Major Sea-level Lowstands Research Articles

Constraining the onset and environmental setting of metazoan biomineralization: The Ediacaran Nama Group of the Tsaus Mountains, Namibia

The advent of animal (metazoan) biomineralization marks a fundamental transition in Earth's history, facilitating the diversification of novel body plans as well as the biological control of carbonate sediment production, resulting in a permanent restructuring of the global carbon cycle. The oldest known skeletal metazoans, the tubular ‘cloudinids’ e.g., Cloudina, appeared during the terminal Ediacaran of the Nama Group, Namibia, but precisely when, where, and why metazoans first acquired the ability to biomineralize and their timing relative to the regional δ13Ccarb profile is undocumented. Assessing possible environmental triggers for this key event therefore requires accurate constraint of the age, paleoenvironmental setting and the geochemical context of the earliest cloudinids.Here we present new stratigraphic, sedimentological, and geochemical (δ13Ccarb and δ18Ocarb) data from the oldest strata (Dabis Formation) of the Nama Group from the Witputs Sub-basin, exposed in the Tsaus Mountains of the Tsau Khaeb National Park (formerly Sperrgebiet). This constrains the oldest confirmed occurrence of Cloudina to limestones of the lower Kliphoek Member, ca. 551–550 Ma, that were deposited laterally-equivalent to the more proximal fossiliferous Kliphoek quartzite. These limestones have dominantly negative δ13Ccarb values that immediately precede recovery from the basal Nama negative δ13Ccarb excursion (BANE), and were deposited in shallow waters after a transition from semi-restricted, evaporitive-dolomitic to open marine carbonate settings. Regional correlation shows that Cloudina first appeared during an interval of dominantly low oxygen and unstable, regional marine redox conditions. We conclude that data do not support a long-term shift towards more stable, oxygenated conditions as a driver for the first appearance of skeletonization in metazoans, but do suggest that open marine carbonate settings with high carbonate supersaturation were required to support the calcifying Cloudina. In the Nama Group, the appearance of Cloudina also notably coincided with a major sea level lowstand, which we hypothesise may have enabled oxygenation of these more distal settings. Such an expansion of shallow marine oxygen may therefore have provided an evolutionary driver for innovations that were not metabolically costly, such as the onset of Cloudina biomineralization.

Cambrian radiation speciation events driven by sea level and redoxcline changes on the Siberian Craton.

The evolutionary processes of speciation during the Cambrian radiation and their potential extrinsic drivers, such as episodic oceanic oxygenation events, remain unconfirmed. High-resolution temporal and spatial distribution of reef-associated archaeocyath sponge species on the Siberian Craton during the early Cambrian [ca. 528 to 510 million years ago] shows that speciation was driven by increased endemism particularly ca. 521 million years (59.7% endemic species) and 514.5 million years (65.25% endemic species) ago. These mark rapid speciation events after dispersal of ancestors from the Aldan-Lena center of origin to other regions. These speciation events coincided with major sea-level lowstands, which we hypothesize were intervals when relative deepening of the shallow redoxcline permitted extensive oxygenation of shallow waters over the entire craton. This provided oxic corridors for dispersal and allowed the formation of new founder communities. Thus, shallow marine oxygen expansion driven by sea-level oscillations provides an evolutionary driver for sucessive speciation events during the Cambrian radiation.

An allostratigraphic framework for the Late Albian to Early Cenomanian upper Fort St. John Group across the foredeep of the Western Canada Foreland Basin, NE British Columbia

