Transgressive Surface Of Erosion Research Articles

Regressive depositional architecture on a Mesoproterozoic siliciclastic ramp: Sequence stratigraphic and Nd isotopic evidences from Bhalukona Formation, Singhora Group, Chhattisgarh Supergroup, central India

A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona Formation, Singhora Group, central India provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the Proterozoic siliciclastic ramp settings under falling and lowstand sea level conditions. Beside delineation of two diachronous surfaces, process-based facies analysis identified fourteen facies types that are grouped under five different facies associations. Paleo-environments range among continental fluvial, beach-foreshore, upper shoreface, lower shoreface and wave-dominated delta front. From the paleocurrent measurement within the fluvial channel sandstones and measurement of crest line orientations of wave generated swash bedforms, it is inferred that the Bhalukona Sea had NNE-SSW shoreline trajectory and the south-easterly flowing river system carried sediment on the shoreline from a source in the west-northwest. Such inference though in clear discordance with the earlier proposed south-southeastern sediment source for the Singhora basin, finds support in the shift in εNdt (t=1.42Ga) values (from −3.5±3.3 to −9.3±2.2) indicating change in sediment provenance at the early Bhalukona sedimentation history; sediments in the Bhalukona Formation derived from more evolved or older continental crustal sources in comparison to those of the underlying Saraipalli Formation. A tectonic forcing behind the shift in sediment provenance and fall in relative sea level is inferred that established the forced regressive and lowstand shoreline in the Singhora basin during the Bhalukona time.Abrupt basin-ward shift of facies tract and incision on shelf is exhibited by the occurrence of poorly sorted, coarse granular Bhalukona fluvial system (carrying rip-up mud clasts) directly above the argillaceous highstand Saraipalli shelf with ∼10m incision and is inferred as the signal for forced regression and development of Type-I sequence boundary. In low-gradient Proterozoic ramp settings without shelf-slope break, we interpret that the Bhalukona fluvial system incised the coastal prism developed on the Saraipalli highstand coastline. The low-gradient of the ramp, however, prompted long distance (∼15km) regression represented by the offlapped and detached delta front lobe away from the shoreline. The slow, steady rise in sea level, onset of lowstand and establishment of a wave-dominated coastline caused reworking of fluvial sediments in the basinal part (within the wave base) and resulted development of ravinement deposit. Basin-ward, the surface grades into correlative conformity. With aggradational and weak retrogradational stacking the beach-foreshore, upper- and lower-shoreface, in order of superposition, record the lowstand depositional history. The basin-scale transgression is witnessed with formation of Transgressive surface of erosion (TSE) and establishment of the Chuipalli shelf system, dominantly beyond storm wave base. Taking into consideration ∼23m preserved shoreface succession, 1m per year eustatic rise consistent with present day rate and average rate of shoreface retreat 0.5m per year, ∼11.5km retreat for the Bhalukona shoreline is estimated in its lowstand history.

Sequence Stratigraphic Analysis of “XB Field”, Central Swamp Depobelt, Niger Delta Basin, Southern Nigeria

Well logs and biostratigraphic data from six wells in the “XB Field”, central Swamp Depobelt, Niger Delta were integrated to carry out a sequence stratigraphic analysis of depositional systems in the field. The analysis revealed four 3rd order depositional sequences (SEQ1 to 4) bounded by three erosional unconformities interpreted as Sequence Boundaries (SB1 to 3). Transgressive Surfaces of Erosion (TSE1 to 3) that mark the onset of marine flooding and turnarounds from progradational facies to retrogradational facies during sequence build-up were delineated. Three 3rd order Maximum Flooding Surfaces (MFS1, MFS2 and MFS3) characterized by marker shales, high faunal abundance and diversity were also delineated and dated 15.9, 17.4 and 19.4 Ma, respectively. The delineated sequences comprised Lowstand Systems Tracts (progradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational packages), which reflect depositional systems deposited during different phases of base level changes. The Lowstand Systems Tract (LST) consists of Basin Floor Fans (BFF), Slope Fans and Channel Sands deposited when sea level was low and accommodation space lower than rate of sediment influx. Transgressive Systems Tract (TST) consists of retrogradational marine shales deposited during high relative sea levels and when accommodation space was higher than rate of sediment influx. Highstand Systems Tracts (HST) consisted of shoreface sands displaying mostly aggradational to progradational stacking patterns. The sands of LST and HST show good reservoir qualities while the shales of the TSTs could form potential reservoir seals. The above recognized sequences, were deposited within the Neritic to Bathyal paleoenvironments and are dated mid-Miocene (15.9 - 20.4 Ma) in age.

