Volcanic Ash Horizons Research Articles

Revision of lithostratigraphic nomenclature in the Koobi Fora region, Kenya

Recent advances in our understanding of the Plio-Pleistocene sedimentary sequence northeast of Lake Turkana in northern Kenya has allowed a revision of the lithostratigraphic nomenclature and a more complete description of the deposits. As redefined, the Koobi Fora Formation encompasses the entire Plio-Pleistocene sedimentary sequence, and is subdivided into eight members. These members are delineated using volcanic ash horizons, which can be identified uniquely on the basis of chemical composition. The revised stratigraphy resolves all earlier conflicts with the biostratigraphic zonations based on vertebrate and molluscan studies. The sedimentary sequence of the Koobi Fora Formation attains an aggregate thickness of 560 m and spans the period from about 4.3 to 0.6 Ma. Detailed correlations have now been established to relate this sequence to other deposits within the Turkana Basin and to those at Hadar and in the Gulf of Aden.

Toward a comprehensive upper quaternary tephra and ignimbrite stratigraphy in New Zealand using electron microprobe analysis of glass shards

New Zealand Quaternary marine and terrestrial sequences contain numerous tephras, or volcanic-ash horizons, that are the distal correlatives of voluminous welded ignimbrite sheets, erupted from central North Island. Electron microprobe analyses of glass shards from the distal tephras demonstrate their homogeneity and are shown to identify each tephra examined. By matching tephras from stratigraphically controlled sequences, the first comprehensive tephra stratigraphy spanning from 50,000 to 700,000 yr ago and covering the New Zealand region is advanced. Analyses on glass shards from the unwelded base of ignimbrite sheets are comparable to the distal tephra analyses and allow correlation between ignimbrites and to the distal tephras. The better exposed tephra record constrains the number of separate eruptive events and the stratigraphy of the ignimbrites, both of which were previously confused by lack of outcrop. Samples from pumiceous marine sediments were found to contain two or more chemically distinct populations of glass. The pumice is in cross-bedded sands or sand lenses within conglomerate, attesting to a shallow high-energy environment where reworking could occur. However, each glass population could be matched to older, known tephras.

THE PETROLOGY OF THE MIDDLE JURASSIC SANDSTONES FROM THE MURCHISON FIELD, NORTH SEA

A petrographical investigation of the sandstones of the Brent Group (Middle Jurassic) from the Murchison field has produced evidence for the existence of two transgressive events within the sequence, one occurring immediately above the Etive Fm., the other within the upper part of the Ness Fm. These transgressions have not been previously recorded in Murchison and their discovery demands that some modification be made to the existing depositional model for the Brent Group of this area. A volcanic ash horizon has been found in a similar stratigraphic position to that recorded from Statfjord, and if this, and the two transgressive horizons noted above, are of more than local extent they could prove to be useful stratigraphic markers.Although the diagenesis of the Brent Group sandstones in Murchison differs little from that recorded elsewhere in the Brent province, the interpretation of the timing of the development of certain phases differs somewhat from those of other authors. In particular, kaolinite development appears to have taken place soon after burial in response to periods of emergence and weathering during Brent Group deposition, rather than through groundwater circulation during the Kimmerian tectonic phase.

The effects of volcanic ash influenced soils and nitrogen fertilization on douglas‐fir seedlings

Abstract An experiment was conducted to help understand the contributions of different soil layers to soil fertility, plant growth, and response to fertilization. Douglas‐fir (Pseudotsuga menziesii) seedlings were used in a pot bioassay to delimit the effects of volcanic ash soils, urea fertilization at 100 and 200 ppm, and the technique of using undisturbed soil as a growth medium. Volcanic ash horizons contained more available phosphorus and mineralized more nitrogen than underlying horizons. Best seedling growth occurred in ash horizons fertilized with 200 ppm of urea‐N. Fertilization decreased soil pH, mycorrhizae formation and foliar‐P levels but increased foliar‐N. The technique of using undisturbed soil had little effect on seedling growth in the weak structured ash horizons but did decrease root weights in the moderate structured sub‐ash layers.

