Clay Minerals (1984) 19, 271-286
O R G A N I C R E A C T I O N S AS I N D I C A T O R S OF T H E
B U R I A L A N D T E M P E R A T U R E H I S T O R I E S OF
SEDIMENTARY SEQUENCES
A. S. M A C K E N Z I E
Geochemistry Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex TW16 7LN
(Received 12 September 1983; revised 28 November 1983)
A B S T R A C T: Three organic reactions which affect individual compounds present in most
organic-rich sedimentary rocks have been identified--configurational isomerization and aromatization of steroid hydrocarbons, and configurational isomerization of hopanes. The extents to
which these reactions have occurred can be assessed by gas chromatographic-mass spectrometric analysis of the organic extracts of the sediment samples; they provide sensitive indicators
of thermal maturation. When combined with the model of sedimentary basin formation by
extension of the lithosphere, this molecular approach to the maturation of organic material
can be used to constrain the temperature-time histories of sedimentary sequences. This
temperature history may be used to integrate the relevant Arrhenius' and rate equations
governing the reaction rates. Each rate equation has three constants--the activation energy,
frequency factor and the ratio of forward reaction to backward reaction. These have been
estimated using analyses of samples whose temperature-time history is well constrained. The
resulting estimates can then be used to reconstruct the temperature-time histories of other
samples from organic geochemical data, e.g. calculation of the amount and timing of uplift.
The availability of oil exploration samples from wells drilled into sediments whose present
temperatures and burial depths are their maximum values has allowed potential
geothermometers, deposited with the sediments, to be calibrated and evaluated. The
temperature-dependent interconversions of organic compounds which occur with the
temperature rise associated with burial have proved particularly promising in this respect
(Mackenzie & Maxwell, 1981). In certain situations, the application of such methods m a y
provide good estimates of the maximum temperatures and depths experienced by
sedimentary rocks. This contribution introduces and reviews briefly this methodology;
only three organic chemical reactions which occur in sediments will be discussed in detail,
although m a n y more exist (Mackenzie & Maxwell, 1981).
THE REACTIONS
During the deposition of sedimentary rocks varying proportions of the remains of dead
organisms present, such as algae, bacteria and plants, are incorporated. On average, 0.1%
o f the annual organic carbon production in the oceans is contributed to the sediments
forming on the seafloor (Degens et al., 1981); the other 99.9% is recycled by the
organisms living in the water column. This microbial reworking continues to considerable
depths as the bottom sediments are buried, perhaps to temperatures as high as 3 5 ~
1984 The Mineralogical Society
A. S. Mackenzie
272
HO
~
~
5
Many steps
k 2
k 1
b
Fro. 1. Representations of the molecules of a sterol, found in living organisms, and ((a) and
(b)) the two isomeric steroid hydrocarbons found in sediments which are used to determine
the extent of isomerization. The lines represent carbon-carbon bonds, and the carbon atoms lie
at the line intersections. All carbon atoms have four bonds, and where these are not shown bonds
to hydrogen atoms are implied. The hexagons are cyclohexane rings which are not planar.
Chiral centres are shown in two different ways, by dots or by drawing a solid triangle (or a
dotted line) with the centre at one apex. A solid (open) dot represents a carbon atom above
(below) the plane of the paper, and with a carbon-hydrogen bond pointing out of (into) the
plane of the paper. A solid triangle (dotted line) shows a -CH 3 group above (below) the plane
of the paper. The configuration at one chiral centre (C-24) is not indicated because the two
isomers were not resolved by the GC-MS systems used. Of the 256 possible arrangements
of the eight chiral centres shown, only two are present after early diagenesis. Hydrogen exchange
then occurs at the site marked 20 by removal and then reattachment of the hydrogen atom.
