Submarine Cementation Research Articles

Cavity‐filling dolomite speleothems and submarine cements in the Ediacaran Dengying microbialites, South China: Responses to high‐frequency sea‐level fluctuations in an ‘aragonite–dolomite sea’

AbstractSpeleothems, mostly composed of calcium carbonate, are widely present in modern karst‐originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee‐deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle‐like pendants and ground‐up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro‐crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg‐calcite). Meanwhile, the carbon‐oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high‐frequency sea‐level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.

Holocene tropical reef accretion and lagoon sedimentation: A quantitative approach to the influence of sea‐level rise, climate and subsidence (Belize, Maldives, French Polynesia)

AbstractAccretion rates of Holocene tropical coral reefs in three areas in the Atlantic, Pacific and Indian Oceans have been quantified in 79 dated core sections in 34 reef cores from Belize, the Maldives and French Polynesia. Holocene vertical reef accretion rate averages 5.05 m/kyr and has decreased during the past 10 kyr. Accretion rates in branched and massive coral facies are statistically similar. Reef accretion rate is positively correlated with the rate of sea‐level rise, that is the degree of creation of accommodation space, and with climate as expressed in a Holocene sea surface temperature anomaly. Accommodation space is also created by subsidence, but at a rate one to two orders of magnitude lower than that created by glacio‐eustasy (0.04 to 0.16 m/kyr). Lagoonal background sedimentation in adjacent reef lagoons averages 0.89 m/kyr as measured in 72 dated core sections in 28 cores. Lagoonal carbonate sedimentation on top of underlying mangrove peat usually starts after a considerable hiatus of ca 3 kyr on average. The lagoonal background sedimentation rate increased during the Holocene, probably due to deepening. The differences between vertical reef accretion and lagoonal background sedimentation rates are a major factor in the production of the widely known saucer shapes typical of tropical reefs and carbonate platforms, that is the creation of unfilled accommodation space. Reef core recovery, used as a proxy for reef consolidation, and core depth exhibit a statistically negative correlation based on data from 326 core barrels. Recovery and marine cement abundance (average volume 8.6%) also decrease from windward to leeward core positions. These observations are presumably a result of both a decrease in the rate of sea‐level rise that is the increase in time available for submarine cementation during the Holocene and the amount of flushing of reef interstices by marine waters.

Submarine canyon morphologies and evolution in modern carbonate settings: The northern slope of Little Bahama Bank, Bahamas

The recent high-quality multibeam echosounder swath bathymetry data and very high-resolution seismic profiles collected along the northern slope of Little Bahama Bank (LBB, Bahamas) constitute a unique dataset that can be used to investigate submarine canyon morphologies in modern carbonate settings. This region represents one of the few examples of submarine canyons that have developed in a purely carbonate system around the world. Our study reveals that there are 18 submarine canyons within the survey area that incise the slope between water depths of 450m and 1000m. Morpho-sedimentary analyses allow us to characterise their geometry and spatial distribution and to interpret their formation and control parameters. Among these canyons we distinguish four types: the amphitheatre-shaped canyons (1), canyons with up-dip linear incisions (2), canyons with internal recent depositional geometries, including levees and aggrading terraces on the side of their talweg (3) and canyons that are partially to completely filled (4). Based on these new morphological and geometrical classifications, the distributions of canyon morphologies and the integration of the full dataset, we propose a model for the formation and evolution of carbonate canyons. The latter emphasises that canyon initiation is solely the result of slope failures, which subsequently evolve into proper canyons through two successive phases of retrogressive erosion processes that are controlled by a downslope submarine cementation gradient. The first phase of retrogressive erosion produces widened canyon shapes at water depths of up to 600m, whereas the second phase forms up dip-propagating linear incisions at water depths of up to 450m. These linear incisions, which are located 5 to 10km away from the platform edge, favour the funnelling of sediment fluxes originating from the platform. Subsequently or contemporaneously with this retrogressive erosion, muddy gravity flows are responsible for canyon-fills and the formation of levees and aggrading terraces, which will eventually lead to the complete infill of the canyons, marking the end of an “erosion-fill” cycle. On a larger scale, the persistence of such canyons on the slope of LBB is of particular interest, as these are considered to be an exception along the present-day accretionary Bahamian slopes. Although similar canyons have been identified along the western slope of Great Bahama Bank (GBB), today these are totally buried, unlike those of LBB. We therefore suggest that the persistence of LBB canyons may be the result of relatively low sedimentation rates linked with the windward orientation of the platform. This low sedimentation rate has prevented the burial of these canyons since the Pliocene.

