Sedimentology and petrography of mass‐emplaced limestone (Orahiri Limestone) on a late Oligocene shelf, western North Island, and tectonic implications for eastern margin development of Taranaki Basin

Abstract

Abstract The Te Kuiti Group in North Wanganui Basin, North Island, New Zealand, of Oligocene ‐ earliest Miocene (Whaingaroan‐Waitakian) age, is dominated by calcareous siltstone, calcareous sandstone, and skeletal limestone. Exposures in the southwestern corner of the basin at Awakino Tunnel are distinctive because, compared with elsewhere, the group is generally thicker (>300 m), has strong dips (25–45°E), exhibits an up‐section decrease in the amount of dip, and the capping Orahiri Limestone includes several thick (up to 3 m) mass‐emplaced units containing a variety of 1–10 cm sized calcareous lithoclasts of older Te Kuiti Group rocks. Petrographic and δI8O and δ13C data suggest that the source deposits of these lithoclasts were cemented at relatively shallow subsurface burial depths (100–500 m) before their uplift and erosion. The lithoclasts so produced were rounded by abrasion in shoal water, often bored profusely by pholad bivalves, and sometimes encrusted by coralline algae, before being periodical...