Subaerial Emergence Research Articles

Paleoenvironments and sequential stratigraphy of Ordovician marginal marine deposits of Sierra de Zapla (Sierras Subandinas, northwestern Argentina) and their relation with the Central Andean Basin

The best and most complete palynologically constrained Ordovician section across the Sierras Subandinas crops out in the southern part of the Sierra de Zapla along the Capillas River (Subandean Ranges, Jujuy Province). The dominantly marginal marine setting explains the scarcity of invertebrates throughout the section. Palynomorphs are the main tool in stratigraphic studies of the area and trace fossils are important aids in paleoenvironmental analysis. Based on a detailed facies analysis, geometry and paleocurrent analysis, and recognition of key surfaces, four depositional sequences (S1-S4) with different internal arrangements and compatible with 3rd order cycles were identified. The Zanjón Formation (upper part of Global Stage 2 and Global Stage 3) with unexposed base, initiates the stratigraphic section and it is characterized by a heterolithic facies association with subordinate storm layers, thin-bedded phosphate-rich micro-conglomerates and inarticulate-rich shell beds. Common subaerial exposure features like truncated ripple tops and mud cracks suggest tidal flat environments. This interval yields a mixed Skolithos-Cruziana ichnofacies. Towards the top, an increase in mottling and a gradual change into dominant purplish-red colors (Laja Morada Member of the Labrado Formation) indicate a protracted subaerial emergence during a Darriwilian (Global Stage 4) relative sea-level drop, and a fluctuating water table in interfluves. This unit is covered in sharp erosive boundary by the Lagunilla Member of the Labrado Formation, suggesting development of composite fluvial incisions. Sandy fluvial to tide-dominated estuarine fills and thoroughly-bioturbated subtidal hetherolithic beds overlie the sequence boundary representing a typical transgressive systems tract. Development of surfaces with dominant Skolithos forms are interpreted as representing Glossifungites ichnofacies typically indicating by-pass erosion and firm-ground development previous to reworking of interfluves by advancing ravinement surface. The Capillas Formation (upper Darriwilian and lower part of Global Stage 5), sharply levels the previous estuarine complexes and represents a fine-grained wedge that gradually coarsens up. This unit contains the only truly marine shelly fauna concentrated in few storm beds above the fine-grained interval that characterizes a maximum flooding stage. This is the only interval that can be positively correlated with sections in the Cordillera Oriental to the west and Bolivia. A gradual coarsening to silty sandstones and the replacement of Cruziana by Skolithos dominated ichnofacies suggest a shallowing-upward section, capped by the thick-bedded, high energy quartz sandstones with pipe-rock structures of the Centinela Formation (middle and upper part of Global Stage 5). A shallowing-upward trend indicates the progradation of deltaic complexes, which in turn were erosively truncated by the waxing stage of the Gondwanan Hirnantian (uppermost Ordovician) ice-cap represented by the Zapla Formation (Global Stage 6) along the Central Andes. Above the diamictite-rich Zapla Formation the Lipeón Formation (Silurian) is interpreted as related to repeated transgressive ravinement surfaces that truncated Fe+2 saturated estuaries after a waning glacial stage and isostatic rebound, favouring the deposition of oolitic ironstones and succeeded by the development of a Zoophycos dominated muddy shelf.

Turbidite Systems in the Inner Forearc Domain of the Hikurangi Convergent Margin (New Zealand): New Constraints on the Development of Trench-Slope Basins

