The geological results of gravity and magnetic surveys in the Malvern Hills and adjacent districts

THE GEOLOGICAL RESULTS OF GRAVITY AND MAGNETIC SURVEYS IN THE MALVERN HILLS AND

ADJACENT DISTRICTS MICHAEL BROOKS

Detailed gravity and vertical force magnetic surveys in and around the Malvern Hills are described, and the results of determinations of densities and magnetic properties of local rocks are presented. The geophysical surveys indicate that the Malvern axis is a broad anticline involving basement rocks and that the folded basement extends northwards at shallow depth under the Storridge anticline; it cannot be traced further north under the Abberley Hills. Areas of surface Pre-Cambrian rock are interpreted as comprising intensely folded and faulted axial portions of the major fold. which is assumed to be of Armorican age. On the basis of the shallow depth to basement directly north of the Hills, and by analogy with the situation in the southern Malverns where Llandovery strata overstep the Local Cambrian succession, it is concluded that the Cambrian rocks were largely removed from the Storridge area as well as from the Malvern Hills area following a period of pre-Llandovery tectonism. To the west, the basement surface dips at about 30" to a depth of at least 5,000 ft.

Cook and Thirlaway's (1955) suggestion that the Eastern Boundary Fault is part of a step-fault complex is accepted, and their interpretation of the configuration of the Worcestershire Triassic basin is not significantly amended in the present account.

1. INTRODUCTION The Malvern Hills and areas immediately north and west form a dissected upland

region of Pre-Cambrian and Lower Palaeozoic rocks bounded by relatively flat areas, at different altitudes, of Trias and Old Red Sandstone (see Fig. 1). Lower Palaeozoic strata of the Malvern and Abberley Hills are characterized by a N-S structural trend (the Malvernoid trend) which is upheld by the eastern and western boundaries of exposed Pre-Cambrian. The eastern boundary is formed by a great discontinuity, the Eastern Boundary Fault, which traverses the area under review.

,The nature and form of the eastern and western boundaries of Pre-Cambrian, the configuration of crystalline basement in depth and the age of the major tectonic events which have shaped the area have been a source of controversy and debate for more than a century. Of course, these problems are interrelated and over the years a number of structural syntheses have been presented which attempted to account at once for many of the surface geological data. Geophysical methods have been em- ployed in the present work in an attempt to elucidate structural problems which, because of the variety of valid interpretations based on surface data, have not been resolved using geological evidence alone. In particular, investigation has been directed towards outlining the form of the Pre-Cambrian surface in the immediate vicinity of the Malvern Hills.

The first important geophysical work in the area was by Cook and Thirlaway (1950) who carried out an extensive gravity survey of the Welsh Borders. Their geological results are important in showing the configuration of the Eastern Boundary Fault in depth. Up to the present time no geophysical surveys have been concerned in detail with events directly west of the Eastern Boundary Fault, and it is mainly in this respect that the present surveys contribute to our understanding of Malvern tectonics. But in the same way that Cook and Thirlaway's work pointed to the desirability of more gravity and magnetic investigations, so the present work needs following up by seismic surveys to check conclusions and provide additional controls for the gravity and magnetic interpretations. Geol. J. Vol. 6, Pt. 1, 1968 13

14 MICHAEL BROOKS

2. MEASUREMENT OF ROCK PROPERTIES

(a) Determinations of rock density

Accurate values of rock density are essential for the calculation and reliable interpretation of Bouguer anomalies. In the present survey, laboratory determinations of density were made on a wide range of rock-types using a modified form of the method described by Parasnis (1952). In addition, special studies were undertaken of Pre- Cambrian and Triassic densities and these are reported elsewhere (Brooks 1963, 1966). Altogether, about 300 determinations of density were made and the results are reviewed below.

The Malvernian granites and acid metamorphic rocks reveal a surprisingly constant density of 2-63 k0.02 g/cm3 (26 specimens). Basic and ultra-basic rocks give significantly higher values, ranging from 2-68 to 3.07 g / m 3 but high densities (>3.0 g/cm3) are very rare. An in situ density measurement of Malvernian rock in the vicinity of the Colwall railway tunnel (National grid reference : SO 770434), involving gravity readings through and over the tunnel (Brooks 1963), produced a density value of 2.67 g/cm3. The assessment of a mean density for crystalline Pre-Cambrian is made difEdt by the shortage of information on the distribution and relative abundance of the various igneous and metamorphic types. However, the results of the laboratory measurements, with an overall mean of 2-70 g/cm3, tend to support Cook and Thirlaway (1955) in ascribing a value of 2.70 g/cm3 to the Malvernian and this value has been adopted for the gravity interpretation.

Using Cambrian member thicknesses given by Groom (1902), a bulk Cambrian density of 2.53 g/cm3 is obtained for the sedimentary succession. The overall Cambrian density is increased by the presence of igneous intrusives, of which 300 ft would raise the bulk density to about 2.55 g/m3. Bulk densities of Silurian strata differ slightly from area to area with changes in individual bed thickness but always fall within the range of 2.56 to 2-59 g/cm3. For the purposes of interpretation Cambrian and Silurian rocks are grouped together with a density of 2-57 g/cm3. Flaggy siltstones of the Red Downtonian, which covers much of the western part of the area under review, produced a density of 2.55 k0.04 g/cm3 (6 specimens).

Numerous measurements of Triassic densities, using surface and borehole samples from a wider region than that of the geophysical surveys, together with compaction tests, suggested that the density of sandstones in the Trias basin does not increase significantly with depth (Brooks 1966). The mean density of 225 g/cm3 derived from measurements of 60 surface and borehole samples is therefore likely to approximate to the bulk density of the entire Trias sandstone succession. Extensive surface and borehole sampling indicated that the Keuper Marl likewise maintains in depth its near-surface density of 2-45 k0-02 g/cm3 (41 specimens).

