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Mourne Plain Complex - Sandpiper PitDown
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Summary Full report
Site Type: Pit
Site Status:
District:
Grid Reference: J32823121
Rocks
Rock Age: Quaternary (Late Midlandian)
Interest
Other interest: No data, Glacial deposit, Glaciomarine deposit, Glaciotectonic, ice lobe, moraine ridge, outwash

Summary of site:

The Sandpiper Pit is located immediately to the east and outside the impressive arc of the Cranfield Moraine and its extensive working faces expose three associated groups of sediments; from the base, stratified sand and gravel, stratified diamict and massive diamict.

The stratified sand and gravel is about 4 m thick with regular cycles of sand, silt and clay interrupted periodically by lenses and more continuous beds of gravel. Some gravels pass laterally into sands. The sands are variable in thickness, up to 40 cm and are either finely bedded or cross-bedded. Ripple surfaces are common, often covered in fine clay. There are many unusual features in this association. Pillar and dish structures of exceptional size, up to 1.8 m across, are present. They are formed by migration of excess pore water between sand grains eventually finding a point of escape, leading to a dish-shaped collapse. There is also a random scatter of larger rock fragments including a boulder over a metre across which breaches the junction between pebbly and laminated sand deforming the laminations beneath. Pebbly sands, almost vertical on the western side, are draped over it. Pods of chaotic gravels and boulders 2 m deep and up to 12 m long lie on and deform laminated sands and cause some small scale faulting. The stratified diamict overlies the stratified sand and gravel along an observed length of 500 m. It is 4 m thick with rather vague bedding up to 10 cm thick. Diamict is a sediment with a mixture of grain sizes, from clay to cobble and is most often associated with glaciation. This one has a matrix of sand containing larger rocks up to boulder size. The cobbles and boulders penetrate and disturb the underlying sediment. Most of the gravel to pebble-sized particles are aligned. The deposit is cut by channels up to 10 m wide and 3 m deep filled with stratified sand and gravel as well as massive (meaning without internal bedding planes) sands. The third sediment division is an unbedded diamict with very few larger particles except for well defined lenses and clumps of cobbles and boulders. A channel from this level penetrates the other two divisions and is sealed by mud laminations in continuous sequence with the diamict. These three facies (associations of sediments) reveal the complex processes operating on the fringe of a melting glacier ploughing across a sea bed covered in its own submarine outwash deposits. The stratified sands and gravels contain a good weight of evidence. The cycles of sand silt with occasional gravels reflect sediment supply and/or varying pulses of flow and their ability to transport particles. The dish and pillar structures suggest rapid deposition trapping excess water that then migrated to an escape vent. At periods of minimal flow, fine, suspended mud settled to form the mud drapes. The sands are fine-grained and well sorted (of even grain size formed in a stable sedimentary regime) which draws attention to the scatter of larger rocks and isolated boulders. Such relationships are well understood and widely interpreted as dropstones, rocks released from melting ice floes containing glacial debris. On impact with the sea floor, any laminated sediment beneath them would deform. The sand-filled channels are typical of submarine outwash from melting glaciers. The pods of gravel and boulders in this setting are, most likely, grounding events as icebergs are driven into shallow water gouging depressions in the sea floor. The faulting could be caused by such loading and then a loss of support as the ice melted or moved in local currents. The stratified diamict was formed by the downslope movement of sediment on a gently sloping seabed. Deep channels can form where flow becomes concentrated. The larger rocks are, again, dropstones from melting surface ice. The massive diamict is the result of a debris-filled meltwater current entering calmer water. Rapidly flowing channelised meltwater is capable of carrying large quantities of mud, sand and gravel, all in suspension at the same time. When such turbid flows enter stiller water they form plumes which spread and rapidly shed their mixed load. The sediments in the Sandpiper Pit conjure a picture of a complex, dynamic, high-energy environment adjacent to the glacier snout that created the Cranfield Moraine (site 441). A wide variety of near shore conditions are reflected in the sediments in close proximity to the ice front, complicated by the effects of ice floes dropping isolated rocks or decanting pods of sediment as they rolled over. The grounding of icebergs also indelibly marked this sea bed. The pit clearly exhibits an unusual, inshore glacial environment and the processes that shaped it and is an ideal teaching and research area.


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