Geology Reference
In-Depth Information
facies studies and has been successfully applied to fora-
miniferal and mollusk limestones.
Biofabrics of foraminiferal limestones: Clues to poten-
tial reservoir qualities
Larger foraminifera from different systematic
groups contribute to the formation of bioclastic lime-
stones, particularly in the Late Carboniferous and Per-
mian, Late Cretaceous and Cenozoic.
Excellent examples of foraminiferal biofabrics are
Early Tertiary limestones (Aigner 1985, Racey 1994).
The life cycle of Tertiary larger foraminifera includes
smaller sexual and larger asexual tests. The ratio be-
tween these tests differs in autochthonous and alloch-
thonous accumulations and can be used to recognize
hydrodynamically different ramp and shelf settings
(Fig. 5.3; Pl. 18/3). Accumulations of Cenozoic larger
foraminifera are particularly common in shallow-ma-
Fig. 5.2.
Many limestone beds exhibit concentrations of fos-
sils on the top surface of the beds.
The photograph shows bra-
chiopods, tentaculitids and ostracods on the bedding surface
of fine-grained shelf limestones. Late Silurian (Ludlow): Got-
land Island, Sweden. Detail from Samtleben et al. 2000. Scale
is 1 cm.
Sediment transport on sedimentary slopes
can be
deciphered from
upside-down geopetals (Pl. 26/7) and
strongly diverging geopetals (Pl. 17/6).
Recognizing tectonically tilted or overturned beds:
Cavity fillings and incomplete fillings in fossils are used
to determine the initial dip of tilted and faulted beds
(Shrock 1948).
Fig. 5.3.
Biofabric of a Tertiary foraminiferal limestone
. The
sediment is characterized by an accumulation of nummulitid
larger foraminifera (predominantly
Nummulites
deserti
and
Nummulites fraasi
) forming a bioclastic packstone. Because
small asexually produced A-forms and large, sexually pro-
duced B-forms respond differently to transport processes,
biofabrics can be used to reconstruct depositional conditions
and settings, and to estimate primary porosity and perme-
ability. Most foraminiferal tests of the sample represent small-
sized A-forms of
Nummulites
and
Assilina
tests of equal size.
The matrix is micrite. Note imbrication of grains (arrow).
The good sorting of the bioclasts is due to transport and indi-
cates redeposition. Eocene (Esna Formation, Ilerdian): Ferafra
Oasis, Western Desert, Egypt. Scale is 5 mm.
5.1.2 Biofabrics and Grain Orientation
Many bioclastic limestones exhibit characteristic bio-
fabrics characterized by specific patterns defined by
orientation, packing and sorting of skeletal grains.
Biofabrics as well as the statistical orientation of bio-
clastic and intraclastic grains reflect the hydrodynamic
and sedimentological history of skeletal carbonates.
Biofabric analysis is a very promising tool for micro-
