Marine Transgressive Surfaces Research Articles

Coastal palaeoenvironmental record of Late Bronze to Iron Age harbour development at Liman Tepe-Clazomenae, western Anatolia, Turkey

Liman Tepe-Clazomenae, located in the southern Bay of Izmir, Turkey, was an important Early Bronze Age to Classical Period trading port and cultural centre in the eastern Aegean. The mainland harbour, now submerged ~1.5–2 m below present sea level, is one of the best-preserved examples of an Iron Age (Archaic Period; ca. 7th–6th c. BCE) semi-enclosed harbour (>5 ha) with engineered breakwater structures. A multi-proxy study (micropalaeontology, micro-XRF core scanning) was conducted on seven harbour sediment cores and integrated with geophysical data to map the harbour structures and document coastal palaeoenvironmental changes. Bathymetry and side-scan mapping revealed two broad (>35 m) rubble-constructed breakwater structures and a submerged headland that divided the harbour into two separate sub-basins. Linear magnetic anomalies within the eastern breakwater indicate a buried pier structure, recording possible augmentation of a Late Bronze Age (LBA) or Early Iron Age (EIA) proto-harbour embayment. The harbour basin stratigraphy comprises foreshore and upper shoreface deposits overlying terrigenous clays across a marine transgressive surface. A distinctive silt-rich chemofacies with increased Ti/Ca and decreased Si marks a transition from a sandy marine shoreface to a low energy, sheltered LBA proto-harbour embayment. The Iron Age harbour construction (ca. 7th–6th c. BCE) is recorded by a rise in Rosalina, decreased Ti/Ca and the appearance of Archaic pottery. The harbour was in use from the Archaic to early Classical periods and served as Clazomenae's mainland commercial port.

Syn-depositional oil seeps in the Late Albian Paddy Member of the Peace River Formation (Early Cretaceous), north-central Alberta

Abstract Controversy exists regarding the timing of emplacement of oil in the giant Athabasca and Peace River oil sands. Bitumen-cemented sandstones are present in the late Albian Paddy Member of the Peace River Formation; some cemented sandstones formed burrowed firmgrounds and reworked intraclasts, showing that oil was reaching surface by about 101–102 Ma. Estuarine and shallow-marine sandstones of the Paddy Member are exposed on the Peace and Heart rivers in the vicinity of the town of Peace River. Bitumen-cemented sandstone occurs in four stratigraphic settings: 1) An in-situ cement forms locally bedding-transgressive, sheet-like masses in cross-bedded estuarine sandstone; 2) A bitumen-cemented and heavily burrowed layer, 30–60 cm thick, lies immediately beneath a marine transgressive surface and shows that oil infiltrated downward into, and cemented, the upper surface of a shoreface sandbody. The firm sand was subsequently exhumed by transgressive erosion, fractured and burrowed by arthropods; 3) Rounded pebble- to cobble-sized clasts of bitumen-cemented sand lie on the floor of a tidal channel, and the channel floor surface is also stained and burrowed. This case suggests floating oil infiltrated the channel floor at low tide, subsequently hardening prior to erosion and burrowing; 4) Rounded cobble-to boulder-size clasts of bitumen-cemented sandstone, up to 1 m wide and 0.5 m thick, lie on a regional marine ravinement surface cut on the uppermost Paddy shoreface sandstone. The sand was permeated with oil, top-down, then oxidized to form a tough bitumen cement. The bituminous sand was subsequently scoured by waves during marine transgression to form a boulder lag that is enclosed in transgressive marine claystone. All four of these examples show that oil was reaching the Earth’s surface during Paddy time where it infiltrated porous sands to a depth of several decimetres, subsequently degrading and oxidizing to form a tough ‘asphalt pavement’ that attracted a burrowing firmground fauna and was eroded into cohesive intraclasts weighing many tens of kilograms. Pyrolysis proved the bitumen cement to be degraded oil, but it was not possible to identify the source-rock using gas chromatography-mass spectrometry. The Paddy ‘asphalt pavements’ are comparable to cements that form in beach sediments in the wake of catastrophic spills from tanker groundings. A direct analogue is provided by Eocene estuarine sediments in Dorset, U.K. There, bitumen-cemented estuarine sands also formed firmgrounds and intraclasts on channel-floor and marine transgressive surfaces.

Palaeoshoreline reconstruction and underwater archaeological potential of Liman Tepe: A long-occupied coastal prehistoric settlement in western Anatolia, Turkey

