Change In Depositional Style Research Articles

Three‐dimensional stratigraphic architecture, secondary pore system development and the Middle Pleistocene transition, Sandy Point area, San Salvador Island, Bahamas

AbstractThe Bahamian Archipelago has been the subject of extensive studies of stratigraphy, carbonate island morphology and diagenetic overprinting for many decades. A recently‐acquired dataset comprised of high‐resolution light detection and ranging, tightly spaced cored boreholes with image logs, thin sections, porosity and permeability measurements, and isotopic geochemical analyses provides a unique opportunity to perform detailed sequence stratigraphic analysis. This multi‐faceted study first establishes a new high‐resolution sequence stratigraphic framework for the Sandy Point area of San Salvador Island, encompassing six unconformity bound sequences (exposure surfaces) representing varying lengths of missing time. The stratigraphically lowest sequence is Late Pliocene in age and is dominated by reef facies capped by an extensive laminar caliche that represents 1.5 million years of exposure. The overlying Early Pleistocene is split into two sequences, EP1 and EP2, which are both dominated by low‐energy subtidal facies. The upper three sequences are tied to the Marine Isotope Stage 11, 9 and 5e interglacials, and are distinctly different in facies composition and architecture, being dominated by higher energy facies distributed in a more complex mosaic. The new stratigraphic framework highlights significant changes in depositional style and facies architecture that can be linked to increasing sea‐level amplitude oscillations and greater climate gradient at the Mid Pleistocene Transition. The well‐constrained framework is then used to provide key context for observed complex diagenetic evolution through repeated sea‐level inundation and subaerial exposure. These results suggest that well‐developed subaerial exposure surfaces drive increases in non‐matrix features in the subsequent stratigraphic package following exposure as the well‐cemented exposure surface acts to concentrate fluid flow and dissolution. Integration of non‐matrix features within this framework highlights the importance of dissolution/cementation patterns on formation of these modified unconformity surfaces for focusing later meteoric diagenesis and creating enhanced fluid flow pathways for continued multicyclic diagenetic events.

A chronology of post-glacial mass-transport deposits on the Canadian Beaufort Slope

Extent and chronology of 24 buried and seabed-exposed mass transport deposits (MTDs) on the continental slope of the Canadian Beaufort Sea were compiled towards a regional geo-hazard assessment of the Beaufort region. A total of 2220 lines of 3.5 kHz sub-bottom profiler (SBP) data (~40,000-line kilometres) covering an area of 9740 km2 were analyzed to allow a new understanding of slope instability in the region. Several sediment cores acquired across the region allow dating of the sediment cover. A regionally representative seismic-stratigraphy (type section, ca. 60 m thick) of the stratified sediments mantling the slope was defined. The MTDs all occur above a marked change in sedimentation style from a deeper-situated slope fan that varies substantially laterally to the above-lying stratified layers that host the MTDs. The type section comprises three sedimentary units bound by two prominent markers. Relative ages for the MTDs were defined by measuring the thickness of sediments overlying each MTD and linking that sediment package to the type section. Two 3-D seismic data volumes across the study region verified interpretations from 2D data by imaging internal deposit character and down-slope continuity of the MTDs. Seismic amplitude and similarity attributes enabled identification of further MTD events, incompletely imaged by the SBP data. A composite chronostratigraphy based on 14C dating of foraminifera and shells was assembled despite the fact that the type section is far thicker than sediment coring limits. Sub-unit thickness varies up- and down-slope such that a selection of cores across the study region enabled its compilation. The marked change in sedimentation style at the base of the type section required substantial extrapolation to date. Simple (quadratic function) age models project that the base of the type section signals the end of the last glacial maximum (Wisconsinan, or marine isotope stage 2 glaciation) and initiation of pro-glacial plume sedimentation (the deglacial), and finally post-glacial (marine) ultimately from the Mackenzie River. The MTD abundance above this contact defines an average theoretical recurrence rate of one MTD per ~1000 yrs.; however, MTDs are clustered temporally with the highest number of events occurring just prior to the onset of the Younger Dryas at around 13 ka BP (cal.).

Sedimentology of the continental end‐Permian extinction event in the Sydney Basin, eastern Australia

