Early Highstand Research Articles

Generation and evolution of oolitic shoal reservoirs in the Permo-Triassic carbonates, the South Pars Field, Iran

Ooid grainstone is the main reservoir rock in the Permo-Triassic Dalan and Kangan formations (Khuff equivalents) in many gas fields of the Persian Gulf and neighbouring areas. Ooids with a dominant aragonite mineralogy accumulated in a series of linear shoals and sand banks parallel to the shoreline, on the shallow parts of a vast epeiric carbonate platform. On the basis of the sequence stratigraphic analysis, these reservoir facies mainly developed during relative sea-level rises and increases in accommodation space. Integrated petrographic and geochemical studies reveal that ooid grainstone was altered through a complex diagenetic history, largely controlled by water chemistry, as a result of relative sea-level fluctuations. Two main types of ooid grainstone are the result: dolomitised grainstone or type H and oomouldic grainstone or type M. Dolomitised grainstone is commonly associated with the transgressive systems tract (TST), and developed under hypersaline conditions. In comparison, oomouldic grainstone is predominant in the early highstand systems tract (HST), which was affected by intensive meteoric diagenesis. Petrophysical study indicates that the reservoir properties of these rocks are largely a function of diagenesis. On the basis of their dominant pore types and diagenetic modifications, the ooid grainstone facies of the studied formations are grouped into five reservoir rock types. The spatial and temporal distribution of these rock types and their diagenetic evolution can be predicted within the sequence stratigraphic framework. Among these rock types, porosity has been greatly enhanced by dolomitisation and dissolution in the dolomitised grainstone (DG) and oomouldic grainstone (MG). On the other hand, it has been reduced by cementation and compaction in tightly cemented and compacted grainstones (CEG and COG rock types). The primary porosity has been relatively well preserved in grainstone with interparticle porosity (IPG rock type). The later porosity reduction during burial was also controlled by rock type. This study shows that despite the great burial depth, a significant amount of porosity is still preserved in oomouldic grainstone.

Lithofacies and depositional characteristics of gas shales in the western area of the Lower Yangtze, China

The Lower Yangtze is considered as a potentially productive shale gas area in China, but only limited research has thus far been carried out there. On the basis of a detailed investigation of fourteen outcrops, eight lithofacies in the Hetang Formation (510–541 Ma), the Gaojiabian Formation (430–443.4 Ma), the Gufeng Formation (270–281 Ma) and the Dalong Formation (252–263 Ma) have been identified: silicalite, siliceous non‐calcareous mudstone, siliceous shale, carbonaceous shale, calcareous mudstone, silty–shaly interlaminated mudstone, siltstone and limestone. Three types of fossils were also found in the four formations: sponge spicules, radiolarians and graptolites. Moreover, four key outcrops of the fourteen were suitable for more additional detailed analyses and interpretation of their depositional environments and plausible sequence of the stratigraphic system tracts. As a result, the four potential shale gas formations were all divided into third‐order sea‐level sequences. Every third‐order sequence was further divided into four system tracts, corresponding to an early transgressive system tract (ETST), a late transgressive system tract (LTST), an early highstand system tract (EHST) and a late highstand system tract (LHST). On the basis of the interpretation of the lithofacies and depositional environments in the western area of the Lower Yangtze, sediments can be related to hydrothermal and biological processes, terrigenous clastic input and calcium compensation. These processes were interpreted to occur in a sequential order that we divide into three stages. The ETST period was the first stage, in which hydrothermal upwelling from the open ocean produced a siliceous lithological combination. During the second stage corresponding to the LTST + EHST, our results suggest that some biological communities in the stable deep water provided plenty of organic‐rich matter. The LHST period was the third stage, in which terrigenous clastic material was deposited close to the land, and some carbonate sediments began appearing near the platform. Copyright © 2014 John Wiley & Sons, Ltd.

Depositional facies and platform architecture of microbialite-dominated carbonate reservoirs, Ediacaran–Cambrian Ara Group, Sultante of Om

