Shallow-marine Carbonate Deposits Research Articles

Funsulinid assemblages and facies of the Bombaso Fm. and basal meledis Fm. (Moscovian-Kasimovian) in the central Carnic Alps (Austria/Italy)

The Bombaso Formation and basal Meledis Formation in the central Carnic Alps near Straniger Alm and Zollnersee (Austria/Italy) unconformably overlie the folded Variscan basement and consist of shallow marine clastic and carbonate sediments which are arranged to form two fining and deepening upward sequences. Particularly limestones and even breccias of the Bombaso Formation yielded a rich fusulinid fauna composed of 34 species which are attributed to the following zones:Quasifusulinoides quasifusulinoides-Protriticites ovatus; Protriticites pseudomontiparus, andMontiparus montiparus.

Diagenesis and remanence acquisition in the Cretaceous carbonate sediments of Monte Raggeto, southern Italy

Abstract Lower Cretaceous limestones and dolomites drilled at Monte Raggeto, north of Naples, show a hierarchical organization in cycles, bundles and superbundles, similar to that found in other Cretaceous shallow-water carbonate rocks exposed in the Southern Apennines. These cycles have been interpreted as being caused by the influences of planetary orbital perturbations of the Earth (precession, obliquity), with the bundles and superbundles being considered to represent the short- and long-term eccentricity orbital cycles, respectively. The palaeomagnetic properties of an 88 m core (corresponding to a total interval of c. 3.2 Ma with a rock accumulation rate averaging 1 cm per 360 ± 16 years) were determined at 2 cm intervals. These show the same spectral periodicities as for the sedimentary properties determined at 1 cm intervals, but there are no correlations between these sedimentological and palaeomagnetic sets of data, when analysed on the scale of the bore-core. An example is given of a 15.50 m length of the bore-core, comprising a single superbundle corresponding to one of the long eccentricity terms, and including eight elementary cycles (each topped by a brief emersion episode) grouped in three bundles. These shallow marine carbonate sediments underwent very early diagenesis accompanied by marine cementation in the pore spaces at or immediately below the water-sediment interface in the same way as in modern-day tropical-subtropical environments. During brief emersions at the top of each cycle, meteoric early diagenesis also occurred. All non-carbonate grains that were present in the original sediment, including ferromagnetic ones, are likely to have been cemented in during these very early phases of diagenesis. It is concluded that the studied interval provides a model of the diagenetic evolution of the whole core, where the early diagenesis controlled the locking-in of a nearly continuous record of the geomagnetic field behaviour for the the entire core. Thus depositional, diagenetic and palaeomagnetic properties can be used in conjunction to provide detailed information on the environmental variations and geomagnetic field behaviour during the evolution of this stratigraphic sequence at an ultra-fine resolution, as 1 cm of rock is equivalent, on average, to 360 ± 16 years.

Abstract: Reversible Sulfate Reduction and Methanogenesis Reactions in Dolomitization of Holocene Shallow-Marine Carbonate Sediments, Belize 

Abstract: Reversible Sulfate Reduction and Methanogenesis Reactions in Dolomitization of Holocene Shallow-Marine Carbonate Sediments, Belize 

Effect of biogeochemical diagenesis on sediment fabric in shallow marine carbonate sediments near the Dry Tortugas, Florida

Shallow marine carbonate sediments near the Dry Tortugas undergo extensive biogeochemical diagenesis upon deposition, resulting in postdepositional fabric comprised of micritic aggregates of clay-sized particles, a matrix of peloidal mud, and intraparticular cementation. Freshly deposited primary skeletons, mostly Halimeda fragments, yield micritic aggregates of clay-sized particles, which then form peloidal mud fabric upon compaction. The cryptocrystalline particles produced by micritization are later replaced by high magnesian calcite. Cementation is observed in the intra-particular microenvironment. However, no inter-particular, grain-binding cement was detected throughout the gravity core samples investigated.

