Fifth-order Sequences Research Articles

Orbital obliquity evolution during the late Paleozoic ice age across the northeastern gondwana: Implications for regional sea-level change trigger and reservoir quality assessment

Orbitally-induced cycles build stratigraphic sequences on time spans ranging from several thousand to several million years by altering depositional conditions. Previous works have shown that the gas-bearing Faraghan Formation (Cisuralian) has been impacted by orbitally-forced climatic change, resulting in the development of sedimentary sequences on astronomical cycle timescales. Obliquity evolution has been observed in the Faraghan Formation, with ∼1.2 Ma and ∼173 ka modulation cyclicities recorded. We attribute a strong sedimentary noise observed in the Faraghan Formation, which is distinct from the other noises, to a transregional sea-level rise, most likely correspond to MFS P10. The prolific gas zones (PGZ) in the Faraghan Formation are in consistent relationship with obliquity maxima and depositional noise events, which are associated with high obliquity power. This constant offers the hypothesis that obliquity altered the sedimentary regime in such a way that favorable conditions for reservoir quality development occurred. The ∼173 ka obliquity modulation cycle is recorded from the Late Paleozoic. It is postulated that this periodicity was the causal mechanism for the creating of the Faraghan Formation's fifth-order sequences. The reconstructed sea-level patterns from sedimentary noise modeling (DYNOT) confirm the three third-order sequences proposed for the Faraghan Formation by displaying three different sedimentary noises. The detected ∼1.2 Ma obliquity modulation cycles match the ∼1.2 Ma periodicities filtered from the DYNOT median values, indicating that the ∼1.2 Ma orbital cycle is one of the primary drivers of regional sea level. ∼1.2 Ma obliquity modulation cycles are correlated with third-order sequences, implying that the aforementioned cycle was one of the key mechanisms causing the creation of these third-order sequences.

Subtle reservoirs and implications for hydrocarbon exploration in terrestrial lacustrine fan-delta deposits: Insights from the Triassic Baikouquan Formation, Mahu Sag, Junggar Basin, western China

Petroleum reserves are abundant in the large-scale lacustrine fan delta reservoirs containing mixed oil in the Baikouquan Formation (MA131 Block, Junggar Basin). However, detecting such subtle reservoirs in the lacustrine foreland basins is challenging. High-resolution sequence stratigraphy, sedimentary basin evolution, and reservoir models are established by integrating the results obtained via wireline log data, cores, thin sections, and seismic data. A fifth-order sequence stratigraphic framework for the Baikouquan Formation is established, which enables the reconstruction of the sedimentary basin evolution and further detailing of the reservoir model, such that the correlation to multistage progradation and retrogradation of the fan delta complex can be determined. The study of core samples led to the identification of eight microfacies, of which sandy debris flows and distributary channels dominate approximately 80% of the depositional system. Stratal slices enable the characterization of the northern Xiazijie and eastern Madong fan-delta systems. The northern Xiazijie fan delta is widely distributed throughout sequences SSQ1–SSQ6, whereas the Xiazijie fan delta underwent, in general, gradual retrogradation during the formation of sequences SSQ6 and SSQ7. The eastern Madong gravity flow fan-delta system only developed during the formation of sequences SSQ1–SSQ5 at the location of an abrupt slope break. There are nine categories of reservoir models in the study area based on the source rocks, faults, and reservoirs. Geomorphology and climate are crucial in controlling the sedimentary infill. Deep faults and an unconformity between the Permian and the Triassic of the Mahu Sag played a decisive role in controlling the hydrocarbon migration pathways. The faults, source rocks, and microfacies together exert considerable control on hydrocarbon accumulation. Within the nearshore fan-delta front, variable reservoir quality can be observed. Crucial factors in improving reservoir quality are fine-grain size and relatively low clay content. This study may facilitate subtle reservoir characterization in other basins.

