Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on February 18, 2016
Porosity reduction, microfabric and resultant lithification in UK
uncemented sands
S. N. Palmer & M. E. Barton
SUMMARY: Studies of the extent of diagenetic change in matrix-free, uncemented,
quartzose sands ranging in age from the Jurassic to the Recent in the UK have been carried
out as part of a geotechnical research programme. All the sands studied are thought to have
experienced only a relatively small depth of burial and the extent of diagenetic change is
consequently small. Previous studies of the in situ fabric of sands in this category have been
limited owing to the sampling difficulties created by their very friable nature. Careful
sampling, however, has succeeded in obtaining undisturbed material and has facilitated
studies of the porosity, microfabric and degree of lithification. Distinctive changes,
progressive with age, include reduction of porosity, an increase in the numbers and
complexity of grain contacts and an increasing degree of lithification. The cause of these
diagenetic changes are discussed and it is concluded that the evidence strongly favours
pressure solution of the detrital quartz grains as the dominant process.
This is a study of very clean, virtually matrixfree, uncemented, mature quartzose sands of
ages ranging from Recent to Jurassic in the UK.
The primary motivation for the research is
geotechnical. Surprisingly, despite other areas of
progress in soil and rock mechanics, very little
geotechnical work has been done on the transition from loosely compacted, unlithified sands to
compact, indurated sandstones. Dusseault &
Morgenstern (1979) coined the phrase 'locked
sand' to denote the intermediate state where the
original depositional porosity is reduced, the
grains are inter-locked (or 'locked') and the sand
has acquired a small but measurable degree of
lithification without true cementation. The authors have investigated the extent to which these
characteristics are present in U K sands (Barton
et al. 1986b, Barton et al. in prep.) The study has
included observations on the porosity, microfabric and lithification and has revealed the
extent of diagenetic changes in these uncemented sands. Because of the friable nature of such
sediments, studies of the in situ fabric of weakly
lithified sands have tended to be neglected and it
is therefore considered that the results are of
significant sedimentological interest.
Sands studied
Samples of matrix-free, uncemented, quartzose
sands have been obtained from numerous quarries and natural exposures. The Mesozoic and
Tertiary sands, although readily disaggregated
by manipulation, with care, can be blocksampled (Barton et al. 1986a) thereby preserving
the in situ fabric and allowing laboratory impregnation (Palmer & Barton 1986). The Quaternary
sands are impregnated in situ. Eight of these
sands (Table 1) have a similar median grain size,
similar sorting (Fig. 1) and quartz dominated
mineralogy as shown in Table 2. The particle
shapes and roundness of the various sands show
some variation, but all eight sands have similar
minimum remoulded (see below) porosities. It is
important to note that all the sands have minor
matrix (not greater than 2.0~ in any case) and
negligible cement content (not greater than
0.5~o). It can be considered therefore that these
sands have sufficient similarities in the sedimentary characteristics influencing depositional and
diagenetic fabrics (Meade 1966, Berner 1971,
Wolf & Chilingarian 1976, Chilingarian 1983) to
permit comparison between them in respect of
their porosity reduction and microfabric.
The uncemented nature of these sands, particularly those of Mesozoic age, is distinctive
since there are other sand deposits of equivalent
burial histories which are considerably more
lithified due to the development of an authigenic
mineral phase. Indeed, as noted in Table 1, the
petrographic features discussed here are not
necessarily present throughout the complete
stratigraphic thickness of the strata detailed. The
Mesozoic strata, for example, contain horizons
possessing the textural features described in this
paper, but also frequently contain zones of
cemented and/or matrix-rich (>5~o in this
context) sands. The reason for the lack of
interstitial cement is unknown and requires
further investigation. There is no evidence of
secondary porosity or cement dissolution. The
From: MARSHALL,J. D. (ed.), 1987, Diagenesis of Sedimentary Sequences,
Geological Society Special Publication No. 36, pp. 29-40.
29
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S. N. Palmer & M. E. Barton
30
TABLE 1.
