Showing papers in "Journal of Sedimentary Research in 1987"
TL;DR: In this article, the authors classified planar and nonplanar dolomite textures according to crystal size distribution and crystal boundary shape, which is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics.
Abstract: Dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape. The classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics, and crystal boundary shape is controlled by growth kinetics. Size distributions are classified as unimodal or polymodal. Crystal boundary shapes are classified as planar or nonplanar. If the evidence permits, a complete classification includes a description of recognizable allochems, matrix, and void filling. Allochems and preexisting cements may be unreplaced, partially replaced, replaced mimically, or replaced nonmimically. Allochems may be dissolved, leaving molds. Matrix can be unreplace, partially replaced, or replaced by a unimodal or polymodal size dolomite. Unimodal size distributions generally indicated a single nucleation event on a unimodal substrate. Polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate. Planar crystal boundaries develop when crystals undergo faceted growth, and nonplanar boundaries develop when crystals undergo nonfaceted growth. Nonplanar boundaries are characteristic of growth at elevated temperature (> 50°C) and/or high supersaturation. Both planar and nonplanar dolomite can form as a cement, replacement of CaCO3, or neomorphism of a precursor dolomite.
735 citations
TL;DR: Brines and salt were sampled at the Morton Bahamas solar salt production facility on Great Inagua Island in the Bahamas and analyzed by ion chromatography to define more precisely than heretofore the evaporation path of seawater to the end of the halite facies.
Abstract: Brines and salt were sampled at the Morton Bahamas solar salt production facility on Great Inagua Island in the Bahamas. The brines were analyzed by ion chromatography to define more precisely than heretofore the evaporation path of seawater to the end of the halite facies. At Inagua, calcium carbonate begins to precipitate at a brine concentration factor of 1.8 times that of seawater. Gypsum begins to precipitate at a brine concentration of 3.8 times seawater, and halite at a concentration factor of 10.6. Three of the most concentrated brines from Inagua (40 times seawater) were evaporated further in the laboratory. Magnesium sulfate first precipitated at brine concentrations about 70 times those of seawater, and potassium-bearing phases began to precipitate for these brines at concentrations greater than 90 times those of seawater. The distribution of coefficients of Br- and K+ between brines and halite were determined by combining analytical data for the Inagua brines with measurements of the Br- and K+ content of halites from Inagua and of halite which had precipitated from Inagua brines during storage. The observed average value of DBr- is 0.032, in good agreement with some of the previous measurements. The measured values of DK+ are highly variable (0.001 to 0.021); DK+ for halite precipitated early in the halite facies is in the vicinity of 0.015.
505 citations
TL;DR: In this article, it is suggested that, in general, contemporaneous dolomite will form at low temperatures only by direct precipitation, a mechanism that requires special conditions of highly super-saturated waters of high Mg/Ca ratio and elevated CO 3 -HCO 3 concentrations.
Abstract: The principal models in vogue today for dolomitization are the mixing zone and the sabkha models. Despite the wide acceptance of these models, there has been little critical assessment of their validity. Such an assessment is the objective of the present paper. A close look at the "Dorag" mixing-zone model (Badiozamani 1973) reveals several serious weaknesses: 1) the range of freshwater-seawater mixtures that meet the Dorag requirement for dolomitization shrinks to a small "window" if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based; 2) in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed; 3) in a number of dolomites interpreted to be of mixing-zone origin, dolomite has precipitated without dissolution of the calcite substrate, evidence that negates the fundamental premise of the Dorag model. In addition to these weaknesses, isotope and trace-element data used to identify mixed-water dolomite are inadmissible because 1) isotopic fractionation factors for dolomites remain unresolved; 2) isotope values (uncorrected for temperature) for a host of dolomites, interpreted to be of different origins, overlap; 3) nonisomorphous trace elements, such as Na, in dolomite cannot, on theoretical grounds, be relied on to identify dolomitizing fluids. Similar overall objections can be brought against Folk and Land9s (1975) "schizohaline" version of mixing-zone dolomitization. In summary, mixing-zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization. Contemporaneous dolomite formation in modern sabkhas is well documented, but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved. In the well-studied sabkha at Abu Dhabi, brine chemistry changes have been used as evidence of a replacement origin for the dolomite, but it is shown here that this evidence is far from unequivocal. An alternative origin of direct precipitation of dolomite is offered, an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes, and in line with our laboratory experience with dolomite synthesis. Moreover, it is suggested here that, in general, contemporaneous dolomite will form at low temperatures only by direct precipitation, a mechanism that requires special conditions of highly super-saturated waters of high Mg/Ca ratio and elevated CO 3 -HCO 3 concentrations. This may explain why modern dolomite is mainly restricted to evaporitic environments, a bias not shared by ancient dolomites. In contrast, replacement dolomite appears to require, at low temperatures, long reaction times > or = 10 4 yr?), a requirement that is mainly to be met in large, regional groundwater-flow systems, both marine and nonmarine. A third dolomitization model considered here is that of Baker and Kastner (1981), based on the experimental finding that sulfate ions inhibit or retard dolomitization. A number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations, 2 to 70 times that of seawater. Thus, the Baker-Kastner model should be held in abeyance until these serious contradictions are resolved. The current emphasis on mixing-zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem. Four of these avenues, each of which deemphasizes the "special water" approach, are briefly addressed and are as follows: 1) influence of temperature and time; 2) mass transfer processes; 3) burial diagenesis of epigenetic dolomites; 4) fluid-inclusion studies. At elevated temperatures the dolomite problem essentially disappears (ordered dolomite can be made in the laboratory in days at 100 degrees C). What is more, at temperatures above 60 degrees C, Ca-rich waters become dolomitizing fluids, which makes most natural subsurface waters capable of dolomitization. At low temperatures, time may be the key element, so that seawater will become a major dolomitizing fluid only where stable circulation systems, such as Kohout convention, can drive seawater through carbonate platforms for many thousands to millions of years. This paper has tried to show that currently favored models of dolomitization carry serious uncertainties, enough to warn us to look more critically at the validity of these models, and also, it is hoped, enough to spur new efforts to find new solutions to the problems of dolomitization.
