Destructive Diagenesis Research Articles

Pleistocene aragonite crust diagenesis mimics microbialite fabrics (Danakil Depression, Ethiopia)

Fibrous aragonite crusts occur in two consecutive Pleistocene successions in the Danakil Depression (Afar, Ethiopia). Lateral transitions between pristine and altered fibrous aragonite crusts document changes in texture associated with diagenesis. Crusts formed as essentially abiotic seafloor precipitates at the transition from marine to evaporitic conditions. Diagenesis started with the dissolution of aragonite fans at the interface between single fans in non-laminated crusts and along lamination planes in isopachous, irregular, or crudely laminated crusts. Incomplete dissolution resulted in the development of secondary porosity within a matrix of undissolved aragonite fibers. Subsequently, the porosity was filled with calcite that systematically encased remaining aragonite crystals. This was followed by the dissolution of remnant aragonite fibers, producing a network of elongated inter- and intracrystalline pores that were eventually filled with low-Mg calcite. The stepwise substitution of fibrous aragonite by low-Mg calcite resulted in sparry, sparry-cloudy, sparry-micritic (including clotted micrite), and peloidal textures, which obscure the fibrous nature of the original deposits. Stable C- and O-isotope compositions suggest that early diagenesis was driven by meteoric and evaporative fluids. These observations unequivocally demonstrate destructive diagenesis, resulting in secondary textures, which mimic micritic and grumous (peloidal and clotted) textures associated with sparry microfabrics.This suggests that these textures, classically interpreted as primary microbial precipitates and used as evidence of biogenicity in ancient microbialites, might be diagenetic products in some cases, even though at some stage, microbial processes and/or degradation of organic matter could have been involved in the diagenetic process.

Reservoir properties and hydrocarbon enrichment law of Chang 1 oil layer group in Yanchang Formation, Wanhua area, Ordos Basin

Evaluation of tight oil reservoir properties is of great significance to the exploration of oil and gas in tight reservoirs. The Chang 1 Member of the Yanchang Formation in the Wanhua Area, Ordos Basin is a new exploration stratum for tight sandstone oil. The lack of understanding of reservoir characteristics and crude oil enrichment rules has seriously restricted the efficient development of oil and gas resources in this stratum. In this study, the reservoir characteristics of the Chang 1 Member in the Wanhua area and the effects of superimposed sand bodies, structures and paleogeomorphology on accumulation of hydrocarbons were systematically studied. The Chang 1 sandstone is a typical ultra-low porosity-ultra-low permeability reservoir, and it has experienced destructive diagenesis of mechanical compaction, pressure solution and cementation, and constructive diagenesis of dissolution. Strong pressure solution caused the secondary enlargement of quartz and feldspar and the formation of patchy dense mosaic structures. The target layer has experienced argillaceous, siliceous and carbonate cementations. Moreover, the sandstone reservoir in the Chang 1 Member also experienced strong dissolution, and it is the main factor for the formation of secondary pores and the improvement of reservoir physical properties. The study also found that the main types of pores in the Chang 1 Member are intergranular dissolved pores and remaining intergranular pores. Superimposed sand bodies, nose-shaped uplifts, dominant facies and eroded paleo-highlands have significant effects on the hydrocarbon accumulation. Based on this study, it was found that the migration and accumulation mode of hydrocarbons in the Chang 1 reservoir belongs to the ladder-like climbing migration + structural ridge accumulation type. In addition, sand body thickness is an important controlling factor for the hydrocarbon accumulation. At present, the discovered crude oil in the Chang 1 Member is always distributed in the areas with thick sand bodies (>20 m), and most of the sand bodies have a thickness in the range of 25–40 m, and the effective thickness is in the range of 2–6 m. In addition, the eroded highlands are the highest topographic units, they are favorable areas for the large-scale accumulation of oil and gas.

