Geometry and architectural associations of co-genetic debrite–turbidite beds in basin-margin strata, Carboniferous Ross Sandstone (Ireland): Applications to reservoirs located on the margins of structurally confined submarine fans

Co-genetic debrite–turbidite beds are most commonly found in distal basin-plain settings and basin margins. This study documents the geometry, architectural association and paleogeographic occurrence of co-genetic debrite–turbidite beds in the Carboniferous Ross Sandstone with the goal of reducing uncertainty in the interpretation of subsurface data in similarly shaped basins where oil and gas is produced.The Ross Sandstone of western Ireland was deposited in a structurally confined submarine basin. Two outcrops contain co-genetic debrite–turbidite beds: Ballybunnion and Inishcorker. Both of the exposures contain strata deposited on the margin of the basin. An integrated dataset was used to characterize the stratigraphy of the Ballybunnion exposure. The exposure is divided into lower, middle, and upper units. The lower unit contains laminated shale with phosphate nodules, structureless siltstone, convolute bedding/slumps, locally contorted shale, and siltstone turbidites. The middle unit contains co-genetic debrite–turbidite beds, siltstone turbidites, and structureless siltstone. Each co-genetic debrite–turbidite bed contains evidence that fluid turbulence and matrix strength operated alternately and possibly simultaneously during deposition by a single sediment-gravity-flow event. The upper unit contains thin-bedded sandy turbidites, amalgamated sandy turbidites, siltstone turbidites, structureless siltstone, and laminated shale. A similar vertical facies pattern is found at Inishcorker.Co-genetic debrite–turbidite beds are only found at the basin-margin. We interpret these distinct beds to have originated as sand-rich, fully turbulent flows that eroded muddy strata on the slope as well as interbedded sandstone and mudstone in axial positions of the basin floor forming channels and associated megaflute erosional surfaces. This erosion caused the axially dispersing flows to laterally evolve to silt- and clay-rich flows suspended by both fluid turbulence and matrix strength due to a relative increase in clay proportions and associated turbulence suppression. The flows were efficient enough to bypass the basin center/floor, physically disconnecting their deposits from coeval lobes, resulting in deposition of co-genetic debrite–turbidite beds on the basin margin. The record of these bypassing flows in axial positions of the basin is erosional surfaces draped by thin siltstone beds with organic debris.A detailed cross-section through the Ross Sandstone reveals a wedge of low net-to-gross, poor reservoir-quality strata that physically separates sandy, basin-floor strata from the basin margin. The wedge of strata is referred to as the transition zone. The transition zone is composed of co-genetic debrite–turbidite beds, structureless siltstone, slumps, locally contorted shale, and laminated shale. Using data from the Ross Sandstone, two equations are defined that predict the size and shape of the transition zone. The equations use three variables (thickness of basin-margin strata, thickness of coeval strata on the basin floor, and angle of the basin margin) to solve for width (w) and trajectory of the basinward side of the low net-to-gross wedge (β). Beta is not a time line, but a facies boundary that separates sandy basin floor strata from silty basin-margin strata. The transition zone is interpreted to exist on lateral and distal margins of the structurally confined basin.Seismic examples from Gulf of Mexico minibasins reveal a wedge of low continuity, low amplitude seismic facies adjacent to the basin margin. Strata in this wedge are interpreted as transition-zone sediments, similar to those in the Ross Sandstone. Besides defining the size and shape of the transition zone, the variables “w” and “β” define two important drilling parameters. The variable “w” corresponds to the minimum distance a well bore should be positioned from the lateral basin margin to intersect sandy strata, and “β” corresponds to the deviation (from horizontal) of the well bore to follow the interface between sandy and low net-to-gross strata. Calculations reveal that “w” and “β” are related to the relative amount of draping, condensed strata on the margin and the angle of the basin margin. Basins with shallowly dipping margins and relatively high proportions of draping, clay-rich strata have wider transition zones compared to basins with steeply dipping margins with little draping strata. These concepts can reduce uncertainty when interpreting subsurface data in other structurally confined basins including those in Gulf of Mexico, offshore West Africa, and Brunei.     