Rising post-glacial sea levels had a major influence on the prehistoric settlement of the Aegean coastal zone. At Liman Tepe, an important Chalcolithic-Bronze Age coastal settlement on the south coast of the Bay of Izmir, archaeological evidence suggests a Neolithic (ca. 9600-5500 BCE) presence, but no settlement has been discovered on land. Sea levels during the Neolithic period were between 6 and >20 m below present and there is high potential for discovery of submerged prehistoric sites. Marine sediment coring and geophysical investigations (bathymetry, sub-bottom seismic profiling; >600 line-km) were conducted over a 4-km2 inshore area to assess the underwater archaeological potential. Multi-proxy sediment analysis (sedimentary facies, micropalaeontology, micro-XRF geochemistry) was conducted on 20 cores to reconstruct the relative sea level (RSL) history and coastal palaeogeography. Palaeoshoreline positions were estimated by back-stripping of the decompacted sediment thickness from a digital bathymetric model (DBM).The DBM reveals a drowned middle Holocene coastal plain with well-preserved relict river channels, palaeoshorelines and coastal headlands. The inshore stratigraphy consists of shoreface, foreshore and lagoonal deposits overlying terrestrial clay and palaeosols, defining a marine transgressive surface (MTS). The MTS records the inundation of the coastal plain prior to ca. 4000 BCE (transgressive systems tract; TST) and is marked in cores by an increasing abundance of foraminifera and a rise in Ca/Ti. During the Early Neolithic (ca. 6700 BCE), the shoreline was >500 m seaward (RSL ∼ −14 to −16 m) and Karantina Island was a broad coastal headland with a sheltered western embayment. By the Middle Chalcolithic (ca. 4800 BCE), the coastline had transgressed ∼800 m inland of the present shoreline and the Liman Tepe headland was separated from the mainland by a shallow coastal wetland. The maximum transgression (∼1 km inland at ca. 4000 BCE) was followed by a shift to a high-stand systems tract (HST) and rapid coastline progradation by barrier accretion and lagoon development. Palaeogeographic maps identify areas with high underwater archaeological potential: 1) palaeoriver channels and lowland riverine habitats formed during the TST, prior to 4000 BCE, 2) submerged palaeoshorelines and coastal promontories (water depths 10–14 mbsl) with high potential for Neolithic sites, and 3) protected coastal embayments and lagoons representing possible prehistoric anchorage sites.

Sedimentology, diagenesis, and reservoir characterization of the Permian White Rim Sandstone, southern Utah: Implications for carbon capture and sequestration potential

Rising CO2 emissions have caused scientists and policy makers to explore potential solutions to reduce atmospheric CO2 levels, including carbon capture and sequestration. This project, funded through the Department of Energy CarbonSAFE initiative, aims to characterize the potential storage capacity and preferential fluid-flow pathways through the Permian White Rim Sandstone on and near the San Rafael Swell, Utah, to assess the formation’s viability as a possible CO2 sequestration reservoir. A diverse suite of sedimentological observations and diagenetic analyses was used to assess the White Rim Sandstone’s reservoir potential, including detailed outcrop observations, facies descriptions and ichnology, measured stratigraphic sections with associated correlations, permeability measurements, thin-section petrography, and visible and near-infrared spectroscopy. Stratigraphically, the White Rim Sandstone has two major units separated by a marine transgressive surface. The lower eolian unit is characterized by decimeter- to meter-scale trough cross-bedded grain-flow and wind-ripple facies. The upper reworked, shallow-marine unit has a consistent set of facies at all three field sites: (1) soft-sediment deformation facies (e.g., contorted bedding or clastic pipes), (2) symmetric ripple laminations, (3) bioturbated (dominantly horizontal), and (4) bioturbated (dominantly vertical). This set of four facies represents the transgression of the Permian Kaibab sea over the Permian White Rim coastal dune field. The outcrop samples and subsurface core samples (∼1561–2243 m deep and ∼33–50 km away from the outcrop localities) experienced different diagenetic fluid-flow histories that drastically impacted the reservoir properties. Outcrop samples have initial quartz overgrowth cements followed by rim-forming calcite cements, iron oxide cements, and occasional oil staining. In contrast, the core samples show significant compaction, followed by quartz overgrowth cementation and a later stage of patchy, large, pore-filling calcite cement that eliminated the porosity and permeability of the potential reservoir (<0.1 md). Outcrop permeability values tend to reflect textural differences in facies, particularly the presence and spacing of internal laminations and bounding surfaces. Facies with fewer internal laminations, such as the grain-flow facies, have higher permeability values than facies with closely spaced internal laminae, such as the wind-ripple facies. Disruption of original bedding (e.g., soft-sediment deformation or bioturbation) alters original depositional textures and creates preferential flow pathways with the potential to increase permeability. Overall, if the diagenesis and resulting low permeability values of the core samples are representative of the White Rim Sandstone at depth, then the White Rim Sandstone would not make a high-quality storage reservoir despite initial, promising outcrop results.

Rocky-shore unconformities marking the base of Badenian (Middle Miocene) transgressions on Mt. Medvednica basement (North Croatian Basin, Central Paratethys)