Lithofacies and sequence stratigraphy of the Barnett Shale in east-central Fort Worth Basin, Texas

Ten Barnett Shale lithofacies have been recognized in a 223-ft (68-m)-long core from Johnson County, Texas. Eight of these lithofacies match those previously identified in the main producing area of the Newark East (Barnett Shale) field in the northern part of the Fort Worth Basin, but two new lithofacies have been identified in this core, resedimented spiculitic mudstone lithofacies and lag deposits, both of which are indicative of a relatively higher energy environment and downslope resedimentation of shallower water deposits. The recognition of cyclical stacking patterns of the lithofacies, condensed sections (CSs), and transgressive surfaces of erosion were the keys to establishing the sequence-stratigraphic framework in these fine-grained rocks, which consists of seven stratigraphic intervals in the lower Barnett Shale and nine stratigraphic intervals in the upper Barnett Shale. Spectral gamma-ray uranium and thorium logs aided in this objective and are recommended for future sequence-stratigraphic studies of these and other shales. The sequence-stratigraphic framework reveals that the lower Barnett Shale in this area was deposited mainly in a low-energy, relatively deep-water environment, somewhat far from a terrigenous source area, which probably lies to the northwest. By contrast, the upper Barnett Shale was deposited in an oxygenated shallower water environment, which had a source area from the west and southwest sides of the basin. The higher frequency of sea level fluctuation during development of the upper Barnett Shale most probably indicates periodic tectonic activity, perhaps associated with a structural high that was susceptible to sea level fluctuations. Alternatively, it could have resulted from the onset of glaciations in Gondwanaland during this time. This higher frequency may indicate that the upper Barnett is Chesterian in age, because cyclicity was higher than during the Osagean and Meramecian stages. If so, there may be more high-frequency cycles than recognized in this core. Siliceous sponge spicules are more common in this core than in more northerly cores, so more brittle facies might prevail in the southern part of the Fort Worth Basin. High gamma-ray log responses, which are caused by a high total organic carbon, and/or in-situ phosphate minerals are commonly found in CSs and can be used for regional correlations. However, high gamma-ray phosphatic deposits that have been resedimented to downslope positions by sediment gravity flows are an exception to the previous statement. Correlation of the Barnett stratigraphic intervals now provides a north-to-southeast stratigraphic framework along the Fort Worth Basin. Relative hydrocarbon potential (RHP) is an organic geochemical parameter applied to this core and found to provide an indicator of marine transgressions and regressions. We recommend continued testing and use of the RHP parameter for high-frequency sequence-stratigraphic analysis of unconventional gas shales.

Stratigraphic architecture of the fluvio-lacustrine deposits, Bathan Formation, Al-Rehaili area, North Jeddah, Saudi Arabia

A siliciclastic-dominated succession (∼11 m thick) underlying Harrat Rahat, belonging to the Miocene–Pliocene Bathan Formation is recently exposed at Al-Rehaili area, North Jeddah, Saudi Arabia. It covers a wide spectrum of grain sizes varying from clay-rich mudstones to cobble grade conglomerate and consists of a variety of facies vary from fluvial to marginal and open lacustrine deposited in a half-graben basin formed along the eastern margin of the extensional Red Sea Basin. Field-based sedimentologic investigation enables to identify ten facies grouped into three facies associations (A–C). The depositional history is subdivided into two stages. The first stage represents deposition in gravel to sand-dominated fluvial system sourced from a southern source and grade northward into lacustrine delta and open lacustrine setting. The second stage on the other hand includes deposition of fluvial channels running in E–W direction with attached bank sand bar. Sequence stratigraphic interpretations of the lacustrine deposits enable to identify three unconformity-bounded sequences (SQ1–3). The basal sequence is incomplete, consisting of three aggradationally to progradationally stacked delta plain and delta front parasequences. The second sequence is sharply and erosively overlying a red paleosol bed that defines the upper boundary of the first sequence. It includes two system tracts; upward-fining and deepening lacustrine offshore mudstones of the transgressive system tracts unconformably overlain by red paleosol of the regressive systems tracts. The top of this sequence is delineated at the sharp transgressive surface of erosion at the base of delta mouth bar deposits of sequence 3. Changes in the accommodation and sedimentation rates by basin subsidence under the influence of tectonics and sediment compaction and loading as well as climatic oscillation between semi-arid to arid conditions were the major controls on the fluvio-lacustrine sedimentation and their facies distribution. Tectonic reorganization of the drainage system resulted in the formation of E–W flowing fluvial streams in the second stage.