Quantitative characterization of connectivity and conductivity of sandstone carriers during secondary petroleum migration,applied to the Third Member of Eocene Shahejie Formation,Dongying Depression,Eastern China

Studies on migration pathways remain qualitative, albeit extensive quantification of migration forcing. In this study, hydrocarbon carriers are defined as carrier units and their corresponding carrier beds for the Third Member of Eocene Shahejie Formation in the Dongying Depression, Bohai Bay Basin, eastern China, on the basis of their lithofacies and physical properties, spatial relationships, and data availability; secondary migration conduits are then quantified. A carrier unit is defined as a stratigraphic unit that contains microscopically porous and permeable carrier beds and is covered by regional seals. The carrier beds are macroscopically and physically connected to each other within a carrier unit and are hydrodynamically connected during secondary migration. A method of quantifying sandstone carrier units using common physical properties is developed. First, a carrier unit containing potential carrier beds is identified on the basis of lithofacies and their lateral changes. Second, physical connectivity of sandstone carrier beds is assessed on the basis of percolation theory. Third, the hydrodynamic connectivity of a sandstone carrier unit is analyzed using effective parameters that may reflect the hydraulic circulation in the carrier unit. Last, the conductivity of a sandstone carrier bed is quantitatively characterized using appropriate physical property parameters. The results for sandstone carrier units in the Third Member of the Shahejie Formation are used in numerical models addressing Pleistocene secondary migration. The model results explain the discovered accumulation and hydrocarbon shows well; and the model predictions on exploration targets have been confirmed by drilling.     

Deposition of laminated shale: A field and experimental study

Intermittently laminated shale of the Jemtland Formation in Maine is characterized by thin lenticular silt segregations interlaced with argillaceous and organic material (including graptolites). This shale is thinly interbedded with nonlaminated shale, siltstone, and thicker turbidite graywacke beds. Experiments suggest that the intermittently laminated shale was deposited by silt/clay-laden currents and may have been part of an upward turbidite progression from parallel-laminated silt (>60% silt), through intermittently laminated mud (40 to 60% silt), to nonlaminated mud (20 to 40% silt). Intermittently laminated mud may be produced from silt/clay flows that are: decelerating at a constant silt content, losing silt at constant velocity; or both decelerating and losing silt.     

The application of geostatistical inversion in shale lithofacies prediction: a case study of the Lower Silurian Longmaxi marine shale in Fuling area in the southeast Sichuan Basin,China

Based on cores, well logs and seismic data, we established the isochronous sequence stratigraphic framework of the Lower Silurian Longmaxi Formation and predicted the shale lithofacies distribution within the sequence stratigraphic framework using geostatistical inversion. The results of our study show that the Lower Member of the Longmaxi Formation is a third order sequence that includes a transgressive systems tract (TST), an early highstand systems tract (EHST) and a late highstand systems tract (LHST). Four lithofacies units have been recognized, specifically siliceous shale, argillaceous shale, calcareous shale and mixed shale. The results of geostatistical inversion reveal that the TST is characterized by flaky siliceous shale and some sparsely distributed calcareous shale. The EHST is dominated by mixed shale with minor amounts of siliceous shale, which occurs in only a small area. Moreover, in the LHST, argillaceous shale occupies almost the entire study region. Comparing to traditional geological research with geophysical research, the vertical resolution of the predictive results of geostatistical inversion could reach 1–2 m. Geostatistical inversion effectively solves the problem of precisely identifying the lithofacies in the Fuling shale gas field and predicting their spatial distribution. This successful study showcases the potential of this method for carrying out marine shale lithofacies prediction in China and other locations with similar geological backgrounds.     