AbstractUpper Permian to Lower Triassic coastal plain successions of the Sydney Basin in eastern Australia have been investigated in outcrop and continuous drillcores. The purpose of the investigation is to provide an assessment of palaeoenvironmental change at high southern palaeolatitudes in a continental margin context for the late Permian (Lopingian), across the end‐Permian Extinction interval, and into the Early Triassic. These basins were affected by explosive volcanic eruptions during the late Permian and, to a much lesser extent, during the Early Triassic, allowing high‐resolution age determination on the numerous tuff horizons. Palaeobotanical and radiogenic isotope data indicate that the end‐Permian Extinction occurs at the top of the uppermost coal bed, and the Permo‐Triassic boundary either within an immediately overlying mudrock succession or within a succeeding channel sandstone body, depending on locality due to lateral variation. Late Permian depositional environments were initially (during the Wuchiapingian) shallow marine and deltaic, but coastal plain fluvial environments with extensive coal‐forming mires became progressively established during the early late Permian, reflected in numerous preserved coal seams. The fluvial style of coastal plain channel deposits varies geographically. However, apart from the loss of peat‐forming mires, no significant long‐term change in depositional style (grain size, sediment‐body architecture, or sediment dispersal direction) was noted across the end‐Permian Extinction (pinpointed by turnover of the palaeoflora). There is no evidence for immediate aridification across the boundary despite a loss of coal from these successions. Rather, the end‐Permian Extinction marks the base of a long‐term, progressive trend towards better‐drained alluvial conditions into the Early Triassic. Indeed, the floral turnover was immediately followed by a flooding event in basinal depocentres, following which fluvial systems similar to those active prior to the end‐Permian Extinction were re‐established. The age of the floral extinction is constrained to 252.54 ± 0.08 to 252.10 ± 0.06 Ma by a suite of new Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry U‐Pb ages on zircon grains. Another new age indicates that the return to fluvial sedimentation similar to that before the end‐Permian Extinction occurred in the basal Triassic (prior to 251.51 ± 0.14 Ma). The character of the surface separating coal‐bearing pre‐end‐Permian Extinction from coal‐barren post‐end‐Permian Extinction strata varies across the basins. In basin‐central locations, the contact varies from disconformable, where a fluvial channel body has cut down to the level of the top coal, to conformable where the top coal is overlain by mudrocks and interbedded sandstone–siltstone facies. In basin‐marginal locations, however, the contact is a pronounced erosional disconformity with coarse‐grained alluvial facies overlying older Permian rocks. There is no evidence that the contact is everywhere a disconformity or unconformity.

Evolution of gas hydrate–bearing deep-water channel-levee system in abyssal Gulf of Mexico: Levee growth and deformation

Gas hydrate in the Green Canyon area (Block 955) in the northern Gulf of Mexico is hosted by relatively clay-free silty levee deposits bounding a Pleistocene submarine channel. During the initial phase of channel development, the channel axis experienced sediment bypass while silty to sandy flanking levees formed. Development of high levees prompted gravitational collapse along normal faults dipping toward the channel axis. Extension at the levees was accompanied by compression at the channel axis. The gravitational failure caused rotation and displacement of the levee deposits. Later, deposition occurred within channel axis, whereas only fine-grained sediments accumulated on the levees. The change in depositional style is interpreted as the result of reduced height and strength of the turbidity currents moving along the channel at the later stage, which restricted most nonclay fraction sediments to the levee-confined channel. There is no gravitational failure in this stage, probably because relief between the levee crest and channel floor was reduced because of deposition within the channel. The hydrates are almost entirely restricted to the clay-poor levee deposits formed during the early stage of channel development, with minor hydrate accumulation in late-stage channel axial deposits. The silty levee reservoir is compartmentalized because of the presence of gravitational failure surfaces and an extensive network of salt-related normal faults. The results of this predominantly seismic stratigraphic study completed as a part of The University of Texas at Austin/US Department of Energy Deep-Water Methane Hydrate Characterization and Scientific Assessment project should be compared with sedimentological information from analysis of pressurized cores obtained during The University of Texas-Gulf of Mexico 2-1 (UT-GOM2-1) Hydrate Pressure Coring Expedition presented elsewhere in this volume.

Miocene quartz sands redeposited on subaqueous and alluvial fans during the Saalian: Interpretation of the depositional scenario at Ujście, western Poland

Abstract The 14-m-thick sandy succession at Ujście in western Poland formed during the Odranian stadial of the Saalian glaciation, is exceptional in being very well sorted, almost mono-fractional (fine-grained sands) and mono-mineral (mainly quartz grains) and in lacking Scandinavian erratics. The lower sequence (5 metres in thickness) consists of three stacked packages of clinoforms (inclined cross-stratified sands) and is interpreted as having been deposited on a subaqueous fan in a shallow lake during two phases of rising water levels. The upper sand (9 metres in thickness) with (sub) horizontal stratification was redeposited on a subaerial alluvial fan. Distinctive distributary channels that occur in the uppermost part of the subaqueous fan and in the lowermost portion of the alluvial fan may indicate a change in sedimentation style from subaqueous to subaerial. Moreover, the subaerial position of the fan supports the presence of ice-wedge casts that developed under periglacial conditions in the upper part of alluvial fan. The results of granulo-metric analysis, rounding and frosting of grains and mineral analysis indicate that the sands are derived from Gorzów Formation of Early Miocene age. The only feasible explanation is that the 14-m-thick unit must have been redeposited during the Saalian glaciation.

