Eustatic Sea-level Cycles Research Articles

Trench floor depositional response to glacio-eustatic changes over the last 45 ka, northern Hikurangi subduction margin, New Zealand

ABSTRACT Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems.

Tectonic and eustatic control on channel formation, erosion, and deposition along a strike-slip margin, San Diego, California, USA

Active margins are characterized by small mountainous streams, which contribute significantly to global sediment transport from land to sea. The response of active margin streams to base-level change due to eustatic sea-level cycles is less well understood than passive margin streams, which form the basis of most sequence stratigraphic models. The southern California margin offers an opportunity to investigate the evolution of small mountainous streams along a strike-slip margin over a sea-level cycle. High-resolution Chirp subbottom data were examined from the Silver Strand littoral cell in San Diego, California, which is located in a region of localized transtension associated with a step in the Newport Inglewood-Rose Canyon fault zone. Chirp data reveal active faulting on multiple fault strands, a subaerial surface likely formed from MIS 4 to MIS 2, fluvial incision associated with MIS 2, and the transgressive surface of erosion formed during sea-level rise from MIS 2 to MIS 1. Interpreted acoustic units observed in the data include the Cretaceous outcropping Rosario Group, Miocene to Pliocene prograding clinoforms, subaerial deposits associated with MIS 4 to MIS 3, channel fill and transgressive lag deposits formed during the most recent sea-level transgression, and Holocene marine sediment. The Chirp data examined in this study build upon findings from other seismic studies, sediment cores, trenching, and outcrops that have been examined on- and offshore San Diego. Combined, these data suggest that the fault-controlled pull-apart basin formed in this region influences sediment transport and deposition throughout the last glacial cycle. During highstand, the fault-controlled wide shelf, subsiding basin, and circulation patterns result in trapping of sediment in San Diego Bay and nearshore, cutting off sediment delivery to the deep-sea. During lowstand, the fault-controlled morphology creates a drainage divide that deflects streams to the south which lengthens and reduces slope of the drainage pathways. These findings are in contrast to observations in other fault controlled littoral cells along the southern California margin where sediment delivery to the deep-sea remains active during highstands, highlighting the variability of stream response to base-level change along active strike-slip dominated margins.

A geomorphic enthalpy method: Description and application to the evolution of fluvial-deltas under sea-level cycles

Fluvial deltas are composites of two primary sedimentary environments: a depositional fluvial region and an offshore region. The fluvial region is defined by two geomorphic moving boundaries: an alluvial-bedrock transition (ABT), which separates the sediment prism from the non-erodible bedrock basement, and the shoreline (SH), where the delta meets the ocean. The trajectories of these boundaries in time and space define the evolution of the shape of the sedimentary prism, and are often used as stratigraphic indicators, particularly in seismic studies, of changes in relative sea level and the identification of stratigraphic sequences. In order to better understand the relative role of sea-level variations, sediment supply, and basin geometry on the evolution of the ABT and SH, we develop a forward stratigraphic model that accounts for curvature changes of the fluvial surface and treats the SH and ABT as moving boundaries (i.e., internal boundaries that are not known a priori and their location must be calculated as part of the solution to the overall problem). This forward model extends a numerical technique from heat transfer (i.e., enthalpy method), previously applied to the evolution of sedimentary basins, to account for sea-level variations, including eustatic sea-level cycles. In general, model results demonstrate the importance of the dynamics of the fluvial surface on the system response under a large range of input parameter values. Specifically, model results suggest that time lags in the ABT response during sea-level cycles can result in geologically long-lived river incision in the upper and mid portions of the fluvial surface during sea-level rise. These results suggest that the relationship between the coastal onlap configuration of strata and relative changes in sea level is complex, and therefore not necessarily a good indicator of contemporaneous sea-level changes.