Middle Permian sponge–microencruster reefal facies in the mid-Panthalassan Akiyoshi atoll carbonates: observations on a limestone slab

A Murgabian (Middle Permian) reef-core facies in the Akiyoshi Limestone, southwest Japan, which is of mid-Panthalassan atoll origin, is here described from a limestone slab. The reef-core facies is composed mainly of boundstone with submarine cement and matrix. The reef-building community is characterized by a high-diversity biota, including sponges (sphinctozoans, inozoans, and chaetetids), bryozoans (fistuliporids and cryptostomates), crinoids and various microencrusters (Tubiphytes, Archaeolithoporella, reticular and laminar microbialites, encrusting foraminifers and problematic laminar red algae). Among them, sphinctozoan sponges and microencrusters such as Tubiphytes and microbialites are the most abundant. This community is similar to a time-equivalent lagoonal mound (patch-reef) biota on the Akiyoshi atoll in terms of the dominant sponges, Tubiphytes and microbialites, but differs in the additional occurrence of bryozoans, Archaeolithoporella and encrusting foraminifers, which probably preferred higher-energy conditions in the reef-core environment. Among temporally changing reef-building communities on the Carboniferous–Permian Akiyoshi atolls, the sponge-dominated reefal community described here flourished on mid-Panthalassan atolls in a relatively stable warm-water environment during a Middle Permian post-deglacial period.

Jia-2 Member doloarenite reservoir in the Moxi gas field, middle Sichuan Basin

Abstract Based on analysis on the macroscopic and microcosmic features of doloarenite in C layer, sub-member 2, Jia-2 Member of the Jialingjiang Formation in the Moxi gas field, the genetic mechanism of favorable reservoirs in beach facies carbonate rock is established. Primary inter-granular pores are the main reservoir spaces in the beach facies carbonates, and have the following key characteristics and identification signs: reservoir development was strictly controlled by the depositional microfacies; main pore types are residual inter-granular pores and dissolved-enlarged residual inter-granular pores; only submarine cement existed in grain contacts; initial compaction led to grains under framework support in line-concave-convex contact; and the burial cement in some separate inter-granular pores being not dissolved. The conservation mechanism of primary inter-granular pores is described as follows: after submarine cementation, the initial compaction created grains with early rim cement under framework support and the throats decreased accordingly; the compaction fluid cementation compressed the beach body's margin, the limited shallow burial cementation in inter-granular pores plugged the throats, and the cementation terminated after the diagenetic fluid reached dissolution-cementation balance. Late burial-dissolution adjusted and optimized early porous strata, which played a secondary role in increasing the reservoir space of each permeable reservoir bodies and did not change the spatiotemporal distribution of this kind of reservoir.

Multistage dolomitization in the cretaceous carbonates of the east Shahmirzad area, north Semnan, central Alborz, Iran

