Distal Facies Research Articles

Ultra-shallow-marine anoxia in an Early Triassic shallow-marine clastic ramp (Spitsbergen) and the suppression of benthic radiation

AbstractLower Triassic marine strata in Spitsbergen accumulated on a mid-to-high latitude ramp in which high-energy foreshore and shoreface facies passed offshore into sheet sandstones of probable hyperpycnite origin. More distal facies include siltstones, shales and dolomitic limestones. Carbon isotope chemostratigraphy comparison allows improved age dating of the Boreal sections and shows a significant hiatus in the upper Spathian. Two major deepening events, in earliest Griesbachian and late Smithian time, are separated by shallowing-upwards trends that culminated in the Dienerian and Spathian substages. The redox record, revealed by changes in bioturbation, palaeoecology, pyrite framboid content and trace metal concentrations, shows anoxic phases alternating with intervals of better ventilation. Only Dienerian–early Smithian time witnessed persistent oxygenation that was sufficient to support a diverse benthic community. The most intensely anoxic, usually euxinic, conditions are best developed in offshore settings, but at times euxinia also developed in upper offshore settings where it is even recorded in hyperpycnite and storm-origin sandstone beds: an extraordinary facet of Spitsbergen's record. The euxinic phases do not track relative water depth changes. For example, the continuous shallowing upwards from the Griesbachian to lower Dienerian was witness to several euxinic phases separated by intervals of more oxic, bioturbated sediments. It is likely that the euxinia was controlled by climatic oscillations rather than intra-basinal factors. It remains to be seen if all the anoxic phases found in Spitsbergen are seen elsewhere, although the wide spread of anoxic facies in the Smithian/Spathian boundary interval is clearly a global event.

Subsurface lacustrine storm-seiche depositional model in the Eocene Lijin Sag of the Bohai Bay Basin, East China

Recent progress in facies analysis helps to discriminate storm-induced deposits based on interpretation of sedimentary records of combination of oscillatory and unidirectional flows. Located in the southeastern corner of the Bohai Bay Basin in East China, the Lijin Sag is a NE–SW trending Cenozoic half-graben basin. Part of its Eocene deposits (Bindong deposits), which are deposited far away from a contemporary shoreline, consists of thin bedded fine-sandstones and siltstones, interlayed with dark-gray mudstones. New data from drilling wells permit an interpretation of the sedimentary facies. Based on seismic data, well log data, core data and thin-section analyses, storm-dominated deposits were recognized. Petrologic analysis shows that these deposits mainly consist of fine sand- to silt-sized lithic arkose. Detailed sedimentological analyses on lithofacies were conducted to address flowtypes dominant during their geneses. The beds are normal graded and contain Bouma-like sequences. The typical and complete sedimentary sequence consists of fining-upwards successions from an erosive base, followed by gravity flow-induced massive or faint laminated bed or soft sediment deformation structures and unidirectional-combined-oscillatory flow induced beddings, which are attributed to storm wave and seiche processes. From proximal to distal in plane, the Bindong storm deposits exhibit different lithofacies associations and sedimentary processes, i.e., the proximal facies is coarser and dominated by gravity flows, unidirectional flows and combined flows, and formed under strong hydrodynamic conditions; the transitional facies is formed under full range of flow regimes exhibiting a complete Bouma-like sequence; while the distal facies is dominated by gravity flows and pure unidirectional flows without influence of waves. During the deposition period of the Bindong deposits, the paleo-environmental characteristics, such as paleogeographic position, paleoclimate, provenance, paleo-water depth and paleotopography, provided an advantageous source and site for the formation of storm deposits.

Paleoenvironmental reconstruction of the coastal Monte Léon and Santa Cruz formations (Early Miocene) at Rincón del Buque, Southern Patagonia: A revisited locality