These values conform well where comparisons are available with those obtained by Parasnis (1952) from sampling a wider area of the English Midlands. There are dis- agreements, however, with the values selected by Cook and Thirlaway (1955) for use in their earlier interpretation of Malvern structures. In particular, they assumed a density of contrast of only 0-34 g/cm3 between Pre-Cambrian and Triassic rocks, whereas the above results indicate that 0.45 g/cm3 is a more likely value (see p. 20).

Fig. 1 . Sketch map of the geology of the Malvern Hills and adjacent districts based largely upon Groom’s maps of 1899 and 1900.

H

. . .'.'.*/- 1 . . I . . . . . . . . .

Silurian . ' . 'd-

Cambrian . . . .

Warren House Series * .

Ma lver n i a n . . . . . . .

. . . . * . . . . . . . . .

. . . . . . . .

. . . .

I . . . / I . . . . I ::::(-$;;/- . . . . . . . . . . . . . . . . . . . . . .

I . . . \

R

16 MICHAEL BROOKS

(b) Magnetic susceptibility and remanence determinations

Rocks exhibit both induced and permanent magnetization. Often the permanent, or remanent, magnetization forms a significant proportion of the total and for a full interpretation of magnetic data both types must be estimated.

Fifty-six determinations of susceptibility have been made on a wide range of rock- types existing in the area, together with 23 determinations of the direction and intensity of remanence in Pre-Cambrian rocks of theMalvern Hills. In addition, measurements of remanence intensity only were made on unoriented cores of Pre-Cambrian prepared for susceptibility determinations. Most of the measurements were carried out using a Blackett astatic magnetometer. All sedimentary rocks in the area, with the exception of the Cowleigh Park Beds (Llandovery), are negligibly magnetic. Specimens of the latter exhibited a mean volume susceptibility of 1.9 x emu. Malvernian rocks showed a wide range with some types exceeding 4.5 x loM4 emu.

Remanent magnetization values for the Warren House Series and Malvernian rocks are reported elsewhere (Brooks 1963). Both rock groups have a steeply-inclined direction of remanent magnetization (77"). Malvernian intensities ranged from 1.6 to 3.0 x emu. The near-verticality of the remanent magnetization in both groups means that it can be added to the vertical component of the induced magnetization for the purposes of interpreting the local vertical force magnetic field. This procedure produces an equivalent volume magnetic susceptibility (Grant and West 1965 337) of approximately 7 x emu for the Malvernian. It was considered, however, that sampling was biased towards acid types and that consequently the observed mean value would be lower than the true mean. In the interpretation a value of emu was therefore adopted for Malvernian and buried crystalline basement rock.

emu, and Warren House Series intensities from 2.4 to 8.0 x

3. GEOPHYSICAL SURVEYS

(a) Gravity survey

A Worden geodetic meter (No. 307) was used to make 650 gravity observations within the area, distributed at an average of 1.5 stations per square mile. Gravity values were connected to at least one of five reference stationsformingaclosed network connected to Cook and Thirlaway's reference station at Worcester Cathedral. A datum value of -27.53 mgal was used for this station, which has subsequently been connected to a Geological Survey reference station in Worcester and has been given a revised value of -27.68 mgal.

In the reduction of gravity observations to sea-level, elevation corrections were made on the basis of the density prevailing in the vicinity of the gravity station. When the station was situated near to a geological boundary involving a change in density a method described by Colley (1956) was used to calculate an effective density and to modify the elevation correction accordingly. Terrain corrections were applied to all the survey data using Hammer charts and tables (1939). Corrections were insignificant away from the Malvern Hills but large corrections were necessary at stations situated on the flanks of the Hills.

PLATE 1

Bouguer anomaly map of the Malvern Hills and adjacent districts. Gravity values have been reduced to sea-level using local densities, and anomalies are expressed against theoretical gravity at sea-level given by the International Gravity Formula of 1930. Observation points are marked by dots. Contour interval: 1 rngal.

GRAVITY AND MAGNETIC SURVEYS IN THE MALVERN HILLS 17

The Bouguer anomalies of PI. 1 and Figs. 2b and 3a have been derived using the the theoretical gravity field at sea-level given by the International Gravity Formula of 1930 (Heiskanen and Vening-Meinesz 1958 74).

@) Magnetic survey

Nearly 900 vertical force magnetic measurements were made in the same general area as that of the gravity survey, using an Askania torsion magnetometer and a vertical balance. Observations were connected to local base stations of which one, at Bransford (SO 795527), was given an arbitrary anomaly value of +lo0 gammas. Corrections were applied for diurnal variation and for geomagnetic field variation, the latter given by Walker (1919). Bock (1960) derived a single normal geomagnetic field for Europe which differs slightly from that of Walker. However, over the length of the present survey area the difference between the two fields is only 20 gammas. The vertical force anomalies are presented in PI. 2 and Figs. 2a and 4a.

(c) Outline of results

The map of Bouguer anomalies closely resembles that of Cook and Thirlaway (1955 pl. IV), and is dominated by an anomaly parallel to tke Eastein Ecurdaiy Fault. Peak anomaly values follow the line of the Malvern Hills and continue northwards along the belt of surface Silurian rocks extending towards the Abberley Hills. Anomaly values fall westwards over Old Red Sandstone strata.

The most obvious feature of the magnetic map is a positive anomaly overlying the Malvern Hills and extending northwards along the anticline through Storridge (SO 750487) towards the Abberley Hills. From its axis, values fall to similar levels both westwards on to Downtonian strata and eastwards on to Triassic rocks. A second large positive anomaly flanked by a wide zone of uniform gradient occurs in the Trias country east of Newent (SO 723260). Elsewhere, anomalies are small and the magnetic field is generally rather featureless.

