Geology Reference
In-Depth Information
5 Microfacies Data: Fabrics
Having discussed matrix and grain types of carbonate
rocks in previous chapters, this chapter focuses on the
fabrics constructed by rock constituents. The term
fab-
ric
includes textural as well as structural criteria. Most
fabrics reflect depositional controls or early diagenetic
processes. This chapter starts with the description of
common depositional and diagenetic fabrics (Sect. 5.1),
continues with criteria on the recognition and signifi-
cance of discontinuity structures (Sect. 5.2) which are
of major importance in sequence stratigraphy, and
closes with a review of predominantly postdepositional
features (Sect. 5.3).
scaled fabrics that are the topic of microfacies studies.
The most abundant microscopic geopetal criteria are
summarized in Fig. 5.1. These fabrics are produced by
sedimentological, biological and diagenetic processes.
Bedding plane surfaces:
The top and bottom sur-
faces of limestone beds exhibit many geopetal sedi-
mentary structures, e.g. ripple marks, mud cracks, and
trace fossils (see Sect. 3.1.1.2). A simple but compel-
ling example are fossils concentrated on the surface of
bedding planes (Fig. 5.2).
Sedimentary structures:
Thin sections of fine-
grained limestones sometimes show fine-scaled cross-
bedding and cross-lamination which attenuate and flat-
ten downdip and are truncated above (Harbaugh 1959).
Unconformity planes may truncate underlying features.
Mechanical deposition on free surfaces:
Sand-sized
grains can be trapped in bottom reliefs, saucer-shaped
shells (Pl. 127/2), or internal cavities in the sediment
(
Sandfang
: Sander 1936). Some examples are shown
in Pl. 17/1, 2. Grains may also settle on flat surfaces of
oncoids (Pl. 17/2), on flat surfaces of shells, or on lithi-
fied discontinuity surfaces (Sect. 5.2).
Grain grading
characterized by coarse grains, e.g.
bioclasts and lithoclasts, at the base of the bed and fine
grains at the top is an often used, but not always un-
equivocal geopetal criterion. Normally graded beds are
most commonly found at the base of turbidites (Pl. 17/
1), but are also produced by waning currents (e.g. ebb
tides), burrowing organisms in intertidal and shallow
subtidal environments (Rhoads and Stanley 1965), trap-
ping of sediment by sea grass (Wanless 1981), and the
effect of hurricanes. Reverse (inverse) grading with the
finest material at the base and the coarsest at the top is
not uncommon (Pl. 17/1) and is a typical feature of
high-concentration gravity flows. It is common in grain-
and debris-flow deposits, but is also found in numer-
ous kinds of deposits including beach, base-surge and
overwash sediments. A frequently employed explana-
tion for the origin of reverse grading is the Bernoulli
principle, whereby dispersive pressures near the bound-
ary of deposition tend to drive large particles upward
(see Fisher and Mattinson 1968; Sallenger 1979; Naylor
5.1 Depositional and Diagenetic
Fabrics
The order of the discussion in this chapter can be used
as guideline for studying fabrics in thin sections. A typi-
cal investigation should start with an evaluation of top-
and-bottom criteria, then grain orientation, packing pat-
terns, and indications of lamination and bedding, and
finally an assessment of burrowing textures, which in
turn may contribute to the formation of nodular struc-
tures, along with various diagenetic processes.
5.1.1 Geopetal Fabrics
Geopetal fabrics (Sander 1936) indicate the relation-
ship between top and bottom at the time the rock was
formed. Recognizing geopetal fabrics in limestones is
crucial for understanding the depositional and post-
depositional history of carbonate rocks, particularly
changes in original sedimentary dips of beds by com-
paction and tectonic tilting of beds. The comprehen-
sive manual for clarifying top-and-bottom criteria pub-
lished by Shrock in 1948 is still a very valuable refer-
ence.
Common geopetal features in carbonate rocks
Geopetal structures occur on different scales rang-
ing from meter-scaled crossbedding to millimeter-
