Nodular Chert Research Articles

Morphology of sea-floor landslides on Horizon Guyot: application of steady-state geotechnical analysis

Mass movement and erosion have been identified on the pelagic sediment cap of Horizon Guyot, a seamount in the Mid-Pacific Mountains. Trends in the size, shape and preservation of bedforms and sediment textural trends on the pelagic cap indicate that bottom-current-generated sediment transport direction is upslope. Slumping of the sediment cap occurred on and that the net bedload transport direction is upslope. Slumping of the sediment cap occurred on the northwest side of the guyot on a 1.6° to 2.0° slope in the zone of enhanced bottom-current activity. Submersible investigations of these slump blocks show them to be discrete and to have a relief of 6–15 m, with nodular chert beds cropping out along the headwall of individual rotated blocks. An evaluation of the stability of the sediment cap suggests that the combination of the current-induced beveling of the sea floor and infrequent earthquake loading accompanied by cyclic strength reduction is responsible for the initiation of slumps. The sediment in the area of slumping moved short distances in relatively coherent masses, whereas sediment that has moved beyond the summit cap perimeter has fully mobilized into sediment gravity flows and traveled large distances. A steady-state geotechnical analysis of Horizon Guyot sediment indicates the predisposition of deeply buried sediment towards disintegrative flow failure on appropriately steep slopes. Thus, slope failure in this deeper zone would include large amounts of internal deformation. However, gravitational stress in the near-surface sediment of the summit cap (sub-bottom depth< 14 m) is insufficient to maintain downslope movement after initial failure occurs. The predicted morphology of coherent slump blocks displaced and rafted upon a weakened zone at depth corresponds well with seismic-reflection data and submersible observations.

Chertification histories of some Late Mesozoic and Middle Palaeozoic platform carbonates

ABSTRACTA consistent pattern for the silica sources, depositional environments and timing of chertification was observed in a diverse suite of five Late Mesozoic and Middle Palaeozoic carbonate sequences; the (1) Upper Greensand (Cretaceous) and (2) Portland Limestone (Jurassic) of southern England, (3) the Ramp Creek Formation (Mississippian) of southern Indiana, and the (4) lower Helderberg Group (Devonian) and (5) Onondaga Limestone (Devonian) of New York State. Nodular chert formation in all five limestone sequences occurred in sediments that were largely uncemented. Ghosts of pre‐chertification carbonate cements are present in some chert nodules but are volumetrically minor. In every limestone sequence except the Upper Greensand, chertification occurred after burial to a depth sufficient for intergranular pressure solution and mechanical grain deformation of carbonate sand.Nodular chert is most abundant in subtidal, normal marine wackestones and mudstones that were deposited at or below fair‐weather wave base, and is absent or rare in supratidal, intertidal and high‐energy subtidal limestones and dolomites. An intraformational sponge spicule silica source for chert nodules is suggested by direct evidence, such as calcitized sponge spicules in the host limestone, and circumstantial evidence, such as ghosts of sponge spicules in chert nodules and a correlation of chert abundance with depositional environment. Most molds of siliceous sponge spicules were apparently obliterated by post‐chertification intergranular compaction. We propose that these general trends for the depositional environments, silica sources and timing of chertification are representative of most Mesozoic to Middle Palaeozoic platform limestones.

Nodular Chert Formation in Carbonate Rocks

Mapping of carbonate dissolution in nodular chert-bearing strata indicates that nodular chert formation in a diverse suite of Phanerozoic limestones occurred in bulk pore waters that were at calcite saturation. Opal-CT and quartz supersaturation was achieved in most chert-bearing strata by the intraformational dissolution of amorphous silica skeletal material. Chertification occurred by a combination of force of crystallization-controlled replacement of the host carbonate, whereby non-hydrostatic stresses resulting from opal-CT and quartz crystal growth caused calcite dissolution, and the isomineralic heterogenous nucleation of new opal-CT and quartz crystal at nodule peripheries. The restriction of the solution phase at silica-carbonate contacts to thin films permitted the preservation of ghosts of micron-sized features in the chert. Heterogeneities in sediment organic matter content, porosity, and biogenic silica concentrations promote the nucleation of chert nodules in some formations.