Study on the characteristics and influencing factors of Chang 7 ultralow-porosity and low-permeability reservoirs in the Heshui area, Ordos Basin

Recently, the exploration and development of the Chang 7 shale oil reserves and production have increased in the Ordos Basin. However, the characteristics of the Chang 7 reservoir vary greatly in different areas, which affect the exploration and development of shale oil. We used various analytical methods such as core observation, casting section, high-pressure mercury injection, nuclear magnetic resonance, X-ray diffraction (XRD), and logging interpretation to study characteristics of the reservoir in the Heshui area. The lithology of the Chang 7 member is mainly feldspathic quartz sandstone suggesting that the content of quartz is higher than that of feldspar, and it has relatively low carbonates. In addition, the kinds of feldspar are mainly plagioclase and potassium feldspar, and the concentration of clay minerals is 80%, mainly comprising illite and chlorite. The reservoir of the Chang 7 member is chiefly comprised of submicron pores, such as feldspar dissolved pores, intergranular pores, dissolved pores, microfractures, and intergranular pores. The porosity ranges from 6% to 12%, whereas permeability is less than 0.2 × 10−3 [Formula: see text]. The Chang 7 reservoir has a strong heterogeneity. Specifically, the heterogeneity of Chang 71 is weaker than that of Chang 72. There is a complex diagenesis such as compaction, dissolution, and cementation, and the compaction and cementation are relatively more and the dissolution is dominated by constructive diagenesis of feldspar dissolution. Sedimentary microfacies are one of the main factors controlling reservoir development. The physical properties of the reservoir in the branch channel are better than the edges of the branch channel and the lacustrine. The diagenesis affects the development and distribution of good reservoirs, and destructive diagenesis such as compaction and cementation reduces porosity and permeability. Conversely, dissolution increases the physical properties of the reservoir. Tectonics has no obvious effects on reservoir but plays a positive role in the migration and accumulation of hydrocarbon when microfractures developed.

Quantitative evaluation of tight oil reservoirs in the Chang 8 Member of the Yanchang Formation in southern Ordos Basin

The precise and quantitative characterization of reservoir properties is the key to efficient development of tight oil reservoirs. In this paper, taking the Chang 8 Member of the Yanchang Formation in the Shuimogou area in the southern Ordos Basin as an example, the sedimentary facies types, microscopic pore structures, diagenesis, influencing factors of physical properties, and hydrocarbon enrichment law of tight oil reservoirs are systematically studied. The research results show that the Chang 8 Member in the study area is a typical delta front subfacies deposit, including distributary channel and inter-distributary bay microfacies. The constructive diagenesis of the Chang 8 Member include dissolution, metasomatism and rupture; while the destructive diagenesis include mechanical compaction and cementation. The Chang 8 reservoir has entered the middle diagenetic stage A. The factors affecting the physical properties of tight oil reservoirs include deposition, compaction, cementation and dissolution. The secondary pores formed by dissolution account for 10–40% of the total surface porosity, with an average value of 24%. Local structures and sediments have significant control over hydrocarbon accumulation. The westward dipping tectonic setting of the northern Shaanxi Slope provides the basic conditions for the migration of oil and gas to the eastern updip areas. The changes of lithology and physical properties in the updip direction of the structure form the blocking conditions for the continued migration of oil and gas, which is conducive to the accumulation of oil and gas. In addition, the main oil reservoirs are mainly distributed along the distributary channel of the delta front, and most of the distributary channel sandstone is more than 20 m thick. The areas of thick sand body with multiple layers in the lateral direction is the main part of oil and gas accumulation. However, the reservoirs formed on the flanks of underwater distributary channels are generally thin and poorly sorted, which is not conducive to the accumulation of hydrocarbons.