Sands
studied
Age
Stratigraphic
Modern-day
beach sand
Holocene
Norwich Crag sand
Absolute
Ma (bp)
Estimated
maximum
depth of
burial
(m)
Approx.
thickness
of sand
unit*
(m)
Location
NGR
Site
Holkham Beach,
Wells-next-the-Sea,
Norfolk
0.0
0.2
>2
TF 892 455
Pleistocene
c. 1.65
20
c. 3
Eastern Bavents,
Southwold,
Suffolk
Barton sand,
Barton Beds (Bed K),
Barton Formation
Late
Eocene
c. 42
169
c. 5
Becton Bunny,
Barton-on-Sea,
Essex
SZ 425 092
Thanet sand,
Late
Thanet Formation
Palaeocene
c. 57
295
c. 30
Linford Pit,
Thurrock,
Essex
TQ 667 799
Woburn Sands
Albian/Aptian
L. Cretaceous
c. 100
302
c. 60
Nine Acres Pit,
Leighton Buzzard,
Bedfordshire
SP 939 277
Folkestone sand,
Foikestone Beds
Albian
L. Cretaceous
c. 100
474
c. 78
West Heath Commons,
West Harting,
West Sussex
SU 785 227
Kellaways Sand
Kellaway_ Beds
Callovian,
M. Jurassic
c. 139
700
c. 10
South Cave Pit,
South Cave,
Humberside
SE 920 330
Grantham sand,
Grantham Formation
Aalenian,
M. Jurassic
c. 170
780
c. 7
Wittering Grange Pit,
Wittering,
Cambridgeshire
TL 048 101
TM 518 780
* The petrographic features detailed for each sand in this paper do not necessarily occur throughout the sand's total thickness.
(2) 5
3
4
2
Very Fine Sand
Coarse Silt
Medium Sand
Fine Sand
100,
m
/
t /f
,///~
.//,'/
!
/,'" / . W
/
0
/
O3
/
./," ,,A 3"
/.-"
../' ./i"
0
0.032
0.063
0.125
Particle Size (ram)
0.250
FIG. 1. Particle size distribution o f the sands, illustrating their very similar grain sizes a n d sorting.
0.500
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Porosity reduction & UK uncemented sands
31
TABLE 2.
Petrographic
components (%)
,...-.,
g
Mineralogical
composition (%)
O
Sands studied
X
Modern-day beach sand
Norwich Crag sand
Barton sand
Thanet sand
Woburn sand
Folkestone sand
Kellaways sand
Grantham sand
0.16
0.19
0.15
0.10
0.14
0.18
0.12
0.14
0.58
0.63
0.54
0.47
0.55
0.56
0.51
0.57
99.2
97.7
98.0
97.0
98.4
98.3
97.3
98.6
0.8
1.8
1.7
1.9
1.5
1.2
1.7
1.1
0.0
0.2
0.3
0.5
0.1
0.5
0.4
0.3
47.2
43.1
35.6
35.1
35.0
34.5
34.1
33.6
38.8
38.5
39.0
40.7
38.7
37.9
39.6
38.6
97
93
96
93
94
96
95
94
,~
~
~
O
3
5
3
3
3
2
3
2
0
1
1
2
1
T
1
2
0
T
T
I
1
2
T
1
T
1
T
1
1
T
1
1
* Detrital grains of diameter greater than 20/am.
t Detrital grains of diameter less than 20 Iam.
$ Authentic mineral development.
porosity and microfabric are therefore considered to be primary features.
Ideally, it would be most appropriate to make
comparisons between the sands' porosities and
microfabrics in terms of factors such as depth of
burial, maximum past temperature and other
conditions likely to influence their diagenesis.
Unfortunately, the majority of these factors
0-- Be
C
B
...1:
a
LU
K
1000
J
l
,
,
I
Ma
(b.p.)
