Dolomite: occurrence, evolution and economically important associations

Various types of carbonate platforms are characterized by distinctive profiles, facies, and evolutionary sequences. Ramps may be homoclinal or distally steepened, and may have fringing or barrier shoal-water complexes of ooid-pellet sands or skeletal banks. Homoclinal ramps pass seaward into deeper water without major break in slope, and lack deep-water breccias. Distally steepened ramps may be low energy, and characterized by widespread, shallow, subwave-base mud blankets, or high energy with coastal beach/dune complexes and widespread skeletal sand blankets. Slope facies may contain abundant breccias of slope-derived clasts. Rimmed shelves have relatively flat tops, and marked break in slopes at the high energy, shallow-shelf edge where they pass into deep water. Such shelves may be aggraded with peritidal facies extending over much of the shelf, or incipiently drowned, depending on magnitude of sea level fluctuations. They may be accretionary, or bypass types that include gullied slope, escarpment, and high-relief erosional forms. Intrashelf basins occur on some shelves, controlling distribution of reservoir and source beds. Isolated platforms are surrounded by deeper water and may be located on rifted continental margins, or on submarine volcanoes. Most have high-relief rimmed margins. Platforms that have been subjected to rapid sea level rise may be incipiently drowned, and characterized by raised rims, elevated patch or pinnacle reefs, and widespread subwave-base carbonate or fine clastic blankets. Completely drowned shelves develop where the shelf is submerged to subphotic depths, terminating shallow water deposition, and commonly resulting in blanketing of the shelf by deeper water facies. Some margins show extensive down-to-basin faulting that is contemporaneous with carbonate deposition, or associated with thick prograding clastic sequences. The various types of platforms change in response to variations in sedimentation, subsidence or sea level rise, and may form distinctive evolutionary sequences. The relatively few models presented appear to accommodate most geological examples, some of which contain major reservoir facies.

Carbonate Platform Facies Models

It is generally accepted that the formation of kerogens is the result of the so-called depolymerization-recondensation pathway (Tissot and Welte, 1984; Durand, 1980), Thus, naturally occurring macromolecular substances such as polysaccharides and proteins are enzymatically depolymerized to oligo- and monomers, which for the most part are mineralized. However, a small part of them are thought to condense with other substances such as low-molecular-weight lipids in a random way (Fig. 1). During diagenesis, the “geopolymers” thus formed continuously undergo chemical transformations by which they become more and more insoluble and resistant. Kerogens are being formed, which—depending on the nature of the original organic matter contributions—can generate various amounts and sorts of oil under thermal stress.

A Review of Macromolecular Organic Compounds That Comprise Living Organisms and Their Role in Kerogen, Coal, and Petroleum Formation

Sedimentation is a widespread and increasing process on most rocky coasts. The literature on its effects is reviewed and support is found for the general conclusion that sedimen- tation is an important ecological factor for hard bottom organisms. Sediments deeply affect the composition, structure and dynamics of rocky coast assemblages, and increased sediment load as a consequence of anthropogenic activities can be a threat to their diversity and functioning. Sediments that accumulate on rocky substrata are important agents of stress and disturbance. They can cause burial, scour and profound modifications to the characteristics of the bottom surface, and interact with other important physical and biological processes. The effects of sedi- mentation are complex, because they involve both direct outcomes on settlement, recruitment, growth or survival of individual species and indirect outcomes through mediation of competitive and/or predator-prey interactions. Not all species and assemblages are equally affected by sedi- mentation and responses vary over space and time, depending on the characteristics of the depo- sitional environment, life histories of species and the stage of development of individuals and assemblages, and in relation to variable physical factors, including hydrodynamics, light intens- ity and bottom topography. Recent studies have much improved our ability to detect and under- stand the effects of sedimentation on rocky coast assemblages. However, little is still known about the underlying mechanisms. Overall, our present ability to make generalisations and pre- dictions is limited by a paucity of quantitative and experimental research, and by the scant atten- tion devoted to measuring the regime of perturbation by sediments and responses of organisms at relevant spatial and temporal scales. Predicting the magnitude of the effects that different sed- imentation regimes have on rocky coast organisms and the critical levels above which detrimen- tal effects become manifest remains a key issue for the ecology of rocky coasts and a challenge for future studies.