Pre-Cenozoic Nodular Cherts; Evidence for opal-CT Precursors and Direct Quartz Replacement

Pre-Cenozoic Nodular Cherts; Evidence for opal-CT Precursors and Direct Quartz Replacement

Pre-Cenozoic nodular cherts; evidence for opal-CT precursors and direct quartz replacement

Pre-Cenozoic nodular cherts; evidence for opal-CT precursors and direct quartz replacement

Chert occurrences in carbonate turbidites: examples from the Upper Jurassic of the Betic Mountains (southern Spain)

ABSTRACTIn Upper Jurassic carbonate turbidites of the Betic mountains (southern Spain), chert occurs in three morphologies: bedded chert, nodular chert and mottled chert. The last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock. The three types of chert are formed by replacement of limestones and are associated with different calcareous facies. Turbidite packstones of Saccocoma and peloids, and turbidite lime mudstones of pelagic material contain bedded and nodular cherts. The silicification textures are mainly micro‐ and cryptocrystalline quartz, with local chalcedonic quartz (both length‐fast and length‐slow) which is more common in the packstones. Only mottled chert is produced where calcareous breccia beds are silicified. Mottled chert consists of micro‐ and cryptocrystalline quartz, length‐slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz. Beds and nodules are the result of early diagenetic silicification, with silica derived from the calcitization and dissolution of radiolarians and, subordinately, sponge spicules, whereas mottled chert is the consequence of later silicification in a probably Mg‐rich environment.Early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones. Turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water. A silica‐rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert.

RbSr and SmNd systematics of cherts and other siliceous deposits

We have analyzed a suite of marine and lacustrine cherts to determine the concentration of Rb, Sr, Sm and Nd and the isotopic composition of Sr and Nd. The samples ranged from Pliocene to Silurian in age and included nodular cherts in carbonates as well as bedded cherts. Massive samples were leached in acid to remove carbonate and both the leach and residue analyzed. The residues contained 0.1 to 17 ppm Rb and 0.5 to 20 ppm Sr. All samples have high Rb Sr ratios (~0.1 to 0.8) and thus show enrichment of Rb relative to Sr of 6 to 60 times that of seawater. The Rb and Sr in pegmatitic quartz are much lower by factors of 10 to 10 3, although these also may have high Rb Sr . The site of Rb and Sr in the cherts was not established but appears to be correlated with Al content. The high Rb Sr ratios permit the use of cherts for dating in favorable circumstances. The Nd contents range from 9 to 1000 ppb with Sm Nd ~ 0.16 to 0.19, typical of continental sources. The initial 87Sr 86Sr ratio for a Pacific deep sea chert is equal to that of associated carbonates and hence reflects the seawater environment. In shelf and marginal basin environments, the initial 87Sr 86Sr ratios of chert are distinctly more radiogenic than seawater. This is most obvious where a substantial clay component is present, but is observed even in samples with Al 2O 3 contents less than 0.1%. These results show the Rb-Sr system in chert to be rather isolated from the associated carbonate with very little Sr exchange. This separate behavior appears to persist through both early and late recrystallization stages. These data suggest that, in many cases, cherts are formed by “dissolved” SiO 2 nucleating on clay-like material which preserves the isotopic characteristics of the detrital source. The detrital component required is less than 1% of the total chert. In some cases, it is possible that the cherts were formed from silica-saturated waters from a nonmarine source, possibly in coastal environments or during uplift. These waters contained radiogenic Sr in solution that was not exchanged with the carbonate matrix. This latter mechanism is also indicated by oxygen isotopic studies ( Knauth and Epstein, 1976; Kolodny and Epstein, 1976). The Nd isotopic signature of the deep sea samples reflects the seawater composition in agreement with the initial 87Sr 86Sr data on the same samples. Samples with excesses of 87Sr relative to seawater show low ϵ Nd, both sets of data thus indicating inheritance of an old detrital component. The results demonstrate that the Rb-Sr and Sm-Nd systems may be used in elucidating the origin and evolution of cherts and that the Rb-Sr system may be used for dating, if consideration is given to the initial Sr composition and to diagenetic changes. This opens up numerous possibilities for sedimentological-geochemical studies.

