Subaerial Erosion Research Articles

Quaternary atoll development: New insights from the two‐dimensional stratigraphic forward modelling of Mururoa Island (Central Pacific Ocean)

AbstractKnowledge about the Quaternary evolution of mid‐ocean atolls comes mainly from drilling and field observations carried out on a number of Pacific carbonate islands. However, little is known about the early to mid Pleistocene atoll development history, especially at margin and foreslope settings. Using previous field and subsurface data from Mururoa Atoll and a process‐based modelling software (DIONISOS), a two‐dimensional forward stratigraphic model of atoll development is proposed for the past 1·8 million years (Myr). Observational data from vertical to inclined coring, seismic and bathymetric surveys indicate that, from approximately 0·45 to 0·40 million years before present (Ma), carbonate deposition at Mururoa Atoll resulted in a series of mostly prograding reef units. The model is first constrained at the base by the shape and topography of the pre‐Quaternary basement. A number of sensitivity tests were performed to define the respective influence of variant parameters. The best‐fit development scenario that accounts for the overall geometry and stratigraphic architecture of the Quaternary sediment packages is obtained by using the sea‐level curve by Milleret al. (2005), uniform subsidence rate of 105 m Myr−1, and carbonate production rates gradually increasing from 0·50 to 8 mm yr−1between 1·80 Ma and the present. Additional controlling parameters include subaerial erosion (at a constant rate of 0·25 m/kyr), wave‐energy and sediment‐transport processes. The stratigraphic forward model predicts a succession of three distinct types of carbonate systems that have developed since the earliest Pleistocene: toe of slope systems from 1·80 Ma to about 0·80 Ma, open‐platform systems from 0·80 Ma to 0·50 Ma, and framework‐reef systems from about 0·50 Ma to the present. The development of these different systems is most likely to be controlled by climate and changes in sea‐level cycles. During the low‐amplitude 41 kyr cycle periods of the earliest Pleistocene, ambient conditions were not conducive to framework‐reef growth; shallow‐water carbonate sedimentation was dominantly gravity‐driven, operating along the platform foreslopes only. During the Mid‐Pleistocene Climate Transition, narrow, open‐platform units have developed at the upper parts of the pre‐Quaternary basement flanks. With the onset of the high‐amplitude 100 kyr sea‐level modes and climate restoration, reef frameworks started to be generated. These models from Mururoa agree with a number of previous studies suggesting that most of the true framework reefs were not initiated prior to 0·50 Ma. Mururoa Atoll is demonstrated to be a robust analogue for providing more realistic interpretations of the development history of Pacific atolls. Further modelling with three‐dimensionalDIONISOScould generate better predictions by taking into account hydrodynamic and transport parameters more accurately.

Microplaty hematite ore in the Yilgarn Province of Western Australia: The geology and genesis of the Wiluna West iron ore deposits

The Wiluna West small (~130Mt) high-grade bedded hematite ore deposits, consisting of anhedral hematite mesobands interbedded with porous layers of acicular hematite, show similar textural and mineralogical properties to the premium high-grade low-phosphorous direct-shipping ore from Pilbara sites such as Mt Tom Price, Mt Whaleback, etc., in the Hamersley Province and Goldsworthy, Shay Gap and Yarrie on the northern margin of the Pilbara craton. Both margins of the Pilbara Craton and the northern margin of the Yilgarn craton were subjected to sub-aerial erosion in the Paleoproterozoic era followed by marine transgressions but unlike the Hamersley Basin, the JFGB was covered by comparatively thin epeirogenic sediments and not subjected to Proterozoic deformation or burial metamorphism. The Joyner's Find greenstone belt (JFGB) in the Yilgarn region of Western Australia was exhumed by middle to late Cenozoic erosion of a cover of unmetamorphosed and relatively undeformed Paleoproterozoic epeirogenic sedimentary rocks that preserved the JFGB unaltered for nearly 2Ga; thus providing a unique snapshot of the early Proterozoic environment.Acicular hematite, pseudomorphous after acicular iron silicate, is only found in iron ore and BIF that was exposed to subaerial deep-weathering in early Paleoproterozoic times (pre 2.2Ga) and in the overlying unconformable Paleoproterozoic conglomerate derived from these rocks and is absent from unweathered rocks (Lascelles, 2002). High-grade ore and BIF weathered during later subaerial erosion cycles contain anhedral hematite and acicular pseudomorphous goethite. The acicular hematite was formed from goethite pseudomorphs of silicate minerals by dehydration in the vadose zone under extreme aridity during early Paleoproterozoic subaerial weathering.The principal high-grade hematite deposits at Wiluna West are interpreted as bedded ore bodies that formed from BIF by loss of chert bands during diagenesis and have been locally enriched to massive hematite by the introduction of hydrothermal specular hematite. No trace of chert bands are present in the deep saprolitic hematite and hematite–goethite ore in direct contrast to shallow supergene ore in which the trace of chert bands is clearly defined by goethite replacement, voids and detrital fill. Abundant hydrothermal microplaty hematite at Wiluna West is readily distinguished by its crystallinity.The genesis of the premium ore from the Pilbara Region has been much discussed in the literature and the discovery at Wiluna West provides a unique opportunity to compare the features that are common to both districts and to test genetic models.