499 citations
Journal Article•
TL;DR: The main models in vogue today for dolomitization are the mixing zone and the sabkha models Despite the wide acceptance of these models, there has been little critical assessment of their validity as mentioned in this paper.
Abstract: The principal models in vogue today for dolomitization are the mixing zone and the sabkha models Despite the wide acceptance of these models, there has been little critical assessment of their validity Such an assessment is the objective of the present paper A close look at the "Dorag" mixing-zone model (Badiozamani 1973) reveals several serious weaknesses: 1) the range of freshwater-seawater mixtures that meet the Dorag requirement for dolomitization shrinks to a small "window" if the geologically more realistic disordered dolomite is used in the calculations instead of the ordered dolomite on which the model is based; 2) in none of the known modern coastal mixing zones in limestone or lime sediments has replacement by dolomite been observed; 3) in a number of dolomites interpreted to be of mixing-zone origin, dolomite has precipitated without dissolution of the calcite substrate, evidence that negates the fundamental premise of the Dorag model In addition to these weaknesses, isotope and trace-element data used to identify mixed-water olomite are inadmissible because 1) isotopic fractionation factors for dolomites remain unresolved; 2) isotope values (uncorrected for temperature) for a host of dolomites, interpreted to be of different origins, overlap; 3) nonisomorphous trace elements, such as Na, in dolomite cannot, on theoretical grounds, be relied on to identify dolomitizing fluids Similar overall objections can be brought against Folk and Land's (1975) "schizohaline" version of mixing-zone dolomitization In summary, mixing-zone models have such weak underpinnings that they should be questioned as viable explanations for massive dolomitization Contemporaneous dolomite formation in modern sabkhas is well documented, but the important question of whether the mechanism of dolomite formation is replacement or direct precipitation remains to be resolved In the well-studied sabkha at Abu Dhabi, brine chemistry changes have been used as evidence of a replacement origin for the dolomite, but it is shown here that this evidence is far from unequivocal An alternative origin of direct precipitation of dolomite is offered, an origin in keeping with dolomite precipitation known from other modern sabkhas and saline lakes, and in line with our laboratory experience with dolomite synthesis Moreover, it is suggested here that, in general, contemporaneous dolomite will form at low temperatures only by direct precipitation, a mechanism tha requires special conditions of highly super-saturated waters of high Mg/Ca ratio and elevated CO3-HCO3 concentrations This may explain why modern dolomite is mainly restricted to evaporitic environments, a bias not shared by ancient dolomites In contrast, replacement dolomite appears to require, at low temperatures, long reaction times 104 yr?), a requirement that is mainly to be met in large, regional groundwater-flow systems, both marine and nonmarine A third dolomitization model considered here is that of Baker and Kastner (1981), based on the experimental finding that sulfate ions inhibit or retard dolomitization A number of modern sedimentary dolomites are not in accord with this model in that they are forming from brines with large sulfate concentrations, 2 to 70 times that of seawater Thus, the Baker-Kastner model should be held in abeyance until these serious contradictions are resolved The current emphasis on mixing-zone and sabkha dolomitization has diverted attention from other promising avenues of approach to the dolomite problem Four of these avenues, each of which deemphasizes the "special water" approach, are briefly addressed and are as follows: 1) influence of temperature and time; 2) mass transfer processes; 3) burial diagenesis of epigenetic dolomites; 4) fluid-inclusion studies At elevated temperatures the dolomite problem essentially disappears (ordered dolomite can be made in the laboratory in days at 100°C) What is more, at temperatures above 60°C, Ca-rich waters become dolomitizing fluids, which makes most natural subsurface waters capable of dolomitization At low temperatures, time may be the key element, so that seawater will become a ma or dolomitizing fluid only where stable circulation systems, such as Kohout convention, can drive seawater through carbonate platforms for many thousands to millions of years This paper has tried to show that currently favored models of dolomitization carry serious uncertainties, enough to warn us to look more critically at the validity of these models, and also, it is hoped, enough to spur new efforts to find new solutions to the problems of dolomitization
426 citations
TL;DR: The lower Eocene Willwood Formation of the Bighorn basin, northwest Wyoming, consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from Eocene pedogenesis.
Abstract: The lower Eocene Willwood Formation of the Bighorn Basin, northwest Wyoming, consists of about 770 m of alluvial rocks that exhibit extensive mechanical and geochemical modifications resulting from Eocene pedogenesis. Willwood paleosols vary considerably in their relative degrees of maturity; maturity is defined as stage of development as a function of the amount of time required to form. Five arbitrary stages are proposed to distinguish these soils of different maturities in the Willwood Formation. Stage 1 soils, the least mature, are entisols; stage 2 and stage 3 soils are intermediate in maturity and are probably alfisols; and stage 4 and stage 5 soils, the most mature, are spodosols. These stages are not only time-progressive elements of an in situ maturation sequence for W llwood soil formation, but, in the lateral dimension, they are also usually distributed sequentially. Study of Willwood paleosols indicates that an inverse relationship exists between soil maturity and short-term sediment accumulation rate. The least mature Willwood paleosols formed in areas of relatively high net rates of sediment accumulation on 1) channel, levee, and crevasse-splay sediments of the proximal alluvial ridge, and 2) deposits filling large and small paleovalleys formed by major episodes of gullying (lowered baselevels). In contrast, the fine-grained sediments of the distal floodplain, where net sediment accumulation rates were relatively low, experienced development of much more mature soils. Soils of intermediate maturities occur in the order of their stage on intervening proximal floodplain and distal alluvial ridge sediments. Adjacent bodies of sedimentary rock that differ in their ancient soil properties because of distance from areas of relatively high sediment accumulation are denoted by the new term pedofacies. The remarkable sequence of paleosols in the Willwood Formation clearly illustrates several important principles of soil-sediment interrelationships in aggrading alluvial systems that have broad application to other deposits. This is especially true in view of the widespread distribution of paleosols in nearly all ancient fluvial rocks. Further study of Willwood paleosols will not only enable precise lateral correlation of coeval alluvial sediments, and thereby fluvial sedimentary events, from the distal to the proximal realms of the floodplain but will also contribute to increasingly informative evaluations of the nature, tempo, and mode of alluvial succession.
