Incidence of obliquity and precession-forced Milankovitch cycles in the western Mediterranean: early Messinian sedimentation in the Sorbas Basin (Almería, southern Spain)

Spectral analysis of the Messinian Abad marls in the Cariatiz section (Sorbas Basin, south-eastern Spain) reveals three relevant orders of cyclicity. The most significant cycle is in the lowest frequency (average thickness of 365 cm, 4–5 cycles in the section). It is recorded in the composition of planktic foraminiferal assemblages indicative of surface-water temperature, planktic and benthic stable isotope signals, and carbonate proportions. The planktic assemblages, isotope values and carbonate proportions also record a middle-frequency cycle with an average of 177 cm (9–10 cycles in the section). The highest frequency cycle (average of 132 cm, 12–13 cycles in the section) is mainly reflected in siliciclastic and calcite proportions. Age constraints and cycle patterns suggest that the lowest frequency cycle was forced by orbital obliquity, whereas the two higher-frequency ones are related to precession. Obliquity seems to have controlled major changes in surface-water temperature in the Sorbas Basin during the early Messinian. Surface-water temperature was also affected by precession, with changes in weathering and run-off. Spectral analysis has also been applied to vertical shifts of reef facies throughout the progradation of the Cariatiz reef. This reef is coeval with the Cariatiz section. Vertical shifts of reef talus breccias point to the existence of 4–5 major cycles of sea-level change, whereas 7–9 higher-frequency cycles are reflected in the repeated occurrence of lowstand, non-reefal deposits. Correlation with the cycles observed at the Cariatiz section suggests that obliquity forced glacio-eustatic sea-level oscillations in the western Mediterranean during the Late Miocene.     

Reef geometries, erosion surfaces and high-frequency sea-level changes, upper Miocene Reef Complex, Mallorca, Spain

The upper Miocene Reef Complex of Mallorca is a 20-km prograding unit which crops out in sea cliffs along the southern side of the island. These vertical and exceptionally clean outcrops permit: (i) identification of different facies (lagoon, reef front, reef slope and open platform) and their geometries and boundaries at different scales, ranging from metre to kilometre, and (ii) construction of a 6-km-long high-resolution cross-section in the direction of reef progradation. This cross-section shows vertical shifts of the reefal facies and erosion surfaces linked to a general progradational pattern that defines the accretional units. Four hierarchical orders of magnitude (1-M to 4-M) of accretional units are identified by consideration of the vertical facies shifts and by which erosion surfaces are truncated by other erosion surfaces. All these orders show similar patterns: horizontal beds of lagoonal facies in the upper part (landward), reefal and slope facies with sigmoidal bedding in the central part, and open-platform facies with subhorizontal bedding in the lower part (basinwards). The boundaries are erosion surfaces, horizontal over the lagoon facies, dipping basinwards over the reef-front facies and connecting basinwards with their correlative conformities over the reef-slope and open-platform facies. The four orders of accretional units are interpreted in terms of four (1-M to 4-M) hierarchies of sea-level cycles because (i) there is a close relation between the coral growth and the sea surface, (ii) there are vertical shifts in the reefal facies and their relation to the erosion surfaces, and (iii) there was very little tectonic subsidence in the study area during the late Miocene. Additionally, all these units can be described in terms of their position relative to the sea-level cycle: (i) the reefs prograde on the open-platform sediments during low stands of sea-level; (ii) aggradation of the lagoon, reef and open-platform facies dominates during sea-level rises, and the lagoonal beds onlap landwards upon the previous erosion surface; (iii) reefal progradation occurs during high stands of sea-level; and (iv) the 2-M sea-level fall produces an off-lapping reef and there is progradation with downward shifts of the reefal facies and erosion landward on the emerged (older) reefal units (A-erosion surfaces); the 3-M and 4-M sea-level falls produce only erosion (B-and C-erosion surfaces). Although precise age data do not exist at present, some speculations upon the frequency of these Miocene relative sea-level cycles can be made by comparisons with Pleistocene cyclicity. There is a good correlation between the Miocene 2-M cycles and the 100-ka Pleistocene cycles. Consequently, the 1-M cycles can be assigned to a fourth order in relation to previously proposed global cycles and the 2-M to fifth-order cycles. All these accretional units could be defined as ‘sequences’, according to the definition as commonly used in sequence stratigraphy. However, they represent higher than third-order sea-level cycles, but are not parasequences. The term subsequence, therefore, is suggested to define ‘a part of a sequence bounded by erosion surfaces (mostly subaerial) and their correlative conformities basinwards'. A hierarchy of subsequences can be established.     

