Transgressive Surface Research Articles

Seismic and core investigation on the modern Yellow River Delta reveals the development of the uppermost fluvial deposits and the subsequent transgression system since the postglacial period

Postglacial stratigraphy and environmental evolution onshore and offshore the modern Yellow River Delta were investigated and analyzed through ∼1200-km high-resolution seismic profiles and four boreholes together with previous publications. Four seismic units (SU 1–4, top-to-bottom) that are bounded by seismic surfaces (T1-T4) were identified in seismic profiles, while four depositional units (DU 1–4, top-to-bottom) were recognized in representative boreholes. These seismic units and depositional units exhibit good correlation. We interpreted SU 1/DU 1 as the modern Yellow River Deltaic deposits, SU 2/DU 2 as the Holocene neritic sediments, SU 3/DU 3 as a Pleistocene-Holocene transitional layer, and SU 4/DU 4 as the lowstand fluvial sediments. Apart from T1 (seafloor), T2 (deltaic base), T3 (shoreface ravinement) and T4 (transgressive surface) all dip seaward, but their dipping gradients reduced from T4 to T2. Therefore, the thicknesses of SU 2–3 were observed seaward-thicker trends presumably in relationship with different spatial sedimentation rates. Additionally, down-core distributions of environmental proxies (e.g. grain size, microfossils and geochemical characteristics) reveal the transgression system (DU 2 and 3) can be further subdivided into 5 intervals associated with sharp environmental changes.Based on above evidences, we raised an evolutionary model of the postglacial depositional environment at the modern Yellow River Delta and adjacent marine areas, suggesting the study area evolved from riverine, estuarine, coastal, shoreface, neritic to final prodeltaic/deltaic environment since the Post-Last Glacial Maximum (LGM) in relationship with eustatic and climatic events as well as sediment input. In the model, we redefined the two-phase channel systems that exhibited in our previous study (Liu et al., 2014) as the tributaries of the LGM paleo-Yellow River and the tidal/estuarine tidal channels that formed at the early Holocene. Besides, we speculated that two hiatuses/erosions occurred at Younger Dryas and ∼4–3cal.kyr BP, and further divided the study area into three depositional zones based on their different sedimentation rates of the transgression system.

Depositional model and sequential evolution of the upper Quintuco Formation in its type area: implications for paleogeographic reconstructions for the southern Neuquén Basin during the Valanginian

The Quintuco Formation (Berriasian-early Valanginian) in central Neuquén province comprise marine, transitional and continental deposits. These deposits were included in several regional stratigraphic studies since the 80’s but receiving little attention in terms of detailed facies analysis, palaeoenvironmental reconstructions and evolution. Understanding the evolutionary stage corresponding to cusp section of the Quintuco Formation and its relation with basal deposits of the Mulichinco Formation, is key to estimate the magnitude of associated change to the event of basin reconfiguration occurred in the early later Valanginian. In order to reconstruct the final stages of the evolution of the Quintuco Formation in its type locality (Sierra de la Vaca Muerta), a facial, architectural and sequence-stratigraphic analysis of the uppermost interval is presented. The study comprise the sedimentological description and interpretation of facies, combined with architectural analysis of key intervals. This allowed the identification of 7 facies associations, which represent the accumulation in prodelta (FA 1), delta front (FA 2 and FA 3), distal delta plain (FA 4 y FA 5) and proximal delta plain (FA 6 and FA 7). Subsequently, the spatial distribution of the different facies associations were analyzed and key surfaces related to significant changes in the depositional settings were identified across the investigated region. In this context, sequence-stratigraphic analysis of the studied interval was addressed and stacking patterns of the successions and vertical evolution of the interval are discussed. The upper interval of the Quintuco Formation represents the development of a deltaic system, where fluvial processes were dominant, but waves affected the off-axis parts of the system. The proximal areas were located towards the southwest with prodelta settings located toward the east. The delta system was built by successive shallowing-upward successions (15-40 m), bounded by regional transgressive surfaces, and with a long-term progradational staking. The reconstructed paleogeography for the last evolutionary stage of the Quintuco Formation suggest a well-established source area from the west-southwest, that would represent a new insight for paleogeographic settings for the southern Neuquén Basin during the Valanginian.

