Tidal Ravinement Research Articles

The hypertidal Santa Cruz–Chico River estuary (South Patagonia, Argentina): A hybrid ria-type system under extreme tides, arid climate and active uplift

The present study focuses on the morphosedimentary organization and sediment infilling stratigraphy of one of the largest estuaries of southern Patagonia in Argentina. With a tidal range up to 12 m, the area is subject to extreme tidal conditions, combined with moderate offshore wave climate, strong and constant westerly winds, and contrasted water and sediment discharges from the two tributaries of the estuary, the Santa Cruz and Chico rivers. The estuarine valley is entrenched in the Patagonian coastal plateau due to significant uplift. On the basis of sediment facies (sedimentary structures, grain size, geochemistry, mineralogy), meiofauna (foraminifera and testate amoebae), morphological changes and shallow geophysics (high-resolution seismic reflection, ground-penetrating radar) data, the Santa Cruz–Chico River system is defined as a hybrid system comprising a tide-influenced fluvial mouth (the Santa Cruz River) and a tide-dominated estuary (the Chico River estuary), both converging toward an elongated subtidal ria-type estuarine basin. River-supplied sands and muds by-pass the estuarine basin and are exported offshore where they settle and form an ebb-tidal delta. Sediments in the Santa Cruz–Chico River valley mainly consist of Pleistocene lowstand fluvial gravels resting on the regional Miocene substrate, and thin early Holocene transgressive deposits, deeply incised by a tidal ravinement surface that developed during the highest Holocene sea-level at ca 7500 y. BP. After the maximum stillstand, relative sea level fell and a competition occurred between erosion, promoted by water depth decrease, and deposition, favored by tidal prism reduction. At present, sediment by-passing and offshore sediment export are the dominant processes. The very large size of the ebb-tidal delta, which expands on the continental shelf, suggests that this situation has prevailed for a very long time.

Revisiting the stratigraphic development of the postglacial, transgressive paleo-Changjiang (Yangtze River) mouth: A journey from tide-dominated estuary to delta

The transition between a tide-dominated estuary and a tide-dominated delta, which is observed in modern settings and repeatedly throughout the geologic record, is a critical process for understanding sea-level and environmental changes. This transition remains poorly documented, however, leaving a knowledge gap about the sedimentary and stratigraphic processes involved. To help bridge the gap, we reexamined the postglacial transgressive system beneath the modern Changjiang (Yangtze River) Delta, built by one of the world’s largest rivers, using new chirp data supplemented by correlation with previously documented boreholes. Our results revealed a previously undocumented tide-dominated estuary-to-delta transition around the Younger Dryas, with the pretransition evolution dominated by the retrogradation of the tide-dominated estuary, which fills most of the accommodation space in the lowstand incised valley. This transition occurred when the middle and seaward parts of the estuary reached the subaqueous modern Changjiang Delta area and underwent extensive tidal erosion. Overlying the tidal ravinement, a series of ridge-and-swale structures formed due to tidal/fluviotidal channelization on the mud-dominated heterolithic substrate, primarily consisting of delta-front sediments characterized by flat, stratified reflections (bedding) with shallowing-upward trends. Ridge orientation rotates clockwise from north to south, with intervening swales bifurcating from a common apex point, and these features are flanked by inclined and sigmoidal levee-like channel fills dominated by homogeneous mud (acoustically transparent material). We reinterpret these features as tide-dominated delta islands and distributary channels, rather than the previously interpreted erosional mud ridges in the seaward part of the estuary, as they align with patterns observed in classic tide-dominated deltas. These patterns also suggest a shift from delta-front to delta-plain environments coinciding with shoreline advance ca. 10 k.y. B.P., but this was preceded by the evolution of a backstepping delta. Our study highlights the interplay between sediment supply and sea-level fluctuations in controlling the large-scale stratigraphic and environmental evolution of tidal-dominated river mouth systems. In the transgressive, paleo-Changjiang mouth, this interplay drove the system to evolve through stages of retrogradational estuary, backstepping delta, and progradational delta, and each environment differs significantly from the pattern observed in the modern Changjiang Delta. This study provides important insights for evaluating the evolution and sequence-stratigraphic reconstruction of other tidal depositional systems.