Intrasalt carbonates of the Ediacaran–Cambrian Ara Group constitute a significant reservoir element of the intrasalt “stringer” play in Oman, in which dolomitic carbonates are encased in salt at depths of 3 to 7 km (1.9 to 4.3 mi). These reservoir carbonates have significant microbial influences. Although Ara Group reservoirs are mostly latest Precambrian, the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded, in lacustrine settings where calcified invertebrates are not a significant source of carbonate, or after periods of mass extinction before faunal recovery. A broad range of carbonate facies provides the context in which to understand the origin of the microbialite-dominated reservoirs developed across both ramp and rimmed shelf profiles. Major facies associations include carbonate-evaporite transition zone, deep ramp and slope, subtidal microbialites, clastic-textured carbonates, and restricted peritidal carbonates. Microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties, and that help in predicting the location of reservoir fairways. Microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities (>50%), shallow subtidal pustular laminites with cm-scale variability of lamina morphology, deeper subtidal crinkly laminites that show mm-scale variability of lamina morphology, and intertidal tufted laminates that show mm- to cm-scale tufted textures. Other reservoir facies are more conventional grainy carbonates including ripple cross-stratified grainstone–packstone, hummocky cross-stratified grainstone–packstone, flat pebble conglomerate, ooid and intraclast grainstone–packstone, and Cloudina grainstone–packstone. These facies are almost invariably dolomitized and all have moderate to excellent reservoir quality. These facies comprise carbonate platforms, broken up during salt tectonics, that range up to 160 m (525 ft) in thickness and extended laterally, prior to halokinesis, for tens to over 50 km (31 mi). The distribution of reservoir facies follows sequence stratigraphic predictions, with microbialites occurring in every accommodation profile. Late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build-ups, whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies. Pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods. Crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate.

Linked sequence stratigraphy and tectonics in the Sichuan continental foreland basin, Upper Triassic Xujiahe Formation, southwest China

Intracontinental subduction of the South China Block below the North China Block in the Late Triassic resulted in formation of the transpressional Sichuan foreland basin on the South China Block. The Upper Triassic Xujiahe Formation was deposited in this basin and consists of an eastward-tapering wedge of predominantly continental siliciclastic sedimentary rocks that are up to 3.5km thick in the western foredeep depocenter and thin onto the forebulge and into backbulge depocenters.Five facies associations (A–E) make up the Xujiahe Formation and these are interpreted, respectively, as alluvial fan, transverse and longitudinal braided river, meandering river, overbank or shallow lacustrine, and deltaic deposits. This study establishes a sequence stratigraphic framework for the Xujiahe Formation which is subdivided into four sequences (SQ1, 2, 3 and 4). Sequence boundaries are recognized on the basis of facies-tract dislocations and associated fluvial rejuvenation and incision, and systems tracts are identified based on their constituent facies associations and changes in architectural style and sediment body geometries. Typical sequences consist of early to late transgressive systems tract deposits related to a progressive increase in accommodation and represented by Facies Associations A, B and C that grade upwards into Facies Association D. Regionally extensive and vertically stacked coal seams define maximum accommodation and are overlain by early highstand systems tract deposits represented by Facies Associations D, E and C. Late highstand systems tract deposits are rare because of erosion below sequence boundaries. Sequence development in the Xujiahe Formation is attributed to active and quiescent phases of thrust-loading events and is closely related to the tectonic evolution of the basin. The Sichuan Basin experienced three periods of thrust loading and lithospheric flexure (SQ1, lower SQ2 and SQ3), two periods of stress relaxation and basin widening (upper SQ 2 and SQ3) and one phase of isostatic rebound (SQ4). Paleogeographic reconstruction of the Sichuan Basin in the Late Triassic indicates that the Longmen Mountains to the west, consisting of metamorphic, sedimentary and pre-Neoproterozoic basement granitoid rocks, was the major source of sediment to the foredeep depocenter. Subordinate sediment sources were the Xuefeng Mountains to the east to backbulge depocenters, and the Micang Mountains to the northwest during the late history of the basin. This study has demonstrated the viability of sequence stratigraphic analysis in continental successions in a foreland basin, and the influence of thrust loading on sequence development.

Depositional sequences and ammonoid assemblages in the upper Cenomanianlower Santonian of the Iberi

A clear relationship exists between eustatic sea-level rises and falls recorded as cyclical depositional sequences and ammonite faunas during the Cenomanian-Santonian in the Iberian and West Portuguese basins. Most of the faunal turnovers correlate with stratigraphic intervals related to marine transgressions, maximum flooding of the shelf (locally associated to anoxic events), and marine regressions. Specifically, within each depositional sequence, three distinct and identical events of morphological change occur, involving ammonoids belonging to different groups. Transgressive sediments are characterized by moderately ornamented, inflated and evolute morphologies, which are replaced by smooth, involute and compressed oxycones (the most hydrodynamic shells) during maximum flooding (and to a lesser extent at the early highstand) of the sequences. The latter morphologies in turn are replaced by coarsely ornamented and evolute shells during late highstands. We conclude that ammonoid faunal analysis can be used to trace sea-level changes and provides an additional tool for sequence stratigraphy.

