Relative Sea-level Lowstand Research Articles

Sequence-stratigraphic signatures of hemipelagic siltstones in deep-water successions: the Lower Pleistocene Kiwada and Otadai Formations, Boso Peninsula, Japan

Sequence-stratigraphic signatures of hemipelagic siltstones were investigated using profiles of the magnetic susceptibility and selected chemical composition of the Early Pleistocene deep-water successions of the Kiwada and Otadai Formations on the Boso Peninsula, Japan. In the context of an independently developed sequence-stratigraphic framework for the submarine-fan deposits of the Otadai Formation, the magnetic susceptibility and chemical composition, such as the concentrations of TiO 2, MgO and Fe 2O 3, show that the lowstand systems tract deposits have higher values of these parameters than the transgressive and highstand systems tract deposits. In contrast, the CaO contents have inverse relationships with the magnetic susceptibility and are higher in the transgressive and highstand systems tract deposits. The positions of sequence boundaries largely coincide with the horizons from which the magnetic susceptibility and the contents of mafic component increase abruptly. The sequence-stratigraphic variations in the magnetic susceptibility and chemical composition of the submarine-fan hemipelagic siltstones are due to increases in the input of fine-grained, terrigenous clastic sediments from midwater flow suspension, in addition to the direct fluvial supply of relatively unmodified terrigenous clastic sediments during relative sea-level lowstands, although grain size of hemipelagic siltstones does not exhibit any distinct variation through depositional sequences. The Kiwada Formation is characterized by siltstone-dominated basin-plain deposits and its sequence-stratigraphic classification has been difficult when using just lithofacies features. Nevertheless, the profiles of the magnetic susceptibility and chemical composition of the basin-plain deposits are similar to those of the submarine-fan deposits with duration largely equivalent to the 41,000-years obliquity cycle of the Early Pleistocene oxygen isotope sea-level index. This finding indicates that the profiles of the magnetic susceptibility and chemical composition of hemipelagic siltstones reflect sequence-stratigraphic variation in the input of fine-grained terrigenous clastic sediments to the deep-water environments and are crucial for the recognition of cryptic sequence boundaries in hemipelagic successions.

Initiation of turbidity currents: outcrop evidence for Eocene hyperpycnal flow turbidites

The most widely used, although not as widely documented, explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites. Such instantaneous sediment failures generate short-lived surge-type flows that deposit turbidite beds described as Bouma sequences. In this communication, we present data from the Eocene Central Basin of Spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds, deposited by gradual aggradation from sustained flows. Sustained flows are long-lived and more or less continuous. They can be generated by a variety of mechanisms, e.g.: (1) instability during volcanic eruptions and the consequent remobilisation of unconsolidated material; (2) seismically triggered subaerial sliding within the drainage; (3) storm surges; (4) retrogressive slope failure; (5) breaching; and (6) hyperpycnal flows. Our database shows that in the Eocene Central Basin, the sustained flows were generated by hyperpycnal flows, i.e., by direct river effluent. The hyperpycnal flow turbidites have been recognized on the basis of (1) physical connection between fluvial and turbidite channels at the shelf edge, (2) abundance of thick turbidite sandstone beds, (3) sand-prone nature of the turbidite systems, (4) downslope changes of individual thick, sandy turbidite beds, and their collapsed pinch-out segments, (5) great abundance of continental material (leaves, coal fragments) in turbidite beds, (6) low abundance of associated slumped or debris-flow beds, and (7) the occurrence of turbidites in systematically accreted shelf-margins. These features strongly suggest that hyperpycnal flow, generated by direct river effluent, deposited significant number of turbidite beds in the documented slope and basin-floor turbidite successions. We also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level. The turbidite beds were studied in the context of seismic-scale (1×15 km), dip-oriented outcrops by measuring detailed vertical sections, following individual beds and packages of beds downslope for distances over 5 km, lateral mapping, and helicopter-taken photomosaics.

Mass mortality or exceptional fossilization ? The case of the early and middle Toarcian fossiliferous beds from the Digne-Les-Bains area (southeastern France)

