The Waihemo, Blue Lake and Hawkdun Fault Zones initially formed with their northeast side moving downwards, with the faults mainly dipping to the northeast. Some of the faults also dipped to the southwest, and this created a major valley system that drained southeast from Central Otago along the down-dropped faults. Small amounts of sediments from this river system, mainly gravels, are preserved near the present east coast as the Horse Range Formation, and inland as the Kyeburn Formation, shown on the map below. These deposits formed between 112 and 100 million years ago, and have mainly been uplifted and eroded away in the intervening areas as younger mountains formed, with a new drainage pattern. The modern Shag River follows near to the course of the ancient river, the “Waihemo River” as shown in the cartoon below, but the headwaters of the ancient river are now in the headwaters of the Taieri River catchment.
The Horse Range Formation sediments deposited by the Waihemo River were not locally-derived from the nearby schists and semischists that were being uplifted by the adjacent Waihemo Fault Zone. Instead, the formation was derived mainly from greywacke outcrops at least 50 km away, and clasts were transported in a braided river, the “Waihemo River”, see cartoon above). The Horse Range Formation also had input from the Otago Schist belt to the west and/or south, and this appears as quartz and schist pebbles, especially near the base of the formation. Greywacke and schist material in the Horse Range Formation has been substantially altered by groundwater, with addition of calcium carbonate (calcite) from overlying marine sediments, and also from calcite within the greywacke and semischist. Some alteration associated with coaly sediments has resulted in formation of pyrite and Fe2+-bearing clay minerals.
The Kyeburn Formation is a spectacular remnant of the middle Cretaceous river system. It consists of a sequence of river gravels that is several kilometres thick in parts, representing a very large depositional basin that formed in the early stages of Otago Schist uplift. These sediments are locally-derived from nearby semischist and greywacke outcrops. Many of the cobbles are angular or poorly rounded because of shorter transport distances compared to the Horse Range Formation. Groundwater alteration has caused some cementation of these gravels, but this is mostly clay minerals, including some pyrite and Fe2+-bearing clay minerals. The intense calcite cementation seen in the Horse Range Formation is absent from the Kyeburn Formation.
- Mitchell, M, Craw, D, Landis, C A & Frew, R. 2009. Stratigraphy, provenance, and diagenesis of the Cretaceous Horse Range Formation, east Otago, New Zealand. New Zealand Journal of Geology & Geophysics 52: 171-183. doi:10.1080/00288300909509884
- Craw D 2010 Delayed accumulation of placers during exhumation of orogenic gold in southern New Zealand. Ore Geology Reviews 37: 224-235. doi:10.1016/j.oregeorev.2010.03.006
- Crustal structure and topography of the Otago northeast margin
- Initiation of the fault zones on the northeast Otago margin
- Faulting and erosion of the Macraes gold deposit
- Gold-bearing veins at Oturehua
- Blue Lake Fault Zone
- Graphite and gold on the northeast schist margin
- Gold and arsenic in pyrite
- Waipounamu Erosion Surface
- Alluvial gold along the northeast Otago margin
- Rise of modern mountains on Otago's northeast margin
- Patearoa gold: Alluvial concentrations in a dynamic environment