The development of trench-slope basins on subduction wedges is strongly related to subduction processes. Theseconfined basins form on the lower trench slope, and their edges consist of structurally controlled linear bathymetric highs. In this particular setting, sedimentary processes may be controlled both by tectonism and by sea-level changes. Thus, the study of the sedimentary record may permit evaluation of the spatial and temporal contribution of tectonic activity on the development and stratigraphic evolution of trench-slope basins. The recent evolution of the Hikurangi subduction wedge (North Island of New Zealand) is characterized by the uplift and subaerial emergence of the trench­slope break, which has provided good exposure of the Miocene Akitio trench-slope basin. Identification of the main sedimentary discontinuities, reconstruction of the geometry of the sedimentary units, and paleocurrent studies were undertaken in the Akitio basin in order to propose a stratigraphic and structural scheme for the evolution of this emerged trench­slope break. Deep-sea facies associations are representative of three main fine-grained gravity-driven systems that may develop during various stages of development of mature trench-slope basins. These are, from base to top, large submarine slides (olistostrome deposits), fine-grained sand-rich submarine fans, and low-gradient submarine ramps (sheet-like turbidites). This basin-fill megasequence constitutes a lowstand systems tract, which can be subdivided into basin-floor fan, slope fan, and progradingwedge complex. Slope facies associations are common and highlight the important contribution of slope processes to the basin sediment budget. Rapid shifts of facies and important paleoenvironmental changes, reflecting slope creation or modification, characterize five major discontinuities that are related to regional tectonic events (i.e., onset of subduction at 25 Ma, beginning of regional subsidence at 15 Ma) or local tectonic events (i.e., local uplift of structural edges of the Akitio trench-slope basin at 17.5 Ma and 16.5 Ma, acceleration of subsidence at 13.2 Ma). Moreover, we show that the timing of development of some lowstand system tracts (i.e., of slope fans and of prograding-wedge complexes) may also be controlled largely by changes in style and/or amplitude of tectonic activity.

Carbon and oxygen isotopic variations in peritidal stromatolite cycles, Paleoproterozoic Kajrahat Limestone, Vindhyan basin of central India

Variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a Paleoproterozoic peritidal stromatolite succession. The upper division of the Kajrahat Limestone, Vindhyan Supergroup of central India consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size. Normally, larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle. Desiccation cracks are found in all the facies indicating subaerial exposure. We investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence. Carbon and oxygen isotopic values of the limestones, in general, are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level. The δ 13C and δ 18O values of the limestones vary within an individual stromatolite cycle; depleted values characterize the topmost part of the cycles. The isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure. These variations also probably caused the observed scatter in δ 13C and δ 18O values of supratidal microbial laminites.

Facies architecture and Late Pliocene – Pleistocene evolution of a felsic volcanic island, Milos, Greece

The volcanic island of Milos, Greece, comprises an Upper Pliocene –Pleistocene, thick (up to 700 m), compositionally and texturally diverse succession of calc-alkaline, volcanic, and sedimentary rocks that record a transition from a relatively shallow but dominantly below-wave-base submarine setting to a subaerial one. The volcanic activity began at 2.66±0.07 Ma and has been more or less continuous since then. Subaerial emergence probably occurred at 1.44±0.08 Ma, in response to a combination of volcanic constructional processes and fault-controlled volcano-tectonic uplift. The architecture of the dominantly felsic-intermediate volcanic succession reflects contrasts in eruption style, proximity to source, depositional environment and emplacement processes. The juxtaposition of submarine and subaerial facies indicates that for part of the volcanic history, below-wave base to above-wave base, and shoaling to subaerial depositional environments coexisted in most areas. The volcanic facies architecture comprises interfingering proximal (near vent), medial and distal facies associations related to five main volcano types: (1) submarine felsic cryptodome-pumice cone volcanoes; (2) submarine dacitic and andesitic lava domes; (3) submarine-to-subaerial scoria cones; (4) submarine-to-subaerial dacitic and andesitic lava domes and (5) subaerial lava-pumice cone volcanoes. The volcanic facies are interbedded with a sedimentary facies association comprising sandstone and/or fossiliferous mudstone mainly derived from erosion of pre-existing volcanic deposits. The main facies associations are interpreted to have conformable, disconformable, and interfingering contacts, and there are no mappable angular unconformities or disconformities within the volcanic succession.