Ichnology of tidal ravinement omission surfaces in siliciclastic transgressive deposits from the Puesto El Moro formation (Upper Cretaceous), Southern Patagonia, Argentina

Omission surfaces represent a gap in the sedimentary record on a particular environment, constituting an important tool for basin analyses. Nevertheless, not always these surfaces constitute key sequence-stratigraphic surfaces, and their correct interpretation is highly important for the recognition of genetically related strata. Documentation and interpretation of ichnological data from stratigraphic surface can refine the sedimentological model. The Puesto El Moro Formation (lower Upper Cretaceous) in the Austral-Magallanes Basin, southern Patagonia (Argentina), shows excellent examples of several omission surfaces stacked during a transgressive context associated with the beginning of the foreland stage. Each of these surfaces, interpreted as Tidal Ravinement Surfaces, contain Thalassinoides isp. and Sinusichnus isp., which represent opportunistic colonization by crustaceans into a predominantly firm substrate. In this case study, the vertical recurrence of substrate-controlled trace fossil suites is the result of high-frequency repeated coastal conditions during the transgression; the associated surfaces, which have limited lateral extension, have low potential for correlation at basin scale.

Bayhead delta evolution in the context of late Quaternary and Holocene sea-level change, Richards Bay, South Africa.

Richards Bay is part of a back-barrier lagoon fronted by high coastal dunes on the NE, Indian Ocean coast of South Africa. In the early 1970s, a berm was constructed, dividing the original Mhlathuze Estuary into two separate systems; the Richards Bay Harbour and the new Mhlathuze Estuary. This study investigates the stratigraphic evolution of the incised valley system and bayhead delta in the Richards Bay Harbour segment. Seven seismic units (Units 1–7) were imaged. A single regionally developed sequence boundary (SB) along with two tidal ravinement surfaces (tRS1 and tRS2) were identified. Surface SB is associated with the LGM lowstand which developed when sea levels were ~ 130 m below present, until ~18,000 year BP. Cretaceous age siltstones (Unit 1) form the basement. Transgressive material overlying SB (Unit 2) reflects the filling of an incised valley located in the middle segment of a wave-dominated back-barrier system. It is overlain by a bayhead delta (Unit 3), the geometry and seismic signature of which indicate alternating periods of aggradation/progradation and backstepping. The behaviour is attributed to episodic jumps in sea-level, and is tentatively (on the basis of elevations in relation to the regional sea-level curve) linked to periods of rapidly rising sea-level (8.2 ka event and Meltwater Pulse (MWP)-1d). These intervals of rapidly rising sea-level, combined with relatively low gradient settings facilitated backstepping of the delta. Fills (Unit 4) occur within minor incisions along the delta top. These are interpreted as distributary channels that fed sediment to the seaward edge of the bayhead delta system. Elongated mounds on the seafloor (Unit 5) are interpreted as spoil from contemporary port dredging. Slump deposits (Unit 6) along the delta front are attributed to a combination of oversteepening of the delta by dredging, as well as deposition of modern sediments brought into the system by tidal currents. The system is capped by fine-grained, tidally redistributed and deposited sediments (Unit 7) which were possibly sourced from older organic material of an indeterminate source. This site is especially sensitive to episodic rates of sea-level change due to the relatively small Glacial Isostatic Adjustments (GIA) during the postglacial transgression and the flat antecedent gradients of both the subaerial unconformity and the overlying tidal ravinement.

Coarse‐grained deltas approaching shallow‐water canyon heads: A case study from the Lower Pleistocene Messina Strait, Southern Italy

AbstractThe tide‐dominated Messina Strait (southern Italy) is a 3 km wide marine passageway, whose block‐faulted borders form steep subaqueous zones incised by canyons and gullies. These erosional features retreat towards the shorelines and are often in direct connection with subaerial valley‐bounded river deltas. High‐energy density‐flows generated by river floods periodically enter the canyon heads, attaining supercritical‐flow regime and accreting large, upslope‐migrating bedforms. Although these bedforms have been documented in recent studies, little attention has been paid to the definition of the type of delta entering canyon heads, the internal features of river‐influenced deposits accumulated in the nearshore zone, and their interplay with tidal currents flowing axially to the strait. This study focuses on a Lower Pleistocene coarse‐grained succession exposed along the north‐eastern margin of the modern Messina Strait, investigated using conventional facies analysis and sedimentological logging, integrated with photogrammetric techniques and interpretation of drone‐acquired imagery. Facies confinement between basement blocks suggests a subaqueous delta complex shed from the tectonically controlled margin of the ancient strait and entering shallowly submerged canyon heads. Basal breccias, conglomerates and pebbly sandstones exhibiting channel‐form discontinuities and upslope dipping backsets are interpreted as cyclic‐step and antidune deposits. Units composed of these facies are comprised between master erosional surfaces and tidal ravinement surfaces. The tidal ravinements suggest that canyon infill occurred during a major phase of sea‐level rise, punctuated by minor falls and stillstands. These surfaces are overlain by mixed bioclastic–siliciclastic, arenitic, trough and planar cross‐strata, representing dunes migrating roughly parallel to the palaeo‐coastline and originated by tidal currents amplified by the narrowing of the ancient Messina Strait. Tidal‐influenced sedimentation dominated over the fluvial‐influenced processes during the late transgression, overfilling the canyon relief. The exceptionally good exposure of depositional architectures and facies characteristics is key to outline the general features of a specific type of delta system, fed by valley‐bounded rivers and entering canyon heads in the nearshore of tectonically‐controlled, tide‐influenced steep strait margins. The pre‐existing subaqueous incised topography forced the delta front to be split into lobe branches during the canyon infilling, hampering clinoform architectures and preserving large supercritical‐flow sedimentary structures. This study suggests these as possible criteria for the recognition of similar systems in outcrop or subsurface.

