Controls on sedimentation in distal margin palaeovalleys in the Early Tertiary Alpine foreland basin, south-eastern France

The influence of palaeodrainage characteristics, palaeogeography and tectonic setting are rarely considered as controls on stratigraphic organization in palaeovalley or incised valley systems. This study is an examination of the influence of source region vs. downstream base level controls on the sedimentary architecture of a set of bedrock-confined palaeovalleys developed along the distal margin of the Alpine foreland basin in south-eastern France. Three distinct facies associations are observed within the palaeovalley fills. Fluvial facies association A is mainly dominated by poorly sorted, highly disorganized, clast-to-matrix-supported cobble-to-boulder conglomerates that are interpreted as streamflood deposits. Facies association B comprises mainly yellow siltstones and is interpreted as recording deposition in an estuarine basin environment. Estuarine marine facies association C comprises interstratified estuarine siltstones and clean, well-sorted washover sandstones. The sedimentary characteristics of the valley fill successions are related to the proximity of depositional sites to sediment source areas. Palaeovalleys located proximal to structurally controlled basement palaeohighs are entirely dominated by coarse fluvial streamflood deposits. In contrast, distal palaeovalley segments, which are located several kilometres downstream, contain successions showing upward transition from coarse fluvial facies into estuarine central basin fines, and finally into estuarine-marginal marine facies. Facies distributions suggest that the fluvial deposits form wedge-shaped, downstream-thinning sediment bodies, whereas the estuarine deposits form an upstream-thinning wedge. The vertical stacking of fluvial to estuarine to marginal marine depositional environments records the fluvial aggradation and subsequent transgression of relatively small bedrock-confined river valleys, which drained a rugged, upland terrain. Facies geometries suggest that a fluvial sediment wedge initially prograded downvalley, in response to high bed load sediment yields. Subsequently, palaeovalleys became drowned during the passage of a marine transgression, with the establishment of estuarine conditions. Initial fluvial aggradation and subsequent marine flooding of the palaeovalleys is a consequence of the interaction of high local rates of sediment supply and relative sea-level rise driven by flexural subsidence of the basin.     

Sequence stratigraphy of a low productivity carbonate platform succession: the Upper Permian Wegener Halvø Formation, Karstryggen Area, East Greenland

The carbonate platform of the Upper Permian Wegener Halvø Formation in the Karstryggen area of central East Greenland is an example of a carbonate system with low production rates (2–3 cm kyr^–1) and differs from most other carbonate platforms by the lack of well-developed highstand progradation. The platform consists of three depositional sequences that formed in response to Kazanian sea-level cycles. Pinning point curves for the subaerial exposure surfaces separating the depositional sequences quantify the amplitude of the relative sea-level fluctuations in the range of 70–140 m. The platform developed on the karstified surface of an older Permian carbonate platform with a topographic relief locally exceeding 70 m. The predepositional relief influenced deposition in all three sequences. Transgressive systems tracts are thin and commonly dominated by condensed siliciclastic deposits in off-platform areas and palaeo-lows. Over palaeotopographic highs they consist of aggrading cementstones. Highstand deposits are limited to palaeotopographic elevated areas and consist of cementstone build-ups along the basin margin, and shallow subtidal to intertidal carbonates and evaporites in the platform area. Elsewhere, carbonate deposition took place during falling sea-level, and thin laterally extensive units of shallow-marine grainstones rest directly on top of deeper marine shales in the two first sequences, whereas thick prograding units of oolitic grainstones form the forced regressive systems tract of the uppermost sequence.     

Lithofacies and sequence development on an Upper Devonian mixed carbonate-siliciclastic fore-reef slope, Canning Basin, Western Australia