Significance of hypoburrow nodule formation associated with large biogenic sedimentary structures in open-marine bay siliciclastics of the Upper Eocene Birket Qarun Formation, Wadi El-Hitan, Fayum, Egypt

Unusually large biogenic sedimentary structures from the shallow quiescent-marine siliciclastics of the Upper Eocene Birket Qarun Formation in the Fayum area of Egypt display pronounced concretion formation around the trace fossils. The structures are massive, and vary morphologically, forming branched pillars (up to dm-scale), vertical (up to 180 cm height) amphora-like masses, and 3-D box-work “maze”. Bioturbation, mainly Thalassinoides attributable to the Glossifungites ichnofacies, mediated and modified the physical and chemical microenvironments influencing early diagenesis; i.e., burrows promote the precipitation of pervasive calcite-dominated cement. The inferred paragenesis, combined with the negative (light) carbon and oxygen stable-isotopic values of the bulk calcite (δ 13C PDB from −0.94 to −4.98‰ and δ 18O PDB from −4.63 to −7.22‰) and bulk dolomite (δ 13C PDB from −2.05 to −8.23‰ and δ 18O PDB from −1.41 to −11.20‰), imply that the pore-water carbon was derived directly from seawater and dissolution of metastable carbonate, which was mediated by bacterial decomposition of organic matter and mixing of meteoric ground water. Thereby, the carbonate cement precipitated mostly under eodiagenetic conditions near the sediment/water interface (<~3 m in depth). The distribution of these structures is confined to parasequence-bounding flooding surfaces (generally expressed as transgressive surfaces of erosion). Notably, sedimentological, ichnological and paragenetic data can be related to stratigraphic evolution such that geochemical and textural evidence is distinctly associated with (1) early cementation of the host sandstone during highstands of relative sea level, (2) the formation of firmgrounds during low relative sea level, (3) the development of a Glossifungites-demarcated discontinuity during initial relative sea-level rise, and (4) continued cementation with rising relative sea level. This was followed by burial diagenesis, evidence for which is derived from petrographic and isotopic data.

Record of Late Pleistocene (Oxygen Isotopic Stage 5) climate changes during episodes of karst development on the Northern coast of Crete: Sequence stratigraphic implications

In Late Pleistocene times, during Oxygen Isotopic Stage (OIS) 5, aeolian calcareous sand dunes were deposited along the northern coast of Crete, where they now discontinuously crop out wedged between thin shallow marine Tyrrhenian deposits. Late Pleistocene marine sediments are scarce and amalgamated because the combination of slow uplift movement and low elevation of OIS 5 highstand shorelines, close to present mean sea level, promoted their erosion in the shallow marine and subaerial environments. The studied succession is organized in transgressive–regressive (T–R) sequences bounded below and above by transgressive surfaces of erosion superimposed on subaerial unconformities. The vertical stacking of sedimentary facies between consecutive key surfaces demonstrates that the preservation potential of marine transgressive deposits (TST) was higher than that of regressive sediments. Similarly, aeolian strata and palaeosols located landward were well preserved. Detailed sedimentological and petrographic studies of the aeolianites and their framing units from several sites revealed a complex microstratigraphy of diagenetic features that relate to: (1) the succession of highstand shorelines during OIS 5; and (2) episodes of subsurface karstification interpreted to result from soil cover development on limestone substrate during relative sea-level falls. A complex sequence of climate changes during karst formation is suggested by solution pipes that cross-cut or contain calcified root systems, and whose inner walls are covered with carbonate crusts interpreted as calcretes. The repetition during OIS 5 of this transition from rhizocretion formation to karstification to calcretization in the studied deposits is argued to occur in response to contrasting climate conditions at times of sea-level lowstand. It is proposed that palaeosols and rhizocretions formed during the falling limb of the relative sea-level curve under semi-arid conditions, karstification developed at the turn between falling and rising sea level when climate was at its wettest, and calcretization occurred during the rising limb of the relative sea-level curve under arid conditions. Identification of wave-cut surfaces of erosion on top of transgressive marine sediments and below aeolian deposits, suggest that the latter are contemporaneous with falling sea-level marine units deposited basinward below the lowstand shoreline, namely forced regressive systems tracts (FRST), otherwise not recognized in the field. The complex suite of alteration processes recorded within the palaeosols above aeolian deposits are interpreted to represent periods extending from the latest stage of sea-level fall to the earliest stage of sea-level rise, including lowstand systems tracts (LST). This proves a satisfactory complement to reconstruct palaeosea-level fluctuations in settings where the shallow-marine nature of sedimentary environments means that most regressive and lowstand deposits are not recognized in the stratigraphic record.

