Fine-grained Clastic Rocks Research Articles

Geochemistry of metasedimentary rocks in the late Archean Hemlo-Heron Bay greenstone belt, Superior Province, Ontario: implications for provenance and tectonic setting

The late Archean Hemlo-Heron Bay greenstone belt of the Superior Province near the Hemlo gold deposit, Ontario, consists mainly of fine-grained clastic metasedimentary rocks with subordinate amounts of metavolcanic rocks. Cordierite-anthophyllite rocks are also locally present, but restricted to a single metasedimentary sequence, and most likely represent metamorphosed Mg- and Fe-rich sedimentary rocks derived from predominantly mafic-ultramafic sources. The clastic metasedimentary rocks are geochemically highly immature. Some of their geochemical characteristics, such as low Al 2O 3/Na 2O ratio, low CIA, K 2O/Na 2O < 1.0, uniform Th/U ratio (3.6), and strong correlations of transition metals and MgO with Fe 2O 3∗, are similar to those of typical Archean sedimentary rocks. However, their REE compositions (high contents of LREE and steep chondrite-normalized patterns) are different from those of most Archean sedimentary rocks and similar to those of more mature Proterozoic sedimentary rocks. The clastic metasedimentary rocks from the Hemlo-Heron Bay greenstone belt were derived mainly from felsic (or intermediate) volcanic sources, admixed with variable amounts of mafic-ultramafic volcanic components and possibly some continental weathering products, and deposited rapidly as turbidites. The geochemical uniformity of all clastic metasedimentary rocks indicates that the Hemlo-Heron Bay greenstone belt represents a single tectonic environment (possibly equivalent to a modern island-arc system) rather than a juxtaposition of suspect terranes.

Al2O3-REE correlations in sedimentary rocks.

Linear correlations between Al2O3 and ΣREE (14 REEs) are defined for post-Archean fine-grained clastic sedimentary rocks from the Qinling region, Central China and Australia: ΣREE = −16(±71) + 12Al2O3, and for Archean fine-grained clastic sedimentary rocks worldwide: ΣREE = −78(±78) + 12Al2O3. One important property of such correlations is the additivity, i.e., the correlation equation for ΣREE is equal to the sum of the correlation equations for individual REE, and each REE shows a roughly constant slope. These correlations indicate that the REE of fine-grained clastic sedimentary rocks or metamorphosed equivalents are in general dominantly contained in clay minerals or their metamorphosed products. Using these correlations, the REE abundances of the upper continental crust, sedimentary cover and Devonian strata were calculated for different tectonic units of the Qinling region. The results generally agree with the estimate from direct sampling within 18%.

Palynology and paleoenvironment of uranium deposits in the Permian Beaufort Group, South Africa

An unusual suite of marine and continental microfossils was found in fine-grained clastic rocks from the uranium-bearing Lower Formation of the Beaufort Group. These rocks are classically defined as fluvial sandstones in a terrestrial sequence. Reworking of the microfossils from the underlying marine Ecca Formation cannot be ruled out; however, if they are in place, they may show that uranium mineralization followed a hither-to-before unrecognized paleosalinity gradient. If they are reworked, they show uranium mineralization follows a distinct zone that can be identified palynologically.

Cover deformation above a blind duplex: An example from West Virginia, U.S.A.

Pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting, backthrusting and coupling. The role of these responses was examined in the Hanging Rock-Cacapon Mountain anticlinorium of the central Appalachians. This cover fold consists of Middle Ordovician and younger sedimentary rocks above a blind horse of Cambro-Ordovician carbonates. The cover has abundant folding and subsidiary faulting. Cover cleavage in carbonate and fine-grained clastic rocks formed before regional folding in response to layer-parallel shortening (LPS). This 15% LPS also produced grain-suturing and fluid-inclusion trails in quartzarenites. The LPS formed during cover forethrusting, accommodating older blind horses in the hinterland. With the formation of the underlying horse, cover coupling occurred with bending-folding, additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening. This cover coupling was insufficient to accommodate horse formation. No evidence exists for backthrusting, but forelandwards, abundant subsurface evidence exists for cover forethrusting with imbricate thrusting. This thrusting accommodates horse formation and differs in style from the LPS of the older forethrusting event. So, for this blind system, the cover responded with a combination of coupling and forethrusting.

