Editing Lower Jurassic rocks between Staithes and Port Mulgrave - an excursion

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== Geological background ==
 
== Geological background ==
  
During the Lower Jurassic, the Staithes area was part of the Cleveland (or Yorkshire) Basin, bounded by land to the north and at times to the west, by the Market Weighton Block to the south, and passing offshore into the '''fault'''-bounded Sole Pit Trough. Shelf seas covered the area to depths of up to 100–200 m. The Lias Group is a sequence of dominantly mud-grade sediments reaching about 420 m thickness in North Yorkshire ([[:File:YGS_YORKROCK_FIG_16_02.jpg|Figure 16.2]]). The Staithes Sandstone Formation, of Pliensbachian age, lies at the top of a shallowing and coarsening upwards sequence. Deepening seas above led to a return of fine-grained sediments, but the shales of the Cleveland Ironstone Formation are punctuated by a series of ironstone bands, each formed at the top of a small-scale shallowing upward cycle. Renewed '''transgression''' close to the Pliensbachian–Toarcian boundary initiated the Whitby Mudstone Formation, including the distinctive, hydrocarbon-rich Mulgrave Shale Member and the widely worked Alum Shale Member. A final shallowing event led to an influx of sandy material forming the Blea Wyke Sandstone Formation capping the Lias, but over most of the region these younger sediments are missing and the Middle Jurassic rests directly on the Alum Shale Member at Port Mulgrave. The cause of sedimentary cyclicity in the Lias is probably a combination of global sea level rise and fall and local earth movements. The best-known evidence for the latter is the '''unconformity''' of about 25 m amplitude below the Main Seam of the Cleveland Ironstone Formation that causes it to '''overstep''' lower units to the south down to a level below the Avicula Seam.
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During the Lower Jurassic, the Staithes area was part of the Cleveland (or Yorkshire) Basin, bounded by land to the north and at times to the west, by the Market Weighton Block to the south, and passing offshore into the fault-bounded Sole Pit Trough. Shelf seas covered the area to depths of up to 100–200 m. The Lias Group is a sequence of dominantly mud-grade sediments reaching about 420 m thickness in North Yorkshire ([[:File:YGS_YORKROCK_FIG_16_02.jpg|Figure 16.2]]). The Staithes Sandstone Formation, of Pliensbachian age, lies at the top of a shallowing and coarsening upwards sequence. Deepening seas above led to a return of fine-grained sediments, but the shales of the Cleveland Ironstone Formation are punctuated by a series of ironstone bands, each formed at the top of a small-scale shallowing upward cycle. Renewed transgression close to the Pliensbachian–Toarcian boundary initiated the Whitby Mudstone Formation, including the distinctive, hydrocarbon-rich Mulgrave Shale Member and the widely worked Alum Shale Member. A final shallowing event led to an influx of sandy material forming the Blea Wyke Sandstone Formation capping the Lias, but over most of the region these younger sediments are missing and the Middle Jurassic rests directly on the Alum Shale Member at Port Mulgrave. The cause of sedimentary cyclicity in the Lias is probably a combination of global sea level rise and fall and local earth movements. The best-known evidence for the latter is the unconformity of about 25 m amplitude below the Main Seam of the Cleveland Ironstone Formation that causes it to overstep lower units to the south down to a level below the Avicula Seam.
  
 
Ammonites are variably common more or less throughout the sequence, which is thus well dated. Both trace and body fossils are richly represented at various levels, with bivalves the dominant invertebrates.
 
Ammonites are variably common more or less throughout the sequence, which is thus well dated. Both trace and body fossils are richly represented at various levels, with bivalves the dominant invertebrates.
  
