Sedimentology and paleoenvironmental analysis of Castlecliffian strata in the Dannevirke basin : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Quaternary Science at Massey University
Castlecliffian deposits of the Mangatarata Formation are widespread in the Dannevirke Basin, a fault angle depression on the east coast of North Island, New Zealand. The basin is 80km long and 19-24km wide, is bounded to the east by the upthrust Waewaepa-Oruawharo High and to the west by the upthrust front of the Ruahine Ranges. The basin floor is broken by three major axial-trending anticlinal folds. Basin development began during early Pliocene times with subsidence continuing into the Nukumaruan. Uplift during Castlecliffian time was accompanied by deposition of the Mangatarata Formation. Four facies associations are recognised in deposits of the Mangatarata Formation. Facies association one comprises greywacke rubble and gravel deposited in proximal distal alluvial fan environments. Facies association two is dominated by fluvially deposited, cross-bedded sand and gravelly sand with associated overbank fine deposits. Facies association three is generally a fossiliferous, flaser bedded silt to ffine grained sand, deposited in a tidal/estuarine environment. Facies association four is a rarely fossiliferous, fine grained, centimetre bedded silt deposited in a locally subsiding, interfluvial lacustrine environment. Pumice deposits derived from erosion of unwelded ignimbrite originating in the Taupo Volcanic Zone are significant components of facies association two. They show a range of sedimentary structures associated with rapid deposition. The thickest pumice units, up to 30m, are interpreted to have been deposited in a meandering fluvial environment and display features typical of hyperconcentrated flow and streamflood deposits. It is possible, using identified pumice units as marker beds, to express the physical and temporal extent of specified sedimentary environments. The boundary between fluvial and tidal/estuarine environments and thus approximate paleoshoreline shows that the overall sea-level trend during the Castlecliffian was one of regression. Superimposed on this trend are at least ten episodes of transgression related to eustatic sea-level rise. These alternating cycles of tidal/estuarine to terrestrial sediments are interpreted as cyclothems deposited dominantly during 100,000 and 40.000 year long. c.80-130m magnitude, fifth and sixth order orbitally forced glacio- eustatic sea-level cycles and represent Oxygen Isotope stages 21 to 39. Thinning of pumice tuffs younger than the Rewa Pumice (c.1.0 Ma) toward anticlinal crests suggests that strata younger than this were deformed contemporaneously with deposition, while strata older than the Rewa Pumice show no evidence of syndepositional deformation. This coincides with the restriction of alluvial fan deposits to the west of the study area due to the growing Dannevirke Anticline, between deposition of the Rewa Pumice (c.1.0 Ma) and the Potaka Pumice (c.0.80 Ma).