A storm and tidally-influenced prograding shoreline—Upper Cretaceous Milk River Formation of Southern Alberta, Canada

The Santonian-Campanian Milk River Formation of Southern Alberta represents the transition from an open shelf, through a storm-dominated shoreface into a non-marine sequence of shales and sandstones, with coal. The open shelf deposits consist of interbedded bioturbated mudstones with sharp-based hummocky cross-stratified sandstones. There are no indications of fairweather reworking of the sandstones, which are therefore interpreted as having been deposited below fairweather wavebase. The shoreface sequence consists of a 28 m thick sandstone. It has a very sharp, loaded base, and is dominated by swaley cross-stratification, a close relative of hummocky cross-stratification. Angle of repose cross-bedding is preserved in scattered patches only in the top 5 m of the sand body. Channels up to 180 m wide and 7 m deep are cut into this sand body, with channel margins characterized by lateral accretion surfaces. Regional dispersal trends, as well as local palaeocurrent readings suggest flow toward the NW. Within the channels there is some herringbone cross-bedding and at least two examples of neap-spring bundle cycles, suggesting that the channels are tidally-influenced. Above the channels there is a sequence of carbonaceous shales with in situ root casts and lignitic coal seams. No marine, brackish or lagoonal fauna was identified, and the sequence appears to represent a distal floodplain. The sequence from interbedded hummocky cross-stratified sandstones and bioturbated mudstones into a 10–20 m thick, sharp-based shoreface sandstone characterized by swaley cross-stratification is uncommon. The scarcity or absence of angle of repose cross-bedding in the shoreface, and the dominance of swaley cross-stratification suggests that the shoreface was so storm-dominated that almost no fairweather record was preserved. Other examples of swaley cross-stratified shorefaces are reviewed in the paper.     

Internal anatomy of a mixed siliciclastic–carbonate platform: the Late Cenomanian–Mid Turonian at the southern margin of the Spanish Central System

The Late Cenomanian–Mid Turonian succession in central Spain is composed of siliciclastic and carbonate rocks deposited in a variety of coastal and marine shelf environments (alluvial plain–estuarine, lagoon, shoreface, offshore‐hemipelagic and carbonate ramp). Three depositional sequences (third order) are recognized: the Atienza, Patones and El Molar sequences. The Patones sequence contains five fourth‐order parasequence sets, while a single parasequence set is recognized in the Atienza and El Molar sequences. Systems tracts can be recognized both in the sequences and parasequence sets. The lowstand systems tracts (only recognized for Atienza and Patones sequences) are related to erosion and sequence boundary formation. Transgressive systems tracts are related to marine transgression and shoreface retreat. The highstand systems tracts are related to shoreface extension and progradation, and to carbonate production and ramp progradation. Sequences are bounded by erosion or emergence surfaces, whose locations are supported by mineralogical analyses and suggest source area reactivation probably due to a fall in relative sea‐level. Transgressive surfaces are subordinate erosion and/or omission surfaces with a landward facies shift, interpreted as parasequence set boundaries. The co‐existence of siliciclastic and carbonate sediments and environments occurred as facies mixing or as distinct facies belts along the basin. Mixed facies of coastal areas are composed of detrital quartz and clays derived from the hinterland, and dolomite probably derived from bioclastic material. Siliciclastic flux to coastal areas is highly variable, the maximum flux postdates relative sea‐level falls. Carbonate production in these areas may be constant, but the final content is a function of changing inputs in terrigenous sediments and carbonate content diminishes through a dilution effect. Carbonate ramps were detached from the coastal system and separated by a fringe of offshore, fine‐grained muds and silts as distinct facies belts. The growth of carbonate ramp deposits was related to the highstand systems tracts of the fourth‐order parasequence sets. During the growth of these ramps, some sediment starvation occurred basinwards. Progradation and retrogradation of the different belts occur simultaneously, suggesting a sea‐level control on sedimentation. In the study area, the co‐existence of carbonate and siliciclastic facies belts depended on the superimposition of different orders of relative sea‐level cycles, and occurred mainly when the second‐order, third‐order and fourth‐order cycles showed highstand conditions.     

