Sequence Stacking Patterns Research Articles

An integrated sequence stratigraphic analysis of the early Marsdenian substage of the Millstone Grit Group, Central Pennines, UK

The early Marsdenian substage (Millstone Grit Group) of the Pennines comprises repeated deltaic cycles separated by ammonoid-bearing marine bands. This cyclicity, controlled by the combined forces of glacio-eustasy and modulations in sediment supply, provides an important outcrop area for the application of sequence stratigraphic principles. A new correlation framework is presented where two orders of sequence are identified: (1) a low-order sequence, within which marine bands (R 2a 1, R 2b 1, R 2b 2 and R 2b 3) represent a maximum flooding surface, and (2) high-order sequences nested within the low-order sequences. By integrating and correlating key exposures, historic well boreholes and field mapping, lateral changes in facies and facies association are observed, and palaeogeographic trends mapped. This allows the variation in sequence and systems tract stacking patterns to be interpreted. Three orders of Milankovitch cyclicity are inferred to control the sequence stacking patterns; long-duration ( c. 400 ka) eccentricity oscillation controlling maximum flooding events represented by the R 2a 1 and R 2b 2 marine bands, sub-100 ka obliquity oscillations (controlling the R 2b 1, R 2b 2 and R 2b 3 marine bands and intervening low-order sequence boundaries), and precessional frequencies ( c. 25 ka) which may control the periodicity of the high-order sequences. Supplementary material: Locality details, including lithostratigraphic units observed, gross and sand thicknesses within sequences, and palaeocurrent data measured are available at https://doi.org/10.6084/m9.figshare.c.6408203

Sequence architecture and depositional evolution of Eocene to Oligocene, synrift to early postrift strata in the Baiyun Sag, South China Sea

The Pearl River Mouth Basin (PRMB), located on the northern continental margin of the South China Sea, underwent a complex evolution during the Paleogene. We have focused on the Baiyun Sag in the PRMB with the goal of constructing the architecture and depositional evolution of the Eocene–Early Oligocene synrift stage to the Late Oligocene early postrift stage of basin evolution. Based on analysis of 3D seismic data complemented by well logs and cores, the Paleogene basin fills can be classified into three composite sequences bounded by regional unconformities and 14 sequences bounded by local unconformities. We identify seismic facies on the basis of different internal reflection configurations, external shapes, and contact relationships. The distribution of seismic facies and lithologies interpreted from gamma-ray curves reveal that the synrift basin fill consists mainly of fan delta deposits adjacent to the southern fault scarp, braid delta deposits on the adjacent hanging-wall blocks, and lacustrine mudstones and sublacustrine fan deposits in the center of the basin. The overlying early postrift stage is dominated by shelf and shelf-slope environments, with widespread developed southward-prograding deltas and submarine fans. Tectonics is the principal controlling factor on the development and distribution of depositional systems during the synrift stage. In contrast, sea-level changes superimposed on long-term subsidence related to thermal cooling determined the stacking patterns of sequences during the postrift stage. The results provide new insights on synrift and early postrift tectonics and sedimentation patterns along an evolving passive margin.

Rift activity and sequence stratigraphy of the Oligocene Dongying Formation in the Nanpu Sag, eastern China: Implications for rift sequence stratigraphy in lacustrine basins