Field observations from the Socotran platform: their interpretation and correlation to Southern Oman

Recent fieldwork on Socotra and adjacent islands, located along the southern margin of the Gulf of Aden, coupled with extensive biostratigraphic studies, has led to the identification of a number of distinct sedimentary successions in the area. Triassic strata have been identified in a fault-bounded area of easternmost Socotra and Jurassic rocks, previously unrecognised on the island, are preserved unconformably overlying the Triassic in south-east Socotra. In other parts of the Socotran Platform, Cretaceous sedimentary rocks lie unconformably above basement. The oldest and thickest successions are of Barremian to Aptian age and occur on the western islands. A marine transgression is recorded by a transition from estuarine to mixed shallow-marine carbonate and siliciclastic sedimentation. Thin sedimentary sequences of Campanian and Maastrichtian age have been identified in two separate areas of Socotra. Tertiary deposition commenced in latest Palaeocene times and a gentle angular unconformity with the underlying Cretaceous deposits is apparent. Simultaneous with the onset of Gulf of Aden rifting, a new phase of deposition began in the late Oligocene. A wide range of facies reflect strong tectonic controls on deposition. Plate tectonic reconstructions indicate that prior to the Gulf of Aden rifting, Socotra was located adjacent to the Dhofar region of Southern Oman. This interpretation is well-supported by comparisons of the Mesozoic and Tertiary successions in these two areas.

Compaction and Decompaction Algorithms for Sedimentary Carbonates

ABSTRACT The mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration, backstripping analyses, and quantitative models for simulation of carbonate stratigraphy. Deep-water chalks and calcareous oozes are not treated here. Empirical porosity-depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity-depth curves for shallow-water carbonate muds and sands. The curves are exponential in form. This review indicates that mud-rich shallow marine carbonates behave differently than sand-rich, grain-supported sediments with regard to reduction in porosity and thickness. Quantitative analysis of compaction behavior in carbonate sediments reveals that, barring early cementation, carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths, e.g., 50% thickness reduction with about 150-200 m overburden. However, primary porosities of carbonate muds, approximately 70-80%, are reduced only to about 42% under that overburden. Thus, the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity. This indicates that interparticle cementation, with or without chemical compaction, is required to transform muddy carbonate sediments into lithified rock. In the absence of early lithification, carbonate sands compact more slowly than muds. Primary interparticle porosities of about 40-45% can be maintained in the first 200-300 m of burial, with little thickness reduction.

A New Bioevent Stratigraphy of Late Jurassic Arab-D Carbonates of Saudi Arabia

ABSTRACT A series of fine-scale biofacies cycles are recognised by the application of semiquantitative micropalaeontological analysis of cores in eight wells from the reservoir carbonates from the Arab-D and upper Jubaila formations in Saudi Arabia. They are of Kimmeridgian - Tithonian age, and form the primary producing reservoir of the world’s largest single continuous reservoir located in the Ghawar field of Saudi Arabia. Three new composite biozones are identified that include a basal, deep marine Biozone D3 with fining-upwards cycles which equate, in part, with the Jubaila Formation. Biozone D2 was deposited in shallower conditions of the lower Arab-D carbonate, and Biozone D1 deposited in a very shallow marine environment of the upper Arab-D carbonate; both zones contain multiple coarsening-upwards cycles. Numerous additional bio-events, of potential subzonal value, are detected within these broad zones, and are considered to be related to discrete shoaling-upwards responses to transgressive episodes. The proposed scheme differs significantly from that erected for the Arab-D Formation in offshore Abu Dhabi. Carbonates of the Arab-D member are concluded to be composed of a series of high frequency palaeobathymetric changes which were superimposed upon an extensive carbonate platform. Gradual shallowing of the platform resulted possibly from a reduction in subsidence rate, a fall in sea level, rise in carbonate productivity rate or a combination of these. The latest episode of very shallow marine carbonate sedimentation terminated with the deposition of the Arab-D evaporites.

