Geology Reference
In-Depth Information
Plate 10 Peloids and Cortoids
Peloids
are rounded micro- and cryptocrystalline carbonate grains appearing dark in transmitted and white in
direct light. Most peloids except some of fecal origin show no differentiated internal structures. The term 'pellet'
should be reserved for peloids of fecal origin. Peloids are generally smaller than other carbonate grains. The term
peloid includes genetically different grains (see Fig. 4.11) originating from biotic activities (fecal pellets, Fig. 4.12;
algal peloids; bioerosional peloids), reworking of mud and grains (mud peloids or lithic peloids, -> 2; mold
peloids, Fig. 4.12), early and late diagenetic alterations of grains (bahamite peloids, -> 3; pelletoids, Fig. 4.12),
as well grains formed in situ by biochemical and/or chemical processes (microbial peloids, -> 1; precipitated
peloids). Peloids are abundant in tropical and subtropical shallow-marine environments (platforms, ramps, reefs),
but can be also found on slopes and in basins, and contribute to the formation of non-marine carbonates in
pedogenic, freshwater and lacustrine settings as well.
Cortoids
are carbonate grains (frequently bioclasts) exhibiting thin
micrite envelopes
. The micritic coatings
are commonly thinner then those of oncoids which may develop from the overgrowth of cortoids. In the case of
mineralogical differences between the coated bioclast and the micrite rim, cortoids can preserve shape and gross
structure even of diagenetically highly altered grains (-> 4; Pl. 130/3). Destructive and constructive micritization
are probably the most common modes of cortoid formation. Destructive micrite envelopes are formed by the
interplay of repeated microboring and infilling of the tiny little voids by microcrystalline Mg-calcite or aragonite
cements. Constructive micrite envelopes are formed by an intergrowth of externally calcified filamentous algae
on the surface of carbonate grains. Both modes are connected with the activity of microbes, algae and fungi. The
most intensive carbonate grain micritization takes place in warm shallow tropical seas, and is carried out almost
exclusively by cyanobacteria. Micritization of carbonate grains caused by microbes, however, also occurs in the
deep oceans. In non-tropical settings the formation of cortoids predominantly results from chemical dissolution
and recrystallization of carbonate grains in cold, carbonate-undersaturated waters. Cortoids are common in shallow-
marine shelf and platform carbonates, but also originate in non-marine environments. Considering the strong
impact of light on the vertical distribution of microborers, many authors have used marine cortoids as
paleobathymetrical indicators.
More cortoids are shown in Pl. 33/8, Pl. 35/2, Pl. 118/3, Pl. 127/1, and Pl. 130/3.
1
Microbial peloids.
The clotted structure of the peloids and the occurrence as constituents of bindstones point to a microbially
induced origin of the grains formed in situ within mats ('benthic peloids': Gerdes et al. 1994; Kazmierczak et al. 1996).
Bedded lagoonal limestone. Late Permian (Zechstein): Harz, Germany.
2
Mud peloids.
Small, subrounded and subangular micritic grains. Poor sorting and distinct differences in size and shape
indicate that these grains originated from the reworking of weakly lithified carbonate mud. Genetically, these peloids are
small intraclasts, but are conventionally assigned to the peloid group with an arbitary size limit of about 200
m. SMF 16-
N
ON
-
LAMINATED
. Early Carboniferous (Viséan): Nötsch, southern Austria.
3
Bahamite peloids.
Peloids originating from the micritization of ooids ('pseudopeloids': Lakschewitz et al. 1991). The
microstructure of small ooids is completely or highly altered by crystal diminution. Some ooid grains (arrows) still have
preserved relics of the original structure, but most grains exhibit the characteristic criteria of peloids (structureless round
micrite grains). Pseudopeloids are the dominant grains within the grainstone texture (GT) as well within the packstone
texture (PT) of synsedimentary clasts. The ooids and peloids were formed on submarine pelagic swells (as indicated by
pelagic faunal elements such as protoglobigerinids as nuclei of the ooids) and redeposited subsequent to transport in
deeper parts of the slopes. The pseudopeloid grainflows are overlain by micritic limestones with pelagic skeletal grains.
The succession reflects sedimentation on a subsiding swell accompanied by syntectonically caused downslope sediment
movements. Late Jurassic (Red Nodular Limestone): Chiemgau Alps, Bavaria, Germany.
4
Cortoids.
The micrite envelope of the outer part of the shell is caused by the attacks of microborers associated with micrite
precipitation. The boundary between the micrite envelope and the shell exhibits the characteristic irregular outline. Note
the distinct microborings on the surface (black arrows). The infestations have already destroyed the interior part of the
shell fragment (white arrow). Early Carboniferous: Cracow area, Poland.
5
Cortoids,
rounded fossils and redeposited oncoids. Micrite envelopes (ME) caused by microbial microboring and infilling
of the holes by micrite is evident in echinoderm fragments (E). Echinoderm fragments show a characteristic cleavage
structure. The micrite envelopes of this sample are stained by hematite impregnations. This points to contributions of Fe-
bacteria (see Mamet and Boulvain 1990). Note the imbricate structure represented by the parallel and inclined arrangement
of the bioclasts, and indicating current transport. Dasyclad algal fragments (DA), fusulinid tests (F) and oncoids (O) are
conspicuously rounded. In total, these criteria reflect the following steps: (1) Reworking and rounding of the grains
formed within a subtidal environment, (2) formation of microborings and bacterially-induced staining during a quiet-
water phase, followed by (3) final deposition within a current-influenced ramp environment. Early Permian: Carnia,
Southern Alps.
