Miocene Submarine Research Articles

Evolution and stratigraphic architecture of marine slope gully complexes: Monterey Formation (Miocene), Gaviota Beach, California

Three small headlands in the sea cliffs west of Gaviota Beach, California, are the remnant fill of three discrete submarine gullies incised into the late Miocene submarine slope environment. These promontories provide excellent, three-dimensional exposure of the gully fill in outcrop, permitting documentation of their complex internal stratigraphic architecture. Detailed study of these exposures elucidates the sedimentologic processes that occur in the filling of slope gullies, guides interpretation of the acoustic records of otherwise unsampled modern gully systems on continental slopes, and provides insight into the heterogeneity that may characterize slope gully petroleum reservoirs. We develop a comprehensive facies scheme to describe the variability within these intercalated coarse- and fine-grained deposits and use two-dimensional photopans to interpret the overall depositional system. Defined by internal bedforms, sedimentation units, and sediment size, each facies records sedimentation under different hydrodynamic conditions and can be genetically related to a discrete depositional mechanism. We differentiate channel axis, margin, and overbank sub-environments within the gully fill, which together define overall crudely braided, low-sinuosity channels within the gullies. Like many modern gully systems, the Gaviota gullies probably initiated through local slope oversteepening that led to slope failure, slumping, and initiation of sediment flows. Erosion and sedimentation from these high-density turbidity currents formed the primary depositional process for the Gaviota conglomerate units. Once initiated, the gullies acted as sediment conduits from the shelf and possibly onshore regions. Despite their prevalence on modern upper slopes and their pivotal role in shaping shelf margins and transporting sediment to deeper water, the details of submarine slope gully formation and filling remain obscure. The Gaviota gully complexes provide valuable insights into long-term gully fill attributes not easily obtained from modern slope gully systems and rarely preserved in the rock record.

A facies model for a submarine volcaniclastic apron: The Miocene Manukau Subgroup, New Zealand

Submarine volcaniclastic aprons of emergent volcanoes provide a long-term record of their history and are preserved long after the source has been eroded following the cessation of volcanism. A facies model for submarine volcaniclastic aprons is necessary to guide interpretation of ancient uplifted successions and to understand the sedimentary and volcanic processes associated with apron formation. The Manukau Subgroup, New Zealand, includes a well-exposed Miocene submarine apron consisting of the products eroded and erupted from a basaltic-andesite volcanic island complex. The depositional environment, setting, and stratigraphy of the volcaniclastic apron are well constrained, allowing us to conduct a detailed facies analysis and to assess the apron architecture. Lateral downslope and vertical progradational sequences reflect erosion of the subaerial edifice and the direct and subsequent products of contemporaneous volcanism. The volcaniclastic facies vary largely as a function of proximity to source and the types of rock resedimented. The dominant transport method was high-density turbidity currents, with lesser contributions from debris avalanches, debris flows, and dilute turbidity currents. In addition, eruptions from volcanic vents located within the apron are recorded as interbedded water-settled and resedimented pumice and scoria deposits (explosive eruptions), lavas, hyaloclastite, and crosscutting dikes (effusive eruptions). The proximal apron is largely composed of coarse breccia-conglomerate that is locally chaotically disrupted due to the instability promoted by steep depositional slopes and rapid sedimentation rates. The subangular cobbles and boulders originated from nearby (probably coastal) variably vesicular and variably oxidized lavas and were deposited as lithic lags. In this highly energetic environment, very little sandstone was deposited or preserved, and freshly erupted pumice deposits were quickly resedimented. In the medial apron, poorly sorted conglomerate and thickly bedded sandstone, which includes pumice lapilli, built up over the distal apron sequence of thinly bedded, fine-grained volcaniclastic mudstone/sandstone. The presence of pumice lapilli suggests that progradation was a response to voluminous explosive eruptions. Due to their low densities and easy entrainment, pumice lapilli largely bypassed deposition on the steep proximal apron and were sedimented onto the more gently sloping medial apron. Hence, coarse pumice clasts were transported much farther from their source than their dense lithic counterparts. The apron records a shallowing-upward sequence, and the final submarine volcaniclastic facies was deposited within channels that incised the medial and distal apron. The channels record an initial eruption hiatus and erosion of the edifice. Well-rounded clasts recycled from the uplifted breccia-conglomerate were initially deposited followed by pumiceous and scoriaceous pebbly sandstone from renewed explosive eruptions. Submarine volcaniclastic aprons involve rapid and prolific sedimentation from multiple source locations, clasts of different densities and origins, and can include the direct products of contemporaneous volcanism. The facies architecture, however, can be clearly divided into proximal, medial, and distal volcaniclastic apron environments. The proximal apron is dominated by the coarsest clasts. Steep slopes, rapid sedimentation rates, and seismic activity in the steep proximal apron promote slumping. The medial and distal aprons are more likely to record explosive activity, debrisflow events, and eruption hiatuses due to the gentler slopes and depositional rather than erosional setting.

