Progradational Packages Research Articles

Bio-electrosequence interpretation of late cretaceous sediments of the Southern Bornu Basin, Nigeria

Integrated analysis that involves physical sedimentological, standard palynological and electrofacies analyses on ditch cuttings and suite of wireline logs from Gaibu–1 Well, southern Bornu were examined to identify critical sequence elements and construct a bio-sequence stratigraphical framework. Four (4) palynozones consisting of Triorites africaensis, Cretacaeiporites scabratus - Odontochitina costata, Droseridites senonicus and Syncolporites/Milfordia spp Assemblage Zones construed to be Late Cretaceous – younger successions. Nine (9) depositional sequences each with candidate maximum flooding surfaces (375, 900, 1875, 2250, 2600, 3050, 3400, 3800, 4300 m) marked by marker shales with high abundance and diversity of palynomorphs. Thus, equate with the local lithostratigraphy and global large-scale depositional cycles with candidate sequence boundaries (50, 725, 1625, 2175, 2490, 2850, 3300, 3610, 3960, 4470 m) ranging about 96.28 to 70.07 Ma. The delineated transgressive surfaces along the built sequences mark the subjected onset of marine flooding characterised with interchange of progradational to retrogradational facies. Delineated sequence elements generally show up-hole from progradational to retrogradational and aggradational that represents Lowstand Systems Tracts (LSTs), Transgressive Systems Tracts (TSTs) and Highstand Systems Tracts (HSTs) respectively. The LSTs are seen in form of prograding complex and slope fans, suggestive of good reservoirs. The TSTs consist of channel sand units and shales that depict retrogradational marine units, which could serve as both seals and source rocks for the sand units. The HSTs are made up of interplay of aggradational to progradational sediment packages that could serve as a potential source rock. The palaeoenvironmental indices depict the successions are deposited within continental to open marine settings. 

Factors controlling sedimentation and sequence stratigraphy evolution in shallow marine (carbonates) platform: example of Middle Eocene deposits from Gafsa Basin

Based on their facies characteristics and stratal geometries, the carbonate deposits of the Middle Eocene outcropping in the Tamerza area (Western Gafsa Basin) led to the recognition of nine facies that were deposited in shallow marine environment summarized in a carbonate ramp model: homoclinal ramp under an arid climate. The depiction of facies features and the sequential analysis help to determine the depositional environments and establish the linkage between facies evolution, eustasy, climate and tectonics. The vertical evolution of facies shows a regressive trend that records a transition from infratidal to supratidal environments. The stacking pattern revealed several kinds of meter-scale, shallowing-upward cycles. Each cycle starts with relatively open-marine deposits, which are overlain by shallower and more restricted facies (tidal flat progradation). The varied facies, which are described within the Kef Eddour Formation, compose seven depositional sequences, mainly made of carbonates, marls and phosphates. Fauna associations and sedimentary structures imply a shallow subtidal to intertidal, moderately to high-energy platform. Seven sequences are interpreted as depositional sequences showing retrogradational (lowstand systems tract), aggradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The seven depositional sequences which show a hierarchical organization of many cycles, as described above, suggested that eustatic sea-level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Facies analysis and depositional environments of the Miocene syn-rift carbonate–siliciclastic rock packages in the northwest Gulf of Suez, Egypt

The Miocene sedimentary successions in the northwest Gulf of Suez allow insight into the syn-rift deposition. These rocks are subdivided into three formations, namely the Sadat Formation coeval with the Gharra Formation (Early Miocene), the Hommath Formation (Middle Miocene), and the Hagul Formation (Late Miocene). The Miocene rocks in the study area are dominated by carbonate-siliciclastic packages. Detailed microfacies analysis permit discrimination of 14 sedimentary microfacies types, which are represented by continental quartz-arenites and sandy siltstones, as well as shallow-marine gypsiferous laminated shales, litharenites, dolomicrites, wackestones, wacke-/packstones, packestones, pack-/grainstones, ooidal/pelloidal pack-/grainstones, grainstones, rudstones, framestones, and bindstones. These microfacies types are stacked into four facies associations that represent fluvial, and inner ramp, shallow marine environments (facies associations for the later are restricted lagoonal/tidal flat, shoal bar, and open-lagoonal/reefal). In terms of sequence stratigraphy, it is possible to distinguish five depositional sequences in the studied Miocene succession. The retrograditional package displays predominance of open lagoon/reefal facies. The aggradational package shows increase in restricted lagoon tidal flat and shoal bar facies. The progradational package marks occurrence of continental (fluvial) facies. The regional tectonic activity related to the rifting of the Gulf of Suez was the leading control of the Miocene sedimentation in the study area.

