Surface sediments from a transect across the mid-ocean ridge north of Iceland (Kolbeinsey Ridge) have been analyzed according to their compositional, textural and hydromechanical characteristics. The results were used to reconstruct sediment formation and depositional processes. The ridge sediments are dominated by volcaniclastic particles of hyaloclastic and pyroclastic origin. These particles show a wide variety in size, shape and density. Single-grain settling velocities of the different glass types reveal the suitability of this parameter as a reflector of the particle properties of size, shape and density, which are also known to be relevant to grain transport. Observations concerning different current expositions of central ridge sediments, combined with the parameters of settling velocity distribution, grain-size distribution and sediment particle composition, were applied to distinguish between transport association with rare, easily movable glass shards and poorly sorted sediments in sheltered ponds. A bimodal settling velocity distribution of steep ridge-flank sediments probably indicates the effect of sediment admixture from poorly sorted mass flows. Alternating coarse- and fine-grained layers characterize the transition between ridge-glass sands and the ridge-adjacent plain, which is dominated by slow-settling pelagic material.
The coeval Early Silurian platformal Sayabec and La Vieille formations covered 10,000 and 20,000 km 2 , respectively, in the Gaspe Basin in the Appalachian orogenic belt. Field studies, petrography, and carbon and oxygen stable isotopes have been used to document the diagenetic evolution of the carbonate platforms. Significant diagenetic features included marine cements, shallow burial marine to possible meteoric-phreatic cements, burial cements, pressure-solution stylolites, fractures and minor fracture-fill cements, meteoric dissolution and meteoric-vadose cements. Cementation started in the marine environment (LF1). It was minor ( 18 O = -5.3 per thousand ; delta 13 C = +3.8 per thousand ) is taken as the best value for Early Silurian marine composition. The upper part of the radiaxialoptic cement layers is composed of an initial non-luminescent, equant to prismatic spar (LF2) passing into complexly banded luminescent scalenohedral spar (LF3). These two cements constitute the bottom of cement successions in other lithofacies of both the Sayabec and La Vieille formations. They are interpreted as likely shallow marine burial cements, but a possible meteoric-phreatic origin is not ruled out. Both hypotheses rely on CL petrographic similarities with known examples, their depleted delta 18 O signature and wide range of delta 13 C values. Ferroan cements (LF4) fill over 85% of the pore space, occlusion of which was completed at burial depths of around 450 m. LF4 cements are 18 O-depleted (delta 18 O = -8.1 per thousand and -8.7 per thousand for Sayabec and La Vieille formations, respectively) when compared to LF2 (Delta nearly equal 2.3 per thousand ) and LF3 (Delta nearly equal 1.3 per thousand ) cements for both formations. The delta 13 C values for the LF2 to LF4 La Vieille cements are much higher than those of the Sayabec. This is explained by early methanogenesis related to the paleogeographic nature of the platform. Stylolitization, fracturing and minor calcite cementation, minor dolomitization and sulfatization occurred in the burial environment and postdate LF4 cements. Post-initial burial, LF5 (gravitational) and LF6 cements (delta 18 O = -10.7 per thousand and -10.1 per thousand , delta 13 C = +3.7 per thousand and +4.4 per thousand , respectively) are interpreted to be meteoric-vadose cements filling solution vugs of the La Vieille limestones in the western part of the ramp. They are related to Late Silurian uplift in the Gaspe Belt.
ABSTRACT Laramide uplift and erosion of the Uinta Mountains are recorded in a 10-km-long outcrop of the Lower Eocene Wasatch Formation, on the Utah-Wyoming border. This 750-m-thick package of interbedded sandstones and conglomerates is dominated by a coarsening then fining-upward megasequence 650 m thick that records the growth and abandonment of a humid alluvial fun system during a major cycle of uplift and unroofing of the thrust-bounded northern flank of the Uinta mountains. Grain size, thickness, and lateral extent of channel-complex deposits increase upward in the lower 400 m of the sequence, reflecting construction and northward progradation of the fan. Grain size and channel-complex thickness decrease upward in the upper 250 m of the sequence, reflecting gradual reduction of both sedime t yield and sediment caliber during postorogenic lowering of source-area relief. Within the megasequence, coarsening-up sequences 10-100 m thick built mainly of channel-complex deposits reflect progradation of fan lobes, punctuated by periodic fan-head avulsions. These medium-scale sequences in turn comprise small-scale fining-up cycles 1-10 m thick that reflect such fluvial processes on the fan as bar building, discrete flood events, and the filling and lateral migration of braided-stream channels. Both the medium-scale and small-scale sequences are commonly underlain by paleosols. Evidence of debris flows or other mass-movement processes is conspicuously lacking. Above the main megasequence a second, thinner megasequence containing lacustrine mudstones and wave-reworked conglomerates indicates that the fan persisted as a locus of coarse clastic deposition during the first of a series of lacustrine transgressions that began as ratios of sediment flux to subsidence rate decreased toward the end of the Early Eocene. Partial inundation of the fan during this transgression transformed it into a fan delta.