A facies model for a submarine volcaniclastic apron: the Miocene Manukau volcanic complex, Northland, New Zealand

The Manukau Subgroup, New Zealand comprises a Miocene submarine volcaniclastic apron consisting of the products eroded and erupted from a large offshore partially emergent basaltic to andesitic volcanic complex. The submarine volcaniclastic apron is well exposed for over 30 km along the coast. The apron records the depositional history of a marine basin at bathyal water depths and the onset of fan progradation due to volcanism. The exceptional exposure, variation in fan architecture (distal, lobe, upper slope and channel), facies (e.g. sandstone, pumice breccias, conglomerates and pillow lavas) and origin (primary, resedimented or epiclastic), allows analysis of a facies model for submarine volcaniclastic fans. The facies analysis shows that submarine volcaniclastic fans differ from other clastic sedimentary fan models in the origin, supply and types of clasts.

The complex facies architecture and emplacement sequence of a Miocene submarine mega-pillow lava flow system, Muriwai, North Island, New Zealand

The early Miocene Waiatarua Formation at Maori Bay, Muriwai, North Island, New Zealand consists of a complex association of basaltic andesite volcanic facies including entablature-jointed thick massive facies, colonnade-jointed thin massive sheet facies, mega-pillow facies, normal pillow facies and minor associated fragmental facies, including vitric sandstone and breccia interpreted as hyaloclastite and peperite. Field observations suggest that the facies, which form the Waiatarua Formation lava, were emplaced as multiple flow lobes in a single lava flow from one sustained eruption. Magma discharge rate at the vent was high; however magma supply rate to the more distal and lateral portions of the flow, as its surface area increased, varied considerably. Higher magma supply rates produced thick, massive and thin sheet facies, whereas mega-pillow facies and normal pillow facies were produced contemporaneously with the thick and thin massive sheet facies but were restricted to portions of the distal flow subjected to lower magma supply rates. The evolution of the Waiatarua formation lava flow at Maori Bay has been reconstructed from the complex facies architecture. This suggests that the propagation of the lava involved 7 discrete lobes that were emplaced successively.

Detachment folding–related Miocene submarine slope instability in the Romagna Apennines (Italy)

The relationships between slope failure and growing thrust‐related structures have been explored in the Miocene Marnoso‐Arenacea foredeep basin (Romagna Apennines). Impressive fossil slump layers are distributed along the footwall of the regional Mount Nero thrust, which evolved from detachment fold to breakthrough thrust. During the detachment fold stage, development of growth strata on the rotating forelimb was preceded by the emplacement of slump layers that are currently part of the Mount Nero thrust footwall. In this process, sliding is assumed to nucleate along the contacts between different lithologies in the semiconsolidated rock mass. The conditions of sliding, specifically the critical angle of sliding αcrit, are quantified by adding the influence of cohesion of sediments to the equilibrium analysis reported in Hubbert and Rubey's (1959) classical work. The resulting angles αcrit are discussed with respect to the depth of the basal surface of sliding and pore fluid pressure states. On the basis of such an analysis and field work results, an evolutionary model is proposed in which slide sheets come from the crest and upper forelimb of growing detachment folds and attain a deeper detachment depth as the forelimb rotates and steepens. Pore fluid pressure is found to play a major role by radically decreasing the critical angle of sliding. Likewise, localization of overpressures may also control the depth of sliding during progressive forelimb rotation. To allow sliding along gently dipping bedding surfaces, slope failure is interpreted as being related to the reactivation of anisotropies represented by lithological discontinuities.