The Characteristics and Main Controlling Factors of the Upper Paleozoic Sandstone Reservoirs in Dongpu Depression, Bohai Bay Basin

Based on the existing core and logging data, as well as previous research achievements, various experimental data were obtained by means of thin section, casting thin section, scanning electron microscopy, X-ray diffraction analyses, and cathodoluminescence. These data were used for the purposes of examining the features, diagenetic characteristics, and diagenetic evolution sequences of the Upper Paleozoic reservoirs of the Dongpu Depression, and analyzing the main controlling factors. The research results showed that the reservoir lithology was mainly subarkose, sublitharenite, feldspathic litharenite, and lithic arkose. The reservoir was generally characterized by ultra-low porosity and ultra-low permeability, with low porosity and low permeability in some areas. The pore types mainly include primary intergranular residual pores, dissolution pores, intercrystalline micropores, and microfractures. The pore throats are small and the throat channels are poorly sorted. The diagenetic processes mainly include compaction, pressure dissolution, cementation, metasomatism, and dissolution features of the research area. The diagenetic stages were determined to be mainly the middle diagenetic stages (Stage A and Stage B). The delta distributary channel and distributary bay of the Upper and Lower Shihezi Formation, along with the river channel sub-facies of the Shiqianfeng Formation, were considered to be the strata with good reservoir conditions. Moreover, the sand grains are dominated by medium-to-good sorting properties, sub-roundness and sub-angular, which provided a good spatial basis for the development of the primary pores. The calcite and kaolinite in the interstitial materials were corroded, which played a constructive role in the generation of the secondary pores. The results of the compaction, pressure dissolution, and cementation analyses indicate destructive diagenesis, which potentially reduced the porosity and permeability values. However, the dissolution, metasomatism, and chlorite lining cementation indicate improved diagenesis, which potentially increased the porosity and permeability values.

Characteristics of heterogeneous diagenesis and modification to physical properties of Upper Paleozoic tight gas reservoir in eastern Ordos Basin

To correctly evaluate reservoir quality and determine promising tight gas zones, the characteristics of heterogeneous diagenesis and modifications to physical properties in the Upper Paleozoic tight gas reservoir in the eastern Ordos Basin (UPTGREOB) were assessed by comprehensively analyzing lithology, petrophysics, and organic geochemistry data. It was found that the types of sandstones in UPTGREOB mainly include (feldspathic) detritus sandstones and detritus feldspathic sandstone in low–ultralow porosity and permeability, with complex pore structure. Sandstone pores can be divided into two types: primary (primary intergranular and residual primary intergranular pores), secondary (intercrystalline, dissolved intragranular, dissolved intergranular, and dissolved cementing pores and microfractures). The main types of diagenesis are compaction, cementation, dissolution, and metasomatic alteration. Shiqianfeng Formation and Upper Shihezi Formation belong to period B of the syndiagenesis stage and period A1 and A2 of the mesodiagenesis stage; Lower Shihezi Formation and Shanxi Formation are entirely in the mesodiagenesis stage; Taiyuan Formation can be regarded to be in periods A2 and B of the mesodiagenesis stage and period A of the telodiagenesis stage. The original porosity of UPTGREOB was approximately 34%. Compaction was the primary destructive diagenesis rather than cementation, while dissolution was found to be the primary constructive diagenesis. The heterogeneous diagenesis played different roles in the UPTGREOB: in syndiagenesis B, the UPTGREOB began to densify under the compaction and early cementation; in mesodiagenesis A, dissolution and metasomatic alteration produced massive secondary dissolved pores and intercrystalline pores, improving the porosity and permeability; in mesodiagenesis B, dissolved pores were filled by new cements, resulting in decreased porosity and permeability; in telodiagenesis A, intensified compaction and cementation made the UPTGREOB retain extra low porosity and permeability. In general, the reservoirs of Shiqianfeng Formation and Upper/Lower Shihezi Formation in the mesodiagenesis stage A have preferable physical properties for tight gas accumulation.

Main diagenesis of middle-lower Ordovician of Yubei Area in Tarim Basin

According to analysis of cores, rock sections, casting sections, logging data of Ordovician carbonate reservoirs in the Yubei area, reservoir rock in the study area include 4 types: mud micrite limestone, grain limestone, biological limestone and dolomitic limestone. In combination with core photos and imaging logging analysis, reservoir can be divided into pores and cracks. Diagenesis mainly includes compaction, pressolution, cementation, filling, recrystallization, dolomitization, dissolution. To sum up, the main constructive diagenesis includes recrystallization, dolomitization, dissolution, otherwise the main destructive diagenesis includes compaction, pressolution, cementation, filling.

