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1.
A sloping travertine mound, approximately 85 m across and a few metres thick is actively forming from cool temperature waters issuing out of Crystal Geyser, east‐central Utah, USA. Older travertine deposits exist at the site, the waters having used the Little Grand Wash Fault system as conduits. In contrast, the present Crystal Geyser travertine mound forms from 18°C waters which have been erupting for the last 80 years from an abandoned oil well. The present Crystal Geyser travertine accumulation forms from a ‘man‐made’ cool temperature geyser system; nevertheless, the constituents are an analogue for ancient geyser‐fed carbonate deposits. The travertine primary fabric is composed of couplets of highly porous, thin micritic laminae intercalated with thicker iron oxide rich laminae. Low Mg‐calcite is the dominant mineralogy; however, aragonite is a major constituent in deposits proximal to the vent and decreases in abundance distally. Cements exhibit a variety of fabrics, isopachous being common. Constituents include micro‐stromatolites, clasts, pisoids and the common occurrence of Frutexites‐like iron oxide precipitates. Leptothrix, a common iron‐oxidizing bacterium, is believed to be responsible for the production of the dense iron‐rich laminae. Pisoids litter the ground around the vent and rapidly decrease distally in abundance and size.  相似文献   

2.
Microbial dolomite crusts from the carbonate platform off western India   总被引：1，自引：1，他引：1  
Abstract The occurrence of Late Pleistocene dolomite crusts that occur at 64 m depth on the carbonate platform off western India is documented. Dolomite is the most predominant mineral in the crusts. In thin section, the crust consists of dolomitized microlaminae interspersed with detrital particles. Under scanning electron microscopy, these laminae are made up of tubular filaments or cellular structures of probable cyanobacterial origin. Dolomite crystals encrust or overgrow the surfaces of the microbial filaments and/or cells; progressive mineralization obliterates their morphology. Well-preserved microbial mats, sulphide minerals (pyrrhotite and marcasite) and the stable isotope composition of dolomite in the crusts indicate hypersaline and anoxic conditions during dolomite formation. The crusts are similar to dolomite stromatolites, and biogeochemical processes related to decaying microbial mats under anoxic conditions probably played an important role in dolomite precipitation. The dolomite is therefore primary and/or very early diagenetic in origin. The dolomite crusts are interpreted to be a composite of microbial dolomite overprinted by early burial organic dolomite. The results of this study suggest that a microbial model for dolomite formation may be relevant for the origin of ancient massive dolomites in marine successions characterized by cryptalgal laminites. The age of the crusts further suggests that the platform was situated at shallow subtidal depths during the Last Glacial Maximum.  相似文献   

3.
The combination patterns and depositional characteristics of the carbonate banks are investigated based on outcrop sections, thin sections, and carbon isotopes of Ordovician in the western Tarim Basin, China. Four carbonate bank combination patterns are deposited in the Ordovician, western Tarim Basin, including: Reef-Bank Complex (RBC), Algae-Reef-Bank Interbed (ARBI), Thick-Layer Cake Aggradation Bank (TLCAB), and Thin-Layer Cake Retrogradation Bank (TLCRB). All combination patterns show clear periods vertically. The RBC is mainly composed of reefs and bioclastic banks, and the dimension of the RBC depends on the scale of the reefs. Bioclastic banks deposits surround the reefs. The range of the ARBI is determined by the scale of algae-reefs, algae peloid dolomite microfacies and algal dolomite microfacies deposit alternating vertically. TLCAB and TLCRB are deposited as layer-cakes stacking in cycles and extending widely with cross bedding developed. The grains of TLCAB and TLCRB are diverse and multi-source. With the impacting of relative sea level change, biological development and geomorphology, the ARBI, TLCAB or TLCRB, RBC are successively developed from the Lower Ordovician Penglaiba Formation to the Middle Ordovician Yijianfang Formation. The depositional environment analysis of Ordovician indicates that the RBC and ARBI can form effective oil and gas reservoirs, and the TLCAB and TLCRB have the potential to form the huge scale oil and gas reservoirs and to be the crucial targets of exploration for the Ordovician carbonate banks in the future.  相似文献   

4.
The origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ18O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite.  相似文献   