Sequence stratigraphic interpretation of parts of Anambra Basin, Nigeria using geophysical well logs and biostratigraphic data

The Anambra basin constitutes the southeastern lower portion of the Benue Trough, which is a large structural depression that is divided into lower, middle and upper parts; and is one of the least studied inland sedimentary basins in Nigeria. Sequence stratigraphic interpretation had been carried out in parts of the Anambra Basin using data from three wells (Alo-1 Igbariam-1 and Ajire-1). Geophysical well logs and biostratigraphic data were integrated in order to identify key bounding surfaces, subdivide the sediment packages, correlate sand continuity and interpret the environment of deposition in the fields. Biostratigraphic interpretation, using foraminifera and plankton population and diversity, reveals five maximum flooding surfaces (MFS) in the fields. Five sequence boundaries (SB) were also identified using the well log analysis. Four 3rd order genetic sequences bounded by maximum flooding surfaces (MFS-1 to MFS-6) were identified in the areas; four complete sequences and one incomplete sequence were identified in both Alo-1 and Igbariam-1 wells while Ajire-1 has an no complete sequence. The identified system tracts delineated comprises Lowstand Systems Tracts (progradational to aggradational to retrogradational packages), Transgressive Systems Tracts (retrogradational packages) and Highstand Systems Tracts (aggradational to progradational packages) in each well. The sand continuity across the fields reveal sands S1 to S5 where S1 is present in Ajire-1 well and Igbariam-1 well but not in Alo-1 well. The sands S4 to S5 run across the three fields at different depths. The formations penetrated by the wells starting from the base are; Nkporo Formation (Campanian), Mamu Formation (Late Campanian to Early Maastrichtian), Ajali Sandstone (Maastrichtian), Nsukka Formation (Late Maastrichtian to Early Palaeocene), Imo Formation (Palaeocene) and Nanka Sand (Eocene). The environments of deposition revealed are from coastal to bathyal. The sands of lowstand system tract and highstand system tract found in Ajali, Nsukka, Nkporo and Imo (Ebenebe Sandstone) Formations show good continuity and as such good reservoir qualities while the shales of the transgressive system tracts which includes the Imo Formation, Mamu, and Nkporo Formations where most of the maximum flooding surfaces were delineated, can serve as seals to the numerous reservoir units. Combinations of the reservoir sands of the lowstand system tract and highstand system tract and the shale units of the transgressive system tract can form good stratigraphic traps for hydrocarbon and hence should be hydrocarbon exploration targets.

Glacial and submarine processes on the shelf margin of the Disko Bay Trough Mouth Fan

Fast-flowing ice streams and outlet glaciers exert a major control on glacial discharge from contemporary and past ice sheets. Improving our understanding of the extent and dynamic behaviour of palaeo-ice streams is crucial for predictions of how the cryosphere will respond to climate warming and the associated implications for global sea level. This paper presents results from two 3D-seismic surveys located on the continental shelf adjoining the Disko Bay Trough Mouth Fan (TMF), one of the largest glacial outlet systems in Greenland. Located at the seaward terminus of the c. 370km long cross-shelf Disko Trough, the Disko Bay TMF was generated by highly efficient subglacial sediment delivery onto the continental slope during repeated ice stream advances. A variety of submarine glacial landform assemblages are recognised on the seabed reflecting past ice stream activity. The 3D-seismic study covers the shallow banks located north and south of the Disko Trough and sheds focus on the seabed and the uppermost stratigraphic interval associated with the late Pleistocene development. The buried section (probably of Saalian age) contains a prominent grounding-zone wedge (GZW) in the northern and low-angle progradational packages in the southern area, indicating a period of major glacial advances to the shelf margin. Subsequently, the outer margin was influenced by glacimarine sedimentation, localized shelf-edge ice advances and sediment transport by contour currents that possibly began in the last interglacial period (Eemian). During the last (de)glaciation, the northern bank appears to have been covered by passive ice leaving a field of dead-ice deposits. In contrast, multiple sets of terminal moraine ridges observed on the southern bank suggest a slow retreat of active, grounded ice from the Last Glacial Maximum (LGM) position on the outer shelf. Isostatic and tectonic influences on relative sea level may have played a role in generating the divergent glacial configurations of the northern and southern bank areas.