Rifting plays an important role in accommodation variations in rift basins. However, fluvial discharge as another important factor affecting lacustrine rift sequence stratigraphy is not well explored. In this work, seismic and well data are used to investigate the stratigraphic features, rift activity, and effect of fluvial discharge in the Nanpu Sag, Bohai Bay Basin. The syn‐rift deposition of the Dongying Formation (28.5–23.8 Ma) is divided into four sequences (SQ1, SQ2, SQ3, and SQ4), and the rift‐related subsidence rate along two parallel boundary faults is calculated. The results indicate that the rift activity weakens from SQ1 to SQ4 and that the two parallel boundary faults consist of a total of four fault segments. As the fault segmented, three propagation folds formed intrabasin highs on which the Beipu, Laoyemiao, and Gaoliu outlets developed. Well logs and 3D seismic data show that from SQ1 to SQ3, retrogradational stacking patterns occur in Beipu, Laoyemiao, and the hanging‐wall ramp, while progradational stacking patterns occur in the Gaoliu outlet. In SQ4, the sequence stacking patterns are progradation‐dominated. By evaluating channel erosion and sediment supply, the fluvial discharge can be estimated, and the result indicates that the Gaoliu outlet has the strongest fluvial discharge from SQ1 to SQ3 with strong channel erosion, and in SQ4, the fluvial discharge of each outlet is comparable and relatively low. The high fluvial discharge of the Gaoliu outlet from SQ1 to SQ3 is considered to be the primary reason for the lake level rise in the intervals between the pulses of rift activities. Sequence stacking patterns and depositional trends in our study intervals are not in line with the rift subsidence rate; this indicates that rifting is not the only factor controlling the rift sequence stratigraphy and variations in the accommodation of lacustrine rift basins. While the Gaoliu outlet has predominant fluvial discharge from SQ1 to SQ3, progradational parasequence sets deposit in the Gaoliu outlet. In contrast, the retrogradational parasequence sets occurred in the Beipu outlet, Laoyemiao outlet, and the hanging‐wall ramp. As the fluvial discharge of the Gaoliu outlet decreased in SQ4, the fluvial discharge in each outlet became low, and the stacking patterns were dominated by progradation. Consequently, this research suggests that fluvial discharge is a significant factor controlling the accommodation creation in lacustrine rift basins, and a sequence model which gives a full consideration of fluvial discharge is constructed for lacustrine rift basins.

Regional Controls On Depositional Trends In Tidally Modified Deltas: Insights From Sequence Stratigraphic Correlation and Mapping of the Loyd And Sego Sandstones, Uinta And Piceance Basins of Utah and Colorado, U.S.A.

Abstract Conditions for deposition of tidally influenced facies are most commonly associated with transgression; however, tidal influence can also be observed in regressive deposits. Herein we present a regional study that correlates the Loyd and Sego sandstones across the Uinta–Piceance basins, an area rich in both outcrop and subsurface data. Variable energy conditions in these tidally modified deltas result in a complex arrangement of facies and environments, making it difficult to correlate sequence stratigraphic surfaces. We highlight the controls, including tectonics, eustasy, and sediment supply, on depositional environments and sand distribution in these regressive tidally modified deposits. Five environments of deposition (EOD) were identified from a combination of vertical and lateral facies trends and stratal geometries found in outcrops and core. EODs were assigned unique well-log signatures and identified, correlated, and mapped in subsurface logs and outcrop from the Uinta to the Piceance Basin. From a proximal to distal position along shorelines these EODs are: 1) coastal-plain channels and floodplains, 2) tidally influenced valley fills, 3) tidally modified deltas, 4) delta front, and 5) open marine. The regional distribution and stacking of these environments were used to identify ten flooding surfaces and map nine genetic sequences: two in the Loyd Sandstone, four in the lower Sego Sandstone, and three in the upper Sego Sandstone. The interplay of sediment supply, sea-level fluctuations, and active tectonics contribute to the distribution of environments of deposition and sequence stacking patterns. This study is the first to identify the Loyd Sandstone in outcrops along the Rangely Anticline. During deposition of the Loyd Sandstone, sediment supply from up-dip fluvial systems was high. The Loyd Sandstone is interpreted as being deposited in river-flood-dominated but tidally modified deltas that exhibit a generally lobate geometry. In contrast, the lower Sego Sandstone comprises tidally influenced deltas and valley fills, and forms regionally extensive shore-parallel sandstone belts. An increase in tidal energy prevalent in the lower Sego Sandstone is attributed to a combination of high sediment supply, slow relative base-level rise, and active faulting over the Laramide-age Douglas Creek Arch. This process formed complex coastline geometries, which likely amplified tides. The upper Sego Sandstone is interpreted to comprise aggradationally stacked sequences composed of tide- and wave-modified deltas. During the deposition of the upper Sego, sediment supply and accommodation were balanced, causing deltaic depocenters to remain stationary over time. Decreased tectonic activity in the upper Sego diminished the influence of tides on sedimentation. This study correlates the Loyd and Sego from the Uinta to the Piceance basins and sheds light on the applicability of this interval for use as analogs for tidally influenced systems in regressive sequences.