Erratum to: Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada

The Early Cretaceous separation of Newfoundland from Iberia–Ireland is a classic example of a magma-poor continental margin with hyperextension and with widespread minor magmatism resulting in seamounts. This study defines the distribution of seamounts east of Orphan Knoll, and documents and interprets the geochemical character of the one recovered lava sample. Video imagery of lava outcrops, and the sample, were obtained by ROV from Orphan seamount, one of a linear series of small seamounts overlying transitional thinned continental crust on the seaward side of Orphan Knoll. New multibeam bathymetry and legacy seismic data show several seamounts that extend irregularly along the fault-bound NE margin of Orphan Knoll. Whole rock geochemistry shows the sample to be highly alkaline basanite or possibly tephrite. Diopside–hedenbergite, kaersutite and K-feldspar phenocrysts were analyzed by electron microprobe and scanning electron microscope, and alteration minerals including kaolinite were identified by X-ray diffraction. The highly alkaline character of the basanite is similar only to Early Cretaceous volcanic and sub-volcanic rocks erupted through thick continental crust of the Mesoproterozoic Grenville Orogeny. The location of the linear set of seamounts is related to margin-parallel faults on the seaward side of Orphan Knoll that provided a pathway for magma, although ENE-trending lineaments in individual seamounts or seamount groups suggest the influence of oceanic fracture zones. A lower gradient crest to Orphan seamount above 2,200 m suggests subaerial erosion, consistent with the presence of kaolinite as an alteration product and the absence of lava pillows at and above this depth.

Submerged karst landforms observed by multibeam bathymetric survey in Nagura Bay, Ishigaki Island, southwestern Japan

Submerged tropical karst features were discovered in Nagura Bay on Ishigaki Island in the southern Ryukyu Islands, Japan. The coastal seafloor at depths shallower than ~130m has been subjected to repeated and alternating subaerial erosion and sedimentation during periods of Quaternary sea-level lowstands. We conducted a broadband multibeam survey in the central area of Nagura Bay (1.85×2.7km) and visualized the high-resolution bathymetric results over a depth range of 1.6–58.5m. Various types of humid tropical karst landforms were found to coexist within the bay, including fluviokarst, doline karst, cockpit karst, polygonal karst, uvalas, and mega-dolines. Although these submerged karst landforms are covered by thick postglacial reef and reef sediments, their shapes and sizes are distinct from those associated with coral reef geomorphology. The submerged landscape of Nagura Bay likely formed during multiple glacial and interglacial periods. According to our bathymetric results and the aerial photographs of the coastal area, this submerged karst landscape appears to have developed throughout Nagura Bay (i.e., over an area of approximately 6×5km) and represents the largest submerged karst in Japan.

Chapter 14 The rock coast of Australia

Abstract The rocky coast of Australia has been the focus of scientific enquiry since Charles Darwin visited Sydney during the voyage of the Beagle (1831–1836). As the philosophy of geomorphological enquiry has evolved from Davisian landscape theory to modern questions of landscape response to climate change, so has the understanding of Australian shore platforms. Early work was focused on penultimate levels of coastal planation; however, the majority of research has focused on the balance between marine and subaerial erosion in forming shore platforms. As field instrumentation has advanced there has been a greater variety of data available to researchers which has led to studies of microerosion and the distribution of wave energy across platform surfaces. Platforms formed in Quaternary carbonates and volcanics as well as Mesozoic sediments have been the focus of research. Those found in softer carbonates form closer to low-tide level while those in resistant basalt form at high-tide elevation. Rock hardness and structure are the primary drivers of platform morphology; however at an individual field site the surface morphology, such as the presence of a ramp, will be determined by the balance between wave and subaerial processes as well as local variations in lithology.

