Ocean Basin Floor Research Articles

Do the deformed alkaline rocks always serve as a marker of continental suture zone? A case study from parts of the Chotanagpur Granite Gneissic complex, India

In Santuri- Kankarkiari area of Purulia, West Bengal, India, a nepheline syenite body cuts across the regional Grenvillian (∼1000–950 Ma) fabric (S2) of the host Chotanagpur Granite Gneissic Complex (CGGC). Subsequently, the nepheline syenite of the studied area and its basement are metamorphosed and develop a pervasive regional fabrics (S3). Isoclinal folding of S2 fabric of the host gneissic rocks resulted in axial planar S3 fabric. Field relations, ‘frozen-in’ textural feature, petrology and geochemistry of the studied rock and the published geological and geochronological information from the adjoining areas are consistent with (a) derivation of its magmatic protolith from the then sub-continental lithospheric mantle and emplacement of the melt in a rift basin (b) the rifting of the Grenvillian basement seems to have occurred during (∼950–900 Ma) and (c) burial, deformation and metamorphism (peaked at 9–10 kbar, 690–770 °C) of the studied alkaline rocks and its Grenvillian basement seem to have occurred in a continent-continent collisional tectonic setting during ∼900–780Ma. The commonly perceived notion that the deformed alkaline rocks (DARs) are invariably linked to continental suture along which a former ocean basin floor was consumed is not supported in this study. Integrating all the information, we propose that the nepheline syenite of the studied area and also from parts of the Granulite Terrane of South India represent metamorphosed and deformed aulacogen rock ensemble that should not be associated with opening and closure of an oceanic crust. Combining the information of this study and the published works, it is demonstrated that the deformed alkaline rocks (DARs) can also form in a coherent crustal block without any connection with a continental suture zone. A plausible tectonic model for the evolution of the CGGC has also been presented.

Role of gas hydrates in slope failure on frontal ridge of northern Cascadia margin

Several slope failures are observed near the deformation front on the frontal ridges of the northern Cascadia accretionary margin off Vancouver Island. The cause for these events is not clear, although several lines of evidence indicate a possible connection between the occurrence of gas hydrate and submarine landslide features. The presence of gas hydrate is indicated by a prominent bottom-simulating reflector (BSR), at a depth of ∼265–275 m beneath the seafloor (mbsf), as interpreted from vertical-incidence and wide-angle seismic data beneath the ridge crests of the frontal ridges. For one slide, informally called Slipstream Slide, the velocity structure inferred from tomography analyses shows anomalous high velocities values of about 2.0 km s−1 at shallow depths of 100 mbsf. The estimated depth of the glide plane (100 ± 10 m) closely matches the depth of these shallow high velocities. In contrast, at a frontal ridge slide just to the northwest (informally called Orca Slide), the glide plane occurs at the same depth as the current BSR. Our new results indicate that the glide plane of the Slipstream slope failure is associated with the contrast between sediments strengthened by gas hydrate and overlying sediments where little or no hydrate is present. In contrast, the glide plane of Orca Slide is between sediment strengthened by hydrate underlain by sediments beneath the gas hydrate stability zone, possibly containing free gas. Additionally, a set of margin perpendicular normal faults are imaged from seafloor down to BSR depth at both frontal ridges. As inferred from the multibeam bathymetry, the estimated volume of the material lost during the slope failure at Slipstream Slide is about 0.33 km3, and ∼0.24 km3 of this volume is present as debris material on the ocean basin floor. The 20 per cent difference is likely due to more widely distributed fine sediments not easily detectable as bathymetric anomalies. These volume estimates on the Cascadia margin are approaching the mass failure volume for other slides that have generated large tsunamis—for example 1–3 km3 for a 1998 Papua New Guinea slide.

DFTopoSim: Modeling Topographically-Controlled Deposition of Subseismic Scale Sandstone Packages Within a Mass Transport Dominated Deep-Water Channel Belt

Facies bodies in geostatistical models of deep-water depositional environments generally represent channel-levee-overbank-lobe morphologies. Such models adequately capture one set of the erosional and depositional processes resulting from turbidity currents traveling downslope to the ocean basin floor. However, depositional morphologies diverge from the straight forward channel-levee-overbank-lobe paradigm when the topography of the slope or the shape of the basin impacts the timing and magnitude of turbidity current deposition. Subaqueous mass-transport-deposits (MTDs) present the need for an exception to the channel-levee-overbank-lobe archetype. Irregular surface topography of subaqueous MTDs can play a primary role in controlling sand deposition from turbidity currents. MTD topography creates mini-basins in which sand accumulates in irregularly-shaped deposits. These accumulations are difficult to laterally correlate using well-log data due to their variable and unpredictable shape and size. Prediction is further complicated because sandstone bodies typical of this setting are difficult to resolve in seismic-reflection data. An event-based model is presented, called DFTopoSim, which simulates debris flows and turbidity currents. The accommodation space on top of and between debris flow lobes is filled in by sand from turbidity currents. When applied to a subsurface case in the Molasse Basin of Upper Austria, DFTopoSim predicts sand packages consistent with observations from core, well, and seismic data and the interpretation of the sedimentologic processes. DFTopoSim expands the set of available geostatistical deep-water depositional models beyond the standard channel-levee-overbank-lobe model.