(Coleman et al., 1979). Despite these microbial and chemical alterations, some of the
original organic structures produced by enzymatic biosynthesis in living biota can be
recognized in whole or in part to considerable temperatures and depths: organic molecules
whose three-dimensional structures bear the unambiguous hallmarks o f enzymatic
biosynthesis can be detected in sediments o f m a x i m u m depths equivalent to m a x i m u m
temperatures of about 2 3 0 ~
the limit o f the techniques discussed here is, however,
~130~
Examples of such molecules are the steroid and h o p a n o i d h y d r o c a r b o n s found in
sedimentary rocks and petroleums (see Ourisson et al. (1979) for a review). Fig. 1 shows
the chemical structure o f a sterol found in living organisms. Structures (a) and (b) in the
Figure are c o m m o n components o f mature sediments and petroleums; they are saturated
steroid h y d r o c a r b o n s , known as steranes. The specificity of the sterane structures links
them clearly to the sterol. After the sterol is incorporated into sediments, as part of a dead
organism, microbial and chemical reactions combine to convert the sterol into the more
stable sterane by removing the O H group attached to carbon-3 (C-3) and reduction of
(addition o f hydrogen to) the double bond which connects C-5 to C-6 ( R h e a d et al., 1971;
Dastillung & Albrecht, 1977). These reactions are complete by about 5 0 ~ (Mackenzie et
al., 1982).
The only difference between structures (a) and (b) is that the bond which connects the
methyl group to C-20 in (a) points into the page, whereas in (b) it points out of the page.
Organic reactions as indicators of temperature
273
2.0O
O
O
2.5'
0
i
O
~
0
3.00
O
3.5O
50
PANNONIAN
BASIN
PLIOCENE
40
0
20
1
% 20S/(20R + 20S) STERANES
0
FIG. 2. R a t i o o f ( p r o d u c t s ) / ( p r o d u c t s + r e a c t a n t s ) for s t e r a n e i s o m e r i z a t i o n as f u n c t i o n o f d e p t h
in the PannonianBasin.
The two compounds are stereoisomers at C-20. For complicated reasons (Mackenzie &
Mackenzie, 1983; Gunstone, 1974), (a) in Fig. 1 is called the 20R isomer and (b) the 20S
isomer. The sterol shown has the 20R configuration. Nearly all sterols isolated from living
organisms have this configuration; it is required for the function of the sterols in the cells of
living systems (Nes, 1974). Consequently, the sterane composition of sediments is
dominated initally by the 20R stereochemistry.
The 20S form is, however, of similar stability to the 20R form (Van Graas et al., 1982).
At temperatures >65~
the hydrogen atom connected to C-20 is removed by an
unknown mechanism, and the resulting intermediate can regain hydrogen on either side of
the molecule with near equal probability. Thus 20R steranes are converted to 20S forms
until the predicted equilibrium composition (20S/(20R + 20S) = 0.54 (Van Graas et aL,
1982)) is reached. This type of reaction is called configurational isomerization. Plots of the
ratio 20S/(20R + 20S) against depth in a sedimentary basin which has not been uplifted
frequently show a steady change from near zero (mainly 20R) to the equilibrium value.
This is the case in the Pliocene of the Pannonian Basin of SE Hungary (Fig. 2).
The relative amounts of the 20S and 20R forms are determined by gas chromatographic-mass spectrometric (GC-MS) analysis of the hydrocarbon fraction which can
be extracted from the sediment samples of interest; similar techniques are employed for the
other reactions discussed here (Mackenzie et aL, 1983).
An analogous reaction to the sterane isomerization is the isomerization of hopanes at
C-22 (Ensminger et al., 1974, 1977; Fig. 3). Hopanoids such as tetrahydroxy-
274
A. S. Mackenzie
hydroxybacteriohopane
I
Many steps
~
22R
~
a
22S
b
FIG. 3. Representations of a hopanoid molecule found in living organisms (tetrahydroxybacteriohopane) and ((a) and (b)) the two isomeric hopanes found in sediments which were used
to determine the extent of isomerization.
kA
H
a
>
b
FIG. 4. (a) Two monoaromatic steroid hydrocarbons which are isomeric at C-5 (shown by the
sinuous C-H bond), with the aromatic ring shown by an inscribed circle. The letters A-D are
used to identify the rings, numbers to identify the carbon atoms (see Fig. 4). With increasing
temperature, hydrogen and a methyl group are lost to produce a triaromatic hydrocarbon (b).
