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1.
M. T. HARRIS 《Sedimentology》1993,40(3):383-401
The Latemar reef buildup of the central Dolomites (northern Italy) provides a rare opportunity to examine an in-place Middle Triassic (Upper Anisian to Lower Ladinian) platform margin that is not strongly deformed or dolomitized. The margin lies between the flat lying platform interior and steeply dipping foreslope clinoforms. Across this transition, the depositional profile relates directly to a consistent lateral facies pattern: (1) restricted-biota grainstone of the platform interior, (2) ‘Tubiphytes’-rich boundstone and (3) diverse-biota grainstone that grades into (4) foreslope breccia beds. The boundstone and diverse-biota grainstone facies comprise the platform margin. The boundstone facies consists of a framework of small (< 10 cm) skeletal remains (< 10% by volume) with associated biotic crusts, internal sediments and syndepositional cements. Crusts and cements constitute most of the rock volume and created the boundstone fabric. Biotic crusts exhibit gravity-defying geometries and range from a light grey, ‘structure grumeleuse’ rind to dark grey, micritic laminae. Both cements and biotic crusts occur as redeposited talus in the foreslope talus deposits, indicating a syndepositional origin. The diverse-biota grainstone facies primarily consists of skeletal-peloidal grainstone with a diverse open marine biotic assemblage, in contrast to the restricted biota grainstones of the platform interior that have a low diversity, restricted marine biota. Metre scale hexacoral boundstone and centimetre-scale sponge boundstone and microbial boundstone occur as isolated patches (tens to hundreds of metres apart) within the diverse-biota grainstone facies. The depositional profile, facies zonation and biotic constituents all indicate that the Latemar buildup had a shallow water reef margin, in contrast to previous interpretations that these were upper slope reefs. The syndepositional biotic crusts and inorganic cementation played key roles in stabilizing the boundstone fabric to form a wave-resistant reef fabric.  相似文献   

2.
A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock‐Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north–south transect, from the Jura through the Alpine Tethys (Sub‐Briançonnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon‐isotope excursion in carbonate (ca 2 to 4‰) and organic matter (ca 4 to 5‰) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (≤11 wt.%) in the Jura, to thin‐bedded marl (≤5 wt.% total organic carbon) in the Sub‐Briançonnais Basin and to hemipelagic reddish marly limestone (total organic carbon <0·05 wt.%) in equivalent levels from the Lombardian Basin. The carbon‐isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub‐Briançonnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely‐bedded laminae and homogeneous mud layers containing rip‐up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian–Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea‐level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings.  相似文献   

3.
The sedimentary record of the Arabian Shelf offers a unique opportunity to study the Cretaceous (Albian–Turonian) greenhouse climate from a palaeoequatorial perspective. In particular, hemipelagic to pelagic carbonate successions from the extensive Shilaif intra‐shelf basin have the potential to produce an excellent record of carbon cycle perturbations during this interval. This study presents a 269 m thick chemostratigraphic (carbonate δ13C and δ18O) record from the Middle Albian to Early Turonian of central Abu Dhabi (United Arab Emirates), representing over 14 Myr of uninterrupted carbonate sedimentation. The Mauddud to Shilaif formations represent outer ramp to basinal intra‐shelf carbonates with variations from laminated organic‐rich to clean bioturbated intervals. Isotopic evidence of the latest Albian Anoxic Event (Oceanic Anoxic Event 1d), Middle Cenomanian Event I and the Cenomanian–Turonian Anoxic Event (Oceanic Anoxic Event 2) are confirmed and biostratigraphically calibrated by means of calcareous nannofossils. The carbon isotope record allows correlation with other regional records and well‐calibrated records across the Tethyan Ocean and represents a significant improvement of the chronostratigraphic framework of the United Arab Emirates (Shilaif) and Oman (Natih) intra‐shelf basins. The study further confirms that low carbon isotope values corresponding to the two source rock intervals in the Shilaif Formation clearly precede the isotopic expressions of Oceanic Anoxic Event 1d and Oceanic Anoxic Event 2.  相似文献   