Sedimentological, ichnological and paleontological analyses of the Early Miocene uppermost Monte León Formation and the lower part of the Santa Cruz Formation were carried out in Rincón del Buque (RDB), a fossiliferous locality north of Río Coyle in Santa Cruz Province, Patagonia, Argentina. This locality is of special importance because it contains the basal contact between the Monte Léon (MLF) and the Santa Cruz (SCF) formations and because it preserves a rich fossil assemblage of marine invertebrates and marine trace fossils, and terrestrial vertebrates and plants, which has not been extensively studied. A ∼90 m-thick section of the MLF and the SCF that crops out at RDB was selected for this study. Eleven facies associations (FA) are described, which are, from base to top: subtidal–intertidal deposits with Crassotrea orbignyi and bioturbation of the Skolithos-Cruziana ichnofacies (FA1); tidal creek deposits with terrestrial fossil mammals and Ophiomorpha isp. burrows (FA2); tidal flat deposits with Glossifungites ichnofacies (FA3); deposits of tidal channels (FA4) and tidal sand flats (FA5) both with and impoverish Skolithos ichnofacies associated; marsh deposits (FA6); tidal point bar deposits recording a depauperate mixture of both the Skolithos and Cruziana ichnofacies (FA7); fluvial channel deposits (FA8); fluvial point bar deposits (FA9); floodplain deposits (FA10); and pyroclastic and volcaniclastic deposits of the floodplain where terrestrial fossil mammal remains occur (FA11).The transition of the MLF–SCF at RDB reflects a changing depositional environment from the outer part of an estuary (FA1) through the central (FA2–6) to inner part of a tide-dominated estuary (FA7). Finally a fluvial system occurs with single channels of relatively low energy and low sinuosity enclosed by a broad, low-energy floodplain dominated by partially edaphized ash-fall, sheet-flood, and overbank deposits (FA8–11). Pyroclastic and volcaniclastic materials throughout the succession must have been deposited as ash-fall distal facies in a fluvial setting and also were carried by fluvial streams and redeposited in both estuarine and fluvial settings. These materials preserve most of the analyzed terrestrial fossil mammals that characterize the Santacrucian age of the RDB's succession. Episodic sedimentation under volcanic influence, high sedimentation rates and a relatively warm and seasonal climate are inferred for the MLF and SCF section.Lateral continuity of the marker horizons at RDB serve for correlation with other coastal localities such as the lower part of the coastal SCF south of Río Coyle (∼17.6–17.4 Ma) belonging to the Estancia La Costa Member of the SCF.

K-Ar dating of bentonite diagenesis in accretionary-wedge turbidites: case study from Western Outer Carpathians

Sixteen bentonite layers of Eocene–Oligocene age were sampled at thirteen sites in accretionary-wedge turbidite sequences, in the three innermost nappes of the Western Outer Carpathians. K-Ar dating was carried out for five of these layers to obtain the maximum burial ages. All of the ages obtained are older than the stratigraphic ages of the host strata. This relationship is due to significant contamination of the bentonites with non-authigenic minerals. This contamination resulted from rapid sedimentation during synsedimentary folding, which is a common feature in accretionary wedges. It follows that the K-Ar dating of bentonite layers in the turbidite sequences of accretionary wedges should be largely restricted to the very distal facies of turbidites or to pelitic intercalations within the turbidites.

Lower Carboniferous ramp sedimentation of the Central Alborz Basin, northern Iran: integrated sedimentological and rock–magnetic studies

Abstract The Lower Carboniferous Mobarak Formation in the Alborz Basin (northern Iran) was deposited along the northeastern margin of Gondwana in a carbonate ramp setting. This paper focuses on the Tournaisian stratigraphic interval of this formation that crops out at the Jaban section in the southwestern Central Alborz Basin. The following facies associations, representing different ramp palaeoenvironments, have been identified: (1) mudstone–wackestone outer-ramp facies; (2) crinoidal to skeletal grainstone–packstone mid-ramp facies; (3) peloidal to crinoidal grainstone–packstone inner-ramp facies; and (4) coastal facies, which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs. This ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non-skeletal shoals in the distal mid-ramp to the most proximal inner-ramp areas. The development of the skeletal to non-skeletal shoals along the sea side of the ramp formed a semi-enclosed lagoon sensitive to the influence of both high tides and storm surges. The magnetic susceptibility (χ in ) of all the samples was measured and compared with that of the facies from which the sample was taken. There is a clear link between χ in and the facies; the average χ in values were higher for the distal facies than for the proximal facies. The χ in profile of this Lower Carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials. In the context of the sequence stratigraphic framework, the average χ in values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits. The clear link between χ in and facies indicates at least a partly preserved primary χ in signal related to the detrital inputs. However, to obtain a better understanding of the nature and origin of the minerals carrying the χ in , we performed hysteresis measurements on selected samples. It appears that the χ in signal is mainly carried by low-coercivity ferromagnetic minerals such as magnetite, with a mixture of relatively coarse grains (detrital fraction) and ultra-fine grains (probably formed during diagenesis).