The reaction is assumed to be irreversible under geological conditions. As in (a) the chiral
centre at C-24 is not shown
bacteriohopane are contributed to sediments as part of bacterial cell membranes (Ourisson
et aI., 1979). Early microbial and chemical reactions convert these components to hopanes
like (a) in Fig. 3. Most hopanoids in living systems have the 22R configuration: the bond
which joins the hydrogen atom to carbon-22 points into the page (Fig. 3). This preferred
stereochemistry is inherited by the hopanes of shallow sediments but, with the increasing
temperature associated with burial, hydrogen exchange at C-22 steadily converts mixtures
dominated by the 22R stereochemistry to the 22S/(22R + 22S) equilibrium ratio of
0 . 5 5 - 0 . 6 (Fig. 3; Seifert & Moldowan, 1980). In this study an equilibrium value of 0.61
has been used because it is the average value recorded by the present author for mature
sediments.
Another early diagenetic product of sterols (Fig. 1) is monoaromatic steroid
hydrocarbons (structure (a) in Fig. 4). These compounds contain one aromatic ring; more
stable compounds which contain three such r i n g s - - t r i a r o m a t i c - - a l s o occur in sedimentary rocks (structure (b) in Fig. 3). The triaromatic structures are thought to be
products of the aromatization of monoaromatic compounds which occurs in most
Organic reactions as indicators of temperature
275
0
2.50
0
00
#
3.0-
a
3.5EAST
SHETLAND
BASIN
JURASSIC
100
80
60
0
0
% TRI/(TRI- + MONOAROMATIC) STEROID HYDROCARBONS
FIG. 5. Ratio of (products)/(products + reactants) for the aromatization of monoaromatic
steroid hydrocarbonsas a functionof depth in the Jurassic of the East Shetland Basin.
sedimentary sequences between 80 and 120~ (Ludwig et al., 1981; Mackenzie et al.,
1981). An example of this is shown in Fig. 5. Jurassic shales from the East Shetland Basin
at depths of 2 km contain many monoaromatic components; by 3 km triaromatic species
dominate.
ORIGIN
AND FATE OF STEROID
HYDROCARBONS
AND HOPANOID
The steroid and hopanoid hydrocarbons derive from oxygen-containing natural product
compounds. Nes (1974) and Ourisson & Rohmer (1982) believe that these natural
products perform an architectural function in cell membanes. Hopanoids do not require
molecular oxygen for their biosynthesis; they occur principally in the membranes of
procaryotic cells, e.g. bacteria. Molecular oxygen appears necessary for the manufacture
of sterols, which have chiefly been isolated from the eukaryotic cells of higher life forms
than bacteria. It can be reasonably presumed that most steroid hydrocarbons of sediments
came initially from the algae present during deposition.
Since algae and bacteria have contributed to the organic matter of most sediments,
these sediments now contain sufficient steranes and hopanes to make the measurements
necessary for the assessment of maturity. Typically a sample containing at least 50 mg of
sedimentary organic matter is required. Sediments heated to temperatures greater than
150~ do not contain sufficient material. Beyond 150~ the steroid and hopanoid
molecules are degraded: their specific structures are broken apart by the thermal cleavage
276
A. S. Mackenz ie
of their carbon-carbon bonds. A rapid decrease in concentration is often apparent
between 130 and 150~ (Mackenzie et al., 1983b). However, the changes in steroid
hydrocarbon concentration at lower temperatures are entirely consistent with the simple
rearrangements outlined above (unpublished results). As aromatization proceeds, the
concentration of monoaromatic steroid hydrocarbons decreases and that of triaromatic
steroid hydrocarbons increases; as isomerization proceeds the concentration of 20R
steranes declines and that of 20S steranes rises. It is not possible to test the integrity of
the hopane isomerization since the reactions coincide with very early stages of oil
generation by the thermal breakdown of kerogen. Hopanes, but not steroids, are produced
during the early stages of oil generation.