Sequence development, depositional filling evolution, and prospect forecast in northern Aryskum Depression of South Turgay Basin, Kazakstan

Jurassic to Cretaceous clastic rocks of the South Turgay Basin were deposited in the typical Mesozoic rift basin formed during the late Triassic collision between the Kazakstan and Siberia plates. In this study, we used more than 140 wells and 2400 km2 of 3D seismic data in the northern Aryskum Depression to produce a detailed sequence stratigraphic interpretation of the South Turgay Basin. Guided by sequence stratigraphy, sedimentology, and structural geology, lower Jurassic-lower Cretaceous strata of the northern Aryskum Depression in the South Turgay Basin, Kazakhstan were subdivided into 10 third-order sequences based on geological and geophysical data. Combined with tectonic evolution characteristics, sequence developments in the basin can be divided into four stages: early rift stage (SQ1–SQ3), late rift stage (SQ4–SQ6), fault to depression transition stage (SQ7–SQ8), and depression stage (SQ9–SQ10). Through comprehensive analysis of seismic sequence configuration, sequence stacking pattern, and depositional filling characteristics, we established the depositional model of the Aryskum Depression in the South Turgay Basin. It is indicated that there are differences in depositional compositions of sequences formed in different stages. Four stages can be clearly identified: filling stage of fan delta facies–lacustrine facies (Stage I) corresponding to the rapid filling in the early rift stage, filling stage of fan delta facies–lacustrine facies–normal delta facies (Stage II) corresponding to trichotomous characteristics of internal systems tracts in the late rift stage, filling stage of braided river delta facies–normal delta facies–lacustrine facies (Stage III) corresponding to the development of high-stand systems tracts in the fault to depression transition stage, and filling stage of fluvial facies–normal delta facies–lacustrine facies (Stage IV) corresponding to binary characteristics of internal systems tracts in the depression stage. Finally, optimization of favorable exploration strata and prospects in the Aryskum Depression are proposed.

Middle–Upper Pleistocene record of 100-ka depositional cycles on the Southern Tuscany continental margin (Tyrrhenian Sea, Italy): Sequence architecture and margin growth pattern

Five unconformity-bounded depositional sequences have been seismically defined on the Tyrrhenian margin offshore Southern Tuscany, Italy. The set of sequences encompasses a ~500ka interval, each sequence recording a ~100-ka orbitally-driven climate-eustatic cycle. Similarly to most of Quaternary continental margins, sequences on the Southern Tuscany margin show an overall regressive motif reflecting shelf sediment progradation basically during the prolonged phases of sea level fall that characterise the Middle–Upper Pleistocene 100-ka cyclicity. On seismic profiles, however, progradational units within each sequence display a variable clinoform geometry, interpreted as a record of the alternation of highstand, falling and lowstand intervals within 100-kacycles. The observed geometric variability is thus used as a key for separating systems tracts otherwise merging into a general progradational/regressive sedimentary body composing the bulk of each sequence. On a longer term, the interplay between sea level changes modulated by 100-kacycles and regional subsidence controls the stratigraphic architecture and the growth pattern of the continental margin. This effect is documented by the overall aggradational/retrogradational stacking pattern of sequences, superimposed on shelf margin progradation and reflecting the renewal of accommodation space landward of the shelf margin and its exploitation at each cycle.

