380
8 ALLOCHTHONOUS SEDIMENTS
Fig. 8.37. Illustrations of sandstone diagenetic motifs and their porosity responses. Porosity (th) increases to
the left. All sand:shale boundaries may be seismic reflectors, but R = seismic reflecting horizons due to diagenetic fronts within sandstone. (A) Envelope effect, showing carbonate cement at sand:shale contacts.
(B) Unconformity motif, showing zones of enhanced and diminished porosity analogous to "gossan" ore bodies (see Section 2.3.4.2). (C) Petroleum:water contact diagenetic effects. C1 illustrates a valid trap, still containing petroleum, which inhibits cementation above the petroleum:water contact, whereas C2 illustrates a
trap which once contained petroleum that inhibited cementation, but which has since leaked, leaving a phantom flat spot at the diagenetic front of the paleo-petroleum:water contact. Examples of all the above motifs
are cited in the text.
the very coarse sandstones. Traditionally the rudaceous deposits are divided into breccias, whose particles are angular, and conglomerates, whose particles are rounded.
Breccias are rare rocks that occur principally in faults (tectonic breccias) and in s o m e
screes. The majority of rudaceous rocks are c o n g l o m e r a t e s (Koster and Steel, 1984).
8.6.1 C o n g l o m e r a t e s
T h e c o n g l o m e r a t e s can be divided into three groups by their composition. The volcaniclastic conglomerates, t e r m e d agglomerates, have already b e e n described (see Sec-
8 ALLOCHTHONOUS SEDIMENTS
Fig. 8.37. Illustrations of sandstone diagenetic motifs and their porosity responses. Porosity (th) increases to
the left. All sand:shale boundaries may be seismic reflectors, but R = seismic reflecting horizons due to diagenetic fronts within sandstone. (A) Envelope effect, showing carbonate cement at sand:shale contacts.
(B) Unconformity motif, showing zones of enhanced and diminished porosity analogous to "gossan" ore bodies (see Section 2.3.4.2). (C) Petroleum:water contact diagenetic effects. C1 illustrates a valid trap, still containing petroleum, which inhibits cementation above the petroleum:water contact, whereas C2 illustrates a
trap which once contained petroleum that inhibited cementation, but which has since leaked, leaving a phantom flat spot at the diagenetic front of the paleo-petroleum:water contact. Examples of all the above motifs
are cited in the text.
the very coarse sandstones. Traditionally the rudaceous deposits are divided into breccias, whose particles are angular, and conglomerates, whose particles are rounded.
Breccias are rare rocks that occur principally in faults (tectonic breccias) and in s o m e
screes. The majority of rudaceous rocks are c o n g l o m e r a t e s (Koster and Steel, 1984).
8.6.1 C o n g l o m e r a t e s
T h e c o n g l o m e r a t e s can be divided into three groups by their composition. The volcaniclastic conglomerates, t e r m e d agglomerates, have already b e e n described (see Sec-