/pdf/the-effects-of-sedimentation-on-rocky-coast-assemblages-3q5jc2s5yj.pdf

The effects of sedimentation on rocky coast assemblages

Mangroves are among the most well described and widely studied wetland communities in the world. The greatest threats to mangrove persistence are deforestation and other anthropogenic disturbances that can compromise habitat stability and resilience to sea-level rise. To persist, mangrove ecosystems must adjust to rising sea level by building vertically or become submerged. Mangroves may directly or indirectly influence soil accretion processes through the production and accumulation of organic matter, as well as the trapping and retention of mineral sediment. In this review, we provide a general overview of research on mangrove elevation dynamics, emphasizing the role of the vegetation in maintaining soil surface elevations (i.e. position of the soil surface in the vertical plane). We summarize the primary ways in which mangroves may influence sediment accretion and vertical land development, for example, through root contributions to soil volume and upward expansion of the soil surface. We also examine how hydrological, geomorphological and climatic processes may interact with plant processes to influence mangrove capacity to keep pace with rising sea level. We draw on a variety of studies to describe the important, and often under-appreciated, role that plants play in shaping the trajectory of an ecosystem undergoing change.

/pdf/how-mangrove-forests-adjust-to-rising-sea-level-wnmuwpbe0t.pdf

How mangrove forests adjust to rising sea level

The composition and microstructure of widespread subtidal biological mats binding sandy carbonate sediments in the Rock Harbour Cays vicinity of Little Bahama Bank were examined in detail; these mats were subjected to in situ flume experiments. The mats consist of various assemblages of green algae, red algae, blue-green algae, diatoms and animal-built sand grain tubes. Green algae, red algae and/or sand grain tubes provide a rigid open network into which grains infiltrate and are trapped. The mucilaginous secretions of both blue-green algae and diatoms in association with the fine filaments of blue-green algae bind the grains to each other and to the mat network. On the basis of composition and microstructure, three basic mat types were recognized: a fibrous, rigid, Cladoph ropsis mat; a thin, gelatinous, Lyngbya mat; a cohesive, aggregated, Schizothrix mat. The erosion by artificial currents of initially undisturbed mats was studied in the field using an underwater flume, and the complex manner in which the mats disintegrated was recorded. The surface sediment from each mat area was then treated with NaOCl to remove the organic matter and erosion tests repeated in a tank in the laboratory with the same apparatus. The natural, in situ, mat-bound sediment could withstand current velocities at least twice as high and, in some cases, as much as five times as high as those that eroded the treated, unbound sediment. The intact mat surface could withstand direct current velocities three to nine times as high as the maximum tidal currents 13 cm/sec) recorded in the mat environment. Each mat type eroded in a characteristic manner and sequence dependent upon the mat composition and microstructure. This breakdown process differed markedly from the erosion behavior of loose sediment. Observations indicate that grain size, sorting, packing, structure and sediment surface morphology are influenced by mat formation. This study demonstrates the need for close consideration of interfacial biological communities when examining depositional and erosional processes at the sediment-water interface, or when making interpretations from the products of these processes in ancient rocks.

