Editing Geology of the Llanidloes area: Geological description - Late Ordovician and Silurian

Jump to navigation Jump to search

Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.

The edit can be undone. Please check the comparison below to verify that this is what you want to do, and then save the changes below to finish undoing the edit.

This page supports semantic in-text annotations (e.g. "[[Is specified as::World Heritage Site]]") to build structured and queryable content provided by Semantic MediaWiki. For a comprehensive description on how to use annotations or the #ask parser function, please have a look at the getting started, in-text annotation, or inline queries help pages.

Latest revision Your text
Line 12: Line 12:
 
[[Image:P775111.jpg|thumb|center|500px|  '''Plate P775111'''    Rusty weathering anoxic mudstones of the Cwmere Formation overlain in the upper part of the scarp by more resistant bioturbated mudstones of the Derwenlas Formation, Uwch-y-coed [SN 8300 9502].          ]]
 
[[Image:P775111.jpg|thumb|center|500px|  '''Plate P775111'''    Rusty weathering anoxic mudstones of the Cwmere Formation overlain in the upper part of the scarp by more resistant bioturbated mudstones of the Derwenlas Formation, Uwch-y-coed [SN 8300 9502].          ]]
  
[[Image:P775112.jpg|thumb|center|500px|  '''Plate P775112'''    Feature (slack) formed by Monograptus Sedgwickii Shales Member at the base of Rhayader Mudstones Formation (right of picture), with the underlying Derwenlas Formation exposed in left foreground, Uwch-y-coed [SN 8300 9513].    ]]
+
[[Image:P775112.jpg|thumb|center|500px|  '''Plate P775112'''    Feature (slack) formed by Monograptus Sedgwickii Shales Member at base of Rhayader Mudstones Formation (right of picture), with the underlying Derwenlas Formation exposed in left foreground, Uwch-y-coed [SN 8300 9513].    ]]
  
 
The Rhayader Mudstones Formation is succeeded diachronously by the '''Devil’s Bridge Formation (DBF)''' and '''Foel Fadian Formation (FoF)''', two broadly comparable early Telychian divisions of thinly interbedded, Bouma-type, turbidite sandstones and mudstones ([[Media:P775113.jpg |'''Plate P775113''']]), which crop out in the south-west and north-west of the district respectively.  The sandstones are parallel and cross-laminated, between 1 and 5&nbsp;cm thick, and form up to 30 per cent of the succession; widely scattered thicker sandstone beds range up to 30&nbsp;cm.  The interbedded turbidite mudstones are generally pale green-grey, colour banded and somewhat thicker.  They are commonly capped by a thin burrow-mottled hemipelagic mudstone, although parts of each succession include laminated hemipelagites containing graptolites indicative of the ''turriculatus'' (s.l.) Biozone.  Deposition of both the Devil’s Bridge and Foel Fadian formations is thought to result from the overall increase in sediment supplied to the basin in response to tectonic uplift of the source areas during the Telychian (Davies et al., 1997<ref name="Davies 1997"></ref>; Soper and Woodcock, 1990<ref name="Soper 1990">Soper, N J, and Woodcock, N H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, Vol. 127, 527–542.</ref>).  Palaeocurrent indicators suggest that the Devil’s Bridge Formation was derived from south-easterly quadrants whereas those of the Foel Fadian Formation are indicative of a north-westerly source. Both source areas lay outside the Llanidloes district.  The Devil’s Bridge Formation thickens to over 500&nbsp;m in the extreme south-west, coincident with the basinward thinning of the underlying Rhayader Mudstones Formation.  At its maximum development within the district the Foel Fadian Formation is 430&nbsp;m thick, and it appears to thin in a general eastward direction.  The reason for these thickness variations is equivocal, although the distribution of both formations was influenced by a sea-floor topography that may have been affected by contemporaneous movement on component structures of the Central Wales Lineament which runs through the western part of the district (Davies et al., 1997<ref name="Davies 1997"></ref>; see below).
 
The Rhayader Mudstones Formation is succeeded diachronously by the '''Devil’s Bridge Formation (DBF)''' and '''Foel Fadian Formation (FoF)''', two broadly comparable early Telychian divisions of thinly interbedded, Bouma-type, turbidite sandstones and mudstones ([[Media:P775113.jpg |'''Plate P775113''']]), which crop out in the south-west and north-west of the district respectively.  The sandstones are parallel and cross-laminated, between 1 and 5&nbsp;cm thick, and form up to 30 per cent of the succession; widely scattered thicker sandstone beds range up to 30&nbsp;cm.  The interbedded turbidite mudstones are generally pale green-grey, colour banded and somewhat thicker.  They are commonly capped by a thin burrow-mottled hemipelagic mudstone, although parts of each succession include laminated hemipelagites containing graptolites indicative of the ''turriculatus'' (s.l.) Biozone.  Deposition of both the Devil’s Bridge and Foel Fadian formations is thought to result from the overall increase in sediment supplied to the basin in response to tectonic uplift of the source areas during the Telychian (Davies et al., 1997<ref name="Davies 1997"></ref>; Soper and Woodcock, 1990<ref name="Soper 1990">Soper, N J, and Woodcock, N H. 1990. Silurian collision and sediment dispersal patterns in southern Britain. Geological Magazine, Vol. 127, 527–542.</ref>).  Palaeocurrent indicators suggest that the Devil’s Bridge Formation was derived from south-easterly quadrants whereas those of the Foel Fadian Formation are indicative of a north-westerly source. Both source areas lay outside the Llanidloes district.  The Devil’s Bridge Formation thickens to over 500&nbsp;m in the extreme south-west, coincident with the basinward thinning of the underlying Rhayader Mudstones Formation.  At its maximum development within the district the Foel Fadian Formation is 430&nbsp;m thick, and it appears to thin in a general eastward direction.  The reason for these thickness variations is equivocal, although the distribution of both formations was influenced by a sea-floor topography that may have been affected by contemporaneous movement on component structures of the Central Wales Lineament which runs through the western part of the district (Davies et al., 1997<ref name="Davies 1997"></ref>; see below).

Please note that all contributions to Earthwise may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here.
You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see Earthwise:Copyrights for details). Do not submit copyrighted work without permission!

Cancel Editing help (opens in new window)

  [] · [[]] · [[|]] · {{}} · · “” ‘’ «» ‹› „“ ‚‘ · ~ | ° &nbsp; · ± × ÷ ² ³ ½ · §
[[Category:]] · [[:File:]] · <code></code> · <syntaxhighlight></syntaxhighlight> · <includeonly></includeonly> · <noinclude></noinclude> · #REDIRECT[[]] · <translate></translate> · <languages/> · ==References== · {{reflist}} · ==Footnote== · {{reflist|group=note}} · <ref group=note> · __notoc__ · {{DEFAULTSORT:}} <div class="someclass noprint"></div> {{clear}} <br>

Templates used on this page: