Chert Deposits Research Articles

Growth fault control of Early Archaean cherts, barite mounds and chert-barite veins, North Pole Dome, Eastern Pilbara, Western Australia

This paper describes the results of detailed mapping and combined structural and sedimentologic investigation of the ∼ 3.5 Ga old North Pole Chert along the eastern and southern flanks of the North Pole granitoid dome of the Pilbara craton. The following relationships have been found. (1) Stepwise and repeated thickening over north block-down listric normal growth faults, now tilted to 120 E. Apart from the thickness distribution, the growth fault character has been inferred from progressive upward decrease of throw towards gentle flexuring, and the presence of roll-over anticlines, in a restored pre-tilt cross-section. The faults are unrelated to the actual geometry of the dome, and overprinted by pre-doming compressive deformation. (2) A reconsideration of the sedimentary model of the chert-barite unit with emphasis on vertical grainsize distribution and facies architecture shows direct synsedimentary fault control. (3) Black chert veins, clustered in the hangingwall blocks of the growth faults follow a conjugate fracture pattern in and immediately beneath the North Pole Chert, symmetrical with respect to the synsedimentary fault geometry. Upwards, the veins terminate in the lowermost chert-barite unit of the North Pole Chert; downwards, the majority of the veins converge towards centres at the growth faults in the underlying basalt. (5) Barite occurs: in veins, in association with black chert; as large synsedimentary mounds (15 m high, 50 m wide), formed on the original shallow subaqueous basin floor; as commonly silicified sinters and sedimentary or early diagenetic evaporite associated with littoral and/or stromatoloidal facies. The barite mounds are directly connected with chert-barite veins, and are uplapped by banded sedimentary chert and diamictite with barite clasts. Field relationships suggest precipitation of barite, particularly in the mounds, may have been primary and synsedimentary. Vein barite at or near the original depositional surface of the chert is associated with base metal sulphide at depth. The tectono-sedimentary relationships between normal faults, sedimentation patterns and thickness distribution, primary and secondary chert, synsedimentary barite mounds, stratiform barite and chert-barite veins, support a tensional fault-controlled, hydrothermal model with Ba, Si and sulphide emanation in ca 50 m depth of water. Such a model envisages boiling and/or degassing above vents to locally mix a normally stratified water body, causing instantaneous oxidization into sulphate. The sedimentary model of the North Pole Chert indicates repeated uplift and subsidence, with development of faults and fractures, the intensity of which diminished from the lowermost chert-barite unit of the North Pole Chert to the uppermost, fifth unit.

Turbidites and contourites of the Palaeogene Lefkara Formation, southern Cyprus

Deposition of the Palaeogene to early Neogene calcareous and siliceous deep-water sediments of the Lefkara Formation in Cyprus was successively dominated by pelagic, turbidity current, a return to pelagic and then bottom-current processes. Sedimentation occurred on the distal parts of a carbonate slope-apron to basin plain setting located in the western arm of the Tethys Ocean. The early phase of sedimentation directly overlying ocean crust, ridge-derived volcaniclastics and chemogenic sediments, was dominated by pelagic deposition of marl, marly chalk and radiolarian-rich calcilutite. This was followed by a gradual increase in the influx of biogenic turbidites (forming both chalk and chert deposits) from the north during the Early and Middle Eocene period, primarily as a response to tectonic uplift of the Kyrenia Range. Slower rates of sedimentation returned during the Late Eocene to Early Miocene, together with significant hiatuses in the sedimentary record. Together these reflect the cessation of turbidity current input, continued pelagic deposition and the onset of bottom-current influence on sedimentation. A combination of subtle features and supporting evidence allows the recognition of contourites in all the sections studied, together with the interaction of turbidity current and bottom-current processes in parts of the Lefkara Formation.