Influence of antecedent geology on the Holocene formation and evolution of Horn Island, Mississippi, USA

Horn Island, one of the two most stable barriers along the Mississippi-Alabama chain (Cat, East and West Ship, Horn, West Petit Bois, Petit Bois, and Dauphin), provides critical habitat, helps regulate estuarine conditions in the Mississippi Sound, and reduces wave energy and storm surge before they reach the mainland shore. However, important details of the formation and evolution of the island in response to sea-level rise, storms, and antecedent geology remain unclear. This study integrates 2200 km of high-resolution geophysical data, 35 sediment cores, and 18 radiocarbon ages to better understand the geologic history of the island. Incised valleys of the Biloxi and Pascagoula Rivers underlie Horn Island and played a profound role in the evolution of the system. Within the incised valleys, sandy paleochannel deposits represent potential sediment sources during island development. Scour associated with wave and tidal ravinement processes liberated sand from the paleochannels and along with numerous other sizable sand sources on the shelf contributed to the formation and continued maintenance of Horn Island. Based on radiocarbon ages, transgressive ephemeral islands/shoals with no preserved shoreface existed at least 8000 cal yr BP and were frequently overwashed when sea-level rise rates were ~ 4–5 mm/yr. Approximately 5000 cal yr BP, coinciding with a deceleration in sea-level rise to about 1.4 mm/yr and attendant increased sand supply, radiocarbon ages associated with Horn Island's barrier complex and lower shoreface indicate a period of island stabilization. Seismic and sediment core data show a long history of westward lateral migration by longshore currents through tidal ravinement and inlet fill. Subsurface sand packages associated with tidal inlet fill and paleochannels are available for ravinement and may be important sand sources for Horn Island to maintain subaerial exposure with the expected accelerated future sea-level rise.

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

Geological inheritance and its role in the geomorphological and sedimentological evolution of bedrock-hosted incised valleys, lake St Lucia, South Africa

The morphology of Lake St Lucia, Africa's largest estuary, has been strongly influenced by antecedent topography throughout its Quaternary evolution. The 5 × 20 km southern basin is bounded to landward by a 50 m-high Early/Mid-Pleistocene dune ridge and to seaward by a 100 m-high Late Pleistocene/Holocene dune barrier. Nine seismic units (A-I) are identified in the enclosed basin, along with two regionally developed sequence boundaries (SB1 and SB2) and two tidal ravinement surfaces (tRS1 and tRS2). The lowermost sequence boundary (SB1) is related to regional late-Pliocene hinterland uplift. SB2 is related to the last glacial maximum (LGM) when sea levels fell ∼130 m below present. Lowstand fluvial sediments occupy the base of both SB1 and SB2 incisions. Thick central basin deposits dominate the valley fills, more so in the Pliocene age valleys, the occurrence and thickness of which is attributed to sheltering by a seaward barrier-dune system as well as to the low gradient settings. Prograding tidal bedforms occupy the minor incisions formed within the tidal ravinement surfaces, marking the onset of lagoonal conditions in the area. The system is capped by lagoonal deposits, deposited above tidal flats. A series of prograding spits mark the early stages of lagoonal segmentation in the Holocene. The preservation of only one incision and fill sequence prior to the development of an LGM-aged valley succession is attributed to geological inheritance whereby older valleys were repeatedly re-incised by younger valleys throughout the Pleistocene, causing partial preservation of Pleistocene fills up to the most recent cycle. The older fill remnant relates to a larger magnitude base level fall prior to the Quaternary, which survived erosion during subsequent sea-level falls. Adjacent topography in the form of rocky cliffs and the seaward fronting barrier limit the accommodation space in which infilling material can be hosted outside the major valley network. The competent basement rock into which these valleys have incised protected lowstand fluvial sediments from subsequent tidal ravinement.

