Aspects of late quaternary stratigraphy and evolution of some coastal embayments on the east Coromandel Peninsular, New Zealand
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/15058
The stratigraphy, morphology and sedimentology of the late Quaternary deposits of eastern Coromandel coastal embayments are described, and depicted on a series of ~1:12 500 scale geological and morphological maps. Five stratigraphic units are recognised on the basis of morphology, chronology and depositional environment. The sequence consists of sediments deposited as barrier shorelines, and in the lee of barriers in estuarine and fluvial settings. Prograded, stationary and, rarely, receded barriers are identified, giving rise to three major coastal embayment categories, namely prograded barrier spit embayments, bay barrier embayments, and strand plain embayments. The various barrier types formed in response to different environmental controls, mainly sediment supply rates, coastal configuration and shelf dimensions. The barrier sediments are mainly mature quartzofeldspathic-rich medium sands derived from reworking within the nearshore system. The fluvial sediments are typically immature, poorly sorted gravels, sand and muds transported from the hinterland by rivers. Estuarine sediments are commonly organic- or shell-rich medium to fine sand containing in situ plant remains and in situ estuarine shells. Ultimately, the sediments have been derived from stream and coastal cliff erosion of surrounding Whitianga and Coromandel Group Volcanics. Distinctive tephras (Rotoehu Ash, c. 50 ky BP; Tuhua Tephra Formation, c. 6.2 ky BP; Taupo Pumice Formation, c. 1.8 ky BP; and Loisels Pumice, c. 650 y BP) constitute marker horizons in the deposits and, along with radiocarbon dates, have enabled the age structure of the barriers to be determined. Other relative age methods, including vegetation succession, soil development and weathering products, support the coastal chronosequence established from the interbedded tephras . A model is synthesised to explain the accumulation of sand within the coastal embayments during the late Quaternary. Sea level changes are inferred to be the major control on sedimentation. Two main depositional phases are recorded by the barrier deposits (Pleistocene and Holocene barriers), and are associated with the last interglacial (c. 125 ky BP) and Holocene (6.5 ky BP to present) sea level stillstands. Since sea level reached its present position 6.5 ky BP, the Holocene barriers have experienced several phases of accretion and erosion due to variations in wave regime and sand supply into the embayments. Depositional environments are inferred for Pleistocene barrier samples from multivariate discriminant function analyses of sample textural data, and enable the estimation of the·last interglacial sea level position and subsequent tectonic movements along the east Coromandel coast from the relative displacement of this level.
The University of Waikato
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