High-amplitude Sea-level Changes Research Articles

The Bakken Formation—understanding the sequence stratigraphic record of low-gradient sedimentary systems, shale depositional environments, and sea-level changes in an icehouse world

The Bakken Formation is a major petroleum producer in the continental US. However, its deposition in an intracratonic, low-gradient setting has often been mistakenly described as “layer-cake”. This contribution is designed to highlight the time-transgressive nature of its main petroleum-producer, the middle Bakken member. Correlation of individual parasequences reveal the subtle nature of otherwise invisible low-angle stratigraphic geometries. Sequence stratigraphically-relevant surfaces occur throughout the unit and subdivide the entire Bakken into 5 third-order sequences; one of them is a hidden sequence at the base of the petroleum-producing middle Bakken indicating both a lowstand and a subsequent transgression. The organic-rich shales above and below the middle Bakken were deposited in an oxygen-deficient environment and show several burrow/fecal string types and indications of active currents during deposition. The Bakken records high amplitude sea-level changes during sequences compared to relative low amplitude sea-level changes of parasequences. This, coupled with a likely mismatch in timing of Bakken deposition relative to world-wide ice-age-induced cyclicity makes it unlikely that the Bakken sea-level fluctuations were dominated by glaciation.

Numerical modeling of icehouse and greenhouse sea-level changes on a continental margin: Sea-level modulation of deltaic avulsion processes

Abstract Sequence stratigraphic models emphasize changes in accommodation driven by relative sea level as a principal control on continental-margin development. Therefore, it has been hypothesized that there are differences in the volume and distribution of sand delivered to deep-water during the high-amplitude sea-level changes of icehouse settings versus the low-amplitude sea-level changes of greenhouse settings. Previous numerical modeling and field studies suggest an alternative hypothesis: that there would be minimal differences between each setting. Here, we used a three-dimensional numerical stratigraphic forward model to test the impact of greenhouse and icehouse sea-level changes on sand distribution in a passive margin setting. Model results were analyzed to evaluate the impacts of the rate and amplitude of relative sea-level changes on individual deep-water sand delivery events, total deep-water sand volumes, and sand distribution. Model results show that total deep-water sand volume is almost independent of the rate of relative sea-level change, which we attribute to the interaction of this sea-level change rate and the time required by the delta to reach a dynamic equilibrium state as a result of avulsion processes. However, there are major differences in the large-scale spatial distribution of sand between the icehouse and greenhouse conditions. The high amplitude sea-level changes in the icehouse model caused significant shelfal incision, which minimized deltaic avulsion processes and localized deep-water sedimentation. The relatively low amplitude sea-level changes in the greenhouse model allowed avulsion of the fluvial-deltaic system, thus causing more laterally extensive deep-water deposits. Our experimental results demonstrate that the rate of deep-water sand delivery is largest during periods of low sea level in both icehouse and greenhouse conditions, but show that there is a poor relationship between the rate of deep-water sediment delivery and the rate of sea-level change. We attribute both of these results to the nature of avulsion processes on the delta: more frequent avulsions of greenhouse deltas spread sediment across the shelf and limit sediment delivery to deep water until a dynamic equilibrium state is achieved, whereas the development of an icehouse incised valley locks the fluvial-deltaic system in place and localizes deep-water sedimentation. Moreover, our results indicate that the total volume of deep-water sand is controlled by the total sediment supply to the basin. These results provide a framework for the timing and quantity of continental-margin sedimentation and deep-water sediment delivery, with applications to sequence-stratigraphic interpretations.

Past and Future Ecosystem Change in the Coastal Zone

The coastal zone is in a constant state of flux. Long term records of change attest to high amplitude sea level changes. Relative stability though the Late Holocene has allowed for the evolution of barrier dune systems, estuaries and coastal lakes with associated plant and faunal associations. This evolution has been interspersed with changes in the balance between climate driven changes in outflow from catchments. These interactions have been considerably disturbed through the impacts of industrialised people who have diverted and consumed water and invested in infrastructure that has impacted on river flows and the tidal prism in estuaries. This has impacted their provisioning services to humans. It has also impacted their regulating services in that development along the coastline has impacted on the resilience of the littoral zone to absorb natural climate extremes. Looking from the past we can see the pathway to the future and more easily recognise the steps needed to avoid further coastal degradation. This will increasingly need to accommodate the impacts of future climate trends, increased climate extremes and rising seas. Coastal societies would do well to identify their long term pathway to adaptation to the challenges that lie ahead and plan to invest accordingly.