Depositional sequences of the Mekong river delta and adjacent shelf over the past 140 kyr, southern Vietnam

Two 40 m-long sediment cores BL01 and SC02 were drilled in the lower Mekong River Delta plain and compared to high-resolution seismic profiles from the adjacent shelf documenting the presence of two depositional sequences (SQ1, SQ2) for the past 140 kyr. SQ2 is a fourth-order sequence and covers a complete sea-level cycle of 120 kyr. SQ1 is a fifth-order sequence comprising a sea-level half cycle in the past 20 kyr. These two sequences are bounded by the sequence boundaries (SB1, SB2) and the topographic surface. The transgressive systems tract (TST2) of SQ2 was probably formed during the sea-level rise from the Marine Isotope Stage (MIS) 6 lowstand to the MIS 5e highstand, indicated by riverine-estuarine and shallow-marine sediments. The regressive systems tract (RST2) of SQ2 corresponds to the sea-level lowering from MIS 5e to 3 with deltaic and alluvial plain sediments prevailing. These sediments were then gradually exposed to tropical subaerial processes to generate the formation of patchy reddish-orange stiff sandy silt and clay. The overlaying sequence SQ1 contains the lowstand systems tract (LST1), TST1, and RST1. LST1 consists of deltaic sediments at the shelf edge deposited during the MIS 2 sea-level lowstand. TST1 includes retrograding-fluvial, estuarine, and shallow-marine sediments deposited during the deglacial sea-level rise 19–8 cal kyr BP and the maximum flooding period 8–6 cal kyr BP before the advance of the modern Mekong River Delta. RST1 comprises the modern delta-facies association formed during the past 8 kyr and is restricted to the water depth of 25 m.

Application of seismic sedimentology in lithostratigraphic trap exploration: A case study from Banqiao Sag, Bohai Bay Basin, China

A method of identifying lithostratigraphic traps based on seismic sedimentology is proposed. We first establish a 3D high-resolution (fifth-order) sequence stratigraphic framework by using the stratal slices. Then, the reservoir distribution and reservoir-seal assemblage are investigated within the high-resolution sequence framework. This method turns the interpretation of lithostratigraphic traps from traditional seismic facies-based approach to the dynamic analysis of high-resolution seismic geomorphic information. We divide the lower Sha-1 member in the Banqiao Sag, Bohai Bay Basin, East China, into fourth- and fifth-order sequences by applying our method. The fifth-order sequence corresponding to Sha1-2 shows that the fan delta-distal subaqueous fan depositional system can be observed at the bottom of Sha1-2. The bounding fault and paleogeomorphology control the deposition of sand, whereas the sand bodies in the fan delta and distal subaqueous fan are developed near the bounding fault and the open lake basin, respectively. We then predict the sand thickness based on the well logs and seismic amplitudes. Moreover, according to the structural features, at least four lithostratigraphic traps are identified. These traps form a good reservoir-seal assemblage with overlying mudstones deposited during the period of lacustrine level rising. The drilling results in Trap-3 suggest that, our method can be a method of choice for effectively identifying the lithostratigraphic traps, a significant measure for hydrocarbon exploration.

Facies and sequence analysis of Miocene open-shelf warm-temperate carbonates in Portimão (Lagos-Portimão Formation, Portugal)

The Lower–Middle Miocene Lagos-Portimao Formation at Portimao, southern Portugal, consists of three main heterozoan carbonate facies: mollusc-rich rudstone in a matrix of skeletal packstone/grainstone (MR), fine-to-medium skeletal packstone/grainstone (SPG), and quartz-rich skeletal packstone (QSP). These facies occur in different arrangements and indicate deposition on a wave-dominated open shelf with a well-established heterozoan fauna, most likely related to increased trophic resources where continent supply and upwelling contributed nutrients. These facies form belts inferred to have lain roughly parallel to the strike lines of the shelf, with the MR occupying the more landward, shallower areas and the SPG and QSP extending progressively offshore on the open shelf. Bryozoan abundance in the QSP facies indicates a deep setting on the middle/outer shelf, whereas the absence of rhodalgal components is attributed to a combination of temperature decrease and seafloor eutrophication during the late Langhian and Serravallian, coeval with the “Middle Miocene climatic cooling”. The large photosymbiont-bearing foraminifers and the absence of calcareous algae and zooxanthellate corals point to deposition under warm-temperate conditions. Two main orders of cyclicity are present. Small-scale sequences, 40–60 cm thick, may represent minor sea-level changes related to fifth-order sequences modulated by (~ 100-kyr) eccentricity forcing. Six large-scale sequences, 3–9 m in thickness, are recognized. Based on LAD and FAD of planktic foraminifers and the occurrence of two erosive surfaces related to major sea-level falls (Lang2/Ser1 and Ser4/Tor1), five Miocene sequences (Bu, L1, S1, S2, and S3) are correlated with third-order eustatic cycles.