The active submarine NW termination of the South Aegean Active Volcanic Arc: The Submarine Pausanias Volcanic Field

Methana peninsula shows the longest recorded volcanic history at the western end of the South Aegean Active Volcanic Arc, including volcanic products from the Upper Pliocene to recent times. The volcanic rocks comprise widespread dacite domes and andesite lava flows from several small volcanic centers and are only imprecisely dated. In this paper, the integrated analysis of swath bathymetry, side scan sonar data, and high resolution seismic reflection profiles correlated with core samples, has allowed detailed mapping, characterization and precise chronological identification of the Pausanias submarine volcanic field activity offshore northern Methana. Six volcanic cones or domes are recognized, typically 1–3 km in diameter, some elongated NE–SW and some with a small central crater. On their flanks, the acoustically reflective volcanic rocks pass laterally into incoherent transparent seismic facies interpreted as volcaniclastic deposits, possibly including hyaloclastites, that interfinger with the regional basin sediments. A sea-bottom hummocky field, is interpreted as volcanic avalanche and appears to be the submarine continuation of the volcaniclastic apron of northern Methana peninsula. A robust chronostratigraphic framework has been established, based on the recognition of shoreline progradational units and their connection with Quaternary eustatic sea level cycles. Relative dating of the different phases of submarine volcanic activity during the Upper Quaternary has been achieved by correlating the imaged volcaniclastic flows, interlayered within the chronostratigraphically dated sediments. Dating by stratigraphic position, relative to 2D imaged eustatic sea level clinoform wedges appears to be more precise than radiometric methods on land. Three main submarine Volcanic Events (VE) are recognized: VE3 at ~450 ka, a less precisely dated interval at 200–130 ka (VE2), and VE1 at ~14 ka. Based on chronostratigraphic constraints, subsidence rates of 0.16 (±0.008) m/ka in-between Marine Isotopic Stages 6 and 12 and 0.19 (±0.009) m/ka in-between Marine Isotopic Stages 12 and 16 were estimated for the marine basin north of Methana. The morphological similarity to the onshore volcanoes of Methana Peninsula implies that magmatic constructive processes were dominant, regardless of whether in air or in water. The Upper Quaternary submarine volcanic rocks of Methana differ from those known from stratovolcanoes elsewhere in the Mediterranean, (e.g. Kos-Nisyros, Stromboli) and in other volcanic arcs (e.g., Montserrat, St Vincent), in the submarine development of domes or small cones, the paucity of volcano flank failure deposits and the lack of explosive events. Pausanias volcanic products date the onset of NE–SW faulting as well as the following tectonic phase of E-W striking faults, possibly related to basin inversion, caused by a major rifting phase that also affected most of the South Aegean Arc and the adjacent Gulfs of Corinth and Argolikos.

Seismic sequence stratigraphic analysis of the carbonate platform, north offshore Taiping Island, Dangerous Grounds, South China Sea

Taiping Island, also known as Itu Aba, is the largest natural terrestrial landmass in the South China Sea and is centrally located. Using bathymetry and marine multi-channel seismic data, we explored the seismic stratigraphic features of the offshore and isolated carbonate platform north of Taiping Island. The western flank of the carbonate platform is characterized by an intercalation between high-amplitude and low-amplitude reflections, showing the landward and seaward migration of the platform foreslope deposits. In addition, there are two offshore carbonate build-ups that are underlain by normal faults. Six sequence boundaries and five depositional sequences caused by eustatic sea level cycles are identified and correlated with the eustatic sea level change chart. Although the evolution of the seismic sequences is partly controlled by local tectonics, the overall stacking pattern of the sedimentary strata in our study area reveals five third-order cycles and one second-order cycle, which is in accordance with the eustatic sea level chart. Additionally, the formations of the Western Taiping Seamount Group and the Zhenghe-Daoming Trough are preliminarily analyzed based on seismic data.

The origin and depositional architecture of Paleogene quartz-glauconite sands in the Lubartów area, eastern Poland.

The study uses quantitative methods to analyse the latest Bartonian to Early Rupelian sedimentary succession at the SE outskirts of the Polish Lowland Paleogene Basin, in the back-bulge zone of the Carpathian orogenorebulge. The vertical lithotype proportion diagrams from a large number of well logs are compiled to reveal the area’s sequence stratigraphy. Six sequences are recognized and correlated with 3rd-order eustatic sea level cycles. The basal sequence of type 1 is overlain by three sequences of type 2 and followed by a fifth sequence of type 1, whose depositional forced-regressive and lowstand systems tracts brought the main volume of quartz-glauconite sand to the study area. The study focuses further on the deposits of this fifth sequence, exposed and surveyed with GPR in the Nowodwor-Piaski sand pit. Their sedimentary facies analysis reveals the local spatial pattern of a wave-dominated and tidally-influenced sedimentation, supporting the earlier notions of a southern palaeoshoreline and a tectonically-controlled sedimentation.The analysis, aided by multidimensional GPR survey, indicates syndepositional development of a tectonic graben filled laterally by fault scarp-attached large sand bars and an axial action of tidal ebb currents. The bars were formed of shore-derived sand swept by littoral waves from the graben footwall areas. As the graben’s tectonic activity ceased, it became buried by the lowstand regressive sands overlain by gravelly foreshore deposits, most of which were later removed by the Pleistocene glacial erosion. A 3D model of the deposits in the Nowodwor-Piaski area is constructed on the basis of outcrop and GPR data with the use of multiple-point statistical methodology to depict the internal architecture, heterogeneity and spatial relationships of main sedimentary facies. The model can serve as a guide for the future exploration and exploitation of the quartz-glauconite sands in the area and as instructive example of how a petroleum reservoir model of a complex sedimentary succession can be constructed with the use of modern statistical methods.