The Great Barrier Reef: The Chronological Record from a New Borehole

ABSTRACT A new borehole, 210 mbsf (meters below sea floor) deep, drilled in Ribbon Reef 5 on the Great Barrier Reef off Cooktown, NE Australia, reveals a shallowing-upwards succession, the younger part of which is punctuated by a series of erosion surfaces. Nine depositional units have been defined by lithological changes and are numbered sequentially from the base of the hole upwards. Aminostratigraphy, magnetostratigraphy, radiocarbon dating, uranium series dating, and modeling together with strontium ratios have been applied in an attempt to establish a chronology of accumulation. Carbonate deposition began about 770 ka ago in a relatively deep-water slope environment and is represented by a series of debris flows. Lithoclasts within these rocks, indicate that older limestones already existed in the area. Subsequent accretion involved the downslope accumulation of grainstones and wackestones, sometimes cross-laminated, characterized by intervals with abundant rhodoliths and scattered, probably reworked, corals. Four units at the base of the hole reflect deposition that probably began during isotope stage 16 and continued through stage 15 from about 770 to about 564 ka. Unit 5 probably extended to stage 11 (about 400 ka), and unit 6 to stage 9 ( 330 ka). Typical reefal associations of corals and calcareous algae were established in this area only above depths of about 100 m in the borehole, units 5-4. The succession is apparently unbroken to an erosion surface at 36 mbsf indicating subaerial emergence. The lack of evidence of emergence below this surface reflects progressive accretion or progradation or both. Two younger erosion surfaces define further periods of lowered sea level. Unit 7 is attributed to deposition during isotope stage 7, but erosion during stage 8 resulted in the preservation of only 8 m of unit 7 limestones. Unit 8 is correlated with stage 5 (125 ka), and unit 9 is interpreted as Holocene (post 7,700 ka). The limited thicknesses of units 7, 8, and 9 are considered to reflect erosion. The progressive shallowing brought the depositional surface within the zone exposed during lowstands, and there is no sedimentological evidence that aggradation was restricted by a lack of accommodation.

Geomorphological evolution of Montserrat (West Indies): importance of flank collapse and erosional processes

Analysis of topography and new swath bathymetry as well as geophysical data provides information about aerial and submarine morphological features and mass transfer processes on Montserrat. The island has a characteristic shallow (<100 m) submarine shelf, interpreted as having been formed through erosion with a depth controlled by glacio-eustatic sea-level variation. Several debris avalanche deposits are identified on the lower submarine flanks of Soufrière Hills volcano, and there is evidence of lateral collapses at the older volcanic centres. The morphological evolution of Montserrat is interpreted in terms of three stages. The first stage comprises submarine growth. The second stage, subaerial growth, is represented by the active South Soufrière Hills–Soufrière Hills volcanic centre. During the current eruption of Soufrière Hills volcano (1995–2002) more than half of the lava erupted was transported into the sea. Flank collapses occurred several times during this stage, such as the English's Crater event ( c . 4000 years ago) or the Boxing Day event during the current eruption (26 December 1997). Montserrat's older volcanic centres, the Centre Hills and Silver Hills, illustrate the third stage of evolution, extinction and erosion. Magma production, long-term erosion and total sedimentation rates on Montserrat have been estimated as 0.17 km 3 ka − 1 , 0.0125 km 3 ka − 1 and 0.11 km 3 ka − 1 (i.e. 1.1 cm ka − 1 ), respectively.

Stress fields associated with the growth of a large shield volcano (La Palma, Canary Islands)

Measurements of dyke orientations and fault slip data have been taken in 39 stations located in the northern part of the island of La Palma (Canary Archipelago). These structures affect the materials belonging to the submarine growth stage and to the lower units of the subaerial shield volcano stage. Four sets of dykes have been distinguished (Groups I, II, III‐1, and III‐2). The two younger dyke groups (III‐1 and III‐2), with highly variable strikes, are clearly related to the subaerial growth stage. Most of the measured faults are normal. Field relationships and the use of stress inversion methods allowed us to separate three different stress tensors. Tensor 2 is associated with the dykes of Group III‐1. Given the spatial distribution of the stations with faults of tensor 2 and dykes of its coeval Group III‐1, it has been possible to obtain a map of trajectories of the maximum (SH) and minimum (Sh) compressive stress axes in the horizontal. Comparison between this map and the results of theoretical models suggests that the stress field active during, at least, the early stages of growth of the shield volcano in La Palma (from near 1.7 Myr to, at least, 1.2 Myr ago), includes a compressive radial (volcanic) component plus a regional tensile stress of a tectonic origin. This second component might be the imprint in the Atlantic oceanic lithosphere of the Cenozoic convergence between the African and Eurasian plates.