Lower Mississippian Carbonate Stratigraphy of Southeastern Idaho and Northeastern Utah: ABSTRACT

Lower Mississippian carbonates of southeastern Idaho and northeastern Utah have been referred to the Madison Group or Lodgepole Limestone since the early part of this century. Although time equivalent to the Lodgepole and basal part of the Mission Canyon Limestones of central Montana, the southeastern Idaho and northeastern Utah strata are lithologically distinct as a result of deposition in a more distal shelf edge to basin margin environment. Furthermore, the strata may be sub-divided into three members that are traceable throughout the region. The basal member, informally known as the Chinese Wall, consists of thin-bedded wackestones and mudstones with a few grainstones. Graded beds are common throughout the unit, as are chert nodules and discontinuous nodular chert bands. Where dipping less than 30/sup 0/ it weathers as a sheer cliff 16 to 84 m thick. Conodont faunas from the unit belong to the Lower and Upper Siphonodella crenulata Zone, of middle to late Kinderhookian age. The middle member is thin-bedded wackestones and mudstones with fewer graded beds and rarely contains chert nodules. The middle member weathers as a slope 47-90 m thick and is of late Kinderhookian age, S. isosticha-Upper S. crenulata Zone. The upper member consists of grainstones withmore » a few wackestones, thin to medium bedded, moderately common graded beds, chert nodules present in the upper half, weathers as a cliff or cliff capped with a receding slope, and the thickness ranges from 30 to 58 m. Conodont faunas belong to the S. isosticha-Upper S. crenulata Zone in the lower part and the Gnathodus typicus Zone in the upper part. New stratigraphic names are being proposed for each of the units discussed.« less

Sedimentología y petrología de los abanicos aluviales y facies adyacentes en el Neogeno de Paracuellos de Jarama (Madrid)

Lateral and vertical variations of middle Miocene lithofacies (Lower and Intermediate Units of the Miocene of the Madrid Basin) have been analyzed in the Paracuellos de Jarama area, near Madrid. A complete transition from medial to distal alluvial fan facies and palustrinelshaltow 1acustrine deposits can be observed. This sedimentary evolution is inferred from the detailed sedimentological. analysis carried out in each of the previously defined lithostratigraphic units: a 1 ) Green shale and dolomite Unit, a 2 ) Brown clays, arkosic and carbonate Unit; these two units are linked together into a Lower Group. b) Coarse Arkosic Unit (Upper Group). Facies relationsbips within the Lower Group show a lateral change pattern between alluvial fan deposits, and palustrine deposits. Also the former ones gradually prograde over the palustrine deposits, as deduced from vertical evolution in the southernmost parts of the area. Upper Group represents a sudden progradation of the alluvial fan systems, that extensively overlie the distal facies observed in the Lower Group. Wide development of calcretes, sepiolite deposits, and nodular chert is an outstanding feature in the most distal parts of the alluvial fan bodies. They are studied in some detail provided the scarcity of well described examples of facies relationships in the kinds of deposits.

