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of acidic in the larger basins also of intermediate to basic silica-
saturated magmas. One of the best exposed of these grabens is
the Carboniferous-Permian Saar-Nahe Basin in southwestern
Germany. The diatremes studied in the Saar-Nahe Basin clearly
show that their tephra, in addition to its juvenile ash grains and
lapilli (Fig. 5) only contains the individual minerals (quartz,
feldspar, mica, clay-minerals) and rock pebbles (milky quartz,
quartzite, black chert, rhyolite pebbles) of the surrounding pres-
ently indurated sediments. In addition, blocks of sedimentary
material like sandstones subsided in the diatremes, and contain
dykelets and stringers of juvenile clasts clearly showing that
the presently indurated sandstones were still unconsolidated
sediments (sands) at the time of subsidence of these blocks
inside the diatremes (Figs. 6, 7). From the known downward
penetration of the diatremes below their original surface it is
evident that at the time of volcanic activity the sediments had
not been indurated yet to depths below the syneruptive surface
of one to possibly two km (Lorenz 1986, Lorenz and Haneke
2002). The Carboniferous Midland Valley is also a classic area
for volcanism having interacted with Carboniferous water-
saturated unconsolidated sediments (Francis 1962, 1970, Leys
1982). As in the late Variscan Saar-Nahe Basin, a great many
diatremes, containing within their tephra the minerals and peb-
bles from the surrounding sediments but hardly any indurated
sedimentary rock clasts, are associated with sills, laccoliths and
intrusive-extrusive domes. A similar environment existed for
the volcanism in the Old Red basins in northwestern Europe
which represent a late Caledonian basin-and-range-like prov-
ince. The Basin and Range Province in the western U.S.A. (Hei-
ken 1971), the continental rift zones of the Upper Rhine Graben
and the Limagne Graben (de Goer et al. 1998) as well as the
main part of the Miocene Hegau Volcanic Field in the Alpine
Molasse Basin (Lorenz 1982a, Keller et al. 1990), they all show
features related to former explosive interaction of magma with
water-saturated unconsolidated sediments.
The Upper Proterozoic diatreme of Argyle, in northern
Western Australia formed when diamondiferous lamproite mag-
ma interacted explosively with thick water-saturated uncon-
solidated littoral/coastal sands and muds (Boxer et al. 1989,
Stachel et al. 1994).
The diamondiferous kimberlite pipes in northern European
Russia are surrounded by very friable and water-saturated sand-
stones of Vendian (Upper Proterozoic) age (Sinitsin and Grib
1995). At the time of the explosive phreatomagmatic activity,
probably in Devonian times, the sandstones were in all prob-
ability also unconsolidated and water-saturated. The tephra con-
tains the individual minerals from the sandstones.
The Miocene Ellendale maar-diatreme volcanoes in West-
ern Australia, discussed in more detail in the following chapter,
show gently sloping former inner crater walls within uncon-
solidated sands from the Permian Grant Group. These gently
dipping crater walls must have been the result of the soft sedi-
ment behaviour of the water-saturated unconsolidated sands
which tend to flow when an open crater is formed and thus
cannot form walls with steeper angles typical for newly formed
maar craters in much more consolidated rocks. The Ellendale
maar craters have been filled with younger pyroclastic and re-
worked deposits from the crater walls as well with lava lakes
during the active eruptive period. Maar-diatremes with craters
of such low angle walls and steep-walled diatremes are said to
be champagne glass-shaped (Smith and Lorenz 1989, Stachel
et al. 1994).
Combined hard-rock – soft-rock environments
Hard rocks overlain by soft rocks. A combined hard-rock –
soft-rock environment exists when water-saturated unconsoli-
dated sediments overlie jointed hard rocks.
In the Miocene lamproitic Ellendale Volcanic Field, uncon-
solidated sands of the Permian Grant Group overlie Carboni-
ferous and Devonian jointed shales and limestones (Smith and
Lorenz 1989, Stachel et al. 1994). The sands which still today
are water-saturated and unconsolidated caused the maar craters
to have evolved with very low wall angles and with collapse
structures and sand flows towards the crater centre. In contrast,
the underlying Carboniferous and Devonian hard rocks (shales
and limestones) gave rise to steep-sided diatremes. In almost all
of the 48 Ellendale maar-diatreme volcanoes in a second mag-
matic phase, magma of leucite lamproite or olivine lamproite
composition erupted magmatically and formed lava domes (in
the case of the more viscous leucite lamproites) or lava lakes
(in the case of the less viscous olivine lamproites). The reason
for this systematic change from phreatomagmatic to magmatic
eruptive activity must have been that during the growth of the
maar-diatreme volcanoes, the downward penetration of the dia-
treme root zones to the levels of Carboniferous and Devonian
hard rocks (of known very low water yield) prevented suffi-
cient access of groundwater to the rising magma thus causing a
change to magmatic eruptions (Smith and Lorenz 1989, Stachel
et al. 1994).
Fig. 7. A block of feldspar-bearing sandstone in the Rödern
diatreme in the Carboniferous-Permian Saar-Nahe
Basin, SW Germany (Lorenz 1971). Tephra contain-
ing juvenile ash grains and lapilli (bright in colour)
on the margin of this part of the block, following an
irregular serrated contact, shows that the presently
indurated sandstone blocks were not indurated yet
at the time the diatreme formed in Carboniferous–
Permian times.