The history of sediment flux to Atchafalaya Bay, Louisiana

Abstract The Atchafalaya became a free-flowing river system in the mid-nineteenth century. It progressively increased its diversion of waters from the Mississippi until, by the mid-twentieth century, it threatened to capture the total flow of the main river. The installation of control structures has, temporarily at least, prevented total diversion, limiting the flows to 30% of the Mississippi; water discharge and sediment transport into the Atchafalaya have led to deposition throughout the basin. A major system of lakes and bayous has become largely filled with deltaic sands and layered silts, so that the waters currently drain almost directly into the Atchafalaya Bay. The natural outlet and one artificially created outlet each carry waters from the system. As a result of major flooding in 1973, sediment was flushed from the Atchafalaya Basin and began the subaerial emergence of deltas, which had been slowly building below the bay water surface since the 1950s. The two deltas have provided, parallel growth one largely in natural form, and the other substantially modified by the dredging needed to maintain a navigable routeway to Morgan City and the interior of the basin. The sub-aerial deltas initially grew rapidly, a process apparently slowing, but the sediment transported to the depositional areas has not diminished, rather the area of deposition within the receptor basin is increasing, so that apparent growth inevitably slows. By 1994 the two deltas occupied an area of more than 153 km 2 above the −0.6 m measuring datum, created in approximately 30 years. In each case the sediments show upward passage from fine clays to laminated silts and fine sands, within a succession rarely more that 3 m in thickness, as defined by the original 1890 bathymetry of the Bay. Within the Bay itself resuspension of deposited fine sediment due to wave activity and water set-up and set-down in response to variations in the directions of coastal winds creates plumes of material that travel through the basin and may leave to form suspended sediment plumes on the inner parts of the continental shelf. A total of 20–25 such plumes annually drift initially east of the Bay outlet but later move west towards the chenier coasts of western Louisiana and Texas.

Sedimentology of the Triassic-Jurassic boundary beds in Pinhay Bay (Devon, SW England)

New exposures in Pinhay Bay (SE Devon) of the White Lias (Langport Member of the Lilstock Formation) and basal Blue Lias reveal rapidly changing palaeoenvironments during the Triassic-Jurassic (T- J) boundary interval. During deposition of the topmost White Lias a soft seafloor of micritic mudstone was lithified and bored. The resultant hardground was locally eroded, probably in a shallow marine setting, to form a spectacular intraformational conglomerate that was itself lithified. Brief subaerial emergence then followed and produced a fissured and pitted top surface to the White Lias. The regression was short lived and rapid transgression at the base of the Blue Lias established organic-rich shale deposition with a small framboidal pyrite population and low Th/U ratios indicative of a stable, sulphidic lower water column (euxinic conditions). The White Lias/Blue Lias contact thus records a short duration, high amplitude relative sea-level change. This sea-level oscillation has also been postulated for other T -J boundary sections in Europe, although the failure to identify it in regional-scale sequence stratigraphic studies is probably due to its brief duration. Deposition of the basal beds of the Blue Lias was marked by a discrete phase of syn-sedimentary folding and small growth fault activity that may record a regional pulse of extensional tectonic activity.

Computer simulations of tsunamis due to sector collapse at Stromboli, Italy

Stromboli is an island volcano of the Aeolian Volcanic Arc, characterised by persistent activity. The cone rises about 2500–3000 m from its submarine base with very steep slopes; its summit at 924 m above the sea level. The subaerial growth of Stromboli, occurred in the last 100 ka, has been marked by repeated episodes of large gravitational collapses especially affecting the NW flank of the island in the last 13 ka. The last one occurred less than 5000 years ago forming the deep depression on the NW seaward flank, named Sciara del Fuoco (SdF), and it produced very likely large water waves. This paper envisages a scenario where a huge mass of volcanic material collapses into the sea in the same sector in which the SdF collapse took place in Holocenic times and it computes possible tsunami evolutions assuming the present-day bathymetry. Numerical simulations are performed by means of two distinct models; one for the mass collapse and one for the tsunami. Slope failure dynamics are calculated with the aid of a Lagrangian model: the landslide is subdivided into blocks, and the motion of each constituent block is calculated by applying the basic principle of mechanical momentum conservation, with block–block and block–ambient interactions being taken into account. Water waves are computed by solving a system of shallow-water equations including a forcing term dependent on the sliding mass motion. The finite-element (FE) technique is employed since it permits the use of non-uniform grids, which are adequate to account for marine basins with irregular coastlines. In addition to the sensitivity analysis concerning the main parameters governing the slide motion, two main cases are explored, differing in the slide path followed by the mass. The resulting tsunami is very large, with giant waves as high as several meters (tens of meters in the worst cases) impinging the coast. Due to the strong wave refraction induced by bathymetry, waves travel around the island, affecting even the island coast opposite the source.