0
'
'
'
'
l
200
FIG. 2. Estimate of the maximum depth of burial
(based on regional stratigraphy) versus chronological
age. The plot suggests that there is an approximately
linear relationship between maximum burial depth
and age for these deposits.
remain speculative or unknown. It is considered
necessary, however, in such a study as this to
make some estimate of the burial depth for each
deposit to serve as a rough guide for comparative
purposes.
With the exception of the modern beach
material, the sands are shallow water deposits
which, in the majority of cases, have been
preserved as the deposits of stable, shelf areas
(Whittaker 1985) and, while the Tertiary deposits are associated with basins, the samples
studied are either from the upper stratigraphic
levels or from the marginal areas and are
similarly unlikely to have experienced any great
depth of burial. Estimates of the maximum
burial depths are given in Table 1. These figures
are maximum thicknesses based on the lithostratigraphy presented in the relevant Geological
Society Report volumes (Mitchell et al. 1973,
Curry et al. 1978, Rawson et al. 1978, Cope et al.
1980). No attempt has been made to allow for
possible thicknesses of sediments eroded prior to
subsequent periods of deposition and hence the
figures must be treated as speculative estimates
of the regional burial depths for each sand.
Since the chronological age of the sands is well
documented (Geological Society Special Report
volumes cited above and Anderton et al. 1979), it
is considered a more satisfactory parameter for
detailed comparison between the sands. The
chronological age does, of course, represent a
time-scale and, in terms of slow diagenetic
processes requiring extended time periods, such
comparisons will be germane. It is also possible,
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32
S. N . P a l m e r & M . E. B a r t o n
O--
9C m
,Beach
iB
o..
W
"F
100--
RANGE OF MIN. REMOULDED
POROSITIES
G-!
I
I
200
I
i
[
I
30
iI
I
I
I
40
I
I
50
Porosity %
FIG. 3. Mean porosities of sands versus chronological age. The curve shows an initially rapid decrease in
porosity with time, followed by a continuing, slow reduction over a long time period. The Folkestone (F) and
Woburn (W) sands both have an approximate age of 100 Ma and have been separated purely for clarity.
as suggested by Fig. 2, that the chronological age
of the sediments may represent a scale of
generally increasing depth of burial.
Porosity
The natural intact or in situ porosities of the
sands, as measured by standard petrographic
and soil engineering techniques, are given in
Table 2. They are plotted versus age in Fig. 3
which shows a rapid decrease in porosity from
the recent beach sand (with a porosity equal to
the deposited value) to the Mesozoic sands,
where significant porosity reduction has occurred. With sands such as these (well sorted,
quartz dominated, matrix-free) the in situ porosities can usefully be compared with (i) the likely
original deposited porosity and (ii) the minimum
porosity obtained in the laboratory by recompacting the disaggregated grains (producing
a remoulded, dense sand deposit).
Original depositional porosities of sands are
generally in the range of 40-50% and more
specifically 46-50% for fine sands such as those
of this study (Pryor 1973, Friedman & Sanders
1978, North 1985). There are apparently no
records of denser packing being achieved in
sands by the normal, natural processes of
sedimentation and thus no reason to believe that
the depositional porosities of the sands detailed
here were any different to the figures quoted
above. Thus, as seen in Fig. 3, apart from the
modern Beach and the Pleistocene Sand, the
existing in situ porosities are considerably reduced (by about 12-13% on average).
Comparisons of in situ porosities with recompacted (or 'remoulded') porosities are common practice in soil engineering (Kolbuszewski
1948). Various techniques can be used to obtain a
range of porosities from a maximum possible at
one end of the scale to a minimum (without grain
fracturing or crushing) at the other. In this study
the minimum porosity was achieved using a
device applying a lateral vibration during sedimentation (Barton & Brookes, in press); giving
values in the range from 38 to 41%. The existing
in situ porosities, as shown in Fig. 3, are
significantly less than these values (by about 45% on average).
Associated with these reductions in porosity
are changes in the microfabric and the degree of
lithification.