Characteristics of Rare Earth and Trace Element Patterns in Bedded Cherts from the Bottom of the Lower Cambrian in the Northern Tarim Basin, Northwest China and Their Genetic Significance

Abstract The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo, As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, Tl, Bi and Th are highly depleted in the cherts. These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondrite-normalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu* ∼10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. ΣREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu)N ratio decreases from 2.72 at the bottom to 0.67 at the top. NASC-normalized (La/Ce)N ratio increases from 1.36 at the bottom to 3.13 at the top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean-basin floor in the Franciscan Complex exposed at Marin Headlands, California (F-MH chert) (Murray et al., 1991). These geochemical signatures are inconsistent with our previous sedimentological data, which suggests a continental shelf setting. Based on multiple lines of evidence including high TOC content in the concomitant black shales, phosphorite at the bottom of black rock series, regional rise of sea level, and beginning of the southern Tianshan Ocean geotectonic cycle, the authors infer that the hydrothermal fluid was carried to the continental shelf by upwelling from a divergent pelagic ocean floor setting.

Differences in geochemical features between Permian and Triassic cherts from the Southern Chichibu terrane, southwest Japan: REE abundances, major element compositions and Sr isotopic ratios

REE abundances, Sr isotopic data and major element compositions are reported for the Permian and Triassic cherts from the Kuma and Saiki areas in the Southern Chichibu (Sambosan) terrane, the Outer Zone of southwest Japan, in order to elucidate the nature of sources, depositional environment and any changes that occurred from the Permian to the Triassic. A higher contribution of detrital components in the Triassic Sambosan cherts than in the Permian ones is suggested by the Ce-anomaly seen in the REE pattern, MnO/TiO 2 and Al 2O 3/(Al 2O 3 + Fe 2O 3) ratios and initial 87Sr/ 86Sr ratio of these cherts. Small negative Ce-anomalies (Ce/Ce * = 0.75–0.95) are observed in the Permian cherts from both Kuma and Saiki areas, in contrast to small positive Ce-anomalies (Ce/Ce * = 1.1–1.3) for the Triassic cherts from both areas. It should be noted that the Sambosan cherts show neither large negative Ce-anomalies (Ce/Ce * = 0.2–0.5) commonly observed in deep-sea cherts nor large variation in Ce-anomalies (Ce/Ce * = 0.3–1.5) observed for cherts from the Franciscan terrane of California. Al 2O 3/(Al 2O 3 + Fe 2O 3) ratios are 0.5–0.8 for all Sambosan cherts. MnO/TiO 2 ratios are larger than 0.5 for most of the Permian Kuma cherts and the Early Permian Saiki cherts, while the ratios are smaller than 0.5 for the Triassic Kuma cherts and Late Permian and Triassic cherts from the Saiki area. Initial 87Sr/ 86Sr ratios for the Permian Saiki cherts are 0.706–0.707 and are slightly lower than those for the Triassic Kuma cherts (0.707–0.709). These 87Sr/ 86Sr ratios for the Permian and Triassic Sambosan cherts are close to the coeval marine 87Sr/ 86Sr ratios of Burke et al. (1982)and are clearly lower than that for cherts from the Mino terrane (0.713), Inner Zone of southwest Japan ( Shibata and Mizutani, 1982). Our geochemical and isotopic data of the Sambosan cherts suggest a change from ocean-basin floor deposition for the Permian to hemipelagic for the Triassic cherts. However, geochemical features showing deposition on a typical deep-ocean floor are not observed even for the Permian cherts. Some difference in depositional location between the Kuma and Saiki cherts in the Sambosan terrane is also implied from the difference in temporal variation of geochemical features between these two cherts (e.g., increase of REE content with age for the Kuma cherts, in contrast to a decrease for the Saiki cherts). In addition, it is suggested that the Permian and Triassic Sambosan cherts were deposited in a more open-ocean environment than the Triassic cherts in the Mino terrane. This study demonstrates that geochemical and isotopic data could be used to distinguish differences of depositional environment of cherts and those data from the Permian to the Triassic cherts suggest increasing proximity to land in their depositional environments.