Visions of ice sheets in the early Ordovician greenhouse world: Evidence from the Peninsula Formation, Cape Peninsula, South Africa

The depositional architecture within the Early Ordovician, Peninsula Formation, South Africa, indicates a three-tier stratal sequence hierarchy comprising likely 4th order autogenic, 3rd order allogenic and 2nd order regressive–transgressive sequence stacking pattern. Based on correlation with the global eustatic sea level curve, we interpret the 3rd order sequences, comprising predominantly lowstand braided fluvial deposits with thin early transgressive system tract marginally marine sandstones, as orbitally induced, glacioeustacy. Eustacy affected sand delivery, base level changes and discharge patterns. The Cape Peninsula lay at ~ 30°S during the Early Ordovician yet the primary control on sedimentation was the waxing and waning of ice sheets in the source area. We hypothesize the presence of an Early Ordovician ice sheet of 8–12 × 10 6 km 3 and a Polar Front at ~ 40°S.

High-resolution transgressive-regressive sequence stratigraphy of Chang 8 Member of Yanchang Formation in southwestern Ordos basin, northern China

The reservoirs within the Chang 8 Member of Yanchang (延长) Formation in the southwestern Ordos basin are characterized by low permeability. It is significant to build a high-resolution sequence stratigraphic framework for the exploration and development of these reservoirs. Based on an integrated investigation of well logs, seismic data, cores and outcrops, the Chang 8 Member is interpreted as a 3rd-order transgressive-regressive (T-R) sequence, which consists of six 4th-order T-R sequences, namely, K1, K2, K3, K4, K5 and K6, respectively, from base to top. Each 4th-order sequence comprises a transgressive systems tract overlain by a regressive systems tract. From K1 to transgressive systems tract of K4, it is characterized by the landward shifts of facies and overall retrogradational sequence stacking pattern due to accommodation increase more than sediment supply during the 3rd-order base-level rise. The distributary channels are mainly deposited in the transgressive systems tracts, and the interdistributary bays, sheet sands, and small-scale mouth bars are primarily developed in the regressive systems tracts in response to the 4th-order base-level fluctuations. From the regressive systems tracts of K4 to K6, it is characterized by the basinward shifts of facies and overall progradational sequence stacking pattern because of accommodation increase less than sediment supply during a fall in 3rd-order base level. The distributary channels are mainly accumulated in the transgressive systems tracts, and distributary channels and mouth bars are deposited in the regressive systems tracts as a result of the 4th-order base-level changes. Detailed stratigraphic and sedimentological analyses indicate that sequences K1, K2, K5 and K6, as well as the basin edge of K3 and K4, remain potential for exploration and development due to their high reservoir quality in response to the low accommodation and more sediment supply.

Facies stacking patterns in high-frequency sequences influenced by long-term sea-level change on a Permian Panthalassan oceanic atoll: An example from the Akiyoshi Limestone, SW Japan

Detailed core examination of Middle Permian (Guadalupian) lagoonal successions of mid-Panthalassan atoll carbonates (Akiyoshi Limestone) revealed distinct stratigraphic variation of facies and sequence stacking patterns. The long-term sea-level change reconstructed from the atoll stratigraphy of the Akiyoshi includes a Bolorian to Midian gentle sea-level fall, punctuated by a short-lived Murgabian sea-level rise. These sea levels can be regarded as eustasy, judging from the monotonous subsidence of the Akiyoshi atoll and its good correspondence with the sea-level curves of other areas. Two sequence types are defined. Sequence type A consisting of shoal to lagoon grainstone occurred in low accommodation during the sea-level highstand-to-falling stages of the late Murgabian to Midian. This sequence type is bounded by two successive subaerial exposure surfaces and exhibits the uneven but generally low δ 13C values throughout the section. In contrast, sequence type B occurred in increasing accommodation during the sea-level rise of the early Murgabian. This sequence type represents a composite sequence comprising parasequences and consists of various types of facies such as peritidal lime-mudstone, lagoonal packstone and boundstone, and shoal grainstone. Sequence type B is characterized by consistently high δ 13C values except for the negative shift immediately below the sequence-top surface. These lines of evidence demonstrated that long-term eustatic sea-level change obviously controls the variations of sequence types in atoll carbonates.