Badenian (Middle Miocene) transgressive deepening-upward successions located in the NE part of Mt. Medvednica (North Croatian Basin, Pannonian Basin System) unconformably overlie Mesozoic basement. Triassic and Upper Cretaceous limestone pebbles, cobbles, and boulders of the Badenian basal conglomerates display abundant in situ bivalve borings of Gastrochaenolites and sponge borings of Entobia. This Gastrochaenolites-Entobia ichnoassemblage is related to the Entobia subichnofacies of the Trypanites ichnofacies, characterizing littoral rocky-shore environments (wave-cut platforms and marine transgressive surfaces with a low or null rate of sedimentation). Gastrochaenolites torpedo, Gastrochaenolites lapidicus, and Entobia recorded in Badenian basal conglomerates (compared with modern Northern Adriatic rocky-shore environments), enabled more precise palaeoenvironmental interpretations. The occurrence of G. torpedo (produced by lithophaginid bivalves) on all sides of individual limestone lithoclasts in the Gornje Oresje basal conglomerate, coupled with truncation of the formation (possibly indicating multiphase colonization), reflect gravel transport, roll-over, overturning and erosion by wave action in high-energy rocky-shore settings. Gornje Psarjevo-2 basal conglomerate boulders were probably not subjected to significant movement and abrasion, as suggested by good preservation of both G. lapidicus (potentially produced by gastrochaenid bivalves), associated G. torpedo, as well as abundant shallow Entobia borings. The Badenian Gastrochaenolites-Entobia ichnoassemblage also could be the result of a composite development. However, direct cross-cutting relationships between G. torpedo and G. lapidicus and/or Entobia were rarely observed. In addition, Badenian boring tracemakers might have coexisted at the same water depth. Northeast Mt. Medvednica Badenian successions probably formed during different Central Paratethys transgressive pulses (NN5 and NN6 Zones). However, exact timing of Badenian transgressions, stratigraphic correlations and tectono-eustatic implications are unresolved, due to sparsely integrated biostratigraphic and high-precision geochronological studies of Early–Middle Miocene North Croatian Basin deposits and due to the absence of a uniform biostratigraphic zonation and regional chronostratigraphic division of Central Paratethys.

Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud‐dominated incised valley – an example from the Red River (Gulf of Tonkin)

AbstractThe Gulf of Tonkin coastline migrated at an average rate of ca 60 m year−1 landward during Holocene sea‐level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full‐marine deposits. Thus, the river mouth represented a mud‐dominated open funnel‐shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea‐level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base‐level rise is documented by non‐bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short‐term seawater incursions. Siderite in overlying deposits points to low‐salinity estuarine conditions. The open funnel‐shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short‐term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr−1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (&gt;0·5 m) and pronounced erosion (&lt;1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full‐marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated.

Climatically Versus Eustatically Controlled, Sediment-Supply-Driven Cycles: Carbonate–Siliciclastic, High-Frequency Sequences In the Valanginian of the Neuquén Basin (Argentina)

Carbonate–siliciclastic high-frequency sequences are formed by reciprocal sedimentation. In siliciclastic-dominated successions, the carbonate-rich interval is typically associated with transgressive conditions and relatively low terrigenous influx, particularly in distal parts of the marine basin. However, key questions for each individual case are: What sort of depositional system developed during transgressions, and what controls sediment-supply fluctuations? This paper integrates outcrop and subsurface data to present a high-resolution sequence-stratigraphic study of small-scale (2–18 m), carbonate–siliciclastic cycles deposited during the Valanginian in a proximal–distal transect of the Neuquen Basin (western Argentina) and discusses those key issues. Carbonate hemicycles in this study are composed of cross-bedded oolitic–skeletal grainstones and packstones to massive skeletal floatstones, which accumulated in carbonate ramps with high biogenic production in their distal sector (bivalves, serpulids, echinoids), and significant contribution of non-biogenic (ooids) material in inner-ramp settings. Siliciclastic material is present only at the base of the hemicycles, but rapidly diminishes upwards. These carbonate hemicycles have retrogradational staking patterns in proximal sectors and less obvious trends in distal areas. The siliciclastic hemicycles comprise shallowing-upward successions that include mudstones, bioturbated muddy sandstones, sandstones with HCS and SCS, and bioturbated sandstones. These facies were collectively deposited in a storm- and wave-influenced offshore-shoreface depositional system during normal regressions. These shoreface systems were likely fed by littoral drift from riverine inputs located tens of kilometers to the south. The duration of the investigated small-scale, transgressive–regressive cycles can be estimated in 50 to 150 kyr, and they are bounded by marine transgressive surfaces placed at the bases of carbonate hemicycles. The bounding discontinuities together with the sequence architecture do not suggest periods of sea-level falls between cycles, nor high-amplitude relative sea-level changes during a cycle formation. Additionally, the low proportion of mixed sediments suggests rapid changes in the rate between terrigenous supply and carbonate productivity. The resulting sequence architecture cannot be reasonably explained by autocyclic controls or eustatic changes alone. It is argued here that high-frequency changes in sediment supply, driven by orbitally induced climate fluctuations, could be a better explanation for these sequences, particularly in the context of a long-term relative sea-level rise. In this scenario, periods of low siliciclastic influx during extreme arid conditions would develop a negative balance with accommodation and prompt carbonate systems and transgression. On the other hand, during more humid conditions relatively high sediment flux would temporarily overcome long-term accommodation, producing the regressive siliciclastic hemicycles. As this scenario does not require high-frequency sea-level falls, it would satisfactorily explain the observed sequence architecture. The distal Mulichinco carbonate hemicycles would be hard to distinguish from transgressive, fossiliferous-rich deposits that characterize transgressive–regressive cycles in clastic systems. In the Mulichinco case, however, these carbonate hemicycles are recording the onset of carbonate ramps, not the trapping of siliciclastic sediments in transgressive coasts (e.g., filling estuaries). This alternative scenario, ultimately controlled by reciprocal sedimentation, could be common in epicontinental basins developed under arid to semiarid conditions, and its identification could provide additional templates for basin-scale correlations and hydrocarbon exploration within carbonate–siliciclastic small-scale cycles.