303 citations
TL;DR: In this article, a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids.
Abstract: Some pockmarks on the Norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms. Detailed examination of a sandstone sample from a pockmark in Norwegian Block 25/7 in the North Sea has revealed the presence of Mg calcite and aragonite cements, some of the latter forming botryoids. All cement types are characterized by extremely light carbon isotopic compositions, with a mean 13C value of -56.1 PDB, which shows that the cements contain carbonate produced by oxidation of biogenic methane. Oxidation occurred in both the oxic and anoxic diagenetic zones. Trace amounts of interstitial methane (mean 13C = -40.8%) and higher hydrocarbon gases (up to C5) with a C1/Cn ratio of 0.855-0.874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas.
285 citations
TL;DR: The selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain-sorting processes that lead to the formation of placers in sands and to armored beds in gravels as discussed by the authors.
Abstract: The selective entrainment of grains by a current from a bed of mixed sizes and densities is important to grain-sorting processes that lead to the formation of placers in sands and to armored beds in gravels. Existing field and laboratory measurements of selective entrainment depart systematically from the standard threshold curves (such as that of Shields) which are based on experiments with nearly uniform grain sizes. Entrainment measurements from beds of mixed sizes form trends which obliquely cross the threshold curves for uniform grains, the crossing point being roughly at the median diameter of the size distribution. Due to this crossover, the coarser size fractions of the distribution require lower-flow stresses for their entrainment than if they formed uniform beds, while the finer-size fractions require higher stresses than uniform beds. With deposits having medians in the range of medium sand through gravel, the larger the individual particle within the mixed sizes, the greater the flow stress required for its entrainment. In this case the smaller grains are preferentially entrained, possibly leading to bed armoring. In contrast, if the median is in the fine-to-medium sand range, the smallest grains in the bed of mixed sizes are the most difficult to entrain due to sheltering by the larger grains; where grain density is also involved, this sorting can lead to the formation of placers. A variety of empirical relationships is considered to quantify the evaluation of selective entrainment from deposits of mixed sizes, but no single formulation is satisfactory for all data sets.
175 citations
TL;DR: The age and fabric of 347 Phanerozoic marine oolites have been compiled in order to evaluate secular variations in the original mineralogy of these physicochemically precipitated sedimentary carbonates.
Abstract: The age and fabric of 347 Phanerozoic marine oolites have been compiled in order to evaluate secular variations in the original mineralogy of these physicochemically precipitated sedimentary carbonates. Variation in the abundance of oolite correlates with positions of global sea level; ooids were common during transgressions and regressions but were rare during lowstands and highstands. The rarity of oolite during continental emergence (early Cambrian, Permo-Triassic, late Tertiary-Quaternary) probably relates to a diminished areal extent of shallow-carbonate environments. Causes for a similar paucity during continental submergence (Silurian-Devonian, late Cretaceous) are more problematic; higher atmospheric CO2 and lower hydrospheric CO3=concentration during highstands may have inhibited a biotic carbonate precipitation. Ooids coeval with highstands display a preponderance of fabrics evincing a primary calcite mineralogy; ooids formed during lowstands are more diverse but record dominance of aragonite. Variations in cortical mineralogy relate more closely to flooded continental areas than to sea-level elevations, supporting the suggestion of MacKenzie and Pigott ( 1981 ) that variations in hydrospheric carbonate concentrations were more important in determining Phanerozoic ooid mineralogy than were magnesium-calcium concentrations. These data on oolite abundance and mineralogy demonstrate that abiotic Phanerozoic marine carbonates have been responsive to secular changes in atmospheric-hydrospheric chemistry and global eustasy, and suggest that these changes are directly related to past rates of submarine weathering and sea-floor hydrothermal activity.
174 citations
TL;DR: The Roaring River alluvial fan formed on 15 July 1982, in Rocky Mountain National Park, Colorado, by a catastrophic flood that was generated by a dam failure.
Abstract: The Roaring River alluvial fan formed on 15 July 1982, in Rocky Mountain National Park, Colorado, by a catastrophic flood that was generated by a dam failure. The fan covers an area of 0.25 square km, has a radial length of 0.7 km, and is up to 14 m thick. Sedimentation occurred in three phases, each producing a distinct fan lobe. Initial sedimentation was by a noncohesive sediment-gravity flow which deposited two levees on the proximal boundaries of Lobe I. The levees consist of a poorly sorted mixture of logs, sand, pebbles, cobbles, and boulders. The first two lobes were built primarily by sheetflooding, which deposited imbricated boulders in trains behind obstacles that formed as jams between boulders or logs and upright trees. Horizontally laminated granule and sand sedimentation took place down-fan from the boulders due to deceleration of the expanding sheetflood. Thin-to-medium interbedded sand and cobble-pebble gravel couplets were deposited by sheetflooding on the third lobe. Gravel was transported as bedload by supercritical flow and deposited locally where antidunes broke. Sand was transported as suspended load and deposited where flood velocity locally decreased due to destruction of antidunes, increased roughness, or flow separation around the low-amplitude gravel bed forms. The flood rechannelized at the distal end of Lobe III due to constriction between the fan and the valley margin. Deposits in the upper rechannelized reaches consist of crudely bedded cobble and pebble gravel, and interstratified pebble gravel and backset-bedded sand. Deposition was by supercritical flow. In the lower reaches, planar-cross-bedded, sandy pebble gravel and climbing ripple, horizontal, trough-cross-bedded, and backset-bedded sand were deposited by supercritical and subcritical flow. The flood deposit was modified during waning flood stage and during the three years following the flood by noncatastrophic discharge events. These events formed braided distributary channels by erosion into the top of the sheet flood deposits. Fan building took place mostly by catastrophic unconfined discharge, whereas much of the present fan surface consists of braided channels that formed by erosion into the sheetflood deposits by noncatastrophic discharge.