Evidence for evaporites in the ORS of northern Scotland: replaced gypsum horizons in Easter Ross

The thick lacustrine sequence of the Old Red Sandstone (ORS) Orcadian Basin might be expected to yield evidence of evaporites. Lithological evidence suggests sabkha/playa-type environments in the Lower and Middle ORS, with widespread deposition of algal carbonates ([Donovan 1978][1]). However records of pseudomorphs after evaporites in the basin ([Donovan 1978][1]; [Westoll 1979][2]) are scant and uncertain, and the extensive lacustrine deposits of the Middle ORS are generally considered to have been laid down in a non-evaporative environment (e.g. [Leeder 1982][3]). Pseudomorphs after gypsum (usually moulds due to differential weathering) are in fact widespread and occur in each of the Upper, Middle and Lower divisions of the ORS. Scarce pseudomorphs occur in Upper ORS green siltstones at Dunnet Head and Hoy (A. McAlpine pers. com. 1984). Within the Middle ORS, dolomicrites, siltstones and sandstones at most stratigraphic levels yield sparite or dolosparite pseudomorphs after gypsum. Examples in Orkney occur in the Eday Marls at Hoxa [ND 424 931] and Eday [HY 551 365], Rousay Group siltstones at Gorseness [HY 421 187], and in Stromness Group stromatolitic dolomites at Hesta Geo [HY 252 287], Stellar quartz pseudomorphs, possibly after a sodium carbonate mineral, occur in Middle ORS nodular cherts in Orkney and Caithness, interpreted as Magadi-type cherts ([Parnell 1983][4]), and in thin chert bands within the Sandwick Fish Bed west of Stromness [HY 242 081]. Pseudomorphs after gypsum are particularly prevalent in the Lower ORS of Easter Ross and are the main subject of this communication. The Lower ORS of Easter Ross . . . [1]: #ref-1 [2]: #ref-9 [3]: #ref-4 [4]: #ref-8

Mississippian Wood-grained Chert and its Significance in the Western Interior United States

ABSTRACT Wood-grained chert is nodular chert which has internal light- and dark-colored, concentric, three-dimensional, compositional banding resembling grain in wood. The light-colored bands are dolomitic and the dark-colored bands are quartzose with carbonaceous matter. Wood-grained chert is recognized in the Mississippian Lodgepole, Deseret, and Great Blue Limestones and some correlatives of these formations in Montana, Utah, and Idaho. The Deseret starved basin model (Sandberg and Gutschick 1980; Gutschick and Sandberg 1983) provides a paleogeograpic framework that restricts the wood-grained cherts in this basin to the foreslope between the oxygenated carbonate platform margin ( 300 m) basin. Wood-grained chert may mark th approximate position of the lower limit of the pycnocline in the Deseret starved basin. Upwelling of nutrient-rich water along the foreslope generated high biological productivity which in turn supplied abundant biogenic silica and organic matter to the sediments of the foreslope and basin. Siliceous microfossils dissolved and provided a source of silica for the formation of porcellanite (opal-CT) nodules within the foreslope sediments during burial and compaction. Calcium carbonate diffused into the nodules from the surrounding mixed calcareous/siliceous muds and early diagenetic conversion of opal-CT to quartz released magnesium. Reduction of sulfate, a dolomite inhibitor, in the associated anoxic, organic-rich sediments below the pycnocline permitted dolomite formation. Dolomite precipitated in bands as a result of a periodic supersaturation-precipitation-nucleation-d pletion process (Ostwald-Prager theory of Liesegang band formation), possibly augmented by competitive particle growth. Petrographic evidence and the apparent inverse relationship between band spacing and time support our theory.

The sedimentology and environment of deposition of the Mallacoota Beds, eastern Victoria

Abstract The Mallacoota Beds of Ordovician age in eastern Victoria include: (1) a succession of greywackes, sandstones, siltstones and shales that show a variety of sedimentary structures and bedding features, including graded bedding, lateral continuity of beds, convolute lamination, slumps, flute casts, load structures and all divisions of the Bouma sequence; (2) radiolarian cherts and black shales, representing pelagic and hemipelagic deposition. These former radiolarian‐rich oozes, which occur between turbidites, have been diagenetically converted to bedded and nodular chert, whereas thick (>100 m) ribbon‐chert sequences probably accumulated in areas such as topographic highs, away from dilution by turbidite deposits. Six separate lithofacies can be defined, and bed thickness analysis indicates the presence of thinning and/or fining‐upward facies sequences and thickening and/or coarsening‐upward facies sequences, suggesting the variable proximity of channel axes with time. Palaeocurrent measurements i...