Origin and evolution of a submarine large igneous province: the Kerguelen Plateau and Broken Ridge, southern Indian Ocean

Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998–99) and four from Legs 119 and 120 (1987–88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at ∼110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatile-rich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet–biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet–biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate.

The sub-basaltic surface in northeast Ireland and its significance for interpreting the Tertiary history of the region

In northeast Ireland a striking unconformity exists between late Cretaceous, marine limestones (Ulster White Limestone Formation) and Paleocene, subaerially emplaced, basalt flows (Antrim Lava Group) above. The well-cemented nature of the Ulster White Limestone has been attributed to thermal or loading effects associated with the Antrim Lava Group. However, the geomorphology of the palaeokarst beneath can be used to demonstrate that both cementation and joint emplacement in the limestone significantly preceded burial beneath the basalts. A thin, irregularly developed palaeosol, dominated by red-brown or grey clay above a relatively thin basal lag of flint debris, is present on the palaeokarst surface. It has been interpreted previously as a Terra Rossa, derived from the accumulation of insoluble residues remaining after limestone dissolution. However, the colour, mineralogy and geochemistry of the clay, and the low proportion of flints to clay, indicate instead that the clay fraction was derived from ash falls, usually basaltic but occasionally rhyolitic, prior to the first lava flows. In the long term the survival of the Permian and Mesozoic sediments in northeastern Ireland cannot be ascribed solely to the supposed ‘carapace’ effect of the Tertiary basalts, with continued basin subsidence from Mesozoic into Tertiary times being of greater significance. However, it did play a role in the shorter term. Variation in the preserved thickness of limestone beneath the basalts at any one site was controlled by three factors; original thickness of the limestone, its elevation in Paleocene times, and the time elapsed between subaerial emergence and burial by the advancing lava flows. Preservation of significant thicknesses of Ulster White Limestone in the western part of the Antrim Plateau, where previous work has indicated that the onset of volcanism was later than in the east, suggests that limestone uplift preceded lava emplacement by perhaps only 10 4 -10 5 years and was due to local thermal doming on a scale of only a few tens of kilometres. Tertiary subsidence must also be invoked for the Kingscourt half-graben further to the south, where friable, autobrecciated basalts overlie a spectacular palaeokarst surface developed on Permian gypsum.

Tide-Dominated Estuarine Facies in the Hollin and Napo (T"and "U") Formations (Cretaceous), Sacha Field, Oriente Basin, Ecuador"

The article by Shanmugam et al. (2000) is a welcome addition to the scant sedimentological literature on the Oriente Basin and its continuation in Colombia, the Putumayo Basin (Higgs, 1997a). Shanmugam et al. (2000) reinterpreted the Hollin and Napo T-U sands as tide-dominated estuarine deposits, rejecting a previous deltaic model. Neither model, however, accords with the stated lack of evidence for subaerial emergence. An alternative, tidal shelf interpretation is promoted here, and petroleum reservoir implications discussed. Also, although Shanmugam et al. (2000) did not report any sequence boundaries, their data suggest that angular (tectonic) sequence boundaries occur at the base of the A and B limestones, signifying possible incised valleys cutting into the U and T sands, again important for oil exploration. ### Lack of Emergence Indicators The 516 ft (157 m) of cores from seven wells examined by Shanmugam et al. (2000) show a simple facies association of shales, heterolithics, and cross-bedded sands. [N]o evidence for subaerial exposure exists (Shanmugam et al., 2000, p. 674); for example, rooting is lacking (Shanmugam et al., 2000, p. 660) and no coal beds, paleosoils, or desiccation cracks were reported. Sparse interpreted marsh deposits (<1% of one 58 ft core) (Shanmugam et al., 2000, table 2) are based on nondiagnostic interpretation criteria, that is, sandstone with intervals of concentrated carbonaceous fragments … may be interpreted as a marsh environment (Shanmugam et al., 2000, p. 660). Alternatively, such plant-flake concentrations could occur on a shelf because of tidal-current segregation of particles. An interpreted fluvial channel is based solely on cross-stratification and basal lags (Shanmugam et al., 2000, p. 656), both of which can also apply to submarine tidal bars (e.g., Stride et al., 1982, figure 5.22; Dalrymple, 1992, figure 29C). The same facies association and absence or scarcity of emergence indicators characterizes the Hollin and Napo T-U …