Sequence stratigraphy of an argillaceous,deepwater basin-plain succession: Vischkuil Formation (Permian), Karoo Basin,South Africa

The 380 m thick fine-grained Vischkuil Formation comprises laterally extensive hemipelagic mudstones, separated by packages of graded sandstone and siltstone turbidites, and volcanic ash beds, and is an argillaceous precursor to a 1 km thick sand-prone basin floor fan to shelf succession. The Vischkuil Formation provides an insight into the process by which regional sand supply is initiated and for testing sequence stratigraphic principles in a basin plain setting. Regionally mapped 1–2 m thick hemipelagic mudstone units are interpreted as condensed drapes that represent the starved basin plain equivalents of transgressive systems tracts and maximum flooding surface on the coeval shelf (now removed during later uplift). The section above each mudstone drape comprises siltstone turbidites interpreted as highstand systems tract deposits and a surface of regional extent, marked by an abrupt grain size shift to fine sandstone. These surfaces are interpreted as sequence boundaries, related to abrupt increases in flow volume and delivery of sand grade material to the basin-plain. The interpreted lowstand systems tract comprises sandstone-dominated turbidites and is overlain by another hemipelagic mudstone drape. The upper Vischkuil Formation is marked by three 20–45 m thick debrites, with intraformational sandstone clasts up to 20 cm in diameter that can be mapped over 3000 km². In each case, debrite emplacement resulted in widespread deformation of the immediately underlying 3–10 m of silty turbidites. A sequence boundary is interpreted at the base of each deformation/debrite package. Six depositional sequences are recognised and the interfered energy shift across each successive sequence boundary and LSTs include a larger volume of sandstone increases up section. The lower two sequences thin to the NW and show NW-directed palaeocurrents. The four overlying sequences show a polarity switch in palaeocurrent directions and thinning, to the E and SE. Sequence 6 is overlain sharply by the 300 m thick sandstone dominated Fan A of the Laingsburg Formation. The LST debrites may indicate gradual development of major routing conduits that subsequently fed Fan A. The polarity shift from westward flowing turbidity currents to an eastward prograding deepwater to shelf system represents establishment of a long term feeder system from the west. Sand supply to the Karoo basin floor was established in an incremental, stepwise manner. Given the early post-glacial setting in an icehouse climate, glacio-eustatic sea-level changes are considered to have been the main control on sequence development.     

Growth faults at the prodelta to delta-front transition,Cretaceous Ferron sandstone,Utah

Cliff exposures of synsedimentary growth faults at the base of the Cretaceous Ferron sandstone in central Utah represent outcrop analogs to subsurface growth faults. Delta front sands prograded over and deformed less dense prodelta muds of the underlying Tununk Shale. Detailed fault patterns and associated facies changes demonstrate a complex fault history and style for growth fault development rather than a simple progressive development of faults in a basinward position. The most proximal and most distal fault sets were the earliest active faults. Growth faulting was initiated by deposition of cross-bedded distributary channel and mouth bar sandstones that reach 9 m thick in the hangingwalls of the faults. Curvature of the beds in the hangingwall of the faults nucleates smaller conjugate fault sets. Cross-bed sets in the hangingwalls of faults decrease from meter to decimeter scale away from the faults suggesting decreasing flow velocity or decreased preservation of cross sets as a result of decreasing accommodation in distal hangingwalls. Shifts in depositional loci, including upstream and downstream accretion of mouth bar sands contribute to the complex faults history and internal heterogeneity and development of potentially isolated sandy reservoir compartments.     