Silica-replaced fossils through the Phanerozoic

A systematic survey of 1863 papers on macrobenthic assemblages reveals that an average of 21% of published Paleozoic papers concern silicified fossils, but that average drops to just 4% for post-Paleozoic papers. During the Paleozoic, silicified fossil occurrences do not significantly correlate with the amount of shelf chert, outcrop area, time, duration of geologic intervals, or carbonate rock volume. This substantial drop in numbers of silicified fossils coincides temporally with increased importance of aragonite faunas following the end-Permian extinctions. However, qualitative measurements of secular changes in abundance and diversity of siliceous sponges are consistent not only with the post-Paleozoic decline in fossil silicification, but also with fluctuations in the amount of silicified fossils throughout the Paleozoic. The facies distribution of silicified fossils in the Permian of West Texas also suggests that the distribution of silicified fossils may reflect the occurrence of siliceous sponges. The decline in silicified fossils after the Permian may be related to a concomitant rise in offshore bedded chert deposition and movement of the locus of biogenic silica formation from nearshore to offshore regions beginning in the Triassic, rather than with the expansion of diatoms in the Cretaceous.

Tectonic implications of early silurian thrust imbrication of the northern exploits subzone, Central Newfoundland

Central Newfoundland's Dunnage Zone contains a composite assemblage of island arc and oceanic rocks formed in the Paleozoic Iapetus Ocean. Two different and perhaps unrelated Late Cambrian-Middle Ordovician volcanic belts (Lush's Bight and Robert's Arm belts, and their correlatives) are preserved in the Notre Dame Subzone, and are interpreted to represent arc and back-arc basin sequences developed; (1) adjacent to the Laurentian margin of Iapetus and (2) in the intraoceanic realm of the 4000 km wide Paleozoic ocean. Paleozoic paleogeographic reconstructions suggest that the Exploits Subzone contains volcanic and sedimentary rocks originally deposited in a third arc located between the intraoceanic arc (Robert's Arm Belt) and the Peruvian promontory of Gondwana. The New Bay Pond area is located within the northern Exploits Subzone, and preserves an imbricate thrust stack formed during Late Ordovician/Early Silurian southeastward-directed thrusting. Thrust-load related subsidence led to deposition of cherts and argillites overlain by a southeastward-prograding wedge of orogenic flysch and molasse in a foreland trough. Late Ordovician-Early Silurian structural disruption of the flysch/molasse sequence and older strata of the northern Exploits Subzone, by a series of thrust faults, place Middle Ordovician rocks over the younger flysch, and formed a new, southeast-vergent accretionary wedge. This event records the emplacement of the Exploits Subzone over Gondwanan continental margin rocks of the Gander Zone, soon after the Notre Dame Subzone collided with the Appalachian margin of Laurentia. Continued closure of Iapetus brought together the collisionally modified margins of Gondwana and Laurentia. The Exploits Subzone is cut by numerous Silurian-mid Devonian predominantly dextral strike-slip faults, suggesting that this convergence involved a significant component of dextral transpression. Several pull-apart basins are located along these strike-slip faults, and are filled by Silurian volcano-sedimentary sequences. These faults formed in response to the counter-clockwise rotation of Gondwana, juxtaposing the Avolonian margin of Gondwana with the Appalachian margin of Laurentia by the Carboniferous.

Mesozoic radiolarians and radiolarian‐bearing sequences in the circum‐Pacific regions: A report of the symposium‐‘Radiolarians and Orogenic Belts’