Abstract The latest Domerian to late Toarcian sedimentary series (from −190 Ma up to −180 Ma) from the « Réserve Géologique de Haute-Provence » (southeastern France) yields two kinds of remarkable fossiliferous beds. The greatest interest of the early Toarcian type is the occurrence of ichtyosaur remains (at least in six sites) among many other fossils such as ammonites, belemnites, bivalves, wood. The middle Toarcian type is specifically rich in ammonites and nautiluses. Litho- and biostratigraphical, palaeontological, sedimentological and geochemical analyses allow us to determine whether these fossiliferous beds are the results of mass mortalities, linked or not to biological crisis, or of exceptional fossilizations of organisms after normal mortality. The early Toarcian accumulations of fossils have been accurately dated from the middle part of the Serpentinum zone, Exaratum sub-zone pro-parte and Falciferum sub-zone pro-parte, strangewaysi and falciferum horizons. Thus, they were not connected to the so called Toarcian biological crisis which occurred previously (during the Semicelatum sub-zone). Organisms likely died according to a normal mortality rate during a time span that lasted around 700 to 800 k.y. All organisms appear to have been removed from their life environments and buried within siliciclastic sediments. Organisms and sediments were trapped owing to the creation of hemigrabens in an extensional tectonic regime, as evidenced by synsedimentary normal faults. Burying and good preservation, i. e. exceptional fossilization, were favoured by the hypoxic or anoxic conditions which prevailed at the sea floor, specifically during these times. High organic carbon content (up to 2.49 per cent) in the fossiliferous silty-quartzose marls proves hypoxia if not anoxia. All these facts were linked to a deepening and transgressive systems tract which succeeded a relative sea-level lowstand at the Pliensbachian-Toarcian boundary. The marine transgression probably reworked and shifted basinwards the siliciclastic sediments and wood remains which formed previously on the exposed land. Thus, continental remains were mixed with the marine fossils. Small scale hummocky cross stratifications in the terrigenous deposits show episodic occurrence of high hydrodynamics events. Such events, likely storms and associated currents, may have provoked the accumulations of dead marine organisms. The middle Toarcian accumulations of cephalopods are dated from the upper part of the Bifrons zone, Bifrons sub-zone pro-parte, lusitanicum and bifrons horizons. They are contained within 3 to 4 calcareous beds, 60 up to 85 centimetres thick in the whole. Since the corresponding deposition time span lasted around 850 k.y. up to 1 m.y., the sedimentation rate was very weak : about 1 centimetre or less each 1 k.y. Subsequently, these accumulations are regarded firstly as faunal condensations. The weak sedimentation rate is considered as linked to a major deepening and flooding event which led to sedimentary starvation in the involved Vocontian basin. Moreover, this phenomenon was probably favoured by a well known crisis of the carbonate production at that time. High values of manganese at the base of these condensed limestones are useful in correlating lithostratigraphic units of the same age in the entire basin. Previous studies gave rise to similar interpretations of these units, i. e. records of major flooding and wide connection to the oceanic domain. In addition, the taphonomic study indicates that many fossils were reworked due to episodes of high hydrodynamism, as indicated by erosion of internal molds of ammonites, and in addition to bioturbation. So, the accumulations may come also from faunal concentrations. Finally, this study shows that the inventory of the local and general conditions that govern fossilization must be done before interpreting all exceptional and widespread fossiliferous beds in terms of mass mortality or extinction.

Shelf-margin deltas: their stratigraphic significance and relation to deepwater sands