5.
We report here new field and analytical data from Precambrian rocks on Hainan Island of the Cathaysia Block, south China, and examine its probable connection to Laurentia. Granitoids and newly discovered felsic volcanic rocks dated at 1433 ± 6 Ma and 1439 ± 9 Ma (SHRIMP U‐Pb zircon) on Hainan Island are coeval with, and isotopically similar to the 1500–1350 Ma trans‐continental granite‐rhyolite province in southern Laurentia. Quartzites unconformably overlying the ca. 1430 Ma volcaniclastic rocks on Hainan Island are interpreted as locally‐sourced Grenvillian foreland basin deposits that can be correlated with the Deer Trail Group of south‐western Laurentia. The detrital provenance of the quartzites contains age populations comparable to the 1610–1490 Ma, westerly‐sourced non‐Laurentian detrital grains reported in the Belt Basin of south‐western Laurentia. Our new data thus make Cathaysia the most likely continental block next to western Laurentia before and during the late Mesoproterozoic assembly of Rodinia.  相似文献   

6.
In south Cumbria, Permo-Triassic breccias and conglomerates (‘brockram’) are exposed only at Rougholme Point on the Cartmel peninsula. In 1973 the Institute of Geological Sciences Humphrey Head borehole penetrated 257 m of brockram before entering probable Upper Carboniferous sediments. The brockram consists of pebbles of carbonate, chert and basalt in a matrix of haematite-stained quartz sand. Carbonate and chert fragments were derived from the upper part of the Carboniferous Limestone sequence exposed today nearby. Basalt clasts were derived from lavas, which appear to have cooled in a subaqueous environment, at least in part. They were locally derived and are the only certain evidence for Carboniferous volcanic activity in south Cumbria. As volcanic fragments increase in abundance towards the base of the borehole they must have been derived from the top of the succession being eroded and are probably of Brigantian age. Carbonate fragments were dolomitized soon after incorporation in the brockram, probably by saline fluids derived from the evaporative Zechstein Sea. The dolomitization was incomplete, leaving remnant limestone cores to clasts which were subsequently dissolved. The resultant vugs were infilled by dolomite, calcite and gypsum cements, which have been partially weathered from outcropping brockram, leaving hollow pebbles.  相似文献   

7.
Newly discovered carbonate laminites are described from the Lincolnshire Limestone Formation (Middle Jurassic, Britain). These occur in the upper peloidal unit of fining-upward rhythms which comprise much of the lagoonal lower Lincolnshire Limestone in south Lincolnshire. The flat, millimetre-scale laminations are of three types: (1) alternating peloid-rich, peloid-poor laminae; (2) alternating bioclastic and peloidal laminae; (3) alternating bioclastic and micritic laminae. In all three types, small-scale cross-laminated sets (usually < 40 mm thick) also occur. The laminite horizons are usually < 150 mm thick and have, in some cases, been traced laterally for ～100 m. The close analogy of these carbonate laminites with siliciclastic counterparts favours their interpretation as tidal rhythmites, mechanically deposited in a low intertidal/shallow subtidal setting. The associated sedimentary features and overall stratigraphic-sedimentologic position of the deposits support this conclusion. According to the literature, mechanically deposited as opposed to algally induced carbonate laminites are rare outside the supratidal realm. Possible reasons for the real or imagined scarcity of intertidal/ subtidal carbonate laminites in ancient sedimentary regimes are discussed.  相似文献   

8.
Marine calcite cementation and lithification of Carboniferous carbonate sediments hosting Zn-Pb mineralisation in the Irish orefield occurred at or near the seafloor. A relatively early, fine-grained, grey replacive dolomite, preferentially developed in micrite, is widely developed in the Waulsortian Limestone Formation, the main host to mineralisation, and is pervasive in the southeastern Midlands in proximity to the Leinster Massif. This dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias. Later, coarse-grained white dolomite preferentially replaced coarser components of the Waulsortian Limestone and infilled residual vuggy porosity. Whilst some of this coarse dolomite may be related to the fine replacive dolomite event, a common spatial association with fault zones, coupled with primary fluid inclusion data, suggest that a significant proportion of this phase precipitated during the onset of fault-controlled subsidence and widespread hydrothermal circulation within the Irish Midlands area. Fluids up to ~250 °C and 10–15 wt% NaCl equivalent, sourced from a Lower Palaeozoic basement-equilibrated fluid reservoir, infiltrated the carbonate sequence via faults and fractures. The more localised development of dolomite-cemented breccias (white matrix breccias) that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids. Ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore-fluids and localised, near-surface reservoirs of low-temperature, H2S-rich brine. In the Waulsortian, this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides. Although ore-stage sulphides postdate significant diagenesis of the host rocks, and often display "epigenetic" textures, the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial. In fact, the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris-flow breccias is only consistent with mineralising processes occurring in the near-seafloor environment, relatively soon after host-rock deposition. The regional development of a distinctive pink dolomite associated with faults and fractures was a post-ore event, and is considered to mark a regional brine migration linked to the onset of the Variscan orogeny. The development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in Ireland.Editorial handling: J. Menuge  相似文献   