Lithostratigraphy of the Red Sea Region

ABSTRACT The onshore and offshore Saudi Arabian Red Sea region contains a series of lithostratigraphic units that have not previously been formally defined and described. Based on an intensive study of the succession, a lithostratigraphic scheme is proposed in a lexicon format that integrates biostratigraphic, sedimentological, seismic and field studies from the Midyan Peninsula in the north, to the Jizan Coastal Plain in the south. In view of the economic aspect of the Neogene succession and greater accessibility to Neogene subsurface samples, emphasis has been placed on a revision of the Neogene lithostratigraphy. Resting upon Proterozoic Basement, the sedimentary succession was deposited during the Cretaceous to Pleistocene times. The oldest pre-rift Suqah Group nonconformably overlies the Proterozoic Basement and consists of Upper Cretaceous shales of the Adaffa Formation and Cretaceous to Palaeogene sandstones, shales and thin limestones of the Usfan Formation. A series of volcanics includes the early to middle Oligocene Matiyah Formation and the late Oligocene-early Miocene Jizan Group. The Neogene succession displays a great lithological diversity. The Tayran Group (Al Wajh, Musayr and Yanbu Formations) includes marginal marine siliciclastics of the Al Wajh Formation, and represents the earliest rift-associated sediments deposited during the earliest Miocene. These are conformably overlain by lower Miocene shallow-marine carbonates of the Musayr Formation. In some of the central Red Sea onshore basins, thick lower Miocene submarine evaporites of the Yanbu Formation were deposited under locally restricted conditions and form the third formation of the Tayran Group. Rapid subsidence during the early Miocene caused deposition of deep-marine, planktonic-foraminiferal mudstones and thick submarine fan sandstones of the Burqan Formation. Carbonates, marine mudstones and submarine evaporites of the Maqna Group (Jabal Kibrit and Kial Formations) unconformably overlie the Burqan Formation and were deposited during latest early Miocene to earliest middle Miocene. The Jabal Kibrit Formation consists of an anhydrite-carbonate facies, of which the carbonates form the Wadi Waqb Member. Siliciclastic facies of the Jabal Kibrit Formation are termed the Umm Luj Member. Above the Jabal Kibrit Formation, the Kial Formation is typified by interbedded anhydrites, calcareous siltstones and carbonates, and includes the Sidr, Nakhlah, Yuba, Rayaman and Sabya Members. Within the region, thick evaporites of the Mansiyah Formation were deposited extensively during the middle Miocene, and are overlain by poorly exposed sands, shales and thin anhydrite beds of the middle to upper Miocene Ghawwas Formation. The Lisan Group unconformably overlies the Ghawwas Formation and consists of coarse alluvial sands and gravels of possible Pliocene to Holocene age.

Nd, Sr and Pb isotopic and REE geochemical study of some Miocene submarine hydrothermal deposits (Kuroko deposits) in Japan

Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (ɛNd: −4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, 87Sr/86Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (206Pb/204Pb=18.35–18.84, 207Pb/204Pb=15.59–15.97 and 208Pb/204Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.

