Contorted stratification with clay lobes in volcanic ash beds, Raglan-Hamilton region, New Zealand

Contorted stratification in basal volcanic ash beds of the Pleistocene Hamilton Ash Formation incorporates halloysitic clay lobes which project upward into a bed of predominantly allophanic material. The forms produced are similar to convolute laminations described in other marine and non-marine sedimentary sequences. The halloysitic clay lobes have been described previously as concretions and as the products of differential weathering processes. A third hypothesis is proposed to explain the formation of the clay lobes and associated contorted stratification of these basal ash beds, namely, that the beds were deformed by plastic flowage of halloysitic clay into a sensitive allophanic bed. This deformation was possibly a result of water-saturated beds rapidly losing strength as a result of cyclic reversals of stress and strain produced by earthquake shock waves

Volcanic ash beds in the Waikato district

This report lies somewhere between the "pathfinder" variety and the completed account for the reason that the results of detailed mapping and identification are still being prepared for publication. For the younger beds less than 36,000 years we now know both the source and the distribution, but for the older ashes commonly referred to as the Hamilton ash, sources are unknown and a knowledge of distribution restricted to the Waikato district. The principal source is the Okataina volcanic centre with Taupo as a subsidiary (Healy, 1964; Thompson, 1964 :44), and on this information, current mapping into the Waikato district proceeds from the east. Under the circumstances of partly completed work it seems prudent to discuss relevant ash beds already known (Vucetich and Pullar, 1963:65-6; 1964:45-6) to introduce briefly current work by the same authors and by W. T. Ward, and then to relate all of this to previous work portrayed in a soil-forming ash shower map by Taylor (1953)

Uses of volcanic ash beds in geomorphology

In geomorphology air-fall volcanic ashes possess high value as marker beds. These have proved particularly useful in studies associated with infilling of flood plains, fan building, terrace correlation and chronology, erosion, shoreline and sea level changes, recent tectonics, archaeology and ground surfaces. Ash beds and the community are also discussed

Late Cenozoic tephrostratigraphy offshore the southern Central American Volcanic Arc: 1. Tephra ages and provenance

We studied the tephra inventory of 18 deep sea drill sites from six DSDP/ODP legs (Legs 84, 138, 170, 202, 205, 206) and two IODP legs (Legs 334 and 344) offshore the southern Central American Volcanic Arc (CAVA). Eight drill sites are located on the incoming Cocos plate and ten drill sites on the continental slope of the Caribbean plate. In total we examined ∼840 ash-bearing horizons and identified ∼650 of these as primary ash beds of which 430 originated from the CAVA. Correlations of ash beds were established between marine cores and with terrestrial tephra deposits, using major and trace element glass compositions with respect to relative stratigraphic order. As a prerequisite for marine-terrestrial correlations we present a new geochemical data set for significant Neogene and Quaternary Costa Rican tephras. Moreover, new Ar/Ar ages for marine tephras have been determined and marine ash beds are also dated using the pelagic sedimentation rates. The resulting correlations and provenance analyses build a tephrochronostratigraphic framework for Costa Rica and Nicaragua that covers the last >8 Myr. We define 39 correlations of marine ash beds to specific tephra formations in Costa Rica and Nicaragua; from the 4.15 Ma Lower Sandillal Ignimbrite to the 3.5 ka Rincón de la Vieja Tephra from Costa Rica, as well as another 32 widely distributed tephra layers for which their specific region of origin along Costa Rica and Nicaragua can be constrained

Upper and Middle Miocene Ash Beds in the Central Gulf of Mexico

Volcanic ash beds have been widely used as correlation markers in the Paleogene of the western Gulf of Mexico, but their identification and application as regional events in the Neogene play of the deep water Gulf of Mexico have been underutilized. This study approaches this problem through the organization, identification, and correlation of tuffaceous lithologies, supported by available thin sections, SEM images, biostratigraphy, log analysis, and published studies. Tuffaceous lithologies can be recognized with great confidence by calibrating sedimentological evidence with petrophysical properties as observed in well logs. Various beds have been encountered in the Miocene of the central Gulf of Mexico, specifically in the Walker Ridge and Green Canyon protraction areas. Direct or semi-direct evidence of amorphous volcanic glass has been documented from petrographic thin-sections, SEM, and XRD analyses. Tuffaceous lithologies have also been described in mud logs of numerous wells. The next best tool to infer ash beds is the spectral gamma-ray log tool, which reveals a distinct increase in thorium content relative to uranium or potassium in both shales and sandstones. An almost unequivocal petrophysical response to the presence of ash beds is the combination of high GR and low bulk density. Integration of these various lines of evidence within a robust chronostratigraphic framework revealed 11 correlative ash beds in Middle and Late Miocene sandstones and mudstones. Tuffaceous sandstones are preserved in deep-water turbidites and are probably linked to continental ash fall and transport into the Gulf of Mexico. These beds range in thickness from 120 to 250 ft. Tuffaceous mudstones are more likely the result of direct ash fall and preservation in the deep-water environment, and their average thickness is 23 ft. Volcanic activity of the Mid- Late Miocene Yellowstone calderas was the likely source of the volcanic glass found in the tuffaceous lithologies of the central Gulf of Mexico, providing both a long-distance, wind-driven ash fall, and a source-to-sink transport mechanism involving fluvial, deltaic, and deep-water processes. Direct correlation between specific tuffaceous lithologies in the Gulf of Mexico to their specific Yellowstone sources will enhance the Miocene stratigraphic framework of the Gulf of Mexico.Statoil Gulf Services LLC.No embargoAcademic Major: Earth Science