Environmental and sequence stratigraphic implications of anhydrite textures: A case from the Lower Triassic of the Central Persian Gulf

The Lower Triassic Kangan Formation in the Persian Gulf (South Pars Gas Field) and its adjacent areas are composed of carbonate–evaporite sequences. These sediments were deposited in a shallow marine homoclinal ramp. Study of the anhydrite-bearing intervals shows various structures and textures. The anhydrite structures are mainly bedded, massive, chicken-wire and nodular type and the main textures are felted, sparse crystal, needle shape, lath shape, equant and fibrous. Pervasive and poikilotopic cement together with replacement and porphyroblastic gypsum are accounted as the most common diagenetic features in anhydrite. Evaluation of anhydrite occurrences and features support both primary and secondary formations. The nodular to chicken-wire anhydrite formed under synsedimentary sabkha conditions, whereas anhydrite cements occurred during the late stages of diagenesis (shallow burial stage). Massive to bedded anhydrite could have been formed under subaqueous conditions or originated by coalescing and continued growth of anhydrite nodules in the sabkha zone. Anhydrite fabrics impose a significant control on the reservoir quality of the Kangan carbonates at the South Pars Gas Field. Thick massive and bedded anhydrite could have been formed as an intraformational seals and anhydrite cements occluded pore spaces and reduced the poroperm values. The sequence stratigraphic analysis revealed two depositional sequences in the studied intervals, which are composed of TST and HST. Investigation of anhydrite throughout depositional sequences indicates a change in the content and style of anhydrite texture. Anhydrite content (volume) decreases upward through transgressive system tract (sea-level rise) whereas, it enhances during highstand system tract (sea-level fall). Pervasive and poikilotopic anhydrite cements together with replacement by anhydrite are prevalent features during transgressive and early highstand system tract. At the late HST, with a progradational stacking pattern, anhydrite value increases and felted, radial, equant, crystalline and mosaic texture are the most common anhydrite fabrics. Sequence boundaries that indicate maximum sea level fall and exposure of successions are marked by the broad anhydrite deposits with massive to bedded and chicken-wire structures and various textures that located in late HST package. There is an unambiguous relationship between the microfacies associations, the evaporite textures, and the sea-level fluctuations. This relationship could lead to a predictable pattern that can be of use as a general guide for the sequence stratigraphic interpretations in the area.

Aggradation and progradation controlled clinothems and deep-water sand delivery model in the Neogene Lake Pannon, Makó Trough, Pannonian Basin, SE Hungary

In the Late Miocene–Early Pliocene Lake Pannon, regression went on for about 6Ma. Sediments arriving from the Alpine–Carpathian source area were partly accumulated on the flat-lying morphological shelf of the lake, whereas other portions of the sediment were passing through to the slope and deposited on the deep basin floor. The height of the slope exceeded 400–500m based on correlated well and seismic data. An extended 3D seismic volume covering the Makó Trough, one of the largest and deepest depressions within the Pannonian Basin, provided an opportunity to study sequences and shelf-margin trajectories generated as a result of continuous slope advancement. The lithology of these shelf, slope and basin centre deposits was inferred from seven well logs and 220m core material.In the Makó Trough the southeastward migrating shelf-margin was formed by alternating aggradational and progradational clinothems. Aggradational clinothems, i.e. aggradation accompanied by subordinate progradation, are characterised by rising shelf-margin trajectories. The shelf built up from inner-shelf to shelf-edge deltaic lobes which compose a few dozen metre thick coarsening-up units. The majority of the sand, however, was transported by effective turbidity currents through leveed channels into the basin, and deposited as thick, extended slope-detached turbidite lobes up to a distance of 30km from the shelf edge. In aggradational clinothems both the shelf and the basin floor accreted vertically. Development of progradational clinothems resulted in horizontal (flat) shelf-margin trajectories. Corresponding reflections toplap at the shelf edge and downlap within a distance of few kilometres from the toe of the slope. The shelf was bypassed, sediments accumulated on the slope and directly at the slope–toe region as small simple lobes. Short-distance transport was the result of clay-poor, non-effective turbidity currents. Consequently, the thickness of coeval basin-centre sediments remained negligible in progradational clinothems.Alternations of rising and horizontal shelf margin trajectories indicate that the climate- and subsidence-controlled lacustrine base-level rose continuously, though at varying rates. Descending trajectories were not observed. It means that base-level drops larger in amplitude than the seismic resolution (20–30m), did not occur during the studied time interval, i.e. at 7–5Ma ago, approximately corresponding to the Messinian age. As a result, major forced-regressive wedges or lowstand fans did not develop. This is in contrast with former stratigraphic models claiming that several 3rd-order sequences, including the intra-Messinian unconformity supposedly induced by hundred metres large lake-level drop, developed in Lake Pannon with significant volume of lowstand deposits as turbidites. Instead, aggradational and progradational clinothems are interpreted as fourth-order transgressive, early and late highstand systems tracts. These incomplete sequences represent less than 100kyr time intervals.Due to climate control both on high rate of sediment supply and the water budget of Lake Pannon, conditions were more favourable for deposition of large volumes of well-developed turbidite systems during base-level rise than during stagnation or minor base-level fall. Therefore, sand delivery to the basin centre was at maximum during the early highstand aggradational stage and at minimum during the late highstand progradational stage. The timing and position of sand accumulation in the Makó Trough of Lake Pannon is different from those predicted by “traditional” sequence stratigraphic considerations.