Detailed sedimentological and stratigraphical analysis coupled with conodont biostratigraphy of a fore-reef slope succession in the Napier Range (Napier Formation) is used to develop a depositional model and relative sea-level history for late Frasnian to late Famennian reef evolution in the Canning Basin of north-western Australia. Changes in sedimentary style on the slope, reflecting differing rates of carbonate production on the platform, are linked to third- and higher order relative sea-level fluctuations. Overlapping slope aprons accumulated along the base of a steep-walled platform margin. Coarse carbonate debris was deposited adjacent to the margin as talus breccias (via rockfall) and debris-flow breccias. Depositional slopes up to 45°, and locally steeper, are demonstrated using rotated geopetal cavity fills. The predominance of channel-filling lithofacies throughout the slope succession indicates the highly channelized nature of the aprons. The middle slope is dominated by sandy oolitic-peloidal turbiditic grainstones interpreted as sediment exported from an active platform. The turbidites and associated debris-flow breccias contrast with condensed carbonate intervals and deep-water, non-fenestral stromatolites that record times of very low platform production. Lower slope turbidites and associated intraclastic breccias indicate widespread redeposition of sediment eroded from lithified and semi-lithified limestones higher up the slope. Several third-order sequences are recognized in the fore-reef succession and these are composed primarily of transgressive and highstand deposits. Carbonate production was severely restricted in the early Famennian coinciding with development of onlapping siliciclastic aprons during a relative sea-level lowstand. Evidence for a subaerial exposure event is also preserved within the siliciclastic strata. Controls on sequence development are difficult to constrain. Although two sequence boundaries can be correlated with falls on the global sea-level curve, the reef complexes evolved in an active extensional regime and it is highly likely that tectonism, in conjunction with eustasy, controlled accommodation on the platform and therefore carbonate productivity.     

Palaeovalleys and protozoan assemblages in a Late Carboniferous cyclothem, Sydney Basin, Nova Scotia

The early Stephanian Bonar Cyclothem of the Sydney Basin, Nova Scotia, contains an erosional surface cut through coastal plain strata with economic coals and distributary channel bodies. The erosion surface is interpreted as a palaeovalley 20 m deep and at least 7 km wide that marks a sequence boundary formed during relative fall in sea level. The palaeovalley is filled with stacked alluvial channel bodies which become more isolated as the valley fill passes upward into red, alluvial plain deposits, probably laid down in an anastomosed river system. In an adjacent, interfluve area, calcretes and red, vertic palaeosols cap coastal strata. Assemblage analysis of agglutinated foraminifera and thecamoebians indicates that the palaeovalley was filled with freshwater sediments before an initial marine transgression flooded the alluvial surface and adjacent interfluve. Valley incision probably reflects glacioeustatic sea level fall. However, the alluvial nature of the valley deposits suggests that valley filling reflects an abundant sediment supply during lowstand and/or transgressive stages and was not a direct consequence of sea level rise. During the subsequent transgression phase, aggradation was rapid as sediment supply apparently kept pace with rising sea level. Features of both channel and extra-channel facies suggest that seasonality intensified during the transition from coastal plain to palaeovalley and alluvial plain deposition.     

鄂尔多斯盆地南缘奥陶系平凉组深水沉积特征及其与古环境关系--以陕西富平赵老峪地区为例

深水沉积及其与古地理的关系一直是沉积学领域研究的重点,也是薄弱环节之一。鄂尔多斯盆地南部富平赵老峪地区奥陶系平凉组深水沉积发育,岩性以泥晶灰岩及砾屑灰岩为主,局部可见泥灰岩、泥岩、硅岩及凝灰岩。沉积构造以流水波痕和小型交错层理为主,生物扰动极为发育。典型岩相有泥晶石灰岩夹硅岩和泥岩相、生物扰动泥晶灰岩相及块状层理灰岩相,分别代表斜坡原地沉积、等深流沉积及碎屑流沉积。三种沉积的δ13C-δ18O、Th-U、Ga-Cu、B-Cr、B-Ga、B-Sr、Ni/Co-U/Th、B/Ga-U/Th及87Sr/86Sr-Sr/Ba等地化指标差异明显。从下至上,斜坡原地沉积及碎屑流沉积规模逐渐减小,等深流沉积规模不断增加。另外,平凉组沉积时期,相对海平面整体上升,古盐度呈多个高-低旋回变化,古气候逐渐干燥,还原作用增强。其中,相对海平面升高,古盐度变化明显,湿润气候及较强还原作用有利于等深流沉积的发育;而相对海平面降低、干燥气候及构造运动有利于碎屑流沉积发育。     

Formation and fate of sedimentary depocentres on Southeast Asia's Sunda Shelf over the past sea-level cycle and biogeographic implications