Silurian marginal marine sedimentation and the anatomy of the marine-Old Red Sandstone transition in Pembrokeshire, SW Wales

Abstract Deposition of the Silurian (Wenlock) siliciclastic Gray Sandstone Group of southwest Pembrokeshire took place within littoral environments close to the palaeogeographical shelf-margin of the Welsh Basin. Sedimentation described a northerly, basinward progradation across an earlier Late Ordovician? to Aeronian rift basin. Changes in relative sea level (rsl) had profound effects on depositional environments, and five depositional sequences are recognized. During highstands of rsl, the area was influenced by wave-dominated, shallow-marine conditions. During lowstands of rsl, shelf incision and sediment bypass occurred. Associated valley fills vary in nature from high-sinuosity estuarine channels, tidal flats and tidally-influenced, high-sinuosity fluvial channels. The last of these predominate within the youngest sequence, with abundant subaerial emergence indicators heralding the onset of true continental deposition, and the conformable transition into the overlying Old Red Sandstone Red Cliff Formation within the Marloes Peninsula. A renewal of tectonic activity ensued within the Lower Old Red Sandstone, with pebbly low-sinuosity alluvium of the Albion Sands Formation, and fanglomerates of the Lindsway Bay Formation refecting reactivation of earlier rift-margin faults, probably within a transtensional tectonic regime.

The emergence of a Galápagos shield volcano, Roca Redonda

Roca Redonda volcano is a mostly submarine shield volcano that rises nearly 3 km from the adjacent seafloor. Over twenty lava flows and palagonite tuff are exposed in a 60 meter high oblong outcrop above sea level, and several other flows are exposed in the shallow water surrounding the islet. Thick, slightly alkaline picritic flows form the base of the section. Thinner picrites interbedded with sparsely porphyritic alkali-olivine basaltic pahoehoe toes characterize the upper section. The subaerial section probably records the filling of a palagonite tuff cone with younger lavas. Numerous fumaroles that may have a magmatic component are present in the shallow ( MgO > 14%, high-Mg basalts with MgO of about 9%, and low-Mg basalts with MgO of about 6%. The Sr and Nd isotopic ratios of the three lava types are within analytical uncertainty. Olivine compositions indicate that the picrites are basaltic liquids that have accumulated olivine whose composition is in equilibrium with the host basaltic liquid. Apparently, basaltic magmas percolated through dunite and troctolite that had crystallized from slightly older Roca Redonda basaltic magma. Lavas from Roca Redonda have enriched trace element contents and isotopic ratios relative to nearby Wolf volcano, but they are quite similar to lavas from Cerro Azul and Ecuador volcanoes. The common characteristic of these volcanoes is that they lie on the periphery of the archipelago and are in a stage of subaerial growth. This suggests that Galapagos volcanoes may go through a juvenile alkaline stage before a mature tholeiitic stage, analogous to the Loihi stage of Hawaiian volcanism. A low 3He/4He ratio in olivine from one of the picrites indicates a small contribution by the Galapagos mantle plume.

