Geology Reference
In-Depth Information
cementation as well as for partial dolomitization and
silicification.
see Pl. 20/2, Pl. 41/2). Two major types of open-space
structures - Stromatactis (Dupont 1881) and birdseyes
(Ham 1952) - can be distinguished using the follow-
ing criteria:
5.1.5 Birdseyes, Fenestral Fabrics and
Stromatactis
Stromatactis
Birdseyes
Shape
Flat to undulose base,
Circular, oval,
Many limestones are characterized by open-space struc-
tures (Wolf 1965). The term has been suggested for
variously sized cavities in carbonate rocks filled with
calcite and/or internal sediment. The term is a non-ge-
netic, designating specific sedimentary and diagenetic
voids usually in fine-grained micritic limestones (pre-
dominantly mudstones, packstones and bindstones).
The voids are conspicuous constituents of the lime-
stones and form characteristic fabrics, some of which
resemble blind windows within a dark wall and are,
therefore, called fenestral fabrics (Tebbutt et al. 1965;
digitate and dentate roof
irregular
Filling
Mud at the bottom
Calcite spar or
(often planar) and/or
evaporite mineral
calcite spar at the top
Orientation Subhorizontal or
Irregularly or
irregular
parallel to bedding
planes
Geometry
Network consisting of
Single individuals
interconnected cavities
or single individuals
Size
Several mm to tens of cm,
A few millimeters
larger than birdseyes
Plate 20 OpenSpace Structures: Birdseyes, Stromatactis, and Fenestral Fabrics
Open-space structure is a non-genetic term designating specific sedimentary and diagenetic voids in carbonate
rocks (predominantly mudstones, packstones and bindstones). The mm- to cm-sized voids are conspicuous con-
stituents of the carbonate rocks and form specific fabrics. Some of these fabrics resemble windows and are,
therefore, called fenestral fabrics (Tebbutt et al. 1965). The voids are differentiated according to shape and size.
Millimeter-sized, irregularly distributed spheroidal spar-filled voids are called birdseyes (-> 7, 8), often associ-
ated with microbial bindstones. Larger millimeter- to centimeter-sized voids with flat bases and digitate roofs
are known as stromatactis. These voids are commonly filled with fine-grained sediment at the base and sparry
cements at the top. Birdseye limestones are formed preferentially in supratidal and intertidal environments,
stromatactis-type voids occur more commonly in subtidal environments. Both types are formed by various
processes (Box 5.1 and Box 5.2). Significance: Open-space structures are valuable microfacies criteria for indi-
cating ancient depositional settings and environmental conditions, and controlling the porosity and permeability
of carbonate rocks.
1 Stromatactis structure in peloidal wackestone. The cm-sized structures occur within a 1.5 m thick horizon in the upper-
most part of slope sediments close to a microbial-sponge reef. They are interpreted as the result of an internal erosion of
a laminar organic framework (Matyszkiewicz 1993). The roofs of the cavities are lined with thin, marine phreatic isopachous
cements, the center with blocky calcite crystals formed under burial or meteoric conditions. Note the basal internal sedi-
ment consisting of tiny peloids. Late Jurassic (Oxfordian): Mlynka quarry NW Crakow, southern Poland.
2 Laminoid fenestral fabric (LF-A fabric) Laterally elongated open spaces within pisoid layers. Supratidal environment.
SMF 21. Late Triassic (Carnian): Nesza quarry, Budapest, Hungary.
3 LF-B II fabric in bindstones with reworked pisoids. The fabric is characterized by a large amount of detrital sediment.
Supratidal/intertidal environment, inner carbonate platform. Early Jurassic (Pantokrator Limestone): Peloponnesus, Greece.
4 Fenestral fabrics within cyanobacterial mats. Note the occurrence of filaments (arrow). The association of cryptalgal
structures and fenestral fabrics indicates a peritidal environment. Early Carboniferous (Late Tournaisian): Czatkovice
quarry, Crakow Upland, southern Poland.
5 Fenestral fabric. Note pisoids, black pebbles (center), and internal micrite (gray). Supratidal environment. Early Jurassic
(Pantokrator Limestone): Peloponnesus, Greece.
6 Not birdseyes! Open bifurcated voids (plant roots and rootlets) within recent caliche. Root molds can be mistaken for
burrows but differ in downward branching and decrease in diameter. French Reef, Bahamas.
7 Birdseyes. Small calcite-filled voids in a laminated dolomite mudstone. Planar fine laminae composed of couplets of
peloid-rich layers alternating with continuous dark mudstone layers. This is a common texture in peritidal carbonates.
Lagoonal intertidal environment. Late Triassic (Hauptdolomit, Norian): Steinplatte, Tyrol, Austria.
8 Close-up of birdseyes. Note sediment infilling. Late Triassic (Hauptdolomit, Norian): Steinplatte, Austria.
-> 1: Matyszkiewicz 1993
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