珠江口盆地白云凹陷珠江组深水牵引流沉积特征及其地质意义

研究深水牵引流对古大洋环流重建、陆坡位置判断及沉积矿产和能源资源勘探具有重要意义。珠江口盆地白云凹陷中新统珠江组取心段发育丰富的、与远洋低密度浊流和半远洋-远洋悬移沉积有明显区别的深水牵引流沉积,产物包括平行层理细砂岩、双泥岩纹层、双向交错层粉-细粒砂岩、波状层理和脉状层理灰质粉-细粒砂岩、波状层理和脉状层理抱球虫灰岩、透镜状沙纹层理粉砂岩等岩石相类型。珠江组深水牵引流沉积的发现具有重要地质意义,由其可以推测自渐新世末(23.8 Ma)白云凹陷陆架坡折带形成之后就开始存在沿着陆架坡折带活动的深水牵引流,并可能影响和改变沉积物的物质组成和区域分布,这将为南海北部古大洋洋流循环重建起到重要启示作用。     

Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy)

The Upper Cretaceous Bordighera Sandstone of NW Italy is a coarse-grained, sand-rich elongated turbidite system (ca. 15 × 45 km in outcrop) up to 250 m thick, interpreted to have been deposited in a trench setting. The siliciclastic succession interfingers with muddy calcareous turbidites, which become more abundant toward the lateral and distal domains. Bed type associations allow the distinction of a proximal channelized domain which transitions to a more distal lobe domain, characterized by abundant mudclast-rich sandstones and by bipartite and tripartite beds with a mud-rich middle or upper division (hybrid event beds). The transition between the proximal and distal domains occurs over a relatively limited spatial extent (ca. 5 km). The presence of lenticular bed-sets made up of coarse grained and mud-poor sandstones throughout the distal domain suggests that distributary channels were present, indicating sediment bypass further down-dip toward the most distal and not preserved parts of the system. Hybrid event beds - commonly associated with distal and marginal fan environments such as fan fringes - are present throughout the lobe domain and extend for up to ca. 30 km in down-dip distance. They are more abundant in the proximal and axial depositional lobe domain and their appearance occurs within a short basin-ward distance from the inferred channel-lobe transition zone. Flow expansion at the termination of the channelized domain and the enhanced availability of cohesive substrate due to the presence of intra-basinal muddy calcareous beds are interpreted as the key controls on the widespread occurrence of mudclast-rich and argillaceous sandstone beds. The abrupt appearance and the persistent occurrence of such beds across an extensive domain have implications for characterizing bed-scale (sub-seismic) heterogeneity of deep-water clastic hydrocarbon reservoirs.     

A dual-porosity model for evaluating petroleum resource potential in unconventional tight-shale plays with application to Utica Shale,Quebec (Canada)

Unconventional tight to shale reservoirs vary from tight sandstone/siltstone to organic-rich mudstone/shale, commonly with mixed lithologies. In such reservoir systems, matrix pores and organic pores with different origins and distinct physical and chemical properties co-exist for hydrocarbon storage. Traditional resource assessment methods, designed for conventional reservoirs, cannot handle the two pore systems properly. This study proposes a dual-porosity model to respond to the need for a new method in assessing hydrocarbon resource potential in such reservoir systems. The dual-porosity model treats the two types of pores separately and derives the resource estimates from different sources of data, thus better characterizing unconventional reservoirs with complicated pore systems. The new method also has the flexibility of assessing resource potential for the entire spectrum of mixed lithologies ranging from a complete tight to a pure source rock (organic–rich shale/mudstone) reservoir. The proposed method is illustrated through the assessment of the in-place petroleum resource potential in the Upper Ordovician Utica Shale of southern Quebec, Canada. The results of the application suggest that the proposed approach effectively handles the two pore systems in tight-shale reservoirs effectively and provides a useful tool for estimating resource potential in unconventional plays.     