たい積物中の火山ガラスの研究 大阪平野の更新〜完新統について

The Pleistocene to Holocene sediments underlying the Osaka Plain consist of unconsolidated gravels, sands, silt and clay beds of limnic, fluvial and marine origin.IKEBE et al. (1970) and FURUTANI (1978) described the subsurface geology of the Osaka Plain is through the investigation of the drilling cores. From these studies the subsurface stratigraphy of the Osaka Plain summarized as the following sequence, the latest Pleistocene to Holocene formation (Nanba Formation), the upper Pleistocene formation (Ma 11 bed, Ma 12 bed and Tenma Formation) and the Osaka Group in descending order (Figs. 1 and 2).The writer investigated the content of glass fragments in these sediments and also studied the features such as shape and refractive index of glass fragments.The results obtained are the following:1) The content and the features of glass fragments from the Pleistocene to Holocene sediments are shown in Fig. 3, 4, 5, 6, 7, 8, 9, and 10.2) On the basis of the vertical variation-diagram (Figs. 3…10), volcanic ash- horizons can be found in these sediments, because the vertical variations of content and features of glass fragments are controlled mainly by the fall of volcanic ash.3) Five volcanic ash-horizons occur in these sediments and are widely traceable. Among these five horizons, three horizons are in the Ma 12 bed, and two horizons are in the Nanba Formation.4) The lower ash-horizon in the Nanba Formation can be correlated with the ash layer described by MAEDA (1976) in Core Site 2. The upper ash-horizon in the Nanba Formation can be correlated with the ash layer described by ISHIDA et al. (1969) in the Kyoto Basin.

Physical, Chemical, and Mineralogical Characteristics of Soils from Volcanic Ash in Northern Idaho: I. Morphology and Genesis

AbstractMission soils, coarse‐silty, mixed, frigid Andic Fragiochrepts, are representative of volcanic ash soils found extensively in northern Idaho, northeastern Washington, and northwestern Montana. They have developed under a western hemlock (Tusuga heterophylla) habitat type with 740 mm of precipitation, > 8°C mean summer soil temperature and 7.5°C mean annual soil temperature (weather data, Univ. of Idaho. Res. Stn., Sandpoint). The B2ir or cambic horizons have developed from volcanic ash and the underlying IIA'&B' and IIB'2x horizons have fragipans developed in glacial outwash sediments. Horizons from volcanic ash have higher values of estimated allophane, organic C, extractable Fe, Al, and Mn, exchangeable H, 1/3‐ and 15‐bar moisture, and CEC than do underlying horizons. Bulk densities range from 0.80 to 0.90 in volcanic ash horizons compared with 1.41 to 1.65 for glacial sediments. X‐ray patterns show little indication of crystalline minerals in the ash‐related horizons. Concretions have greater contents of Mn and Fe whereas Al remains relatively constant in relation to the total soil. Development of a B2ir, without an Al horizon under the O horizon, is characteristic of soils associated with these climatic and vegetational zones. Diagnostic properties make the cambic gradational to a spodic horizon.

Paleomagnetic study of sediments around the Komyoike Volcanic Ash Horizon in Osaka Group, Senpoku Area, Osaka Prefecture, Japan

The Osaka Group is typical of the Plio‐Pleistocene Series in Japan. The correlation of paleomagnetic stratigraphy and biostratigraphy gives a clear picture of the Quaternary history of the Group. The Komyoike volcanic ash layer distributed only in the Senpoku area, in the southern part of Osaka Prefecture is shown to have normal polarity at 1.1 million years (fission track age). The remanent magnetizations of more than one hundred oriented samples collected in sequence from the out‐crop sediments intercalated with the Komyoike volcanic ash bed were measured. Paleomagnetic results after magnetic cleaning show that the sedimentary sequence is divided into three distinct zones by changes in the remanent polarity.

Tectonic control of Late Pliocene molasse sedimentation in a portion of the Jhelum Re-Entrant, Pakistan

A Late Pliocene interval of the Neogene Siwalik (Himalayan) molasse exposed in the Jhelum Re-entrant, Pakistan, is examined for lateral variations in the environment and rate of fluvial sedimentation. A correlative pair of volcanic ash horizons within the Upper Siwalik section defines an isochronous interval of fluvial deposition. Stratigraphic sections measured through the interval at various localities in the Jhelum Re-entrant are analyzed in terms of fluvial stratigraphy and interval thickness, or net sedimentation rate. The stratigraphy indicates a fluvial system of laterally migrating streams leaving behind fining upward sedimentary cycles of lateral and vertical accretion deposits. Fully developed sedimentary cycles display point bar sands (lateral accretion deposits) of the stream channel environment topped by mudstones (vertical accretion deposits) of the overbank (floodplain) environment. The detailed sequence of vertical accretion deposits in these cycles indicates that the floodplain environment consisted of a stream-proximal, sedimentation-dominant zone and a stream-distal, pedogenesis-dominant zone. Broad lateral variations in the rate of molasse sedimentation appear to be related to the large-scale syntaxial tectonics of the northwestern Himalaya. Sedimentation rate increases toward the apex of the re-entrant, indicating that the syntaxial structure controlled the focus of molasse sedimentation during the examined interval of Late Pliocene time. In addition, sedimentation may have been influenced by mild flexures within the molasse basin.

