Geology Reference
In-Depth Information
ments, inner shelf settings as well as at high latitudes.
For a long time impure limestones with admixtures of
siliciclastics have been neglected, in the context of both
sediment classification as well as facies interpretation.
A descriptive classification system (Fig. 8.8) was pro-
posed by Mount (1985) using four components: (1) Si-
liciclastic sand (sand-sized quartz, feldspar etc.),
(2) non-carbonate mud (mixtures of silt and clay),
(3) carbonate grains or allochems (peloids, ooids, bio-
clasts and intraclasts >20
m in size) and (4) carbonate
mud (or micrite, defined as <20
m in size). The name
of the sediment type reflects both the dominant grain
type and the most abundant antithetic component.
Fig. 8.8.
Classification of mixed siliciclastic-carbonate rocks.
Modified from Mount (1985).
Plate 47 Classification of Carbonates That are Marked by Diagenesis
The textures of most limestones result from an interplay of depositional, biological, and diagenetic factors.
Carbonates classifiable as calcimudstones, wackestones, packstones, or grainstones may be products of diage-
netic processes that obliterate the depositional textures or create new non-obliterative textures (Fig. 8.5; Wright
1992). Obliterative diagenetic textures can be caused by recrystallization and/or replacement and result in a
complete loss of the original fabrics and the formation of
sparstone
or
microsparstone
(-> 2). Sparstones consist
of sparry calcite crystals (>10
m in diameter) forming inequant blocky mosaics. Microsparstones are composed
of calcite crystals within the range of 4-10
m. Non-obliterative textures include
diagenetic packstones
(-> 1, 4)
and
condensed or fitted grainstones
(-> 3) caused by modification of originally grain-supported textures by
pressure solution during burial. However, pressure solution can also completely transform textures into new
ones (-> 5, 6) and create new fabrics (see Pl. 37). Displacive and replacive growth of micrite cements in pe-
dogenic carbonates can result in a complete textural inversion.
Diagenetic and biological factors are responsible for the formation of
cementstones
. The term refers to reef
limestones consisting largely of replaced but recognizable marine cements occurring in association with pre-
dominantly encrusting organisms (see Pl. 145). Conspicuous cementstones forming reefs on the paleoshelf oc-
cur in the Neoproterozoic Cap Carbonates, which worldwide cover the glacigene sediments of Early and Late
Proterozoic age of the 'snowball earth' (James 2001).
1
Diagenetic echinoderm packstone
. Pressure solution has transformed a depositional wackestone into a diagenetic pack-
stone fabric (Pl. 144). Bioclasts are separated by microstylolites. Note the almost complete absence of matrix. Mississip-
pian: Muleshoe Mound, Sacramento Mountains, New Mexico, U.S.A.
2
Microsparstone
formed by meteoric recrystallization of a laminated micrite. Note the equant uniform crystals. Mississip-
pian: Big Hill, Cairn, Canada.
3
Condensed grainstone
caused by compaction. Grains are flattened and compacted micrite clasts. Clasts are surrounded by
thin rims of burial calcite cement. Both rims and grains were dolomitized. The mechanical compaction also affected the
cements (arrows). Early Cretaceous (Purbeck facies, Berriasian): Subsurface, Kinsau well, Bavaria, Germany.
4
Diagenetic stromatoporoid packstone.
The original close packing of grains has been enhanced by pressure solution. Note
stylolite seams (black arrow), grain penetration (GP) and irregular grain boundaries (white arrow). Most fossils are stro-
matoporoids (
Stachyodes
, S;
Stromatoporella
, ST) and thamnoporid tabulate corals (T). C: Rugose corals; B: Brachiopod
shells. The matrix is a black argillaceous mudstone. Back-reef environment. Middle Devonian: Brilon, Sauerland, Ger-
many.
5
Mudstone
(M), grading into argillaceous lithoclast-bearing mudstone (LM), caused by pressure solution. Lithoclasts are
relicts of former 'tuberoids'. Stylolaminated fabric (see Sect. 7.5.2). Late Jurassic: Subsurface, Saulgau well, southern
Germany.
6
Marly diagenetic lithoclast wackestone/floatstone
, caused by continued pressure solution of a former mudstone (-> 5).
The former micrite matrix was converted into a marly matrix with up to 60 % insoluble residue. Note concentration of
stylolite swarms (arrows) near pressure-solution resistant components floating within the marly matrix. Stylolaminated to
stylocumulate fabric. Same locality as -> 5.
-> 5, 6: Huber 1987
