Geology Reference
In-Depth Information
and shape is used in classifying
laminoid-fenestral fab-
rics
(LF fabrics) characterized by elongate horizontal
fenestrae within fine-grained or grain-supported sedi-
ment (Fig. 5.5): (a) LF-A fabrics - with distinct lam-
inoid fenestrae (Pl. 132/6, Pl. 142/13), and (b) LF-B
fabrics - with irregular laminoid fabrics. The latter type
is subdivided into low-detrital fabrics (LB-1) with voids
embedded in micrite, and high-detrital fabrics with
voids occurring in sediment rich in grains (LB-2, Pl. 8/6,
Pl. 122/1).
develop (as do cyanobacterial mats) in a variety of sub-
aqueous marine and non-marine environments, too.
(2)
Tubular fenestrae
originate in modern intertidal
and shallow-subtidal environments as burrows or as
root holes, but are also formed by upward escape of
gas bubbles biogenically produced within the sediment.
(3)
Spherical to subspherical fenestrae
are produced
by air and gas bubbles trapped during the deposition of
the host sediment or they are generated by post-depo-
sitional decay of organic matter. Air bubbles can be trans-
ported within the sediment by rising groundwater.
A particular type of mm- to cm-sized spherical and
subspherical open fenestrae are
keystone vugs
(Emery
1945), see Pl. 29/9. The vugs are larger than ordinary
birdseyes and are produced by trapping of air-bubbles
during storm deposition in the squash zone on beaches
or in the sheetwash zone on tidal flats or playas (Shinn
1968; Dunham 1970; Deelmann 1972).
(4)
Irregularly formed and distributed fenestrae
(Pl.
40/4) may form by (a) desiccation, (b) soft-sediment
deformation of other originally more regular-shaped
fenestrae, (c) gas bubbles, (d) evaporite molds, (e) bur-
rows, (f) burial of pustular cyanobacterial mats, (g) de-
watering of gel-like carbonate muds, and (h) replace-
ment of grains.
Some of these 'irregular' structures are not fenes-
trae but rather intergranular pores of grain- and pack-
stones. Shinn (1983) proposed the term 'pseudo-fenes-
trae' for these structures. Note that shelter pores in sub-
tidal lithoclast grainstones can strongly resemble vuggy
laminoid fenestrae (Goldhammer et al. 1987, Hardie et
al. 1986).
Origin and distribution of fenestral fabrics
The shape and orientation of fenestrae reflect dif-
ferent origins:
(1)
Laminoid fenestrae
(Pl. 41/2) are of polygenic
origin and may be caused by (a) wetting and drying of
carbonate mud in supratidal settings (Shinn 1968),
(b) drying out of the surface of cyanobacterial mats re-
sulting in wrinkling, lifting and separation from the ad-
jacent sediment (Hardie and Ginsburg 1977; Davies
1970; Logan et al. 1974), and (c) degassing of decay-
ing organic material connected with compaction of sub-
spherical gas bubbles.
Laminoid fenestrae are common in modern inter-
tidal to supratidal settings. However, these structures
5.1.5.3
Stromatactis
These structures, characterized by elongated cavities
with irregular tops and flat bases, were first observed
in Frasnian and Early Carboniferous carbonate mounds
of Belgium (Dupont 1881, 1883) and were considered
as recrystallized fossils.
Most authors now agree, that the name stromatactis
should refer to a spar-filled (and sometimes mud-filled)
body commonly embedded in micritic limestones that
originated from centripetal cementation of cavities by
fibrous or radiaxial cements. Some authors restrict the
term to a spar network formed by interconnected cavi-
ties and exhibiting a reticulate distribution (Bourque
and Boulvain 1993), others also incorporate non-inter-
connected spar bodies occurring underneath platy skel-
etons of stromatoporoids or corals (Tsien 1983; Flajs
and Hüssner 1993). These structures resulted from ce-
mentation of synsedimentary shelter cavities or cavi-
Fig. 5.5.
Subtypes of laminoid-fenestral fabrics
. Type A and
type B are distinguished by shape and distribution of the spar-
filled fenestral voids (here white), type B-I and B-II by the
relative amount of voids and sediment. Adapted from Tebbutt
et al. (1965) and Müller-Jungbluth and Toschek (1969).