Ichnology and Sedimentology of a Mud-Dominated Deltaic Coast: Upper Cretaceous Alderson Member (Lea Park Fm), Western Canada

Abstract Current depositional models largely promote the perception that all open-coastal distal (sea)–proximal (land) gradients are reflected by upward-coarsening grain-size trends, and that shoreline deposits are represented by prominent sand bodies. Although commonly the case, significant departures from this model may occur when the availability of coarser sediment calibers (sand-sized and larger) is limited. This is especially true where alongshore sediment-transport-influenced depositional systems are associated with rivers that supply abundant suspended sediments. Underestimating the role of grain-size segregation may lead to misinterpretations of energy levels and water depths, especially in some shale-dominated sedimentary units. The Upper Cretaceous Alderson Member (Lea Park Fm) is an up to 180-m-thick, gas-charged shale unit that we interpret as an ancient analogue for modern offshore and mud-dominated deltaic coasts. Sedimentological and ichnological data collected from 27 cores indicate that much of the sediment volume of the Alderson Member was deposited in relatively shallow water under the influence of tidal and wave processes in a deltaic coastal setting. Characteristic features reflecting these depositional affinities include: (1) increased proportions of terrestrially derived organic matter; (2) indications of thixotropic to soupy substrates (e.g., fluid mud) coupled with rapid depositional rates; (3) an impoverished ichnological signal (Planolites-dominated suites); (4) micro-laminated shale; (5) shale-on-shale erosional contacts; (6) scour-and-fill structures; and (7) intervals of low-angle cross-stratification. The interpretation of relatively shallow-water settings is also supported by recurring root-bearing horizons, Glossifungites Ichnofacies-demarcated transgressive surfaces of erosion, and conglomeratic surfaces at particular stratigraphic levels. The deposits are interpreted to include offshore, subaqueous deltas, muddy shoreface and/or tidal flat, and aggradational muddy coastal plain (chenier plain) sub-environments. The results of this study improve our knowledge of ichnological and sedimentological characteristics of shallow-marine shale units, and are potentially useful for recognition of similar nearshore mud-prone deposits elsewhere.

Ichnological analysis in high-resolution sequence stratigraphy: The Glossifungites ichnofacies in Triassic successions from the Betic Cordillera (southern Spain)

This study integrates ichnological and sedimentological data in order to refine sequence stratigraphy and interpretations of sea-level dynamics for the Ladinian (Middle Triassic), Muschelkalk succession (Siles Formation) in the Betic Cordillera (southern Spain). Facies analysis was integrated with a detailed ichnological study, focused on the middle part of the lower member of the Muschelkalk succession (transgressive systems tract), which is characterized by an abundant and conspicuous trace fossil assemblage. Seven lithofacies were recognized, recording sediment accumulation in tidal flat, and inner to outer marine carbonate ramp, depositional environments. Thin-bedded marly limestones with bioclastic shelly beds (Facies E: middle ramp, with storm influence), are characterized by Diplocraterion and Rhizocorallium. Diplocraterion is protrusive, usually eroded on top, and mainly recorded in the marly limestone intervals. Rhizocorallium preserves well defined scratch-marks, and is commonly emplaced in bioclastic, shelly beds. The assemblage represents the Glossifungites ichnofacies. Sedimentological and ichnological data are interpreted to record a complex transgressive context, associated with high-frequency sea-level dynamics that allowed formation of transgressive surfaces of erosion (TSE, i.e., ravinement surfaces) of different orders. Major TSE, associated with continuous bioclastic shelly beds, delimit parasequences; the absence of the Glossifungites suite reveals that there was little time between erosion and deposition. Intermediate TSE, associated with discontinuous shell beds, are related to comparatively less significant sea-level rises and occur within parasequences. The Glossifungites suite reveals colonization of firmgrounds during relatively prolonged times between erosion and deposition related to intermediate TSE. Minor order TSE, recorded between the intermediate TSE, are related to punctuated, highest frequency sea-level changes; phases of colonization by the Diplocraterion tracemaker and protrusive, capped structures reveal more or less stillstand phases (TSE/stillstand phases), and minor order erosional phases.

The early Pliocene Titiokura Formation: Stratigraphy of a thick, mixed carbonate‐siliciclastic shelf succession in Hawke's Bay Basin, New Zealand

