Geology Reference
In-Depth Information
rine carbonate environments. Larger foraminifera are
'biologically standardized', e.g. the size and shape of
their tests depends on biological factors (Seilacher
1973). Since larger foraminiferal tests have high initial
porosity, low bulk density, and are easily transported,
they can form ideal hydrocarbon reservoirs as seen in
the Eocene nummulitid limestones in Tunisia, Libya
and northern Oman where reservoir quality is prima-
rily controlled by biofabric type and the degree of frac-
turing.
current information on local and regional scales, but
only elongated fossils respond systematically to the
aligning effects of currents, and are therefore usually
used in basin analysis studies (Potter and Pettijohn
1977). For many fossils ambiguities exist as to whether
they are oriented transversely or perpendicularly to cur-
rent patterns (e.g. brachiopods, gastropods). The rela-
tionship between fossil orientation and current direc-
tion is mainly controlled by the shape and uniformity
of particle density. Common fossils used in field-based
orientation studies are bivalve and gastropod shells,
orthocone cephalopods (Sundquist 1982; Wendt 1995),
belemnites, and crinoid columnals (Schwarzacher
1963).
Systematic mapping of fossil orientation is com-
monly based on bedding plane studies, but observa-
tions of microfossils in thin sections following the plane
of stratification or the bedding-perpendicular plane are
also useful in recognizing hydrodynamic conditions on
the sea bottom (Stauffer 1962). An impressive example
of the contribution of microfossils to basin analysis was
Orientation patterns of bioclastic and intraclastic car-
bonate grains
Orientation patterns of grains seen in thin sections
reveal depositional factors (e.g. hydrodynamic vari-
ables) as well as diagenetic changes during compac-
tion (Bathurst 1987).
Fossil orientation:
Fossils generally exhibit good
orientation patterns and may provide valuable paleo-
Plate 18 Biofabrics, Grain Orientation, Bedding and Lamination: Indications of Depositional and Climatic
Controls
Biofabric (-> 3) and grain orientation patterns (-> 1, 5, 7) as well as bedding and lamination (-> 2, 4, 6) reflect
depositional and climatic conditions and indicate biological and diagenetic controls during sedimentation. Inter-
nal bedding of limestones is recorded by distinct compositional changes (-> 2) and current-aligned orientation
patterns of elongate particles (e.g. imbrications, cross-structures, -> 1, 3). Biolaminated fabrics are character-
ized by thin laminae differing in grain size, texture, biota (-> 4, Pl. 50/2), mineral composition, clay content, and
the amount of organic material. Ideally, laminae represent 'sedimentation units' formed in short-term intervals.
The classification of bedding and lamination is based on structural and textural criteria (Demicco and Hardie
1994, see Sect. 3.1.1.2).
1
Grain orientation:
Imbrication structure, indicating current energy. The current 'direction' is from right to left. Most
grains are cortoids comprising coated shells (CS), fusulinids (F), and echinoderms (E). Open marine platform. Early
Permian: Forni Avoltri, Carnia, Italy.
2
Bedding:
Interlayering of fine wavy laminae consisting of sand-sized grains (peloid packstone, P) and carbonate mud
(mudstone, M). This texture is called 'heterolithic bedding' and indicates a depositional environment where current flow
varies considerably. Jurassic: Northern Alps.
3
Cross-bedding
of foraminiferal shells (Nummulitidae:
Assilina
and
Nummulites
). Note the radial hyaline wall structures.
The biofabric characterized by chaotically stacked tests, linear contact imbrication und cross-bedding indicate deposition
by short-term high energy events. Early Tertiary (Eocene): Agathazell, southern Bavaria, Germany.
4
Biolamination:
Alternation of fine wavy laminae corresponding to microbialites (MB), sponges (S), and sediment. Late
Triassic (Dachstein limestone, Norian): Begunjscica Mountains, northern Slovenia.
5
Grain orientation:
Parallel orientation of tabular limestone clasts caused by current transport. Smaller grains are micrite
intraclasts and peloids. Precambrian (Altyn Formation): Logan Pass, Glacier Park, Montana, U.S.A.
6
Lamination
consisting of microspar laminae and peloidal laminae. Equant coating of the small peloids by cement rims
perhaps points to a bacterial origin of these grains occurring within a peloid/micrite crust (see Pl. 8/5). Reefal limestones.
Middle Triassic (Ladinian): Seiser Alm, Dolomites, Italy.
7
Lamination
due to parallel oriented densely-packed 'filaments' (arrows, see Pl. 113/2). The microfabric of undisturbed
hemipelagic laminae is often characterized by a preferred horizontal orientation of elongated particles (Krinsley et al.
1998). Early Jurassic: Djebel Assiz, Northern Tunisia.