Tectonic and eustatic control on a mixed siliciclastic–carbonate platform during the Late Oxfordian–Kimmeridgian (La Rochelle platform,western France)

Boreal and Tethyan realms of Western Europe present significant sedimentological, paleontological, and stratigraphic differences. The purpose of this study is to constrain regional versus global controls on the dynamics of a sedimentary system located at the interface of these two realms in order to better understand the origin of their differences. Detailed sedimentological, palynofacies and calcareous nannofossil analyses were performed on two sections from the La Rochelle platform (western France). The Pas section includes part of the Late Oxfordian and Early Kimmeridgian, and the Rocher d'Yves section is assigned to the Late Kimmeridgian. They correspond to monotonous marl–argillaceous limestone alternations. Limestones are essentially mudstones with echinoderms, bivalves and foraminifera that suggest low-energy, open-marine conditions. Highly bioclastic and/or peloidal deposits occur commonly, and show wackestones to wacke-pack-grainstones textures. These deposits indicate frequent high-energy events, and are interpreted as storm deposits. Marls dominate in the most proximal depositional environments, while calcareous deposits are more important in more distal environments. The Rocher d'Yves section is globally more marly than the Pas section, suggesting a more proximal setting. Palynofacies are dominated by woody particles, suggesting shallow-water, proximal depositional environments. Calcareous nannofossils are ascidian spicules, coccoliths, and schizospheres. Watznaueria britannica dominate calcareous nannofossil assemblages in the Pas section. The Rocher d'Yves assemblages are quasi-exclusively composed of Cyclagelosphaera margerelii, and indicate more proximal paleoenvironments than those of the Pas section. Different orders of depositional sequences are defined, with sequence boundaries corresponding to the most rapid relative sea-level falls. They are hierarchically stacked, and correlate, on the basis of ammonite zones, with the sequences of contemporaneous sections from Tethyan and boreal realms. The stacking pattern of these sequences suggests an orbital control on sedimentation. Small-, medium- and large-scale sequences correspond to precession (20 ky) cycles and to 100 ky and 400 ky eccentricity cycles, respectively. The elementary sequences have durations shorter than 20 ky. The Kimmeridgian was a period of global sea-level rise that ended in the Late Kimmeridgian. More proximal depositional environments in the Rocher d'Yves section (Late Kimmeridgian) than in the Pas section (Early Kimmeridgian) imply a progradation of the La Rochelle platform during the Kimmeridgian. This progradation resulted from a slowdown of the subsidence in the Aquitaine Basin during the Kimmeridgian, corresponding to the first steps of Atlantic Ocean opening. High-frequency cycles on the La Rochelle platform formed in sync with Milankovitch orbital cycles, while tectonics controlled the formation of the low-frequency cycles.     

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

Geological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year⁻¹ compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.     

Postglacial growth history of a French Polynesian barrier reef tract, Tahiti, central Pacific

The internal structure and growth pattern of Tahiti reefs over the last 14 ka is reconstructed using sedimentological, morphological and palaeobiological data coupled with radiometric dates in drill cores through the modern barrier reef. Flooding of the volcaniclastic deposits or the karst surface of a Pleistocene reef started at ≈ 14 ka BP, and coral growth began shortly after inundation. The sequence in the Tahiti barrier-reef edge has formed predominantly through long-term keep-up growth controlled by stable environmental conditions, while the adjacent backreef deposits did not start to accumulate before sea-level stabilization, around 6 ka. The dominance of Porites communities and the coeval occurrence of branching gracile Lithophyllum in the lowermost part of the postglacial reef sequence (14–11 ka) suggest the prevalence of uniformly moderate- to low-energy conditions and/or growth in slightly deeper waters all over the drilled area during the early reef stages. During the last 11 ka, the reef frameworks developed in a high-energy environment, at maximum water depths of 5–6 m, and were dominated by an Acropora robusta/danai–Hydrolithon onkodes association; the local interlayering of other coralgal assemblages (dominated by tabular Acropora or domal Porites ) reflects distinct diversification stages, resulting either from the palaeotopographic control of the substrate or from slight and episodic environmental changes.     