167 citations
TL;DR: The Todos Santos Formation of central Chiapas, Mexico was deposited in half-graben basins formed during the early rifting stage of the southern Gulf of Mexico Basin this article.
Abstract: The nonmarine Upper Jurassic-lowermost Cretaceous Todos Santos Formation of central Chiapas, Mexico, was deposited in half-graben basins formed during the early rifting stage of the southern Gulf of Mexico Basin. This formation consists of alluvial-fan, fluvial, and lacustrine sequences that were deposited under arid climatic conditions. Alluvial fans were built outwardly from the elongate basin margins by runoff that flowed transverse to the basin axis, whereas fluvial units were deposited by rivers that flowed parallel to the basin axis. Lacustrine deposition took place in a topographic depression adjacent to the basin margin. The presence of alluvial-fan and fluvial or lacustrine deposits in vertically stacked, cyclic megasequences hundreds of meters thick indicates that the basin opography changed through time. This cyclicity denotes that periodically fluvial or lacustrine environments occupied the basin-margin position and at other times alluvial-fan sedimentation occurred there. Cyclicity was caused by periodic changes from high to low rates of basin subsidene. The response of these three depositional environments to basin subsidence differed due to the unique hydrologic controls on sediment transport and deposition in each environment. Lakes were maintained by springs that emanated from the fractured fault margin and by fluvial discharge. Fluvial discharge and deposition resulted from precipitation or annual snowmelt anywhere in the river's expansive drainage basin, whereas alluvial-fan sedimentation occurred only when there was infrequent, significant precipi ation in the small drainage basin from which fan sediment was derived. As a result of these hydrologic controls, the lacustrine or fluvial environments responded more quickly to periods of active tectonic subsidence and migrated over the fans to occupy the basin-margin depression. Aided by a decrease in basin subsidence, alluvial fans eventually prograded and displaced the fluvial or lacustrine environments away from the basin margin.
146 citations
TL;DR: In this paper, a reevaluation of data in support of this assumption indicates that, although relative dolomite abundances do change significantly throughout Phanerozoic carbonate sequences, no data set demonstrates any direct correlation between dolomerite content and age, whereas tabulation of available data does suggest greater amounts of dolombite formation during periods of continental flooding.
Abstract: Increasing dolomite abundance with age (relative to limestone) has long been accepted as one of the general truisms of sedimentary geology, a relationship which suggests that dolomitization is a cumulative process occurring slowly over the full burial history of a carbonate sequence. However, reevaluation of data in support of this assumption indicates that, although relative dolomite abundances do change significantly throughout Phanerozoic carbonate sequences, no data set demonstrates any direct correlation between dolomite content and age, whereas tabulation of available data does suggest greater amounts of dolomite formation during periods of continental flooding. Global mass balance calculations of potential magnesium sources also indicate that most dolomite has formed from marine or marine-derived fluids at the Earth9s surface. This conclusion is indirectly supported by significant short-term (period scale) changes in Phanerozoic dolomite abundance, a variation which would be more attenuated if dolomitization were dominantly a burial process. Dolomitization may be enhanced during times of global transgression, higher atmospheric pCO 2 , and lower calcite saturation state in shallow-marine settings.
TL;DR: In this paper, the lower Eocene Willwood Formation was subdivided into different pedofacies or developmental stages, over which a grosser scale cyclicity is superposed, and the pedogenic maturity of the multistorey paleosols progressively increases and then decreases upwards in response to episodic avulsion.
Abstract: Extrachannel sediments of the lower Eocene Willwood Formation, which were subdivided into different pedofacies or developmental stages, exhibit two well-developed orders of pedofacies sequences, over which a grosser scale cyclicity is superposed. Simple sequences, with thicknesses measured in meters, are comprised of one or more soil profiles that are bounded above and below by pedogenically unmodified crevasse-splay deposits. On a larger scale, compound pedofacies sequences are tens of meters thick and consist of several simple sequences sandwiched between channel sandstones. Between these sandbodies, pedogenic maturity of the multistorey paleosols progressively increases and then decreases upwards in response to episodic avulsion. Formation of both compound and simple pedofacies seq ences was largely controlled by local patterns of deposition and erosion that produced vertical variability in both rate of sediment accumulation and parent material. Because paleosol maturity also reflects the effects of extrabasinal factors, a pedofacies megasequence, which is hundreds of meters thick, developed in response to changes in tectonic activity through Willwood time. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin also reveals that areally differing sediment accumulation rates and thereby basin subsidence rates can be interpreted from large-scale pedofacies sequences. Consequently, paleosols are useful in determining changes in relative rates of sediment accumulation that are controlled by both intrabasinal and extrabasinal factors.
TL;DR: The Deschutes Formation of central Oregon is a sequence of interstratified volcanic and sedimentary rocks, as much as 700 m thick, largely derived from the adjacent Cascade Range as discussed by the authors.