The Liguride Complex is an ophiolite-bearing tectonic unit related to westward subduction of the lithosphere of the Neo-Tethys Ocean beneath the European margin. The complex includes a tectonic melange (the Calabro-Lucanian Flysch Unit) containing broken formations of ophiolites and related pelagic and clastic sediments (radiolarian chert. limestone, black shale, and quartzarenite), olistoliths or ''tectonic slices'' of continental crust (gneiss, granite, and amphibolite), Middle Eocene to Upper Oligocene siliciclastics, and a less deformed turbidite unit (the Saraceno Formation) that caps the succession.Detrital modes of Upper Cretaceous to lower Miocene sandstones from the Liguride Complex accretionary wedge are grouped into four distinct petrofacies that reflect the geodynamic evolution of Southern Italy. An Upper Cretaceous to Middle Eocene quartzose petrofacies (Q90F9L1) is composed of deep-water quartzarenitic and subarkosic sandstones that appear to have undergone long-distance transport by turbidity currents from the European or African foreland. A Middle Eocene to Upper Oligocene quartzofeldspathic petrofacies (Q62F21L17) was produced by mixing of sediment from ophiolitic and pelagic sedimentary sources with volcanic debris. An Upper Oligocene volcanolithic petrofacies (Q16F24L60) is interbedded with the quartzofeldspathic strata. The andesitic volcanic debris and the fact that the particles are generally fresh and larger than other terrigenous clasts suggests that the volcanic debris was derived from co-eruptive products produced by Oligocene calc-alkaline andesitic volcanism in Sardinia. An Upper Oligocene to Lower Miocene quartzolithic petrofacies (Q54F10L36) is present in the turbidites of the Saraceno Formation.The accretionary-wedge sandstones of the Liguride Complex may record long-distance longitudinal transport for hundreds of kilometers from distal sources (preexisting forelands and volcanic arc) and short-distance transverse transport from local sources (accreted oceanic terranes and collisional-belt terranes). In accretionary-wedge sandstone assemblages, sediments from local and distal sources may be mixed or may record a consistent evolution from one type of transport to the other.
The morphological and optical properties of crystal aggregates which develop by nucleated growth from a plane substrate are modelled by crystal growth diagrams. Such diagrams show that fabric maturation occurs through three growth stages: 1) isolated, 2) competitive, and 3) parallel.Diagrams are presented which are constructed using calcite rhombohedra of equant {1011BAR}, obtuse {0112BAR} and acute {4041BAR} form. The acute form develops, in the most mature parallel growth stage, length-fast columnar crystals. This graphical model is matched by a natural, calcite-cemented neptunian dyke. The obtuse form develops, in the most mature parallel growth stage, approximately length-slow, bladed crystals. This graphical model is matched by a natural, calcite-cemented dilational vein.Diagrams are presented which show seeded, in contrast to nucleated, growth. Seeding produces overgrowths which are divisible into an early epitaxial stage of rapid growth and a later idiomorphic stage. The volume of epitaxial growth depends on surface area, shape and lattice perfection of the seed, and on the crystal habit of the overgrowth. The overriding importance of echinoderm skeletal elements in stimulating epitaxial growth in limestones is related to their large size relative to other seeds and to their lattice properties. Polycrystalline seeds with parallel c-axis orientation develop overgrowth aggregates which omit the competitive stage because the crystals are constrained to develop normally to seed surfaces even when these are not the directions of most rapid growth. Growth from polycrystalline seeds with random c-axis orientation produce crystal aggregates similar to those developed by nucleation from a passive substrate.Crystal growth diagrams are presented to show the effect of a change in crystal habit during continuous growth. The acute to obtuse change examined results in the crystal's greatest growth vector(s) being reorientated, which in turn causes a new episode of fabric maturation. This graphical model is matched by a calcite cement from a South Wales grainstone. T-shaped crystals, growth hiatuses on some crystal surfaces, and enfacial junctions are created in both model and natural example.The use of crystal fabrics in the diagnosis of cement, the correlation between crystal fabric and saturation state, and the standard of crystallographic observation in studies of calcite cement are criticized.The match between accurately drawn crystal growth diagrams and natural crystal aggregates is excellent. These diagrams are an underused aid in the stereological assessment of crystal aggregates and reveal features that are not intuitively obvious.