Sea-level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late Quaternary times. Fluctuations on millennial time-scales produce locally complex deposits in coasts and on shelves, associated with short-term influence on species development. This article reviews the sedimentary and biogeographic history of the tropical siliciclastic Sunda Shelf as an end-member of continental shelves regarding extreme width, an enormous sediment supply, and highest biodiversity in response to rapid sea-level fluctuations. We describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data, field observations, and calculation of hydro-isostatic adjustment effects on changing relative sea level. These segments are characterized by individual sedimentary processes and deposits, and by a specific potential for material storage and re-mobilization.Long-term regressive intervals led to overall sigmoidal-promoting, extremely thick, and wide succeeding units. In contrast, rapid lateral shifts of defined depocentres over long distances took place in response to short-term sea level fluctuations. Fully isolated small-scale sediment bodies formed when sea level changed at exceptionally high rates. As a result of the high availability of organic-rich sediments, mangrove and freshwater peats formed frequently over late Quaternary times. The appearance of thick, massive and widespread peats is mainly linked to time intervals of a sea-level rise at slow rates, whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level.The preservation potential of the regressive units is generally high due to highest initial sediment supply, stabilizing soil formation during exposure and rapid subsidence. Preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change. Besides complex channel incision, an overall lowering of the sediment surface related to erosion, as deep as 20 m or more, over wide areas took place mainly during sea level lowering. The final export of shelf material is documented by enormous mass-wasting packages on the associated continental slope.From a palaeogeographic perspective, the rapid formation or disappearance of special habitat zones, such as mangrove fringes and extended mud flats, led to species establishment or truncation in distribution. In addition, the opening or closure of ocean passages, as narrow bridges allowing limited species crossing or as fully colonized corridors, had severe impact on eco-fragmentation and the expansion or contraction of species. Independent of such particular conditions, sea-level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast-related ecosystems.     

High-frequency cyclicity (Milankovitch and millennial-scale) in slope-apron carbonates: Zechstein (Upper Permian), North-east England

The Upper Permian (Zechstein) slope carbonates in the Roker Formation (Zechstein 2nd‐cycle Carbonate) in North‐east England consist of turbidites interbedded with laminated lime‐mudstone. Studies of turbidite bed thickness and relative proportion of turbidites (percentage turbidites in 20 cm of section) reveal well‐developed cyclicities consisting of thinning‐upward and thickening‐upward packages of turbidite beds. These packages are on four scales, from less than a metre, up to 50 m in thickness. Assuming that the laminae of the hemipelagic background sediment are annual allows the durations of the cycles to be estimated. In addition, counting the number and thickness of turbidite beds in 20 cm of laminated lime‐mudstone, which is approximately equivalent to 1000 years (each lamina is 200 μm), gives the frequencies of the turbidite beds, the average thicknesses and the overall sedimentation rates through the succession for 1000 year time‐slots. Figures obtained are comparable with modern rates of deposition on carbonate slopes. The cyclicity present in the Roker Formation can be shown to include: Milankovitch‐band ca 100 kyr short‐eccentricity, ca 20 kyr precession and ca 10 kyr semi‐precession cycles and sub‐Milankovitch millennial‐scale cycles (0·7 to 4·3 kyr). Eccentricity and precession‐scale cycles are related to ‘highstand‐shedding’ and relative sea‐level change caused by Milankovitch‐band orbital forcing controlling carbonate productivity. The millennial‐scale cycles, which are quasi‐periodic, probably are produced by environmental changes controlled by solar forcing, i.e. variations in solar irradiance, or volcanic activity. Most probable here are fluctuations in carbonate productivity related to aridity–humidity and/or temperature changes. Precession and millennial‐scale cycles are defined most strongly in early transgressive and highstand parts of the larger‐scale short‐eccentricity cycles. The duration of the Roker Formation as a whole can be estimated from the thickness of the laminated lithotype as ca 0·3 Myr.     