Late glacial-Holocene shelf evolution of the Sea of Marmara west of Istanbul

We present an investigation the Late Quaternary seismic stratigraphy of the shelf area of the northern the Sea of Marmara extending from its northern coast (between Silivri and Kumkapı) to approximately 100m depth, using shallow high-resolution seismic reflection data along 73N–S and 15 E–W lines. Seismic sequence analysis is used to identify the depositional systems, associated sedimentation conditions, and relative sea level changes. Seismic stratigraphic interpretations indicate the presence of four distinct seismic units (SU I, II, III and IV) underlying the shelf area. Seismic units are bounded by erosional unconformities overlying an acoustic basement. Seismic unit I constitutes the acoustic basement (AB), which is characterized by chaotic, subparallel, and wavy reflections that out locally off the rocky shorelines and along the crests of the positive structures where the sedimentary cover is absent. Seismic unit II is interpreted to represent the pre-Holocene deposits and exhibits subparallel reflections. Seismic unit II is interpreted to have been subjected to sub-aerial erosion during the Last Glacial Maximum. Seismic unit III–IV are interpreted to have formed during the Holocene (Flandrian) transgression and have parallel/subparallel internal reflection patterns. The top of seismic unit IV forms the present-day sea floor. As a result of the presence of fill, seismic facies within seismic unit IV reflect differences in depositional processes. The bathymetry of the study area has a close relation with sedimentation dynamics, tectonic, wave and flow dynamics and palaeotopograpy. Particularly, sudden dip changes determined at the shelf area might have been due to fault and/or fault systems that control the bottom topography. Seismic activity in the Sea of Marmara region has a key role the northern branch of the North Anatolian Fault Zone (NAFZ) affecting on the tectonic activity of the study area. The last two earthquakes in İzmit and Duzce, Turkey, in 1999 have been originated from NAFZ. The presence of several faults on various scales in the seismic profiles of the study area is identified. The active normal faults in the Holocene units that disturb the seafloor are determined. Most of these faults are intense in the eastern part of research area. The isopach map of Holocene sediment (seismic unit III and IV) indicates that the maximum sediment accumulations (up to 36m) occur in the offshore areas of Bakırköy and Kumkapı. Young sediment thicknesses are common on the shoreline areas in the investigated area.

Dense shelf water cascading and Messinian Canyons: A new scenario for the Mediterranean salinity crisis

The salt giant beneath the deep Mediterranean seafloor is the impressive record of the “Messinian salinity crisis,” a dramatic event that occurred about 6 Ma ago following the reduction of the connections with the Atlantic Ocean. According to the shallow-water deep-basin model, developed for these deposits ([Hsü and others, 1973a][1], [1973b][2], [1978a][3], [1978b][4]), the Messinian evaporites formed in a deep but desiccated Mediterranean, while shelves and slopes underwent subaerial erosion due to fluvial rejuvenation triggered by a 1500 m sea level drawdown. Deeply incised Messinian canyons in the continental slopes surrounding the Mediterranean are the main argument supporting this scenario. Using a state of the art model and idealized but realistic numerical simulations, here we demonstrate that the activation of downslope flows of hypersaline, dense waters, in a process similar to present-day “dense shelf water cascading,” but much more energetic, may account for both slope erosion and progressive salinity rise leading to the formation of deep-seated supersaturated brines. Our findings support a deep-water deep-basin model ([Schmalz, 1969][5], [1991][6]; [De Benedetti, 1976][7], [1982][8]; [Dietz and Woodhouse, 1988][9]), thus implying that evaporite deposition may have occurred in a non-desiccated basin with strongly reduced ocean connections. [1]: #ref-57 [2]: #ref-58 [3]: #ref-59 [4]: #ref-60 [5]: #ref-103 [6]: #ref-104 [7]: #ref-26 [8]: #ref-27 [9]: #ref-29