9.
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10.
For a petroleum geologist knowledge of the density of the distribution of subsurface Cretaceous reefs is a matter of practical interest. Hence, the discovery in the Judean Desert of Israel, near the western margin of the Dead Sea, of an exhumed sea bottom below an erosional unconformity is of particular interest, because it reveals the original distribution of such reefs, and thus may provide a clue to their subsurface distribution. In this desert, newly described Turonian (Upper Cretaceous) reefs are the dominant geomorphic features, as in their original habitat on the Cretaceous sea bottom. These reefs can be traced on aerial photographs: they are generally 5-15 m in diameter and 1-2.5 m in vertical dimension. Generally, reefs occur within 100-200 m of one another; in many places they are less than 50 m apart. The shapes of the patch or pinnacle reefs are almost circular, as are those occurring in the Edwards Limestone (Comanchean, Lower Cretaceous) in central Texas (Roberson, 1972). The hard, resistant, ring-like outer rims of these exposed reefs weather out as raised rims. A central depression within such structures consists, in places, of Senonian soft chalky or friable material that has been interpreted as a diagenetic product of the vadose zone. The massive reef core consists of porous dolomite. The flanking strata which dip away from the reef core at angles of approximately 15-25° are composed of a probable original grainstone which has been diagenetically changed to a micritic fabric.  相似文献   

11.
Accumulation of microbial mats and stromatolites dominate in the crystallization ponds of solar salt works west of Alexandria, Egypt. These microbial mats are laminar in the permanent submerged part of the ponds. The microbial mats commonly form sites for growth of gypsum crystals during periods having higher salinity. In the dominant submerged part of the pond, domal stromatolites are common around groundwater seepage holes. In the shallow, intermittent margin of the ponds, the laminated microbial structure forms laterally close-linked hemispheroidal stromatolite type, with unidirectional and multidirectional ripple mark-like morphology on their surface. The microbial laminite and stromatolite types in the modern solar salt works are similar to the organic-rich Miocene gypsum beds of El-Barqan (west Alexandria, Egypt) and Rabigh (north Jeddah, Saudi Arabia). The Miocene organic-rich beds consist of interlayered dark-colored microbial laminae and light-colored gypsum laminae. These beds may have three different variations: regular even lamination, laterally closed-linked hemispheroidal stromatolites, and/or discrete hemispheroidal stromatolites. Petrographic examination of the microbial laminites and stromatolites in the solar salt works and the Miocene gypsum beds indicate that the dark-colored, organic-rich laminae are composed of micritized microbial laminae and/or brown organic filaments. In El-Barqan area, the light-colored gypsum-rich laminae are composed of either gypsum crystal fragments, or lenticular and prismatic gypsum. These gypsum crystals are either entrapped within the microbial filaments or are nucleated at the surface of the microbial laminae to form a radial pattern, whereas in Rabigh area, the light-colored gypsum-rich laminae are composed of secondary porphyrotopic, poikilotopic, or granular gypsum crystals. By comparison of the microbial structure in the Miocene gypsum beds with the recent occurrence of the microbial laminites and stromatolites in the solar salt works, it is demonstrated that the organic-rich Miocene gypsum beds were formed in a very shallow salina with slightly fluctuating brine levels.  相似文献   