Abstract This paper contains extended abstracts of the seven papers presented at the symposium ‘Radiolarians and Orogenic Belts’ held at the seventh meeting of the International Association of Radiolarian Paleontologists (INTERRAD). Important results of the symposium include the following: (1) Upper Paleozoic and Mesozoic cherts are widely distributed within accretionary complexes in the circum‐Pacific orogenic belt. Radiolarian dating reveals that long durations of chert sedimentation in a pelagic environment are recorded on both sides of Pacific‐rim accretionary complexes (e.g. New Zealand, Japan, Russian Far East, Canadian Cordillera). (2) Triassic radiolarian faunas from New Zealand and the Omolon Massif, northeast Siberia are similar in composition and are characterized by the absence of typical Tethyan elements. This suggests that radiolarian faunal provincialism may have been established as early as the Triassic. High‐latitude radiolarian taxa exhibit a bi‐polar distribution pattern. (3) The Lower Triassic interval in chert dominant pelagic sequences is mechanically weaker than other levels and acted as a décollement in accretionary events. This lithologic. contrast in physical property is considered to reflect radiolarian evolution, such as the end‐Permian mass extinction.

Stratigraphy and depositional evolution of the Upper Palaeozoic sedimentary succession in eastern Peary Land, North Greenland

The Upper Palaeozoic Foldedal and Kim Fjelde formations in eastern Peary Land are redefined on the basis of new biostratigraphic data, including fusulinids, conodonts, palynomorps and small foraminifera. The Foldedal Formation in its new definition includes all late Moscovian to Gzelian deposits in the region. It is separated by a major hiatus from the redefined Kim Fjelde Formation which includes mid-Permian (late Artinskian - Kungurian) carbonates and chert deposits. The Upper Carboniferous succession is dominated by cyclically interbedded siliciclastics and carbonates with minor tabular build-ups. The mid and Upper Permian succession consists of cool-water carbonates, spiculitic chert and shales.

Tectonic evolution of the Bantimala Complex, South Sulawesi, Indonesia

Abstract The Bantimala Complex of South Sulawesi, Indonesia is an assemblage of northeast-dipping tectonically stacked slices. The slices consist mainly of high pressure metamorphic rocks, radiolarian chert, breccia, sandstone and shale, and melange. In order to understand the tectonic evolution of the Bantimala Complex, we have investigated the lithology, age, stratigraphy, structure and relationships of the components. The K-Ar ages of high P-low T metamorphic rocks suggest that an oceanic plate subducted beneath the Sundaland continent during the Late Jurassic or earliest Cretaceous. The subduction ceased during the Albian, and the high pressure schists were exhumed and eroded at the surface before and during the deposition of middle Cretaceous radiolarian chert. The exhumation of the schists was related to the collision of microcontinents derived from Gondwanaland. The Jurassic shallow marine sedimentary rocks in the Bantimala Complex are possibly remnant fragments of the collided microcontinent. Tectonic stacking of the Bantimala Complex was caused by Neogene subduction and collision of another continental fragment further to the east.

Statistical Modelling of Exploration Data for Grade Potential of the Phosphorite Deposit, Khatamba Blocks, Jhabua District, Madhya Pradesh

Abstract The Phosphorite deposit (74°25' to 74°26'E : 22°58' to 23°00'N), Jhabua district, Madhya Pradesh, is one of the productive deposits in India, and has evoked great interest with regard to its phosphogenesis, stromatolite association and resource potential. It is a sedimentary-stromatolite type of deposit within the Precambrian metasediments of the Aravalli Supergroup and represents the southern extension of the well known Jhamarkotra phosphorite deposit of Rajasthan. The geological set-up shows that the phosphorite occurs in two separate host rocks of older carbonate and younger chert sediments. Geological Survey of India (Khan et al. 1979), using conventional cross-section method, estimated the weighted average grade of the Khatamba Blocks from the available 21 intersections in 12 trenches as 28.60% P2O5. In the present study, statistical modelling of exploration data of the Khatamba Blocks, using histogram plots, probability diagrams and skewness and kurtosis parameters for 20 intersections reveals a 3-parameter log-normal distribution for accumulation (grade-thickness) and thickness values. The graphical estimate of the average grade from the probability diagrams is 29.59% P2O5 and that estimated by computer is 29.40% P2O5. A computer programme ONEPOP.FOR in interactive FORTRAN77 has been developed for statistical modelling of the exploration data. Using this approach the grade. estimated for phosphorite deposit of the Khatamba Blocks shows a marginal increase in the value of average grade of P2O5, content as compared to the estimate made by conventional method.