APPLICATION
OF REACTION
KINETICS
All three reactions discussed (Figs 1, 3 and 4) can be considered as conversion of some
reactants to some products. In addition, for the isomerization reactions there is a
backward reaction; equilibrium is established when the rate of the forward reaction equals
the rate of the backward reaction. The aromatization reaction proceeds to completion: all
monoaromatic steroid hydrocarbons are converted to triaromatic steroid hydrocarbons.
Each reaction rate can be expressed by
rate = k • [reactants]
(1)
where k is the rate coefficient in reciprocal seconds. The concentration of products at time
(t) is obtained from the equation
[products] t = [reactants]0 e kt
(2)
when the reactions are first-order. Mackenzie & McKenzie (1983) have shown that there
is some evidence that this is the case for the isomerization reactions, but in the case of the
aromatization reactions this assumption is based on theoretical considerations for which
there is no proof at present. Rearranging equation (2) gives
[products] t
= e kt
(3)
[reactants] t + [products]~
The left-hand side can be measured using G C - M S and k calculated when t is known.
Equation (3) is appropriate for the aromatization reaction but more complicated equations
are necessary for the isomerization reactions since there are two reactions involved: a
forward and backward component (Mackenzie & McKenzie, 1983).
The parameter k is related to absolute temperature (T) using the Arrhenius' equation
k = A e e/RT
(4)
where A and E may be regarded as constants; A is the frequency factor, E is the activation
energy and R is the gas constant.
Equation (4) cannot be used directly for reactions which take place in sedimentary
rocks since T varies with time. Instead, the temperature history of a sediment must be
broken into short time steps where T may be regarded as constant. A quicker method is to
use an 'effective heating time' (called t,,~j). This term was introduced by Hood et al. (1975).
Their calculations showed that the kinetics of oil generation could be modelled by
assuming that all the reactions occurred at the maximum temperature experienced by the
Organic reactions as indicators of temperature
277
6.[.....
21i~ ~
~
on
oNorth Sea
/
~
1o
216
\
1
i
i
i
I/T x 1000
FIo. 6. Generalized plot of logek as a function of 1/T for the isomerization and aromatization
reactions of steroid and triterpenoid hydrocarbons, k - 1/(1 + 7)t loge [1 - (1 + 7)f/fJ. t for
isomerization, 7 - 1.174 for isomerization of steranes at C-20; 7 = 1-564 for isomerization of
hopanes at C-22, k - -1/tloge (1 -- x) for aromatization, t can be considered as the time
spent within 15~C of the maximumtemperature which is approximatelyequivalentto 0.4 x time
since the stretching event which caused the subsidence, x and y refer to the relevant ratios of
reactants to (reactants + products). T is the maximum temperature, which must be the present
temperature to apply this figure. The straight line marked sterane isomerization corresponds to
a reaction rate coefficientfrom 20R to 20S whose activation energy is 91 kJ mol i and whose
frequency factor is 0.006 s 1. That marked hopane isomerization corresponds to a reaction
rate coefficientwhose activation energy is 91 kJ mo1-1and whose frequency factor is 0.016 s 1.
The straight line marked aromatization corresponds to a reaction rate coefficient with an
activation energy of 200 kJ mo1-1 and frequency factor of 1.8 x 1014 s 1. The points shown
are from the aromatization reaction; those numbered are heating experiments with the time of
heating in days (Mackenzie& McKenzie, 1983).
host sediment over a timespan equivalent to the time spent by the sediment within
approximately 15~ of the m a x i m u m temperature. This timespan is k n o w n as effective
heating time. Fig. 6 is a plot of logek as a function of the reciprocal of m a x i m u m
temperature, where k was calculated from equation (3) using the effective heating time for
each sample for t. F r o m equation (4) it can be seen that the slope of the type of plot shown
in Fig. 6 will be --E/R and its intercept will be logeA. Thus E and A m a y be estimated.
However, a more rigorous approach would require a better attempt to reconstruct the
thermal history of the sediments.