Depositional sequences and correlation of middle(?) to late Miocene carbonate complexes,Las Negras and Nijar areas,southeastern Spain

During Serravallian through Messinian time, marine carbonates flanked topographic highs that rimmed Neogene basins in the Western Mediterranean. Middle to upper Miocene carbonate strata in the Las Negras and Nijar areas (southeastern Spain) are 50-150 m thick and display 50-200 m of shelf-to-basin relief over 1-2 km. Detailed studies in those areas document the effects of relative sea-level change on sedimentation, biotic composition, and reef development. We identify three previously unrecognized, regionally correlatable depositional sequences (DS1, DS2, DS3) that occur between the underlying basement and the overlying Terminal Carbonate Complex. The lower depositional sequences (DS1, DS2) are mostly normal marine shelf (ramp) carbonates deposited on the flanks of basement highs. The basal part of DS2 locally contains some megabreccia reef blocks composed of Tarbellastraea and Porites. These blocks are the first evidence of reef growth in the area and represent a previously unrecognized period of reef development prior to the fringing reef development. The reef blocks probably formed as upslope patch reefs that were eroded and transported to distal slope locations. The upper sequence (DS3) is characterized by clinoform strata of a Porites-dominated fringing reef complex that prograded basinward in a downstepping style with successively younger reefs forming in a topographically lower and more basinward position as a result of a net sea-level drop. Regional correlation of Miocene shallow-marine strata between basins in Spain and elsewhere in the western Mediterranean is complicated because basins were semi-isolated from adjacent basins making physical correlation impossible. In addition, age-definitive biostratigraphic markers are poorly preserved in most of the Miocene shallow-water strata; basinal sediments that are more easily dated by microfossils do not typically interfinger with the shallow-marine strata in outcrop. Even where datable microfossils are found, resolution of dating is poor. Our studies in the Las Negras and Nijar areas illustrate the usefulness of integrating sedimentological, geometric and biotic data with locally derived relative sea-level (accommodation space) curves for correlation. The relative sea-level curves for each area show remarkable similarities in shape and magnitude of sea-level changes. These curves indicate several relative sea-level fluctuations during Miocene carbonate deposition prior to the major sea-level drop at the end of DS3 deposition that culminated in the exposure of the basin margin deposits and the deposition of evaporites in basinal areas during the Messinian. The depositional sequences in the Las Negras and Nijar areas may correlate with depositional sequences of similar age throughout the southern Cabo de Gata area, in Mallorca some 600 km to the northeast, and possibly in other Mediterranean locations. The widespread occurrence and possible correlation of the depositional sequences suggest regional processes such as eustacy or tectonism for their formation. The integration of sedimentological, palaeontological and sequence stratigraphic studies, and the construction of relative sea-level (accommodation space) curves may help in the interpretation of depositional histories of shallow-marine carbonate complexes and correlation of these strata between isolated areas. Other dating methods, in addition to microfossil dating, may allow for better age determination of the sequences and aid in identifying the importance of eustacy and tectonism in sequence development.     

Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands,south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp ; accretion rates average 5·65 m kyr^?1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr^?1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp , i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr^?1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.     