Different types of Cretaceous limestone and dolostone are the host for lead–zinc deposits and occurrences in the northeast of Shahmirzad, central Alborz. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include (1) very fine-crystalline, non-planar, xenotopic-a (Rd1); (2) fine- to medium-crystalline, planar-e dolomite rhombs, idiotopic-e, scattered dolomite crystals in micrite matrix (Rd2); (3) fine- to medium-crystalline, planar-e (porphyrotopic) (Rd3); (4) medium-crystalline, planar-s, subhedral to euhedral mosaic dolomites, idiotopic-s (Rd4); they post-date early submarine cements. Two types of dolomite cements were identified: medium- to coarse-crystalline, planar-e, euhedral to subhedral dolomite cements, idiotopic-c (Cd1); coarse-crystalline, non-planar, saddle dolomite cements (Cd2). They post-date replacement dolomites and predate late-stage calcite cements that line mouldic vugs and fractures. The association of Rd1 with stromatolites and evaporites, combined with δ18O positive values, postdating early diagenetic features such as micrites and marine cements, could evidence the importance of evaporitic brines for the dolomitization in sabkha (salt flats) environment. The emersion of the carbonate successions at the end of Early Cretaceous caused an ingression of waters of mixed meteoric-marine composition. This caused a wide dissolution of the carbonates and the generation of a first karstic phase. Rd2 and Rd3 dolomites were formed in this environment, as well. A second generation of karstic dissolution happened during the Laramide orogeny in Late Cretaceous, when the carbonate successions were uplifted till emersion. This stage wsas followed by a shallow burial phase and by the hydrothermal formation of Rd4, Cd1 and Cd2 dolomites. These dolomite types are associated epigenetic base-metal mineralization (MVT). The δ18OPDB and δ13CPDB isotopic ratios of the dolomites range from −5.80 to 2.14‰ and from −0.03 to 2.94‰, respectively. The δ18OPDB values are suggesting that all the studied dolomites have been deposited in low-temperature conditions. The measured δ18O values of fine-grained dolomites are heavier than those occurring in the hydrothermal types. This is pointing to lower formation temperatures in respect to other dolomite types in the study area. The remaining porosity of vugs and veins was filled by banded calcites and/or macrocrystals of sparry calcite. The δ18OPDB values of these calcites vary between −12.46 and −7.45‰ and δ13CPDB is in a range between −3.28 and 1.56‰. The O-isotopic values of calcites are lower than those of dolomites and may therefore indicate higher temperatures of precipitation. The different generations of dolomite contain also variable amounts of calcium carbonate, ranging from 49.5 to 55.5%. Fe, Mn and Sr in the dolostones are also variably concentrated: 280–1,500, 75–101 and 71–178 ppm, respectively. Fe, Mn and Sr values can also reflect the complexity of the diagenetic processes and the hydrothermal activity, which was responsible for Pb–Zn mineralization throughout the area as well as for the late dolomite generations. The hydrothermal flow might have been caused by Eocene volcanism, which affected the Cretaceous succession. Late dolomite distribution suggests that the dolomitizing fluids were probably diffused from the tectonic network and were expanded laterally following the permeability conduits.

Late Quaternary reef growth and sea level in the Maldives (Indian Ocean)

Based on rotary drilling and radiometric and U-series dating, we present the first comprehensive data on Holocene reef anatomy and sea-level rise as well as nature and age of underlying Pleistocene limestone in the Maldives. Holocene reefs in Rasdhoo Atoll, central Maldives, are composed of four facies including (1) robust-branching coral facies, (2) coralline algal facies, (3) domal coral facies, and (4) detrital sand and rubble facies. Branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef. In addition, microbialite crusts are found in lower core sections of marginal reefs. Microbialites formed during the early Holocene in reef cavities. Holocene reef thickness ranges from 14.5 m to > 22 m. Reef growth started as early as 8.5 kyr BP. Marginal reefs accreted in the keep-up mode with rates of > 15 m/kyr. Rate of sea-level rise significantly slowed down from 7–6 kyr BP and subsequently gradually rose with rates < 1 m/kyr. The lagoon reef accreted in the catch-up mode with rates of around 4 m/kyr. Even though no indications of a higher than present sea level were found during this study, it is not entirely clear from the data whether the sea gradually rose to or exceeded present level in the late Holocene. Submarine cementation in Holocene reefs studied is rather weak, presumably as a consequence of high accretion-rates, i.e., short time available for consolidation. Pleistocene coral grainstone was encountered in one core at 14.5 m below present level and three U-series dates indicate deposition during marine isotope stage 5e ca. 135 kyr BP.