PALEOECOLOGICAL DYNAMICS OF FURONGIAN (LATE CAMBRIAN) TRILOBITE-DOMINATED COMMUNITIES FROM NORTHWESTERN ARGENTINA

Abstract The Cambrian–Ordovician boundary interval is a critical moment in the ecology of trilobite communities. To understand this transition, we studied—at three different spatial scales—changes in the structure of olenid-dominated communities included in the Parabolina fauna, which flourished in the latest Cambrian, largely storm-dominated, successions of northwestern Argentina. At the local (∼meter) scale, species-poor communities occur in shoreface deposits. Relatively flat species-abundance distributions (SADs) and high evenness characterize upper offshore to offshore transition settings of the early highstand systems tract (HST), whereas uneven SADs in species-poor communities are typical of the lower offshore and shelf environments of the transgressive systems tract (TST). This pattern is unlikely to be caused by a change in time averaging and is consistent with the intermediate disturbance hypothesis predicting unimodal diversity gradients. The pattern is thus interpreted to be related to a trend...

Depositional systems and evolution of the Upper Carboniferous Huanglong Formation in the eastern Sichuan Basin

Abstract New research on basic geology has been done for Upper Carboniferous carbonate lithologic gas pools in the Huanglong Formation in the eastern Sichuan Basin. Based on the development characteristics of terrigenous clastic deposit and the development location of suspending gravels with some psephicity, the transgression direction and tidal depositional system distribution are deduced. It is believed that, two transgression channels were developed in southeast of Yunyang and northwest of Dazhou in the area, two tidal-flat depositional systems in Chongqing-Wanzhou and southeast of Dazhou respectively were formed, and a suit of complete type-I sequence composed of lowstand systems tract (C 2 h 1 ), transgressive systems tract (C 2 h 2 ) and early highstand systems tract (C 2 h 3 ) was developed. Sedimentary microfacies features of each member of the Huanglong Formation were delineated finely. In the middle stage of late Carboniferous, transgressive systems tract was developed in eastern Sichuan, where shallow shoal and algal dolomite-flat microfacies were developed, which are favorable for the development of carbonate lithologic traps. In contrast, in the early and late stages of late Carboniferous, lowstand systems tract and highstand systems tract were developed, respectively, where shallow shoal microfacies were poorly developed, which limited the large scale development of carbonate lithologic traps.

Evidence for the episodic “turn on” and “turn off” of turbid-zone coral reefs during the late Holocene sea-level highstand

Terrigenous sediment accumulation within nearshore marine environments is regarded as a major factor inhibiting carbonate production and coral reef accretion. While recent ecological and geological research into reef development under long-term terrigenous sediment influence questions the overly simplistic nature of such views, understanding of the time scales of reef initiation and growth and the morphodynamics of reef accretion in these settings remains limited. Here we present evidence to support recent suggestions that, once established, rapid reef accretion and progradation is possible, but that the restricted accommodation windows in which such reefs develop often result in short-lived (ephemeral) phases of reef building. Specifically, we describe two discrete periods of reef growth within one small (∼600 m wide) coastal embayment around a high island on the terrigenous sediment–dominated inner shelf of the central Great Barrier Reef, Australia. These reef-building phases occurred at either end of the Holocene sea-level highstand, the first during the late postglacial marine transgression and early highstand (∼6900–4500 calibrated (cal) yr B.P.), the second following the late Holocene regression and stillstand (∼1600 cal yr B.P. to present). An ∼3000 yr hiatus occurred between these events, probably as a function of subtle changes in sea level and associated shoreline morphodynamics.

