Lacustrine Dolomite Research Articles

Lacustrine carbonates in the southwest margin of the Qikou Sag, Huanghua Depression, Bohai Bay Basin

Abstract The southwest margin of the Qikou Sag is a flexural gentle slope formed by the deformation of a drape structure due to its underlying basement lifting. The sedimentary period of the Lower Member 1 of Shahejie Formation is the maximum extensional time of the Paleogene lacustrine basin. Debris from source areas is in shortage. In the broad littoral shallow lake, hundreds of square kilometers of lacustrine carbonates are deposited and distributed stably. The main lithologies of the sediments are biolithite and dolomite (micritic dolomite and dolomitic limestone), and the dolomites are mainly middle late buried dolomite and penecontemporaneous dolomite. Their distribution area is more than 1 650 m 2 , the cumulative thickness is between 5 and 25 meters, and the average thickness is about 10 meters. Intercrystal pores, intergranular pores, corrosion pores and frequent fissures are developed in the medium porosity and low permeability dolomite reservoirs. The dolomites alternate with oil shale and dark mudstone, favorable for the charge of low mature hydrocarbons and the formation of large scale accumulations. Drilling exploration practices show that the lacustrine dolomite reservoirs are characterized by medium burial depth and regional petroleum accumulation and are a new exploration area for Paleogene oil and gas.

Characteristics of Cenozoic lacustrine dolomite in central Tibetan Plateau and its environmental implications

青藏高原中部新生代湖泊发育普遍.对该时期形成的湖相白云岩特征的研究对于了解当时及此后青藏高原的环境演化进程具有重要意义.笔者主要利用XRD、SEM及薄片观察方法对沱沱河地区两个剖面的白云石特征进行了研究,分析了白云石有序度、晶形及白云石中碳酸钙摩尔浓度三者之间的关系,并根据白云石的矿物学和岩石学特征以及野外宏观岩层组合特征对白云岩的成因作出了判断,进而联系同一时期高原内部以及周边地区的构造运动导致的环境变化,对剖面所代表地区沉积时的环境变化作了解释.;Lakes developed widely during Cenozoic era in central Tibetan Plateau. Researches on those lacustrine dolomite formed in that period is highly significaive to analyze the evolutionary process of Tibetan Plateau environment from then on. XRD, SEM and thin section analytical approaches were applied to analyze the relationship among Degree Order, crystal shape and the CaCO3 molal fraction of the dolomite samples took from Tuotuohe area. Based on the mineralogy and petrology features, investigation depends on the field work on the outcrops, and the origin mode of dolomite are diagnosed. Furthermore, the environmental changes recorded within the sedimentary sections are interpreted in connection with some tectonic movements which led environmental changes internal or external the Plateau.

Petrographic and geochemical evidence for the formation of primary, bacterially induced lacustrine dolomite: La Roda ‘white earth’ (Pliocene, central Spain)