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

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

The morphology and distribution of submerged reefs in the Maui-Nui Complex, Hawaii: New insights into their evolution since the Early Pleistocene

Reef drowning and backstepping have long been recognised as reef responses to sea-level rise on subsiding margins. During the Late Pleistocene (~ 500–14 ka) Hawaiian reefs grew in response to rapid subsidence and 120 m 100 kyr sea-level cycles, with recent work on the submerged drowned reefs around the big island of Hawaii, and in other locations from the last deglacial, providing insight into reef development under these conditions. In contrast, reefs of the Early Pleistocene (~ 1.8–0.8 Ma) remain largely unexplored despite developing in response to significantly different 60–70 m 41 kyr sea-level cycles. The Maui-Nui Complex (MNC — forming the islands of Maui, Molokai, Lanai and Kahoolawe), provides a natural laboratory to study reef evolution throughout this time period as recent data indicate the reefs grew from 1.1 to 0.5 Ma. We use new high resolution bathymetric and backscatter data as well as sub-bottom profiling seismic data and field observations from ROV and submersible dives to make a detailed analysis of reef morphology and structure around the MNC. We focus specifically on the south-central region of the complex that provides the best reef exposure and find that the morphology of the reefs varies both regionally and temporally within this region. Barrier and pinnacle features dominate the steeper margins in the north of the study area whilst broad backstepping of the reefs is observed in the south. Within the Au'au channel in the central region between the islands, closely spaced reef and karst morphology indicates repeated subaerial exposure. We propose that this variation in the morphology and structure of the reefs within the MNC has been controlled by three main factors; the subsidence rate of the complex, the amplitude and period of eustatic sea-level cycles, and the slope and continuity of the basement substrate. We provide a model of reef development within the MNC over the last 1.2 Ma highlighting the effect that the interaction of these factors had on reef morphology.

A new geological framework for the Middle Ordovician Carillon Formation (uppermost Beekmantown Group, Ottawa Embayment): onset of Taconic foreland deposition and tectonism within the Laurentian platform interior

The Carillon Formation underlies the Ottawa Embayment, a craton-interior extension of the regional Laurentian paleoshelf, defined today by the Ottawa–Bonnechere Graben. Facies within the embayment during the Middle Ordovician identify a mixed-sediment (carbonate, siliciclastic), tide-influenced, peritidal and schizohaline basin, host to a mosaic of dolomitic tidal mudflats, shallow lagoons or ponds, and channels. Skeletal metazoans were rare, but bioturbation increased with episodic influx of less saline water carrying sand and mud, possibly in response to regional climate change. Stratigraphic analysis reveals that the formation coincides with a third-order eustatic sea level cycle, but higher order stratigraphic patterns show no obvious eustatic link. Instead, a stepwise onlap geometry coincided with local faulting that produced widespread syndepositional sediment deformation. In particular, stages of faulting and post-Carillon erosion are mapped along the present-day trace of a northwest–southeast-oriented lineament, the Gloucester Fault, within the present graben. Other present-day northwest–southeast-oriented faults of the graben may also possess Middle Ordovician histories. Platform-interior tectonism and development of the Carillon Formation coincided with the onset of Taconic deformation along the outer paleoshelf (western Newfoundland). The formation defines the initial stage of foreland basin sedimentation in the embayment. Platform-interior faulting identifies a predisposed structural weakness associated with underlying Neoproterozoic structure, possibly enhanced by a regional structural connection (an ocean-to-onland transform fault system) with the distal (~1200 km), convergent, plate boundary.

