Geoscience Reference
In-Depth Information
10.3 Sedimentary basins and continental margins
10.3.1 Introduction
Sedimentary basins are of great economic importance because fossil fuels were
formed there. Knowledge of the origin of a basin can provide information about
the location and the type of thermal maturation of hydrocarbon deposits. A num-
ber of tectonic mechanisms can cause the formation of sedimentary basins. These
are discussed in the following sections.
LOADED
UNLOADED
h
SL
h
w
ρ
w
d
w
ρ
w
ρ
s
d
s
ρ
c
h
c
10.3.2 The loading effect of sediments
Forasedimentary basin to form, some factor must cause the basement to subside.
However, let us initially suppose that a subaqueous depression of depth
d
w
in the
basement exists (Fig. 10.31). Because the Earth has rain and winds, erosion of
surrounding terrain automatically fills this depression with sediments. Assuming
that it is completely filled with sediments and that isostatic equilibrium is main-
tained, we can use the methods of Section 5.5.2 to calculate the total thickness of
these sediments. This thickness
d
s
is calculated by equating the mass of the water
in the depression plus the mass of displaced mantle to the mass of the sediments:
h
c
ρ
c
ρ
m
ρ
m
d
w
ρ
w
+
(
d
s
−
d
w
)
ρ
m
=
d
s
ρ
s
which, on rearranging terms, gives
d
s
=
d
w
ρ
m
−
ρ
w
ρ
m
−
ρ
s
Figure 10.31.
The loading
effect of sediments.
ρ
w
is
the density of water;
(10.1)
ρ
s
,
the density of sediment;
ρ
c
, the density of crust;
and
ρ
m
, the density of
mantle.
h
w
is the depth of
water in which sediments
were deposited;
d
s
, the
thickness of sediments;
h
c
, the thickness of crust;
d
, the backstripped depth
of basement; and
h
SL
, the
change in sea level from
the time at which
sediments were deposited
to the present day.
where
ρ
m
is the density of the mantle,
ρ
w
the density of water and
ρ
s
the density
10
3
kg m
−
3
,
10
3
kg m
−
3
of the sediments. If we take
ρ
m
as 3.3
×
ρ
s
as 2.5
×
10
3
kg m
−
3
, the presence of the sediments amplifies the original
subaqueous depression by a factor
d
s
/
and
ρ
w
as 1.0
×
d
w
of 2.9. Thus, a basin filled with 5 km of
sediments represents a subaqueous depression of the crust of about 1.5-2.0 km,
the exact value depending on the density of the infilling sediments. To make
comparisons between one sedimentary basin and another, we must first calculate
the loading effect of the sediments so that this amplification can be removed.
This process is called
backstripping
. The depth at which the basement would
have been if there had been no infilling sediments,
d
w
,isgivenby Eq. (10.1)as
d
w
=
d
s
ρ
m
−
ρ
s
ρ
m
−
ρ
w
(10.2)
However, suppose that the sediments were deposited in water of depth
h
w
instead
of at sea level (Fig. 10.31). In this case, Eq. (10.2) should be written as
d
w
=
h
w
+
d
s
ρ
m
−
ρ
s
ρ
m
−
ρ
w
(10.3)
Sea level has not remained constant throughout geological history but has risen
and fallen many times. Figure 10.32 illustrates one model of the variation of