Microfabric
Standard petrographic techniques show all the
sands to have remarkably similar microfabrics,
i.e. high intergranular porosities, grain-supporting, matrix-free, uncemented micro-textures.
Similarly to the reduction in porosity, the
numbers of grain contacts per grain (Taylor
1950) increase with age, as shown in Fig. 4.
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Porosity reduction in UK uncemented sands
Concomitantly, a change in the type of contacts
occurs, principally as a reduction in the numbers
of tangential contacts with an increase in straight
and concavo/convex contacts (Fig. 5). The
microfabrics are illustrated in the photomicrographs (Fig. 6) and SEM photographs (Fig. 7). A
small number of sutured contacts are seen in the
Cretaceous and Jurassic sands and although a
few of these are clearly inherited (in polycrystalline grains) most appear to be primary diagenetic
features. A search has been made, using cathodoluminescence petrography, SEM and X R D techniques, for evidence of cementation but none has
been found by the authors in any of these sands.
Confirmation of a lack of quartz overgrowths in
the Grantham, Folkestone and Barton Sands has
been made by GAPS Geological Services. Grain
contact areas and grain surface textures seen
under the SEM in the older sands (Fig. 6b) show
features strongly suggestive of pressure solution.
Both the reduced porosities and the types of
0--
T
100--
F
/
!
2OO
I
I
3
4
r
2
1
1
33
0
N~- of Contacts per Grain
FIG. 4. Number of grain contacts per grain (contact
index) versus age, illustrating an increase in the state
of packing with time.
100
Tangential
Long Strt.
Short St(t.
Long C - C
Short C - C
o,<,~
~
...............
S
_
SS
Sutured
3Z
"J
..jj.<-/
ss
. /
T
Y
Z
_ ~s
._.._._..__.--------~
~ ' ~ " ~ T ~
s
IS
.......SC
__
I~S ...--- ' ~
IS IS
~
~__~..~.~.
%0
k
0
~..~Is/I.~.,S C
_._...~
....
I
..~.~"
~
....--. IC
SC
i
__.--S c--- SC
Ic IC
1
50
l
i
~_----------/~~'-"--'-"
I
1
100
1
i
.~S
I
I
~
I
I
150
Ma (b.p.)
FIG. 5. Grain contact morphology versus chronological age. The plot illustrates a progressive decrease of
tangential contacts and increase in planar-type contacts with age. The latter causes gradual increase in the
grain-grain contact areas and the state of fabric interlocking with age. *The Woburn and Folkestone sands
both have an approximate age of 100 Ma, and have been separated purely for clarity.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on February 18, 2016
34
S. N. Palmer & M. E. Barton
FIG. 6. Photomicrographs of five of the sands to illustrate the progressive porosity reduction, increase in the
interlocking of the fabric (tightness of packing) and change in contact type morphologies: (a) modern-day
beach sand, with high porosity, very low number of grain contacts per grain, tangential contacts dominating;
(b) Barton Sand; (c) Folkestone Sand, tightness of packing increasing, planar contacts important; (d)
Kellaways Sand; (e) and (f) Grantham Sand, marked porosity reduction, high number of grain contacts per
grain, interlocked fabric, predominantly straight and concavo/convex contacts giving high grain-grain contact
areas. With the exception of the modern-day beach sand, which was impregnated in situ, all the sands were
block-sampled and impregnated in the laboratory using the Drip Method and the epoxy Epo-tec 301, spiked
with the dye Waxoline Blue AP-FW in a 1~ wt concentration (Palmer & Barton 1986). (Scale in (f) applies to
all photomicrographs.)
grain contacts are consistent with features
known to be produced by pressure solution.
Lithification
Lithification is thought of in geological terms as
a change from unconsolidated to indurated rock.
A quantitative measure of lithification can be
obtained either from the shear strength
measured on dry samples in the direct shear box
(Fig. 8) or by taking values of unconfined
compressive strength (Fig. 9). Samples for these
tests were prepared by hand carving (Fig. 10).
