Aggradational Stacking Pattern Research Articles

Shelf-edge trajectories and stratal stacking patterns: Their sequence-stratigraphic significance and relation to styles of deep-water sedimentation and amount of deep-water sandstone

Using a seismic database from the Qiongdongnan Basin in the South China Sea, this study demonstrates that shelf-edge trajectories and stratal stacking patterns are reliable, but understated, predictors of deep-water sedimentation styles and volumes of deep-water sand deposits, assisting greatly in locating sand-rich environments and in developing a more predictive and dynamic stratigraphy. Three main types of shelf-edge trajectories and their associated stratal stacking patterns were recognized: (1) flat to slightly falling trajectories with negative trajectory angles () (−2° to 0°) and negative shelf-edge aggradation to progradation ratios () (−0.04 to 0) and associated progradational and downstepping stacking patterns with low clinoform relief () (150–550 m [492–1804 ft]) and negative differential sedimentation on the shelf and basin () (−0.6 to 0); (2) slightly rising trajectories with moderate (0°–2°) and medium (0–0.04), and associated progradational and aggradational stacking patterns with intermediate (250–400 m [820–1312 ft]) and intermediate (0–0.6); and (3) steeply rising trajectories with high (2°–6°) and high (0.04–0.10) and associated dominantly aggradational stacking patterns with high (350–650 m [1148–2132 ft]) and high (1–2). Each trajectory regime represents a specific stratal stacking patterns, providing new tools to define a model-independent methodology for sequence stratigraphy. Flat to slightly falling shelf-edge trajectories and progradational and downstepping stacking patterns are empirically related to large-scale, sand-rich gravity flows and associated bigger and thicker sand-rich submarine fan systems. Slightly rising shelf-edge trajectories and progradational and aggradational stacking patterns are associated with mixed sand/mud gravity flows and moderate-scale slope-sand deposits. Steeply rising shelf-edge trajectories and dominantly aggradational stacking patterns are fronted by large-scale mass-wasting processes and associated areally extensive mass-transport systems. Therefore, given a constant sediment supply, then , , , and are all proportional to intensity of mass-wasting processes and to amounts of mass-transport deposits, and are inversely proportional to the intensity of sand-rich gravity flows and to amounts of deep-water sandstone. These relationships can be employed to relate quantitative characteristics of shelf-edge trajectories and stratal stacking patterns to deep-water sedimentation styles.

Constraining the sedimentology and stratigraphy of submarine intraslope lobe deposits using exhumed examples from the Karoo Basin, South Africa

Intraslope lobe deposits provide a process record of the infill of accommodation on submarine slopes and their recognition enables the accurate reconstruction of the stratigraphic evolution of submarine slope systems. Extensive exposures of discrete sand-prone packages in Units D/E and E, Fort Brown Formation, Karoo Basin, South Africa, permit analysis of the sedimentology and stacking patterns of three intraslope lobe complexes and their palaeogeographic reconstruction via bed-scale analysis and physical correlation of key stratal surfaces. The sand-prone packages comprise tabular, aggradationally to slightly compensationally stacked lobe deposits with constituent facies associations that can be attributed to lobe axis, lobe off-axis, lobe-fringe and distal lobe-fringe environments. Locally, intraslope lobe deposits are incised by low aspect ratio channels that mark basinward progradation of the deepwater system. The origin of accommodation on the slope for lobe deposition is interpreted to be due to differential compaction or healing of scars from mass wasting processes. The stacking patterns and sedimentary facies arrangement identified in this study are distinct from those of more commonly recognized basin-floor lobe deposits, thereby enabling the establishment of recognition criteria for intraslope lobe deposits in other less well exposed and studied fine-grained systems. Compared to basin floor lobes, intraslope lobes are smaller in volume, influenced by higher degrees of confinement, and tend to show aggradational stacking patterns.