Bedform facies in western Torres Strait and the influence of hydrodynamics, coastal geometry, and sediment supply on their distribution

This study uses outputs from hydrodynamic and bedload transport models combined with satellite imagery, aerial photography, and bathymetric data to understand the distributions and mechanisms maintaining six spatially extensive bedform facies in western Torres Strait. Changes in bedform morphology occur along north–south variations in coastal geometry and east–west variations in hydrodynamic regime. Numerous islands create an environment that favours the formation of banner banks in the south, while other sandbank morphologies occur on an extensive and comparatively ‘flat’ basement to the north. The western side of Torres Strait experiences net bedload transport that is directed to the west throughout the year and favours the formation of bedforms that prefer unidirectional bedload transport regimes (i.e. barchan-shaped sandbanks and sand ribbons). The eastern side of the strait experiences seasonal changes in the direction of net bedload transport and maintains ‘S’ and ‘V’ shaped sandbanks.Sediment availability also influences the bedform facies. Western Torres Strait experiences net westward bedload transport through the Strait. However, sandbanks (indicating high sediment availability) are found in the central shallow and high current velocity areas, while sand ribbons (indicating low sediment availability) are a distal facies on the western side of the Strait. This sequence of bedforms indicates that sediments are preferentially trapped within the central portion of the Strait and not transported further west into the Gulf of Carpentaria.

Origin of Fossiliferous Limestone Beds inside the Upper Part of Tanjero Formation at the Northwest of Sulaimani Area, Kurdistan Region, NE-Iraq

The present study is focused on origin of fossiliferous limestone beds that are exposed in the Chaqchaq Valley at 10 kms to northwest of Sulaimani city. These beds are exposed inside upper part of the Tanjero Formation (Upper part of Gurpi Formation in Iran) and, in this study, they are call Chaqchaq limestone. The petrography and age determination proved that they aged Late Maastrichtian and belongs to Aqra Formation (Seymare or Tarbur Formation in Iran). These beds represent the extreme distal facies of the formation. The fossil contents of these beds include large foraminiferas (Omphalocyclus, Orbitoids and Loftusia), fragments of Rudists in addition to solitary corals, echinoderm and gastropods.These fossils are all allochthous and reworked by submarine mass wasting from their original life position in Chwatra–Mawat area. 
 As the discovered outcrops are located, stratigraphically, near the boundary of Cretaceous and Tertiary ages, it has important contribution on changing paleo-geography and tectonism of northeastern Iraq during Maastrichtian and Paleocene. The paleogeography of the area was consisted of large foreland basin that was covered all north eastern Iraq without any compartmentation. Previously Aqra Formation is recorded in the Imbricated and Thrust Zones while the present study records it in the High Folded Zone for the first time in the Northeastern Iraq. This record will remove the uncertainty that associated with the relations of the stratigraphic units in the Imbricated and Thrust Zones with those of High Folded Zones. In these zones, Tanjero Formation (with Aqra Formation) and Red Bed Series are deposited during Maastrichtian and Paleocene. The occurrence of these beds inside hemiplelagite (upper part of Tanjero
 Formation), make the age determination of the beds and their equivalent (Aqra Formation) in the Chwarta Mawat area are possible by planktonic forms. The planktonic foraminiferas show that the age of these beds (Chachaq Limestone) is late Maastrichtian.

Fluvio-lacustrine deposits reveal precipitation pattern in SE Arabia during early MIS 3

On a global scale, Marine Isotope Stage (MIS) 3 (ca. 55–30 ka) was characterised by frequent and abrupt changes in climate. At the same time, humans may have dispersed out of Africa and populated the planet. For the Arabian Peninsula, a key region in both contexts, the paucity of suitable terrestrial records leaves our knowledge of the environmental conditions during this important period enigmatic. Here, we elucidate the environmental history for part of the Oman Mountains by analysing luminescence-dated fine-grained playa-like sediments from Wadi Mistal that formed in a landslide-dammed ephemeral lake. We identified 112 individual fining-upward sequences in the distal facies of the lake that are interpreted to represent major precipitation events during early MIS 3. The nature of sedimentation together with the presence of desiccation cracks and the absence of any faunal remains imply that the lakes had a temporary nature. It appears that the time of deposition was characterised by strong short-lived precipitation events, rather than continuous rainfall under a monsoonal regime. This points towards a climatic situation during early MIS 3 that was not so different from that of the present, at least in this part of Arabia.