Rare earth elements as indicators of different marine depositional environments in chert and shale

Research Article| March 01, 1990 Rare earth elements as indicators of different marine depositional environments in chert and shale Richard W. Murray; Richard W. Murray 1Department of Geology and Geophysics, University of California, Berkeley,California 94720 Search for other works by this author on: GSW Google Scholar Marilyn R. Buchholtz ten Brink; Marilyn R. Buchholtz ten Brink 2Earth Sciences Department, Lawrence Livermore National Laboratory, Livermore California 94550 Search for other works by this author on: GSW Google Scholar David L. Jones; David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley,California 94720 Search for other works by this author on: GSW Google Scholar David C. Gerlach; David C. Gerlach 3Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore California 94550 Search for other works by this author on: GSW Google Scholar G.Price Russ, III G.Price Russ, III 3Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore California 94550 Search for other works by this author on: GSW Google Scholar Author and Article Information Richard W. Murray 1Department of Geology and Geophysics, University of California, Berkeley,California 94720 Marilyn R. Buchholtz ten Brink 2Earth Sciences Department, Lawrence Livermore National Laboratory, Livermore California 94550 David L. Jones 1Department of Geology and Geophysics, University of California, Berkeley,California 94720 David C. Gerlach 3Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore California 94550 G.Price Russ, III 3Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore California 94550 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (3): 268–271. https://doi.org/10.1130/0091-7613(1990)018<0268:REEAIO>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Richard W. Murray, Marilyn R. Buchholtz ten Brink, David L. Jones, David C. Gerlach, G.Price Russ; Rare earth elements as indicators of different marine depositional environments in chert and shale. Geology 1990;; 18 (3): 268–271. doi: https://doi.org/10.1130/0091-7613(1990)018<0268:REEAIO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Differences in rare earth element (REE) relative fractionations and total abundances in marine cherts and shales reflect the depositional location of the sediments. Based on cerium anomaly (Ce/Ce*) and total REE abundance (ΣREE) variations preserved in interbedded chert and shale sequences of the Franciscan assemblage, the Claremont Formation, and the Monterey Formation of coastal California, we can resolve three depositional regimes: spreading ridge proximal, ocean-basin floor, and continental margin. This resolution of depositional environment, previously unattainable in otherwise physically indistinct marine sedimentary rocks, provides a potentially powerful new tool for tectonic and stratigraphic reconstructions. Chert and shale deposited near the spreading ridge (within ∼400 km) under the influence of metalliferous activity are characterized by extremely low Ce anomalies (Ce/Ce* ∼0.29), those from an ocean-basin floor setting by well-developed yet less extreme Ce anomalies (Ce/Ce* ∼0.55), and those from continental margin regimes by no or slight anomalies (Ce/Ce* ∼0.90 to 1.30). ΣREE in chert and shale are extremely low in regimes greatly influenced by continental input, owing to the high sedimentation rate, which minimizes exposure time to seawater and, thus, the adsorption of REE. Preliminary results indicate that the effect of diagenesis does not mask the primary REE depositional signature. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A Basin Plain Sand-Layer Geometry Classification: A Predictive Tool: ABSTRACT

The areal distribution of sand layers in the basin plains of 13 modern deep ocean basins has been studied. Fundamentally, sand-layer distribution is controlled by the volume of sand input in single events relative to the basin plain size and by the areal distribution of basin entry points on the ocean basin floor. Although tectonic setting and sea level position may play a role in rate of basin infilling, these factors are relatively unimportant in the control of sand-layer geometry. A basin plain sand-layer classification has been devised. The classification is comprised of a hierarchy with three ranks or categories. The first category describes the distribution of basin entry points as being one of the following; radial, semiradial, longitudinal, or lateral. The second ategory separates basin plains with similar entry point distributions on the basis of the extent of sand coverage on the flat basin floor. Sand coverage is classified as either overall or partial. The final category further separates basin plains according to the general shape of sand bodies on the basin floor--either wedge, sheet, or axial. The classification has application to the study of both modern and ancient deposits. If the distribution of sand layer characteristics is known, it may be possible to locate probable basin entry points and to predict the size of the drainage area relative to the size of the flat basin floor. Alternatively, if the approximate size of the drainage area relative to the basin plain area and the distribution of basin entry points are known, it may be poss ble to predict the nature of the sand distribution on the basin plain. Inherent in each basin plain type is a specific distinctive areal distribution of sand layer characteristics and particular manifestations of proximal/distal relations. For example, a basin with sand layer geometry classified as radial fill, overall coverage, and wedge-shaped (the Hispaniola-Caicos plain) would have its thickest sand and lutite (or shale) layers located around the periphery of the basin plain. The highest frequency of sand layers would probably be found in the basin plain center and the ratio of drainage area to basin plain area would be relatively large (greater than 3). End_of_Article - Last_Page 534------------