Prediction of stratigraphic compartmentalization in marginal marine reservoirs

Abstract Marginal marine depositional systems exhibit stratigraphic reservoir compartmentalization potential at three hierarchical scales. At each of these scales, stratigraphic compartmentalization potential can be related to the dominant depositional processes and accommodation:coarse sediment supply ratio (A/S) that are acting at the time of deposition. All three orders of compartmentalization potential must be considered in order to define optimal field development plans and completion strategies. The lowest order of compartmentalization is usually at the inter-parasequence scale. The parasequence is represented by a conformable succession of strata separated by marine flooding surfaces and as such it generally defines the basic flow unit in marginal marine systems. In systems tracts associated with relatively high A/S ratios, for example late Lowstand, Transgressive and early Highstand (steeply rising shoreline trajectories), vertical compartmentalization potential is relatively high because of the enhanced preservation potential of flooding surface shales under these conditions. In systems tracts associated with relatively low A/S ratios, for example late Highstand, Falling-stage and early Lowstand (flat, slightly rising and falling shoreline trajectories), vertical compartmentalization potential of parasequences is reduced because the potential for erosion of flooding surface shales by overlying deposits is high and hence potential for vertical sand–sand contact between parasequences is enhanced. The second level of compartmentalization hierarchy is the inter sand-body scale. Individual sand bodies are defined within parasequences. The lateral connectivity of these sand bodies is a product of the dominant depositional processes active at the time of their deposition (wave, tidal, fluvial). Wave-dominated systems tend to produce more laterally continuous sand bodies, fluvial-dominated systems more laterally restricted sand bodies and tide-dominated systems both laterally continuous and laterally restricted sand bodies. Vertical compartmentalization potential of these reservoir sand bodies is related to A/S regime. In high A/S regimes, sand bodies are more likely to be disconnected or compartmentalized. In low A/S regimes, erosional amalgamation of sand bodies is more likely thereby leading to lower compartmentalization potential. The third order of potential stratigraphic compartmentalization is the intra sand-body scale. This scale is represented by intra sand-body heterogeneities such as dipping or horizontal shales, carbonaceous-rich beds or laminae, shale abandonment plugs of channels and carbonate concretions. In high A/S regimes the preservation potential of these heterogeneities is relatively high leading to an enhanced potential for intra sand-body compartmentalization. Lower A/S regimes result in a greater likelihood of lateral and vertical erosion of these heterogeneities leading to a higher potential for reservoir connectivity.

Active synsedimentary tectonism on a mixed carbonate-siliciclastic continental margin: third-order sequence stratigraphy of a ramp to basin transition, lower Sekwi Formation, Selwyn Basin, Northwest Territories, Canada

The lower part of the Early Cambrian Sekwi Formation in the Selwyn Basin of the Northwest Territories, Canada, is composed of two regional, unconformity-bounded sequences, S0 and S1, which record the first widespread carbonate deposition during the initial Palaeozoic transgression onto the western margin of Laurentia. These Early Cambrian sequences are unique to the western North American Cordillera, representing the only record of primarily deep-water deposition on a tectonically active, mixed carbonate–siliciclastic ramp during this period. More specifically, the geometry of the Sekwi ramp changed during deposition of S0 and S1, from a shallowly dipping homoclinal ramp during the S0 transgressive systems tract to a steeply dipping tectonically modified ramp during the early highstand systems tract of S0. The steeply dipping ramp profile of S0 was preserved into the early transgressive systems tract of S1. The Sekwi ramp returned to a gently sloping ramp during the late highstand systems tract of S1 and remained so throughout the remainder of Sekwi deposition. The evolving shape of the Sekwi ramp is attributed to syndepositional ‘down to the basin’ faulting during deposition of both S0 and S1 and is recorded by: (i) the westward thickening, irregular geometries of S0 and S1; (ii) geographical restriction of deep-water facies (including sediment gravity flow deposits); (iii) the presence of large allochthonous blocks; and (iv) the clast composition of sediment gravity flow deposits. Sediment gravity flow deposits play an unusually important role in the sequence stratigraphic interpretation of the lower Sekwi Formation, as they delineate depositional packages, including the maximum flooding zone, the transitions between portions of systems tracts, and the inferred locations of syntectonic extensional faults. Syntectonic faults increased accommodation basinward of an extensive ooid-shoal complex that developed along the Sekwi ramp crest, greatly influencing sequence geometry and initiating the downslope motion of sediment gravity flows. The syndepositional faulting probably was a continuation of extension that began during the latest Neoproterozoic rifting of western Laurentia. The composition of sediment gravity flow deposits track changing accommodation space on the lower Sekwi ramp and can be used to differentiate systems tracts that probably were related more to tectonism than eustasy.

Sequence stratigraphy, cyclic facies, and lagerstätten in the Middle Cambrian Wheeler and Marjum Formations, Great Basin, Utah