Stratigraphy, facies analysis and sequence stratigraphy of the Eocene succession in the Shabrawet area (north Eastern Desert, Egypt): an example for a tectonically influenced inner ramp carbonate platform

The Eocene rocks in the Shabrawet area (north Eastern Desert, Egypt) have been subdivided into three formations that are from base to top: the Minia Formation (Late Ypresian), the Sannor Formation (Bartonian) and the Maadi Formation (Priabonian). These three formations are disconformable with each other. In addition, the whole Eocene succession is disconformably overlain by the Oligocene rocks and is underlain by the Cretaceous rocks with an angular unconformity surface. The Sannor Formation has been subdivided into three informal units: lower, middle and upper units. The overall sedimentary nature of the Eocene rocks in the study area is dominated by carbonate-siliciclastic rocks. Detailed microfacies analysis has enabled discrimination of 22 sedimentary microfacies types, which are grouped into six facies associations, equivalent to six depositional environments sited on an inner ramp setting. These depositional environments are floodplain-dominated fluvial, lacustrine/palustrine, tidal flat, restricted inner lagoon, shoal bar and outer lagoon with open circulation. A suitable depositional model of the Eocene rocks is given. In addition, four discrete benthonic foraminiferal biozones were reported in the Eocene succession, arranged from base to top: (1) Alveolina frumentiformis biozone (Late Ypresian), (2) Orthoplecta clavata biozone (Bartonian), (3) Dictyoconus aegyptiensis biozone (Bartonian) and (4) Discorbis vesicularis biozone (Priabonian). In terms of sequence stratigraphy, the studied Eocene succession exhibits three superimposed depositional sequences, each of which shows retrogradational (transgressive systems tract) and aggradational (highstand systems tract) to progradational (lowstand systems tract) packages of facies. The retrogradational package displays a predominance of outer lagoon and restricted inner lagoon facies. The aggradational package shows an increase of shoal bar and tidal flat facies, whereas the progradational package marks the occurrence of continental facies (floodplain-dominated fluvial and lacustrine deposits). In this study, it is noticed that there are lateral and vertical changes in the depositional environments of the Eocene rocks, between shallow marine and continental environments. Also, many unconformities have been recorded in between and within these rock units. Such remarks indicate that the Eocene rocks were deposited within a highly tectonically active area that was consequently influenced by the transgression and regression of the Neo-Tethys. Such tectonic activity could be related to the Syrian Arc System that was renewed and enlarged several times during the Late Cretaceous up to the Neogene in the eastern and southeastern Mediterranean domain.

Beach dynamics and oscillations of shoreline position in recent years at Miramar Beach, Goa, India: a study from a GPR survey

