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1.
The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45 Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO2-rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume.  相似文献   

2.
Oceanic islands – such as the Azores in the mid‐North Atlantic – are periodically exposed to large storms that often remobilize and transport marine sediments along coastlines, and into deeper environments. Such disruptive events create deposits – denominated tempestites – whose characteristics reflect the highly dynamic environment in which they were formed. Tempestites from oceanic islands, however, are seldom described in the literature and little is known about storm‐related sediment dynamics affecting oceanic island shelves. Therefore, the geological record of tempestite deposits at oceanic islands can provide invaluable information on the processes of sediment remobilization, transport and deposition taking place on insular shelves during and after major storms. In Santa Maria Island (Azores), a sequence of Neogene tempestite deposits was incorporated in the island edifice by the ongoing volcanic activity (thus preserved) and later exposed through uplift and erosion. Because it was overlain by a contemporary coastal lava delta, the water depth at the time of deposition could be inferred, constituting an excellent case‐study to gain insight on the still enigmatic processes of insular shelf deposition. Sedimentological, palaeontological, petrographic and palaeo‐water depth information allowed the reconstruction of the depositional environment of these sediments. The sequence typifies the characteristics of a tempestite (or successive tempestites) formed at ca 50 m depth, in a steep, energetic open insular shelf, and with evidence for massive sediment remobilization from the nearshore to the middle or outer shelf. The authors claim that cross‐shelf transport induced by storm events is the main process of sediment deposition acting on steep and narrow shelves subjected to high‐energetic environments, such as the insular shelves of open‐sea volcanic islands.  相似文献   

3.
The presence of decapod crustaceans in the Pliocene and Pleistocene (MIS 5e) fossil record of Santa Maria Island (Azores Archipelago) is herein reviewed. Our study raises the number of fossil decapods from this island from one species to 10 taxa (three for the Pliocene and seven for the Last Interglacial). Four of these 10 taxa are reported for the first time in the fossil record, worldwide. A new species of a mud shrimp is also described (Upogebia azorensis n. sp.). Our study suggests that the Plio–Pleistocene decapod assemblages of the Azores did not differ significantly from modern ones, being dominated by species that are today widespread across the Webbnesia ecoregion, the Mediterranean Sea, and the eastern Atlantic shores, including the Azores. As far as can be judged from the limited fossil record, apparently no tropical crab species with a Cabo Verdean/Senegalese provenance reached the Azores during windows of opportunity associated with Glacial Termination 2 or with the initial setting of the Last Interglacial period. This contribution increases the total number of marine taxa reported for the Pliocene and Pleistocene outcrops of Santa Maria Island to 218 and 155, respectively, highlighting the scientific relevance of its palaeontological heritage.  相似文献   

4.
Present estimates of the age of volcanic activity in the eastern Azores group are based on shelly fauna in a coquina zone separating the two basaltic sequences on Santa Maria. A Vindobonian (Middle Miocene) age has been assigned to the coquina zone. Our K-Ar data suggests that it was deposited 4–6 m.y. ago, i.e. Mio-Pliocene in age.The basaltic series exposed below the coquina zone on Santa Maria is 6–8 m.y. old and probably much older. The post-coquina basaltic complex is 4 m.y. old and younger. The Nordeste basaltic complex in the NE part of São Miguel and the basaltic flows of the Formigas Bank are also 4 m.y. old and younger. We suggest that the three latter basaltic sequences are chronostratigraphic equivalents.  相似文献   

5.
The petrology and geochemistry of the Azores Islands   总被引：7，自引：0，他引：7  
Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.  相似文献   