Sequence stratigraphic architecture of marine to fluvial deposits across a passive margin (Cenomanian, Atlantic margin, Morocco, Agadir transect)

Seven sections, covering the upper Albian to lowermost Turonian, have been correlated from full-marine to continental-dominated deposits across a passive margin, along a transect 425 km long, from the present-day Atlantic coast to the ``Pre-African Trough'' between the Anti-Atlas and the High-Atlas. The thickness of the Cenomanian succession changes from around 500 metres in the fully marine sections to 250 metres in mostly continental facies in the western High-Atlas, about 150 km updip, to a few tens of metres in the Bou Tazoult area. The strata thicken again eastwards into the Pre-African Trough where they can be traced without major facies changes to the Kem Kem embayment and to the Bechar area in Algeria. Over all this eastern area, continental facies are overlain by the fully-marine shallow-water deposits of the Cenomanian-Turonian boundary interval. A first major conclusion is that fluvial aggradation in high-frequency transgressive-regressive sequences is coeval with the seaward-shift of the shoreline, in accordance with the genetic sequence stratigraphic model of GALLOWAY (1989). Both the flatness of the depositional profile and the corresponding very low energy of the marine environment during the transgressions account for the blanket of red continental clays on top of marine facies in updip depositional sequences, which is then preserved under the marine transgressive surface of the next sequence. A second major conclusion is that the high-frequency transgressive-regressive (T-R) sequences do not look like classical parasequences bounded by transgression surfaces. They usually exhibit a surface created by a sea-level fall within the regressive half-cycle. This is interpreted in the following way: regressions did not operate through a regular seaward-shift of the shoreline, but through stepped sea-level falls. The very low slope of the depositional ramp is thought to have enhanced the sequence stratigraphic record of such stepped regressions. Short-term, high-frequency sequences are organized into medium-frequency T-R sequences (seven in the Cenomanian) which show an overall aggrading and slowly retrograding pattern along the whole transect. Comparisons with other basins show that medium-frequency sequences do not fit the third-order depositional sequences described elsewhere, casting doubts about a eustatic mechanism for their deposition.

Allostratigraphy and paleogeography of the Harmon and Cadotte members of the Peace River Formation (Middle Albian), west-central Alberta and adjacent British Columbia

Abstract The mudstone-dominated Harmon and sandstone-dominated Cadotte members of the Peace River Formation were deposited during the middle Albian in the southern end of a Boreal marine embayment that occupied the foredeep of the Western Canada Foreland Basin. Sandstone of the Cadotte Member represents a shoreface environment and has traditionally been interpreted as the updip equivalent of offshore Harmon mudstone. Allostratigraphic correlation, based on marine flooding surfaces and unconformities shows that the Harmon alloformation can be divided into five regionally-mappable allomembers. Similar erosion surfaces allow the Cadotte alloformation to be divided into three allomembers. The new allostratigraphy shows that the informal Harmon and Cadotte alloformations do not have an interfingering relationship and hence are not genetically related. Harmon and Cadotte allomembers have been mapped over about 100 000 km2, but show no evidence of stratal lap-out or depositional topography; it is concluded that deposition took place on a shallow marine ramp of very low relief. The vertical stacking of Harmon and Cadotte allomembers across the foredeep indicates that rates of accommodation and sediment supply remained in approximate equilibrium throughout deposition of these units. In the far southern and eastern parts of the study area, changing gamma ray log response suggests that marine deposits of both the Harmon and Cadotte alloformations grade laterally into coeval coastal plain deposits that are mapped in outcrop as Gates Formation whereas in subsurface wireline log picks, these rocks are usually included in the Upper Mannville Group. The lateral boundary between offshore mudstone and coeval nearshore muddy sandstone can be mapped within individual Harmon allomembers; the mudstone to sandstone boundary has a lobate to cuspate pattern in plan view, suggesting that, at the southern extremity of the ‘Harmon Sea’ rivers built elongate deltas in a very shallow-water, low wave-energy setting. Lenticular, sharp-based sandstone bodies 17–42 m thick hang from allomember bounding surfaces in both the Harmon and Cadotte alloformations; these sandbodies probably represent paleovalley fills. Valley incision may have been linked to relative sea-level changes that also gave rise to the marine transgressive surfaces that are traceable over the entire study area. In the east, both the Cadotte and Harmon alloformations are completely truncated by a bevelling unconformity at the base of the Paddy and the laterally-contiguous Joli Fou alloformation.