Sequence stacking patterns in the Western Canada foredeep: influence of tectonics, sediment loading and eustasy on deposition of the Upper Cretaceous Kaskapau and Cardium Formations

AbstractThe Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low‐gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3·5 Myr and forms a mudstone‐dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2·1 Myr, is about 100 m thick, sandstone‐rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge‐derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back‐step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW–SE, suggesting that the load shifted several hundred kilometres to the south. This re‐orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime.Within the longer‐term offlap–onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher‐frequency onlap–offlap cycles, each spanning ca 0·5 to 0·7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge‐shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distribution is explained readily in terms of current elastic flexural models. Most shoreface sandstones in the proximal foredeep show evidence of forced regression. Eustasy provides the most plausible explanation for relative sea‐level rise–fall cycles on the 125 kyr allomember timescale. Geometric relationships suggest eustatic oscillations of about 10 m. Forced regressive shoreface development was suppressed during Kaskapau allomembers 1 to 10 when the rate of flexural subsidence was at its highest.

Tectonic controls on sequence stacking pattern and along-strike architecture in the Pleistocene Mejillones Formation, northern Chile: Implications for sequence stratigraphic models

In the Mejillones Formation, a shallow-marine Pleistocene succession of northern Chile, the cyclic stratigraphic record is the result of the complex interaction of regional uplift, glacio-eustasy, local tectonics, sediment supply, and sedimentary processes. Stratal geometries, characteristics of sedimentary facies, and nature of sequence-bounding unconformities have been investigated to evaluate the influence of: (i) intrabasinal, short-term normal faulting on both along-strike variations in sequence architecture and genetic complexity of key stratal surfaces; and (ii) long-term regional uplift on sequence stacking pattern. The stratigraphic succession, dissected by small-displacement (few meters) normal faults striking obliquely with respect to the palaeo-shoreline trends, displays systematic variations in sequence architecture and the nature of bounding surfaces across them. Indeed, depending on position with respect to the fault plane, two basic types of internal organisation can be recognised in the examined shallow-marine, almost clastic-starved sequence. Within grabens it consists of a siliciclastic-rich transgressive systems tract (TST), which is bounded beneath by a transgressively modified, Glossifungites-demarcated sequence boundary (SB/RS), overlain by a mollusc-bearing falling-stage systems tract (FSST). The erosional downlap surface that separates the TST from the FSST is the regressive surface of marine erosion (RSME). On the footwall crests the combination of marine regressive erosion, during falls in relative sea-level, and uplift has resulted in complete removal of the sediments of the TST from these sites, leading to the formation of a tectonically enhanced basal unconformity composed of the RSME superimposed onto the previous SB/RS (SB/RS/RSME). The prominent lateral change in component units (systems tracts) and nature of bounding surfaces within the studied sequence is directly related to the presence of normal faults and indicates that fault activity had a major impact on the sequence stratigraphic evolution of the Mejillones Formation, enhancing subsidence within the grabens and promoting unconformities in the horsts. Overall, the Mejillones Formation records a long-term sea-level fall driven by the contemporaneous regional uplift, punctuated by repeated, high-frequency eustatic sea-level changes. The effect of this superimposition was that glacio-eustatic sequences were displaced progressively downward and basinward and stacked in a distinct downstepping, tectonically enhanced falling-stage sequence set, which reflects basin-wide loss in accommodation space. The sequence set is underlain by a composite RSME that becomes progressively younger basinward and is made up by the lateral and down-dip connection of a series of lower-rank sequence boundaries including hanging-wall SB/RSs and footwall SB/RS/RSMEs of successive sequences.

Differentiating the effect of episodic tectonism and eustatic sea‐level fluctuations in foreland basins filled by alluvial fans and axial deltaic systems: insights from a three‐dimensional stratigraphic forward model