Origin and evolution processes of hybrid event beds in the Lower Cretaceous of the Lingshan Island,Eastern China

On the basis of detailed sedimentological investigation, three types of hybrid event beds (HEBs) together with debrites and turbidites were distinguished in the Lower Cretaceous sedimentary sequence on the Lingshan Island in the Yellow Sea, China. HEB 1, with a total thickness of 63–80 cm and internal bipartite structures, is characterised by a basal massive sandstone sharply overlain by a muddy sandstone interval. It is interpreted to have been formed by particle rearrangement at the base of cohesive debris flows. HEB 2, with a total thickness of 10–71 cm and an internal tripartite structure, is characterised by a normal grading sandstone base, followed by muddy siltstone middle unit and capped with siltstones; the top unit of HEB 2 may in places be partly or completely eroded. The boundary between the lowest unit and the middle unit is gradual, whereas that between the middle unit and the top unit is sharp. HEB 2 may be developed by up-dip muddy substrate erosion. HEB 3, with a total thickness up to 10 cm and an internal bipartite structure, is characterised by a basal massive sandstone sharply overlain by a muddy siltstone interval. The upper unit was probably deposited by cohesive debris flow with some plant fragments and rare mud clasts. HEB 3 may be formed by the deceleration of low-density turbidity currents. The distribution of HEBs together with debrites and turbidites implies a continuous evolution process of sediment gravity flows: debris flow → hybrid flow caused by particle rearrangement → high-density turbidity current → hybrid flow caused by muddy substrate erosion → low-density turbidity current → hybrid flow caused by deceleration.     

塔里木盆地北部寒武纪海平面变化研究

在以往的沉积学研究中，常常用剖面中的相对水深变化来讨论海平面的变化规律。本文通过对塔里木盆地北部寒武纪上超点变化曲线与肖尔布拉克地区寒武系露头剖面中相对水深变化曲线的对比研究发现，上超点变化曲线与相对水深变化曲线具有明显的差异，前者在寒武纪呈持续上升趋势，而后者除早期有一快速上升外，总体呈持续下降趋势，这说明相对水深变化虽与海平面变化存在着本质的联系，但单凭露头剖面中相对水深变化的研究是难以对海平面变化作出正确估价的，还必须综合考虑基底沉降、沉积物供给等多种因素的影响。计算机模拟结果也证实了，在碳酸盐台地的浅水处，当基底沉降速率较低时，无论是海平面上升还是下降时期，相对水深均呈现持续下降趋势。为了能从露头剖面的地层层序中来了解海平面的变化规律，本文应用作者改进后的Ｆｉｓｃｈｅｒ图解和数学方法对肖尔布拉克寒武系剖面进行了海平面变化的重建，结果发现，它们与上超点变化曲线的变化趋势完全一致。这充分说明了海平面变化对沉积层序的控制作用，同时也告诉我们，不能简单地利用露头剖面中的相对水深变化来研究地质历史时期的海平面变化规律。只有在排除了不同沉积背景上基底沉降速率和沉积物供给速率对沉积层序的控制效应后，才能从露头剖面?     

四川江油马角坝地区上石炭统层序地层研究

应用露头层序地层学基本原理和方法、结合碳酸盐岩微相、古生物学、成岩作用、沉积相和沉积事件等理论,对江油马角坝地区上石炭统碳酸盐岩地层进行了露头层序地层研究。研究表明,研究区晚石炭世为典型的陆表海沉积,地形平坦,地壳下降相对缓慢,海水较浅,沉积物界面与海平面接近,主体为碳酸盐岩开阔台地和浅滩沉积环境。水体主体深度为0~50m,为低幅高频低速的海平面变化。根据沉积层序界面的露头和镜下标志,共划分出7个三级层序,反映了研究区晚石炭世具有7次相对较大的海平面升降旋回。发育TST和HST,普遍缺失LST,局部CS难以识别。平均每个层序时限约为2.7Ma,并伴随着多次四级、五级乃至高频海平面变化,共同组成了研究区简单而复杂的碳酸盐岩地层格架。动力机制主要源于晚古生代冰川型全球海平面变化,其原因可能为晚古生代冰期(大洋水体积的周期型变化或大陆冰盖的增长和衰减)和风暴事件共同作用的结果,同时受到古地形、物源和沉积物供应量变化的制约。其中,CSQ1、CSQ3和CSQ4对应的海平面变化幅度较大,海平面变化速率主要为突发性的快递上升到缓慢下降或缓慢上升到相对下降的特点。CSQ2、CSQ5、CSQ6和CSQ7对应的海平面变化幅度不大,为缓慢上升和缓慢下降的特点。     

Quantification of input and compositional variations of calciturbidites in a Middle Triassic basinal succession (Seceda, Dolomites, Southern Alps)