Whanganui Basin, an archive of prehistoric earthquakes? Waitapu Shell Conglomerate (c. 0.9 Ma), North Island, New Zealand

ABSTRACTThe Waitapu Shell Conglomerate is an important marker horizon in the eastern Whanganui Basin, occurring within a Pleistocene volcaniclastic record that contains early eruption products from the Taupo Volcanic Zone. The unit comprises a cross-bedded pebbly-shell conglomerate containing the first influx of Kaukatea Pumice (c. 0.9 Ma) within the Rangitikei succession. We document soft sediment deformation structures that occur in close stratigraphic proximity to the Waitapu Shell Conglomerate and other laterally equivalent units within the basins Castlecliffian outcrop belt. Soft sediment deformation structures formed through a combination of liquefaction and fluidisation, triggered by a range of mechanisms, including evidence of high sedimentation rates, loading, slope instability and potential for wave and earthquake-induced seismicity. Lateral changes in depositional style toward the basins eastern margin relate to relative position on the paleo-shelf, reduction of accommodation space, intermittent preservation of low stand deposits and proximity to the uplifting paleo-axial range.

Tectonic and oceanographic process interactions archived in Late Cretaceous to Present deep‐marine stratigraphy on the Exmouth Plateau, offshore NW Australia

AbstractDeep‐marine deposits provide a valuable archive of process interactions between sediment gravity flows, pelagic sedimentation and thermohaline bottom‐currents. Stratigraphic successions can also record plate‐scale tectonic processes (e.g. continental breakup and shortening) that impact long‐term ocean circulation patterns, including changes in climate and biodiversity. One such setting is the Exmouth Plateau, offshore NW Australia, which has been a relatively stable, fine‐grained carbonate‐dominated continental margin from the Late Cretaceous to Present. We combine extensive 2D (~40,000 km) and 3D (3,627 km2) seismic reflection data with lithologic and biostratigraphic information from wells to reconstruct the tectonic and oceanographic evolution of this margin. We identified three large‐scale seismic units (SUs): (a) SU‐1 (Late Cretaceous)—500 m‐thick, and characterised by NE‐SW‐trending, slope‐normal elongate depocentres (c. 200 km long and 70 km wide), with erosional surfaces at their bases and tops, which are interpreted as the result of contour‐parallel bottom‐currents, coeval with the onset of opening of the Southern Ocean; (b) SU‐2 (Palaeocene—Late Miocene)—800 m‐thick and characterised by: (a) very large (amplitude, c. 40 m and wavelength, c. 3 km), SW‐migrating, NW‐SE‐trending sediment waves, (b) large (4 km‐wide, 100 m‐deep), NE‐trending scours that flank the sediment waves and (c) NW‐trending, 4 km‐wide and 80 m‐deep turbidite channel, infilled by NE‐dipping reflectors, which together may reflect an intensification of NE‐flowing bottom currents during a relative sea‐level fall following the establishment of circumpolar‐ocean current around Antarctica; and (c) SU‐3 (Late Miocene—Present)—1,000 m‐thick and is dominated by large (up to 100 km3) mass‐transport complexes (MTCs) derived from the continental margin (to the east) and the Exmouth Plateau Arch (to the west), and accumulated mainly in the adjacent Kangaroo Syncline. This change in depositional style may be linked to tectonically‐induced seabed tilting and folding caused by collision and subduction along the northern margin of the Australian plate. Hence, the stratigraphic record of the Exmouth Plateau provides a rich archive of plate‐scale regional geological events occurring along the distant southern (2,000 km away) and northern (1,500 km away) margins of the Australian plate.

Palaeoenvironments and palaeocommunities from Lower Cretaceous high-latitude sites, Otway Basin, southeastern Australia

Lower Cretaceous (Barremian to Albian) fossil plant assemblages are preserved in sediments of the Otway Group, Otway Basin, and contemporaneous Strzelecki Group, Gippsland Basin, southeastern Australia. Detailed lithofacies and biofacies analyses of terrestrial strata within the upper Eumeralla Formation (Albian), Otway Group, allow fine-scale interpretation of braided fluvial and paludal depositional environments throughout the succession. The previously described flora is re-assessed in light of changes in depositional style and plant communities to describe six Albian biofacies. Forests in the highlands are dominated by Araucariaceae conifers, which turn over to Podocarpaceae and Cheirolepidiaceae forests on the dry, raised areas in the lowlands. Ferns and angiosperms inhabit the moist floodplains and water ferns and lycophytes dwell in the ox-bow lakes.Significant changes occur between floral communities characteristic of riparian, levee and floodbasin settings through the Early Cretaceous. Albian floras are characterized by the dominance of broad-leafed araucarian conifers, an understory of diverse ferns and a dearth of seedferns and angiosperms. There is a notable absence of macrofossil ginkgoaleans in the Eumeralla Formation, although they reappear in younger (Turonian) deposits in southeastern Australia, but angiosperms are extremely scarce as macrofossils compared to the diversity recently recorded in the pollen record. Abundant charcoal demonstrates that fire continued to be a significant environmental factor at high latitudes during the middle to late Albian. The discovery of dinoflagellate species supports an earlier marine incursion and increased coastal environments, probably inhabited by cheirolepids, across the Otway Basin. Palaeontological, palynological and sedimentological data has provided a synthesis of the region's warm, high-latitude, palaeoclimatic setting in the Albian stage of the Early Cretaceous when compared to the cooler Barremian to Aptian.