Stepped Holocene sea-level rise and its influence on sedimentation in a large marine embayment: Maputo Bay, Mozambique

The sedimentary record in large marine embayments holds important information on Holocene environmental and sea-level changes. This paper investigates the evolution of a large subtropical marine embayment, Maputo Bay, in southern Mozambique. Using a combination of sediment cores, stable isotope data, seismic reflection profiling and 14C dating, we show complex back-barrier changes during the Holocene and link these to stepped rises in sea level. Our data reveal a sea-level “slowstand” followed by an abrupt rise that we correlate with the 8.2 ka global rise in sea level. Sedimentological, isotope and seismic evidence point to a subsequent slowstand interspersed by two punctuated rises in sea level that formed clear tidal ravinement surfaces. Sedimentary changes in the embayment are primarily forced by sea-level change, whereby tidal ravinement surfaces sandwich normal regressive packages formed via marginal progradation and shallowing of the system during “slow-” or still-stands. Large marine embayment systems hold great potential in unravelling relative sea-level change. Axiomatically, this reduces their utility as archives of climate-change.

Incised channel morphology and depositional fill of the paleo-Seomjin River in the continental shelf of the South Sea, Korea

This study is the first attempt at describing and interpreting incised-channels on the continental shelf of the South Sea, Korea. The distribution, pattern, shape, and infilling of submerged channels incised across the continental shelf of the South Sea have been investigated based on approximately 1940 line-km of chirp and sparker data. The incised-channel system of the paleo-Seomjin River on the continental shelf is approximately 109 km long, 800–5000 m wide, and is more than 890 km2 in area. Meandering and straight types of channel are dominant on the inner shelf, with a change to straight and braided planforms on the mid to outer shelf. Sparker seismic data suggest the paleo-channels formed under fluvial systems when the shelf was exposed during the Last Glacial Maximum (LGM). Channel fills are divided into five types, based on erosional surfaces and internal seismic reflectors: (I) transparent to semi-transparent incised channel fill; (II) parallel to sub-parallel incised channel fill; (III) complex incised channel fill; (IV) divergent incised channel fill; and (V) chaotic incised channel fill. The acoustically transparent facies of the channel fill type (Type I) suggest the presence of basin muddy deposits in a low energy environment, which were dominant in the inner shelf. These muddy sediments were likely during a period of higher sea level during the Holocene transgression. Types II, III, and IV are characterized by onlap, downlap, and wave or tidal ravinement surfaces, respectively, indicating that they were deposited during a transgressive stage, which type V represent basal incision surface. The evolution of paleo Seomjin River has been strongly controlled by sea level change and sediment transport processes since the LGM.

Sequential development of tidal ravinement surfaces in macro- to hypertidal estuaries with high volcaniclastic input: the Miocene Puerto Madryn Formation (Patagonia, Argentina)

The late Miocene beds of the Puerto Madryn Formation (Provincia del Chubut, Argentina) are formed by shallow marine and estuarine sediments. The latter include several tidal-channel infills well exposed on the cliffy coast of the Peninsula Valdes. The Bahia Punta Fosil and Cerro Olazabal paleochannels are end members of these tidal channels and show a fining-upward infilling starting with intraformational channel lag conglomerates above deeply erosional surfaces interpreted as fluvial ravinement surfaces (the erosion surface formed in the purely fluvial or the fluvially dominated part of the estuary, where erosion is driven by fluvial processes). These are overlain and eventually truncated (and suppressed) by the tidal ravinement surface (TRS), in turn covered with high-energy, bioclastic conglomerates mostly formed in the “tidally dominated/fluvially influenced” part of an estuary. Above, large straight or arcuate point bars with alternatively sandy/muddy seasonal beds and varying trace and body fossil contents were deposited from the freshwater fluvially dominated to saline-water tidally dominated part of the estuary. The upper channel infill is formed by cross-bedded sands with mud drapes and seaward-directed paleocurrents, together with barren, volcaniclastic sandy to muddy heterolithic seasonal rhythmites, both deposited in the fluvially dominated part of the estuary. Volcanic ash driven by the rivers after large explosive volcanic eruptions on land resulted in sedimentation rates as high as 0.9 m per year, preserving (through burial) the morphology of tidal channels and TRSs. The channel deposits were formed in a tide-dominated, macrotidal to hypertidal open estuary with well-developed TRSs resulting from strong tidal currents deeply scouring into the transgressive filling of the channels and eventually cutting the fluvial ravinement surface. The TRSs extended upstream to the inner part of the estuary during long periods of low sedimentation rates, extended channel migration and sediment bypass, interrupted by transient, high volcaniclastic input. The tidal channels of the Puerto Madryn Formation constitute a unique example of estuary sedimentation with pulsed sediment supply in a macrotidal to hypertidal estuary.