Origin of nodular chert in the carbonate rocks of the Kaladgi group (younger precambrian), Karnataka, India

Lensoid, irregular nodules and laminations of chert appear along the bedding planes and laminations of the Vidyanagar Dolomite Member of the Kaladgi Group, Karnataka, India. The field relations of the chert nodules with the host dolomite and evidence on polished hand-specimens clearly demonstrate that the cherts are secondary after dolomite. This contention finds extensive support from thin-sections which reveal floating relict dolomite fragments on a microcrystalline quartz mat, dolomite rhombs in various stages of progressive replacement, polycrystalline quartz rhombs apparently after dolomite. Chertification has also resulted in an aggrading neomorphic recrystallization of dolomicrite into dolomicrospar which rims the dislodged floating fragments of dolomite. Scanning electron microscopic studies revealed three distinct types of surfaces of chert; the polyhedral surface characteristic of microcrystalline quartz, the spongy surface characteristic of chalcedonic quartz (documented in thin-sections as semi-radiating to radiating aggregates of elongated crystals growing on the rhombic surfaces of dolomite, and the intermediate type showing both. The microcrystalline quartz seems to have originated through multinucleation at equally spaced centres on the dolomite mat, whereas the chalcedonic quartz originated by slow but direct and unhindered precipitation into cavities and residual pore-spaces, following dolomitisation. The silica invasion seems to have been accomplished by alkaline interstitial waters charged with silica. The occurrence and relation of chert-nodules with the host dolomite not only helps in unravelling their own origin, but also aids in drawing the dolomitisation curtain and in turn the post-dolomitisation diagenetic modifications of the host carbonates. It may be added, however, that the Kaladgi Carbonates have undergone mineralogical/textural modifications during diagenesis with superimposition of changes affected later, i.e. during tectonism which folded and cross-folded these sediments.

Features and Origin of Italian Jurassic Radiolarites Deposited on Continental Crust

ABSTRACT Jurassic radiolarites that were deposited on continental crust and which are overlain and underlain by pelagic limestones were examined in four basins. Radiolarites studied range from bedded chert, where chert occurs in even, continuous beds separated by conscpicuous rhythmic shale interbeds, to nodular-lumpy chert, where chert occurs as isolated nodules in limestone at one extreme to lumpy beds of irregular thickness at the other extreme. Bedded chert formations are composed of microcrystalline quartz, clay minerals, and minor amounts of either hematite or organic matter pigment; nodular cherts contain in addition considerable micrite (altered coccolith ooze). Chert nodules, lenses, and beds all formed by the diagenetic reorganization of silica almost entirely of biogenic origin, chiefly from Radiolaria. Nodular-lumpy chert beds and even some bedded chert formed by partial to complete replacement of limestone. Lumpy beds formed because compaction, dissolution of calcite, and replacement of calcite were not uniform in either time or place within a bed. Many even-bedded chert layers were originally silts or sands composed almost entirely of current-deposited Radiolaria. The rhythmic bedding of radiolarite is the product of 1) episodes of rapid current-deposition (e.g., turbidity currents) of radiolarian sediment alternating with slow deposition of hemipelagic mud and 2) episodic growth of Radiolaria. Bioturbation blurs the evidence of these two different origins, but the following features indicate that many chert-precursor beds were current-deposited: sharp chert-shale bedding contacts, flute casts, graded beds, clay clasts, laminations and rapidly buried animal tracks and trails. Water depths during radiolarite deposition are uncertain. Regional stratigraphic data, evidence of redeposited radiolarian sand, and acceptance of the Bosellini-Winterer model of carbonate dissolution surfaces during Late Jurassic time leads EFM to conclude that well-bedded radiolarite was deposited at depths close to the CCD (2500 m) and that lumpy-bedded chert was deposited between the ACD and the CCD (1500 to 2000 m). Features indicating the presence of evaporites (evaporite pseudomorphs, breccias formed by crystal growth, quartzine, lutecite), paleosoil fabrics, stalactite-stalagmite fracture fillings, and disbelief in the validity of the CCD in Late Jurassic time in the Tethys leads RLF to conclude that the Lombardy radiolarites were deposited in environments that were, in part, shallow intertidal mudflats where local precipitation and solution of evaporite minerals and occasional subaerial exposure took place.