By dividing the temperature history into short timesteps of constant temperature, the
ratio of products to (products + reactants) can be calculated for each time step using the
correct form of equation (3), and k calculated from (4) and assumed values of E and A.
Thus the current (cumulative) ratio of products to (products + reactants) predicted for
given E and A values can be calculated.
E and A are adjusted until the fit between the observed and predicted reaction extents is
acceptable and best-fit values emerge. The most convenient way to compare these
278
A. S. Mackenzie
observed and predicted values for a number of samples from the same area has proved to
be plots of one reaction extent (products/(reactants + products)) against another (Figs 7
and 8). Here maturity increases from the bottom left-hand corner to the top right-hand
corner and the fit can be tested using the shape of the curve predicted and observed, and
the predicted and observed present-day temperatures and depths which correspond to a
position on these plots.
ESTIMATION
OF REACTION
CONSTANTS
In practice it is only possible to be reasonably certain of the burial history of a sediment
when burial has been near-continuous; this requires that no uplift of significance has
occurred. Estimation of the amount and timing of significant uplift is problematic, and has
been avoided for the calibration of the organic reactions. Thus sequences where the present
depth corresponds to the maximum depth have been sought.
For the approach outlined above, it is also preferable that the present temperatures
correspond to the maximum temperature. This is predicted to be the case in the sediments
of sedimentary basins formed by extension of the lithosphere whose present depths do not
exceed 4 km (McKenzie, 1981) and the temperature-time histories of sediments from such
sequences can be calculated when the amount and timing of lithospheric stretching are
known (McKenzie, 1978, 1981). In such sequences, the time spent within 15~ of the
maximum temperature is approximately 0-4 • time since stretching (Mackenzie &
McKenzie, 1983).
Three sequences were chosen:
(i) A series of Jurassic shales from the area of the North Sea where the Tertiary section
is complete--the North Sea is thought to have been formed by lithospheric stretching
which ended ~100 Ma ago (Sclater & Christie, 1980; Christie & Sclater, 1980; Wood
& Barton, 1983).
(ii) A sequence of Pliocene shales and siltstones for a single well in the Pannonian Basin
of SE H u n g a r y - - t h e Pannonian Basin can be regarded in simplified terms as having
been formed by sudden lithospheric stretching ~ 15 Ma ago (Sclater et al., 1980).
(iii) A sequence of sediments from the Nova Scotian continental margin which
originates from the stretching event associated with the opening of the North Atlantic
~180 Ma ago (Royden & Keen, 1980).
The plots of logek against the reciprocal of maximum absolute temperature have been
published elsewhere (Mackenzie & McKenzie, 1983; also Mackenzie et al., in prep.) but
Fig. 6 acts as a summary. The estimates of E and A for the three reactions which emerged
are listed in Table 1.
TABLE1. Estimates of reaction rate constants.
Isomerization of steranes at C-20, 20R ~ 20S
Isomerization of hopanes at C-22, 22R ~ 22S
Aromatization of monoaromatic steroid hydrocarbons
monoaromatic -, triaromatic
E
(kJ mol ~)
A
(s l)
y
91
91
0.007
0-025
1-174
1-564
200
1.8 X 1014
E = activation energy; A -- frequency factor; y = forward reaction rate coefficient/backwardreaction rate
coefficient.
279
Organic reactions as indicators of temperature
0
"
%
0
0)
~.\.~
.o
0
~
I~
o
o\
0
~"
.~
~.~
~
,,
e-
~\
~
"
o
x o :
>o
zO~
o
.~~ .~ o ~
-
~-I
~,~
o
qD
~
0
~-
r
!
o
o
o
~
..~.~ o ~ 0 ~
0 ~
0
~.
~ ,~.: ~ -
9~ . ~tr,, g ~
~.~ ~ ~ . o ~ o o o
~
~
~
.~
o~
Sterane
6
oO~
oi ~
:
Isom.
ool
~"
"i-
g
II
~.
o
Sterone
Isom.