4.
Well‐exposed Mesozoic sections of the Bahama‐like Adriatic Platform along the Dalmatian coast (southern Croatia) reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding Late Jurassic (Tithonian) shallow platform‐interior (over 750 m thick, ca 5–6 Myr duration). Facies within parasequences include dasyclad‐oncoid mudstone‐wackestone‐floatstone and skeletal‐peloid wackestone‐packstone (shallow lagoon), intraclast‐peloid packstone and grainstone (shoal), radial‐ooid grainstone (hypersaline shallow subtidal/intertidal shoals and ponds), lime mudstone (restricted lagoon), fenestral carbonates and microbial laminites (tidal flat). Parasequences in the overall transgressive Lower Tithonian sections are 1–4·5 m thick, and dominated by subtidal facies, some of which are capped by very shallow‐water grainstone‐packstone or restricted lime mudstone; laminated tidal caps become common only towards the interior of the platform. Parasequences in the regressive Upper Tithonian are dominated by peritidal facies with distinctive basal oolite units and well‐developed laminate caps. Maximum water depths of facies within parasequences (estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences) were generally <4 m, and facies show strongly overlapping depth ranges suggesting facies mosaics. Parasequences were formed by precessional (20 kyr) orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles. Parasequences are arranged in third‐order sequences that lack significant bounding disconformities, and are evident on accommodation (Fischer) plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position. Modelling suggests that precessional sea‐level changes were small (several metres) as were eccentricity sea‐level changes (or precessional sea‐level changes modulated by eccentricity), supporting a global, hot greenhouse climate for the Late Jurassic (Tithonian) within the overall ‘cool’ mode of the Middle Jurassic to Early Cretaceous.  相似文献   

5.
The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontia initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework.Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous faunal transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks.  相似文献   

6.
Sequence stratigraphic analysis of four widely spaced outcrops of middle Cenomanian to middle Turonian strata deposited in the Western Interior foreland basin in southern New Mexico, USA, defines ten sequence boundaries in a marine shale‐rich interval ca 200 m thick. The majority of sequence boundaries are based on basinward shifts in lithofacies characterized by either a non‐Waltherian contact between distal‐bar or lower shoreface sandstone and underlying lower offshore shale, or an erosional contact between distal‐bar or lower shoreface sandstone and underlying upper offshore shale. The sequence boundaries commonly correlate basinward to packages of storm‐deposited sandstone and to beds of sandy grainstone composed of winnowed inoceramid shell fragments. In several cases, however, the sequence boundaries pass basinward into presumably conformable successions of lower offshore shale. Maximum flooding surfaces within the sequences are represented by one or more beds of locally phosphatized globiginerid wackestone and packstone or exist within a conformable succession of lower offshore shale. Following initial south/south‐westward transgression into the study area, the regional trend of palaeoeshorelines was north‐west to south‐east, although isopach data indicate that lobes of sandstone periodically spread south‐eastward across the study area. The ten sequences in the study area are arranged into a third‐order composite megasequence that is characterized by overall upward‐deepening followed by upward‐shallowing of sequences. The composite megasequence is similar but not identical to the previously established T‐1 transgression and R‐1 regression in New Mexico. Based on radioisotopic dates of bentonites, the average frequency of the sequences within the study area was ca 327 kyr, which is consistent with fourth‐order cycles of ca 400 kyr interpreted in coeval marine strata elsewhere in the world.  相似文献   