Multistage hydrothermal dolomites in the Middle Devonian (Givetian) carbonates from the Guilin area, South China

AbstractPervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ18O values from −13·7 to −9·7‰ VPDB, δ13C values from −2·7 to + 1·5‰ VPDB and 87Sr/86Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (Th) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ18O values (−14·2 to −9·3‰ VPDB), lower δ13C values (−3·0 to −0·7‰ VPDB), higher 87Sr/86Sr ratios (≈ 0·7100) and higher Th values (171–209 °C), which correspond to trapping temperatures (Tr) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in 87Sr/86Sr and δ18O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ18O values (−18·5 to −14·3‰ VPDB), variable δ13C values (−11·3 to −1·2‰ VPDB) and high Th values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).

Lithofacies and depositional processes on a high, steep-margined Carboniferous (Bashkirian–Moscovian) carbonate platform slope, Sierra del Cuera, NW Spain

The depositional slope of the Sierra del Cuera, a Carboniferous (Bashkirian–Moscovian), high, steep-margined carbonate platform, provides excellent cross-sections of lithofacies zonations and associated stratal geometries. The steep (30–40°) and nearly planar upper slope is dominated by massive sheet-like layers of microbial, cement-rich boundstone, which alternate with intercalations of red-stained bryozoan cementstone with crinoids and brachiopods. The slightly gentler (20–26°) and concave-upward lower slope is characterized by clast-supported resedimented deposits. The upper slope extended from platform break to ca. 300 m water depth, whereas lower slope sediments were deposited in water depths up to 600–700 m, at which level the slope beds flatten to a few degrees (toe-of-slope) and interfinger with spiculitic and argillaceous basinal sediments. Between 250 and 450 m water depth, boundstones and breccias alternated in a transitional zone. The lower slope sediments include clast-supported breccias with radiaxial fibrous cement in interparticle space, mud- to clast-supported breccias with red-stained carbonate mud matrix and packstone to grainstone and rudstone beds. Most of the clasts comprise boundstone reworked from an upper slope setting and smaller grains are platform and slope derived. A pervasive submarine cementation occurs along the upper two-thirds of the flank and this stabilized the slope. Slope deposition is interpreted as follows. During active boundstone accretion, microbial boundstone layers slid off and formed breccia tongues extending from the lower upper slope down to the toe-of-slope. Rock falls and avalanches were generated whenever the shear strength of the substrate of loose (or partly lithified) sediment was exceeded. Upper-slope boundstone accretion and shedding, independent of the depth of light penetration, controlled most of the depositional processes on the slope. Cement-dominated intervals are considered to be related to early highstand (and/or flooding) phases. Relative sea-level fluctuations and/or associated changes in the water conditions are believed to be responsible for intervals of low boundstone production or cement precipitation. Whether the in situ boundstone and breccia are preferentially related to lowstand or highstand periods is, as yet, unclear.

Kilop Cretaceous Hardground (Kale, Gümüshane, NE Turkey):description and origin

A hardground surface is well exposed in the Kilop area of Kale (Gümüshane, NE Turkey) which forms part of the Eastern Pontides. Here, the hardground is underlain by shallow water Lower Cretaceous limestones, and overlain by Upper Cretaceous red limestones/marls which contains a planktonic microfauna including Globotruncanidae. In the field, the recognition of the hardground is based on the presence of extensive burrows (especially vertical burrows), the encrusting rudistid bivalve Requienia, neptunian-dykes with infills of pelagic sediments and synsedimentary faults. Skolithos and Thalassinoides-type burrows are present. Some burrow walls show iron hydroxide-staining. The extensive burrowing occurred prior to lithification. On the other hand, the neptunian-dykes and synsedimentary faults, which cut the hard ground, occurred after the lithification. These features indicate the progressive hardening of the substrate.The burrowed limestone consists of an intrabioclastic peloidal grainstone which was deposited in an intertidal to shallow, subtidal, moderate to relatively high energy environment. The peloidal limestone shows little or no evidence of submarine cementation, characterized by only scarce relics of isopachous cement rims of bladed calcite spar. The grainstone cement is composed predominantly of blocky calcite and overgrowth calcite cements on the echinoid-fragments. The origin of this cement is controversial.Biostratigraphic analysis of the limestones demonstrates that there is a marked stratigraphic gap (hiatus), spanning the Aptian to the Santonian, in the Cretaceous of the Kilop area.The formation of the Kilop Hardground is related to the break-up and subsidence of the Eastern Pontides carbonate platform during the formation of the Black Sea backarc basin. Hardground development was initiated in a shallow marine environment of slow sedimentation and with moderate to high energy indicating slow subsidence. Later, the hardground subsided abruptly, as shown by the deposition of pelagic sediments on the hardground surface. During drowning, the Kilop area was converted to a bypass-margin where currents were effective. The formation of the hardground may also have been associated with an eustatic rise in sea-level.

