Middle Cenozoic unconformities in the Waihao Valley, South Canterbury Basin

Abstract

This thesis presents descriptions and interpretations of stratigraphic sections in the lower Waihao Valley, southern Canterbury Basin. The aim of this reconnaissance study was to describe and interpret the age, origin and significance of previously identified unconformities in the Eocene – Oligocene sedimentary succession. Four sections were logged and sampled for foraminiferal biostratigraphy. Sedimentary petrological – and petrophysical analysis was carried out to understand the provenance and lithological variations to either confirm or contradict previous local paleoenvironmental interpretations. Eocene strata in the lower Waihao Valley consist of Bortonian to Kaiatan age (ca. 42.6 – 36.7 Ma) Waihao Greensand and Kaiatan to Runangan? age (ca. 39.1 – 34.6 Ma) Ashley Mudstone. Oligocene strata in the lower Waihao Valley consist of lower Whaingaroan age (ca. 34.6 – 29.8 Ma) Earthquakes Marl, upper Whaingaroan to Duntroonian age (ca. 29.8 – 25.2 Ma) Kokoamu Greensand, and Duntroonian to Waitakian age (ca. 27.3 – 21.7 Ma) Otekaike Limestone. Four locations were investigated in the study area: Dons Hole, McCullochs Bridge, Dyer Farm quarry, and Waihao River Walkway (“Cabbagetree Gully”). Local Torlesse metasedimentary basement rocks were investigated at the Waimate Gorge quarry to supplement provenance analysis of the sedimentary strata. Major results are as follows: One Eocene unconformity and two Oligocene unconformities are identified. One Oligocene unconformity is particularly significant as it represents a new location of the regional Mid – Oligocene Marshall unconformity, which was identified with confidence to represent a hiatus of at least 2.5 Ma. One Eocene unconformity was identified at Dons Hole: A burrowed disconformity at the top of the Waihao Greensand (Otaio Limonitic Member) is established to be of Kaiatan age (ca. 39.1 – 36.7 Ma). Two Oligocene unconformities were identified in a measured section at Dyer Farm quarry: (1) A strongly burrowed disconformity, separating the lower Whaingaroan Earthquakes Marl (ca. 29.8 – 27.3 Ma) from the Duntroonian Kokoamu Greensand (ca. 27.3 – 25.2 Ma), shows significant age – and lithological changes, and confidently represents a new location of the regional Mid – Oligocene Marshall unconformity in Zealandia. (2) A fossiliferous horizon in the basal Duntroonian to Waitakian Otekaike Limestone (ca. 27.3 – 21.7 Ma) was identified as a diastem. It represents a short-time, high – energy storm - wave deposit (tempestite).

Local Duntroonian Kokoamu Greensand bed thickness and facies variations were recorded at Dyer Farm quarry and Waihao River Walkway (“Cabbagetree Gully”) - ca. 3 km apart. These lateral variations in deposition rate and facies are associated with either local changes in water depth or sediment supply. Provenance analysis shows that the terrigenous influx in the Eocene – Oligocene strata was probably derived from nearby Torlesse Terrane metasedimentary rocks of Permian age and Cretaceous to Eocene Taratu Formation, suggested by the occurrence of abundant polycrystalline quartz grains. The parautochthonous glauconite in the Eocene – Oligocene strata in the lower Waihao Valley indicates a slow sediment deposition rate in a marine shelf environment and confirms previous local paleoenvironmental interpretations of the southern Canterbury Basin. The outcome of this thesis is the lithostratigraphy and chronostratigraphy of the lower Waihao Valley, a confident identification of a new location of the regional Mid – Oligocene Marshall unconformity representing a hiatus of at least 2.5 Ma, and the confirmation of local Eocene – Oligocene paleoenvironmental interpretations, which add valuable contributions to the wider southern Canterbury Basin geology.