7.
Severe global climate change led to the deterioration of environmental conditions in the oceans during the Toarcian Stage of the Jurassic. Carbonate platforms of the Western Tethys Ocean exposed in Alpine Tethyan mountain ranges today offer insight into this period of environmental upheaval. In addition to informing understanding of climate change in deep time, the effect of ancient carbon cycle perturbations on carbonate platforms has important implications for anthropogenic climate change; the patterns of early Toarcian environmental deterioration are similar to those occurring in modern oceans. This study focuses on the record of the early Toarcian Oceanic Anoxic Event (ca 183.1 Ma) in outcrops of the north‐west Adriatic Carbonate Platform in Slovenia. Amidst environmental deterioration, the north‐west Adriatic Platform abruptly transitioned from a healthy, shallow‐water environment with diverse metazoan ecosystems to a partially drowned setting with low diversity biota and diminished sedimentation. An organic carbon‐isotope excursion of ?2.2‰ reflects the massive injection of CO2 into the ocean‐atmosphere system and marks the stratigraphic position of the Toarcian Oceanic Anoxic Event. A prominent dissolution horizon and suppressed carbonate deposition on the platform are interpreted to reflect transient shoaling of the carbonate compensation depth to unprecedentedly shallow levels as the dramatic influx of CO2 overwhelmed the ocean’s buffering capacity, causing ocean acidification. Trace metal geochemistry and palaeoecology highlight water column deoxygenation, including the development of photic‐zone anoxia, preceding and during the Toarcian Oceanic Anoxic Event. Ocean acidification and reduced oxygen levels likely had a profoundly negative effect on carbonate‐producing biota and growth of the Adriatic Platform. These effects are consistent with the approximate doubling of the concentration of CO2 in the ocean‐atmosphere system from pre‐event levels, which has previously been linked to a volcanic triggering mechanism. Mercury enrichments discovered in this study support a temporal and genetic link between volcanism, the Toarcian Oceanic Anoxic Event and the carbonate crisis.  相似文献   

8.
Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.  相似文献   

9.
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10.
Limestones containing radiaxial fibrous cements were sampled along the southern slope of the late Anisian (Middle Triassic) Latemar carbonate platform in the Dolomites, northern Italy. The Latemar upper slopes comprise massive microbial boundstone, whereas lower slopes are made of clinostratified grainstone, rudstone and breccia. Samples are representative of a seawater column from near sea‐level to an aphotic zone at about 500 m water depth. Radiaxial fibrous cements were analyzed for carbon (δ13C) and oxygen (δ18O) stable isotopic composition, as well as major and trace element content, to shed light on the origin of the slope facies zonation. The δ13C vary between 1·7‰ and 2·3‰ (Vienna Pee‐Dee Belemnite), with lowest values at palaeo‐water depths between 70 m and 300 m. Radiaxial fibrous cements yielded seawater‐like rare earth element patterns with light rare earth element depletion (NdSN/YbSN ≈ 0·4), superchondritic yttrium/holmium ratios (≈55) and negative cerium anomalies. Cadmium reaches maximum values of ca 0·5 to 0·7 μg/g at palaeo‐water depths between 70 m and 300 m; barium contents (0·8 to 1·8 μg/g) increase linearly with depth. The downslope patterns of δ13C and cadmium suggest increased nutrient and organic matter contents at depths between ca 70 m and 300 m and point to an active biological pump. The peak in cadmium and the minimum of δ13C mark a zone of maximum organic matter respiration and high nutrient and organic matter availability. The base of this zone at ca 300 m depth corresponds with the transition from massive microbial boundstone to clinostratified grainstone, rudstone and breccia. The microbial boundstone facies apparently formed only in seawater enriched in organic matter, possibly because this organic matter sustained benthic microbial communities at Latemar. The base of slope microbialites on high‐relief microbial carbonate platforms may be a proxy for the depth to maximum respiration zones of Palaeozoic and Mesozoic periplatform basins.  相似文献   

11.
The Rottnest Shelf is a narrow, wave-dominated open shelf on the passive continental margin of southwest Australia, adjacent to a hinterland of low relief and sluggish drainage. High physical energy, low nutrients in cool subtropical waters, and rapid postglacial transgression have limited carbonate productivity, restricted grain types, and reworked the transgressed surface to form only a thin ( < 1 m) blanket of carbonate and relict sediment, with little terrigenous influx. Subaerial weathering of the shelf during Late Pleistocene emergence was followed by postglacial drowning, erosional shoreface retreat, and generation of a transgressive lag deposit. Establishment of the warm temperate biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf disconformity surface and generation of hardground veneers and thin skeletal carbonate sheets. Linear topographic ridges of Pleistocene limestone partition the shelf into systems with varying physical energy, biota and sediment supply. The Holocene sediments are a shallowing-upward coastal sequence; wave-ripple cross-stratified grainstone (Inner Shelf); and bioturbated bryozoan grainstone to skeletal wackestone (Outer Shelf to Upper Continental Slope), characterised by seaward fining and increasing percentages of planktic carbonate sediment.