井一震结合进行高分辨率层序地层划分——以南苏门答腊盆地N油田U组为例

N油田U组上段地层发育粉砂岩和钙质泥页岩的三角洲前缘一浅海过渡相,多变的沉积相加大了小层对比的难度。针对此特点,结合层序地层学理论以及地震地层学的理论与研究方法,进行井一震结合对比划分地层,利用地震资料和标准层控制,构建大套地层等时格架;然后运用地震反射轴的等时性结合沉积旋回在地层格架内对比小层,确定研究区80口井6个分界界面。建立了区域高分辨率层序地层格架,形成了可容纳空间控制下的高级次层序格架建立的技术方法。     

西藏比如盆地生储盖条件分析

根据野外地质调查资料和采样测试分析,对比如盆地的生油岩、储集层、盖层条件及其生储盖组合特征进行了分析。认为盆地中发育的主要烃源岩有两套,岩石类型主要为板岩,有机质类型好,热演化程度高,以生气为主;储集层主要为碎屑岩,岩性以含砾砂岩、粉砂岩、中-细砂岩为主,属于低孔、低渗型;盖层较发育,岩性主要为泥晶灰岩、板岩,厚度大;最有利的生储盖组合型式为中上侏罗统拉贡塘组自生自储自盖式。     

Utility of GOI,QGF, and QGF-E for interpreting reservoir geohistory and oil remigration in the Hudson oilfield,Tarim basin,northwest China

The physical mechanisms responsible for hydrocarbon migration in carrier beds are well understood. However, secondary migration is one of poorly understood facets in petroleum system. The Carboniferous Donghe sandstone reservoir in the Tarim Basin's Hudson oilfield is an example of a secondary (or unsteady) reservoir; that is, oil in this reservoir is in the process of remigration, making it a suitable geologic system for studying hydrocarbon remigration in carrier beds. Experimental methods including grains containing oil inclusions (GOI), quantitative grain fluorescence (QGF) and quantitative grain fluorescence on extract (QGF-E) -- together with the results from drilling, logging and testing data -- were used to characterize the nature of oil remigration in the Donghe sandstone. The results show that (1) significant differences exist between paleo- and current-oil reservoirs in the Donghe sandstone, which implies that oil has remigrated a significant distance following primary accumulation; (2) due to tectonic inversion, oil remigration is slowly driven by buoyancy force, but the oil has not entered into the trap entirely because of the weak driving force. Oil scarcely enters into the interlayers, where the resistance is relatively large; (3) the oil-remigration pathway, located in the upper part of the Donghe sandstone, is planar in nature and oil moving along this pathway is primarily distributed in those areas of the sandstone having suitable properties. Residual oil is also present in the paleo-oil reservoirs, which results in their abnormal QGF-E. A better understanding of the characteristics of oil remigration in the Donghe sandstone in the Hudson oilfield can contribute to more effective oil exploration and development in the study area.     

Stratigraphic and provenance evolution of the Southern Apennines foreland basin system during the Middle Miocene to Pliocene (Irpinia-Sannio successions,Italy)

The paper deals with original stratigraphic, petrographic and structural data concerning the evolution of the southern Apennines chain (Italy). The main Langhian to Pliocene deposits cropping out in the northern sector of the southern Apennines foreland basin system (Sannio-Irpinia area) have been studied and correlated in order to document the effects of tectonic changes on the evolution of sandstone detrital modes and stratigraphic architecture. The studied sandstone units can be grouped in five key intervals: a) Numidian Flysch, mostly formed by Langhian mature quartzarenitic deposits and conformable Serravallian post-Numidian successions, formed by arkosic and calciclastic arenaceous-pelitic beds (foreland depozones); b) Langhian to Tortonian San Giorgio Fm., mostly composed of quartzofeldspatic sandstones (foredeep depozone); c) Tortonian to Early Messinian, quartz-feldspatic and partly sedimentary-carbonatoclastic petrofacies, thrust-top successions (Vallone Ponticello, Villanova del Battista and San Bartolomeo fms.); d) Late Messinian quartzolithic to quartzofeldspatic sandstones (Torrente Fiumarella, Anzano Molasse and Tufo-Altavilla unit), which can be referred to infilled thrust-top basins; e) unconformity-bounded Pliocene quartzofeldspatic sandstone strata (wedge-top depozones), characterized by synsedimentary tectonic activity.Detrital modes of the Serravallian through Middle Pliocene sandstones of the southern Apennines foreland basin system testify clear provenance relations from the accreted terranes forming the southern Apennine thrust-belt. The studied clastics show almost the same blended (quartz-feldspatic) composition; this condition could be related to the tectonic transport over thrust ramp of source rocks, as suggested by the tectonic evolutionary model. This study, dealing with sedimentary provenance analysis and tectonostratigraphic evolution, provides an example of the close relations between clastic compositions and foreland basin system development in southern Apennines.     

