Sediment transport in analogue flume models compared with real‐world sedimentary systems: a new look at scaling evolution of sedimentary systems in a flume

Evolution of sedimentary systems at large temporal and spatial scales cannot be scaled down to laboratory dimensions by conventional hydraulic Froude scaling. Therefore, many researchers question the validity of experiments aiming to simulate this evolution. Yet, it has been shown that laboratory experiments yield stratigraphic responses to allocyclic forcing that are remarkably similar to those in real‐world prototypes, hinting at scale independency with strong dependence on boundary conditions but weak dependence on the actual sediment transport dynamics. This paper addresses the dilemma by contrasting sediment transport rules that apply in the laboratory with those that apply in real‐world geological systems. It is demonstrated that the generation of two‐dimensional stratigraphy in a flume can be simulated numerically by the non‐linear diffusion equation. Sediment transport theory is used to demonstrate that only suspension‐dominated meandering rivers should be simulated with linear diffusion. With increasing grain‐size (coarse sand to gravel) and shallowness of river systems, the prediction of long‐term transport must be simulated by non‐linear, slope‐dependent diffusion to allow for increasing transport rates and thus change in stratigraphic style. To point out these differences in stratigraphic style, three stages in infill of accommodation have been defined here: (i) a start‐up stage, when the system is prograding to base level (e.g. the shelf edge) with no sediment flux beyond the base‐level point; (ii) a fill‐up stage, when the system is further aggrading while progressively more sediment is bypassing base level with the progression of the infill; and (iii) a keep‐up stage, when more than 90% of the input is bypassing the base level and less than 10% is used for filling the accommodation. By plotting the rate of change in flux for various degrees of non‐linearity (varying the exponent in the diffusion equation) it was found that the error between model and real‐world prototype is largest for the suspension‐dominated prototypes, although never more than 30% and only at the beginning of the fill‐up stage. The error reduces to only 10% for the non‐linear sandy‐gravelly and gravelly systems. These results are very encouraging and open up ways to calibrate numerical models of sedimentary system evolution by such experiments.     

Shaler: in situ analysis of a fluvial sedimentary deposit on Mars

This paper characterizes the detailed sedimentology of a fluvial sandbody on Mars for the first time and interprets its depositional processes and palaeoenvironmental setting. Despite numerous orbital observations of fluvial landforms on the surface of Mars, ground‐based characterization of the sedimentology of such fluvial deposits has not previously been possible. Results from the NASA Mars Science Laboratory Curiosity rover provide an opportunity to reconstruct at fine scale the sedimentary architecture and palaeomorphology of a fluvial environment on Mars. This work describes the grain size, texture and sedimentary facies of the Shaler outcrop, reconstructs the bedding architecture, and analyses cross‐stratification to determine palaeocurrents. On the basis of bedset geometry and inclination, grain‐size distribution and bedform migration direction, this study concludes that the Shaler outcrop probably records the accretion of a fluvial barform. The majority of the outcrop consists of large‐scale trough cross‐bedding of coarse sand and granules. Palaeocurrent analyses and bedform reconstruction indicate that the beds were deposited by bedforms that migrated towards the north‐east, across the surface of a bar that migrated south‐east. Stacked cosets of dune cross‐bedding suggest aggradation of multiple bedforms, which provides evidence for short periods of sustained flow during Shaler deposition. However, local evidence for aeolian reworking and the presence of potential desiccation cracks within the outcrop suggest that fluvial deposition may have been intermittent. The uppermost strata at Shaler are distinct in terms of texture and chemistry and are inferred to record deposition from a different sediment dispersal system with a contrasting provenance. The outcrop as a whole is a testament to the availability of liquid water on the surface of Mars in its early history.     

