Xenotopic Dolomite Research Articles

Epigenetic dolomitization of the Přaídolí formation (Upper Silurian), the Barrandian basin, Czech Republic: implications for burial history of Lower Paleozoic strata

Stratabound epigenetic dolomite occurs in carbonate facies of the Barrandian basin (Silurian and Devonian), Czech Republic. The most intense dolomitization is developed in bioclastic calcarenites within the transition between micritic limestone and shaledominated Přidoli and Lochkov formations deposited on a carbonate slope. Medium-crystalline (100–400 µm), inclusion-rich, xenotopic matrix dolomite (δ18O=−4.64 to −3.40‰ PDB;δ13C=+1.05 to +1.85‰ PDB) which selectively replaced most of the bioclastic precursor is volumetrically the most important dolomite type. Coarse crystalline saddle dolomite (δ18O=−8.04 to −5.14‰ PDB;δ18C=+0.49 to +1.49 PDB) which precipitated in fractures and vugs within the matrix dolomite represents a later diagenetic dolomitization event. In some vugs, saddle dolomite coprecipitated with petroleum inclusion-rich authigenic quartz crystals and minor sulfides which, in turn, were post-dated by semisolid asphaltic bitumen. The interpretation of the dolomitization remains equivocal. Massive xenotopic dolomite, although generally characteristic of a deeper burial setting, may have been formed by a recrystallization of an earlier, possibly shallow burial dolomite. Deeper burial recrystallization by reactive basinal pore fluids that presumably migrated through the more permeable upper portion of the Přidoli sequence appears as a viable explanation for this dolomitization overprint. Saddle dolomite cement of the matrix dolomite is interpreted as the last dolomitization event that occurred during deep burial at the depth of the oil window zone. The presence of saddle dolomite, the fluid inclusion composition of associated quartz crystals, and vitrinite paleogeothermometry of adjacent sediments imply diagenetic burial temperatures as high as 160°C. Although high geothermal gradients in the past or the involvement of hydrothermally influenced basinal fluids can account for these elevated temperatures, burial heating beneath approximately 3-km-thick sedimentary overburden of presumably post-Givetian strata, no longer preserved in the basin, appears to be the most likely interpretation. This interpretaion may imply that the magnitude of post-Variscan erosion in the Barrandian area was substantially greater than previously thought.

Deep-Burial Dolomitization in the Ordovician Ellenburger Group Carbonates, West Texas and Southeastern New Mexico

ABSTRACT From western Texas and southeastern New Mexico, core samples of shallow subtidal and peritidal carbonates in the Ellenburger Group (Lower Ordovician) were examined to investigate the effects of deep burial on diagenesis. At present, the burial depth of samples ranges from 6,000 ft. (1.5 km) to 23,000 ft. (7.0 km). Below 10,000 ft. (3.0 km) carbonates are exclusively dolostones, showing evidence of deep-burial dolomitization such as the presence of coarse crystalline dolomite, xenotopic texture, homogeneous cathodoluminescence, high fluid-inclusion homogenization temperatures, and light oxygen-isotope compositions. Locally, neomorphism of older, fine crystalline dolomite occurs; however, the preservation of depositional textures in coarse crystalline, xenotopic dolomite indicates a nonobliterative replacement origin in deep-burial environments. Millimeter-size crystals of euhedral saddle dolomite commonly fill vugs and fractures. The Mg+2 ions in the nonferrous dolomites are probably derived from basinal brines via high porosities and fractures. Higher temperatures and longer reaction times coupled with the addition of Mg+2 ions from circulating waters are apparently responsible for deep-burial dolomitization.

Epigenetic dolomitization and the origin of xenotopic dolomite texture; discussion and reply

Epigenetic dolomitization and the origin of xenotopic dolomite texture; discussion and reply

Epigenetic Dolomitization and the Origin of Xenotopic Dolomite Texture: REPLY

Epigenetic Dolomitization and the Origin of Xenotopic Dolomite Texture: REPLY

Reservoir Development in Ellenburger Group of West Texas--a Diagenetic Jambalaya: ABSTRACT

Porosity and permeability in the Ellenburger Group of west Texas result from a complex interaction of early to late diagenetic processes. Porosity formation occurred in at least seven stages: (1) early marine phreatic calcite cementation, (2) fabric-selective (mixing-zone?) dolomitization, which created intercrystalline porosity, (3) episodic subaerial exposure and karstification, which created vuggy, cavern, solution-channel, moldic, fenestral, breccia, and fracture porosity, (4) mineralogy-selective meteoric phreatic or mixing-zone silicification, which preserved existing porosity by preventing further carbonate cementation, (5) deep-burial xenotopic dolomite recrystallization, which destroyed nearly all of the precursor intercrystalline porosity, (6) deep-burial dolomi e, calcite, and anhydrite cementation of some vugs and fractures, and (7) late-stage tectonic fracturing, which created most of the reservoir permeability. The Ellenburger Group consists of numerous vertically stacked subaerial exposure cycles 1-20 ft thick. Porosity within each cycle is laterally discontinuous and patchy. The complete cycle is composed of four zones (from top to bottom): (1) glauconitic shale, which is interpreted to be a paleosoil horizon, (2) brecciated dolomite, cherty dolomite, or chert, which formed from solution collapse, (3) nonbrecciated dolomite containing abundant dissolution-generated porosity, and (4) nonporous dolomite, which was largely unaffected by karstification. Zones 1 and 4 are nonporous with very low permeability; zones 2 and 3 have high porosity and permeability. The presence of subaerial exposure cycles throughout the Ellenburger Group has resulted in numerous vertical permeability barriers, which may be the cause of reservoir stratification in some fields. End_of_Article - Last_Page 312------------