Mesoproterozoic deep-water reefs from Borden Peninsula, Arctic Canada

A superbly exposed stromatolite reef complex occurs in the Victor Bay Formation near Strathcona River on northern Baffin Island. Individual reefs are up to 130 m thick and nearly 1 km in length, and their development was clearly related to their position in the facies spectrum and to sea-level dynamics. In the first sea-level cycle, metre-scale reefs grew amongst mid-ramp calcarenites and outer-ramp shales during slow sea-level rise; a 25-m-thick oblate reef tract, separating mid-ramp and outer-ramp facies, formed during the highstand. The greatest period of reef growth was during the second sea-level cycle. Pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea-level, producing structures with more than 75 m of depositional relief. A gradual symmetrical succession of stromatolite growth forms, from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform, is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing. The tops of these high-standing reefs were karsted during the following regression, while dolomite ‘cryptodomes’ grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs. Continued sea-level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst. These Proterozoic slope buildups are similar to Phanerozoic deep-water reefs in size, shape, prevalence of synsedimentary lithification, presence of Neptunian dykes and in their well-developed vertical zonation of reefbuilders. However, they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements, and in lacking halos of perireefal sand- and gravel-sized calcareous debris. Their responses to changes in sea-level were strikingly similar to those shown by their younger counterparts, and suggest that sequence-stratigraphic concepts derived from studies of Phanerozoic reefs can also be applied to the Proterozoic.     

Fringing reef growth on a terrigenous mud foundation, Fantome Island, central Great Barrier reef, Australia

The leeward fringing reef at Fantome Island (central Great Barrier Reef province) is a carbonate body which has developed under the influence of terrigenous sedimentation. The reef flat is up to 1000 m wide and is surfaced by mobile sand and gravel, with almost all live corals restricted to the seaward rim. The reef slope has coral columns and heads on the upper part, but below 5 m water depth it is a muddy substrate with scattered mounds of branching corals. Three high recovery cores show the reef is up to 10 m thick and developed over a gently sloping terrace of weathered Pleistocene alluvium. Three post-glacial stratigraphic units are recognised: (1) carbonate reef top unit of coral rudstone and framestone including Sinularia spiculite; (2) lower slope unit of coral floatstone in a terrigenous muddy matrix; and (3) transgressive basal unit of skeletal arkosic sand. The acid insoluble content of matrix and of individual corals increases downwards. Coral growth rates decrease downwards, reflecting slower growth in muddier environments. Radiocarbon dating shows that the reef prograded seaward at almost stable sea level. An average vertical accumulation rate of 6.7 mm yr^-1 is indicated. Two age reversals are interpreted as material transported by storms or by erosion in response to a late Holocene sea-level fall. The carbonate reef top unit has developed adjacent to, and is environmentally compatible with a muddy terrigenous, lower slope unit. Terrigenous influx has not changed during the Holocene, and terrigenous content of sediments is controlled by deposition on the reef slope of fine sediment winnowed from the reef flat and concentration of coarse sediment in the transgressive basal sheet.     

The effects of sea level and palaeotopography on lithofacies distribution and geometries in heterozoan carbonates, south-eastern Spain

This study utilized three-dimensional exposures to evaluate how sea-level position and palaeotopography control the facies and geometries of heterozoan carbonates. Heterozoan carbonates were deposited on top of a Neogene volcanic substrate characterized by palaeotopographic highs, palaeovalleys, and straits that were formed by subaerial erosion, possibly original volcanic topography, and faults prior to carbonate deposition. The depositional sequence that is the focus of this study (DS1B) consists of 7–10 fining upward cycles that developed in response to relative sea-level fluctuations. A complete cycle has a basal erosion surface overlain by deposits of debrisflows and high-density turbidity currents, which formed during relative sea-level fall. Overlying tractive deposits most likely formed during the lowest relative position of sea level. Overlying these are debrites grading upward to high-density turbidites and low-density turbidites that formed during relative sea-level rise. The tops of the cycles consist of hemipelagic deposits that formed during the highest relative position of sea level. The cycles fine upward because upslope carbonate production decreased as relative sea level rose due to less surface area available for shallow-water carbonate production and partial drowning of substrates. The cycles are dominated by two end-member types of facies associations and stratal geometries that formed in response to fluctuating sea-level position over variable substrate palaeotopography. One end-member is termed ‘flank flow cycle’ because this type of cycle indicates dominant sediment transport down the flanks of palaeovalleys. Those cycles drape the substrate, have more debrites, high-density turbidites and erosion on palaeovalley flanks, and in general, the lithofacies fine down the palaeovalley flanks into the palaeovalley axes. The second end-member is termed ‘axial flow cycle’ because it indicates a dominance of sediment transport down the axes of palaeovalleys. Those cycles are characterized by debrites and high-density turbidites in palaeovalley axes, and lap out of strata against the flanks of palaeovalleys. Where and when an axial flow cycle or flank flow cycle developed appears to be related to the intersection of sea level with areas of gentle or steep substrate slopes, during an overall relative rise in sea level. Results from this study provide a model for similar systems that must combine carbonate principles for sediment production, palaeotopographic controls, and physical principles of sediment remobilization into deep water.     