Geologic characteristics of deep water deposits and exploration discoveries in slope zones of fault lake basin: A case study of Paleogene Shahejie Formation in Banqiao-Qibei slope, Qikou sag, Bohai Bay Basin

Abstract Based on seismic, logging, formation testing, core and lab test data, this study analyzed the sequence division, facies features of deep water deposits and modes, development of large-scale gravity flow, reservoir physical properties and their main controlling factors, and proposed a classification standard and prediction method of favorable exploration areas in deep water area of the Bin1 oil layers of the lower sub-member of the first member of Paleogene Shahejie Formation in Banqiao-Qibei slope zone of Qikou sag, Bohai Bay Basin. The Bin1 oil layers can be divided into three fifth-order sequences, each less than 100 m thick; a set of gravity flow deposits were formed under deep water background in the slope zone, which contains sedimentary micro-facies such as main channel, distributary channel, channel margin, inter-channel mudstone, and turbidite sand sheet in areas without channels, and, in space, has inherited and constructive development features of multistages. It is a sedimentary sequence of fan delta – distal subaqueous fan – deep lake, and every distal subaqueous fan formed by gravity flow can be divided into inner-, middle- and outer fans. The cross-facies transported sands which are sourced from higher-sand-content major sands of delta front can form high quality reservoirs with an average porosity of 15.1% and geometric average permeability of 5.1×10−3 μm2. The main channel and distributary channel of distal subaqueous fan are the most favorable exploration zones.

Seismic geomorphology and depositional system of delta and terminal fan: A case study of the Neogene Shawan Formation in the Chepaizi Uplift, Junggar Basin, China

The mainpurpose of this article is to demonstrate the utility of stratal slice images for exploring the sequence stratigraphy and sedimentology of complex depositional systems. A seismic sedimentological study was performed to map sediment dispersal characteristics of the Neogene Shawan Formation in the Chepaizi Uplift of the Junggar Basin, China. The Chepaizi Uplift is developed on the Carboniferous igneous rock basement that lies at the western boundary of the Junggar Basin. The data sources primarily include lithology, well-logging and seismic data. In the main target strata, the Neogene Shawan Formation can be divided into three fourth-order sequences (SQN1s1, SQN1s2, and SQN1s3), and the sequence SQN1s1 is subdivided into three fifth-order sequences (SQN1s11, SQN1s12, and SQN1s13). Based on the established fine-sequence stratigraphic framework, the sedimentary facies types have been identified, they are shallow braided-river deltas, fan deltas, littoral and sublittoral lakes, braided rivers, and terminal fans. Then, stratal slices have been used to clearly depict the boundaries of sedimentary facies. Accurate results have been obtained that characterize braided river channels, terminal fans, littoral and sublittoral lake beaches, and subaqueous distributary channels in the braided-river delta front. Additionally, this seismic sedimentology study reflects variations in source area and evolution history.

A new perspective on sequence stratigraphy of syn-orogenic basins: insights from the Tertiary Piedmont Basin (Italy) and implications for play concepts and reservoir heterogeneity

Abstract The Tertiary Piedmont Basin was reshaped as the Apennines progressively overrode the Alpine retroforeland. Nine basin-wide unconformities were followed by abrupt accommodation turnarounds or high-accommodation/high sediment supply intervals. Unusual stratigraphic discontinuities require a different perspective to reconcile allostratigraphy and sequence stratigraphy, highlighting a wider range of tectonically driven discontinuities (subaerial/subaqueous/regressive/transgressive), diachronism of accommodation and sediment–fairway turnarounds, unusual routing to deepwater, and relationships between fourth- and fifth-order sequences. This approach allows innovative play concepts and prediction of reservoir heterogeneity. Steep gradients and high sediment fluxes favour the development of coarse-grained deltas dominated by hyperpycnal processes and large-scale instability/sediment failures, often expressed by retrogressive slump scars, owing to over-steepening generated by drowning unconformities related to orogenic collapse or basin inversion. During increasing-accommodation intervals, subaqueous erosion leaves remnants of drowned deltas forming stratigraphic traps sealed by prodelta-slope muds. The sediments removed feed turbidites, generated during transgressive intervals and deposited at the toe of oversteepened slopes. These may eventually back-fill the slump scar system and show a fining-upward stacking-pattern owing to the re-establishment of the equilibrium profile. Another play concept relies on the combination of onlapping non-marine to marginal marine deposits against fault-bounded basement highs, sealed by marine muds deposited during the subsequent drowning.