Quantifying rates of syndepositional marine cementation in deeper platform environments-new insight into a fundamental process

ABSTRACT Syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments, as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids. Rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry, especially related to the preservation of steep slopes, but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural. In an attempt to quantify the rates of syndepositional cementation in marginal platform environments, substrates containing samples of carbonate sand (ooids) suspended above the sea floor were emplaced at various depths across a modern platform in the Bahamas to evaluate the rates of cement growth through direct observation. Initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m. These results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments, and confirms earlier hypotheses that very rapid, syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments.

Synsedimentary cementation in the Lower Miocene reefal carbonates of the central basin, Iran

A systematic petrographic investigation is used to answer the question of occurrence, distribution and reasons for variation of cementation within reefal carbonates of Lower-Miocene age. Petrographic evidences indicate that submarine cementation occurred at or near the sediment/rock-water interface, but unlike some modern counterparts (i.e. Belize and the Great Barrier Reef) it is not restricted to local areas along the seaward margins of reefs. The f-Member of the Qum Formation is a classic example in which cementation occurs in relatively deeper basinal water as well as a wide variety of lagoonal, reef and fore-reef facies.

Where's the reef: The role of framework in the Holocene

Holocene reef models generally emphasize the role of in-place and interlocking framework in the creation of a rigid structure that rises above its surroundings. By extension, a number of ancient biohermal deposits have been disqualified as “true reefs” owing to their lack of recognizable framework. Fifty-four cores from several eastern Caribbean sites (Fig. 1) clearly demonstrate that in-place and interlocking framework is not common in these reefs that are comprised of varying mixtures of recognizable coral (primary framework), loose sediment/rubble and secondary framework made up mostly of coralgal fragments bound together by submarine cementation and biological encrustation. Recovery of primary and secondary framework ranged from 22% (avg.) in branching-coral facies to 33% in intervals dominated by head corals. Accretion rate decreased as expected with water depth. However, the recovery of recognizable coral generally increased with water depth, inversely to presumed coral-growth rates.

Submarine Cementation and Subaerial Exposure in Oligo-Miocene Temperate Carbonates, Torquay Basin, Australia