Consideration of human influences is crucial to understanding the coastal sediment supply and associated shoreline responses prior to undertaking coastal hazard management studies. Observation of the widening of some selected Indian beaches, especially over the last 6 decades, is of significance. From this perspective, Miramar Beach, Goa, India, was studied using three ground-penetrating radar shallow subsurface profiles (4 m depth). Based on a series of depositional siliciclastic packages, six progradational packages were recognised, which were interrupted by sharp erosional boundaries. These erosional boundaries represent transgressive phases of the shoreline migration. It was observed that the shoreline migration is coupled with the deposition and erosion of sediments, and this is supported by the historical admiralty charts. The optically simulated luminescence dating of the sediments collected at the first progradation period reveals that the age corresponds to the years 1952–1957, which also corroborates the information provided by the local populace. In the past 6 decades, the shoreline growth has been rapid because of the heavy sediment influx from the Mandovi River caused by increased mining activities (since the 1950s) in upstream areas. Since the 1950s, the shoreline has prograded rapidly, building a beach from ~40 to ~280 m wide (average rate of 4 m/year) in response to enhanced sediment supply from the Mandovi River created by mining activities upstream. Superimposed on this overall regressive trend is a series of deposition and erosion cycles. Perhaps, if a similar trend continues, then there will possibly be a further widening of the beach in the future. A close monitoring network is needed to understand the causes of the cycles in shoreline position and to predict their future behaviour. The present investigation on the nature of the coastal response to anthropogenic activities in a river basin as well as the role of short-time cycles on shoreline behaviour in the last 6 decades could be an ideal reference study and motivate the search for similar areas along other coastal locations.

Facies architecture, depositional environments, and sequence stratigraphy of the Middle Cambrian Fasham and Deh-Sufiyan Formations in the central Alborz, Iran

Based on their lithologic characteristics and stratal geometries, the Middle Cambrian Fasham and Deh-Sufiyan Formations of the lower Mila Group in the Central Alborz, northern Iran, exhibit 39 lithofacies representing several supratidal to deep subtidal facies belts. The siliciclastic successions of the Fasham Formation are divided into two facies associations, suggesting deposition in a tide-dominated, open-mouthed estuarine setting. The mixed, predominantly carbonate successions of the Deh-Sufiyan Formation are grouped into ten facies associations. Four depositional zones are recognized on the Deh-Sufiyan ramp: basinal, outer ramp (deep subtidal associations), mid ramp (shallow subtidal to lower intertidal associations), and inner ramp (shoal and upper intertidal to supratidal associations). These facies associations are arranged in small-scale sedimentary cycles, i.e., peritidal, shallow subtidal, and deep subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. Systematic changes in stacking pattern (cycle thickness, cycle type, and facies proportion) allow to reconstruct long-term changes in sea-level. Six large-scale cycles (S1–S6) have been identified and are interpreted as depositional sequences showing retrogradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The six depositional sequences provide the basis for inter-regional sequence stratigraphic correlations and have been controlled by eustatic sea-level changes.

The changing architecture of sea-level lowstand deposits across the Mid-Pleistocene Transition: South Evoikos Gulf, Greece

On subsiding continental shelves, the style of stacked coastal and deltaic progradational packages is directly dependent on relative sea-level changes. In the past ∼0.6 Ma, sea-level change has been dominated by asymmetric 100 ka eustatic sea-level cycles, whereas the record of sea-level changes in earlier Pleistocene progradational sequences is less clear. In a steadily subsiding basin in which accommodation balances sediment flux, the depth of a eustatic lowstand determines the paleo-depth of the deepest clinoform inflection point and the seaward limit of the erosional transgressive surface, whereas the duration of a lowstand controls the amount of progradation that takes place. We report high-resolution seismic profiles of an exceptionally preserved coastal progradational sequence from a coastal embayment in the Aegean Sea that is subsiding at ∼100 m/Ma. The seismic profiles show clinoforms of smaller amplitude and volume that were deposited before the 100 ka cyclic progradational units. This contrasts with literature reports of complexity in progradational sequences at that time. We assume that published stacked benthic foram O-isotope records are a good proxy for the duration and a reasonable proxy for the amplitude of Pleistocene eustatic sea-level cycles. The MIS 6–5 eustatic sea-level rise is recognised based on sedimentation rates from cores. The underlying major progradational units are correlated with the long-duration, extreme lowstand 100 ka cycles of MIS 6, 12 and 16. Changes in the elevation of lowstand inflection points in clinoforms are used to tentatively extend the record back to MIS 38, by comparison with the benthic foram proxy. The deposits of the higher and longer highstands of MIS 25, 31 and 37 are also recognised. This record provides a testable template for future studies of short early Pleistocene sections on land and raises questions of detail about parts of the published δ18O records. It independently supports the recent evidence that the Mid-Pleistocene transition was an abrupt event, with an extreme lowstand in MIS 22.