Spectral properties and ASTER-based alteration mapping of Masahim volcano facies, SE Iran

This study applies Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data and the Mixture Tuned Matched Filtering (MTMF) algorithm to map the sub-pixel distribution of alteration minerals associated with the Masahim volcano, SE Iran for understanding the spatial relationship between alteration minerals and volcano facies. Investigations of the alteration mineralogy were conducted using field-spectroscopy, X-ray diffraction (XRD) analysis and ASTER Short Wave Infrared (SWIR) spectral data. In order to spectrally characterize the stratovolcano deposits, lithological units and alteration minerals, the volcano was divided into three facies: the Central, Proximal, and Medial–distal facies. The reflectance spectra of rock samples show absorption features of a number of minerals including white mica, kaolinite, montmorillonite, illite, goethite, hematite, jarosite, opal, and chlorite. The end-members of key alteration minerals including sericite (phyllic zone), kaolinite (argillic zone) and chlorite (propylitic zone) were extracted from imagery using the Pixel Purity Index (PPI) method and were used to map alteration minerals. Accuracy assessment through field observations was used to verify the fraction maps. The results showed that most prominent altered rocks situated at the central facies of volcano. The alteration minerals were discriminated with the coefficient of determination (R2) of 0.74, 0.81, and 0.68 for kaolinite, sericite, and chlorite, respectively. The results of this study have the potential to refine the map of alteration zones in the Masahim volcano.

The early Cambrian Chahmir shale-hosted Zn–Pb deposit, Central Iran: an example of vent-proximal SEDEX mineralization

The Chahmir zinc–lead deposit (1.5 Mt @ 6 % Zn + 2 % Pb) in Central Iran is one among several sedimentary-exhalative Zn–Pb deposits in the Early Cambrian Zarigan–Chahmir basin (e.g., Koushk, Darreh-Dehu, and Zarigan). The deposit is hosted by carbonaceous, fine-grained black siltstones, and shales interlayered with volcaniclastic sandstone beds. It corresponds to the upper part of the Early Cambrian volcano-sedimentary sequence (ECVSS), which was deposited on the Posht-e-Badam Block during back-arc rifting of the continental margin of Central Iran. Based on crosscutting relationships, mineralogy, and texture of sulfide mineralization, four different facies can be distinguished: stockwork (feeder zone), massive ore, bedded ore, and distal facies (exhalites with barite). Silicification, carbonatization, sericitization, and chloritization are the main wall-rock alteration styles; alteration intensity increases toward the proximal feeder zone. Fluid inclusion microthermometry was carried out on quartz associated with sulfides of the massive ore. Homogenization temperatures are in the range of 170–226 °C, and salinity is around 9 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore facies suggests anoxic to locally suboxic event for the host basin. δ34S(V-CDT) values of pyrite, sphalerite, and galena range from +10.9 to +29.8 ‰. The highest δ34S values correspond to the bedded ore (+28.6 to +29.8 ‰), and the lowest to the massive ore (+10.9 to +14.7 ‰) and the feeder zone (+11.3 and +12.1 ‰). The overall range of δ34S is consistent with a sedimentary environment where sulfide sulfur was derived from two sources. One of them was corresponding to early ore-stage sulfides in bedded ore and distal facies, consistent with bacterial reduction from coeval seawater sulfate in a closed or semiclosed basin. However, the δ34S values of late ore-stage sulfides, observed mainly in massive ore, interpreted as a hydrothermal sulfur component, leached from the lower part of the ECVSS. Sulfur isotopes, along with the sedimentological, textural, mineralogical, fluid inclusion, and geochemical characteristics of the Chahmir deposit are in agreement with a vent-proximal (Selwyn type) SEDEX ore deposit model.