Control factors of Holocene sedimentary infilling in a semi-closed tidal estuarine-like system: the bay of Brest (France)

This study details the sedimentary infilling of an original tidal-dominated estuary system during the final stage of the last marine transgression. The Bay of Brest is confined and connects the rivers Elorn and Aulne, to the sea of Iroise by a narrow strait encasing a well preserved paleo-channel. The compilation of high- and very-high-resolution bathymetric and seismic data, constrained by sediments datations, allows us to classify the paleo-morphology of the bay into three stepped domains: the paleo-valley floor surrounded by fluvial terraces, the central plateau, and the shallow embayments. Taking into account the main factors controlling the infilling, including sea-level rise, substratum morphology, and hydrodynamics, the stratigraphic scheme of the bay has been reconstructed. The lowstand system track (LST) is assumed to correspond to relict Pleistocene continental deposits associated with the last low sea level around 21,000calyrB.P. The transgressive phase, represented by the transgressive system track (TST), is divided into two parts. The lower part (TST1) constitutes the first stage of transgressive deposition in the bay. Starting around 9000calyrB.P. and currently conserved in the shallowest parts, it is characterized by tidal flats associated with deposits in the inner estuary. This stage ends at about 7000calyrB.P. and is separated from the upper part (TST2) by a tidal ravinement surface that occurs around 7700calyrB.P. at the foot slope. TST2 formed sand bodies in the central part of the bay between 6800 and 3000calyrB.P. These deposits, interpreted as tidal banks, are associated with the outer estuarine environment. The maximum flooding surface (MFS), dated around 3000 and 2000calyrB.P., marked the installation of a highstand system track (HST) under the combined influence of tidal currents, storms events, and anthropogenic activity. The geometry deposition of each system track is controlled at the first order by the combination of the irregular Holocene sea level rise and the inherited rocky substratum morphology. The retreat of the shoreline, and estuarine environments, is function of the successive flooding of stepped domains. The direct evolution of the volume of the assumed accumulated sea water in the bay favoured the gradual installation of the present-day hydrodynamic circulation.

The stratigraphic evolution of a large back-barrier lagoon system with a non-migrating barrier

Lake St Lucia, the largest estuarine system in Africa, is enclosed by a 120m-high compound Quaternary barrier-dune system in northern KwaZulu-Natal, South Africa. It comprises several discrete sedimentary basins within a single shallow back-barrier water body. This paper reports the first very-high-resolution seismic study of the system. Seven seismic units (A–G) are identified and interpreted based on their geometry, acoustic properties and a sediment coring programme. The units are bounded by regionally developed sequence boundaries and lower order unconformity surfaces corresponding to bay and tidal ravinement and hiatus surfaces. The lowermost subaerial unconformity formed during regression related to late-Pliocene hinterland uplift. Initial infilling of this surface in the proximal areas reflects estuarine sedimentation in a mixed wave- and tide-dominated system during the subsequent lowstand and early transgressive systems tracts, overlain by prograding highstand deltaic deposits that developed as sea-levels began to stabilise. Distally, this cycle of sedimentation is initially reflected by the infilling of a similar estuarine system that gradually transitioned to a back-barrier lagoon. Superimposed stillstands drove imbalances in the rate of sediment supply and accommodation space, and this, coupled with the development of an early barrier complex, caused lagoon aggradation and shallowing. A second similar trend is repeated for the recent postglacial sea-level cycle. There appears to be no preservation of the intervening Pleistocene cycles of erosion and deposition. The system is capped by estuarine-lacustrine deposits. The onset of lower energy estuarine-lake conditions began ~6235cal BP in North Lake and started earlier in the proximal False Bay owing to sheltering by the antecedent barrier of a rocky peninsula. This study reveals a markedly different evolutionary history in adjacent basins of the same back-barrier system during two full sea-level cycles. These are attributed to a long established, non-migrating barrier, and to the influence of antecedent conditions on the system response to transgression. Unlike the US Atlantic margin, where migrating barriers truncate the infilling sequences, the main stratigraphic unconformities within each fill sequence correspond to tidal and bay ravinement surfaces.

