Contrasting heavy mineral populations rich in 1) ilmenite and zircon, and 2) ilmenite, epidote and semi-opaque debris, exist in both indurated Mesozoic (Torlesse) basement sandstones and in overlying transgressive Cretaceous-Tertiary sandstones around Ohuriawa Gorge. The ilmenite-zircon population forms a zone over 100 m thick below the unconformity at the top of the Torlesse on its eastern boundary northeast of the gorge. The great structural complexity of the Torlesse contrasts with the parallelism between the ilmenite-zircon zone and the unconformity and, together with the lack of any change in overall detrital composition paralleling the change in heavy minerals, shows that the population contrast in the Torlesse rocks must result from alteration that post-dates deformation. The ilmenite-zircon zone in the overlying Cretaceous-Tertiary sediments extends up to near the Paleocene-Eocene boundary. Apatite in coal measures within this zone has been leached early during diagenesis and may be being affected by acidic pore fluids now associated with weathering of pyrite. The nature and distribution of solution effects on epidote grains in Tertiary samples and the absence of the same effects in Torlesse samples show that the change in the Tertiary sediments has not been inherited from the Torlesse but formed soon after deposition by leaching in a dominantly alkaline pore-fluid regime.

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