6.
Groundwater salinization in the Azores archipelago (Portugal)   总被引：1，自引：1，他引：0  
Groundwater salinization in coastal regions causes severe constraints to water supply and economic losses to society worldwide. In the Azores archipelago, groundwater abstraction in wells drilled in coastal aquifers is very important for water supply, and quality problems have been reported. Therefore, a groundwater chemistry dataset from wells was compiled to study groundwater salinization in these aquifers. Waters are mainly of the Na–Cl type, presenting a slightly acidic to slightly alkaline character, with a pH between 5.63 and 8.50 (median 7.40). Electrical conductivity measurements range from 127 to 9,670 μS/cm (median 862), suggesting highly variable mineralization, with higher values observed on Santa Maria, São Miguel, Pico, Graciosa, and São Jorge islands. The major-ion composition reflects the contribution of seawater to the groundwater compositional evolution, which is essentially explained by seawater intrusion into wells. In many samples, exchange reactions of Na+ + K+ for Ca2+ + Mg2+ are associated with salinization. The seawater fraction in groundwater composition reaches a maximum of 22.5%. These results are extremely challenging to water managers in the Azores because failure to comply with national water quality regulations and with European Union groundwater directive requirements often occurs.  相似文献   

7.
X-ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal-A), (2) cristobalitic silica (opal-CT), and (3) microcrystalline quartz. Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal-CT zone, the d(101)-spacing of opal-CT decreases in a smooth non-linear fashion from about 4 10 Å to 4-04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal-CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal-CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients. Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal-CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal-CT and quartz, extrapolated experimental quartz-water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal-CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method. Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.  相似文献   

8.
Marine shelf strata of the Quinault Formation reflect the influences of storm–flood processes and convergent margin tectonism on sedimentation and palaeocommunity distributions in an active forearc basin of Early Pliocene age, western Washington, USA. The sedimentologic, ichnologic and invertebrate megafaunal character of coastal sea cliff exposures in the Pratt Cliff–Duck Creek area, Quinault Indian Nation, reveal five different sedimentary facies – scoured, Rosselia, bioturbated, mixed and Acharax. These facies document the shifting interplay and intensities among storms, waves and river‐flood plumes during transgression in inner to mid‐shelf settings. Storm sedimentation on the inner shelf is recorded north of Pratt Cliff by amalgamated, proximal tempestites of the scoured facies, which grade up‐section to thick deposits of hummocky cross‐stratified sandstone, indicative of strong wave influences. These hummocky beds alternate, in metre‐scale packages, with banded mudstone and siltstone that have distinctive sedimentologic and ichnofaunal characteristics (Rosselia facies). In particular the mudstone and siltstone occur as 1–15 cm‐thick, rhythmic, parallel beds that are laterally continuous, internally homogeneous to faintly laminated, and thus similar in nature to fine‐grained, oceanic flood deposits reported from shelf settings offshore the modern Eel River, northern California. The Quinault flood deposits are dominated by the ubiquitous trace fossil Rosselia socialis, comprising vertical, mud‐packed, flaring burrows with a sand‐filled central shaft which has been inferred as the feeding‐dwelling structure of a vermiform invertebrate adapted to high sedimentation rates in inner‐shelf settings. Fairweather conditions in between the higher energy periods of storms, waves and floods are recorded north of Pratt Cliff by the mixed facies, which is interpreted as representing the sand and mud zone of the inner‐ to mid‐shelf transition. Quieter, deeper, mid‐shelf, fairweather settings are typified by the bioturbated facies south of Pratt Cliff, where lower sedimentation rates and lower physical energies produced extensively bioturbated deposits of sandy siltstone punctuated, in places, by isolated sandy beds of distal tempestites. Quinault strata also chronicle stratigraphic signatures of subduction of the Juan de Fuca plate beneath western Washington during the Pliocene. For example, the imprint of geochemically unusual authigenic carbonates and a chemosynthetic palaeocommunity (Acharax facies) have been interpreted as a methane seep on the Quinault seafloor. Furthermore, a mobile rockground epifauna of pholadid bivalves became established on abundant, dark mudstone cobbles and pebbles sourced from the Hoh Assemblage, a Miocene accretionary prism that was actively deforming as well as interacting with Quinault forearc sediments during the Pliocene. Hoh mudstone clasts were supplied to the Quinault shelf via seafloor‐piercing diapirs and eroding mélange shear zones, exposures of which today occur in fault contact with Quinault strata along the coast from Taholah to the Raft River.  相似文献   