Controls on Reservoir Development in Devonian Chert: Permian Basin, Texas

Chert reservoirs of the Lower Devonian Thirty-one Formation contain a significant portion of the hydrocarbon resource in the Permian basin. More than 700 million bbl of oil have been produced from these rocks, and an equivalent amount of mobile oil remains. Effective exploitation of this sizable remaining resource, however, demands a comprehensive appreciation of the complex factors that have contributed to reservoir development. Analysis of Thirtyone Formation chert deposits in Three Bar field and elsewhere in the Permian basin indicates that reservoirs display substantial heterogeneity resulting from depositional, diagenetic, and structural processes. Large-scale reservoir geometries and finer scale, intra-reservoir heterogeneity are primarily attributable to original depositional processes. These cherts, which were deposited by relatively deep-water sediment gravity processes along a north-trending depocenter, exhibit relatively continuous stratal geometries in northern, more proximal parts of the depocenter and more discontinuous geometries in the center of the depocenter. Despite facies variations, porosity development in these cherts is principally a result of variations in rates and products of early si ica diagenesis. Because this diagenesis was in part a function of depositional facies architecture, porosity development follows original depositional patterns. In reservoirs such as Three Bar field, where the Thirtyone Formation has been unroofed by Pennsylvanian deformation, meteoric diagenesis has created additional heterogeneity by causing dissolution of chert and carbonate, especially in areas of higher density fracturing and faulting and along truncated reservoir margins. Structural deformation also has exerted direct controls on heterogeneity that are particularly noteworthy in reservoirs under waterflood. High-density fracture zones create preferred flow paths that result in nonuniform sweep through the reservoir. Faulting locally creates compartments by offsetting reservoir flow units. Three Bar field exhibits many of the major styles of heterogeneity that contribute to inefficient recovery in the Thirtyone Formation. As such, the processes and models defined here improve understanding of the causes of heterogeneity in all Thirtyone chert reservoirs in the Permian basin and aid recovery of the sizable hydrocarbon resource remaining in these rocks.

Cyclic Volcanism and Sedimentation in the Mahakoshal Greenstone Belt, District Sidhi, Madhya Pradesh

Abstract Several ordered cycles of volcanism and sedimentation have been recorded from the Mahakoshal greenstone belt in the eastern part of the Sidhi district, Madhya Pradesh. Each cycle is characterised by pillow lava-pahoehoe toes, bomb agglomerate. volcanic breccia and tuff, culminating with the deposition of massive chert under quiescent conditions. The volcanics and the sediments exhibit linear distribution and were deposited as a result of eruptions along ENE-WSW trending fissures, parallel to the margins of the belt. The deposition of volcanics and sediments took place over thin Archaean sialic crust where tholeiitic magma differentiated at a depth of less than 15 km.

Long-lived Panthalassic remnant: The Bridge River accretionary complex, Canadian Cordillera

Newly identified radiolarians from ribbon chert of the Bridge River complex in the southeastern Canadian Coast Mountains range in age from Mississippian to late Middle Jurassic. The Bridge River complex and the associated Cadwallader arc and Tyaughton and Methow basins lie between the Intermontane superterrane to the east and the Insular superterrane to the west. Triassic-Middle Jurassic development of the Bridge River subduction-accretion complex records an important component of convergence between these superterranes. The time span represented in the Bridge River complex (∼170 m.y.), one of the longest known age ranges for chert sedimentation, suggests that the Bridge River complex contains remnants of a long-lived, potentially far-traveled Panthalassic oceanic domain.