Ba'id al Jimalah tungsten prospect, Najd region, Kingdom of Saudi Arabia

The Ba'id al Jimalah tungsten prospect is located in the NE of the Arabian Shield, at 25°09′N, 42°41′E. Mineralization is associated with a late-Proterozoic, porphyritic microgranite emplaced in folded, fine-grained clastic rocks of the Murdama group, within an aureole of biotite-rich hornfels. The microgranite forms a 30 m-thick sill and numerous smaller sills and dikes cropping out along two low, sub-parallel ridges and several small hills in an area 700 m square. The form of the intrusion at depth is uncertain. It is slightly to intensely sericitized, in places greisenized, and is enriched in Li, F and Rb. Wolframite occurs with minor cassiterite, scheelite and sulfides in quartz veins cutting both microgranite and hornfelsed wall-rock. The veins have a dominant trend of 110–115°, and are thicker and more numerous in the microgranite. Gangue minerals include plagioclase and potassium feldspar, muscovite, sericite, fluorite and minor siderite. A major Najd fault trending 130–135° probably controlled magma emplacement and subsequent hydrothermal and pneumatolytic activity. A percussion drilling program, restricted to the outcrop of the sill on the north ridge, has outlined 800,000 tonnes grading 0.10% WO 3 and 0.01% Sn.

Tectonic Significance of Currant Creek Formation, North-Central Utah: ABSTRACT

The Currant Creek Formation is composed of conglomerate, sandstone, and fine-grained clastic rocks that crop out along the northwestern margin of the Uinta basin in north-central Utah. Lateral gradations in grain size define proximal, medial, and distal parts of coalescing alluvial-fan deposits that prograded eastward from the active Sevier-Laramide orogenic belt during Maestrichtian through Paleocene(?) time. Paleocurrent directions indicate a dominant southerly transport direction and a minor easterly component. Strong east and southeasterly directions, measured in imbricated clasts and in sand lenses in conglomerate, indicate multiple source areas for the detritus. Source of the coarse-grained detritus in the Currant Creek Formation was the Charleston thrust sheet. Conglomeratic clasts are composed of Precambrian and Cambrian quartzite, chert derived from Cambrian and Mississippian carbonate beds, and Pennsylvanian sandstone. These rocks are exposed in the upper plate of the Charleston thrust near Deer Creek Reservoir, Mount Timpanogos, and Strawberry Reservoir. At Big and Little Canyons, the same rocks are exposed in the lower plate. A large basement-cored east-verging anticline that is refolded around the Uinta Mountain structure is present in the region. By the time of its final emplacement, the Cottonwood fold likely formed a small mountain range. Erosional dissection was well under way and Carboniferous, Cambrian, and Precambrian beds were exposed. The Charleston-Absaroka thrust rises in the stratigraphic section farther east, displacing Mesozoic rocks. Reworking of Cretaceous sandstone and shale flanking the newly uplifted western end of the Uinta arch also contributed sediment to southward-flowing streams. End_of_Article - Last_Page 939------------

Microfabric Analysis of Fine-Grained Clastic Rocks as an Independent Criterion for Determining Depositional Environment: ABSTRACT

SEM examination of clay fabric in siltstones as old as Pliocene has proved useful in distinguishing rapidly deposited muds from hemipelagic clays: turbiditic siltstones are characterized by random orientation of clay minerals and hemipelagic units show preferred (parallel) orientation of clay minerals. Random orientation of clay minerals has been recognized in Paleozoic shales, indicating that postdepositional compaction of shales does not always result in parallel alignment of clay minerals. Therefore, the possibility exists that original depositional fabric of clay minerals is retained in rocks of Paleozoic age, and that clay-fabric analysis would provide an independent tool for interpreting the depositional environment of fine-grained clastic rocks. This study shows that original depositional fabric of clay minerals is preserved in texturally homogeneous, nonbioturbated mudstones as old as Middle Cambrian. Clay fabric may be used in determining whether the mud deposition was rapid or slow (e.g., pelagic). Rapid mud deposition is characterized by a random clay fabric; slow sedimentation is characterized by preferred alignment of clay minerals. Bioturbation and bimodal grain size within a sample (e.g., a clayey siltstone) also are associated with a disordered clay fabric, so this analysis is most confidently used with mudstones that have a homogeneous texture and show no evidence of bioturbation. Therefore, environmental interpretations based on SEM analysis of clay fabric can be used with confidence provided that the caveats of bi odal grain size, bioturbation, and diagenesis are anticipated. End_of_Article - Last_Page 564------------