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.

Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems

In offshore East Kalimantan, Indonesia, three-dimensional seismic reflectors can be traced downslope from a lowstand delta to a basin-floor fan, giving insight into depositional processes controlling the distribution of sands that serve as hydrocarbon reservoirs in many ancient deep-water settings. The studied interval includes the last three Pleistocene cycles (10–330 ka; each 110 k.y. in duration). Cycles on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. Progradational packages are separated by parallel reflectors and carbonate buildups of the transgressive systems tracts. During the last two lowstands of sea level (18 and 130 ka), coarse clastics were not deposited in deep-water environments because growth faults and regional subsidence prevented lowstand deltas from reaching the slope. During the lowstand of sea level that ended at about 240 ka, a delta prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.Strata on the slope and basin floor show how a deep-water depositional system evolved during a single cycle of eustatic sea level. A canyon on the slope connects the 240-ka lowstand delta to a coeval basin-floor fan. The canyon has a sinuous, bipartite fill that consists of a lower, amalgamated channel complex and an upper channel-levee complex. The basin-floor fan formed at the toe of the slope also has two parts. The stratigraphically lower part of the basin-floor fan has broad lobes with relatively continuous reflectors. The stratigraphically higher part has a sinuous channel-levee complex that prograded over the lower fan and fed sheetlike lobes on the outermost fan. The amalgamated channel fills on the slope and sheetlike lobes on the basin-floor fan have moderate- to high-amplitude reflectors and are inferred to represent sand-rich, early lowstand deposits. The channel-levee complexes on the slope and basin floor are dominated by low-amplitude reflectors and are inferred to be mud-rich strata deposited during the late lowstand. Unlike classic sequence-stratigraphic models, these lowstand strata do not onlap the slope; instead, deep-water clastics extend from the last clinoforms of lowstand deltas. In this system, lowstand deltas determined when and where sand-rich sediments entered preexisting canyons on the slope to feed basin-floor fans.

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.

New Exploration Concepts for the Edwards and Sligo Margins, Cretaceous of Onshore Texas (E&P Notes)

We examined the potential in onshore Texas of the most prolific reservoirs found to date in the Gulf of Mexico basin, Cretaceous carbonates, and in particular the Edwards and Sligo formations. Two-dimensional (2-D) and three-dimensional (3-D) seismic data with lithologic and biostratigraphic information led to a detailed sequence stratigraphic framework. This framework resulted in a concentrated exploration effort in Lavaca County, a redefinition of the Edwards shelf margin, and confirmation of a major sequence boundary in the Sligo. The youngest Edwards margin appears to have stepped seaward a distance of more than 3 mi (4.8 km) from the position of the margin as indicated by the Word field. This seaward shift and repositioning of the margin is shown in 3-D seismic and well data. Recognized within the progradational package are distal slope wackestones, reef and bank complexes, and back-reef lagoonal deposits that are offset seaward across sequence boundaries. Reef and grainstone deposits are located far seaward of the commonly recognized margin and numerous exposure surfaces occur in the shelf deposits. Proximity to faulting after burial ensures the development of a plumbing system that enhances secondary porosity and provides a migration pathway for hydrocarbons. A Sligo debris play is based on a sequence boundary within the upper part of the Sligo in ferred from seismic geometry and surface exposures in Mexico. The seismic portrays a wedge geometry positioned downslope from the Sligo margin. Base-level change about the sequence boundary would have initiated coarse carbonate debris- and grain-flow deposition seaward of the Sligo shelf margin. Rapid deposition may have helped preserve porosity within the thick debris wedge. Data from reservoir analogs confirm that downslope carbonates can retain reservoir-quality porosity. Facies variation and slump faulting on the foreslope creates the potential for trapping, and juxtaposition to deep-water carbonates sets up the source and migration pathway. This undrilled wedge extends for hundreds of miles along the Sligo margin.