~~
280
A. S. Mackenzie
The lines in Fig. 6 suggest that aromatization will proceed more rapidly than the
isomerization reactions at elevated source temperatures. At temperatures <100~ the
hopane isomerization reaction is faster than steroid aromatization; at temperatures <
90~ the sterane isomerization reaction is faster than steroid aromatization. Generally kt
~ 1 if a reaction is to be measurable. Hence measurements in basins younger than about
20 Ma (teyI <8 Ma) will show isomerization and aromatization ratios, based on the
left-hand side of equation (3), which are appreciably different from zero and the reaction
end-point, when logek lies between - 3 1 and - 3 5 . In this range temperatures are greater
than 90~ and aromatization proceeds faster than sterane isomerization, and at similar
rates to hopane isomerization. In basins older than about 100 Ma (tesr >40 Ma), the
appropriate values of logek are less than - 3 4 . In this range temperatures are ~90~
hopane isomerization proceeds faster than aromatization and at similar rates to sterane
isomerization.
The estimates of the reaction constants obtained from plots like Fig. 6 can be used with
the temperature-time histories predicted by the basin model. In this paper three ratios are
discussed, of the form of the left-hand side of equation (3), which measure the extents of
the reactions. Two types of plot are used to test the values of the reaction constants listed
in Table 1 : extent of aromatization against extent of sterane isomerization, and extent of
aromatization against hopane isomerization. These plots are referred to as A - I diagrams.
The A - I diagrams for sterane isomerization are shown in Figs 7(a)-(c). The points on
these diagrams are the observations, labelled with the present temperatures of the samples
to which they refer. Superimposed on the plots are the curves predicted by the basin model
adopted in each case. The reasons for choosing the shown fl factors and times since
stretching have been discussed elsewhere (Mackenzie & McKenzie, 1983; also Mackenzie
et al., in prep.). The predicted curves are marked with the predicted present-day
temperatures.
.s~
0"61
r
115~ 123"
Pannonian Basin
15 M o
P~=2.0
(Hod)
o ;2~~ e.s~
0.6:
S8
2o"
9
IT2o IIS ~
--
J
113'e
--r-I-
el01"
n
IO0"
LO~
9 $1~
3
m9g"
3
io
7~
100Ma
N o r t h Sea
80'
Atom.
(a)
Atom
(b)
FIG. 8. (a) The extent ofhopane isomerization as a function of steroid hydrocarbon aromatization for Pannonian Basin samples. The continuous curves were generated using a reaction rate
of 0.016 exp (91 kJ mol-'/RT) for the hopane isomerization reaction. See Fig. 7. (b) As for
(a) but for the North Sea.
Organic reactions as indicators of temperature
281
Overall the fit between observed and predicted reaction extents is good, both in terms of
the shapes of the curves and the temperatures. The general behaviour discussed above is
apparent. In the young Pannonian Basin (Fig. 7a) the reactions occur at higher
temperatures, and aromatization is appreciably faster than sterane isomerization: the
aromatization reaction is complete before major isomerization. As a basin ages, significant
reaction occurs at lower temperatures and the rate of sterane isomerization relative to
aromatization increases. This is apparent when comparing the North Sea (Fig. 7b) and the
Scotian Shelf (Fig. 7c) with the Pannonian Basin (Fig. 7a). If anything, by the age of the
Scotian Shelf, isomerization is faster.
The North Sea fit is the least convincing. The most likely reason is that the
aromatization reaction does not go to completion and, in this case, the assumptions
regarding the mechanism of the aromatization reaction are not quite correct.
Fig. 8 displays two of the A-I diagrams for the hopane isomerization. Their format is
the same as the sterane isomerization A-I diagrams. The fit between the observed and
predicted trends is good, as is the general behaviour predicted previously. In the young
Pannonian Basin the two reactions proceed at similar rates (Fig. 8a); however, in the
older North Sea basin, hopane isomerization has occurred significantly faster than
aromatization.
