Clay Minerals (1988) 23, 439-446
THE RELATION BETWEEN THE TERRA ROSSA
AND THE CARBONATE-FREE
R E S I D U E OF T H E
UNDERLYING
LIMESTONES AND DOLOSTONES
IN APULIA, ITALY
M. M O R E S I
AND G . M O N G E L L I
Dipartimento Geomineralogico, Universith di BarL Tray. 200 via Re David, 4 Bari, Italy
(Received October 1987; revised 3 May 1988)
ABSTRACT : A statistical comparison has been made of chemical data for terra rossa and
carbonate-free residues of Cretaceous limestones and dolostones in Apulia in order to evaluate
the hypothesis that the terra rossa is a product of weathering of the underlying carbonate rocks.
It has been shown that the differences in chemical composition between the residue of the
carbonate rocks and the terra rossa are consistent with the former being the parent material of
the latter, The transformation from carbonate rock residue to terra rossa was governed mainly
by chemical weathering which produced a marked decrease in the K20/AI203 (i.e.
illite/kaolinite) ratio. The geochemical pattern of the Apulian terra rossa has been influenced by
sedimentary processes which have led to a characteristic distribution of mineralogical
components and a moderate contamination by biogenic silica.
Terra rossa is a reddish clay soil which is almost always present on the surface of carbonate
formations in regions with a Mediterranean-type climate. The most widely accepted theory
for its origin is that it is the residue of the dissolution of the carbonate rocks, but various
authors have emphasized the important contamination of terra rossa by eolian materials
from volcanoes in the surrounding area, or from the desert regions of N o r t h Africa (B~trdossy,
1982; Lippi-Boncampi et al., 1955; Yaalon & Ganor, 1973; Jackson et al., 1982).
Apulia (Southern Italy) is a typical example of the association between terra rossa a n d
carbonate rocks. The A p u l i a n Carbonate Platform consists of a thick sequence of Cretaceous
limestones and dolostones outcropping in the Gargano, Murge and Salento areas. The top of
this formation is extensively covered by deposits of terra rossa which can be up to 10 m thick
in morphologically depressed areas (Baldassarre & Francescangeli, 1985). The terra rossa
covering the Cretaceous formations m a y in turn be covered by Pliocene-Pleistocene
sediments of mainly carbonate composition (Dell'Anna, 1967; Iannone & Pieri, 1982;
Cotecchia et al., 1985). Deposits of terra rossa completely enclosed in the Cretaceous
carbonate rocks (as materials accumulated in K a r s t cavities) are also common.
The aim of this research is to compare the geochemical patterns of the terra rossa and the
Cretaceous carbonate rock residue in order to gain information which will help to evaluate
(i) the hypothesis that the residue of the carbonate rocks is the parent material of the terra
rossa; (ii) the nature of the processes which determined the composition o f the terra rossa.
9 1988 The Mineralogical Society
M. Moresi and G. Mongelli
440
MATERIALS
AND
SOURCE
OF ANALYTICAL
DATA
All the analytical d a t a are taken from the papers referred to in this section. The statistics were
calculated using the chemical composition of the residues from acid dissolution of 69 samples
of Cretaceous carbonate rocks, and 82 samples of carbonate-free terra rossa (MnO contents
were not used as figures were available for only a few samples). The carbonate rocks are from
the Murge and Salento areas (Dell'Anna, 1963; D e l l ' A n n a & De Fino, 1965; D e l l ' A n n a &
Nuovo, 1967). The samples of terra rossa, both from deep deposits (entirely enclosed in
Cretaceous carbonate rocks, or covered by Pliocene-Pleistocene calcarenites) and superficial
deposits, are from the Murge, Salento and G a r g a n o areas (Mascolo, 1960; Bottini, 1965;
Dell'Anna, 1967; D e l l ' A n n a & Garavelli, 1968; D e l l ' A n n a et al., 1973).
These papers indicated that the m a i n minerals (as determined by X-ray diffraction and
optical analyses) in the carbonate rock residue are illite, kaolinite (the former being more
a b u n d a n t than the latter), and oxides and hydroxides of iron; minor components are quartz,
feldspars, micas, oxides and hydroxides of aluminium, and oxides of titanium. The terra
rossa consists of the same minerals, but the kaolinite content is higher than that of the illite.
RESULTS
The statistical treatment of the analytical data shows that the chemical composition of the
terra rossa differs from that of the carbonate rock residue in various respects. The main
differences are:
(i) The relative amounts of the chemical components and their relative deviation (Table
1). The terra rossa is richer in SiO2 and A1203, and poorer in MgO, CaO, K 2 0 , N a 2 0 and
P205 than the carbonate rock residue; there is no significant difference between the average
TiO2, Fe203 and H 2 0 contents. The m120 3 and Fe203 contents are more homogeneous in the
terra rossa than in the carbonate rock residue, but the opposite is the case for MgO, K 2 0 and
TiO2.