Eocene oil shales from Jordan – Paleoenvironmental implications from reworked microfossils

Reworked microfossils, common in Paleogene sediments in Jordan, are here used to reconstruct the depositional environment. The reworked taxa, which include both calcareous nannofossils and foraminifera of Cretaceous and Paleocene age, were found in Eocene oil shales. The potential provenance of the reworked material and the underlying processes of the reworking are discussed. We differentiate between a subaerial erosion of exposed hinterland strata and a submarine abrasion of sediments.A total of 73 smear slides have been prepared to identify calcareous nannofossils, another fifteen samples were analyzed for foraminifera. The allochthonous calcareous nannofossil and foraminifera taxa can be linked to a lithified source, which was eroded and transported with both calcitic lithic fragments and organic matter. Multiple factors controlling the transport of the reworked taxa during the time of deposition have been investigated for the Jordanian oil shales. Climate changes are thought to be the cause for changes in the abundance patterns and in the composition of the reworked taxa. The input of common autochthonous components during arid phases and more allochthonous sediment particles during humid periods filled the Eocene sink. A fall in relative sea level, perhaps in combination with increased storm activity, caused a transport of reworked material to deeper parts of the Azraq Hamza Sub-basin. The relative sea level changes in turn were related to syndepositional movements, redefining the shape of the Azraq Hamza Sub-basin and its internal fault-block architecture.

Geochemistry, U–Pb geochronology, and genesis of granitoid clasts in transported volcanogenic massive sulfide ore deposits, Buchans, Newfoundland

The Buchans Group, central Newfoundland, represents an Ordovician continental bimodal calc-alkaline arc sequence that hosts numerous volcanogenic massive sulfide (VMS) occurrences, including both in situ and mechanically transported sulfide breccia–conglomerate orebodies. Diverse lithic clasts associated with transported deposits include rounded granitoid clasts. Earlier workers have suggested that Buchans Group VMS-hosting felsic extrusive units, small granodiorite intrusions (e.g., Wiley’s Brook), and granitoid cobbles associated with transported ore represent co-genetic products of the same magmatic system. The granitoid cobbles and small granodiorite intrusions are geochemically similar and closely resemble Buchans Group felsic volcanic units. U–Pb zircon age determinations show a (i) 466.7 ± 0.5 Ma crystallization age for the Wiley’s Brook granodiorite (WBG), (ii) 464 ± 4 Ma crystallization age for a granitoid cobble, and (iii) 466 ± 4 Ma maximum deposition age for a conglomerate–sandstone sequence associated with transported ore. Thus, Buchans Group felsic plutonic rocks are within experimental error of felsic volcanism and VMS deposition. Furthermore, εNd (T) (T, time of crystallization) values of four granitoid cobbles (–1.95 to –4.0) overlap values obtained from Buchans Group felsic volcanic units. Our results are compatible with plutonic and volcanic rocks being related through fractional crystallization or partial melting processes but do not support a petrogenetic link between VMS deposition and exposed felsic plutons. Comparisons to modern arc analogues favour exhumation of plutonic rocks by extension along caldera or rift walls and (or) subaerial erosion. Enigmatic rounding of Buchans granitoid clasts was likely accomplished in a subaerial or shallow marine environment, and the clasts transported into a VMS-active basin by mass flows.

Accommodation-based coal cycles and significant surface correlation of low-accommodation Lower Cretaceous coal seams, Lloydminster heavy oil field, Alberta, Canada: Implications for coal quality distribution