Stratigraphic evolution of the Paleozoic Stikine assemblage in the Stikine and Iskut rivers area, northwestern British Columbia

The Stikine assemblage, the "basement" of Stikinia, extends 500 km along the western flank of the Intermontane Belt, east of younger Coast Belt plutons. Four different stratigraphic successions are characteristic of Lower to Middle Devonian, Carboniferous and Permian rocks in the Stikine and Iskut rivers area. West of Forrest Kerr Creek are penetratively deformed Lower to Middle Devonian island-arc volcaniclastic rocks, coralline limestone, and felsic tuff. Fringing carbonate buildups in an arc setting are best illustrated in the sequence at Round Lake where Lower Carboniferous mafic-dominated, bimodal submarine volcanic rocks grade upward into two distinctive coarse echinoderm limestone units and medial siliceous siltstone and limestone conglomerate. Conodont colour alteration indices for Lower Carboniferous rocks near Newmont Lake indicate an anomalously low-temperature thermal history. Upper Carboniferous–Permian polymictic volcanic conglomerate and Lower Permian limestone overlie these strata there. The Scud River sequence is distinguished by subgreenschist- to greenschist-grade Carboniferous(?) volcanic and sedimentary rocks overlain by a structurally thickened package (greater than 1000 m) of Lower Permian limestone. Local calcalkaline pyroclastic rocks interfinger with limestone near the top of the Scud River sequence. Basinal, shelf, and shallow-water carbonate facies developed in the Early Permian, giving way to calcalkaline volcanism in Late Permian followed by deposition of deep-water chert and argillite.The tectonic setting during the Devonian and Carboniferous is comparable with modern Pacific volcanic arcs and atolls, but there is no modern analogue for the shelf-carbonate accumulation during the Early Permian which characterizes the Stikine assemblage and permits Cordilleran-scale correlations. Permian fusulinid and coral species have very close affinity to those of the McCloud Limestone of the eastern Klamath Mountains, California. Other geologic events common to both Stikinia and the Eastern Klamath terrane are Devonian limestone breccia deposition, Lower Permian limestone accumulation with McCloud faunal affinity, Carboniferous and Permian calcalkaline volcanism, and Upper Permian tuffaceous limestone. Stratigraphic differences include the absence of quartz detritus in Devonian strata and lack of thick Upper Permian volcanic rocks in the Stikine River area.

Assessment of the Sensitivity of Missouri Reservoirs to Acidification

ABSTRACT We used water quality information from 103 Missouri reservoirs to assess their sensitivity to acidification from acidic atmospheric deposition. Total alkalinity less than 20 mg/L or a Calcite Saturation Index (CSI) of 1.0 or greater were the criteria to classify reservoirs as sensitive to acidification. Most of the reservoirs (83 of 103) would be considered circumneutral, hard water systems with high acid neutralizing capacity. Thirteen reservoirs were sensitive to acidification based on alkalinity or CSI. Four reservoirs with both low alkalinity and high CSI are located in southeast Missouri in areas of igneous rock outcroppings or chert deposits in limestone formations. These reservoirs are small (<40 ha) and have limited watersheds (<600 ha) and site-specific edaphic factors that likely determined their water chemistry. Site-specific factors also influenced water chemistry of acid-sensitive reservoirs that were located in regions expected to yield highly buffered water. No reservoir classified as acid-sensitive presently seems affected by acidification.

Sedimentary evolution and palaeogeographical reconstruction of the Mesozoic continental rise in Oman: evidence from the Hamrat Duru Group