The Burnside Formation records the shift from marine-shelf to largely alluvial conditions in the Paleoproterozoic (almost-equal-to 1.9 Ga) Kilohigok foreland basin in the northwest Canadian Shield. The Burnside is a thick (up to 3.5 km), northwest-tapering siliciclastic wedge representing a sandy braid-delta and braided-river system that prograded northwest across the Archean Slave craton. Criteria are developed to distinguish shallow-marine facies from alluvial facies, allowing construction of a stratigraphic framework for this unfossiliferous, alluvially dominated succession.The sandy alluvial system prograded over a storm-influenced fine-grained siliciclastic shelf. Shelf associations can be divided into siltstone-dominated and mixed siltstone-sandstone. The siltstone-dominated association comprises facies ranging from mudstones deposited below storm-wave influence to siltstones deposited under storm and fair-weather wave conditions. Mixed siltstone-sandstone facies form coarsening-upward sequences on the scale of several meters to a few tens of meters that appear to represent prograding marine sand bars. Initial alluvial influence is manifested by associations of mixed siltstone and sandstone with shallow-water wave-formed and current-formed structures, abundant soft-sediment deformation, and isolated meter-size channels filled with trough-cross-bedded sandstone and associated rip-up clasts. Thick (tens to hundreds of meters) tabular-bedded fine sandstones represent a delta-platform environment between coarser-grained trough-cross-bedded braid-delta distributary channels. Unidirectional paleocurrents in the transitional alluvial facies indicate predominantly west to southwest transport.The bulk of the Burnside comprises several associations representing sandy alluvial braid-plain facies. Conglomeratic lithologies are subordinate to sandy ones and are restricted largely to the proximal (southeastern) part of the basin. Braid-plain facies, occupying hundreds of meters of section, consist mainly of hierarchies of trough cross-bedding on scales of a few meters to many tens of meters. Unidirectional paleocurrents from the main alluvial associations cluster with a single narrow west-northwest mode.Associations of sedimentary facies and paleocurrents indicate that initially the alluvial system was restricted to the proximal part of the basin and drained longitudinally between the orogenic hinterland and a synsedimentary flexural arch created by thrust loading. Contemporaneous shallow-marine-shelf facies were present to the northwest (over and beyond the flexural arch. An abrupt shift to transverse drainage and the superposition of medial-braid-plain facies directly on marine-shelf and deltaic facies in the foreland mark an abrupt, unconformable contact representing the progradation of the alluvial system over the cratonic foreland. The area of maximum disparity between marine and nonmarine facies coincides with the area of greatest stratigraphic thinning and erosion, and marks the crest of the flexural arch. Only the transverse paleocurrent mode is recorded over the flexural arch and in the distal foreland. Stratigraphic relations suggest that marginal-marine and braid-delta facies tracts in the distal foreland shifted abruptly over large distances and were sensitive to changes in relative sea level. In contrast, braid-plain facies tracts shifted very little, and more gradually. The large lateral extent of alluvial facies suggests that the source area had a humid or monsoon climate.