Pore-fillings of dolomite reservoirs in Sinian Dengying Formation in Sichuan basin

Abstract The Sinian Dengying Formation in the Sichuan basin is an important potential reservoir for oil and gas exploration in the periphery of the Sichuan basin. Its reservoirs were well developed. And many kinds of fillings which lead to the decrease of pores can be seen in reservoir pores and caves. In this paper, the pore fillings of the Dengying Formation reservoir in Sichuan basin are the object of study. The types and characteristics of the fillings are studied in detail by means of rock slices analysis, cathode luminescence, trace elements and inclusion temperature measurement. The study shows that the fillings include dolomite fillings and siliceous quartz fillings and bitumen fillings. Among them, dolomite fillings can be divided into botryoidal dolomite fillings, foliated dolomite fillings, finely-medium crystalline dolomite fillings and coarse crystalline dolomite fillings. 50%–80% of the pores are filled by the dolomite fillings with botryoidal structure, and the pores can be partially full filled; 0%–20% of pores are filled by foliated dolomite fillings; 20–80% of the pores are filled by finely-medium dolomite fillings; 20–80% of the pores are filled by coarse crystalline dolomite fillings. 1%–5% of secondary pores are often filled by siliceous quartzs, and the pores can be partially full filled. Bitumen fillings often occur after the above-mentioned fillings, and the residual pores are often semi-filled by bitumens, which makes the original pores and throat smaller and reduces the effective porosity and permeability of the reservoir. So, it is a destructive diagenesis. The pore fillings make the secondary porosity in the reservoir significantly lower, which is not conducive to the formation and preservation of the Dengying Formation reservoir.

Main Diagenesis of Middle-lower Ordovician of Yubei Area in Tarim Basin

The Ordovician in Yubei area is an important target and production layer of marine carbonate exploration in Tarim Basin. In order to understand the reservoir distribution and therefore to promote exploration, by means of analysis of cores, rock sections, casting sections, logging data of Ordovician carbonate reservoirs in the Yubei area, the reservoir petrology, diagenesis types and characteristics of the carbonate rocks in the study area are systematically studied, and the identification marks, development rules of various diagenesis and their influence on the formation of the reservoir are summarized. The results show that the carbonate reservoir in the study area is mainly composed of 4 types: mud micrite limestone, grain limestone, biological limestone and dolomitic limestone. In combination with core photos and imaging logging analysis，reservoir can be divided into pores and cracks, which are fracture type and fracture pore type. The diagenesis types of carbonate rocks in study area are complex and diverse, mainly including compaction, pressolution, cementation, filling, recrystallization, dolomitization, dissolution. To sum up, the main constructive diagenesis include recrystallization, dolomitization, dissolution, otherwise the main destructive diagenesis include compaction, pressolution, cementation, filling. The above research results on the diagenesis of carbonate rocks has a certain role in promoting the development of oil and gas exploration in the central Tarim Basin.

Ordovician deep dolomite reservoirs in the intracratonic Ordos Basin, China: Depositional model and Diagenetic evolution

Recent natural gas discoveries indicate that non-karstification-dominated reservoirs exist in the intracratonic Ordos Basin. This study examines the sedimentological and geochemical characteristics needed to clarify the depositional model and diagenetic evolution process of this newly discovered reservoir type. The depositional environment of the dolomite reservoir can be characterized as a tidal flat that grew from the Central Paleo-uplift to the eastern depression by cyclic progradation on an epeiric platform. A tidal flat sequence can extend laterally as a progradational wedge in each cycle of sea level fluctuation. The sheet-shaped peritidal shoal facies associations patched on the wedge represent potential dolomite reservoirs and can be recognized by the presence of doloarenite that has been altered into a vaguely relict grained-texture by diagenesis. Although continuing destructive diagenesis has led to reservoir densification, burial dolomitization and burial dissolution with facies selectivity have tended to occur in peritidal shoal facies associations, thus improving the quality of the dolomite reservoirs. These models provide new insights for targeting deep dolomite hydrocarbon reservoirs in intracratonic basins.