Seismic stratigraphic interpretation of megasequence and sequence framework in the Upper Cambrian through Devonian formations in the central part of the Darling Basin, western New South Wales, Australia

Upper Cambrian through Devonian formations in the central part of the Darling Basin, western New South Wales were investigated using seismic data, complemented by well and outcrop information. Detailed seismic stratigraphic interpretation of four main second-order seismic megasequence buildups in the central-northern Blantyre and southern Pondie Range sub-basins, shows that they are constructed of seventeen principal third-order seismic sequences. The four seismic megasequences are characterized as follows: Seismic megasequence A corresponds to the Upper Cambrian through Middle Ordovician strata and is equivalent to the Scropes Range Formation or Mutawintji Group, Seismic megasequence B corresponds to uppermost Silurian through Lower Devonian strata and is equivalent to the Winduck Interval, Seismic megasequence C corresponds to upper Lower Devonian through lower Middle Devonian strata and is equivalent to the Snake Cave Interval and Seismic megasequence D corresponds to upper Middle Devonian through Upper Devonian strata and is equivalent to the Ravendale Interval. Five regional seismic-stratigraphic boundaries have been identified in the three seismic sections to show the continuity of the Upper Cambrian through Devonian formations throughout the central-northern Blantyre and southern Pondie Range sub-basins. All seismic-stratigraphic boundaries are based on good continuous markers, with strong amplitudes throughout the whole sub-basins.Detailed sedimentological analysis was applied using lithologic samples, wireline-log and limited biostratigraphic information, to aid in the development of a paleodepositional environment for the Upper Cambrian through Devonian sequence which displays ten different sedimentary facies successions. Seismic facies analysis was also applied to some of the seismic lines, comparing well log observation with reflection configuration and other seismic characteristics for the Upper Cambrian through Devonian successions. The seismic facies patterns show continuous, semicontinuous and discontinuous reflections with moderate amplitude and high to moderate frequency. The sedimentary sequence from the well data can also be recognized in the seismic lines, with features such as subparallel to divergent high amplitude reflections, parallel to subparallel medium to low amplitude reflectors with high-amplitude and high-continuity reflections, sediment waves with hummocky clinoforms and sometimes high-angle clinoforms, indicating fluvial channels complex and shallow channel-fill complex. As a result, a sequence stratigraphic framework was constructed successfully, consisting of nine seismic sequences. Six of these contain three systems tracts (lowstand, transgressive to highstand) and two sequence surfaces (transgressive surface and marine flooding surface). Another seismic sequence is composed of two systems tracts, a transgressive systems tract and a highstand systems tract separated by a maximum flooding surface. The other two, within the Blantyre 1 well and the Pondie Range 1 well, consist of one stratal pattern that has a highstand systems tract capped by a seismic sequence boundary.

Mid-Miocene (Badenian) transgression on Mesozoic basement rocks in the Mt. Medvednica area of northern Croatia

Rocky shorelines in the geological record, which represent major transgressive surfaces and provide us with crucial information about paleoshorelines and ancient sea levels, often involve a basal conglomerate associated with an unconformity. The unconformity between the Mesozoic basement and the overlying Middle Miocene (Badenian) deposits (which belong paleo-geographically to the southwestern margins of the Central Paratethys, and geotectonically to the Pannonian Basin System) in the Gornje Oresje section (northeast Mt. Medvednica, northern Croatia) is marked by basal Badenian conglomerates. The Upper Cretaceous limestone lithoclasts occurring in basal conglomerates show abundant truncated Gastrochaenolites and Entobia borings (represented by an in situ rocky substrate community of bivalves and sponges, respectively), with Gastrochaenolites being the dominant ichnogenus. Gastrochaenolites-Entobia ichnofossil assemblage related to the Entobia subichnofacies and in turn assignable to the Trypanites Ichnofacies, is very typical of Neogene rocky shores. This association characterizes littoral rockground environments, indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) with a low or null rate of sedimentation. Erosion of a pre-existing Mesozoic basement rocky shore during a marine transgressive phase in these high-energy littoral conditions, which formed basal conglomerates analyzed here, is also evidenced by truncation and the occurrence of Gastrochaenolites borings on all sides of limestone clasts. Calcareous red algae encrusting bored lithoclasts and often-forming rhodoliths also imply that the basal conglomerates occur at the start of a deepening-upwards succession of a transgressive systems tract deposits. The biostratigraphic correlation of the transgressive deepening-upward sequence from the Gornje Oresje section (ascribed to a single Badenian transgressive event that affected the Mesozoic basement) is based on the presence of the orbulinas in the uppermost outer shelf marl interval. The co-occurrence of Praeorbulina glomerosa circularis and Orbulina suturalis points to the short interval between FO (first occurrence) of Orbulina suturalis and LO (last occurrence) of Praeorbulina glomerosa circularis. These samples belong to the plankton zone M6 (Orbulina suturalis Zone) or the base of NN5 zone, marking the third-order sequence TB 2.4, which represents the second, main Badenian transgression in the Central Paratethys.