Upper Pliocene dolomites (‘white earth’) from La Roda, Spain, offer a good opportunity to evaluate the process of dolomite formation in lakes. The relatively young nature of the deposits could allow a link between dolomites precipitated in modern lake systems and those present in older lacustrine formations. The La Roda Mg‐carbonates (dolomite unit) occur as a 3·5‐ to 4‐m‐thick package of poorly indurated, white, massive dolomite beds with interbedded thin deposits of porous carbonate displaying root and desiccation traces as well as local lenticular gypsum moulds. The massive dolomite beds consist mainly of loosely packed 1‐ to 2‐μm‐sized aggregates of dolomite crystals exhibiting poorly developed faces, which usually results in a subrounded morphology of the crystals. Minute rhombs of dolomite are sparse within the aggregates. Both knobbly textures and clumps of spherical bodies covering the crystal surfaces indicate that bacteria were involved in the formation of the dolomites. In addition, aggregates of euhedral dolomite crystals are usually present in some more clayey (sepiolite) interbeds. The thin porous carbonate (mostly dolomite) beds exhibit both euhedral and subrounded, bacterially induced dolomite crystals. The carbonate is mainly Ca‐dolomite (51–54 mol% CaCO3), showing a low degree of ordering (degree of ordering ranges from 0·27 to 0·48). Calcite is present as a subordinate mineral in some samples. Sr, Mn and Fe contents show very low correlation coefficients with Mg/Ca ratios, whereas SiO2 and K contents are highly correlated. δ18O‐ and δ13C‐values in dolomites range from −3·07‰ to 5·40‰ PDB (mean=0·06, σ=1·75) and from −6·34‰ to −0·39‰ PDB (mean=−3·55, σ=1·33) respectively. Samples containing significant amounts of both dolomite and calcite do not in general show significant enrichment or depletion in 18O and 13C between the two minerals. The correlation coefficient between δ18O and δ13C for dolomite is extremely low and negative (r=−0·05), whereas it is higher and positive (r=0·47) for calcite. The lacustrine dolomite deposit from La Roda is interpreted mainly as a result of primary precipitation of dolomite in a shallow, hydrologically closed perennial lake. The lake was supplied by highly saturated HCO3−/CO32− groundwater that leached dolomitic Mesozoic formations. Precipitation of dolomite from alkaline lake waters took place under a semi‐arid to arid climate. However, according to our isotopic data, strong evaporative conditions were not required for the formation of the La Roda dolomite. A significant contribution by bacteria to the formation of the dolomites is assumed in view of both petrographic and geochemical evidence.

Diagenetic transformation of dolomite into stevensite in lacustrine sediments from Jbel Rhassoul, Morocco

ABSTRACT Stevensite in the Tertiary lacustrine basin of the Jbel Rhassoul in Morocco is used as fuller's earth. Observations of the structure and textures of the deposits provide evidence of diagenetic replacement of sedimentary lacustrine dolomite by stevensite. Carbon and oxygen isotopic compositions confirm the precipitation of primary sedimentary dolomite and the diagenetic formation of secondary stevensite. According to thermodynamic calculations, stevensite is stable at the expense of dolomite in a silica-rich and CO2-poor environment. Under such conditions, destabilized detrital illite, chlorite, and palygorskite release Al, Fe, Li, and trace elements, which are incorporated into the structure of stevensite. Compared with other lacustrine stevensite occurrences, the Jbel Rhassoul deposit seems to correspond to a new mode for the formation of diagenetic stevensite after dolomite.

Dedolomites associated with karstification. An example of early dedolomitization in lacustrine sequences from the Tertiary Madrid basin, central Spain

A variety of calcite fabrics formed by dedolomitization of middle to upper Miocene continental sediments of the Madrid basin, central Spain. The dedolomites are associated with other carbonates that show abundant diagenetic features. The diagenetic carbonates (Diagenetic Carbonate Zone, DCZ) overlies and grades laterally into lacustrine dolomite (Lower Dolomite Unit, LDU) and evaporites (Evaporitic Unit, EU), and in turn is capped by a paleokarst surface.

Lacustrine dolomite—an overview of modern, Holocene, and Pleistocene occurrences

Dolomite formation and dolomitization in sedimentary environments are subjects of long-standing scientific interest. Lacustrine sediments occupy a unique position in the realm of dolomitization studies. Although lake sediments have formed the basis for a number of landmark investigations of dolomite generation, in general, lacustrine dolomites have been poorly documented. There are over 40 reported occurrences of non-detrital lacustrine dolomite of Quaternary age. Most are from either North America or Australia. Although the lacustrine environments in which these dolomites are found span a wide range of geochemical and hydrological conditions, nearly all are from saline or hypersaline lakes. About half of the occurrences are from modern settings; only 30% have been reported from Pleistocene sediments. In contrast to Holocene dolomites found in marine environments, most of the lacustrine CaMg(CO 3) 2is interpreted to be primary in origin. Few studies have adequately documented secondary (i.e., replacement) dolomitization in lake sediments.