4. GEOLOGICAL INTERPRETATION Important controls in the interpretation of gravity and magnetic anomalies include

surface geological information, borehole data and knowledge of the densities and magnetic properties of all rock-types available for sampling. In the area under review the surface geology is well documented; the writer has had access to unpublished Ph.D. theses (Phipps 1957; Reeve 1953) relevant to the geology and available in the University of Birmingham. In addition, physical properties of a complete range of relevant rock-types have been determined and for the purposes of interpretation the rock succession has been grouped into units of similar physical property. However, the possibility of unforeseen variations in rock properties at depth leads to uncertainty in the interpretations presented.

Various interpretation methods have been used. Where controls were sufficient to allow a preferred structural configuration to be given, interpretation proceeded by the method of .successive approximation using standard two-dimensional formulae, interpretation charts or a combination of both. Where controls were lacking analytical methods were used to estimate limiting values of certain parameters

18 MICHAEL BROOKS

such as depth to causal body. The following stratigraphic groupings and rock properties were used in the interpretation :- Densities

New Red Sandstones . . . . . . . . . . . . . . . . . . . . . . . . 2-25 g/cma Old Red Sandstones . . . . . . . . . . . . . . . . . . . . . . . . 2-55 g/cma Lower Palaeozoic . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-57 g/cm" Malvernian and local crystalline basement . . . . . . . . . . . . . . . 2-70 g/cma

Malvernian and local crystalline basement . . . . . . . . . . . . . . . 10'" emu All other rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

Magnetic properties (equivalent volume magnetic susceptibility)

(a) Area north of the Malvern Hills (i) Magnetic profile rhrough Storridge (Fig. 2a). The magnetic anomaly is sym-

metrical, with an amplitude of 1250 gammas, and is essentially positive with respect to the background field, suggesting a culmination in magnetic rocks under the Storridge anticline. The anomaly can be accounted for by the structural model shown in Fig. 2c. A core of crystalline basement extending to within 900 ft of the surface accounts for the amplitude of the observed anomaly. The western flank has been matched closely by the effect of a basement surface sloping at 30" to the west and extending to at least -5,000 ft O.D. Continuing westward fall of anomaly values is probably explained by a further lowering of the basement surface. The eastern flank of the observed anomaly has not been matched so closely. The magnetic effect of an Eastern Boundary Fault with the configuration suggested by the gravity data gives rise to a residual anomaly reaching +30 gammas two miles east of the surface fault. Accepting the gravity interpretation, the magnetic residual may be explained by a reduction of Malvernian magnetization in depth locally or alternatively by the presence of Lower Palaeozoic, or even Eocambrian sediments, below the Trias basin.

Certain modifications to this proposed structure can be accommodated without disturbing the good fit between measured and calculated fields. In particular, stepping the basement surface with the faults of 500 ft amplitude shown in Fig. 2c produces variations not exceeding about 10 gammas. The main factor governing the interpreted amplitude of the proposed basement structure is the magnetization value used in the interpretation. A reduction of this value requires a western limb extending to greater depth. To make the gravity interpretation consistent would involve raising the postulated Lower Palaeozoic density. The amplitude of the basement structure cannot be reduced without increasing both the magnetization contrast and the density contrast between crystalline basement and overlying Lower Palaeozoic rocks, and this would be inconsistent with the laboratory measurements of rock properties. Moreover, the postulation of shallower depths to basement west of the Malvern axis leads to problems in accommodating known or likely thicknesses of rock above the basement. For example, a complete succession of Cambrian and Silurian rocks approaches 6,000 ft in thickness.

Fig. 2. Interpretation of anomalies along an east-north-easterley line through Storridge (PIS. 1 and 2, AA').

(a) Vertical force magnetic anomaly. (b) Bouguer anomaly. (c) Two-dimensional structural model.

90 3%

30(

2%

20c

1%

m 1c

C

-10

- 20

ma

II A

m - 1

0 Kms 2

20 MICHAEL BROOKS

(ii) Gravityprofle through Storridge (Fig. 2b). The Eastern Boundary Fault anomaly in this district has an amplitude of 36 mgal, with a maximum measured horizontal gradient of 15 mgal per mile close to the surface fault. Maximum anomaly values overlie Silurian strata and values fall slowly westward. The observed anomaly can be matched using the structural model shown in Fig. 2c. In this, the interface between Pre-Cambrian and Triassic rocks dips at 45" to -2,500 ft O.D. and at 20" to -8,000 ft O.D. (In the Worcestershire basin, the Keuper Marl varies in thickness from zero to over

1,000 ft and gives rise to a gravity effect by virtue of its density contrast with the underlying Triassic sandstones. Though this effect exceeds two milligals locally it is normally considerably less than one milligal. In View of the large number of factors which could cause gravity effects of this order, as for example, low density basal beds in the New Red Sandstone, or Carboniferous or Devonian strata above Lower Palaeo- zoic and Pre-Cambrian under the Trias basin, the effect of Keuper Marl thickness variations has been ignored in the present interpretations of the Eastern Boundary Fault.)

The density contrast between Malvernian and Lower Palaeozoic rocks is small, and hence the gravity anomalies also are small over structures involving these rock-types. To produce the observed westerly fall of the gravity field so near to the Eastern Boundary Fault, using the small density contrast prevailing between Pre-Cambrian and Lower Palaeozoic rocks, a major basement structure needs to be inferred. The gravity anomaly is consistent with the presence of a basement fold with a western limb dipping at 30" and bringing Pre-Cambrian rocks to within 1,000 ft of the surface less than two miles west of the Eastern Boundary Fault. This result agrees with the magnetic interpretation presented above. Continued fall of the gravity anomaly westward may be explained partly by thickening Downtonian in that direction and partly by a continuing fall in basement level since magnetic values also continue to fall westwards.