High-frequency stacking pattern and stages of canyon/gully evolution across a forced regressive shelf-edge delta-front

The succession of varying facies tracts filling within a canyon, or a deep incised gully, cutting across the delta-front lobes at the shelf-edge requires to be explained by high-frequency allocyclic and/or autocyclic events. A ‘fifth- and sixth-order’ sequence-stratigraphic model for the shelf-edge canyon-fill or gully-fill is here proposed for the first time. The first Gelasian cold phase and the renewal of transpressional uplift of basin-margin occurring at the same time enforced regressive transit of the inner-shelf paleo-Orinoco delta onto the paleo-shelf-edge. Its delta-front developed along the shoulders of the major successive basin-bounding growth faults. The delivery of clastic sediments into the deeper slope and beyond in the Columbus Basin was through the gullies and canyons that incised and cannibalized across the shelf-edge delta. The Mayaro Formation of southeast Trinidad furnishes the role of growth faulting during the first phase of delta-front development at the paleo-shelf-edge. Thereby, it serves as an example of several ‘fifth-order sequences’, within a ‘fourth-order’ equivalent episode, which is contemporaneous to both the glacio-eustatic episode, the Pretiglian, and the renewal of transpressional uplift after a pause during the Pliocene. Thus, large incision and subsequent filling within the Mayaro Formation cover a passage of time (‘fifth-order’ equivalent) that is unequivocally shorter than the span of the falling Pretiglian eustatic sea-level and second renewed phase of transpression tectonism. The categorization of sedimentary facies of the canyon-fill or gully-fill and the encasing delta-front outside the canyon/gully cut into a series of ‘sixth-order’ genetic facies tracts shows variations of geomorphic stages of the canyon/gully evolution – from accommodation creation to the stages of filling. These stages, therefore, should be attributed to the ‘sixth-order’ equivalent autogenic changes influenced by high-frequency growth-tectonics, instead of assuming the stages to be related to the relatively lower-frequency glacio-eustatic allocyclicity of the earliest Gelasian.

蒙古国塔木察格盆地塔南凹陷下白垩统构造层序地层学的初步研究

基于钻井地质分层、地震反射界面波组特征并结合沉积学的成果，以构造层序地层学为指导，研究并划分了塔南凹陷下白垩统的地层层序。铜钵庙组是初始裂陷期的沉积，岩性上分为两段，对应2个三级层序SQ1和SQ2。南屯组是主裂陷期的记录，对应1个三级层序，可划分出3个四级层序(Sq31、Sq32和Sq33)，进一步划分出7个五级层序。Sq32的顶面是最大湖泛面，标志凹陷范围在此时达到最大，7个次级构造单元在南屯组沉积期形成。大磨拐河组是断坳转换期的沉积，对应1个三级层序SQ4，划分出2个四级层序。伊敏组是坳陷期的记录，对应的三级层序为SQ5。同生断层活动在层序发育中起了重要作用，按规模分为三级，按展布分为两套。铜钵庙组–南屯组沉积期北东走向的正断层活动，其上盘可容空间明显增大；大磨拐河组–伊敏组沉积早期北北东–近南北向的走滑(或张扭)断层活动。剖面上这两套断层以T22界面(南屯组顶面)为分界。 Based on the drilling data, seismic reflecting horizons, together with sedimentological achievements, in the light of the tectonic sequence stratigraphy, the sequences in the Lower Cretaceous of the Tanan Sag are studied and discriminated. The Tongbomiao Formation, as deposits of the initial rifting stage and being divided into two members lithologically, is correlated to two third-order sequences (SQ1 and SQ2). The Nantun Formation, record of the main rifting stage, is correlated to third-order sequence SQ3, in which three forth-order sequences (Sq31, Sq32 and Sq33), and furthermore, seven fifth-order sequences, are identified. The superface of Sq32 represents the largest lacustrine overflow, which marks blossom phase of the basin developing, and the seven suborder tectonic units formed during this period. The Damoguaihe Formation, as deposits of the stage from fault-depressing to down-warping, is correlated to third-order sequence SQ4, in which two forth- order sequences are identified. The Yimin Formation, record of down-warping stage, is correlated to third-order sequence SQ5. The contemporaneous faulting plays an important role in the sequence developing, which can be separated into three grades and two systems. The NE-orientated normal faulting activates during sedimentary period of the Tongbomiao and Nantun Formations, which results in the containing space enlarged on the upper side. The strike-slip (or transtensile) faulting, with NEN-striking or approximate longitude-striking, moves during sedimentary period of the Damoguaihe Formation and lower part of the Yimin Formation. Vertically, the both fault systems are bounded by the T22 seismic reflecting horizon (the top surface of Nantun Formation).