12.
Middle to Late Ordovician subtidal carbonates in the Manitoulin Island area of Ontario are predominantly limestone in composition, but non-ferroan and ferroan dolomite is a common cement as well as a selective or locally pervasive replacement phase. Integration of field, petrographic, geochemical (δ13C, δ18O) and fluid inclusion data indicates that lithification of these carbonates occurred during burial diagenesis, with much of the alteration controlled by regional fracturing and hydrothermal influences. Aqueous (type 1) fluid inclusions in early calcite (pre-dolomite) and dolomite are saline (> 29 wt% NaCl eq.) solutions with Ca and/or Mg in excess of Na and display homogenization temperatures with modes of 95 and 101°C, respectively. These temperatures can be explained by significantly more burial than can be accounted for either by the available stratigraphic information or by an unusually high palaeogeothermal gradient, which also is not well supported. The fluid inclusion temperatures are interpreted to have resulted from hydrothermal fluids which circulated during the burial diagenesis of these strata. Type 1 inclusions in late (post-dolomite) calcite are less saline (<19 wt% NaCl eq.) and have a bimodal distribution of homogenization temperatures with a relatively well defined low temperature peak similar to those in early calcite and dolomite and a broad higher temperature grouping with a mode at 183°C. A small proportion of methane and light hydrocarbon-bearing fluid inclusions (type 2) are present in all stages of carbonate. Dolomitizing fluids were derived from burial compaction of argillaceous sediments in the more central parts of the Michigan Basin and the updip migration of these brines along fractures to the basin margin where the carbonates of the Manitoulin Island area were dolomitized. Alternatively, migration of dolomitizing brines downward from the overlying pervasively dolomitized Silurian sequence into fractures in the Ordovician carbonates may have occurred. Integration of the aqueous fluid inclusion data into the diagenetic history of these carbonates remains equivocal because most of the inclusions are secondary or indeterminate in origin. Nevertheless, high salinities resulting from interaction with evaporitic strata and hydrothermal effects are clearly implicated although the origin of the latter remains unclear. The alteration styles of the Ordovician carbonates in the Manitoulin area are similar to those of Ordovician hydrocarbon reservoirs described from other parts of the Michigan Basin. They indicate that fracture-related diagenesis occurred on a basin-wide scale and that hydrothermal effects were important.  相似文献   

13.
Both the mineralogy and facies of lacustrine bio‐induced carbonates are controlled largely by hydrological factors that are highly dependent upon climatic influence. As such they are useful tools in characterizing ancient lake environments. In this way, the study of the sedimentary record from the small ancient Sarliève Lake (Limagne, Massif Central, France) aims to reconstruct the hydrological evolution during the Holocene, using petrographical, mineralogical and geochemical analyses. The fine‐grained marls, mainly calcitic, display numerous layers rich in pristine Ca‐dolomite, with small amounts of aragonite, which are clearly autochthonous. As these minerals are rather unusual in the temperate climatic context of western Europe, the question arises about their forming conditions, and therefore that of the lacustrine environment. Ca‐dolomite prevails at the base of the sequence as a massive dolomicrite layer and, in the middle part, it builds up most of the numerous laminae closely associated with organic matter. Scanning electron microscope observations reveal the abundance of tiny crystals (tens to hundreds of nanometres) mainly organized as microspheres looking like cocci or bacilli. Such a facies is interpreted as resulting from the fossilization of benthic microbial communities by dolomite precipitation following organic matter consumption and extracellular polymeric substance degradation. These microbial dolomites were precipitated in a saline environment, as a consequence of excess evaporation from the system, as is also suggested by their positive ?18O values. The facies sequence expresses the following evolution: (i) saline pan, i.e. endorheic stage with a perennial lowstand in lake level (Boreal to early Atlantic periods); (ii) large fluctuations in lake level with sporadic freshening of the system (Atlantic); (iii) open lake stage (sub‐boreal); and (iv) anthropogenic drainage (sub‐Atlantic).  相似文献   

14.
Numerous shallow‐marine limestone layers of the Furongian (Late Cambrian) Chaomidian Formation in the Jiulongshan section (Shandong Province, China) are breccias. Some of these breccias show abundant vertical to sub‐vertical clasts. Typically, these clasts end abruptly at the contact plane with the overlying deposit, either abutting the overlying sedimentary bed or via an erosional plane that truncates the clasts. A few exposures show concentrations of clasts that must have been uplifted to the extent that they transgressed the then sedimentary surface or (possibly) penetrated the overlying sediment which, in this case, consists of muds or marls. The clasts tend to show clusters with respect to the enclosing fabric. All clasts are parallel to each other in a specific cluster, while the various clusters may show different orientations of the clasts. It is deduced that both the exceptional position and the exceptional orientation of the clasts must be ascribed to the upward movement of the clasts under the influence of pore water escaping under high pressure through fluidized sediment.  相似文献   