Diagenesis of the Triassic Yanchang Formation Tight Sandstone Reservoir in the Xifeng–Ansai Area of Ordos Basin and its Porosity Evolution

AbstractThis work aims to reveal the evolution of the porosity in the Triassic Yanchang Formation tight sandstone reservoir of the Xifeng–Ansai area of Ordos Basin. Based on destructive diagenesis (compaction and cementation) and constructive diagenesis (dissolution) of sandstone reservoirs, this study analyzed the diagenesis characteristics of the tight sandstone reservoirs in this area, and discussed the relationship between sandstone diagenesis and porosity evolution in combination with present porosity profile characteristics of sandstone reservoir. The effect simulation principle was employed for the mathematical derivation and simulation of the evolution of porosity in the Yanchang Formation tight sandstone reservoirs. The result shows that compaction always occurs in tight sandstone reservoirs in the Yanchang Formation, and cementation occurs when the burial depth increases to a certain value and remains ever since. Dissolution occurs only at a certain stage of the evolution with window features. In the corresponding present porosity profile, diagenesis is characterized by segmentation. From the shallow to the deep, compaction, compaction, cementation and dissolution, compaction and cementation occur successively. Therefore, the evolution of sandstone porosity can be divided into normal compaction section, acidification and incremental porosity section, and normal compaction section after dissolution. The results show that the evolution of sandstone porosity can be decomposed into porosity reduction model and porosity increase model. The superposition of the two models at the same depth in the three stages or in the same geological time can constitute the evolution simulation of the total porosity in sandstone reservoirs. By simulating the evolution of sandstone reservoir porosity of the eighth member in Xifeng area and the sixth member in Ansai area, it shows that they are similar in the evolution process and trend. The difference is caused by the regional uplift or subsidence and burial depth.

Characteristics of hydrothermal activity in the Tarim Basin and its reworking effect on carbonate reservoirs

Hydrothermal activity is common in the Tarim Basin. In this paper, petrologic and geochemical features of the hydrothermal products in the carbonate rocks in this basin were analyzed so as to figure out the effect of hydrothermal activity on the Lower Palaeozoic carbonate reservoirs in this area. It is shown that the hydrothermal fluids in the Tarim Basin are generally high in CO2 content, but very low in Mg2+ content. Then, the reworking effect of hydrothermal activity on reservoir was further discussed. It is indicated that the component of hydrothermal fluid is the deciding factor for the reworking of reservoirs. The study provided the following findings. Firstly, hydrothermal fluid is rich in CO2. Dissolution is intense in the vicinity of fracture zones and dissolved pores of different sizes are developed. Therefore, small-scale high-quality reservoirs with good porosity and permeability are formed. Secondly, hydrothermal fluid itself is low in magnesium content and some additional magnesium can be produced from Cambrian dolomites by means of dissolution, but the scale of hydrothermal dolomitization is small. Thirdly, the hydrothermal fluids rich in CO2 form high-quality dissolved vug typed reservoirs by means of dissolving surrounding rocks and their distribution are controlled by faulting. Fourthly, the saddle-like dolomite with hydrothermal origin fills dissolved vugs. Furthermore, the hydrothermal activity in the vicinity of faults results in the recrystallization or excessive growth of dolomite crystal, blocking existed pores, which is a type of destructive diagenesis as a whole. The study results can provide a reference for carbonate reservoir prediction of deep basin.