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.

Seismic and Sequence Stratigraphic Analysis of Ewan and Oloye Fields (Middle Miocene), Northwestern Niger Delta: Implications for Deltaic Depositional Sequences

The main aim of this works is to define local sequence variability and its implications for deltaic depositional sequences within the studied area. This research work makes use of depth converted three dimensional seismic cube, well logs, core photography descriptions and biostratigraphic reports. Recognition and interpretation of stratal surfaces were based on facies shifts and stacking patterns and reflection terminations derived from the interpretation of seismic data and wireline logs integrated with core photography and biostratigraphic reports. Seismic stratigraphic analysis of well and seismic data suggests that middle Miocene- early Pliocene strata within the studied area were made of six depositional sequences. Sequences are made up aggradational to progradational HST that makes up about 50% of a sequence. TST are thin and widespread while LST are restricted to within WNWESE trending valley fills typically 4 km wide and an average 120 m deep. Outside the LST valleys, transgressive surfaces, maximum flooding surfaces and sequence boundaries are coincident on interfluves. This suggests that during LST times most of the area was periodically subaerially exposed and several incised valleys/canyons were developed. These incisions/canyons acted as a sand feeder that led to the deposition of high amplitude deposits (sandstones) on the slope and within the basin floor setting. Sequences mapped over the study area show local variation associated with growth faults. There is a strong local growth fault control on sequence variability. Further local controls on sequence geometry are shelf instability and slope scars which have a strong geomorphological impact on the area. The depth of incised valleys within the uppermost HST (up to 350 m) is difficult to explain in terms of sea level fall alone because eustatically-driven sea level variations during the Miocene are generally reported to be less than 100 m (330 ft). This suggests that the character of sequence development and depositional facies preserved within the study area depends on relative sea level changes, relative rates of regional structural collapse and sedimentation rate. The occurrence of several incisions in the middle Miocene succession is interpreted as evidence of significant relative sea level fluctuations, and the presence of type-1 sequence boundaries may be the stratigraphic signature of major drops in relative sea level during Miocene and Pliocene. Variability within the depositional sequences is said to be largely controlled by local growth faults, rapid relative sea level changes, basin physiography (shelf edge) and high rate of sediment influx.

The effects of tectonic deformation and sediment allocation on shelf habitats and megabenthic distribution and diversity in southern California

Landscape and seascape structures are typically complex and manifest as patch mosaics within characteristic biomes, bordering one another in gradual or abrupt ecotones. The underlying patch structure in coastal shelf ecosystems is driven by the interaction of tectonic, sedimentary, and sea level dynamic processes. Animals and plants occupy and interact within these mosaics. Terrestrial landscape ecological studies have shown that patch structure is important for ecological processes such as foraging, connectivity, predation, and species dynamics. The importance of patch structure for marine systems is less clear because far fewer pattern-process studies have been conducted in these systems. For many coastal shelf systems, there is a paucity of information on how species occupy shelf seascapes, particularly for seascapes imbued with complex patch structure and ecotones that are common globally due to tectonic activity. Here, we present the results of a study conducted along a myriameter-scale gradient of bottom and sub-bottom geological forcing altered by tectonic deformation, sea level transgression and sediment allocation. The resulting seascape is dominated by unconsolidated sediments throughout, but also exhibits increasing density and size of outcropping patches along a habitat patch gradient forced by the erosion of a sea level transgressive surface that has been deformed and tilted by tectonic forcing. A combination of sub-bottom profiling, multibeam bathymetry, and ROV surveys of the habitats and the demersal megafauna occupying the habitats indicate (1) significant beta diversity along this gradient, (2) biological diversity does not scale with habitat diversity, and (3) species occupy the patches disproportionately (non-linearly) with regard to the proportional availability of their preferred habitats. These results indicate that shelf habitat patch structure modulates species specific processes and interactions with other species. Further studies are needed to examine experimentally the mechanics of how patch structure modulates ecological processes in shelf systems. Our results also provide further support for including multiple spatial scales of patch structure for the application of remote habitat sensing as a surrogate for biological community structure.