El Cretácico del borde meridional del Sistema Central: unidades litoestratigráficas y secuencias deposicionales

The Central System is an alpine rejuvenation of the hercinian base of the Iberian Plateo The landward ends of the cretaceous carbonate platforms of the Iberian Basin croup out along the southern margino Near to the Iberian Range (NE), the cretaceous sediments are composed of shallow marine and litoral carbonate deposits. They grade towards the basin boundary (Central System) into a litoral and continental terrigenous facies. An important reduction southward of the thickness of the sedimentary succession is recognized. The stratigraphic record is organized in sixteen lithostratigraphic units and in nine depositional sequence, that can be correlationed with the chart of cycles global of Haq et al. The age of the studied deposits are Cenomanian to Campanian. The stacking pattern is agradational, with a onlap not very marked in the base, a truncation and an internal uncorformity in the Santonian age. The thickness and facies changing is due to sinsedimentary faults within a distensive context, that have a little vertical displacement. They are younger outward of the sedimentary basin.

Variable rates of phosphate uptake by shallow marine carbonate sediments: Mechanisms and ecological significance

We determined phosphate uptake by calcareous sediments at two locations within a shallow lagoon in Bermuda that varied in trophic status, with one site being mesotrophic and the other being more eutrophic. Phosphate adsorption over a six hour period was significantly faster in sediments from the mesotrophic site. Uptake at both sites was significantly less than that reported for a similar experiment on calcareous sediments in an oligotrophic lagoon in the Bahamas. The difference in phosphorus adsorption between our sites did not appear to be related to sediment characteristics often cited as important, such as differences in surface area (as inferred from grain size distributions), total organic matter content, or iron content. However, the sediment total phosphorus contents were inversely related to phosphorus uptake at our sites in Bermuda, and at the previously studied Bahamas site. We hypothesize that phosphate uptake in these calcareous sediments is a multi-step process, as previously described for fluvial sediments or pure calcium carbonate solids, with rapid initial surface chemisorption followed by a slower incorporation into the carbonate solid-phase matrix. Accordingly, sediments already richer in solid phase phosphorus take up additional phosphate more slowly since the slower incorporation of surface-adsorbed phosphate into the carbonate matrix limits the rate of renewal of surface-reactive adsorption sites. Although carbonate sediments are a sink for phosphate, and thereby reduce the availability of phosphorus for benthic macrophytes and phytoplankton in the shallow overlying water, phosphate uptake by these sediments appears to decrease along a gradient from oligotrophic to eutrophic sites. If our result is general, it implies a positive feedback in phosphorus availability, with a proportionately greater percentage of phosphorus loading being biologically available longer as phosphorus loading increases. This pattern is supported by the significantly higher tissue phosphorus content of the seagrass,Thalassia testudinum, collected from the eutrophic inner bay site. Over time, this effect may tend to cause a shift from phosphorus to nitrogen limitation in some calcareous marine environments.

Upper Cenomanian-lower Turonian sea-level rise and its consequences on the Adriatic-Dinaric carbonate platform

The Adriatic carbonate platform existed throughout most of the Mesozoic as an isolated (‘intraoceanic’) carbonate platform. It is characterized by what has been traditionally labelled a ‘continuous’ shallow marine carbonate sedimentation, without terrigenous influences. However, global sea-level changes left unequivocal imprints in the ‘continuous’ carbonate record, thus proving that the carbonate platform sensitively responded to sea-level fluctuations. This paper focuses on the relatively short Cenomanian — Turonian drowning event, which temporarily submerged the platform below the euphotic zone, and which is related here to the well known global Cenomanian — Turonian sea-level highstand. This drowning episode is characterized by the abrupt disappearance of numerous shallow water benthic invertebrate taxa and by an abundant occurrence of planktonic organisms, including calcispheres and ‘primitive’ planktonic foraminifera. Near the middle of the Turonian, the platform regime was re-established, correlating with the global fall in sea level. The situation is different only at the south-west margin of the platform, where pelagic sedimentation, initiated near the end of the Cenomanian, continues into the Santonian and probably marks the Adriatic trough. The drowning began earlier (in the Middle Cenomanian) in those areas nearer to the north-east margin of the platform (the area of what is today central Croatia) and then progressed gradually towards the more interior areas of the platform (the middle Dalmatian islands), which were drowned near the end of the Cenomanian.

