Pacific offshore record of plinian arc volcanism in Central America: 3. Application to forearc geology

[1] Sediment gravity cores collected on the Pacific slope and incoming plate offshore Central America reach up to 400 ka back in time and contain numerous ash layers from plinian eruptions at the Central American Volcanic Arc. The compositionally distinct widespread ash layers form a framework of marker horizons that allow us to stratigraphically correlate the sediment successions along and across the Middle America Trench. Moreover, ash layers correlated with 26 known eruptions on land provide absolute time lines through these successions. Having demonstrated the correlations in part 1, we here investigate implications for submarine sedimentary processes. Average accumulation rates of pelagic sediment packages constrained by bracketing tephras of known age range from ∼1–6 cm/ka on the incoming plate to 30–40 cm/ka on the continental slope. There are time intervals in which the apparent pelagic sedimentation rates significantly vary laterally both on the forearc and on the incoming plate where steady conditions are usually expected. A period of unsteadiness at 17–25 ka on the forearc coincides with a period of intense erosion on land probably triggered by tectonic processes. Unsteady conditions on the incoming plate are attributed to bend faulting across the outer rise triggering erosion and resedimentation. Extremely low apparent sedimentation rates at time intervals >50–80 ka suggest stronger tectonic activity than during younger times and indicate bend faulting is unsteady on a longer timescale. Submarine landslides are often associated with ash layers forming structurally weak zones used for detachment. Ash beds constrain ages of >60 ka, ∼19 ka, and <6 ka for three landslides offshore Nicaragua. Phases of intense fluid venting at mud mounds produce typical sediments around the mound that become covered by normal pelagic sediment during phases of weak or no activity. Using intercalated ash layers, we determine for the first time the durations (several hundred to 9000 years) of highly active periods in the multistage growth history of mud mounds offshore Central America, which is essential to understand general mud-mound dynamics

Geotechnical Offshore Seabed Tool (GOST): CPTu measurements and operations in New Zealand.

The Geotechnical Offshore Seabed Tool (GOST) was used in three field campaigns in on-and offshore areas in Tauranga, Bay of Plenty, New Zealand. Offshore measurements were performed along the main shipping channel in the Tauranga Harbor and onshore measurements were conducted at the locations of two landslides at Pyes Pa and Omokoroa. From each of these sites a sample static CPTu profile is described and reviewed. Additionally, a vibratory CPTu from the Pyes Pa landslide is presented. The CPTu results were used for subsurface sediments investigations at the tested locations

Pacific offshore record of plinian arc volcanism in Central America: 1. Along-arc correlations

We collected 56 marine gravity cores from the Pacific seafloor offshore Central America which contain a total of 213 volcanic ash beds. Ash-layer correlations between cores and with their parental tephras on land use stratigraphic, lithologic, and compositional criteria. In particular, we make use of our newly built database of bulk-rock, mineral, and glass major and trace element compositions of plinian and similarly widespread tephras erupted since the Pleistocene along the Central American Volcanic Arc. We thus identify the distal ashes of 11 Nicaraguan, 8 El Salvadorian, 6 Guatemalan, and 1 Costa Rican eruptions. Relatively uniform pelagic sedimentation rates allow us to determine ages of 10 previously undated tephras by their relative position between ash layers of known age. Linking the marine and terrestrial records yields a tephrostratigraphic framework for the Central American volcanic arc from Costa Rica to Guatemala. This is a useful tool and prerequisite to understand the evolution of volcanism at a whole-arc scale

From the inner shelf to the deep sea : depositional environments on the West Antarctic Peninsula margin - a sedimentological and seismostratigraphic study (ODP leg 178) = Vom inneren Schelf zur Tiefsee: Ablagerungsräume am Westrand der Antarktischen Halbinsel - eine sedimentologische und seismostratigraphische Studie (ODP Leg 178)

