Editing Upper Carboniferous of the Halifax area - an excursion

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In Namurian times the Huddersfield Basin was filled initially with sediment supplied by northerly sourced, '''turbidite'''-fronted, deep water delta systems. As the basin shallowed the deltas, now lacking turbidites, assumed a sheet-like geometry and the basin gradually evolved into a low relief alluvial plain by the beginning of the Westphalian. Deposition was characterized by a cyclic repetition of strata, each cycle being bounded by laterally persistent marine bands related to '''eustatic''' sea level changes. In late Namurian times braided rivers flowed to the southwest, past Leeds, and to the south and southeast as far as Sheffield, located at this time on the northern margin of the Gainsborough Trough which was occupied by a standing body of fresh or brackish water characterized by low wave and tide energy.
 
In Namurian times the Huddersfield Basin was filled initially with sediment supplied by northerly sourced, '''turbidite'''-fronted, deep water delta systems. As the basin shallowed the deltas, now lacking turbidites, assumed a sheet-like geometry and the basin gradually evolved into a low relief alluvial plain by the beginning of the Westphalian. Deposition was characterized by a cyclic repetition of strata, each cycle being bounded by laterally persistent marine bands related to '''eustatic''' sea level changes. In late Namurian times braided rivers flowed to the southwest, past Leeds, and to the south and southeast as far as Sheffield, located at this time on the northern margin of the Gainsborough Trough which was occupied by a standing body of fresh or brackish water characterized by low wave and tide energy.
  
The upper part of the Namurian Millstone Grit seen on this excursion includes the Midgley Grit and the Rough Rock unit which are equivalent in age to the middle part of the Marsdenian and the upper part of the Yeadonian Stages respectively ([[:File:YGS_YORKROCK_FIG_09_01.jpg|Figure 9.1]]). In the Halifax area, the Rough rock unit comprises a braided channel sheet sandbody, known as the Rough Rock, scoured into sandstones and siltstones of the underlying Rough Rock Flags. Halifax is largely built of locally quarried Rough Rock sandstone, and the River Calder on the south side of the town was cut down through these sandstones forming a deep, well-wooded valley in the underlying shales. When traced westwards into the deeper, more rapidly subsiding Rossendale Basin, the Rough Rock is underlain by the Upper and Lower Haslingden Flags, interpreted as the bar finger sands of an easterly '''prograding''' birdsfoot delta similar to the present-day Mississippi delta. In contrast, the Rough Rock Flags, which are equivalent to the Upper Haslingden Flags, are interpreted as the distal deposits of a lobate, shallow water delta prograding to the southwest.
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The upper part of the Namurian Millstone Grit seen on this excursion includes the Midgley Grit and the Rough Rock unit which are equivalent in age to the middle part of the Marsdenian and the upper part of the Yeadonian Stages respectively ([[:File:YGS_YORKROCK_FIG_09_01.jpg|Figure 9.1]]). In the Halifax area, the Rough rock unit comprises a braided channel sheet sandbody, known as the Rough Rock, scoured into sandstones and siltstones of the underlying Rough Rock Flags. Halifax is largely built of locally quarried Rough Rock sandstone, and the River Calder on the south side of the town was cut down through these sandstones forming a deep, well-wooded valley in the underlying shales. When traced westwards into the deeper, more rapidly subsiding Rossendale Basin, the Rough Rock is underlain by the Upper and Lower Haslingden Flags, interpreted as the bar finger sands of an easterly prograding birdsfoot delta similar to the present-day Mississippi delta. In contrast, the Rough Rock Flags, which are equivalent to the Upper Haslingden Flags, are interpreted as the distal deposits of a lobate, shallow water delta prograding to the southwest.
  
Coal Measures '''conformably''' overly the Millstone Grit and show a similar pattern of deposition, except that sandstones are thinner and finer grained, coal seams are thicker and more abundant, non-marine '''bivalve''' bands are common, and marine bands are rare. The coals are mostly bituminous coals, underlain by '''seatearths''' comprising soft '''fireclay''' and hard '''ganister''', some of which have been exploited commercially between Halifax and Sheffield for making firebricks and furnaces. The most prominent sandstones in the lower part of the Coal Measures, some 165 m above the base, are the locally named Elland Flags (Figure 9.1). These are still worked extensively around Halifax for flags and roofing slates.
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Coal Measures conformably overly the Millstone Grit and show a similar pattern of deposition, except that sandstones are thinner and finer grained, coal seams are thicker and more abundant, non-marine bivalve bands are common, and marine bands are rare. The coals are mostly bituminous coals, underlain by seatearths comprising soft fireclay and hard ganister, some of which have been exploited commercially between Halifax and Sheffield for making firebricks and furnaces. The most prominent sandstones in the lower part of the Coal Measures, some 165 m above the base, are the locally named Elland Flags (Figure 9.1). These are still worked extensively around Halifax for flags and roofing slates.
  
 
The description of the outcrop in Greetland Quarry and the Elland Road Cutting is based on the work of Bristow (1993 and earlier work) augmented by the author's own field observations.
 
The description of the outcrop in Greetland Quarry and the Elland Road Cutting is based on the work of Bristow (1993 and earlier work) augmented by the author's own field observations.

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