Carboniferous volcaniclastic-arc deposits of the eastern Klamath terrane, California, include Late Visean/Namurian limestone lenses that formed as small carbonate banks. The limestone lenses, within the Bragdon and Baird formations, reach 17 m in thickness and 1.2 km in length. Slope deposits consist of argillaceous spiculitic wackestone, and bank-edge deposits include ooid grainstone, Striatifera packstone, argillaceous phylloid algal packstone, and argillaceous skeletal packstone. Bank-interior deposits include skeletal wackestone/packstone and argillaceous sandy mudstone. The limestone lenses overlie proximal deltaic deposits of thick-bedded volcaniclastic sandstone and conglomerate. Carbonate banks developed on delta lobes during intervals of minimal clastic sedimentation, possibly related to sea-level rise and volcanic quiescence. The carbonate banks were short-lived depositional systems, and they were covered by prograding deposits of younger volcaniclastic sands.
Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numerical models are used to evaluate the effects of several processes influencing ooid size. The first demonstrates that low supply of new ooid nuclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and velocity gradient also enhance ooid growth. Several Neoproterozoic oolites contain unusually large ooids, some reaching diameters of up to 16 mm. While lower nuclei supply and higher ooid growth rate may have prevailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher growth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a prevalence of carbonate ramps over rimmed shelves during Neoproterozoic time.
A comparison of sandstone cores from Trestakk Field was carried out to better understand the difference in permeability between a well near the oil-filled crest (average 26 mD) and a second well located mainly within the water zone (average 4 mD). Plug samples distributed throughout similar 80-90 m cored intervals in each well were analyzed by X-ray diffraction, bulk chemical analysis, point counting of thin sections, and SEM image analysis of mineralogy and porosity. No differences were found between the two wells in either primary sand quality or the degree of diagenetic alteration, including illitization. However, permeability is strongly correlated with both pore-size distribution and abundance of intergranular macroporosity, which in turn appear to be controlled by the volume occupied by illitic clay. Despite the similar degree of illitization in the two wells, the illite may be more widely distributed in the intergranular pore system in the lower-permeability well, thus reducing intergranular macroporosity and permeability. This difference is suggested to result from growth of illite in completely water-filled porosity in the flank position, as opposed to illite growth only in residual-water-saturated porosity on the oil-filled structural crest.
ABSTRACT Detailed analysis of compositionally unaltered marine fibrous cements (MFC) from a single core through the Mississippian Irish Waulsortian Limestone indicates that the variation of seawater 87Sr/86Sr is nonmonotonic across the Ivorian-Chadian boundary. This nonmonotonic variation has not been recognized by previous studies. Furthermore, marine cements yielded 87Sr/86Sr ratios lower than previously reported values for the Ivorian-Chadian (Osagean). Marine fibrous cements are interpreted to be compositionally unaltered on the basis of nonluminescent character and stable isotope (C, O) composition comparable to previous estimates of Mississippian marine calcite. The isotope chemistry (C, O, Sr) and cathodoluminescent character of the marine fibrous cements therefore remained intact during their conversion from high-Mg calcite to low-Mg calcite + microdolomite, a conversion that probably took place in marine water during precipitation of Zone 1 calcite cement, the oldest non-MFC cement. High stratigraphic resolution was obtained by restricting the sample set to a single core, 429 m long, thereby eliminating chronostratigraphic correlation errors. The MFC cements are pre-Arundian (based on previous study), and are interpreted to be in stratigraphic order and to be approximately the same age as the host limestones. The core is estimated to represent about 9.8 million years of Waulsortian Limestone deposition. 87Sr/86Sr ratios of pristine MFC samples range from a high of 0.707908 in the early Ivorian to a low of 0.707650 in the late Ivorian and mid-Chadian, with an early Chadian maximum at 0.707800. The maximum rate of change in seawater 87Sr/86Sr is -0.00012/Ma, comparable in magnitude to Tertiary values. Our data document the presence of fine-scale seawater 87Sr/86Sr modulations for the Ivorian/Chadian, in contrast to the previously published monotonic seawater 87Sr/86Sr curve for this interval, and emphasize the importance of well characterized intraformational isotopic baselines.