Growth Styles of Shelf-Margin Clinoforms: Prediction of Sand- and Sediment-Budget Partitioning Into and Across The Shelf

Abstract Growth styles of shelf-margin clinoforms are reliable but understudied predictors of sand- and sediment-budget partitioning into and across the shelf. Three discrete clinoform-growth styles were recognized, including (1) strongly progradational shelf-margin clinoforms with low growth-trajectory angles (Gct,), low aggradation to progradation ratios (A/P), low clinoform heights (Hc), and long clinoform length (Lc); (2) mixed progradational and aggradational shelf-margin clinoforms with moderate Gct, moderate A/P, intermediate Hc, and moderate Lc; and (3) strongly aggradational clinoforms with high Gct, high A/P, high Hc, and short Lc. In the South China Sea dataset considered, strongly progradational shelf-margin clinoforms exhibit flat progradational and at times a mildly aggrading stacking patterns, whereas mixed progradational and aggradational clinoforms display stacking patterns with significant progradation and aggradation. Strongly aggradational shelf-margin clinoforms are dominated by aggradational stacking patterns. Each clinoform-growth style therefore represents a specific stratal stacking pattern, providing an important tool for approaching a model-independent methodology in sequence stratigraphy. In the study dataset, strongly progradational and strongly aggradational shelf-margin clinoforms are fronted by sand-prone submarine fan systems with high sand–shale ratios and mud-dominated mass-transport systems with low sand–shale ratios, respectively. Mixed progradational and aggradational clinoforms are associated with mixed sand–mud submarine canyon systems with moderate sand–shale ratios. Additionally, strongly progradational clinoforms partitioned great volumes of sediment into deep-water areas, as reflected by high rates of shelf-edge progradation and apparent toplap. Strongly aggradational clinoforms, in contrast, stored great volumes of sediment on the shelf itself, as indicated by high rates of shelf-edge aggradation and very thick clinoform topsets. Gct and Hc therefore increase linearly with sediment volumes partitioned into the shelf, but decrease linearly with sand- and sediment-budget partitioning into deep-water areas, given a constant sediment-supply condition. Growth styles of shelf-margin clinoforms are thus good predictors of source-to-sink sand- and sediment-volume partitioning into and across the shelf, assisting greatly in developing a more dynamic stratigraphy.

Sequence stratigraphy and architectural variability in Late Eocene lacustrine strata of the Dongying Depression, Bohai Bay Basin, Eastern China

Stratigraphic sequences and architectural variability in the Late Eocene lacustrine strata of the Dongying Depression, eastern China, were investigated using the interpretation of 2-D and 3-D high-resolution seismic profiles, analysis of spontaneous potential and resistivity curves, and observation of drill cores. Four third-order sequences controlled by syndepositional faults or fault slope break zones were identified, based on the characteristics of sequence boundaries and sedimentary successions. The architecture of the sequences in the different structural belts of the depression is complicated by the relationship between the rate at which fault-controlled accommodation was created and the rate of sediment supply. At fault margins, the rate of sediment supply exceeded accommodation space. Here, lowstand systems tracts consist of lowstand fan deltas with small progradational to retrogradation stacking patterns controlled by steeply dipping, parallel and cross-shaped syndepositional faults or fault slope-break zones; transgressive systems tracts consist of fan deltas with retrogradational to aggradational stacking patterns; and highstand systems tracts consist of fan deltas with normal regressive or progradational stacking pattern. At hinged margins, the rate of sediment supply was equal to or exceeded accommodation controlled by faults. Lowstand systems tracts at hinged margins consist of incised channel fills deposited on the landward side of gently dipping parallel and broom-shaped syndepositional faults or fault slope break zones and lowstand fans or sublacustrine fans deposited on the shores of lakes. Transgressive systems tracts consist of delta systems and shore to shallow-lake subfacies with retrogradational stacking patterns. Highstand systems tracts consist of braided deltas and fluvial delta systems with progradational or normal regressive and aggradational stacking patterns. Along the axis, the rate of sediment supply far exceeded accommodation. Only the lowstand systems tracts developed, consisting of lowstand deltas deposited on the landward side of the syndepositional faults or fault slope break zones, and lowstand fans or sublacustrine fans deposited on the lakeward side of the zones. Here, transgressive systems tracts consist of thin, deep lacustrine deposits and fluvial delta systems with retrogradational or transgressive stacking patterns; and highstand systems tracts consist of thick fluvial delta systems with a progradational configuration or normal regressive stacking patterns.The four kinds of syndepositional fault slope-break zones controlled the stratal architecture of sequences and the distribution of lowstand systems tracts. Sand bodies within lowstand systems tracts provide suitable conditions for the formation of hydrocarbon reservoirs when they are overlain by sediments from transgressive systems tracts, and are therefore favorable sites for lithostratigraphic trap exploration.

Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wave-dominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave–tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Sedimentary growth pattern on the northern Argentine slope: The impact of North Atlantic Deep Water on southern hemisphere slope architecture

Large sedimentary deposits consisting of several major contourite drifts were studied by means of high-resolution multichannel seismic data at the middle slope along the Northern Argentina Continental Margin to determine their evolutionary stages as well as to identify and assess the possible impact of Northern Source Deep Water (NSDW) on the slope architecture. The imaged contouritic sediments allow decoding on the regional paleo-oceanographic setting of the last 32Ma.Earliest contouritic sedimentation can be observed close to the Eocene/Oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies, pointing toward a hydrodynamically turbulent flow pattern. This facies is most likely related to the opening of the Drake Passage associated with global cooling and a strengthening of surface, intermediate and deep ocean currents in the Southern Ocean. During the Middle Miocene plastered drift sequences with an aggradational reflection pattern were deposited. Their depositional style indicates weak, non-turbulent current conditions, which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of NSDW during the Mid-Miocene climatic optimum. On top, the formation of plastered drift sequences led to the modern extent of the Ewing Terrace, which was probably controlled by the continuous strengthening and thickening of NSDW until the final closure of the Central American Seaway (CAS). During the Pliocene and Quaternary, after the complete closure of the CAS and under the influence of the full force of the NSDW, mounded plastered drift sequences are built upon the Ewing Terrace generating the modern slope morphology. Therefore, we suggest that deep-water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern South Atlantic since the Middle Miocene.

Fluvial Sequence Stratigraphy: The Wapiti Formation, West-Central Alberta, Canada

Abstract Outcrop exposures and high-resolution subcrop data allow the description and mapping of four stratigraphic discontinuities in the fully nonmarine strata of the Campanian–Maastrichtian Wapiti Formation (Western Interior foreland basin, Alberta, Canada). This framework defines four unconformity-bounded depositional sequences (A–D), based on sequence stratigraphic methodology, well-log patterns, and facies analysis. In ascending order, sequence A records the transition from the underlying marine facies of the Puskwaskau Formation into the Wapiti fluvial facies, and consists of strongly progradational and aggradational stacking patterns. Sequences B and C show a similar pattern of basal amalgamated channel-fill deposits that grade upwards into floodplain-dominated strata. Finally, sequence D is dominated by channelized sediments and extensive overbank facies. An aggradational stacking pattern suggests deposition under high-accommodation conditions. Maximum flooding surfaces are interpreted within fine-grained deposits in the upper portions of sequences C and D. They are tied to regionally extensive coals that accumulated more than 250 km away from the coeval shoreline. This study provides new evidence that major coal seams may represent the extension of marine maximum flooding surfaces into the downstream-controlled fluvial realm. Furthermore, the new stratigraphic framework of subaerial unconformities and maximum flooding surfaces provides the means of subdividing the previously undifferentiated fully nonmarine Wapiti Formation into depositional sequences and component systems tracts.

Process Regime Changes Across a Regressive to Transgressive Turnaround in a Shelf-Slope Basin, Eocene Central Basin of Spitsbergen

Abstract The sedimentological, architectural, and stratigraphic database from one investigated shelf-margin clinothem complex in the Eocene Central Basin of Spitsbergen suggests that dominating process regimes and facies products may change several times during a sea-level cycle, even though the basin-wide processes are constant. Continuous mountainside exposures were investigated by a combination of “walking-out” of stratigraphic surfaces and geometrical elements, closely spaced detailed vertical sections, and tracing on helicopter photo mosaics. The clinothem complex represents a regressive to transgressive shoreline complex and consists of five clinothem sets, recognized by specific stacking patterns and sedimentary facies. The regressive part preserves wave- and river-dominated deltaic deposits in a progradational to aggradational stacking pattern. The delta clinothems consist of tide-influenced distributary channels, wave- and hyperpycnal-flow-dominated mouth bars that on the shelf sourced turbidite-filled slope channels, and slope lobes. The transgressive part of the clinothem complex preserves strongly tide-dominated estuarine deposits underlain by tidal ravinement surfaces that cut down into deposits of wave-dominated deltas. This change in facies product on the inner shelf from wave to tidal at the regressive to transgressive turnaround, was however not a basin-wide process change, inasmuch as the distributary channels also reflect tidal influence. This suggests that tidal currents were present only in areas protected from waves and amplified by the lateral confinement of the estuary. There was a process-regime change on the middle to outer shelf, from wave-dominated highstand and falling-stage delta complex to river-dominated (and wave-influenced) early-lowstand delta complex. The delta complex prograded across the shelf during highstand, and during falling stage the relative sea-level fall forced the deltas out to shelf edge. The early-lowstand delta mouth bars are volumetrically dominated by hyperpycnites, indicating dominance of riverine processes. The proportion of the hyperpycnal-flow deposits decreased during the late lowstand, and the delta complex became wave-dominated anew. The process-regime change from wave dominance to fluvial dominance and back to wave dominance is interpreted to have occurred due to the seaward, and then landward, shift of the feeding river system, whereas the tidal effects were dependent on shelter from waves and on the shoreline configuration.