Last millennium development and dynamics of vegetated linear dunes inferred from ground‐penetrating radar and optically stimulated luminescence ages

Using ground‐penetrating radar, optically stimulated luminescence dating, particle‐size distribution and morphological analysis, the study of the construction phases of a vegetated linear dune in the arid north‐western Negev dunefield of Israel during the last millennium improves current knowledge about vegetated linear dunes that developed in the late Pleistocene. Vertical accretion in rapid pulses forming horizontally bedded units along the axis of vegetated linear dunes, regardless of their age, was found to be characteristic of vegetated linear dunes. The combination of the unique topographic feature of a longitudinal 5 m step‐like fall in dune crest elevation with the substantial narrowing of dune width constitutes a distinct morphological marker for interpreting local dune growth and stabilization of the last, albeit localized, dune mobilization episode at ca 0·5 ka. Evidence for lateral dune migration was not observed. Where local sediment supply exists, short episodes of powerful winds within the Holocene (with recurrence intervals separated by hundreds of years) can lead to the construction of vegetated linear dunes. The spatially constrained extent of such young dunes in the north‐western Negev may be due to limited sand availability because most of the Negev dunes were stable during the Holocene. These findings imply that vegetated linear dune construction can occur in glacial and interglacial (including Holocene) environments in semi‐arid to arid climates if certain conditions are met. In times of increased wind power during the Anthropocene, a period characterized by simultaneous rises in the human impact on the landscape and in climate variability (i.e. drought), local growth of vegetated linear dunes can be expected. This study demonstrates that ground‐penetrating radar is a reliable tool for interpreting the shallow internal structure of young vegetated linear dunes.     

Onset of natural terrain landslides modelled by linear stability analysis of creeping slopes with a two-state variable friction law

This paper further examines the possibility of modelling landslide as a consequence of the unstable slip in a steadily creeping slope when it is subject to perturbations, such as those induced by rainfall and earthquakes. In particular, the one-state variable friction law used in the landslide analysis by Chau is extended to a two-state variable friction law. According to this state variable friction law, the shear strength (τ) along the slip surface depends on the creeping velocity (V) as well as the two state variables (θ₁ and θ₂), which evolve with the ongoing slip. For translational slides, a system of three coupled non-linear first-order ordinary differential equations is formulated, and a linear stability analysis is applied to study the stability in the neighbourhood of the equilibrium solution of the system. By employing the stability classification of Reyn for three-dimensional space, it is found that equilibrium state (or critical point) of a slope may change from a ‘stable spiral’ to a ‘saddle spiral with unstable plane focus’ through a transitional state called ‘converging vortex spiral’ (i.e. bifurcation occurs), as the non-linear parameters of the slip surface evolve with its environmental changes (such as those induced by rainfall or human activities). If the one-state variable friction law is used in landslide modelling, velocity strengthening (i.e. dτ_ss/dV > 0, where τ_ss is the steady-state shear stress) in the laboratory always implies the stability of a creeping slope containing the same slip surface under gravitational pull. This conclusion, however, does not apply if a two-state variable friction law is employed to model the sliding along the slip surface. In particular, neither the region of stable creeping slopes in the non-linear parameter space can be inferred by that of velocity strengthening, nor the unstable region by that of velocity weakening. Copyright © 1999 John Wiley & Sons, Ltd.     

Stratigraphy,sedimentary development and palaeoenvironmental context of a naturally accumulated pitfall cave deposit from southeastern Australia

McEachern's Deathtrap Cave (G—49/50) is located in the Lower Glenelg region of southeastern Australia and records a Late Pleistocene to Holocene sedimentary record that has been directly influenced by surface processes during its formation. The sedimentary sequence contained within the cave is divided into lower, middle and upper sequences consisting of eight facies. The lower sequence represents the earliest phase of sedimentation, and groundwater fluctuations during the Last Interglacial period resulted in its erosion and redistribution deeper into the cave system. A decrease in the magnitude and frequency of flood events in the cave during the formation of the middle sequence indicates increasingly drier surface conditions prior to the Last Glacial Maximum. The middle sequence has a minimum age of 9840 ± 290 a BP. Moving sand sheets during the Last Glacial blocked the entrance to the cave allowing flowstones to develop on the cave floor. The surface environment surrounding the cave was probably not as dry as contemporaneous inland sites because sedimentation continued to be dominated by flowing water during this period. Holocene sedimentation is represented by the upper sequence and reflects wetter cave conditions between 7680 ± 160 a BP and 5700 ± 110 a BP. A major phase of sediment accretion occurs after 5700 a BP and correlates to a phase of dune instability in the Lower Glenelg region. Flowing water remodelled the sediment cone sometime after 2240 ± 100 a BP, which represents a period of increased surface runoff, although it is not clear whether this is due to climatic or anthropogenic influences.     

