Highstand Sequence Set Research Articles

Disentangling tectonic and eustatic controls on forearc basin stratigraphy, Talara Basin, Peru

The controls on basin-fill stratigraphy such as sediment supply, climate, eustasy and tectonics are well understood in many basin types, but there are few examples documented from forearc basins. Traditional evolutionary models for forearc basins emphasise the importance of accretionary processes under a largely contractional tectonic regime. The Talara Basin of northwestern Peru preserves a lower Eocene stratigraphic record of dominantly extensional collapse tectonics punctuated by periods of uplift, driven by variability of the subduction process. The margin is dominated by subduction erosion and only minor accretion processes. In this geodynamic setting, we present for the first time a sedimentological and sequence stratigraphic model for the 350 m-thick fluvio-deltaic succession of the Pariñas Formation. Three depositional sequences are stacked into a highstand sequence set comprising proximal fluvial strata that correlate down dip into marginal marine and marine deposits. Syn-sedimentary normal faulting related to episodes of subduction erosion restricted much of the Pariñas to hanging-wall locations subject to minor-scale transgressions. The succession evolved from shelf-confined clinoforms to later basin margin scale clinoforms, related to long-term sea-level rise. The interaction of eustasy, sediment supply and episodes of subduction erosion are integrated into an evolutionary model that may have application in other non-accretionary forearc basins worldwide. Although the model shares characteristics with extensional models for rift basins, the episodic subduction-driven uplift events add another component of relative sea-level complexity.

Depositional environment and source rock quality of the Woodbine and Eagle Ford Groups, southern East Texas (Brazos) Basin: An integrated geochemical, sequence stratigraphic, and petrographic approach

X-ray fluorescence chemostratigraphy of the Cenomanian–Turonian Woodbine and Eagle Ford Groups in the southern parts of the East Texas Basin highlights significant mudstone chemical heterogeneities that commonly are difficult to observe or quantify at the macroscale. Several key elements, namely, Ca, Si, Mo, Mn, and Ni, were correlated to depositional conditions and used in a hierarchical cluster analysis to characterize five chemical facies (i.e., “chemofacies”) across 10 cored intervals of the Woodbine and Eagle Ford Groups: (1) argillaceous, organic-matter poor; (2) transitional, organic-matter poor; (3) transitional, organic-matter moderate; (4) calcareous, organic-matter rich; and (5) calcareous, organic-matter moderate. Characterizations of organic matter richness, mineralogy, and environmental conditions of deposition were established by correlating between key element abundances, total organic carbon measurements, x-ray diffraction measurements, and petrographic observations of lithologic composition, bioturbation, and sedimentary textures. Combined analysis of elemental geochemistry, stratigraphy, and petrographically observed sedimentary textures indicates that all chemofacies were deposited in an intrashelf basin above storm wave base. The most organic-rich chemofacies was deposited on a dominantly dysoxic distal shelf. Mudstone organic matter enrichment is driven dominantly by the minimization of siliciclastic dilution and secondarily enhanced by oxygen restriction. Regional correlations of chemofacies within a sequence stratigraphic framework developed from previous outcrop and subsurface work indicate a clear relationship between interpreted stratigraphy and chemofacies deposition. Generally, the highstand sequence sets of the Woodbine Group and upper Eagle Ford formation are dominated by mineralogically clay-rich, organic matter–lean, siliciclastic sedimentation and contain poor-quality source rock. Conversely, the transgressive sequence set of the lower Eagle Ford formation is dominated by organic matter–rich pelagic carbonate accumulation and contains excellent-quality source rock.