9.
Sedimentary rocks are rarely preserved on reefless volcanic oceanic islands because their sediments are mostly exported from coastal areas towards the deep sea and such islands typically undergo subsidence. In contrast, the exceptional geological record of the uplifted Santa Maria Island (Azores) provides a unique opportunity to gain insight on such coastal systems. This study focuses on a locality at Ponta do Cedro (eastern Santa Maria Island), which features a series of marine fossiliferous sediments wedged between steep lava deltas. As demonstrated by local structure, these sediments correspond to clinoforms deposited on the steep submarine slope of an active volcanic island, implying transport from shallow waters to greater depths and subsequent colonization by benthic communities. Rapid volcanic progradation eventually sealed the deposits, allowing for their preservation and providing a rare snapshot of the ecology during those intervals, in addition to insights on sedimentary dynamics along submarine island slopes. This study reveals spatial relationships between wedges of sedimentary bodies encapsulated by lavas in the Ponta do Cedro section, and interprets depositional processes preserved in those strata based on sedimentological and palaeontological data. The dynamics of the environment are mostly related to relative sea-level changes, intense volcanic activity and regional uplift during the Neogene.  相似文献   

10.
Core 2011804‐0010 from easternmost Lancaster Sound provides important insights into deglacial timing and style at the marine margin of the NE Laurentide Ice Sheet (LIS). Spanning 13.2–11.0 cal. ka BP and investigated for ice‐rafted debris (IRD), foraminifera, biogenic silica and total organic carbon, the stratigraphy comprises a lithofacies progression from proximal grounding line and sub‐ice shelf environments to open glaciomarine deposition; a sequence similar to deposits from Antarctic ice shelves. These results are the first marine evidence of a former ice shelf in the eastern Northwest Passage and are consistent with a preceding phase of ice streaming in eastern Lancaster Sound. Initial glacial float‐off and retreat occurred >13.2 cal. ka BP, followed by formation of an extensive deglacial ice shelf during the Younger Dryas, which acted to stabilize the retreating margin of the NE LIS until 12.5 cal. ka BP. IRD analyses of sub‐ice shelf facies indicate initial high input from source areas on northern Baffin Island delivered to Lancaster Sound by a tributary ice stream in Admiralty Inlet. After ice shelf break‐up, Bylot Island became the dominant source area. Foraminifera are dominated by characteristic ice‐proximal glaciomarine benthics (Cassidulina reniforme, Elphidium excavatum f. clavata), complemented by advected Atlantic water (Cassidulina neoteretis, Neogloboquadrina pachyderma) and enhanced current indicators (Lobatula lobatula). The biostratigraphy further supports the ice shelf model, with advection of sparse faunas beneath the ice shelf, followed by increased productivity under open water glaciomarine conditions. The absence of Holocene sediments in the core suggests that the uppermost deposits were removed, most likely due to mass transport resulting from the site's proximity to modern tidewater glacier margins. Collectively, this study presents important new constraints on the deglacial behaviour of the NE Laurentide Ice Sheet, with implications for past ice sheet stability, ice‐rafted sediment delivery, and ice−ocean interactions in this complex archipelago setting.  相似文献   

