Journal of the Geological Society - Earth Dynamics Group @ UiO

SILURIAN MAGNETOSTRATIGRAPHY 825Silurian Palaeomagnetic DataEssential palaeomagnetic and stratigraphic details of each ofthe Silurian data sets displaying single and mixed polarityare recounted below. The stratigraphic positions of Silurianpalaeomagnetic data are shown in Fig. 3. Data sets withineach category are discussed in order of decreasing numberof palaeomagnetic sampling sites.Data sets of single magnetic polarity(i) Western Ireland sediments, Galway and Mayo,Eire. Smethurst & Briden (1988) describe multicomponentmagnetizations from 39 palaeomagnetic sites within LateSilurian sediments of northwest Galway (Salrock Formation)and South Mayo (Clare Island succession), westernIreland. A Silurian magnetization revealed by detailedstepwise-thermal demagnetization predates late-Caledoniandeformation of both areas. The Silurian component, termedH, displays an exclusively normal polarity within all sitesyielding stable directions. Unpublished data from the latestLlandovery Lough Mask Formation also display normalpolarity (M. A. Smethurst pers. comm. 1992; stratigraphicage after Graham et al. 1989).A Wenlock age is probable for the Salrock Formation(Laird & McKerrow 1970). The Clare Island Silurian rocksare probably Wenlock or Ludlow in age (Palmer et al. 1989)and may be contemporaneous with the Salrock Formation(Phillips 1974; Smethurst & Briden 1988).(ii) Andesitic lavas, East Mendips lnlier, SW England. Piper(1975) reported initial palaeomagnetic results from four siteswithin andesitic lavas of the East Mendips Silurian inlier.All samples were subjected to blanket alternating-field demagnetizationto peak fields of 50-100 mT. Following tectoniccorrection, the lower two sites were regarded as ofreverse polarity whilst the upper two sites were of normalpolarity. In resampling the sequence (15 sites, 2 in agglomerates),Torsvik et al. (1993) identified two stable magnetizationcomponents during stepwise thermal demagnetization.These magnetizations were attributed Permo-Carboniferous and Silurian remanence ages respectively. Apositive intra-formational palaeomagnetic agglomerate testconfirms the primary nature of the Silurian magnetization.Only normal-polarity Silurian magnetizations were identified.The previous 'reverse' polarity sites of Piper (1975)are interpreted by Torsvik et al. (1993) as being Permo-Carboniferous overprints which had been erroneously correctedfor tectonic dip.The interpretation of Hancock (1982) suggests that thevolcanics (in Moon's Hill Quarry) all young to the north andare underlain by lower Wenlock shales (M. riccartonensisZone) with probable conformity: they are thus nowconsidered to be Wenlock in age.(ifi) Ainslie Volcanics, Australian Capital Territory,Australia. Seven palaeomagnetic sites from the Ainslie volcanicsyielded stable magnetizations of normal magneticpolarity (Luck 1973). The sample collection spanned approximately170 m of a 210-thick sequence of interbeddedacid to intermediate flows and pyroclastic rocks. The rocksuccession is 'mostly sub-horizontal' (Luck 1973). Sampleswere demagnetized to peak fields/temperatures of50mT/550°C. Although originally attributed to the EarlyPridoliLudlowWenlockLlandoveryLudfordianGorstianGleedonHomerianWhitwellSheinwoodianTelychianAeronian'~"~Wabash Reef(11)~ Hemse Beds(6")~F~_-~-~-~ ~d,a. vo~.i.(6)Ainslie Voleanics(7)Duoro Volcanics (6)Beds (29")Increased ~BloomsburgDominance ! Formation (4) (2)atRingedkeReverseSandstonePolarity (50) (6) (8)(A)(1) / ~ YuejiashanQ I~m~lmlm HuixingshaoSalro~k +Clare IslandSuccession.•(39) (15)MendipMask Fm.U.Visb fH8 klint Beds Volcanics (13)(5) ..m..m~.m~ LoughBotwood King } ~ ' ~~'--"~"~'-~ V OICS + Seals (11)L~'~._'~ George ~',,.~-~,'~Lake 4. \ \ \ \ ] Rose HillArea (11Formation(9): £-'ZZ--3 Lower Trap:Tortworth (1) [~_'~_"~_'~_.](B)KEY"-----] ApproximateStratigraphicLevel & RangeBoth PolaritiesRhuddanianLRed Mountain~ ~ F .... tion (4)[ ]ReverseNormalNo DataFig. 3 Magnetic polarity data for the Silurian Period. Individual datasets are discussed in the text. Stratigraphic divisions taken from Harland etal. (1990). Numbers in brackets next to each data set indicate the number of palaeomagnetic sample sites; (those with asterisks refer tonumbers of samples). Black, normal polarity; hatched, mixed polarity.