Geology Reference
In-Depth Information
ties formed by the collapse of mud underneath a rigid
object acting as an umbrella during water escape and
mud compaction. Stromatactis is abundant in micritic
limestones, but geometrically similar open-space-
structures also occur in cemented grainstone crusts
(Macdonald et al. 1994).
The
internal sediment
in the cavities of stromatac-
tis commonly is fine-grained micrite, sometimes with
microfossils, peloidal micrite, or laminated silt-sized
sediment. Genetically it corresponds to the infill of fine-
grained sediment in a reticulated cavity system or to
diagenetic products caused by recrystallization of mud
and deposition of solution products (Aissaoui and
Purser 1983; Flajs and Hüssner 1993; Vigener 1996;
Aubrecht and Szulc 2000; see Sect. 7.4). A common
feature of internal sediments is a vague and indistinct
lower boundary.
Common
cements
occurring within stromatactis
cavities are radiaxial, fibrous and granular calcite. Fi-
brous and radiaxial cements are considered as being of
early synsedimentary origin, formed a few centimeters
or decacentimeters beneath the depositional surface.
Early diagenetic fibrous cements first originate in small
voids enlarged by erosional and solution processes.
Other cement types are formed during shallow and
deeper burial phases. Early cementation contributes
greatly to rapid lithification and stabilization of car-
bonates formed at the flanks of mounds.
Stromatactis fabrics occur in patchy distribution or
as layers which alternate with limestones consisting pre-
dominantly of micrite (Fig. 5.6). These alternations, ob-
served in bedded Devonian stromatactis carbonates,
have been interpreted as reflecting Milankovitch cycles
Box 5.2.
Proposed origin of stromatactis fabrics.
Organic
1 Recrystallized fossils: Dupont 1881; Scoffin 1987
2 Permineralized, unfossilizable organism (e.g. algae): Lecompte1937; Tsien 1983
3 Cavities formed after the decay of various soft-bodied organisms in the sediment: Bathurst 1959; Philcox 1963;
Otte and Parks 1963; Lees 1964 ; Davies and Nassichuk 1990; Lees and Miller 1985; Warnke and Meischner 1995
4 Burrow network such as
Callianassa:
Shinn 1968
5 Action of various burrowing organisms: Shinn 1968
6 Non-skeletal algae: Coron and Textoris 1974
7 Thrombolites, microbial: Pratt 1982
8 Diagenetic CaCO
3
replacement of large microbial accretions: Tsien 1985; Weller 1989
9 Winnowing of sediment between microbial framework mats on the sea floor: Pratt 1982, 1986
10 Cementation of cavities left after the decay of microbial mats: Flajs and Hüssner 1993; Kaufmann 1998; Vigener
1996
11 Collapse of material within organic tissue of probable microbial origin: Lees 1964, 1988; Lees and Miller 1985
12 Partial cementation of cavities created by collapse of unlithified sponge tissue: Bourque and Gignac 1983; 1986;
Beauchamp 1989; Bourque and Boulvain 1993; Vigener 1996
13 Early marine cementation of cavity networks created by exacavation of uncemented material in partly indurated,
decaying sponges and organic mats: Bourque and Boulvain 1993; Dicken 1996
14 Recrystallized bryozoans: Lowenstam 1950; Textoris and Carozzi 1964
15 Sheltering of sediment by bryozoan sheets: Schwarzacher 1961; Lees and Miller 1995
16 Action of plant roots: Bechstädt 1974
Inorganic
17
Primary cavities and/or dissolution cavities: Kukal 1971
18
Cavities produced by dewatering and compaction of a thixotropic mud: Heckel 1972; Macdonald et al. 1994;
Bernet-Rollande et al. 1982
19
Excavations between layers of early-cemented crusts on the sea floor, differential cementation of host peloidal
mudstone into successive, closely-spaced hardgrounds: Bathurst 1980, 1982; Wallace 1987
20
Downslope creeping and slumping of partially cemented mud on mound flanks: Schwarzacher 1961; Bathurst
1982; MacDonald et al. 1994
21
Internal erosion of cyanobacterial growth cavities on a partly lithified slope: Matyszkiewicz 1993
22)
Enlargement of pre-existing cavities (e.g. vertical water and gas escape structures) in unconsolidated bioclastic
lime mud by internal erosion and internal reworking: Wallace 1987; Shen and Zhang 1997; Vennin 1997
23
Forcing apart and fracturing of variably bound and lithified sediment by episodic earthquakes: Pratt 1995
24
Subaerial solution (e.g. karstification) resulting in the formation of voids subsequently filled with carbonate silt
and cement: Semeniuk 1971
25
Recrystallized patches of the host lime mud: Black 1952; Orme and Brown 1963; Wolf 1965; Ross et al. 1975
26
Pressure solution connected with low-grade metamorphism: Logan and Semeniuk 1976
27
Creation of cavity systems by hydrothermal endo-upwelling: Belka 1998
