Stratigraphy, provenance, and diagenesis of the Cretaceous Horse Range Formation, east Otago, New Zealand

Abstract

The Horse Range Formation is a structurally controlled late Early Cretaceous to early Late Cretaceous nonmarine unit in east Otago, South Island, New Zealand, containing immature lithic debris. Clasts are generally rounded, with only minor subangular material. The formation contains clasts derived from two principal basement sources: schist and greywacke. Schist debris is most abundant at the base of the described section, and this material is dominated (>60%) by quartz from the greenschist facies core of the Otago Schist belt. Conglomerates with >70% greywacke clasts constitute most of the upper part of the Horse Range Formation. These greywacke conglomerates have a matrix of sand derived mainly from schist. A 60 m thick wedge of quartz‐rich, locally carbonaceous sand occurs interlayered with greywacke conglomerates. The Horse Range Formation rests on sub‐greenschist facies semischist, which forms only a small proportion (<10%) of conglomerate clasts, and most sediment underwent considerable transport (tens of kilometres). Paleocurrent indicators suggest that sediment was derived mainly from the northwest, and drainage was probably controlled by the ongoing development of the northwest‐striking Waihemo Fault System, which had normal sense of motion at that time. The formation is developed from the remnants of a braided axial river flowing southwest from active uplift of greywacke ridges along the Waihemo Fault System at least 50 km inland. Some lateral input occurred from the adjacent schist belt that was being exhumed by the extensional tectonics. Post‐depositional diagenesis of the sediments has resulted in widespread replacement of silicate minerals, particularly albite, by calcite that forms up to 40% of the rock locally. The calcite has ä18OSMOW near +24‰ and 813CPDB near ‐2‰, and was partly dissolved and redeposited from the immature basement debris (metamorphic calcite) and partly introduced from overlying Late Cretaceous and Teriary marine sediments by groundwater.