Triassic calciturbidites were studied in a 100-m long core and nearby outcrops of the basinal Buchenstein Formation to determine composition and thickness variations. The quantity of recognized turbidite sediment relative to background sediment changes from 15% (by volume) in the lower part to 60% in the upper part, reflecting the steady progradation of nearby platforms. The composition of the sand fraction of 214 turbidites was point-counted in thin sections. Micrite peloids (average 23%) and lithoclasts (16%) are by far the most dominant constituents. They are interpreted as two different varieties of in-situ precipitated micrite (automicrite), which probably formed under the influence of microbes and constitute the principal building material of the adjacent platforms. Platform-derived skeletal grains amount to only 0.5%. Variations in turbidite composition were quantified using Spearman's rank correlation and cluster analysis. The most significant compositional variations seem to be related to hydrodynamic sorting in the turbidity currents and to the gradual shift from distal to more proximal turbidites in the core as the platforms prograded basinward. Cluster analysis of the 214 samples shows a major subdivision into micrite and sparite dominated turbidites. Clusters associated with micrite-dominated turbidites are enriched in Radiolaria and thin-shelled bivalves, whereas the clusters related to sparite-dominated turbidites show an abundance of lithoclasts. This subdivision seems strongly related to sorting effects in a turbidity current. Point-counting of turbidites in nearby outcrops revealed a lateral variation in composition. Proximal turbidites are sparite-dominated and enriched in lithoclasts, distal portions are chiefly micrite with an open-ocean biota (thin-shelled bivalves, Radiolaria). This differentiation resembles the vertical change in composition of thick turbidite beds, and is attributed to different settling rates of the various grains in the turbidity current. There is no indication that turbidite composition fluctuated significantly under the influence of sea-level fluctuations. This is not surprising because the dominant automicrite facies of the platforms only migrates laterally, but does not change much during sea-level cycles.     

Depositional framework and controls on mixed carbonate-siliciclastic gravity flows: Pennsylvanian-Permian shelf to basin transect, south-western Great Basin, USA

Mixed carbonate-siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley - Owens Valley region of east-central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate-siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non-channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles. Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz-rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point-sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta-fed submarine ramp facies model, which is a line-sourced, shelf-fed system that is not supply limited. The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.     

The unusual cyclicity of the Triassic (Carnian) Bleiberg facies of the Wetterstein Formation (Drau Range,Austria)

The carbonate platform of the upper Wetterstein Formation (Cordevol) of the Drau Range (Austria) shows a distinct facies differentiation which depends on the palaeotopography. Although most areas of the platform were permanently flooded, a periodically emergent palaeotopographic high (Bleiberg facies) occurred near to the centre of the platform. The resulting cyclic sequences are composed of subtidal and intertidal to supratidal carbonates and emersion horizons. Typical shallowing upward cycles occur, as well as deepening upward, symmetrical and incomplete cycles. These types of cycles, with long-term emersion phases, can be explained by eustatic sea-level oscillations. It can be shown, due to an excellent lithostratigraphy based on distinct emersion horizons, that the cycle-type varies along a marker bed while the vertical distance between the emersion horizons is retained. Therefore the cycles cannot be explained by eustatic sea-level oscillations alone; local events which produce a local relief are necessary.A compiled profile of the Bleiberg facies is simulated with the help of a one-dimensional, parametric computer model. The quantification of some parameters which result from this simulation is used to verify and quantify the effects of relief on sedimentation.     

一种低海平面时期的沉积岩相——深水盆地相碳酸盐砾岩层序

作者认为北美阿巴拉契亚地区和我国贺兰山地区奥陶系的一套碳酸盐砾岩层序,就是低海平面时期的一种特殊沉积岩相。这种岩相不仅是古海洋条件变化的一种标志,而且也是良好的油气储集层。文中主要从沉积学的角度解释了它们的形成机制。     

塔里木盆地寒武-奥陶纪海平面升降变化规律研究

利用元素地球化学特征和地震几何构型特征,对塔里木盆地寒武-奥陶纪时期海平面升降变化规律进行了研究,建立了寒武-奥陶纪相对海平面变化曲线。研究发现,塔里木盆地寒武-奥陶纪可划分出3个一级旋回,8个二级旋回,17个三级旋回和多期高频震荡旋回。其中,寒武纪经历了3期二级旋回,早中奥陶世经历了2期旋回。相对于寒武纪,早奥陶世海平面升幅较大。晚奥陶世经历了3期二级旋回：第一期处于海平面低位期,幅度较小;后二期快速上升,直达最高水位状态。海平面相对升降变化控制着盆地的沉积充填和层序的形成演化,其旋回性与盆地沉积作用的旋回性相一致,并可通过沉积相的演变表现出来。     