Sequence stratigraphy of the subaqueous Changjiang (Yangtze River) delta since the Last Glacial Maximum

This study focuses on sedimentary research at the subaqueous Changjiang (Yangtze River) delta, based on five high-resolution seismic profiles and seven borehole cores with accurate AMS 14C datings. Three distinct seismic units were identified from the seismic profiles according to seismic reflection characteristics, and five sedimentary facies were recognized from borehole cores. These facies constituted a fining upward sedimentary sequence in relation to postglacial sea-level transgression. Three sequence surfaces (sequence boundary (SB), transgressive surface (TS), and maximum flooding surface (MFS)) demarcate the boundaries between early transgressive system tract (E-TST), late transgressive system tract (L-TST), early highstand system tract (E-HST) and late highstand system tract (L-HST), which constitute the sixth order sequence. These system tracts were developed coevally with postglacial sea-level rise. E-TST (~19–12kaBP) corresponds to an incised-valley infilling in the early stages of postglacial transgression whereas L-TST (~12–7.5kaBP) was formed during the last stage of postglacial transgression. The progradational structure of L-TST reflected in seismic profiles is possibly related to the intensification of the East Asian summer monsoon. E-HST (~7.5–2kaBP) was deposited in response to the highstand after maximum postglacial transgression was reached, while L-HST (~2kaBP–present) was initiated by accelerated progradation of the Changjiang delta.

Stratigraphic vs structural contacts in a late orogenic basin: the case of the Tertiary Piedmont Basin in the Sassello area (Ligurian Alps, Italy)

This geological map (1:10.000 scale) of the ‘Sassello Basin’ remnant covers an area of about 33.4 km2 of Liguria (NW Italy); it highlights the occurrence of two main types of contacts between the sediments of the Tertiary Piedmont Basin and the metamorphic substratum (Voltri Unit): (i) stratigraphic and (ii) structural (thrust or steeply dipping faults). (i) Stratigraphic contacts are represented by the main transgressive surface and the nonconformity between the metamorphic rocks of the substratum and the subaerial deposits. They are locally folded and occur along the steeply dipping short limbs of asymmetric folds related to the late-alpine/apennine tectonics. (ii) Structural contacts are related to the late-alpine/apennine tectonics (thrust faults) or (mostly) to Plio-Quaternary extensional/transtensional faulting.

Tubular tempestites from Jurassic mudstones of southern Poland

This paper presents tubular tempestites from southern Poland and their application for environmental and sequence stratigraphy interpretation. Tubular tempestites are present in muddy successions of the Lower Jurassic Ciechocinek Formation and in the Middle Jurassic Częstochowa Ore-Bearing Clay Formation in the Silesian-Kraków region. They occur as sand-filled tubes of Spongeliomorpha and Thalassinoides entrenched in the mudstone, which form two characteristic horizons. Tubes were emplaced in semi-consolidated substrate and filled with sand brought by storm-generated bottom currents. The trace fossils reveal features typical of Glossifungites ichnofacies associated firmgrounds, which record discontinuities in the stratigraphical record, linked with a depositional hiatus or condensation and erosion of the sea-floor. The coincidence of these horizons with early phases of regional transgressions suggests that they represent transgressive surfaces of erosion. Their limited spatial extent probably resulted from varying intensity of erosion, which locally scoured deeply down to already consolidated substrate, whereas in other places erosion was weaker or even absent, and thus it is not marked in the sedimentary record there.

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.

Continental shelf morphology and stratigraphy offshore San Onofre, California: The interplay between rates of eustatic change and sediment supply

Abstract New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from ~ 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure.