Valleys, Estuaries, and Lagoons: Paleoenvironments and Regressive–Transgressive Architecture of the Upper Cretaceous Straight Cliffs Formation, Utah, U.S.A

Abstract: The John Henry Member of the Upper Cretaceous Straight Cliffs Formation preserves deposition of four regressive–transgressive (R-T) cycles in 350 m of strata of the Sevier foredeep in south-central Utah, USA. Each cycle is discussed in detail, with emphasis on the transgressive phases of deposition. Regressive intervals comprise wave-dominated shorefaces and coastal-plain strata, whereas transgressive intervals record tide-influenced coastal-margin and low-energy-bay and lagoonal deposits. One R-T cycle in the lower John Henry Member preserves a compound incised-valley system filled with a complex assemblage of tidal and estuarine facies. In contrast, overlying R-T cycles are not associated with valley formation, but instead preserve sandstone-rich back-barrier platform deposits that transition landward into tidal-creek, tidal-flat, and marsh depositional settings. Excellent outcrop expression permits detailed examination of the complex internal architecture of the compound incised-valley, and demonstrates that: 1) tidal ravinement significantly modified the initial valley shape during transgression, a process not fully recognized in most conceptual models of valley formation and fill; 2) the valley system incised in a basin-axial position (NNE–SSW), subparallel to the thrust front and oblique to the orientation of pre-valley-formation shorefaces, which prograded from west to east. Axial systems are well-known transporters of large volumes of sediment in foreland basins, and yet most incised-valley models imply a direct and oversimplified relationship between up-dip (source area and tectonics) and down-dip (base level) controls; 3) the major subaerial unconformity and bypass surface occurred at a higher (younger) stratigraphic position than previously interpreted, and is herein renamed the lower John Henry Member sequence boundary. The changes in regional correlations necessitated by this discovery have several broader implications for sequence stratigraphic models; 4) finally, correlations down dip along the axial valley system indicate a steep topographic gradient of 0.011, with 47% vertical, compacted expansion of the whole John Henry Member over 14 km from south to the north. This suggests structural control on sediment transport and deposition, with significant lateral variability in accommodation parallel to the fold-thrust belt. This study adds to the growing body of literature documenting the complex nature of transgressive deposits, which will aid in the interpretation, prediction, and management of analogous subsurface reservoirs.

Facies architecture and depositional model of a macrotidal incised-valley succession (Qiantang River estuary, eastern China), and differences from other macrotidal systems

Filling of the modern coastal-zone portion of the Qiantang River (QR) in eastern China was initiated by marine inundation during the transgression after the Last Glacial Maximum and has continued during the Holocene sea-level stillstand. The early part of the fill is transgressive in character, while the younger part is regressive. This paper deals with the sedimentary facies, surfaces, architecture, and depositional model of the QR incised-valley fill based on the detailed analysis of the newly drilled core SE2 and its correlation with more than 800 boreholes. The incised-valley deposits are grouped into five stacked facies: amalgamated channel, floodplain and channel, paleo-estuary, offshore shallow marine, and present-day estuary. A paleo-estuary facies had never been identified before, making these observations novel. This facies is characterized by a sedimentary succession and sediment distribution that are distinct from those of the present-day estuary because of a change in sediment supply from the sea floor beyond the estuary mouth. It also contains large numbers of tidal-channel sand bodies that are significant reservoirs for shallow biogenic gas. Since the last glaciation, there have been three stages in the development of the QR incised-valley fill sequence: (1) a formation stage associated with sea-level fall, (2) a filling stage during the early transgression with rapid sea-level rise, and (3) a burial stage corresponding to the slowing of sea-level rise and the onset of progradation. This fill consists of four longitudinal segments, each of which is distinguished by a distinct stratigraphic succession and different degrees of marine and fluvial influence. The basal erosion surface and sidewalls of the incised valley, the top of stiff clay on the interfluves, and the top of falling-stage fluvial-terrace deposits compose the sequence boundary. The initial flooding surface and maximum flooding surface are located within the amalgamated channel and offshore shallow-marine sediments, respectively. These observations indicate that: (1) relative sea-level changes determine the stratal stacking patterns, but local environmental factors, such as physical processes, accommodation, sediment supply, and coastal configuration control the nature of the facies, surfaces, and sediment-distribution patterns; and (2) the tripartite facies organization and wave ravinement surfaces typifying wave-dominated or mixed-energy (wave and tide) incised-valley fills are absent in the macrotidal QR succession, and the tidal ravinement and erosion surfaces of the macrotidal QR incised valley are more extensive and numerous than those of the wave-dominated or mixed-energy succession.