The Application of Seismic Sedimentology in Prediction of Channel Sand in Pu I Formation, Gaotaizi Oilfield

Pu I formation in Gaotaizi oilfield mainly developed delta distributary channels. The sand body is narrow and has a complex superimposition in vertical, which caused the sand body hard to forecast. To solve this problem the method of seismic sedimentology was applied to building high frequency stratigraphic framework and extracting high-precision stratal slice. The technique of 90-degree phasing conversion and multi-attribute comprehensive analysis to the channel sand was also performed. At last Pu I formation in Gaotaizi Oilfield is divided into fifteen fifth-order sequences. The attributes of Rms amplitude and Arc length are considered as the best attribute to reflect the characteristics of the channel sand in study area. By a multiple linear regression to the attributes, the prediction accuracy between wells is improved from 61% to 78%.

Evolution of sedimentary environments of the middle Jiangsu coast, South Yellow Sea since late MIS 3

An evolutionary model of sedimentary environments since late Marine Isotope Stage 3 (late MIS 3, i.e., ca. 39 cal ka BP) along the middle Jiangsu coast is presented based upon a reinterpretation of core 07SR01, new correlations between adjacent published cores, and shallow seismic profiles recovered in the Xiyang tidal channel and adjacent northern sea areas. Geomorphology, sedimentology, radiocarbon dating and seismic and sequence stratigraphy are combined to confirm that environmental changes since late MIS 3 in the study area were controlled primarily by sea-level fluctuations, sediment discharge of paleo-rivers into the South Yellow Sea (SYS), and minor tectonic subsidence, all of which impacted the progression of regional geomorphic and sedimentary environments (i.e., coastal barrier island, freshwater lacustrine swamp, river floodplain, coastal marsh, tidal sand ridge, and tidal channel). This resulted in the formation of a fifth-order sequence stratigraphy, comprised of the parasequence of the late stage of the last interstadial (Para-Sq2), including the highstand and forced regressive wedge system tracts (HST and FRWST), and the parasequence of the postglacial period (Para-Sq1), including the transgressive and highstand system tracts (TST and HST). The tidal sand ridges likely began to develop during the postglacial transgression as sea-level rise covered the middle Jiangsu coast at ca. 9.0 cal ka BP. These initially submerged tidal sand ridges were constantly migrating until the southward migration of the Yellow River mouth to the northern Jiangsu coast during AD 1128 to 1855. The paleo-Xiyang tidal channel that was determined by the paleo-tidal current field and significantly different from the modern one, was in existence during the Holocene transgressive maxima and lasted until AD 1128. Following the capture of the Huaihe River in AD 1128 by the Yellow River, the paleo-Xiyang tidal channel was infilled with a large amount of river-derived sediments from AD 1128 to 1855, causing the emergence of some of the previously submerged tidal sand ridges. From AD 1855 to the present, the infilled paleo-Xiyang tidal channel has undergone scouring, resulting in its modern form. The modern Xiyang tidal channel continues to widen and deepen, due both to strong tidal current scouring and anthropogenic activities.

Sedimentary Environment and Sequence Stratigraphy of Late Quaternary Deposits in the East China Sea

High-resolution (Chirp and Sparker system) seismic profiles were analyzed to investigate the sedimentary environment and sequence stratigraphy of the late Quaternary deposits in the East China Sea. Echo types in the study are divided into three major echo types which are classified as flat (transgressive sediment sheets, relict sands, and Holocene muds), mounded (tidal ridges), and irregular echoes (channels). Sequence analysis of high-resolution seismic profiles reveals that the shelf deposits form a succession of high-frequency (fifth-order) sequences consisting of three depositional sequences (DI, DII, and DIII) developed during the late Quaternary. The depositional sequence includes five sedimentary units, each with different seismic facies and geometry: (1) regressive estuarine/deltaic wedge (unit J1), (2) regressive deltaic/offshore deposits (unit J2), (3) low-stand deltaic wedge with incised-channel fill (unit J3), (4) transgressive estuarine/deltaic complex (unit J4-a) and sand ridges/sheet (unit J4-b), and (5) recent mud (unit J5). The results (correlation of echo types and depositional sequences, sedimentary features, and sequence stratigraphic interpretation etc.) of this study suggest that the late Quaternary deposits of the East China Sea were formed by complex sedimentary processes and systems under the influence of tidal action, sea-level change, and an enormous bulk of sediment inflow from adjacent rivers.

