376
8 ALLOCHTHONOUS SEDIMENTS
the relative shortage of free oxygen. This is largely because of bacterial action. Organic
matter may be preserved, and iron and sulfur combine to form pyrites. This combination of substances, together with the lack of red ferric oxide imparts an overall drab graygreen color to the sediment. Early pore-filling cements of siderite, calcite, and quartz
may develop. A considerable amount of work has been done on the mineralogy of these
oxidizing and reducing iron reactions (Van Houten, 1973; Folk, 1976; Walker, 1976; Allen, 1986). The consensus of opinion is that the iron soon achieves equilibrium with the
diagenetic environment, irrespective of whether it was deposited in the ferric or ferrous
state. Field relationships and petrographic studies show that these reactions are completed at an early stage of diagenesis. A clear example of this occurs in the Pre-Cambrian
Torridon Group of northwest Scotland. Here there is a sequence of three deposits: scree,
lacustrine, and braided alluvium. These deposits are banked against an irregular basement topography with marginal fanglomerate development. The three facies are red,
gray-green, and red in vertical sequence. These colors extend into the marginal fanglomerate, indicating that the iron coloration is a primary feature. The reducing waterlogged conditions of the lake allowed gray-green chloritic cement to develop in both it
and the adjacent fanglomerate. The oxidizing environment of the earlier scree and later
braided deposits caused a red ferric oxide to develop in them and their lateral conglomeratic equivalents (Fig. 8.34). Other examples are described by Friend (1966), Walker
(1967), and Van Houten (1968). Early red ferric oxide cements can change to graygreen ferrous iron during later burial. This is very characteristic around oil fields in red
sandstone reservoirs. The strongly reducing oil field brines often form a gray-green halo
in the sandstones of the water zone around the field (e.g., the Lyons Sand of the Denver
Basin, Levandowski et al., 1973).
Throughout the redoxomorphic phase of diagenesis porosity of a sand is lost slowly,
but this is due primarily to the effects of compaction and dehydration rather than to the
Fig. 8.34. Diagram showing the correlation between facies and diagenesis in Torridonian (Pre-Cambrian)
sediments of northwest Scotland. This suggests that cementation was early and directly related to the chemistry of syndepositional pore fluids. (From Selley, 1966. Courtesy of the Geologists' Association.)
8 ALLOCHTHONOUS SEDIMENTS
the relative shortage of free oxygen. This is largely because of bacterial action. Organic
matter may be preserved, and iron and sulfur combine to form pyrites. This combination of substances, together with the lack of red ferric oxide imparts an overall drab graygreen color to the sediment. Early pore-filling cements of siderite, calcite, and quartz
may develop. A considerable amount of work has been done on the mineralogy of these
oxidizing and reducing iron reactions (Van Houten, 1973; Folk, 1976; Walker, 1976; Allen, 1986). The consensus of opinion is that the iron soon achieves equilibrium with the
diagenetic environment, irrespective of whether it was deposited in the ferric or ferrous
state. Field relationships and petrographic studies show that these reactions are completed at an early stage of diagenesis. A clear example of this occurs in the Pre-Cambrian
Torridon Group of northwest Scotland. Here there is a sequence of three deposits: scree,
lacustrine, and braided alluvium. These deposits are banked against an irregular basement topography with marginal fanglomerate development. The three facies are red,
gray-green, and red in vertical sequence. These colors extend into the marginal fanglomerate, indicating that the iron coloration is a primary feature. The reducing waterlogged conditions of the lake allowed gray-green chloritic cement to develop in both it
and the adjacent fanglomerate. The oxidizing environment of the earlier scree and later
braided deposits caused a red ferric oxide to develop in them and their lateral conglomeratic equivalents (Fig. 8.34). Other examples are described by Friend (1966), Walker
(1967), and Van Houten (1968). Early red ferric oxide cements can change to graygreen ferrous iron during later burial. This is very characteristic around oil fields in red
sandstone reservoirs. The strongly reducing oil field brines often form a gray-green halo
in the sandstones of the water zone around the field (e.g., the Lyons Sand of the Denver
Basin, Levandowski et al., 1973).
Throughout the redoxomorphic phase of diagenesis porosity of a sand is lost slowly,
but this is due primarily to the effects of compaction and dehydration rather than to the
Fig. 8.34. Diagram showing the correlation between facies and diagenesis in Torridonian (Pre-Cambrian)
sediments of northwest Scotland. This suggests that cementation was early and directly related to the chemistry of syndepositional pore fluids. (From Selley, 1966. Courtesy of the Geologists' Association.)