Ages on weathered Plio-Pleistocene tephra sequences, western North Island, New Zealand

Using the zircon fission-track method, we have obtained five ages on members of two strongly-weathered silicic, Pliocene-Pleistocene tephra sequences, the Kauroa and Hamilton Ash formations, in western North Island, New Zealand. These are the first numerical ages to be obtained directly on these deposits. Of the Kauroa Ash sequence, member K1 (basal unit) was dated at 2.24 ± 0.29 Ma, confirming a previous age of c. 2.25 Ma obtained (via tephrochronology)from K/Ar ages on associated basalt lava. Members K2 and K3 gave indistinguishable ages between 1.68 ± 0.12 and 1.43 ± 0.17 Ma. Member K12, a correlative of Oparau Tephra and probably also Ongatiti Ignimbrite, was dated at 1.28 ± 0.11 Ma, consistent with an age of 1.23 ± 0.02 Ma obtained by various methods on Ongatiti Ignimbrite. Palaeomagnetic measurements indicated that members K13 to K15 (top unit, Waiterimu Ash) are aged between c. 1.2 Ma and 0.78 Ma. Possible sources of the Kauroa Ash Formation include younger volcanic centres in the southern Coromandel Volcanic Zone or older volcanic centres in the Taupo Volcanic Zone, or both. Of the Hamilton Ash sequence, the basal member Ohinewai Ash (HI) was dated at 0.38 ± 0.04 Ma. This age matches those obtained by various methods on Rangitawa Tephra of 0.34-0.35 Ma, supporting correlation with this Whakamaru-caldera derived deposit. The origin of the other Hamilton Ash beds is unknown but various younger volcanic centres in the Taupo Volcanic Zone are possible sources. The topmost member, Tikotiko Ash (H6-H7), is estimated to be aged between c. 0.18 and 0.08 Ma. Various silicic pyroclastic deposits documented in North Island and in marine cores may be co-eval with members of the Kauroa Ash and Hamilton Ash sequences on the basis of their age

The age of quaternary surfaces at Waihi Beach

The Waihi Beach surfaces were originally mapped and correlated with European surfaces of similar altitude by Kear and Waterhouse (1961). Exposures along the edges of the surfaces indicate that they are covered with volcanic ashes, the younger of which are of known age. It is the sub-ash surface which should be used for height correlations, and it is the most seaward ash-covered part of the surface which is preferred as the reference point for altitude studies. The heights of the surfaces may not correlate with positions of sea-level at the ages indicated by the ash beds

High level sill and dyke intrusions initiated from rapidly buried mafic lava flows in scoria cones of Tongoa, Vanuatu (New Hebrides), South Pacific

Scoria cones are generally considered to grow rapidly in days to weeks or months. During their growth lava flows may be fed onto the cone surface from lava-lake breaches, or form by coalescence of spatter; such flows are preserved interbedded with scoria lapilli and ash beds. On Tongoa, an island of the Vanuatu volcanic arc in the South Pacific, a series of scoria cones developed during the Holocene, forming a widespread monogenetic volcanic field. Half sections of scoria cones along the coast expose complex interior architecture cone architectures. On the western side of Tongoa Island a scoria cone remnant with steeply crater-ward dipping beds of scoria ash and lapilli contains various dm-to-m thick lava flows, which are connected by irregular dikes cutting obliquely across the beds of the cone. The lava flows are coherent igneous bodies with well-developed flow top and basal breccias. The lavas interbedded with the cone-forming layers are part of a larger (up to 7 m thick) body that is connected to dykes and sills of irregular geometries that intrude the cone's pyroclastic layers. This 3D relationship suggests that the lava flows were buried quickly under the accumulating scoriaceous deposits. This allowed subsequent escape of magma from the fluid interiors of flows, with the magma then squeezed upward or laterally into the accumulating pyroclastic pile. Movement of the pile above the partly mobile lava, and potential destabilisation during intrusion into the pile of lava squeezed from the flows, may signal the onset of localised cone failures, and could be implicated in development of major cone breaches (e.g. Paricutin)

Chronology for climate change: Developing age models for the biogeochemical ocean flux study cores

We construct age models for a suite of cores from the northeast Atlantic Ocean by means of accelerator mass spectrometer dating of a key core, BOFS 5K, and correlation with the rest of the suite. The effects of bioturbation and foraminiferal species abundance gradients upon the age record are modeled using a simple equation. The degree of bioturbation is estimated by comparing modeled profiles with dispersal of the Vedde Ash layer in core 5K, and we find a mixing depth of roughly 8 cm for sand-sized material. Using this value, we estimate that age offsets between unbioturbated sediment and some foraminifera species after mixing may be up to 2500 years, with lesser effect on fine carbonate (<10 mu m) ages. The bioturbation model illustrates problems associated with the dating of ''instantaneous'' events such as ash layers and the ''Heinrich'' peaks of ice-rafted detritus. Correlations between core 5K and the other cores from the BOFS suite are made on the basis of similarities in the downcore profiles of oxygen and carbon isotopes, magnetic susceptibility, water and carbonate content, and via marker horizons in X radiographs and ash beds

Further comments-Waihi Terrace and Hamilton Ash Ages

The authors of the previous paper (Selby et al. 1971) kindly allowed us to see their manuscript, prior to publication. They have made a significant contribution to Bay of Plenty late Quaternary stratigraphy in recognising established ash beds in the coastal terrace sequence at Waihi Beach (Kear & Waterhouse, 1961). This brief note acknowledges their work, and uses their data to produce an alternative age interpretation, that implies broad dates for the formation of each of the coastal terraces and for the Hamilton Ash