ETAPA DE APROBACIÓN

much less internal variation in lithology and can be compared with each other. Grain size curves for 4 such profiles are given in Fig. 6.5.ii. The important differences in granulometry between the profiles are due largely to contrasting lithologies. The Mormond Quarry and Whitestones parent rocks originally contained higher feldspar contents and greater proportions of coarse quartz

grains than the finer grained Rannas and Caimbarrow quartzites. The low fines contents of the weathered Rannas and Cairnbarrow quartzites are a direct result of their originally low contents of unresistant primary minerals.

The dependence of the granulometry of the quartzitic saprolites on the content of non-quartz minerals creates problems in

interpretation. The granulometric evolution of the saprolite will effectively cease^once the alteration of all available primary minerals is completed. In rocks predominantly composed of quartz,

thorough alteration of other minerals can be achieved fairly quickly. Continued weathering may lead to Si mobilisation or to further changes in clay mineralogy but these changes will have little effect on the granulometry of the saprolite. Granulometry therefore offers a poor guide to the degree of evolution of quartzose

saprolites.

6.6 Other metasedimentary saprolites 6.6.1 Variation within a section

The deep section in weathered biotite metapsammite at Northseat shows only modest vertical changes in granulometry (Fig. 6.6.i). The content of fines decreases downwards but the gradual coarsening with depth is interrupted by bands of blocky disintegration.

C u m u la tiv e

cryoturbated till gruss blocky disintegration gruss talus

Fig. 6.6.i. Vertical variations in granulometry in a weathered biotite metapsammite at Northseat, Auchnagatt.

Fig. 6.6.ii. Granulometry of the weathered metasediments.

Note : Weathered metasediments below the Buchan Gravels are not plotted.

6.6.2 Variation between sections

A total of 16 sections in saprolites derived from other Dalradian metasediments were sampled for particle size analysis. The sections represent a wide range of predominantly fine to medium grained rock types (Table 6.6.i)jyet the grain size distributions of the saprolites indicate a broad similarity in both the manner and stage of decomposition (Figs. 6.6.ii and iii).

The proportions of silt generally lie between 15 and 35% (Md 27.4%) whilst the clay contents range from 2.4 to 8.2% (Md 4.5%). The size of the sand fractions are more variable with several saprolites having predominant fine sand fractions

(Boyne, Cairnhill and Montgrew). The median grain sizes of 10 of the 16 samples fall within the fine sand size range.

The samples of weathered schists and gneisses form beneath the Buchan Gravels at Moss of Cruden and Windyhills possess distinctive

granulometries and are dominated by fines.

A notable characteristic of the weathered metasediments is that, unlike certain granitic and basic igneous saprolites, there are no samples which have very low fines and clay contents. In less thoroughly decomposed samples, such as Howford 1, Ythanbank and Bonnykelly, pebble and cobble sized fragments remain intact yet fines and clay contents already exceed 20% and 3% respectively. The transition from coherent rock to thoroughly decomposed

saprolite can be observed in parts of the Howford and Northseat sections. The rock at the base of the profiles or within resistant bands is broken into angular blocks and alteration of the blocks

Table 6.6.i Weathered Metasediments - Granulometry

Site Rock Type CS FS Fines Clay

Auchintoul Moss Quartz- mica schist 44.8 35.2 20.0 2.1

Boyne Quarry Calc schist 20.3 40.3 36.1 4.8

Bonnykelly Biotite meta- psammite

42.1 21.8 36.1 3.8

Bruxie Quartz- mica schist 20.2 32.7 47.1 2.4

Cairnhill Mica schist 32.3 40.3 27.4 3.5

Forglen Macduff Slate 49.0 16.6 34.4 3.9

Hillfoot Quartz- mica schist 45.1 19.2 35.6 A.l Howford 1.3m Quartz- mica schist 36.9 21.8 41.3 8.2

Kinmuck Sericite- Chlorite 63.5 16.3 20.2 7.1

schistose meta- pelite

Montgrew Findlater Flags 27.5 45.2 27.2 5.8

Northseat Biotite meta- psammite

62.7 <14.5 22.8 4.5 Oldmeldrum Mafic metapsammite 62.0 22.0 16.0 4.0

Toddlehills Metapsammite 52.1 21.4 26.5 5.3

Wardhead Quartz- mica schist 24.0 34.6 40.4 6.3 Ythanbank Mafic metapsammite 31.1 37.6 31.3 3.0