Three‐dimensional modelling and sequence stratigraphy of a carbonate ramp‐to‐shelf transition,Permian Upper San Andres Formation

This study highlights three‐dimensional variability of stratigraphic geometries in the ramp crest to basin of mixed carbonate–siliciclastic clinoforms in the Permian San Andres Formation. Standard field techniques and mapping using ground‐based lidar reveal a high degree of architectural complexity in channellized, scoured and mounded outer ramp stratigraphy. Development of these features was a function of location along the ramp profile and fluctuations in relative sea‐level. Deposition of coarse‐grained and fine‐grained turbidites in the distal outer ramp occurred through dilute and high‐density turbidity flows and was the result of highstand carbonate shedding within individual cycles. In this setting, high‐frequency cycles of relative sea‐level are interpreted on the basis of turbidite frequency, lateral extent and composition. Submarine siliciclastic sediment bypass during lowstand cycles resulted in variable degrees of siliciclastic preservation. Abundant siliciclastic material is preserved in the basin and distal outer ramp as point‐sourced lowstand wedges and line‐sourced early transgressive blankets. In mounded topography of the outer ramp, siliciclastic preservation is minimal to absent, and rare incised channels offer the best opportunity for recognition of a sequence boundary. Growth of mounded topography in the outer ramp began with scouring, followed by a combination of bioherm construction, fusulinid mound construction and isopachous draping. Intermound areas were then filled with sediment and continued mound growth was prevented by an accommodation limit. Mound growth was independent of high‐frequency cycles in relative sea‐level but was dependent on available accommodation dictated by low‐frequency cyclicity. Low‐angle ramp clinoforms with mounded topography in the outer ramp developed during the transgressive part of a composite sequence. Mound growth terminated as the ramp transformed into a shelf with oblique clinoform geometries during the highstand of the composite sequence. This example represents a ramp‐to‐shelf transition that is the result of forcing by relative sea‐level fluctuations rather than ecologic or tectonic controls.     

Classification of clastic coastal depositional environments

This paper proposes a new classification for clastic coastal environments which includes the full range of major depositional settings including deltas, strand plains, tidal flats, estuaries and lagoons. This classification includes both morphologic and evolutionary components and is based on dominant coastal processes. It has the potential to predict responses in geomorphology, facies and stratigraphy. The significance of this classification is its evolutionary capability, and its inclusion of all major clastic coastal depositional environments, making it more comprehensive than previous classifications.

We employ a ternary process classification with two axes. The first (horizontal axis) is defined as the relative power of wave versus tidal processes. The second (vertical) axis represents relative fluvial power (increasing upward). A ternary diagram defined by these axes can be used to illustrate the genetic process-response relationships between major coastal environments. The evolutionary classification combines the concept of two sediment sources (river and marine) with a relative sea-level parameter to classify embayed as well as linear and elongate/lobate shorelines. This approach identifies the evolutionary relationships between coastal sedimentary environments.

The new ternary approach to process classification can be applied to estuaries and lagoons to define wave and tide end-member facies models, each consisting of a tripartite facies zonation. The evolutionary classification is compatible with sequence stratigraphy because sediment supply and relative sea level are included, and serves as a starting point for more refined coastal stratigraphic analyses.     

Anatomy and origin of toplap in a mixed carbonate-clastic system, Seven Rivers Formation (Permian, Guadalupian), Guadalupe Mountains, New Mexico, USA

The Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Mountains, New Mexico, reveals complex relations between long- and short-term relative changes in sea-level, shelf configuration and sedimentation, which interacted to create a distinct toplap geometry. At least five sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and create a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well-sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palaeotopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary unit (updip), merging sandstones contain rare sedimentary structures including evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fabrics and mudcracks. Laterally (downdip), the sandstone-bounding peritidal carbonate facies pass into subtidal carbonate facies (ooid-peloid-fusulinid-dasyclad-mollusc pack- and grainstone) and interbedded sandstones contain sedimentary structures such as ripple marks and trough to planar cross-stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continued in still-submerged areas downdip from the shelf crest, and hence represents depositional toplap. Physical tracing of subaerial exposure surfaces suggests that the shoreline migrated up and down palaeoslope several times. The vertical component of five short-term shoreline migrations decreased during formation of the toplap geometry. Sea-level rose to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high-‘frequency’ relative changes of sea-level that combined to produce the larger-scale geometry. We suggest that changing amplitudes of relative sea-level may play a significant role in the stratigraphic evolution of platforms and that separating ‘short-term’and ‘long-term’relative sea-level may be ambiguous in such instances.     