Seismic stratigraphy of the Deglacial deposits of the Rhône prodelta and of the adjacent shelf

In order to achieve a synthesis of the stratigraphic organization of the Deglacial deposits of the inner/middle shelf in front of the Rhône delta plain, we merged high resolution (Mini-sparker and Sparker), and very high resolution (chirp and mud-penetrator) seismic data into a single seismic database. Thus, the merged seismic database improves the lateral correlation between eastern and western parts, separated by the Rhône Incised Valley deposits. As a result the interpretation of seismic units in relation to local and global environmental changes was refined. The Deglacial deposits rest on a basal erosional discontinuity capping a complex of Pleistocene prograding wedges. The identified units make up Transgressive and Highstand Systems Tracts, and are bounded by flooding surfaces. The main flooding surfaces are (1) the transgressive surface (D200) which forms the lower boundary of the Deglacial deposits and (2) the maximum flooding surface which forms the boundary between the Transgressive and Highstand Systems Tracts. Regarding the data set the study area is divided into three parts depending on the stacking pattern and main control factors. In the western area the units present an aggradational stacking pattern and the rate of sediment supply and dynamic conditions seems coupled with glacio-eustacy role on sedimentary units building and evolution. In the central area the units present an overall backstepping pattern mainly controlled by glacio-eustacy. In the eastern area the units present an aggradational stacking pattern and both the glacio-eustacy and rate of sediment supply have an important role. The lower parasequence of the Transgressive Systems Tract (U200) is due to the reworking of Würmian terraces. Above, the parasequence U300 is a transgressive body formed during a rapid sea-level rise. The two upper parasequences are interpreted as ancestral coastal systems with a backstepping pattern. The first coastal system (U400) is due to a decrease in the rate of sea-level rise that occurred during the Younger Dryas. The second coastal system (U500) may be due to a similar event or to an increased rate of sediment supply during a constant sea-level rise. Finally the Highstand Systems Tract is composed of three units: (i) unit U601, located to the West and interpreted as a subaqueous delta, (ii, iii) units U600 and U610, located to the East and interpreted as prograding deltaic lobes.

Architecture of coastal and alluvial deposits in an extensional basin: the Carboniferous Joggins Formation of eastern Canada

Abstract The Joggins Formation was deposited in the Cumberland Basin, which experienced rapid mid‐Carboniferous subsidence on bounding faults. A 600 m measured section of coastal and alluvial plain strata comprises cycles tens to hundreds of metres thick. The cycles commence with coal and fossiliferous limestone/siltstone intervals, interpreted as widespread flooding events. These intervals are overlain by coarsening‐upward successions capped by planar‐based sandstone mounds, up to 100 m in width that represent the progradation of small, river‐generated delta lobes into a standing body of open water developed during transgression. The overlying strata contain sand‐rich heterolithic packages, 1–8 m thick, that are associated with channel bodies 2–3 m thick and 10–50 m wide. Drifted plant debris, Calamites groves and erect lycopsid trees are preserved within these predominantly green‐grey heterolithic sediments, which were deposited on a coastal wetland or deltaic plain traversed by channel systems. The cycles conclude with red siltstones, containing calcareous nodules, that are interbedded with thin sandstones and associated with both single‐storey channel bodies (1–1·5 m thick and 2–3 m wide) and larger, multistorey channels (3–6 m thick) with incised margins. Numerous channel bodies at the same level suggest that multiple‐channel, anastomosed river systems were developed on a well‐drained floodplain. Many minor flooding surfaces divide the strata into parasequences with dominantly progradational and aggradational stacking patterns. Multistorey channel bodies are relatively thin, fine grained and modestly incised, and palaeosols are immature and cumulative. The abundance and prominence of flooding surfaces suggests that base‐level rise was enhanced, whereas the lack of evidence for abrupt basinward stepping of facies belts, coupled with the absence of strong fluvial incision and mature palaeosols, suggests that base‐level fall was suppressed. These architectural features are considered to reflect a tectonic architectural signature, in accordance with the high‐subsidence basinal setting. Evidence for restricted marine influence and variation in floral assemblages suggests modulation by eustatic and climatic effects, although their relative importance is uncertain.