Spatial and Temporal Evolution of a Permian Submarine Slope Channel-Levee System, Karoo Basin, South Africa

Abstract The Laingsburg depocenter of the SW Karoo Basin, South Africa, includes a series of sandstone-dominated deepwater clastic deposits (units A and B of the Laingsburg Formation and Unit C to Unit G of the overlying Fort Brown Formation) separated vertically by regional mudstones and records basin-floor to upper-slope deposition during the Permian icehouse climate. Unit C provides nearly continuous exposures over tens of kilometers, and the presence of regionally persistent internal mudstone markers (lower and upper C mudstones) allows the distribution of sedimentary facies and architectural elements of a slope-turbidite system to be documented for more than 30 km downslope and 20 km across slope. The spatial and temporal distribution of architectural elements and interpreted depositional environments (external levees, channel belts confined by a combination of basal erosion and overbank aggradation along the margins, and distributive frontal splays) reveals distinct changes in the sedimentation pattern and stratal architecture of the turbidite system through time. Unit C evolved in a stepwise manner from a weakly incised, levee-confined channel belt and its downdip distributive frontal splays (C1), through a more entrenched and sinuous channel–levee complex set that fed submarine fans farther into the basin (C2), to a regionally backstepping package of thin-bedded deposits of a distal distributive system (C3). Unit C is interpreted as a lowstand sequence set, composed of three depositional sequences, each of which includes a sandstone-dominated lowstand systems tract (C1, C2, and C3) and a mudstone-prone interval that is taken to represent the transgressive and highstand systems tracts. The overlying combined transgressive and highstand sequence set is marked by the 30-m-thick C–D mudstone interposed between Unit C and Unit D. Unit C and the C–D mudstone together form the Unit C composite sequence. At the scale of the Unit C lowstand sequence set, the evolutionary trend from C1 to C3 reflects an overall basinward then landward stepping of the system across the depositional profile, which is interpreted as the product of an overall waxing then waning of flow energy (volume and efficiency).

Sedimentology and sequence stratigraphy of the Lopingian (Late Permian) coal measures in southwestern China

The Lopingian coal measures of southwestern China were deposited within a range of facies associations spanning a spectrum of settings from fluvial to marine carbonate platform. The transitional to terrestrial coal measures are dominated by siliciclastics, but they also contain fifteen laterally extensive marine bands (limestone beds and mudstone). These bands act as marker horizons that enable correlation between fully marine and terrestrial facies. Examination of this range of facies and their sedimentology has enabled the development of a high-resolution sequence stratigraphic framework. Set against the established backdrop of second-order Lopingian transgression, sixteen fourth-order sequences and three composite sequences (third-order) are recognized. Results show that, in the composite sequences, peat accumulation in the seaward parts of the study area predominantly correlates with early transgressive sequence sets (TSS), while in more landward areas it correlates with the middle TSS to late highstand sequence sets (HSS). Differences in peat-accumulation regimes within the sequence stratigraphic framework are attributed to variations in subsidence and background siliciclastic input rates in different depositional settings, with these combining to produce differences in the rate of accommodation change. The preservation of coal resources in the middle to late HSS in this area was most likely related to the rise of the regional base level throughout the Lopingian.

Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain

The syn-tectonic Sobrarbe deltaic complex (Eocene of the Ainsa Basin, Spanish Pyrenees) is confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. Six facies associations have been distinguished within the deltaic complex: (i) slope marls and turbidite sandstones; (ii) distal delta front silty and bioturbated sandstones; (iii) proximal delta front and delta plain deposits (tidal inlet, tidal flat, mouth-bar, bay, distributary channel, and floodplain environments); (iv) biogenic deposits at flooding surfaces; (v) collapse zone sediments; and (vi) Nummulite-dominated shallow-marine carbonates. The deltaic complex comprises four composite sequences, which each contain a number of minor sequences of the genetic (maximum flooding surface bounded) type. Regressive unconformities and their correlative surfaces separate the composite sequences (CS), and each CS can be divided into lowstand, transgressive and highstand components. At the base, the Comaron CS is a WNW-prograding deltaic complex containing six minor sequences characterized by basinward-stepping sandstone wedges separated by transgressive mudstones. Double regressive unconformities are often associated with these sandstone wedges proximally in the basin. Growth of the intra-basinal Arcusa anticline caused gentle folding of the Comaron CS, and syn-growth strata at the base of the overlying Las Gorgas CS contain a prominent forced regressive sandstone wedge (the Las Gorgas sandbody) and display thinning of bedsets onto the crest of the anticline. Moreover, it can be demonstrated that relative sea-level rise took place in the growth synclines at the same time as a fall in relative sea level occurred on the growth anticlines. The tectonic deformation caused a shift in the direction of deltaic progradation towards the north-northwest, as demonstrated by the well-developed clinoforms in the highstand sequence set of the Las Gorgas CS. A second phase of tectonic activity, this time triggering large-scale collapse of the delta front, occurred at the transition between the Las Gorgas and Barranco el Solano CS. Tilting of the basin floor led to the development of coeval transgressive and regressive strata separated by a fulcrum (line of zero net-vertical movement of the shoreline). In the post-tilt phase, reduced sediment supply and accelerated subsidence led to widespread transgression and a change from clastic- to carbonate-dominated sedimentation. This phase was terminated by a gradual increase in sediment supply causing progradation of a mixed clastic–carbonate system dominated by large Nummulite banks and bioturbated shoreface marls and sandstones. Finally, a pronounced fall in relative sea level associated with the transition to the Buil CS triggered slope instabilities and eventually established fluvial conditions all over the study area. With the exception of the anticline crests, the relatively high rate of tectonic subsidence in the study area suppressed relative sea-level falls, resulting in development of discontinuous lowstand strata and bounding surfaces. The segmented nature of many regressive unconformities may be related to incremental growth of thrust-related structures, and each segment is separated from neighbouring segments by gaps of limited vertical and lateral displacement. Precursors of progressive unconformities are seen in the upper proximal part of the Comaron CS by downdip `fanning' and updip convergence of minor sequences. These differentially tilted stratal units provide reliable indicators of syn-sedimentary tectonism.

Abstract: Evidence for Hierarchy of Stratigraphic Forcing in the Upper Carboniferous (Virgilian, Wabaunsee Group) in the Anadarko Basin 

Analysis of Upper Carboniferous (Virgilian, Wabaunsee Group) strata cropping out in the North American Midcontinent suggests a hierarchy of stratigraphic forcing. Fourth-order depositional sequences from the Wabaunsee Group represent the latest highstand sequence sets of a composite third-order sequence (1-1 0 m.y.) that encompasses strata from the Douglas, Shawnee, Wabaunsee, and Admire Groups. The composite third-order sequence (1 10 m.y.) is composed of 15 composite fourth-order depositional sequences (0.1-1 m.y.). The composite fourth-order depositional sequences of the Wabaunsee Group contain between two and three fifth-order cycles (0.0 1-0.1 m.y.). These fifth-order cycles form retrogradational transgressive system tracks and aggradational to progradational highstand-system tracks.

Dextral Pan-African shear along the southwestern edge of the Achankovil shear belt, south India: constraints on Gondwana reconstructions : Sacks P.E., Nambiar C.G. & Walters L.J., Journal of Geology, 1997, 105/2 (275–284)