Atlantic extreme wave events during the last four millennia in the Guadalquivir estuary, SW Spain

A multidisciplinary study from a number of drilled cores in the Guadalquivir estuary has made possible to identify as many as three extreme wave events and their facies in the 4th millennium BP (A: ~4000calyr BP, B: ~3550calyr BP, and C: ~3150calyr BP). These events, which caused strong erosion in the Guadalquivir sandy barrier and in the neighboring aeolian systems of El Abalario, brought about significant paleogeographical changes that may have affected human settlements established in the area during the Neolithic and Copper Age periods and during the Middle Bronze Age. The three events can be spatially correlated and their facies differentiated from more proximal to more distal from the coastline. The most proximal facies is characterized by a massive accumulation of shells, a sandy or sandy–muddy matrix, an erosive base, a highly diverse mixture of species (marine and estuarine), and lithoclasts. The most distal facies presents a muddy–sandy matrix, dominance of estuarine fauna, shell accumulation, presence of terrestrial species, mudpebbles, pebbles in a clayey matrix, and bioturbation. The evidence presented will further advance scientific knowledge about the impact of extreme wave events on coastal areas in SW Iberia and NW Africa.

Climate vs. tectonics: the competing roles of Late Oligocene warming and Alpine orogenesis in constructing alluvial megafan sequences in the North Alpine foreland basin

AbstractMegafan conglomerates of foreland basins chronicle the combined effect of palaeoclimate conditions, tectonic processes and the flux and granulometric composition of the supplied sediment. However, the architecture of these deposits is seldom uniquely compatible with a single driving force. This problem is illustrated here with a field‐based analysis of the ca. 30–20 Ma‐old Napf deposits in the north Alpine foreland basin which are coeval with a substantial global warming of ca. 6°C during the Late Oligocene. The observed larger grain sizes and a change in fluvial style from wandering to braided could be explained climatically by a shift to drier conditions with sparse vegetation, but would have resulted in less than 400 m of additional accommodation space during the 1 Ma duration of change. Accordingly, a climate scenario alone is also not compatible with rapid sediment accumulation rates of >1000 m Ma−1 recorded at Napf, or with a lack of any remarkable shifts in the Froude number, which would be expected if water discharge patterns changed substantially. Alternatively, flexural downwarping in response to a tectonic pulse could account for the increase in grain size and the change in fluvial style from wandering (more distal facies) to braided (proximal equivalent). However, a third driving force is required to explain the contemporaneous backstepping of the distal gravel front and progradation of the proximal braided facies. We suggest that the erosional hinterland steepened in response to an inferred tectonic pulse, resulting in a more widespread exposure of lithologies with higher erosional resistance, as inferred from an increasing contribution of crystalline constituents in the clast suites. Such a change would result in a larger D50 and a higher clast size variability in the supplied sediment, which in turn would contribute to the observed change from wandering to braided and the related shift in depositional systems. This study highlights the importance of tectonic processes and the role of changing surface lithologies in the source area for explaining variations in megafan construction even in the light of substantial palaeoclimate shift.

Late Quaternary inner shelf deposits in response to late Pleistocene–Holocene sea level changes: Nakdong River, SE Korea

The high-resolution seismic profiles from the inner shelf off the Nakdong River in SE Korea can be directly correlated with the lithologic characteristics in the borehole data. These correlations offer an opportunity to document the evolution of the inner shelf deposits with sediment supplies in response to the late Quaternary sea level changes. The inner shelf sequence in this area consists of four sedimentary units, which comprise a set of lowstand, transgressive, and highstand systems tracts deposited since the last glacial maximum: incised-channel fill (SU1), estuary (SU2), sand veneer (SU3), and subaqueous delta (SU4). The lowermost unit (SU1), which overlies the sequence boundary, is interpreted as fluvial deposits formed during the last glacial period and the early stage of transgression, and belongs to the lowstand to transgressive systems tract. The lower middle unit (SU2) lying below the ravinement surface represents a paralic component that consists of estuarine sandy mud or muddy sand developed between approximately 13 and 8 cal ka BP, whereas the upper middle unit (SU3) above the ravinement surface corresponds to a marine component that consists of sand veneer produced by the shelf erosion during the ensuing sea-level rise. These two units (SU2 and SU3) belong to the transgressive deposits. The uppermost unit (SU4), regarded as the highstand systems tract, formed the Nakdong subaqueous delta including the proximal and distal systems developed after the highstand sea level at approximately 6 cal ka BP. The lateral transition from the proximal to distal facies suggests a prograding delta system in the Nakdong River.