11.
The Middle and Late Triassic Santa Maria Basin, exposed in southernmost Brazil, and Waterberg Basin, in Namibia, are herein interpreted as part of en échelon small basins in southern West Gondwana. The main structures are the Waterberg-Omaruru Fault which can be connected to a NW-strike anastomosed fault zone in Brazil. Based on field structural and stratigraphic analysis two populations of NW-strike fractures, named A-type and B-type, are recognized. A-type fractures (Az = 280°-290°) occur in the Sanga do Cabral Supersequence, underlying units of Santa Maria Basin, as extension of the Waterberg-Omaruru Fault during the Early Triassic. B-type fractures (Az = 295°-345°) are observed in all studied stratigraphic units, from the Triassic Sanga do Cabral Supersequence to the Early Cretaceous Botucatu/Serra Geral formations. Based on the structural analysis we propose that NNE-SSW extension reactivated structures of the Damara Belt, Namibia, with a propagation towards Rio Grande do Sul State forming an anastomosing normal fault system and related-rift basin by Early-Middle Triassic time. The A-type fractures were preferentially active by this phase and the B-type ones are interpreted as secondary link segments within the anastomosing system. During the Gondwana break-up and South Atlantic opening (rift to proto-oceanic phase, Aptian) the B-type fractures were reactivated as normal faults by N50°E-S50°W extension.  相似文献   

12.
The absence of human occupation sites in southeastern Brazil during the mid Holocene has been referred to as the 'Archaic Gap' (8970–1940 cal. a BP) and is predicted to have resulted from increased aridity. A ca. 14 000 cal. a pollen history from two well‐dated lake sediment cores located within the archeological district of Lagoa Santa, in the State of Minas Gerais, southeastern Brazil, was used to test this hypothesis. Our analyses indicated that the present cerrado and tropical semi‐deciduous forest mosaic persisted throughout the mid Holocene, until ca. 5500 cal. a BP, and the Lagoa Santa region did not experience especially dry conditions during the Holocene period. The early Holocene pollen spectra contained an assemblage of cold‐adapted taxa such as Podocarpus, Myrsine and Araucaria, co‐occurring with taxa from cerrado, e.g. Caryocar. A replacement of cold taxa by the modern cerrado–semi‐deciduous forest vegetation took place progressively, but appears to have been completed by the mid Holocene. No evidence of sustained drought was found in sedimentation or forest composition, nor any prolonged dry event in the study region. Holocene dryness as an explanation for the abandonment of Lagoa Santa region is not supported by the palynological analyses conducted in this study. Rather it is suggested that unpredictable climate may have underlain that abandonment. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
The Lower–Middle Albian coaly clay bed of the Escucha Formation, which is exposed at Rubielos de Mora (eastern Iberian Ranges, Spain), contains a diverse fossil plant assemblage. Among the taxa present in this layer, Mirovia gothanii Gomez sp. nov. differs from other species of the genus by its greater leaf length, margins typically overhanging the depressed stomatal groove, a single short, blunt, papilla borne by each subsidiary cell, non-stomatal cells inside the groove and margins, and a higher number of resin ducts in the mesophyll. Morphological study of the well-preserved cuticles demonstrates that the species also occurs in Lower Cretaceous coals of Santa Maria de Meià (Pyrenees, Spain) where Gothan (1954) described it as Sciadopitytes sp. Both localities constitute the southernmost extent of the genus in Laurasia when the family was likely to have reached its climax in terms of abundance and diversity.  相似文献   

14.
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15.
Nummulites, a particularly abundant and diverse genus of larger benthonic foraminifera, formed huge accumulations (banks) during the Eocene, which are often good hydrocarbon reservoirs, especially in North Africa. Despite their economical interest, these accumulations are not well‐understood and their origin is still under discussion. Reasons for this debate are the absence of living Nummulites accumulations and the high‐variability of facies, including the size, shape and extension of the banks, which reflect the array of processes controlling sediment production and accumulation. The nummulitic banks near Santa María de Buil, in the Ainsa Basin (South Pyrenean Foreland Basin) are composed of recurrent facies associations within mappable bed units bounded by physical surfaces. The depositional processes that produced the Nummulites deshayesi accumulations are interpreted considering the shape of the banks, the type of bounding surfaces, the distribution of sedimentary textures, Nummulites test shapes and the associated skeletal components within the banks. This integrative analysis indicates that nummulitic banks accumulated from mass flows, with very poor sediment sorting. Textural and compositional differences among banks suggest that globose Nummulites thrived in the shallower part of the mesophotic zone with abundant nummulithoclasts, whereas flat nummulitic forms thrived in deeper mesophotic, clay‐dominated settings. Interbedded with nummulitic banks, coral biostromes and coral mounds bearing Operculina, Discocyclina and Asterocyclina, represent in situ accumulation near the base of the photic zone. Internal waves (waves that propagate along the pycnocline) are thought to be the triggering mechanism for the density flows. Internal waves and induced bottom currents are sporadic but strong enough to bring sediments into suspension. In contrast to surface waves (both fair‐weather and storm), whose impact is strongest near the sea surface and decreases with bathymetry, the impact of internal waves is usually strongest in the mid‐shelf region and weaker in shallow water. This explains the compositional character of the nummulitic banks.  相似文献   