Ordovician stratigraphy of the Kovel-1 well (Volkhov–Haljala) in the Volynia region, northwestern Ukraine

The Ordovician succession of the Kovel-1 well in the Volynia region, northwestern Ukraine is composed of a basal 0.6 m thick siliciclastic unit succeeded by 24.7 m Lower and lower Middle Ordovician carbonate sediments. The carbonate rocks are divided into 13 informal lithologic units. The carbonate sediments accumulated in marine shallow water open shelf and shoal or turbulent environs. Biostratigraphically, the succession is referred to seven chitinozoan zones and 12 conodont biozones. Integration, chronostratigraphic position and correlation of the proposed biozones with those from Baltoscandia are briefly discussed. Four major unconformities are recognized within the succession: 1) the Pakerort(?)–Volkhov unconformity, 2) the mid Volkhov unconformity, 3) the early Kunda unconformity and 4) the early Mid Ordovician hiatus. The latter straddles the Oeland–Viru regional Series boundary in the well. The early Mid Ordovician unconformity is prominent and the corresponding hiatus spans the Aseri and Lasnamägi regional stages (= upper Darriwilian). A complex of cyclic transgressive–regressive depositional pattern prevailed and the whole succession is referred to three major depositional cycles. The major depositional cycles are related to global eustatic sea-level cycles in general and hypothetic way to tectonic events caused by collisions of Peri-Gondwanan microcontinents with Baltica.

Ethologic and ecologic interpretation of complex stellate structures in Pleistocene deep-sea sediments (Otadai Formation), Boso Peninsula, central Japan

A large stellate structure, which is characterized by long, horizontal radial structures with a single vertical shaft, occurs in mud-dominated lower submarine fan sediments (Lower Pleistocene Otadai Formation, Boso Peninsula, Japan) deposited on an active plate margin during a highstand interval of a eustatic sea-level cycle. A single radial structure is divided into more than 20 meniscate packages filled with pellets of fecal origin. The Otadai specimens closely resemble the trace fossil Phoebichnus trochoides in its configuration, which has been reported only from Mesozoic rocks of shallow marine origin. However, the radials of the Otadai material have no mantle or lining. Hence, the Otadai material can be discriminated from P. trochoides at the ichnospecies level. Thus the Otadai material is introduced as a new ichnospecies, namely Phoebichnus bosoensis. Considering the anatomy and mode of occurrence of P. bosoensis, the trace-making animal adopted a sedentary life-style and apparently fed on detritus at the seafloor and conveyed the fecal material downward into the radials. Internal structures in the radials of P. bosoensis appear to have been produced by the repetition of systematic excretory behavior of the trace-making animal. The vertical shaft is thought to be a dwelling structure. During a highstand interval of a eustatic sea-level cycle, large amounts of terrigenous sediments were deposited by turbidity currents at a lower submarine fan setting of the Otadai Formation, as suggested by sedimentologic data. Large amounts of terrigenous sediment supply to a lower submarine fan setting of the Otadai Formation was due to a narrower shelf and a steeper slope in comparison to a passive plate margin. Therefore sufficient influx to the seafloor of organic matter as the food for the benthos is assumed for the highstand interval of a eustatic sea-level cycle. This inference means that the nutrient level at the seafloor was suitable for surface deposit-feeders. Sufficient food supply must have been available to support the long-term sedentary life-style for the Otadai Phoebichnus producers, which produced the stellate structure as a dump for fecal matter.

Tectonic and stratigraphic significance of the Middle Ordovician carbonate breccias in the Ogcheon Belt, South Korea