Sequence stratigraphy and coal cycles based on accommodation trends were investigated in the coal-bearing Lower Cretaceous Mannville Group in the Lloydminster heavy oil field, eastern Alberta. The study area is in a low accommodation setting on the cratonic margin of the Western Canada sedimentary basin. Geophysical log correlation of coal seams, shoreface facies, and the identification of incised valleys has produced a sequence-stratigraphic framework for petrographic data from 3 cored and 115 geophysical-logged wells. Maceral analysis, telovitrinite reflectance, and fluorescence measurements were taken from a total of 206 samples. Three terrestrial depositional environments were interpreted from the petrographic data: ombrotrophic mire coal, limnotelmatic mire coal, and carbonaceous shale horizons. Accommodation-based coal (wetting- and drying-upward) cycles represent trends in depositional environment shifts, and these cycles were used to investigate the development and preservation of the coal seams across the study area. The low-accommodation strata are characterized by a high-frequency occurrence of significant surfaces, coal seam splitting, paleosol, and incised-valley development. Three sequence boundary unconformities are identified in only 20 m (66 ft) of strata. Coal cycle correlations illustrate that each coal seam in this study area was not produced by a single peat-accumulation episode but as an amalgamation of a series of depositional events. Complex relations between the Cummings and Lloydminster coal seams are caused by the lateral fragmentation of strata resulting from the removal of sediment by subaerial erosion or periods of nondeposition. Syndepositional faulting of the underlying basement rock changed local accommodation space and increased the complexity of the coal cycle development. This study represents a low-accommodation example from a spectrum of stratigraphic studies that have been used to establish a terrestrial sequence-stratigraphic model. The frequency of changes in coal seam quality is an important control on methane distribution within coalbed methane reservoirs and resource calculations in coal mining. A depositional model based on the coal cycle correlations, as shown by this study, can provide coal quality prediction for coalbed methane exploration, reservoir completions, and coal mining.

Cyclostratigraphic and chronostratigraphic correlations in the Barremian–Aptian shallow-marine carbonates of the central-southern Apennines (Italy)

We present a high-resolution sedimentological analysis of Barremian–Aptian shallow-water carbonates from two cores (S. Maria 6 and 4) that were drilled in the central Apennines (central Italy) and one section (Monte Faito) that crops out in the southern Apennines (southern Italy). The aims of this work are (a) to propose a high-resolution correlation of sections that are located approximately 170 km apart in different tectonic units and paleogeographic domains using a microstratigraphic (cm-scale) approach and (b) to reveal global and regional mechanisms that control the stratigraphic architecture of these carbonate platform strata.A composite S. Maria section was assembled by integrating the sedimentologic and biostratigraphic analyses of the two cores, which overlap each other across the Barremian–Aptian boundary. Both the S. Maria and the Monte Faito sections show repetitive facies patterns that are expressed as elementary cycles, which are hierarchically grouped into bundles and superbundles. The elementary cycles are meter-scale sedimentary units that are bounded by subaerial erosion surfaces, which directly overlie subtidal deposits. This implies that they formed under the influence of relative sea-level fluctuations. In both sections, the superbundles are organized into Transgressive/Regressive Facies Trends (T/RFTs), which are considered to be expressions of lower-frequency relative sea-level changes.These deposits, like their Cretaceous analogues of other areas of the Apennines, show evidence of astronomically controlled eustatic oscillations, which are reflected in the hierarchical organization of their stacking patterns. They also exhibit a sequence-stratigraphic configuration that is best recognizable in the superbundles and T/RFTs. Based on integrated stratigraphic criteria, a high-resolution regional correlation between S. Maria and Monte Faito was developed and compared with the reference section of Monte Raggeto (M. Maggiore, southern Apennines, Italy), where biostratigraphic and cyclostratigraphic studies have been complemented by magneto- and isotope-stratigraphy.We also propose a chronostratigraphic correlation between our T/RFTs and the Tethyan stratigraphic cycles of Hardenbol et al. (1998). Based on the cyclostratigraphic interpretation and orbital chronostratigraphy of the studied interval, we estimate a minimum duration of 5.2 my for the Barremian interval, which is similar to the 4.5 my duration from the Geological Time Scale of Gradstein et al., (2012).

Impacts of flamingos on saline lake margin and shallow lacustrine sediments in the Kenya Rift Valley