The increase in age, and concomitant decrease in
porosity, is associated with an increase in
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Porosity reduction in UK uncemented sands
35
FIG. 7. SEM photomicrographs of (a) Barton (scale bar = 40 p.m) and (b) Grantham Sand (scale bar =
100 ~tm) showing the interlocked fabric, absence of cement and matrix. Features suggestive of pressure
solution are seen in the Grantham Sand.
lithification as measured by these tests of
strength.
The direct shear strength tests were conducted
as slow, drained tests at normal stresses ranging
from 50 to 900 kPa. The samples gave approximately linear envelopes over this stress range and
show increasing values of shearing resistance
angle (4r and increasing values of the cohesive
intercept (c'), with age of the sand, correlating
with porosity reduction and an increase in the
number of grain contacts per grain. The beach
sand, which could not be prepared for shear box
testing of the in situ fabric, and the Pleistocene
sand have values of cohesive intercept (c') equal
to zero. There is some evidence to suggest that at
very low normal stresses the envelopes may
become curvilinear but, owing to the usual
difficulty of obtaining reliable results in this
range, further work will be required to substantiate this.
Measurement of unconfined compressive
strength, using dry cylindrical samples in a
triaxial testing machine, are given in Fig. 9
against values of grain-grain contact areas
(Dobereiner & de Freitas 1986) and numbers of
tangential contacts (a measure of fabric 'locking'). A further confirmation of the increasing
lithification with age is obtained together with
the relationship with the other diagenetic features--a decrease in tangential contacts and an
increase in grain contact areas. The most
'lithified' formation, the Grantham Sand, remains geologically 'a sand' and geotechnically it
is best classed as an engineering soil. Although
none of these sands are sufficiently indurated to
withstand coring, with the exception of the
beach sand they are compact enough to be
sampled and transported as intact blocks in
which the porosity and micro-fabric remain in
the in situ state (Barton et al. 1986a).
Discussion
The sands studied clearly show a gradual series of
diagenetic changes. Admittedly the degree of
diagenetic change can be regarded as small-thus we may compare, for instance, the considerably altered but more deeply buried Brent Sand
of the North Sea with the comparably aged
Grantham Sand as shown here. Comparisons of
porosity with depth have been made by Atwater
& Miller (1965), Hsii (1977), Selley (1978),
Zieglar & Spotts (1978), Magara (1980) and
Loucks et al. (1984) among others, but in all these
cases the depth of burial is considerable and the
porosity much less (especially at the deeper
levels) than found in the sands of this study. Even
so the porosities recorded in Table 2, and as
shown in Fig. 3, are reduced below the original
deposited values for such sands and, therefore,
indicate a measure of diagenetic change.
Similarly we can compare the gradual changes
in packing and particle contact types (as shown
in Figs 3 and 4 respectively) with the wellknown and referenced study by Taylor (1950) of
the deeply buried Jurassic and Cretaceous sands
in Wyoming. In this case, however, the changes
in contact index and contact types are considerably more intense than observed in this study.
Taylor concluded that porosity reduction, increase in contact index, decrease in tangential
contacts and increase in concavo/convex and
sutured contacts relate to an increase in compaction. Selley (1982) and Burley et al. (1985) suggest
that the features in the Wyoming Sands are due
to the development of quartz overgrowths. In
this latter case the observed changes would be
clearly of a different origin from the uncemented
fabrics studied here. Other studies of grain
contacts and packing parameters versus depth of
burial include Beaudry (1950) and Hays (1951),
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on February 18, 2016
36
S. N. Palmer & M. E. Barton
2000--
///"
.d /
oY /
/
9/ ; + /
t~
O_
,,i
I-O
9
/"
.-
~"
o+, /
/..
/
, o..C
T j / /-. 4 . " ,
/',J ,/'.: /
/
r
///-"
#
.
/
~
.
~
/
.
.