Marine and nonmarine systems tracts in fourth-order sequences in the Early-Middle Cenomanian, Dunvegan Alloformation, northeastern British Columbia, Canada

Abstract The Early to Middle Cenomanian Dunvegan Alloformation is a 3rd-order clastic sequence that was deposited on the western margin of the Alberta foreland basin. The alloformation, comprising ten allomembers (A-J) has been mapped over 33 000 km 2 using 1500 well logs and 40 outcrops. Regional flooding surfaces are used to define allomembers because they are more reliably identified in well logs than are Exxon-type unconformities (although in many instances unconformities and flooding surfaces are coplanar). The Dunvegan records episodic deltaic progradation from NW to SE along the axis of the foreland basin. This paper focuses on an 800 km 2 study area in NE British Columbia where both shallow marine and coastal plain strata are exposed in the valleys of the Peace and Beatton rivers. Marine strata comprise sandier-upward successions that grade from platy mudstone through cm-scale interbeds of wave rippled fine sandstone to dm-scale beds of HCS, commonly including large gutter casts. Successions may be capped by SCS and cross-bedded, fine-grained sandstone, sometimes with a rooted top. The succession of facies suggests a strongly wave-influenced deltaic shoreline. Gutter casts are consistently orientated shore-normal and appear to have been formed by oscillatory wave scour during storms. Sandstone distribution varies between successive progradational packages (‘shingles’) in allomember J, suggesting that delta switching was an important control on depositional cyclicity. Coastal plain deposits include small-scale (few metres) coarsening-upward successions of carbonaceous mudstone, siltstone and fine sandstone interpreted as lake and bay-fill deposits. Decimetre scale sharp-based sandstones may represent crevasse splays whereas lenticular, rippled and crossbedded sandstones were probably deposited in distributary channels. Rooted palaeosols are abundant. Both coastal plain and marine strata are locally replaced by erosive-based bodies of cross-bedded sandstone up to 25 m thick and 0.5–3 km wide, interpreted as incised valley fills underlain by type 1 sequence boundaries. Additional evidence of relative sea-level fall is provided by regressive surfaces of marine erosion beneath sharp-based shoreface sandstones. If attached to highstand deposits, these sandstones are assigned to the falling stage systems tract, whereas detached sandstones are assigned to the lowstand systems tract. During the early part of the transgressive systems tract, sedimentation was confined to incised valleys but later, interfluves were buried by sediments that locally contain dense Thalassinoides burrow systems, Teredolites and well-developed mud drapes, suggestive of brackish, tidally-influenced conditions. Overlying coastal plain strata lack evidence of marine influence and are attributed to the highstand systems tract.

The Parasequence Definition--Are Transgressive Deposits Inadequately Addressed?

ABSTRACT In only a few decades sequence stratigraphy has become one of the cornerstones of modern stratigraphy. Although the sequence is the principal stratigraphic unit, parasequences are the fundamental composite building blocks. By definition, parasequences are typically upward-shoaling successions that are bounded by flooding surfaces that form in response to relatively rapid rises of relative sea level. In that definition little provision is made for significant transgressive deposition during the time of flooding, but transgressive deposits do exist in the geological record. Because these deposits lie above the flooding surface and derive much of their sediment from transgressive erosion of the underlying progradational succession, should the parasequence boundary not be moved from the u derlying flooding surface upward to the top of the transgressive unit? By doing so, the underlying progradational unit and the overlying transgressive unit would constitute one complete regressive-transgressive succession. Nonetheless, although appealing, the typical inability to easily, accurately, and consistently identify the uppermost surface of the transgressive unit makes it an untenable surface for stratigraphic purposes. As suggested by earlier workers, therefore, the flooding surface, a readily identifiable surface that caps the progradational package and consistently underlies transgressive deposits, is the most appropriate surface to bound the parasequence. In any event, to ignore transgressive deposits is to ignore an important component of the geological record--a record that not only provides insight into transgressive depositional processes but also is one that can be economically important. As a result, transgressive deposits need to be included in the parasequence definition, possibly only implicitly, but certainly not explicitly excluded. Furthermore, the notion that sequences typically indicate progradational deposition excludes examples where upward-fining transgressive deposits (related to upward deepening) are present and lie below the upward-shoaling (progradational) part of the same parasequence.

