8.6 RUDACEOUS ROCKS
383
where they are sometimes termed "fluxoturbidites." Other diamictites are glacial in
origin. Notable examples are the Pleistocene boulder clays.
The second class of conglomerates, defined by texture, are grain supported. In these
the individual pebbles touch one another, but the intervening spaces are generally infilled by a matrix of poorly sorted sand and clay. In fluvial conglomerates it is quite
likely that this was a primary depositional feature. In marine conglomerates, on the
other hand, such as those beach gravels that mark marine transgressions, it is more
likely that there was no depositional matrix. The matrix that is commonly present is
probably due to ~nfiltration from overlying sediment. Conglomerates are deposited with
high porosities but, because of their large throat passages, they have excellent permeability. This high permeability enables porosity to be quickly destroyed by matrix infiltration even before any cementation can occur.
Two further classes of conglomerate are defined according to the compositions of
their pebbles. Polymictic conglomerates are composed of pebbles of more than one type.
Oligomictic conglomerates have pebbles of only one rock type. Whereas the polymictic conglomerates are of diverse composition, the oligomictic conglomerates are generally quartzose. This is because of the chemical stability of silica. Thus the conglomerate
of polycyclic sediments is commonly made of pebbles of vein quartz, quartzite, and chert.
Polymictic conglomerates are generally the product of aggradation where tectonically
active source areas shed wedges of fanglomerates. Oligomictic conglomerates, by contrast, are generally the product of degradation where tectonic stability allows extensive
reworking to produce the laterally extensive basal conglomerates at unconformities.
A third bipartite division of conglomerates is according to the source of the pebbles.
Extraformational or exotic conglomerates are composed of pebbles that originated outside the depositional basin. Intraformational conglomerates contain pebbles of sediment that originated within the depositional basin. The majority of limestone conglomerates are intraformational in origin for the reason already stated. Intraformational
sand conglomerates are rare because unconsolidated sand lacks cohesion and disaggregates on erosion. Occasionally, however, intraformational conglomerates of sand occur
adjacent to petroleum seeps, where the oil acts as a binding agent, allowing sand cobbles
to be transported for some distance (Fig. 8.40).
Mud intraformational conglomerates are commonly called "shale flake" or "shale
pellet" conglomerates. These are volumetrically insignificant, occurring in beds generally only one or two clasts thick. Sedimentologically they are significant, however, because they indicate penecontemporaneous erosion close to the site of deposition. Intraformational shale pellet conglomerates are often present at the base of turbidite units
and of channels (Fig. 8.41).
This brief review of conglomerates shows that there is no consistent scheme of nomenclature and classification. Table 8.4 shows how descriptive terminology is based on
divisions of texture, composition, and source.
8.6.2 Sedimentary Breccias
Breccias, defined as rocks composed of angular cobbles, are formed from meteoritic impact and tectonic and sedimentary processes. It is not always easy to differentiate their
origin (Reimold, 1997). Only the sedimentary breccias are dealt with here. Sedimentary
383
where they are sometimes termed "fluxoturbidites." Other diamictites are glacial in
origin. Notable examples are the Pleistocene boulder clays.
The second class of conglomerates, defined by texture, are grain supported. In these
the individual pebbles touch one another, but the intervening spaces are generally infilled by a matrix of poorly sorted sand and clay. In fluvial conglomerates it is quite
likely that this was a primary depositional feature. In marine conglomerates, on the
other hand, such as those beach gravels that mark marine transgressions, it is more
likely that there was no depositional matrix. The matrix that is commonly present is
probably due to ~nfiltration from overlying sediment. Conglomerates are deposited with
high porosities but, because of their large throat passages, they have excellent permeability. This high permeability enables porosity to be quickly destroyed by matrix infiltration even before any cementation can occur.
Two further classes of conglomerate are defined according to the compositions of
their pebbles. Polymictic conglomerates are composed of pebbles of more than one type.
Oligomictic conglomerates have pebbles of only one rock type. Whereas the polymictic conglomerates are of diverse composition, the oligomictic conglomerates are generally quartzose. This is because of the chemical stability of silica. Thus the conglomerate
of polycyclic sediments is commonly made of pebbles of vein quartz, quartzite, and chert.
Polymictic conglomerates are generally the product of aggradation where tectonically
active source areas shed wedges of fanglomerates. Oligomictic conglomerates, by contrast, are generally the product of degradation where tectonic stability allows extensive
reworking to produce the laterally extensive basal conglomerates at unconformities.
A third bipartite division of conglomerates is according to the source of the pebbles.
Extraformational or exotic conglomerates are composed of pebbles that originated outside the depositional basin. Intraformational conglomerates contain pebbles of sediment that originated within the depositional basin. The majority of limestone conglomerates are intraformational in origin for the reason already stated. Intraformational
sand conglomerates are rare because unconsolidated sand lacks cohesion and disaggregates on erosion. Occasionally, however, intraformational conglomerates of sand occur
adjacent to petroleum seeps, where the oil acts as a binding agent, allowing sand cobbles
to be transported for some distance (Fig. 8.40).
Mud intraformational conglomerates are commonly called "shale flake" or "shale
pellet" conglomerates. These are volumetrically insignificant, occurring in beds generally only one or two clasts thick. Sedimentologically they are significant, however, because they indicate penecontemporaneous erosion close to the site of deposition. Intraformational shale pellet conglomerates are often present at the base of turbidite units
and of channels (Fig. 8.41).
This brief review of conglomerates shows that there is no consistent scheme of nomenclature and classification. Table 8.4 shows how descriptive terminology is based on
divisions of texture, composition, and source.
8.6.2 Sedimentary Breccias
Breccias, defined as rocks composed of angular cobbles, are formed from meteoritic impact and tectonic and sedimentary processes. It is not always easy to differentiate their
origin (Reimold, 1997). Only the sedimentary breccias are dealt with here. Sedimentary