Cenomanian-Turonian platform to basin integrated stratigraphy in the South Pyrenees (Spain)

The deposits registering the geological events that took place in the South Central Pyrenees during Late Cenomanian and Early Turonian times reflect oceanic changes by the modification of their facies and fossil assemblages. In the basin, where pelagic sedimentation was continuous, the Cenomanian-Turonian boundary is characterized by laminated sediments, rich in 13C and lacking keeled planktonic Foraminifera. Only opportunistic species such as whiteinellids, hedbergellids and heterohelicids survived and reflect high biological productivity in the surface waters. On the inner platform, the shallow marine carbonate sedimentation with abundant large benthic Foraminifera, rudists, and other molluscs producing well defined facies tracts during the Cenomanian is brutally interrupted, and covered by earliest Turonian sediments containing only abundant echinoid debris and calcisphaerids. Interruption of carbonate platform sedimentation and subsequent deposition of echinoids recall the impact of artificial eutrophication in modern coral reef environments.

Dissolution and recrystallization in modern shelf carbonates: evidence from pore water and solid phase chemistry

We present an overview of geochemical data from pore waters and solid phases that clarify earliest diagenetic processes affecting modern, shallow marine carbonate sediments. Acids produced by organic matter decomposition react rapidly with metastable carbonate minerals in pore waters to produce extensive syndepositional dissolution and recrystallization. Stoichiometric relations among pore water solutes suggest that dissolution is related to oxidation of H 2 S which can accumulate in these low-Fe sediments. Sulphide oxidation likely occurs by enhanced diffusion of O 2 mediated by sulphide-oxidizing bacteria which colonize oxic/anoxic interfaces invaginating these intensely bioturbated sediments. Buffering of pore water stable isotopic compositions towards values of bulk sediment and rapid 45 Ca exchange rates during sediment incubations demonstrate that carbonate recrystallization is a significant process. Comparison of average biogenic carbonate production rates with estimated rates of dissolution and recrystallization suggests that over half the gross production is dissolved and/or recrystallized. Thus isotopic and elemental composition of carbonate minerals can experience significant alteration during earliest burial driven by chemical exchange among carbonate minerals and decomposing organic matter. Temporal shifts in palaeo-ocean carbon isotope composition inferred from bulk-rocks may be seriously compromised by facies-dependent differences in dissolution and recrystallization rates.

Sedimentology and Paleogeography of the Natih Carbonate Platform in the Oman Mountains

Field study of the Natih Formation in the Jebel Akhdar and the Oman foothills allows us to establish a new stratigraphical and sedimentological model of this important hydrocarbon reservoir unit. Thanks to the study of rudists and the discovery of ammonites, a new precisions can be given to the chronostratigraphy of the Natih Formation. It was especially demonstrated by the presence of Hippuritids (rudists) that the top of the Natih Formation matches the Cenomanian/Turonian boundary and corresponds to either rudist-rich layers or hard grounds and condensed levels. The stratigraphical correlations between several outcrop section allow one to establish a conceptual sequence stratigraphic model which can be compared to the subsurface by using the Natih subdivisions [open quotes]A to G.[close quotes] The recognition of sequence boundaries, maximum flooding surfaces, and system tracts might help to understand the seismo-stratigraphic expression of the Natih interval in the subsurface. Furthermore, this sequence stratigraphic model clearly illustrates the interfingering of the carbonate reservoir intervals and the organic-rich units (Fitri Mb) at the top of the Natih Formation. We also have identified a clear zonation from deeper marine to shallow-marine carbonate deposits, the most significant of which are the rudistid facies. They form banks, thickets, andmore » biostroms and do not constitute anomalous build ups such as bioherms. Good leaching potentials generally are related to these rudistid facies, especially when they are very rich in skeletal aragonite from the Caprinids shells. Significant primary porosity may be related also to the Hippuritid skeletal cavities at the top of the Natih. Reservoir potentials can be enhanced if these Caprinid-rich intervals are related to exposure surfaces such as the top Natih E and the top Natih A.« less