The possibility of igneous rocks in concealed Cambrian in the core of the Storridge anticline cannot be ruled out. However, the southern Malvern area of exposed Cambrian with intrusive rocks is one of weak negative anomaly. The inference is that such rocks at depth would have little effect upon the form of regional magnetic anomalies but that where thickly developed they may be indicated by weak negative anomalies of the sort which are found further south over Silurian and Downtonian strata (anomalies A and B, PI. 2). In the magnetic interpretation the effect of Cowleigh Park Beds has been ignored. This is not entirely justifiable for they may give rise to an effect reaching about 20 gammas.

An important parameter in the interpretation is the depth to basement north of the Malvern Hills. If the above estimate of about 1,000 ft be correct it follows that there cannot be a thick development of Cambrian under the axial portion of the Storridge anticline, for the Cowleigh Park Beds have a thickness of at least 400 ft. Either Cambrian was not fully developed locally or-it was subsequently largely removed from the vicinity of the fold axis following tectonismpre-datingthe Llandovery transgression. This situation appears similar to that in the southern Malverns where Llandovery

PLATE 2 Vertical component magnetic anomaly map of the Malvern Hills and adjacent districts. Magnetic

values have been corrected for the geomagnetic gradient. Anomalies are expressed against an arbitrary datum. Observation points are marked by dots, except in the immediate vicinity of the Hill region where they are too numerous to be represented. Contour interval: 25 gammas.


deposits overstep a thick Cambrian succession over a distance of one mile and lie, in Gullet Quarry (SO 762381), directly upon Malvernian rock (Brooks and Druce 1965). Reading and Poole (1961) invoked ‘Taconic’ tilting and faulting to explain the unconformity below the Llandovery deposits. This point is discussed further @. 29).

The interpretation presented represents the simplest structure consistent with the surface gravity and magnetic anomalies and does not include hypothetical pre-Middle or Upper Llandovery faults affecting the basement. A negligible effect is introduced into calculated surface fields by stepping the basement on the western limb of the major fold. That is, the basement surface may be smoothly folded or both step- faulted and folded. Pre-Middle or Upper Llandovery faulting thus can readily be tolerated in the interpretation and is shown in an alternative basement model included in Fig. 2c. Such faulting might be expected also along the western side of the Malvern Hills and this point is discussed below (p. 22).

(b) Malvern Hills

The positive magnetic anomaly decreases southwards from Storridge before rising again over the he-Cambrian rocks of North Hill (SO 769464). Cowleigh Park (SO 760475) is represented by a ‘saddle’ in the magnetic anomaly axis which may be explained in terms either of a slightly deeper basement or of a local reduction in Malvernian magnetization. Along the Malvern range magnetic field values are at a maximum over North Hill and Worcest‘ershire Beacon. Values decrease sharply southwards and the field loses the extreme irregularity which it exhibits over the northern part of the range. Over North Hill and Worcestershire Beacon the field is extremely irregular and anomaly values commonly exceed +600 gammas whereas they rarely rise much above +300 gammas to the south. Without an adequate geological map of the Malvernian these considerations cannot be camed far but it is clear that the surface magnetic field is very much influenced by the locally prevailing Malvernian rock-type. The width of the main magnetic anomaly is similar over the northern Malvern Hills and the area directly to the north but it broadens markedly west of Herefordshire Beacon and covers the broad belt of Silurian strata which extends to the vicinity of Ledbury (SO 713377). Further south the magnetic field contains local disturbances brought about by igneous rocks in the Cambrian; these anomalies are not indicated on PI. 2 which shows the area to be one of weak negative anomaly. Along the western side of the Hills there are a number of narrow elongate negative anomalies culminating in a nearly continuous zone (anomaly C, P1. 2) clearly associated with the steep interface forming the western boundary of Malvernian rock. There is no obvious equivalent in the magnetic field north of the Hills but incipient lows in that area may have a similar cause.

The gravity field west of the Eastern Boundary Fault is more regular than the magnetic field of the same area. In considering the Storridge area and Malvern Hills together, a persistent, though weak positive gravity axis can be noted extending throughout the area. However, peak anomaly values decrease from over 7 mgal around Storridge and North Hill to less than 5 mgal south of Wyche Cutting, and climb again to over 6 mgal south of Herefordshire Beacon. These differences may be explained in terms of real structural differences in depth or of density changes within the Malvernian or even within adjacent Lower Palaeozoic rocks. Density measurements on Lower Palaeozoic rocks give no indication of regional differences which could account for these variations in the level of the gravity peak. If density differences within the Malvernian and local basement rock are their cause, these almost certainly do not

22 MICHAEL BROOKS

persist in depth. The Eastern Boundary Fault anomaly remains essentially similar throughout the area, so that significant regional changes in overall Pre-Cambrian density would lead to the conclusion of widely differing thicknesses of New Red Sandstones to the east. It is difficult to accept this as the true situation. Nevertheless, surface Pre-Cambrian rock does undergo marked changes of density from north to south along the Malvern Hills and these doubtless have a small effect, albeit indeter- minate, on the local gravity field.

(i) Gravityprofllesouth ofThe Gullet,south Malverns. (Fig. 3a)~milenorthofHoll~bush (SO 762370) the main fault anomaly has an amplitude of 31 mgal and the maximum measured horizontal gradient is approximately 16 mgal per mile. As with the profile through Storridge a change at depth in the mean slope of the junction between Triassic and earlier rocks has been incorporated to obtain a close fit between observed and calculated profiles. The upper slope is at 45" and extends to -3,000 ft O.D.; below this is a slope of 20" extending to -7,500 ft O.D. The anomaly fall-off to the west is interpreted as being caused by a sloping basement surface extending to more than -5,000 ft O.D. The dip of the lower part of this surface is about 30" but the dip shallows eastwards to 20" before culminating in a 300 ft step which brings Pre- Cambrian to the surface along the Malvern Hills. This interpretation is shown in Fig. 3b and it is consistent with the form of the local magnetic field. As on the Storridge profile step faults in the basement surface west of the Hills can be incorporated.without affecting the good fit between observed and calculated anomalies.

mgal 10 r

-measured anomaly

theoretical values

0 shown.