/
.../
,..L~+,, /
./
/
/
/
/
/
i
/ "/ / // / -"
9 ./ /
/ ///
/ /
,,...//i 4
0
-
l
/@~
/
./
./ r
./t,./..I"
I
I
I
I
I
0
I
I
I
I
1000
O'4n (kPa)
FIG. 8. Shear strength envelopes obtained from direct shear tests on dry samples using a standard 60 mm
square shear box. The envelopes are drawn as best-fit straight lines through the experimental points: there is
evidence that at low stresses the envelopes become markedly curvilinear. The strength of the sands increases
with the age of the deposits. The envelope of the beach sand is obtained from the maximum angle of repose in
the laboratory, since undisturbed samples for shear box testing could not be produced for this sand, it being a
truly unlithified deposit.
as reported by Wolf & Chilingarian (1976), of
deeply buried sandstones showing cementation,
unlike the sands of this study. It would appear
there is an absence of studies in the literature
where porosity reduction in sand is unequivocal-
ly due to compaction (mechanical and/or chemical) alone.
It is not intended to discuss here in detail
the reasons for the diagenetic changes observed
in the sands studied, but simply to comment
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37
Porosity reduction in UK uncemented sands
10-_
- -
_
-
GRANTHAM
-
_
-- - - K E L L A W A Y S
_
Ck
__
FOLKESTONE-
~ - - '
- -
-
W O B U R N
-
~THANET-0.1~
-
-
_
-
-
--
0.01 -
-
-
-
BARTON
CRAG
i
0
I
'
20
I
30
'
I
40
'
I
'
50
Grain Contact & Tan. Index (%)
Fro. 9. Unconfined or uniaxial compressive strength versus number of tangential contacts (tan. index) and
grain contact areas (grain contact). The plot illustrates that the unconfined compressive strengths of the sands
increase with age, and correlates to decreasing numbers of tangential contacts and increasing grain contact
areas, i.e. an increase in the interlocking.
upon the fundamental processes that may have
been operative. The main diagenetic processes
which primarily control porosity reduction
in sediments, and therefore also the development of microfabric in these sands (as
discussed by Fuchtbauer 1967, Bjorlykke 1983
and Houseknecht 1984) are:
(i) Mechanical compaction due to overburden
pressure--causing the rotation, reorientation and fracturing of competent grains, and
the plastic deformation
of ductile
components.
(ii) Mechanical compaction due to seismic
disturbances over a geological time-scale-causing the geometrical rearrangement of
framework components.
(iii) Chemical compaction--the intergranular
pressure solution (dissolution) at contacts
between particles, resulting in material
mass transfer.
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38
S. N. Palmer & M . E. Barton
FIG. 10. Preparation of prismatic samples for direct shear box testing (left) and cylindrical samples (centre
and right) for confined and unconfined compressive tests. These samples are prepared by hand carving from
block samples such as that in the left background.
(iv) Cementation--the precipitation of authigenic mineral matter in interstitial pore
space.
Various studies indicate that the observed
porosities and microfabrics in the sands of this
type could not have been obtained by simple
mechanical compaction. The fabrics produced
by laboratory compaction do not produce features consistent with the microfabrics observed
in natural sands and sandstones (Maxwell 1960).
Berner (1971), Pettijohn et al. (1972), North
(1985) and preliminary studies by the authors all
indicate that minimal porosity reduction will
occur by the mechanical mutual adjustment of
grains under load. This is particularly so since
the sands' compositions are dominated by pure,
stable quartz, there being only very small
proportions of incompetent ductile clasts, thus
very much limiting porosity reduction by plastic
deformation of grains (Nagtegaal 1978, Dodge &
Loucks 1979). There is no petrographic evidence
to suggest that grain fracturing (caused by
mechanical compaction) significantly contributed to porosity reduction. All the sands show
textural parameters, such as good or moderate
sorting, which are similarly known to promote
the retention of porosity under load (Rittenhouse
1971). The textural and mineralogical maturity
are widely recognized as major factors in
determining the mechanical compaction of a
sediment (e.g. North 1985 and Selley 1985).