Abstract The Upper Albian to Lower Cenomanian Hasler and Cruiser shales and intervening Goodrich Sandstone are up to about 800 m thick and constitute most of the upper Fort St. John Group in the foredeep of NE British Columbia. These rocks exhibit pronounced lateral changes in lithology and thickness from west to east, making their lithostratigraphic boundaries highly diachronous. An allostratigraphic approach to correlation, based primarily on discontinuity surfaces represented in wireline well logs, allows subdivision of these broad lithostratigraphic units, and permits correlation of genetic stratal packages across facies transitions, and from subsurface to outcrop. Upper Fort St. John strata spanning the foredeep in British Columbia are shown to be correlative with Viking allomember VD, and with the overlying Westgate and Fish Scales alloformations, previously defined in Alberta on the basis of outcrop, core and wireline log data. Bounding surfaces have also been correlated northward to connect with successions described from the Sikanni and Liard river areas of British Columbia. In the British Columbia study area, pyritic, largely unbioturbated mudstone of allomember VD thickens from 20 to 200 m, indicating syn-depositional flexural subsidence, greatest in the north-western part of the study area. The Westgate alloformation is also dominated by weakly bioturbated mudstone, but includes more common, thin, fine-grained sandstone beds, many with wave- and combined-flow ripples, organized in parasequences up to approximately 20 m thick. Westgate allomembers WA through WD take on a progressively more tabular geometry upward, reflecting diminishing flexural subsidence with time. Allomember WD includes shoreface/delta front sandstones (‘Goodrich Formation’) that extend eastward from the Foothills outcrop for up to 70 km. The Fish Scales alloformation, the base of which is defined by surface FE1, marks an abrupt introduction of sand across a muddy sea floor in response to sea-level fall. The upper boundary of the Fish Scales is the condensed section/downlap surface ‘Fish Scales Upper’ (FSU) beneath the Dunvegan alloformation. The Fish Scales alloformation is a pronounced wedge, thickening westward from approximately 100 to approximately 400 m. It consists mainly of pyritic, unbioturbated mudstone to claystone with a variable content of fish debris, indicative of dysaerobic to anaerobic bottom-water conditions. Ammonites do, however, indicate oxic surface waters. The Fish Scales alloformation comprises allomembers FA and FB, separated by an erosion surface termed the Base Fish Scales Marker (BFSM) which, throughout the Peace River region, has a veneer of chert pebbles that mark a major sea-level lowstand; pebbles were subsequently reworked by marine transgression. A 15 m thick conglomerate-filled channel (or paleovalley?), exposed in Hasler Creek, directly underlies the BFSM surface and is interpreted to be the deposit of a large river that supplied sediment to the lowstand shoreline. In the far west, close to the Foothills, the Fish Scales alloformation becomes sandy and bioturbated, and includes stacked ‘Goodrich’ shoreface sandstone bodies that prograded a few tens of km eastward. Mapped to the SE, allomember FA pinches out close to the Alberta-British Columbia border, whereas allomember FB persists as a thin and highly radioactive phosphatic mudstone, the base of which comprises the merged FE1 and BFSM erosion surfaces. Despite up to about 800 m of subsidence, sedimentary structures throughout the succession show that the sea floor, which formed a homoclinal ramp, remained above storm wave base, attesting to a very high sedimentation rate and effective offshore dispersal of sediment, primarily by storm-driven combined flows. Bivalves of the genus Posidonioceramus have recently been proposed as a marker for the base of the Cenomanian Stage. These fossils are found in the study area as low as the base of Viking allomember VD, which implies that the Albian-Cenomanian boundary may lie at a level significantly lower than the long-accepted ‘Base of Fish Scales’ erosion surface.

Abrupt environmental changes during the last glacial cycle in Western Mediterranean (Formentera Island, Balearic archipelago, Spain)

A sedimentary sequence covering the entire last glacial cycle (period between Terminations I and II) outcrops along the south-eastern coast of Formentera Island. A detailed geomorphological, geological and sedimentological study, supported by geochemical, soil and soil-morphology analyses, magnetic susceptibility, phytolite content and luminescence dating (TL, OSL) allowed to reconstruct the environmental evolution of this coastal setting, and to frame it within the evolutionary pattern of the North Atlantic climate variability. Three highstands of sea level are identified in this island for MIS 5e, and a fourth one is attributed to MIS5a. MIS5 – MIS4 transition is characterized by soil development under a moist-warm climate and a descending sea level scenario. Aeolian units (72 ± 7 ka BP) developed during MIS4 under prevailing northerly winds that persisted until the beginning of MIS3, when new aeolian dunes (54 ± 5 ka BP) developed after a major sea-level lowstand. A sudden shift in prevailing winds occur within MIS3, when aeolian units (51 ± 4 ka BP) grew under the influence of S-SW winds and moister climate, evidenced by a dense root bioturbation. The greater influence of northerly winds is attributed to the weakening of Atlantic Meridional Overturning Current (AMOC) in North Atlantic, and enhancement of westerlies in NW Europe during colder periods. Periods of prevailing southerly winds and moister climate correlate well with warm Greenland Interstadials, and reinforcement of AMOC. Between 50 and 40ka, alluvial/colluvial sedimentary units punctuated by soil and calcrete development, witness the climatic variability recorded along this period in the North Atlantic. A sedimentary hiatus with erosion and calcrete development characterizes the transit between MIS3 and MIS2. Finally, a reddish alluvial sedimentary unit records a short span of humid and warm climate (soil development, peak in magnetic susceptibility, phytolite content) within MIS 2 (20 ± 2 ka BP – 17 + 2.4/-2.2) that contrast with the general climatic trend recorded in Western Mediterranean during the Last Glacial Maximum.