The Archaeocyaths of Ifrane (Western Anti-Atlas-Morocco): Geological Value and Geotourism Development

In the western part of the Anti Atlas, the Cambrian formations known since long (Nice, 1924; Bourcard, 1927; Bung & Netlner, 1933), have been the subject of a very detailed stratigraphic approach. The sedimentological work affecting the lower Cambrian of the Western Anti Atlas is rare. It concerns mainly: a lithostratigraphic description and a summary interpretation of the environmental sedimentary formations. Indeed, recent sedimentological data allowed a new lithostratigraphic division of the Lower Cambrian succession in three lithostratigraphic series. These, are characterized, each one, by a particular microbialitic builds ups, resulted, probably, from Lower Cambrian marine ecosystem variations. The first step, the stromatolitic series, represents a long period in which a large area of the Anti-Atlasic platform was occupied by peritidal cyanobacterial mats. The second step, represented by the Thrombolites series, marks a global sea level rise (Thrombolites proliferation) relayed by a massive silico-clastic supply inputs. And the third stage of evolution during which the medium sufficiently deepened to allow the installation of the distal facies platform. The development of the Archaeocyaths seems to be one of the precursors of dendritic form acquired by certain microbial communities. Dating Neoproterozoic, the Archaeocyaths of Ifrane, whose morphological diversity and the microbial communities associated with them, are a focus for the industry of ornamental rocks in Ifrane in the Moroccan western Anti-Atlas. In this area, the structural diversity of the Archaeocyaths is very pronounced, which facilitates the systematic division and their use in stratigraphy. The perforations or pores vary in number, size and arrangement, those whose walls are stretched in the subdivision pore channels or tubes which may be linear, bent, or inclined in the case of thicker walls. Indeed, the main objective of this work is to highlight the “Archaeocyathans mounds” as a geological heritage through the study of varied silico-clastic and carbonate predominate components facies. Regrettably, the surexploitation of the Archaeocyaths in the ornamental industry and mining activities may lead to their death in the absence of regulatory laws of their exploitation. Clues are offered to make the Archaeocyaths a geological site, a geological destination, thing that necessitate the conduct of a number of approaches, the mobilization of various networks of actors but also the establishment or adaptation of infrastructure, home or mediation.

Upper Devonian–Mississippian stratigraphic framework of the Arkoma Basin and distribution of potential source-rock facies in the Woodford–Chattanooga and Fayetteville–Caney shale-gas systems

Wireline logs were used to document the stratigraphic framework of Upper Devonian–Mississippian strata in the Arkoma Basin, and maps of high-gamma ray (HGR) log response were used to analyze the spatial distribution of potential source rocks in the Woodford–Chattanooga and Fayetteville–Caney shale-gas systems. The Woodford–Chattanooga shale is a transgressive deposit that accumulated on an arid continental margin influenced by marine upwelling and minimal sediment influx. A broad HGR depocenter along the southwestern margin of the basin includes two areas of higher accommodation containing the thickest HGR concentrations. Basin-wide patterns of HGR likely reflect broad tectonic influence on accommodation. The proportion of chert in the formation increases eastward and southward, likely reflecting latitudinal and bathymetric influence on the accumulation of siliceous ooze. The Lower Mississippian Burlington sequence, which lies between the two shale-gas systems, comprises carbonate ramp and distal shale deposits. Proximal ramp facies form an apron around the southern flank of the Ozark uplift and grade radially basinward into distal facies. An Upper Mississippian succession in the east includes lowstand deposits of the Batesville delta, which onlap the relict Burlington ramp. Basinwide, the succession includes the transgressive Fayetteville–Caney shale overlain by regressive deposits of the proximal Pitkin Limestone and distal upper Fayetteville (Arkansas) and “false” Caney (Oklahoma) shale. The HGR shale is concentrated in an area of intermediate accommodation on the western margin of the Mississippi Embayment and just basinward of the Pitkin Limestone pinchout in Arkansas, and in an area of relatively high accommodation in Oklahoma.