Neogene to Quaternary stratigraphic evolution of the Antarctic Peninsula, Pacific Margin offshore of Adelaide Island: Transitions from a non-glacial, through glacially-influenced to a fully glacial state

A detailed morphologic and seismic stratigraphic analysis of the continental margin offshore of Adelaide Island on the Pacific Margin of the Antarctic Peninsula (PMAP) is described based on the study of a regular network of reflection multichannel seismic profiles and swath bathymetry. We present an integrated study of the margin spanning the shelf to the continental rise, establish novel chronologic constraints and offer new interpretations on tectonic evolution and environmental changes affecting the PMAP. The stratigraphic stacking patterns record major shifts in the depositional style of the margin that outline three intervals in its evolution. The first non-glacial interval (Early Cretaceous to middle Miocene) encompasses a transition from an active to a passive margin (early Miocene). The second glacially-influenced interval (middle to late Miocene) is marked by pronounced aggradational sedimentary stacking and subsidence. Ice sheets advanced over the middle shelf of the margin at the end of this second interval, while the outer shelf experienced rare progradational events. The third, fully glaciated interval shows clear evidence of glacially dominated conditions on the margin. This interval divides into three minor stages. During the first stage (late Miocene to the beginning of the Pliocene), frequent grounded ice advances to the shelf break began, depositing an initial progradational unit. A major truncation surface marked the end of this stage, which coincided with extensive mass transport deposits at the base of the slope. During the second progradational glacial margin stage (early Pliocene to middle Pleistocene), stacking patterns record clearly prograding glacial sequences. The beginning of the third aggradational glacial margin stage (middle Pleistocene to present) corresponded to an important shift in global climate during the Mid-Pleistocene Transition. Morphosedimentary characteristics observed along the margin today began to develop during the latest Miocene but did not become fully established until sometime during the interval between the end of the Pliocene and middle Pleistocene. Between these two time intervals, the northeastward lateral migration of the Marguerite Trough also played a critical role in margin evolution, as it controlled ice sheet drainage pathways across the shelf, which in turn influenced development of slope and rise morphologies. Areas offshore from Adelaide Island differ from other areas of the PMAP due to changes in sedimentary processes that resulted from migration of the trough. This study confirms that the PMAP represents an exceptional locality for decoding, reconstructing and linking past tectonic and climatic changes. The study area specifically records not only the most relevant changes in depositional style, but also the relative importance of persistent along- and down-slope sedimentary processes. Our study approach can be extended to other areas and integrated with additional techniques to understand the evolution and the global linkages of the entire Antarctic continental margin and the ice sheets.

Cenozoic North Atlantic deep circulation history recorded in contourite drifts, offshore Newfoundland, Canada

In the North Atlantic Ocean, contour-following deep currents have created regional erosional unconformities and deposited contourite drifts that exceed 2km in thickness and extend for 100s of km. The stratigraphic records in the drifts have been used to reconstruct variations in North Atlantic deep-water circulation throughout the Cenozoic; however, uncertainties remain about certain aspects of the timing, intensity, depth distribution, and regional impact of these currents. Here, we use an integrated dataset of seismic-reflection profiles and IODP core data (lithology, biostratigraphy, and magnetostratigraphy) to document sedimentation history and the development of current effects in the Cretaceous to present sedimentary record on the J-Anomaly Ridge and Southeast Newfoundland Ridge, offshore Newfoundland, Canada. The Newfoundland ridges are in a key location, lying between well-studied areas in the northern and western North Atlantic and under the path of both the modern Deep Western Boundary Current and the Gulf Stream. Late Cretaceous through Early Eocene sedimentation on the ridges was dominated by biogenic pelagic sedimentation, but at ~47Ma, near the Early-Middle Eocene boundary, well developed contourite drifts began to accrete in paleo-water depths of ~4000–4500m, accompanied by an order-of-magnitude increase in terrigenous sediment mass accumulation rates. From this time forward, drift deposition, interrupted by brief episodes of erosion, continued unabated. This timing for the onset of persistent deep currents is coincident with reorganization of Atlantic circulation inferred from a change from biosiliceous to non-biosiliceous sedimentation in the western North Atlantic (Horizon AC) and with the current-eroded Intra-Eocene Unconformity (IEU) in the northern North Atlantic. A change in sedimentation style occurred within the Middle Eocene to upper Oligocene drift sequence, and it likely was related to a shift to deeper, more intense currents that eroded the widespread Horizon AU along the margin of eastern North America about Early Oligocene time. Beginning in the Late Oligocene (~25Ma) a thick drift exhibiting seismically laminated mudwaves was deposited in a distinct belt at ~3500–4500m paleodepth on the Southeast Newfoundland Ridge. This development correlates with widespread Late Oligocene through Miocene-Pliocene drift accumulation throughout the North Atlantic. The most recent phase of drift deposition, since Late Pliocene time (~3Ma), occurred after a shift to the ‘modern’ circulation system of deeper, swifter currents, and it includes mixed pelagic-hemipelagic sediments and ice-rafted debris that reflect glacial-interglacial influences on sedimentation.