This paper presents a systematic stratigraphic description of the architecture of the early Pliocene Titiokura Formation (emended) in the Te Waka and Maungaharuru Ranges of western Hawke's Bay, and presents a facies, sequence stratigraphic, and paleoenvironmental analysis of the sedimentary succession. The Titiokura Formation is of early Pliocene (Opoitian‐Waipipian) age, and unconformably overlies Mokonui Formation, which is a regressive late Miocene and early Pliocene (Kapitean to early Opoitian) succession. In the Te Waka Range and the southern parts of the Maungaharuru Range, the Titiokura Formation comprises a single limestone sheet 20–50 m thick, with calcareous sandstone parts. In the vicinity of Taraponui Trig, and to the northeast, the results of 1:50 000 mapping show that the limestone gradually partitions into five members, which thicken markedly to the northeast to total thicknesses of c. 730 m, and concomitantly become dominated by siliciclastic sandstone. The members (all new) from lower to upper are: Naumai Member, Te Rangi Member, Taraponui Member, Bellbird Bush Member, and Opouahi Member. The lower four members are inferred to each comprise an obliquity‐controlled 41 000 yr 6th order sequence, and the Opouahi Member at least two such sequences. The sequences typically have the following architectural elements from bottom to top: disconformable sequence boundary that formed as a transgressive surface of erosion; thin transgressive systems tracts (TSTs) with onlap and backlap shellbeds, or alternatively, a single compound shellbed; downlap surface; and very thick (70–200 m) highstand (HST) and regressive systems tracts (RST) composed of fine sandstone. The sequences in the Opouahi Member have cryptic TSTs, sandy siltstone to silty sandstone HSTs, and cross‐bedded, differentially cemented, fine sandstone RSTs; a separate variant is an 11 m thick bioclastic limestone (grainstone and packstone) at the top of the member that crops out in the vicinity of Lake Opouahi. Lithostratigraphic correlations along the crest of the ranges suggest that the Titiokura Formation, and its correlatives to the south around Puketitiri, represent a shoreline‐to‐shelf linked depositional system. Carbonate production was focused around a rocky seascape as the system onlapped basement in the south, with dispersal and deposition of the comminuted carbonate on an inner shelf to the north, which was devoid of siliciclastic sediment input. The rates of both subsidence and siliciclastic sediment flux increased rapidly to the northeast of the carbonate “platform”, with active progradation and offlap of the depositional system into more axial parts of Hawke's Bay Basin.

Neotectonics and the Kos Plateau Tuff eruption of 161 ka, South Aegean arc

A high-resolution seismic-reflection survey of the area between Kos and Tilos islands is used to constrain the nature of the Kos Plateau Tuff (KPT) eruption and post-eruptive subsidence. A unique acoustically incoherent unit tens of metres thick at a subbottom depth of 15–35 m is recognised throughout the West Kos basin, which lies between Pachia, Yali and Kos. It commonly unconformably overlies folded, stratified sediment, but in places is concordant with stratified sediment more than 100 m thick. In places south of Kos, the acoustically incoherent unit is overlain by an unconformity and irregularly stratified sediment interpreted as terrestrial or shallow marine. Southeast of Nisyros, a correlative acoustically incoherent unit overlies a planar marine transgression erosion surface that extends almost to Tilos. The stratigraphic level of this unit is dated by comparison with the global eustatic sea-level record and the presence of major transgressive erosion surfaces on adjacent continental shelves, constrained by regional sedimentation rates, and indicates that it is of similar age to the Kos Plateau Tuff eruption. The relationship of this unit to coastal erosion surfaces, and its absence in many areas where seismic-reflection profiles show continuous marine sedimentation, suggests that it is a pyroclastic deposit of subaerial, or at most very shallow marine, origin from the Kos Plateau Tuff eruption. This presence of transgressive unconformities implies that a coastal plain or shallow sea extended southeast of Nisyros to Tilos and the Datça peninsula, and thus it is unlikely that pyroclastic flows crossed large stretches of deep water towards Tilos, as proposed by Allen and Cas (Allen, S.R., Cas, R.A.F., 2001. Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece. Bull. Volcanol. 62, 441-456). Late Pleistocene tectonic subsidence has taken place throughout the whole region between Kos and Tilos, in places at an average rate of at least 3 mm/a. The older dacites of Pyrgousa and Pachia and the post-KPT volcanics of Yali, Strongili and Nisyros lie on a NE–SW-trending lineament marked by major faults and abrupt changes in bathymetry. This NE–SW faulting was initiated in the early Pleistocene and parallels prominent lineaments in the east Cretan Sea. ENE–WSW sinistral strike-slip faulting initiated in the middle Pleistocene in the area from Santorini to Kos would have produced extension on this older lineament, playing a major role in channelling magma to the surface and permitting the ingress of water to the magma conduit.

Transgressive surfaces of erosion as sequence boundary markers in cool-water shelf carbonates