APPLICATIONS
The most powerful application of this technique is the determination of uplift. Following
the rules of Hood et al. (1975), rapid uplift which causes cooling of sediments by more
than 15~ will be sufficient to freeze the progress of most organic reactions. Thus such
uplift will preserve the imprint of past heating events on sedimentary organic matter,
provided it is not followed by further major subsidence.
Measurement of uplift requires first that the temperatures shown in Figs 7 and 8 are
converted into depths. The depths are dependent on the thermal conductivities of the
sediments. Mackenzie & McKenzie (1983) observed a reasonable correlation between
present depths of the samples on Figs 7 and 8 and the values predicted by the models in the
Pannonian Basin and the North Sea when both basins were considered to have been filled
with compacting clay or carbonate--these matrices have similar thermal conductivities
and the model allows for changes in overall conductivity which arise from the porosity
reduction associated with compaction. When the sediment fill is rich in quartz or anhydrite
the conductivities increase and the geothermal gradient is reduced: it is necessary to go
deeper to reach the same temperature of maturity..The sediments on the Nova Scotian
shelf are rich in quartz, consequently better fits between observed and predicted depths
corresponded to a sediment fill with about 50% quartz (Mackenzie et al., in prep.);
improved fits with North Sea data were obtained by adding small amounts of quartz
(Mackenzie & McKenzie, 1983).
Despite the complications in modelling thermal conductivity, the applications to be
discussed here assume the sedimentary fill is low in quartz and anhydrite. The
mathematical model was used to construct Figs 9(a) and (b). These are composite
diagrams for basins formed by a stretching event, with a/~ factor of 1- 5. The families of
curves labelled in Ma correspond to increasing age since stretching. Using the thermal
conductivities of compacting clay and carbonate, the depths along these curves were
calculated, and depth contours produced. In the Figures, the age of the basin increases
from bottom right to top left; depth increases from left to right. If major uplift occurs the
282
A. S. Mackenzie
0 54
13 =15
9
53//
Paris Basin
O L. . . .
F
0o*
0~
,o
2111b;~A/
....
02o
0(),, 02~
0
Arorn
(a)
~061
)6
2.2km
o~ ~
I
,,
9 .....
~-/2"~m20
2~
//m'~j~
/ ~.,y~o~/~.-'/2.~..~~
3
o.oof o9/
.o ~'
o(
jok~j
07.0
ors
~ ~ ~
I~:~z~
"
13 =1.5
9 Paris
Basin
Arom.
(b)
FIG. 9. (a) Curves generated from the sterane isomerization and aromatization rates at the
times marked after extension by a factor of 1.5, with a surface temperature of IO~ The
heavy lines join points with the same depth. If both reactions are frozen by uplift, the position
of a point obtained from analysis of a sample will show the time which elapsed between the
stretching and the uplift events and the difference between the calculated and observed depth
will give the amount of uplift. The points shown are from the Paris and Lower Saxony Basins.
The uplift estimates are shown in Fig. 10. (b) As for (a) but for hopane isomerization.
283
Organic reactions as indicators of temperature
Lower Saxony Basin
L
IOOkm
i
8"
I0~
f- 54"
"'-.,
i
"E19833" ""
eclonce Contours
9 E9819
~ * E9818
E9827~"-*"}0"5]
i
./'E ~821L.....~, ~ E9824
I
../~
1"7 E9800""-o "SS
E9825.~" no~ ~ 1 0
~/1.5>E.o~---~2o-O~ " ~ . ~ 8 4 2
--'--
Maim
Rhaetic / Lias
*
E9812
:
'
~r
,o E9823
0"2
E982.
" 0"7
]\
t52"
(a)
-2
Paris Basin
0
2
4
6
50
49
§ ++
+
~bO O
;
48
[]
Pre-Jurassic
[~
[ ~ ] Jurassic & younger
L
lOOkm
I
Outcrop of Lower Jurassic (Lias)
\
Isobaths of bottom of Toarcian Shales
o.. (metres) below sea level.