-I0 t I

- - i

0 Kms 2

Fig. 3. -Gravity interpretation along an east-west line across the south Malverns near The Gullet (PI. 1, BB'J.

(a) Bouguer anomaly. (b) Two-dimensional structural model.


Except for the thin Malvern Quartzite exposed in The Gullet (SO 760380), surface rocks along the gravity profile west of the Hills are everywhere May Hill Sandstones. Nevertheless, the interpreted depth to basement one mile west of the Gullet is only 2,000 ft. The underlying Cambrian succession therefore appears to be only thinly developed, for May Hill Sandstones probably extend in depth for a few hundred feet at this point. Peak anomaly values fall rapidly southwards along the belt of Cambrian strata south of Hollybush, at least partly due to a thickening Cambrian succession, for Groom (1902) estimated a thickness of 2,500 ft in the vicinity of Whiteleaved Oak (SO 760360). This indication that the Cambrian thickness is locally variable, in conjunction with the geological information that the Cambrian is overstepped locally from west to east by the Llandovery (Reading and Poole 1961), draws attention to the importance of pre-Middle or Upper Llandovery movements in this vicinity. On the basis of differences of strike, Phipps and Reeve (1967 357) infer that the Cambrian was folded along an easterly trend before the overlying Silurian was deposited. Variations in peak anomaly values further north may similarly be influenced by variations in the local Cambrian thickness on the western limb of the major Armorican fold, brought about by this earlier folding.

(ii) Magnetic profile across the north Malvern Hills (Fig. 4a). It is not possible to present a representative magnetic profile across the Malvern Hills because of the irregular magnetic field over exposed Pre-Cambrian rock. In some areas of surface Pre-Cambrian rock the magnetometer was read in a vertical magnetic gradient of about 100 gammas per foot. Consequently small variations in tripod height and thickness of soil, among other factors, must greatly have influenced the actual reading obtained. It is thus unwarranted to strive for very close agreement between observed and calculated fields in areas of surface Pre-Cambrian rock and unjustifiable to place much reliance on any interpretation which provides close agreement in such areas, for this agreement will be at least partly fortuitous.

Fig. 4a shows the form of the magnetic field along a profile extending across the Hills. For the reasons given above, calculated values are omitted across the area of surface Pre-Cambrian. More important than anomaly variations on Pre-Cambrian rock are the form of the anomaly flanks west and east of the Malvern axis. Inter- pretation based on a vertical magnetization fails to account for the zone of negative anomalies lying close to the western boundary of Pre-Cambrian (see PI. 2 and Fig. 5). This zone is explicable h two ways. Either the Malvernian exhibits, in places near to its western margin, an east-west component of remanence (Fig. 5a) or there is a shallow boundary under the Hills such that the surface anomaly is due in part to a thin slab of magnetic material of which the top face may be the ground surface (Fig. 5b). Such a boundary could occur within the Malvernian but the negative anomalies may alternatively be taken to suggest the presence of a slab of Lower Palaeozoic strata under surface Pre-Cambrian rocks, insufficiently thick to give rise to a measur- able gravity effect.

Although Raw’s (1952) ideas on major overthrusting are difficult to reconcile with the fact that the Lower Palaeozoic unconformably overlies the Malvernian in places, local thrusting may possibly occur. Reading and Poole (1961) suggested that low- angle thrusting may have taken place along earlier reverse faults after they had been tilted during Armorican folding. The zone of negative anomalies marginal to Pre- Cambrian rock could possibly represent part of the edge effect of a thin layer of Re-Cambrian truncated to the west by a steep boundary and underlain by Lower Palaeozoic rocks (see Fig. 5). A layer of normal basement 250 ft thick would produce a negative anomaly of the right order of size and width. This layer would produce a complementary positive anomaly overlying its marginal zone and although such an

gamma

me a su red anomaly t h eoretica I values using structure shown.

30

200

0- W E - Lr: Palaeozoic I I I 1u3

400 "g 451ft Pre-Cam brian 7 m f t

Tri-r I

by a pprox.1000 f t .

anomaly along AA' (through Storridge)


anomaly cannot be seen in the irregular magnetic field over the Pre-Cambrian its existence cannot be ruled out.

An important aspect of the structural model (Fig. 4b) derived from interpreting the broader features of the Malvern magnetic profile and disregarding the local negative anomaly is the indication that the western margin extends nearly vertically for some few hundred feet and is followed in depth by a westerly basement slope similar to that postulated in other parts of the area. To the east there is a small discrepancy (reaching c. 10 gammas) which may be explained in terms of a local change in Malvernian magnetization at depth.

The structural model closely resembles that at Storridge, but two main differences are apparent. Firstly, the Malvernian of the Hill region seems to be in the form of a protuberance on the crest of the major fold which has no equivalent in the Storridge fold. This difference may in part be more apparent than real and result only from the insensitivity of the geophysical interpretation of the Storridge area. Structural complexity affecting Silurian strata exposed north of the Hills suggests that, as in the Hill region, basement rock in the axial part of the broad major fold may be disrupted by faulting and intense folding; but a marked protuberance in the case of the Storridge anticline is inconsistent with the interpretation presented.

Secondly, the highest point on the basement profile lies further away from the Eastern Boundary Fault in the Storridge area so that the basement level falls eastwards to a considerable depth before being truncated by the fault. This eastern limb normally has no counterpart in the Malvern Hills where the axis of the major fold lies close to (or east of) the Eastern Boundary Fault. However, the eastern side of Swinyard Hill (SO 762385) is composed of Upper Llandovery strata and though these were considered by Groom (1900) to be faulted into place, they may alternatively be regarded as occupying a steep portion of the eastern limb of the major fold. In spite of these differences between the Storridge and Maivern structures, a close

similarity in basement form exists. It follows that the north Malvern magnetic field continued upwards by 1,000 ft should resemble closely the surface field in the Stomdge area. The testing of this hypothesis forms a useful alternative way of utilizing the magnetic data, particularly in the present case where surface magnetic rock giving rise to considerable topographic relief weakens the basis of direct interpretation.