A model for the origin of chert in limestone

Research Article| June 01, 1979 A model for the origin of chert in limestone L. Paul Knauth L. Paul Knauth 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 708032Present address: Department of Geology, Arizona State University, Tempe, Arizona 85281 Search for other works by this author on: GSW Google Scholar Author and Article Information L. Paul Knauth 1Department of Geology, Louisiana State University, Baton Rouge, Louisiana 708032Present address: Department of Geology, Arizona State University, Tempe, Arizona 85281 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1979) 7 (6): 274–277. https://doi.org/10.1130/0091-7613(1979)7<274:AMFTOO>2.0.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 L. Paul Knauth; A model for the origin of chert in limestone. Geology 1979;; 7 (6): 274–277. doi: https://doi.org/10.1130/0091-7613(1979)7<274:AMFTOO>2.0.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 It is proposed that many nodular cherts in limestone have formed in the ground water of mixed meteoric-marine coastal systems where dissolution of biogenic opal and mixing of marine and fresh waters can produce waters highly supersaturated with respect to quartz and undersaturated with respect to calcite and aragonite. Aspects of cherts readily explained by this model include the observed isotopic ratios in cherts, typical field relationships, the relative resistance of dolomite to silicification, the source-of-silica problem, the preservation of siliceous fossils in cherts, and aspects of chert morphology and mineralogy. 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.

Microfossils from the Dismal Lakes Group (Neohelikian), District of Mackenzie

Microfossils have been discovered in thin sections cut from a sample of black, laminated, nodular chert collected from the Neohelikian Dismal Lakes Group of the Dismal Lakes area, District of Mackenzie. Two distinct morphological forms are differentiated: filaments and spheroids. The filaments are further subdivided, on the basis of morphology, into four basic varieties: (1) septate, unbranched, uniseriate filaments composed of chains of spheroidal to ellipsoidal cells showing marked constriction between cells, (2) septate, unbranched, uniseriate filaments characteristically lacking constrictions between cells, (3) nonseptate, unbranched filaments of large diameter (&gt; 1.8 μm), (4) nonseptate, unbranched filaments of small diameter (&lt; 1.3 μm). The spheroidal structures occur singly, in pairs and in clusters, and some of them contain blebs of carbonaceous matter which may represent remnants of organelles, reflecting eucaryotic affinities.

S.E.M. Observations of Quartz in Cherts: Etched vs. Unetched Surfaces

Cherts, rocks composed essentially of microcrystalline quartz, display a wide variety of crystal forms. Fractured and polished surfaces of a bedded and a nodular chert were examined with the S.E.M. before and after being etched in various solutions (HF, KOH, NaOH). The bedded chert is a member of the Arkansas Novaculite containing pyrite, organic matter, and silicified organic spherical bodies (possibly spores) 100-500 microns in diameter. The chert nodule is from a Cretaceous carbonate, and displays a white patina which is interpreted as a weathering rind.Unetched surfaces generally do not display crystal morphologies as well as etched surfaces. However, crystal forms were observed on unetched surfaces where the fracture exposed cavities within the rock.