(b)
FIG. 10. (a) Map of the Lower Saxony Basin showing the location of the samples examined,
contours of equal vitrinite reflectance for Jurassic sediments and the amount of uplift estimated
from Fig. 9. (b) Map of the Paris Basin, showing the location of the samples examined and the
uplift estimates from Fig. 9.
284
A. S. Mackenzie
position of a point on Figs 9(a) and (b) will be frozen. Thus the time since stretching when
uplift occurred and the maximum burial depth are recorded. The difference between a
sample's present depth and its maximum depth is the amount of uplift.
This technique has been applied to two basins where major uplift has occurred--the
Lower Saxony Basin and the Paris Basin. In both cases, measurements of the reaction
extents in organic-rich Lias shales at various localities in the basins were available. In the
Lower Saxony Basin, Pliensbachian (Lias fi) shales were analysed; in the Paris Basin
Toarcian shales were considered. The results are plotted on Figs 9(a) and (b), and the
average amounts of uplift is shown on maps of the basins in Figs 10(a) and (b)
The Lower Saxony Basin is an excellent example of an inversion structure (Ziegler,
1980); inversion occurred by thrusting, and the thrusts are probably reactivated normal
faults. The burial curves of this basin (Ziegler, 1980) imply that major subsidence, as a
result of lithospheric stretching, occurred during early and middle Cretaceous times, and
that uplift and inversion were the result of late Cretaceous compression. The uplift map
(Fig. 10(a)) suggests that the inversion, amounted to at least 2.5 km in places. The
positions of the Lower Saxony points on' Figs 9(a) and (b) are compatible with uplift
taking place in the late Cretaceous, or ~ 50 Ma after the extensional event.
The history of uplift in the Paris Basin is more controversial. The points for the Paris
Basin which lie close to the origins on Figs 9(a) and (b) are of little use. Those that plot
near the top right-hand corner can provide estimates of the amount, but not the time, of
uplift. Probably only the two samples with significant aromatization and isomerization,
and marked 2.0 and 1.2 on Figs 9(a) and (b), provide reliable estimates of the timing of
uplift. They suggest that the event occurred within 40 Ma of the extensional event. The
geographical locations of the uplift estimates obtained by subtracting the present depth of
the Toarcian shales from the predicted maximum depths based on the organic reaction
extents are shown in Fig. 10(b). In the centre of the basin no vertical motion can be
detected, but the uplift increases eastward towards the eastern margin of the basin.
The amount of uplift predicted is in broad agreement with Goy's (1979) study of sonic
and resistivity log responses, which suggested that the uplift along the eastern margin of
the basin exceeded 1 km. Both the timing and amounts of uplift contradict the conclusions
of Deroo (1967), who suggested that no more than 700 m of uplift had occurred gradually
on the eastern margin of the basin since the end of the Jurassic to the present day. Since
the crustal stretching which formed the Paris Basin ended 180 Ma ago (Brunet & Le
Pichon, 1982), the geochemical arguments imply major uplift at the end of the Jurassic on
the eastern margin. Pomerol (1974) believed that a widespread regression occurred at this
time, which could have been associated with major uplift and erosion. The cross-section of
the basin (Pomerol, 1974) suggests an asymmetry between E and W, which is compatible
with the eastern margin being the result of uplift and erosion. The amount of uplift on the
eastern margin determined from the geochemical measurements of the Toarcian shales is
similar to the present depths of the Toarcian shales in the deepest part of the basin.
Therefore if the estimates are correct, the original basin extended E - W as a deep trough
and the approximately circular shape of the present basin is the result of later uplift.
CONCLUSIONS
The use of organic molecules for studying the maturation of organic matter, when
combined with the extension model of sedimentary basin formation, provides a powerful
Organic reactions as indicators of temperature
285
insight into the temperature, burial and uplift histories of organic-rich sedimentary rocks.
Thus the tectonic history of the underlying continental crust can be better examined. The
improved models of temperature-time history should be applied to the overall study of
sedimentary rock diagenesis. In this way the importance of the time-temperature histories,
and their relationship to diagenetic sequences involving changes in mineralogy and
porosity, may be better investigated.
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