AbstractMany studies of foreland basins have recognized a hierarchical organization in the stacking of sequences deposited by axial‐deltaic and alluvial fan systems. The hierarchy is often explained in terms of the competing control of eustasy and pulsed tectonic subsidence and the different frequencies at which these processes operate. Unravelling the relative contributions of tectonic and eustatic controls on the sequence stacking pattern is a fundamental question in foreland basin analysis, yet this is difficult because of the lack of independent stratigraphic evidence. In this study, a three‐dimensional numerical model is presented, which aids in the interpretation of alluvial successions in foreland basins filled by transverse and axial depositional systems, under conditions of variable tectonism and eustatic sea‐level change. The tectono‐sedimentary model is capable of simulating the hierarchical stratigraphic response to both eustatic and tectonic forcing, and is of higher resolution than previous models of foreland basin filling. Numerical results indicate that the onset of tectonic activity is reflected by rapid retrogradation of both depositional systems and by widespread flooding and onlap of carbonate sediments. Syntectonic fluvial patterns on the axial‐deltaic plain are dominated by bifurcating channels, swiftly relocating in response to the general rise in relative sea level induced by flexural subsidence. The resulting surface morphology of the axial delta is convex upwards. Syntectonic eustatic sea‐level fluctuations result in parasequence‐scale packages of retrograding and prograding fan and delta sediments bounded by minor flooding surfaces and type 2 sequence boundaries. Incised channels are rare within the syntectonic parasequences and are formed only during phases of tectonic quiescence when eustatic falls are no longer compensated by the subsidence component in the rise in relative sea level. Suites of amalgamating, axial channels corresponding to multiple eustatic falls delineate the resulting type 1 unconformities. Coarse‐grained, incised‐channel fills are found in the zone between the alluvial fan fringes and the convex‐upward body of the axial delta, as the axial streams tend to migrate towards this zone of maximum accommodation.

Stratigraphic correlation of Upper Cretaceous to Paleocene forearc basin sediments in Northeast Japan: cyclic sedimentation and basin evolution

Cretaceous to Paleocene sediments are distributed across a 200 km-wide and 1400 km-long belt in Northeast Japan to Sakhalin. Twenty-six stratigraphic sections including land-based and offshore drill holes and surface sections are correlated to delineate the sedimentary history and basin evolution of the Yezo forearc basin. Two second-order shallowing-upward cycles are recognized in Hokkaido but are obscure in north Honshu. Shallow-marine to paralic sediments of the first cycle (early Albian to Turonian) are represented by the Mikasa Formation (late Albian to Turonian) as the western marginal facies, while the eustatic curves (e.g. Haq curves) reach the highstand maximum at the late Cenomanian. The Campanian to Maastrichtian shallow- and non-marine facies of the second cycle (Coniacian to Maastrichtian) are characterized by the Hakobuchi Group in Hokkaido and its correlatives in north Honshu. This large-scale shallowing may have reflected directional changes of the Izanagi-Kula plate motion near the early Campanian. The uppermost Maastrichtian to Upper Paleocene is possibly absent everywhere in the basin. The shallow-marine to non-marine facies often show several third-order depositional sequences with upward-coarsening facies successions. They may have been controlled by global eustasy even in an active-margin setting, since the stacking patterns of sequences and the timing of sequence boundaries appear considerably concordant with the oscillation patterns of the Haq curves.

Characteristics, stratigraphic architecture, and time framework of multi-order mixed siliciclastic and carbonate depositional sequences, outcropping Cisco Group (Late Pennsylvanian and Early Permian), Eastern Shelf, north-central Texas, USA

The Cisco Group on the Eastern Shelf of the Midland Basin is composed of fluvial, deltaic, shelf, shelf-margin, and slope-to-basin carbonate and siliciclastic rocks. Sedimentologic and stratigraphic analyses of 181 meter-to-decimeter-scale depositional sequences exposed in the up-dip shelf indicated that the siliciclastic and carbonate parasequences in the transgressive systems tracts (TST) are thin and upward deepening, whereas those in highstand systems tracts (HST) are thick and upward shallowing. The sequences can be subdivided into five types on the basis of principal lithofacies, and exhibit variable magnitude of facies shift corresponding to variable extents of marine transgression and regression on the shelf. The sequence stacking patterns and their regional persistence suggest a three-level sequence hierarchy controlled by eustasy, whereas local and regional changes in lithology, thickness, and sequence type, magnitude, and absence were controlled by interplay of eustasy, differential shelf subsidence, depositional topography, and pattern of siliciclastic supply. The outcropping Cisco Group is highly incomplete with an estimated 6–11% stratigraphic completeness. The average duration of deposition of the major (third-order) sequences is estimated as 67–102 ka on the up-dip shelf and increases down dip, while the average duration of the major sequence boundaries (SB) is estimated as 831–1066 ka and decreases down dip. The nondepositional and erosional hiatus on the up-dip shelf was represented by lowstand deltaic systems in the basin and slope.

