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Tempo Valley Delta Complex - Tempo and PubbleTyrone
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Summary Full report
Site Type: Inland exposure, Pit
Site Status:
District:
Grid Reference: H340450, H322472, H366494
Rocks
Rock Age: Quaternary
Rock Type: Gravel, Sand
Interest
Other interest: No data, Glacial deposit, Shallow water sediments, outwash, proglacial

Summary of site:

Within this broadly triangular area, 5 km long to its apex at Tattinweer in the north east and 3km wide along the north west/south east orientated base (between Ratoran and Drumderg), is an area of sand and gravel. It stands on glacially scoured bedrock, predominantly the Topped Mountain Sandstone Formation of the Carboniferous period. These deposits are eroded by deep meltwater channels into hills 200 m long and 15 m high. There are also occasional kettle holes puncturing these spreads. The only good exposures in the area are the 2 in the sand pit at Pubble, about 1 km north of Drumderg.

The first is dominated by well sorted (of similar grain size) cross-bedded sands in beds ranging from fine to coarse grained, separated by thin beds of well-sorted shingle and pebbles. Some beds are in unbroken sequence but there are a few erosional breaks in the succession. This exposure can be divided into 3 units. The lowest is 2 m thick, composed of cross-bedded sands later carved by channels that filled with medium-grained sand. It merges upwards into well-sorted sands with sets of superimposed, migratory ripples (climbing ripples). The flow direction is slightly west of north. The middle unit is wedge-shaped, 30 m long and up to 2.5 m thick and consists of well-sorted shingle and pebbles arranged in beds varying between 20 and 50 cm thick. It thins down flow i.e. to the north. The top unit is essentially similar to the lowest but at 3 m is slightly thicker and contains a layer of cemented sand concretions in the same plane as the bedding. The second exposure in the pit is about 60 m long and 6 m high and consists of matrix-free shingle and pebbles and rhythms of sands. The lowest assemblage is of the shingle/pebble type and around 1.5 m thick flowing roughly to the north west. It passes upwards without break into well sorted sands in beds up to 20 cm thick with channels infilled by coarser sands. Then follow rhythms of alternating thick-bedded, medium-grained sands and fine grained, rippled, cross-bedded sands which continue to the top of the exposure. Each rhythm is between 30 and 40 cm thick and the rhythmic sequence totals about 4 m. These sediments were deposited in a shallow water environment at a late stage of the ice retreat. There is no indication of the presence of an active ice front, suggesting that the deposition occurred beyond its influence. Long unbroken sequences of sediments and their relatively fine grain further support this interpretation suggesting low and fairly constant levels of water flow from a southern source. The kettle holes show that some masses of stranded ice were buried in these deposits, melting after the sediments were consolidated.


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