Palaeoenvironmental changes in the Pliocene Intra-Apenninic Basin, near Bologna (Northern Italy)

Combined palaeoecologic and sedimentologic studies reveal remarkable palaeoenvironmental changes in the Pliocene Intra-Apenninic Basin (PIB) of the Bologna Apennines (Northern Italy). At the northern margin of PIB, the stratigraphic succession is approximately 1000 m thick, and displays an overall coarsening-upward trend, including a lower mudstone-dominated succession (Argille Azzurre Formation) of Zanclean age, overlain by a sandstone-rich unit (M. Adone Formation), assigned to the Zanclean-Piacenzian. The vertical succession of biofacies within the Argille Azzurre Fm. indicates an obvious shallowing-upward tendency, from slope/outer-shelf (150–300 m water depths) to inner-shelf (30–50 m) environments. A vertical cyclic pattern of facies, with alternating sandy siltstones and sandstone bodies, characterizes M. Adone Formation. Each elementary cycle, about 30–50 m thick, includes a couple of siltstone-dominated inner-shelf and prodelta deposits, passing upwards into progradational delta front, tabular sand bodies. These high-rank cycles, spanning an interval of time in the order of magnitude of 40 ky, are separated by flooding surfaces, clearly recognizable on the basis of palaeoecologic analysis, and reflect an aggradational stacking pattern of parasequences superposed to the general shallowing-upward trend.

Changes in depositional systems and sequences in response to basin evolution in a rifted and inverted basin: an example from the Neogene Niigata-Shin'etsu basin, Northern Fossa Magna, central Japan

The Niigata-Shin'etsu basin, which occurs along the eastern margin of the Sea of Japan, was generated in Middle Miocene time as a failed rift basin in relation to the backarc opening of the Japan arc, and was converted to a compressional basin in Late Miocene time. The basin-filling succession can be divided into four tectonostratigraphic units (stages), based on characteristics of depositional systems, depositional sequences, tectonic-subsidence rate, and sediment supply. These units coincide in time with major tectonic events during backarc rifting and subsequent basin inversion. Unit I (16–13.5 Ma) represents a syn-rift stage, characterized by rapid basin subsidence. The rate of accommodation exceeded the rate of accumulation, resulting in a retrogradational stacking pattern of muddy slope to basin floor systems. Unit II (13.5–6.5 Ma) represents a post-rift thermal-subsidence stage. The rate of accommodation nearly equaled the rate of accumulation, resulting in an aggradational stacking pattern of five third-order depositional sequences, mainly comprising submarine-fan turbidite systems affected by supply of coarse clastics. Unit III (6.5–1 Ma) represents the basin-inversion stage, showing no subsidence or uplift. The accumulation rate exceeded the rate of accommodation, resulting in rapid progradation of depositional sequences composed of slope, shelf, shallow-marine, delta, and fluvial systems. Unit IV (1 Ma–present) represents an intense compressional stress field; sedimentation has been restricted to a synclinal area due to syndepositional folding and uplift. Changes in basin subsidence rate and sediment supply, resulting from tectonics and climate change in the basin and the provenance, strongly affected characteristics of the depositional systems and sequences. The change in basin subsidence also created a basinwide depositional cycle, whereas eustasy-affected relative sea-level changes created higher-order depositional sequences during relatively stable phases of tectonics.

中東地域下部白亜系炭酸塩岩油層における５点水攻法下の水付き現象とその地質学的要因

Premature water breakthrough was observed in a Lower Cretaceous carbonate reservoir under five spot pattern water injection in a Middle East oil field. Openhole log resistivity anomalies in recently drilled wells and time lapse pulsed neutron logs suggested preferential water movement in certain horizons. Reservoir heterogeneity depending on lithological variation is identified as a cause of the water breakthrough.Two high permeable lithofacies were defined by core and thin section observation. One is altered bioclast dominant coarse grainstone/packstone in the middle part of the reservoir. The other is very fine altered bioclastic peloidal grainstone/packstone in the upper part of the reservoir. These lithofacies are composed mainly of rarely micritized bioclastic grains with rigid framework preserving intergranuler porosity. Large molds and vugs associated with rudists in the upper part of the reservoir are locally connected and contribute to the fluid flow.Relative sea level change and paleo-topography are recognized as important factors to control lithofacies distribution. Areal distribution of the lithofacies shows aggradational stacking pattern and abrupt lateral lithofacies change particularly in the middle part of the reservoir. This suggests that simple carbonate ramp model currently applied is not appropriate for reservoir characterization purpose in this reservoir.

