Te Kuiti group in the Waitomo-Te Anga area

Abstract

It is proposed to divide the Te Kuiti Group, a transgressional sequence of shallow-water, dominantly marine beds of Oligocene age, into two subgroups. The lower Te Anga Subgroup is mostly calcareous siltstone and sandstone, and includes the Aotea Sandstone, Whaingaroa Siltstone, and thin basal conglomerates, sandstones, and limestones; the overlying Castle Craig Subgroup, dominantly f1aggy shell limestone, includes the Orahiri Limestone, Waitomo Sandstone, and Otorohanga Limestone, all of which grade into one limestone unit of comparable thickness south and west of Waitomo. The boundary between the two subgroups is defined as the pebbly horizon at the base of the main flaggy limestone sequence. Thin conglomerates, sandstones, and limestones were deposited on a gently undulating erosion surface. Transgression of the sea from the north formed an embayment at least 30 miles wide in which dominantly fine grained sediments were deposited below wave base. Relatively strong currents near the centre of the embayment are evidenced by coarser sediments in the Te Anga and lower Castle Craig Subgroups there. Gradual elevation of the sea floor above wave base, together with increased current action, gave rise to a coarser deposit containing a high proportion of shell material. The thin pebble band at the base of the Castle Craig Subgroup in the Te Anga area has some features suggesting deposition by a high density “gravel flow”, and was probably derived from the Triassic Moeatoa Conglomerate in the west. Pebbles at this horizon near Waitomo were probably derived from the east and deposited under a tractive current regime. The large oysters, which supplied much of the shell debris in the lower and middle parts of the Castle Craig Subgroup, form extensive beds as thick at 30 ft, and suggest that the sea floor at this time may have been only several metres deep.