Climate changes in the pre-Hirnantian Late Ordovician based on δ18Ophos studies from Estonia

Results of conodont-based δ18Ophos studies from two core sections in Estonia (Mehikoorma-421 and Valga-10) indicate that the pre-Hirnantian Late Ordovician was characterized by a general cooling trend: a rapid climate cooling in the early Sandbian and a less intense cooling in the late Sandbian through Katian. During this time interval, sea-surface temperatures probably decreased by up to 9o C. This fits with earlier ideas of a colder Late Ordovician climate before the Hirnantian. On the background of general cooling trend in the pre-Hirnantian Late Ordovician, tendencies in the temperature changes were quite variable. Several episodes of SST decrease and increase of different magnitudes and durations are evident from the δ18Ophos curves in the studied sections. In the interval sampled in detail, 7 cooling events are recognized and tentatively called (in stratigraphical order): Early Kukruse, Haljala, Keila, Early Nabala, Vormsi, Early Pirgu and Middle Pirgu CE-s. Duration of these events and amplitudes of changes in seawater temperatures increased from Sandbian to late Katian. All cooling events may have triggered the waxing of polar and subpolar Gondwana icecaps and are reflected in the sedimentary succession by major sea-level lowstands recognized also outside the study region. Results of our research do not support a simple, straightforward correlation between temperature trends inferred from δ18Ophos measurements and changes in the δ13C record. Comparison of δ18Ophos and δ13C curves reveal that these two variables behave quite independently: in some intervals their trends fit quite well, in others they might show completely the opposite. For this comparison we provide also a new, detailed δ13C curve from the Valga-10 core section.

Sequence stratigraphy and onlap history of the Donets Basin, Ukraine: Insight into Carboniferous icehouse dynamics

The degree to which Permo-Carboniferous cyclothemic successions archive evidence for long-term variations in ice volume during the Late Paleozoic Ice Age is insufficiently resolved. Here we develop the sequence stratigraphy and onlap–offlap history for a 33-my interval of the Carboniferous using the U–Pb calibrated succession of the Donets Basin, Ukraine, in order to assess the relationship between sea-level, high-latitude changes in glacial extent, and climate. Integrated subsurface and outcrop data permit meter-scale correlation of 242 biostratigraphically constrained limestones and coals, and in turn individual cyclothems, across ~ 250 km of the Donets Basin. Rapid uniform subsidence and basinwide continuity of marker beds indicate Pennsylvanian deposition under relatively stable tectonic conditions. Three scales of sequences (avg. durations of ~ 140 ky, ~ 480 ky and 1.6 my) are recognized on the basis of stratigraphic stacking patterns and basinwide architecture of marine to terrestrial facies assemblages. The hierarchy of sequences and the geographic and stratigraphic positions of shifts in base-level sensitive facies across the Donets ramp permit the construction of an onlap–offlap history at a sub-400 ky scale. Major sea-level lowstands occur across the mid-Carboniferous boundary and during the early Moscovian. These lowstands coincide with glacial maxima inferred from high-latitude glacigenic deposits. The middle to late Pennsylvanian is characterized by a stepwise onlap, culminating in an earliest Gzhelian highstand, suggesting contraction of Carboniferous ice sheets prior to the initiation of Early Permian glaciation. The stratigraphic position of climate sensitive facies within individual Donets cyclothems indicates a turnover from seasonal sub-humid or semi-arid climate to everwet conditions during the late lowstand and maximum ice sheet accumulation. Comparison of the stratigraphic and aerial distribution of coals and evaporites in the Donets Basin with the onlap–offlap history further indicates everwet conditions during lowstands and inferred glacial maxima and drier climate during onlap and inferred ice sheet contraction at the intermediate (~ 0.8 to 1.6 my) and long (10 6 yr) time-scales. Taken together, the relationship between inferred climate and glacioeustasy suggests a likely teleconnection between high-latitude ice sheet behavior and low-latitude atmospheric dynamics.