Integrated brachiopod-based bioevents and sequence-stratigraphic framework for a Late Ordovician subpolar platform, eastern Anti-Atlas, Morocco

AbstractThe Upper Ordovician (Katian–Hirnantian) brachiopods of Tafilalt, eastern Anti-Atlas, are locally abundant, diverse and well preserved, providing a near-continuous record of faunal change on a high-latitude siliciclastic-dominated platform. A chronostratigraphic framework, based on brachiopod distribution and preservation in shell accumulation events and integrated with sequence stratigraphy, has been generated for the Katian interval, which has allowed correlation with the chitinozoan-based chronostratigraphic and sequence-stratigraphic framework erected for the central Anti-Atlas. In Tafilalt, two Katian (transgressive–regressive) composite depositional sequences,c. 60 and 170 m thick and related to third-order fluctuations in sea level, were unaffected by Hirnantian glaciogenic erosion. They were deposited on a mixed platform with a bryonoderm association dominated by brachiopods, bryozoans and echinoderms. Brachiopods developed in high-energy inner shelf areas, whereas bryozoans (mainly trepostomates and fenestrates) and pelmatozoans (cystoids and crinoids) dominated in low-energy outer shelf areas. Brachiopod accumulations mark distinct event surfaces, such as lag and event concentrations, hydraulic simple and composite concentrations related to transgressive surfaces, and hiatal condensed concentrations marking maximum flooding surfaces. The taphonomic condensation displayed by the Hirnantian Alnif Member, which onlaps the erosive base of glaciogenic tunnel channels, is explained as reworking and resedimentation of allochthonous, robust, biogenic hard parts sourced from the underlying (Katian) Ktaoua Group.

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

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.

Non‐unique stratal geometries: implications for sequence stratigraphic interpretations

AbstractThere is now strong evidence that stratal geometries on basin margins are most likely a consequence of multiple controls, not just variations in accommodation. Consequently, correct sequence stratigraphic interpretation of stratal geometries requires an understanding of how multiple different controls may generate similar geometries. Using a simple numerical stratigraphic forward model, we explore the impact of time variable sediment supply and different sediment transport rates on stratal geometries. We demonstrate how four common types of stratal geometry can form by more than one set of controlling parameter values and are thus likely to be non‐unique, meaning that there may be several sets of controlling factors that can plausibly explain their formation. For example, a maximum transgressive surface can occur in the model due to an increase in rate of relative sea‐level rise during constant sediment supply, and due to a reduction in rate of sediment supply during a constant rate of relative sea‐level rise. Sequence boundaries, topset aggradation and shoreline trajectories are also examples of non‐unique stratal geometries. If the model simulations in this work are sufficiently realistic, then the modelled stratal geometries are important examples of non‐uniqueness, suggesting the need for a shift towards sequence stratigraphic methods based on constructing and evaluating multiple hypotheses and scenarios.

Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: Paleoenvironments and sequence stratigraphy

The Paleoproterozoic Sokoman Formation (ca. 1.88Ga) of the Labrador Trough, eastern Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained, terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and lateral distribution of redox-sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore.Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favorably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline.As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces frame iron formation sequences, but with important differences. The absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult. Transgressive surfaces can also be easily mistaken for Phanerozoic-style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement. Outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus. Understanding these differences is essential for establishing an accurate sequence stratigraphic framework. Such context is the backdrop for properly interpreting the sedimentology, oceanography, microbial ecology, and geochemistry of continental margin iron formations.

Devonian–Carboniferous transition containing a Hangenberg Black Shale equivalent in the Pho Han Formation on Cat Ba Island, northeastern Vietnam

On Cat Ba Island in northeastern Vietnam, the Devonian to Carboniferous (D–C) transition consists mainly of ramp carbonates intercalated with black shale beds (Beds 1 to 176) in the Pho Han Formation and is one of the few records of the D–C transition of the eastern Paleotethys. The three main facies of the sequence are Facies 1 (alternations of whitish gray to gray limestone and marl), Facies 2 (calcirudite, Bed 115b), and Facies 3 (alternations of dark gray limestone and organic-carbon-rich black shale, Beds 115c–120 and 126–129). The latest Famennian (Siphonodella praesulcata Subzone) conodont assemblage of S. praesulcata, Palmatolepis gracilis, Palmatolepis sigmoidalis, and Rhodalepis polylophodontiformis was recognized in Beds 113–115c. Beds 105–112 commonly contain Palmatolepis expansa, P. gracilis, and P. sigmoidalis. Bed 119 yielded a basal Carboniferous index conodont Siphonodella sulcata. In Beds 116–118, solenoporids such as Pseudochaetetes elliotti and Parachaetetes sp. were characteristic species in organic-carbon-rich dark gray limestone.Facies 1 is characterized by bioclastic, peloidal, and intraclastic grainstone and packstone containing massive normal grading and cross-laminations, and is interpreted to represent deep ramp carbonates above storm wave base. Facies 2 is represented by typical lag deposits overlying a transgressive surface. Facies 3 comprises organic-carbon-rich black shale and minor scour-filling bioclastic, peloidal, and intraclastic packstone, and may represent a marginal basin plain environment surrounding a carbonate ramp. The alternations of organic-carbon-rich black shale and dark gray packstone (Facies 3) show no evidence of bioturbation and have high TOC contents (0.18–5.73wt.%). A minor succession within the transgressive lag deposits (from Bed 115b of Facies 2 to Beds 115c–120 in the lower part of Facies 3) is equivalent to the Hangenberg Black Shale (s. l.) in the middle part of the Siphonodella praesulcata to Siphonodella sulcata zones, because Beds 115b–120 characterized by no evidence of bioturbation and high TOC contents are interpreted to be accumulated in anoxic to dysoxic conditions.

