Geology Reference
In-Depth Information
grains was reported from many Holocene carbonates
of tropical and nontropical marginal marine environ-
ments (Florida, Bahamas, Persian Gulf, Great Barrier
Reef/Australia, Tunisia; North Sea) as well as from hy-
persaline lakes (Ward et al. 1970). Both organic matter
and/or iron or manganese sulfide as well as clay min-
erals have been quoted as the blackening agents (Lang
and Tucci 1997; Maiklem 1967). Pilkey et al. (1969)
proved the necessity of reducing conditions for black-
ening in an experiment. The salt-and-pepper texture
may be caused by laterally migrating tidal channels that
erode blackened grains and redeposit them together with
nonblackened grains, or it may be the result of land-
ward migrating mudbanks. Erosion and redeposition
transport blackened grains and lithoclasts into various
sedimentary environments and, therefore, they are
found in near coastal areas and supratidal and inland
ponds (Fig. 4.29, Box 4.21 and Box 4.22).
and show evidence of having been fractured. The clast
occur at the top of pedogenic sequences and are not
graded. The organic matter absorbed on and infiltrat-
ing into the grains is derived from rotted plants (high
terrestrial plants, algae) and affects mainly unconsoli-
dated carbonate sediments deposited in terrestrial, su-
pratidal, intertidal and subtidal settings (Platt 1992).
Black pebbles are associated with their original envi-
ronment, such as soils (Bechstädt and Döhler-Hirner
1983; Francis 1986). They are also known from tem-
pestites (Aigner 1982) and beach rocks (Pl. 33/3).
Blackening of lithoclasts derived from soilstone crusts
in the Florida Keys has been explained as being caused
by 'instantaneous' forest fire heating (Shinn and Lidz
1988). Heating experiments show that limestone
pebbles can be blackened at temperatures between
400 °C
and 500 °C.
Cementation of subtidal sediments within the va-
dose zone, followed by brecciation and resedimenta-
tion can result in the formation of 'black pebbles'
(Bechstädt 1975). Lithoclasts deposited in depressions
Black lithoclasts:
Black lithoclasts originate in sub-
aerial environments. Most black lithoclasts are angular
Box 4.22.
Significance of intraclasts in microfacies analysis.
Paleoenvironmental proxies
Paleocurrent analysis:
Intraclast orientation patterns are useful in evaluating wave action and tidal currents in shal-
low subtidal and peritidal environments (Pl. 18/5). It seems likely that the long axes of intraclasts are aligned trans-
versely to the flow (Lindholm 1980). Imbrication of flat pebbles indicates upcurrent orientation. Orientation patterns of
mud clasts in turbidites allow bottom currents to be recognized.
Seismic shocks:
Intercalations of intraformational flat-pebble conglomerates and laterally continuous intraclastic
carbonates in subtidal shelf carbonates were interpreted as indicators of possible tsunami effects related to seismic
shocks (Kazmierczak and Goldring 1978; Rüffer 1996).
Sedimentation patterns
Discontinuous sedimentation:
Repeatedly bored and encrusted lime clasts in micritic shelf carbonates can be used to
reconstruct sea-bottom conditions (e.g. soft-, firm- or hardground) and lithification stages (Chudziekiewicz and Wieszorek
1985). Hardground intraclasts indicate repeated breaks in sedimentation and may mark discontinuity surfaces.
Significance of 'black pebbles':
The occurrence of 'black grains' indicates submarine diastems and unconformities.
'Black lithoclasts' in multicolored breccias point to subaerial exposure and the existence of a vegetation cover. Black
pebbles occurring within breccias associated with marls and marly dolomites of lagoonal sequences point to cyclic
emersion and karstification producing residual sediments. In general, evidence of blackened carbonate material in lime-
stones should stimulate one to think of breaks in marine sedimentation, partial to complete subaerial exposure, rework-
ing of carbonate strata, changes in sea-level fluctuations, existence of terrestrial plant growth and possibly even forest
paleo-fires, as well as the existence of nearby terrestrial environments.
Reconstruction of lost platforms and sealevel fluctuations
Clast analysis
(see Sect. 16.3) is of particular value in evaluating platform- and reef-derived lithoclastic carbonates
and breccias deposited on the slope or at the base-of-slope (Pl. 17/1). The analyses require field studies (abundance and
compositional variations of clasts), microfacies differentiation, and biostratigraphical dating of clasts and enclosing
matrix (Pohler and James 1989; Stock 1994; Belka et al. 1996).
Clues to diagenetic processes
Intraclasts are common in Proterozoic shelf and ramps. The influence of storm wave erosion and subsequent erosion
is more probable in the formation of these intraclasts than fair-weather processes, because of the extensive sea-floor
cementation caused by microbialites carbonates (Sami and James 1993).
Reworked dolomite crusts in tidal carbonates may be indicative of early supratidal dolomitization (Germann 1965).