Physical sedimentation processes on a mid-proterozoic (1800 Ma) shelf: the reynolds range group, arunta block, central Australia

Abstract Shallow-marine siliciclastic and carbonate deposits belonging to the 1800 Ma old Reynolds Range Group, central Australia, represent a transgressive sequence deposited on a low-gradient shelf, that was affected by longshore currents, tidal currents, storm wave action, and fair-weather wave action. The sediments comprise a 1100 m thick sequence in which three main depositional facies associations can be distinguished; (i) a near-shore and inner shelf quartz-arenite facies association is overlain by (ii) a middle shelf mudstone facies association which contains lenses of (iii) a carbonate facies association. The quartz-arenite facies association mainly occurs along the northeast side of the range, and grades into the mudstone facies association towards the southwest. This suggests that the palaeo-shelf extended towards the southwest, which is in accordance with a concentration of southwest-directed palaeo-current directions within the quartz-arenite facies. A sudden increase in thickness of quartz-arenite in the Mount Thomas area, suggests this area was the locus for fluvial sediment input. Vertical and lateral stratigraphic transitions between the facies associations are generally gradational, which may reflect coexistence of all facies on the shelf and small, net, relative sea level changes. The quartz-arenite facies association contains tide-influenced, bidirectional, small-scale trough cross-bedded, tabular cross-bedded and parallel-laminated sets that were deposited on the shoreface. These are generally overlain by large-scale trough and tabular cross-bedded sets, which were deposited on the inner shelf and record a southeast-directed longshore current. Numerous truncation surfaces indicate that frequent storms eroded the shoreface and carried large volumes of sediment onto the shelf. The mudstone facies association consists of alternating mudstone beds and generally thin, sheetlike quartz-sandstone beds that lack grading and traction structures. They represent storm-induced suspension deposits laid down on the middle shelf (inferred water depths $ 100 m ). Longshore and tidal currents appear to have had little influence in this environment. Stromatolite bioherms, and rippled marls and grainstones constitute the carbonate facies association which formed on the middle shelf in shallow marine areas of low siliciclastic sediment input. Ripples in storm sands throughout the mudstone facies association and a lack of turbidites, reflect the shallow depths and low-gradient of the shelf slope. A general lack of tidal deposits and the non-barred nature of the palaeo-coastline, indicate a small tidal range (0–4 m). The unidirectional southeast-directed longshore current, appears to have only influenced sedimentation on the inner shelf which suggests it resulted from prevailing (westerly) winds. The described facies in the Reynolds Range Group occur in strongly deformed and highly metamorphosed rocks, and the paper serves as an example that even from such tectonised rocks valuable sedimentological information can be extracted.

Environmental gradients in carbonate sediments and rocks detected by correspondence analysis: examples from the Recent of Norway and the Dinantian of southwest England