Abstract: The upper Miocene to lower Pliocene Deschutes Formation of central Oregon is a sequence of interstratified volcanic and sedimentary rocks, as much as 700 m thick, largely derived from the adjacent Cascade Range. The formation is correlative with the early High Cascade eruptive episode that included voluminous pyroclastic eruptions and culminated in the development of an intra-arc graben in the central Oregon High Cascades. Facies associations and paleocurrent data define three depositional settings. 1) The eastward-sloping arc-adjacent alluvial plain was the depositional site by sheetfloods, debris flows, and hyperconcentrated flood flows. Their deposits from 10- to 70-m-thick sequences separated by erosion surfaces with up to 60 m of relief. Widespread superimposed paleosols overlie these facies and constitute the upper 10 to 50 m of the section. Sedimentation was episodic with pulses of rapid aggradation, corresponding to periods of explosive volcanism and abundant sediment supply, separated by periods of nondeposition when streams incised to reestablish grade as sediment loads diminished. 2) A gravel-bedload river flowed northward through the basin at the foot of the alluvial plain and is represented by channel-fill conglomerates and overbank sandstones and siltstones. 3) A thin sequence derived from older Tertiary highlands was deposited on the east side of the basin and was little affected by Cascade volcanism. Extensive development of thick, superimposed paleosols reflects the inactive nature of the eastern basin margin as a sediment source. Deschutes Formation deposition records the response of a fluvial system to large, volcanism-induced sediment loads. Popular facies models do not adequately allow for the influence of volcanism on fluvial systems because sediment is mobilized episodically and on a scale unparalleled in nonvolcanic settings.
Journal Article•
TL;DR: In this article, a reevaluation of data in support of this assumption indicates that, although relative dolomite abundances do change significantly throughout Phanerozoic carbonate sequences, no data set demonstrates any direct correlation between dolomerite content and age, whereas tabulation of available data does suggest greater amounts of dolombite formation during periods of continental flooding.
Abstract: Increasing dolomite abundance with age (relative to limestone) has long been accepted as one of the general truisms of sedimentary geology, a relationship which suggests that dolomitization is a cumulative process occurring slowly over the full burial history of a carbonate sequence. However, reevaluation of data in support of this assumption indicates that, although relative dolomite abundances do change significantly throughout Phanerozoic carbonate sequences, no data set demonstrates any direct correlation between dolomite content and age, whereas tabulation of available data does suggest greater amounts of dolomite formation during periods of continental flooding. Global mass balance calculations of potential magnesium sources also indicate that most dolomite has formed from marine or marine-derived fluids at the Earth's surface. This conclusion is indirectly supported by significant short-term (period scale) changes in Phanerozoic dolomite abundance, a variation which would be more attenuated if dolomitization were dominantly a burial process. Dolomitization may be enhanced during times of global transgression, higher atmospheric pCO2, and lower calcite saturation state in shallow-marine settings.
TL;DR: Sugarloaf Key is an area where Holocene dolomitization is occurring as discussed by the authors, and calcium-rich Dolomite is found in a 0.25- to 10-cm-thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted Pleistocene Miami oolite.
Abstract: Sugarloaf Key is an area where Holocene dolomitization is occurring Calcium-rich dolomite is found in a 0.25- to 10-cm-thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted Pleistocene Miami oolite. Radiocarbon ages of the crust range from 160 years B.P. to 1420 years B.P., with a corresponding increase in dolomite content from 0% to 80%. The relatively high permeability of the underlying Pleistocene oolite and low permeability of the Holocene carbonate mud results in a tidal lag between surface water and the partly confined aquifer. Consequently, seawater is pumped upward and downward through the Holocene sediment during spring tides. The highest concentration of dolomite is found in areas where the sediment layer is thinnest and tidal pumping is most effective. Limited analyses of surface and subsurface water taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially Florida Bay water. The earliest diagenesis is caused by precipitation of dolomite cement, which occurs as 0.1- to 0.3-micron, subrounded crystallites that show no distinct crystal form. During further cementation, and somewhat later, during replacement of preexisting crystallites, poorly ordered dolomite forms as 1- to 5-micron euhedral rhombs. X-ray diffraction data indicate that the crystallized dolomite is better ordered and less calcium-rich than the dolomite composed of crystallites.
TL;DR: The Hawkesbury sandstone is a Triassic sheet sandstone, extensively exposed in the Sydney Basin, New South Wales, particularly along the coast near Sydney as discussed by the authors, which is attributed primarily to floodplain deposition, but abandoned channel fills are also present.
Abstract: The Hawkesbury Sandstone is a Triassic sheet sandstone, extensively exposed in the Sydney Basin, New South Wales, particularly along the coast near Sydney. Unidirectional paleoflow in the sandstone, its freshwater biota, and abundant mudrock intraclasts indicate fluvial deposition. Sheet morphology, low paleocurrent variance, abundant erosion surfaces, and the paucity of in situ mudrocks point to a braided fluvial system. Three facies assemblages have been recognized: stratified sandstone, massive sandstone, and a minor mudrock assemblage. The stratified sandstone assemblage is dominated by stacked sets of planar cross-strata and minor trough cosets in sequences 6-23 m thick, bounded by erosion surfaces. Significant paleocurrent changes between channel sequences indicate that they were initiated by the avulsion of major channel systems. In some cases the channel sequences fine upward, with planar cross-stratal sets overlain by trough sets that decrease in magnitude upwards, fining up into mudrocks. The massive sandstone assemblage occurs principally as massive sandstone in elongate erosional features oriented transverse to paleoflow. The massive sandstone commonly contains large mudrock intraclasts and is attributed to failure of high channel banks and/or large bedforms during falling-water stages. The mudrock assemblage comprises rippled and horizontally laminated fine sandstone, siltstone, and shale, with minor mudstone. It is attributed primarily to floodplain deposition, but abandoned channel fills are also present. The Hawkesbury Sandstone does not conform to existing models for braided-fluvial deposition in that planar cross-strata accumulated in deeper parts of channels, whereas trough cross-strata formed in shallower water. The large scale of planar cross-strata (sets up to 7.5 m thick), mudrock intraclasts (up to 38 m long), bank-collapse scars (up to 11 m deep) and abandoned channel fills (up to 18 m deep) show that the Hawkesbury River was very large, with deep main channels and high but variable discharge.