Natural Clay-Sized Glauconite in the Neogene Deposits of the Campine Basin (Belgium)

AbstractNatural clay-sized glauconite has the same mineralogical composition as sand-sized glauconite pellets but occurs in <2 μm clay fractions. This particular glauconite habit has been described previously from soil environments resulting from pelletal weathering but is rarely reported in higher-energy sedimentary environments. In the present study, clay-sized glauconite was identified as a common constituent in transgressive Neogene glauconite pellet-rich deposits of the southern North Sea in Belgium. X-ray diffraction results revealed that the characteristics of the clay-sized glauconite are very similar to the associated glauconite pellets in sand deposits. Both glauconite types consisted of two glauconite-smectite R1 phases with generally small percentages of expandable layers (<30%) with d060 values ranging between 1.513 Å and 1.519 Å. Clay-sized glauconite was not neoformed but formed by the disintegration of sand-sized glauconite pellets which were abraded or broken up during short-distance transport within the sedimentary basin or over the hinterland. Even in an environment where authigenic glauconite pellets occur, minimal transport over transgressive surfaces is sufficient to produce clay-sized glauconite. Furthermore, clay-sized glauconite can be eroded from marine deposits and subsequently resedimented in estuarine deposits. Clay-sized glauconite is, therefore, a proxy for the transport intensity of pelletal glauconite in energetic depositional environments and, moreover, indicates reworking in such deposits which lack pelletal glauconite.

Facies analysis and depositional architecture of shelf-type deltas in the Crati Basin (Calabrian Arc, south Italy)

This study focuses on facies analysis and depositional architecture of the Arente shelf-type delta, located at the southernmost margin of the Pleistocene Crati Basin (northern Calabria, southern Italy). Tectonically-controlled Gilbert deltas are extensively discussed in the literature, whereas few works document shelf-type deltas and factors controlling sedimentation. This paper contributes to these issues, with a special focus on evaluating eustatic and tectonic control on depositional architecture also by considering the occurrence of coeval Gilbert-delta systems.The Arente delta complex represents part of the coarse-grained sediments presently exposed along the eastern margin of the Crati Basin. It was sourced from the rising Sila Massif to the E and shows an overall westerly progradation.Conventional sedimentological analysis allowed to distinguish seven different facies associations. Well-exposed sections, at the basin margin, generally show an overall upward transition, locally basinward, from mouth-bar conglomerates and sandstones, to deltafront sandstone, to offshore mudstones. Facies distribution and arrangement are interpreted as the record of a shoal-water fan delta.Depositional processes were clearly influenced by the high sediment input, with a dominance of hyperpycnal deposits.Based on the occurrence of a series of bounding surfaces indicating abrupt drowning stages, the Arente deltaic succession can be divided into 5 distinct transgressive-regressive sequences.The fining and the thinning upward trend of the whole succession and the stacked landward-stepping architecture of the transgressiveregressive sequences, i.e. the E-directed retrogradational behavior of the Arente delta complex indicate that.Tectonic subsidence provided the accommodation space for the deposition of the delta complex, for the marked local backstepping of the system, and for the progressive drowning of bedrock. Highfrequency sea level fluctuations led to the repeated arrangement of the facies associations; in particular, the stacking of the individual deltaic sequences separated by transgressive surfaces suggests amplitudes of eustatic sea-level changes of a few tens of metres, thus representative of a higher-order cyclicity. The differing internal architecture, if compared to the steeply-dipping foreset beds of the Gilbert-type delta cropping out to the N of the study area, is interpreted as due to a decrease in displacement toward the southern tip of the Crati system fault.