Although sequence stratigraphy was first conceptualized in, and applied to, siliciclastic successions, its methodology and concepts are applicable to the carbonate regimen, provided that potential modifications to the existing models are appreciated that take into account the specificities of carbonate systems. We demonstrate that some properties inherent to isolated (i.e. continent-detached), shallow-water, temperate carbonate systems, including their typical low diagenetic potential due to the prevalance of calcitic mineralogies, the critical role exerted by storm waves and tidal currents on sediment dynamics (e.g. shedding of sediments at wave abrasion depth during highstand and lowstand conditions), and uneven sea-bed topographies cause both the genesis and preservation of subaerial unconformities to be uncommon, thereby rendering their use as sequence boundaries inappropriate. Instead, transgressive surfaces of erosion (TSEs) that formed as wave base shifts upwards during early rise of relative sea level, are readily identifiable in the field, extensive and, by their very nature, are preserved in the rock record, typically at the base of upward-deepening stacked facies. In Pliocene cool-water carbonate successions from New Zealand, the TSEs are characterized by starved sedimentation and burrowed network development, and are overlain by coarse bioclastic shoreface sediments and shell beds. In most cases, the surfaces top-truncate shoaling-upward deposits, but also deepening-upward stacking facies in places. Emersive events rarely correspond to actual physical surfaces, but diagenetic evidence preserved beneath TSEs indicates that occasionally subaerial surfaces had previously developed and were later modified into TSEs. Therefore, we propose that TSEs are the most adequate unconformities to define high-order depositional sequences (20–250 ka) in the studied successions because, by contrast with emersion surfaces, their formation and preservation are not dependent upon cool-water sedimentary facies and mineralogy, and are favoured by the dynamics of the setting (i.e. wave base shift and accommodation creation during relative sea-level rise).

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.

Anatomy of a transgressive lag: Panther Tongue Sandstone, Star Point Formation, central Utah

Abstract The Panther Tongue of the Star Point Formation in central Utah contains a variety of transgressive lag deposits that, when mapped regionally, show a sensitive dependence upon pre‐existing topography of the palaeoshoreline. The Panther Tongue consists of a coarsening‐upward sandstone wedge that prograded into the Western Interior Seaway during Late Cretaceous (Santonian) time. High‐resolution sequence stratigraphic analysis revealed that this member was deposited during the long‐distance (&gt;50 km) regression and transgression of a delta into shallow‐marine environments, containing basal highstand, forced regression, lowstand and transgressive systems tracts. Based on grain size, clast composition, lateral extent and stratigraphic position, the coarse sandstones on top of the Panther Tongue were classified into four types: (1) simple; (2) dispersed; (3) oxidized; and (4) local lags. The simple lag is composed of dark grey coarse sandstone with oyster fragments and shark teeth. This lag is typically extensively bioturbated and massive. Laminated and cross‐bedded units are also common. This type of coarse sandstone is interpreted as a typical transgressive lag. The dispersed lag differs in that it contains abundant mud and commonly occurs as multiple beds in thick intervals of muddy sandstone. Mixing of bay/estuarine and shallow‐marine mud with simple lag sand may be responsible for deposition of this type of coarse sandstone. The oxidized lag is distinctive in its reddish colour with extensive bioturbation and is commonly overlain by a simple lag. The local lag is composed of thin‐bedded, dark grey, coarse sandstone, occurring locally between the mouth bar and distributary channel. The variation in types, grain size and bed thickness of the coarse‐grained lags was mainly controlled by antecedent topography as suggested by immediately underlying lithofacies. Relatively thick (≈30 cm) simple lags are present on top of mouth‐bar sandstones, whereas dispersed lags are common on top of the distributary channel sandstone and in bay/estuarine and shallow‐marine mudstones. Erosion of topographic highs (mouth bar) resulted in relatively thick accumulation of simple lags. In topographic low areas such as distributary channel, estuary, bay and shallow‐marine environments, fine‐grained muddy sands that were eroded from the nearby topographic highs were redeposited. Intermittent storm waves transported coarse sands both landward and seaward, forming a dispersed lag. The net effect was reworking of local topographic relief during overall transgression, forming an apparently planar transgressive surface of erosion.

Tectonic Control of Facies Architecture in Falling-Stage Deposits in A Forearc Basin: Upper Miocene Senhata Formation, Boso Peninsula, Japan

ABSTRACT The Upper Miocene Senhata Formation is the infill of the Middle Miocene-Middle Pliocene forearc basin that developed in response to the north ward to northwestward subduction of the Philippine Sea plate beneath the Eurasia plate at the Sagami Trough. The formation has been affected by anticlinal and synclinal deformation and provides an opportunity for analyzing three-dimensional variations in thickness, stacking patterns, and bounding-surface features of component lithofacies associations. The lower part of the formation consists mainly of shoreface and shelf deposits and indicate falling-stage deposits that are overlain by transgressive shoreface and shelf deposits of the upper Senhata Formation, with the contact defined as a wave-ravinement surface. Forced-regressive shoreface-to-shelf deposits of the lower Senhata Formation are characterized by an erosional basal surface of a Type 1 sequence boundary and lateral change to multiply stacked, thicker regressive deposits with a gradational basal contact with the underlying shelf and upper slope deposits, indicating progradational and aggradational stacking of shoreface and shelf deposits, which are interpreted to be equivalent to the shelf-margin systems tract of a Type 2 sequence. The erosional sequence boundary at the base of forced-regressive deposits coincides laterally with a transgressive surface of erosion to the northwestern upslope area, where the forced-regressive deposits are absent. The lateral variation in the falling-stage regressive deposits of the lower Senhata Formation is interpreted to indicate that falling-stage and lowstand-stage deposits of Types 1 and 2 sequences are largely contemporaneous and their development is controlled by spatial variation in the rate of basin subsidence in both depositional-dip and depositional-strike directions.