Aim of this study was to describe and characterize three typical West Antarctic Peninsula continental margin depositional environments. Seismic, downhole and laboratory tools and techniques were applied to record physical and chemical proxies for a mulifacetted and differenciated understanding of the particular environments. 1) A detailed seismostratigraphic characterisation of the unique Antarctic inner and outer shelf along the passive margin of the West Antarctic Peninsula has been complemented by logging informations that are tied to the seismic profiles via a new continuous depth vs. velocity profile. Sealevel fall overcompensated by increased isostatic subsidence due to ice build-up that affected the continent and the adjacent shelfs is the most likely explanation for time synchronous start of topset preservation at ~2.8 Ma observed in Antarctic Peninsula and East Antarctic shelf records in conjunction with Nothern Hemnisphere Glaciation. 2) Seismostratigraphic Model of Palmer Deep based on seismic Deep Tow Boomer records. 3) Large mounts on the continental rise along the Antarctic Peninsula margin between 63°S and 69°S and within 200 km of the shelf break are interpreted as sediment drifts, separated by turbidity current channels. Using a special standardized moving window counting routine the spatial facies information data has been transformed into a regional Antarctic Peninsula ice volume indicator curve that is in good agreement with the Lear et al., 2000 data over the past 10 Ma. This regional ice volume curve for the Antarctic Peninsula is the most significant finding of this thesis. All evaluated proxy data prove that the early Pliocene was a time of reduced global- and Antarctic ice volume (-70%) and sea ice extent. Never before and after during the last 10 Ma was palaeo productivity higher at the rise than during this time. During the late Miocene the ice sheet was highly dynamic with frequent advances and retreats. Starting at 3.2 Ma in concert with the Northern Hemisphere Glaciation the ice sheet became a permanent feature occupying the shelf during most of the glacial half cycles

From the inner shelf to the deep sea: Depositional environments on the West Antarctic Peninsula Margin - A sedimentological and seismostratigraphic study (ODP Leg 178

To be entered later

Potential of estimating relative density by measuring shear wave velocity in field and in laboratory

This study presents empirical correlation between shear wave velocity and relative density established from triaxial tests with bender elements on North Sea sands. The correlation is applied on in-situ locations with available P-S-Logging and CPT data. The resulting relative densities are compared. The results are generally in good agreement and reasons for some differences are discussed

Résumé des séances de la société Murithienne du Valais, Partie administrative, Activité de la Murithienne durant 1973

In this study we present a late Miocene–early Pliocene record of sixty-four zones with prominent losses in the magnetic susceptibility signal, taken on a sediment drift (ODP Site 1095) on the Pacific continental rise of the West Antarctic Peninsula. The zones are comparable in shape and magnitude and occur commonly at glacial-to-interglacial transitions. High resolution records of organic matter, magnetic susceptibility and clay mineral composition from early Pliocene intervals demonstrate that neither dilution effects nor provenance changes of the sediments have caused the magnetic susceptibility losses. Instead, reductive dissolution of magnetite under suboxic conditions seems to be the most likely explanation. We propose that during the deglaciation exceptionally high organic fluxes in combination with weak bottom water currents and prominent sediment draping diatom ooze layers produced temporary suboxic conditions in the uppermost sediments. It is remarkable that synsedimentary suboxic conditions can be observed in one of the best ventilated open ocean regions of the World

Relative density prediction based on in-situ and laboratory measurements of shear wave velocity

Extensive laboratory and in-situ testing has been carried out in an onshore test site on the North Sea Coast near Cuxhaven, Germany. The test site was chosen due to initially high cone resistance (>30 MPa) in sand and close stratigraphic links to soil conditions for North Sea offshore wind projects. The in-situ program included cone penetration testing, geophysical downhole logging and sampling. A correlation between relative density, shear wave velocity and mean effective stress is proposed based on laboratory measurements with bender elements under anisotropic stress conditions. The proposed correlation is applied to in-situ shear wave velocity data and compared with relative density estimated based on density logs and recognized empirical CPT correlations. Taking the density logging as a reference, the new correlation provides comparable ac-curacy range as the existing empirical CPT-based relations although with a slight tendency for underestimation. This novel correlation could be used as an alternative and conservative verification procedure for the relative density of sands

Settlements of sand under cycling loading: Experiments on samples from the North Sea

In the current study strain controlled cyclic simple shear tests were involved into the determination of the cyclic compaction. It is shown that the preparation method used to achieve a range of soil fabrics affects the settlement under cyclic loading significantly. The influence of the preparation procedure becomes more considerable with the increasing of the shear strain amplitude