The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden MARTIN H. P. BOTT CONTENTS Rock densities and methods of survey . 3Io Interpretation of the Lake District negative gravity anomaly 313 (x) Eskdale granite and Ennerdale granophyre 3x3 (n) Skiddaw granite 3t5 (e) Shap granite 3x7 (I)) Hidden granite beneath the Lake District 3~8 (~) Other gravity anomalies 3x9 Uplift of the Lake District 320 Vale of Eden . 321 (A) Granite ridge at "depth joining Shap and Weardale granites 32x (13) The Permo-Triassic trough . 322 (c) Structural history of the Pennine fault line 324 References . 326 SUMMARY The Lake District is dominated by an east- is probably connected to the Weardale granite west belt of relatively low Bouguer anomaly by a deep granite ridge. which attains individual minima over the Present and past uplift of the Lake District exposed Eskdale, Shap and Skiddaw granites. may be attributed to the granite mass deficiency, The negative anomaly is attributed to a which is estimated to be x.x × xolSg and is composite granite batholith which underlies approximately equal to the present elevation of the central and northern parts of the Lake the Lake District above a 27 °ft (82 m) datum. District, connecting the exposed granites at The low gravity values along the Vale of depth. Interpreting on the basis of surface Eden suggest that the Permo-Triassic rocks density measurements, the granites appear to reach a maximum thickness of at least 1 km extend to a depth of about 7 to xo km and the northeast of Penrith, and that these rocks contacts with the country rocks generally slope formed during contemporaneous subsidence. outwards. There are substantial variations in The detailed gravity interpretation of the density within the composite granite body. The structure of the Vale of Eden allows a new roof region of the granite includes a series of assessment to be made of the structural history shallow granite 'ridges,' one of these connecting of the Pennine line which reconciles the the Eskdale and Shap granites, and another Hercynian structures with the occurrence of connecting the Ennerdale, St. John's, Threl- Whin Sill or dyke pebbles in the Upper keld and Skiddaw granites. The Shap granite Brockram. TH E L A ~p. D I ST R I CT is formed of Lower Palaeozoic rocks which were strongly folded in late Silurian or early Devonian time, and were intruded by post tectonic granites of early Devonian age. Faulted and tilted Carboniferous strata overlie the earlier beds with strong unconformity; these generally dip away from the Lake District and their outcrop forms a discontinuous belt around it. The Carboniferous rocks are unconformably overlain by Permo-Triassic beds, which are faulted against the Lower Palaeozoic rocks along the western margin of the Jl. geol. Soc. Lond. vol. x3o, I974, pp. 3o9-331 , 8 figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/309/4884781/gsjgs.130.4.0309.pdf by guest on 29 September 2021 3 i o M.H.P. Bott Lake District. The region suffered earth movements in Hercynian and post Triassic times which caused doming of the Lake District and faulting and tilting in the surrounding regions. General accounts of the stratigraphy and structure are given by Hollingworth (i954) , Mitchell (I956), Taylor et al. (1971) and Moseley (i972), with lists of references. This gravity survey was made primarily to investigate the deep structure of the Lake District granites and of the Permo-Triassic trough of the Vale of Eden. I. Rock densities and methods of survey Densities measurements on Lower Palaeozoic and Permo-Triassic rocks are shown in Table I. The densities of these formations at depth probably lie between the saturated and grain density values. The density of Carboniferous formations is based on measurements in NE England (Bott & Masson-Smith I957) , and this depends critically on the propor- tions of limestone (mean saturated density 2.68 g cm -3, mean grain density 2"7 I), shale (2.56, 2"69) and sandstone (2"42, 2"65). A typical Yoredale sequence has a density of about 2.60 g