16.
Autostratigraphy is the stratigraphy generated by large‐scale autogenesis, developed based on the full recognition of the non‐equilibrium behaviour of depositional systems in response to steady external forcing. The existing autostratigraphic concepts were derived mostly from studies of river deltas growing during a single rise or fall of base level (or relative sea level). The present study challenges to extend the autostratigraphic framework to the alluvial‐shelf system growing through steady base‐level cycles by two‐dimensional tank experiments. During each experimental run, the base level was changed symmetrically, wherein through cycles, the rise and fall had the same constant rate (|Rbl|) and period (Tbl), and thus the same constant amplitude (Abl), but with no basin tectonism. In total, nine runs with different combinations of |Rbl| and Abl were performed. The experimental results brought the following implications. (i) The shelf‐transiting active depositional system takes non‐equilibrium responses in earlier base‐level cycles, during each of which the system experiences episodes of degradation with base‐level fall. (ii) After the system has sufficiently grown through cycles, non‐equilibrium responses change into equilibrium responses, whereby the shelf‐transiting alluvial system, whether retrogradational or progradational, is free from degradation and continues to aggrade but with a gradually decreasing rate of aggradation. (iii) The alluvial topset river tentatively but autogenically attains a graded state during the falling limb of an intermediate cycle, which separates the earlier degradation‐inclusive and later aggradation‐sustainable cycles. (iv) The number (or duration) of cycles elapsed prior to this phase‐transition is linearly proportional to the amplitude (or the square of the period) of base‐level cycles, with a coefficient defined by the rates of base‐level change and sediment supply. Such a growth pattern does not necessarily hold when considering long‐term tectonic subsidence or uplift. These notions help to understand the stratigraphic architectures of natural alluvial‐shelf systems evolved through base level cycles.  相似文献   

17.
The palaeoceanographic evolution of the SW Svalbard shelf west of Hornsund over the last 14 000 years was reconstructed using benthic foraminiferal assemblages, stable oxygen and carbon isotopes, and grain‐size and ice‐rafted debris data. The results reveal the complexity of the feedbacks influencing the shelf environment: the inflow of Atlantic and Arctic waters (AW and ArW, respectively), and the influence of sea ice and tidewater glaciers. The inflow of subsurface AW onto the shelf gradually increased with the first major intrusion at the end of the Bølling‐Allerød. During the Younger Dryas, the shelf was affected by fresh water originating from sea ice and glacier discharge. Glaciomarine conditions prevailed until the earliest Holocene with the intense deliveries of icebergs and meltwater from retreating glaciers and the occasional penetration of AW onto the shelf. Other major intrusions of AW occurred before and after the Preboreal oscillation (early Holocene), which resulted in more dynamic and open‐water conditions. Between 10.5 and 9.7 cal. ka BP, the shelf environment transformed from glaciomarine to open marine conditions. Between c. 9.7 and 6.1 cal. ka BP the AW advection reached its maximum, resulting in a highly dynamic and productive environment. At c. 6.1 cal. ka BP, the inflow of AW onto the Svalbard shelf decreased due to the intensification of the Greenland Gyre and the subduction of AW under the sea‐ice‐bearing ArW. Bioproductivity decreased over the next c. 5500 years. During the Little Ice Age, bioproductivity increased due to favourable conditions in the marginal sea‐ice zone despite the effects of cooling. The renewed advection of AW after AD 1850 started the climate warming trend observed presently. Our findings show that δ18O can be used to reconstruct the dominances of different water‐masses and, with some caution, as a proxy for the presence of sea ice in frontal areas over the northwestern Eurasian shelves.  相似文献   