TL;DR: The paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the Paris Basin and the Central Massif of France as mentioned in this paper.
Abstract: The paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the Paris Basin and the Central Massif of France The initial silica deposits are most frequently opal with varying degrees of structural organization Subsequently, recrystallization produced varieties of progressively better-organized quartz; namely, fibrous quartz (chalcedony and lutecite), which developed from deposits of microlaminated opal, and microcrystalline quartz, which formed from nodular opal However, even during the formation of microcrystalline quartz facies, dissolution and recrystallization processes continued, resulting in progressively bigger quartz crystals In the final stages of silcrete development, hyaline quartz crystals grew in voids These mineral sequences are determined by the solubility of each of the component parts; solubility is controlled by the "purity" and the degree of order of the crystalline structure The most disordered minerals are the most soluble The degree of order is controlled, in part, by foreign ions incorporated into the lattice, but also by the degree of supersaturation of silica in solution in the system which, in turn, controls the rate of formation of nuclei Dissolution and recrystallization processes are dependent on the percolation of dilute solutions through the silcretes The concentrations of silica and other cations in these solutions is a function of the solubility of the silica phases in the host material and of the rate of percolation Thus, the porosity of the host material is important Rapid percolation favors dissolution, but in zones with slow percolation, equilibrium is attained and no transformation of existing silica phases occurs It is in the zones of intermediate porosity that progressive recrystallization of quartz occurs The effect of rate of percolation on dissolution and recrystallization explains the vertical distribution of different silica polymorphs observed in numerous silcrete duricrusts
TL;DR: A large planimetric exposure of an exhumed, high-sinuosity channel sandstone in the Permian Beaufort Group of the southwestern Karoo, South Africa, reveals the original geometry and eroded accretion topography of four consecutive point bars as discussed by the authors.
Abstract: A large planimetric exposure of an exhumed, high-sinuosity channel sandstone in the Permian Beaufort Group of the southwestern Karoo, South Africa, reveals the original geometry and eroded accretion topography of four consecutive point bars. An abandoned channel fill separates the crescentic point-bar sandstone bodies, clearly defining the paleomeander loops. The point-bar coarse member is made up of trough-cross-bedded and horizontally laminated, fine- to medium-grained sandstone with patchily developed basal mudrock conglomerate. Low-angle (< 10°) discontinuities divide the single-storied sandstone into epsilon units that are expressed on the eroded upper surface as irregular "accretion ridges." Remnant channel-fill deposits comprise two units, a lower "active" fill similar to the basal point-bar sequence, overlain by a "passive" fill of numerous thin beds of bioturbated, ripple-cross-laminated, fine-grained sandstone separated by maroon mudrock veneers. A tabular crevasse-splay sandstone sheet emanating from the apex of meander 2 is composed of horizontally laminated, fine-grained sandstone. The irregularity of lateral accretion units and the continuity of discontinuity surfaces to the base of the point-bar coarse member is indicative of highly fluctuating, possibly ephemeral discharge. Measurements of meander wavelength (2,400-4,000 m), being one of the least altered paleomorphological features, are used to calculate average bankfull width (approx. 277 m), depth (approx. 11 m), mean annual discharge (approx. 850 m3/s) and mean annual flood (approx. 12,000 m3/s). Paleocurrent data and the pattern of accretion ridges are considered in the reconstruction of migration behavior. The meanders expanded and increased in curvature while migrating at a rate of approximately 0.85 m/year over a period of some 1,500 years before an upstream avulsion caused their abandonment.
TL;DR: TEM/AEM study of a sequence of shales from the Salton Sea Geothermal Field revealed that the mechanism for the reaction of smectite to form illite is markedly different from that for Gulf Coast sediments as discussed by the authors.
Abstract: TEM/AEM study of a sequence of shales from the Salton Sea Geothermal Field revealed that the mechanism for the reaction of smectite to form illite is markedly different from that for Gulf Coast sediments. Illite in Salton Sea shales occurs principally as individual, euhedral-to-subhedral, pseudohexagonal crystals in open pore space. Smectite was not observed by TEM in proximity to such illite and is shown by XRD data to decrease in amount with depth as the proportion of illite increases. Illite is thus implied to be derived by complete dissolution of smectite (and other phases?), transport of components, and crystallization of illite from solution. However, in shallow sediments detrital smectite is also observed to be in part directly replaced by illite in a manner duplicating Gulf Coast textures. The mechanism by which smectite undergoes complete dissolution to allow precipitation of illite at a distant site is compatible with an open system such as the Salton Sea sediments, characterized by high water/rock ratio and relatively high permeability. In contrast, the mechanism involving a reaction front where the smectite is directly replaced by illite, such as occurred in Gulf Coast shales, is compatible with relatively impermeable shales.
TL;DR: The geochemistry of detrital garnets in the Upper Paleocene Forties Formation has been studied by electron microprobe techniques as mentioned in this paper, and two separate sand fans can be mapped using the technique, the dominant Forties-Montrose Fan being supplied from the metamorphic terrain of the Northern Highlands of Scotland and the subsidiary Gannet Fan supplied from mainly granitic terrain of Grampian Highlands.
Abstract: The geochemistry of detrital garnets in the Upper Paleocene Forties Formation has been studied by electron microprobe techniques. In this sand body, variations in garnet geochemistry can be related to provenance and diagenetic factors. Two separate sand fans can be mapped using the technique, the dominant Forties-Montrose Fan being supplied from the metamorphic terrain of the Northern Highlands of Scotland and the subsidiary Gannet Fan supplied from the mainly granitic terrain of the Grampian Highlands. Intrastratal solution, caused by high-temperature pore fluids circulating at depth, has removed some of the garnets from the assemblages. Garnets with the highest calcium contents are the least stable and are dissolved at comparatively shallow depths. The dependence of garnet stability on calcium content in deep burial must be taken into account when assessing provenance by this technique, but nevertheless garnet is far less sensitive to diagenetic and hydraulic controls than is the total heavy mineral suite.
