Geology Reference
In-Depth Information
Fig. 5.10.
Based on thin-section observations of Early and Middle Jurassic limestones. Note variously preserved ammonite
shells, various-sized Fe/Mn nodules and sutured discontinuity surfaces above the hardground. The latter exhibits typical
micrite-filled microborings covered by microbial microstromatolites that are often impregnated with Fe and/or manganese
oxides. The crusts alternate with micrite layers. A common feature of these crusts in Triassic and Jurassic limestones are
attached foraminifera. After Wendt (1970).
hibiting specific trace fossil association on omission
surfaces (Bertling 1999).
• Reworked hardgrounds and occurrence of poorly
sorted lithoclasts.
• Bored and encrusted, early diagenetic, clay-hosted
concretions (Hesselbo and Palmer 1992).
• Ammonite pavements, occurring together with man-
ganese and iron crusts. Primarily aragonitic cephalo-
pod shells are preserved as calcite spar, micrite or goe-
thite pseudomorphs. High percentages of broken or
partly dissolved shells (Pl. 90/3).
• Highly variable microfacies types occurring only
within a distance of a few centimeters and ranging from
mudstones, wackestones, packstones and grainstones
to floatstones and rudstones.
• Common microbiota: Attached foraminifera, ser-
pulids, small gastropods, thin-shelled bivalves, and os-
tracods. High concentrations of planktic and benthic
fossils (sometimes constituting mixed assemblages
(Gehring 1986)
• Authigenic minerals (glauconite, phosphorite, sid-
erite etc.).
• Ferruginous 'microstromatolite' crusts (Fig. 5.10),
nodules and Fe-ooids (Pl. 22/3): Goethite crusts (mas-
sive or consisting of up to 50
m thick laminae, alter-
nating with calcitic layers and forming flat to dome-
shaped digitate structures. Centimeter-sized goethite
oncoids (Sect. 4.2.4.1). Nuclei are lithoclasts from ad-
jacent hardgrounds or rockgrounds. Laminae contain
large amounts of sessile foraminifera. Oncoids occur
isolated or densely packed within a single layer.
• Manganese crusts and scattered manganese nodules
(Banerjee and Iyer 1991).
• Phosphatic 'microstromatolites', composed of strati-
form and columnar sheets consisting of alternating dark
(phosphatic, with microbes) and light laminae (non-
phosphatic, sediment). Phosphoritized calcareous shells
(Pl. 22/2). Current induced condensation accompanied
by phosphogenesis is often associated with platform
drowning (Föllmi 1989).
•
Accumulations of organic matter.
The history of many condensed units often exhibits
a three-stage sequence:
(1) Reduced sedimentation stage, recorded by shell
concentrations (e.g. ammonite pavements), bored and
encrusted bio- and lithoclasts, reworked iron crusts,
sometimes indication of current transport,
(2) Omission stage, characterized by irregular, bored
and encrusted hardgrounds as well as scoured surfaces
exhibiting truncation of pre-omission burrows. Rework-
ing breccias and large cavities may be common,
(3) Fe and Fe/Mn crust stage.
Time involved in deep-marine condensation horizons
and hardgrounds
Geological condensation refers to a long-lasting
break in the rock-stratigraphic record commonly asso-
ciated with a hiatus (Fig. 5.11). Very thin rock units
may represent very large timespans ranging from hun-
dreds of thousands to several millions of years. Excel-
lent case studies dealing with the processes and the time
involved in the formation of condensed carbonate se-
quences in the Jurassic of Spain give some idea of the
immense time spans associated with deep-marine con-
densation (Seyfried 1978, 1980, 1981; Fels and Seyfried
1993; Fels 1995). The authors described an instructive
example showing that at least 10 million years may be
concentrated in a one-meter thick condensed carbon-
ate succession.
Because the carbonate microfacies types were de-
posited relatively rapidly, most of the time should be
hidden within the goethite crusts, hardgrounds and the
erosional discontinuities.