Stratigraphy and sedimentology of the Great Oolite Group in the Humbly Grove Oilfield,Hampshire

The main reservoir of the Humbly Grove Oilfield comprises variably dolomitic grainstones and packstones representing the Bathonian Great Oolite Group. The Bathonian sequence commences in Lower Fuller's Earth claystones which coarsen upwards into oncolitic claystones and skeletal packstones probably equivalent to the Fuller's Earth Rock. Above is a variable succession of wackestones and thin packstones which have a distinctive sandstone at their base. This sequence is named here the Hester's Copse Formation. The succeeding Great Oolite Limestone is predominantly oolitic and cross-bedded on a variety of scales. It exhibits both coarsening and fining sequences which have locally well-developed capping hardgrounds and burrowed horizons. The Great Oolite Limestone is subdivided into three Members: the lowest (the Humbly Grove Member), and the highest (the Herriard Member) begin with massive shoal oolite deposition, but each then pass upward into more interbedded sequences representing a more transgressive environment. The middle member (the Hoddington) is a thin but widely correlatable wackestone. The overlying Forest Marble commences abruptly in claystones, but there is an upward increase in both the incidence and thickness of discrete oolitic limestones. Both the Great Oolite Limestone and Forest Marble were affected by early fresh-water dissolution and cementation in addition to the localized development of submarine cements. The top of the Great Oolite Group is represented by the Cornbrash. The Lower Cornbrash is a thin micritic limestone while the Upper Cornbrash is a calcareous claystone which passes upwards into the Kellaways Clay. The Bathonian sequence overlies the dolomitic limestones of the Inferior Oolite, the Lower Fuller's Earth claystones being interpreted as a basinal marine mudstone sequence, marking a substantial deepening and transgressive phase at the opening of the Bathonian. These mudstones shoal upwards into the quiet, but photic, water deposits of the Fuller's Earth Rock. The Hester's Copse Formation represents the temporary development of wave-dominated terrigenous shoreface and lagoonal conditions. Renewed transgression established a high-energy, tide-dominated, carbonate shelf upon which the Great Oolite Limestone was deposited as a series of shoal oolites, channels, tidal deltas and spill-overs. Periodic exposure of the carbonate sand-bodies led to the production of early dissolutional and cementation fabrics that post-date (and largely obliterate) submarine cements. The Forest Marble opened with a further phase of deepening, and the temporary establishment of muddy facies. Subsequently discrete tide-dominated ridges and linear channelized oolitic sands prograded into the area. The latest Bathonian is marked by subsidence of the carbonate ramp to the south of the London Platform, the Cornbrash-Kellaways Clay sequence accumulating under progressively deepening waters.     

胶莱坳陷莱阳组碎屑岩的成岩作用

莱阳组由砾岩，砂岩，泥岩，页岩等岩石组成。砂岩中出现赤铁矿，粘土矿物，方解石，石英及斜长石等五种主要的胶结物，它们形成于不同的成岩作用阶段。早期成岩阶段形成赤铁矿，粘土矿物，方解石和石英，晚期成岩阶段形成斜长石，而表生成岩阶段则再次沉淀出赤铁矿。     

Continuous resistivity profiling survey in Mersin Harbour, Northeastern Mediterranean Sea