(iii) Upward continuation of the Malvern magneticfield. Field continuation upwards by about 1,000 ft was carried out along the north Malvern magnetic profile line using the method of Peters (1949). The field at the higher level is presented in Fig. 4c together with the surface field through Storridge for direct comparison, and it can be seen that the fields are closely similar. In particular the negative anomaly due to the western margin of Malvernian rock, which is pronounced at ground level, is very indistinct at the higher level. Incipient lows in the Storridge area may therefore represent the equivalents, at the higher structural level exposed there, of the intense lows along the west side of the Malvern Hills. There is a marked difference in peak values between the ground and upward continued profiles but these are dependent in the latter case upon local irregularity in the field directly overlying Pre-Cambrian rock.

Fig. 4. Magnetic interpretation along an east-west line across the north MaIverns (PI: 2, CC’). (a) Vertical force magnetic anomaly. (b) Two-dimensional structural model. (c) Comparison of the Storridge anomaly at ground level with the north Malvern anomaly

continued upwards by approximately 1,000 ft.

26 MICHAEL BROOKS

t h eoret ica I field due to structure

\ 'j L + + + + + D Lr I

t 250 f t

t

150 c 'c ga rn ma

- - - - - Fig. 5. Possible explanations of the cause of negative anomalies along the western side of the

(a) Vertical force anomaly along a portion of the line AB due to a horizontal component of Malvern Hills.

magnetization in the block I. (b) Vertical force anomaly along a portion of the line CD due to a vertical component of

magnetization in the slab 11. N.B. The negative anomaly shown is taken from the western flank of Worcestershire Beacon. The

precise position of the Lower Palaeozoic: Malvernian boundary is not known.

(iv) Note on the magnetic anomaly southeast of Newent (anomaly D. PI. 2). A magnetic anomaly with an amplitude of 300 gammas and a width of 7 miles is located in Trias country east of Newent. The southeastern part of this anomaly was picked up by Falcon and Tarrant (1951) and the equivalent total force anomaly is apparent on the aeromagnetic map of part of Great Britain and Northern Ireland, Sheet 5 (Bullerwell 1964). There appears to be no associated gravity effect, suggesting that the magnetic anomaly is not associated with pronounced basement relief.


The causal structure was assumed to be vertical-sided basement zone and its approximate depth estimated using a method described by Peters (1949). Estimates using both flanks of the anomaly produced a depth of 7,100 ft to the top face of the structure. Interpretation was also carried out using an aeromagnetic proiile along the same line, kindly supplied by Dr. Bullerwell of the Institute of Geological Sciences. Using the Henderson and Zietz (1948) half-width method and a line-of-poles approximation a depth of 8,000 ft was obtained which, subtracting the flying height of 1,000 ft, gives a depth to basement of 7,000 ft.

5. THE FORM AND POSSIBLE NATURE OF THE MALVERN AXIS AND ITS CONTINUATION NORTHWARDS

On the basis of a similarity in gravity and magnetic fields it is concluded that the structure of the area directly north of the Malvern Hills is essentially the sameasthat ofthe Hill region. The major structure is a basement fold of large amplitude bounded to the east by a fault complex extending to great depth and having a steep western limb which is also probably in part faulted. On this view, the essential difference between the Malverns and the area to the north is the lower structural level exposed in the former area. This is no new idea, for Phillips (1848 38) wrote of “the rocky axis of Malvern, prolonged to the north beneath the strata of the Martley and Abberley Hills”. Phillips’ view, however, was based on an interpretation of the significance of small patches of Pre-Cambrian rock north of the Malvern Hills which was later dis- puted by Groom (1900). These patches, at three localities in the Cowleigh Park area and at a fourth west of Martley (SO 756599), were assumed by Phillips (1848) and Holl(l865) to be peaks piercing through Silurian strata but connected in depth with the crystalline rocks of the Malvern Hills. Groom (1900) fust drew attention to the structural complexity of Cowleigh Park and claimed these patches of Pre-Cambrian to be bounded by thrusts; and his mapping revealed other thrusts and transverse faults (with respect to the axis trend) in the vicinity.

Nevertheless, geological evidence strongly supports a straightforward link between the Malvern and Abberley ranges. Firstly, the vertical Malvern fold involving Palaeozoic strata passes insensibly northwards into the western limb of the Storridge anticline and the zone of dip inversion adjacent to the Malvern Hills persists into the Abberleys where overturned Silurian and Old Red Sandstone strata are encountered. Secondly, the extreme complexity of structure in the axial region of Lower Palaeozoic rocks north of the Hills is matched closely by the nature of structures affecting Malvernian rocks to the south (see, e.g. sections given by Groom 1899 and 1900).

In the vicinity of Stomdge, north of the structural complications of Cowleigh Park, the depth to basement is estimated to be about 1,000 ft. Judging from a fall of gravity and magnetic anomalies northwards, this depth increases rapidly north of M r k k (SO 749530). In the Abberley Hills the effects of the Malvern axis are obscure, seemingly because of its greater depth and the influence of other deep-seated effects in the geophysical results. In view of the structural complications north of the Malvern Hills, involving possible tectonic thickening or thinning in depth, the calculated depths to basement cannot confidently be translated into a stratigraphical succession.

