Results of shallow geophysical surveys, Cainozoic of north-east Scotland
|Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
Results of shallow geophysical surveys
During surveys of the Cainozoic deposits in north-east Scotland various shallow geophysical techniques have been used to provide data on the nature, thickness and lateral extent of concealed sedimentary units (Applied geology). Interpretation of electrical conductivity and resistivity measurements, in particular, has also provided insights into the lateral heterogeneity typical of the surface deposits of the district.
The methods employed and the results from a conductivity survey in the vicinity of the Houff of Ury (NO 856 889), near Stonehaven, and the resistivity soundings from the Inverurie–Stonehaven sand and gravel assessment area (MAR 148 on P915334), are fully described in Auton et al. (1988) and Auton (1992). Both techniques provided insights into the three dimensional distribution of workable deposits of sand and gravel, augmenting the surface mapping information. For example, interpretation of a contoured plot of ground conductivity values in the Houff of Ury area (P915337 a), in conjunction with detailed geological mapping, resulted in a 60 per cent reduction in the estimated extent of sand and gravel, compared with that recorded during the primary geological survey of the area in 1884. It also led to the recognition of minor, but notable amounts of workable sand and gravel (P915337 b) concealed beneath thin till overburden.
Forty three resistivity depth soundings were taken at 25 sites during the Inverurie–Stonehaven sand and gravel resource survey. The soundings, together with data obtained from 173 sample points (trial pits, exposures and boreholes) were used to characterise the Quaternary sequences in the area. Eight of the resistivity sites were positioned close to sample points to allow calibration of resistivity values. The calibrated results enhanced extrapolation of the nature and thickness of deposits between sample points, particularly in the major river valleys, where the full thickness of water saturated material was commonly not proved by pitting and shallow drilling.
The soundings also provided data on the typical range of resistivities for each type of Quaternary deposit encountered (P915338). For example, resistivity values for ‘clayey’ materials, such as till and hummocky glacial deposits, were low, ranging from about 18 to 300 ohm m, whereas values for sand and gravel were high, ranging from 400 to 10 000 ohm m. The range of resistivity values obtained from 44 soundings taken in the adjacent Strachan-Auchenblae–Catterline assessment area (Auton et al., 1990) was somewhat larger. Values of 175 to 700 ohm m were obtained for tills, and 10 000 to 17 000 ohm m for sands and gravels of the East Grampian Drift Group in the Strachan area; values of 150 to 900 ohm m (till) and 300 to 6000 ohm m (sand and gravel) were recorded for deposits of the Mearns Drift Group, between Auchenblae and Catterline.
Differences between the ranges of values obtained from the two assessment areas were probably, in part, a reflection of moisture content, particularly in the permeable, sandy Quaternary strata, at the time of survey. The Inverurie–Stonehaven soundings were taken when the ground was wet, in early spring, whereas the Strachan–Auchenblae–Catterline soundings were made when the ground was dry, at the end of a notably dry summer. The resistivity values for tills of the East Grampian and Mearns drift groups are very similar in the Strachan–Auchenblae–Catterline area, and comparable to many of those recorded from tills in the Inverurie–Stonehaven area. The resistivity values from sands and gravels of the Mearns Drift Group are generally lower than those from the East Grampian Drift Group and probably reflect the finer grained and more silty nature of the former deposits.
The resistivity contrast between permeable sand and gravel, and less permeable deposits (till, glaciolacustrine silt and clay) was evident in both assessment surveys allowing clear subdivision of the Quaternary sequences, based on their geophysical signature. However, the contrast between Quaternary deposits and bedrock was generally less evident and in places accurate rockhead depth was difficult to determine without nearby borehole control. The resistivity values obtained for the Quaternary sediments in north-east Scotland are generally much higher than those in southern Britain (Auton, 1992). This reflects, in part, the crystalline or sandy nature of the resistant bedrock parent material, incorporated in the Quaternary deposits of the district, and also the high proportion of boulders and cobbles in the sediments. In southern England, in particular, much of the fine-grained material in glacigenic sediments is derived from mudstone bedrock and large clasts are normally less numerous.
Ground probing radar traverses
The two ground probing radar (GPR) traverses (Greenwood and Raines, 1994), each about 750 m in length, were undertaken in the Houff of Ury area (sited on P915337 a). They provided data on the lateral extent and sedimentary architecture of the Quaternary deposits beyond what could be gleaned from surface mapping, conductivity measurements and the sinking of boreholes and trial pits (Greenwood et al., 1995). Gently inclined reflectors were interpreted as corresponding with large-scale foreset bedding typical of glaciofluvial deltaic deposits within the Drumlithie Sand and Gravel Formation in the area. These deltas form rounded and flat-topped mounds. Organised, quiet, layered patterns of reflectors, are thought to be typical of flat-lying glaciolacustrine deposits (Ury Silts Formation) infilling shallow ice-scoured rock basins. The glaciolacustrine sediments typically comprise silty, fine-grained sand and sandy silt. Multiple diffractions, typical of pebble and cobble gravel, were seen beneath topographic highs. These correspond to gravelly deltaic topset beds; the gently inclined reflectors indicating foreset bedding are developed on the flanks of the topographic highs. Lower amplitude diffraction patterns, indicating sediments containing scattered boulders and cobbles, were seen to be characteristic of till overlying Dalradian metamorphic bedrock.