Allostratigraphy of the Peace River Formation (Albian) in north-western Alberta and adjacent British Columbia

Abstract The middle to upper Albian Peace River Formation was deposited at the southern end of the ‘Hulcross Sea’, an embayment of the Boreal Ocean that occupied the foredeep of the Western Canada foreland basin. The Peace River Formation can be allostratigraphically divided by numerous, regionally-mappable erosion surfaces, recognizable both in wireline logs, core, and outcrop. We propose recognition of the informal Harmon, Cadotte and Paddy alloformations, bounded by four regionally-mappable marine transgressive surfaces, HE0, CE0, PE0 and VE3 in ascending order. The Harmon alloformation consists predominantly of marine mudstone, and is divided into five allomembers bounded by marine flooding surfaces and/or thick bentonites. The Harmon forms a wedge thinning from >145 m in the SW to 5 m in the NE over approximately 300 km. There is no geometric evidence of a sigmoidal geometry, or that allomembers downlap onto a single, distinct surface. Deposition took place on a very low-gradient and low-relief marine ramp, strongly influenced by storms. The Cadotte alloformation consists of well-sorted swaley cross-stratified sandstone and was deposited on a wave-dominated strandplain shoreface. Ravinement surfaces divide the Cadotte into three, sheet-like allomembers, each typically about 10 m thick. Allomembers CA and CB are mainly fine-grained sandstone whereas allomember CC is dominantly medium-grained sandstone with abundant pebble conglomerate. Towards the north, sandstone grades laterally into heterolithic facies indicative of an offshore setting. The Paddy alloformation lies unconformably on the Cadotte, and comprises an array of facies, with alluvial and lagoonal deposits predominant in the south. Further north, sandstone indicative of a wave-influenced strandplain shoreline forms a belt trending WSW-ENE across the middle of the area; heterolithic offshore marine deposits lie to the north. The Paddy alloformation comprises nine allomembers, PA to PI, that onlap progressively onto the basal unconformity PE0 from SW to NE. The pronounced wedge shape and muddy lithology of the Harmon implies rapid flexural subsidence and largely vertical aggradation whereas the sheet shape of the Cadotte suggests a lower rate of subsidence when high-frequency eustatic change may have driven rapid regressions and transgressions. The lower Paddy allomembers PA to PF are strongly wedge-shaped, suggestive of renewed tectonically-driven subsidence whereas upper allomembers PG to PI are more sheet-like, suggesting that a tectonic load exerted a less important influence and instead, more uniform regional isostatic subsidence took place, driven by the load of the eastward-encroaching sediment body. Sandstone-rich Paddy strata are abruptly overlain, at the VE3 unconformity, by a blanket of dark marine mudstone that forms the basal part of the lithostratigraphic Shaftesbury Formation; the mudstone between the VE3 and VE4 surfaces is of upper Viking age, and has a pronounced wedge geometry which implies a major new pulse of flexural subsidence in the west.

An allostratigraphic framework for a retrogradational delta complex: the uppermost Dunvegan Formation (Cenomanian) in subsurface and outcrop, Alberta and British Columbia

Abstract Allomembers A and B, re-defined herein, are the uppermost components of the Dunvegan Formation, and are of ?late Middle to early Late Cenomanian age. They record the final drowning of the huge Dunvegan delta complex that occupied the western margin of the Cretaceous foreland basin in Alberta and British Columbia. During deposition of allomembers A and B, the delta-front back-stepped north-westward by about 220 km relative to allomember C, on a timescale estimated at 105–440 k.y. Allomembers A and B are now divided into three sequences A1–A3 and B1–B3, bounded by marine transgressive surfaces; each sequence is estimated to span approximately 18–72 k.y. Sequences shoal upward and represent wave-influenced delta, prodelta, and coastal plain environments. Sequence B3 is incised by an extensive system of narrow (much less than 1 km) sandstone-filled paleo-valleys, up to 16 m deep. Although sequences rarely exceed 10 m thick, they are mappable for hundreds of km and appear to be primarily of allogenic (?eustatic) origin. Sequences were probably deposited in water no more than approximately 15 m deep and each records at least several metres of relative sea-level fall. Eustatic changes on a vertical scale of several metres, over a few tens of thousands of years are potentially attributable to thermal expansion/contraction of seawater, water storage on land, and glacio-eustasy. All three mechanisms may have contributed to the observed sea-level changes, and were probably driven by orbitally-modulated climate cycles. A regional isopach map of allomember C shows negligible updip (westward) thickening, suggesting that contemporaneous thrust loading of the basin margin was minimal. In contrast, allomembers A and B collectively show dramatic thickening to the west, suggestive of renewed loading in that direction. Although all sequences in allomembers A and B thicken westward, this trend is most prominent in sequences B3 to A3, suggesting that the rate of tilting accelerated over time. The transgressive limit of the shoreline mapped for successive sequences shows about 20 km of backstep per sequence, with the principal transgressive direction being towards the NW. This is oblique to the pattern of flexural subsidence inferred from isopach mapping and suggests that the depositional surface (coastal plain and prodelta) was inclined down to the SE, despite maximum tectonic subsidence being down to the west. This implies that the rate of sediment supply exceeded that of subsidence, and that depositional systems were able to maintain a slope inclined from NW to SE. The overall transgression recorded by allomembers A and B appears to have been strongly influenced by an accelerating rate of flexural subsidence, and, probably by slow eustatic rise during the Greenhorn transgression. However, this trend was modulated by higher-rate eustatic oscillations of at least several metres that caused corresponding shoreline excursions of several tens of kilometres within an overall transgressive regime.