Miocene Kareem Sequence, Gulf of Suez, Egypt

ABSTRACTThe Miocene Kareem Formation in the Egyptian Gulf of Suez, and its equivalent formations throughout the Red Sea (250–550 m thick), contain one of the most important petroleum reservoirs in these highly faulted rift basins. They present a difficult exploration target, particularly over the shelves of the sparsely explored Red Sea for several reasons: (1) water depth exceeds one kilometer, (2) they underlie thick evaporites (including salt exceeding one kilometer in thickness), (3) they are difficult to image by conventional seismic techniques, and (4) their lithology is laterally variable and difficult to predict (anhydrite, carbonate, sandstone, shale and marl). The target Red Sea formations are best controlled by boreholes in the Gulf of Suez, where the Kareem Formation and its members are characterized by various synonymous units. A review of representative data and interpretations shows that the formation and its members are better understood when considered as a third-order, transgressive-regressive (T-R) depositional sequence, named the Kareem Sequence in the Middle East Geologic Time Scale (ME GTS). The Sequence is bounded above by the Belayim Sequence Boundary (Sub-Belayim Unconformity) and below by the Kareem Sequence Boundary (Sub-Kareem Unconformity), both corresponding to major sea-level lowstands. It contains the Arabian Plate Langhian Maximum Flooding Surface Neogene 30 (MFS Ng30) at the top of the Kareem Maximum Flooding Interval (MFI). Its lower Rahmi Member forms the majority of the transgressive systems tract (TST). The Kareem MFI and regressive systems tract (RST or HST) occur within the upper Shagar Member. The paleontology of the Formation is characterized by Planktonic Foraminiferal Zone N9 and in recent papers also N8, and Calcareous Nannofossil Biozone NN5, but the Formation’s assignment to Miocene stages (Burdigalian, Langhian and Serravallian) is unresolved in the literature.In this paper, the Kareem Sequence is interpreted in terms of Kareem subsequences 1 to 6. At semi-regional scales (10s of km), the older three are each represented by an anhydrite bed (Rahmi Anhydrite 1 to 3, each c. 10 m thick) overlain by deep-marine deposits (shale, marl and carbonate, 10s of meters thick). Subsequences 4 to 6 are represented in El Morgan field (Kareem A to C units), and in representative boreholes, by three deep-marine shale/marl units, each of which is overlain by a regressive shallow-marine sandstone unit. The Kareem Sequence is correlated to third-order orbital sequence DS3 1.1 with a depositional period of ca. 2.43 million years between ca. 16.1 and 13.7 million years before present (Ma), or numerically the latest Burdigalian, Langhian and earliest Serravallian (Langhian: 15.97–13.65 Ma in GTS 2004; 15.97–13.82 Ma in GTS 2009). The six subsequences are correlated to the orbital 405,000 year eccentricity cycle (referred to as Stratons 40–35 or DS4 1.1.1 to 1.1.6). The older three subsequences form the transgressive systems tract; the fourth contains the maximum flooding interval MFI (ca. 14.9–14.7 Ma) in its lower part. The regressive systems tract starts in the upper part of the fourth subsequence and encompasses subsequences 5 and 6. The orbital architecture of the Sequence provides a simplified framework for predicting lithology and reservoir development.The six Kareem subsequences carry the orbital-forcing glacio-eustatic signal. During low eccentricity, Antarctic ice-making and global sea-level drops, the northernmost Gulf of Suez and Bab Al Mandeb Strait restricted marine circulation in the Gulf and Red Sea rift basins. The resulting evaporitic setting was associated with the deposition of the Rahmi Anhydrite 1 to 3 beds and exposure over paleohighs. The deeper-marine deposits above the three Rahmi Anhydrite beds, and those of subsequences 4 to 6 reflect high eccentricity, Antarctic ice-melting, global sea-level rises, pluvial conditions at low latitudes (10–30oN), and open-marine circulation in the Red Sea. During pluvial periods, fluvio-deltaic systems prevailed over the mountainous rift shoulders and coastal plains and carried massive clastics into the Gulf and Red Sea Basins.

Palaeohydrogeological control of palaeokarst macro-porosity genesis during a major sea-level lowstand: Danian of the Urbasa–Andia plateau, Navarra, North Spain