Recurrent taphofacies, including conservation lagerstätten, are identified within a spectrum of facies in fourth-order sequences in the Middle Cambrian Wheeler, and Marjum formations of the Drum Mountains and House Range, west-central Utah. These sequences are 3–20 m thick and commence with sharply-based compact, oncolitic, oolitic, or pelletal pack- and grainstones with sharply-defined, corroded and mineralized upper contacts that record drowning discontinuities and early transgressive systems tracts (TSTs). Overlying intervals of calcareous shale and thin-bedded wacke- to packstones with abundant, disarticulated polymerid and agnostoid trilobites represent late TSTs. These are commonly followed by lavender-gray mudstones rich in sponge spicules and comminuted fossil debris that reflect condensed maximum flooding zones. The overlying early highstand (HST) intervals of black, fissile shales are typically barren except for indistinct, circular carbonized algae, but in rare instances, include soft-bodied animal remains. A combination of lower dysoxic–anoxic conditions, with a fluctuating oxycline, and relatively rapid episodic influx of fine-grained detrital sediment favored repeated burial and preservation of abundant organic detritus and rarely soft-bodied animals. Interbedded dark gray, shales include abundant articulated agnostoid trilobites and diminutive polymerids (e.g., Jenkinsonia). Overlying platy, calcareous bedding planes covered with articulated bodies and molts of the polymerid Elrathia indicate rapid blanketing of undisturbed seafloors by calcareous mud layers. These beds grade upward successively into interbedded, sparsely fossiliferous platy to flaggy shale and thin, pale gray weathering calcisiltites, and burrow-mottled to nodular limestones, recording late HST to falling stage (FSST) carbonate shedding. Thin calcisiltites include fossil debris and articulated larger polymerid trilobites and the eocrinoid Gogia, preserved by obrutionary deposits. The repeated recurrence of these patterns provides the rudiments of a predictive model that not only explains the differing modes of preservation but may also aid in prospecting for new lagerstätten.

Volcanic Tephra Bed Formation and Condensation Processes: A Review and Examination from Devonian Stratigraphic Sequences

The formation and preservation of airfall volcanogenic tephras is the result of various interacting factors. Explanations of the depositional history of individual tephra beds lie along a spectrum between two end-member models of a primary single event and a complex depositional history. Nonrecognition of the history of a tephra bed may lead to errors in geochronology, correlation, and interpretation of the record of explosive volcanism in sedimentary successions. Increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history. This is supported by studies that indicate many tephras in the rock record result from amalgamation and/or reworking in the depositional environment. A prediction of increased tephra bed formation/preservation within transgressive systems tracts due to condensation processes is tested through 11 Lochkovian to Eifelian (Lower to lower Middle Devonian) depositional sequences. The character and distribution of over 80 Devonian K-bentonites and tuffs in the Appalachian Basin (eastern United States) indicates a complex history of tephra deposition and preservation. Of 11 sequences, four have numerous tephra beds, one sequence yields one tephra bed, and six sequences yield no tephra beds. In total, 38 individual beds cumulatively occur within transgressive systems tracts, 39 in early highstand systems tracts, one in late highstand systems tracts, and one in lowstand systems tracts, noting that lowstands are poorly preserved or not recognized along the outcrop belt. This indicates that airfall tephras are concentrated in the transgressive and early highstand systems tracts, not just in condensed intervals of transgressive systems tracts; they do not consistently occur through all sequences. A simple sequence stratigraphic condensation model for the deposition and preservation of volcanic tephras is not applicable.

Palynofacies and sea-level changes in the Upper Cretaceous of the Vocontian Basin, southeast France

Upper Cretaceous platform carbonates of the Vocontian Basin (southeastern France) have been investigated in a cross-section from the proximal deposits exposed in the lower Rhône Valley to the distal part of the basin in the Southern Subalpine Ranges north of Nice. The stratigraphic interval studied in detail spans the uppermost Turonian and Coniacian. Palynofacies patterns were used to detect eustatic signals at a third-order scale and are the tool for correlation of proximal and distal platform deposits. The organic constituents observed in the studied samples have been grouped into a continental fraction, including higher plant debris (phytoclasts) and sporomorphs, and a marine fraction with dinoflagellate cysts, acritarchs, prasinophytes, and foraminiferal test linings. The main factors influencing the stratigraphic and spatial distribution of land-derived, allochthonous, and marine, relatively autochthonous, organic particles are the proximity of land, the organic productivity, the degree of biodegradation and the hydrodynamic conditions of the depositional system. Palynofacies parameters used for the sequence stratigraphic interpretation are: (1) the ratio of continental to marine constituents (CONT/MAR ratio); (2) the ratio of opaque to translucent phytoclasts (OP/TR ratio); (3) the phytoclast particle size and shape; and (4) the relative proportion and species diversity of marine plankton. Ternary diagrams illustrating significant proximality changes were used to decipher transgressive-regressive trends within the succession. High amounts of translucent phytoclasts and decreasing values of the CONT/MAR ratio occur during the phase of relative sea-level rise in the upper Turonian. The stratigraphic interval of maximum flooding around the Turonian/Coniacian boundary is marked by the highest abundance and species diversity of dinoflagellate cysts, and by high percentages of opaque, equidimensional particles within the phytoclast group. The OP/TR ratio is still high within the lower Coniacian representing the early highstand deposits, whereas the relative abundance of marine constituents is again decreasing. Sedimentary organic matter of the upper Coniacian is dominated by large, blade-shaped, mainly opaque phytoclasts, which are a characteristic palynofacies signature of late highstand deposits. The present study demonstrates the high potential of palynofacies analysis in high-resolution stratigraphy and correlation of sedimentary series of shallow epeiric seas.