The depositional history of the Eocene-Oligocene Burwash strike-slip basin is characterized by a transition from nonvolcanic clastic sedimentation of the Amphitheatre Formation to deposition of lavas and volcaniclastic rocks of the overlying lower Wrangell volcanic sequence. The lowermost Wrangell volcanic deposits within the Burwash basin consist mostly of 110 m of trachybasalt, basaltic trachyandesite, and trachyte lava flows which directly overlie about 440 m of stream-dominated alluvial fan, fan-delta, and lacustrine deposits of the Amphitheatre Formation. Overlying the lava flows are about 50 m of interbedded volcaniclastic fluvial deposits, pumice-rich, andesitic pyroclastic-fallout deposits, and lithic-rich trachyandesitic pyroclastic-flow deposits. Although volcanism is known to occur within strike-slip basins, the role of volcanism in strike-slip basin evolution and the effects of volcanism on sedimentation in strike-slip basins are largely unknown. The depositional history of the sedimentary rocks (Amphitheatre Formation) and the volcanic rocks (Wrangell volcanic sequence) in the Burwash basin can be divided into four progressive stages: 1) progradational stream-dominated alluvial fan deposition; 2) lacustrine deposition; 3) renewed basin margin faulting, proximal alluvial fan deposition and initial volcanic vent formation; and 4) intrabasinal volcanism resulting in lava flow emplacement, pyroclastic eruptions, and fluvial volcaniclastic sedimentation. This study focuses on the complex interplay between volcanism and fluvial sedimentation that can be demonstrated during stages 3 and 4, and on the overall reorganization of depositional systems in this strike-slip basin during the transition from non-volcanic clastic sedimentation to lava flows and volcaniclastic sedimentation.Paleocurrent data collected from the Burwash basin document major reorganization of paleodrainage during the clastic to volcaniclastic transition. Prior to volcanism, fluvial systems flowed westward toward and along the basin axis, whereas after the onset of volcanism fluvial systems drained northeastward, outward from the basin center. In addition, conglomerate compositions in the basin change at the clastic to volcaniclastic transition from extrabasinally-derived basement clasts eroded from uplifted source areas along the strike-slip basin margin to intrabasinally-derived volcanic clasts.Fluvial depositional style was also strongly influenced by contemporaneous explosive volcanism during the late stage of Burwash basin evolution. Fluvial aggradation, which occurred in the form of sand-dominated hyperconcentrated flood-flows and braided streams, was contemporaneous with episodes of pyroclastic activity. These syneruption fluvial deposits are sheet-form and contain abundant juvenile detritus (vitric grains and pumice), euhedral to subhedral and angular crystal fragments of plagioclase and hornblende, and a small percentage of basaltic lithic fragments. Fluvial degradation, marked by the incision of steep valleys, occurred during periods between eruptions. The inter-eruption fluvial deposits are characterized by lenticular beds of clast-supported conglomerate and trough-crossbedded sandstone and are composed predominantly of basaltic lithic grains with small amounts of crystal and vitric fragments. Subsequent eruptions generated pyroclastic flows which ponded in the fluvial valleys. These pyroclastic-flow deposits exhibit crystal-rich bases, lithic-rich lapilli-sized middle zones with out-sized lithic blocks and gas-escape structures, upper pumice-rich zones, and interbedded stratified surge deposits.Our results show that the onset of volcanism in the Burwash basin produced a major reorganization of fluvial drainage systems and sediment source areas. In addition, changes in fluvial depositional style were closely linked to alternating periods of explosive volcanic activity and quiescence during the late stages of strike-slip basin evolution. The excellent exposures in the Burwash basin provide an apt opportunity to examine the role of volcanism in the depositional history of an intracontinental strike-slip basin.
The Iberian Pyrite Belt (South Portuguese Zone, Hercynian Chain), an important metallogenic province, has a volcanic-sedimentary origin. Overlying this volcanic succession is a thick sedimentary unit, the Culm Group, which represents postvolcanic Paleozoic sediments of the Iberian Pyrite Belt, It consists of three stratigraphic units: the Basal Shaly Formation (BSF), the Culm Facies Turbiditic Formation (CFTF), and the Shallow-platform Sandy Unit (SPSU). The BSF is represented by fine-grained sediments of volcanic to nonvolcanic origin that mark the end of volcanism in the region, the reworking of volcanic products in a shallow-marine basin, and the beginning of autochthonous sedimentation of pelagic clay. It constitutes a depositional sequence sensu Mitchum et al. (1977). The distribution, facies, and facies associations of the turbidites of the CFTF are related to the configuration of the basin, which controlled the mechanisms of deposition and distribution of the detritus from different source areas. The SPSU represents sediments eroded from the volcanic upland and redistributed over the shelf.Detailed study of these three units and their relationships provides the means to define the geometry and evolution of the postvolcanic Carboniferous basin of the Iberian Pyrite Belt. The basin was roughly subdivided by the Paymogo and the Puebla de Guzman paleoridges. Together with the allochthonous Ossa-Morena Zone, north of the study area, they formed three topographic barriers that delineated two interconnected subbasins.Sedimentation in the postvolcanic basin of the Iberian Pyrite Belt can be related to a model of oscillating sea level. Synorogenic characteristics of the Culm Group sediments support a tectonic origin for the sea-level oscillations, particularly since the sediments had already been affected by the first pulses of the Hercynian Orogeny.
