Geology Reference
In-Depth Information
provided by King (1948) who used fusulinid orienta-
tions for interpretating the directions of ancient shore-
lines. Tentaculites and styliolinids are often current-
aligned, but preferred orientations can vary consider-
ably (Wendt and Belka 1991, Hladil et al. 1996). Other
current-aligned microfossils are shown in Pl. 74/8 (fora-
minifera), Pl. 78/2 (sponge spicules), Pl. 94/7 (trilo-
bites), Pl. 57/7 (gymnocodiacean algae), Pl. 112/1
(sponge spicules), Pl. 102/6 (shells), Pl. 105/2 (dasy-
clad algae), and bivalve shells.
thin sections that are important in microfacies analyses
(see Fig. 3.4).
Depositional controls
on fine-scaled lamination and
bedding are recorded by conspicuously planar laminae
(Fig. 3.3), distinct changes in texture, composition and
grain size (Pl. 18/2), parallel orientation of grains (Pl.
18/7), or abrupt changes in microfacies (Pl. 18/6). The
latter can reflect short-term depositional events (e.g.
storms, turbidites, mass flows) causing event stratifi-
cation (Wheatcroft 1990; Seilacher 1991). Depositional
processes leading to the formation of laminae are set-
tling-out of grains from suspension, bottom flows and
bedload deposition, and debris and grain flows. Com-
mon depositional bedding types are parallel bedding,
cross-bedding (Pl. 18/3) and normal or reverse graded
bedding (Pl. 17/1). For reverse grading see Sect. 5.1.1.
Orientation of intraclasts:
Penecontemporaneous
erosion and redeposition of sediment, indicated by in-
traclastic beds, is a common feature of peritidal and
shallow subtidal carbonates. Measurements of the azi-
muths of the long axes of intraclasts on bedding planes
and in oriented samples assist in reconstructing tidal
currents and wave movement (Lindholm 1980).
Biological controls
are indicated by wavy or crinkled
laminae, irregular alternations of micritic and sparry
layers (Fig. 3.3, Fig. 3.5, Pl. 18/4, Pl. 50/1), intercala-
tions of sedimentary layers and peloidal fabrics (Pl. 18/
6, Pl. 50/2), and relicts of cyanobacteria (Pl. 53) and
other microbes. Microbial mats form on bedding sur-
faces. Most mats have a characteristic filamentous mor-
phology, the filaments have rigid walls and polymeric
sheets which act as sites of agglutination of sedimen-
tary grains and external calcification.
Of major importance for the creation of a laminated
pattern (
biolaminations
, Gerdes and Krumbein 1987;
Gerdes et al. 1991) is the capacity of the mat-construct-
ing biota to migrate vertically to escape burial by sedi-
ments and to recolonize the newly deposited surface.
In terms of ecology, biolaminites correspond to suc-
cessions with formative and consuming stages. Forma-
tive stages include primary production via photosyn-
thesis and chemosynthesis. Consuming stages are char-
acterized by anaerobic microbial communities with het-
erotrophic and anoxygenic phototrophic bacteria.
Heterotrophic bacteria trigger the formation of car-
bonates, silicates and sulfides, and the growth of ce-
ments which increase the preservation potential of bio-
lamination structures. The thickness of biolaminated
stacks ranges from millimeters to centimeters and even
decimeters. Resulting biolaminated structures have
been called 'cryptalgal' structures (Aitken 1967; Monty
1976).
Imbrication of intraclasts or elongated skeletal grains
characterized by the preferred dipping of disk-shaped
or elongate fragments at an angle to the bedding can be
used as indicators of current-flow directions (Pl. 18/1,
3). Flat pebble conglomerates containing imbricate in-
traclasts are typical in modern shallow-marine carbon-
ates, and also known from ancient carbonate platforms
(Whisonant 1987). Imbricate intraclasts document epi-
sodic events. Microfacies analysis reflects the source
sediments of the intraclasts and may indicate whether
the material was transported landward or seaward.
5.1.3 Bedding and Lamination Fabrics
Bedding and lamination are caused by changes in depo-
sitional, biological and diagenetic controls. Depositional
factors primarily include changes in sedimentation rates
and in the composition of the sediment, and alterna-
tions of sedimentation and non-sedimentation phases.
Biological controls are predominantly a result of the
interaction of microbes and microbial mats with their
physical and chemical environment and their influence
on binding and trapping of the sediment. Diagenetic
processes leading to bedding structures are dissolution
and cementation within the sediment.
Types, descriptive criteria and the significance of
bedding and lamination structures are discussed in Sect.
3.1.1.2. Although bedding and lamination must be stud-
ied on an outcrop scale, a thorough investigation of ori-
ented thin sections is important for recognizing hidden
sedimentary structures (e.g. small-scale cross bedding)
and cyclic patterns. This chapter describes some of the
more common lamination and bedding types seen in
Bioturbation commonly alters or destroys bedding
and lamination structures and contributes to a homog-
enization of the sediment (Reineck 1963, see Sect.
5.1.4). Exceptions exist however: A specific type of bio-
logically controlled bedding is
biogenic stratification