The Nama foreland basin developed in Neoproterozoic times as a peripheral foreland basin between the Kalahari Craton in the east and the partly contemporaneous Damara and Gariep orogenic belts in the north and west. The foreland basin is subdivided into three basins, from north to south the Zaris, Witputs, and Vanrhynsdorp basins. Deposits of the Nama Group proper fill the first two basins, whereas the third contains the Vanrhynsdorp Group which differs in many aspects from the Nama Group. Only during its latest phase of development did the foreland basin form one continuous depository.The stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis, flexural bulging and sea-level changes that affected the basin throughout its history. Palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin. As mountain belts emerged along the interactive northern and western orogenic margins, transport directions switched to southerly and southeasterly. Foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens.Three major unconformity bounded sequences (composite sequences), probably of second-order cyclicity, can be recognised. The uppermost composite sequence (CS3) comprises the Fish River Subgroup, whereas the lower composite sequence (CS1) is composed of the Flaminkberg Formation and the Kwanous, Kuibis and lower and middle Schwarzrand subgroups. CS2 comprises the Knersvlakte, Brandkop and upper Schwarzrand subgroups. CS1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a “Type 1” unconformity at the base of the Nomtsas Formation. CS2 contains a conformable lowstand flysch delta partly derived from CS1 deposits eroded on flexural bulges as well as an orogenic source in the northwest. CS3 is entirely composed of orogen-derived molasse onlapping a basal “Type 2” unconformity and overstepping older sequences in a cratonward direction. A number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading.The age of the Nama and Vanrhynsdorp groups is estimated at between 600 Ma and 530 Ma as determined by the age of the basement, the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast, which affected Nama and Vanrhynsdorp rocks along the active basin margins. Marginal tectonogenesis of the Nama foreland basin resulted from northwesterly subduction of the Kalahari plate underneath the Congo and South American plates through a process of sinistral transpression along the western Kalahari margin leading to sequential closure of the proto-Atlantic (Adamastor) Ocean from north to south.

Le plateau Sud-Tasman (Australie): collage de deux fragments du Gondwana oriental? (The South Tasman Rise (south of Tasmania): a collage of two fragments of eastern Gondwana?) : Rollet N., Royer J.-Y., Exon N.F. & Hill P.J., Comptes Rendus-Academie des Sciences, Serie II: Sciences de la Terre et des Planetes, 1996, 323/10 (865–872)

The Nama foreland basin developed in Neoproterozoic times as a peripheral foreland basin between the Kalahari Craton in the east and the partly contemporaneous Damara and Gariep orogenic belts in the north and west. The foreland basin is subdivided into three basins, from north to south the Zaris, Witputs, and Vanrhynsdorp basins. Deposits of the Nama Group proper fill the first two basins, whereas the third contains the Vanrhynsdorp Group which differs in many aspects from the Nama Group. Only during its latest phase of development did the foreland basin form one continuous depository.The stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis, flexural bulging and sea-level changes that affected the basin throughout its history. Palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin. As mountain belts emerged along the interactive northern and western orogenic margins, transport directions switched to southerly and southeasterly. Foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens.Three major unconformity bounded sequences (composite sequences), probably of second-order cyclicity, can be recognised. The uppermost composite sequence (CS3) comprises the Fish River Subgroup, whereas the lower composite sequence (CS1) is composed of the Flaminkberg Formation and the Kwanous, Kuibis and lower and middle Schwarzrand subgroups. CS2 comprises the Knersvlakte, Brandkop and upper Schwarzrand subgroups. CS1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a “Type 1” unconformity at the base of the Nomtsas Formation. CS2 contains a conformable lowstand flysch delta partly derived from CS1 deposits eroded on flexural bulges as well as an orogenic source in the northwest. CS3 is entirely composed of orogen-derived molasse onlapping a basal “Type 2” unconformity and overstepping older sequences in a cratonward direction. A number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading.The age of the Nama and Vanrhynsdorp groups is estimated at between 600 Ma and 530 Ma as determined by the age of the basement, the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast, which affected Nama and Vanrhynsdorp rocks along the active basin margins. Marginal tectonogenesis of the Nama foreland basin resulted from northwesterly subduction of the Kalahari plate underneath the Congo and South American plates through a process of sinistral transpression along the western Kalahari margin leading to sequential closure of the proto-Atlantic (Adamastor) Ocean from north to south.