Sedimentology and stratigraphy of a transgressive, muddy gravel beach: Waterside Beach, Bay of Fundy, Canada

Sediments exposed at low tide on the transgressive, hypertidal (>6 m tidal range) Waterside Beach, New Brunswick, Canada permit the scrutiny of sedimentary structures and textures that develop at water depths equivalent to the upper and lower shoreface. Waterside Beach sediments are grouped into eleven sedimentologically distinct deposits that represent three depositional environments: (1) sandy foreshore and shoreface; (2) tidal‐creek braid‐plain and delta; and, (3) wave‐formed gravel and sand bars, and associated deposits. The sandy foreshore and shoreface depositional environment encompasses the backshore; moderately dipping beachface; and a shallowly seaward‐dipping terrace of sandy middle and lower intertidal, and muddy sub‐tidal sediments. Intertidal sediments reworked and deposited by tidal creeks comprise the tidal‐creek braid plain and delta. Wave‐formed sand and gravel bars and associated deposits include: sediment sourced from low‐amplitude, unstable sand bars; gravel deposited from large (up to 5·5 m high, 800 m long), landward‐migrating gravel bars; and zones of mud deposition developed on the landward side of the gravel bars. The relationship between the gravel bars and mud deposits, and between mud‐laden sea water and beach gravels provides mechanisms for the deposition of mud beds, and muddy clast‐ and matrix‐supported conglomerates in ancient conglomeratic successions. Idealized sections are presented as analogues for ancient conglomerates deposited in transgressive systems. Where tidal creeks do not influence sedimentation on the beach, the preserved sequence consists of a gravel lag overlain by increasingly finer‐grained shoreface sediments. Conversely, where tidal creeks debouch onto the beach, erosion of the underlying salt marsh results in deposition of a thicker, more complex beach succession. The thickness of this package is controlled by tidal range, sedimentation rate, and rate of transgression. The tidal‐creek influenced succession comprises repeated sequences of: a thin mud bed overlain by muddy conglomerate, sandy conglomerate, a coarse lag, and capped by trough cross‐bedded sand and gravel.     

Facies and sequence stratigraphy of a Late Barremian wave-dominated deltaic deposit, Agadir Basin, Morocco

The late Barremian succession in the Agadir Basin of the Moroccan Western High Atlas represents wave-dominated deltaic deposits. The succession is represented by stacked thickening and coarsening upwards parasequences 5–15 m thick formed during fifth- or fourth-order regression and building a third-order highstand systems tract. Vertical facies transitions in parasequences reflect flooding followed by shoaling of diverse shelf environments ranging from offshore transition interbedded mudstones, siltstones and thin sandstones, lower shoreface/lower delta front hummocky bedforms to upper shoreface/upper delta front cross-bedded sandstones. The regional configuration reflects the progradation of wave-dominated deltas over an offshore setting. The maximum sea-level fall led to the development of a sequence boundary that is an unconformity. The subsequent early Aptian relative sea-level rise contributes to the development of an extensive conglomerate lagged transgressive surface of erosion. The latter and the sequence boundary are amalgamated forming a composite surface.     

Sedimentology,sequence stratigraphy and syn-rift model of younger part of Washtawa Formation and early part of Kanthkot Formation,Wagad, Kachchh basin,Gujarat