The systematic evaluation of landslide incidence and susceptibility in the United States

A landslide overview map of the United States published in 1976, at a scale of 1∶7,500,000, shows areas of landslides and areas susceptible to future landsliding. The map was prepared by classifying geologic units shown on the geologic map of the United States according to high, medium, or low incidence of landslides and susceptibility to landsliding. The compilation showed that landsliding was most prevalent in fine-grained clastic rocks, particularly where they are interbedded with more resistant rocks; in highly sheared rocks, particularly melanges; and in loose slope accumulations of fine-grained surface debris. The influence of climate, structure, and lithology on landsliding in various parts of the country could not be shown on the map. These factors are compared for the Colorado Plateau in the western United States and the Appalachian Highlands in the eastern United States.

Landslides — Cause and effect

Landslides can cause seismic disturbances; landslides can also result from seismic disturbances, and earthquake-induced slides have caused loss of life in many countries. Slides can cause disastrous flooding, particularly when landslide dams across streams are breached, and flooding may trigger slides. Slope movement in general is a major process of the geologic environment that places constraints on engineering development. In order to understand and foresee both the causes and effects of slope movement, studies must be made on a regional scale, at individual sites, and in the laboratory. Areal studies — some embracing entire countries — have shown that certain geologic conditions on slopes facilitate landsliding; these conditions include intensely sheared rocks; poorly consolidated, fine-grained clastic rocks; hard fractured rocks underlain by less resistant rocks; or loose accumulations of fine-grained surface debris. Field investigations as well as mathematical- and physical-model studies are increasing our understanding of the mechanism of slope movement in fractured rock, and assist in arriving at practical solutions to landslide problems related to all kinds of land development for human use. Progressive failure of slopes has been studied in both soil and rock mechanics. New procedures have been developed to evaluate earthquake response of embankments and slopes. The finite element method of analysis is being extensively used in the calculation of slope stability in rock broken by joints, faults, and other discontinuities.

Primary and Secondary Sedimentary Structures in Fine-grained Lacustrine Rocks of Green River Formation (Eocene), Piceance Creek Basin, Colorado: ABSTRACT

A study of 285 polished slabs (59.0% oil shale, 23.0% carbonate, and 18.0% fine-grained clastic rock) collected from four measured sections along the southern and eastern edge of the Piceance basin reveals important sedimentologic information on the distribution of primary and secondary sedimentary structures. The slabs were studied under low-power binocular magnification, and individual stratification characteristics were noted. A total of 528 primary structures and 334 secondary structures were observed in the slabs. Eleven descriptive classes of primary structures are important: (1) even parallel stratification; (2) discontinuous even parallel stratification; (3) wavy parallel and nonparallel stratification; (4) discontinuous wavy parallel and nonparallel stratification; (5) discontinuous curved parallel stratification; (6) curved nonparallel stratification; (7) structureless; (8) mottled; (9) brecciated; (10) algal stratification; and (11) graded stratification. Of these classes, the oil shale is dominated by classes 1, 2, 3, and 4, and the carbonate and fine-grained clastic rocks by classes 6, 7, 8, and 10. Classes 5 and 9 are rarely represented. Apparently there is a correlation between the organic content and the stratification type of the oil shale. As oil shale increases in organic content, classes 2 and 4 become more abundant and classes 1 and 3 are less so. In the oil shale of the Parachute Creek Member of the two easternmost measured sections, class 1 decreases, whereas classes 2 and 4 increase upward through the sections. The older classes remain approximately the same. These vertical changes correlate with indications of desiccation in the depositional environment in the upper parts of the Parachute Creek Member. Six classes of secondary sedimentary structures are common: (1) loop structure; (2) fault displacement; (3) crystal-growth displacement; (4) bioturbation; (5) contortion; and (6) total disruption. Most of these classes are restricted to oil shale, and loop, fault and crystal-growth types are most abundant. The frequency End_Page 912------------------------------ of occurrence of the secondary structures, like the primary structures, varies with lithology. Crystal-growth disruption (sulfides and carbonate clots) in the oil shale increases with increasing organic content. In a vertical sequence of oil shale in the Parachute Creek Member, crystal-growth disruption of laminae increases upward through the section, and loop and fault structures decrease. Contortion of laminae is almost exclusive to the oil shale, and bioturbation is restricted to claystone and very limy claystone. End_of_Article - Last_Page 913------------