Warm‐temperate, marine, carbonate sedimentation in an Early Miocene, tide‐influenced, incised valley; Provence, south‐east France

AbstractThe Saumane‐Venasque compound palaeovalley succession accumulated in a strongly tide‐influenced embayment or estuary. Warm‐temperate normal marine to brackish conditions led to deposition of extensive cross‐bedded biofragmental calcarenites. Echinoids, bryozoans, coralline algae, barnacles and benthic foraminifera were produced in seagrass meadows, on rocky substrates colonized by macroalgae and within subaqueous dune fields. There are two sequences, S1 and S2, the first of which contains three high‐frequency sequences (S1a, S1b and S1c). Sequence 1 is largely confined to the palaeovalley with its upper part covering interfluves. Each of these has a similar upward succession of deposits that includes: (i) a basal erosional surface that is bored and glauconitized; (ii) a discontinuous lagoonal lime mudstone or wackestone; (iii) a thin conglomerate generated by tidal ravinement; (iv) a transgressive systems tract series of cross‐bedded calcarenites; (v) a maximum flooding interval of argillaceous, muddy quartzose, open‐marine limestones; and (vi) a thin highstand systems tract of fine‐grained calcarenite. Tidal currents during stages S1a, S1b and S1c were accentuated by the constricted valley topography, whereas basin‐scale factors enhanced tidal currents during the deposition of S2. The upper part of the succession in all but S1c has been removed by later erosion. There is an overall upward temporal change with quartz, barnacles, encrusting corallines and epifaunal echinoids decreasing but bryozoans, articulated corallines and infaunal echinoids increasing. This trend is interpreted to be the result of changing oceanographic conditions as the valley was filled, bathymetric relief was reduced, rocky substrates were replaced as carbonate factories by seagrass meadows and subaqueous dunes, and the setting became progressively less confined and more open marine. These limestones are characteristic of a suite of similar cool‐water calcareous sand bodies in environments with little siliciclastic or fresh water input during times of high‐amplitude sea‐level change wherein complex inboard antecedent topography was flooded by a rising ocean.

Facies model of a fine‐grained, tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert, Egypt

AbstractExisting facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.

The paleo-Etel River incised valley on the Southern Brittany inner shelf (Atlantic coast, France): Preservation of Holocene transgression within the remnant of a middle Pleistocene incision?

The study of a dense and regular grid of 59 High Resolution seismic profiles supplemented by 23 superficial vibro-cores has led to propose a three-dimensional detailed sedimentary model of the paleo-Etel River buried valley located on the inner shelf of South Brittany (Atlantic Coast, France).This fluvial valley (S2) lies at a mean depth of −37m below mean sea level above Paleozoic meta-sediments (U1) and middle to late Eocene formations (U2). No fluvial deposits seem to have been preserved. Its sedimentary record reflects the retrogradation of estuarine environments (tidal flats (U3), Tidal Ravinement Surface (S3), and intra to pro-estuarine sand bars (U4/S4/U5)) during the Holocene transgression (ca. 9000 to 7500calyr BP). Open marine environments seal up this estuary with the onset of a flat Wave Ravinement Surface (S5) overlain by thin muddy-sands deposits (U6-F2) interpreted as low-energy open marine conditions (4250 to 1800calyr BP). Shoreface sands, made up of coarse sands bodies migrating landwards (U6-F3, F4), cover this sedimentary sequence since 1800calyr BP.A numerical interpolation of the picked seimic horizons was proceed to produce seismic isopach maps of sedimentary units and isobath maps of their boundary surfaces. Furthermore, the isobath map of the basal channel topography has enabled us to calculate some geomorphological features (W/D ratio, mean slope, sinuosity index) that characterized the type of channel pattern. The Etel River geomorphological indexes correspond to a straight channel that involves bed-load transportation processes, potentially associated to braided deposits if the sediment load is sufficient. Compared to the Vilaine River, which braided regime is dated at middle Pleistocene in several zones, we assume a possible mainly climatic control for river incisions in Southern Brittany linked to drastical changes in climate regimes occurring during the “Middle Pleistocene Transition”.