The 600 m thick prograding sedimentary succession of Wagad ranging in age from Callovian to Early Kimmeridgian has been divided into three formations namely, Washtawa, Kanthkot and Gamdau. Present study is confined to younger part of the Washtawa Formation and early part of the Kanthkot Formation exposed around Kanthkot, Washtawa, Chitrod and Rapar. The depositional architecture and sedimentation processes of these deposits have been studied applying sequence stratigraphic context. Facies studies have led to identification of five upward stacking facies associations (A, B, C, D, and E) which reflect that deposition was controlled by one single transgressive — regressive cycle. The transgressive deposit is characterized by fining and thinning upward succession of facies consisting of two facies associations: (1) Association A: medium — to coarse-grained calcareous sandstone — mudrocks alternations (2) Association B: fine-grained calcareous sandstone — mudrocks alternations. The top of this association marks maximum flooding surface as identified by bioturbational fabrics and abundance of deep marine fauna (ammonites). Association A is interpreted as high energy transgressive deposit deposited during relative sea level rise. Whereas, facies association B indicates its deposition in low energy marine environment deposited during stand-still period with low supply of sediments. Regressive sedimentary package has been divided into three facies associations consisting of: (1) Association C: gypsiferous mudstone-siltstone/fine sandstone (2) Association D: laminated, medium-grained sandstone — siltstone (3) Association E: well laminated (coarse and fine mode) sandstone interbedded with coarse grained sandstone with trough cross stratification. Regressive succession of facies association C, D and E is interpreted as wave dominated shoreface, foreshore to backshore and dune environment respectively. Sequence stratigraphic concepts have been applied to subdivide these deposits into two genetic sequences: (i) the lower carbonate dominated (25 m) transgressive deposits (TST) include facies association A and B and the upper thick (75m) regressive deposits (HST) include facies association C, D and E. The two sequences are separated by maximum flooding surface (MFS) identified by sudden shift in facies association from B to C. The transgressive facies association A and B represent the sediments deposited during the syn-rift climax followed by regressive sediments comprising association C, D and E deposited during late syn-rift stage.     

Facies analysis of a Toarcian–Bajocian shallow marine/coastal succession (Bardas Blancas Formation) in northern Neuquén Basin,Mendoza province,Argentina

Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest.     

Quaternary depositional patterns and sea-level fluctuations, northeastern North Carolina

A detailed record of late Quaternary sea-level oscillations is preserved within the upper 45 m of deposits along an eight km transect across Croatan Sound, a drowned tributary of the Roanoke/Albemarle drainage system, northeastern North Carolina. Drill-hole and seismic data reveal nine relatively complete sequences filling an antecedent valley comprised of discontinuous middle and early Pleistocene deposits. On interfluves, lithologically similar marine deposits of different sequences occur stacked in vertical succession and separated by ravinement surfaces. Within the paleo-drainage, marine deposits are separated by fluvial and/or estuarine sediments deposited during periods of lowered sea level. Foraminiferal and molluscan fossil assemblages indicate that marine facies were deposited in a shallow-marine embayment with open connection to shelf waters. Each sequence modifies or truncates portions of the preceding sequence or sequences. Sequence boundaries are the product of a combination of fluvial, estuarine, and marine erosional processes. Stratigraphic and age analyses constrain the ages of sequences to late Marine Isotope Stage (MIS) 6 and younger (∼ 140 ka to present), indicating multiple sea-level oscillations during this interval. Elevations of highstand deposits associated with late MIS 5 and MIS 3 imply that sea level was either similar to present during those times, or that the region may have been influenced by glacio-isostatic uplift and subsidence.     

Architecture and formation of transgressive–regressive cycles in marginal marine strata of the John Henry Member,Straight Cliffs Formation,Upper Cretaceous of Southern Utah,USA

Marginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive – regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.     

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

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

Upper Cretaceous Gongila Formation in the Hawal Basin, northeast Benue Trough: a storm and wave dominated regressive shoreline complex

The Gongila Formation in the Hawal Basin displays lithological characteristics, textural variations and sedimentary structures that facilitate palaeoenvironmental reconstruction. The 41 m thick Gongila succession is divisible into: (i) a mudstone facies association (at the bottom) composed of fossiliferous limestone, clay shale, and sharp-based, graded and swaly-bedded shell debris; and (ii) a cross-stratified sandstone facies association that constitutes the uppermost 60% of the entire succession. The cross-stratified sandstone facies association is further subdivided, on the basis of sedimentary structures, into: (i) a lower interval represented by a coarsening upward fine- to medium-grained sandstone, siltstone and shale in which units characterised by parallel lamination and hummocky cross-stratification pass upward through a zone of small-scale low angle cross-stratification into units characterised by planar cross-stratification and sparse Teichichnus and Skolithos burrow traces; and (ii) an upper interval dominated by fine- to medium-grained sandstone and bioturbated siltstone characterised by erosive based, high angle tangential foresets, subhorizontal laminations and burrow structures belonging to the Thalassinoides, Ophiomorpha and Skolithos ichnogenera.The overall sequence of the Gongila Formation represents progradation on a wave influenced coast, passing from shelf mudstone at the base to lower and upper shoreface sandstones at the top. Each facies association displays an alternation between relatively high energy conditions when sediment was mainly deposited by decelerating suspension laden currents, and relatively low energy conditions when wave reworked fine-grained sediment as it was deposited from suspension. The influence of storms in these conditions is inferred from the associated lithofacies, textural characteristics and sedimentary structures.     