A new method for recovering paleoporosity of sandstone: case study of middle Es3 member of Paleogene formation in Niuzhuang Sag, Dongying Depression, Bohai Bay Basin in China

This paper presents a new method for recovering paleoporosity of sandstone reservoirs and quantitatively defines the evolution process of porosity. This method is based on the principle that the present is the key to the past. We take the middle Es3 member in Niuzhuang Sag, Dongying Depression, and Bohai Bay Basin as an example. The method used in this study considers the present porosity as a constraint condition, and the influences of both constructive diagenesis and destructive diagenesis to divide the porosity evolution process into two independent processes, namely porosity increase and porosity decrease. An evolution model of sandstone porosity can be established by combining both the pore increase and pore decrease effects. Our study reveals that the porosity decrease model is a continuous function of burial depth and burial time, whereas the porosity increase model mainly occurs in an acidified window for paleotemperature of 70°C to 90°C. The porosity evolution process can be divided into the following phases: normal compaction, acidification and pore increase, and post-acidification compaction. Thus, the porosity evolution model becomes a piecewise function of three subsections. Examples show that the method can be applied effectively in recovering the paleoporosity of sandstone reservoirs and simulating the porosity evolution process.

Developing concepts of high-resolution diagenetic stratigraphy for Pliocene cool-water limestones in New Zealand, and their sequence stratigraphy

The diagenetic evolution of calcitic cool-water limestones can be markedly different from that for aragonite-dominated tropical carbonates. This petrographic study develops a "sequence stratigraphic" approach to cool-water carbonate diagenesis based on the chronological occurrence of all diagenetic features (not only cement phases) within sedimentary successions, their vertical and lateral distribution, and their grouping into discontinuity-bounded (by dissolution surfaces, fractures, renucleation events) suites at a thin-section scale. The progression of diagenetic environments that moved through a succession is related to diagenetic discontinuities correlatable throughout the study area. In our limestones a critical discontinuity event to decipher is the first evidence of mechanically-induced skeletal fracturing, enabling the definition of precompaction and post-compaction diagenetic features. A comparison of just the pre-compaction diagenetic suites in samples from above and below significant outcrop disconformities enables isolation of the "specific diagenesis" of unconformity-bounded depositional units, which is that suite of diagenetic events associated with the evolution of the sedimentation realm of a unit, exclusive of diagenetic overprints shared with any overlying unit through an unconformity since these relate to some subsequent, different depositional stage. In turn, the specific diagenesis of a succession of depositional facies between consecutive unconformities may then be interpreted within a sequence stratigraphic framework. Sections of cool-water Pliocene skeletal limestones in eastern North Island, New Zealand, comprise deepening-and then sometimes shallowing-upwards, unconformity-bounded depositional packages, or sequences. We recognize ten different specific diagenetic signatures in these rocks that are correlatable to depositional lithofacies. The vertical distribution of specific diagenetic signatures within the sequences exhibits a trend from mainly constructive to mainly destructive diagenesis in the deepening-upwards member of the sequences, and constructive in their shallowing-upwards portion, that can be integrated in a sequence stratigraphic framework. Constructive diagenesis occurs in the lower part of the transgressive system tract (TST) and in the upper part of the regressive system tract (RST), whereas destructive diagenesis is dominant in the upper part of the TST, the highstand system tract (HST), and the lower part of the RST.

Magnetic Ilmenite-Hematite Detritus in Mesozoic-Tertiary Placer and Sandstone-Hosted Uranium Deposits of the Rocky Mountains

Mesozoic and Tertiary sandstones throughout the Rocky Mountains region of the United States contain a de-trital Fe-Ti oxide mineral that is generally regarded as unusual and is preserved wherever these sandstones have escaped destructive diagenesis. This mineral, a medial ilmenite-hematite solid solution, is highly magnetic and has sometimes been mistaken for magnetite. We have encountered this mineral in paleomagnetic studies and in investigations of sandstone-hosted uranium deposits and Cretaceous shoreline placer deposits. It is known to occur elsewhere in intermediate volcanic rocks, consistent with quenching from hypersolvus high-temperature conditions. The detrital occurrences we describe are of economic importance in uranium deposits, where they are among the reductants that precipitate uranium, and in Cretaceous placer deposits, whose exploitation would require a separation procedure tailored to this mineral. The distribution of detrital hematite-ilmenite sandstones in Cretaceous rocks suggests a contemporaneous volcanic source in the northern Rocky Mountains.