Abstract Carbonate breccias occur sporadically in the Lower–Middle Ordovician Maggol Limestone exposed in the Taebacksan Basin in the northeastern part of the northeast–southwest‐trending Ogcheon Belt, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault‐related breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these carbonate breccias. The present study, however, indicates that the majority of these carbonate breccias are solution–collapse breccias, which are causally linked to paleokarstification. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician (earliest Darriwilian). Extensive subaerial exposure of platform carbonates resulted in paleokarst‐related solution–collapse breccias in the upper Maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk–Tippecanoe sequence boundary elsewhere beneath the Middle Ordovician succession and its equivalents, most notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second‐ or third‐order eustatic sealevel drop during the early Middle Ordovician. Although a paleokarst breccia zone is recognized beneath the Middle Ordovician succession in South Korea, the Sauk–Tippecanoe sequence boundary appears to be a conformable transgressive surface on the top of the paleokarst breccia zone in the upper Maggol Limestone. The paleokarst breccia zone beneath the conformable transgressive surface is represented by a thinning‐upward stack of exposure‐capped tidal flat‐dominated cycles that are closely associated with multiple occurrences of paleokarst‐related solution–collapse breccias. This paleokarst breccia zone was a likely consequence of repeated fourth‐ and fifth‐order sealevel fluctuations. It suggests that second‐ and third‐order eustatic sealevel drop may have been significantly tempered by substantial tectonic subsidence near the end of the Maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off‐platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e. the Yemi Breccia). With the continued tectonic subsidence, subsequent rise in the eustatic cycle caused drowning and deep flooding of carbonate platform, forming a transgressive surface on the top of the paleokarst breccia zone. This tectonic implication contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously interpreted. Thus, it is proposed that the Taebacksan Basin in the northeastern part of the Ogcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician. The proposed tectonic model in the basin gives much better insight to unravel the stratigraphic response to tectonic evolution of the Ogcheon Belt, which remains an enigmatic feature in formulating a tectonic framework of the Korean peninsula. The present study also provides a good example that the falling part of the eustatic sealevel cycle may not produce a significant event in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence.

Sequence aggradation and systems tracts partitioning in the mid-eastern Yellow Sea: roles of glacio-eustasy, subsidence and tidal dynamics

In the mid-eastern Yellow Sea, late Pleistocene–Holocene glacio-eustatic sea-level fluctuations and a local subsidence regime have formed an aggradational set of high-resolution sequences (sequences I–V in descending order). Extensive tidal scour-and-fill processes during transgressions played a significant role in marked partitioning of non-marine to paralic clast-rich lowstand systems tract (LST) and tidal fine-grained transgressive–highstand systems tract (TST–HST) within each sequence. The lateral extent and thickness of sequences and component systems tracts reflect differences in magnitude and duration between eustatic sea-level cycles. Sequence V is characterized by extensive LST, indicating sea-level fall to the shelf-margin area. Sequences IV and III display a limited extent of LST, reflecting small-scale sea-level falls during the late Pleistocene. Sequence II displays seaward restriction of TST–HST owing to a small-scale sea-level rise prior to the last glacial maximum (LGM). Sequence I has formed since the LGM and is characterized by discontinuous TST–HST over the entire shelf area, reflecting rapid retreat of sediment sources during the post-glacial sea-level rise. Along with the predominant controls of local subsidence, glacio-eustasy and transgressive tidal dynamics on the overall stratal architecture, minor variations in architectural characteristics were added by other local factors such as changing sediment flux, antecedent topography and basin physiography.

ABSTRACT: Three-Dimensional Numerical Modelling of Eustatic vs Local Controls on Depositional Sequences

A three-dimensional numerical model of sediment transport and deposition in coarsegrained deltas is used to investigate the controls on depositional sequence variability. We consider the stratigraphic response to eustatic sea-level (amplitude and rates) and local controls, such as physiography, sediment supply and local subsidence. The development of key stratal surfaces, stratal geometry and facies stacking patterns show a systematic development with respect to stages in a eustatic sea-level cycle. However, even with constant sediment supply and a simple sinusoidal eustatic sea-level variation many of the features developed in the models are, by their nature, three-dimensional and twodimensional analysis of the model results can lead to erroneous interpretations of the causes of along-strike variability. Local controls generate significant variability in depositional sequence development, including the timing and amount of fluvial incision during relative sea-level fall, and the timing and nature of transgression during rapid phases of relative sea-level rise. The model results suggest that different systems tracts may be coeval and that key stratal surfaces defining and subdividing depositional sequences may be of local extent. Furthermore, the results highlight pitfalls in sequence stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.