Seismic stratigraphy and Quaternary sedimentary history of the northeast Aegean Sea

Seismic profiles show that the upper Quaternary–Recent stratigraphic architecture of the NE Aegean Sea is characterized by several vertically stacked depositional sequences which are developed within three NE–SW-trending basins. The depositional sequences are separated from one another by prominent shelf-crossing unconformities, formed during periods of falling and low sea level and reworked further during the subsequent sea-level rise by shoreface erosion. Each depositional sequence consists of a lower mainly parallel-stratified subunit, and an upper oblique to sigmoid-oblique prograded subunit. The lower subunit represents hemipelagic and prodeltaic deposition of the transgressive and early highstand systems tracts. The upper subunit represents deltas of the forced regressive systems tract. The chronology of the depositional sequences is constrained by radiocarbon and U/Th dates on cores. An average sedimentation rate of ∼ 30 cm/ka is calculated for basinal muds of the region. Correlations with global sea-level curves based on oxygen-isotope stratigraphy reveal that the seaward-prograded delta sequences developed on a rapidly subsiding shelf during successive glacio-eustatic sea-level falls associated with late Quaternary glaciations. A set of paleogeographical maps shows changing basin configuration since ∼ 330 ka. Volumes of the depositional sequences compare well with the volumes of sediment estimated to have been eroded from the incised valleys of the Tuzla, Karamenderes, and Dümbek rivers draining the Biga Peninsula.

Benthic Foraminiferal response to sea level change in the mixed siliciclastic‐carbonate system of southern Ashmore Trough (Gulf of Papua)

Ashmore Trough in the western Gulf of Papua (GoP) represents an outstanding modern example of a tropical mixed siliciclastic‐carbonate depositional system where significant masses of both river‐borne silicates and bank‐derived neritic carbonates accumulate. In this study, we examine how benthic foraminiferal populations within Ashmore Trough vary in response to sea level–driven paleoenvironmental changes, particularly organic matter and sediment supply. Two 11.3‐m‐long piston cores and a trigger core were collected from the slope of Ashmore Trough and dated using radiocarbon and oxygen isotope measurements of planktic foraminifera. Relative abundances, principal component analyses, and cluster analyses of benthic foraminiferal assemblages in sediment samples identify three distinct assemblages whose proportions changed over time. Assemblage 1, with high abundances of Uvigerina peregrina and Bolivina robusta, dominated between ∼83 and 70 ka (early regression); assemblage 2, with high abundances of Globocassidulina subglobosa, dominated between ∼70 and 11 ka (late regression through lowstand and early transgression); and assemblage 3, with high abundances of neritic benthic species such as Planorbulina mediterranensis, dominated from ∼11 ka to the present (late transgression through early highstand). Assemblage 1 represents heightened organic carbon flux or lowered bottom water oxygen concentration, and corresponds to a time of maximum siliciclastic fluxes to the slope with falling sea level. Assemblage 2 reflects lowered organic carbon flux or elevated bottom water oxygen concentration, and corresponds to an interval of lowered siliciclastic fluxes to the slope due to sediment bypass during sea level lowstand. Assemblage 3 signals increased off‐shelf delivery of neritic carbonates, likely when carbonate productivity on the outer shelf (Great Barrier Reef) increased significantly when it was reflooded. Benthic foraminiferal assemblages in the sediment sink (slopes of Ashmore Trough) likely respond to the amount and type of sediment supplied from the proximal source (outer GoP shelf).

Sub-orbital sea-level change in early MIS 5e: New evidence from the Gulf of Corinth, Greece