Facies analysis and depositional model of late Permian Raniganj formation: Study from Raniganj coal bed methane block

The Raniganj Formation (late Permian) forms the uppermost economic coal-bearing unit of the Gondwana succession. The dominant facies interpreted from analysis of cores from the Raniganj formation are classified as Sandstone dominated facies, Sandstone - shale heterolith facies, Shale facies and Coal facies. The natural Gamma response of Raniganj Formation shows predominance of repetitive fining upwards cycles. Integration of core analysis and geophysical log data of the Raniganj formation indicates meandering fluvial environment. The lower part of Raniganj Formation is channel dominated which corresponds to thick amalgamated sand bodies while the upper part represent overbank shows predominance of channel avulsion indicating a gradual change in accommodation space. Five major fining upward depositional sequences, bounded by sub-aerial unconformities (sequence boundaries) have been dentified in Raniganj formation, based on changes in depositional style that are correlated regionally. Each sequence comprises of Low accommodation system tract (LAST) at base and high accommodation system tract (HAST) at top. LAST is characterized by vertically stacked, multistory amalgamated channel sandstone dominated facies, while floodplain dominated facies characterize HAST. The coal seams deposited in LAST are thicker and relatively more continuous than the frequent thin seams of HAST. Such facies distribution study would be helpful for the development strategy for CBM blocks based on production priority.

Stratigraphic framework and evolution of the Cretaceous continental sequences of the Bauru, Sanfranciscana, and Parecis basins, Brazil

With the breakup of the supercontinent Gondwana, the South American Plate has undergone an intense process of tectonic restructuring that led to the genesis of the interior basins that encompassed continental sedimentary sequences. The Brazilian Bauru, Sanfranciscana and Parecis basins during Late Cretaceous have had their evolution linked to this process of structuring and therefore have very similar sedimentary characteristics. The purpose of this study is to establish a detailed understanding of alluvial sedimentary processes and architecture within a stratigraphic sequence framework using the concept of the stratigraphic base level or the ratio between the accommodation space and sediment supply. The integration of the stratigraphic and facies data contributed to defining the stratigraphic architecture of the Bauru, Sanfranciscana and Parecis Basins, supporting a model for continental sequences that depicts qualitative changes in the sedimentation rate (S) and accommodation space (A) that occurred during the Cretaceous. This study discusses the origin of the unconformity surfaces (K-0, K-1 and K-1A) that separate Sequences 1, 2A and 2B and the sedimentary characteristics of the Bauru, Sanfranciscana and Parecis Basins from the Aptian to the Maastrichtian, comparing the results with other Cretaceous Brazilian basins. The lower Cretaceous Sequence 1 (Caiuá and Areado groups) is interpreted as a low-accommodation systems tract compound by fluvial and aeolian systems. The upper Cretaceous lacustrine, braided river-dominated alluvial fan and aeolian systems display characteristics of the evolution from high-to low-accommodation systems tracts (Sequences 2A and 2B). Unconformity K-0 is related to the origin of the Bauru Basin itself in the Early Cretaceous. In Sanfranciscana and Parecis basins, the unconformity K-0 marks the contact between aeolian deposits from Lower Cretaceous and Upper Cretaceous alluvial systems (Sequences 1 and 2). Unconformity K-1, which was generated in the Late Cretaceous, is related to an increase of the A/S ratio, whereas Unconformity K-1A is the result of the decrease in the A/S ratio. Unconformity K-1A bound Sequence 2A (lacustrine and fluvial systems) and Sequence 2B (alluvial deposits) in Bauru Basin whereas in the Sanfranciscana and Parecis basins this unconformity marks the transition from alluvial system to aeolian system (Sequences 2A and 2B). Changes in depositional style in both basins correspond to two distinct tectonic moments occurring within the South American plate. The first associated with post-volcanic thermal subsidence of the Early Cretaceous (Serra Geral and Tapirapuã volcanismos), and the second moment associated with the uplift occurred in the Late Cretaceous (Alto Paranaíba, Vilhena and Serra Formosa Arcs).

Revealing the hidden Milankovitch record from Pennsylvanian cyclothem successions and implications regarding late Paleozoic chronology and terrestrial-carbon (coal) storage