Nevertheless it can be stated that a thick succession of Cambrian strata under the Storridge anticline is incompatible with the geophysical results. This finding is consistent with the scarcity of recorded Cambrian strata from the Malvern Hills. Their only representatives are faulted patches (though interpreted by Groom (1899 fig. 18) as tightly folded with the Malvemian) of Lower Cambrian on Raggedstone and Mid- summer Hills (SO 760375). This is in notable contrast to the fairly common and

28 MICHAEL BROOKS

widespread occurrence of Llandovery rocks. Indeed in the example cited above May Hill Sandstones also occur, without the stratigraphically intermediate shales of Upper Cambrian or Tremadocian age. Groom suggested (1899 153) that these shales had been squeezed out during a phase of intense folding but it is at least as reasonable to suppose that the Llandovery strata here overlie unconformably an incomplete Cambrian succession. It should be noted that according to Groom (1900 160) a small amount of Cambrian quartzite is found associated with the Re-Cambrian inliers of both Cowleigh Park and Martley. This may be used to suggest a thin development of Lower Cambrian strata under these areas but its thickness cannot exceed a few hundred feet.

The basement near Martley is interpreted to be at considerable depth (probably more than 3,000 ft) and it is difficult to envisage the derivation of basement rocks through such a thick Lower Palaeozoic succession. It is tentatively suggest,ed that the axis of the major fold passed east of Martley in Armorican times, subsequently to be lowered by movements along the Eastern Boundary Fault. On this view, the Martley inlier may comprise material thrust westward from the axial zone of the major fold prior to the movements which gave rise to the Worcestershire Trias basin. The structure of the Martley mass is then closely similar to that of Herefordshire Beacon and Chase End Hill (SO 761355).

In the Malvern area the western boundary of Pre-Cambrian rock has been variously interpreted as the steep limb of a fold (Phillips 1848; Butcher 1962), a vertical or easterly-hading fault (Groom 1899, 1900; Phipps and Reeve 1964) and a line along which both faulting and folding have at different times prevailed (Reading and Poole 1961 ; Ziegler 1964). To the extent that these various views do not necessarily involve different geometric configurations of rock in depth, geophysical methods are rendered largely ineffectual in resolving the problem. Only insofar as the results indicate the amplitude of the basement structure and its possible extent do the present surveys contribute to our structural knowledge. The amplitude of the western limb is probably greater than 5,000 ft. So far as the extent of the fold is concerned, the geophysical anomalies suggest continuity throughout the Malvern Hills and the area immediately to the north. If the western boundary of Malvernian rock were a major fault or part of a fault-complex of Armorican age, surface faulting would be expected to course west of Cowleigh Park and extend up the western side of the Abberley range. In fact there is, in the Abberley Hills, a “western boundary fault” which brings Red Marls against Ludlow strata (see 1”:l mile Geol. Surv. Map: Droitwich, Sheet 182) and which might be related genetically to basement faulting at depth. But nowhere between the Malvern and Abberley Hills is there a major fault which could be the surface expression of a continuation of Groom’s Western Boundary Fault sensu stricto. The absence of a universal western boundary fault in the Malverns and the demonstrable folding of the Pre-Cambrian basement and Palaeozoic strata to a high angle (as in Gullet Quarry) indicate clearly that post-Lower Old Red Sandstone folding is responsible in large measure for the disposition of rocks in this area. That there was subsidiary faulting associated with this folding is generally accepted. Butcher (1962) drew attention to its existence and showed that its form is compatible with folding of the basement surface into an essentially monoclinal form. Subsidiary faulting is represented along the western boundary by shearing of the sort visible in Gullet Quarry (see Reading and Poole 1961; Whitworth 1962) and possibly also by the larger scale faulting recorded in the new railway tunnel at Colwall by Robertson (1926). Its importance in depth cannot be gauged but in view of the amplitude of the basement culmination, with a vertical limb possibly approaching 1,000 ft in places, it is likely to be well developed.

GRAVITY AND MAGNETIC SURVEYS IN THE MALVERN HILLS

With regard to earlier movements, Reading and Poole (1961 298) drew attention to their necessary existence to explain the removal from the Malvern axis at Gullet Quarry of the thick Cambrian succession which is preserved only a mile away from the quarry. Reading and Poole suggested tilting and faulting of Taconic age and assumed that it took place along an embryonic Malvern line which was further folded in Armorican times..Such a phase of early movement along the whole length of the Malvern and Abberley Hills would provide an explanatibn for the shortage of Cambrian strata on the Malvern axis in terms of post-Taconic erosion. As stated above, faulting of this age can readily be incorporated into the geophysical interpretation though it seems not to exist as a single major step. In searching for evidence of the actual date of earlier movements in the Malvern area the similar structural situation around the Longmynd in Shropshire may hold a clue. In both areas Pre-Cambrian blocks and flanking Cambrian deposits are overstepped by Llandovery strata. At the west side of the Shelve Inlier west of the Longmynd, folded Caradoc beds are preserved beneath the Llandovery cover. It is tempting to suggest that the post-Caradoc movements of the Shelve area acted also in the Malverns to produce upstanding Cambrian that was largely eroded before or during the Llandovery transgressive phase. The igneous intrusions of the south Malverns may also be ascribed to this period of late Ordovician tectonism.

The suggestion of Cook and Thirlaway (1955) that the junction between Triassic and earlier rocks is in the form of a step-faulted unconformity is accepted here. The mean slope of the junction is too steep for it to be unfaulted, for Triassic dips are generally very low away from the immediate vicinity of surface faulting. The reduction of the mean slope of the junction at depth, from 45” to 20”, may reflect a reduction, away from the Malvern axis, of the abundance or amplitude of faults affecting the unconformity. The maximum depth of Triassic deposits, assumed homogeneous, is calculated to be 8,000 ft. This is somewhat less than Cook and Thirlaway estimated, but not significantly so in view of uncertainties as to Triassic densities in depth and the nature of the underlying Palaeozgic floor.