An extensive palaeokarst porosity system, developed during a pronounced mid-Paleocene third-order lowstand of sea level, is hosted in Danian limestones of the Urbasa–Andia plateau in north Spain. These limestones were deposited on a 40–50 km wide rimmed shelf with a margin characterised by coralgal buildups and coarse-grained bioclastic accumulations. The sea-level fall that caused karstification was of approximately 80–90 m magnitude and 2.5 Ma in duration. During the exposure, a 450 m wide belt of sub-vertical margin-parallel fractures developed a few hundred metres inboard of the shelf edge. Most fractures are 90–100 m deep, average 1 m in width, and are associated with large erosional features created by collapse of the reefal margin. Inland from the fracture belt, three superimposed laterally extensive cave systems were formed over a distance of 3.5 km perpendicular to shelf edge, at depths ranging from 8–31 m below the exposure surface. The palaeocaves range from 0.3 to 2 m in height, average 1.5 m high. They show no evidence of meteoric processes and are filled with Thanetian grainstones rich in reworked Microcodium, a lithology that also occurs infilling the fractures. The caves are interpreted as due to active corrosion at the saline water–fresh-water mixing zone. Caves are missing from the shelf edge zone probably because the fractures beheaded the meteoroic lens preventing mixing-zone cave development beyond the fracture zone. Towards the platform interior, each cave system passes into a prominent horizon, averaging 1 m in thickness, of spongy porosity with crystal silt infills and red Fe-oxide coatings. The spongy horizons can be traced for 5.5 km inboard from the cave zone and occur at 10.5 m, 25 m and 32 m below the exposure surface. In the inland zone, two additional horizons with the same spongy dissolution have been recognised at depths of 50 m and 95 m. All are analogous to Swiss-cheese mixing-zone corrosion in modern carbonate aquifers and probably owe their origins to microbially-mediated dissolution effects associated with a zone of reduced circulation in marine phreatic water. In the most landward sections a number of collapse breccia zones are identified, but their origin is unclear. The palaeokarst system as a whole formed during the pulsed rise that followed the initial sea-level drop, with the three main cave-spongy zones representing three successive sea-level stillstands, recorded by stacked parasequences infilling large erosional scallops along the shelf margin. The geometry of the palaeo-mixing zones indicates a low discharge system, and together with the lack of meteoric karstic features favours a semi-arid to arid climatic regime, which is further supported by extensive calcrete-bearing palaeosols occurring in coeval continental deposits.

Rocky shoreline deposits in the Lower Silurian (upper Llandovery, Telychian) Chicotte Formation, Anticosti Island, Quebec

The upper Llandovery (Telychian) Chicotte Formation is a regionally extensive crinoid-rich unit exposed in the south-central part of Anticosti Island in the Gulf of St. Lawrence. The Chicotte encrinites represent an inner ramp, crinoidal sand-shoal complex (about 80 m thick) that prograded over deeper middle to outer ramp facies of the underlying Jupiter Formation in response to a long-term sea-level fall. The typical depositional unit in the Chicotte Formation is a metre-scale subtidal cycle indicating that higher frequency sea-level changes were also present. Metre-scale cycles are typically characterized by coarsening-upward, locally cross-bedded encrinitic material capped by a sharp erosional surface. These bounding surfaces change progressively upsection from simple planar to low-relief scalloped erosional surfaces to complex polyphase surfaces formed by distinct but superimposed erosional events. Three-dimensional paleomorphological features with local relief up to 50 m associated with erosional surfaces are exposed along coastal and river sections and in places have been partially exhumed by modern erosion along coastal exposures to expose a prominent intraformational unconformity. Paleolandforms include large-scale irregular sea stacks and shallow cliffs, similar to those present along modern rocky shorelines. Reefal limestones with small pockets of well-washed crinoidal sand and brachiopod banks form a narrow shoreline sediment wedge piled against the intra-Chicotte unconformity. The sea-level lowstand recorded within the Chicotte Formation coincides with a major sea-level lowstand recognized elsewhere on several continents during the early to middle Telychian time, indicating a eustatic rather than tectonic origin.

Dry climate near the Western Pacific Warm Pool: Pleistocene caliches of the Nansha Islands, South China Sea

The Nansha (Spratly) Islands are located in the middle of the South China Sea (at about 10°N) near the northwestern margin of the Western Pacific Warm Pool (WPWP). A borehole was drilled at an atoll at Nansha, and cores were taken. The upper 165.4 m of the borehole consists entirely of limestone of reef facies and lagoon facies, which had been influenced by extensive meteoric diagenesis. Petrographic and geochemical studies have identified at least four subaerial exposure surfaces (SES) in the Pleistocene carbonate sequence. These SES are thought to have resulted from global sea-level changes and characterized by caliche formation. When developed in limestone, caliches typically form in areas where annual precipitation ranges from 500 mm up to about 1200 mm, compared with 1800–2200 mm of annual rainfall of the present-day Nansha Islands. The Nansha caliche therefore indicates the existence of several dry climate episodes during Pleistocene sea-level lowstands. Lower rainfall and higher evaporation during such dry conditions may explain the higher sea-surface salinities reported elsewhere in the South China Sea. The Nansha caliche may also indicate reduced extent of the WPWP and southerly shift of the Intertropical Convergence Zone during major sea-level lowstands.