Epigenetic Dolomitization and the Origin of Xenotopic Dolomite Texture

ABSTRACT Xenotopic texture, which is commonly observed in pre-Cenozoic rocks, is defined here as a mosaic of anhedral crystals with irregular or curved intercrystalline boundaries and, usually, undulatory extinction. Xenotopic dolomite texture is similar in appearance to neomorphic limestone textures. Idiotopic dolomite texture (euhedral to subhedral crystals with straight, intercrystalline boundaries) contrasts with xenotopic texture and is commonly observed in both Cenozoic and more ancient dolomites. Texture may be controlled by the at which crystals grow. Crystal growth theory predicts that at low a smooth crystal surface is energetically favored, and atoms are added to crystal faces layer by layer with dislocations acting as nucleation sites. This results in faceted crystals and euhedral to subhedral crystal mosaics. Above a critical roughening temperature (CRT), a rough surface is energetically favored, surface nucleation does not require dislocations, and atoms are randomly added to the crystal surface, resulting in nonfaceted growth and an anhedral crystal mosaic. It is hypothesized that a critical roughening temperature exists for dolomite above 50°C. Xenotopic dolomites are produced by dolomitization of limestone and/or neomorphic recrystallization of dolomite at elevated (above the CRT) after burial. Idiotopic dolomites are produced below the CRT by near-surface processes and are occasionally produced above CRT if growth-inhibiting effects of the solution or impurities stabilize crystal faces. Calcite has a CRT below 25°C and, therefore, develops anhedral crystal mosaics (characteristic of neomorphic texture) both at near-surface and at elevated temperatures. Synthetic xenotopic dolomite was produced in the laboratory at 250°C and 300°C by dolomitization of aragonite and calcite skeletal fragments and by recrystallization (neomorphism) of nonstoichiometric Cenozoic dolomites. Xenotopic dolomite resulted from the metamorphic recrystallization of the idiotopic Hueco dolomite (Permian, Texas) near the Marble Canyon intrusion, at temperatures between 350°C and 600°C. Hydrothermal dolomitization of periclase-calcite marble near the intrusion also resulted in a xenotopic texture. Xenotopic dolomite in the Galena Group (Ordovician, Wisconsin) was produced by neomorphism of a preexisting dolomite during the emplacement of lead-zinc sulfides at temperatures between 80°C and 227°C. In the Trenton Limestone (Ordovician, Michig n), xenotopic dolomite replaced limestone during the migration of hot (> 50°C) fluids along fracture systems. Xenotopic dolomite was not observed in Cenozoic dolomites that were subjected to temperatures near surface values only.

Dolomite Selectivity, An Experimental Approach: ABSTRACT

Hydrothermal dolomitization of corals, gastropods, pelecypods, echinoderms, forams, and coralline algae indicates that grain size is more important than mineralogy in determining (1) whether or not a fossil will be dolomitized and (2) whether or not the dolomite will pseudomorphically replace a fossil. Dolomite commonly selectively replaces matrix and/or specific fossils. When dolomite replaces fossils, certain fossils retain their optical characteristics (i.e., pseudomorphic replacement). These selective characteristics have been attributed to both grain size and mineralogy and have been used to make inferences about the predolomitization diagenetic history of sediments. End_Page 433------------------------------ Fossils were dolomitized at 250°C (482°F) in Ca/MgCl2 solutions for periods of time from 4.5 to 320 hours. Aragonitic corals, gastropods, and pelecypods formed stoichiometric, microcrystalline, xenotopic dolomite and low Mg-calcite (LMC). The dolomite was not pseudomorphic after the aragonite. The conversion of AR ^rarr LMC is more rapid than the AR ^rarr DOL in these experiments. For instance, gastropods run for 23 hours formed dolomite and LMC in a ratio of 1/10, at 170 hours the ratio was 1/4, and at 340 hours the ratio was 1/1. HMC coralline algae, forams, and echinoderm fragments were dolomitized before and after conversion to LMC. The dolomite formed was cryptocrystalline and pseudomorphic after the forams and echinoderms regardless of the mineralogy. We attributed this to the cryptocrystalline nature of the substrate. Oyster fragments composed of microcrystalline LMC formed non-stoichiometric, poorly ordered dolomite even after 320 hours. None of the other reactants were as resistant to dolomitization. Our results indicate that grain size is more important than mineralogy in determining the fabric of dolomite replacement crystals. Both HMC and LMC can be pseudomorphically replaced. Pseudormophic replacement requires (1) abundant nucleation sites and (2) a regular crystallographic relationship between the calcite and dolomite. Argonite was not pseudomorphically replaced, probably because it was microcrystalline rather than cryptocrystalline. Also, most of the aragonite converted to LMC prior to dolomitization. Selective replacement characteristics of many natural dolomites are readily explained as being an effect of the grain size of the material replaced. Freshwater diagenesis of a sediment prior to dolomitization may retard dolomitization if the grain size of the CaCO3 is increased. However, conversion of LMC without appreciable increase in grain size may not retard dolomitization. End_of_Article - Last_Page 434------------