TL;DR: In this paper, core samples of shallow subtidal and peritidal carbonates in the Ellenburger Group (Lower Ordovician) were examined to investigate the effects of deep burial on diagenesis.
Abstract: From western Texas and southeastern New Mexico, core samples of shallow subtidal and peritidal carbonates in the Ellenburger Group (Lower Ordovician) were examined to investigate the effects of deep burial on diagenesis. At present, the burial depth of samples ranges from 6,000 ft. (1.5 km) to 23,000 ft. (7.0 km). Below 10,000 ft. (3.0 km) carbonates are exclusively dolostones, showing evidence of deep-burial dolomitization such as the presence of coarse crystalline dolomite, xenotopic texture, homogeneous cathodoluminescence, high fluid-inclusion homogenization temperatures, and light oxygen-isotope compositions. Locally, neomorphism of older, fine crystalline dolomite occurs; however, the preservation of depositional textures in coarse crystalline, xenotopic dolomite indicates a nonobliterative replacement origin in deep-burial environments. Millimeter-size crystals of euhedral saddle dolomite commonly fill vugs and fractures. The Mg+2 ions in the nonferrous dolomites are probably derived from basinal brines via high porosities and fractures. Higher temperatures and longer reaction times coupled with the addition of Mg+2 ions from circulating waters are apparently responsible for deep-burial dolomitization.
TL;DR: In this article, the color characteristics of mixtures of synthetic hematites and kaolinite with the color of red beds from different geologic ages and locations were studied and compared, and the presence of clusters of hematite crystals having an optical behavior different from that of individual particles could be responsible for hue differences between red bed samples.
Abstract: We have studied and compared the color characteristics of mixtures of synthetic hematites and kaolinite with the color of red beds from different geologic ages and locations. The color of synthetic mixtures depends on the particle size of hematites; increasing size results in more purplish hues. In contrast, the color of the red beds is not related to the size of the hematites. We hypothesize that the presence of clusters of hematite crystals having an optical behavior different from that of individual particles could be responsible for hue differences between red bed samples. This is supported by the fact that, after prolonged grinding, implying, presumably, cluster destruction, the hue of all red beds becomes yellowish red, even when the hue of the unground red beds ranges from purp ish red to yellowish red.
TL;DR: Isotopic and trace-element analyses of dolomites from the Miocene Monterey Formation of California show that there is a strong correlation between the 13C values and the iron, manganese, and strontium contents as mentioned in this paper.
Abstract: Isotopic and trace-element analyses of dolomites from the Miocene Monterey Formation of California show that there is a strong correlation between the 13C values and the iron, manganese, and strontium contents of dolomites from different localities. Dolomites with negative 13C values have low trace-element contents, averaging less than 1 mole % iron, 300 to 400 ppm manganese, and 200 to 250 ppm strontium. Those with positive 13C values have much higher trace-element contents, averaging greater than 2 mole % iron, with values as high as 10 mole %, 1,200-1,400 ppm manganese, and 600 to 800 pm strontium. Monterey sections with dolomites with low trace-element contents contain higher percentages of dolomite and have lower sedimentation rates and lower detrital mineral contents than sections with dolomites with high trace-element contents. Differences in iron and manganese contents of dolomites from different sections are probably attributable to variation in the amount of readily available iron and possibly manganese oxide coatings on detrital minerals. Whether a dolomite forms in or below the zone of organic-matter oxidation by microbial sulfate reduction also may affect the availability of iron and manganese. In the zone of sulfate reduction, reduced iron, and possibly manganese, may be precipitated as sulfide minerals rather than be incorporated into dolomite. Differences in the strontium contents of Monterey dolomites are probably the result of different reaction stoichiometries of the dolomitization process. Calculations of the maximum depth of dolomite formation based on a diffusion-limited seawater source of Mg2+ for dolomitization show that for sections with high percentages of dolomite (10-20%), dolomitization must take place within the uppermost few meters of the sediment column.
TL;DR: In this article, the Caicos ooids are formed in the beach and shoreface environment in response to wave and current agitation generated by prevailing southeasterly trade winds.
Abstract: Modern ooids are forming and accumulating as beach-dune complexes which face into and prograde across the very shallow, broad, low-energy Caicos Bank. This contrasts with other modern oolite occurrences that are located at shelf margins or at the heads of steep ramps facing deeper water where tidal energy plays an important role in ooid generation. Ooid quality and distribution coupled with radiocarbon dating indicates that the Caicos ooids are forming in the beach and shoreface environment in response to wave and current agitation generated by prevailing southeasterly trade winds. The Caicos model is a useful analog for many ancient sheetlike oolite accumulations deposited on broad, epeiric shelves or gently sloping ramps.
TL;DR: In this article, it was concluded that the latter interpretation is correct because published isotopic analyses of metamorphic rocks consider only the coarser-grained rocks, whose quartz is nearly all of silt and clay size and has more positive delta 18 O values than quartz from schists and gneisses, probably supplies the bulk of quartz to mudrocks.
Abstract: Published data indicate that in igneous rocks the delta 18 O of quartz averages about +9 per thousand ; in metamorphic rocks, +13 to +14 per thousand ; in sandstones, +11 per thousand ; in shales, +19 per thousand ; quartz overgrowths in sandstones, +20 per thousand ; and cherts, +28 per thousand . Either a very large proportion of the silt and clay-size quartz in shales (about 90% of the total quartz in shales) is secondary, or the isotopic data are incomplete. It is concluded that the latter interpretation is correct because published isotopic analyses of metamorphic rocks consider only the coarser-grained rocks. Phyllites and slates, whose quartz is nearly all of silt and clay size and has more positive delta 18 O values than quartz from schists and gneisses, probably supplies the bulk of quartz to mudrocks.