New evidence from uplifted marine isotope stage (MIS) 5e carbonate deposits in the Gulf of Corinth demonstrate two rapid, sub-orbitally forced, sea-level oscillations in the early part of MIS 5e. Microbial bioherms with inter-grown marine coralline algae are interpreted as early highstand deposits. Presence of thin vadose flowstone (speleothem) coating inter-bioherm surfaces mark a short-lived regression of > 10 m, followed by sea-level recovery and re-establishment of the highstand, marked by coralline algae coating inter-bioherm cavity surfaces. These marine algae are then coated by a younger vadose flowstone, thick enough to provide an uncontaminated U/Th date of 134.8 ± 2.0 ka. The dated flowstone is itself encrusted by marine fauna and the entire sequence overlain by highstand marine sediments and marine aragonite cements dated to 114–118 ka, part of the sustained MIS 5e highstand. The age of the younger flowstone demonstrates that the early highstand occurred before 134.8 ± 2.0 ka, and uplift arguments suggest that the bioherms are unlikely to be older than ∼ 136 ka. These data are consistent with the notion that most of termination II (TII) sea-level rise had occurred before 135 ka; indeed they suggest sea-level at this time reached about 2–4 (± 4) m below present sea-level, 6–18 (± 4) m higher than previous estimates. This early highstand was itself punctuated by a rapid sea-level oscillation of > 10 m (as yet undated), and this oscillation, supported by new TII sea-level data from the Red Sea [Siddall, M., Bard, E., Rohling, E.J., Hemleben, C., 2006, Sea-level reversal during termination II, Geology, 34, 817–820.], probably occurred in about 1000 yr. The flowstone dated at 134.8 ± 2.0 ka is interpreted to record the early part of the ‘Aladdin's Cave' regressive event from Papua New Guinea, although in Greece only the first 16 m of the event is recorded. The new data presented here support a recently published Red Sea TII sea-level curve: they also highlight minor discrepancies in the absolute dating of these early MIS 5e sea-level events worldwide.

Sequence stratigraphy and orbital cyclostratigraphy of the Mooreville Chalk (Santonian–Campanian), northeastern Gulf of Mexico area, USA

Cycles from the depositional sequence level to decimeter-scale rhythmic alteration of chalk and marl beds in the Mooreville Chalk (Santonian–Campanian) in the northeastern Gulf of Mexico area are studied employing an integrated biostratigraphic, cyclostratigraphic, and sequence stratigraphic approach. Two dominant frequencies, identified from the natural gamma-ray log from the Alabama Power Selma Site Test well, site 3 hole 3, are attributed to two of the Milankovitch cycles, the long eccentricity (413 ka) and eccentricity (100 ka) cycles. Parasequences, the building blocks of systems tracts, appear to be modulated by long eccentricity cycles. Individual eccentricity cycles were counted to establish a high-resolution orbital chronology, which agrees with established biostratigraphic chronology. A high-resolution record of temporal changes in sedimentation rate is established on the basis of cyclostratigraphic chronology. Two condensed sections are identified. The lower condensed section is found to correspond to a downlap surface recognized in seismic data in the offshore Alabama and Mississippi area. This condensed section and downlap surface are interpreted to represent the maximum flooding event of a third-order depositional sequence. The maximum flooding event is also represented by a marine flooding surface in eastern Mississippi and by the Mooreville tongue in eastern Alabama. The upper condensed section is found to correspond to a faunal abundance peak recognized in data from outcrop studies, and is interpreted to represent the maximum bathometric surface that was achieved as a result of a condition of rising sea level in association with low sediment input during the deposition of the early highstand systems tract.

Accommodation and Sedimentary Architecture of Isolated Icehouse Carbonate Platforms: Insights from Forward Modeling with CARB3D+

Abstract A fundamental control on the evolution of carbonate platforms is accommodation, the space available for sediment deposition. Here we adopt a sensitivity-analysis approach, using the process-based forward model CARB3D+, to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy, subsidence, and surface dissolution. Sediment production, the mechanism invoked to fill accommodation, is also investigated. Fourth-order Milankovich-driven sea-level cycles dominate icehouse carbonate sedimentology, with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present. Both overwhelm higher-frequency, lower-amplitude fifth-order cycles, which are rarely preserved on the platform top. Subsidence is a fundamental control on sedimentary stacking patterns, sequence-boundary frequency (SBF), and platform geometry, with rapid subsidence increasing the preservation of platform-top sequences. Most icehouse platforms are subject to extended periods of platform-top exposure. Missed beats, when the platform top remains emergent at a sea-level maximum, are common and create non-Milankovich SBFs. Furthermore, during platform emergence, surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event, with surface dissolution rates in humid climates comparable to moderate rates of subsidence. However, accommodation generation by surface dissolution is self-limiting, in as much as accommodation generated by dissolution is rapidly refilled on flooding. Sediment production is a key control on unfilled accommodation, which our simulations suggest is considerable on icehouse platforms. Increased shallow-water (< 3 m depth) sediment production enhances intra-platform bathymetric relief between the margin and interior; this is driven in large part by a higher sedimentation rate at the margin than in the interior. Platforms can be flooded to depths up to 45 m during marine transgressions, and accommodation often remains unfilled. There are two distinct phases of platform-top sedimentation. During the transgression and early highstand, platform margins accumulate at a greater rate than interior sediments, generating differential intra-platform bathymetry. Subsequently, accommodation in the interior fills by forced regression and progradation from the margins into the platform interior. Numerical forward models can augment our understanding of icehouse carbonate-platform evolution by identifying the nature of the sedimentary response to individual controls. They can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms, such as the large hydrocarbon fields in the North Caspian and S.E. Asia.