Abstract The mainly Mesozoic Hamrat Duru Group represents deep-water, primarily continental rise Neo-Tethyan sediments derived from the Oman continental margin and later emplaced onto the Arabian carbonate platform during the Late Cretaceous obduction of the Semail ophiolite. Sediment patterns reflect fluctuations in sea level relative to the platform, with carbonate input dominant during periods of platform stability. High and low sea-level stands, controlled by eustacy or tectonism, were accompanied by significant reductions of carbonate production and input into the basins. Raised calcite compensation depths associated with high sea levels resulted in the deposition of cherts whereas, during low stands, terrigenous clastics prograded across or by-passed the platform into the Hawasina basin. The Hawasina basin was divided into two sedimentary basins separated by a submarine high. The shale-rich ‘Al Ayn sub-basin’ contained coarse-grained sediments that formed either small fans, or laterally coalescing sediment bodies that were fed by multi-source input of carbonate and terrigenous clastics. This is preserved as the Hamrat Duru Group of the northern and central Oman Mountains. The carbonate-rich ‘Duru sub-basin’ was fed exclusively from the south and large sediment bodies with sheet-like morphologies extended northwards parallel to the margin, outboard from the Al Ayn sub-basin, with thinning and fining trends to the NW and NE. Duru basin units are found in the central and southern Oman Mountains. Contrasting sedimentation patterns in the Al Ayn subbasin and the Duru sub-basin may reflect the influence of prevailing SE trade winds on the margin.

Petrography and Origin of Bedded Barite and Phosphate in the Devonian Slaven Chert of Central Nevada

ABSTRACT Bedded barite deposits in the Slaven Chert in East Northumberland Canyon, Toquima Range, central Nevada, are part of the Roberts Mountains Allochthon which was thrust eastward onto the carbonate shelf by the Late Devonian to Early Mississippian Antler Orogeny. High concentrations of organic matter (up to 6.2%), well-preserved microfossil skeletal hash, and the presence of phosphate in interbedded barite and chert deposits indicate that the rocks formed in an anoxic environment. Rare earth element values from Early Paleozoic Cordilleran margin phosphates resemble patterns of phosphorites formed in Holocene zones of coastal upwelling at low latitudes versus phosphate formed in non-upwelling areas. These facts suggest that the barite, chert, and phosphate formed within an oxygen minimum one associated with coastal upwelling currents. The barite displays three main morphologies: massive, nodular, and composite. Massive barite is fine-grained, lacks matrix chert, and contains phosphate nodules. The massive beds formed as a rapid primary precipitate at or above the sediment/water interface. Nodular barite is composed of barite nodules associated with matrix chert. Chert laminations are bent by the barite nodules, indicating that the nodules grew prior to lithification of the chert and are therefore not a later diagenetic replacement of a precursor. Composite barite consists of barite nodules within a fine-grained barite matrix and lacks brown matrix chert, indicating that the nodules formed as a synsedimentary precipitate within a previously deposited fine-grained barite bed. Phosphate nodules are common to abundant in the chert beds and rare to common in the barite beds. Phosphate nodules within massive barite beds commonly contain abundant fine-grained barite rimmed by phosphate masses, occasionally the opposite relationship of phosphate rimmed by fine-grained barite was present. Similar relationships were observed between nodular barite and phosphate. Therefore, the phosphate and barite were contemporaneous to penecontemporaneous precipitates. Rare microbial mats exhibit a range in growth orientation within barite samples. The various orientations indicate that the mats were not light dependent, and geochemical analyses support a bacterial origin. Lab experiments on barite precipitates from sulfuric acid solutions demonstrate that the fine-grained barite precipitates rapidly out of solution, whereas the dipyramidal and bladed crystals precipitate more slowly, after the fine-grained barite. Although the medium in which the barite precipitated is different from any natural fluid, this sequence is very similar to the patterns observed in the rocks.

Barite Nodules in Bedded Barite Within Phosphatic Devonian Slaven Chert, Toquima Range, Central Nevada: ABSTRACT