The widely held view that Pennsylvanian cyclothems formed in response to Milankovitch-controlled, glacio-eustatic, sea-level oscillations lacks unambiguous quantitative support and is challenged by models that are based on climate-controlled precipitation-driven changes in depositional style. This study shows that cyclothem successions do in fact contain a clear record of Milankovitch-controlled oscillating sea level, but that it is prerequisite that besides cyclothem thickness, cyclothem composition is taken into account. A simple subdivision of cyclothems into subaqueous and subaerial facies is sufficient to reveal the signal, provided that sufficiently long and complete successions are studied. Two Duckmantian–Bolsovian (Westphalian B–C) successions were studied—one from a high-accommodation setting in the Netherlands and another from a medium-accommodation setting in Kentucky in the United States. The Dutch record comprises an exceptional, 1728-m-long, continuously cored interval, and it shows a distinct twofold cyclicity in the subaerial-facies ratio of subsequent cyclothems at wavelengths of ∼256 m and ∼59 m, which is confirmed by power-spectral analysis. The signal is not present in the Kentucky succession due to subsidence-controlled low preservation of only one out of three to four cyclothems, and that explains why many cyclothem studies have yielded inconclusive results. Recent U/Pb ages indicate that the 256 m cycle represents ∼395 k.y., which matches with long eccentricity (413 k.y.). This then gives a 95 k.y. duration for the 59 m cycle (short eccentricity). Individual cyclothems in the high-accommodation Dutch succession are mostly between 5 and 35 m thick, which points to a sub-eccentricity duration (mean 21 k.y.). The highly variable thickness may be due to interference of precession-, obliquity-, and eccentricity-driven sea-level fluctuations or alternatively to autocyclic or climate-controlled variations in sediment supply. Integration of the results with U/Pb calibrated radiometric ages for “tonstein” ash layers from North America and Europe allowed refinements of the chronology of the main Westphalian (Moskovian–Bashkirian) coal interval; these refinements are consistent across Euramerica. An analysis of cumulative coal-bed thickness further indicates that terrestrial-carbon (coal) storage patterns are comparable in the two remote areas: in the Netherlands ∼5 m coal per m.y. during the Langsettian (Westphalian A) and increasing abruptly to ∼20 m/m.y. at the start of the Duckmantian substage (Westphalian B). In Kentucky, storage rates were lower, but when standardized to Dutch subsidence, the pattern is identical. This suggests that burial of terrestrial carbon during the Late Paleozoic Ice Age was globally controlled and possibly very predictable.

The influence of long-wavelength tilting and climatic change on sediment accumulation

The elevation of continental interiors over time is demonstrably variable. A major part of change in elevation within the continental interior is likely driven by density changes within the upper mantle and by global mantle convection. For example, upper-mantle flow has been invoked as the cause of Neogene uplift of the interior Rocky Mountains and Colorado Plateau, warping and tilting sediment transport slopes that link to the widespread deposition of gravel units within the Great Plains. These geomorphic and sedimentologic features, however, can also be generated by an increase in runoff, since erosion will promote change in elevation due to isostatic compensation and the loading of the lithosphere by the deposition of sediment. To explore the consequences of change in topography and climate, we use a general length-dependent diffusive sediment transport law to model both erosion and deposition that includes the concentrative effects of river systems. The simplicity of the approach means that we can collapse sediment transport to one dimension and couple erosion and deposition with plate flexure. We find that for a landscape that is gently tilted (slope of order of 10 –3 ), a change in runoff has a minor effect on transport gradient, as sediment transport and associated flexural response maintain topography at a similar elevation. However, there can be a significant change in depositional style when the degree of tilt is altered by, for example, a local change in upper-mantle density. An increase in buoyancy within the upper mantle, which increases slopes, leads to a transient reduction in grain sizes deposited at a fixed location. This behavior is due to a temporary retreat of the zone of erosion into the catchment and a transient increase in accommodation space relative to sediment supply. A reduction in tilt has the opposite effect, the older deposits are eroded, and the erosion-deposition transition rapidly moves down system. There is convincing evidence that the formation of thin and laterally extensive conglomeratic units of the Great Plains was due to a reduced rate of subsidence. Based on the results of our model, we suggest that the deposition of widespread conglomeratic units within continental interiors is generally a consequence of a reduction in slope, as the dynamic support for regions of high topography is reduced.

The Kamoa Copper Deposit, Democratic Republic of Congo: Stratigraphy, Diagenetic and Hydrothermal Alteration, and Mineralization

The Kamoa copper deposit is located in the Katanga Province of the southern Democratic Republic of Congo outside of the area previously considered prospective for large copper deposits. With a 1% Cu cutoff, the hypogene ore zone of Kamoa spans over 81 km2, varying in thickness from 2 to 15 m, and is currently laterally unconstrained. The deposit is hosted at the stratigraphic contact between hematitic and locally pyritic Mwashya Subgroup sandstones and overlying fine-grained pyritic diamictites of the Grand Conglomerat unit of the Nguba Group. The Mwashya sandstones appear to have been deposited in a marginal marine to fluvial environment. The Grand Conglomerat diamictite contains glacially derived mass transport and sediment gravity flow deposits. The unit appears to have been deposited in a tectonically active, locally anoxic marine environment. The contact between the Mwashya and Grand Conglomerat units represents a major change in depositional style during what was probably a period of rapid subsidence or sea level rise. The majority of the Kamoa orebody occurs within the lowermost portion of the Grand Conglomerat unit that contains siltstones with high concentrations of diagenetic framboidal and later cubic pyrite that may be indicative of early hydrothermal activity. Later hydrothermal alteration mineral assemblages within the Grand Conglomerat are stratigraphically zoned, changing from a potassic alteration and silicification assemblage in the lowermost stratigraphic units to a dominantly magnesian alteration assemblage higher in the stratigraphy. Ore stage sulfide minerals are zoned vertically away from the Mwashya-Grand Conglomerat contact, changing from chalcocite to bornite to chalcopyrite to pyrite with increased stratigraphic height. Copper sulfide minerals occur as fine-grained disseminations within the diamictite matrix that probably represent replaced diagenetic pyrite as well as in coarse-grained mineral rims on diamictite clasts. The rims are best developed in the lowermost stratigraphic units and gradually lessen in size, vertical elongation, and abundance up stratigraphic section. Sulfur isotope studies indicate that most of the sulfur in the copper sulfides was derived from earlier formed diagenetic iron sulfide. Fluid inclusion analyses indicate that the ore-forming fluid was saline, ~23 to 26 wt % NaCl wt equiv, and had homogenization temperatures (T h ) ranging from 210° to 240°C.