cm -8. Significant density contrasts occur at the following interfaces: Ordovician/ Silurian, Lower Palaeozoic/granite, Lower Palaeozoic/Carboniferous, Carboni- ferous/Permo-Triassic. Although some density variation occurs within formations, the main gravity anomalies can essentially be interpreted in terms of the above density contrasts. The survey of 874 new gravity observations was made using Worden gravimeter No. 138. The calibration factor was verified by measuring between pendulum stations at York and Newcastle-upon-Tyne. The calibration error is probably less than +o. 1%. The local base network (Fig. I) is connected to I.G.S. gravity base stations at Kendal, Shap and Penrith and is related to a value of 98I'265oo cm s -* at Pendulum House, Cambridge. The maximum closing error within the base net- work is 0.04 mgal. During the survey, base connections were normally made at not more than two hour intervals. The standard error of a single gravity obser- vation is estimated to be +0.05 mgal. Correction for latitude was made using the I.G.F. and the combined elevation correction was applied using the following formation density values: 2"75 g c m-3 (Skiddaw Slates and Borrowdale Volcanics), 2"72 (Silurian), 2"65 (granite and Carboniferous except Coal Measures), 2.50 (Coal Measures), 2.33 (Permo- Triassic) and 2.oo (alluvium). Terrain corrections were applied using the Hammer zone chart method. The largest unsystematic source of error in the Bouguer anomalies comes from the terrain corrections, and may be as much as +0"5 mgal in regions where the corrections approach i o mgal, although more usually this error will be about 4-o-15 mgal. A small systematic error is introduced by uncertainty in rock density, but is unlikely to exceed zko.5 mgal even at elevated stations. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/130/4/309/4884781/gsjgs.130.4.0309.pdf by guest on 29 September 2021 TABLZ I : Sample density measurements Saturated density Grain density Locality (grid Number of with s.d. with s.d. Formation reference) specimens (g cm -s) (g cm -3) Source Skiddaw Slates NY218277 to 2"78 -4- 0-02 2.8I 4- 0.02 (i) NY217279 I9 2"78 4- 0-02 2.8I 4- 0.02 NY244236 i7 2"76 4- o-o2 2.78 4- o.o2 NY2419 7 2.76 4- o.oi 2"77 4- o-o2 Mean of localities 2"77 4- o.oI 2"79 4- o-o2 Borrowdale NY244135 20 2"76 4- 0"03 2"77 4- 0.02 (I) Volcanics NY216o72 I t 2"80 4- 0"02 2"8I 4- 0"02 NY2 i8o74 2 2.8o 2.82 NY22oo76 24 2"70 4- o.03 2"70 4- o.o2 NY222o95 22 2"77 4- 0"03 2"77 4- 0"03 NY22IO 2I 2.7i 4- 0.02 2.71 4- 0.02 NY253167 24 2.76 4- o.oi 2.78 4- o.o2 NY249199 21 2"76 4- o.oi 2"79 4- 0.02 SD29698o 2I 2"69 4- 0-02 2"7 ° 4- 0"03 SD28199I 24 2.66 4- o-o3 2"67 4- o'o3 SD29o98o 36 2"75 4- o.o2 2"79 4- o.o2 SD278982 2i 2.78 4- o.oi 2.78 4- o.ol Mean of localities 2"74 4- o'o4 2"76 4- o-o5 Brathay Flags SD299975 20 2"74 4- o.oi 2"77 4- 0.02 (I) NY358oI6 2I 2.75 4- 0.02 2"77 4- o.oI Bannisdale Slates 2I 2"72 4- o-oi 2"73 4- 0.02 (I) Kirkby Moor Flags SD5892 20 2-69 4- 0.02 2"74 4- o.o1 (x) Penrith Sandstone NY54231o 2I 2"40 4- 0"05 2"63 4- o.oi (i) NY528493 I3 2"44 4- 0"05 2.64 4- o.o1 St. Bees Shale NY485673 13 2"45 4- o'o4 2"77 4- o'o3 (I) NY4565o9 4 2"47 4- o-o5 2.71 4- o.o2 St. Bees Sandstone NY46o534 2I 2.26 4- 0"03 2.61 4- o.oi (1) NY528587 I6 2.28 4- 0"05 2-62 4- 0.02 NY527589 I3 2.29 4- 0-06 2.62 4- 0.02 NY61°372 3 2.I2 4- o'o3 2-63 4- o.o2 NY613326 4 2.I 5 4- 0.08 2"64 4- 0"03 Mean of localities 2-22 4- o'o7 2.62 4- o.oi Eskdale granite NYI49O5O 5 2.64 4- o.oi (2) NYI22oi 3 6 2.63 4- O'OI NYI9x°I3 9 2.6I + o.ox NY162oo3 9 2.62 4- o.oi NYI47OO4 5 2-62 4- o.o1 SDx6199o 6 2.6I 4- o.oI $D17o979 4 2.63 -4- o-o2 SDI I2943' 7 2'69 4- 0.02 SDI339o7" 8 2.73 4- 0.03 Ennerdale NYI i2152 5 2-63 4- o.oI granophrye NYI24145 9 2.62 4- 0"03 Shap granite NY557o84 9 2.66 4- o-oi (2) Skiddaw granite NY3233 8 2.58 4- 0.02 2.63 + o.oi (I) Threlkeld NY328243 I7 2"67 4- 0"03 2.7 ° 4- o-ol (,) microgranite Sources of data: (I) Collected by Bott and Hadand, measured by Masson-Smith (I 958). (2) Collected by Bott, measured at Durham.