Shallow marine syn-rift sedimentation: Middle Jurassic Pelion Formation, Jameson Land, East Greenland

The Middle Jurassic Pelion Formation – Fossilbjerget Formation couplet of Jameson Land, East Greenland, is a well-exposed example of the Middle Jurassic inshore–offshore successions characteristic of the rifted seaways in the Northwest European – North Atlantic region. Early Jurassic deposition took place under relatively quiet tectonic conditions following Late Permian – earliest Triassic and Early Triassic rift phases and the Lower Jurassic stratal package shows an overall layer-cake geometry. A long-term extensional phase was initiated in Middle Jurassic (Late Bajocian) time, culminated in the Late Jurassic (Kimmeridgian–Volgian), and petered out in the earliest Cretaceous (Valanginian). The Upper Bajocian – Middle Callovian early-rift succession comprises shallow marine sandstones of the Pelion Formation and correlative offshore siltstones of the Fossilbjerget Formation. Deposition was initiated by southwards progradation of shallow marine sands of the Pelion Formation in the Late Bajocian followed by major backstepping in Bathonian–Callovian times and drowning of the sandy depositional system in the Middle–Late Callovian. Six facies associations are recognised in the Pelion–Fossilbjerget couplet, representing estuarine, shoreface, offshore transition zone and offshore environments. The north–southtrending axis of the Jameson Land Basin had a low inclination, and deposition was sensitive to even small changes in relative sea level which caused the shorelines to advance or retreat over tens to several hundreds of kilometres. Eight composite sequences, termed P1–P8, are recognised and are subdivided into a total of 28 depositional sequences. The duration of the two orders of sequences was about 1–2 Ma and 360,000 years, respectively. The Upper Bajocian P1–2 sequences include the most basinally positioned shallow marine sandstones, deposited during major sealevel lowstands. The lowstands were terminated by significant marine flooding events, during which sandstone deposition was restricted to northern, more proximal parts of the basin. The Upper Bajocian – Middle Bathonian P3–4 sequences show an overall progradational stacking pattern. The sequence boundary at the top of P4 marks a significant shift in stacking pattern, and the Upper Bathonian – Middle Callovian P5–8 sequences show large-scale backstepping, terminating in a widespread condensed succession at the distal, southern end of the basin. The largescale backstepping was governed by combined tectonically-induced subsidence, reflecting increased rates of extension, and eustatic sea-level rise. The depositional trends of the Pelion Formation – Fossilbjerget Formation couplet provide a well-exposed analogue to contemporaneous subsurface deposits which form major hydrocarbon reservoirs on the west Norway shelf, and in the Northern North Sea.

Stratigraphic framework for the Cambrian–Ordovician rift and passive margin successions from southern Quebec to western Newfoundland

The Taconian Humber Zone stretches from western Newfoundland to southern Quebec. The Early Cambrian slope succession in Newfoundland is found in the Curling Group, whereas in Quebec, various units were deposited during that first time slice. Biostratigraphic data allow correlation of the Curling Group with the Labrador Group in Newfoundland and with the newly time-constrained slope succession in Quebec. The end of the rift–drift transition is marked by a sea-level lowstand at the end of the Early Cambrian. The Middle Cambrian to latest Early Ordovician passive margin history recorded five cyclic sea-level fluctuations. Three of these cycles are recorded in the shallow-marine Middle to Late Cambrian platform (Port au Port Group) and slope sediments preserved in the Cow Head and Northern Head groups in Newfoundland. The biostratigraphic information assists correlation with Cambrian passive margin units in Quebec. Major sea-level lowstands are recognized along the continental margin in early–middle Late Cambrian (Steptoan) and in late Late Cambrian (Sunwaptan). Even if the Quebec succession can be tied with its Newfoundland correlative, some significant differences in the nature of Upper Cambrian slope conglomerates argue for a tectonic control on the depth of erosion of the Cambrian continental margin. The Lower Ordovician record of the passive margin consists of two depositional cycles (Tremadocian–Arenigian) separated by a sea-level lowstand. This last event is well expressed in platform succession and is also recognized in conglomerate units found in the slope succession.

Tropical view of Quaternary sequence stratigraphy: Siliciclastic accumulation on slopes east of the Great Barrier Reef since the Last Glacial Maximum

Generic models of continental-margin evolution predict that siliciclastic fluxes to slopes should be maximal and minimal during major sea-level lowstands and transgressions, respectively. Here we document the opposite for the northeast Australian margin, the largest extant mixed siliciclastic-carbonate depositional system. Cores from slopes of this margin consistently contain siliciclastic-rich intervals, ~0.3–1 m thick, in the upper few meters. Radiocarbon dates of planktonic foraminifera show that this interval was deposited between 12 and 7 ka and represents greatly increased siliciclastic fluxes during late transgression. This massive terrigenous discharge to slopes occurred along at least 450 km of the margin, irrespective of modern variations in bathymetry or climate. Although we cannot dismiss a significantly different early Holocene climate with greatly enhanced sediment discharge, available data instead suggest that rivers aggraded on the shelf during lowstand because of an extensive subaerially exposed reef system. This phenomenon may occur on other margins rimmed by reefs, requiring a major revision of concepts used to interpret mixed siliciclastic-carbonate systems.