Sedimentary history of the Tethyan basin in the Tibetan Himalayas

After an epicontinental phase, the sedimentary rocks in the Tibetan Himalayas document a complete Wilson cycle of the Neo-Tethyan (Tethys Ill) evolution between the Gondwana supercontinent and its northward drifting margin (Lhasa block) from the Late Permian to the Eocene. During the Triassic rift stage, the basin was filled with a huge, clastic-dominated sediment wedge with up to > 5 000 m of flysch in the northern zone. Widespread deltaic clastics and shallow-water carbonates of late Norian to earliest Jurassic age in the southern zone mark, in conjunction with decreasing tectonic subsidence, the transition to the drift stage. Some 4 500 m of Jurassic and Early Cretaceous shallow-water carbonates and siliciclastics accumulated on the Tethyan Indian passive margin. Deepening-upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments. Extensive abyssal sediments with basaltic volcanics in the northern deep-water zone reflect continued ocean spreading and thermal subsidence. Paleomagnetic data, gained separately for the northern Indian plate and the Lhasa block, indicate that the Neo-Tethys reached its maximum width about 110 Ma ago with a spreading rate of 4.8 cm/year, before it commenced to close again. During the remnant basin stage in the Late Cretaceous and Paleogene, a shallowing-upward megasequence, capped by a carbonate platform, developed in the southern inner shelf realm. In the northern slope/basin plain zone, turbidites and chaotic sediments, derived from both the acretionary wedge and the steepening slope of the passive margin, accumulated. The depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting. The sediments from the Triassic to the Paleogene are tentatively subdivided into five mega-sequences, which are controlled mainly by regional tectonics. Climatic influence (e.g., carbonate deposition), due to northward plate motion, is partially subdued by terrigenous input and/or increased water depth. During the Oligocene and Miocene, crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins.

Sedimentary evolution of the northwestern Alboran Sea during the Quaternary

Five depositional bodies occur within the Quaternary deposits of the northwestern Alboran Sea: Guadalmedina-Guadalhorce prodelta, shelf-edge wedges, progradational packages, Guadiaro channel-levee complex, and debris flow deposits. The sedimentary structure reflects two styles of margin growth characterized: 1) by an essentially sediment-starved outer, shelf and upper slope and by divergent slope seismic facies; 2) by a prograding sediment outer shelf, and parallel slope seismic facies. Eustatic oscillations, sediment supply, and tectonic tilting have controlled the type of growth pattern, and the occurrence of the depositional bodies. Debris flows were also controlled locally by diapirism.

Large‐scale stratigraphic architecture, eustatic variation, and unsteady tectonism: A theoretical evaluation

We incorporate a process‐based depositional model with basin subsidence models to predict stratigraphic records. This allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins. The models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin, the record of episodic tectonism is distinct from that of eustatic sea level change. In the model, sediment transport is approximated by slope‐controlled diffusion; nonmarine transport is treated as more efficient diffusion than marine transport. Three different subsidence and sediment supply models are examined: a simple passive margin basin, a simple foreland basin, and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief. Predicted passive margin stratigraphies, for cases of varying eustatic sea level, are similar to those of natural basins and include progradational packages and subaerial unconformities, which are used to define sequence boundaries that form during sea level fall. We explore the timing relationships between stratigraphic features and a sinusoidal sea level history, showing that the phase relationship depends on subsidence, sediment flux, efficiency of sediment transport, and period and amplitude of sea level. When the basic geometry of the basin is inverted, placing the sediment supply on the side with maximum subsidence as is the case in foreland basins, the sequence character changes markedly: subaerial erosion does not generate unconformities. In the models of a dynamic foreland basin, sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant. For steady thrusting and variable sea level, unconformities that define sequence boundaries form only on the distal or forebulge side of the basin, and the ages of the sequence boundaries correlate to limes of rising sea level. In cases of constant sea level but variable thrusting, subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin, yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle: erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side.