Reservoir Distribution and Exploration Potential of the Spiro Sandstone in the Choctaw Trend, Arkoma Basin, Oklahoma and Arkansas

A multidisciplinary study of the Spiro Sandstone shows that in Pittsburg and Latimer counties, Oklahoma, along the Choctaw fault trend, the thrusted Spiro is predominantly a barrier island deposit consisting of reservoir-quality progradational and aggradational sandstones; however, to the east from Le Flore County, Oklahoma, through Yell County, Arkansas, along the structural trend, the Spiro is composed predominantly of tight, nonproductive, retrogradational sandstones. Petrographic observations of the progradational Spiro show that sands in the west are medium to fine grained, and in the east they are very fine grained. Chlorite coatings that inhibit quartz cementation are present only in the west. Vitrinite reflectance data indicate that the Spiro in the west is within the gas window (~1.5% R0), but in the east it is (>2.5% R0). The term overmature applies to reservoirs that have insufficient porosity for commercial production due to destructive diagenesis caused by thermal stress. The simultaneous multidisciplinary approach is economically significant because exploration analysis time for this study was decreased, and decisions were made with more reliability.

Surficial cool-water carbonate sediments on the Otway continental margin, southeastern Australia

The Otway continental margin of southern Australia is a swell-dominated, open, cool-water carbonate platform. It is partitioned into a series of depth-related zones of biological sediment production and accumulation by three hydrodynamic interfaces: (1) abrasion depth (−70 m), (2) swell wavebase (−130 m), and (3) storm wavebase (> −250 m). The Otway shelf profile drops steeply from the strandline. The shallow shelf lies largely between −30 and −70 m, and is a high energy zone of carbonate sediment production, intensive destructive diagenesis and erosion. The seafloor is characterized by widespread exhumed limestone substrates which host dense encrusting assemblages of molluscs, sponges, bryozoans and red algae. Loose sediment accumulation is patchy, and consists of a thin, quartz-rich, palimpsest gravel veneer. The middle shelf (−70 to −130 m) is a zone of swell wave shoaling, and production and accumulation of coarse mega-rippled (2-D dunes) bryozoan sands. Deep shelf areas (−130 to −180 m) accumulate finer bioclastic sands (foraminiferal/echinoid/bryozoan), that are intensely bioturbated and episodically reworked by storms. Grains are variably fragmented and marginally bored. At the shelf edge and top of slope (−180 to −350 m) nutrient-rich upwelling currents support laterally extensive aphotic biostromal/hermal bryozoan/sponge/coral communities which generate and accumulate thick successions of muddy skeletal sand. Sediments of the upper slope (−350 to −500 m) are a bioturbated mixture of periplatform bioclastic debris and pelleted foraminiferal/nannofossil mud. Turbidites and resedimentation features are common. Bioturbation and shelf-derived skeletal content decrease progressively downslope and pelagic muds dominate below −500 m. Lower slope areas are cross-cut by gullies and have low accumulation rates. The base of slope is characterized by sediment apron deposits consisting of interbedded shelf-derived, coarse-grained turbidites, and pelagic ooze.

Preliminary steps toward formation of a generalized budget for cold-water carbonates

