Geology Reference
In-Depth Information
Plate 23 Discontinuities: Criteria of Submarine Hardgrounds and Condensed Carbonate Rocks
A hardground is a usually only centimeter-thick zone at the sea bottom, which is lithified to form a hardened
surface. It implies a gap or retardation of sedimentation and may be preserved as an unconformity. Common
criteria of submarine hardgrounds are sharp upper and diffuse lower contacts, mineralizations (iron hydroxides,
manganese oxides, phosphates, glauconite) of upper surfaces in the form of crusts or microstromatolites (-> 2,
4, 6; Massari 1981; Préat et al. 1999), ferruginous impregnations of grains, cements or within peloids and on-
coids, encrustations by microbes, foraminifera, serpulids, brachiopods, bivalves et al., borings and burrows in
endured substrates (-> 1), e.g. omission surfaces, condensed fauna, lithoclasts resulting from intraformational
reworking of hardgrounds, reworked hiatus concretions, interlayers of marine sediment and cements, dissolu-
tion features, e.g. steinkerns of primarily aragonitic shells. Hardgrounds originate in 'deep-marine' pelagic envi-
ronments (slopes, basins, ridges) as shown in the examples on this plate, but are also abundant on the sea
bottoms of shelves and ramps.
1
Break in sedimentation.
A fine-grained biomicrite (BM) is covered by dark red argillaceous microbioclastic micrite,
penetrating into a microrelief. Some skeletal grains enter into the lower region as well. Bioturbation (right upper corner)
is shown by the whorled alignment of shell fragments. Deposition of the dark sediment probably took place subsequent to
borings by bioeroding organisms (cf. Ekdale and Bromley 2001) into hardgrounds or firmgrounds. Differentiation of
hardgrounds and firmground borings requires criteria such as the shape of endolithic structures (better seen in the field
than in non-oriented thin sections as in this example) and the type of the boundary between the host sediment and the
borings (sharp, truncating fossils or discontinuities surfaces, or indistinct). A break in sedimentation is indicated by
accumulations of glauconite grains. These '
Orthoceras
limestones' represent a condensed ca. 50 m thick pelagic sedi-
ment, deposited under relatively cool-water marine conditions, on the Baltic Shield, extending one-half million square
kilometers (Lindstrom 1963). The rocks are predominantly micritic, fossiliferous and thinly bedded. Complex ichnofabrics
indicate that the sea floor ranged from softground to firmground to hardground. Early Ordovician (Limbata limestone,
Arenigian): Öland Island, southern Sweden.
2
Hardground:
The echinoderm fragments in the bottom half of the picture have been dissolved along a discontinuity and
subsequently truncated. Hardground criteria: Limonitic cauliflower structures (CS, 'deep-water stromatolite') above the
discontinuity surface. Decimeter-sized, dome-shaped microbial microstromatolites growing on discontinuity surfaces,
lithoclasts or shells, are common features of deep-marine pelagic limestones (Massari 1981). Early Jurassic (Adnet lime-
stone): Adnet near Salzburg, Austria. The Adnet limestone is an example of widely distributed Tethyan 'deep-water
limestones' generally summarized as 'Ammonitico rosso' because of the common occurrence of ammonites and the red
color of the rock (see
Pl.
139). This facies represents the post-drowning facies of 'pelagic carbonate ramps'. The Adnet
limestone originated on drowned late Triassic carbonate platforms in upper and lower slope settings.
3
Hardground:
Fe/Mn mineralizations upon and within mm-sized columns constructed by agglutinated sessile foraminifera
(
Placopsilina
d'Orbigny, arrows) growing on lithified microbial substrates and forming minute 'reef' structures (Wendt
1969). Encrusting agglutinated foraminifera are abundant in modern manganese crusts and nodules. Late Triassic (Hall-
statt limestone, Carnian): Feuerkogel near Bad Aussee, Styria, Austria. The Hallstatt limestone originated on the top of
fault-bounded ridges in water depths of several hundreds of meters.
4 Detail of
'deep-water stromatolites'
with abundant sessile foraminifera (arrows). Late Triassic (Carnian): Feuerkogel
near Bad Aussee, Styria, Austria.
5
Limonitic
(brownish or yellowish) '
cauliflower structure
' (originating by migration of colloidal Fe-solutions, CS) into a
cavity below a discontinuity surface (arrows). Note differences in sediment composition: Gray biomicrite with foramin-
ifera and shells (GB), and red fine-bioclastic biomicrite with ostracods and other skeletal debris (RB) also occurring in the
cavity below the discontinuity (arrows). E - echinoid spine. Late Triassic (Norian): Hallstatt, Upper Austria.
6
Pelagic hardground.
Note the thin upward growing stromatolitic Fe-Mn crust (arrow). Post-drowning micrites above a
sunken atoll. Late Cretaceous?: Isakov Guyot, northwestern Pacific.
7
Hardground
at the top of a drowned carbonate platform. Microbial Fe-Mn microstromatolite (arrow) growing perpen-
dicular to the wall of a dissolved component within a completely phosphatized rock. Middle Cretaceous (Late Albian):
Charly Johnson Guyot, northwestern Pacific.
8
Phosphatization:
Resedimented hemipelagic mudstone clasts with Fe-Mn crusts in a phosphorized matrix. Clasts were
transported by submarine currents prior to the formation of the crusts. Cretaceous: Heezen Guyot, northwestern Pacific.
-> 6, 7: Grötsch and Flügel 1992