Discussion of “Neoproterozoic glacial and gravitational sedimentation on a continental rifted margin: the Jequitaı́-Macaúbas sequence (Minas Gerais, Brazil)” by Uhlein et al.

Five unconformity-bounded depositional sequences have been seismically defined on the Tyrrhenian margin offshore Southern Tuscany, Italy. The set of sequences encompasses a ~ 500 ka interval, each sequence recording a ~ 100-ka orbitally-driven climate-eustatic cycle. Similarly to most of Quaternary continental margins, sequences on the Southern Tuscany margin show an overall regressive motif reflecting shelf sediment progradation basically during the prolonged phases of sea level fall that characterise the Middle–Upper Pleistocene 100-ka cyclicity. On seismic profiles, however, progradational units within each sequence display a variable clinoform geometry, interpreted as a record of the alternation of highstand, falling and lowstand intervals within 100-ka cycles. The observed geometric variability is thus used as a key for separating systems tracts otherwise merging into a general progradational/regressive sedimentary body composing the bulk of each sequence. On a longer term, the interplay between sea level changes modulated by 100-ka cycles and regional subsidence controls the stratigraphic architecture and the growth pattern of the continental margin. This effect is documented by the overall aggradational/retrogradational stacking pattern of sequences, superimposed on shelf margin progradation and reflecting the renewal of accommodation space landward of the shelf margin and its exploitation at each cycle.

Strike Variability of Carbonate Platform Margin Stratal Architecture and Cycle Stacking Patterns: Outcrop and Seismic Examples from Lower Permian Depositional Sequences of the Permian Basin, U.S.A.: ABSTRACT

Strike variability in stratal architecture, cycle stacking patterns and facies differentiation within sequences exemplifies the potential for differential stratigraphic response of platforms to eustasy. Variability exists within base-level cycles of several scales within a compound stratigraphic hierarchy, though the causes may vary among different scales. Cross-sections from outcrops in the Sierra Diablo document the framework and stacking pattern of 3rd-order sequences (HFS), including: two progradational middle Wolfcampian HFS (mW1-2), one backstepped upper Wolfcampian HFS (uW1), one progradational lower Leonardin HFS (L1), aggradational middle (L2) and upper Leonardian HFS (L3), three progradatic upper Leonardian HFS (L4-6), and two backstepped, aggradational upper Leonardian HFS (L7-8). Seismic lines from the northern Delaware and Midland Basins and San Simon Channel area document the regional consistency of 3rd-order sequence stacking patterns (a response to eustasy), but show variability related to local subsidence, antecedent topography (owing to deeper structures and platform margin erosion), windward vs. leeward facing, and siliciclastic sediment supply. Sequences L2 and L3 appear to exhibit the greatest variability in stacking pattern. Strike variability in 4th/5th-order cycle stacking patterns within 3rd-order sequences as studied in outcrop is greatest in sequences L2 and L3, in which headland-bight margin trends are developed on a lateral scalemore » of 1-2 miles. Aggrational to backstepping reef-margin facies with steep ([le]35[degrees]) foreslopes developed along headlands. Mudstones abut these margin facies abruptly along headlands and may contain megabreccias at the toe-of-slope. More gently sloping (10-15[degrees]) [open quotes]ramp[close quotes] margin strata composed of fusulinid packstones characterize bights.« less

Sequence stratigraphy in active extensional basins: implications for the interpretation of ancient basin-fills