ABSTRACTContinuous sedimentary gradients are only crudely expressed by standard facies and microfacies methods which are more appropriate to situations where changes occur in relatively discrete steps. In carbonate sediments and rocks, continuous gradients are often represented by the arrangement of component grain types in a relay, that is, a systematic shifting of the relative importance of the components. Subdivision of such relays into (micro)facies can only be arbitrary.Correspondence analysis is shown to be useful for detecting, isolating and describing relays. Particular use is made of the arch effect in which samples and components from data sets with a strong unidimensional structure (a relay) plot in the form of an arch in the plane of the first two factor axes. A relay index, indicating the position of samples in the relay, can be extracted from the analysis and plotted on maps and stratigraphic logs to reveal details of the sedimentary gradient in areal and/or stratigraphic context.Examples are given from: (i) Recent shallow‐marine carbonate sediments from northern Norway, illustrating a relatively simple depositional setting where surface sediments are viewed in plan; and (ii) Lower Carboniferous carbonates of southwest England, representing a more complex regional study of a particular stratigraphic interval viewed in cross‐section. In both examples the relays can be related to identifiable environmental gradients.

Shelf to basin transition in the Cambrian-Lower Ordovician of Sardinia (Italy) reply

In southern and central Sardinia two different Cambrian-Lower Ordovician sedimentary belts occur. The former is exposed in southwestern Sardinia (Iglesiente and Sulcis), and is characterized by shallow-marine siliciclastic and carbonate sediments settled on a continental shelf with lowangle morphology. The latter, exposed in the Nappe Zone (central and southeastern Sardinia), consists of a thick monotonous sequence of siliciclastic deposits settled in a deeper-water basin.

Dispersed remnants of a northeast Pacific fringing arc: Upper Paleozoic terranes of Permian McCloud Faunal affinity, western U.S.

Two fragmentary, subparallel belts of terranes within the western North American Cordillera contain upper Paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting Permian faunal affinity. These two belts are (A) volcanic‐arc related successions of Permian McCloud faunal affinity (McCloud belt) and (B) subduction‐related accretionary complexes of Permian Tethy an faunal affinity (Cache Creek belt). This paper supports the hypothesis that the fragmentary terranes of the McCloud belt once constituted parts of a northeast Pacific fringing‐arc system and, in constrast to some earlier interpretations, concludes that the volcanic arc evolved above an eastward dipping subduction zone. The absolute distance between this arc and western North America during the late Paleozoic cannot be constrained, however, there is little evidence to suggest closure of a major (>10³ km) ocean basin or protracted periods of westward dipping subduction. Parts of Devonian to Permian volcanic island arc sequences of the western U.S. Cordillera are represented in the northern Sierra, eastern Klamath, Bilk Creek, Grindstone, and Chilliwack terranes. These scattered volcanic arc remnants share several fundamental characteristics: (1) The sequences were constructed across continental‐affinity basement assemblages. (2) They underwent similar tectonic evolution during late Paleozoic time, such as coeval pulses in volcanism and related depositional histories. (3) They contain Early Permian McCloud‐type fauna, of distinctive biogeographic affinity. (4) McCloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the Cache Creek belt which contain fragments of Upper Triassic blueschist and Permian limestone blocks bearing Tethyan Permian fusulinids and corals. Based on the presence of distinctive Early Permian McCloud fauna, the island arc remnants discussed in this paper are referred to as the McCloud belt. The similarities between McCloud belt terranes suggest that they were once parts of a long‐lived late Paleozoic volcanic arc complex. The stratigraphy of these terranes records the tectonic evolution of the volcanic arc. Volcanism within the Paleozoic arc began as early as the Early Devonian, and was succeeded or accompanied by carbonate and epiclastic sedimentation during the Middle and Late Devonian. An influx of detritus derived from sedimentary and metasedimentary rocks during the Late Devonian and Mississippian records the uplift and erosion of arc basement sequences, an event that was, at least locally, accompanied by ongoing volcanism. A pulse in volcanism during the Pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and, locally, subaerial exposure which persisted into the early Early Permian. Elsewhere during the Early Permian, shallow‐water carbonate deposition was established, with only minor influx of volcanogenic debris. During the Late Permian, and locally as early as late Early Permian, a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas. Most of these sequences are overlain by Upper Triassic carbonate platformal sequences. Arc volcanism was recorded, at least locally, until the Middle Jurassic. Paleomagnetic data are used to constrain a schematic pre‐Cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages. The inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone, with a long‐lived paleogeographic relation to North America. The current structural position of these terranes is largely the result of Mesozoic and locally younger deformation.