江汉平原及邻区海相碳酸盐岩的古岩溶特征及控制因素

对于露头剖面的观察、相关样品分析及钻井资料的研究表明,区内碳酸盐台地在形成、发展及消亡过程中,由于不同级次的相对海平面下降,台地沉积过程中断,出现暴露并伴随溶蚀作用,在台地埋藏后还受到地下热水的溶蚀改造。将区内古岩溶划分为同生期层间岩溶、裸露期古风化壳岩溶、埋藏期热水岩溶等三类。同生期层间岩溶多与相对短时间的海平面下降有关,以发育溶蚀残积物、溶孔(铸模孔)等为特征。裸露期古风化壳岩溶表现为台地长时间的暴露及溶蚀,一般可见明显的古风化壳岩溶垂向分带;埋藏期热水岩溶以非选择性溶蚀为特征。控制碳酸盐台地古岩溶发育的主要因素有气候、全球性海平面变化、古地形、古构造等因素。初步探讨了区内古岩溶与储层发育的关系。     

The frequency and periodicity of preserved turbidites in submarine fans as a quantitative record of tectonic uplift in collision zones

The frequency and periodicity of preserved graded turbidite cycles in submarine fans in the Coral Sea and Sea of Japan are correlated to times of tectonic uplift in response to major collisions in the Owen-Stanley Range of Papua and the Hida Range of Japan, respectively. Large frequencies and shorter-term periodicities of turbidites at DSDP Site 210 were coeval with early Pliocene maximum tectonic-uplift rates which occurred in the Owen-Stanley Range in response to obduction. Similarly, large frequencies and shorter-term periodicities of turbidites at Site 299 (Toyama Submarine Fan) were coeval with the late Pleistocene uplift in the Hida Range; this uplift of 1000 to 1500 m occurred in response to collision tectonics. In both cases, trends of increasing frequencies and towards shorter-term periodicities of preserved turbidite depositional events correlate to trends of increasing rates of tectonic uplift.The role of sea-level fluctuations on changing denudation rates in these two collision zones is secondary. At Site 210, larger frequencies and short-term periodicities of preserved turbidites were coeval with early Pliocene high stands of sea level, whereas at Site 299, Pleistocene sea-level fluctuations are considered minor because at low stands of sea level, both relief and denudation rates were increased by about ten to 14%. At Site 286 (New Hebrides Basin), Eocene turbidite deposition is coeval with high stands of sea level, whereas at Site 297 (Northern Shikoku Basin), turbidite deposition was coeval with both rising and falling sea level.Analysis of both frequency and periodicity of turbidites by fan subenvironment at Site 299 indicates a record of continuous deposition, and maintainance of frequency and periodicity trends controlled by tectonic uplift. Late Pleistocene channel and overbank deposits showed periodicity differences of less than 28% of an order of magnitude, whereas Miocene-Pliocene overbank and distal turbidite periodicities differed by a 19% order of magnitude. Greater differences in magnitude occurred between distal turbidites or early Pleistocene age and Pliocene age than between Miocene-Pliocene overbank and distal turbidite deposition with a magnitude difference of 860%. These findings suggest that shifting depocenters and differences in sedimentation history in subenvironments of submarine fans are secondary to the role of tectonic uplift in this particular case.The minimal rate of tectonic uplift required to generate deep-sea fan turbidities appears to be approximately 400 m/million years. This figure is tentative and is based on very few observation points.Frequency and periodicity of preserved turbidite cycles in submarine fans in active continental margins and ancient counterparts should provide an independent measurement of rates and timing of tectonic uplift, particularly in collision terrains. Because this sediment parameter is a record of a single process from a single source and a record of “event stratigraphy”, its usage is preferable over standard and bulk sediment accumulation rates determined from age depth curves.