Late Dinantian (Lower Carboniferous) platform carbonate stratigraphy of the Buttevant area North Co. Cork, Ireland

A thick sequence of late Dinantian (Asbian–Brigantian) carbonates crop out in the Buttevant area, North Co. Cork, Ireland. A mud-mound unit of early Asbian age (the Hazelwood Formation) is the oldest unit described in this work. This formation is partly laterally equivalent to, and is overlain by, over 500 m of bedded platform carbonates which belong to the Ballyclogh and Liscarroll Limestone Formations. Four new lithostratigraphic units are described within the platform carbonates: (i) the early Asbian Cecilstown Member and (ii) the late Asbian Dromdowney Member in the Ballyclogh Limestone Formation; (iii) the Brigantian Templemary Member and (iv) the Coolbane Member in the Liscarroll Limestone Formation. The Cecilstown Member consists of cherty packstones and wackestones that are inferred to have been deposited below fair-weather wavebase. This unit overlies and is laterally equivalent to the mud-mound build-up facies of the Hazelwood Formation. The Dromdowney Member is typified by cyclic-bedded kamaenid-rich limestones possessing shell bands, capped by palaeokarst surfaces, with alveolar textures below and shales above these surfaces. The carbonates of this unit were deposited at or just below fair-weather wavebase, the top of each cycle culminated in subaerial emergence. The Templemary Member consists of cyclic alternations of subtidal crinoidal limestones capped by subtidal lagoonal crinoid-poor, peloidal limestones possessing coral thickets. Intraclastic cherty packstones and wackestones characterize the Coolbane Member, which is inferred to have been deposited below fair-weather wavebase but above storm wavebase. The early Asbian Cecilstown Member has a relatively sparse micro- and macrofauna, typified by scattered Siphonodendron thickets, archaediscids at angulatus stage and common Vissariotaxis. Conversely, macro- and microfauna is abundant in the late Asbian Dromdowney Member. Typical late Asbian macrofossils include the coral Dibunophyllum bipartitum and the brachiopod Davidsonina septosa. The base of the late Asbian (Cf6γ Subzone) is recognized by the first appearance of the foraminifers Cribrostomum lecompteii, Koskinobigenerina and the alga Ungdarella. The Cf6γ Subzone can be subdivided into two biostratigraphic divisions, Cf6γ1 and Cf6γ2, that can be correlated throughout Ireland. Relatively common gigantoproductid brachiopods and the coral Lonsdaleia duplicata occur in the Brigantian units. The base of the Brigantian stage (Cf6δ Subzone) is marked by an increase in the abundance of stellate archaediscids, the presence of Saccamminopsis-rich horizons, Loeblichia paraammonoides, Howchinia bradyana and the rarity of Koninckopora species. Changes in facies at the Cecilstown/Dromdowney Member and the Ballyclogh/Liscarroll Formation boundaries coincide closely with the changes in fossil assemblages that correspond to the early/late Asbian and the Asbian/Brigantian boundaries. These facies changes are believed to reflect major changes in relative sea-level on the Irish platforms. The sea-level variations that are inferred to have caused the facies changes at lithostratigraphic boundaries also brought in the new taxa that define biostratigraphic boundaries. Moreover, many of the Dinantian stage boundaries that are defined biostratigraphically in Great Britain, Belgium and the Russian Platform also coincide with major facies boundaries caused by regressive and transgressive episodes. The integration of detailed biostratigraphic analyses with facies studies will lead to better stratigraphic correlations of Dinantian rocks in northwest Europe. © 1997 John Wiley & Sons, Ltd.

Deposition and diagenesis of the windward reef of Enewetak Atoll

Six cores from the northeast reef of Enewetak Atoll reveal that 25 to 50 ft (7.5 to 15 m) of Holocene deposits unconformably overlie at least 200 ft (60 m) of Pleistocene limestone. The Holocene-Pleistocene boundary and four other unconformities within the Pleistocene section are distinguished by paleosols. The unconformities record repeated periods of subaerial emergence and death of the Pleistocene reef associated with low sea-level stands. During emergent periods, meteoric-water diagenesis altered the Pleistocene section. The resulting mineralogy and diagenetic textures are characteristic of the meteoric-vadose diagenetic environment. Aragonite generally composes from 30 to 70% by weight of the Pleistocene limestone and is absent only locally; magnesium calcite is not present. Cementation by calcite is variable but generally increases with depth. Much of the section is poorly cemented and retains high primary and secondary porosity. Micrite and mixed micrite-microspar are the most abundant cements throughout the section. They occur as meniscus, uneven, and irregularly distributed (“patchy”) pore-fill cement. In the upper parts of depositional units, the following paleosol-associated cements are common: caliche crusts, rhizocretions, needle-fiber cement, and lath-crystal cement. Phreatic-type cements are present, but generally are unimportant volumetrically. A reconstruction of the Pleistocene paleohydrology of the atoll indicates that the section has occupied the meteoric-vadose environment during emergent periods. The similarity of Pleistocene limestones beneath Enewetak and other Pacific atolls studied by drilling suggests that for many atolls, most alteration of shallow subsurface Pleistocene limestones has occurred in the meteoric-vadose diagenetic environment.

