Transcript Landform terrains

LANDFORM TERRAINS OF
HB/EAST COAST
LANDFORM TERRAINS OF HB/EAST COAST
4 distinctive terrains
1. Axial ranges
•
Comprise Ruahine, Kaweka, Huiarau and Raukumara Ranges of Triassic and
Jurassic age.(170mybp)
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Made up of sandstones, mudstones and greywacke caused by crustal
stresses at the plate boundary. Deposited in deep marine basins then
eroded to a lowlying area and transported to ocean basins by rivers. 1mybp
uplifted to almost 2000m in places and aligned mostly NNE/SSW
•
Fractured, faulted, both laterally (e.g. Ruahine reverse Fault) and
transcurrently (e.g. Mohaka Fault, Kaweka Fault) to form ‘horst and graben’
terrain (e.g. Ohara Depression and Wakarara range)
2. Inland Depression (low hills)
•
Extends from Wairarapa. Low hills and terraces. 20km wide at Hastings to
12km at woodville.
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Terraces are up to 500m asl and dip eastward. Formed after present rivers
were. Lower younger ones form Ruataniwha Plains and border main rivers.
Made up of mostly greywacke and sands, silts and gravels from ranges up
to 1700m thick and have a loess and tephra cover bed up to 1m thick.
• Depression filled with thick (up to 1700m) Pleistocene/Pliocene sediments
of mudstone, sandstone and limestone. Shows there was a huge seaway in
Pliocene, but conglomerates and pumice (from volcanic eruptions) show it
became a lake and riverway in Pleistocene when sea levels dropped and
slow uplift had begun by Ruahine and Wakarara ranges which dragged the
depression up too. Has become dissected as this uplift has continued.
• The Depression is also folded and faulted NE/SW which has influenced
river directions (e.g. Tukituki follows Tukituki Fault and would once have
flowed into sea near Kairakau, Ngaruroro follows Kaweka fault) and
deposition which has covered faults. Many towns and cities sit on these
faults (e.g. Hastings sits on the Poukawa fault, Napier sits on the Awanui
and Napier faults). Lakes Oingo and Runanga were formed when movement
along the Puketapu Fault caused rivers to reverse.
• The 1931 earthquake caused uplift of 2m which drained the Ahuriri lagoon
and caused the Tutaekuri River to discharge at Awatoto.
3. Coastal Uplands
• Made of same material as depression/low hills/terraces terrain but have been
block-faulted. Differential erosion of sandstone, limestones and impervious
siltstones has resulted in cuesta and Karst topography e.g. Te Mata Peak,
Maraetotara plateau, Kahuranaki, Craggy Range (all consist of more resistant
limestone) with northeast trending ridges, their steepest slopes are usually fault
scarps facing east, separated by broad valleys. Highest elevation 600m nearest
coasts. 20 to 40 km wide but continues off shore.
• Much very soft mudstone/clays in coastal areas e.g. Cape kidnappers, Young
Nicks head, Mahia Peninsula leads to massive slope failure especially when
lubricated by heavy cyclonic easterlies which the region is prone to. The clays
are in 5m thick beds and are bright red and green and have been known to ooze
up through faults. They can result in marshy and lake landscapes and rounded
humpy hill formations.
4 Northern Hawkes Bay
• Comprises an uplifted gently dipping and dissected plateau up to 1000m at the
Mangaharuru Range. Generally summits about the same (200m) height. Mostly
mudstone from Tutira to Wairoa and beyond. Easily incised with many landslips
and slides. Some harder limestones e.g. Tangoio Bluff, Napier Hill.