Evaluation of Hydrocarbon Potential from Stratigraphic Analysis of Mesozoic Clastic Sequence, Saskatchewan: ABSTRACT

Jurassic and Cretaceous sediments form a northward-thinning wedge of 5,000 ft maximum thickness that occupies 125,000 sq mi in south Saskatchewan. Significant petroleum production began in the 1940s. New reservoirs were located each year to total, by the end of 1969, 73 main pools in 6 principal producing units, yielding 337,089,144 bbl of crude oil and 435,851,579 Mscf of natural gas from depths of 750 to 4,700 ft. Middle Jurassic beach and channel-fill, marine sandstones and carbonates, enclosed in less permeable carbonates and fine-grained clastic rocks yield medium-gravity oil in southwestern Saskatchewan and are prospective both west of the oil-field trend and in southeastern Saskatchewan. Medium-gravity oil also is produced in southwestern Saskatchewan from Upper Jurassic marine sandstones forming updip mesas, buttes, and interfluves beneath a basal Cretaceous cover of locally permeable and productive continental deposits. Production of heavy oil and nonassociated natural gas is obtained where deltaic sandstones of the Cretaceous interdigitate with marine shales. The sequence is prospective throughout central Saskatchewan, particularly where sandstone-body trends may be related to major str ctural features. In west-central Saskatchewan, light oil and nonassociated gas are produced from sandstone bodies of good economic potential. These sandstones are hydraulically isolated within a thick sequence of Lower Cretaceous marine shales and exhibit structural features that closely reflect the texture of the dissected End_Page 608------------------------------ pre-Cretaceous surface. Nearshore marine and continental-fluviomarine sandstone bodies of Late Cretaceous age yield nonassociated natural gas in western Saskatchewan; the best prospects appear to be at the base of this sequence in southwestern Saskatchewan. End_of_Article - Last_Page 609------------

Classification of Fine-grained Sedimentary Rocks

ABSTRACT Classifications of fine-grained sediments and sedimentary rocks are based on combinations of several criteria. These include: (1) texture, (2) fissility, (3) tectonic association or environment of deposition, (4) mineral composition, (5) color, (6) chemical composition and (7) degree of metamorphism. None of the classifications are generally used, however. For field studies, texture is the most significant property. The proportions of clay, silt and sand are used herein to define four major groups: claystone (clay), siltstone (silt), mudstone (mud) and sandstone (sand). Claystone, siltstone and sandstone are rocks containing more than 50 percent of clay-, silt- or sand-sized material, respectively. Mudstone contains less than 50 percent of clay, slit or sand. Silty and sandy are useful modifiers of claystone when clay-sized material is less than 75 percent but more than 50 percent. Clayey and sandy are used to modify siltstone when silt-sized material is less than 75 percent but more than 50 percent. The expression good sorting is applied to fine-grained clastic rocks that contain more than 90 percent silt or clay-sized material. Fair-sorted rocks contain 75 to 90 percent silt or clay. Poorly sorted siltstone or claystone contains less than 75 percent silt or clay. If fissile, fine-grained clastic rocks are so designated. The mineral composition of most fine-grained clastic rocks can be determined reliably with the petrographic microscope and by X-ray diffraction. Therefore, where possible, they should also be named according to one of the sandstone classifications and on the dominant clay mineral present. Color, bedding types and sedimentary structures, fossil content, type of cement and fissility should be included in complete descriptions. An example of a good field and laboratory description is: clayey siltstone, illite-subarkose, poor sorting, grayish red (5R4/2), dolomite cement, slabby, horizontal lamination, pelecypods.