TABLE 1. Chemical composition and statistical parameters.
Carbonate rock residue
SiO2
TiO2
A1203
Fe203
MgO
CaO
KzO
Na20
P205
H20 §
41.74
1.78
22.67
13.38
2.09
1.58
3.44
0-48
0.43
12.17
Terra rossa
a
C%
~
a
C%
t
7.53
0.46
5.75
10.71
0-70
0.89
1.19
0.37
0.34
2.72
18.04
25.84
24.57
80.04
33.49
56.33
34.59
77.08
79.07
22.35
44.18
1.67
28.80
11.60
0.61
1.30
1.39
0.28
0-18
11.85
6.93
0-74
4.47
2.40
0.57
0.72
0.87
0.22
0.15
2.19
15.68
44-31
16.68
20.69
93.44
55.38
62.59
78.57
83.33
18.48
2.0716
- 1.0641
4.9627
-1-4628
- 14.3188
-2.1368
- 12.2009
-4.1082
-5.9727
-0.7985
~: average; tr: standard deviation; C%: relative deviation; t: Student's function
(statistical significance = 95% for t = 1.9761).
Geochemistry of terra rossa and underlying rocks
441
(ii) The general pattern of correlations between the various chemical components (Table 2).
The carbonate rock residue shows a low number of correlations; the least ambiguous of these
concerns the distribution of F e 2 0 3 which correlates negatively with some other components
(SiO2, TiO2, A1203, KzO), among which few positive correlations exist. On the other hand,
the terra rossa shows a large number of correlations; the chemical components can clearly be
divided into two groups, the first including SiO2 and K20, and the other comprising TiO2,
A1203, Fe203 and H20. The components belonging to the same group correlate positively,
while those belonging to different groups correlate negatively.
(iii) The trends of the individual oxides vs. the K 2 0 x 1 0 0 / A 1 2 0 3 ratio (which reflects the
illite/kaolinite ratio). Comparing the carbonate rock residue and the terra rossa (as the
K 2 0 / A 1 2 0 3 ratio decreases), the differences in composition, in order of decreasing values of
the Student's function (Table 1), are in the following sequence:
A1 < Si < Ti < Fe < Ca < Na < P < K < Mg
(A)
On the other hand, the rate at which each oxide varies (as the K20/A1203 ratio decreases),
when calculated by means of least-square equations (Table 3), is in the following sequences,
in order of decreasing values:
AI < T i < S i < Fe < Mg < K < P < N a < C a
(B)
for carbonate rock residue, and
T i < M g < A I < Fe < P < Si < Ca < N a < K
(C)
for the terra rossa.
INTERPRETATION
The increase in A1203 and decrease in K 2 0 in the terra rossa compared to the carbonate rock
residue are consistent with the main mineralogical difference (increase in kaolinite and
TABLE 2. Relationships between oxides (values of correlation coefficient r).
Terra rossa
SiO2
2
O
SiO2
TiO:
A1203
FezO 3
MgO
CaO
K20
Na20
P205
H20 +
0.1712
0.3093
-0.8749
0.2004
-0.0701
0-4600
0.0985
-0.3216
-0.1414
T i O 2 A1203 F e 2 0 3
- 0.6024 - 0.8929
0.6592
0.3090
-0.3450 -0.6342
-0-1370 -0.3215
-0.2888 -0-5306
0.1449
0.0624
0.1986 -0.2670
-0.2222 -0-6034
0.1064 -0.1302
MgO
CaO
K20
Na20
P 2 0 5 H20 +
- 0.5779 - 0.1332
0.0589
0.6775 - 0.0947 - 0.0947 - 0.7652
0.1776 -0-0076 -0.3669 -0.7758 -0.0741
0.0085
0-5394
0-2719
0.1336 -0.2460 -0.7523 -0.0779
0.0885
0.6198
-0.1087
0.0023 -0.3405 -0.2269
0.1956
0.2109
-0.1119
0.3214 -0.1993
0.0179 -0.1060
0.1471
0-1760
0-4544
0-3736
0-1737 -0-1396 -0-0212
- 0.4476
0.4090
0.0466
0.2712 - 0.1485 - 0.4286
-0.0695
0.1093
0-1215
0.1821
-0-1158
0-3101
0.4646
0-0317
0.4685 -0.2933
0.2273
-0.0310
-0.0659
0-0488
0.1557 -0.1302
0.2484
0-1448
Statistical significance = 95% for r = 0.2369 referred to carbonate rock residue, and for r = 0.2172 referred
to terra rossa.