Abstract Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate concerning three versus four systems-tract systematics and the positioning of a sequence boundary. Shelf-margin deltas create wide (tens of kilometers), high (hundreds of meters) and steep (3–6°) clinoforms, because they build across the relatively deepwater shelf margin. Such bodies tend to form strike-elongate wedges that initially thicken basinwards, attaining a maximum thickness of 50–200 m just outboard of the shelf edge, then thin on the middle/lower slope, whereas they commonly pinch out landwards by lapping back onto shelf shales. Prior to reaching the shelf edge, small-scale (tens of meters), tangential foresets (usually The key facies association in shelf-edge deltas is the mouth bar-to-delta front association. Mouth-bar facies, landwards of the shelf edge, consist mainly of thick, clean, flat to low-angle and ripple-laminated medium to fine sands. Basinwards from the shelf edge, such sands commonly alternate with heterolithic slumped unit, creating characteristic slumped to laminated couplets. Because the delta front is superimposed on a preexisting, steep and extended shelf margin, it commonly contains (beyond the mouth bars) thick successions (up to many tens of meters) of sandy, slope turbidites. Shelf-margin deltas differ from inner shelf deltas in showing: (1) an order of magnitude higher clinoforms (hundreds instead of tens of meters), and strike-elongated, locally pod-like sand bodies commonly affected and augmented by growth faulting; (2) paleoecological evidence of abrupt shallowing (foreshortened stratigraphy); (3) turbidite-prone delta-fronts; (4) larger scale and greater abundance of slope-controlled soft-sediment deformation and (5) the general absence of a paralic ‘tail’ along the trailing edge of the delta front. Shelf-margin deltas can be classified into two types. Stable shelf-margin deltas are usually tens of meters thick and are not associated with shelf-edge incision or major slope collapse/disruption features. Slope turbidites are common and occur as unconfined sheets and lobes. Such deltas form when relative sea level falls no lower than the level of the shelf platform or, after longer sea-level falls, when rivers reestablish at the shelf edge on the rise. Unstable shelf-margin deltas are associated with large-scale slope collapse, listric growth faults and sometimes with salt diapirs. Slides are common on the upper slope, whereas imbricated sediment packages and compressional ridges affect the slope toe. Slope turbidites can be ponded within structurally controlled mini-basins on the slope. Such deltas are often related to unusually great sand influx to the shelf edge, or simply to prolonged or large fall of relative sea level below the shelf edge. The diachronous erosional unconformity atop the shelf-to-shelf-edge deltaic wedge is viewed by some researchers as the most easily recognizable and persistent surface on the shelf and slope. This surface contrasts with the downlap erosional surface developed from the beginning of sea-level fall, which is viewed by other researchers as the key boundary surface within this complex. It should be noted that sea level can fall for a long period of time (tens of thousands of years) before deltas even reach the shelf margin and, therefore, before significant volumes of sand are delivered across the shelf break. Hence, the time of incision of the shelf edge and emplacement of deepwater sand is commonly long after the initial fall so the time of (maximum) relative lowstand of sea level may be a more practical choice for the timing of the sequence boundary. Recognition of shelf-margin deltas and analysis of their architecture help in the prediction of presence or absence of basin-floor fans. Deltaic complexes that are aggradational to backstepping, downlap onto disrupted or complex slopes, and overlie an incised shelf and shelf edge, predict that there should be basin-floor fans present. In contrast, delta complexes that show a prolonged and preserved progradational to downstepping architecture implies the presence of turbidite accumulations on the slope but not on the basin floor.

In Defense of Shelf‐Edge Delta Development during Falling and Lowstand of Relative Sea Level

There is a continued debate about the sequence stratigraphic interpretation of deep‐marine sand accumulations. The conventional notion states that it is preferably (but not only) falling stage and lowstand of relative sea level that favor the formation of the deep‐marine systems because, at these times, the fluvial and deltaic transport system tends to extend relatively easily across a shallow‐marine shelf surface and hence has potential for sand delivery into deeper‐marine environments. It has been argued recently, however, that sand delivery across the shelf to the shelf edge is equally likely during highstand, as gauged from the estimated time periods for modern natural rivers to build their deltas to the shelf edge, during the modest relative sea level rise of the past 7000 yr. This argument and counterargument are examined here quantitatively by using, as a basis, the concept of “shoreline autoretreat.” The autoretreat theory predicts that under conditions of constant sediment supply ($$S> 0$$) to the basin and a steady rate of rise ($$A> 0$$) in relative sea level, a prograding delta inevitably attains an “autobreak” state (time: tb) after which the delta's subaqueous slope begins to be starved of sediment. This happens because the finite sediment supply becomes inadequate to fill the greatly increased accommodation (sensu Muto and Steel 2000) on the delta plain created by the rising sea level and the extended delta front slope as the delta attempts to move into deeper water. It is impossible for a delta to reach a shelf edge if the autobreak state is attained before this position is reached. Using published data for modern river‐shelf systems, numerical simulations to estimate tb were carried out under different sets of assumed conditions. We conclude that during development of highstand systems tracts along many coastlines during the past 7000 yr, tb can take a considerably small value, which tends to prevent rivers from reaching their shelf edges, especially if the delta's subaqueous slope is of relatively high inclination (ca. ≥1°). Simulations for stable and falling stages of relative sea level, however, indicate that deltas can reach a shelf‐edge location much more easily and quickly than with even modest relative sea level rise. Thus, this study defends the notion of shelf‐edge delta development and potential sand delivery to deepwater areas preferentially during falling or lowstand stages of the relative sea level cycle.

Lower Pleistocene to present structural deformation and sequence stratigraphy of the continental shelf, offshore Eel River Basin, northern California

Seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore Eel River Basin of northern California, using a closely spaced, high-resolution multichannel seismic grid. Geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences, producing shifting sedimentation patterns in the offshore basin during the last ∼1 Myr. Estimated timing of unconformity formation correlates generally with a deep-ocean global δ 18O curve, both before and after a ∼500-kyr transition from 41-kyr to 100-kyr dominated cycles. We suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide. However, regional tectonic influences on sequence development are also observed. Folding associated with Gorda–North America Plate convergence ended progressively from south to north between ∼1.0 Ma and ∼500 ka, in response to northward migration of the Mendocino Triple Junction (MTJ). Since ∼500 ka, continued encroachment of the MTJ produces rotation of preexisting structures, uplift of the Table Bluff Anticline (TBA), related periods of channel incision south of the TBA, and generally reduced sediment preservation across the shelf. MTJ-related uplift may also have induced formation of Eel Canyon. Over the past ∼1.0 Myr, a dominant northern sediment source becomes progressively less important; a southern source, probably the paleo-Eel River, has become dominant since ∼750 ka. Seismic unconformities fall into two categories: (1) irregular surfaces of limited mappable extent, interpreted as incision/exposure surfaces forming during relative sea-level lowstands, and (2) smooth, laterally extensive regional unconformities, interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea-level transgressions. A sequence stratigraphic model for sediment deposition and preservation on the Eel River shelf predicts that preserved sediment on the Eel margin is dominated by fluvially derived highstand silts and muds, deposited by longshore-directed currents and waves. Lowstand sediments are preserved only in fluvial channels infilled during late lowstand/early transgression. Preserved transgressive sediments are likely limited to a thin veneer of well-sorted coarse sediment or shell debris, deposited above the transgressive ravinement. Future deep coring will be required to confirm these predictions.

Depositional Environment and Sequence Stratigraphy of Miocene Deltaic Cycles Exposed along the Jerudong Anticline, Brunei Darussalam

ABSTRACT Kilometer-scale prograding clinoforms associated with deltas are rarely seen in outcrop; however, one such example is found in a Miocene sand-shale sequence exposed along the Jerudong anticline in Brunei Darussalam. Regional sequence stratigraphic interpretation shows that large clinoforms at the base of the Miocene Belait delta represent a succession of at least three major sand-shale sequences. The stratigraphically highest and best exposed sequence exhibits large slumps and sharp-based detached sand bodies at its base. Accumulation of these units most likely occurred during a relative sea-level lowstand. An overlying 1-1.5 km thick shale unit is interpreted to have developed during subsequent transgressive and early highstand conditions. Rapid progradation of thick sand-dominated shoreface deposits characterizes the late highstand systems tract. The clinoforms below show similar depositional geometries: slumps and thin blankets of shallow-marine sandstones mark the individual bases, shales and mudstones succeed, and progradational shoreface and tidal deposits form the top of each clinoform. New sedimentological and micropaleontological data document that all sediments (regardless of whether sand- or shale-dominated) formed in a shoreface to shelfal setting in front of a mud-rich delta. This differs from previous studies interpreting a continental-slope to deep-marine depositional environment for all shale-dominated units, and FIG. 1. A) Landsat satellite composite image of NE Brunei Darussalam (after Sandal 1996) and B) geological map (after Wilford 1961). The Belait Formation is dominated by shallow-marine shoreface and tidal sandstones. The Setap Shale Formation (Setap Shales) is mainly composed of shelfal mudstones and shales. Note pronounced progradational geometries at the base of the Belait Formation on the eastern margin of the Belait Syncline. Black boxes indicate locations of Figures 2 and 4. End_Page 913------------------------ indicates that kilometer-scale clinoforms can develop entirely on the continental shelf in water depths less than 200 m.

Deposition of sheet-like turbidite packets and migration of channel-overbank systems on a sandy submarine fan: an example from the Late Miocene–Early Pliocene forearc basin, Boso Peninsula, Japan

Abstract Sheet-like turbidite packets and channel-overbank systems are interpreted to be major components of submarine fans. Spatial and temporal relationships between the two major components, however, are not yet clearly understood, in particular for sand-dominated submarine fans in active-margin basins. Three-dimensional outcrop analysis of an ancient sandy submarine fan succession in a Late Miocene–Early Pliocene forearc basin on the Boso Peninsula, Japan, together with mapping of chronostratigraphic markers defined by volcanic ash beds, indicates that deposition of sheet-like turbidite packets was intimately associated with the lateral migration of depocenters of channel-overbank systems and distinct entrenchment of fan channels in response to upfan avulsion. Deposition of a sheet-like turbidite packet is not restricted in a single interchannel low, but rather is spread out over most of the channel-overbank area, possibly reflecting the high width to depth ratio of channel-overbank systems on sandy submarine fans. Largely contemporaneous deposition of sheet-like turbidite packets and channel-overbank systems further indicates that the “basin-floor fan” and “slope fan” of the sequence-stratigraphic model do not necessarily characterize different parts of a relative sea-level lowstand or temporally different types of depositional systems.

