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Gravity anomaly between Dartmoor and Bodmin Moor: an alternative explanation
143
NEW TERTIARY BASIN IN SW ENGLAND
Gravity anomaly between Dartmoor and Bodmin Moor: an alternative explanation
c. M.
Bristow
ECC International Ltd., John Keay House, St. Austell, Cornwall, PL254DJ
I have just read with some interest Rollin's (1988) paper on the gravity 'basin' at Lamerton, having carried out geophysical surveys on the Dutson Basin long before anyone else was aware of its existence! There is one other explanation that ought to be considered for Lamerton. In the course of a gravity survey over the southern half of Goss Moor, north of the St. Austell granite, a number of gravity 'lows' were discovered, at least as large as the Lamerton one. We interpreted these to be cusps of granite, probably kaolinised, which reached up to within 100 m of the surface. Model studies were entirely consistent with this interpretation. However, on drilling these anomalies, especially at Gaveriggan, we found a different picture (Fig. 1) with about 200 m of completely kaolinised slaty material (density about 2.0 Mg m- 3 ) overlying slaty material impregnated with blebs of SUlphide. Further investigation revealed that there was a causal connection between the mineralisation and the unusually deep kaolinisation of the killas, as the acids liberated by the weathering of the sulphide, perhaps during deep Tertiary weathering, had caused accelerated kaolinisation of the slates (Fig. 1). Peter Sabine (1968) described a similar effect alongside the Perran iron lode. As mention is made of a mineralised horizon below the clay deposit, as indicated by the geophysics, I do believe that an explanation along the lines of what we found under Goss Moor is well worth consideration. Clearly the only way to resolve matters is a borehole about 100-150 m deep!
GOSSMOOR Our model interpretation SURFACE
CIrca
100m
Kille.
Klolinised granite
0=2.7-2.8
Unkaolinised granite
o • 2.65
What we found!
Gavenggan
ltlbo..hole
SURFACE
. . . . . - ---- --
r--
Unkaolinisad .Iat•• . , \
o • 2.7-2.8
1/ .-f
Kaolini.ad .1010. ,
0 • 2.0
"
___________ J.
/
. Slate.
mj~.·r.Ii~.d· .'
.
' . . with' . blabs of pyntes .
~ G,.nite @3OOm
Fig. 1. Model interpretation and dril1ing results at gravity anomalies on Goss Moor.
References ROLLIN, K. E. 1988. A detailed gravity survey between Dartmoor and Bodmin Moor: the shape of the Cornubian granite ridge and a new Tertiary basin. Proc. Geol. Ass., 99,15-25.
SABINE, P. A. 1968. Origin and age of solutions causing the wall rock alteration of the Perran Iron Lode, Cornwall. Trans. Insc. Min. Metall., 877, 1-5.
NEW TERTIARY BASIN IN SW ENGLAND
Gravity anomaly between Dartmoor and Bodmin Moor: an alternative explanation
c. M.
Bristow
ECC International Ltd., John Keay House, St. Austell, Cornwall, PL254DJ
I have just read with some interest Rollin's (1988) paper on the gravity 'basin' at Lamerton, having carried out geophysical surveys on the Dutson Basin long before anyone else was aware of its existence! There is one other explanation that ought to be considered for Lamerton. In the course of a gravity survey over the southern half of Goss Moor, north of the St. Austell granite, a number of gravity 'lows' were discovered, at least as large as the Lamerton one. We interpreted these to be cusps of granite, probably kaolinised, which reached up to within 100 m of the surface. Model studies were entirely consistent with this interpretation. However, on drilling these anomalies, especially at Gaveriggan, we found a different picture (Fig. 1) with about 200 m of completely kaolinised slaty material (density about 2.0 Mg m- 3 ) overlying slaty material impregnated with blebs of SUlphide. Further investigation revealed that there was a causal connection between the mineralisation and the unusually deep kaolinisation of the killas, as the acids liberated by the weathering of the sulphide, perhaps during deep Tertiary weathering, had caused accelerated kaolinisation of the slates (Fig. 1). Peter Sabine (1968) described a similar effect alongside the Perran iron lode. As mention is made of a mineralised horizon below the clay deposit, as indicated by the geophysics, I do believe that an explanation along the lines of what we found under Goss Moor is well worth consideration. Clearly the only way to resolve matters is a borehole about 100-150 m deep!
GOSSMOOR Our model interpretation SURFACE
CIrca
100m
Kille.
Klolinised granite
0=2.7-2.8
Unkaolinised granite
o • 2.65
What we found!
Gavenggan
ltlbo..hole
SURFACE
. . . . . - ---- --
r--
Unkaolinisad .Iat•• . , \
o • 2.7-2.8
1/ .-f
Kaolini.ad .1010. ,
0 • 2.0
"
___________ J.
/
. Slate.
mj~.·r.Ii~.d· .'
.
' . . with' . blabs of pyntes .
~ G,.nite @3OOm
Fig. 1. Model interpretation and dril1ing results at gravity anomalies on Goss Moor.
References ROLLIN, K. E. 1988. A detailed gravity survey between Dartmoor and Bodmin Moor: the shape of the Cornubian granite ridge and a new Tertiary basin. Proc. Geol. Ass., 99,15-25.
SABINE, P. A. 1968. Origin and age of solutions causing the wall rock alteration of the Perran Iron Lode, Cornwall. Trans. Insc. Min. Metall., 877, 1-5.