Morphology and Sediments of Three Ocean Basin Entry Points, Hatteras Abyssal Plain

ABSTRACT Three canyon systems which enter into the northern one third of the Hatteras Abyssal Plain are the principal sources of Late Pleistocene and Holocene sediment for the entire abyssal plain. The Hatteras Canyon System enters as a point source via a small fan built directly on the ocean basin floor. The Wilmington Canyon mouth is somewhat wider and disgorges its load from a lower continental rise fan. The Hudson Canyon empties across the 110-km wide Sohm Gap, a feature which is on the true ocean basin floor. Because sedimentary processes are different, each basin entry point has produced unique topographic features, as well as unique sediment distribution patterns and sedimentary structures. The relative importance of turbidity current, pelagic and contour current processes, is variable rom system to system. Southward flowing contour currents contribute very little, if any, sediment to the Hatteras Fan, but are an important sediment source for fine materials on the seafloor at the mouth of the Wilmington Canyon and Sohm Gap. Provenance is also responsible for differences in the sediments in each canyon system. As an example, the Hatteras Canyon material is much more calcareous than material from the other two basin entry points, because of the relatively calcareous nature of the continental shelf source area near Cape Hatteras.

Natural hazards to submarine cables

Submarine cable failures from representative marine environments are examined. Sea-floor regions covered are: current-swept restricted passages, continental and insular shelves, inter-island oceanic passages, continental and insular slopes, oceanic ridges and the ocean basin floor. Chafe by bottom currents is found to be a potential danger to a depth of 300 fathoms in certain locales. Continental slopes, especially when seaward of a river mouth, are especially hazardous.

Evolution of Continental Margin: ABSTRACT

In the past few years, new geologic and geophysical data from the continental margin and the ocean-basin floor have permitted a more detailed reconstruction of the evolution of the continental margin off the east coast of the United States. Application of the Vine-Mathews hypothesis of sea-floor spreading suggests that continental rifting in the North Atlantic began in Late Triassic or Jurassic time. Triassic redbeds of the Newark Series may represent the stage of rifting presently seen in the African Rift Valley. The continental block or coastal plain has prograded slowly seaward (15 km during Tertiary time) and its surface has subsided slowly (5 cm/1,000 years) but remained near sea level during Cretaceous-Tertiary time. Seismic reflection profiles, bottom morphology studies, long sediment cores (more than 300), and bottom photographs (350 stations) demonstrate that southerly flowing deep-ocean currents have constructed and shaped the large margin sedimentary wedges of the continental rise and outer ridges. This southerly flowing North Atlantic deep water is--and has been--eroding, transporting, and depositing sediment parallel with the regional contours. The present sediment surface of the lower continental rise (0-10 m) is marked by a distinct sedimentary facies of current-produced alternating thin silt laminations with hemipelagic lutites. Seismic reflectors horizon A (Upper Cretaceous) can be traced beneath the large (1-2-km thick) sedimentary wedge of the Blake-Bahama outer ridge and also beneath the continental rise on the north, and shows that these features have been lapped adjacent to the continental block in post-Cretaceous time. The initiation of this unique sedimentary process can be linked to the growth of a strong pattern of thermohaline circulation in the North Atlantic with the widening of a growing North Atlantic basin during the Cenozoic. End_of_Article - Last_Page 740------------

Deep-sea current evidence from abyssal sediments

Relatively strong bottom currents in the deep-sea can be inferred from sedimentary structures observed in bottom photographs and cores. The majority of photographs of high and steep topographic prominences, such as seamounts, escarpments and the crests of major ridges, show ripples, scour and rock outcrops. Although photographic current evidence is uncommon in the deeper waters of the ocean basin floor, striking and significant examples do occur. Current scour and ripples are observed beneath the Antarctic bottom current in the western South Atlantic, below the Gulf Stream in the Florida Straits and on the Blake Plateau, beneath the outflowing Mediterranean water west of Gibraltar and beneath the deep currents in the Drake Passage. The three types of deep-sea sands and silts are turbidite, accretionary, and residual. The latter two types are always associated with currents and in some areas turbidites are reworked by bottom currents. Traction velocities necessary for the transport of deep-sea sediment particles probably range from 4–60 cm/sec, velocities similar to those found through dynamic computations of geostrophic currents and observed by recent deep-sea direct current measurements.