Late Pleistocene channel incisions in the southern Eel River Basin, northern California: implications for tectonic vs. eustatic influences on shelf sedimentation patterns

High-resolution multichannel seismic (MCS) profiles from the southern offshore Eel River Basin off northern California show multiple, superimposed buried channels beneath the continental shelf. These channels display consistent southwest orientations. Their dendritic map pattern and low but consistent cross-shelf gradients suggest that they formed by fluvial erosion during periods of shelf exposure associated with relative sea-level lowstands. All of the channels were incised after ∼500ka, the age of the onshore–offshore Hookton Datum. Their frequency of incision exceeds that of known glacioeustatic lowstands since ∼500ka, confirming that some of the surfaces must have formed during relative lowstands initiated by local tectonic uplift.Sedimentation patterns in the Eel River Basin shift dramatically in response to relative sea-level fluctuations. Today, sediments from the Eel River move north and west on the shelf, in response mainly to large winter storm events. In contrast, fluvial incision on the exposed shelf during relative lowstands directed sediments to the southwest, most likely into the head of the Eel Canyon. Shelf channels were probably fed not only by ancestral rivers, but also by local uplifts like the Table Bluff Anticline. Increases in gradient and channel depths at the southern end of the MCS coverage indicate that canyon-forming processes modified fluvial channels as they converge towards embayments near the head of the Eel Canyon. Cyclic recurrence and consistent orientation of these channels demonstrate that the Eel Canyon has been extant through numerous relative sea level cycles, and is perhaps as old as ∼500ka.

Pinch‐out style and position of tidally influenced strata in a regressive–transgressive wave‐dominated deltaic sandbody, Twentymile Sandstone, Mesaverde Group, NW Colorado

The Upper Cretaceous Twentymile Sandstone of the Mesaverde Group in NW Colorado, USA, has been analysed with respect to its pinch‐out style and the stratigraphic position of tidally influenced facies within the sandstone tongue. Detailed sedimentological analysis has revealed that the Twentymile Sandstone as a whole is a deltaic shoreface sandstone tongue up to 50 m thick proximally. Facies change character vertically from very fine‐grained, storm wave‐dominated shelf sandstones and mudstones to fine‐grained, wave‐dominated sandstones and, finally, to fine‐ to coarse‐grained tidally dominated sandstones. The pinch‐out style is characterized by a basinward splitting of the massive proximal sandbody into seven coarsening‐upward fourth‐order sequences consisting of a lower shaly part and an upper sandy part (sandstone tongue). These are stacked overall to reflect the regressive‐to‐transgressive development of the tongue. Each of the lower sandstone tongues 1–3 are gradationally based, very fine‐grained and dominated by hummocky cross‐stratification and were deposited on the lower to upper shoreface. Sandstone tongues 4 and 5 prograded further basinwards than the underlying tongues, are erosively based, fine‐ to coarse‐grained and mainly hummocky, herringbone and trough cross‐stratified. Especially in tongue 5, tidal indicators, such as bipolar foresets and double mud drapes, are common. These tongues were deposited as upper shoreface and tidal channel sandstones respectively. Sandstone tongues 6 and 7 retrograded in relation to tongue 5, are very fine‐ to fine‐grained and hummocky cross‐stratified. These tongues were deposited in lower shoreface to offshore transition environments. The two lower fourth‐order sequences were deposited during normal regressions during slowly rising or stable relative sea level and represent the highstand systems tract. The three succeeding fourth‐order sequences, which show succeedingly increasing evidence of tidal influence, were deposited during falling and lowstand of relative sea level and represent the falling stage (forced regressive) and lowstand systems tracts. The uppermost two fourth‐order sequences were deposited during rapidly rising sea level in the transgressive systems tract. The maximum tidal influence occurred during lowstand progradation, in contrast to most other published examples reporting maximum tidal influence during transgression.

A depositional and sequence stratigraphic model for cold-water, spiculitic strata based on the Kapp Starostin Formation (Permian) of Spitsbergen and equivalent deposits from the Barents Sea

This article summarizes the lithostratigraphy of two sections through the Kapp Starostin Formation on Spitsbergen having contrasting distal and proximal depositional settings. These outcrop sections are compared with an approximately age-equivalent spiculitic unit penetrated in two exploration wells 800 km to the southeast on the Finnmark Platform. Eight main facies associations comprise these strata, including limestones (matrix-rich, matrix-poor, and mudstone), siliciclastics (shale and sandstone), spiculite (dark/tight and light-colored/porous), and phosphatized surfaces. A sequence stratigraphic model is proposed wherein siliciclastic-limestone intervals represent lowstands of relative sea level, and the spiculites are mainly highstand deposits. Based on this model, seven depositional cycles (sequences) are recognized in the distal, high-accommodation locality on Spitsbergen (Akseloya), four in the proximal locality (Dickson Land), and two on the Finnmark Platform. A speculative model for correlation between these localities is presented, wherein lateral amalgamation of sequences, due to landward pinch-out of some lowstand intervals, results in recognition of fewer sequences in areas of lower accommodation. On both Dickson Land and the Finnmark Platform, an upward change in spiculite lithology from dark/tight to light/porous is shown, and Dickson Land also shows an upward change in siliciclastic lithology from shale to glauconitic sandstone. These changes record a long-term shoaling trend approaching the Permian-Triassic boundary. This model provides a basis for interpreting the basin-scale (Begin page 2062) distribution of lithofacies and predicting the occurrence of porous zones representing potential hydrocarbon reservoirs.