Studies of modern, Holocene, and Pleistocene sediments around saline to hypersaline, alkaline Lake Bogoria and Lake Magadi show that evidence of flamingo activity in marginal areas of these lakes is nearly ubiquitous. Flamingos produce discrete structures such as webbed footprints (~9cm long, ~11cm wide) and nest mounds (~30cm wide, ~20cm high), and they also extensively rework sediments in delta front, delta plain, and shoreline areas. Large (~0.5–2cm in diameter), pinched, ‘bubble pores’ and ped-like mud clumps are formed by the trampling and churning of wet clay-rich sediments in these settings. Flamingo nest mounds, although superficially similar to some thrombolite mounds, are typically internally structureless, unless formed on pre-existing sediments that preserve internal structures. The flamingo mounds consist of a dense, packed oval-shaped core, a surrounding ‘body’ of packed sediment, and an external layer with a ped-like texture of clumped mud. The nests may contain open holes from roots or feather shafts incorporated into the nest, and (or) burrows produced once the nests are abandoned. In areas with high densities of flamingos, lake margin sediments may be preferentially compacted, particularly at breeding sites, and become resistant to subaerial erosion and the effects of transgressive ravinement on time scales ranging from seasons to tens of thousands of years. The relatively well-compacted nest mounds and associated sediments also contribute to the stability of delta distributary channels during regressive-transgressive cycles, and can lead to the minor channelization of unconfined flows where currents are diverted around nest mounds. Pleistocene exhumed surfaces of relatively well-indurated lake margin sediments at Lake Bogoria and Lake Magadi that are interpreted as combined regressive and transgressive surfaces (flooding surface/sequence boundary) preserve evidence of flamingo activities, and are overlain by younger, porous lacustrine silts that preserve large bubble pores produced by flamingos.

Sedimentology, palaeoenvironments and biostratigraphy of the Pliocene–Pleistocene carbonate platform of Grande‐Terre (Guadeloupe, Lesser Antilles forearc)

AbstractPliocene and Pleistocene deposits from Grande‐Terre (Guadeloupe archipelago, French Lesser Antilles) provide a remarkable example of an isolated carbonate system built in an active margin setting, with sedimentation controlled by both rapid sea‐level changes and tectonic movements. Based on new field, sedimentological and palaeontological analyses, these deposits have been organized into four sedimentary sequences (S1 to S4) separated by three subaerial erosion surfaces (SB0, SB1 and SB2). Sequences S1 and S2 (‘Calcaires inférieurs à rhodolithes’) deposited during the Late Zanclean to Early Gelasian (planktonic foraminiferal Zones PL2 to PL5) in low subsidence conditions, on a distally steepened ramp dipping eastward. Red algal‐rich deposits, which dominate the western part of Grande‐Terre, change to planktonic foraminifer‐rich deposits eastward. Vertical movements of tens of metres were responsible for the formation of SB0 and SB1. Sequence S3 (‘Formation volcano‐sédimentaire’, ‘Calcaires supérieurs à rhodolithes’ and ‘Calcaires à Agaricia’) was deposited during the Late Piacenzian to Early Calabrian (Zones PL5 to PT1a) on a distally steepened, red algal‐dominated ramp that changes upward into a homoclinal, coral‐dominated ramp. Deposition of Sequence S3 occurred during a eustatic cycle in quiet tectonic conditions. Its uppermost boundary, the major erosion surface SB2, is related to the Cala1 eustatic sea‐level fall. Finally, Sequence S4 (‘Calcaires à Acropora’) probably formed during the Calabrian, developing as a coral‐dominated platform during a eustatic cycle in quiet tectonic conditions. The final emergence of the island could then have occurred in Late Calabrian times.

Scale independence of basin hypsometry and steady state topography

Basin hypsometry has long been used as an indicator of stages in landscape evolution. It has also served as a tool for detecting tectonically active regions. Whether hypsometric curve and its integral are independent of differences in basin size and relief has been discussed in many recent studies. The Taiwan Mountain Range, the result of an oblique collision between the Phillipine Sea Plate and the Eurasian Plate, provides an excellent opportunity to study landscape evolution in relation to steady state conditions. Taking advantage of the space-for-time substitution concerning the building process of Taiwanese mountains, this study sampled major drainage basins for hypsometric analysis, from the southern tip of the island to the northern end. This study found that the area previously known as steady state topography is characterized by 1) the hypsometric integral (HI) close to 0.5, 2) S-shaped hypsometric curves, and 3) normally distributed elevations. The response time required for a drainage basin of various Strahler orders to evolve from exposure of sub-aerial erosion to steady state topography ranges from 0.5 to 2.0My, and is longer for basins of a higher Strahler order. The HI value of 0.5 for a drainage basin seems critical in terms of landscape evolution. The scale dependence of basin hypsometry is manifested mainly by the fact that HI of basins increases as basin order and/or basin size decreases. This study also found that basin hypsometry is scale independent where topography is in a steady state; scale dependence occurs only when drainage basins are in a non-steady state. The basin hypsometry may therefore be useful to probe into the nature of steady-state topography.