Given sufficient time, the Rottnest Shelf could recover from drowning, and form blanket-like skeletal carbonates. Thin ( < 1 m) lags overlying disconformities, which underlie shallowing-upward coastal and shelf sediments a few metres thick, will be generated by glacio-eustatic cycles of sedimentation (105 y duration). Thick (several tens of metres) sediment bodies, composed of wave-rippled to bioturbated skeletal carbonate sediment with a temperate biota, will be formed during longer term (1–10 My) sedimentation cycles. Such cycles have characterised passive margins during the Cenozoic. The Rottnest Shelf thus provides a facies model for temperate shelf sedimentation along passive continental margins.  相似文献   


12.
A rich foraminiferal assemblage, consisting of abundant Aulotortus sinuosus and A. tumidus in association with Lamelliconus multispirus, Endoteba ex gr. controversa, E. ex gr. badouxi, Endotebanella kocaeliensis, Endotriada tyrrhenica, Endotriadella wirzi, Malayspirina fontainei, Ammobaculites rhaeticus, Diplotremina astrofimbriata, Agathammina austroalpina, and others, was found in the Doi Long Formation of the Triassic Lampang Group, Northern Thailand. These foraminifers suggest that the formation is referable to the Carnian (early Late Triassic), which is consistent with the age estimated by ammonoids. In microfacies, peloidal grainstone, bioclastic grainstone, sponge-microbial boundstone, and oolitic grainstone were recognized in the Doi Long Formation. Based on microfacies, three depositional facies, the lagoon facies, reef facies, and shoal facies, are recognized in this formation. These lines of evidence are suggestive that the Doi Long Formation was deposited in a reef or bank setting rather than a ramp setting.  相似文献   

13.
Holocene fringing reef development around Bora Bora is controlled by variations in accommodation space (as a function of sea‐level and antecedent topography) and exposure to waves and currents. Subsidence ranged from 0 to 0·11 m kyr?1, and did not create significant accommodation space. A windward fringing reef started to grow 8·7 kyr bp , retrograded towards the coast over a Pleistocene fringing reef until ca 6·0 kyr bp , and then prograded towards the lagoon after sea‐level had reached its present level. The retrograding portion of the reef is dominated by corals, calcareous algae and microbialite frameworks; the prograding portion is largely detrital. The reef is up to 13·5 m thick and accreted vertically with an average rate of 3·12 m kyr?1. Lateral growth amounts to 13·3 m kyr?1. Reef corals are dominated by an inner Pocillopora assemblage and an outer Acropora assemblage. Both assemblages comprise thick crusts of coralline algae. Palaeobathymetry suggests deposition in 0 to 10 m depth. An underlying Pleistocene fringing reef formed during the sea‐level highstand of Marine Isotope Stage 5e, and is also characterized by the occurrence of corals, coralline algal crusts and microbialites. A previously investigated, leeward fringing reef started to form contemporaneously (8·78 kyr bp ), but is thicker (up to 20 m) and solely prograded throughout the Holocene. A shallow Pocillopora assemblage and a deeper water Montipora assemblage were identified, but detrital facies dominate. At the Holocene reef base, only basalt was recovered. The Holocene windward–leeward differences are a consequence of less accommodation space on the eastern island side that eventually led to a more complex reef architecture. As a result of higher rates of exposure and flushing, the reef framework on the windward island side is more abundant and experienced stronger cementation. In the Pleistocene, the environmental conditions on the leeward island side were presumably unfavourable for fringing reef growth.  相似文献   