Morphology and distribution of Miocene slope incisions off New Jersey: Are they diagnostic of sequence boundaries?

High-resolution multichannel seismic reflection profiles confirm that middle–late Miocene continental slope canyons off New Jersey are rare, in contrast to their prevalence on the slope today. Localized canyon incision occurred in two physiographic settings bounded by declivity breaks: steeper clinoform fronts and more shallowly dipping regions seaward of clinoform toes. Canyons incising Miocene clinoform fronts are U and V shaped, relatively small, closely spaced, and localized along strike. They apparently developed independently of known fluvial inputs, although rivers probably discharged sediment near paleo-shelf edges (clinoform breakpoints) during Miocene relative sea-level lowstands. However, not all clinoform fronts are incised; nonchannelized downslope sediment transport also appears to have been a common process. V-shaped canyons are the most common type recognized seaward of clinoform toes; occasional amphitheater-shaped failures also occur. Rare canyons with partly aggradational banks may reflect either overbank and/or levee or contour-current deposition. Many Miocene canyons definitely incise sequence boundaries; they may all do so. This implies that canyons formed during relative sea-level lowstands, in accord with the classic sequence stratigraphic model of siliciclastic depositional systems. However, depth of incision on clinoform fronts decreases downslope, whereas seaward of toes it decreases upslope; development of canyon systems in the two settings is not linked by a single set of processes. Therefore, the presence or absence of canyon incision on this margin must be dictated not only by fluctuations in sea level, but by subtle changes in local conditions or regime variables. These include the efficiency of downslope sediment transport, rate of sediment supply, grain size, and possible slope collapse related to fluid escape.

Facies relationships and cyclicity of high-latitude, Late Permian coal measures, Bowen Basin, Australia

The Late Permian Moranbah Coal Measures are developed throughout the northern and central Bowen Basin, eastern Australia, and are subject to large-scale opencut coal mining activity. They were deposited in a rapidly subsiding retroarc foreland basin, during frequent base-level changes. Contemporaneous compressive deformation and volcanism facilitated the supply of abundant volcanoclastic sediments, resulting in rapid, but punctuated, basinal aggradation. To the south and east, transgressive–regressive cycles were marked by the inter-digitation of paralic–shallow marine and alluvial strata. Lonestones (dropstones) are common within the marine deposits and are attributed to ice-rafting indicative of a cold-climate depositional regime. Two basin-wide chronostratigraphic units, the P-tuff bed and the Harrow Creek superseam, have been used to evaluate the facies architecture of the stratigraphic interval they enclose. The lithofacies mosaic of the 25–75 m thick sequence has been examined by means of 26 widely distributed coreholes and two detailed district-scale studies based around the Moranbah–Goonyella and German Creek minesites. Eight alluvial–shallow marine facies associations have been recognised. Their architectural and temporal relationships allow the accurate reconstruction of the coal-bearing depositional system. Two dissimilar, stacked, depositional systems tracts, A (organics) and B (clastics) are recognised. The P-tuff bed is enclosed by facies of tract A, forming a thick, low-ash, coal-bearing interval which was emplaced essentially synchronously across the basin. It represents a time of clastic sediment starvation when basin-wide peat mire accumulation prevailed. Four similar extensive coal-bearing intervals within the Moranbah Coal Measures are also considered to represent basin-wide episodes of peat accumulation. This pattern is in strong contrast to traditional views of coal measure development, which argue that coals formed within a contemporary facies mosaic, in which fluvial channels, levees and crevasse splays were also represented. The second depositional systems tract (B) is characterised by clastic packages, dominated by large-scale (up to 4 km wide and 50 m thick), sandstone bodies deposited by low sinuosity trunk river systems, flanked by substantial crevasse splay deposits (up to 3 km wide and 30 m thick). It represents the product of catastrophic flooding events. The cyclothemic succession of systems tracts A and B is interpreted in terms of eustatic sealevel cycles. The tract A coal-bearing intervals represent relative sealevel lowstands. Systems tract B represents relative sea level highstand with its lower interval at German Creek to the south consisting of paralic and marine facies. The cycles had durations in the order of 400 to 600 ky and are considered to represent 4th order sealevel cycles of the Permian Period.