Mid-Eocene deep water, the Late Palaeocene Thermal Maximum and continental slope mass wasting during the Cretaceous-Palaeogene impact

Abstract A series of widespread Maastrichtian and Palaeogene reflectors in the western North Atlantic have been interpreted to record episodes of vigorous bottom-water circulation produced by periodic flooding of the deep North Atlantic basins with southern source waters. In general, the ages of these reflectors have been poorly known with estimated ages spanning several million years. New seismic and core data from Ocean Drilling Program Leg 171B tightly constrain the ages of several of the most prominent reflectors and demonstrate that several of them are associated with geologically short-lived events associated with major palaeoclimatic, palaeoceanographic and evolutionary transitions. On Blake Nose in the western North Atlantic, Reflector A c formed shortly after the close of the early Eocene warm period between 48 and 49 Ma. The reflector corresponds to an abrupt inception of vigorous deep-water circulation that winnowed foraminiferal sands at 2000–2500 m water depth and caused mass wasting into the deep basins of the Bermuda Rise. Reflector A c is correlative with a sequence of unconformities present in nearly every part of the global ocean from the shallow shelf to the deep sea, suggesting that this time interval is associated with a global change in ocean circulation, including a major sea-level lowstand. The reflector and unconformities are roughly equivalent in age to glacial tillites on the Antarctic Peninsula, suggesting a link to an early phase of southern hemisphere glaciation. Another widespread reflector, A b , has a late Palaeocene to earliest Eocene age on the Bermuda Rise. On Blake Nose, the equivalents of Reflector A b consist of a stack of three closely spaced hiatuses ranging from early late Palaeocene (58.5–60.5 Ma) to latest Palaeocene ( c. 55.5 Ma) age. The youngest of these hiatuses is associated with the carbon isotope excursion at the Late Palaeocene Thermal Maximum (LPTM), when there was a major reorganization of deep-water circulation and dramatic, transient warming of high latitudes. Bottom currents appear to have prevented the widespread deposition of sediments at water depths shallower than c. 2200 m from the LPTM until early mid-Eocene time. Erosion on Blake Nose was produced by a strengthened, southward-flowing deep western boundary current at the same time that a southern source watermass produced extensive erosion on the Bermuda Rise. We suggest that the increased flow of the deep western boundary current reflects a stronger outflow of warm intermediate waters shallower than 2000 m from Tethys. The combination of warmer intermediate waters and erosion along the margin may have helped to trigger slope failure of gas hydrate reservoirs around the North Atlantic margin and set the LPTM-greenhouse feedback system in motion. Reflector A* is correlative with highly deformed Maastrichtian sediments on Blake Nose and Maastrichtian chalk interbedded with red claystone on Bermuda Rise. Seismic and coring evidence from Blake Nose shows that the K-P boundary slumping was associated with the magnitude c. 11–13 Richter Scale earthquake generated by the Chicxulub impact event. The chalk sequence on the Bermuda Rise appears to represent the distal turbidites produced by slumping of the margin. Correlation of the chalk beds with Reflector A* shows that the mass wasting deposits are found over nearly the entire western North Atlantic basin. Apparently, much of the eastern seaboard of North American must have catastrophically failed during the K-P impact event, creating one of the largest submarine landslides on the face of the Earth.

Origin of the Strait of Çanakkale (Dardanelles): regional tectonics and the Mediterranean–Marmara incursion

Abstract The regional tectonics and the mid to late Quaternary seismic stratigraphy of the Strait of Canakkale (Dardanelles) and its opening towards the Aegean Sea were interpreted by using shallow seismic profiles. This strait is a narrow valley that resulted from regional uplift of the Gelibolu and Biga Peninsulas in the Pliocene as a consequence of compressional events generated by strike-slip faulting activity of the Ganos Fault, a northern strand of the North Anatolian Fault. A NNE–SSW trending fault system between Eceabat and Canakkale, believed to result from the left lateral displacement of the Ganos fault on the Gelibolu Peninsula, cut this valley into different segments. The connection between the Marmara Basin and the Aegean Sea has been achieved after flooding of the connected valleys during the mid to Late Pleistocene. Three major sea level lowstands (−130/−150 m) during this period (∼600 kyear BP) interrupted the sea water connection at least twice. Three main seismic unconformities detected in the sediments of the Strait of Canakkale represent these lowstands. Minor sea level lowstands could have induced narrowing of the strait and erosion of depositional units. The recent geological history of the Strait of Canakkale appears, as deduced from the Gelibolu Peninsula and its western regions, strongly related to escape tectonics influenced by the compressional forcing of the Anatolian Block. In addition, it also shows a morphological evolution, which carries the effects of the global sea-level variations.