The composition, structure and erodability of subtidal mats, Abaco, Bahamas

Many ancient carbonate rocks consist of dolomite interlayered with limestone on a centimeter or millimeter scale. Many such rocks contain cryptalgal laminites or stromatolites. Comparable Recent stromatolitic and flat laminated algal sediments are composed of alternating layers of particulate carbonate sediment and algal mats. The characteristics of algal-rich and sediment-rich laminae are identical to those of the dolomite and calcite laminae, respectively, of ancient forms. On this basis, dolomite laminae in ancient interlaminated sediments are considered equivalent to, and hence derived from, algal-rich lamina in Recent stromatolitic sediments; calcite laminae are derived from sediment-rich laminae. Modern algal sediments of this type, however, are not known to contain thin layers f dolomite. It is suggested that the dolomite layers are secondary. During deposition, magnesium ions are complexed organically in the algal mat layers. The algal sheath material, in which the magnesium is concentrated, is very stable and does not decompose until long after deposition, or even lithification. Only when the organic matter is decomposed is the magnesium released to form dolomite in the micro-environment of the relict algal mat layers. Thus the dolomite, although secondary, conforms to the primary algal mat layers of the sediment. Laboratory experiments support the hypothesis. Sheath material of the major stromatolite-forming blue-green alga Schizothrix calcicola grown in sea water shows a three to four fold increase in Mg/Ca ratio relative to that in the sea water medium. Sufficient magnesium is complexed in a single 2 mm thick algal mat layer to produce a layer of dolomite 1 mm thick. No dolomite was precipitated in the algal mat layers experimentally, but crystallization of 17-20 mole percent-Mg calcite in contact with the sheath material was achieved, showing the influence of the organic layer micro-environment on the composition of the carbonate mineral produced. The actual formation of dolomite may be a long term process, probably achieved by the partial replacement of the carbonate matrix in conta t with the relict algal mat layers, and/or by selective replacement of high-Mg calcite within the algal mat layers. The hypothesis explains why thinly interlayered dolomite is not found abundantly in Recent sediments, and yet why the dolomite layers so faithfully conform to the primary sedimentary structures in ancient rocks. The hypothesis refers specifically to laminated cryptalgal rocks, but should be considered for other problematical types of ancient dolomite where the distribution of dolomite may be related to the original distribution of organic matter in the sediment.

Algal origin of dolomite laminations in stromatolitic limestone

Following on the research presented in AAPG Memoir 13, which focused on environment and Quaternary history of Shark Bay, this publication examines the same area again, but with a strong stratigraphic emphasis running as a common thread through all 7 papers in this volume.

Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia

Subsurface Dolomitization Beneath the Tidal Flats of Central West Andros Island, Bahamas

The central-west coast of Andros Island is a complex carbonate facies mosaic, deposited at or near mean sea level. The shore has prograded intermittently during the past 3,000 years to produce a seaward-thickening wedge of sediments up to 25 km wide and about 4 m thick beneath the present shoreline. Old channel levees and beach ridges have been preserved during progradation as topographically high ridges which allow the development of freshwater lenses beneath. These lenses show considerable seasonal variation, both in geometry and pore-water chemistry. Mixing zones, extending laterally a few hundred meters around the lenses and down to Pleistocene bedrock, separate the fresh waters of the lenses (with high alkalinity and low ionic concentration), from adjacent saline and hypersaline waters (with low alkalinity and high ionic concentration). Fresh waters are generally undersaturated with aragonite, calcite, and dolomite, but calcite saturation may occur locally. The ends of aragonite needles, skeletal grains, and pellets are commonly corroded, although low-magnesian calcite crystals may locally enclose and replace aragonite by dissolution-reprecipitation. Water in the mixing zone, between 2,500 and 15,000 ppm Cl-, is undersaturated with respect to aragonite and low-magnesian calcite, but supersaturated with respect to calcium magnesium carbonates. X-ray diffraction (XRD) studies indicate small amounts of (< 6%) of protodolomite (38 to 44 mole % of MgCO3) distributed in patches across whichever sedimentary facies are intersected by the mixing zone. The mixing-zone origin of the protodolomite is however equivocal, as similar compositions occur more rarely with saline pore waters not associated with present or past mixing zones. SEM reveals 1µm euhedral rhombic protodolomite crystals between and engulfing aragonite needles. The needles may later dissolve. Chemical data from mixing zones indicate precipitation of a magnesium-rich phase frequently in excess of observed protodolomite concentrations; a huntite phase is indicated by considerations of stability, but is unsupported by XRD results. End_of_Article - Last_Page 457------------

/pdf/mixing-zone-dolomite-in-tidal-flat-sediments-of-central-west-3xvlwoj4yh.pdf

Conrad D. Gebelein

Papers

The composition, structure and erodability of subtidal mats, Abaco, Bahamas

Algal origin of dolomite laminations in stromatolitic limestone

Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia

Subsurface Dolomitization Beneath the Tidal Flats of Central West Andros Island, Bahamas

Mixing Zone Dolomite in Tidal-Flat Sediments of Central-West Andros Island, Bahamas: ABSTRACT