J. geol. Soc. London, Vol. 138, 1981, pp. 653-659, 7 figs, 2 tables. Printed in Northern Ireland. Seismic refraction evidence for a basement ridge between the Derbyshire Dome and the W of Charnwood Forest D. N. Whitcornbe & P. K. H. Maguire SUMMARY:Results from a 40-kmseismic refraction profile crossing a series of positive gravity anomalies extending across the postulated mouthof the Widmerpool Gulf are reported. A refractor with a similar velocity to the Precambrian rocks of Charnwood Forest dips away from Charnwood to the southern flank of the Derbyshire Dome to a depth of 2 km.This refractor is faulted into horst and graben structures. The horst blocks correlatewith the positive gravity anomalies. A northern boundary fault to the Precambrian outcrops of Charnwood is defined. This profile was the third in a series of seismic refrac- proved Ordovician mudstonesat a depth of 1.8 km tion experimentscarried out by the University of beneath CarboniferousLimestone (Dunham 1973). Leicester to investigate the contact of the Precambrian The Ashby borehole BH5 (Fig. l), sunk by the IGS in rocks of Charwood Forest with the surrounding 1978, reachedpre-Tremadoc strata of probable Mesozoic sediments, and to define the nature of the basement to these sediments. The results of the first two profiles have been reported in Whitcombe & Maguire 1981. This northern profile extendedfrom Charnwood Forest to the southern edge of the Derby- 381 shire Dome, crossing apostulated basement ridge (Maroof 1973) associated with a series of positive Dome gravity anomaliesextending across the proposed mouth of the Widmerpool Gulf (Falcon & Kent 1960). 36 .E PEAK CLLIDON \ CALDO . Geology and previous .-- -_ geophysical work 3L . Widmorpool Solsmic Gulf The position of the seismic profile, quarry shot points, \ Profile and relevant boreholes, in relation to Charnwood For- est and the Derbyshire Dome is shown in Fig. 1. Fig. 2 32 shows the simplified surface geology of the area cros- sed by the seismic profile and the positions of the individual recording stations. The oldest rocks in the areaare the ‘Charnian’ late Precambrian rocks of Charnwood Forekt Charnwood Forest, folded into a south easterly asym- 301 I I I 1 metrical plunging anticline (Watts 1947) and consi- L0 0 L20 L L0 L60 EASTING dered to have a faulted northern boundary (Moseley 1979). An analysis of the velocity structure of these BOREHOLES m BH1 - Eyam, BH2 - Woodale, rocks gave a P wave velocity of 5.65 km/s forthe BH3 - Caldon Low, BHL-Ropton. olderBlackbrook ‘Series’ (Whitcombe & Maguire I BH5 - Ashby 1980). An underlying basement refractor with a veloc- QUARRIES A ity of c. 6.4 km/shas been mapped at a depth of FAULTS F1 - Boothorpc, F2 - Thrlngstonr, >2 km beneath Charnwood Forest. Seismic refraction F3- New Brook Volley. FL - Worthlngton profiles radiating to the SW and ENE of Charnwood CARBONIFEROUSLIMESTONE OUTCROPS 0 age D2DI age ago Forest indicated refracting horizons with P wave vel- a.. ocities of 5.76 and 5.64 kmls, respectively (Whit- . Older C2 S1 ago Wldmorpool foclos combe & Maguire 1981), considered to be associated with Charnian type basement. The velocity structure FIG. 1. Seismicprofile, quarry shotpoints and of the upper crust to the N of Buxton has been defined boreholes in the study area, showing the relation- by the LISPB experiment (Bamford et al. 1977). The ship of CharnwoodForest to the Derbyshire Eyamborehole to the E of Buxton (BH1 in Fig. 1) Dome. 0016-7649/81/1100653%02.00 @ 1981 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/6/653/4887010/gsjgs.138.6.0653.pdf by guest on 01 October 2021 654 D. N. Whitcornbe & P. K. H. Maguire deredthe Woodale volcanic rocks tobe contem- poraneous with the Carboniferous Limestone, a view supported by Maroof’s gravity work. The dominant feature of the surface geology is the Derbyshire Dome, an area of outcropping Carbonifer- ous Limestone.From a study of gravity anomalies, Maroof (1973) concluded thata thickness of 130- 300m of CarboniferousLimestone occurs in the southernpart of theDome, and 200-500m in the north. George (1963) considered the uplift of the Derbyshire Dome to haveoccurred during the Ter- tiary. The limestones of the Derbyshire Dome are mainly massif facies. Gulf or ‘Widmerpool’ facies limestones are found outcropping to the SW and S of the Dome. The Repton borehole BH4 (Fig. 1) proved Gulf facies Carboniferous Limestone beneath Trias and Millstone Gritat a depth of 290 m. Falcon & Kent (1960) postulated that the Widmerpool Gulf extends between Charnwood Forest and the Derbyshire Dome into the N Staffordshire Gulf (Fig. 1). The Bouguer gravity map is shown in Fig. 3. This map is dominated by two positive anomalies, denoted L20 c30 L L0 L50 Contoursin rnlllloals EASTING Prccambrion m Widmerpoolfocies Carboniferous Limestone 8 MillstoneGrit. m CorbonifcrousLimestone H Cool Measures m Perrno -Triassic 0 Post - Jurossic Igneousrocks BAL: Ballidonquarry M : MiddlePeokquorry C : Cloud Hill quarryW: Whitwick quorry B : BardonHill quorry CL1 to CL3Corboniforous Llrnestone inlters FIG. 2. Geological map of the study area showing the seismic recording site locations. Cambrianage at a depth of 174111 beneath Coal Measures. The Caldon Low borehole, BH3, situated in the Weaver Hills on the SW edge of the Derbyshire Domeand also sunk by the IGS in 1978, reached L20 L30 LLO L50 EASTING sandstone of either Carboniferous or Devonian age at BAL . Bollidon quarry.M Middle Peak quarry a depth of 365 m. Old Red Sandstone underlies the C ’ Cloud Hillquarry. W: Whltwtckquorry. Coal Measures of the Staffordshire Coalfield (Hains & B ’ Bordon Hill quarry. Horton 1969). The Woodale borehole BH2 (Fig. 1) reached volcanic rocks at a depth of 312 m;these were G’, G’ onornollosdiscussed In text originallycorrelated with the PrecambrianCharnian FIG. 3. Bougueranomaly map of thestudy area rocks(Cope 1949). However, Le Bas(1972) consi- showing the seismic recording site locations. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/138/6/653/4887010/gsjgs.138.6.0653.pdf by guest on 01 October 2021 Basement ridge betweenDerbyshire Dome andCharnwood Forest 655 G’ and G’. Modelling of theseanomalies indicates at all the other stations were placed in shallow pits dug into Mesozoic thicknesses of 200-700 m under the centres soil. of each anomaly, with pre-Carboniferous basement at Quarry blasts from the 10 quarries (Fig.1) were used in the depths of 1.0-1.7 km on their flanks (Maroof 1973). analysis. Good quality signals could generally be obtainedup The anomalies could bedue to either a smoothly to a distance of c. 35 km from the various quarries, but good signalswere recorded at c. 60 km fromTunstead quarry. undulating basement or upfaulted blocks. The latter Table 1 gives the number of traveltime observations re- interpretation was favoured for anomaly G’. The steep corded from each shot point. The unfltered analogue tape contours of this anomaly correlate with the positions recordings were replayed directly onto paper records. The of the Hercynian Boothorpe Fault and the New Brook relative arrival times of the signals could in most cases be Valley Fault (Fig. 1). The Worthington and Thring- picked to accuracies of better than 10ms. stone faults also occur in this area (Fig. l), but from the Bouguer anomaly map (Fig. 3) do not appear to be major structuralfeatures. Several Carboniferous Data analysis Limestone inliers also occur in this structurally com- plex area, and may be considered in 3 groups (CL1- CL3, Fig. 2). The first group (CL1) lie outside the limits The method of the Widmerpool gulf and also to the SW of the New A fuller description of the method used forthe Brook Valley fault and are therefore located on the analysis is given in Whitcombe & Maguire (1981). In postulated horst block. The second group (CL2) are brief, the reduced travel times were calculated using situated nearthe Worthingtonfault and have large reducing velocities similar to the expected basement dips, unlike the CL1inliers. The.beds of the CL3inlier, refractor velocity. Changes in the reduced travel times situated immediately to the NW of the Precambrian between adjacent stations are a good first approxima- outcrops, are almost horizontal. tion tothe changes in recordingstation time-terms between these stations. A study of the reduced travel Data collection times therefore reveals the approximate topography of the basement refractor (Whitcombe & Maguire 1979). Theseismic profile was dog-legged to avoid the city of Arrivals which donot originate from the basement Derby. The profile was positioned to pass through Cloud Hill refractor may be easily identified from a study of the quarry which is situated ina group CL2 Carboniferous Limes- reduced travel times, and excluded from the data set tone inlier, and which acted as a shotpoint for the experi- ment. The profile crossed the Repton borehole, BH4, (Fig. 1) used todetermine a leastsquares velocity estimate (station 24), and terminatedat Ballidon quarry in the S of the using the time-termmethod (Willmore & Bancroft Derbyshire Dome. Because of the large number of stations 1960). requiredfor this seismic experiment it wasnecessary to undertake the profile in sections. It later proved necessary to supplementthe data byreoccupying 8 of theoriginal 39 The reduced travel times stations, and continuously recording for a further 3 weeks. Each station consisted of a short period Willmore Mk I11 In view of the basement velocities obtained in the seismometer. The signals were recorded on two ‘Geostore’ Charnwood area (-5.6 km/s) and in the Buxton area analogue tape recorders. The Rugby MSF radio clock was by the LISPBexperiment (-5.8 km/s), it was ex- recorded as the time base for the experiment. Stations 35-39 pected that a similar velocity refractor would be weresituated on or near outcropping Precambrian rocks. defined beneath this seismic profile.