Carbonate shedding and sedimentary cyclicities of a distally steepened carbonate ramp (Miocene, Great Bahama Bank)

Received: 10 May 1999 / Accepted: 16 October 1999     

低可容纳空间三角洲层序地层及其控砂模式

为了揭示低可容纳空间三角洲沉积背景下层序发育及砂体分布特征,运用层序地层学的相关理论和方法,结合岩心、测井、录井和地震等资料,以珠江口盆地白云凹陷北坡珠江组下段为例进行了层序地层和控砂模式的研究。识别出强制海退和正常海退造成的暴露侵蚀不整合面、与暴露侵蚀界面相对应的整合面、海泛面、最大海泛面、强制海退底面和最大海退面等6种界面,将研究区珠江组下段划分为一个半三级层序;并指出低可容纳空间三角洲沉积背景下沉积界面与基准面一致或略低于基准面,准层序以与海泛面对应的暴露侵蚀面为界面,且主要形成于基准面上升半旋回,区域基准面相对海平面的下降是控制层序边界形成的主要因素;平面上砂体赋存在三角洲前缘和下三角洲平原的沉积地区,新增可容纳空间的形成速率及剩余可容纳空间的原有规模是控制砂体横向连续分布的关键因素;垂向上砂体分布在SQ1层序基准面缓慢上升到下降之前形成的高位体系域和SQ2层序基准面下降最低点到缓慢上升至最大海退面形成的低位体系域,这两套砂体横向连续性较好,存在良好的生储盖组合和油气输导条件,是形成大型油气藏的有利部位。     

Calcite formation in microbial mats: modeling and quantification of inhomogeneous distribution patterns by a cellular automaton model and multifractal measures

The evolution of early diagenetic calcite cements in microbial mats of recent supratidal sediments of the southern North Sea is modeled in a two-dimensional microscale approach by a cellular automaton model (CAM). Calcite is traced out in the model by virtual calcium distribution patterns obtained from runs under different assumptions concerning sediment-intrinsic conditions. For justification of the CAM, real calcium distribution patterns, documented by scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDX), are quantitatively compared with the virtual patterns on the basis of multifractal analyses. The formation of high magnesian calcite as a consequence of biogenic anaerobic decomposition of organic matter starts at certain initial calcite domains. In this stage an inhomogeneous and multifractal calcium distribution is characteristic. Nearly complete remineralization of organic matter leads to monofractal behavior of generalized fractal dimensions (D_B(q) ±1.84). The CAM results confirm that calcite formation is a self-determining morphogenetical process and diffusive transport processes of reactants within the mat affect the biogenic calcite formation.     

遗迹化石对显生宙5大生物-环境事件的响应

通过系统梳理与奥陶纪-志留纪、晚泥盆世弗拉期-法门期、二叠纪-三叠纪、三叠纪-侏罗纪、白垩纪-古近纪之交 的5次生物大灭绝期遗迹化石记录相关的生物和环境事件,发现遗迹化石对5次大灭绝事件为负响应,即在大灭绝事件之后 的残存期和复苏期期间,遗迹化石的多样性、丰度、潜穴直径、生物扰动强度、遗迹组构阶层都大为减小.遗迹化石反映的造迹 生物行为习性和觅食策略在5次生物大灭绝事件后也各有不同,食沉积物性觅食策略在晚奥陶世和晚白垩世大灭绝事件之 后占据主导,滤食性觅食策略在晚三叠世大灭绝事件之后占据主导,机会主义遗迹(如Planolites)、食沉积物性和滤食性等多 种觅食策略和行为习性在晚泥盆世F-F和晚二叠世两次大灭绝事件之后占据主导.晚泥盆世F-F和晚二叠世两次大灭绝事件 之后,遗迹化石记录了底栖生物系统由简单向复杂、由二维向三维生态空间拓展的变化趋势.      