14.
A sequence of shallow reef cores from Heron Reef, Great Barrier Reef, provides new insights into Holocene reef growth models. Isochron analysis of a leeward core transect suggests that the north‐western end of Heron Reef reached current sea‐level by ca 6·5 kyr bp and then prograded leeward at a rate of ca 19·6 m/kyr between 5·1 kyr and 4·1 kyr bp (pre‐1950) to the present reef margin. A single short core on the opposing margin of the reef is consistent with greater and more recent progradation there. Further to the east, one windward core reached modern sea‐level by ca 6·3 kyr bp , suggesting near ‘keep‐up’ behaviour at that location, but the opposing leeward margin behind the lagoon reached sea‐level much more recently. Hence, Heron Reef exhibited significantly different reef growth behaviour on different parts of the same margin. Mean reef accretion rates calculated from within 20 m of one another in the leeward core transect varied between ca 2·9 m and 4·7 m/kyr depending on relative position in the prograding wedge. These cores serve as a warning regarding the use of isolated cores to inform reef growth rates because apparent aggradation at any given location on a reef varies depending on its location relative to a prograding margin. Only transects of closely spaced cores can document reef behaviour adequately so as to inform reef growth models and sea‐level curves. The cores also emphasize potential problems in U‐series dates for corals within a shallow (ca 1·5 m) zone beneath the reef flat. Apparent age inversions restricted to that active diagenetic zone may reflect remobilization and concentration of Th in irregularly distributed microbialites or biofilms that were missed during sample vetting. Importantly, the Th‐containing contaminant causes ages to appear too old, rather than too young, as would be expected from younger cement.  相似文献   

15.
The Lower Cretaceous sections in northern Sinai are composed of the Risan Aneiza (upper Barremian-middle Albian) and the Halal (middle Albian-lower Cenomanian) formations. The facies reflect subtle paleobathymetry from inner to outer ramp facies. The inner ramp facies are peritidal, protected to open marine lagoons, shoals and rudist biostrome facies. The inner ramp facies grade northward into outer ramp deposits. The upper Barremian-lower Cenomanian succession is subdivided into nine depositional sequences correlated with those recognized in the neighbouring Tethyan areas. These sequences are subdivided into 19 medium-scale sequences based on the facies evolution, the recorded hardgrounds and flooding surfaces, interpreted as the result of eustatic sea level changes and local tectonic activities of the early Syrian Arc rifting stage. Each sequence contains a lower retrogradational parasequence set that constituted the transgressive systems tracts and an upper progradational parasequence set that formed the highstand systems tracts. Nine rudist levels are recorded in the upper Barremian through lower Cenomanian succession at Gabal Raghawi. At Gabal Yelleg two rudist levels are found in the Albian. The rudist levels are associated with the highstand systems tract deposits because of the suitability of the trophic conditions in the rudist-dominated ramp.  相似文献   

16.
The Mid-Cenomanian Event was a positive carbon-isotope (δ13C) excursion recorded in hemipelagic basins of the western Tethyan Sea, North to Tropical Atlantic Ocean, and Japan. It is thought of as a prelude to the Oceanic Anoxic Event 2. However, the Mid-Cenomanian Event has never been studied in detail in shallow marine platform deposits and it is not known how it relates to carbonate production and stratigraphic geometry. To better understand how this carbon cycle disruption influenced the neritic biological communities in shallow carbonates during the Cenomanian, a facies, geochemical, diagenetic, and sequence stratigraphic study of the northern Aquitaine platform has been conducted. Seventy-six δ13C and δ18O measurements have been made on micrite, rudists, and diagenetic cements. Fifteen sedimentary facies have been arranged into four depositional environments. Three third-order sequences (CB, CC, CD) are defined from late early Cenomanian to early late Cenomanian and are well correlated with eustatic cycles in European basins. Two peaks of the Mid-Cenomanian Event (MCE1a, +1.2‰, and MCE1b, +1.7‰) have been identified for the first time in shallow marine carbonates. Analysis of diagenetic blocky calcite cements suggests that diagenesis did not affect the δ13C of micrite, which can be discussed in terms of the initial signal. The Mid-Cenomanian Event was synchronous with a turnover in neritic carbonate producers marking a transition from photozoan to heterozoan facies. This facies change resulted from the establishment of mesotrophic to eutrophic conditions at the early/mid-Cenomanian transition, reflecting a clear connection between the Mid-Cenomanian Event and neritic biological communities. Depositional geometry and carbonate production varied with δ13C during the Mid-Cenomanian Event on the Aquitaine platform. When δ13C values were between 2.5‰ and 3‰, the geometry was a flat platform with a high carbonate sedimentation rate leading to the formation of sandbars and rudist bioherms (Accommodation/Sedimentation ratio less than 1, A/S < 1). When the δ13C value exceeded 3‰, a carbonate demise occurred and clays and marls were deposited in the lower offshore environment (A/S >> 1). The general carbonate demise affecting the northern Aquitaine platform during the mid-Cenomanian can be explained by both a eustatic sea-level rise and the establishment of eutrophic conditions. The coincidence of the Mid-Cenomanian Event with both (1) the occurrence of mesotrophic to eutrophic conditions marked by carbonate producer turnover from photozoan to heterozoan facies and (2) the transgressive cycles, suggests that eustatic sea-level rise leading to high trophic conditions could explain this positive δ13C excursion in the Atlantic and western Tethyan domain. During the mid-Cenomanian, carbon cycle perturbations largely controlled the neritic biological communities on shallow carbonate platforms in a part of the western Tethyan domain.  相似文献   