442
M . M o r e s i a n d G. M o n g e l l i
TABLE 3. Correlation: oxides vs. K20.100/A1203 ratio.
Carbonate rock residue
SiO2
TiO2
A1203
Fe203
MgO
CaO
K20
Na20
P205
H2O§
Terra rossa
a
b
r
a
b
r
42.19
1.95
31.76
7.87
1.13
0.50
1.82
0-19
0.22
11.66
-0.027
-0.010
-0.541
0.332
0.048
0.060
0.089
0.011
0.010
0.022
-.0252
-.1521
--6390
.2118
.5684
.5031
.5720
-3301
.2575
.0612
36-28
2.28
32.38
13.03
0.83
1.02
0.25
0.21
0.19
13.43
1.380
-0.109
-0.981
-0.238
-0.030
0.051
0.192
0.006
-0-004
-0.271
-8250
-.7346
-.8919
-.4192
-.2403
.2933
.9411
.2730
-.1104
-.5253
a and b: least-square equation coefficients; r: correlation coefficients (statistical significance as
in Table 2).
TABLE4. Changes in the oxide concentrations as the K20.100/A1203 ratio decreases.
Trends in
carbonate rock
residue
Differences from
carbonate rock residue
to terra rossa
Trends in
terra rossa
Differences from sand
to silt + clay of
clay sediments
SiO2
NO CHANGE
INCREASE
[ DECREASE
DECREASE]
TiO 2
NO CHANGE
NO CHANGE]
[INCREASE
INCREASE ]
A1203
INCREASE
INCREASE
INCREASE
INCREASE I
Fe203
NO CHANGE
NO CHANGE[
[INCREASE
INCREASE 1
MgO
DECREASE
DECREASE
[ INCREASE
INCREASE
CaO
DECREASE
DECREASE
DECREASE I
INCREASE
K20
DECREASE
DECREASE
DECREASE
DECREASE
Na20
DECREASE
DECREASE
DECREASE
DECREASE
P205
DECREASE
DECREASE
NO CHANGE
INCREASE
H2O+
NO CHANGE
NO CHANGE I
]
I
t INCREASE
INCREASE [
decrease m illite). Moreover, the results o b t a i n e d p e r m i t a n affirmative answer to the
q u e s t i o n as to w h e t h e r the c a r b o n a t e rock residue is, or is not, the p a r e n t m a t e r i a l of the terra
rossa. T h e sequences (A) a n d (B) are similar to one a n o t h e r (see also T a b l e 4), a n d
furthermore, the majority of the trends (oxides vs. K20/A1203) o b t a i n e d for the c a r b o n a t e
rock residue p o i n t to c o n c e n t r a t i o n values very similar to the average values calculated for
Geochemistry o f terra rossa and underlying rocks
5t
10
..... 0...
443
---O-----
HeO
5
I
0
15
....... j ~ w
10
0
t
Fe203
"~-
I
0"4 t
5
0.2
0
0
..... Q) .........
z~
~r-I
30
.....m<.......~...o......
20
I-1
A'2%
10
0
2
......0
1
----0----.........
0
ri%
A~it.l_
t"
1
K20
O
80
..."
70.
.o ...........
~
OL
60
s~o z
--
50
......|
40
...I
J
0
0
CaO
I
10
20
30
K2o. 7 o o 1 , % %
10
20
30
K 2 o . IoolAI2%
FIG. 1. Oxides (%) vs. K20.100/AlzO3 ratio. Full circle and dashed line: average values and
trends from carbonate rock residue. Open circle and dotted line: average values and trends from
terra rossa. The full line connects the average values of sand fractions (squares) and silt-clay
fractions (triangles) from several clay sediments from Southern Italy (see text). The asterisk
shows the average values of 4 sand fractions from terra rossa.
the terra rossa (Fig. 1). This means that the average chemical composition of the terra rossa
agrees well with the continuation of the geochemical trends which characterize the carbonate
rock residue. Therefore the terra rossa can be considered as a carbonate rock residue with a
depressed K20/A1203 ratio.
In order to establish the processes responsible for the decrease in the K20/A1203 ratio, two
possibilities have been taken into consideration: (i) mixing with materials characterized by a
low illite/kaolinite ratio; (ii) transformation of the illite into kaolinite by weathering. The
first possibility seems very unlikely as the materials which m a y have been a d d e d to the terra
rossa are generally richer in iUite than kaolinite. This mineralogical feature is common to
most of the post-Cretaceous sediments in Southern Italy (Dell'Anna, 1968; D e l l ' A n n a et al.,
1968; Garavelli & Nuovo, 1975; De Marco et al., 1981), and the eolian dusts that m a y have
been transported into the Mediterranean area (Tomadin et al., 1984). However, this does not
indicate that the terra rossa was not contaminated by extraneous material, but it certainly did
444
M. Moresi and G. Mongelli
not have any significant influence on the chemical and mineralogical character of the terra
rossa. The second possibility, however, is acceptable because sequences (A) and (B) agree
with the effects of chemical alteration in an early diagenetic environment. The sequences are
in fact quite similar to the mobility scheme of the elements:
(Ti,AI,Fe) < Si < (Mg,K,Ca,Na)
according to several authors (Smith, 1913; Anderson & Hawkes, 1958; Loughnan, 1969;
Fornaseri, 1974; All6gre & Michard, 1977).