Growth and demise of Late Carboniferous carbonate platforms in the eastern Cantabrian Zone, Asturias, northwestern Spain

Geometric relationships of seismic-scale stratal patterns observed on aerial photographs of Late Carboniferous carbonate platforms in the Cuera Region in Asturias, northwestern Spain, allow the identification of two discrete stages of platform development with different styles of deposition. The first phase, represented by the Bashkirian Valdeteja Formation, is characterized by progradation of more than 10 km with steep clinoforms of up to 35°, and slope heights up to 650 m. During the second phase, corresponding to the Moscovian Picos de Europa Formation, the platform aggraded mainly vertically, nearly 850 m. Both phases of platform growth were terminated by major relative sea-level lowstands in conjunction with local filling of the adjacent basins by shales and mudstones that onlap the platform flanks. Calibration with outcrops suggests a subdivision into the following lithofacies, directly related to the depositional profile: (1) platform interior, parallel, well- to thick-bedded skeletal pack- to grainstones; (2) margin, algal- and sponge-dominated bioherms with little internal structure; (3) slope, clinothems of medium- to thick-bedded margin-derived breccia beds; (4) toe-of-slope, alternating grain- and packstones with spiculites; and (5) basin, marly shales, shales, sandstones, and calcareous mudstones. Relative sea-level changes caused by regional tectonic movements are suggested to be responsible for the generation of the two platform phases; sea-level highstands triggered the nucleation and growth of the platforms, whereas major sea-level lowstands in combination with terrigenous infill of the basins terminated the platform growth. In addition, it is proposed that the termination of Late Carboniferous carbonate platforms is caused by the combined effect of a sea-level fall, subaerial exposure and erosion, and, finally, a rapid transgression that drowned and polluted the platform with shales and mudstones. Local variations in the infilling of the basins controlled the variations in the style of progradation. The subsidence rates for the vertically aggrading phase (Picos de Europa Formation) were approximately twice that for the progradational phase (Valdeteja Formation) and may explain the aggradational versus progradational character.

The record of Pliocene sea-level change at Enewetak Atoll

Detailed seismic stratigraphy, lithostratigraphy, and chemostratigraphy indicate that atoll-wide subaerial exposure surfaces (major disconformities) developed during major sea-level lowstands form prominent seismic reflectors and are coincident with biostratigraphic breaks in the Plio-Pleistocene on Enewetak Atoll. Sea-level models based on the stratigraphic position and age of major disconformities suggest a maximum sea-level highstand elevation of 36 m above present sea level and a maximum sea-level lowstand elevation of 63 m below present sea level for the Pliocene.

Cretaceous and Tertiary sedimentation on the western margin of the Eucla Basin

Sedimentation on the northwestern margin of the Eucla Basin has been defined by drilling and palaeontological data to include Early Cretaceous to Miocene sequences. Channel, floodplain and lacustrine deposits of the Loongana Sandstone and Madura Formation indicate that the Early Cretaceous sea did not reach the Kitchener area. Middle to Late Eocene deposits unconformably overlie the Cretaceous sequence and extend up the Lefroy Palaeoriver towards Norseman where they merge with the Bremer Basin sediments. The Hampton Sandstone extends up the Lefroy Palaeoriver and was deposited under shallow high energy marine conditions probably in a well‐flushed large estuary. An erosional break at the top of the Hampton Sandstone and a wedge of fluvial sand in the Chifley palaeochannel provide evidence for a regression. The succeeding Late Eocene transgression covered wide areas around the Lefroy Palaeoriver and led to deposition of the Princess Royal Spongolite. Low oxygen levels and abundant silica, possibly derived from volcanic ash, allowed sponges to thrive in areas with restricted circulation on the basin margin while limestone accumulated in the sediment starved centre of the basin. The break between the Hampton Sandstone and Princess Royal Spongolite in the Harris Lake area is probably equivalent to the major sea level lowstand at 39.5 Ma.