17.
The San Pedro River (SPR) is located in northern Sonora (Mexico) and southeastern Arizona (USA). SPR is a transboundary river that develops along the Sonora (Mexico) and Arizona (USA) border, and is considered the main source of water for a variety of users (human settlements, agriculture, livestock, and industry). The SPR originates in the historic Cananea mining area, which hosts some of the most important copper mineralizations in Mexico. Acid mine drainage derived from mine tailings is currently reaching a tributary of the SPR near Cananea City, resulting in the contamination of the SPR with heavy metals and sulfates in water and sediments. This study documents the accumulation and distribution of heavy metals in surface water along a segment of the SPR from 1993 to 2005. Total concentrations of Cd, Cu, Fe, Mn, Pb, and Zn in surface waters are above maximum permissible levels in sampling sites near mine tailing deposits. Nevertheless, a significant decrease in the Fe and SO4 2− in surface water (SO4 2−: 7,180–460.39 mg/L; Fe: 1,600–9.51 mg/L) as well as a gradual decrease in the heavy and transition metal content were observed during the period from 1994 to 2005. Approximately 2.3 km downstream of the mine tailings, the heavy metal content of the water drops quickly following an increase in pH values due to the discharging of wastewater into the river. The attenuation of the heavy metal content in surface waters is related to stream sediment precipitation (accompanied by metal coprecipitation and sorption) and water dilution. Determining the heavy metal concentration led to the conclusion that the Cananea mining area and the San Pedro River are ecosystems that are impacted by the mining industry and by untreated wastewater discharges arising from the city of Cananea (Sonora, Mexico).  相似文献   

18.
Geologic mapping in the northern Sierra Los Ajos reveals new stratigraphic and structural data relevant to deciphering the Mesozoic–Cenozoic tectonic evolution of the range. The northern Sierra Los Ajos is cored by Proterozoic, Cambrian, Devonian, Mississippian, and Pennsylvanian strata, equivalent respectively to the Pinal Schist, Bolsa Quartzite and Abrigo Limestone, Martin Formation, Escabrosa Limestone, and Horquilla Limestone. The Proterozoic–Paleozoic sequence is mantled by Upper Cretaceous rocks partly equivalent to the Fort Crittenden and Salero Formations in Arizona, and the Cabullona Group in Sonora, Mexico.Absence of the Upper Jurassic–Lower Cretaceous Bisbee Group below the Upper Cretaceous rocks and above the Proterozoic–Paleozoic rocks indicates that the Sierra Los Ajos was part of the Cananea high, a topographic highland during the Late Jurassic and Early Cretaceous. Deposition of Upper Cretaceous rocks directly on Paleozoic and Proterozoic rocks indicates that the Sierra Los Ajos area had subsided as part of the Laramide Cabullona basin during Late Cretaceous time. Basal beds of the Upper Cretaceous sequence are clast-supported conglomerate composed locally of basement (Paleozoic) clasts. The conglomerate represents erosion of Paleozoic basement in the Sierra Los Ajos area coincident with development of the Cabullona basin.The present-day Sierra Los Ajos reaches elevations of greater than 2600 m, and was uplifted during Tertiary basin-and-range extension. Upper Cretaceous rocks are exposed at higher elevations in the northern Sierra Los Ajos and represent an uplifted part of the inverted Cabullona basin. Tertiary uplift of the Sierra Los Ajos was largely accommodated by vertical movement along the north-to-northwest-striking Sierra Los Ajos fault zone flanking the west side of the range. This fault zone structurally controls the configuration of the headwaters of the San Pedro River basin, an important bi-national water resource in the US-Mexico border region.  相似文献   