フィリピン共和国，イロシン火砕流噴火の推移と特徴

フィリピン共和国，ルソン島南東端のソルソゴン地域での41 cal kBPの大規模火砕流噴火の結果，11 × 11 kmのイロシン（Irosin）カルデラが形成された。このときの噴火は，カルデラを形成する大規模噴火とその先駆的な噴火の2つに区分される。 先駆的噴火は，現在のカルデラ縁の南東斜面の小規模溶岩ドームであるマロバゴ（Malobago）溶岩ドームの形成と細粒火山灰の噴出からなる。ほかにも類似した溶岩の流出がカルデラ内にあったかもしれない。 地質学的短期間（およそ10年以内）の休止後，3つの噴火フェーズからなるカルデラ形成噴火がはじまった。最初の噴火フェーズは，現在のカルデラ内でおこったプリニー式噴火であり，バルク体積約 20 km3の軽石質テフラが北方に拡散した。プリニー式噴火の早い段階で噴煙柱の崩壊が繰り返し発生し，intra-plinian火砕流が薄い細粒火山灰と降下軽石層に挟在している。プリニー式フェーズの終わりごろにソルソゴン地域に強い地盤振動があり，プリニー式噴火による堆積物の上下面に撹乱構造を形成した。 イロシン火砕流の噴火は，グランドレーヤーを基底部に伴う下部ユニット火砕流の噴出ではじまった。そして，それに続く破局的な噴火によって上部ユニット火砕流が噴出し，現在のカルデラ地形を形成した。上部ユニットは，厚く粗粒な火砕流堆積物であり，基底部に粗粒な岩片濃集層（地点Dで厚さ約 2 m）が認められる。イロシン火砕流のバルク体積は，およそ 25 km3と見積もられる。 マヨン（Mayon）火山西麓のイナスカン（Inascan）スコリア丘の山頂部で，イロシン火砕流に伴う降下火山灰（co-ignimbrite ash-falls）が発見されている。ここでの火山灰層は，層厚約1mで細粒火山ガラスと少量の斑晶鉱物からなる。比較的粗粒の最下部層は，プリニー式降下軽石層の遠方相に対比される。これらの降下テフラも含めたイロシンカルデラから噴出した火砕物全体のバルク体積は，ほぼ 70 km3に達する。

A model of tephra dispersal from an early Palaeogene shallow submarine Surtseyan-style eruption(s), the Red Bluff Tuff Formation, Chatham Island, New Zealand

The Red Bluff Tuff Formation, an early Palaeogene volcano-sedimentary shallow marine succession from the Chatham Islands (New Zealand), provides a unique framework, in eastern ‘Zealandia’, to explore tephra dispersal processes associated with ancient small phreatomagmatic explosions (i.e. Surtseyan-style eruptions). Detailed sedimentological mapping, logging and sampling integrated with the results of extensive laboratory analyses (i.e. grain-size, componentry and applied palaeontological methods) elucidated the complex mechanisms of transport and deposition of nine identified resedimented fossiliferous volcaniclastic facies. These facies record the subaqueous reworking and deposition of tephra from the erosion and degradation of a proximal, entirely submerged ancient Surtseyan volcanic edifice (Cone II). South of this volcanic cone, the lowermost distal facies provides significant evidence of deposition as water-supported volcanic- or storm-driven mass flows (e.g. turbidity currents and mud/debris flows) of volcaniclastic and bioclastic debris, whereas the uppermost distal facies exhibit features of tractional sedimentary processes caused by shallow subaqueous currents. Further north, within the proximity of the volcanic edifice, the uppermost facies are represented by an abundant, diverse, large, and well preserved in situ fauna of shallow marine sessile invertebrates (e.g. corals and sponges) that reflect the protracted biotic stabiliszation and rebound following pulsed volcanic events. Over a period of time, these stable and wave-eroded volcanic platforms were inhabited by a flourishing and diversifying marine community of benthic and sessile pioneers (corals, bryozoans, molluscs, brachiopods, barnacles, sponges, foraminifera, etc.). This succession exhibits a vertical progression of sedimentary structures (i.e. density, cohesive and mass flows, and cross-bedding) and our interpretations indicate a shallowing upwards succession. This study reports for the first time mechanisms of degradation of a Surtseyan volcano on Chatham Islands and contributes to a better understanding of complex ancient volcano-sedimentary subaqueous terrains. This model of deposition (i.e. onlapping/overlapping features onto the remains of volcanic edifice(s), a vertical transition of structures from deeper- to shallower-marine environments, disaster faunas and subsequent preferential colonisation of diverse biota, including large in situ sessile invertebrates, on the summit), characterises an extraordinary example to be applied to other ancient subaqueous volcanic environments.