Sedimentologic development of a Late Oligocene–Miocene forearc embayment, Valdivia Basin Complex, southern Chile

Abstract Deposition of Tertiary sediments in the southeastern part of the Valdivia Basin commenced during the Late Oligocene with the rapid incision of rivers draining the Palaeo-Coastal Range. Alluvial fans developing along the steep valley flanks joined gravelly braided streams transporting bedload to the coast. Tectonic and eustatic stability lasting into the Early Miocene subsequently allowed lateral erosion of the valley flanks and the development of extensive fluvial and estuarine peat swamps. This stable period was interrupted briefly by a volcanic eruption at 23.5 Ma. Shortly after 23 Ma, rapid uplift caused by an acceleration in plate convergence resulted in renewed erosion of the landscape, as reflected in the deposition of coarse river gravels. Basin subsidence and marine transgression proceeding up the river valleys subsequently formed still, deep embayments, occasionally disturbed by debris flows and turbidity currents originating on the steep, wooded valley flanks. At the upper limits of the inlets, bayhead deltas, tidal flats and beaches existed. In some areas, fan deltas prograded into the embayments, as reflected in the interfingering relationship between conglomerates and marine mudrocks. In the deeper parts of the embayments, the frequency of debris flows and turbidity currents increased markedly during periods of relative sea-level lowstand. The landscape was eventually inundated by continued subsidence and marine transgression, which lasted until a possibly eustatic sea-level fall in the early Tortonian.

Shelfbreak gullies; products of sea-level lowstand and sediment failure; examples from Bowser Basin, northern British Columbia

ABSTRACT The transition from cyclothemic shelf to shale-dominated slope deposits (Callovian to Oxfordian) that accumulated along the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies. The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topography, which determined (2) the course of fluvial, distributary-type channels and focused gravel-dominated sediment transport during relative sea-level lowstands. Gully fill consists predominantly of debris-flow deposits, locally incorporated into small (lowstand) fan deltas that prograded from gully margins. Lowstand fluvial channels overlie shelf cycles, incise the underlying highstand deposits, and are overlain by fossiliferous, transgressive sandstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the outer shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea-level fall.

The South Pyrenean Eocene carbonate megabreccias revisited: new interpretation based on evidence from the Pamplona Basin

The South Pyrenean Foreland Basin contains numerous units of Eocene carbonate megabreccias intercalated with siliciclastic turbidites and derived by resedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland trough, sheds new light on the origin and regional significance of these South Pyrenean Eocene carbonate megabreccias (SPECMs). The number of the SPECM units in the foreland basin is higher than previously recognized and their age is somewhat older than originally assumed. The SPECM units appear to occur as time–stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megabreccias were derived from a carbonate-platform system hosted by the foreland basin's southern (passive) margin. The episodic instability and mass wasting were triggered by phases of structural steepening (forebulge uplift) accompanied by high-magnitude earthquakes, with the former causing platform emergence, increased load stresses and excess pore-water pressure in the carbonate ramp. The SPECM deposits were emplaced by cohesive debris flows evolving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differentiated, bipartite debrite and turbidite in the medial part; and (3) an incomplete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the SPECM units, and the original terms `megaturbidite' and `seismoturbidite' thus seem to be inappropriate for these deposits.

Compositional variations during phases of progradation and retrogradation of a Triassic carbonate platform (Picco di Vallandro/Dürrenstein, dolomites, Italy)

In this paper data are presented on the composition of sediments deposited at the toe of slope during progradation or retreat of Triassic carbonate platforms in the Dolomites (Italy). For this purpose a succession was studied from the toe of slope of a Triassic (Carnian) carbonate platform (Picco di Vallendro/Durrenstein, Dolomites, Italy). The microfacies analysis of selected calciturbidites sequences revealed a reduced input of oncolites and ooids during progradation and an increase in clasts. The main input, however, was derived from the reefs on the platform. Retrogradation of the platform showed an increase of filaments and radiolarians (open ocean biota) as well as carbonate mud and a reduced input of grains that originated within the reefs on the platform. Both during progradation and retrogradation parts of the platform were flooded and could produce excess sediment that could be exported to the surrounding basins. However, the absence of platform interior biota documents that progradation occurred from sediments of the reefal belt, probably during relative sea-level lowstands. Carbonate composition varies systematically with toe-of-slope progradation/retrogradation and, thus, argues for carbonate production as the main driver of the geometries observed at the toe of slope.