Biostratigraphy,palaeoenvironments and sequence stratigraphy of the Aptian sedimentary succession at Jebel Bir Oum Ali (Northern Chain of Chotts,South Tunisia): Comparison with contemporaneous Tethyan series

The Aptian sedimentary succession of the Chott region in southern Tunisia was deposited on the margin of the Saharan shield, and is punctuated by numerous hiatuses that separate seven 3rd-order depositional sequences. Early Aptian deposits correspond to the Berrani Member (early Bedoulian), which was deposited contemporaneously with the large carbonate platforms with rudists that developed under oligo-mesotrophic, tropical environmental conditions on both margins of the Tethys. Late Bedoulian sediments were deposited under mesotrophic conditions characterized by seagrass, algae, abundant orbitolinids and aragonite-producing organisms. The early to late Aptian transition was marked by the temporary disappearance of carbonate platforms and an important renewal of the microfauna, whose tests and skeletons became less and less aragonite-rich and more and more calcite-rich and arenaceous. This episode is reported from both Tethyan margins. The platform was subsequently flooded, and dysoxic environments with annelids marked the end of the early Aptian carbonate platform development. An arid and probably colder episode (earliest late Aptian) induced the deposition of gypsum in an intrashelf basin. Following on top, the return to more humid conditions triggered an enhanced input of detrital material in fluvio-deltaic environments (late Aptian). Finally, the return to oligo-mesotrophic, marine conditions allowed the temporary installation of wide lagoons with rudists in the latest Aptian and probably in the earliest Albian. Long-distance correlations have been established by means of benthic foraminiferal occurrences. They highlight the importance of stratigraphic gaps linked to low sea levels, which have been tentatively estimated.     

Internal structure of a trough blowout, determined from migrated ground-penetrating radar profiles

Ground‐penetrating radar (GPR) was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits. Although analogous, many of the data‐processing techniques routinely applied in seismic reflection are very rarely applied in GPR studies. In this study, a simple migration program was used that significantly enhanced the quality of GPR images from a large trough blowout at Raven Meols on the Sefton coast, northwest England. These improvements aided subsequent data interpretation, which was achieved through application of the principles of radar stratigraphy. GPR shows the pre‐blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover. Subsequent to stabilization of these dunes a thin soil developed. This dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout. The internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition, as derived from contemporary process studies reported in the literature. Instead, the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand‐transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology. Trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high‐angle cross‐strata, which often display a spatially‐variable radial dip pattern that is only very poorly or partially developed. In addition, a soil, or other surface representing a significant hiatus in dune deposition, is likely to underlie the blowout deposits, the topography of which will show a clear relationship to the dip and orientation of the overlying cross‐strata.     

Dynamics of sedimentary environments in the accelerated siltation of a reservoir: the case of Alhama de Granada,southern Spain

An analysis has been made of sedimentary systems involved in the rapid silting of a reservoir constructed in 1974 in Alhama de Granada (S. Spain); in only 30 years the storage capacity of the reservoir has shrunk by 80% and its perimeter has decreased by 64%. A study of sediment lithofacies identified in a series of shallow trenches and of georadar facies identified in a series of almost 900 m lines of ground penetrating radar (GPR) images, together with a survey of surface geology, has identified 3 alluvial systems (2 transversal systems and a longitudinal system) whose deltas have filled in the reservoir. Thus, there are three phases in the evolution of the reservoir siltation: (1) an initial stage (1974–1977) typified by northward progradation of the longitudinal river delta of about 100 m year⁻¹ and an eastward progradation of the transversal system delta of about 20 m year⁻¹; (2) an intermediate stage (1977–1984) in which the longitudinal river delta progradation slowed to 25 m year⁻¹ and the axial drainage became obstructed due to the considerable eastward progradation of the transversal delta; and (3) a final phase (1984–present) in which there have been few changes in the areal distribution of the deltas apart from a southeastward expansion of the transversal delta. Generally, aggradational growth patterns (vertical accretion) have dominated in this final phase. The lithology of the source area, the slope and precipitation distribution has a significant effect not only on the sediment supply, but also indirectly on the creation of accommodation space and on the evolution of stratal growth patterns.     

Seismic source mechanism inversion from a linear array of receivers reveals non-double-couple seismic events induced by hydraulic fracturing in sedimentary formation

We developed a new method to invert source mechanisms of seismic events observed from a single (vertical or near vertical) array of receivers. This method was tested on a synthetic dataset and applied to a seismic dataset acquired during hydraulic fracturing of a sedimentary formation. We show that a crack-opening seismic event recorded on a single vertical array can be inverted as a pure shear seismic event, indicating that the best fitting shear mechanisms inverted from seismic data acquired along a vertical borehole provide misleading information. We found that the majority of the analyzed source mechanisms of the induced events are not consistent with pure shear faulting as previously thought. The non-shear source mechanisms are required even when seismic noise, location uncertainty, and uncertainty in the attenuation structural model are considered.     

