Geology Reference
In-Depth Information
sence of larger paleokarst solution cavities is explained
by the formation of epikarst underneath a soil cover
during seasonally humid climates (Wright 1994;
Mylroie and Carew 1995) and limited duration of sub-
aerial exposure (d'Argenio et al. 1997). Other karst fea-
tures are the gravitational infill of cavities by stained
and reworked lithoclasts and clayey calcareous silt in
association with micrite crusts and stalactite cements,
• Pedogenic features (Pl. 128/4, Pl. 141/9, 10): Dis-
solution processes and rhizoturbation during exposure
as well as erosion and reworking of the soil during the
subsequent marine flooding, can produce a brecciated
to pebbly appearance of exposure surfaces. Other cri-
teria are microscopic root molds, soil pisoids and cal-
crete crusts (Sect. 15.1.1). The absence of a vertical
zonation of pedogenic fabrics at these surfaces points
to generally short-lived exposures.
All these methods have limits and pitfalls and must
be combined with a detailed analysis of sequence
stratigraphy, biostratigraphy, facies and diagenetic his-
tory in a regional context (Saller et al. 1994; Fouke et
al. 1996).
• Caverns, breccia and vugs can form in deep subsur-
face because of dissolution by basinal fluids or the con-
trols of shales overlying carbonates.
Ore deposits:
Emersion surface-hosted ore depos-
its associated with paleokarst are known from the Pa-
leozoic and Mesozoic (Parnell et al. 1990; Scott et al.
1993; Ruffell et al. 1998).
Pb-Zn-Fe sulfide mineralizations are common be-
low and/or along and above subaerial exposure surfaces
of carbonate platforms. One such example, interesting
in economic terms, is the 'Metallifero' in the south-
western part of Sardinia, a Cambrian carbonate com-
plex, unconformably overlain by Ordovician conglom-
erates (Fanni et al. 1981; Bechstädt and Boni 1994).
Other examples are emersion-bound mineralizations at
Devonian-Carboniferous and Triassic disconformities
in the Alps (Assereto et al. 1976; Hentschel and Kern
1992).
5.2.4 Microfacies Criteria and Significance
of Condensation Surfaces and Hardgrounds
Practical significance of exposure surfaces and sub-
aerial unconformities:
Reservoir rocks:
Identifying and predicting subaerial
exposure surfaces is essential for understanding the
modification of porosity beneath sequence-bounding
unconformities and parasequence disconformities on
carbonate platforms, the reservoir degradation below
subaerial unconformities, and the development of per-
meability patterns (Read and Horbury 1993; Saller et
al. 1994; Budd et al. 1995; Wagner et al. 1995).
Controls of subaerial unconformities on porosity and
permeability include the following points:
• Carbonate diagenesis during subaerial exposure re-
arranges the pore network, but does not necessarily in-
crease total porosity. In many cases porosity is reduced
under unconformity, but remarkable exceptions exist,
• Permeability is more strongly changed by subaerial
discontinuities than porosity, and may increase or de-
crease,
• Pore systems evolve with the time of subaerial ex-
posure. Shorter periods (10 000-400 000 yr) are often
associated with higher porosity than long intervals of
subaerial exposure (1-20 million yr),
• Many carbonates subjected to subaerial exposure
have little or no porosity in the deep surface due to
compaction, cementation and stratal collapse reducing
burial porosity. But unconformity-related diagenesis
may also enhance reservoir porosity by creating pore
systems that are resistant to deeper burial compaction,
Stratigraphically reduced marine carbonates originate
from very low sedimentation and alternating deposi-
tion and erosion, leading to the formation of condensed
sequences. Modern oceanic equivalents have been iden-
tified on seamounts. Breaks in the sedimentation caused
by non-deposition result in the formation of hard-
grounds both in shallow-marine settings and deep-ma-
rine oceanic environments. In many ancient open-ma-
rine carbonates hardgrounds and condensed sequences
are intimately associated.
5.2.4.1 Hardgrounds
Hardgrounds are centimeter-sized discontinuous
surfaces of synsedimentary lithification, having existed
as hardened sea floor prior to the deposition of the over-
lying sediment. They are related to a combination of
non-deposition or low sedimentation rates, and con-
densation. Hardgrounds denote a submarine sediment
surface that became lithified in the ambient depositional
environment before the next sediment layer was de-
posited (Demicco and Hardie 1994). Hardground sur-
faces are the result of submarine cementation by ara-
gonite and magnesian calcite precipitation directly from
seawater circulating through the uppermost few centi-
meters or tens of centimeters of a porous sandy sea
bottom. The basic requirement is a longer-lasting sta-
bility of the sediment-water interface. The time involved