An iron-bearing wave-dominated siliciclastic shelf: Facies analysis and palaeogeographic implications (Silurian-Lower Devonian Sierra Grande Formation,Southern Argentina)

The Sierra Grande Formation (Silurian-Early Devonian) consists of quartz arenites associated with clast supported conglomerates, mudstones, shales and ironstones. Eight sedimentary facies are recognized: cross-stratified and massive sandstone, plane bedded sandstone, ripple laminated sandstone, interstratified sandstone and mudstone, laminated mudstone and shale, oolitic ironstone, massive conglomerate and sheet conglomerate lags. These facies are interpreted as shallow marine deposits, ranging from foreshore to inner platform environments. Facies associations, based on vertical relationships among lithofacies, suggest several depositional zones: (a) beach to upper shoreface, with abundant plane bedded and massive bioturbated sandstones; (b) upper shoreface to breaker zone, characterized by multistorey cross-stratified and massive sandstone bodies interpreted as subtidal longshore-flow induced sand bars; (c) subtidal, nearshore tidal sand bars, consisting of upward fining sandstone sequences; (d) lower shoreface zone, dominated by ripple laminated sandstone, associated with cross-stratified and horizontal laminated sandstone, formed by translatory and oscillatory flows; and (e) transitional nearshore-offshore and inner platform zones, with heterolithic and pelitic successions, and oolitic ironstone horizons. Tidal currents, fair weather waves and storm events interacted during the deposition of the Sierra Grande Formation. However, the relevant features of the siliciclastics suggest that fair weather and storm waves were the most important mechanisms in sediment accumulation. The Silurian-Lower Devonian platform was part of a continental interior sag located between southern South America and southern Africa. The Sierra Grande Formation was deposited during a second order sea level rise, in which a shallow epeiric sea flooded a deeply weathered low relief continent.     

Shelf construction in a foreland basin: storm beds, shelf sandbodies, and shelf-slope depositional sequences in the Upper Cretaceous Mesaverde Group, Book Cliffs, Utah

