Geology Reference
In-Depth Information
in the formation of hardgrounds is unclear, Shinn (1969)
reported antique Greek pottery incorporated into hard-
ground of the Persian Gulf (a few thousand years).
Hardgrounds are common but not confined to
deeper-marine settings. They also occur in the terres-
trial onshore and estuarine zone (tropical lateritic hard-
pans) throughout supra- and intertidal (beachrocks) and
various marine regions. Marine settings include shal-
low subtidal environments (Pl. 127/4), off-platform pe-
lagic settings, shallow slopes (Mullins et al. 1980), and
deep basins. Holocene hardgrounds have been de-
scribed from the Bahamas (Dravies 1979), the Persian
Gulf (Shinn 1969), the Red Sea, and the Mediterra-
nean Sea (Aghib et al. 1991; Bromley and Allouc 1992).
The majority lies less than a few tens of meters under
water. In the Bahamas and in the Persian Gulf the de-
velopment of hardgrounds is probably related to sur-
face water saturation relative to calcium carbonate. In
these shallow-water environments, cessation of sedi-
mentation and development of hardgrounds can form
in the space of a few months. Some of the hardgrounds,
found in oceanic settings, occur in great depths, some-
times on oceanic ridges and guyots (Pl. 23/6-8), be-
tween several hundreds of meters to more than 3000
meters (Fischer and Garrison 1967; Bernoulli and
McKenzie 1981; Malfait and Van Andel 1980).
Fig. 5.9.
Common criteria of carbonate hardgrounds.
Modi-
fied after James and Choquette (1983).
3, 6). The composition, form and intensity of mineral-
ization depend on the environment: Shelf hardgrounds
are usually mineralized by glauconite (occurring within
grains, burrows - Pl. 22/4, borings and cracks), cal-
cium phosphate salts (forming phosphate cements as
well as phosphatic stromatolites - Pomoni-Papaioannou
1994) and iron hydroxides (goethite, limonite), whereas
pelagic hardgrounds in addition also contain a high per-
centage of manganese oxides (Fürsich 1979; Lindström
1979; Marshall and Ashton 1980). In some hard-
grounds, calcium carbonate has been replaced by baryte
(Lindström 1979).
Ferruginous crusts covering hardgrounds frequently
consist of irregular laminae of iron hydroxide, occa-
sionally forming a cauliflower structure (Pl. 23/5), al-
ternating with laminae of calcareous skeletons of en-
crusting organisms (e.g. foraminifera - Pl. 23/3, 4, ser-
pulids). Current interpretations favor a strong micro-
bial control on the formation of laminated Fe-Mn crusts.
In-place dissolution within microbial carbonates with
thin limonitic laminae is explained by a process in
which iron crusts acted as a cathode, sea water as an
electrolyte, and the underlying carbonate as an anode
(Videla 1989; Reitner et al. 1995). The existence of
limonite crusts on hardground surfaces is often regarded
as indication of relatively long omission phases and
low-energy hydrodynamic conditions.
Biogenic encrustations:
The upper surfaces of hard-
grounds are frequently encrusted by organisms who
require a hard substrate to settle. Encrusting bryozo-
ans, brachiopods and oysters occur also on the lower
surface of hardgrounds. Differences in encrustation in-
dicate the polarization of the fauna according to light
Recognizing ancient hardgrounds
Fig. 5.9 summarizes some of the most common fea-
tures of hardgrounds. Important criteria are:
Contacts:
Sharp upper contact and diffuse lower
contact. Grains and cement crystals may be truncated
along the upper contacts (Pl. 23/2). Upper surface some-
times has a thin micrite rind. Some hardground sur-
faces are covered by micritic microbial crusts. The
overlying sediment may infill cavities (Pl. 22/5), bor-
ings or undercut recesses in the hardground.
Surfaces:
The upper contact is characterized by
smooth or irregular surfaces. Many surfaces exhibit ir-
regular protuberances separated by pits and grooves
(Pl. 23/3). Smooth planar surfaces are formed by abra-
sion and are more common in shallow-marine high-
energy environments. Angular surfaces related to so-
lution processes are more common in deep-marine shelf
and basinal settings. The upper surface of shelf hard-
grounds may be buckled into tepee-like expansion
ridges or may show signs of corrosion. The lack of en-
crustations on these surfaces indicates relatively short
omission phases and high-energy conditions.
Mineralization:
Uppermost layers may be mineral-
ized in the form of crusts or as impregnations (Pl. 23/
