Abstract:
The Lower Ururoan to Lower Heterian rocks of the Western limb of the Kawhia Regional Syncline are mapped lithostratigraphically in terms of units based on type sections at Kawhia, and the Waikawau Area. The Otamaehu Formation (32+ m) and its lateral equivalent the Tewharau Formation (90+ m), the Ururoa Shellbed (160-3m) and Ururoa Formation 300-600m) of the Newcastle Group; the Rengarenga Group (370-826m); and the Oraka Formation (72-1040m) of the Kirikiri Group.
Fine to very fine sandstones and mudstones are the dominant lithology of the Newcastle and Kirikiri Groups. Medium to coarse sandstones dominate, and conglomerates are common in the northern exposures of the Rengarenga Group which fines southwards.
A large fan-delta prograded into the northern part of the area during the uppermost Ururoan and Temaikan, resulted in at least temporary non-marine conditions in the Kawhia to Marokopa Valley area. South of Marokopa fully marine conditions prevailed. Rapid thickness variations in both biostratigraphic and lithostratigraphic units suggest localised rapidly downsinking basins. Paleoecologic data show shallowing and deepening trends supporting those indicated by the lithostratigraphic data.
One rhynchonellid brachiopod, two gastropods, four bivalves and one belemnite are described, and a large number of informal species are discussed.
Marwick's Ururoan, Temaikan and Heterian Stages are discussed and a zonal subdivision proposed for each.
A major brachiopod extinction event is recognised and followed soon after by an influx of Tethyan and Cosmopolitan elements (e.g. Families Belemnitidae and Trigoniidae). This faunal change is related to the change in environment, especially water depth, which is associated with a major regression in the uppermost Ururoan and Temaikan.