Response of reef coral communities to sea-level rise: a Holocene model from Mauritius (Western Indian Ocean)

The fringing reef at Pointe-au-Sable (Mauritius, Indian Ocean) was used to examine the effects of Holocene sea-level rise on coral growth. This reef is about 1000 m wide and comprises a forereef slope (30 m maximum depth), a narrow reef crest and a very shallow backreef (1·5 m maximum depth). Four major coral communities were recognized, which developed within relatively narrow depth ranges: a Pachyseris/Oulophyllia community (deeper than 20 m), an Acropora‘tabulate’Faviid community (20–6 m); a robust branching Acropora community (less than 6 m) and a Pavona community (less than 10m). Three high-recovery cores show the Holocene reef sequence is a maximum of 19·3 m thick and comprises four coral biofacies which are similar to counterparts identified in modern communities: robust branching, tabular-branching, robust branching-domal and foliaceous coral facies. A minimum sea-level curve for the past 7500 years was constructed. Using distribution patterns of coral biofacies and radiocarbon dates from corals, reconstruction of reef growth history indicates that both offshore and onshore reef zones were developing coevally, aggrading at rates of 4·3 mm year^?1 from 6900 years B.P. The reef caught up with sea-level only after sea-level stabilized. Changes in coral community and reef growth rates were driven principally by increasing water agitation due to the decrease in accommodation space. Based on the composition of the successive coral assemblages, the reef appears to have grown through successive equilibrium stages.     

Marine transgression and regression in Miocene sequences of northern Pegu (Bago) Yoma,Central Myanmar

Neogene strata of the northern part of the Pegu (Bago) Yoma Range, Central Myanmar, contain a series of shallow marine clastic sediments with stratigraphic ages ranging from the Early to Late Miocene. The studied succession (around 750 m thick) is composed of three major stratigraphic units deposited during a major regression and four major transgressive cycles in the Early to Late Miocene. The transgressive deposits consist of elongate sand-bars and broad sand-sheets that pass headward into mixed-flats of tidal environments. Marine flooding in transgressive deposits is associated with coquina beds and allochthonous coral-bearing sandy limestone bands. Major marine regressions are associated with lowstand progradation of thick estuary point-bars passing up into upper sand-flat sand bodies encased within the tidal flat sequences and lower shoreface deposits with local unconformities. The succession initially formed in a large scale incised-valley system, and was later interrupted by two major marine transgressions in the generally regressive or basinward-stepping stratigraphic sequences. Successive sandbodies were formed during a sea-level lowstand and early stage of the subsequent relative rise of sea level in a tide-dominated estuary system in the eastern part of the Central Myanmar Tertiary Basin during Early to Late Miocene times.     

青藏高原羌塘盆地东缘贡日地区中侏罗统布曲组碳酸盐岩微相与沉积环境

对西藏羌塘盆地东部边缘贡日地区产出石油沥青脉的中侏罗统布曲组的沉积相和沉积环境进行了详细研究,共区分出9种碳酸盐岩微相类型,即灰泥灰岩、纹层状粉泥晶灰岩、砂屑灰岩、球粒泥亮晶泥粒灰岩、含核形石和团块泥粒灰岩、生物碎屑泥粒灰岩、生物碎屑粒泥灰岩、含鲕粒生屑颗粒灰岩、含生物碎屑灰泥灰岩。根据微相类型组合特征和野外沉积构造,认为布曲组灰岩主要形成于碳酸盐台地浅缓坡、中缓坡和深缓坡3种沉积环境,在布曲组内共识别出4个沉积旋回,组成一个相对海平面逐渐上升的海进序列,与上覆地层布曲组岩层构成一个完整的三级海平面变化旋回。     