山西晋中盆地晚新生代地层划分、沉积环境及其先秦以来气候和湖泊演化

【研究目的】晋中盆地位于汾渭断陷盆地带的中部,新生代最大沉积厚度超过3800 m,建立晋中盆地精确地层年代框架和高分辨率沉积层序对于理解汾渭断陷盆地带的演化、华北地区气候变化具有重要意义。【研究方法】本文对晋中盆地东北部晚新生代标准孔ZK01（钻孔深度870.5 m）岩心开展了年代学和沉积学研究。【研究结果】将ZK01孔揭示的松散沉积物分为6个岩性组。首次通过磁性地层学研究,划分出13个正极性时段和12个负极性时段,建立了ZK01孔地层年代框架。晋中盆地新生代沉积物底部最老年龄约8.1 Ma,将晋中盆地活动的起始时间从前人一般认为的上新世初期,确定为中新世晚期,提前了约3 Ma。区内晚新生代地层划分为新近系中新统灞河阶（N₁⁴）和保德阶（N₁⁵）,上新统高庄阶（N₂¹）和麻则沟阶（N₂²2）,第四系下更新统泥河湾阶（Qp¹）,中、上更新统和全新统。【结论】构造活动与气候变化是盆地沉积环境的主要控制因素,共同导致了上新世早期和早更新世中晚期的两次湖泊大规模发育。全新世时期,早中期即先秦（约2.5 ka以前）,盆地中仍是河湖共存阶段;晚期（2.5 ka以来）,出现湖消河长的局面,直至明、清时期湖泊全部消失,气候变化和人类活动影响是盆地中湖泊最终消失的主要原因。创新点：首次通过磁性地层学将晋中盆地发育时间确定为中新世晚期约8.1 Ma;盆地的湖泊发育早期主要受构造和气候控制,全新世以来气候变化和人类活动造成湖泊最终消亡。     

Numerical experiment with modelled return echo of a satellite altimeter from a rough ocean surface and a simple iterative algorithm for the estimation of significant wave height

We have simulated the return echo of a satellite altimeter from a rough ocean surface using an analytical formula and have studied its sensitivity with respect to various oceanic and altimeter parameters. Our numerical experiment shows that for normally observed significant wave heights (SWH) the effect of off-nadir angle (ONA) up to 0.5° on the leading edge is not severe. Also, small surface roughness skewness seems to have little effect on the overall shape of the echo. Newton’s iterative scheme has been used to retrieve SWH from the mean return waveform without noise and with additive Gaussian noise typical of Seasat and Geosat altimeters. It has been observed that SWH can be retrieved in the presence of noise with an accuracy of ±0.6 m for ONA less than 0.5°. For higher ONA, accurate retrieval requires the use of precomputed look-up table along with our scheme.     

辽东古元古代菱镁矿矿床与硼酸盐矿床——同期异相沉积成矿探讨

辽东地区古元古代辽河群火山-沉积建造中发育有大规模的菱镁矿矿床和硼酸盐矿床,是中国非金属矿产镁、硼的主产地。文章对这两类非金属矿床进行了主微量元素、稳定同位素(硫和碳)和LA-MC-ICP-MS锆石UPb定年测试。同时,结合部分前人的同位素测试结果对成矿作用进行了探讨,研究表明:1菱镁矿矿石及内部的石膏岩的δ34SV-CDT为15.6‰~26.5‰,硼矿石及其周缘硬石膏和大理岩围岩的δ34SV-CDT值和δ11B值分别为11.6‰~24.3‰和8.8‰~13.9‰,显示为海相蒸发沉积的特征;2菱镁矿矿体内黄铁矿δ34SV-CDT值为16.0‰~20.7‰,硼酸盐矿体内黄铁矿δ34SV-CDT值为12.3‰~13.2‰,显示海相硫酸盐热还原成因的特征;3菱镁矿矿石的δ13CV-PDB值为0.3‰~1.6‰,硼酸盐矿床周缘新鲜大理岩围岩的δ13CV-PDB值为-1.5‰~4.6‰,记录有受古元古代Lomagundi海相蒸发事件影响的碳同位素"正异常"现象;4菱镁矿矿石和硼矿体周缘的镁质大理岩均具有相似的页岩标准化稀土元素配分图和较高的硼含量;5菱镁矿和硼矿矿体上、下盘中的各类变粒岩和浅粒岩的岩浆锆石阴极发光图像相似,均具有的2.2 Ga左右的峰期岩浆锆石年龄。因此,辽吉裂谷北缘斜坡区的菱镁矿矿床和中央凹陷区的硼酸盐矿床可能均形成于2.2 Ga左右的海相蒸发沉积环境,形成于大规模火山喷发沉积活动的间歇期,属裂谷内南北两侧同期异相海相蒸发沉积的产物。     