Earth's oldest preserved unconformity — Prospect of a beginning in the tectono-sedimentary continental cycle?

Earth's oldest preserved conglomerates and basaltic pillow lavas at Isua, Greenland, provide robust field evidence for deep- and shallow-water environments on our planet within the first billion years of its formation. The conglomerates represent the first Paleoarchaean archive of sub-aerial erosion and shallow water sedimentation. These ca. 3700million year old sedimentary rocks, now metamorphosed at amphibolite grade, comprise units of rounded quartz pebbles set in a sandy to muddy matrix that unconformably overlie an ophiolite sequence. The pillow lavas of the ophiolite are variolitic and essentially vesicle-free, indicative of formation in a deep water environment. Locally, an unconformity separates the conglomerates from the deformed ophiolite-related rocks; elsewhere the contact between these units is tectonic. Such field relationship between deep and shallow water environments resemble those preserved in younger orogenic belts, where obducted oceanic crust has been tectonically emplaced across terrestrial platforms or subaerial parts of forearc and backarc basins.

Modelling depositional shifts between sedimentary basins: Sediment pathways in Paratethys basins during the Messinian Salinity Crisis

The evolution of sedimentary basins separated by uplifted topographic barriers is characterised by gradual regressive deposition until one of the basins is filled and its sedimentation bypasses to the adjacent basin, defining a depositional shift. One of the critical parameters controlling these depositional shifts is sea level variation, its drop potentially triggering a depositional shift by cancelling the available accommodation space. Conversely, a sea level rise can create new accommodation space resulting in a depositional shift towards a previously overfilled basin. Here we use a three dimensional numerical model to study the sedimentary response to sea level variations of a system of two basins. In this model, a single mountainous source area is feeding an intra-continental basin that is separated by a submarine barrier from another basin with normal marine bathymetry. The sedimentary response is modelled during a cycle of sea-level drop and subsequent rebound that exposes the barrier to sub-aerial erosion. The examined parameters are the barrier height, magnitude and duration of sea level change, climate and flexural rigidity. Modelling demonstrates that shifting the bulk of sedimentation from the continental basin to the open marine environment requires some minimum magnitudes and durations of sea level drop. Moreover, given the specific geometry and parameters of our model, an intervening barrier causes a delay of up to 0.35Myr, depending on the magnitude and duration of sea level change, to the onset of an outward depositional shift when compared to a situation without a barrier. These depositional shifts depend on changes in climate, magnitude and duration of sea level change. Model results are applied to the connectivity between the Black Sea and the Dacic Basin, suggesting that depositional shifts observed during the Messinian Salinity Crisis can be explained by a sea level drop of >1000m in the Black Sea.

Nature and significance of the Neoproterozoic Sturtian–Marinoan Boundary, Northern Adelaide Geosyncline, South Australia

The Cryogenian succession of the Northern Flinders Ranges reveals a complex sedimentary record between the Sturtian and Marinoan glacial deposits. A major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a subaerial erosion surface with terrestrial and marginal marine infill directly above the Angepena and Balcanoona Formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with thepreviously defined Sturtian–Marinoan Time Series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta ReefComplex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow-water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation.

The Catalan margin during the Messinian Salinity Crisis: Physiography, morphology and sedimentary record

The Messinian Salinity Crisis (MSC) drastically affected the physiography, morphology and sedimentation all along the Mediterranean area at the end of the Miocene. This paper presents an analysis of the effects of the MSC on the Catalan continental margin, based on a broad database of seismic reflection profiles from the oil industry and academy. We study the sedimentary processes and their controlling factors and the correlation with the nearby and well-known Gulf of Lions. Because of the complexity of the MSC, we define the Margin Erosion Surface/Top Erosion Surface (MES/TES) as the surface marking the end of the Messinian event, which allows differentiating three domains along the Catalan margin: the Creus, Girona and Barcelona domains, separated by the Palamós and Blanes canyons. The Creus domain shows an important structural control on the MES/TES physiography, while the Girona and Barcelona domains are characterized by wide and flat MSC platforms below the present-day shelf. The entire margin developed a series of complex drainage systems as the result of the Messinian sea level drop. The location of the main valleys (Cap de Creus paleo-drainage system, Palamós and Blanes paleo-canyons and the downslope-trending valleys on the Barcelona domain) was directly controlled by the structural framework. In contrast, the Barcelona paleo-drainage system and the tributaries of the Blanes and Palamós systems were excavated by direct subaerial erosion by meteoric rain in the basins limited by basement blocks, and were afterwards totally filled by the Pliocene–Quaternary sedimentation. Regarding the MSC depositional features, the major part of the eroded sediment was deposited as detrital deposits on the foot of the slope. The distribution of detrital bodies and evaporites was also structurally controlled. A Zanclean sea level rise occurring in at least two phases is confirmed by this study, in correlation with interpretations from the Gulf of Lions.

