Abstract

Modern depositional environments of the Canterbury Plains provide an analog for ancient conglomeratic depositional systems in nonmarine and coastal zone settings. The Canterbury Plains are up to 70 km wide and 185 km long, covering approximately 8000 km2. The Plains are dominated by extreme geological and geomorphological processes — primarily climate, tectonics, and wave climate. The Canterbury gravels are primarily the deposits resulting from the coalescence of broad fluvial megafans that flowed into a high-energy coastal environment. Most fluvial channels in the basin are gravelly braided rivers. During the present sea-level highstand, a large portion of the plains can be considered as being situated in a low accommodation setting. Most sediment progradation and aggradation occurred and took place near the end of multiple glacial maxima and their associated sea level lowstands. As such, the fluvial megafans are nonmarine lowstand to early transgressive deposits. Valley entrenchment subsequently took place on the seaward side of the plains during the ensuing sea level rise and highstand. On the inner plains, tectonic uplift of the Southern Alps caused fluvial incision during highstand with coastal retreat concomitant with valley incision.

The modern coastline is highly variable, being, at different places, progradational, retrogradational, and stable. The southern coast is dominated by powerful southerly swell conditions, which are highly erosive, and set up high rates of net northwards longshore drift. Consequently, the southern coast, south of Rakaia River, is transgressive, retreating at about 1 m/yr. All southern rivers are incising into the floodplain. In contrast, the northern coastline is situated in the wave shadow of a large, natural groyne formed by volcanic rocks of Banks Peninsula, resulting in lower wave conditions affecting that coast. The Pegasus Bay coastline progrades as a highstand wave-dominated delta.

Since the last deglaciation, sea level has risen until a relatively stable period was reached about 6500 years ago. During this period of sea-level rise and subsequent highstand, the Waimakariri, Ashburton, Rangitata, Raikaia, and Hinds rivers became entrenched, forming sets of relatively steep terraces in proximal areas, which decrease in relief downstream from the mountains until a zone of no or minimal erosion is reached 8 to 15 km inland of the coast. Other environments of the Canterbury Plains include estuaries, the large brackish and shallow Lake Ellesmere into which a birds foot delta is prograding, a progradational strandplain, and localized occurrences of aeolian sediments on the progradational coast and on the plains. Steep gradients of rivers that are incised into the coastline prevent extensive estuarine conditions from developing on the transgressive coastline. One of the dominant controls on sedimentation is climate, mainly extreme amounts of precipitation. Large rivers commonly flood as a result of extreme precipitation derived from advecting moist, subtropical air from the west, rising over the Southern Alps, combined with spring and summer snow melt. In contrast, smaller rivers flood during winter months as a result of stationary easterly frontal systems. Maximum coastal erosion and littoral sediment transport occurs from large oceanic storms originating from the south and southeast.

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