Oceanic Sediments and their Diagenesis: Some Examples from Deep-sea Drilling

Carbonate sediments dominant except during Eocene (siliceous sediments); progressive breakdown of coccoliths, common occurrence of dolomite rhombs, nodular chert biogenic, bedded chert source unknown; enrichment of basal sediments in iron attributed to sea-floor spreading

Sediments on the Tyrrhenian abyssal plain : Ryan William B. F., Fifield Workum Jr., and J. B. Hersey, 1965. Bull. geol. Soc., Am., 76 (11): 1261–1282.

Mass movement and erosion have been identified on the pelagic sediment cap of Horizon Guyot, a seamount in the Mid-Pacific Mountains. Trends in the size, shape and preservation of bedforms and sediment textural trends on the pelagic cap indicate that bottom-current-generated sediment transport direction is upslope. Slumping of the sediment cap occurred on and that the net bedload transport direction is upslope. Slumping of the sediment cap occurred on the northwest side of the guyot on a 1.6° to 2.0° slope in the zone of enhanced bottom-current activity. Submersible investigations of these slump blocks show them to be discrete and to have a relief of 6–15 m, with nodular chert beds cropping out along the headwall of individual rotated blocks. An evaluation of the stability of the sediment cap suggests that the combination of the current-induced beveling of the sea floor and infrequent earthquake loading accompanied by cyclic strength reduction is responsible for the initiation of slumps. The sediment in the area of slumping moved short distances in relatively coherent masses, whereas sediment that has moved beyond the summit cap perimeter has fully mobilized into sediment gravity flows and traveled large distances. A steady-state geotechnical analysis of Horizon Guyot sediment indicates the predisposition of deeply buried sediment towards disintegrative flow failure on appropriately steep slopes. Thus, slope failure in this deeper zone would include large amounts of internal deformation. However, gravitational stress in the near-surface sediment of the summit cap (sub-bottom depth< 14 m) is insufficient to maintain downslope movement after initial failure occurs. The predicted morphology of coherent slump blocks displaced and rafted upon a weakened zone at depth corresponds well with seismic-reflection data and submersible observations.

Cyclic Patterns in Mechanically Deposited Pennsylvanian Limestones of Northeastern Nevada

ABSTRACT Pennsylvanian sediments in northeast Nevada consist of limestone (80-90%) with smaller quantities of quartz sandstone, siltstone, and chert conglomerate. Outcrop patterns and details of lithology show cyclic alternations. Mechanically deposited limestone exceeds 25% of the total limestone, and includes bioclastic calcarenite and calcirudite and some oolite. Detrital quartz impurities are characteristic. Non-mechanical coquinoid limestones alternate with mechanical varieties. Characteristic nodular chert is secondary (diagenetic) and occurs in very cherry units alternating with non-cherty units. Fossils are not well preserved except where silicified. Algal remains, fragmented fossils, detrital quartz, oolite, and numerous diastems indicate shallow water, frequent agitation, winnowing, and redeposition. Alternating with these conditions were periods of less intense stirring of accumulating sediment, producing cyclic patterns. Orogeny in the eugeosyncline produced lands which shed quartz and chert detritus eastward into the miogeosyncline. Presence of notable quartz sandstones and limestones in the eugeosyncline contradicts popular concepts of only impure graywacke sands in such provinces to the near exclusion of all other sediment types. Local upwarps and platforms within the miogeosyncline subsided less rapidly and furnished clastics to adjacent negative areas. The limestones and associated quartzose sandstones lithologically qu lify as stable shelf deposits like typical sequences of the craton. The Nevada ones are much thicker however, indicating more rapid subsidence in a relatively unstable geosynclinal belt. While depositional environments in the two provinces were identical, tectonic environments differed considerably as evidenced by significantly thicker deposits, local angular unconformities and appreciable chert detritus in Nevada.