Systematic variations in sequences, their component units and their stacking patterns exist within rift basins. These variations can be related to displacement gradients associated with the large-scale (12–50 km) segmentation of normal fault zones, which control accommodation, sediment supply and basin physiography. Sediment supply is also strongly influenced by the nature of pre-rift drainage networks and variations in bedrock lithology. High rates of hangingwall subsidence close to the centre of normal fault segments may cancel out the effects of glacio-eustatic sea-level fall, so that accommodation development is normally characterized by the continual addition of new space. The resulting sequences lack type 1 sequence boundaries and lowstand systems tracts, and stack into aggradational sequence sets. The adjacent footwall is subject to uplift, which may lead to subaerial exposure and incision, generating a composite type 1 sequence boundary. Away from the fault zone and near segment boundaries, slip rates are much lower and hence relative sea-level change is dominated by eustasy. Here, falls in relative sea level are important, resulting in type 1 sequences. Sequence stacking patterns reflect not only eustasy and local fault-controlled subsidence, but also the interaction of these with adjacent uplifting footwalls and/or regional uplift. Tectonic influence on sequence development will be more pronounced during greenhouse times than icehouse times.

Forward Modeling of High-Frequency, Glacio-Eustatic, Depositional Sequences: An Example from Middle Pennsylvanian Shelf Carbonates of the SW Paradox Basin, Honaker Trail, Utah: ABSTRACT

Middle Pennsylvanian (Desmoinesian) shelf carbonates in the southwest Paradox basin display three superimposed orders of stratigraphic cyclicity with a systematic, vertical succession of facies, parasequence, and sequence stacking patterns. Fifth-order parasequences (34 parasequences in 645 ft section; averaging 20 ft thick, 40 k.y. duration) are grouped into fourth-order sequences (averaging 100 ft thick approximately 400 ky. duration), which in turn vertically stack to define a third-order accommodation cycle ({>=} 650 ft thick 2.4 m.y. duration). Fifth-order parasequences are comprised of shallowing-upward packages of dominantly subtidal shelf carbonates with sharp parasequence boundaries (exposure or flooding surfaces). Fifth-order parasequences are packaged into fourth-order sequences (type 1) bounded by regionally correlative subaerial exposure surfaces. These type 1 sequences contain a downdip, basinally restricted lowstand wedge of evaporites and quartz clastics, a regionally correlative transgressive shaly mudstone (condensed section), and a highstand systems tract composed of thinning-upward, aggradational, fifth-order parasequences. Systematic variation in the thickness of fourth-order sequences (thinning followed by upward thickening), as well as the number of fifth-order parasequences/sequences (decreasing followed by increasing number), define a third-order accommodation trend, which is also regionally correlative. High-frequency parasequences and sequences are interpreted as aggradational allocycles generated in response to glacio-eustatic sea level fluctuations drivenmore » by Milankovitch climatic forcing. The fifth-order parasequences record the earth's orbital eccentricity cycle (mean period of 41 ky.), and the fourth-order sequences result from the long eccentricity cycle (mean period of 413 k.y.).« less

Editorial

Systematic variations in sequences, their component units and their stacking patterns exist within rift basins. These variations can be related to displacement gradients associated with the large-scale (12–50 km) segmentation of normal fault zones, which control accommodation, sediment supply and basin physiography. Sediment supply is also strongly influenced by the nature of pre-rift drainage networks and variations in bedrock lithology. High rates of hangingwall subsidence close to the centre of normal fault segments may cancel out the effects of glacio-eustatic sea-level fall, so that accommodation development is normally characterized by the continual addition of new space. The resulting sequences lack type 1 sequence boundaries and lowstand systems tracts, and stack into aggradational sequence sets. The adjacent footwall is subject to uplift, which may lead to subaerial exposure and incision, generating a composite type 1 sequence boundary. Away from the fault zone and near segment boundaries, slip rates are much lower and hence relative sea-level change is dominated by eustasy. Here, falls in relative sea level are important, resulting in type 1 sequences. Sequence stacking patterns reflect not only eustasy and local fault-controlled subsidence, but also the interaction of these with adjacent uplifting footwalls and/or regional uplift. Tectonic influence on sequence development will be more pronounced during greenhouse times than icehouse times.