Geology Reference
In-Depth Information
cause binding organisms (e.g. microbes) may also fix
mud within higher-energy settings.
Evidence of this can be seen in geopetal fabrics. These
packstones may exhibit a micritic matrix occurring to-
gether with sparite formed as cement within the remain-
ing open pores.
• Precipitation of microcrystalline carbonate cement
within intergranular pores may result in a packstone
fabric. This is common in calcretes or beachrocks but
may also occur in marine carbonates.
• Compaction of wackestones, facilitated by dewater-
ing may produce a 'diagenetic packstone' fabric (Shinn
and Robbin 1983).
• Uncemented grains may be welded together during
pressure dissolution resulting in a fabric that is com-
monly classified as 'packstone', even though 'matrix'
is very rare (Pl. 47/1).
What is a 'packstone'?
The term packstone is am-
biguous with regard to its genetic implications. Pack-
stones are characterized by grain-support as well as by
mud matrix, indicating different energy levels during
deposition. How can this contradiction be explained?
How can the matrix be introduced into the grain-sup-
ported fabric? Do all packstones represent depositional
textures?
• Mud from low-energy deposits may filter into empty
pores of underlying, grain-supported high-energy de-
posits or settle out of the water flowing through pores.
Plate 44 Dunham's Classification of Carbonate Rocks: Mudstone, Wackestone and Packstone
Mud- and grain-supported limestones (Pl. 43) are subdivided into textural categories according to the abundance
of grains (Fig. 8.4). The complete rock name requires the combination of the name of the category with adjec-
tives referring to grain kinds. Texture, mineralogy and frequency criteria can be included in the rock name.
Mudstones
are muddy carbonate rocks containing less than 10% grains measured as grain-bulk percent. The
name is more or less synonymous with calcilutite. Wright (1992) replaced the name mudstone by
calcimudstone
because mudstone is also used for rocks consisting of silicate silt and clay material. This term refers to 'lithified
material composed of greater than 90% by volume, silt and clay-grade calcite'. It is not synonymous with 'micri-
te'. The usual interpretation of mudstones is that they represent deposition of fine-grained sediment under low-
energy conditions allowing carbonate mud to settle in calm and quiet waters. However, it is now clear, that fine-
grained carbonates can be also formed by precipitation of microcrystalline carbonate cements (Reid et al. 1990).
Examples are known from microbial carbonates (see Pl. 8), terrestrial carbonates (caliche, Pl. 128/6) as well as
seep and vent carbonates (Pl. 148). Mudstone is a useful term in the field and also for thin-section studies. It
should be noted, however, that the ultrastructure of many mudstones corresponds to extremely fine-grained
bioclastic or peloidal packstones and wackestones.
Wackestones
are mud-supported carbonate rocks containing more than 10% grains. High variations in abun-
dance and type of grains call for a breakdown of this category into informal subcategories (e.g. packed or sparse
wackestone, > or < 50% grains; -> 2 and Pl. 118/1).
Packstones
are grain-supported muddy carbonate rocks exhibiting features pointing to deposition in agitated
waters (grain-support) and criteria pointing to quiet-water deposition. Packstones have different origins includ-
ing infiltrating of previously deposited mud-free grainstones, prolific production of grains in calm waters, mix-
ing of sediment by burrowers, partial leaching of mud or compaction and dewatering of original wackestones
(Shinn and Robbin 1983). A rather common feature of packstones in thin sections is the co-occurrence of mud
(micrite) and irregularly distributed areas with sparry calcite.
1
Mudstone
. Radiolaria-spicula mudstone (MS). Note the homogeneous texture of the matrix. The matrix is a fine-bioclas-
tic micrite, corresponding to type 7 in Fig. 4.1. A different fabric is shown near the bottom of the picture (packed radi-
olaria-spicula wackestone; WS). The sample comes from well-bedded lithographic limestones formed in restricted parts
of an open shelf environment. SMF 1. Late Jurassic (Vaca Muerta Formation, Tithonian): Zapala, Neuquén Basin, Argen-
tina.
2
Wackestone
. Packed echinoderm wackestone. Most grains are crinoid plates. White arrows point to ophiurid elements, the
black arrow to a foraminifera. The echinoderm grains are very worn. Straight elements are bivalve shell debris. The
echinoderm limestone is part of a transgressive unit formed after drowning of Triassic reefs (Böhm et al. 1999). SMF 9.
Early Jurassic (Hettangian): Adnet near Salzburg, Austria.
3
Packstone
. Bioclastic coral-algal packstone composed of corals (C), solenoporacean red algae (
Pseudochaetetes asvapatii
Pia; PS), bivalve shells, and gastropods (G). Arrows point to sediment-binding squamariacean red algae (
Pseudolitho-
thamnium album
Pfender). The infilling of mud and fine skeletal debris in interstices between larger grains indicates
grain-support. SMF 8. Early Tertiary (Kammbühel limestone, Paleocene): Mooshuben near Mariazell, Styria/Austria.