Evolution of a fold-thrust belt deforming a unit with pre-existing linear asperities: Insights from analog models

Heterogeneity, whether geometric or rheologic, in crustal material undergoing compression affects the geometry of the structures produced. This study documents the thrust fault geometries produced when discrete linear asperities are introduced into an analog model, scaled to represent bulk upper crustal properties, and compressed. Varying obliquities of the asperities are used, relative to the imposed compression, and the resultant development of thrust fault traces and branch lines in map view is tracked. Once the model runs are completed, cross-sections are created and analyzed. The models show that asperities confined to the base layer promote the clustering of branch lines in the surface thrusts. Strong clustering in branch lines is also noted where several asperities are in close proximity or cross. Slight reverse-sense reactivation of asperities cut through the sedimentary sequence is noted in cross-section, where the asperity and the subsequent thrust belt interact. The model results are comparable to the situation in the Dinaric Alps, where pre-existing faults to the SW of the NE Adriatic Fault Zone contribute to the clustering of branch lines developed in the surface fold-thrust belt. These results can therefore be used to evaluate the evolution of other basement-involved fold-thrust belts worldwide.     

Undrained stability of an active planar trapdoor in non-homogeneous clays with a linear increase of strength with depth

Finite element limit analysis was employed to determine the upper and lower bound solutions of the active failure of a planar trapdoor in non-homogeneous clays that have a linear increase of strength with depth. Influences of cover ratio, dimensionless strength gradient and trapdoor roughness on predicted failure mechanisms and stability factors were determined. In all cases, the exact stability factors were accurately bracketed by computed bound solutions within 1%. Accurate closed-form equations to predict the exact estimates of stability factors, trapdoor pressure and factor of safety using the new proposed factors for the cohesion and strength gradient are presented.     

Bed thickness characteristics of inner-shelf storm deposits associated with a transgressive to regressive Holocene wave-dominated shelf, Sendai coastal plain, Japan

Holocene inner-shelf storm deposits preserved beneath the Sendai coastal plain facing the Pacific coast of north-eastern Japan were formed during a transgressive–regressive cycle. The evolution of the Holocene wave-dominated depositional system along the Sendai coast is reconstructed using 76 AMS (accelerator mass spectrometers) ¹⁴C ages and the origin of bed thickness variations in the inner-shelf storm deposits is explored. The Holocene succession is <30 m thick and overlies latest Pleistocene to early Holocene non-marine deposits above a transgressive ravinement surface. It comprises transgressive ravinement and inner-shelf deposits, and regressive inner shelf, shoreface, and coastal plain deposits. The inner-shelf deposits comprise alternating sand and mud layers interpreted as stacked storm beds. The average preservation interval of a single storm bed is shortest during the transgression (5·7–20·6 years), and then increases to a maximum during the early regression (83·3–250·0 years), decreasing to 7·7–31·3 years with shoreline progradation. Average accumulation rates decreased during the transgression and then increased during the regression, but the sand/mud ratio varies little, reflecting inefficient sediment segregation downdip on the inner shelf. The vertical pattern of sand-layer thicknesses also shows no relationship to position within the cycle, although small-scale intervals of upward thickening and thinning probably relate to lateral switching of river mouths and/or random storm processes. The average thickness of storm beds is the highest in the interval deposited during the period from maximum flooding to early regression. This is probably because of the low preservation potential of thin beds associated with frequent, low-magnitude storms during this period of low accumulation rates and extensive reworking. This preservation bias and the nature of the Sendai inner shelf resulted in an absence of characteristic bed thickness trends in the preserved storm deposits.