The Upper Cretaceous (Campanian) Kenilworth Member of the Blackhawk Formation (Mesaverde Group) is part of a series of strand plain sandstones that intertongue with and overstep the shelfal shales of the western interior basin of North America. Analysis of this section at a combination of small (sedimentological) and large (stratigraphical) scales reveals the dynamics of progradation of a shelf-slope sequence into a subsiding foreland basin. Four major lithofacies are present in the upper Mancos and Kenilworth beds of the Book Cliffs. A lag sandstone and channel-fill shale lithofacies constitutes the thin, basal, transgressive sequence, which rests on a marine erosion surface. It was deposited in an outer shelf environment. Shale, interbedded sandstone and shale, and amalgamated sandstone lithofacies were deposited over the transgressive lag sandstone lithofacies as a wave-dominated delta and its flanking strand plains prograded seaward. Analysis of grain size and primary structures in Kenilworth beds indicates that there are four basic strata types which combine to build the observed lithofacies. The fine- to very fine-grained graded strata of the interbedded facies are tempestites, deposited out of suspension by alongshelf storm flows (geostrophic flows). There is no need to call on cross-shelf turbidity currents (density underflows) to explain their presence. Very fine- to fine-grained hummocky strata are likewise suspension deposits created by waning storm flows, but were deposited under conditions of more intense wave agitation on the middle shoreface. Cross-strata sets in this region are bed-load deposits that accumulated on the upper shore-face, in the surf zone. Lag strata are multi-event, bed-load deposits that are the product of prolonged storm winnowing. They occur on transgressive surfaces. While the graded beds are tempestites in the strict sense, all four classes of strata are storm deposits. The distribution of strata types and their palaeocurrent orientations suggests a model of the Kenilworth transport system driven by downwelling coastal storm flows, and probably by a northeasterly alongshore pressure gradient. The stratification patterns shift systematically from upper shoreface to lower shoreface and inner shelf lithofacies partly because of a reduction in fluid power expenditure with increasing water depth, but also because of progressive sorting, which resulted in a decrease in grain size in the sediment load delivered to successive downstream environments. The Kenilworth Member and an isolated outlier, the Hatch Mesa lentil, constitute a delta-prodelta shelf depositional system. Their rhythmically bedded, lenticular, sandstone and shale successions are a prodelta shelf facies, and may be prodelta plume deposits. Major Upper Cretaceous sandstone tongues in the Book Cliffs are underlain by erosional surfaces like that beneath the Blackhawk Formation, which extend for many tens of kilometres into the Mancos shale. These surfaces are the boundaries of Upper Cretaceous depositional sequences. The sequences are large-scale genetic stratigraphic units. They result from the arranging of facies into depositional systems; the depositional systems are in turn stacked in repeating arrays, which constitute the depositional sequences. The anatomy of these foreland basin sequences differs     

Early carbonate diagenesis in slope sequences and at their boundaries, Cenozoic, offshore New Jersey (ODP Leg 150)

This paper reports the genetic links among the depth distribution, mineralogy, and stable isotopic composition of diagenetic carbonates with sedimentation rates and types and preservation of organic matter in the terrigenous and biogenic sediments of Oligocene and Miocene age on the New Jersey slope. Calcites formed close to the sediment surface at sequence boundaries and maximum flooding surfaces, when the profile of early-diagenetic reactions was stabilized in the sediment column for extended periods. Dolomites precipitated in the sulfate reduction zone when diagenetic profiles stabilized during truncation, sequence boundary formation, and the deposition of lowstand sediments that overlie the sequence boundaries. Most dolomites occur in distal slope sediments that were deposited before the shelf had prograded into the study area. Siderites formed during a later stage of burial in the methanogenic zone; they are not directly genetically related to the sequence stratigraphy of the New Jersey slope. The diagene-tic dolomites and siderites occur in widely separated depth intervals below the present sea floor. The distribution of the diagenetic carbonates and their preferential occurrence in separated depth intervals resulted from different combinations of sedimentation rates and organic matter types and preservation.     

Facies analysis and sequence stratigraphy of a Miocene warm-temperate carbonate ramp,Montagna della Maiella,Italy

The uppermost Oligocene/Lower Miocene to Upper Miocene ramp carbonates from Montagna della Maiella (Italy) form a supersequence bounded by deeply incised truncation surfaces. This supersequence is subdivided into four sequences. Each sequence is composed of skeletal limestones in its lower part and marly limestones in its upper part. The lower parts of the sequences are foramol limestones, which suggest deposition in the warm-temperate climate zone. Changes in climate, oceanography and relative sea level combined to control sedimentation in the four sequences. In the lower parts of the two older sequences, the skeletal sands built dunes, suggesting high-energy conditions. The dominant skeletal grains in the oldest sequence are larger foraminifers and in the next sequence they are bryozoans; this change reflects cooling around the time of the Aquitanian/Burdigalian boundary. In the lower parts of the two younger sequences, of Middle and Late Miocene age, sediment sheets with red-algal–bryozoan oncoids suggest deposition under calmer conditions. Transgressive and highstand systems tracts are recognized in all sequences; a shelf margin systems tract may be exposed in the second oldest sequence. In contrast to the situation that exists when warm-water carbonates are deposited, sedimentation of the foramol limestones on this isolated ramp was unable to balance accommodation during sea-level rise; this led to hemipelagic sedimentation during sea-level highstands. Conglomerates resulted from reworking along flooding surfaces.     