15.
Following the Frasnian–Famennian mass extinction, which eliminated most skeletal reef-building fauna, the early Famennian reefs of the Canning Basin were constructed primarily by reef-framework microbial communities. In the Napier and Oscar Ranges, the Famennian reef complexes had high-energy, reef-flat depositional environments on a reef-rimmed platform that transitioned into low-energy, deep-water reefs growing in excess of 50 m below sea level. High-energy, reef-flat depositional environments contain doming fenestral stromatolites that grade into porous thrombolites and are associated with coarse-grained sandstones and grainstones. The reef-margin subfacies contains mounds of microdigitate thrombolites, which are more delicate than the reef-flat thrombolites and locally contain abundant red algae, Girvanella, renalcids and sediment-filled tubes. Within the thrombolites, the red algae are in upright growth positions, suggesting that the thrombolites are largely composed of carbonate that precipitated in situ. Reefal-slope environments are dominated by Wetheredella and Rothpletzella with locally abundant Girvanella, renalcids and Uralinella. In reefal-slope strata, delicate fans and microdigitate stromatolites of Wetheredella and Rothpletzella are often oriented horizontal or diagonal to bedding and are interpreted as syndepositionally toppled over. Most mesoscale microbial community structures contain several species of microbial fossils, and no single microbial species appears to have controlled the morphology of the community structure. Therefore, the depositional environment must have determined the distribution and morphology of the stromatolites, thrombolites and other microbial community structures. The adaptability of microbial communities to various reef environments allowed them to fill ecological niches opportunistically after the Frasnian–Famennian mass extinction.  相似文献   

16.
The Gojusan fan delta is a coarse‐grained delta complex on the western margin of the Miocene Pohang Basin, SE Korea. The deposits consist of five facies associations (FA): alluvial‐fan conglomerates (FA I), shallow‐marine mouth‐bar sandstones (FA II), fluvial and mouth‐bar conglomerates (FA III), Gilbert‐type foreset conglomerates (FA IV) and hemipelagic mudstones (FA V). Different facies associations characterize the northern, central and southern parts of the delta complex. To the north, FA IV is laterally juxtaposed with FA I or the basement, with scarp‐derived breccias along the contact. Centrally, and to the south, FA I is laterally juxtaposed with FA II, with an abrupt facies change and local inliers of basement rocks along the contact. Early alluvial fan and mouth‐bar deposits are overlain by the topset (FA III) and truncated by the foreset (FA IV) of a Gilbert‐type delta in the central part of the fan delta complex, whereas FA II passes transitionally upward into FA III in the south, with the latter extending basinward into a gently inclined shoal water delta front. Gilbert‐type and shoal‐water geometries are therefore developed in the same delta complex. The composite delta geometry is interpreted as reflecting (1) its development near an extensional transfer zone where the hangingwall relief was variable over short distances along strike, and (2) the operation of intrabasinal faults. This interpretation contrasts with previous studies that viewed the delta complex as having formed along a pull‐apart basin margin.  相似文献   

17.
The basal unit of the Amadeus Basin sequence is the Heavitree Quartzite, and this formation usually forms a single east‐west ridge along the northern side of the MacDonnell Ranges. However, at Alice Springs there are two such ridges. Basement rocks crop out on the northern side of each ridge, and dolomite and shale of the Bitter Springs Formation crop out on their southern sides. The northern outcrop of dolomite and shale is tightly folded, and is separated from the southern outcrop of basement by a major fault. The bedding of the sediments, the axial plane of the fold, and the fault all dip south at about 45°. Inverted facings on parasitic folds indicate that the northern outcrop of quartzite and dolomite plus shale is an antiform in inverted rocks. Hence the southern outcrop of basement and quartzite is synformal, and is interpreted as the frontal part of a fold nappe. The nappe started as a recumbent anticline whose middle limb of quartzite sheared out as the anticline travelled several kilometres southwards relative to the dolomite and shale below, which formed a tight recumbent syncline. Later monoclinal uplift of the northern half of the area tilted the nappe into its present south‐dipping attitude, thus converting the recumbent anticline into a synform and the recumbent syncline into an antiform.  相似文献   