High-latitude carbonate sediments were once considered isolated exceptions to the tropical rule. They are now being studied throughout the world and preliminary descriptive models have been proposed. Constituents, composition and distribution of cold-water carbonate sediments have been described, and they are diagnostically different from their tropical counterparts. The rates of depositional and post-depositional processes may differ as well. The present study of the rates of gross production, net production and diagenesis was undertaken at Shell Beach in the Gulf of Maine (43°45′N, 69°45′W), a shelly pocket beach. Surface and subsurface sediments were collected and analyzed for CaCO 3 content, assemblage and alteration. Population density of dominant organisms and the weight of skeletal material in typical adults were determined. Transport of sediment was observed. Gross production by dominant organisms was calculated to be 1600 g CaCO 3/m 2 y over the area of production. Net production, while less exact, was on the order of 13–130 g CaCO 3/m 2 y. The difference between these is explained by strong destructive diagenesis, dominantly biological, and at a rate of 80–1300 g CaCO 3/m 3 y. The large gap between gross and net production seems to be characteristic of cold-water carbonate sediments, as does the strongly destructive diagenesis. The gap must be remembered when assessing fossil cold-water populations.

Criteria for Recognition of Cold-Water Carbonate Sedimentation: Berriedale Limestone (Lower Permian), Tasmania, Australia

ABSTRACT The Permian Berriedale Limestone is an alternating sequence of limestone and calcareous shale associated with marine glacial terrigenous sediments. It was deposited at a paleolatitude near 80° S during the Gondwanan Permo-Carboniferous Ice Age. Dropstones commonly disrupt laminae, range widely in size and composition, and reveal glacially-derived shapes and textures under the SEM. There is a lower diversity of fauna (mostly bryozoans, brachiopods and pelecypods) and non-skeletal grains (mostly intraclasts) than in warm-water limestones. The vertical variation in depositional microfacies, through shale-limestone-shale beds, indicates that the shallow-marine limestones formed during the lowering of sea level in colder periods. As the sea level rose during warmer periods, a shower of dropstones preceded shale deposition. For this mason dropstones are more numerous around the contacts between limestone beds and overlying shale beds but rare in the middle or base of limestone beds. Icebergs, calved from adjacent glaciers, were floating, and the surface water temperature was near freezing during limestone deposition. Upwelling cold currents provided nutrients and maintained calcium carbonate at saturation. Well-developed rhombs in micrite, microspar and spar, and length-slow elongate rhombs and poikilotopic rhombic plates (both formed by accretion of rhombohedra) indicate low Mg-calcite precipitated from seawater at a temperature of <3° C. These cements are overlain by micrite, marine-mud, dropstones and geopetal fillings of well-sorted internal sediments, and are cut by borings, all proving both synsedimentary ages and submarine origin of these cements. The calcium carbonate content in these cold marine waters fluctuated, ranging from undersaturated (destructive diagenesis: pits, etchings, rhombic pores, and dissolution features) to saturated (constructive diagenesis). As the sea level rose during warmer periods, large volumes of melt-waters mixed with marine waters. The mixing zone cements. succeeding marine cements, are cloudy, inclusion-rich, coarse plates (up to a few mm). These crystals grew, embedding or nucleating over early-formed crystals and previously-cemented (both fillings and encrustations) bioclasts. The mixing zone cementation is preceded by erosion of early-formed crystals. The eroded-crystals now occur as inclusions in mixing zone cements. The field, microfacies, and diagenetic criteria evolved in this study might be used for recognition of a wide spectrum of cold-water carbonates (temperate, subpolar, and glacio-marine).

Destructive diagenesis of carbonate sediments in the eastern Skagerrak, North Sea

Marine diagenesis of carbonate sediments in shallow waters in the eastern Skagerrak is characterized by dissolution and breakdown. The processes leave a diagnostic petrographic record, consisting of etch patterns on grain surfaces, partial dissolution of fabrics in the interior of grains, and finally, complete structural breakdown of biogenic carbonate constituents. Because of the structural breakdown, great numbers of calcareous organisms are lost from the fossil record. A residual amount of their microscopic structural elements occurs as a very typical lime-mud fraction in the sediments. Dissolution of carbonate constituents seems to be an ordinary ingredient in the normal, healthy marine environment in the eastern Skagerrak. Spot-checks indicate the presence of dissolution also in the Kattegat and the brackish Baltic Sea—in total, an area of 450,000 km 2 . This is supposed to be the largest occurrence of carbonate dissolution so far reported from shallow waters.