Stratigraphic Architecture of a Net-Transgressive Marginal- to Shallow-Marine Succession: Upper Almond Formation, Rock Springs Uplift, Wyoming, U.S.A.

Abstract The upper part of the Almond Formation records the overall retreat of a wave-dominated shoreline and associated lagoons or bays. Exposures of these strata on the eastern flank of the Rock Springs Uplift, Wyoming, U.S.A., enable analysis of their stratigraphic architectures along sections oriented oblique to depositional strike. The upper Almond Formation comprises at least nine vertically stacked regressive–transgressive cycles. The regressive component of each cycle consists of thick (up to 22 m), laterally continuous wave-dominated shoreface and overlying coastal-plain deposits that occur in paleoseaward locations and have abrupt (< 400 m) paleolandward pinchouts. The transgressive component of each cycle consists of one or more bay-fill successions that occur in paleolandward locations and gradually thin in a paleoseaward direction. Transgressive bay-fill deposits in each cycle are thick (up to 18 m) and associated with preservation of surfaces that record, in progressively paleoseaward locations: initiation of a lagoon or bay (transgressive surface), erosional retreat of tidal-inlet channels (tidal ravinement surface) and the shoreface (wave ravinement surface), and marine flooding (marine flooding surface). This architecture records regression of a strandplain or wave-dominated delta, and subsequent transgression of a barrier island and spit with associated lagoon or bay. The occurrence of such thick and fully preserved bay-fill successions indicates that accretionary transgressive shoreline trajectories were developed. Strongly-aggradational-to-weakly-retrogradational stacking of successive regressive–transgressive cycles results in a layered stratigraphic architecture, with laterally continuous shoreface sandstone layers interbedded with bay-fill shale layers. Shoreface sandstones layers pinch out up-dip abruptly (< 400 m) into bay-fill shales and have limited vertical connectivity. Sandstones within bay-fill and coastal-plain deposits occur as small, laterally discontinuous bodies of variable geometry and connectivity. However, these sandstones may provide additional connectivity where they erode through bay-fill shales between two shoreface sandstone layers.

Two Cryogenian glacial successions compared: Aspects of the Sturt and Elatina sediment records of South Australia

Two Cryogenian glacial sequences, the Sturt and Elatina, occur within the Adelaide Rift Complex, S Australia, and are part of an ∼8km thick succession recording Rodinia fragmentation and subsequent passive margin development. We compare and contrast these glacial successions, presenting new data, including detailed logged sections, for both glacial sequences. Facies analysis reveals a total of six facies associations, recording a range of glacial and glacially related processes. These are a diamictite facies association (ice-proximal debris flows), a sheet sandstone facies association (fluvial deposits), a flaser and rhythmically bedded facies association (tidal flat), a hummocky cross-stratified sandstone facies association (shoreface to offshore transition zone deposits) and an underflow facies association (recording offshore deposition). A sixth facies association, a ferruginous lonestone-bearing siltstone, is restricted to the top of the Elatina Formation and records rapid environmental amelioration and emergence from glaciation. In the central Flinders Ranges, the Sturt succession commences with a glacial erosion surface (GES), which in agreement with previous workers, was probably enhanced by rifting. This GES is overlain, in ascending order, by the Pualco Tillite, Holowilena Ironstone and Wilyerpa Formation, with the Warcowie Dolomite Member at the base of this latter formation. A second GES is preserved within this latter unit, and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits. Significant base level falls within the Wilyerpa Formation are identified by fluvial deposits sitting disconformably upon offshore deposits. In contrast, within the Elatina Formation, a single GES is proposed, which defines the base of the formation. The Elatina Formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation. Apart from a locally preserved basal diamictite, the lower member consists of fluvial deposits, whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits. The boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others. Deglaciation left contrasting deposits: the Sturt deglaciation culminated in deposition of black shale of the Tindelpina Shale Member of the Tapley Hill Formation, seemingly as a sheet-like deposit over the Adelaide Rift Complex and beyond into the Amadeus Basin of central Australia. The Elatina deglaciation, meanwhile, began first with deposition of the sixth facies association (ferruginous dropstone-bearing siltstone), then proceeded with deposition of cap dolostone of the Nuccaleena Formation.