Sequence stratigraphy of Cenomanian–Turonian carbonate platform margins (Sarvak Formation) in the High Zagros, SW Iran: an outcrop reference model for the Arabian Plate

Abstract A high resolution sequence stratigraphic model has been constructed for the mid-Cretaceous Sarvak Formation (in the High Zagros region of SW Iran) which was deposited close to the eastern margin of the Arabian Plate. The exceptional outcrop quality, displaying the detailed facies patterns in the transition zone from carbonate platform to intra-shelf basin, offers the rare opportunity to distinguish between the relative control of carbonate sediment supply (S) and accommodation (A) on the depositional geometries of third- and fourth-order depositional sequences. Four third-order sequences have been distinguished in the Sarvak Formation, with a duration varying between 1.5 and 3 Ma, and a thickness of 50–150 m. These are in turn composed of fourth- and fifth-order sequences that form the stratigraphic building blocks of this carbonate system. A significant distinction has been made in the third-order sequences between the early transgression (e-TST) when the system was still flat, and corresponds to a ramp setting, and the late transgression (l-TST) when the carbonate platform to intra-shelf basin topography was created. The rate of accommodation creation is identified as the dominant factor controlling the morphology of the depositional profile, and, as such, the driving motor behind the dynamics of this type of carbonate system. The dip angle of the depositional profile has a major influence on: (1) the hydrodynamics of the system; (2) the type of carbonate sediment; and (3) the volume of carbonate sediment produced. A good correlation with the third-order sequences of the Natih Formation in Oman is demonstrated, which supports a dominant control by eustatic sea-level changes and a similar response of the carbonate system to changes in the rate of sea-level rise on the southern part of the Arabian Plate. This outcrop analogue can be considered as a good reference model for the Cenomanian–Turonian carbonate platform margins of the Arabian Plate, but also as a textbook example of the response of carbonate systems to sea-level fluctuations (relative influence of accommodation and sediment supply).

Mapping sediment-dispersal patterns and associated systems tracts in fourth- and fifth-order sequences using seismic sedimentology: Example from Corpus Christi Bay, Texas

A seismic-sedimentologic study was performed to map fourth- and fifth-order systems tracts in Oligocene (Frio) strata in Corpus Christi Bay, south Texas. Guided by third-order sequence-stratigraphic correlations from seismic and wire-line-log data, we prepared stratal slices from a three-dimensional seismic volume to reveal high-resolution (10-m [33-ft] levels) sediment dispersal patterns and associated systems tracts in a relative geologic-time domain. On average, 1200 m (3940 ft) of sediments were deposited in the third-order lowstand expansion cycle, and at least 16 higher order sequences (fourth- and fifth-order sequences) were recognized. Three types of depositional systems were identified in the Frio stratigraphic section: (1) offshelf lowstand slope fans that are best characterized by submarine channel and levee systems inside and outside incised submarine channels and by fan-shaped sand body geometry; (2) lowstand prograding deltaic systems that are composed of strike-oriented and lobate deltaic sand bodies; and (3) highstand systems that are represented by onshelf barrier, lagoon, and deltaic systems. Higher order sequence development was controlled by the interaction of relative sea level change, sediment supply, and gravity tectonics. The top of sediment ridges was eroded or decapitated during many of the higher order sequences. Sand dispersal patterns are primarily controlled by accommodation resulting from rollover topography associated with growth faulting. Between the boundary fault and the hinge line atop rollover structures, strike-oriented sandstone bodies dominate; within submarine channels and incised valleys and beyond the hinge line to the distal basin, dip-oriented sandstone bodies prevail. Sandstone thickness and dispersal patterns can be predicted by integrating wire-line-log measurements and seismic amplitude patterns.