Microbes, mud and methane: cause and consequence of recurrent Early Jurassic anoxia following the end‐Triassic mass extinction

AbstractThe end‐Triassic mass extinction (c. 201.6 Ma) was one of the five largest mass‐extinction events in the history of animal life. It was also associated with a dramatic, long‐lasting change in sedimentation style along the margins of the Tethys Ocean, from generally organic‐matter‐poor sediments during the Triassic to generally organic‐matter‐rich black shales during the Jurassic. New core material from Germany provides biomarker evidence of persistent photic‐zone euxinia during the Hettangian, the onset of which is associated with a series of both negative and positive carbon isotope excursions. Combined inorganic and organic geochemical and micropalaeontological analyses reveal strong similarities between the Hettangian and the better‐known Toarcian anoxic event. These events appear to be the most clearly expressed events within a series of anoxic episodes that also include poorly studied black shale intervals during the Sinemurian and Pliensbachian. Both the Hettangian and Toarcian events are marked by important changes in phytoplankton assemblages from chromophyte‐ to chlorophyte‐dominated assemblages within the European Epicontinental Seaway. Phytoplankton changes occurred in association with the establishment of photic‐zone euxinia, driven by a general increase in salinity stratification and warming of surface waters. For both events, the causes of large negative carbon isotope excursions remain incompletely understood; evidence exists for both variation in the δ13C of atmospheric CO2 and variation in the sources of organic carbon. Regardless of the causes of δ13C variability, long‐term ocean anoxia during the Early Jurassic can be attributed to greenhouse warming and increased nutrient delivery to the oceans triggered by flood basalt volcanism.

Sedimentological constraints on the evolution of the Cordilleran arc: New insights from the Sonsela Member, Upper Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA)

The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, forms a distinctive part of an extensive fluviolacustrine complex that blanketed the Colorado Plateau region and surrounding areas. Upper Triassic sediments were deposited within a continental backarc setting, synchronous with the development of the Cordilleran magmatic arc. Although the Chinle Formation was derived from a broad variety of source terranes, petrographic and geochemical data clearly fingerprint the Cordilleran arc as a region important to provenance. The Sonsela Member exhibits a stratigraphically abrupt change in depositional style and sedimentary architecture that likely reflects the dynamics of the evolving backarc fluvial system and syndepositional arc magmatism. The stratigraphic boundary between the two disparate depositional styles is marked by a laterally persistent red silcrete horizon that is also associated with a biostratigraphically significant faunal turnover. The lower Sonsela Member—a coarse-grained assemblage dominated by persistent sheet sandstone and conglomerate bodies—reflects a coarse-grained progradation of sediment that suggests low rates of dynamic subsidence and high rates of sediment input. The upper Sonsela Member—characterized by an overall finer-grained assemblage and ribbon sand bodies with an increased proportion of encasing mudstone—reflects an increasing rate of basin subsidence that likely persisted throughout the deposition of the overlying Petrified Forest Member. These packages reflect a sharp change from lower- to higher-sinuosity river systems that closely coincides with the increased rate of subsidence across the regional stratigraphic marker. Previous studies have suggested the importance of the silcrete horizon as a key biostratigraphic marker associated with a significant faunal turnover. This study supports the hypothesis that this horizon may represent a regional unconformity as opposed to local channeling, perhaps the product of an abrupt change in basin geometry followed by an increase in basin subsidence. Because the Sonsela Member is the stratigraphically lowest unit of the Chinle Formation dominated by volcanic clastic detritus on the Colorado Plateau, architectural analysis of the Sonsela Member serves as the most reliable indicator of short-term fluctuations in arc dynamics of the poorly preserved and disjointed Mesozoic portion of the Cordilleran magmatic arc.