Four GPR traverses (P915339 a) were made across the type area of the Buchan Ridge Gravel Member at Moss of Cruden, during 1994, to image the internal geometry of the deposit and the form of its basal contact with the underlying bedrock. A short traverse was also undertaken along a track through a forestry plantation at Moss of Auquharney (NJ 023 397), some 150 m south of the type area. The resulting profiles (Greenwood and Raines, 1994), together with data from trial pits, boreholes, and resistivity soundings provided important evidence bearing on the origin of the gravel (Palaeogene and Neogene deposits; Site 14 Moss of Cruden).
The most informative GPR profile (line 1) is almost coincident with the pitting transect A1–A of Hall and Jarvis (1994, fig.3), across the northern slope of the ridge at Moss of Cruden. Part of the north-west to south-east profile, (162–364 m) is shown in P915339 b. The series of shallow-dipping, well-ordered reflections (without diffractions) indicates undisturbed Palaeogene to Neogene gravel (Buchan Ridge Gravel Member), with low-angle cross-stratification dipping towards the axis of the ridge. Farther up-slope (276–342 m), the GPR reflectors are distorted by diffractions that may indicate periglacial and glacitectonic disturbance of the upper part of the gravel. The profile shows the cross-stratified gravel unit thinning north-westwards, confirming that its feather edge rests on weathered Lower Cretaceous sandstone and Caledonian granitic bedrock.
The Cretaceous Moreseat Sandstone, which is largely decomposed to soft sandy silt and silty sand, is characterised by a diminution in radar reflectivity. This was only clearly imaged toward the north-western end of the profile (Greenwood et al.,1995, fig.5, 80–100 m). The resistivity depth probes and radar indicate that around 25 m of Buchan Ridge Gravel lies beneath the crest of the ridge, infilling a channel running parallel to the line of the ridge.
Interpretation of resistivity data from Moss of Cruden has allowed modelling of resistivity values for parts of the sequence encountered in the GPR profile 1 (P915340). The topsoil is characterised by high resistivity values (2675–5940 ohm m) and the upper parts of the Buchan Ridge Gravel display a range of lower resistivities (447–2534 ohm m). This is thought to be due, in part, to variations in the proportions of kaolinised granitic clasts throughout the unit, the presence of interbeds of clayey pebbly sand (known from the upper part of the gravel sequence in Borehole NK 04 SW3) and the disturbance of bedding, indicated by distorted diffractions in the GPR profile. A layer, about 10 m thick, displaying relatively uniform low resistivities (307–355 ohm m) occurs below the ‘disturbed’ gravel at sounding sites MOSS and M1-285; it reaches the surface at M1-225. Its uniform resistivity response is taken to indicate that this gravel (with its shallow-dipping, well-ordered GPR reflections) is little disturbed and generally contains more kaolinitic material, in the form of decomposed granite cobbles and as fine-grained matrix, than the overlying ‘disturbed’ material. The absence of the latter in sounding M1-225, suggests that the upper ‘disturbed’ units are only well preserved where the gravel is thickest (towards the crest of the ridge); they have probably been removed on the flanks of the ridge by glacial erosion.
The presence of Moreseat Sandstone, infilling a topographic depression in deeply decomposed granite bedrock (between M1-225 and MOSS), is suggested by a 120 ohm m layer at the base of sounding M1-285 and corroborated by the trial pitting results reported by Hall and Jarvis (1994). A value of 120 ohm m is lower than might be expected for sandstone, but is comparable to values of 76–97 ohm m obtained for decomposed Devonian sandy siltstone from the Stonehaven–Auchenblae area (Auton et al., 1990). The very low resistivity (22 ohm m) layer from about 112 to 95 m above OD in sounding M-225, corresponds to grussified granitic bedrock recorded at the bottom of trial pit 15 in Hall and Jarvis (1994, fig.3). Similar (29.5 ohm m) deeply weathered material, some 23 m thick, overlies fresh granite bedrock (3640 ohm m) in sounding MOSS on the ridge crest.
Five GPR traverses were also made (in 1994) across the type area of the Windy Hills Gravel Member (Greenwood et al., 1995). The interpreted GPR data are incorporated in the description of the Windy Hills site (see also Palaeogene and Neogene deposits).