Immediate post-c. 1800a Taupo eruption secondary deposits and shorelines

Clarkson, Roger Allen.

Immediate post-c.l800a Taupo eruption secondary deposits occur on and around the margins of the sub-aerial terrace. This terrace formed in response to the transgression and relatively rapid regression of the shoreline after the products of a plinian ultraplinian eruption temporarily dammed the only outlet for Lake Taupo. Between 0.1-0.2 km3 of lacustrine sediments were deposited on the sub-aerial terrace, which covers an area of -80 km2. Over 95% of the lacustrine sediments on the terrace were deposited during the transgression ofthe shoreline. The highest paleoshoreline is marked by truncated Taupo ignimbrite which represents the limit of the landward erosion of between 2-5 vertical meters of Taupo ignimbrite. In some cases the upper paleoshoreline is marked by the presence of cliffs, particularly in the headland regions of northern Lake Taupo. Many intermediate terraces occur on the sub-aerial terrace. Three closely spaced intermediate terraces at heights of between 5-11 m above lake level occur at Whakaipo Bay and Five Mile Beach. These terraces were formed during temporary still-' stands with the relatively rapid regression of the shoreline. The thickest shoreline regression deposits occur at the tops of these intermediate terraces and are up to -1 m thick. Thicknesses of transgressional deposits also vary, with the thickest deposits occurring on steeper slopes and within fluvial channels filled by shoreline transgression. Most of the dry gullies around Lake Taupo were formed immediately after the eruption with many forming before the maximum "filling" of the lake and were either eroded, partially eroded, or partially or fully filled with the transgression of the shoreline. Dry gully formation continued after the partial "emptying" of the lake with many gullies dissecting the sub-aerial terrace and overlying lacustrine sediments. Most dry gullies end at the three closely spaced intermediate terraces. Since the formation of these intermediate terraces recession of the shoreline has continued to the present day. Several zones of deposition are identified from sediments deposited with shoreline transgression. These include lacustrine shoreline, where swash zone and beach berm are both identified. Fully lacustrine deposits include three main zones: surf to build-up breaker zone; nearshore zone (wave-ripple zone); and offshore zone. Appearance and lithofacies assemblages differ between exposed and sheltered shorelines. Sheltered shorelines such as Rotongaio Bay are composed mainly of suspension and slightly reworked Taupo ignimbrite (layer 2). More exposed shorelines contain traction, intermittent suspension and suspension deposits such as found at Tapuaeharuru Bay. The more exposed shorelines show a stronger development of , composition and grainsize variations from the base to the top of lacustrine deposits. For example rhyolite-lithic content decreases from almost pure lithics at the base of some stratigraphic sections to no lithics or 100% pumice at the top. Grainsize also initially decreases from pebbly sands to fine-medium sands from the basal to middle parts of stratigraphic sections, however it becomes coarser near the tops of sections, with lithofacies of planar bedded sands, granules and pebbles common. Within some lacustrine sediments storm deposits are identified as massive rhyolite-lithic sheet deposits. Braided stream deposits were the most common type of fluvial deposits to occur in the northern margins of Lake Taupo and in most cases were formed during intermittent stream flow and erosion. Slumping was also common, and is associated with layer 2 of the Taupo ignimbrite. Slumps were identified in sub-aerial deposits and within lacustrine sediments. Rare turbidite deposits were identified on one steep part of the sub-aerial terrace at Te Hapua Bay. The erosion of the Taupo ignimbrite by fluvial and lacustrine processes resulted in the separation of pumice from the denser rhyolite lithics. These lithics were deposited as lags in higher energy environments such as fluvial and lacustrine shorelines. Pumice dominates in fully lacustrine deposits and were in most cases floated into position and then reworked to produce planar bedded and normally graded pumice pebble units. The comparison of modern Lake Taupo shorelines and immediate post-Taupo eruption shorelines shows that sediments from the paleoshorelines are poorly rounded and have a lower crystal content but a greater pumice content. These differences reflect the transitory nature of the shoreline. Mega-clasts of gray pumice from eruption z are identified within transgressive deposits and therefore were deposited before the maximum filling of the lake. For this reason it is suggested that the initiation of eruption z occurred earlier than previously recognised.

vi, 157 leaves : ill. (some col.), maps ; 30 cm.

1996Clarkson

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