Sedimentation and basin-fill history of the Neogene clastic succession exposed in the southeastern fold belt of the Bengal Basin, Bangladesh: a high-resolution sequence stratigraphic approach

The Tertiary basin-fill history of the Bengal Basin suffers from oversimplification. The interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo-tectonic provinces, namely, western, northeastern and eastern. Each province has its own basin generation and sediment-fill history related mainly to the Indo-Burmese and subordinately to the Indo-Tibetan plate convergence. This paper is mainly concerned with facies and facies sequence analysis of the Neogene clastic succession within the subduction-related active margin setting (oblique convergence) in the southeastern fold belt of the Bengal Basin. Detailed fieldwork was carried out in the Sitapahar anticline of the Rangamati area and the Mirinja anticline of the Lama area. The study shows that the exposed Neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental–fluvial environments. Based on regionally correlatable erosion surfaces the entire succession (3000+ m thick) has been grouped into three composite sequences C, B and A, from oldest to youngest. Composite sequence C begins with deep-water base-of-slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics. Composite sequence B characteristically depicts tide-dominated open-marine to coastal depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits is separated by shelfal mudstone. Most of the sandbodies contain a full spectrum of tide-generated structures (e.g. herringbone cross-bedding, bundle structure, mud couplet, bipolar cross-lamination with reactivation surfaces, ‘tidal’ bedding). Storm activities appear to have played a subordinate role in the mid and inner shelf region. Rizocorallium, Rosselia, Planolites and Zoophycos are the dominant ichnofacies within the shelfal mudstone. This paralic sedimentation of Neogene succession in the study area can serve as a good point of reference for tide-dominated regressive shelf depositional systems. The top of the composite sequence B is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental–fluvial depositional systems represented by composite sequence A. In this composite sequence, stacked channel bars of low-sinuosity braided rivers gradually pass upsequence into high-sinuosity meandering river deposits. A sequence stratigraphic approach has been adopted to interpret the basin-fill history with respect to relative sea-level changes; and to subdivide the rock record into several sequences and units (systems tracts and parasequences) based on identified bounding discontinuities, such as transgressive erosion surface (TES), regressive erosion surface (RES), marine flooding surface (MFS), and incised valley floor (IVF). This approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the Bengal Basin. It is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin, which is based on the Assam stratigraphy, be abandoned or at least revised. A tentative allostratigraphic scheme is presented, and it is suggested that to formalize the scheme further study, both surface and subsurface, is needed.

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

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

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.

Sequence stratigraphy of the Precambrian Rooihoogte–Timeball Hill rift succession, Transvaal Basin, South Africa

Third-order sequence stratigraphic analysis is performed on the Rooihoogte–Timeball Hill second-order rift succession of the Paleoproterozoic Transvaal Basin, South Africa. This provides a case study for systems tract and sequence development during a time of glacio-eustatic fall, when accommodation was generated by subsidence related to syn-rift and post-rift tectonic processes. Two third-order depositional sequences have been identified, separated by a basin-wide subaerial unconformity. The lower third-order sequence includes the complete succession of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST), whereas the upper third-order sequence only preserves lowstand and transgressive systems tracts. This indicates that the fall in base level associated with the upper second-order boundary of the Rooihoogte–Timeball Hill sequence was of higher magnitude relative to the third-order subaerial unconformity, which is in agreement with the principles of boundary hierarchy based on the magnitude of base-level changes. The position of the lower boundary of the Rooihoogte–Timeball Hill second-order sequence has been revised from the base of the chert breccias to the contact between the breccias and the overlying chert conglomerates. This is because a major tilting event occurred between the deposition of the two facies, which are genetically unrelated, and which are separated by a subaerial unconformity. The lithostratigraphic contact between the Rooihoogte and Timeball Hill formations is interpreted as a diachronous transgressive surface of erosion. In this interpretation, the Polo Ground Member of the Rooihoogte Formation may be coeval with the basal black shales of the Timeball Hill Formation, the two facies (fluvial and marine, respectively) forming together a transgressive systems tract.