Interfluve sequence boundaries in the Grassy Member, Book Cliffs, Utah: criteria for recognition and implications for subsurface correlation

Abstract The Late Campanian Grassy Member of the Blackhawk Formation, Mesaverde Group, Utah consists of two unconformity-bounded fourth-order sequences within a third-order highstand sequence set. Several orders of bounding surface define the internal architecture of each sequence and component parasequences. Incised valley fill deposits, dominated by braided channel-fills updip and laterally accreted fluvial and estuarine channel-fills down dip, overlie the fourth-order sequence boundaries, truncating shoreface and shelf facies of underlying sequences. Incised valleys are separated by extensive interfluves with evidence of subaerial exposure and minor tributary channel systems. The interfluve gullies are interpreted to have transported very little bedload during lowstand and acted primarily as rainwater run-off or bypass channels lateral to the main incised valley system. They are filled with bioturbated fine-grained sandstone and siltstone and capped by marine shale. Interfluve areas provide subtle but crucial evidence for base level falls, including rare examples of roots penetrating formerly offshore marine shale, partially eroded hematitic hardpan sediments and sand or silt filled gullies containing coal and shell debris. Interfluve sequence boundaries have powerful predictive application in respect to recognition of along-strike incised valley fills. Recognition of these surfaces is commonly hampered by removal of subaerial indicators as a result of subsequent transgressive erosion. There is indirect stratigraphic evidence of an emerging salt or basement fault driven structural high, orthogonal to the Late Campanian palaeoshoreline, which controlled the locus of valley incision and associated interfluves in two successive high frequency sequences.

The application of high‐resolution sequence stratigraphy to fluvial systems: a case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA

ABSTRACTThe Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine‐grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills.Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th‐order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle.In the lowstand sequence set, incision associated with successive 4th‐order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th‐order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine‐grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack discretely.

The Nama foreland basin: sedimentation, major unconformity bounded sequences and multisided active margin advance

Abstract The Nama foreland basin developed in Neoproterozoic times as a peripheral foreland basin between the Kalahari Craton in the east and the partly contemporaneous Damara and Gariep orogenic belts in the north and west. The foreland basin is subdivided into three basins, from north to south the Zaris, Witputs, and Vanrhynsdorp basins. Deposits of the Nama Group proper fill the first two basins, whereas the third contains the Vanrhynsdorp Group which differs in many aspects from the Nama Group. Only during its latest phase of development did the foreland basin form one continuous depository. The stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis, flexural bulging and sea-level changes that affected the basin throughout its history. Palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin. As mountain belts emerged along the interactive northern and western orogenic margins, transport directions switched to southerly and southeasterly. Foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens. Three major unconformity bounded sequences (composite sequences), probably of second-order cyclicity, can be recognised. The uppermost composite sequence (CS3) comprises the Fish River Subgroup, whereas the lower composite sequence (CS1) is composed of the Flaminkberg Formation and the Kwanous, Kuibis and lower and middle Schwarzrand subgroups. CS2 comprises the Knersvlakte, Brandkop and upper Schwarzrand subgroups. CS1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a “Type 1” unconformity at the base of the Nomtsas Formation. CS2 contains a conformable lowstand flysch delta partly derived from CS1 deposits eroded on flexural bulges as well as an orogenic source in the northwest. CS3 is entirely composed of orogen-derived molasse onlapping a basal “Type 2” unconformity and overstepping older sequences in a cratonward direction. A number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading. The age of the Nama and Vanrhynsdorp groups is estimated at between 600 Ma and 530 Ma as determined by the age of the basement, the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast, which affected Nama and Vanrhynsdorp rocks along the active basin margins. Marginal tectonogenesis of the Nama foreland basin resulted from northwesterly subduction of the Kalahari plate underneath the Congo and South American plates through a process of sinistral transpression along the western Kalahari margin leading to sequential closure of the proto-Atlantic (Adamastor) Ocean from north to south.