Greenhouse shoreline migration: Wilcox deltas

ABSTRACT In contrast to the high-frequency and high-amplitude sea level changes of icehouse times, eustatic sea level changes in greenhouse times are now generally accepted as significantly lower frequency and amplitude. As a corollary of this, frequency and extent of cross-shelf shoreline transits in greenhouse times are also likely to have been modest by comparison, and it has been suggested that greenhouse deltas may have been docked at the shelf edge for long periods, thus delivering sediment to deep-water areas more frequently. A revisit of upper Paleocene–lower Eocene Wilcox data across south Texas shows repeated regressive–transgressive shoreline migrations longer than 50 km (31 mi) at a time scale of some 300 k.y. This style of repeated shoreline transits is documented from well logs and is supported by the repeated presence of transgressive estuarine deposits with strong tidal evidence as interpreted from core. We argue, therefore, that Wilcox paleogeography was more varied than commonly portrayed and that the greenhouse shoreline transits were caused by greenhouse sea level change but severely modulated by variable sediment discharge caused by Paleogene hyperthermals. Periodic climate warming during Wilcox deposition and Laramide relief generation in the drainage areas were also responsible for unusually high sediment flux into the Gulf of Mexico. The factor of sediment supply in shoreline growth and retreat has been understated in the literature, partly because of an overemphasis on accommodation as the main driver of stratigraphic sequences.

Physical and chemical stratigraphy suggest small or absent glacioeustatic variation during formation of the Paradox Basin cyclothems

The Paradox Basin cyclothems previously have been interpreted as Milankovitch style glacial–interglacial cycles from the Late Paleozoic Ice Age, but an unambiguous test for a glacioeustatic origin has not been conducted. A high resolution coupled chemical and physical stratigraphic analysis of two outcrop sections and three core segments provides new evidence that supports either minor sea level change of several meters or an autocyclic mechanism for parasequence formation. High amplitude sea level change is ruled out by the scale of thin top-negative isotopic meteoric diagenesis trends associated with parasequence tops and subaerial exposure fabrics. Isotopic gradients from shelf (light) to basin (heavy) indicate that parasequences are deposited diachronously, with isotopes of more distal sections recording increased basin restriction. These results support the idea that the late Pennsylvanian was a prolonged period of relatively static eustasy, agreeing with recent studies in the western USA. The methods provide a new set of tools and context for extracting environmental information from cyclic upward shallowing carbonate parasequences.

Chemostratigraphy of the Late Mesoproterozoic Atar Group, Taoudeni Basin, Mauritania: Muted isotopic variability, facies correlation, and global isotopic trends