Facies Development of Atchafalaya Delta, Louisiana: A Modern Bayhead Delta

Early studies of Atchafalaya delta focused on large-scale characteristics of accretion in Atchafalaya Bay. This study concentrates on sedimentation processes and stratigraphic as well as plan-view facies development in a rapidly prograding bayhead delta. Since initiation of subaqueous delta growth (early 1950s), the bay has become shallower with accumulation of progressively coarser grained sediment. Stratigraphic relationships indicate a buildup from the pre-1950 bay bottom through prodelta, distal bar, distributary-mouth bar, levee, back-bar algal flat, and channel-fill deposits. At the present stage of delta development, two distinct processes of subaerial growth are recognizable. Initial growth relates to channel extension, bifurcation, and development of sinuous overbank channels. A later, more mature response of lobe fusion and upstream lobe growth becomes dominant as progradation through bifurcation diminishes in older areas of the delta. Atchafalaya delta growth to 1982 serves as a well-documented sedimentary example for understanding bayhead deltas, which commonly form in protected environments.

Wetland loss and the subdelta life cycle

The rapid deterioration of marsh habitat observed during recent years in the modern Mississippi River Delta is a consequence, at least in part, of the natural life cycle of subdeltas. With life spans typically less than 200 years, subdeltas or bay-fill deposits are scaled-down versions of major delta lobes, yet provide, through pulses of sediment, nearly all the subaerial land in an active delta. Using maps, charts, and aerial photographs, curves were constructed for rates of change in land area, sediment volume, and linear progradation in the four subdeltas that have formed on the modern Mississippi River Delta since the first accurate survey in 1838. Results indicate that each subdelta (1) lasted for approximately 115–175 years, (2) included both periods of growth and deterioration, (3) was initiated by a crevasse or break in the natural levee system, (4) showed linear advancement and volumetric growth during subaerial deterioration, and (5) displayed a new pulse of subaerial growth during the high discharge decade of the 1970s. Contrary to popular accounts, demise of the Mississippi River Delta through deterioration of its subdeltas is not a result of the construction of artificial levees upstream or discharge of sediment off the continental shelf edge. Rather, it is attributable to a substantial decrease and fining of sediments being transported downstream to depositional sites within a delta that has developed, through natural processes, a complex and inefficient channel network for delivering these sediments.

Syntaxial overgrowths in muddy crinoidal limestones: cathodoluminescence sheds new light on an old problem

ABSTRACTLower Carboniferous shallow water limestones of Asbian and Brigantian age in Britain commonly contain abundant interparticle micrite and characteristically display large syntaxial calcite overgrowths on crinoid and echinoid grains. These overgrowths appear to have developed at the expense of the micrite and are widely regarded as neomorphic replacements. However, cathodoluminescence of these has revealed growth features which indicate that they are not neomorphic but originated as passive cement fills of solution voids surrounding echinoderm grains.We introduce the term solution corona for these grain selective voids and consider that three processes may have contributed to their development, namely: high‐Mg calcite stabilization of host grains, crystal ripening, and meteoric dissolution. Cyclic subaerial emergence was a critical factor in these processes, and we contrast the morphology of the overgrowths in question with forms produced in basinal limestones which never experienced comparable early meteoric conditions.These early‐formed solution coronas around echinoderm grains are therefore a useful indicator of meteoric diagenesis and have important implications for porosity evolution. The syntaxial cements which fill the solution coronas show distinct phases of growth in cathodoluminescence which reflect a progression from near‐surface meteoric conditions to deep burial with pressure solution.