Editing Upper Carboniferous of the Halifax area - an excursion

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== Geological background ==
 
== Geological background ==
  
Halifax lies within the central Pennines, which form a prominent topographic feature throughout northern England, comprising steep, flat-topped hills and plateaux up to 600 m above sea level, dissected by deep river valleys and cloughs. Most valleys were incised into preexisting broader, higher-level valley features during late Tertiary times. A good example of this is the River Calder, which flows eastwards through the area ([[:File:YGS_YORKROCK_FIG_09_01.jpg|Figure 9.1]]) in a steep-sided valley incised between prominent shoulders defining the edges of an older, high valley floor.
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Halifax lies within the central Pennines, which form a prominent topographic feature throughout northern England, comprising steep, flat-topped hills and plateaux up to 600 m above sea level, dissected by deep river valleys and cloughs. Most valleys were incised into preexisting broader, higher-level valley features during late Tertiary times. A good example of this is the River Calder, which flows eastwards through the area ([[:File:YGS_YORKROCK_FIG_09_01.jpg|Figure 9.1]]) in a steep-sided valley incised between prominent shoulders defining the edges of an older, high valley floor.
  
Geologically the Pennines consist predominantly of Carboniferous sandstones, shales and limestones folded, during the '''Variscan''' '''Orogeny''', into an asymmetric '''anticline''' with the steeper limb in the west. In the central Pennines, late Carboniferous (Namurian and Westphalian) sediments form a low scarp on either side of the north–south trending Pennine anticlinal ridge. Apart from a few exposures along the scarp crest, most outcrops are confined to old quarries.
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Geologically the Pennines consist predominantly of Carboniferous sandstones, shales and limestones folded, during the Variscan Orogeny, into an asymmetric anticline with the steeper limb in the west. In the central Pennines, late Carboniferous (Namurian and Westphalian) sediments form a low scarp on either side of the north–south trending Pennine anticlinal ridge. Apart from a few exposures along the scarp crest, most outcrops are confined to old quarries.
  
In early Carboniferous (Dinantian) times the structure of northern England consisted of a number of basins and blocks, initiated in response to reactivation of pre-existing lines of crustal weakness by a south-southeast–north-northwest directed extensional stress field. In the Pennine Basin, continued extension led to its further break-up into a number of small, tilted fault blocks, forming a series of '''half-graben''' basins. Thus the north Pennine Basin can be divided into the Huddersfield Basin in the east, and the Rossendale Basin in the west, separated by the Rossendale Ridge. Rapid '''fault''' controlled basin subsidence continued until late Namurian times, followed by a gradual change to a more general phase of thermal subsidence. Thicker, more argillaceous sediments accumulated within the more rapidly subsiding basins with thinner or condensed sequences over the more stable blocks.
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In early Carboniferous (Dinantian) times the structure of northern England consisted of a number of basins and blocks, initiated in response to reactivation of pre-existing lines of crustal weakness by a south-southeast–north-northwest directed extensional stress field. In the Pennine Basin, continued extension led to its further break-up into a number of small, tilted fault blocks, forming a series of half-graben basins. Thus the north Pennine Basin can be divided into the Huddersfield Basin in the east, and the Rossendale Basin in the west, separated by the Rossendale Ridge. Rapid fault controlled basin subsidence continued until late Namurian times, followed by a gradual change to a more general phase of thermal subsidence. Thicker, more argillaceous sediments accumulated within the more rapidly subsiding basins with thinner or condensed sequences over the more stable blocks.
  
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.
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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.
  
 
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.
 
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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