18.
Geologic, geomorphic and seismologic data indicate that west of Lake Cachuma the Santa Ynez fault branches into several major W- and NW-trending splay faults. Two of the faults bracket the wedge-shaped Santa Maria basin. The most compelling evidence for the existence of these two faults is the fact that the Santa Maria basin is floored by Franciscan basement overlain only by Miocene and younger sedimentary rocks, whereas across the inferred traces of each of these faults, the adjacent terrains consist of Franciscan basement overlain by thick sequences of Early Tertiary strata, as well as by Miocene and younger rocks. The third splay fault strikes northwestward through the central Santa Maria basin. Narrow zones of tightly appressed, left-stepping en-echelon folds are locally adjacent to the faults along the south edge, and through the center of the basin. The geometrical arrangement of these folds is indicative of formation over buried sinistral wrench faults. Evidence for Holocene surface rupturing is lacking or nebulous at best, but epicenters of damaging historical earthquakes are spatially, and by inference, genetically related to the central Santa Maria basin faults, indicating that they comprise the presently active strands among the several splay faults.  相似文献   

19.
The Australian Permian bivalves Megadesmus, Astartila, Pyramtis, and Myonia have previously been grouped in either the Family Pachydomidae (Fischer, 1887) or the Family Edmondiidae (King, 1850). Because of similarities in their musculature (in addition to those in dentition, micro‐ornament, and to some extent, shape) they can now be included in the same family as the deep‐burrowing bivalve Vacunella (previously Chaenomya). Vacunella is closely related to the living genus Pholadomya which has existed at least since the Mesozoic, and the two can be placed in the same family — Pholadomyidae (Fleming, 1828). Because of these similarities between Pholadomya, Vacunella, and the “Pachydomidae” it seems likely that the middle and upper Palaeozoic edmondioid bivalves gave rise to the Mesozoic and Cainozoic pholadomyoids, and that these two groups together constitute a major division of the Class Bivalvia — a division that may have been relatively distinct since Ordovician time.  相似文献   

20.
Late Quaternary (MIS 3 to Recent) oceanographic evolution of the Basque shelf has been analysed for the first time based on the sedimentological analysis of three cores obtained from the middle and outer shelves. The cores are located in two interfluves separated by the San Sebastian canyon. The variability of the percentage of the planktonic foraminifera species Neogloboquadrina pachyderma sin. and of δ18Obull allowed us to identify the influence of colder and warmer waters in the Basque shelf during the late Quaternary. From ～56 cal. ka BP to the end of the Last Glacial Maximum (19 cal. ka BP) the sedimentary record shows a decreasing trend in the mean grain size that correlates with the eustatic sea‐level fall. The last Deglaciation (19–11.5 cal. ka BP) is characterized by a sea‐level rise that produced an important hiatus in the western outer shelf. During the Holocene, the middle and outer shelves present different behaviours. From 11.5 to 6.7 cal. ka BP, in the outer shelf the sea‐level rise that started during the Deglaciation produced a hiatus, whereas in the middle shelf the sedimentary succession records the presence of warm to temperate waters. Between 6.7–4.9 cal. ka BP, the entrance of cold surface water‐masses that only affected the middle shelf has been identified, and temperate to warm waters occurred in the outer shelf. The cold surface water‐masses retreated during 4.9–4.3 cal. ka BP in the middle shelf. Finally, from 4.3 cal. ka BP to Recent, the middle shelf registers a hiatus due to sea‐level stabilization after a generalized transgression, synchronous to a decrease in the energy of the water‐masses in the outer shelf. In conclusion, the environmental changes detected in the Basque shelf are attributed to both regional and eustatic sea‐level changes.  相似文献   

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