Sedimentary strata of the Atar/El Mreiti groups, Taoudeni Basin, Mauritania, are characterized by carbon isotope values that fall largely between −2.5‰ and +4.0‰, and contain stratigraphic trends that are strikingly similar to those recorded worldwide in strata with depositional ages from ∼1.2Ga to 1.1Ga. Chemostratigraphic results are consistent with recent Re–Os dates of ∼1.1Ga for Atar/El Mreiti group strata (Rooney et al., 2010) and support an emerging dataset that identifies the Late Mesoproterozoic as an isotopically distinctive interval in Earth history. The relatively low-amplitude carbon isotope variability limits the use of chemostratigraphy in detailed correlation during this time interval. In this study, we combine chemostratigraphic data with key chronostratigraphic horizons and detailed analysis of depositional facies to construct a rigorous and testable model for the intrabasinal correlation of Atar and el Mreiti group strata. Data support a depositional model that results in broadly uniform stratigraphic thicknesses across the West African craton, with predominantly stromatolitic facies of the Atar Group (Mauritania) and the Hank and Dar Cheikh groups (Algeria) representing deposition on shallow-water craton margins, and clayey-carbonate and shale facies of the El Mreiti Group (Mauritania) representing deposition on a broad, shallow-water, epicratonic platform. Within this stratigraphic framework, regional differences in carbon isotope composition are interpreted to reflect spatial differences in water chemistry likely associated with local carbon cycle dynamics in cratonic environments that have limited exchange with the open ocean. Similarly, distinct differences in trace element composition recorded in multilayered marine cements within deeper-water stromatolitic reef facies are interpreted to reflect vertical differences in water chemistry associated with redox-gradients in the water column.We use this multifaceted approach to test a recent hypothesis that suggests correlation of Atar/el Mreiti group strata with glaciogenic strata of the Vazante Group, São Francisco craton. Critically, preservation of shallow-water carbonate and evaporite strata across more than 1200km of the predominantly epicratonic Taoudeni Basin—with minimal evidence for high-amplitude sea level change—is consistent with inferences of late Mesoproterozoic highstand conditions in a global greenhouse climate. Such characteristics are inconsistent with suggestions of a time-equivalency with glaciogenic strata of the Vazante Group, São Francisco craton, despite broad similarities in Re–Os geochronology and carbon isotope composition. Together, these results emphasize both the utility and limitations of carbon isotope stratigraphy to inform hypotheses regarding global correlation and biospheric evolution.

Early Hettangian benthic–planktonic coupling at Doniford (SW England): Palaeoenvironmental implications for the aftermath of the end-Triassic crisis

This paper presents a high-resolution micropalaeontological study, integrated with geochemical and sedimentological analyses of the Triassic–Jurassic (Tr–J) transition at the Doniford section (SW England). Our analyses indicate the renewal of surface carbonate production in the Blue Lias Formation (early Hettangian) after the end-Triassic mass extinction. Based on the high abundance of surface carbonate producers (schizosphaerellids), which occurred in phase with development at sea-floor of calcareous hyaline benthic foraminifers, we deduce that a benthic–pelagic coupling resulted in both the initial restoration of the biological carbonate pump, and also a more efficient export production. The presence of monospecific phytoplankton assemblages and benthic foraminiferal assemblages typical of poorly oxygenated environments suggest that the environmental stress continued after the end-Triassic maximal perturbation. Complete environmental recovery was prevented by a succession of alternating dysaerobic and anoxic phases that favoured the deposition of black-shale sediments. The hypothesis of a methane-release mechanism as a trigger for the global carbon cycle perturbation and greenhouse conditions during the early Jurassic is discounted, based on evidence from the decoupled carbon isotope records of carbonate and organic matter. Apart from local control of a high amplitude sea-level change across the Tr–J boundary, the succession and nature of events at Doniford are consistent with a scenario of global environmental and biotic changes triggered by the physical and chemical effects of the Central Atlantic Magmatic Province (CAMP) volcanism. A super greenhouse climate in the aftermath of the end-Triassic mass extinction appears to be the results of the combination of the long-term CO 2 accumulation in the atmosphere system due to the CAMP volcanic activity, as well as the inefficiency of the biological pump due to reduced export production.

Benthic carbonate facies of the Phanerozoic: Review and example from the carboniferous of the Russian platform

General classifications of Phanerozoic carbonate facies and controlling them factors are reviewed. Three principal carbonate factories distinguished by W. Schlager (2000, 2003) are the tropical shallow-water, the cool-water, and the mudmound factories. The general term for facies associations in the first factory is photozoan carbonates. The cool-water factory encompasses environments producing heterozoan carbonate facies. The mudmound factory is a non-actualistic sedimentary system producing mound-shape buildups of non-skeletal microbial micrites (also termed automicrites). The benthic carbonate production is controlled by light, bottom temperature, eutrophication, siliciclastic influx, and the evolution of marine ecosystems. The cyclic alternation of skeletal associations (“biofacies”) formed under the control of high-amplitude sea level changes is exemplified by the Moscovian (Carboniferous) epeiric carbonates of the East European Craton. Three principal biofacies associations in this example are bryonoderm extended (heterozoan), staffellid-syphonean (photozoan). and Meekella-Ortonella (intertidal flat to stagnant lagoon).