Therefore it may be assumed that the same type of chemical alteration conditioned first the
composition of the carbonate rock residue, and then (after Karst dissolution of the carbonate
rocks) the composition of the terra rossa. In both cases, the main effect of the chemical
weathering is the transformation of illite to kaolinite.
Only the relative position of Si in sequences (A) and (B) appears slightly anomalous
compared to its position in the mobility scheme of the elements. This would suggest that the
chemical weathering was "disturbed" by a moderate enrichment of SiO2. However, this
geochemical feature cannot be explained as a process of contamination by eolian dusts or
other sediments from Southern Italy. In fact the addition of these materials in such quantity
as to cause enrichment of SiO2 would have resulted in other changes in composition (e.g.
AlzO3, TiO2, Fe203 impoverishment), which is in contrast to the results obtained. The
enrichment of SiO2, therefore, can be accounted for only by considering a modest addition of
pure SiO2. This could be linked to the production of biogenic SiOz in the aqueous
environment which affected both the carbonate rock residue and the terra rossa for a certain
period. Sedimentation of the carbonate rock in the Cretaceous sea is obvious, but the terra
rossa also had long periods of interaction with an aqueous environment as indicated by the
fact that the sand fractions of some of the terra rossa samples contain siliceous remains of
marine organisms (Dell'Anna et al., 1973).
Furthermore, the sedimentary character of many terra rossa deposits is easily recognizable
from their stratigraphic position. The deposition of terra rossa in morphologically depressed
areas or in Karst cavities could have taken place either in a continental environment, due to
the action of superficial waters, or in a marine environment in which the deepening of the
Apulian Carbonate Platform caused the transgression of the Pliocene-Pleistocene sea and the
subsequent sedimentation ofcalcarenites (Iannone & Pieri, 1982). The results of this research
also show that the geochemical pattern of the Apulian terra rossa suggests a mineralogical
distribution (following from its granulometric distribution) governed by transport and
sedimentation processes in an aqueous environment. This may be clearly observed by
comparing (Fig. 1, Table 4) the geochemical trends obtained for the terra rossa with the
differences in chemical composition between the sand fractions and the silt-clay fractions
separated from various silicate sediments (analytical data from Dell'Anna, 1968; Garavelli &
Nuovo, 1975; De Marco et al., 1981). Nevertheless, interaction with the aqueous
environment (apart from the biogenic SiO2 enrichment) led to quite different mineralogical
distributions in the terra rossa and the carbonate rock residue.
The correlations between the chemical components in the terra rossa suggest an
antithetical distribution between two different mineralogical associations: the first
consisting of illite and quartz, and the second of kaolinite and oxides and hydroxides of Fe,
AI, and Ti. In the carbonate rock residue, on the other hand, only the distribution of oxides
and hydroxides of Fe seem to condition that of the other minerals which appear to be
Geochemistry o f terra rossa and underlying rocks
445
randomly associated. These differences are probably linked to different energy intensity of
the processes that governed the transport and sedimentation of the two materials.
CONCLUSIONS
This research has established that the geochemical pattern of the A p u l i a n terra rossa
confirms that it is derived from the re-working of the residue of the underlying Cretaceous
carbonate rocks in an early diagenetic environment.
The differences in chemical composition between the two materials can be attributed
mainly to two processes: one of chemical weathering which caused an appreciable
transformation of illite into kaolinite, and the other of moderate SiO2 contamination, mostly
of organic origin. Both these processes conditioned, with similar effects, not only the
chemical evolution of the terra rossa, but also that of the carbonate rock residues before or
during Cretaceous sedimentation.
However, there is an important difference in the energy intensity of the processes which
regulated the distribution of the mineralogical components of the two materials: only in the
terra rossa is this distribution clearly conditioned by transport and sedimentation processes.
The Apulian terra rossa seems to have retained its individuality throughout its long
evolution due to the fact that the A p u l i a n Carbonate Platform has always been somewhat
isolated from other sources of terrigenous materials.
The addition of eolian dusts does not seem to be sufficiently important to alter significantly
the chemical composition and evolution of the terra rossa.
ACKNOWLEDGMENTS
The authors acknowledge the finance provided by the Italian "Ministero Pubblica Istruzione."
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