The term “Cenozoic” is a rather formal one; the Paleocene to Pliocene rocks are usually called the “Tertiary” rocks. In many parts of the world a major break in the succession of strata marks the passage from the Mesozoic Era (“the age of reptiles”) to the Cenozoic Era (“the age of mammals”): New Zealand, however, was one of the regions where sedimentation continued without a break. The diagram opposite shows stages in the slow advance of the sea during the Cenozoic, reaching a maximum in the early Oligocene; followed by a retreat.
Cenozoic strata, although now limited in area in the South Island, are widespread in the North. Economically they are important; they contain almost all of New Zealand's coal and useful limestone; moreover, they and the late Cretaceous sedimentary strata are the only New Zealand rocks that constitute likely sources of petroleum and natural gas.
The late Cretaceous and Cenozoic rocks of this country are wholly sedimentary and volcanic in origin; none of the deep intrusive or metamorphic rocks that may have been forming during this time has yet been exposed at the surface. Volcanic outbursts occurred at intervals during the Cenozoic both in North and South Islands. Some of these volcanic rocks were poured out on to the floors of basins in which the marine sedimentary rocks were accumulating; others were poured out on to land. The huge accumulation of volcanic materials in the central volcanic region was not erupted until very late in the Cenozoic.