Dolomitization and dedolomitization in the Senonian chalk of Northern Ireland

SummaryA dolomite locality in the Senonian chalk at Mullaghcarton shows a sequence of fabric variations between the unconformity with the Trias and a discontinuity surface marking the top of the basal beds. The four main variations are (1) xenotopic dolomite at the base, (2) hypidiotopic dolomite with zoned crystals and a calcite sealed intercrystalline porosity, (3) zoned dolomite rhombs scattered in a microspar micrite matrix, and at the top (4) micrite with rhombohedral pores. The xenotopic fabric is interpreted as being due to distant source dolomitization and the hypidiotopic fabric with zoned crystals to local source dolomitization. The dedolomitization phenomena are related to the mechanisms of dolomitization; for distant source dolomitization the dedolomitization fabric mimics the dolomite fabric, whereas for local source dolomitization the dedolomite regenerates the pre-dolomitization fabric.

Petrology and Structure of a Salina (Silurian) Dolomitized Algal Stromatolite Complex, Northwestern Ohio: ABSTRACT

A unique algal stromatolite complex is located southwest of Maumee, Ohio, on the Niagaran reef-bank system that contributed to the isolation of the Michigan basin during the Cayugan Epoch. Overlying strata have been eroded so that the individual features of the complex may be studied in essentially their original paleogeographic setting. Major features in the study area include tidal channels, natural carbonate levees, algal mounds, scour channels, laminations, ripple marks, desiccation cracks, and gypsum and anhydrite molds. The dolomite rocks in the area may be subdivided into eight microfacies as follows: (1) mound; (2) End_Page 534------------------------------ mound scour channel; (3) mound transition; (4) intermound; (5) blanket; (6) laterally linked hemispheroid; (7) mud flat; and (8) tidal channel. Each of these microfacies is characterized by a particular algal mat frequency, the degree of development and the nature of laminations, and by the amount of mechanically deposited sediment. Major rock types are stromatolite-constructed dolomite mudstone, wackestone, and packstone. Nearly all samples are characterized by the partial or pervasive development of a filamentous (and presumably algal) microstructure. Burrows and pellets are uncommon in all of the microfacies. Ostracods are common. Laminations are well developed in all of the microfacies except the mound microfacies. Breccias are common especially in algal mounds and are due to mineralization by sulfate minerals, and dedolomitization, desiccation, and solution collapse. The general sequence of mineralization was: (1) formation of microcrystalline xenotopic dolomite, probably by replacement of an initial CaCO3 mud, accompanied by the formation of gypsum and anhydrite; (2) dedolomitization, primarily in the mound microfacies; (3) partial replacement of calcite and dolomite by celestite; (4) replacement of some calcite, dolomite, and celestite by fluorite; and (5) limited formation of medium crystalline, idiotopic, and hypidiotopic dolomite by a process of local source dolomite crystallization and recrystallization. Comparison with several Recent carbonate analogs in the Persian Gulf and Bahamas suggests that most of the eight microfacies were formed in a supratidal environment. End_of_Article - Last_Page 535------------

Terminology of Crystallization Textures and Fabrics in Sedimentary Rocks

ABSTRACT A need exists for concise terminology to describe crystallization textures and fabrics in sedimentary rocks. The rocks which display these textures and fabrics are (a) limestones and dolomites in which the depositional textures and fabrics have been obscured or obliterated, (b) sediments which have formed by crystallization in their environment of deposition, such as evaporites, and (c) chemically deposited cements. To date, no textural and fabric terms have been proposed specifically for these rocks and terms in use have been borrowed from igneous and metamorphic petrology with resultant confusion and inconsistencies. Moreover, in part, confusion is compounded by the chaotic tradition of igneous petrological nomenclature. In this paper, the history and background of var ous terms presently in use have been traced, and a tentative classification is offered to serve as a framework for more extensive textural and fabric analyses. For the size and mutual relations of crystals (crystallization fabrics), the terms xenotopic, hypidiotopic, and idiotopic describe both equigranular and inequigranular fabrics, and the terms porphyrotopic and poikilotopic describe inequigranular fabrics. For the shape of mineral crystals (crystallization textures), the igneous terms of the C.I.P.W. system (anhedral, subhedral, euhedral), which are already widely used by North American carbonate petrologists, are recommended for more extensive usage. Idiotopic dolomite commonly has porosity and makes a good reservoir rock for oil and gas. Hypidiotopic dolomite may or may not be porous, and xenotopic dolomite normally has little or no porosity.