Stratigraphic framework of gas accumulations in the Ría de Pontevedra (NW Spain)

Knowledge of marine geological environments in which shallow gas is accumulating is becoming increasingly important in global studies of climate change because a measurable proportion of the total methane source comes from continental margins. Previous studies have revealed that coastal environments represent important geological environments where microbial methane is being generated, is accumulating, and is being released. In the Ria de Pontevedra, at least 4.5 km2 of seafloor in the innermost part of the ria is underlain by sediments containing natural gas. Seismic interpretation contributes new findings for the definition of periods and geological environments in which the gas could have been generated, and is accumulating and released in the Ria de Pontevedra. Groundtruthing the seismic data (facies, environments) makes it possible to identify favourable geological environments for gas generation in the sedimentary infill of the Ria de Pontevedra. Sequence stratigraphy based on high-resolution seismic profiles and post-Last Glacial Maximum sea-level records makes it possible to establish the stratigraphic architecture of the ria and to define the periods in which gas could have been generated. The results of this study show that the sedimentary infill is composed of a fifth-order sequence developed since the Last Glacial Maximum. Within this sequence, gas appears to have accumulated in the Holocene deposits associated with the latest transgressive and highstand system tracts. Seismic analysis shows that gas could have been generated in different geological environments in the Ria de Pontevedra. If coastal environments at times of lower sea level were similar to those of the present, organic-rich mud deposits (deposited mainly in lakes, estuaries and floodplains) could have survived transgression and remained buried as potential gas sources in the inner part of the ria.

Stratigraphic Evolution of Fine-Grained Submarine Fan Systems, Tanqua Depocenter, Karoo Basin, South Africa

Abstract The integration of correlated outcrop and newly acquired core and wireline logs, extensive paleocurrent data, and accurately mapped surfaces has enabled a common model of the stratigraphic evolution to be developed of four Permian fine-grained submarine fan systems (Fans 1–4) from the Tanqua depocenter, SW Karoo Basin, South Africa. Additionally, this data has revealed the influence of subtle seabed topography on the fan systems' boundaries, internal facies architecture, and paleocurrent directions. Furthermore, the internal stratigraphy of individual fan systems are now known to be more complex than first believed. Mapping high-frequency intrafan units reveals a progradational–aggradational–retrogradational stacking pattern common to each submarine fan, which allows the geographic and stratigraphic distribution of lithofacies and architectural elements to be predicted. Basinward of the main feeder system, fan axes are dominated architecturally by sheet turbidites with discrete zones of high amalgamation during progradational and aggradational phases, whereas isolated channel forms that cut thin-bedded turbidites are more common in the retrogradational phase. This change is interpreted to be due to local increase in the lower slope gradient through sediment aggradation increasing the potential for channel avulsion and the development of splay lobes during decreasing sediment supply. The progradational, aggradational, and retrogradational phases are assigned to the early, middle, and late lowstand systems tract of a fifth-order sequence respectively. Each phase is built of higher-frequency (sixth-order) sequences so that each fan is a composite sequence. The overall basinward-to-landward stepping of the individual fans means that moving down dip in any basin-floor fan system, the lowermost sandstone preserved is progressively younger, whilst the uppermost sandstone is progressively older. This stacking pattern imparts an important predictable control on reservoir and seal geometries. This study aids the development of predictive models for fine-grained submarine fan initiation, growth, and abandonment, and lithofacies and architectural element distributions in space and time.

The cyclic hierarchy of the ‘Purbeckian’ Sierra del Pozo Section, Lower Cretaceous (Berriasian), southern Spain

The lowermost Cretaceous (Berriasian) Sierra del Pozo Formation is divisible into metre-scale cycles that are bundled in a four-tiered hierarchy of cycles and cyclic sequences. At the smallest scale, sixth-order cycles, thought to be the product of precessionally forced sea-level fluctuations (c. 20 ka), average less than a metre in thickness, shallow upward and are bounded by surfaces where deeper facies abruptly overlie shallower facies. Bundles of sixth-order cycles, called fifth-order sequences, are recognized by two types of asymmetric patterns in facies distribution. First, more pronounced facies changes occur at sixth-order cycle boundaries lower in a sequence, whereas smaller facies changes occur at cycle boundaries higher in a sequence. Secondly, subtidal marine limestones (i.e. biomicrites and biosparites) are the dominant facies lower in a cyclic sequence, whereas more restricted or shallower water facies (i.e. mud-cracked microbial laminites, dolomites, shale and clay) are more predominant higher in a cyclic sequence. The bundling of sixth-order cycles is explained as a product of periodic change (100 ka) in the eccentricity of the earth’s orbit. The degree of orbital eccentricity modulates the magnitude of precessional sea-level fluctuations, which in turn determine the relative facies contrasts at sixth-order cycle boundaries. Larger scale fourth- and third-order sequences are defined by similar patterns in facies contrast at successive sequence boundaries and by a change in predominant facies type from bottom to top. These patterns are explained as the product of 400 ka and 2 Ma periodic variations in the eccentricity of the earth’s orbit. In summary, the strength of the precessional signal varies in consort with periodic changes in orbital eccentricity at three time scales producing a stacked hierarchy of cycles and sequences of cycles. Change in obliquity of the earth’s axis may modulate the effects of the precessional signal and thereby modify patterns of preserved cyclic structure. This interpretation of cyclicity in the Sierra del Pozo section is markedly different from that of