Evidence for Pre-Cambrian activity along the Malvern axis comes from isotopic age determinations. Lambert and Rex (1966) reported ages grouped closely about 590 _+20 m.y. for a variety of igneous Malvernian rock-types. Though this date may have been lowered by a subsequent event (e.g. Ordovician vulcanism or tectonism of Lower Palaeozoic or Armorican age) the results suggest an “isotopic event” of late he-Cambrian age which may have involved uplift along an early Malvern axis. The development of the Malvern axis may thus have begun in Pre-Cambrian times. Following this, late Ordovician tectonism resulted in movement along the vicinity of the present-day western boundary of Pre-Cambrian rock. The Malvern axis was a line of positive basement relief during Llandovery times and this relief was greatly accentuated by Armorican folding which produced a broad major fold with intense folding and faulting along its crest. The date of the earliest movements along the Eastern Boundary Fault is unknown, but faulting was probably active following Armorican folding and gave rise to a basin in which New Red Sandstones accumu- lated to a great thickness. The evidence is therefore of a succession of major tectonic events along the Malvern axis, and these may owe their origin to a deep fault or zone of weakness in the underlying basement.

Acknowledgements. Most of the field work on which the above account is based was carried out during the period of tenure of a D.S.I.R. Research Studentship at the University of Birmingham. Particular thanks are due to Professor D. H. Griffiths for supervising the work and offering constructive criticism of the manuscript. Dr. Bullerwell very kindly supplied information held by the Institute of Geological

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30 MICHAEL BROOKS

Sciences and also read the manuscript. Colleagues at Swansea are thanked for valuable discussions and interest shown. Invaluable assistance in the field was given by people too numerous to mention but whose help is gratefully acknowledged. The University College of Swansea provided a grant towards publishing costs.

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variation. Geophys. Prospect. 7 (for 1959), 389. BROOKS, M. 1963. Rock groups of the Malvern Hills. Nature, Lond. 198, 567. - 1966. A study of density variations in New. Red Sandstones from the English Midlands.

Geol. Mag. 103, 61. - and DRUCE, E. C. 1965. A Llandovery conglomeratic limestone in Gullet Quarry, Malvern

Hills, and its conodont fauna. Geol. Mag. 102, 370. BULLERWELL, W. 1964. Aeromagnetic map of part of Great Britain and Northern Ireland at

1 :250,000. Sheet 5. Geological Survey of Great Britain. BUTCHER, N. E. 1962. The tectonic structure of the Malvern Hills. Proc. geol. Ass. 73, 103. COLLEY, G. C. 1956. Gravity variations in surveys across geological boundaries. Geophys. Prospecr.

3 (for 1959,403. COOK, A. H. and TFIIRLAWAY, H. I. S. 1950. Recent observations of gravity in Wales and the

Borders. Rep. 18th lnt. geol. Congr. Gt. Br. 1948, part V, 33. - and - 1955. The geological results of measurements of gravity in the Welsh Borders.

Q. J. geol. SOC. Lond. 111,47. FALCON, N. L. and TARRANT, L. H. 1951. The gravitational and magnetic exploration of parts of

the Mesozoic-covered areas of south-central England. Q. J . geol. SOC. Lond. 106 (for 1950), 141. GRANT, F. S. and WEST, G. D. 1965. Interpretation theory in appliedgeophysics. (1st edn) New York

(McGraw-Hill). GROOM, T. T. 1899. The geological structure of the southern Malverns, and of the adjacent district

to the west. Q. J . geol. SOC. Lond. 55, 129. - 1900. On the geological structure of portions of the Malvern and Abberley Hills. Q. J. geol. Sac. Lond. 56, 138. - 1902. The sequence of the Cambrian and associated beds of the Malvern Hills. Q. J. geol.'

SOC. Lond. 58, 89. HAMMER, S. 1939. Terrain corrections for gravimeter stations. Geophysics 4, 184. HEISKANEN, W. A. and VENING-MEINESZ, F. A. 1958. The Earth and itsgravityfield. (1st edn) New

York (McGraw-Hill). HENDERSON, R. G. and ZIETZ, I. 1948. Analysis of total magnetic intensity anomalies produced by

point and line sources. Geophysics 13, 428. HOLL, H. B. 1865. On the geological structure of the Malvern Hills and adjacent districts. Q. J.

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of the Malverns. Nature, Lond. 209, 605. PARASNIS, D. S. 1952. A study of rocks densities in the English Midlands. Mon. Not. R. astr.

SOC. geophys. Suppl. 6,252. PETERS, L. J. 1949. The direct approach to magnetic interpretation and its practical application,

Geophysics 14, 290. PHILLIPS, I. 1848. The Malvern Hills, compared with the Palaeozoic districts of Abberley,

Woolhope, May Hill, Tortworth, and Usk. Mern. geol. Surv. U.K. 2 (l), 1. PHIPPS, C. B. and REEVE, F. A. E. 1964. The Pre-Cambrian-Palaeozoic boundary of the Malverns.

Geol. Mag. 101, 397. - and - 1967. Stratigraphy and geological history of the Malvern, Abberley and Ledbury

Hills. Geol. J. 5, 339. RAW, F. 1952. Structure and origin of the Malvern Hills. Proc. geol. Ass. 63, 227. READING, H. G. and POOLE, A. B. 1961. A Llandovery shoreline from the southern Malverns.

Geol. Mag. 98, 295. ROBERTSON, T. 1926. The section of the new railway tunnel through the Malvern Hills at Colwall.

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Phil. Trans. R . SOC. 219A, 1. WmTwoRm, T. 1962. Correspondence. Geol. Mag. 99, 375. ZIEOLER, A. M.11964. Correspondence. Geol. Mag. 101,467.

4

M. BROOKS,

UNIVERSITY COLLEGE, DEPARTMENT OF GEOLOGY,

SINGLETON PARK, SWANSEA, WALES.

Documents

The geological results of gravity and magnetic surveys in the Malvern Hills and adjacent districts