Geology Reference
In-Depth Information
50% fine-grained carbonate mud (micrite and peloidal
micrite, silt-sized carbonate). In contrast to modern mud
banks, many of the Paleozoic and Mesozoic mud
mounds are characterized by organically formed tex-
tures, originally called 'cryptalgal' and today 'micro-
bial', stressing the predominant role of procaryotic or-
ganisms (e.g. bacteria) and eucaryonts. Siliceous
sponges, various 'crusts' as well as mm- to cm-sized
sediment- and spar-filled cavities (stromatactis) are
commonly associated with these textures (see Reitner
and Neuweiler 1995). Frame-building and baffling or-
ganisms seem to be of minor importance in the forma-
tion of most ancient mud mounds.
Pivotal to the understanding of the origin of ancient
mud mounds is the genetic interpretation of the micro-
crystalline carbonate (automicrite and/or allomicrite;
see Sect. 4.1.2) and of the processes controlling the
formation of these micrites. Mud mounds are formed
by very different biological processes triggering the ac-
cumulation of fine-grained sediment (Pratt 1995). Ma-
jor controls on the formation of mud mounds are the
shelf configuration (position in mid ramp to outer ramp
part of homoclinal settings; imported mud can be dis-
tributed toward the mud mounds), reduced allochtho-
nous sedimentation rates during rising sea level (trans-
gression and early highstand), increased alkalinity of
seawater facilitating calcification and allowing low to
moderate grazing (facilitating growth of biofilms and
mats).
16.2.6.1 Mud Mounds: The Early Devonian
'Kess Kess' Mounds in the AntiAtlas, Southern
Morocco
The Kess Kess mounds are unique in comparison to
other Paleozoic mud mounds. Many studies indicate
that Paleozoic mound accretion is biologically con-
trolled, either by sponges or microbes that may induce
or trigger CaCO
3
precipitation, or by delicate mud-trap-
Plate 143 Mud Mounds: The Early Devonian 'Kess Kess' Mounds in the AntiAtlas, Southern Morocco
Spectacular conical mounds crop out in the Hamar Lakhdad Ridge, 15 km southeast of Erfoud in the Tafilalt
province. Mound and intermound carbonates are crudely bedded. They differ only slightly with regard to biotic
composition and microfacies. Dominating texture types are lime mudstone and skeletal wackestone and pack-
stone with crinoids, trilobites, tentaculitids and brachiopods and a scarce tabulate coral fauna consisting of
auloporids, a few thamnoporids and favositids. Stromatactis-like spar-filled cavities and sheet cracks occur in
the mound and in the intermound facies. Neptunian dikes crosscut the mound and the intermound facies. The
mounds show no differentiation into specific core and flank facies. Intercalations of skeletal rudstones and
grainstones are interpreted as storm-controlled eventstones. Slumping criteria are absent.
1
Mound facies.
Densely packed crinoid wackestone. The role of benthic organisms for the origin of the mounds was most
probably that of a parautochthonous accumulation of bioclastic sediment produced by the degradation of crinoids, trilo-
bites and thamnoporid corals.
2
Tentaculitid wackestone
with trilobite fragments. Intermound facies. Tentaculitids increase in frequency distally from the
mounds toward the more pelagic open-marine basin.
3
Bioerosion
is weak, but some boring occurs in larger crinoid columnals. Skeletal grains other than crinoids are trilobite
fragments and a few auloporids.
4
Trilobite lumachelle and calcite cements
. Bioclastic rudstones intercalated both in the intermound and the mound facies
are interpreted as high-energy current deposition caused by storm events. Evidence of storm deposits are significant
breaks in the size of skeletal grains from bed to bed. Fine-grained sediment is winnowed and interskeletal pores are filled
with marine radiaxial cement.
5
Open-space fabric.
Spar-filled, laterally elongated cavities occur both in the intermound and the mound facies, but are
more common in the mound facies. Many of these stromatactoid cavities have been enlarged by dissolution between
skeletal grains. The structures range from millimeters to tens of centimeters laterally.
6
Open-space fabric.
Typical stromatactis cavity filled with basal micritic sediment and recrystallized sparry calcite. These
millimeter- to centimeter-sized cavities are interpreted very differently (see Box 5.2). Following the hydrothermal venting
model, the cavities might have acted as conduits. Mound facies.
7
Tabulate corals.
Auloporid wackestone, a common microfacies of the mound carbonates.
8
Encrusting corals.
Auloporids encrusting echinoderm fragment. Auloporids preferred firm and hard substrates; their
occurrence provides a clue to rapid synsedimentary lithification of the fine-grained sediment of the mounds.
9
Auloporid corals and carbonate cement.
Auloporids seem to have acted as initial points for early marine cementation.
This is indicated by the frequent patches consisting of densely spaced auloporids surrounded by radiaxial and fibrous
carbonate cements.