ABSTRACT At least four hardgrounds are present throughout the 27-m-thick exposed, Oligo-Miocene, cool-water Point Addis Limestone of the Torquay Basin in southeastern Australia. They range from thin (a few centimeters thick), incipient layers that have a nodular fabric to thick (up to 1 m thick) continuous hardgrounds. The hardgrounds tend to be at the top of coarsening-upward cycles and are typically overlain by low-relief erosion surfaces. The uppermost hardground is the best developed and most widespread in the Point Addis Limestone, and can be observed in all exposures of the succession. At some localities, it shows evidence of karstification such as dissolution pits, clints and grikes, and dissolution pinnacles. Whether karstified or not, well-developed hardgrounds are overlain by a lag c nglomerate that consists of hardground intraclasts with marine Fe-oxide crusts. The hardgrounds are cemented by a first-generation isopachous, inclusion-rich, columnar and fibrous radiaxial calcite of marine origin with trace-element compositions (Mg2+ = 0.68-1.74 mole % MgCO3; Fe2+ = 800-4690 ppm; Sr2+ = 0-260 ppm; Mn2+ = 0-230 ppm) and cathodoluminescence (dull/blotchy) indicative of stabilized Mg-calcite. The isopachous cements are invariably overlain by homogeneous, peloidal, or microbioclastic micrite, having trace-element compositions (Mg2+ = 0.77-1.54 mole % MgCO3; Fe2+ = 760-10,030 ppm; Sr2+ = 0-240 ppm; Mn2+ = 60-340 ppm) and cathodoluminescence (dull/blotchy) again indicative of stabilized Mg-calcite. This micrite is always closely associated wi h the isopachous cements and appears to be of marine origin, perhaps being analogous to the micritic precipitates described from reefal settings. Clear calcite is the last cement generation in the hardgrounds and has attributes of meteoric cements (nonluminescent; Mg2+ = 0.12-0.87 mole % MgCO3; Fe2+ = 0-230 ppm; Sr2+ and Mn2+ below detection limits). The whole-rock stable-isotope compositions of the hardgrounds and host limestone indicate that the whole unit has been subjected to pervasive alteration by meteoric fluids. The least altered carbonates analyzed from the Point Addis Limestone are brachiopods (d13C = -1.5 to +2.2 PDB; d18O = -1.7 to +0.9 PDB) and the unkarstified hardgrounds (d13C = -3.8 to -1.0 PDB; d18O = -0.6 to +0.6 PDB). These hardground occurrences help promote the validity of a unique sea-level-driven model for the formation of hardgrounds in cool-water settings. We propose that the development of these hardgrounds was an entirely marine process, produced by relative sea-level drop and entry of the sea floor into the zone of wave reworking. Marine cementation may begin in the form of nodules, at and below the seawater/sediment interface, and involves both slow sedimentation and shallowing of the cool waters. The nodules can later merge to form continuous hardgrounds. In such a high-energy environment, nondeposition and erosion are the dominant processes and marine cementation can occur. Further sea-level drop would lead to subaerial exposure of the previously formed hardgrounds, as found in one inst nce in the Point Addis Limestone at coastal exposures.

Abstract :Submarine Cementation in Chalks of the Vaihall and Hod Fields, Norwegian North Sea: Origin of Dense Zones"as Submarine Hardgrounds"

Abstract :Submarine Cementation in Chalks of the Vaihall and Hod Fields, Norwegian North Sea: Origin of Dense Zones"as Submarine Hardgrounds"

Dolomitization of the Leduc Formation (Upper Devonian), Southern Rimbey-Meadowbrook Reef Trend, Alberta