Morphology and growth style of a Miocene submarine dacite lava dome at Atsumi, northeast Japan

A Miocene submarine lava dome at Tate-iwa, Atsumi, Yamagata, Japan, displays well-preserved primary morphological features. The dome currently rises above a wave-cut coastal platform and is 55 m high and 90–180 m across. It comprises a massive core and a lava lobe–hyaloclastite rim, both of which are composed of compositionally uniform, feldspar–phyric dacite (SiO 2=64 wt.%). The boundary between these two zones is distinct but gradational. The massive core consists of homogeneous, coherent dacite and is characterized by flow banding along the margin and by columnar joints radiating from the centre to the margin. The lava lobe–hyaloclastite rim encircles the massive core and consists of a complex of 80–90% dacitic lava lobes and 10–20% hyaloclastite. The lava lobes are 1–6 m thick, 3–12 m wide and more than 5 m long. Each lobe consists of a radially columnar-jointed core and a glassy rim 10–30 cm thick. The hyaloclastite comprises polyhedral dacite clasts 5–100 cm across in a matrix of dacite fragments up to 5 mm across. The lava lobes and hyaloclastite have gradational contacts in places. Paleobathymetric studies based on foraminifera in sediment beneath the dome, and the K–Ar age of the dome, suggest that it was extruded in a middle-bathyal environment (1500–2000 m below sea level) at 12.9±0.6 Ma. The internal structures of the dome suggest that it formed by a combination of exogenous growth involving extrusions of small dacite lobes, and endogenous growth involving a continuous magma supply and simple expansion from the interior. The presence of small lava lobes along the rim suggests that the magma had relatively low viscosity at the time of extrusion, in spite of its high silica content. Magma temperatures calculated by two-pyroxene and Fe–Ti oxide geothermometers were 999–1042 and 957–1005 °C, respectively. The inferred low viscosity may be attributed to the high temperature of the magma and/or a high confining pressure resulting from the deep-sea environment.

Morphology and propagation styles of Miocene submarine basanite lavas at Stanley, northwestern Tasmania, Australia

Miocene submarine basanite pillows, lava lobes, megapillows and sheet lavas in the Stanley Peninsula, northwestern Tasmania, Australia, are well-preserved in three dimensions. The pillows have ropy wrinkles, transverse wrinkles, symmetrical wrinkles, contraction cracks and three types of spreading cracks on their surfaces, and concentric and radial joints in the interior. The lava lobes have ropy wrinkles and contraction cracks on their surfaces. The megapillows are cylindrical with a smoothly curved upper surface and steep sides, and are characterized by distinct radial columnar joints in the interior. They are connected to pillows that propagate radially from its basal margin. The sheet lavas are tabular and have vertical columnar joints in the interior. The largest sheet lava shows a remarkable gradation from a lower 5-m-thick pillow facies to an upper massive facies. The pillows, lava lobes, megapillows and sheet lavas are inferred to have been emplaced completely below sea level but in a shallow marine environment. Their morphological features suggest that the pillows grew by episodic rupture of a near-solid crust and emergence of hot lava, whereas the lava lobes propagated by continuous stretching of the outer skin at the flow front. The megapillows and sheet lavas were master feeder channels by which molten lava was conveyed to the advancing pillows. The sheet lavas propagated by repeated processes of pillow formation and overriding by an upper massive part. Alternating pillow and massive facies commonly found in ocean-floor drill cores and exposed in cross-section in many subaqueous volcanic successions may have formed by propagation of pillows from the basal margins of advancing sheet lavas.

Absolute palaeointensity results from the Trans-Mexican Volcanic Belt: implications for the late Miocene geomagnetic field strength

From a large collection (238 orientated cores) of Central Mexico Miocene (8–11 Ma) volcanic units sampled for magnetostratigraphy and tectonics, 36 samples were selected for Thellier palaeointensity experiments. Only 14 samples, coming from five individual lava flows, yielded reliable palaeointensity estimates, with the flow‐mean virtual dipole moments (VDMs) ranging from 5.6 to 8.5 × 1022 A m2. Our results, although not numerous, are of high technical quality and are comparable with other palaeointensity data recently obtained on younger lava flows. The NRM fractions used for palaeointensity determination range from 31 to 72 per cent, and the quality factors vary between 4.8 and 17.5, usually being greater than 6. The VDM values obtained are significantly higher than those recently reported from Miocene submarine basaltic glasses but are very similar to the mean reported from the Steens Mountain continental basaltic flows. More data are needed in order to better understand the behaviour of the Miocene geomagnetic field strength and to constrain the transition mode between the Mesozoic low and Neogene high field.