Effect of orthophosphate on the dissolution kinetics of biogenic magnesian calcites

The dissolution behavior of two biogenic Mg-calcites, the echinoid, Tripneustes (12 mol% MgCO 3), and the red alga, Neogoniolithon (18 mol% MgCO 3 plus brucite), was studied using free-drift methods in distilled water and phosphate-spiked solutions at 25°C and P CO2 = 1 atm. Small concentrations of phosphate strongly inhibit dissolution rates. Inhibition increases with increased phosphate levels and with approach toward saturation. Near saturation, dissolution rates are reduced by 10 3 by the presence of 0.6 μmol adsorbed-P/m 2. The magnitude of phosphate inhibition is similar to that observed for low-Mg calcite, and like calcite, the mechanism of inhibition is probably by adsorption of P at surface kink sites. Phosphate appears to inhibit removal of Mg and Ca equally during Mg-calcite dissolution. Rates of brucite dissolution are unaffected by phosphate. Mg-calcites containing >8.5 mol% MgCO 3 should be thermodynamically unstable relative to aragonite in most natural waters. Recent work, however, shows that in contrast to its effect on the behavior of Mg-calcites. phosphate does not inhibit aragonite dissolution. The presence of phosphate might thus reverse the relative stability of these two minerals during diagenesis of shallow marine carbonate sediments.

Temperate shelf carbonate sediments in the Cenozoic of New Zealand

ABSTRACTShelf limestones are widely distributed in New Zealand Cenozoic sequences and are especially well developed in the Oligocene. Detailed field and laboratory work on several Oligocene occurrences, and reconnaissance field‐work at most other sections have elucidated the major characteristics of the environment, texture, composition and diagenesis of these sediments. Several generalizations emerge which contrast with the commonly accepted characteristics of shallow marine carbonate sedimentation established from studies of tropical and subtropical deposits.The limestones are either calcarenites or, less commonly, calcilutites and, in general, these two lithologies are mutually exclusive, both in time and space. The allochems and interparticle carbonate mud (where developed) in calcarenitic limestones consist almost exclusively of fragmented skeletal material derived primarily from bryozoan, echinodermal, benthic foraminiferal, barnacle, brachiopod, bivalve and coralline red algal tests. The calcilutitic limestones consist mainly of whole and disintegrated tests of pelagic foraminifers and coccolithophorids. Non‐skeletal carbonate components such as ooids, pellets and aggregates are conspicuously absent from both lithologies. Reefal structures are also absent or rare and are mainly oyster reefs.The limestones commonly contain a significant content of terrigenous material and/or glauconite and at the stratigraphic level the limestones are intimately associated with terrigenous formations. The distribution of the carbonate sediments has been governed mainly by rate of supply of river‐derived terrigenous material, by subsequent dispersal patterns of this material over the shelf, and by current sorting. As a consequence of selective grain transport, bedding in the limestones is often defined by the cyclic alternation on a wide range of scales of carbonate units that are relatively enriched and relatively impoverished in terrigenous material.The primary (carbonate) mineralogy of the carbonate sediments was completely dominated by magnesium calcite and/or calcite with only small amounts of aragonite and no dolomite or associated evaporite minerals. The metastable magnesium calcite and aragonite grains were probably altered on, or close below, the shallow sea‐floor. Among other factors, transformation was encouraged by the absorption of magnesium in pore waters by montmorillonitic clays and by the complete oxidation of all organic matter in the bottom sediments. Magnesium calcite grains were stabilized by texturally non‐destructive incongruent dissolution, but aragonite was often dissolved without trace from the sediment, especially in grainstones. Thus submarine diagenesis has been characterized by selective dissolution phenomena.Cementation by granular and syntaxial rim orthosparite of calcite and/or ferroan calcite composition occurred mainly during shallow subsurface burial and was associated with the intergranular solution of calcitic skeletal fragments, especially at those levels in the sediment relatively enriched in terrigenous material. This lithification process has worked to accentuate and modify original litho‐logic differences and sedimentary structures in the primary sediments and has produced a kind of rhythmic vertical alternation of less well cemented, microstylolitized, impure limestone beds (‘cement‐donor’ beds) and well cemented, more open textured, purer limestone beds (‘cement‐receptor’ beds).The New Zealand limestones formed between latitudes 60° S and 35° S under generally cool temperate to warm temperate climate conditions. Oxygen isotopes suggest that surface waters were mainly significantly cooler than 20°C, so that shelf waters may have experienced extended periods of undersaturation with respect to calcium carbonate. Generally open circulation patterns maintained near normal salinity values over the entire shelf platform. Calculated sedimentation rates for the New Zealand carbonate sediments are generally very low (< 5 cm/1000 years). Periods of more active deposition commonly alternated with longer periods of non‐deposition and by‐passing or erosion. It is concluded that many characteristics of the New Zealand shelf limestone occurrences are explained best by a temperate latitude model of shallow marine carbonate sedimentation.