Isotopic Evidence for Glaciation During the Cretaceous Supergreenhouse

The Turonian (93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35 degrees C. High-amplitude sea-level changes and positive delta18O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New delta18O data from the tropical Atlantic show synchronous shifts approximately 91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming.

Palaeoenvironmental implications of Palaeomicrocodium in Upper Devonian microbial mounds of the Chernyshev Swell, Timan-northern Ural Region

Nine major microfacies and eight transgressive-regressive cycles have been recognized in the middle Frasnian-Famennian microbial mound of the Shar'yu River section in the Chernyshev Swell, Timan-northern Ural region. Rocks including Palaeomicrocodium at the top of each cycle show signs of brecciation, freshwater leaching, and vadose cementation. Palaeomicrocodium, an enigmatic structure, being close in nature to Microcodium, is useful as a palaeoenvironmental indicator and can also be used in regional correlations. Seven levels of subaerial expose surfaces with Palaeomicrocodium correspond to seven breaks in growth of the microbial mound in the Upper Devonian reef-like formation. The high-amplitude sea-level changes recognized are in good correlation with those in the Moscow Syneclise and are also fixed on the global eustatic curve.

A radiometric calibration of the SPECMAP timescale

The astronomical theory of climate change asserts that Earth's climate is affected by changes in its orbit, which vary the seasonal and latitudinal distribution of solar radiation. This theory is the basis of the orbitally tuned SPECMAP timescale. A key constraint for this important chronology was the mid-point of the Penultimate Deglaciation, initially dated to 127,000 years ago. Recent work suggests this event may be considerably older, casting doubt on the astronomical theory, the SPECMAP timescale, and the accuracy of orbitally tuned chronologies. Difficulties with U/Th coral dating of sea-level events have impeded progress on this problem, because most corals are not closed systems. Here, we use a new approach to U/Th dating that corrects for open-system behavior and produces a sea-level curve of sufficient resolution to confidently correlate with SPECMAP over the last 240,000 years, permitting a reassessment of both this critical chronology and a central tenet of climate change theory. High-precision ages for 24 oxygen isotope events provide a 240,000-year chronology for marine δ 18O records that is independent of orbital tuning assumptions. Although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the Marine Isotope Stage 7/6 boundary, a comparison of radiometric and SPECMAP ages for identical isotope events suggest that the SPECMAP timescale is quite accurate and that its errors were, in general, overestimated. Despite suborbital complexity, orbital cyclicity is clearly evident in our record. High-amplitude sea-level oscillations at periods greater than ∼20,000 years are very close in phase to summer insolation in the Northern Hemisphere. Although sea-level changes cannot be uniquely tied to a specific season or latitude of insolation forcing, the simplest explanation is that long-period, high-amplitude sea-level change is linked to Northern Hemisphere insolation forcing. These results validate the principles of orbital tuning and suggest such timescales are generally robust.

Importance of high-frequency tectonic sequences during greenhouse times of Earth history

The relative importance of tectonism in the stratigraphic record should be more clearly expressed during greenhouse times of Earth history due to the lack of overmasking effects of high-frequency and high-amplitude sea-level changes typical for icehouse periods. Establishment of the importance of tectonism, especially at temporal scales comparable to durations of typical transgressive-regressive cycles, has been plagued by poor temporal resolution. Our outcrop and subsurface-based study of a Cenomanian shallow-marine siliciclastic interval, constrained by bentonite-based geochronology and detailed biostratigraphy, examines this problem. In a 2.2 m.y. interval, we identified four tectonically driven erosional surfaces that dominate preserved stratigraphy. Biostratigraphic correlation to a sea-level curve for the Cenomanian—where coeval high-frequency lowamplitude eustatic cyclicity has been demonstrated—allows the first direct comparison of the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years during a global greenhouse time. We suggest that minor tectonic pulses locally overshadow the effects of eustasy and exert the dominant control over preserved stratigraphy. While subtle tectonic control on sedimentation has been documented throughout the Cretaceous Western Interior, the results of this study suggest that much of that deformation occurred at sub-million-year frequencies and at time scales comparable to eustatic transgressiveregressive cycles.