Endogenous growth of a Miocene submarine dacite cryptodome, Rebun Island, Hokkaido, Japan

Momo-iwa, Rebun Island, Hokkaido, Japan, is a dacite cryptodome 200–300 m across and 190 m high. The dome is inferred to have intruded wet, poorly consolidated sediment in a shallow marine environment. The internal structure of the dome is concentric, with a massive core, banded rim, and narrow brecciated border, all of which are composed of compositionally uniform feldspar-phyric dacite. Boundaries between each of the zones are distinct but gradational. The massive core consists of homogeneous coherent (unfractured) dacite and is characterized by radial columnar joints 60–200 cm across. The banded rim encircles the massive core and is 40 m wide. It is characterized by large-scale flow banding parallel to the dome surface. The flow banding comprises alternating partly crystalline and more glassy bands 80–150 cm thick. The outermost brecciated border is up to 80 cm thick, and consists of in situ breccia and blocky peperite. The in situ breccia comprises polyhedral dacite clasts 5–20 cm across and a cogenetic granular matrix. The blocky peperite consists of polyhedral dacite clasts 0.5–2 cm across separated by the host sediment (mudstone). The internal structures of the dome suggest endogenous growth involving a continuous magma supply during a single intrusive phase and simple expansion from the interior. Although much larger, the internal structures of Momo-iwa closely resemble those of lobes in subaqueous felsic lobe-hyaloclastite lavas.

Ichnocoenoses of the Mount Messenger Formation, a Miocene submarine fan system, Taranaki Basin, New Zealand

The late Miocene Mount Messenger Formation along the North Taranaki coast comprises a fining‐upwards succession of interbedded sandstones and mudstones previously interpreted as deposits of a series of submarine fans in bathyal to upper bathyal waters. The sequence was divided into three lithofacies associations: thick‐bedded basin floor fan (BFFTK); thin‐bedded basin floor fan (BFFTN); and proximal to distal channel levee complex (CLC). This study recognises a fourth unit, the thick‐bedded Ophiomorpha sandstone lithofacies association (TBOS), as another depositional environment. Ichnology is an important tool in distinguishing the lithofacies assemblages and interpreting original depositional conditions. Each of the lithofacies associations shows a distinctive relationship between lithologic parameters and ichnofauna and ichnofabric. Four ichnocoenoses are recognised, corresponding well with the four lithofacies assemblages. Ichnocoenose I, dominated by fodinichnia, coincides with BFFTK and falls into the Cruziana ichnofacies. Ichnocoenose II, dominated by fodinichnia, coincides with BFFTN and falls into the Zoophycos ichnofacies. Ichnocoenose HI, dominated by fugichnia, coincides with CLC and falls into the Cruziana ichnofacies. Ichnocoenose IV, dominated by domichnia, coincides with TBOS and falls into the Skolithos ichnofacies. Although ichnocoenoses I and III are classed as Cruziana ichnofacies, they are discrete in trace fossil diversity, density, and ethology. Ichnocoenose IV has a unique, monospecific Ophiomorpha assemblage; together with the well‐sorted sand texture, absence of mud, and thick massive bedding, the TBOS lithofacies appears to reflect a different environmental setting to the Mount Messenger Formation with which it is associated.