Chalk Diagenesis and Its Relation to Petroleum Exploration: Oil from Chalks, a Modern Miracle?

Chalks consist largely of stable low-magnesium calcite. Thus, they undergo diagenetic alteration different from that of more widely studied aragonite and high-magnesium calcite-bearing, shallow-marine carbonate deposits. Examination of outcrop and subsurface samples of chalks from the North Sea, onshore Europe, the Scotian Shelf, Gulf Coast, and the U.S. Western Interior indicates that chalks undergo significant diagenetic changes during their postdepositional history. Scanning-electron microscopy, light microscopy, oxygen-isotopic analysis, and trace-element analysis outline the major factors that control the patterns of chalk alteration. The major mechanism of chalk cementation is pressure solution and local reprecipitation. Although small variations in initial grain size, faunal composition, or clay content can lead to significant bed-to-bed variations in cementation, overall patterns of chalk diagenesis appear to be related to two main factors: (1) maximum depth of burial, and (2) pore-water chemistry. With a few notable exceptions, the porosity (and permeability) of chalks decreases as a direct function of burial depth. The exceptions include cases where: (1) oil entered the rock, reducing or terminating carbonate reactions; (2) chalks are overpressured and therefore are not subject to the normal grain-to-grain stresses expected at those depths; and (3) tectonic stresses increase solution and cementation. In areas here fresh water entered the pores before major burial, chalks show a much steeper gradient of porosity loss versus burial depth as compared with regions where marine pore fluids were retained. Under normal circumstances, a typical nannofossil chalk ooze will have 70% porosity at the sediment-water interface. At a depth of 1 km, porosity will be reduced to about 35%; at 2 km, to about 15%; and at 3 km, to essentially 0. Thus, one observes progressive lithification of chalks (and their isotopic alteration) as one moves downhole or toward areas of greater burial. Petrophysical and isotopic studies can predict maximum depths of burial, paleogeothermal gradients, and proximity to zones of deformation. In areas such as the Ekofisk field in the North Sea, however, major quantities of oil are produced from chalks having as much as 40% porosity (largely primary) at depths greater than 3 km. This appears to be related largely to the widespread overpressuring of the Central graben in that area. Other such areas of anomalous porosity in thick chalk sections should be detectable by seismic methods. Significant hydrocarbon production from chalks can occur in three major settings: (1) overpressured or oil-saturated zones where these phenomena were initiated early in the subsidence history (e.g., the North Sea); (2) areas where chalks never have been buried deeply (e.g., the Scotian Shelf); and (3) cemented and fractured chalks in several possible settings (e.g., the Gulf Coast).