ABSTRACT Flow structure and sediment movement over the beach step, commonly present at the foot of the beach face, were studied in a laboratory wave tank. Two sets of experiments were made: a preliminary run using 2-s waves and a colored mixture of fine, medium, and coarse sand to observe details of sediment movement over the step and the beach face, and a set of runs with periods of 1.5 s, 2.0 s, and 2.5 s and coarse sand, in which the flow field was visualized and measured using small, neutrally buoyant suspended particles. By the flow characteristics over the step, six distinct phases of the swash cycle were identified: (1) uprush of incident wave, (2) flow reversal, (3) backwash, (4) occurrence of critical flow, (5) development of a hydraulic jump, and (6) development of a backwash vortex. The time history of local flow velocity at the top of the step during a swash cycle has a saw-tooth shape, with maximum onshore and offshore speeds of the same magnitude. The velocity on the beach face was quite uniform through the water column during most of the swash cycle. The maximum Froude number over the step during the backwash was about 1.4. A backwash vortex formed after the onset of flow separation at the beach step during the final phase of the swash cycle. The backwash vortex traps suspended fine sediment at the step, and the flow up the step slope induced by the vortex impedes avalanching of coarse sediment down the slope, facilitating high-angle sedimentation. Once caught by the vortex, most of the coarse sediment is eventually deposited either on the step or on the lower part of the beach face, and only a small amount may be deposited farther up on the beach face. The dip of the seaward-dipping stratification in the step (25-32°) is somewhat higher than in the field (about 20°), probably because of the presence of irregular waves in the field together with tidal fluctuation in water level, two factors not included in the experiments.
Mudrock samples from a well penetrating Eocene and Paleocene strata (1235-4455 m) of the Texas Gulf Coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis, and the mechanism by which detrital smectitic illite/smectite (I/S) is transformed into diagenetic illitic I/S.Idealized smectite and illite end-member compositions for interstratified I/S are estimated to be [K0.00X+10.56Mg0.39Fe0.57Al1.13Si3.90O10(OH)2] and [K0.53X+10.18Mg0.17Fe0.16Al2.28 Si3.40O10(OH)2], respectively. There is no obvious difference in the amount of K addition to I/S in random- and ordered-interstratified I/S. Aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite-to-illite reaction is a complete dissolution-precipitation reaction rather than a solid-state [K+1 + Al+3] for Si+4 substitution reaction. Depth-related mineral trends and mass-balance calculations suggest that illitic I/S may form from kaolinite, and possibly illite or mica, as well as from smectitic I/S.Although most whole-rock element abundances are invariant over the sampled depth interval, K2O content increases from ca. 2.0 weight percent at 1500 m of burial to ca. 3.8 weight percent at 4000 m. Some of this increase is probably caused by import of K into the shale via fluids derived from interbedded sandstones, implying that Gulf Coast mudrocks behave as open chemical systems during burial diagenesis. If this is the case, a minimum of 10(3) pore volumes of fluid must have passed through the most K-enriched shales to have introduced the added potassium, provided that the K content of present-day formation water is representative of that of ancient formation water.