No detailed information has previously been available on the geological and geophysical characteristics of the sea floor and the underlying strata of Mersin Harbour, Northeastern Mediterranean Sea (Turkey). Continuous resistivity profiling (CRP) and borehole data from Mersin Harbour were used to interpret geoelectric stratigraphy of Neogene-Quaternary sediments in the area. This represents one of few such detailed case studies that have applied these valuable CRP techniques for the purpose of marine stratigraphic imaging. It was found that the Neogene-Quaternary sedimentary succession in the area consists of three geoelectric units (GU1, GU2, and GU3 from base to top). The lowest unit, GU1, has a resistivity value of greater than 20.0 ohm-m and consists of Miocene aged limestone and marl. The middle unit, GU2, is characterized by resistivity values ranging from 3.0 to 20.0 ohm-m. Its thickness is greater than 90 m, with the upper section being composed of stiff clay sequences which are Plio-Pleistocene in age. The uppermost unit, GU3, has resistivity values varying from 1.0 to 3.0 ohm-m. This unit displays a maximum thickness of 15 m, and is composed of Holocene muds together with gravel, sand, silt and clay (sometimes incorporating shells) materials of the Plio-Pleistocene age and their various mixtures, silty/clay limestone, and conglomerate sandstone. Comparisons of the geoelectric units with the depositional sequences interpreted from the available seismic data outwith, but close to, Mersin Harbour reveal that the geoelectric unit GU3 corresponds to the depositional sequences C (mainly Holocene) and B (mainly Plio-Pleistocene). The geoelectric unit GU2 partly correlates with the depositional sequence B which appears to be Plio-Pleistocene in age. The geoelectric unit GU1, which has not been encountered in previous seismic surveys, is a new discovery within Mersin Harbour. Limited correlation between the seismic and resistivity structures in the study area is attributed to differences in the acoustic impedance and resistivity contrasts of sub-bottom layers, as well as the penetration versus resolution performance of the systems.     

Structure and texture of a gravelly barrier island in the Fitzroy estuary,Western Australia,and the role of mangroves in the shore dynamics

In the stratification of the most coarse-textured barrier island of a system of sand and gravel barrier spits and islands on Point Torment peninsula, King Sound, W.A., foreslope beds due to swash-backwash on the beach face are unimportant. The main units are steeply-dipping backslope beds and topslope beds of gentle, chiefly landward inclination. Their bimodal sediments are poorly sorted for wave-built features. A very coarse mode usually at ?1.5 to ?2 ?i; belongs to a population of well-rounded but platy, impure limestone, of local but unlocated source, whereas the fine mode, almost invariably 1.00 to 1.25 ?i;, represents a population of quartz sand fed from low and sub-tidal estuarine shoals. The two populations are thought to derive from the surface creep and saltation cloud components of “traction carpets”. Swash of intermediate energy waves forms the topslope beds through percolation. Washover by waves of translation generated by occasional hurricanes erodes the top of the feature and deposits the backslope beds on the rear face in standing water at abnormally high levels through storm set-up. In both, unidirectional currents produce sediments with certain resemblances to fluvial “traction clog” deposits.Dated by underlying mangrove wood of 1200 B.P. in front and of 500 B.P. in the rear, this barrier probably retreated through mangrove swamp in the manner of some presently active members of the barrier system. The seaward mangrove swamps neither protect the embankments behind nor suffer seriously from the waves rolling the latter landward.     

Submarine slope mudstone facies,cozy dell formation (Middle Eocene), California

A 120 m thick section of the Cozy Dell Formation (middle Eocene), southem California, consists of a graylaminated mudstone and a tanungraded mudstone; sandstone beds are associated with the laminated mudstone. Sedimentary structures, stratification sequences, foraminiferal distributions, and composition indicate that the ungraded mudstone is an upper slope hemipelagic deposit, while the laminated mudstone is an overbank deposit associated with shallow channels or gullies in which the sandstone beds were deposited. This depositional setting may be analogous to that of the modern Mississippi River delta front.