The Pliocene-Pleistocene Development, Uplift and Emergence History of the Manawatu Strait, New Zealand
The Manawatu Saddle is located within the structural and topographical low separating the Ruahine and Tararua axial ranges of the lower North Island. Pliocene-Pleistocene sedimentary rocks unconformably drape over Cretaceous aged basement rock in the structural low and record the existence of a paleo seaway known as the Manawatu Strait, which connected the West Coast (Whanganui Basin) to the East Coast (Ruataniwha Strait). The sedimentary sequence shows a succession of alternating marine and terrestrial units recording the development of the Manawatu Strait. These sedimentary rock formations range in age from Opoitian to Castlecliffian. This study investigates the stratigraphy, lithofacies and resulting geological history of the Manawatu Strait spanning the development, uplift and final emergence history of the strait. Five key measured sections were constructed to take advantage of new outcrop exposure allowed detailed descriptions of the Manawatu Saddle geology to be presented. Four formations are identified and the formation boundary overlaps between past authors is constrained based on field observations. The age range for each formation is also constrained. Based on these results for the first time a detailed lithofacies scheme is applied to the sedimentary rocks within the Manawatu Saddle to understand the changing depositional environments within the Manawatu Strait throughout its development and uplift. A series of 3D schematic paleogeographic figures are presented showing the depositional environments within the Manawatu Strait, at key time intervals. Results highlighted by this thesis show four major formations within the Manawatu Saddle. The oldest formation, the Mangatoro Formation (Opoitian), records the initial formation of the Manawatu Strait attributed to a regional subsidence event known as the Tangahoe pull down event. The Mangatoro Formation also shows sedimentary deposited during peak marine transgression within the Manawatu Strait. The Te Aute Formation (Waipipian-Mangapanian) provides an insight into the uplift timing of the axial ranges and the resulting effect on the Manawatu Strait. The Kumeroa Formation (Nukumaruan) shows the influence of eustatic sea level change in the Manawatu Saddle. The youngest formation within the Manawatu Saddle is the Mangatarata Formation (Castlecliffian), and marks the final uplift and emergence of the Manawatu Strait, indicated by the presence of marginal marine lithofacies this also marks the final separation of the West Coast (Rauataniwha Strait) and West Coast (Whanganui Basin).