湘西王村剖面寒武系花桥组浊积岩特征及其大地构造意义

湘西王村地区的寒武系代表江南斜坡相沉积序列,传统的下寒武统划分为杷榔组、清虚洞组;中寒武统划分为敖溪组、花桥组（下部）;上寒武统划分为花桥组（上部）、追屯组。其中,花桥组中浊流沉积发育,可识别出7次规模较大的浊流活动期,第一期浊积岩包括69个浊积序列,分别归属于11个副层序;第二期浊积岩包括37个浊积序列,分别归属于7个副层序。每个副层序的下部表现为近源浊积岩,上部表现为远源浊积岩,所代表的海水深度变化由浅变深。第一期浊流活动发生在海平面上升时期,形成三级层序的海侵体系域;第二期浊流活动发生在海平面较低、斜坡较陡时期,形成三级层序的低水位体系域。江南斜坡带中—上寒武世浊积岩频繁出现反映台地边缘深大断裂周期性活动强,断裂两盘升降差异运动显著。     

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga.The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale.Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs.The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp.Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.     

渤海湾西北岸牡蛎礁体对区域性构造活动与水动型海面变化的响应

渤海湾地区广泛分布的古海面标志物,为建立该地区的相对海面变化曲线提供了良好的基础。过去20年来该地区已相继建立了一些相对海面变化曲线,但在讨论影响古海面标志物高程的因素时,未将构造活动与水动型海面变化及均衡作用对高程的贡献量加以区分。文章以该地区7个点的古海面标志物及相关层位的高程测量与年代学测定结果,经与地区性预测海面变化曲线进行对比后,定量讨论了该地区海面变化(包括绝对海面变化与冰川、水均衡作用)与构造活动对古海面标志物高程的贡献;并以壳体的稳定同位素、地层记录与古地震研究资料讨论了牡蛎礁体在建礁过程中所记录的构造下沉。     

Cambrian- Ordovician Deep - Water Deposits in Southwest Jiangxi, China

The Cambrian-Ordovician rocks in southwestern Jiangxi are mainly composed of deep-water deposits, in which 5 facies have been recognized: sandstone facies, sandstone-mudstone facies, siltstone-mudstone facies, mudstone (slate)facies, and chert facies. They are of turbidity current origin and are related to pelagic and hemipelagic deposits. In the light of facies distribution, the Cambrian-Ordovician deposits can be classified into 3 facies associations formed in middle fan, outer fan and deep-sea plain environments respectively. The 3 different orders of vertical cycles in the stratigraphic sequence are considered to be controlled by factors such as sea-level fluctuation, basin subsidence and submarine fan progradation. The tectonic setting of the sedimentary basin is interpreted as passive continental margin based on the chemical composition analysis of the sandstone.     

Early diagenesis in Pleistocene coral reefs, southern Sinai, Egypt: response to tectonics, sea-level and climate

The uplifted Pleistocene terraces along the coast of southern Sinai exhibit a well developed reef system formed during isotope stage 9, and a younger one formed during isotope stage 5. An intermediate reef corresponding to isotope stage 7 occurs only as an erosional relic in the study area. The sediments comprise reefal framestones, peri-reefal facies, coral rubble, and siliciclastic-dominated beach and aeolian facies. The compositional and textural complexity of the sediments leads to a highly variable spatial distribution of diagenetic features. However, the geometric relationships and elemental analyses allow a reconstruction of the general diagenetic evolution: during the major eustatic sea-level highstand of isotope stage 9, the Older Reef was constructed and cemented with aragonite and high-Mg calcite. Climate was probably semiarid with some rainy periods which permitted the installation of ephemeral freshwater lenses, especially during the minor sea-level lowstand within isotope stage 9. In these lenses, and during the subsequent major sea-level lowstand, some freshwater dissolution occurred. The highstand during isotope stage 7 led to the construction of the Intermediate Reef. In the Older Reef, some high-Mg calcite precipitated at that time. Dolomite cement formed either in marine interstitial waters modified by some freshwater input, or in a hypersaline context. Phreatic-meteoric low-Mg calcite cement covers, and partly replaces, previous marine cements and dolomite, but is still attributed to the major highstand of isotope stage 7 when freshwater lenses could develop during minor sea-level lowstands. The subsequent major sea-level lowstand was dominated by an arid climate, and only a little freshwater corrosion occurred. The Younger Reef formed during the major highstand of isotope stage 5. Aragonite and high-Mg calcite cements, as well as some dolomite, are common within the reef, whereas freshwater cements are limited to beach and aeolian facies. Due to tectonic uplift, only the lower part of the Older Reef was reflooded during isotope stage 5, and only some aragonite crystals precipitated on top of dolomite or low-Mg calcite. The interrelationships between tectonics, sea-level variations of different orders, and climatic changes thus had a profound impact on the diagenetic history of these reef systems.     