Sedimentology, microfacies and stratigraphic distribution of foraminifers of the lower “Pseudoschwagerina” Limestone (Rattendorf Group, late carboniferous), carnic alps (Austria/Italy)

The Lower “Pseudoschwagerina” Limestone (Rattendorf Group) is a cyclic sequence composed of fossiliferous, shallow marine limestones and thin clastic beds. In the Schulterkofel section three depositional cycles are well developed. Clastic intervals at the base of all depositional cycles formed during relative sea-level lowstands, during transgression well bedded fossiliferous limestones and massive algal mounds were deposited. Bedded cherty limestones with marl intercalations are interpreted as deposits formed during relative sea-level highstands with water depths of some tens of meters. In the Lower “Pseudoschwagerina” Limestone (LPL) fusulinid rich limestone beds occur in different stratigraphic levels, particularly at the base and on top of clastic intervals. Taphonomic features and composition of fusulinid assemblages indicate that reworking and even redistribution of fusulinids play a subordinate role. They are considered as parauthochthonous assemblages, accumulated during periods of low sediment input. Based on fusulinid biostratigraphy, the lower part of the LPL (depositional sequence 1) is correlated with thesokensis Zone, because of the occurrence ofRuzhenzevites ferganensis without species of the genusDaixina (Bosbytauella). The main part of the LPL (uppermost part of depositional sequence 1 to sequence 3) belongs to thebosbytauensis-robusta Zone. For the Carboniferous/Permian (C/P) boundary only an interval can be estimated between the upper part of sequence 3 (Schwagerina versabilis-Rugosofusulina arianica) and tectonically isolated limestone beds of the overlying, predominantly clastic Grenzland Formation (Sphaeroschwagerina-Pseudoschwagerina = middle Asselian). In the Carnic Alps the C/P boundary lies therefore immediately below the base or within the lowermost clastic Grenzland Formation.

Identification of Sequence Boundaries Within Cyclic Strata of the Lower Permian of Kansas, Usa: Problems and Alternatives

In the past, midcontinent cyclothems have been defined on the basis of cyclic facies patterns, rather than discontinuity surfaces. Sequence stratigraphic concepts, although offering a potential means of recognizing and correlating cycle boundaries, have proven difficult to apply to the lower Permian cyclothems of the midcontinent. The cycles of the lower Permian of Kansas are dominated by very shallow marine and paralic facies with a multitude of subaerial exposure surfaces ranging from desiccation cracks to well‐developed paleosols. Meter‐scale shallowing‐upward cycles (para‐sequences) are prominent, and most are capped by subaerial exposure surfaces. Furthermore, incised valleys and valley fills are conspicuously absent. Sequence boundaries are usually equated with surfaces recording maximum relative sealevel lowstand. Within a stacked series of paleosols this would presumably coincide with the most mature paleosol profile. However, relative maturity is difficult to assess if the paleosols were developed under differing topographic or climatic conditions and/or truncated by subsequent transgression. Alternatively, cyclothem boundaries could be defined at the prominent marine flooding surfaces that overlie these paleosol‐bearing intervals. These surfaces are overlain by fossiliferous marine limestones and are typically associated with intraclastic beds. They are equivalent to the transgressive surfaces of sequence terminology and provide more easily recognized and correlated boundaries. The use of transgressive surfaces rather than lowstand unconformities may provide a more practical application of the sequence model to midcontinent Permian cycles.

Controls on Reservoir Quality of an Upper Jurassic Reef Mound in the Palmers Wood Field Area, Weald Basin, Southern England

An Upper Jurassic (Oxfordian) reef mound in the Palmers Wood field area of the Weald basin, southern England, clearly shows the relationship of facies, diagenesis, and porosity development to relative sea level changes. The coral-microbial reef mound was initiated over a drowned oolite shoal during a third-order marine transgression and exhibits changes in coral morphology from base to top as the reef mound caught up with sea level during the subsequent highstand. During the highstand, extensive encrustation of the reef mound took place by microbial, stromatolitic cements, with subsequent porosity loss. During the following lowstand of relative sea level, extensive leaching of the aragonitic corals took place, and a lowstand wedge accumulated down-ramp. Porosity was lost during burial-related cementation and compaction. Coral-microbial reef mounds of this type have moderate potential for porosity formation, unlike tighter, deeper ramp reefs in the Jurassic, which are less prone to subaerial leaching and have less potential for secondary porosity development because they are less rich in aragonitic components. Although the Upper Jurassic reef mound is only a minor contributor to reservoirs in the Palmers Wood field, it may provide a model for other Upper Jurassic reef plays.