Geology Reference
In-Depth Information
Fig. 16.10.
'Cement reef '.
This reef limestone is characterized by the dominance of only two main constituents - cement and
encrusting reefbuilders. Reef limestones formed by aggregates of serpulid worms (S), green (?) algae (A) and volumetrically
dominant synsedimentary carbonate cements correspond to a specific reef type and denote specific environmental condi-
tions. Laminated cement crusts (C), probably caused by microbial precipitation, provide substrate for encrusting organisms,
e.g. microbes, sponges or serpulids. Rapid synsedimentary cementation imparts extra strength and stability to reefs. The
microfacies of the sample is characteristic of meter- to decameter-sized mounds formed in particular paleogeographic loca-
tions in the Late Triassic of the western Mediterranean Tethys. Disaerobic, mesohaline and eutrophic conditions have been
considered as important controls for this reef type. Cement reefs were common in Permian and Triassic, but are also known
from other time intervals (e.g. Cambrian and Precambrian). Late Triassic (Norian): Cahorros de Monachil, Sierra Nevada
near Granada, Betic Cordillera, Southern Spain. After Flügel et al. (1984).
16.2.5.2 Describing Reef Carbonates:
A Practical Guide
Mountains of New Mexico was a key for the creation
of mud mound models, but as Lloyd Pray said, 'there
is an enormous gulf between seeing and perceiving'.
The Muleshoe model has been remodeled again and
again. The same happened to the world-famous Capitan
Reef and the Permian Reef Complex in Western Texas
and New Mexico.
Mounds:
The term 'mud mound' was originally used
for biodetrital carbonate bodies in Florida Bay, whose
origin is related to baffling of sediment by marine
grasses and a surplus of carbonate production by
epibiontic organisms. Other modern carbonate mud
mounds represent hydrodynamic accumulations of mud
produced by various algae and the breakdown of skel-
etal material. The formation of these mounds is not
controlled by framework-building or accretionary or-
ganisms.
Phanerozoic mud mounds are found in various set-
tings, including ramp, foreslope, deeper shelf and sea-
mount positions. They generally consist of more than
Box 16.4 summarizes the criteria of reef limestones
that should be looked for carefully in thin sections and
larger rock samples when evaluating the formation of
ancient reefs and their diagenetic history.
16.2.6 Case Studies of Some Ancient Reefs
The case studies demonstrate diagnostic microfacies
criteria of well-known ancient mud mounds and reefs
and discuss the problems involved in interpreting the
formation of reefs and reef environments. Different ge-
netic interpretations are possible for the spectacular
Devonian Kess Kess mounds in Morocco, depending
on the weighting given to microfacies, paleontological
features or geochemical data. The Early Carbonifer-
ous Waulsortian Muleshoe Mound in the Sacramento