Facies relationships and cyclicity of high-latitude, Late Permian coal measures, Bowen Basin, Australia

The Late Permian Moranbah Coal Measures are developed throughout the northern and central Bowen Basin, eastern Australia, and are subject to large-scale opencut coal mining activity. They were deposited in a rapidly subsiding retroarc foreland basin, during frequent base-level changes. Contemporaneous compressive deformation and volcanism facilitated the supply of abundant volcanoclastic sediments, resulting in rapid, but punctuated, basinal aggradation. To the south and east, transgressive–regressive cycles were marked by the inter-digitation of paralic–shallow marine and alluvial strata. Lonestones (dropstones) are common within the marine deposits and are attributed to ice-rafting indicative of a cold-climate depositional regime. Two basin-wide chronostratigraphic units, the P-tuff bed and the Harrow Creek superseam, have been used to evaluate the facies architecture of the stratigraphic interval they enclose. The lithofacies mosaic of the 25–75 m thick sequence has been examined by means of 26 widely distributed coreholes and two detailed district-scale studies based around the Moranbah–Goonyella and German Creek minesites. Eight alluvial–shallow marine facies associations have been recognised. Their architectural and temporal relationships allow the accurate reconstruction of the coal-bearing depositional system. Two dissimilar, stacked, depositional systems tracts, A (organics) and B (clastics) are recognised. The P-tuff bed is enclosed by facies of tract A, forming a thick, low-ash, coal-bearing interval which was emplaced essentially synchronously across the basin. It represents a time of clastic sediment starvation when basin-wide peat mire accumulation prevailed. Four similar extensive coal-bearing intervals within the Moranbah Coal Measures are also considered to represent basin-wide episodes of peat accumulation. This pattern is in strong contrast to traditional views of coal measure development, which argue that coals formed within a contemporary facies mosaic, in which fluvial channels, levees and crevasse splays were also represented. The second depositional systems tract (B) is characterised by clastic packages, dominated by large-scale (up to 4 km wide and 50 m thick), sandstone bodies deposited by low sinuosity trunk river systems, flanked by substantial crevasse splay deposits (up to 3 km wide and 30 m thick). It represents the product of catastrophic flooding events. The cyclothemic succession of systems tracts A and B is interpreted in terms of eustatic sealevel cycles. The tract A coal-bearing intervals represent relative sealevel lowstands. Systems tract B represents relative sea level highstand with its lower interval at German Creek to the south consisting of paralic and marine facies. The cycles had durations in the order of 400 to 600 ky and are considered to represent 4th order sealevel cycles of the Permian Period.

Abstract: Historical development of the coastal onlap chart and eustatic sea level cycles

Abstract: Historical development of the coastal onlap chart and eustatic sea level cycles

Reservoir biostratigraphy of the Pedernales Field, Venezuela

Abstract The geographic and geological settings of the Pedernales Field, and its history from discovery to development, are briefly outlined. Biostratigraphy and palaeoenvironmental interpretation, and their impact on integrated reservoir description (IRD) and production geology, are discussed in detail. Historical and modern vintage micropalaeontological data enable the recognition of a number of ‘zones’ over the reservoir interval. These ‘zones’ are diachronous, although they do have correlative value when integrated into the framework of isochronous tectonostratigraphic sequence boundaries and flooding surfaces identified on seismic sections and well logs. The ‘zones’ and their containing sediments can be tentatively calibrated against global standard biozones and hence sequence boundaries and flooding surfaces on the eustatic sea-level cycle chart. Micropalaeontological and palynological data also enable the recognition of a number of biofacies interpreted on the basis of modern analogy as indicating a range of delta-top, delta-front and prodelta environments. The principal reservoir is interpreted as having been deposited in a range of delta-top to delta-front subenvironments, the seal in a range of distal prodelta subenvironments. The ongoing high-resolution biostratigraphic work programme has revealed the existence of a large number of correlatable events over the reservoir interval. Integrated high-resolution biostratigraphic, well log and seismic correlation can be achieved both within and between the compartments of the field. High-resolution biostratigraphic/biofacies data demonstrate the existence of a bathymetric gradient at the time the reservoir were being deposited. Gravity gliding along the observed gradient provides a possible mechanism for the locally observed section repetition and enhanced reservoir development, and also for the up-dip trapping mechanism.

