Daniel A. Hepp

A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Mörz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealands export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), quartz-feldspar sand and silt (UNIT5), middle pumiceous sand and silt (UNIT4), silt–sand–clay (UNIT3), upper pumiceous sand and silt (UNIT2), and Holocene marine (UNIT1) sediments forming the modern seafloor. Three paleovalleys filled with sediments from UNIT2 were identified extending west to east across the channel. The CPT profiles were correlated with the seismostratigraphic units and their corresponding soil behaviour type to characterise each units sediment stiffness. The UNIT6 is unlikely to be encountered in current and future capital dredging operations because it is the lowest observed unit. A special dredging methodology may be required for UNIT5 and UNIT4 as they have high CPT tip resistance and undrained shear strength. For UNIT3, UNIT2, and UNIT1, which have very low CPT tip resistance and undrained shear strength, trailing suction and cutter suction dredging is appropriate. The potential turbidity of the water column, however, could impose a significant threat to marine biota and has to be taken into account.

Tributaries of the Elbe Palaeovalley: Features of a Hidden Palaeolandscape in the German Bight, North Sea

Prior to postglacial global sea-level rise in the present North Sea area, Mesolithic hunters and gatherers were able to settle in the coastal lowland landscape between England, Germany and Denmark, commonly known as Doggerland. Regarding the reconstruction of this now drowned palaeolandscape, the German exclusive economic zone (EEZ) sector is still ‘terra incognita’. Recent discoveries of two ancient fluvial systems, both of which were tributaries of the Elbe Palaeovalley, give new insights into the formation of the Mesolithic Doggerland landscape in the German EEZ. One of these fluvial systems developed during the last glaciation and connected the Dogger Hills with the Elbe Palaeovalley. The second river structure discovered in the south seems to be slightly younger and can be identified as the drowned extension of the modern Ems River.

Geophysical and Geotechnical Investigation of a Complex Offshore Foundation Ground Affected by Glacial/Postglacial Sedimentation Processes (German North Sea)

The complexity of the North Sea sediments resulting from the Quaternary glacial history requires a thorough assessment of the local depositional environment to determine appropriate foundation dimensions or to even exclude specific areas from construction. In this study geophysical and geotechnical methods were combined to assess a potential offshore foundation ground in the southern German North Sea. Due to a dense net of multichannel seismic lines the very heterogeneous subsurface structures were mapped and prominent glacial buried valleys were identified in the study area. Geotechnical methods applied at selected locations were used to investigate the bearing capabilities of the soil. The results show, that the infill types of glacial buried valleys have to be taken into account regarding the suitability of foundation ground.

An approach to quantifying Pliocene ice sheet dynamics via slope failure frequencies recorded in Antarctic Peninsula rise sediments

Abstract Understanding of glacially driven sedimentary transport systems across the shelf to the slope and subsequently to deep sea sediment bodies along the Pacific continental margin of Antarctic Peninsula is crucial for interpreting ice sheet dynamics. Here we quantify slope-failure frequencies recorded in Pliocene core intervals of ODP Site 1095. We used the relationship between long-term sedimentation rate and marine carbon burial efficiency to calculate glacial or interglacial specific sedimentation rates. Using the decompacted average length of glacial-interglacial cycles it was possible to solve a set of linear equations to derive average half-periods of 61.59 and 59.77 kyr respectively for the time interval 5.8–3.2 Ma. The resulting frequency distribution of slope failures reflects short and rapid but cyclic ice advances every ∼375 years. Short retention times between slope loading and slope failure are supported by biogenic silica dissolution analyses. This study demonstrates the potential of the rise record to improve models of orbitally controlled size variations of the West Antarctic ice sheet and confirms the hypothesis of a highly dynamic ice sheet during the early Pliocene warm period.