Geology Reference
In-Depth Information
intensity, rates of sedimentation and turbulence levels.
Common encrusters restricted to hard substrates includ-
ing hardgrounds are encrusting calcareous algae, fora-
minifera - Pl. 23/4, bryozoans, corals, serpulid poly-
chaetes, cirripeds, and some brachiopods. Note, that
some encrusting organisms occurring on hard substrates
can also settle on firm substrates (e.g. some oysters,
crinoids and brachiopods). Bryozoans have proved to
be of particular value for differentiating Cretaceous
hardgrounds (Voigt 1968). Some authors consider the
existence of only thin biogenic crusts as an indication
of relatively short omission periods.
Borings:
Macro- and microborings (Pl. 52) are com-
mon along the upper surface of many hardgrounds (Pl.
22/4). Borings provide sedimentological information
about the consistency of the ancient sea floor as well
as paleoecological information about environmental
and depositional conditions (Ekdale 1985; Bromley
1994; Ekdale and Bromley 2001). Borers restricted to
hardgrounds and other hard substrates (e.g. rock
cobbles, shells, coral colonies) include some bivalves,
phoronids, sponges, algae and fungi. Several animals
can penetrate both firm and cemented substrates (e.g.
bivalves and polychaete worms).
Burrowing:
The strata underlying and overlying the
hardground are characterized by specific associations
of trace fossils indicating the changes in substrate types.
Changes from firmground to hardground stages are re-
flected by the temporal relationship of bioturbation, bor-
ing and encrustation.
Clasts of cemented limestones:
Lithoclasts similar
in composition to the hardground occur above the up-
per surface, along the discontinuities or as infill of de-
pressions. The clasts have sharp edges and are often
bored, encrusted or coated with Fe/Mn crusts (Pl. 23/
8). These clasts (e.g. 'subsolution lithoclasts', Sect.
Plate 22 Discontinuities and Unconformities
Recognizing discontinuities and unconformities is a major goal in microfacies studies, because unconformities
represent a key element in sequence stratigraphy and subaerial disconformities have a major impact on the
porosity and permeability of carbonate rocks. Stratigraphic discontinuities manifest breaks in sedimentation at
unconformities in which bedding planes above and below the break are essentially parallel ('disconformity').
Interruptions can result from various processes including erosion, significant reduction in sedimentation rates,
non-deposition, bioerosion, or dissolution below or above carbonate compensation depths. Disconformity sur-
faces are often connected with variously colored condensed beds and hardgrounds. The latter denote a subma-
rine sediment surface that became lithified in the ambient depositional environment before the next sediment
layer was deposited. Hardgrounds are the result of submarine cementation by aragonite or Mg-calcite precipita-
tion directly from seawater circulating through the uppermost part of the sea floor. They are formed in various
marine settings (e.g. supratidal and intertidal, shallow subtidal, shallow and deep slopes, deep open-marine
environments) but also originate in subaerial settings (e.g. on emersion surfaces). Because of the abundant hiati
in deposition and rock-stratigraphic record, very thin condensed beds can represent very large time spans (100 000s
to millions of years).
1
Hardground
(arrows) separating deep-marine quartz-bearing wackestones with filaments and a wackestone characterized
by phosphoritized bioclasts and a glauconite-bearing micrite matrix. The discontinuity surface is marked by Fe/Mn min-
eralization. Deep subtidal shelf limestones. Jurassic (Callovian-Oxfordian): near Pamplona, Spain.
2
Phosphoritized ammonites
, indicating solution and replacement of the aragonitic shell by phosphate during times of
starved sedimentation rates. Most glauconite grains within the micritic matrix are replaced by Fe-calcite. Deep subtidal
shelf limestones. Jurassic (early Oxfordian): Ricla southwest of Zaragoza, Spain.
3
Hardground
in a condensed shelf limestone with ferruginous ooids (O). The concentric layers consist of goethite, the
nucleus of broken ooids, lithoclasts and quartz grains. Note the encrusting foraminifera (F). Jurassic (Early Oxfordian):
near Pamplona, Spain.
4
Hardground.
Distinct borings within an erosional relief were filled with glauconitized peloids. Deep shelf. Jurassic (Ox-
fordian): near Pamplona, Spain.
5
Microfacies
about 50 cm below a distinct disconformity separating two rock units. The thin section exhibits a cavity
within a foraminiferal
limestone. The cavity walls crosscut the matrix and some of the foraminifera (
Lepidocyclina
,
arrows) indicating that the cavity was formed by dissolution below the discontinuity plane, after lithification of the rock.
The cavity is filled with laminated mudstone and skeletal wackestone. The uppermost layer contains imbricated
Lepido-
cyclina
tests. Tertiary (Brac Formation and Cayman Formation, Oligocene/Miocene): Cayman Brac Island, British West
Indies.
-> 1-4: Behnke 1981, 5: Jones and Hunter 1994