19.
紫云晚石炭世叶状藻礁发育在碳酸盐岩台地边缘内。研究以碳酸盐岩微相分析为主要手段,对叶状藻礁剖面中岩石的微相特征和沉积环境研究。主要总结出6个微相类型,为生物碎屑粒泥灰岩、生物碎屑泥粒灰岩、球粒生屑泥粒灰岩、叶状藻粒泥灰岩—叶状藻泥粒灰岩、腕足泥粒灰岩、生物碎屑颗粒灰岩。总结了一个在开阔台地与台地边缘过渡带上的局部相模式,由较深水的开阔台地相、叶状藻礁相、台内斜坡相和台内较浅水的斜坡相、较浅水的开阔台地相、台内浅滩相过渡,分析总结了叶状藻礁的沉积环境。微相组合及其在剖面纵向分布规律显示,剖面的总体沉积环境为潮下带浅水开阔台地环境。   相似文献   

20.
S.J. MAZZULLO 《Sedimentology》2006,53(5):1015-1047
Lithostratigraphy, depositional facies architecture, and diagenesis of upper Pliocene to Holocene carbonates in northern Belize are evaluated based on a ca 290 m, continuous section of samples from a well drilled on Ambergris Caye that can be linked directly to outcrops of Pleistocene limestone, and of overlying Holocene sediments. Upper Pliocene outer‐ramp deposits are overlain unconformably by Pleistocene and Holocene reef‐rimmed platforms devoid of lowstand siliciclastics. Tectonism controlled the location of the oldest Pleistocene platform margin and coralgal barrier reef, and periodically affected deposition in the Holocene. A shallow, flat‐topped, mostly aggradational platform was maintained in the Holocene by alternating periods of highstand barrier‐reef growth and lowstand karstification, differential subsidence, and the low magnitude of accommodation space increases during highstands. Facies in Pleistocene rocks to the lee of the barrier reef include: (i) outer‐shelf coralgal sands with scattered coral patch reefs; (ii) a shoal–water transition zone comprising nearshore skeletal and oolitic sands amidst scattered islands and tidal flats; and (iii) micritic inner‐shelf deposits. Four glacio‐eustatically forced sequences are recognized in the Pleistocene section, and component subtidal cycles probably include forced cycles and autocycles. Excluding oolites, Holocene facies are similar to those in the Pleistocene and include mud‐mounds, foraminiferal sand shoals in the inner shelf, and within the interiors of Ambergris and surrounding cayes, mangrove swamps, shallow lagoons, and tidal and sea‐marginal flats. Meteoric diagenesis of Pliocene and Pleistocene rocks is indicated by variable degrees of mineralogic stabilization, generally depleted whole‐rock δ18O and δ13C values, and meniscus and whisker‐crystal cements. Differences in the mineralogy and geochemistry of the Pliocene and Pleistocene rocks are attributed to variable extent of meteoric alteration. Dolomitization in the Pliocene carbonates may have begun syndepositionally and continued into the marine shallow‐burial environment. Positive dolomite δ18O and δ13C values suggest precipitation from circulating, near‐normal marine fluids that probably were modified somewhat by methanogenesis. Sedimentologic and diagenetic attributes of the Pliocene–Pleistocene rocks in the study area are similar to those in the Bahamas with which they share a common history of sea‐level fluctuations and climate change.  相似文献   

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