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Wenlock age turbidites in central Powys are subdivided into the mudstone rich Llanbadarn Formation and the underlying more sandy Castle Vale Formation. Sand turbidites were deposited by NNE directed flows along the base of a fault controlled basin slope dipping WNW. The resulting Castle Vale Formation thins laterally onto the slope, first onlapping it then offlapping it to be overlain diachronously by silt–mud turbidites of the Llanbadarn Formation. A finely laminated hemipelagic facies occurs between the turbidites throughout. The fine laminae are interpreted as having an annual origin. Bioturbation is entirely absent suggesting that sediment pore-waters may have been anoxic.
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New mapping of the southern part of the Dent Fault reveals three segments, each 5–6 km long, overlapping at two left-stepping zones 1–2 km wide. The main fault strands probably dip steeply WNW. A faulted footwall syncline in Carboniferous strata indicates reverse dip-slip, with a stratigraphic throw of at least 750 m. Locally developed plunging folds and imbricate fault duplexes developed at fault bends reveal a strike-slip component, indicated to be sinistral from limited slickenline data. Silurian strata in the hanging wall lack the Variscan folds observed further north. The northern overstep hosts up-faulted slivers of older Silurian and Ordovician rocks. The southern overstep zone hosts a younger faulted block compatible with releasing kinematics in sinistral strike-slip. The Dent Fault converges at its southern end with the Barbon Fault; an upfaulted wedge of Silurian strata lies between them near the branch point. The two faults swing southeastward, joining the Craven fault system via splays and linkages. Regionally, the Dent and Barbon faults form the innermost pair of a fan of ~N–S striking faults splaying off the northwest end of the South Craven–Morley-Campsall Fault System around the southwestern corner of the Askrigg Block. The kinematics of the Dent, Barbon and Craven faults fit shortening orientated NNW–SSE during late Carboniferous Variscan deformation. The rigid Askrigg Block focussed displacements around its west and south margins where fault and fold orientations were influenced by pre-existing structures, at least Acadian in age to the west and early Carboniferous to the south.
The mainly Gorstian (Lower Ludlow, Silurian) Bailey Hill Formation, defined here, has previously been interpreted as a 'bathyal' turbidite sequence deposited in the north–south elongated Montgomery Trough. It has been seen as the Ludlow equivalent of the Wenlock and Llandovery turbidite systems preserved further west. Slowly deposited background sediments are punctuated by event deposits interpreted here not as turbidites but as distal storm deposits accumulated in an outer shelf setting. The model envisages entrainment of sediment above storm wave base on the inner shelf, lateral transport in the storm forced flow, and redeposition below storm wave base under the residual influence of this same flow. The distribution of the formation reflects the interplay between the palaeoslope of a broad shelf and palaeooceanographic factors such as storm wavebase, during a period of high sea level. Contemporaneous true turbidites to the northwest suggest that a depositional low existed outboard of the area of Bailey Hill Formation deposition at this time, in continuity with earlier Silurian turbidite systems. The revised palaeogeography is consistent with other aspects of Ludlow regional geology. By mid-Ludlow time a tectonic modification of palaeogeography was underway, marked by extensive submarine slides.
Research Article| May 01, 2005 Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England Jon P.T. Tarasewicz; Jon P.T. Tarasewicz 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Nigel H. Woodcock; Nigel H. Woodcock 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar J. Anthony D. Dickson J. Anthony D. Dickson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Jon P.T. Tarasewicz 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Nigel H. Woodcock 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK J. Anthony D. Dickson 1Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Publisher: Geological Society of America Received: 19 Jan 2004 Revision Received: 15 Jun 2004 Accepted: 23 Jun 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (5-6): 736–745. https://doi.org/10.1130/B25568.1 Article history Received: 19 Jan 2004 Revision Received: 15 Jun 2004 Accepted: 23 Jun 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jon P.T. Tarasewicz, Nigel H. Woodcock, J. Anthony D. Dickson; Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England. GSA Bulletin 2005;; 117 (5-6): 736–745. doi: https://doi.org/10.1130/B25568.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The obliqu-reverse Dent Fault, northwest England, throws Carboniferous limestone units in the footwall against mudston-dominated lower Paleozoic rocks in the hanging wall. The fault zone cuts the kilometer-wide steep limb of a precursory forced monocline. However, individual fault strands comprise centimeter-scale cataclasite cores fringed in the footwall carbonates by damage zones, some meters to tens of meters wide, composed of random-fabric dilation breccias. Breccia texture and microstructure, revealed by stained thin sections and peels, imply rapid coseismic fragmentation and then interseismic resealing by void-filling cements. The cements varied in composition through time from calcite to dolomite and then to ferroan calcite. Pervasive dolomitization of the protolith is common in the breccia zones. A key observation is that each volume of dilation breccia shows only limited refracture. This tendency to singl-phase brecciation suggests that cementation caused reseal-hardening of breccia with respect to intact protolith. Breccia thickness and refracture are greatest at jogs in the Dent Fault, but breccia distribution suggests that damage also accumulated in fault walls and at propagating fault tips. Dilation breccias are a common but poorly documented product of brittle deformation of limestone. Their reseal histories can provide valuable general clues to fault zone evolution. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract The boundary between the Rhoscolyn and New Harbour formations on Holy Island, Anglesey, has been described as a high strain zone or as a thrust. The boundary is here described at four localities, with reference to the contrasting sedimentary and deformational character of the two formations. At one of these localities, Borth Wen, sandstones and conglomerates at the top of the Rhoscolyn Formation are followed, without any break, by tuffs and then mudstones of the New Harbour Formation. It is concluded that there is clear evidence of depositional continuity across the boundary here, and that both formations subsequently shared a common two-phase deformation. The first (D 1 ) was manifestly different in intensity and scale in the two formations, whereas the second (D 2 ) produced very similar structures in both. The other three localities provide continuity of sedimentary and tectonic features at this boundary in a traverse along the length of Holy Island, leading us to identify two previously unrecognized major D 1 folds in addition to the Rhoscolyn Anticline. At one of these localities (Holyhead), we confirm the presence of Skolithos just below the boundary, supporting radiometric evidence for a lower Cambrian or later age for the Rhoscolyn Formation. A turbidite interpretation for both the Rhoscolyn and New Harbour formations best fits the available evidence. A deep-water depositional environment is still compatible with the sporadic presence of Skolithos burrows, but less so with reported observations of hummocky and swaley cross-stratification lower down the South Stack Group.
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