Geology of The Hawkes Bay Area

Lee, J.M.; Bland, K.J.; Townsend, D.B.; Kamp, P.J.J. (comps) 2011
Geology of the Hawke's Bay area. Lower Hutt: GNS Science.
Institute of Geological & Nuclear Sciences 1:250,000 geological map 8
93 p. + 1 folded map

Abstract: Overview of the Hawke's Bay Geological Map

The Hawke’s Bay geological map spans 18,800 km² of the central and eastern North Island. It features diverse landscapes, including:

• The southern part of Lake Taupo,
• Active volcanoes of the Tongariro National Park,
• Hill country of the Manawatu region.

The area is bisected by the Ruahine, Kaweka, Kaimanawa, and Ahimanawa ranges, which form part of the North Island axial ranges. East of these ranges, the map covers:

• Heretaunga, Ruataniwha, and Takapau plains,
• Hill country,
• A coastline stretching from the Mohaka River mouth in the north to the rugged shore east of Dannevirke in the south.

Offshore, the continental shelf extends 50–150 km eastward towards the Hikurangi Trough, where the Pacific Plate subducts beneath the Australian Plate, deforming the seafloor into submarine highs and depressions.

Geological History and Rock Formations

Oldest Rocks: Late Jurassic to Early Cretaceous

• Belong to the Torlesse (composite) terrane, exposed in the axial ranges.
• Comprise the Kaweka terrane, Esk Head belt, Pahau terrane, and Pahaoa Group.
• These units were accreted onto Gondwana’s eastern edge along a west-dipping subduction zone.
• The boundary between the Torlesse (composite) terrane and the older Waipapa (composite) terrane is hidden beneath volcanic rocks of Tongariro National Park.

Late Early Cretaceous to Oligocene Rocks

• Found east of the axial ranges.
• Mangapurupuru Group olistostromes, sandstone, and mudstone were deposited during the transition from subduction to an extensional tectonic regime.
• Glenburn Formation was deposited in a submarine fan setting.
• Gradual decrease in coarse-grained sediments occurred from the Late Cretaceous through the Paleogene, with mudstone-dominated Tinui and Mangatu groups forming in a passive margin tectonic setting.

Miocene Developments

• Early Miocene Tolaga Group rocks mark the development of a new subduction system between the Pacific and Australian plates.
  • Features include increased sedimentation rates, olistostromes, higher sandstone content, uplift, facies changes, and sub-basin formation.
• In eastern areas, Late Cretaceous to Paleogene rocks were thrust over Early Miocene rocks.
• Late Miocene saw subsidence of localised sub-basins and deposition of deep-water mudstone and turbidites.
• Latest Miocene-earliest Pliocene: Shallow marine sandstone, limestone, and conglomerate were deposited around the emerging Kaimanawa Mountains and Kaweka Range.

Western and Eastern Areas

• Western regions: Middle to Late Miocene Whangamomona Group sandstone, mudstone, and limestone were deposited in a continental shelf-slope system.
• Eastern regions: Uplift created a narrow coastal area and axial ranges by Early Pliocene.

Geological Hazards in the Hawke's Bay Map Area

The region is vulnerable to various natural hazards, including:

• Seismic activity:
  • The M7.8 1931 Hawke’s Bay Earthquake remains the deadliest in New Zealand’s history.
  • Active faults are widespread throughout the area.
• Volcanic hazards:
  • Mount Ruapehu erupted in 1995/96, disrupting air travel, with its most recent eruption occurring in September 2007.
  • Lahars from Crater Lake, such as the March 2007 event down the Whangaehu River valley, pose risks to infrastructure.
• Coastal erosion and tsunamis: Significant threats to coastal areas.
• Landslides: Common in elevated areas underlain by Miocene and Pliocene mudstone.

The Tongariro Power Scheme contributes 4% of New Zealand’s electricity production, though full exploitation of geothermal energy resources remains unlikely due to the area’s popularity as a tourist destination.