High-resolution sequence stratigraphy of a complex, incised valley succession, Cobequid Bay — Salmon River estuary, Bay of Fundy, Canada

The post-glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio-marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave-dominated estuarine environment under micro-mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun. The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial-estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments. These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical configurations.     

Sedimentology of magmatically and structurally controlled outburst valleys along rifted volcanic margins: examples from the Nuussuaq Basin, West Greenland

ABSTRACT After a period of early Palaeocene faulting and uplift of the Nuussuaq Basin, West Greenland, two valley systems were incised into the underlying sediments. Incision of the older Tupaasat valley took place during a single drainage event of large water masses, which resulted in catastrophic deposition. The valley was cut along early Palaeocene NW‐ to SE‐trending normal faults, clearly showing that the trend and the relief of the valley were structurally controlled. The valley fill is up to 120 m thick and consists of a lower part of sandstones and conglomerates deposited from catastrophic flows characterized by very high concentrations of suspended coarse‐grained sediment load. Catastrophic deposition was followed by rapid decrease in flow discharge and the establishment of a lacustrine environment within the valley characterized by the deposition of heterolithic sediments. The younger Paatuutkløften valley system was mainly cut into the Tupaasat valley fill, which was completely or nearly completely eroded away in many places. The younger valley is 1–2 km wide and up to 190 m deep. Incision of the Paatuutkløften valley probably reflected renewed tectonic activity and uplift of the basin. This phase was shortly followed by rapid major subsidence. The valley‐fill deposits comprise a uniform succession of fluvial and estuarine sandstones. The valley fill is topped by shoreface sandstones, which are succeeded abruptly by offshore mudstones deposited shortly before and during the initial extrusion of a thick hyaloclastite succession. The Paatuutkløften valley fill is attributed to a very rapid rise in relative sea level contemporary with extensive volcanism. It is suggested that this sequence of events coincided with the arrival of the North Atlantic mantle plume. In several respects, the early Palaeocene valley‐fill deposits of the Nuussuaq Basin are different from idealized facies models for incised valley systems and represent very special cases of incised valleys. Major differences from published examples include the dominance of catastrophic deposits and indications of large changes in relative sea level of several hundreds of metres taking place rapidly in less than 1 Myr. These changes were governed by the rise of the North Atlantic mantle plume.     

Lithofacies,palynofacies, and sequence stratigraphy of Palaeogene strata in Southeastern Nigeria

Integrated sedimentologic, macrofossil, trace fossil, and palynofacies data from Paleocene-Middle Eocene outcrops document a comprehensive sequence stratigraphy in the Anambra Basin/Afikpo Syncline complex of southeastern Nigeria. Four lithofacies associations occur: (1) lithofacies association I is characterized by fluvial channel and/or tidally influenced fluvial channel sediments; (2) lithofacies association II (Glossifungites and Skolithos ichnofacies) is estuarine and/or proximal lagoonal in origin; (3) lithofacies association III (Skolithos and Cruziana ichnofacies) is from the distal lagoon to shallow shelf; and (4) shoreface and foreshore sediments (Skolithos ichnofacies) comprise lithofacies association IV. Five depositional sequences, one in the Upper Nsukka Formation (Paleocene), two in the Imo Formation (Paleocene), and one each in the Ameki Group and Ogwashi-Asaba Formation (Eocene), are identified. Each sequence is bounded by a type-1 sequence boundary, and contains a basal fluvio-marine portion representing the transgressive systems tract, which is succeeded by shoreface and foreshore deposits of the highstand systems tract. In the study area, the outcropping Ogwashi-Asaba Formation is composed of non-marine/coastal aggradational deposits representing the early transgressive systems tract. The occurrence of the estuarine cycles in the Palaeogene succession is interpreted as evidence of significant relative sea level fluctuations, and the presence of type-1 sequence boundaries may well be the stratigraphic signature of major drops in relative sea level during the Paleocene and Eocene. Sequence architecture appears to have been tectono-eustatically controlled.     