Fringing carbonate platforms at the Arabian Plate margin in northern Oman during the Late Aptian–Middle Albian: Evidence for high-amplitude sea-level changes

The Aptian–early Middle Albian margin of the Arabian Platform is exposed in Wadi al Assyi (northeast of Jebel Akhdar in northern Oman), and exhibits remarkable geometries testifying to the sedimentary history during the subaerial exposure of most of the Arabian Platform. Detailed analysis of the depositional geometries allows the reconstruction of sea-level fluctuations during the studied time interval, and an estimation of their amplitude. The final progradation of the Lower Cretaceous Arabian carbonate Platform (uppermost part of Kahmah Group) coincides with the deposition of the uppermost Salil Formation whereas the Al Hassanat Formation mostly developed at the margin during the exposure of the Arabian Platform in the Late Aptian to Middle Albian, and thus corresponds to a 2nd-order shelf-margin wedge. During the Early Aptian, km-scaled clinoforms constituted the prograding part of the partly bioconstructed Lower Shuaiba Member. Close to the Early–Late Aptian boundary, the Arabian Platform became exposed and a forced regression took place as evidenced by strong erosion and deposition of a channelized clinoform. These preceded the formation of at least seven, km-scaled, carbonate sequences in the Upper Aptian–lowermost Middle Albian Al Hassanat Formation that, by their geometries and palaeoenvironmental setting, look like fringing coral reefs. These carbonate sequences are therefore described as fringing carbonate platforms that developed laterally and/or above each other. Their internal architecture is mainly retrogradational and aggradational due to rapid, 3rd-order sea-level fluctuations of high amplitude, which forced the carbonate systems to migrate basinward after a sea-level fall and aggraded during sea-level rise, and to the fact that these systems were partly bioconstructed. Five major sea-level falls (30 to 100 m) possibly correlate to sequence boundaries identified in other palaeotectonic/palaeogeographic settings, and are interpreted as resulting from glacio-eustatism or, alternatively, an unknown mechanism.

The stratigraphic architecture and evolution of the Burdigalian carbonate—siliciclastic sedimentary systems of the Mut Basin, Turkey