Miocene carbonates of the Luconia province, offshore Sarawak: implications for regional geology and reservoir properties from Strontium-isotope stratigraphy

The Luconia Province, offshore Sarawak, Malaysia, is one of the largest SE-Asian carbonate provinces with more than 200 platforms and gas reserves exceeding 40 Tscf. The application of the Sr­ isotope technique integrated with core, log and seismic data significantly refines the stratigraphy of the province despite extensive diagenetic alteration. Based on the new data, a unified concept of platform evolution is tied to global sea-level variations and the regional distribution of reservoir architecture. Evidence is presented for major karstification during and after platform growth and its influence on the regional distribution of reservoir properties. Sr-isotope analyses (ntotaJ = 137; data range: O.7084~.70903 for NBS987 = 0.710230) suggest that the carbonate platforms of the Luconia province are Early to Middle Miocene in age. The Sr-isotope signature is apparently unaffected by diagenetic stabilization from metastable carbonates to low-Mg calcite while dolomitization caused alteration of the isotope ratio. Growth and demise of the province can be correlated with a second-order eustatic sea-level cycle (TB2). Major karst horizons and flooding, aggradation and progradation packages are linked via step-changes in isotope signature to third order eustatic sea-level fluctuations. Simultaneous with the second order sea-level drop (late Middle Miocene) the influx of siliciclastics split the province into a southern part with low relief carbonate banks and a northern part with high relief platforms. All growth terminated at the end of the Middle Miocene. Low relief banks were buried while high relief platforms were karstified prior to drowning (Late Miocene­ Pliocene). The regional distribution of pore types, porosity and permeability is linked to the duration of exposure and burial diagenesis. In the karstified platforms of the northern part drilling losses are common.

Sequence concepts at seismic and outcrop scale: the distinction between physical and conceptual stratigraphic surfaces

The sequence stratigraphic terms maximum flooding surface and downlap surface, as currently applied, are ambiguous. Examples of these intra-sequence surfaces are summarised from high frequency mid-Pleistocene sequences and from a Cretaceous–Recent seismic megasequence, both from New Zealand. At any one locality, a mid-Pleistocene sequence contains up to four stratal discontinuities, in ascending order: the sequence boundary, ravinement surface, local flooding surface and downlap surface. These physical surfaces, which occur in outcrop, are regionally diachronous and should be differentiated from theoretical isochronous horizons such as the time of maximum flooding (horizon corresponding to maximum shoreline transgression) and the time of peak eustatic or local relative sea-level (horizons corresponding to the highpoint of the eustatic and relative sea-level cycles, respectively). In seismic studies, the boundary between the transgressive and highstand systems tracts is usually located at the downlap surface. On the basis of a major thermo-tectonic sea-level cycle in the Canterbury Basin, it is shown that the downlap surface is not a single regional surface, and that the change in slope associated with toes of successive prograding clinoforms rises in stratigraphic height basinwards. The downlap surface therefore does not usually coincide with the maximum flooding horizon. In Plio–Pleistocene cyclothems, a discrete unit — the mid-cycle shellbed — straddles the contact between the transgressive and highstand systems tracts. This unit might be classified within its own systems tract (the condensed section systems tract; CSST). Alternatively, the position of the boundary between the transgressive and highstand systems tracts can remain unspecified or unknowable.

The relationship between glacio-eustatic parasequences and a third-order sequence in the Kakegawa Group, central Japan

The Plio–Pleistocene Kakegawa Group, central Japan, consists of a third-order depositional sequence (2.6–1.0 Ma). The northwestern part of the Kakegawa sequence consists of up to 500 m of alluvial, shoreface, shelf, slope and submarine-channel facies. It contains at least sixteen upward-shallowing cycles (parasequences), the deposition of which was affected by high-frequency eustatic sea-level cycles. The lower part of the sequence is characterized by a retrogradational parasequence set, which formed a transgressive systems tract (2.2–1.75 Ma) followed by a progradational parasequence set comprising a highstand systems tract (1.75–1.4 Ma). Subsidence analysis and evaluation of changes in the shelf sedimentation rate estimated from cross-sections, suggest that formation of the third-order sequence was controlled by tectonic subsidence and variation in the sedimentation rate. Rapid subsidence and a high rate of sedimentation during 2.2–2.0 Ma resulted in deposition of the lower part of the transgressive systems tract, characterized by thick backstepping successions. The rate of subsidence decreased in the period 2.0–1.75 Ma. The sedimentation rate also decreased due to a high rate of sediment bypassing. However, subsidence was still the dominant factor, leading to the formation of thin backstepping successions. The 1.75–1.4 Ma progradational succession resulted from a combination of a low rate of subsidence and moderate sedimentation. The progradational units become thicker basinward owing to faster subsidence in the basin center. The maximum flooding surface was formed around 1.75 Ma even though subsidence was slow at this time.