TL;DR: The detrital composition of siliciclastic rocks from six localities in the Taconic and Acadian sections of New York and Quebec can be described as mixtures of two or more differentiable sedimentary components as mentioned in this paper.
Abstract: The detrital composition of siliciclastic rocks from six localities in the Taconic and Acadian sections of New York and Quebec can be described as mixtures of two or more differentiable sedimentary components. A sedimentary component is defined as a mineral or group of minerals that sort independently of other sedimentary constituents. Because of grain morphology, phyllosilicates usually dominate the finer-grained component, and quartz or mixtures of quartz and feldspar dominate the coarser-grained components. The chemical composition of a sedimentary component is distinctive, and the differentiation of sedimentary components usually results in the concomitant separation of associated elements. Typical separations include the enrichment of K2O and Al2O3 in the finer-grained, phyllosilicate-rich fraction and the enrichment of SiO2 and Na2O in the coarser-grained, tectosilicate-rich fraction. Fe2O3 and MgO are also enriched in the fine fraction, if they are abundant constituents in the phyllosilicates. The chemical composition of the graywackes from North America was compared to the chemical composition of unlithified turbidite sediments from the Black Sea. The chemical variation trends found in the deeply buried sedimentary rocks are similar to the trends found in the suite of slightly buried and unlithified deep-sea sediments. This indicates that burial alteration does not necessarily result in large-scale redistribution of the constituent elements. It is possible, therefore, to use the chemical variations observed in a suite of deeply buried and mineralogically altered sedimentary rock to infer the original clastic assemblage and to help clarify the processes that produced the sediment.
TL;DR: In this article, the authors studied upper-flow-regime deposition in Medano Creek, a straight single-channel stream where flow is largely confined to a sequence of upstream-migrating topographic steps (pools and riffles), a stable channel geometry.
Abstract: Medano Creek in Colorado provides a unique opportunity to study upper-flow-regime deposition. It is a steep, aggrading fluvial system in which antidunes are the dominant bedform. Medano Creek is a straight, single-channel stream where flow is largely confined to a sequence of upstream-migrating topographic steps (pools and riffles), a stable channel geometry. Antidunes and upperflow-regime plane beds are arranged in an organized pattern on the surface of pool and riffle sequences as a response to spatially varied flow conditions. In the creek, antidunes form sedimentary structures consisting of low-angle backset and foreset beds and subhorizontal laminae grouped into lenses. Laminae in a lens are often subparallel to the lens-bounding surfaces on the downcurrent end and are truncated y the upper bounding surface on the upcurrent end. Upper-flow-regime plane bed and rhomboid ripples produce planar laminae. Through lateral shifting of the active channel and upstream migration of pools and riffles, Medano Creek produces a facies sequence consisting of alternating 10-40-cm-thick layers of (planar) laminae and low-angle, cross-stratified, lenticular bedding.
TL;DR: In this paper, the authors show that CO2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach, especially in upper horizons of the cliff faces.
Abstract: Active precipitation of high-magnesian cryptocrystalline calcite cement (15.5 mole % MgCO3) in the intertidal zone on the north shore of Maui, Hawaii, results in formation of an extensive beachrock exposure at Hookipa Park. Framework grains in the beachrock are coarse algal, molluscan, and intraclast-bearing sands. Meniscus fabric is dominant in the cement, indicating that most lithification occurs in the vadose zone. The cryptocrystalline cement is generally brownish gold in color, but very thin, clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement, especially in upper horizons of the beachrock. Peloidal texture is scarce in the cement but is present in a few samples. Silt and fine-sand-sized skeletal fragments have been trapped in he evolving cement, resulting in cement-supported fabric. Hollow rims of microcrystalline calcite occupy some pore spaces. These hollow rims may have precipitated at the air-water interface of bubbles that formed in pores, and they may therefore be additional evidence of cementation in the vadose zone. Interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of CO2 degassing in the formation of beachrock cement. When chemical and tidal cycles fluctuate with the same period, PCO2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand. Increasing values of PCO2 most often occur during semi-daily rising tides and decreasing values of PCO2 occur during falling tides. Saturation ratios with respect to calcite vary in direct proportion to PCO2. Saturation levels generally increase during rising tides and decrease during falling tides. T ese data suggest that CO2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach.
TL;DR: In this paper, the average depositional rate of mud in the Thalassia seagrass bed was 4.96 g trap-1 week-1, and in the sandy area, it was 3.04 g trap 1 week -1 in the summer.
Abstract: Sediment traps were used to measure the rate of deposition in a Thalassia seagrass meadow and in an adjacent (sandy) grass-free area. The average depositional rate of mud in the Thalassia bed was 4.96 g trap-1 week-1, and in the sandy area, it was 3.04 g trap-1 week-1 in the summer. In winter, these rates were 1.60 and 1.50 g mud trap-1 week -1, respectively. Electromagnetic current-meter measurements showed that the mean flow velocity in the grass-free area is higher than in the seagrass area. Therefore, the increased depositional rate of mud within the seagrass is due to slowing of water currents by the grass blades. Confirmation of the blade baffle effect was obtained by placing a 1-m2 plot of artificial Thalassia in a grass-free area. Sediment traps in the artificial Thalassia contained 5.45 g mud trap-1 week-1 as compared to 3.04 g mud trap-1 week-1 in the adjacent grassfree area in summer. In winter, these rates were 1.82 and 1.50 g mud trap-1 week-1, respectively. Epiphytes in the Indian River Lagoon had no detectable effect on the amount of mud deposited in Thalassia beds.