Bedded barite deposits in the Devonian Slaven Chert in East Northumberland Canyon, Nevada, are part of the Roberts Mountains allochthon. The barite was deposited in an oxygen minimum zone as indicated by abundant phosphate, organic matter, and radiolarian microfossils. The barite displays three main morphologies: massive, nodular, and composite. Petrographic analyses indicate that massive fine-grained (0.01-0.15 mm in diameter) barite formed as a rapid primary sedimentary precipitate at or above the sediment-water interface, whereas barite nodules formed as early diagenetic precipitates within the top few meters of barite and/or chert deposits. The barite nodules display three internal morphologies: fine grained, fiber bundle, and spherulitic. Fine-grained nodules (0.04-6 mm in diameter) are composed of fine-grained barite similar to the massive barite beds. Fiber bundle nodules (0.75-1.3 mm in diameter) are composed of randomly oriented dipyramidal crystal bundles and lack a differentiated nucleus. Spherulitic nodules (0.1-2.5 cm) are composed of concentric layers of radially arranged dipyramidal crystal bundles around a nucleus of fine-grained barite and organic matter. The spherulites form through episodes of crystallization as indicated by remnant organic matter rims and patches of fine-grained barite between the concentric layers. Spherulites often contain phosphate, silt grains, mica, and radiolarians. SEM analyses indicatemore » that the dipyramidal bundles consist of bladed barite crystals oriented in the same direction. Lab experiments indicate that fine-grained barite precipitated rapidly (in seconds) out of solution, whereas the dipyramidal crystals precipitated more slowly (2 to 3 days) after the fine-grained barite.« less

Late Triassic transition from biogenic to arc sedimentation on the Peninsular terrane: Puale Bay, Alaska Peninsula

The thick (∼700 m) section of Upper Triassic (upper Norian) carbonates, cherts, and volcaniclastics exposed at Puale Bay, Alaska Peninsula, preserves an excellent record of early Mesozoic arc-related sedimentation on the Peninsular Terrane. The Puale Bay sedimentary basin was probably part of a back-arc basin with two stages of evolution: (1) an initial upward-deepening sequence, as reflected in shallow-water carbonates succeeded by spicular chert deposits; and (2) increased magmatic-arc activity and an upward-shallowing sequence near the Triassic-Jurassic boundary, as shown by increased influx of volcaniclastics and deposition of thick, coarse-grained turbidites. This deepening-shallowing cycle of the basin is recorded in kerogen types and in authigenic carbonates that precipitated under marine pore-water influence during the deepening phase, and under meteoric or hydrothermal pore-fluid influence during the upward-shallowing phase. The marine-dominant phase is characterized by types I and II kerogen, δ 13 C (calcite) ratios near 0.0‰ PDB, and Sr values near 700 ppm. The upward-shallowing phase is characterized by type III kerogen, δ 13 C (calcite) ratios with large negative excursions (up to -7.2‰ PDB), and low Sr values down to 33 ppm. The increased volcanic activity, the transition to a terrestrial kerogen source, and the dominance of nonmarine pore fluids near the Triassic-Jurassic boundary are all suggestive of the development of a nearby volcanic arc. This early Mesozoic arc volcanism may reflect closure of ocean basins, between the Peninsular, Wrangellia, and Alexander terranes. Hence, this initiation of arc volcanism may represent an early phase of tectonic amalgamation in southern Alaska.

Sumeini group, Oman—evolution of a Mesozoic carbonate slope on a South Tethyan continental margin