DOĞU KARADENİZ BÖLGESİ (KD TÜRKİYE) VOLKANOJENİK MASİF SÜLFİD YATAKLARI CEVHER FASİYESLERİNE AİT BAZI ÖZGÜN BULGULAR

In the massive sulphide deposits of the eastern Black Sea region, there are ore facies and ore-bearing sedimentary facies. The former are subdivided into hydrothermal-metasomatic, seafloor hydrothermal, and biological facies. Hydrothermal-metasomatic facies refer to sub-seafloor processes and include network-disseminated, massive vein, and massive lens facies. The precipitation of sulphide minerals within pre-existing volcano-sedimentary rocks occurs largely beneath the seafloor, and these ores form an important component of some deposits. The term seafloor hydrothermal facies refers to sulphide accumulation on the seafloor and is characterised by hydrothermal chimneys and clastic sulphide ores. Clastic sulphide ores can be subdivided into proximal and distal facies. Ore-bearing sedimentary facies are characterised by relatively thin ferruginous chert (exhalite?) beds that occur along the uppermost part of the ore horizon. The biological facies is represented by fossil vent fauna, which are diagnostic for seafloor sulphide formation. The VMS ores of the Eastern Black Sea region have well-preserved facies characteristics, especially in terms of texture and components. Some massive sulphide deposits accumulated via molasse/mass flow on the seafloor, whereas others formed via sub-seafloor replacement processes. Clastic sulphide textures and some sedimentary structures indicate that these deposits formed in a highly active setting. Clastic sulphide textures, sulphide chimney fragments, ferruginous cherts and fossil vent fauna (tube worms) recognised in the Pontide deposits are diagnostic and support the seafloor origin of VMS deposits

Depositional, diagenetic and stratigraphic aspects of microfacies from Riachuelo Formation, Albian, Sergipe Basin, Brazil

The rocks of the Riachuelo Formation, Sergipe Basin, Brazil, represent an example of carbonate sedimentation related to the drift phase during the opening of the South Atlantic Ocean. The Carapeba and Brejo quarries exhibit the best onshore outcrops of the drift carbonate section along the Brazilian continental edge. Field studies and microfacies analysis of the outcropped sedimentary section showed six sedimentary deposits related to the physiography of a carbonate shelf. Proximal mixed deposits are represented by the rich-terrigenous dolostone. Levels with alternate layers of fine grained sandstones and siltstones are here related to distal facies of submarine fans deposits. Mudstones with miliolids and textularids represent a lagoonal environment in a semi-restricted middle shelf. Packstones, grainstones and occasionally wackestones with oncoids, intraclasts and peloids represent sedimentary deposits related to the back of shallow sandy bars and environments at the interface with the lagoon. Grainstones whit ooliths, oncoids, intraclasts and bioclasts, with trough cross-bedding, represent a shallower shoreface environment over the shallow carbonate back on outer shelf. Cements and other post-depositional features suggest four different diagenetic environments: a) marine phreatic diagenetic environment with active water circulation; b) marine phreatic diagenetic environment with stagnant water; c) freshwater phreatic diagenetic environment; d) burial diagenetic environment. The sedimentary succession is formed by shallowing upward cycles overlain by a possible transgressive surface, which may indicate the passage of a lowstand to a transgressive system tract.

Architecture and evolution of calciclastic marginal slope fans of the Ordovician carbonate platform in the Yijianfang outcrop of the Bachu area, west Tarim Basin

The Ordovician carbonate platform at the Yijianfang outcrop of the Bachu uplift region in the western Tarim Basin contains four types of genetic facies associations developed in the calciclastic slope-fan depositional system: an olistostrome zone, fan channels, lobes, and a marginal slope. The olistostrome zone is characterized by olistoliths and slump fans, whereas the fan channels and lobes are further divided into proximal and distal facies. The marginal slope deposits constitute the background sedimentation in which the calciclastic slope fans are intercalated. From proximal to distal parts of the fan channels and lobes, their scale gradually becomes smaller, and the size and sorting of grains become finer and better, respectively. Analysis of the stratigraphic framework indicates that the fans formed in the lower strata of the Upper Ordovician Lianglitage Formation in four high-frequency sequences (i.e., Pss1–Pss4). Field paleocurrent measurements indicate northeast-southwest depositional strike for the early platform margin of the Lianglitage Formation. Sediments in the calciclastic slope fans were derived from the platform margin, and evolution of the calciclastic slope fans was generally progradational from Pss1 to Pss2 and then continuously retrogradational from Pss2 to Pss4. The calciclastic slope fans in the outcrop area are not reservoir-prone rocks, but interpretation on these fans can provide useful information about potential hydrocarbon reservoirs along the platform margin. The P-wave velocity, S-wave velocity, and density variations in each genetic facies may be used to identify the subsurface calciclastic slope-fan depositional system.