18.
Stromatolitic crusts on stick-like and platy Porites corals forming Messinian reefs in Almería played an important role in supporting and binding the brittle corals. The crusts were previously regarded as probable marine cements. However, their clotted, peloidal, and micritic fabrics are directly comparable with those of stromatolites. They accreted allochthonous grains even on vertical faces, and include bushy fabrics closely comparable with those produced by cyanobacterial calcification. They contain numerous fenestrae, exhibit rapid fabric variation, and locally form micro-columns and laminated domes. Their similarities to peloid micrite crusts in Recent reefs suggest that at least some of these Recent crusts are microbial in origin, even though they have widely been interpreted as marine cements. The sedimentary effects of crust development substantially affected both the morphology and relief of the reefs and the generation of reefal clasts. Binding of the reef-frame in the Pinnacle and Thicket zones in the lower and middle parts of the reef created a rigid margin which shed large (commonly up to 5 m) cuboidal blocks of coral-stromatolite frame. The blocks broke along planes of weakness provided by the vertical Porites sticks and were deposited on the Fore-Reef Slope. In the uppermost parts of the reefs crusts dominate the structure, constituting 80% or more of the rock. Veneers up to 15 cm thick encrust thin irregular Porites plates to create a solid Reef Crest Zone which has not been recognized before. The coral-stromatolite framework is associated with echinoids, crustose corallines and halimedacean algae which, together with the scleractinians, indicate normal marine salinity throughout reef growth.  相似文献   

19.
Speleothems, mostly composed of calcium carbonate, are widely present in modern karst‐originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee‐deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle‐like pendants and ground‐up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro‐crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg‐calcite). Meanwhile, the carbon‐oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high‐frequency sea‐level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.  相似文献   

20.
This research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms, and describes the involvement of highly structured extracellular polymeric secretion (EPS) templates in dolomite nucleation. The structure of EPS is shown to match the hexagonal–trigonal lattice geometry of dolomite, which favoured the epitaxial crystallization of dolomite on the organic substrate. This structure of EPS also matches the arrangement of silica nanospheres in opal, which further accounts for the organically‐templated formation of opal enabling the non‐replacive co‐existence of dolomite and silica. The study is focused on a 50 m thick dolomite succession that is exposed in central areas of the Tertiary Duero Basin and was deposited in a mudflat‐saline lake sedimentary complex during the Middle to Late Miocene (9 to 15 Ma). In the intermediate intervals of the succession, poorly indurated dolomite beds pass gradually into silica beds. On the basis of sedimentological, compositional, geochemical and petrographic data, silica and dolomite beds have been interpreted as mineralized microbial mats. The silica beds formed in marginal areas of the lake in response to intense evaporative concentrations; this resulted in the rapid and early precipitation of opal. Silicification accounted for the exceptional preservation of the microbial mat structure, including biofilms, filamentous and coccoid microbes, and EPS. Extracellular polymeric secretions have a layered structure, each layer being composed of fibres which are arranged in accordance with a reticular pattern, with frequent intersection angles at 120° and 60°. Therefore, the structure of EPS matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal. Additionally, EPS binds different elements, with preference to Si and Mg. The concurrence of suitable composition and surface lattice morphologies in the EPS favoured the crystallization of dolomite on the substrate. In some cases, dolomite nucleation took place epicellularly on coccoid micro‐organisms, which gave way to spheroid crystals. Organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation. Most of the microbial mats probably developed on the lake floor, under sub‐aqueous conditions, where the decomposition of organic matter took place. The subsequent formation of openly packed dolomite crystals, with inter‐related Si‐enriched fibrils throughout, is evidence for the pre‐existence of fibrillar structures in the mats. Miocene dolomite crystals are poorly ordered and non‐stoichiometric, with a slight Ca‐excess (up to 5%), which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure; this confirms that microbial imprints can be preserved in the geological record, and validates their use as biosignatures.  相似文献   

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