Origin and significance of mud-filled incised valleys (Upper Cretaceous) in southern Alberta, Canada

Seven mud-filled incised valleys (MFIVs) in the paralic facies of the Dinosaur Park and Horseshoe Canyon formations (Upper Cretaceous) of southern Alberta were studied to better understand their morphology, geometry and depositional histories in an estuarine context. Two preservational geometries occur: simple, U-shaped forms; and internally complex forms. Both types of MFIV record deposition in the central zone of low energy (turbidity) in an estuarine setting. Simple, U-shaped MFIVs have sharp basal erosional surfaces and consist of mudstone-dominated heterolithic fills of channel-wide, concave-up laminae. Associated fossil assemblages are marine to brackish. Each simple MFIV records a cut-and-fill history associated with a cycle of relative sea-level drop and rise. Low-energy depositional settings, loss of channel form during infilling, and associated shoreface deposits, as well as the absence of clear tidal indicators suggest a coastal plain estuarine setting, along a wave-dominated, barred coastline. Complex MFIVs are rarer, and consist of imbricated, wedge-shaped sets of inclined-to-horizontal heterolithic strata. Tidal deposits and/or nonmarine-to-marine macrofossils occur locally. Complex MFIVs were infilled in meandering reaches of the central zone of low energy in tide-dominated estuaries. Their rarity compared to simple MFIVs and their freshwater palaeontological content suggest that they were contiguous landward with extensive fluvial channels. A complex MFIV near Onefour comprises three in-channel depositional cycles. Each cycle consists of an erosional surface overlain by lateral accretion bedding and a conformable transition to vertically aggraded strata. Each cycle reflects a cut-and-fill event under the control of changes in relative sea-level that culminated in overbank flooding. All MFIVs formed in low-gradient settings (≤0.03%) where estuarine zones were stretched out over many tens of kilometres. Tide-dominated estuaries apparently exhibited simple, straight-to-meandering upstream transitions and extensive landward penetration (≥200 km) of tidal backwater effects. Few modern estuaries serve as adequate modern analogues to these ancient, tide-dominated estuaries. Radiometric data indicate that MFIV cut-and-fill cycles were 100 000-400 000 years in maximum duration and thus, equivalent to 4th order sea-level cycles. However, negative evidence tentatively suggests that these cycles took place over time intervals 1-2 orders of magnitude smaller (5th order or higher sea-level cycles).     

Interglacial carbonates of the Cryogenian Umberatana Group,northern Flinders Ranges,South Australia

A detailed sedimentological and chronostratigraphic analysis of the Umberatana Group in the northern Adelaide Geosyncline has uncovered a depositional history involving the rapid progradation (at least 20 km) of a giant reef complex (up to 1.1 km relief) during mid-Cryogenian interglacial times. The reef complex, which occurs in the Balcanoona Formation, displays facies similar to Phanerozoic reefs. These include a basal forereef (slope) facies, overlain by a reef-margin facies (consisting of both stromatolitic and non-stromatolitic frameworks), and an upper backreef (platform) facies consisting of shallow-water peloidal and oolitic carbonate. The thickening of the reef complex in a basinward direction, and the distribution of the key facies are consistent with the progradation of the platform into deep water. Progradation was contemporaneous with deposition of the upper Tapley Hill Formation and had largely ceased after a major margin failure event. Following this event, reef growth continued for a short time before becoming extinct, possibly as a result of global climatic cooling and/or eustatic sea-level fall.