Deposition of the mostly Mesozoic Sumeini Group occurred on the slope between the shallow-marine, Arabian carbonate platform and the deep-oceanic Hawasina Basin or South Tethys Sea. These strata record the evolution of the northeast Arabian continental margin from Permian(?) and Triassic rifting to ocean basin closing with the Late Cretaceous obduction of the Semail Ophiolite. Platform margin type and its evolution can be inferred from exposures of these slope sediments at Jebel Sumeini in the Western Oman Mountains. In the Lower Permian base of the sequence, bedded, spicular limestones suggest deep-marine conditions. This is followed by an enigmatic, thick sequence of bedded dolomite. In the Early to Middle Triassic (?), small carbonate submarine fans formed along a distally steepened slope. A subsequent interval of terrigenous, clastic sedimentation was followed by development of a steep escarpment margin during Ladinian to Norian time, with coralgal reefs at the shelf break and a debris apron at the base of the scarp. The Late Triassic is marked by another influx of terrigenous sediment which accompanied widespread emergence of the platform. In the Jurassic, a thick apron of thin-bedded limestone formed on the slope; this apron was cut by channelized gullies that now include blocks of older reefal material and oolitic sands. A well-documented submergence of the platform during the Tithonian and Early Cretaceous led to the deposition of radiolarian cherts on the slope and basin. Upper Lower Cretaceous megabreccias, up to > 200 m thick, suggest an interval of localized slope instability and/or tectonism. Continued deposition of bedded limestone and marl with lesser calcirudite marks the top of stratigraphic sequence at Jebel Sumeini. The Early to Middle Triassic (?) carbonate submarine fan deposits occur as discrete, lenticular sequences of bedded calciturbidite, modified grain flow and debris flow deposits which were derived from mostly slope sources. The Ladinian to Norian base-of-slope debris apron deposits were derived from both slope and shallow-marine sources (including reefs) and appear to form a laterally extensive but narrow belt of thick lenticular to sheet-like beds of debris which accumulated over a long interval of time. Coarse limestones in the Jurassic gullied slope deposits are distinct in that they occur randomly as individual channelized beds in a sequence dominated by thin-bedded lime mudstones. Thick Lower Cretaceous megabreccias are distinguished by their great lateral distribution and general lack of matrix.

Significance of lower and middle Paleozoic phosphatic chert in the Toquima Range, central Nevada

Ordovician to Lower Mississippian cherts of the Roberts Mountains allochthon in central Nevada contain nodules, lenses, and streaks of phosphate as apatite. Major marine phosphate deposition is limited to latitudes less than 50° on margins facing large oceans to the east or west, or of Tethyan (equatorial sea) type; the Nevada margin was suitably located in the lower to middle Paleozoic. The Roberts Mountains allochthon overthrust the shelf of western North America very soon after deposition of the youngest phosphatic cherts, implying that they are most likely not far-traveled or exotic to North America. Phosphates of similar age and nature in the northern Sierra Nevada suggest proximity of Nevada and the northern Sierra Nevada in the early to middle Paleozoic.

Displaced dolomites in radiolarian cherts of the Chichibu Belt on Shikoku Island, Southwest Japan

The northern tract of the Chichibu Belt on Shikoku Island, Southwest Japan, in places contains dolomites of Late Carboniferous age displaced into radiolarian cherts. The sections here examined are along the Niyodo gorge, central Shikoku. The sequence begins with thinly interbedded dolomitized radiolarian cherts and dolomites with a small amount of dolomitized radiolarian claystone and calcisiltite beds. These rocks, 5–10 m thick, are succeeded by a thick section of bedded and massive dolomites, commonly 40–50 m thick, which have thin intercalations of radiolarian claystone in the upper part. This dolomite sequence is depositionally overlain by a sequence, up to 50 m thick, comprising thinly interbedded radiolarian cherts and claystones, which, in turn, contain lenticular bodies of dolomite. Thin-section examination reveals that most of dolomites of the area have an arenitic or lithic texture, and should be termed doloarenite and dololithite. This means that dolomites are detrital. All lines of evidence suggest that the dolomites were not formed in the same depositional site in which the radiolarian cherts were being accumulated, but formed instead as allochthonous bodies which were displaced into a deep oceanic basin of chert deposition. The following sequence of events is postulated: (1) deposition of shallow-water calcareous sediments in a subtidal area; (2) dolomitization in a very shallow-water to supratidal environment; (3) displacement of dolomitized sediments, possibly mainly as debris flows into a deep-water, truely pelagic realm, in which siliceous radiolarian sediments were accumulating; (4) continued accumulation of siliceous sediments after the major influx of dolomitized sediments; and (5) minor influxes of dolomitized sediments during the continuous accumulation of siliceous sediments.