Stratigraphy of Black Shale Facies of Green River Formation (Eocene), Uinta Basin, Utah: ABSTRACT

In the Uinta basin, Utah, the black-shale facies of the Green River Formation (Eocene) is divided into 5 rock units: 4 lacustrine units designated by the letters A-D, and a fluvial unit, the Wasatch tongue of the black-shale facies. The Wasatch tongue occupies the same stratigraphic position as unit C. Unit A contains the oldest lacustrine rocks of the black-shale facies, and is transgressive on the underlying fluvial Wasatch Formation (Paleocene-Eocene). All of the lacustrine units contain black fine-grained clastic rocks. Units B and D contain more carbonate rocks than do other units. Units B and D are also the most extensive of the lacustrine units. Units A, B, and C range in thickness from 100 to 300 ft, whereas the Wasatch tongue is 100-400 ft thick. Unit D has the g eatest thickness range, from about 100 to 500 ft. Rocks contained in the 4 lacustrine units vary in composition depending upon where they were deposited in relation to the center of the basin. The central lake environment of deposition produced mostly dark-gray to black, fine-grained clastic rocks and finely crystalline, brown to dark-brown carbonate rocks. The total clastic content of the lacustrine rocks and their model grain size increase toward the peripheries of the depositional basin and sandstone and siltstone become more abundant. Near the edges of the basin carbonate rocks are more saccharoidal in texture and contain larger amounts of silt- and sand-sized grains, oolite, pisolite, and shell fragments. The depositional axis trends east-west and the depositional center of the lacustrine units is south of Duchesne, Utah, except for unit D where the center is farther south. Well control is sparse in the western part of the Uinta basin. Lake Uinta was initiated by the coalescing of several small freshwater lakes on a broad alluvial plain. Downwarping led to the formation of the first moderately deep Green River Lake and the deposition of the black fine clastics and other rocks in unit A. The lake was thermally or chemically stratified, which is suggested by the preserved organic material and the presence of pyrite and salt crystals. As the lake transgressed over the fluvial sediments and became larger, the sediments of unit B were deposited. Unit B contains abundant carbonate rocks that were deposited over the entire lake, but particularly in the shallower part where the temperature was highest. A change in climate and/or increasing downwarping caused the lake to diminish its total area and unit C was deposited. Duri g the deposition of unit C, the fluvial sediments of the Wasatch tongue of the black-shale facies were deposited along the southern part of the basin. The lake then transgressed back across the fluvial sediments and deposited unit D. The dominant rock types of unit D are very similar to those of unit B, indicating a similar environment of deposition. Much of the oil and gas production in the Green End_Page 543------------------------------ River Formation in the Uinta basin is from the black-shale facies or equivalent units. End_of_Article - Last_Page 544------------

Significance of Palynomorphs as Sedimentation Indicators in Cretaceous Straight Cliffs Sandstone, Utah: ABSTRACT

Palynomorphs, which are abundant in maceration residues of many fine-grained clastic rocks, commonly include spore and pollen exines, waxy cuticles, resinous bodies, and vitrinized and fusinized material which is incorporated with mineral grains in sediments. Differences in occurrence and relative abundance of the palynomorphs may be attributed to differential response of the particles to transport and sedimentation because of these parameters: (1) size: range from silt to fine sand; (2) shape: spheroidal, ovoidal, irregular, tabular, platy, or bladdered; (3) specific gravity: range from about 1.2 to 1.6. Interpretation of the sedimentary environment of the Upper Cretaceous Straight Cliffs Sandstone is based on application of results of studies of the palynology of several modern sedimentary areas, specifically by comparison of fossil and modern palynomorph residues. The following patterns are indicated for the paralic Straight Cliffs Sandstone: 1. In nearshore neritic beds, species of displaced terrigenous plants are numerous, marine microplankton abundant, resinous bodies and cuticles less abundant, and sorting poor. Apparently, once carried into the site of deposition, organic particles were winnowed little by offshore currents. End_Page 734------------------------------ 2. In nearshore continental beds, species of microfossils are less numerous generally; many beds contain mainly one kind of palynomorph, such as resinous bodies or fusinized material. Possibly this reflects local conditions which favor minimum particle transport and mixing. 3. Bladdered pollen grains occur in varying abundance in almost every bed regardless of other palynomorph abundance. Differing hydrodynamic response of these large grains may govern their anomalous occurrence, in contrast to smaller nonbladdered forms. End_of_Article - Last_Page 735------------