ABSTRACT Upper Devonian Leduc carbonate buildups along the southern part of the Rimbey-Meadowbrook reef trend in Alberta (buried at depths of 2300-3500 m) have been extensively dolomitized. Dolomitization occurred first by pervasive replacement and later by minor cementation. In general, replacement dolomites are composed of fabric-destructive, fine to medium (60-250 µm) planar subhedral to anhedral crystals. Replacement dolomitization postdates submarine cementation and deposition of shales and carbonates from the overlying Ireton Formation, and overlaps stylolitization. It predates dolomite cementation that is interpreted to have originated from hydrothermal fluids in a shallow burial environment prior to basin tilting. This, combined with the d18O (mean = -4.5 ± 0.4 PDB) and 87Sr/86Sr (mean = 0.70853) values in the replacement dolomites, suggests that replacement dolomitization occurred also prior to basin tilting, probably during the Antler Orogeny in the subsurface at a temperature of about 45°C and a depth of about 500 m, from slightly modified Late Devonian seawater. Two generations of dolomite cements postdate replacement dolomites: a coarse and planar-e(s) cement (Dolomite-C1), and a later coarser nonplanar cement (Dolomite-C2). Dolomite-C1 has oxygen (mean d18O = -4.73 PDB) and strontium (mean 87Sr/86Sr = 0.70855) isotopic compositions similar to replacement dolomites. These similarities, combined with the local association of Dolomite-C1 with stylolites, suggest that some Dolomite-C1 precipitated from fluids whose composition reflects pressure solution of the preexisting replacement dolomites. The composition of fluid inclusions in Dolomite-C1 (mean Th = 117°C; mean salinity = 19 wt % aCl equivalent) indicates that most Dolomite-C1 precipitated from warm hydrothermal brines. Little variation in fluid-inclusion homogenization temperatures occur with burial depth, suggesting that Dolomite-C1 precipitated prior to basin tilting, and may be related to Antler orogenic events. Dolomite-C2 postdates Dolomite-C1, is relatively more depleted in 18O (mean d18O = -9.06 PDB), and has a more radiogenic 87Sr/86Sr ratio (0.71087). Preliminary fluid-inclusion data from the Crimson Field suggest precipitation temperatures of about 150°C. Thus, Dolomite-C2 originated from fluids of higher temperatures and different composition than Do omite-C1. The petrographic, paragenetic, and geochemical characteristics of replacement dolomites in this study are similar to those of other Leduc buildups along the reef trend and of other Devonian dolostones of western Canada, supporting a model of large-scale basinwide fluid flow. The pervasive distribution of replacement dolomites along the Rimbey-Meadowbrook reef trend, and the similar petrographic and geochemical character of dolomites from both the Leduc and Cooking Lake Formations, is consistent with the hypothesis that the underlying Cooking Lake platform acted as a subsurface conduit system for the dolomitizing fluids. A potential regional fluid source may have been formation fluids (including residual evaporite brines) expelled tectonically during the Antler orogeny.

Multiple-stage diagenetic alteration and fluid history of Ordovician carbonate-hosted barite mineralization, Southern Quebec Appalachians

Lower Ordovician bioclastic limestone of the Upton Group, southern Quebec Appalachians, hosts stratabound Ba-Zn-Pb mineralization. The Upton Group, a mixed platform carbonate-siliciclastic-volcanic succession, is exposed as windows within the tectonically overlying Cambrian siliciclastics of the Granby Nappe. Mineralization consists mostly of barite and minor amounts of sulfides (sphalerite, pyrite, galena, and chalcopyrite), in addition to calcite, quartz and bitumen cements. It is hosted by a bioclastic limestone which is interbedded with and capped by a black calcareous shale, and underlain by a mudstone-siltstone-volcanic succession and a lower poorly fossiliferous limestone. The lower limestone recorded early extensive dolomitization followed by meteoric alteration (dedolomitization, sulphate dissolution, vadose cements, soil pisoids, etc.), and burial diagenesis (recrystallization, fracturation, and cementation). The vadose gravitational calcite cements yield δ 18O PDB values of −8.4 to −11.0‰ andδ 13C PDB values of +2.4 to +2.8‰. The thin soil profiles with pisoids have a δ 18O PDB value of −8.2‰ and a δ 13C PDB value of +2.0‰. These data suggest an evaporative 18O-enrichment of near-surface trapped soil moisture (vadose water) in a rock-dominated diagenetic system. The recrystallized limestone hasδ 18O PDB values of −11.4 to −15.5‰ and near Early Ordovician marine δ 13C PDB values of −0.2 to +2.5‰. These data suggest a final stabilization of the limestone from high temperature fluids in a rock-dominated diagenetic system. The mineralized bioclastic limestone shows rare evidence of early submarine cementation which is overprinted by significant post-depositional recrystallization and hydrothermal alteration. The latter resulted in the generation of secondary porosity and precipitation of a subhedral barite cement, a bladed barite cement, and fracture-filling barite. Fracture- and void-filling calcite, sulfides, quartz and bitumen cementation followed barite mineralization. Pre-barite syntaxial calcite overgrowths on crinoids yieldδ 13C PDB values of −3.9 to −15.0‰ andδ 18O PDB values of −13.7 to −14.8‰. Post-barite sparry calcite cement and fracture-filling calcite haveδ 13C PDB values of −2.6 to −13.0‰ and −2.4 to −17.9‰, respectively, andδ 18O PDB values of −13.6 to −14.2‰ and −14.0 to −15.8‰, respectively. The δ 18O values suggest relatively high-temperature re-equilibration in a deep-burial environment. The variable and depleted 13C values appear to reflect fluid-rock interaction and addition of significantδ 13C-depleted CO 2 from thermochemical sulphate reduction of organic matter. Mixing of reduced, hot basinal brines with oxidizing sulphate-rich fluids resulted in barite precipitation.