Repetitive stratal patterns in a foreland basin sandstone and their possible tectonic significance

Two distinctive, repetitive stratal patterns within members of the Upper Cretaceous Blackhawk Formation in Utah are recognized by comparison of (1) progradational distances of individual parasequences within each member and (2) the position of the updip termination of marine facies for each parasequence. The oldest parasequence within each member always progrades substantially farther than the younger parasequences within the same member, regardless of whether these parasequences are separated by sequence boundaries. The pattern of progradational distances results in an initial progradational to later aggradational stacking pattern within each member. This pattern is observed in all members of the Blackhawk Formation where there is sufficient outcrop exposure to trace the nonmarine-to-marine transition, which is used to establish the progradational distances. These stratal patterns are interpreted to define the path of relative change of sea level through time within the Cretaceous foreland basin and may provide information characterizing thrust-sheet emplacement. Flooding events bounding parasequences may represent periods of punctuated thrusting, with the main flooding surface (member boundaries) representing the period of greatest thrust-sheet movement.

Recognition of Depositional Sequences and Stacking Patterns, Late Devonian (Frasnian) Carbonate Platforms, Alberta Basin: ABSTRACT

Six depositional sequences bounded by regional unconformities or their correlative equivalents (sequence boundaries) have been recognized in Late Devonian (Frasnian) carbonate platforms in the Alberta basin. These sequences consist of a predictable vertical succession of smaller scale shoaling-upward cycles (parasequences). Parasequences are arranged in retrogradational, aggradational, and progradational stacking patterns that can be modeled as a sediment response to relative changes in sea level. Sequence boundaries are recognized by onlap onto underlying shelf or shelf margin strata. This onlap includes shelf margin wedges and deep marine onlap. In outcrop sections shelf margin wedges exhibit an abrupt juxtaposition of shallow water facies over deeper water deposits with no gradational facies changes at the boundaries. High on the platform, subaerial exposure fabrics may be present. The shelf margin wedges are interpreted to have formed during lowstands in sea level and typically exhibit an aggradational stacking pattern. On the platform, two types of sequences are recognized. A type 1 cycle occurs where the sequence boundary is overlain by a flooding surface and subsequent parasequences exhibit retrogradational stacking. In a type 2 cycle the sequence boundary is overlain by an aggradational package of shallow water parasequences, followed by a retrogradational package. These two typesmore » of sequences can be modeled using a sinusoidal eustatic sea level curve superimposed on thermo-tectonic subsidence.« less

Eustatic Controls on Stratification and Facies Associations in Deep-Water Deposits, Great Valley Sequence, Sacramento Valley, California: ABSTRACT

The Great Valley sequence consists of submarine fan deposits that are divided into laterally persistent sandstones and conglomerates separated by thick shaly intervals. The frequency of sandstone-shale successions in the Great Valley closely corresponds to the occurrence of major eustatic falls observed elsewhere in the world during the Upper Jurassic and Cretaceous. This close correspondence between the number of observed fans and sea level cycles has implications for the timing of fan development and facies models of deep-water deposits. On the basis of seismic expression, deep-water deposits from various basins have been divided by Mitchum into a sand-prone lower fan, which has a sharp basal contact, and a younger upper fan, which exhibits downlap onto and over the lower fan. Sand-prone members of the Great Valley (e.g., Venado and Forbes) are sharp-based, fining-upward units that have an aggradational or retrogradational stacking pattern of fan lobes. Massive sandstone, pebbly sandstone, conglomerate, pebbly mudstone, turbidites, and lenticular turbidites compose the fan lithologies. These rocks are typically referred to as inner fan channel or midfan lobes. In contrast, shale-dominated sections with thin-bedded turbidites (e.g., Boxer and Yolo) that have been variously described as basin plain, outer fan, inner fan levee, and slope correspondmore » to the upper fan. Sharp basal fan contacts, textural contrasts between the lower and upper fans, and encasement of sand-prone fans in thick shaly sections indicate that fan development is an episodic rather than a continuous process. Rapid eustatic fall causing stream incision and shelf bypass is a likely mechanism for basin-wide and interbasinal fan development. Lithofacies encountered in fan deposits are related to grain size in the source area; specific lithologies in Great Valley fans (e.g., conglomerate) may be absent in other basins.« less