Nearshore Marine and Continental Facies in Eocene of North-Central Pakistan: ABSTRACT

Stable-shelf carbonate sedimentation along the northwestern edge of the Indian subcontinent preceded the post-mid-Eocene Indian-Asian collision. The early Eocene section in Pakistan displays very rapid facies changes controlled by a cycle of regression and transgression. Beginning with the Paleocene Patala Formation in the Kohat area, quiet-water offshore dark shales grade up-section to include increasingly thick, extensive, and common marly limestones, becoming a foraminiferal limestone sequence. This section then becomes shalier with progressively thinner and more argillaceous micrites and grades upward into the unfossiliferous green Panoba Shale, which then passes into the nearshore, medium-energy, fossiliferous, and bioturbated limestones of the Shekhan Formation. The upper Shekhan beds are mud-cracked, festoon-bedded, channel-form dolostones, presumably tidal deposits. The topmost dolostones contain zones of small disruptive anhydrite nodules and pass rapidly into gypsum laminated with varicolored clays. To the west, the Panoba Shale and the Shekhan Formation grade into a massive salt deposit; to the northeast they grade into deeper water limestone. The evaporites are interpreted as sabkha deposits. All are covered by the mostly continental, mammal-bearing Kuldana Formation red beds. Drowning of the coastline then caused rapid development of (1) local lacustrine dolomite and chert units, (2) oyster-rich lagoonal or estuarine limestones, and finally (3) open-bay nummulitic limestones and shales of the mid-Eocene Kohat Formation. The succession is trun ated by a regional unconformity that records uplift, erosion, and dolomitization of the underlying carbonate rocks. The unconformity is then buried by continental Himalayan molasse. End_of_Article - Last_Page 802------------

Geological significance of Coorong dolomites

ABSTRACTMicrocrystalline dolomite and related carbonate minerals have been forming throughout the Quaternary in shallow ephemeral alkaline lakes on the coastal plain of the Coorong area in southern Australia. These Coorong dolomites differ significantly from sabkha‐type dolomites. They form in areas where evaporation rates during summer months exceed groundwater inflow rates to a series of alkaline lakes. This results in the lakes becoming desiccated during summer months. Brines resulting from this drying phase are then refluxed out of the system into seaward‐flowing groundwaters of an unconfined coastal aquifer. Dolomites and other fine‐grained carbonates remain behind, whilst saline and sulphate evaporite minerals are flushed out of the system. Progressive restriction by sedimentation in and around the Holocene coastal dolomite lakes results in an upward‐shoaling sedimentary cycle. Basal sediments which formed in a restricted marine environment pass upwards to lacustrine dolomites or magnesites exhibiting desiccation and groundwater resurgence structures such as mudcracks and teepees. The upper Proterozoic Skillogallee Dolomite Formation, an early rift basin unit of the Adelaide Supergroup, contains dolomites which show many of the features characteristic of the peculiar groundwater hydrology which plays an important role in Coorong dolomite genesis. These features include aphanitic dolomites which lack relict saline or sulphate evaporite minerals. The Skillogallee Dolomite Formation in some areas overlies an earlier dolomitic unit, informally named the Callanna Beds, typified by abundant pseudomorphs after sulphate minerals. Sabkha style dolomites characterizing the Callanna Beds are replaced up‐section by the Coorong‐type dolomite of the Skillogallee Dolomite Formation. This implies the development of an increasingly more active groundwater regime. The ultimate source and mode of concentration of the necessary Mg required to form both the modern and ancient dolomites remain imperfectly understood.

Stratigraphy and Formation of Holocene Dolomitic Carbonate Deposits of the Coorong Area, South Australia