Early submarine cementation in fore‐reef carbonate sediments, Barbados, West Indies

Early submarine cementation in fore‐reef carbonate sediments, Barbados, West Indies

Early submarine cementation in fore‐reef carbonate sediments, Barbados, West Indies

ABSTRACTBottom sediments from the sea floor west of Barbados between depths of 110 and 324 m are composed of nodular or crusted carbonate deposits. Individual biogenic sediment grains and the cemented aggregates, nodules and crusts are usually more or less altered by bioerosion and may support one or more generations of encrusting organisms.On the basis of component analysis of the topmost part of the bottom sediments it is possible to recognize three facies: (1) a proximal slope facies down to a depth of about 140 m, rich in mollusc fragments, benthic foraminifera and bryozoans; (2) an upper distal slope facies between about 140 m and about 215 m, rich in benthic foraminifera, molluses and crustaceans; and (3) a lower distal slope facies from about 215 m to at least 300 m, dominated by molluscs, especially pelecypods, with subordinate scleractinians and tubes of the polychaete Lygdamus asteriformis.The appearance and quantitative importance of the cemented aggregates is also related to these facles. In the proximal slope facies, only relatively few irregular and very porous nodules are found, whereas in the lower distal slope facies, aggregates are very common. Most aggregates are crust‐like with a smooth upper surface and a more or less irregular, knobby lower surface. The crusts are massive compared with the nodules of the proximal slope facies and, unlike the latter, the lower surfaces and walls of larger cavities are usually coated with Fe and/or Mn oxides. In the upper distal slope facies a gradual transition between the two types of aggregates is found.Petrographical and morphological evidence, together with carbon and oxygen isotopic data, indicates that the nodules and crusts were formed in situ by submarine lithification processes. Radiocarbon dating of two bulk samples suggests that the cementation took place during late Pleistocene and/or early Holocene.

The geological effects of hurricanes on coral reefs and the interpretation of storm deposits

Hurricanes occur in belts 7° to 25° north and south of the equator. Reefs growing in these belts suffer periodic damage from hurricane-generated waves and storm surge. Corals down to 20m depth may be broken and removed, branching colonies being much more susceptible to breakage than upright massive forms. Sand cays may be washed away and former storm ridges may migrate to leeward across reef flats to link with islands. Reef crest and reef front coral debris accumulate as talus at the foot of the fore-reef slope, on submarine terraces and grooves, on the intertidal reef flat as storm ridges of shingle or boulders and isolated blocks of reef framework, as accreting beach ridges of leeward migrating shingle, as lobes and wedges of debris in back-reef lagoons, as drapes of carbonate sand and mud in deep off-reef locations in the fore-reef and lagoonal areas. In addition to the coarse debris deposited, other features may aid the recognition of former hurricane events, including the assemblage of reef biota, its species composition and the structure of the skeletons; graded internal sediments in framework cavities; characteristic sequences of encrusting organisms; characteristic shapes of reef flat microatoll corals; and submarine cement crusts over truncated reef surfaces. The abundance of reef flat storm deposits whose ages cluster around 3000–4000 y BP in certain parts of the world most likely relate to a slight fall in relative sea level rather than an increase in storminess during that period. A higher frequency of storms need not result in more reef flat storm deposits. The violence of the storm relative to normal fair-weather conditions influences the extent of damage; the length of time since the previous major storm influences the amount of coral debris created; the length of time after the hurricane, and before a subsequent storm influences the degree of stabilization of reef-top storm deposits and hence their chances of preservation.