Fiber and dendrite calcite crystals, formed by abnormal growth conditions, are minor but important components of many limestones that have undergone vadose meteoric diagenesis. Fiber crystals, which have a length:width ratio of greater than 6:1, are divided into hexagonal fibers, rectangular fibers, composite fibers, and rhomb chains according to their crystal form. Dendrite crystals are formed of a main stem, primary branches, and secondary branches. Random and tangential fiber crystals refer to masses of fibers that have a random distribution or tangentially coat a substrate, respectively. Such crystals may be tightly or loosely packed.The shape of fiber and dendrite crystals can be substantially modified by destructive or constructive diagenetic processes. Destructive modification includes physical breakdown, dissolution, or micritization. Constructive modification, which usually involves epitaxial growth around the original fiber or dendrite crystal, may proceed to the point where the original crystals are completely disguised. In extreme cases fiber crystals can be cemented together to form lattices that look like dendrite crystals.Fiber and dendrite calcite crystals generally form from fluids that are supersaturated with respect to calcium carbonate. Although experimental data suggest that both types of crystal can form from the same parent fluid, there are only rare examples where both are present together in nature. Such crystals form through biogenically and abiogenically mediated processes. The common association of these crystals with plant roots or microorganisms suggests that the organisms can produce conditions suitable for growth of the crystals.
The Whirlpool Sandstone (basal Silurian of southern Ontario and New York) consists of two units: a lower fluvial unit, deposited by braided rivers on a paleoslope inclined to the NW, and an upper marine unit, consisting of interbedded sandstone and shale deposited offshore during a marine transgression. The transgressive surface that separates these two units has been mapped across the whole outcrop belt; at outcrop scale it is characterized by scours, several meters across and a few decimeters deep, cutting into the fluvial sandstone below. The scoured transgressive surface locally displays a thin, discontinuous shale-clast lag, marine burrows extending down into the fluvial sandstone, and extensive fields of symmetrical wave ripples. Wave ripples on this surface and higher within the marine unit show a regionally consistent NNW-SSE crest alignment, indicating the trend of the shoreline during transgression.Exceptional exposures of the transgressive surface, produced during quarrying operations near Georgetown, show that the transgressive scours include parabolic, linear, and irregular forms. Parabolic scours are several meters long and have a SW-pointing apex with arms opening towards the NE. Linear scours are cigar-shaped to channel-shaped and trend NE-SW: one channel-like scour deepens towards the NE. The scours clearly were formed by marine erosion (probably by storm-induced, offshore-directed currents) during transgression at a time when the fluvial sandstone was somewhat compacted but not cemented. They indicate flow offshore towards the NE, suggesting that the transgression proceeded from NE to SW. The change from a regional paleoslope to the NW, during deposition of the fluvial lower Whirlpool, to a transgression moving westwards across a shore trending NNW-SSE implies tectonic tilting of the basin towards the east, probably as a result of thrust emplacement at the continental margin.
Cumulative patterns of dolomitization in a subsurface Upper Devonian carbonate platform in the Peace River Arch area, Alberta, are related to progressive burial ( < 1 km), then fracture-controlled processes. The paragenetic sequence and dolomite types are defined using plain-light, cathodoluminescence, and fluorescence microscopy, with further definition provided by isotope (deltaO-18, deltaC-13, Sr-87/Sr-86) and trace-element (Mn, Fe) data. Onset of fracture-related dolomitization, between depths of 700 and 1000 m, is defined by depth profiles of vitrinite reflectance.Dolomitization began with near-surface growth of Ca-rich (54-56 mol % CaCO3), Fe-poor isolated rhombohedra. Their continued growth and coalescence coincided with stylolite formation. This progression was followed by formation of patchily distributed stoichiometric, Fe-poor (< 0.5% Fe) matrix dolomite that formed after onset of chemical compaction and prior to fracture-related dolomitization. A narrow range in deltaO-18 (-4.2 to -5.90 parts per thousand PDB; xBAR = -5.3 parts per thousand) values is defined for all these dolomites; they are more negative than expected for Frasnian marine dolomites. A broad scatter in their deltaC-13 (- 2 to + 4 parts per thousand PDB) suggests that they precipitated in zones of ongoing anaerobic organic diagenesis. Sr isotope values (xBAR = 0.7098) of matrix dolomite are radiogenic compared to Frasnian seawater. Progressive burial dolomitization was paralleled by changing Mg sources: (1) diffusion from seawater; (2) chemical compaction of limestone; and (3), as previously modeled for other Leduc platforms, platform-directed flow related to basin sediment compaction.Deep burial (> 1 km) fracturing focused basin-derived fluids along subvertical structural conduits and into adjacent permeable strata. In the Early Mississippian (< 1 km burial), mixing of intraplatform marine-derived waters and basin-derived brines caused dissolution and replacement of limestone and matrix dolomite by Fe-poor microcrystalline dolomite; additional void-fill dolomite followed. At greater depths (< 2 km), during the Mississippian through the Jurassic, microfractures formed conduits for Mg that was locally derived from chemical compaction of existing dolomites. This fracturing stage is defined by Fe-poor fracture-fill dolomite, as well as sparry and saddle dolomites that formed within interconnected secondary macroporosity. The final stage of dolomitization marks the first appearance of ferroan (> 3 wt % Fe) dolomite in the platform, heralding increased Fe mobilization during burial diagenesis. A negative shift in deltaO-18 values, -6.5 parts per thousand to -15 parts per thousand, is associated with successive fracture-related dolomites, and is interpreted to reflect an increase in fluid temperatures. Fracture-related flow was probably rapid; petrographic and isotopic signatures of fracture-related dolomitization can be traced 300 m upsection from the Leduc Formation.