Depositional sequence architecture and filling response model of the Cretaceous in the Kuqa depression, the Tarim basin

The Cretaceous system of the Kuqa depression is a regional scale (second order) depositional sequence defined by parallel unconformities or minor angular unconformities. It can be divided into four third-order sequence sets, eleven third-order sequences and tens of fourth- and fifth-order sequences. It consists generally of a regional depositional cycle from transgression to regression and is composed of three sets of facies associations: alluvial-fluvial, braided river-deltaic and lacustrine-deltaic facies associations. They represent the lowstand, transgressive and highstand facies tracts within the second-order sequence. The tectonic subsidence curve reconstructed by backstripping technique revealed that the Cretaceous Kuqa depression underwent a subsidence history from early accelerated subsidence, middle rapid subsidence and final slower subsidence phases during the Cretaceous time, with the correspondent tectonic subsidence rates being 30—35 m/Ma, 40—45 m/Ma and 5—10 m/Ma obtained from northern foredeep. This is likely attributed to the foreland dynamic process from early thrust flexural subsidence to late stress relaxation and erosion rebound uplift. The entire sedimentary history and the development of the three facies tracts are a response to the basin subsidence process. The slower subsidence foreland gentle slope was a favorable setting for the formation of braided fluvial deltaic systems during the late period of the Cretaceous, which comprise the important sandstone reservoirs in the depression. Sediment records of impermanent marine transgression were discovered in the Cretaceous and the major marine horizons are correctable to the highstands of the global sea level during the period.

Seismic stratigraphy of the Gulf of Cádiz continental shelf: a model for Late Quaternary very high-resolution sequence stratigraphy and response to sea-level fall

Abstract Single-channel, very high-resolution seismic profiles allow detailed study of the Late Quaternary stratigraphic architecture of the Gulf of Cádiz continental margin, Southern Spain. The Late Quaternary stratigraphy of this area comprises fourth-order Type 1 composite depositional sequences, generated by asymmetric relative sea-level changes of 100–110 ka duration. The composite fourth-order sequences consist of forced regressive, lowstand, transgressive and highstand systems tracts. Volumetrically, the forced regressive and lowstand systems tracts are the most important components. The fourth-order composite sequences are themselves comprised of composite fifth-order sequences formed in response to asymmetric relative sea-level changes with a duration of 22–23 ka. Sediments within the forced regressive and lowstand systems tracts dominate the 5th-order sequences; their transgressive and highstand deposits are either (i) perched above present-day sea-level and so not recorded in marine seismic data, (ii) restricted to outer-mid-shelf positions, or (iii) may be absent from the shelf altogether at the resolution of this study (e.g.<0.5 m thick). The fifth-order sea-level falls were themselves modulated by minor cycles, generating very high-frequency (sixth-order) sequences. These very high-order sequences are recognized for the last 80 ka bp , and their development is attributed to asymmetric relative sea-level cycles operating on time scales of: 10–15 ka (Heinrich events), 4–4.5 ka. (P cycles), 2.3–0.97 ka. (Dansgaard-Oeschger oscillations h cycles) and 500–50 a. (c cycles). We have developed a depositional model that accounts for the very high-frequency hierarchy of Late Quaternary depositional sequences observed in the Gulf of Cadiz marine seismic record and incorporates the age of well-constrained highstand coastal deposits that are exposed along the southern Iberian coastline. The model developed serves to illustrate the evolution and importance of depositional systems during falling relative sea-level and forced regression. Development of the forced regressive systems tract appears to be particularly significant within Quaternary strata. This is because the Quaternary was strongly influenced by a high-amplitude, high-frequency glacioeustatic signal characterized by rapid sea-level rises, very short highstands, and gradual relatively long-term sea-level falls suggesting that forced regressive deposits are likely to predominate in continental margin successions subject to low rates of subsidence.