Stable isotope geochemistry of clay minerals from fossil and active hydrothermal systems, southwestern Hokkaido, Japan

Miocene submarine to Quaternary terrestrial volcanism in southwestern Hokkaido, Japan, is associated with hydrothermal clay alteration and mineralization, including Kuroko-type deposits at Kagenosawa (14.2 Ma, Cu > Zn, Pb > Au) and Minamishiraoi (12.5 Ma, Ba > Zn, Pb, Cu), vein-style mineralization at Date (5.2 Ma, Au-Ag-Cu-Pb-Zn) and Chitose (3.6 Ma, Au-Ag), and geothermal activity at Noboribetsu (≤1.8 Ma). The δD and δ 18O values of mica (sericite), mica-smectite, chlorite, chlorite-smectite, nacrite, dickite, kaolinite, and smectite were used to deduce the type(s) of hydrothermal fluid at each locality. Calculated compositions for Minamishiraoi and Kagenosawa fluids suggest that seawater was dominant, but some mixing with magmatic water is also indicated, particularly for the polymetallic Kagenosawa deposit. Hydrothermal fluids at Date, Chitose, and the Noboribetsu geothermal area were dominated by meteoric water. Minor involvement of magmatic water during mineralization at Date cannot be ruled out, but evolution of local meteoric water along an evaporation trend and/or an 18O-shift due to hydrothermal rock-meteoric water interaction also could have produced appropriate fluid compositions. The δD and δ 18O values of modern hot-spring waters at Noboribetsu closely parallel fluid compositions calculated for the clay alteration at Date, Chitose, and Noboribetsu. Because relatively poor reproducibility was obtained for the δD values of the swelling clays, additional tests were conducted. Stepwise heating showed that, for some smectitic clays, water evolved between 200 and 300°C had anomalously high δD values because of residual interlayer water. This error can be minimized by sufficiently long preheating (in vacuo) at ≤200°C. In vacuo TG patterns of other smectitic clays suggested gradual loss of hydroxyl-groups beginning near 200°C, rather than the more typical distinct separation between interlayer water at <200°C and hydroxyl-groups at > 400°C. This behaviour constrains the maximum temperature that can be used for in vacuo preheating. Furthermore, shifts to lower δD values (by as much as 19‰) were obtained when this smectite was dispersed in low-D water for three weeks, perhaps indicating isotopic exchange. However, with appropriate care, δD values obtained by conventional procedures (including preheating to ≤200°C) normally reproduced natural compositions of the smectitic clays with acceptable accuracy and precision.

The buried Afiq Canyon (eastern Mediterranean, Israel): a case study of a Tertiary submarine canyon exposed in Late Messinian times

The Afiq submarine canyon was one of a series of canyons initially incised in a drowned shelf edge and slope of the eastern Mediterranean margins in early Oligocene times (P19 zone). During most of the Early Miocene submarine erosion or non-deposition prevailed. This was followed by deposition of pelagic marls and debris flows in early Middle Miocene (N8) times. Large-scale sliding in late Middle Miocene times (N14) resulted in the collapse of the slope, the removal of most of the middle Miocene sequence and the formation of a box-shaped scar. Back-cutting incision ultimately resulted in the incision of the shelf in Late Miocene times. A sea-level fall ranging between 50 and 800 m below the canyon's rim resulted in the deposition of the Messinian Lower Evaporites (Mavqiim Formation) within the canyon. A subsequent rise of a similar extent led to the deposition of the Upper Evaporites (Be'eri Gypsum) which are found on the canyon's southern shoulder. The final Messinian sea-level drop, below the canyon's floor resulted in a subaerial environment in the canyon, the erosion of the upper evaporites and the subsequent deposition of fluvial and brackish sediments (Afiq Formation). The latter correspond to the Lago-Mare sediments known throughout the Mediterranean. The rapidly rising sea level during the Pliocene along with the high rate of Nilotic clastic sedimentation resulted in the final burial of the Afiq Canyon, and a pronounced seaward progradation (20–30 km) of the shelf edge.