Depositional and Diagenetic Controls on Gas Production from Frio (Oligocene) Sands at Monte Christo Field, Hidalgo County, Texas

ABSTRACT Monte Christo Field is located in central Hidalgo County on the Gulf Coastal Plain of south Texas. Depositional and structural factors controlling the accumulation and production of hydrocarbons are related to lateral and vertical facies changes as well as syndepositional growth faulting. These factors combine to compartmentalize the pay sandstones into discrete, isolated reservoirs. Rollover into these growth faults, coupled with greater sandstone thickness and quality at preferred points of delta-front sand deposition, result in a combination of structural and stratigraphic entrapment of gas. Frio reservoir sandstones consist of stacked, upward-coarsening, progradational packages. In each is found a vertical succession of basal abandonment facies, offshore mudstones, distal-delta front sands and silts (lower shoreface), and distributary mouth bar (upper shoreface) sandstones. The upper shoreface sandstones comprise the best reservoir facies. Each facies contains trace fossils indicative of the paleo-environment. Cyclic progradational packages at Monte Christo Field are the direct result of episodic movement along the Mission fault, a large down-to-basin growth fault which bounds the field to the west and parallels depositional strike. Within the study area, three smaller splinter faults bifurcate to the east from the Mission fault and compartmentalize the Frio reservoirs. Diagenetic influences on reservoir quality involve extensive cementation and secondary porosity generation. Reservoir sandstones are fine- and very fine-grained feldspathic litharenites and lithic arkoses. The breakdown of unstable volcanic rock fragments results in significant chlorite and illite grain-coating cements which commonly block pore throats and greatly reduce permeability. Calcite and ferroan carbonate cementation occludes porosity significantly. Minor quartz, potassium feldspar, zeolite and titanium oxide authigenesis are also present. These factors result in relatively low porosities of 10 to 15% and very low permeabilities. The generation of secondary porosity by dissolution of feldspars and carbonate material enhances the diminished pore volume. Point counts reveal that a minimum of 30% of the existing porosity is of secondary origin. Even with significant porosity enhancement in some intervals, artificial fracture stimulation is required to achieve an economic rate of production. This procedure also taps into the substantial quantities of gas trapped in the lowest permeability portions of the reservoir facies.

Seismic Stratigraphy and Structural Setting of Early Jurassic to Mid-Cretaceous Sedimentary Rocks in Southern Neuquen Basin, Argentina: ABSTRACT

The structural evolution of the Neuquen, or Huincul, dorsal and its effect on sedimentation and stratigraphy is interpreted from seismic and well data in the southern Neuquen basin, a Mesozoic back-arc basin located in west-central Argentina, Seismic cross sections reveal that dominantly marine Jurassic sediments began filling the basin as a series of thick, discontinuous, progradational packages. Transpressional forces during the Late Jurassic Araucanian phase resulted in the formation of an extensive east-northeast-trending anticlinal structure in the southern Neuquen basin, known as the Huincul dorsal, which is important as a structural trap for hydrocarbons. Regional regression and erosion during this phase resulted in a pronounced angular unconformity, readily identifiable throughout the southern basin. The subsequent deposition of Late Jurassic to mid-Cretaceous sediments is seen on seismic cross sections as a series of low-angle, laterally continuous reflectors. Reactivation of the Huincul dorsal occurred during the mid-Cretaceous Mirano phase. Due to uplift and regression during this phase, a low-angle erosional unconformity, although often subtle, can be observed on several seismic cross sections. Uplift and deformation of the Huincul dorsal originated in basement along dominantly northward-dipping, high-angle thrust faults with up to 2000 m of vertical offset. The initial uplift and compressional deformationmore » of the Andean Cordillera began in the mid-Cretaceous to the west of the Neuquen basin. This resulted in a final stage of primarily continental sedimentation, lasting through the Tertiary.« less