Resedimentation of the 1.8 ka Taupo ignimbrite in the Mohaka and Ngaruroro river catchments, Hawke's Bay, New Zealand

Abstract

Deposition of the 1.8 ka Taupo ignimbrite in the Hawke's Bay buried parts of the Ngaruroro and Mohaka River catchments beneath up to 40 m of loose pyroclastic debris. Re-establishment of the two river systems led to the remobilisation of the loose debris and followed similar patterns in both catchments. An initial period of laharic remobilisation and formation of lahar-deposit-dammed lakes was followed by two phases of fluvial remobilisation. During the first of these, streams were shallow and ephemeral to perennial, with sediment-laden flash-floods eroding headwards through valley-ponded primary pyroclastic and laharic deposits. With gradual re-establishment of vegetation, the sediment:water ratio decreased, and braided rivers with deeper, more stable channels became dominant.Although the general pattern of resedimentation was similar for both river systems, field evidence shows that the volumes of remobilised material, and the likely durations of resedimentation in the different catchments, differed. These variations reflect: (1) the different volumes of initial ignimbrite in the trunk valleys; (2) the different percentage of catchment areas covered by ignimbrite debris; and (3) the different river gradients and valley shapes. However, extremely high sediment yields in the Hawke's Bay region are estimated to have persisted for at least 4-17 yr after the eruption.

Fluvial responses to volcanism: resedimentation of the 1800a Taupo ignimbrite eruption in the Rangitaiki River catchment, North Island, New Zealand

“…(iv) Two ignimbrite layers are recognised (sensu Sparks et al, 1973): Layer 1 is richer in dense components and generally thinner (Walker et al, 1981a;Wilson, 1985); Layer 2 forms the source of the vast majority of remobilised material and is dominated by pumiceous debris (Walker et al, 1981b), including coarse clasts and abundant fine vitric ash, with minor contributions by lithics and crystals (b5 and b3 wt.%, respectively). A co-ignimbrite layer 3 (sensu Sparks et al, 1973) derived from fall-out from the umbrella cloud and material elutriated from the pyroclastic density current is volumetrically insignificant but serves as a vital regional chronostratigraphic marker between syn-and early posteruptive reworking and deposits (e.g., Manville, 2002;Segschneider et al, 2002a).…”

“…The role of these topographic variables may be compared with the results of intensively studied postTaupo 1800a landscape readjustments in diverse depositional settings, including intracaldera fluvial and lacustrine environments (Smith, 1991a,b;Clarkson, 1996;Riggs et al, 2001), ephemeral lake basins (Manville, 2001), extracaldera river systems (Segschneider, 2000;Segschneider et al, 2002a,b), and the outlet channel for intracaldera Lake Taupo (Tilly, 1987;Manville et al, 1999;Manville, 2002). Previous studies (Woolfe and Purdon, 1996), of post-Taupo fluvial processes in the Rangitaiki River (Fig.…”

The Influence of Lateral Confinement Upon the Downstream Gradation in Grain Size of the Lower Ngaruroro River, New Zealand

“…The upstream course of the Ngaruroro is constrained within a deeply dissected, highly faulted landscape [40,41]. Tectonic activity in the catchment reflects its proximity to the Hikurangi margin [40,41]. The Ngaruroro Catchment broadly comprises greywacke of varying ages [41], with a limited distribution of ash, mudstone and limestone.…”

“…Tectonic activity in the catchment reflects its proximity to the Hikurangi margin [40,41]. The Ngaruroro Catchment broadly comprises greywacke of varying ages [41], with a limited distribution of ash, mudstone and limestone. The longitudinal profile has a classic concave-upwards shape (Fig.…”

“…It drains a catchment area of more than 2900 km with a trunk stream length of 173 km. The upstream course of the Ngaruroro is constrained within a deeply dissected, highly faulted landscape [40,41]. Tectonic activity in the catchment reflects its proximity to the Hikurangi margin [40,41].…”