Early-mid Quaternary geology of the Gisborne district, East Coast, North Island, New Zealand

Abstract

The early-mid Pleistocene Mangatuna Formation consists of a conformable sequence of terrestrial and nearshore sediments, exposed in the hills north and east of Gisborne, eastern North Island, New Zealand. The 1 Ma Potaka ignimbrite and coccolith assemblages indicate an age range for the formation of 0.8 - 1.0 Ma. Early-mid Haweran loess and tephra horizons at Kaiti Hill unconformably overlie the Mangatuna Formation, and elsewhere, younger loess and tephra (<30ka) overlie the formation. The Mangatuna Formation unconformably overlies marine Pliocene sediments and is c.120 m thick. It now dips 4-8° to the southwest. Five lithofacies can be distinguished and show rapid vertical and lateral variations. Deposition was largely controlled by paleohighs of Pliocene sediments, and different lithofacies are confined to different valleys. The five lithofacies are (1) Lake Sediments, noncalcareous, highly laminated or massive bioturbated muds; (2) Silts and Sands, lightly indurated and interbedded with tephra horizons; (3) Matokitoki Gravel Member, clast supported conglomerates representative of a distal, braided, and shallow river, (ancient Waipaoa River). The conglomerates are derived from strata to the northwest, in the Main Axial Ranges; (4) Town Hill Member, mudstone and/or sandstone units containing terrestrial flora, and freshwater and estuarine fauna; and (5) Loess and paleosols. Loess within the younger Mangatuna Formation is indicative of cold climate and paleosols indicate warmer episodes. At least forty different tephra beds are recognised. All are calc-alkaline medium K rhyolites (74-78% SiO2,) originating from the Taupo Volcanic Zone. Most are homogeneous in composition. However, c.23% are compositionally mixed from reworking and remobilization of older eruptions. The tephra were emplaced under a range of mechanisms including, pyroclastic flow, fluvial deposition and fallout. A non-welded ignimbrite near the base of formation is correlated to the Potaka ignimbrite (1 Ma) based on lithology, geochemistry, mineralogy and biostratigraphy. The presence of the Potaka ignimbrite in Gisborne indicates a far wider dispersal than previously recognised. The local first appearance of the freshwater mussel Hyridella menziesi can be constrained to 0.8 - 1.0 Ma, as is a moa bone (Anomalopteryx didformis) found within the Matokitoki Gravels. The paleoenvironment reconstructed from fossil flora indicate a coastal, lowland, temperate climate, during the early and later stages of Mangatuna Formation deposition. The formation records a marine transgression that is largely tectonically controlled with climate influencing later stages of deposition, before uplift took place during the Haweran.