The Moutere Gravels
Description and origins
The Moutere Gravels form an extensive suite of coarse gravels, stretching the length of the Waimea Valley. They are probably contemporary in age to the similar gravel suites found in in he Karamea area (the Old Man Group) and the Wairau Valley (Hillersden Gravel).
The Moutere Gravels are generally coarse and poorly sorted, with a clay or silty matrix, yellow in colour, and composed almost entirely of rounded sandstone and schist-like rocks (semi-schist) derived from the Torlesse Group, which make up much of the Alpine range in this part of New Zealand. Occasional clasts of granite and quartzite are also found. The material is deeply weathered, and often shows bright orange mottling, and signs of clay and iron translocation (movement), which suggest weathering in a chemically active and warm environment.
On the evidence of spores and pollen collected from the gravels, they have been dated to the Pliocene and earl Pleistocene (ca. 5 - 1.5 million years before present). Some of the most interesting samples that have been analysed come from the Kina area. These contained several conifer species.
From their distribution and composition, they appear to have been laid down by braided streams, flowing off the rapidly rising Alpine range, as it was uplifted along the Waimea-Flaxmore fault. Once, they were probably far more extensive; now, they form a hummocky spread of gravel covering the floor of the Moutere Depression. At their deepest they are over 1 km thick.
Sedimentary features
Sections in the gravels are abundant throughout the Moutere Depression, and the features that these reveal help us to understand the formation and depositional environment of the gravel. Lenses of silt and sand are common, and these can indicate the approximate shape and scale of the stream channels - up to 1-2 metres in depth and 30-50 metres in width. Because the river is likely to have been braided at this time, several channels must have co-existed, though many may have been active only in times of flood. Cross-bedding is also apparent in some places, showing how gravel bars formed and migrated as successive floods of gravel were swept downstream. Fining upward cycles record the gradual reduction in discharge during the course of single floods (or seasons). As discharge reduces, the size of the material that the water can transport is also reduced, and the gravel gives way to sand, and then in some cases to silt; then the next flood sweeps across the surface, bringing new layers of coarse gravel. The size of the gravels (up to 10 cm in length), and their degree of rounding, suggests that the water was fast-flowing and powerful, with speeds of at least 1 m/sec. The poor sorting of the gravels suggest that deposition was rapid, with little opportunity for winnowing of the material during periods of lower flow.
In several places, stumps and trunks of trees can also be seen buried in the gravels. The remains are usually heavily mineralised but still show clear woody structure. The majority of the trees appear to be Nothofagus (Southern beech). Stumps and trunks are often found in small clusters, and in some places it is possible to see the disturbed and stained gravel made by the root-bowls of vanished trees. The trees and root-bowls thus indicate old floodplain surfaces, where the trees originally grew.
Interpretation
When the Moutere Gravels were being deposited, therefore, the local environment must have consisted of a broad, gravelly floodplain, crossed by a complex web of fast-flowing braided stream channels, carrying heavy sediment loads. In more stable areas, perhaps further away from the active streams, beech forests grew. The surrounding mountains were probably steep-sided and deeply eroded by gullies and landslips, as a result of seismic activity. The climate was warm (probably slightly warmer than today), though the general trend was for a cooling climate as New Zealand entered the Pleistocene glaciations.
The Moutere Gravels form an extensive suite of coarse gravels, stretching the length of the Waimea Valley. They are probably contemporary in age to the similar gravel suites found in in he Karamea area (the Old Man Group) and the Wairau Valley (Hillersden Gravel).
The Moutere Gravels are generally coarse and poorly sorted, with a clay or silty matrix, yellow in colour, and composed almost entirely of rounded sandstone and schist-like rocks (semi-schist) derived from the Torlesse Group, which make up much of the Alpine range in this part of New Zealand. Occasional clasts of granite and quartzite are also found. The material is deeply weathered, and often shows bright orange mottling, and signs of clay and iron translocation (movement), which suggest weathering in a chemically active and warm environment.
On the evidence of spores and pollen collected from the gravels, they have been dated to the Pliocene and earl Pleistocene (ca. 5 - 1.5 million years before present). Some of the most interesting samples that have been analysed come from the Kina area. These contained several conifer species.
From their distribution and composition, they appear to have been laid down by braided streams, flowing off the rapidly rising Alpine range, as it was uplifted along the Waimea-Flaxmore fault. Once, they were probably far more extensive; now, they form a hummocky spread of gravel covering the floor of the Moutere Depression. At their deepest they are over 1 km thick.
Sedimentary features
Sections in the gravels are abundant throughout the Moutere Depression, and the features that these reveal help us to understand the formation and depositional environment of the gravel. Lenses of silt and sand are common, and these can indicate the approximate shape and scale of the stream channels - up to 1-2 metres in depth and 30-50 metres in width. Because the river is likely to have been braided at this time, several channels must have co-existed, though many may have been active only in times of flood. Cross-bedding is also apparent in some places, showing how gravel bars formed and migrated as successive floods of gravel were swept downstream. Fining upward cycles record the gradual reduction in discharge during the course of single floods (or seasons). As discharge reduces, the size of the material that the water can transport is also reduced, and the gravel gives way to sand, and then in some cases to silt; then the next flood sweeps across the surface, bringing new layers of coarse gravel. The size of the gravels (up to 10 cm in length), and their degree of rounding, suggests that the water was fast-flowing and powerful, with speeds of at least 1 m/sec. The poor sorting of the gravels suggest that deposition was rapid, with little opportunity for winnowing of the material during periods of lower flow.
In several places, stumps and trunks of trees can also be seen buried in the gravels. The remains are usually heavily mineralised but still show clear woody structure. The majority of the trees appear to be Nothofagus (Southern beech). Stumps and trunks are often found in small clusters, and in some places it is possible to see the disturbed and stained gravel made by the root-bowls of vanished trees. The trees and root-bowls thus indicate old floodplain surfaces, where the trees originally grew.
Interpretation
When the Moutere Gravels were being deposited, therefore, the local environment must have consisted of a broad, gravelly floodplain, crossed by a complex web of fast-flowing braided stream channels, carrying heavy sediment loads. In more stable areas, perhaps further away from the active streams, beech forests grew. The surrounding mountains were probably steep-sided and deeply eroded by gullies and landslips, as a result of seismic activity. The climate was warm (probably slightly warmer than today), though the general trend was for a cooling climate as New Zealand entered the Pleistocene glaciations.
The cliffs at Kina
Sites to note include:
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What to see at Kina CliffsN.B. The cliffs are dangerous, and great care should always be applied in exploring the site.
The cliffs at Kina provide an exposure through ca. 700 metres of Moutere Gravels, reaching up to 150 in depth in places.
Interesting features include:
A fossil tree root, eroding out of the gravels
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Sand lenses and chanel-cut features in the cliffs at Kina.
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Other places to see the Moutere GravelsCut Hill (1604000,5427900): Sections in roadside show bedded gravels with sand lenses.
Spooners Saddle (15922500, 54078000): Deep sections of coarse gravel, including large sand lenses exposed in roadside |
