Geology of the Urewera Greywacke in Waioeka Gorge, Raukumara Peninsula, New Zealand

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Abstract
The structurally complex rocks exposed throughout Waioeka Gorge are flysch-type redeposited volcaniclastic sedimentary rocks. The massive greywackes, thin-bedded alternating greywacke and argillite sequences, and massive argillites are inferred to have been deposited in a submarine fan-type environment by grain flow, turbidity current, and hemipelagic sedimentation mechanisms. The sand-sized greywacke lithologies have an average composition of 20% quartz, 20% feldspar, 25% rock fragmnts, and 30% matrix. Approximately 60% of the rock fragments are of volcanic origin, and much of the matrix is altered volcanic material. The rocks have been metamorphosed to lower prehnite-pumpellyite facies grade. A new formation within the Urewera Greywacke, the Oponae Mélange, is defined and described. The formation consists of characteristically intermixed and deformed “non-exotic” (greywacke/argillite) and “exotic” (chert/marble/spilite) lithologies within an argillite and fine-grained greywacke matrix. Three deformation phases are identified in the Waioeka Gorge sequence. The first, during the Motuan (Albian), affected sediments with differing degrees of consolidation, and resulted in a variety of structural styles. Subsequently, a Late Cretaceous phase formed gentle and open folds, and a Mid-Late Tertiary phase resulted in widespread faulting and some drag folding. Waioeka Gorge strata are assigned to the Urewera Greywacke, an informal unit within the Torlesse Supergroup. The occurrence of diagnostic Motuan fossils within both the Urewera Greywacke and an unconformably overlying fossiliferous unit gives the uppermost part of the Waioeka Gorge sequence a Motuan age. The lower parts of the sequence may be of Taitai or Oteke Series age, or older. No unconformities are known within or underlying the Urewera Greywacke, and therefore the Waioeka Gorge sequence may represent a period of substantially continuous deposition throughout the time of the Rangitata Orogeny.