Geology Reference
In-Depth Information
ping and sediment-stabilizing organisms, e.g. bryozo-
ans. The Early Devonian Kess Kess mounds in south-
ern Morocco do not fit into this model.
The cone-shaped, steep-sided mounds occur on the
top of a volcanic massif. The mounds are confined to a
0.5 x 3.5 km area above a basaltic high enplaced dur-
ing the earliest Devonian above a shale-dominated Or-
dovician-Silurian succession. The upper surface of the
volcanic high was colonized during the Pragian to Early
Emsian time by crinoids that formed an up to 180 m
thick cover of bioclastic sands and gravels. Mounds
and intermound facies developed on top of the crinoid
beds during the Late Emsian, and were buried by Late
Emsian siliciclastic-rich mud.
thermal venting related to thermal flux above the vol-
canic massif (Mounji et al. 1998) or by carbonate pre-
cipitation from brines comprising a mixture of hydro-
thermal fluids and seawater, associated with the aero-
bic oxidation of thermogenic methane (Belka 1998).
Inorganic carbonate mud production might be possible.
Most mounds developed over crosspoints of radial and
tangential faults resembling a network of thermally
induced joints. These faults could have served as con-
duits for the migration of hydrothermal fluids, which
might have had a positive effect on providing a nutri-
tional source.
16.2.6.2 Waulsortian Mud Mounds: Early
Carboniferous (Lower Mississippian) Muleshoe
Mound, Sacramento Mountains, New Mexico,
U.S.A.
Key questions for understanding the formation of
the Kess Kess mounds are: Why were the mounds
formed where they are? What stimulated and controlled
the carbonate mud production? Which factors con-
trolled the shape of the mounds? And if there was a
control on mound development by hydrothermal vent-
ing, how did venting affect or even kill mound organ-
isms?
The beautifully exposed mounds (Fig. 16.11) have
been studied with respect to field data (mapping, paleo-
current measurements using orthocone nautilids), dis-
tribution and structure of the mounds, biota and micro-
facies (Brachert et al. 1992; Hüssner 1994) and with
regard to isotope geochemistry (Belka 1998; Mounji
et al. 1998). The paleoenvironment of the mounds is
interpreted as deep-water, below fair-weather wave base
but within the range of major storms. The absence of
algae and stromatoporoids indicate non-photic condi-
tions.
The Muleshoe Mound near Alamogordo in New
Mexico is a well-studied example of a 'Waulsortian'
mound (Pl. 144).
Waulsortian banks and mounds
(named after
Waulsort, a locality in Belgium) with tabular, knoll-
and sheet-like geometries developed during mid-
Dinantian (Late Tornaisian to Early Viséan) times in
many parts of the world, mainly in central Europe and
in North America.
Mounds and banks are lime mud-dominated. Car-
bonate muds are ' polymuds', e.g. they include both
automicrites and allomicrites. Most authors assume a
strong microbial contribution in mud formation. Di-
agnostic microfacies criteria are skeletal grains, non-
skeletal grains (predominantly peloids), a wackestone
and packstone matrix, and coarse sparry fabrics within
masses of various size and forms (some of which cor-
respond to stromatactis). The facies is characterized
by three end-members: Fenestrate bryozoan fronds bur-
ied in wackestone or coated with spar, wackestones
with or without 'sparry masses' (some open-space
structures) and crinoid-rich packstones. Major biota,
in order of decreasing volumetric importance, are
crinoids, fenestrate and other bryozoans, brachiopods
and mollusks. Note that the dominance of specific
groups (e.g. crinoids and bryozoans in the Waulsor-
tian mounds) is not necessarily a proof that intensified
growth of these organisms significantly influenced
mound formation (Ahr and Stanton 1994). Calcareous
algae are almost absent, but microbial filaments may
be present. Simple foraminifera are ubiquitous, but
plurilocular forms are absent or rare and restricted to
the shallow upper aphotic and the photic parts of the
Different genetic interpretations have been dis-
cussed, depending on the weighting of these criteria:
• The model derived from the microfacies approach
favors mound formation by a self-sustaining interplay
of physically induced changes in the environment (bot-
tom currents, storms) and intrinsic biological factors
(preferred accumulation of skeletal material in the
mound area because of the increased growth of crinoids
due to better nutrient access). The mounds developed
on shoals originating from the transport of sediment
by storms. Carbonate mud production is explained by
the degradation of skeletons. Stabilization of mound
flanks is explained by rapid synsedimentary marine ce-
mentation starting with the early cementation of
auloporid corals. The asymmetry of the mounds is in-
terpreted as a result of shaping by bottom currents.
• Geochemically-based models explain the formation
of microcrystalline carbonate in the mounds by hydro-