Facies analysis and high‐resolution sequence stratigraphy of the Lower Eocene shallow marine Ametlla Formation, Spanish Pyrenees

ABSTRACTThe Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn‐sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment‐starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway.In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized.(1) At the regional scale, elements of two 3rd‐order composite sequences (sensu Exxon) have been recognized. These include a 3rd‐order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd‐order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward‐stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment‐starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high‐frequency sea‐level fluctuations, and to downslope redistribution of sand.(2) At the subregional scale, detailed mapping indicates the presence of five 4th‐order sequences. The 4th‐order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea‐level fall. Sharp‐based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea‐level. Where 4th‐order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone‐dominated transgressive systems tract and a siltstone‐dominated highstand systems tract.(3) At the local scale, 2–9 5th‐order parasequences are present within the 4th‐order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonly stacked in a landward‐stepping manner.

Reservoir Anatomy within a Sequence Stratigraphic Framework: The Pennsylvanian Breathitt Group of Eastern Kentucky

Detailed quantitative sequence stratigraphic analysis and three-dimensional (3-D) modeling of the fluvio-deltaic Pikeville, Hyden, and four Corners formations of the Breathitt Group, eastern Kentucky, indicate that it is possible to make confident deterministic correlations of sand bodies at offshore development well spacings. The key to high-confidence correlation is the delineation of the 3-D architecture of third-order composite sequences, which are seismically mappable, and the recognition of internal fourth-order sequences, which are stacked into lowstand, transgressive, and highstand sequence sets. Volumetric partitioning of reservoir sand bodies between systems tracts and sequences allows prediction of channel sandstone geometries and channel: crevasse splay ratios. Furthermore, channel-fill type is predictable from the sequence stratigraphy. Within 13 studied fourth-order sequences, incised valley fills represent the main reservoirs, which are laterally continuous over tens of kilometers. Sand bodies within transgressive and early highstand systems tracts are usually filled by siltstones or heteroliths and have geometries less than 1.0 km in width. Within fourth-order sequence sets, incised valley fills are sometimes vertically stacked to produce anaomalously thick fluvial sand bodies. These are the prime potential producing zones within coastal plain reservoirs. High-resolution sequence stratigraphy thus provides a deterministic approach to correlation and 3-D modeling. Use of themore » Stratamodel geocellular modeling system permits circulation of connectivities for different systems tracts and sequence sets, thus allowing the development of conceptual models for perforation policy in complex [open quotes]jigsaw-type[close quotes] reservoirs.« less

Inferred glacio-eustatic control for high-frequency depositional sequences of the Plio-Pleistocene Kazusa Group, a forearc basin fill in Boso Peninsula, Japan

The Plio—Pleistocene forearc basin fill of the Kazusa Group, as much as 3000 m thick, is represented by a series of marine environments ranging from deep-sea basin plain to shallow sea. It developed during about 2.4 to 0.45 Ma, and is well exposed in the Boso Peninsula, Japan. Chronostratigraphic resolution of the Kazusa Group is partly within about 0.05 m.y., based on the combination of fission track dating of volcanic ash layers, and bio- and magneto-stratigraphy. Many volcanic ash layers are intercalated in the Kazusa Group and provide distinct chronostratigraphic surfaces within the heterogeneous lithologies. Detailed outcrop studies on depositional facies discontinuities and lateral terminations of volcanic ash layers allow high-resolution sequence stratigraphic interpretation of the Kazusa Group. Seventeen depositional sequences were identified in the Kazusa Group. Each depositional sequence was tentatively correlated with the stable oxygen isotope sea-level index, integrated with the age framework of the Kazusa Group. This correlation indicates that frequencies of the depositional sequences can be estimated to be in the range of 0.2-0.02 m.y. and are within fourth- and fifth-order stratigraphic cyclicities. Depositional sequences constitute third-order transgressive and highstand sequence sets and the Kazusa Group is interpreted to represent a third-order composite sequence with a duration of about 1.95 m.y. Development of the third-order composite sequence was also controlled by an overall rise and fall of glacio-eustatic sea levels, overprinted by tectonic events related to fluctuations in oceanic plate motion which occurred at about 2.6 Ma and 0.5 Ma, respectively.