Late Cretaceous subduction initiation on the eastern margin of the Caribbean-Colombian Oceanic Plateau: One Great Arc of the Caribbean (?)

Detailed mapping on the Leeward Antilles islands of Aruba, Curacao, Bonaire, and La Blanquilla has led to a reassessment of their stratigraphic, magmatic, and structural evolution. In general, each island preserves its own distinct sequence of geologic events. The Cretaceous geology of Aruba and Curacao consists of a mafic igneous complex, long interpreted to represent exposures of the Caribbean-Colombian Oceanic Plateau (CCOP), intruded by 89–86 Ma arc-related plutons and dikes. The rocks on both islands that are interpreted as remnants of the CCOP underwent a period of subaerial erosion in the Late Cretaceous, but subsequently their geologic histories diverge significantly in terms of their stratigraphic and structural evolution. Mapping on Bonaire has resulted in a major revision to the Cretaceous bedrock geology. Instead of a single stratigraphic unit (Washikemba Formation) the island contains two stratigraphic units separated by a northwest-trending fault. The southwest side of the fault consists of an arc-related Early to Late Cretaceous volcaniclastic section cut by shallow level intrusions, whereas the northeast side is composed of Early to Late Cretaceous epiclastic/hemipelagic strata that are locally cut by small arc-related mafic intrusions. La Blanquilla represents the southernmost exposure of the Aves Ridge which is a remnant arc separated from the modern arc of the Lesser Antilles by the Grenada back arc basin. The bedrock geology consists of two Late Cretaceous arc-related plutons. The geologic evolution of the Leeward Antilles when combined within a broader context of Caribbean tectonics leads us to a tectonic model involving three distinct arcs rather than a single “Great Arc” of the Caribbean as an explanation for the geodynamic evolution of the CCOP and its fringing arc system.

Geodynamic responses to a two-step model of the Messinian salinity crisis

Abstract There is a growing consensus that the sulfates and halite within the massive evaporite deposits on the shallow margin and deep floor of the Mediterranean sea formed in two steps. Both phases had geodynamic consequences. The current evidence indicates that during the first step when the first cycle “Lower Evaporites” were deposited primarily in marginal settings, the surface of the Mediterranean remained at or very close to the level of the external Atlantic. The geodynamic response resulted from the increasing weight of the brine layer as it concentrated from normal marine salinity to the threshold for sulfate precipitation and then to the threshold for halite precipitation. This weight alone significantly deepened the Mediterranean basins by isostatic loading. Flexure of the lithosphere caused a peripheral bulge to appear that may have been the agent to close off the further entry of Atlantic seawater. Step two began with the subsequent evaporative drawdown and the deposition of the second cycle “Upper Evaporites”. As the basins dried out the loss of weight of the water led to regional isostatic uplift that permanently closed the prior inlets. Sediments removed from the margins by early subaqueous mass-wasting and later subaerial erosion and delivered to the basin floors further accentuated uplift of the margins and subsidence of the depocenters. The principal result has been to progressively tilt the Mediterranean substrate downwards from the margin towards the basin centers. This tilting was enhanced with the flooding of the desiccated sea at the climax of the salinity crisis. Consequently the salt layer rests today out of equilibrium on a surface that is more inclined after precipitation than before. The current mobility and flowage of the salt away from its margins and towards the basin centers is therefore not so much a response to the differential thickness and weight of overlying sediments, but to the combination of geodynamic processes that have produced the seaward tilting. Other giant salt deposits seem to have experienced a similar two-step evolution.