Sedimentology and palaeogeomorphology of four large valley systems incising delta plains, western Canada Foreland Basin: implications for mid‐Cretaceous sea‐level changes

AbstractThe mid‐Cenomanian Dunvegan Formation represents a delta complex deposited on a foreland basin ramp over about 2 my. The Dunvegan is divided into 10 transgressive–regressive allomembers, labelled J–A in ascending order, each defined by regional marine transgressive surfaces. Parasequences within allomembers show an aggradational to offlapping stacking pattern that reflects alternate generation and removal of accommodation. The upper surfaces of allomembers H–E are incised by extensive valley systems traceable for up to 320 km and over about 50 000 km2. Valley depths range up to 41 m and can change significantly over short distances. However, the average depth of incision (mean 21 m) shows no systematic variation in longitudinal profiles and no evidence of headward shallowing. Valleys are typically 1–2 km wide, but locally widen to about 8 km. Widening is sometimes associated with confluence zones, but elsewhere it is not. Updip reaches of valleys are dominated by cross‐bedded fluvial sandstone forming multistorey point‐bar deposits. Sandstones contain widespread but uncommon paired carbonaceous drapes recognizable as tidal bundles. Inclined heterolithic stratification is locally well developed at the top of the valley fill. Downdip reaches of valleys, typically within 50 km of the lowstand shoreline, have a sandstone‐dominated lower part and, locally, a mud‐rich upper portion consisting of a variety of laminated heterolithic facies with a clear tidal signature. These heterolithic deposits may represent central basin, tidal flat, bayhead delta and point‐bar environments. Valley filling took place mainly during the transgressive systems tract (TST) when tidally influenced environments migrated upvalley. Semi‐diurnal tidal backwater effects extended at least 30 km landward of the regional maximum transgressive marine shoreline. The aggradational late TST and highstand systems tract (HST) includes deltaic and coastal plain deposits comprising lake and anastomosed river deposits that suggest a very low gradient (≈ 1:3000). Delta parasequences of the falling stage systems tract (FSST) offlap seaward and have no equivalent coastal plain deposits. The FSST has an average width of 60 km and an inferred gradient of 1:2500. The upper surfaces of the HST and FSST are extensively incised by valleys. The lowstand systems tract (LST) is subtly aggradational, lacks valleys and is characterized by large delta lobes fed by major distributaries. The width and inferred slope of the FSST, coupled with the thickness of aggradational TST and HST deposits on the coastal plain, suggest a vertical accommodation of about 35 m per transgressive event. About 11 m of this is attributed to isostatic subsidence resulting from water and sediment loads; the residual 24 m is attributed to eustatic rise. This sea‐level change is of the same order of magnitude as the valley depths. The length of valleys, however, does not seem to be explicable solely in terms of downstream forcing by sea‐level change, and an additional, upstream‐forcing mechanism, possibly related to precipitation cycles in the Milankovitch band, might be inferred.

Sedimentology and sequence stratigraphy of paralic and shallow marine Upper Jurassic sandstones in the northern Danish Central Graben

Paralic and shallow marine sandstones were deposited in the Danish Central Graben during Late Jurassic rifting when half-grabens were developed and the overall eustatic sea level rose. During the Kimmeridgian, an extensive plateau area consisting of the Heno Plateau and the Gertrud Plateau was situated between two highs, the Mandal High to the north, and the combined Inge and Mads Highs to the west. These highs were land areas situated on either side of the plateaus and supplied sand to the Gertrud and Heno Plateaus. Two graben areas, the Feda and Tail End Grabens, flanked the plateau area to the west and east, respectively. The regressive–transgressive succession consists of intensely bioturbated shoreface sandstones, 25–75 m thick. Two widespread unconformities (SB1, SB2) are recognised on the plateaus, forming the base of sequence 1 and sequence 2, respectively. These unconformities were created by a fall in relative sea level during which rivers may have eroded older shoreface sands and transported sediment across the Heno and Gertrud Plateaus, resulting in the accumulation of shoreface sandstones farther out in the Feda and Tail End Grabens, on the south-east Heno Plateau and in the Salt Dome Province. During subsequent transgression, fluvial sediments were reworked by high-energy shoreface processes on the Heno and Gertrud Plateaus, leaving only a lag of granules and pebbles on the marine transgressive surfaces of erosion (MTSE1, MTSE2).&#x0D; The sequence boundary SB1 can be traced to the south-east Heno Plateau and the Salt Dome Province, where it is marked by sharp-based shoreface sandstones. During low sea level, erosion occurred in the southern part of the Feda Graben, which formed part of the Gertrud and Heno Plateaus, and sedimentation occurred in the Norwegian part of the Feda Graben farther to the north. During subsequent transgression, the southern part of the Feda Graben began to subside, and a succession of backstepping back-barrier and shoreface sediments, 90 m thick, was deposited. In the deep Tail End and Feda Grabens and the Salt Dome Province, sequence boundary SB2 is developed as a conformity, indicating that there was not a significant fall in relative sea level in these grabens, probably as a result of high subsidence rates. Backstepping lower shoreface sandstones overlie SB2 and show a gradual fining-upwards to offshore claystones that are referred to the Farsund Formation. On the plateaus, backstepping shoreface sandstones of sequence 2 are abruptly overlain by offshore claystones, indicating a sudden deepening and associated cessation of sand supply, probably caused by drowning of the sediment source areas on the Mandal, Inge and Mads Highs. During the Volgian, the Gertrud Plateau began to subside and became a graben. During the Late Kimmeridgian – Ryazanian, a long-term relative sea-level rise resulted in deposition of a thick succession of offshore claystones forming highstand and transgressive systems tracts on the Heno Plateau, and in the Gertrud, Feda and Tail End Grabens.

