Editing Carboniferous rocks of the Howick shore section - an excursion

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Walk northwestwards across the sands of Howdiemont Bay, which cover easily eroded mudrocks of the Foxton Limestone cycles, although the thin limestones can occasionally be seen. Note that moving northwards along the coast the succession is getting older. After 500 m, outcrops of pale yellow/brown sandstones occur towards the top of the Sugar Sands Limestone cycle. They are notable for well-developed examples of both planar and trough cross-bedding. The exposures are 3-dimensional so that the difference between the two is immediately apparent, especially on bedding planes (parallel straight lines versus curved nested lines). The palaeocurrent direction is towards the southeast and deposition took place in a relatively high-energy mouth bar/distributary channel environment. In the sandstones exposed 20 m farther north (i.e. a few metres lower in the succession), the bedding is contorted into fold structures and locally the sandstone is massive. These features are the result of dewatering and liquefaction of the sand soon after deposition, perhaps induced by seismic shock caused by earthquakes.
 
Walk northwestwards across the sands of Howdiemont Bay, which cover easily eroded mudrocks of the Foxton Limestone cycles, although the thin limestones can occasionally be seen. Note that moving northwards along the coast the succession is getting older. After 500 m, outcrops of pale yellow/brown sandstones occur towards the top of the Sugar Sands Limestone cycle. They are notable for well-developed examples of both planar and trough cross-bedding. The exposures are 3-dimensional so that the difference between the two is immediately apparent, especially on bedding planes (parallel straight lines versus curved nested lines). The palaeocurrent direction is towards the southeast and deposition took place in a relatively high-energy mouth bar/distributary channel environment. In the sandstones exposed 20 m farther north (i.e. a few metres lower in the succession), the bedding is contorted into fold structures and locally the sandstone is massive. These features are the result of dewatering and liquefaction of the sand soon after deposition, perhaps induced by seismic shock caused by earthquakes.
  
=== Locality 3 [NU 260 161] ===
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=== Locality3 [NU 260 161] ===
  
 
Continue northwestwards across Sugar Sands Bay, also underlain by mudrocks, to the northwest corner. Here the Sugar Sands Limestone is exposed, distinctive for the presence of large gigantoproductid brachiopods, up to 15 cm across and mainly in their life position, i.e. concave upwards. The trace fossil ''Zoophycos ''is also present on bedding planes of the limestone, looking like brush marks, 15–30 cm across. Immediately beneath the limestone, and only exposed when the tide is halfway out, is a thin (20 cm) dark grey sandstone, containing brachiopods and many burrows, some going down into the underlying white sandstone. In thin section of this dark sandstone, fish scales and bone fragments are common. The underlying distinctive pale grey to white sandstone, with polygonal structures on the bedding planes and thin dark streaks permeating the rock, is a palaeosol. The cracks are the result of expansion and contraction of the soil and the black streaks are the remains of rootlets. The succession at this locality is the top of one cycle (the palaeosol) and the beginning of the next (the thin dark sandstone and the limestone). After deposition of the white sandstone, perhaps by a river or delta distributary, it was colonized by plants and a soil developed. There was then a transgression, and the sea flooded across the coastal plain, reworking the soil to deposit the dark fossiliferous sandstone. Relative sea-level continued to rise and the sea became deeper and clearer enabling the Sugar Sands Limestone to be deposited, in around 20--4.0 m of water.
 
Continue northwestwards across Sugar Sands Bay, also underlain by mudrocks, to the northwest corner. Here the Sugar Sands Limestone is exposed, distinctive for the presence of large gigantoproductid brachiopods, up to 15 cm across and mainly in their life position, i.e. concave upwards. The trace fossil ''Zoophycos ''is also present on bedding planes of the limestone, looking like brush marks, 15–30 cm across. Immediately beneath the limestone, and only exposed when the tide is halfway out, is a thin (20 cm) dark grey sandstone, containing brachiopods and many burrows, some going down into the underlying white sandstone. In thin section of this dark sandstone, fish scales and bone fragments are common. The underlying distinctive pale grey to white sandstone, with polygonal structures on the bedding planes and thin dark streaks permeating the rock, is a palaeosol. The cracks are the result of expansion and contraction of the soil and the black streaks are the remains of rootlets. The succession at this locality is the top of one cycle (the palaeosol) and the beginning of the next (the thin dark sandstone and the limestone). After deposition of the white sandstone, perhaps by a river or delta distributary, it was colonized by plants and a soil developed. There was then a transgression, and the sea flooded across the coastal plain, reworking the soil to deposit the dark fossiliferous sandstone. Relative sea-level continued to rise and the sea became deeper and clearer enabling the Sugar Sands Limestone to be deposited, in around 20--4.0 m of water.

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