Three units in this sequence are of former economic importance. The Cleveland Ironstone Formation was worked in the 19th and 20th centuries, sporadically on the foreshore between Staithes and Port Mulgrave where the seams are thin, but extensively inland in the Cleveland Hills where the Main Seam reaches 3.8 m thick. The last mine closed in 1964. The ore was the original raw material for the local iron and steel industry. Within the background sequence of dark grey shales, the ironstones are scattered, orangey-brown weathering beds composed principally of '''siderite''' and '''chamosite''' and variably '''oolitic'''. Their method of formation has been controversial. Current opinion is that they formed in shallow marine inshore waters, at times of very slow sedimentation. Large amounts of iron, derived from '''lateritic''' weathering of a landmass in the area of the Pennines, were periodically introduced by river systems transporting ferric oxides and hydroxides as a colloidal suspension, or absorbed on the surface of organic material, or as oxide films on clay minerals. Siderite and chamosite were probably formed under reducing conditions by early '''diagenetic''' processes, with the iron salts replacing and displacing freshly deposited sediments below the sediment-water interface.
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Three units in this sequence are of former economic importance. The Cleveland Ironstone Formation was worked in the 19th and loth centuries, sporadically on the foreshore between Staithes and Port Mulgrave where the seams are thin, but extensively inland in the Cleveland Hills where the Main Seam reaches 3.8 m thick. The last mine closed in 1964. The ore was the original raw material for the local iron and steel industry. Within the background sequence of dark grey shales, the ironstones are scattered, orangey-brown weathering beds composed principally of siderite and chamosite and variably oolitic. Their method of formation has been controversial. Current opinion is that they formed in shallow marine inshore waters, at times of very slow sedimentation. Large amounts of iron, derived from lateritic weathering of a landmass in the area of the Pennines, were periodically introduced by river systems transporting ferric oxides and hydroxides as a colloidal suspension, or absorbed on the surface of organic material, or as oxide films on clay minerals. Siderite and chamosite were probably formed under reducing conditions by early diagenetic processes, with the iron salts replacing and displacing freshly deposited sediments below the sediment-water interface.
  
Scattered masses of a tough, shiny, dense black material called jet, occurring in the Mulgrave Shale Member, formed the basis of local manufacture of personal and domestic ornaments, particularly in the second half of the 19th century. Jet was formed from logs of araucarian wood transported into the sea, which became waterlogged and then sank to the reducing sea floor. Diagenetic alteration and compression under these conditions produced this unusual result. The shales of the Mulgrave Formation are rich in hydrocarbons and are an example of an oil source rock. Laboratory distillation yields 54–86 litres per ton of sulphurous oil (Hemingway ''in ''Rayner & Hemingway, 1974).
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Scattered masses of a tough, shiny, dense black material called jet, occurring in the Mulgrave Shale Member, formed the basis of local manufacture of personal and domestic ornaments, particularly in the second half of the i 9th century. Jet was formed from logs of araucarian wood transported into the sea, which became waterlogged and then sank to the reducing sea floor. Diagenetic alteration and compression under these conditions produced this unusual result. The shales of the Mulgrave Formation are rich in hydrocarbons and are an example of an oil source rock. Laboratory distillation yields 54–86 litres per ton of sulphurous oil (Hemingway ''in ''Rayner & Hemingway, 1974).
  
The Alum Shale Member was extensively worked for alum, a mordant (fixing agent) in the dyeing industry, from the early 17th to mid-19th century. The shale was burnt for a year or more in huge heaps (up to 30 m diameter and 15 m high) over beds of brushwood. '''Pyrite''' in the shale was oxidized to form iron and aluminium sulphates, which were extracted by steeping in tanks of water. Potash alum was produced by adding ashes produced by burning seaweed, and later ammonium alum was made by adding urine. On evaporation, the alum crystallized before the impurities (mainly salts of iron), which could then be pumped off. The precise moment at which to stop heating for the maximum yield of alum was determined by floating an egg in the liquor as a hydrometer!
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The Alum Shale Member was extensively worked for alum, a mordant (fixing agent) in the dyeing industry, from the early 17th to mid-19th century. The shale was burnt for a year or more in huge heaps (up to 30 m diameter and 15 m high) over beds of brushwood. Pyrite in the shale was oxidized to form iron and aluminium sulphates, which were extracted by steeping in tanks of water. Potash alum was produced by adding ashes produced by burning seaweed, and later ammonium alum was made by adding urine. On evaporation, the alum crystallized before the impurities (mainly salts of iron), which could then be pumped off. The precise moment at which to stop heating for the maximum yield of alum was determined by floating an egg in the liquor as a hydrometer!
  
 
This account is based in part on Hemingway ''(in ''Hemingway ''et al., ''1968; Rayner & Hemingway, 1974) and Rawson ''(in ''Rawson & Wright, 1992).
 
This account is based in part on Hemingway ''(in ''Hemingway ''et al., ''1968; Rayner & Hemingway, 1974) and Rawson ''(in ''Rawson & Wright, 1992).

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