The relationship between submarine canyon fill and sea-level change: an example from Middle Miocene offshore Gabon, West Africa

A series of Middle Miocene submarine canyons has been mapped in offshore Gabon, West Africa using a combination of seismic facies analysis and well information. Three distinct seismic facies are recognized in the canyon fill; the evolution of these facies is inferred to be related to sea-level change. During sea-level lowstand, deposition was characterized by lateral accretion on the southern canyon wall whilst progressive erosion occurred on the northern wall. This depositional pattern was controlled by a local coast-parallel current, that flowed northwestwards along West Africa during the Middle Miocene. As relative sea level rose, the erosion on the northern wall ceased and the canyons were filled by sediments showing an aggradational stratal pattern. Finally, during the early part of the transgression, the canyons were filled with horizontal or subhorizontal sedimentary strata. The sedimentary fill is thought to consist of alternating sands and mud within its lower part, but as relative sea level rose, clayey sediments gradually dominated the canyon fill.

Geochemistry of igneous rocks of Shimane Peninsula, formed within a Miocene back-arc rifting zone at the Japan Sea margin.

In the San'in district (back-arc side of the Southwest Japan arc), violent subalkalic magmatism took place related to the opening of the Japan Sea during the Miocene. In the northeastern part of Shimane Peninsula, thick middle Miocene submarine volcanigenic strata crop out. This middle Miocene volcanism took place along with intense subsidence probably related to rifting in a mature arc. In the late Miocene, this volcanism and subsidence were followed by eruption of subaerial alkalic basalt (Matsue basalt). Thirty seven samples of these igneous rocks have been analyzed by XRF and INAA. Middle Miocene igneous activity is divided into three stratigraphic stages, and each stage shows a wide variation in chemistry. Intra-stage chemical variations result from crystal differentiation, whereas variations between Stages II and III rocks are interpreted in terms of various degrees of melting from a common source mantle. Stage I rocks were generated from a more depleted mantle. Most middle Miocene rocks have an arc signature, which resulted from the melting of subarc lithospheric mantle which had been metasomatized by a subduction-related fluid. Associated felsic rocks were formed by magmatic fractionation of basaltic magma.

Provenance of Miocene submarine fans in the northern Adana Basin, southern Turkey: A test of discriminant function analysis

AbstractThe provenance of the deep marine clastics of the Cingöz Formation, which were deposited contemporaneously during the Miocene as two small neighbouring submarine fans (western and eastern fans) in the northern part of the Adana Basin, has been studied by: (1) thin section petrography; (2) the use of the ternary discriminant diagrams (Q‐F‐L, Qm‐F‐Lt) of Dickinson et al. (1983); and (3) the use of discriminant function analysis (DFA) techniques.Thin section petrography confirms that the compositions of sandstones in both the western and eastern fans are broadly similar, but they display subtle differences which are related to their local sources. Both of these submarine fans were ultimately supplied from a source area which contained ophiolitic, sedimentary, metamorphic and some felsic igneous rock types.The point counting results have been plotted on the ternary diagrams of Dickinson et al. (1983). On this basis, most of the samples can be assigned to a magmatic arc provenance, with some recycling. There appears to be no significant difference in provenance between these two fans on the basis of these ternary diagrams.Discriminant function analysis of the same data, utilizing the original Dickinson et al. (1983) data set, reveals some otherwise undetected differences in provenance between the two fans. This test also shows that the DFA of sandstones for petrotectonic purposes should be improved by adding further actual data to the data set originally assembled by Dickinson et al. (1983). Moreover the DFA results confirm that in this instance erroneous petrotectonic inferences can be drawn from the Dickinson et al. ternary diagrams as a result of the complex tectonic history of the source, although clues to this complexity can be discerned in the DFA data, provided that the full set of Dickinson categories is deployed. Taken together with the abundant field data these analyses show that the sands and gravels forming these small Mid‐Miocene submarine fans were supplied to the deep marine Adana Basin via two main separate point sources, together with some minor point sources, all located in the Taurus orogenic belt, on the northern margin of the Adana Basin.

Regional metamorphism and hydrothermal alteration related to Miocene submarine volcanism (‘green tuff’) in the Yurihara oil and gas field, northwest Honshu Island, Japan

Abstract Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin‐fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite‐pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5‐15.5 Ma, determined by planktonic foraminifera assemblages of inter‐bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K‐Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C in zones IIa and IIb.

Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy

Research Article| October 01, 1993 Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy WILLIAM CAVAZZA; WILLIAM CAVAZZA 1Dipartimento di Scienze Mineralogiche, Università di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy Search for other works by this author on: GSW Google Scholar PETER G. DECELLES PETER G. DECELLES 2Department of Geological Sciences, University of Rochester, Rochester, New York 14627 Search for other works by this author on: GSW Google Scholar Author and Article Information WILLIAM CAVAZZA 1Dipartimento di Scienze Mineralogiche, Università di Bologna, Piazza di Porta San Donato 1, 40126 Bologna, Italy PETER G. DECELLES 2Department of Geological Sciences, University of Rochester, Rochester, New York 14627 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (10): 1297–1309. https://doi.org/10.1130/0016-7606(1993)105<1297:GOAMSC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM CAVAZZA, PETER G. DECELLES; Geometry of a Miocene submarine canyon and associated sedimentary facies in southeastern Calabria, southern Italy. GSA Bulletin 1993;; 105 (10): 1297–1309. doi: https://doi.org/10.1130/0016-7606(1993)105<1297:GOAMSC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Outcrops of the Stilo-Capo d'Orlando Formation (latest Oligocene-early Miocene) along the southeastern coast of Calabria (southern Italy) expose a cross section, orthogonal to paleoflow, of the proximal part of a turbidite depositional system. Major erosion surfaces cutting into basement rocks define several submarine paleocanyons. Paleocanyon fills consist of large, lenticular conglomerate bodies that are 200-580 m thick and 3-6 km wide.The best example of these paleocanyons is located near the town of Stilo, where the geometric relationships between paleocanyon fill and adjacent slope deposits are well exposed. At this locality, a 460-m-thick section of conglomerate cuts into the basement and is composed internally of smaller scale channel fills of poorly organized, clast-supported, cobble-to-boulder conglomerate, deposited mainly by high-density turbidity flows. The conglomerate is thickest near Stilo and becomes progressively thinner southward, where a wedge of mudrock intervenes between the basement and the conglomerate. The mudrock wedge is composed of poorly bedded, intensely bioturbated mudstone with slump structures and represents slope deposits. Bedding within the mudrock wedge is better defined in its stratigraphically higher part near the conglomerate body, where thin-bedded, normally graded sandstone layers with climbing ripples, horizontal lamination, and general thickening toward the canyon axis are present. These layers are the deposits of dilute turbidity currents that occasionally spilled over the canyon margins when the depression was nearly filled.The conglomeratic canyon fill and the adjacent muddy slope deposits are both overlain by a laterally continuous sequence, 160 m thick, composed of two units of fine-grained, thin-bedded turbidites alternating with two units of thicker sandstone and minor pebble-conglomerate beds.The paleocanyons probably originated as subaerial valleys in response to a major fall in relative sea level at 30 Ma and were later submerged by a combination of relative sea-level rise and concomitant tectonic activity. The sharp transition between coarse-grained, canyon-confined conglomerate and the overlying fine-grained, unconfined thin-bedded turbidites exists throughout southern Calabria and may represent the effect of a significant rise in relative sea level. Two other cycles of relative sea-level changes, probably resulting from local tectonic control, are indicated by the upper part of the Stilo-Capo d'Orlando Formation. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Microshear zones in a Miocene submarine dacite dome of southwest Japan

Microscopic shear zones have been found in the groundmass of glassy rocks of a Miocene submarine dacite dome in southwest Japan. Similar textures have been reported previously but only in dykes. These textures give a valuable insight into the deformation of the dome during its emplacement by recording the orientations of the principal strains. Detailed textural analysis indicates that the microshear zones formed as a result of flattening and stretching of the magma simultaneously with quenching. Measured stretching directions are near-parallel suggesting the magma flowed in one dominant direction rather than in a radial pattern. The strain is believed to also be influenced by high magma pressure inside the dome being imposed on the high viscosity outer part of the dome.