Agro-industrial alluvium in the Swale catchment, northern England, as an event marker for the Anthropocene

Physically and chemically distinctive late-Prehistoric and historical age alluvial deposits are characteristic of many upland and lowland river systems in the UK. Despite their widespread distribution, there have been few attempts to construct robust chronologies or to identify environmental factors that governed their formation. The Swale catchment in northern England is typical in this respect, with large areas of its valley floor covered by sedimentologically distinctive laminated sands and silts, enriched in organic material and Pb, and underlain by uncontaminated and structureless silts. Using 14C dating, chemostratigraphy, lichenometry and historical maps, a catchment-wide change in sedimentation style has been dated to between the mid 18th and early 19th centuries ad. Several causative factors were responsible for this change in sedimentation style and include the initiation of large-scale, intensive lead mining from the latter half of the 18th century onwards, embankment construction in the lowlands and historical peat erosion in the uplands. Transformation of the Swale floodplain also reflects longer-term land-use and climate change. In particular, deforestation of headwater tributaries by monastic grazing practices in the High Middle Ages (ad 1000–1300) led to a period of fine-grained sedimentation in upland catchments, as well as priming hillslopes for erosion and widespread channel network incision and increased fine sediment flux during the climatic downturn of the ‘Little Ice Age’. Sediment facies of a similar nature have been widely recorded in other northern English river catchments and represent a regional land use–climate signal characteristic of the Anthropocene. We introduce the term ‘agro-industrial alluvium’ to describe these types of deposit. They have similarities to post-settlement alluvium in North America and Australia, where historical land-use change had a similar impact on valley floor sedimentology.

Sedimentology and depositional model for the Early Miocene Oficina Formation in the Petrocedeño Field (Orinoco heavy-oil belt, Venezuela)

The Early Miocene Oficina Formation in the Petrocedeño Field (Venezuela) produces extra-heavy oil with an initial gravity of 8.5° API. Reservoir zonation is successfully based on a 3rd-order sequence stratigraphic framework for which a detailed sedimentological understanding is crucial. This paper presents a, primarily core-based, detailed account of the sedimentology and sequence stratigraphy of the Oficina Formation in the Junín (formerly Zuata) Region of the Orinoco heavy-oil belt. In Petrocedeño, the Oficina Formation is typified by the long-term (approximately 7 Ma; 2nd-order sequence) change of a fluvial dominated deltaic system to a tide-dominated lower delta plain and subsequently a tide-dominated, subtidal, upper delta platform. A fluvial braidplain environment with mostly sand-dominated braided and sinuous rivers (Sequences 1 and 2) changed into a fluvially-dominated but notably tidally-influenced delta plain with straight and sinuous channel belts (Sequences 3–6). Subsequently, a mixed-energy (fluvial and tidal) delta front environment with numerous distributaries formed. This delta front drowned and changed into an estuary system with bay-head delta mouth bars (Sequence 7). Maximum transgression was reached in Sequence 8 with the development of fully marine shale. Subsequently, a low-gradient tide-dominated lower delta plain with distributaries and meandering channel belts established (Sequences 9 and 10). These were gradually transgressed to develop into proximal delta platform environments with a limited number of distal distributaries and large protected subtidal areas (Sequence 11). Together, the lower delta plain and upper delta platform facies form one of the few currently known subsurface examples of a tide-dominated delta. The change in depositional style was controlled by a long-term increase of accommodation space in the developing Eastern Venezuelan Basin. The lower part of the formation was primarily controlled by compressional tectonic activity (third-order Sequences 1 through 6). The upper half, however, was primarily controlled by Early Miocene eustatic changes (Sequences 7 through 11).

Holocene settlement shifts and palaeoenvironments on the Central Iranian Plateau: Investigating linked systems

For thousands of years, humans have inhabited locations that are highly vulnerable to the impacts of climate change, earthquakes, and floods. In order to investigate the extent to which Holocene environmental changes may have impacted on cultural evolution, we present new geologic, geomorphic, and chronologic data from the Qazvin Plain in northwest Iran that provides a backdrop of natural environmental changes for the simultaneous cultural dynamics observed on the Central Iranian Plateau. Well-resolved archaeological data from the neighbouring settlements of Zagheh (7170—6300 yr BP), Ghabristan (6215—4950 yr BP) and Sagzabad (4050—2350 yr BP) indicate that Holocene occupation of the Hajiarab alluvial fan was interrupted by a 900 year settlement hiatus. Multiproxy climate data from nearby lakes in northwest Iran suggest a transition from arid early-Holocene conditions to more humid middle-Holocene conditions from c. 7550 to 6750 yr BP, coinciding with the settlement of Zagheh, and a peak in aridity at c. 4550 yr BP during the settlement hiatus. Palaeoseismic investigations indicate that large active fault systems in close proximity to the tell sites incurred a series of large (MW ~7.1) earthquakes with return periods of ~500—1000 years during human occupation of the tells. Mapping and optically stimulated luminescence (OSL) chronology of the alluvial sequences reveals changes in depositional style from coarse-grained unconfined sheet flow deposits to proximal channel flow and distally prograding alluvial deposits sometime after c. 8830 yr BP, possibly reflecting an increase in moisture following the early-Holocene arid phase. The coincidence of major climate changes, earthquake activity, and varying sedimentation styles with changing patterns of human occupation on the Hajiarab fan indicate links between environmental and anthropogenic systems. However, temporal coincidence does not necessitate a fundamental causative dependency.