Episodes of cenozoic volcanism in the circum-pacific region

Abstract The tempo of Cenozoic volcanism on opposite sides of the Pacific Ocean has been examined by compiling the numbers of radiometric dates reported for terrestrial volcanic sequences and the numbers of volcanic ash (glass) horizons recorded in Neogene deep-sea (DSDP) sedimentary sections. Within certain limits these data are believed to provide a reliable record of extrusive and explosive volcanism. Although terrestrial and marine records for individual regions reveal important differences in the episodicity of volcanism, a correlation is found between activity in the Southwestern Pacific, Central America and the Cascade Range of western North America. Two important pulses of Neogene volcanism (the Cascadian and Columbian episodes) occurred during the Quaternary ( t = ∼ 2 m.y. to present) and within the Middle Miocene ( t = 16 to 14 m.y. ago), with less important episodes in the latest Miocene to Early Pliocene ( t = 6 to 3 m.y. ago) and Late Miocene (11 to 8 m.y. ago). The names Fijian and Andean are proposed to these episodes. Dating of terrestrial sequences indicates that these episodes of intense volcanism took place in relatively short intervals of time, separated by longer more quiescent periods. It has been suggested that synchronous episodic volcanism is related to changes in rates of sea-floor spreading and subduction. If so, volcanism must amplify these changes, because the variations in tempo of volcanism are much too great for proportional rate changes. An apparent correlation of volcanism in orogenic zones of the circum-Pacific region with world-wide changes of sea level and changes of activity in the Hawaiian-Emperor chain suggests that volcanism records fundamental tectonic changes throughout the entire Pacific region.

Late Pleistocene Paleoclimatology, Foraminiferal Biostratigraphy and Tephrochronology, Western Gulf of Mexico

The distribution of planktonic foraminifera has been studied in 28 piston cores of Late Pleistocene age from the western Gulf of Mexico. Detailed correlation between the cores has been made possible by a high degree of similarity of frequency changes within several species; by coiling direction changes within Globorotalia truncatulinoides; by a datum level representing the near extinction of Globorotalia menardii flexuosa, and Globorotaloides hexagona at the end of the last interglacial; by three distinct volcanic ash horizons, and by calcium carbonate dissolution effects at distinct intervals. Almost all species demonstrate distinct frequency oscillations that are correlatable between cores. A high proportion of these are clearly related to paleoclimatic oscillations and reflect rapidly changing water-mass conditions within the Gulf of Mexico during the latest Pleistocene. No interval appears to have been represented by stable environmental conditions. Causes of frequency changes within several other species are not clearly related to inferred paleoenvironmental changes. High similarity of faunas exists at all times between the northwest and southwest Gulf of Mexico, reflecting similarity of water-mass conditions over a wide latitudinal range. High sedimentation rates, which average between 10 cm/1000 years and 30 cm/1000 years, have enabled a detailed paleoclimatic curve to be established for the last 200,000 years. Three interglacials and two glacials are recognized. Distinct foraminiferal assemblages have enabled definition of 18 zones most of which are related to paleoclimatic changes. Most intense coolings occurred at the end of the penultimate glaciation (zone W) and during the middle of the Wisconsin glaciation (zone Y).The Gulf of Mexico curve is somewhat similar to those of other regions based on O18/O16 ratios, except that, despite close control, no intense cooling is apparent near the end of the last glaciation. The most sensitive warm water indicators are the G. menardii complex and Pulleniatina obliquiloculata; the most sensitive cooler water forms are Globorotalia inflata, and Globigerina falconensis. Several species have intermediate temperature tolerances. Much climatic information is lost when only the G. menardii complex is utilized in climatic studies, because these forms are essentially absent during glacial episodes. The most distinct faunal change associated with the Holocene warming does not coincide with the Z-Y boundary defined by the first consistent occurrence of G. menardii but occurs slightly earlier during the latest Wisconsin. The X zone, based on the consistent occurrence of the G. menardii group is of shorter duration in Gulf of Mexico cores than in cores from the central Caribbean Sea and the equatorial Atlantic Ocean.Three major volcanic ash horizons coincide with climatic coolings; almost immediately preceding the last interglacial (135,000 years B.P.); towards the end of the last interglacial (end of zone X; 90–95,000 years B.P.), and in the earliest part of the last glaciation (75,000 years B.P.). A drastic reduction of G. menardii flexuosa and G. hexagona coincides with the middle ash horizon.

Volcanic ash-horizons in deep-sea sediments from the Eastern Mediterranean

1N 1948 the Swedish Deep-Sea Expedition with the Albatross collected a series of cores by means of the Kullenberg piston-core sampler. In some of these one or two ash-horizons were established. As seen from Table 1 the ash-horizons are present in the upper part of each core only. The cores were from 658 cm up to 958 cm long.