Clastic Sediment Partitioning in a Cretaceous Delta System, Western Canada: Responses to Tectonic and Sea-Level Controls

The early–mid Cenomanian Dunvegan Formation represents a largedelta complex that prograded at least 400 km from NW to SE. Aregional stratigraphy based on marine transgressive surfaces andequivalent subaerial interfluves allows the formation to be subdividedinto ten transgressive–regressive allomembers, labelled J to A inascending order, each with an average duration of &lt; 200 ky. Analysisof stacking patterns and facies distributions of parasequenceswithin allomembers allows transgressive, highstand, falling stageand lowstand systems tracts to be identified. Extensive valley systemsthat average 1–2 km wide and 21 m deep can be traced for up to320 km across the top surfaces of allomembers H to E. In their lower20–40 km, valleys are filled with muddy heterolithic tidal facies butthis changes to fluvial-dominated multi-storey channel-fills furtherup-valley. Interfluve surfaces are marked by palaeosols, the characterof which indicate a protracted hiatus with extensive physical, chemicaland biological modification of the parent material.Changes in flexural subsidence rate are indicated by isopach patterns.Allomembers J–F have a sigmoidal prismatic geometry, successivelyofflapping to the SE. There is no evidence of thickening towardthe orogen. In contrast, overlying allomembers E–A show progressivedevelopment of a depocentre along the western margin of the basin.The increasing accommodation rate on the updip coastal plain causedmarine deltas to be starved of sediment, leading to progressive backstepof shorelines. Simultaneously, alluvial deposits within the depocentreshow an upward increase in the proportion of subaqueous tosubaerial facies, culminating in the incursion of brackish and finallymarine waters. Thus tectonic subsidence rate had a first-order affecton both the volume of sediment available to build marine deltas andalso on the local character of facies that accumulated on the coastalplain. The onset of flexural subsidence in allomember E appears tohave resulted in subtle uplift of a forebulge, resulting in dramaticdeflection of river systems.Despite the clear tectonic signature, successive transgressions andregressions involved similar horizontal displacements of the shoreline,regardless of subsidence rate. This suggests that modest eustaticchanges also influenced the accommodation available. Based on themeasured horizontal excursions of the shoreline, the vertical thicknessof alluvial strata, and realistic alluvial gradients, an average eustaticexcursion of about 24 m is calculated. The incision of valley systemsis attributed in part to periods of eustatic fall. However, valleys seemtoo long to be explained by eustasy alone, and hence secular changesin discharge are postulated as an additional forcing factor. Climaticcycles in the Milankovitch band may have been responsible for botheustatic and discharge variations.

Field and Seismic Images of Sharp-Based Shoreface Deposits: Implications for Sequence Stratigraphic Analysis

ABSTRACT Sharp-based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shoreface sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern France. These series consist of tens of meter-thick alternations of sandstones bodies and organic-rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high-resolution (ca. 1 m of sediment) marine-seismic profiles obtained several hundred meters offshore. The units described here comprise two sharp-based sandstone bodies. Each is composed of a basal progradational set of shoreface parasequences overlain by a progradational-aggradational shoreface succession. Each sharp-based sand body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion, which in turn is overlain by a thin retrogradational ravinement lag or coarse-grained, planar-laminated bedset. The two progradational packages are separated by a third surface, a subaerial exposure surface that is interpreted as a sequence boundary. Two distinct types of seismic units, referred to as type A and type B, have been identified in the study area. Seismic unit A has conformable upper and lower boundaries and parallel (aggradational) configurations; seismic unit B is characterized by downlap and toplap boundaries and simple or compound, sigmoid and oblique-tangential (progradational) configurations. A single sharp-based shoreface sandstone body makes up the type B unit and typically consists of two compound superimposed progradational sets (B1, B2). The lowermost set, B1, corresponds in the field to the progradational set sensu stricto, whereas B2 corresponds to a progradational-aggradational set. B1 and B2 are separated in outcrop by a sequence boundary. These observations led us to reevaluate the sequence stratigraphic interpretation of sharp-based shoreface sandstones. It is proposed here that complete, single, sharp-based shoreface sandstones bodies can be separated into two different systems tracts: (1) a progradational set (B1 seismic body) at the base, which corresponds to the forced regressive wedge systems tracts (FRWST) of Hunt and Tucker (1992), and (2) a progradational-aggradational set (B2) at the top, above the sequence boundary, which corresponds to the lowstand systems tract (LST) of Posamentier et al. (1992). A complete sharp-based shoreface sandstone body is bounded below the FRWST by a regressive surface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the top of the LST.