ABSTRACT Geologically significant and spatially extensive deposits of primary lacustrine dolomite are found over a 90 200 km region of the coastal plain in the vicinity of the Coorong Lagoon, in southeastern South Australia. The microcrystalline dolomites and associated cabronates, which range in age from Pleistocene to Recent, occur beneath interdune flats between stranded barriers. The most active but by no means only region of present-day microcrystalline dolomite formation is the linear zone of shallow. ephemeral alkaline lakes which occupies the regional groundwater discharge area, immediately landward of the Coorong Lagoon and along the southeastern extension of its interdune corridor. Comparative studies between sediment cores from th s region and a range of contemporary sedimentary environments show that it is possible to characterize the sedimentary sequence which is being generated by Holocene sedimentary regression. In its ideal form this sequence passes upward from a basal marine or lagoonal unit, through a protodolomite + Mg-calcite lacustrine unit, to culminate in an uppermost dolomite or dolomite + magnesite unit which may be capped by a soil. Hydrological considerations, which imply the presence of a landward-dipping continental groundwater-marine groundwater interface immediately inland from the coast, show the above sedimentary sequence to be the result of a progressive seaward movement of that interface, caused by sediment accretion and consequent shoreline regression. The position of a particular microcry talline carbonate-forming environment at any point in time, with respect to the interface, defines its carbonate mineralogy. These observations clearly indicate that optimal conditions for Coorong-type dolomite formation are found in ephemeral lakes which are fed solely by evaporitically-modified continental groundwater and which occur inland from the interface. The relatively large amounts of Quaternary primary dolomite in the area render it somewhat of a geological anomaly in terms of present-day dolomite-forming environments. This study implies that the main reasons for this anomaly may be a favourable combination of factors which include an abundant, renewable source of magnesium for the seaward-flowing groundwaters, an active grounwater regime, and a climate which seasonally varies rom humid to semiarid. The most likely source of magnesium is hypothesized to be from weathering of areally extensive Quaternary basic volcanics which occur in groundwater source areas marginal to the coastal plain. From a geological viewpoint it is suggested that many microcrystalline dolomite occurrences in the geological record may be of the Coorong-type, formed in shallow migrating ephemeral lakes in a relatively humid climate on regressive coastal plains, overlying beach deposits. Such dolomites, which contain no obvious traces of evaporite minerals, are contrasted with superficially similar sabkha-type dolomites which, at least in part, are early diagenetic replacement dolomites and which contain abundant evaporitic associates such as gypsum and anhydrite.

Petrography and paleo-environment of continental carbonate deposits with particular reference to the upper cretaceous and lower eocene of languedoc (Southern France)

The carbonates studied include several types 1. (1) Lacustrine dolomites which have resulted from the concentration of products leached from neighbouring Paleozoic and/or Mesozoic rocks. Certain well-crystallised limestones contain phantoms of rhombohedra, and are the result of dedolomitisation. 2. (2) Limestones deposited in fluviatile channels, or at the outflow of rivers into lakes, which are always sandy (with corroded quartz) and often contain calcitic intraclasts, bioclasts and oncolites. (Cyanophyceae, Rivulariaceae.) 3. (3) True lacustrine limestones, with an entirely subaqueous diagenetic evolution, which are homogenous, or show evidence of reworking: breccias, gravels, coated gravels. Burrowing animals have left traces of their activity (in particular striotubules ∗) in sediments deposited on the bottom lakes. 4. (4) “Paludine” limestones which have resulted from emersion of lacustrine muds, have been affected by soil processes and show various signs of reworking: penetration of roots, burrows, nodulisation ∗, marmorisation, etc. All these phenomena occur together giving facies and microfacies which may be very complicated. 5. (5) Silicification, which may occur in all continental limestones, including lacustrine or paludine types. It exhibits various morphological aspects; on a microscopic scale, it is an accumulation of opal and/or thin veins of chalcedony. These different facies are not distributed haphazardly, but constitute sequences which are repeated, with some variations, in stratigraphical series. Lataral variation in the style of vertical sequence in a basin enables the reconstitution of the paleo-environment at a given period.

Oxygen and carbon isotopic composition of West Texas lake carbonates

The range in δO 18 (relative to PDB) for fourteen lacustrine dolomites is + 5.8 to − 3.4 per mil and the range in δC 13 is + 5.8 to − 3.1 per mil. The range in δO 18 for nine lacustrine calcites is − 5.6 to − 3.7 per mil and the range in δC 13 is − 6.1 to − 0.9 per mil. Carbon 14 apparent ages range from 12,200 yr B.P. to over 37,000 yr B.P.