Md 221 101 170 72 122 218 197 94 481 155 603 381 279 115 108 Beneath the Buchan Gravels QX48 Pelitic schist 5.2 5.2 89.5 6.1 QX49 Pelitic schist 2.7 26.7 70.6 7.4 QX50 Knotted schist pelitic 3.1 3.2 94.7 22.2 PX61 Biotite gneiss 37.7 18.3 44.0 24.2 35 44 26 212

ooo a o

e __ _ _ /

Fig. 6.6.iii,. Grain size curves for granulometric types : /

biotites often appear dulled or rusty and the larger feldspar grains may be soft under a penknife. Yet the blocks remain coherent and can only be broken by hand along the foliation planes with difficulty.

Further up the profile the rock fragments become smaller and more rounded and will crumble easily in the hand. Surrounding

these isolated, coherent fragments is a matrix of predominantly sandy decomposed rock, usually including an important fines content. Small, crumbly rock fragments persist even at the top of the

Howford weathering profile within a matrix of clayey sandy silt.

6.7 Granulometric variation in weathering profiles 6.7.1 Soils and saprolites

At a few sites, thin soils are found developed directly on weathered bedrock. Striking differences are apparent between

the granulometry of the soils and the saprolitic parent material (Table 6.7.i).

Table 6.7.i Granulometry of soils and saprolites

Site Rock Type Soil

CS Gruss CS FS Fines FS Fines Longhaven Grani te 75.1 10.7 14.2 96.9 1.7 1.4 Blackrigg Granite 70.7 10.0 19.3 79.0 8.3 12.7 Silverford Gabbro 44.7 31.5 23.8 66.1 27.2 6.7

The soils are immature and can be classified as incipient podzols or humic soils. In comparison with the subjacent grusses,

the soils are greatly enriched in fines and deficient in coarse sand. Similar contrasts between soils and saprolitic parent materials have been noted by Dejou and Pedro (1967) and Eden and Green (1971). The relatively advanced alteration in the soils can be attributed to the action of organic acids in promoting mineral breakdown (Dejou and Pedro 1967).

6.7.2 Vertical variation

A number of deep sections in weathered rock show only modest changes in granulometry over considerable depths. Saprolites developed on a number of rock types have this characteristic but it is best displayed in the granitic granular grusses at Mill Maud, Glen Cat and Hill of Longhaven. Similar profiles are represented in boreholes which record gradational changes from decomposed to hard rock.

Other profiles have more complex granulometries. In many igneous rocks, evidence of vertical change is obscured by horizontal variations caused by corestone development. In weathered meta­ sediments, granulometric variation is often a result of rapid changes in rock type within a section. Nevertheless samples from similar locations, such as joint planes, or from similar rock types at different depths in a section often have comparable granulometries. Good examples of similar particle size distributions at different depths can be found in the complex weathered metasediments at

Oldmeldrum and Ythanbank. Such profiles correspond with the thick alterations of fresh and weathered rock encountered in many boreholes.

reported from deep sandy saprolites in several other parts of the world (Lumb 1962; Dixon and Young 1981; Eggler et al 1969).

The modest changes in granulometry with depth found in the saprolites of north-east Scotland imply that any material removed from the

profile by glacial erosion was similar to that now surviving in the truncated profiles. However it remains possible that a thin layer of more highly weathered material has been removed from the profiles

(Basham 1974), although no remnant of such a surface layer has been observed.

6.7.3 Horizontal variations

Many sections show considerable internal variation in grain size characteristics. In extreme cases, such as the Howe of Dens and Daugh of Caimbarrow sections, it becomes difficult to give figures which typify the granulometry of the whole section. Often this variability is a result of abrupt changes in the texture and mineralogy of the parent rock. However significant horizontal variation in grain size characteristics also occurs in saprolites derived from homogeneous rocks.

The amount of lateral variation in granite grusses can be considerable Section Bennachie C.P. Cruden Moss Braeside No. of samples 13 10 6 Md Range 96-550 351-521 191-386

Other homogeneous rock types have not been sampled for lateral variation in grain size characteristics but observations suggest