Comparison between high-resolution seismic and sequence stratigraphic approaches applied to the upper Jurassic deposits of the Dover Strait area (Northern France)

This study is an attempt to interpret very high resolution seismic data, usually devoted to shallow marine surveys, in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir. Seismic data are compared with outcrop-based observations in order to discuss the geometrical relationships between rock bodies, and the nature of the seismic reflections in terms of seismic facies related to type lithologies. Two high-resolution seismic surveys, using a sparker, were performed in the Dover Strait area some 100s of metres away from the Boulonnais coastal cliffs. The aim was to look to the 130 m-thick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Kimmeridgian-Tithonian) cropping out at the sea floor and to compare them to the coastal cliffs exposures. The 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution (upper 30 ms part of the profiles). Four seismic facies are recognised on the seismic profiles. They comprise ten seismic units which can be grouped into three broad families. Group 1 has a parallel/aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts. The seismic facies exhibit either a high continuity-high amplitude (corresponding to clayey-mudstone lithologies of the mid ramp environment) or a high continuity-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configuration, bounded at the base by a downlap surface and at the top by a toplap surface. These boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion. The seismic facies is cross-layered and exhibits a moderate continuity, low to moderate amplitude and moderate frequency. Group 2 corresponds to the bioclastic, sand-prone facies of the inner ramp environment. It is interpreted as lowstand sharp-based shorefaces. Group 3 exhibits a progradational, complex to chaotic configuration. It lies on an erosional unconformity with paleogullies. The seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sandstones in channel fill arrangements of fluvial origin (Wealdian deposits) marking the top of the section. The different systems tracts, sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts (onlap, downlap, toplap …) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile. It can only be identified precisely in the field, which also help to relate lithologies to seismic facies. This study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale. It will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data.

Paleoenvironment and sulfur-boron contents of the Mannville (Lower Cretaceous) coals of southern Alberta, Canada—reply

Abstract Sulfur (S) and boron (B) contents were determined for use as paleoenvironmental indicators in 28 samples of coal from the coal-bearing sequence occupying the upper part of the Lower Cretaceous Mannville Group located in the subsurface of southern Alberta, Canada. Sulfur for these coals range from 0.56 to 23.46% daf (wt. dry-ash-free) and boron between 18 and 1144 ppm. Twenty-three samples show S values > 1 wt.% and B > 100 ppm, indicating a strong brackish to marine influence in most of these coals. A S vs. B plot of these coal samples reveals two distinct suites: (1) with high B (256–1144 ppm) and low to moderate S (0.58–2.71% daf), and (2) with moderate B (18–247 ppm) and low to high S (0.56–23.46% daf). These two suites characterize coals from two different locations with differing stratigraphic-sedimentological settings. Coals in suite 1 occur at the tops of 8–10 m thick, regressive sequences stacked one above the other. Each coal bed is overlain by a marine transgressive surface. Inundation of the underlying peat by seawater may have produced conditions which caused the anomalously high (> 500 ppm) B content of nine samples in the suite. Coals in suite 2 on the other hand, lie on top of a single, thick (20–30 m) prograding shoreline sequence not followed by transgression. Their highly variable S content probably is a reflection of their origin in a coastal swamp characterized by a long period of bacterial activity during slow progradation.

Paleoenvironment and sulfur-boron contents of the Mannville (Lower Cretaceous) coals of southern Alberta, Canada

Sulfur (S) and boron (B) contents were determined for use as paleoenvironmental indicators in 28 samples of coal from the coal-bearing sequence occupying the upper part of the Lower Cretaceous Mannville Group located in the subsurface of southern Alberta, Canada. Sulfur for these coals range from 0.56 to 23.46% daf (wt. dry-ash-free) and boron between 18 and 1144 ppm. Twenty-three samples show S values > 1 wt.% and B > 100 ppm, indicating a strong brackish to marine influence in most of these coals. A S vs. B plot of these coal samples reveals two distinct suites: (1) with high B (256–1144 ppm) and low to moderate S (0.58–2.71% daf), and (2) with moderate B (18–247 ppm) and low to high S (0.56–23.46% daf). These two suites characterize coals from two different locations with differing stratigraphic-sedimentological settings. Coals in suite 1 occur at the tops of 8–10 m thick, regressive sequences stacked one above the other. Each coal bed is overlain by a marine transgressive surface. Inundation of the underlying peat by seawater may have produced conditions which caused the anomalously high (> 500 ppm) B content of nine samples in the suite. Coals in suite 2 on the other hand, lie on top of a single, thick (20–30 m) prograding shoreline sequence not followed by transgression. Their highly variable S content probably is a reflection of their origin in a coastal swamp characterized by a long period of bacterial activity during slow progradation.