GeoLines
15 (2003)
78
GeoLines
15 (2003)
79
The Cretaceous to Tertiary Kimberlite Pipes in the NW
Territories, Canada, formed to a large extent in an area where
the crystalline basement rocks were overlain by Cretaceous
to Lower Tertiary mudstones, sediments from the Western In-
terior Seaway and its lacustrine successor (Graham et al. 1998,
Nassichuk and Dyck 1998), the youngest mudstones of which
were probably still in an unconsolidated water-saturated state
during volcanic activity. Likewise the Pliocene nephelinitic
Hopi Buttes in Arizona formed in an area where the sediments
of Mio-Pleistocene Bidahochi Formation, partly deposited syn-
eruptively, consisted of unconsolidated water-saturated mud-
rocks (White 1991, 2000) and were underlain by Triassic sand-
stones and crystalline basement. Again the activity at the many
vents started phreatomagmatically, producing maar-diatreme
volcanoes, and then terminated when magma intruded the dia-
tremes and rose into the maar craters (White 1991, 2000).
In these volcanic fields in the Northwest Territories and
in Arizona the unconsolidated water-saturated muds overlying
the jointed hard rocks may have become liquefied during syn-
volcanic earthquakes and explosive volcanism releasing water
to interact with the rising magma.
Soft rocks interbedded with hard rocks. Another com-
bined hard-rock – soft-rock environment is formed when joint-
ed lava flows or sills of synsedimentary volcanic activity are
interbedded with unconsolidated water-saturated sediments.
Examples of this environment are the Carboniferous-Permian
Saar-Nahe Basin in southwest Germany and the Carboniferous
Midland Valley in Great Britain. As it has been pointed out
above, the unconsolidated sediments penetrated by the maar-
diatreme volcanoes appear in the form of individual minerals
and individual pebbles. The interbedded hard rocks, i.e., the
volcanics of the interbedded lava flows and sills, occur in the
pyroclastic rocks of the maar-diatreme tephra as volcanic rock
clasts, i.e. accidental lapilli and blocks.
When the diatreme fill and proximal or distal maar tephra
of old volcanic fields contain only the individual minerals and
pebbles from the neighbouring sediments, it may be argued that
the explosive activity caused complete disintegration of the
possibly indurated sediments into their mineral and rock clast
components. Judging from the situation in clear-cut synerup-
tive hard-rock environments, where rock clasts are always in-
corporated in the pyroclastic deposits, and from impact craters
in hard-rock environments where rock clasts also always occur
in the ejecta rim, no explosion reduces hard rocks to their indi-
vidual constituents.
Environments with normal and highly permeable
aquifers
Normal aquifers. With respect to their hydraulic productivity,
both the hard-rock and soft-rock environments are frequently
more or less normal aquifers: here, normal groundwater condi-
tions are considered to exist when a well test would result in
fast or slow formation of a cone of depression of the ground-
water table. Under such normal groundwater conditions, a
maar-diatreme volcano can form because only under these con-
ditions does downward penetration of root zones seem possible
(Lorenz 1985, 1986, 1998).
Highly permeable aquifers. In contrast, both types of
aquifers can be very highly permeable, e.g., in coastal environ-
ments where intensively jointed basalt lava flows, coral reefs,
or former karstic limestones may be penetrated by the seawater,
such as on Oahu where near-coastal rise of magma through
such highly permeable hard rocks gave rise to the tuff-rings and
tuff-cones of the Honolulu series. Two tuff rings of Hverfjell
and Ludent occur in a similar but non-marine environment of
intensively jointed basaltic lava flows close to Lake Myvatn
(Lorenz 1986). Highly permeable gravel deposits occur along
many rivers. Examples of such highly permeable water-satu-
rated gravel beds are the tuff rings of the Menan Buttes at the
Snake River, Idaho (Hamilton and Myer 1963, Lorenz et al.
1970, Lorenz 1985, 1986), and many tuff rings along the riv-
ers in central Iceland. The rise of magma into such water-rich
soft-rock environment also leads to the formation of tuff-rings
which ideally contain only a few percent of clasts of country
rock. The highly permeable and water-rich environments do
not allow a pronounced downward penetration of the root zone.
These phreatomagmatic volcanoes therefore lack a pronounced
diatreme formation by repeated collapse into a downward pen-
etrating root zone with a pronounced subsidence crater. A maar
crater does not form in this kind of environment but instead
a tephra ring or a tephra cone develop, surrounding a wide
or small crater above general ground overlying possibly only
a small diatreme, i.e., the so-called tuff rings and tuff cones.
Even more external water is available in lacustrine and shal-
low marine environments. A typical example is Surtsey Island
which formed off the coast of southern Iceland in the North
Atlantic where the sea floor was lying at a depth of 130 m
(Thorarinsson et al. 1964). These water-rich environments lead
to formation of more wet or moist deposits than in ordinary
maar-diatreme environments, i.e., more vesiculated tuffs and
accretionary lapilli are formed (Lorenz 1974).
Diagenesis of diatreme fill in hard-rock and soft-
rock environments
After the eruptions of a maar-diatreme volcano have ended, the
permeable clastic diatreme fill becomes water-saturated and, as
indicated above, the crater will also fill with water up to the
level of the local or regional groundwater. Then, diagenesis
starts being enabled by the large surface area of the clastic dia-
treme fill of many clasts of country rock, of unstable glass and
high-temperature mineral association of the juvenile clasts.
Diagenesis of diatreme fill in hard-rock environment. In
hard-rock environments, the diatreme, up to 2–2.5 km deep and
of respective diameter, represents a huge volume, emplaced
within weeks to years in the consolidated hard rocks but itself
consisting of this highly unstable mixture, will undergo an ex-
tended diagenetic process chain including hydration, compac-
tion and water escape. Thus for an extended period of time the
diatreme fill will go through these diagenetic processes and,
because of its conical shape, the fill will subside differentially.
Marginal intra-diatreme rocks such as tephra beds will obtain
steep inward dips, and internal differential faulting will also