A sequence stratigraphic model for an intensely bioturbated shallow-marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.     

Allogenic and autogenic controls on facies and stratigraphic architecture of the Lower Jurassic Mashabba Formation,Gebel Al-Maghara,North Sinai,Egypt

The Lower Jurassic Mashabba Formation crops out in the core of the doubly plunging Al-Maghara anticline, North Sinai, Egypt. It represents a marine to terrestrial succession deposited within a rift basin associated with the opening of the Neotethys. Despite being one of the best and the only exposed Lower Jurassic strata in Egypt, its sedimentological and sequence stratigraphic framework has not been addressed yet. The formation is subdivided informally into a lower and upper member with different depositional settings and sequence stratigraphic framework. The sedimentary facies of the lower member include shallow-marine, fluvial, tidal flat and incised valley fill deposits. In contrast, the upper member consists of strata with limited lateral extension including fossiliferous lagoonal limestones alternating with burrowed deltaic sandstones. The lower member contains three incomplete sequences (SQ1-SQ3). The depositional framework shows transgressive middle shoreface to offshore transition deposits sharply overlain by forced regressive upper shoreface sandstones (SQ1), lowstand fluvial to transgressive tidal flat and shallow subtidal sandy limestones (SQ2), and lowstand to transgressive incised valley fills and shallow subtidal sandy limestones (SQ3). In contrast, the upper member consists of eight coarsening-up depositional cycles bounded by marine flooding surfaces. The cycles are classified as carbonate-dominated, siliciclastic-dominated, and mixed siliciclastic-carbonate. The strata record rapid changes in accommodation space. The unpredictable facies stacking pattern, the remarkable rapid facies changes, and chaotic stratigraphic architecture suggest an interplay between allogenic and autogenic processes. Particularly syndepositional tectonic pulses and occasional eustatic sea-level changes controlled the rate and trends of accommodation space, the shoreline morphology, the amount and direction of siliciclastic sediment input and rapid switching and abandonment of delta systems.     

Early Miocene lacustrine deposits and sequence stratigraphy of the Ermenek Basin,Central Taurides,Turkey

The lacustrine Ermenek Basin evolved as a SE-trending intramontane graben affected by strike–slip deformation, with the initial two lakes merging into one and receiving sediment mainly through fan deltas sourced from the basin's southern margin. The northern margin was a high-relief rocky coast with a wave-dominated shoreline. The Early Miocene lacustrine sedimentation was terminated by a late Burdigalian marine invasion that drowned the basin and its surroundings. The lacustrine basin-fill succession is up to 300 m thick and best exposed along the southern margin, where it consists of four sequences bounded by surfaces of forced regression. The offshore architecture of each sequence shows a thin lowstand tract of shoreface sandstones overlain by a thick transgressive systems tract of mudstones interbedded with sandy tempestites and delta-derived turbidites, which form a set of coarsening-upward parasequences representing minor normal regressions. The corresponding nearshore sequence architecture includes a thick lowstand tract of alluvial-fan deposits overlain by either a well-developed transgressive systems tract (backstepping parasequence set or single fan-deltaic parasequence) and poorly preserved highstand tract; or a thin transgressive tract (commonly limited to flooding surface) and a well-developed highstand tract (thick fan-deltaic parasequence). The sequences are poorly recognizable along the northern margin, where steep shoreline trajectory rendered the nearshore system little responsive to lake-level changes. The resolution of local stratigraphic record thus depends strongly upon coastal morphology and the character of the depositional systems involved.The sequential organization of the basin-fill succession reflects syndepositional tectonics and climate fluctuations, whereas the lateral variation in sequence architecture is due to the localized sediment supply (deltaic vs. nondeltaic shoreline), varied coastal topography and differential subsidence. The study points to important differences in the sequence stratigraphy of lacustrine and marine basins, related to the controlling factors. A crucial role in lacustrine basin is played by climate, which controls both the lake water volume and the catchment sediment yield. Consequently, the effects of tectonics and the dynamics of changes in accommodation and sediment supply in a lacustrine basin are different than in marine basins.