Mechanical compaction due to seismic disturbance over geological time-scales is also not
considered as likely to affect the observed
porosity reductions. The work of Selig (1963) and
the authors shows that the presence of an
overburden load inhibits the porosity reduction
resulting from vibration.
It is suggested that sands of this type, with
depositional porosities of 47-48%, have undergone only a small porosity reduction resulting
directly from mechanical compaction owing to
the increasing stability produced in the structure
of the sand as porosity reduces. Furthermore it is
considered that porosities equivalent to the
laboratory minimum values are unlikely to be
achieved diagenetically by mechanical compaction alone at the relatively small burial depths of
these sands. Thus other diagenetic processes
must have been operative to produce the dense
state of packing observed, particularly in the
Mesozoic sands for example. Chemical compaction and cementation are the other main processes which could be invoked to explain the
remaining porosity reduction in the sands.
Petrographic, SEM and CL analyses reveal
insignificant amounts of authigenic mineral
phases (Table 2), so cementation does not
account for the observed reductions in porosity.
There is no evidence to suggest previous higher
grade diagenetic changes: the observed porosities must therefore be considered as primary
porosities.
The simplest and most reasonable explanation
of the porosity reduction and microfabric
changes therefore is that they are the products of
the effects of pressure solution. The geometries
of the planar grain contacts, especially in the
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Porosity reduction in UK uncemented sands
older sands, are those widely known to result
from congruent quartz dissolution. Although
quantitatively the sands show relatively small
amounts of dissolution, minor pressure solution
can cause marked porosity reduction (Ffichtbauer 1967). SEM analysis of the Cretaceous and
Jurassic sands shows micro-pitting and contacthollows consistent with pressure dissolution. It is
therefore tentatively proposed that, with reference to Fig. 3, initial rapid but small porosity
reduction is obtained by mechanical compaction. Slower but more significant porosity loss is
obtained later by chemical compaction. A
similar conclusion for more deeply buried sands
was presented by Ffichtbauer (1967).
It has been shown how the decrease in
porosity and increase in grain contact areas
relates to an increase in lithification, and it
follows from the discussion above that this also
results predominantly from pressure solution.
Any contribution to the lithification by the
cohesion provided by simple clay bonding from
the minor matrix (as discussed by Waldschmidst
1941 and Dapples 1972) and the negligible
amounts of cement are thought to be minimal.
Evidence for this is also provided by the lack of
any correlation between strength and the percentage of either matrix or cement. Pressure
cohesion or intergranular welding of the detrital
grains (Dapples 1967, 1972, Pettijohn et al. 1972,
Fairbridge 1983, North 1985)probably contributes to the lithification, possibly significantly,
39
but further work is required to assess its relative
importance.
The decrease in porosity, but more significantly the progressive change of contact types and
increase of grain contact areas with age, is
thought to explain the increase in the degree of
induration. It is suggested, therefore, that these
sands are primarily lithified by the physical
inter-locking of the detrital grains, mainly due to
the high states of packing obtained through
pressure solution.
ACKNOWLEDGMENTS"The authors wish to express their
thanks to the following companies for giving permission to visit and to sample at their sand-pits: Joseph
Arnold & Sons Ltd; Barker-Mill Estates; S. E. Borrow
Ltd; Ready Mixed Concrete (UK) Ltd; St Albans Sand
and Gravel Co. Ltd and in particular British Industrial
Sand Ltd.
We would sincerely like to thank Dr P. J. Gregson,
Department of Engineering Materials, Southampton
University for the use of the SEM; Drs I. M. West and
T. Clayton, Department of Geology, Southampton
University, for the use of the XRD and CL apparatus
and their helpful comments; Dr G. M. Power,
Department of Geology, Portsmouth Polytechnic for
the use of the photomicroscope, and Mr P. G.
Cambridge, Geological Society of Norfolk. Special
thanks are due to Messrs A. Brookes and Y. L. Wong
for technical assistance with sampling and testing and
for drawing the figures.
Financial support for this research was provided by
the Science and Engineering Research Council.
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