations are lens-shaped and were restricted to lows by shelf topography. Sedimentary structures grade from laminated to massive sandstone above a sharp basal contact, to churned shaly sandstone at the top overlain by Austin micrite. Mean grain size decreases upward with an average composition of 83 percent quartz, 6 percent rock fragments, 9 percent matrix, and abundant carbonate cement. Quartz content decreases and matrix increases upward. Bedding sequence and textural gradation suggest that sand was eroded from preexisting Woodbine sediments to the east, and transported to the shelf during passage of a major storm. In contrast, Woodbine sandstones represent a prograding deltaic environment. The sandstones are highly quartzose (80-94 percent) and show an increasing grain size upward. The C sandstone at Hill field is directly overlain by the Austin and grades from bioturbated mudstone at the base to massive or faintly laminated sandstone near the top. Woodbine sandstones C and D thicken significantly eastward as an extension of a giant prograding deltaic system to the northeast. Conversely, Woodbine A and B sandstones were deposited as offshore bars farther west at Kurten (Bell, 1980; Turner and Conger, 1981). Distribution of both the Sub-Clarksville and Woodbine sandstones was affected by deep-seated salt movement at Hill field and the unconformity associated with this local high. Future exploration will depend on a detailed knowledge of the depositional framework in these downdip areas.

Provenance and Depositional Mode of Upper Cretaceous Chatsworth Formation Conglomerates, Simi Hills, California: ABSTRACT

Conglomerates of the Upper Cretaceous Chatsworth Formation occur as lenses of concentrated clasts in channels and as clasts dispersed at the base of thick, coarse-grained, graded sandstone beds. The matrix of the conglomerates consists of grains ranging from silt to granule size (4 mm) and comprises between 20 and 78% of any one conglomerate unit. The matrix composition ranges from 30 to 80% quartz, 25 to 60% feldspar, and 5 to 20% lithic fragments, with accessory biotite up to 5%. The conglomerate clast population is composed principally of clasts in the pebble size range. Five distinct rock types are recognized within the conglomerate clast population: blue-black argillite, 4 to 20%; felsic volcanites, 4 to 28%; felsic plutonites, 16 to 24%; arkosic sandstones and silts one, 0 to 12%; and a group of genetically related quartz-rich clasts, 17 to 46%. The quartz-rich clasts include sandstones and siltstones with continuous textural gradation from well-preserved sandstone through partially recrystallized sandstone with sutured grains, into metamorphosed, foliated quartz sandstone and quartz schist types. In addition, a conglomerate unit may contain between 0 and 14% authigenic rip-up clasts. The Chatsworth Formation, as a whole, is recognized to be a deep-sea fan complex upon which the primary depositional agent for sand was turbidity currents. Lenses of concentrated pebble conglomerates originated as debris flow, whereas beds of dispersed pebble clasts are of turbidity current origin. Paleocurrent data and the conglomerate clast composition for the Chatsworth Formation indicate that its detritus was derived from a source terrane to the south of the Simi Hills. The Santa Monica Mountains basement complex contains a large mass of argillite and felsic plutonite, but contains no felsic volcanite or quartz-rich suite of rocks. The basement in the northern Peninsular Ranges includes representatives of the principal rock types recognized in the clasts of the Chatsworth Formation conglomerates and, therefore, it is the best possible choice for the provenance. Extensive Franciscan terrane also lies south of the Chatsworth conglomerates, but no Franciscan detritus is recognized in the Chatsworth Formation. End_of_Article - Last_Page 1686------------

Debris flow (olistostromes) and slumping on a distal passive continental margin: the Palombini limestone–shale sequence of the northern Apennines

ABSTRACTAn olistostrome accumulation up to 530 m thick occurs in the Casanova area of the northern Apennines. It lies within or above the calciturbiditic Palombini limestone‐shale sequence, and is part of the allochthonous Vara Complex—sediments originally deposited on oceanic crust. The olistostromes are poorly sorted, monomict, matrix‐supported, submarine debris flow deposits with rigid plugs. They have a compactional foliation and a compaction‐modified, planar clast fabric created during flow. Although diachronous in the main part of the area, the olistostromes have a vertical gradational contact with the overlying slumped Palombini, in which recumbent asymmetric fold hinges and trains and slump boudins are present. Many criteria indicate soft‐sediment deformation. Fold asymmetry indicates a uniformly SW‐dipping palaeoslope. The textural gradation from slumps to olistostrome beds, plus slump folds and boudins as olistostrome clasts show that the olistostromes are dismembered slumps. In vertical sections, variations in limestone petrography, volume percentage and size of clasts confirm that the olistostromes were derived from the Palombini as a series of bed‐by‐bed slumps keeping pace with sedimentation of the Palombini. From olistostrome clast sizes and bed thicknesses, a depositional slope of ∼ 4° is estimated. The olistostromes are not precursor sediments shed from advancing nappes, as in the Bracco ridge model of some authors; rather, they were formed at the foot of a distal, block‐faulted passive continental margin, long before nappe emplacement.

Sedimentology of Tuscaloosa Sandstones from False River and Judge Digby Fields, Pointe Coupee and West Baton Rouge Parishes, Louisiana: ABSTRACT

Significant gas reserves at depths ranging from 19,800 to 20,400 ft (6,035 to 6,218 m) have been established in Tuscaloosa sandstones at False River and Judge Digby fields (Chevron U.S.A. Inc. discoveries) located within the deep Tuscaloosa gas trend. The sandstones are within an Upper Cretaceous clastic wedge that thickens in front of, and to the south of, a Lower Cretaceous carbonate shelf. Reservoirs at Judge Digby field are part of a thick clastic section comprised of gray shales and medium to coarse-grained sandstones, in part conglomeratic. Paleontology, textural grading, bed contacts, and sequential arrangement of sedimentary structures suggest fluvial and deltaic environments. Reservoirs at False River field are stratigraphically younger and are interpreted as part of a barrier bar sequence. These sandstones are fine to medium-grained and occur at the top of upward-coarsening gradational sequences. In both fields, the reservoirs commonly have maximum porosities above 25% and permeabilities in excess of 100 md. Although these reservoirs are presently geopressured, petrographic evidence and density of the associated shales indicate that the sandstones are not underco pacted. A study of the sandstone framework suggests that burial diagenesis dramatically reduced the primary pore system, eliminating significant permeability. Leaching of the grain framework created an important contribution to effective secondary porosity and reestablished lost permeability. The secondary pores are easily identified by relic rims of authigenic, grain-coating chlorite cement which persisted after selective grain removal by dissolution. Commercial End_Page 993------------------------------ Figure production from these deeply buried Tuscaloosa sandstones is dependent upon secondary enhancement of a reduced primary pore system and its subsequent maintenance by geopressure. End_of_Article - Last_Page 994------------