This study describes the coeval development of the depositional environments in three areas across the Mut Basin (Southern Turkey) throughout the Late Burdigalian (early Miocene). Antecedent topography and rapid high-amplitude sea-level change are the main controlling factors on stratigraphic architecture and sediment type. Stratigraphic evidence is observed for two high-amplitude (100–150 m) sea-level cycles in the Late Burdigalian to Langhian. These cycles are interpreted to be eustatic in nature and driven by the long-term 400-Ka orbital eccentricity-cycle-changing ice volumes in the nascent Antarctic icecap. We propose that the Mut Basin is an exemplary case study area for guiding lithostratigraphic predictions in early Miocene shallow-marine carbonate and mixed environments elsewhere in the world. The Late Burdigalian in the Mut Basin was a time of relative tectonic quiescence, during which a complex relict basin topography was flooded by a rapid marine transgression. This area was chosen for study because it presents extraordinary large-scale 3D outcrops and a large diversity of depositional environments throughout the basin. Three study transects were constructed by combining stratal geometries and facies observations into a high-resolution sequence stratigraphic framework. 3346 m of section were logged, 400 thin sections were studied, and 145 biostratigraphic samples were analysed for nannoplankton dates (Bassant, P., 1999. The high-resolution stratigraphic architecture and evolution of the Burdigalian carbonate-siliciclastic sedimentary systems of the Mut Basin, Turkey. PhD Thesis. GeoFocus 3. University of Fribourg, 277 p.). The first transect (Alahan) is on the northwestern basin margin. Here, the siliciclastic input is high due to the presence of a river system. The siliciclastic depocentre migrates landwards during transgressions, creating an ecological window allowing carbonates to develop in the distal part of the delta. Carbonate production shuts down during the regression when siliciclastics return. The second transect (Pirinç) is also situated on the northern basin margin 12 km to the east of the Alahan section. It shows a complete platform-to-basin transition. An isolated carbonate platform complex develops during the initial flooding, which is drowned during a time of rapid sea-level rise and environmental stress, associated with prograding siliciclastics. The shelf margin then retrogrades forming large-scale clinoform geometries and progrades before a major sea-level fall provokes slumping collapse, followed by rebuilding of the shelf margin as sea level rises again. The third transect (Silifke) has a steep asymmetric Pre-Miocene valley-topography, forming a narrow strait, linking the Mut Basin to the Mediterranean. Strong tidal currents are generated in this strait area. Siliciclastic input is low and localised. Eighty metres of cross-bedded bioclastic sands are deposited in a tidal regime at the base. Subsequently, carbonate platforms backstep against the shallow-dipping northern flank, while platforms only develop on the steep southern flank when a firm wide shallow-marine substrate is provided by a bench on the footwall block. The energy of the environment decreases with increased flooding of the strait area. Third-order sequences and higher-order parasequences have been identified in each transect and correlated between transects. Correlations were made using biostratigraphic data and high-resolution sequence stratigraphy in combination with the construction of the relative sea-level curve for each site. The third-order highstands are stacked in a proximal position and separated by exposure surfaces, while the lowstands, deposited in a distal setting, are separated by deep-marine (offshore or subphotic) deposits. The parasequences produce dominantly aggradational and progradational geometries with transgressive ravinement surfaces and exposure surfaces developing at times. Reconstruction of the depositional profile shows that the third-order sequences are driven by relative sea-level oscillations of 100–150 m, and that these may be attributed to 400-Ka orbital eccentricity cycles. The parasequences are driven by eustatic 20–30 m sea-level oscillations, which may be attributed to the 100-Ka orbital eccentricity cycles. The isolated carbonate build-ups in the Pirinç and Alahan transects develop at the same time as bioclastic tidal deposits in the Silifke area during the transgression of sequence 1. This is caused by a difference in hydrodynamic regime: a direct result of basin morphology funneling tidal currents in the Silifke area. We also demonstrate how during the highstands a siliciclastic delta system progrades in the Alahan area, while only 12 km to the east, a fringing carbonate platform develops, showing how siliciclastic input can have a very localised effect on carbonate environments. The exceptional quality of the outcrops with its variety of environments and its location at the Tethyan margin make this site a good candidate for a reference model for Burdigalian reef and platform architectures.

A sedimentary prelude to Marinoan glaciation, Cryogenian (Middle Neoproterozoic) Keele Formation, Mackenzie Mountains, northwestern Canada

Continental margin deposition between the Sturtian- and Marinoan-aged global glacial periods is represented in NW Canada by a siliciclastic-to-carbonate grand cycle. The Keele Formation, a 300–600 m thick carbonate and siliciclastic deposit, comprises the upper part of this interglacial cycle. Sedimentary facies in the lower, carbonate-dominated member of the Keele span a broad spectrum of depositional environments, from below storm-wave-base to peritidal to continental, and are disposed across a relatively narrow paleogeographic belt. Lateral arrangement of facies and the gradual stratigraphic transitions between them, suggest deposition in a ramp setting; local slump structures and occasional debrites in deep-water facies imply that the ramp was distally steepened. Fluvial facies are concentrated at a single locality, indicating that terrigenous-clastic sediment was derived from a point source or sources. Most strata of the carbonates are “Bahamian” in character and similar to Phanerozoic warm-water carbonates in that they contain abundant calcite ooids of interpreted original aragonitic mineralogy and have well-developed muddy peritidal facies. Nevertheless, close juxtaposition of deep- and shallow-water facies in Keele rocks indicates high-amplitude sea-level change during deposition, on the order of ∼100 m, suggesting accumulation in a climatic icehouse period. This interpretation implies that although the globe possessed polar and high-altitude glaciers at this time, sea-surface temperature, at least at tropical latitudes, warmed dramatically between Neoproterozoic glacial episodes. The onset of Marinoan glaciation here is heralded by an abrupt shift to nearshore marine siliciclastic sedimentary rocks. These deposits form the upper member of the Keele Formation (informally called the Keele clastic wedge) and directly underlie Ice Brook Formation diamictites. Carbonates are rare and are restricted to mudstones and flat-pebble conglomerates that lack any indication of tropical conditions.