Origin, genetic interrelationships, and stratigraphy over the continuum of fluvial channel-form bounding surfaces: an illustration from middle Cretaceous strata, southeastern Colorado

Abstract A hierarchy of channel-form bounding surfaces generally partitions fluvial strata, each reflecting river scouring processes at a different scale. Low-order (e.g. channel scours, channel-belt boundaries, etc.) and high-order (i.e. valley-form sequence boundaries) surfaces are discussed extensively in the literature. The intermediate-scale boundaries, however, are largely overlooked. An understanding of the range of processes generating these channel-form surfaces is pivotal for interpreting fluvial architecture and understanding sequence-boundary genesis. This paper examines origin, interrelationships, and significance for the full range of channel-form bounding surfaces (channel-fill to sequence boundary) in middle Cretaceous strata of southeastern Colorado, and assesses the role of such surfaces in sequence and allostratigraphy. Middle Cretaceous strata of the Muddy Sandstone were photographed over the entire length (14 km) of the Huerfano Canyon. Lithofacies were then mapped in detail for all Muddy deposits, and bounding surfaces were identified by architectural-element analysis within Muddy fluvial strata. Two alloformations were established based on recognition of three continuous discontinuities. The Cucharas Canyon Alloformation is floored by one sequence-bounding unconformity that is smooth (surface SB3), and is capped by surface SB4. It is mostly a basal sandstone cliff composed of fluvial lithofacies assemblages, but is characterized commonly by paralic lithofacies assemblages in its upper part. The Huerfano Canyon Alloformation is floored by a locally incised sequence boundary SB4, and is capped by a smooth transgressive surface of erosion. It comprises cliff-forming fluvial lithofacies assemblages. Both alloformations reveal six orders of channel-form bounding surfaces. Each order is bundled by surfaces of the succeedingly higher orders; thus, the surfaces can be considered to fit in a hierarchical or ‘nested’ progression. The surfaces record, in order of ascending rank: nested-channel cuts, channel scours, channel-belt boundaries, nested-valley boundaries, valley-fill boundaries, and sequence-bounding unconformities. Though generated by discrete processes, each of these surfaces is formed as a composite of the lower-order surfaces. These surfaces are thus genetically interrelated into a bounding-surface continuum. This continuum is best constrained by allostratigraphic techniques, with sequence stratigraphy and architectural-element analysis treated as approaches under the auspice of allostratigraphy. Middle-Cretaceous strata reveal a close interrelationship between sequence-boundary morphology and architecture of overlying fluvial deposits. Valley-form sequence boundaries tend to record composite surfaces formed of multiple nested-valley scours. Smooth sequence boundaries tend to reflect composites of multilateral channel-fill elements or valley-fills. These observations promote a set of predictive relationships between sequence boundaries and overlying fluvial sandstone that are here categorized into four sequence architectural styles with valley-form boundaries (simple valley, complex valley, compound valley, and compound-complex valley) and four with smooth boundaries (channel sheet, stacked channel sheet, multivalley, and stacked multivalley). Valley and nested-valley incisional boundaries in middle Cretaceous alloformations most likely result from climatic forcing, and do not necessarily record regional surfaces. Such surfaces are abundant in modern deposits, and should be far more common in ancient strata than previously reported. Because of their potential impact on results, they need to be considered as a factor when estimating ratios of accommodation space versus sediment supply, evaluating regional permeability trends, and assessing up-dip fate of sequence-bounding unconformities.

ABSTRACT: Improved Sequence Stratigraphic and Reservoir Characterization Models: Mesaverde Group, Southwestern, Wyoming

Abstract The combination of excellent outcrops and subsurface data sets makes the Rocky Mountain region a great place to do predictive stratigraphy and reservoir characterization. Several years ago, five behind outcrop core wells were drilled and logged in the Cretaceous Mesaverde Group, Rock Springs Uplift, Wyoming. The Mesaverde is a 2nd order regressive-transgressive clastic wedge that is estimated to contain 52,000 TCF and 776 MMBNGL in a variety of alluvial to open marine reservoirs. Data from these wells and surrounding outcrops, combined with regional studies, have resulted in significant updating of the depositional and reservoir-geological models for this succession. Our studies document numerous 3rd and 4th-order sequence bounding unconformities and transgressive erosion surfaces that occur in both overall progradational and overall retrogradational successions. Synsedimentary faulting and lateral variations in subsidence, in conjunction with the presence of high-frequency unconformities, have resulted in a complex stratigraphy on basin to reservoir scales. Incised valleys have been identified in three of the four Mesaverde formations, including the overall transgressive Almond Formation. Reservoir architecture of the fluvio-estuarine Ericson Sandstone is strongly tied to changes in accommodation / sediment supply that can be linked to repeated transits of shoreline transgression and regression. Because of the ramp setting, shoreline dislocations associated with unconformities can be significant. The Blair Formation, previously interpreted as a slope and basin floor fan deposit, is re-interpreted as a lowstand delta. The sequence stratigraphic and reservoir models developed, have application not just to developing gas resources in the Mesaverde, but to improving our understanding of analogous reservoirs worldwide.