Ten undisturbed samples of scarp-derived colluvium from the 1983 Borah Peak, Idaho, fault scarp were subjected to laboratory grain-size and fabric measurements to characterize typical colluvium from a Basin and Range fault scarp. Colluvial deposits consisted of poorly-sorted pebble gravels (mean diameter -0.3 to -3.75 phi; 1.2 to 13.5 mm) with 25-79% matrix (< 4 mm). Stereograms of long-axis orientations of 115 clasts from each colluvial wedge yielded girdle-type distributions, with typical eigenvalue ratios of In S1/S2 = 0.05-0.50 and In S2/S3 = 0.3-1.0. These girdle distributions indicate preferred orientation in angle-of-repose planes coincident with colluvial wedge depositional planes. Short-axis stereograms display single or dual maxima with mean bearing parallel to transport direction and plunge perpendicular to the colluvial wedge surface. Clast fabric strength is not strongly correlated with scarp aspect (corr. coeff. = -0.03 to -0.68) or with the slope of the faulted geomorphic surface (corr. coeff. = -0.16 to +0.57), but it improves with increasing matrix content (corr. coeff. = -0.04 to -0.42). Fabrics in measured wedges are weaker than those reported for other colluviums and talus and resemble fabrics in debris flows. However, the stronger girdle tendencies and distinct orientation subgroups in scarp-derived colluvium should distinguish it from suspected debris flow deposits in fault-zone exposures.
The seismic facies, facies architecture, and stratigraphy of tidal-inlet and tidal-delta deposits and their relation to the development of adjacent coastal lithosomes are examined using high-resolution seismic profiles, vibracores, and borehole descriptions. The Bolivar Roads tidal inlet/delta complex, along the east Texas Gulf coast, was formed approximately 3.3 ka by spit accretion across a baymouth following a rapid sea-level rise approximately 4 ka. An increase in the tidal prism through time and entrenchment of the tidal inlet over the Trinity River incised valley stabilized the inlet and intensified tidal processes on the tidal inlet/delta complex.The tidal-inlet facies show channel stacking and cut-and-fill structures. Stacked clinoforms dip westward across the inlet. The spit facies is characterized by oblique-tangential clinoforms that build outward and deepen from the edge toward the center of the valley. The flood-tidal delta facies has a base that shallows abruptly bayward. As the flood-tidal delta facies thins bayward, it interfingers with bay sediments. Near the inlet the flood-tidal delta shows channel cut-and-fill with an overall channel stacking pattern. On the seaward side the channels have a trough-like geometry. Bayward, the channels broaden and shallow. The channels show a prograded-fill pattern. The ebb-tidal delta facies exhibits gently inclined clinoforms prograding over a ravinement surface.Tidal-inlet deposits are composed of sand, shell, and mud interbeds. Sand and clay interlaminae are ubiquitous in the tidal deltas, and sand and shell beds are common near the inlet. Overall, the Bolivar Roads tidal inlet/delta complex is mud-dominated as a result of high influx of fine sediment into the bay.
