Geology Reference
In-Depth Information
overwhelm bedload transport and is thus a domi-
nating infl uence on texture of event stratifi cation
on epeiric seas and broad carbonate platforms and
their margins.
Grain-size analysis by sieving is not always a
suffi cient approach when analysing modern car-
bonates. Carbonate grains are commonly hol-
low, porous, and/or platy and their sieved grain
size may not be a good measure of the grains'
behaviour. Settling analysis, the settling of grains
through a column of water, is a better measure of
grain size. Ooids and abraded mollusc fragments
are solid carbonate, equant, and have about the
same density as quartz, so sieve analysis will give
a good assessment. However, crinoid columnals,
other echinodermal plates, foraminiferal tests,
bryozoan fragments, the plates of the calcareous
alga,
Halimeda,
and faecal pellet grain aggre-
gates are very porous and will settle together
with quartz grains half to quarter their size
(carbonates - Braithwaithe, 1973; Wanless
et al
.,
1981; siliciclastics - Haven & Morales-Alamo,
1968). Sieving is, therefore, a poor measure of
their behaviour.
Applying this to ancient limestones is challeng-
ing as the original porosity within grains is not
always apparent. The fi rst time a modern crinoid
fragment is handled it is surprising how little the
grain weighs and how porous it is. This contrasts
strongly with the massive crinoidal limestones
in which they are commonly preserved in the
Palaeozoic.
in defi ning criteria to distinguish them based on
the physical sedimentary structures. It would fi rst
be helpful to dispose of the terms tempestite and
turbidite as they essentially force the observer
into one camp or another in the description
phase. The tempestites in particular are probably
still a lumping of stratifi cation from several dif-
ferent types of storm transport processes, includ-
ing local resuspension and redeposition, strong
lateral cross-shelf transport induced by density of
sediment-laden water (Aigner & Reineck, 1982),
and deposition on subtidal storm deltas (Aigner,
1985). Siliciclastic and carbonate sedimentologists
have characterized storm deposition quite differ-
ently, mostly refl ecting different energy settings
and different preserved components. Siliciclastic
storm units are thought of as being made of hum-
mocky and swaley stratifi cation of sand (Harmes
et al
., 1975; Dott & Bourgeois, 1982). In carbon-
ates, undulating sand stratifi cation is only a part
of the sequence, and is complemented by basal
skeletal shell concentrations and capping muds
and post-event faunal processes and community
remains (Aigner, 1985).
Tsunami deposits must now be added to this
list. There is perhaps hope for differentiating
tsunami deposits as they are commonly a series
of 2-6 very closely spaced, but distinct events,
although not all tsunami deposits show mul-
tiple units (Dawson & Shi, 2000). In addition, it
is important to acknowledge that tsunami depos-
its can vary from thin layers of mud or sand that
extend kilometres inland, or more localized giant
boulders (i.e. Grand Cayman).
Reineck & Wunderlich (1968) helped to dif-
ferentiate sequences built of repetitive sand and
mud alternations by tides from a stacking by
less frequent storm events. In sands, flaser
bedding - repeated, commonly reversing, ripple
stratification separated by thin, partly eroded
mud drapes - represent alternations of frequent,
repetitive transport and quiet water settle out,
commonly tidal in origin. More widely tempor-
ally spaced event stratification commonly tends
to be an alternation of sand layers interbedded
with slightly thicker and more continuous mud
layers, and the sand layers are characteristic-
ally more planar to hummocky stratified with
a thin ripple top (Fig. 3). For skeletal gravels,
storm deposition is generally characterized by
a loose and chaotically packed, flat to undu-
lating layer. Some of the skeletal material is
deposited articulated and most exhibits little
abrasion unless the deposit has been repeatedly
THE POINT BAR-TEMPESTITE-
TURBIDITE-TSUNAMIITE DILEMMA
Great strides have been made in understanding
the meaning of layering in turbidites (Kuenen,
1948, 1950; Bouma, 1962; Walton, 1967; Enos,
1969), tempestites (Ager, 1974; Aigner, 1985), and
fl uvial point-bar sequences (Jackson, 1976). These
processes - turbidity currents, storm transport
and river fl ood transport - actually produce very
similar products which are still easily confused
and not rigorously characterized. Most research-
ers seem best able to defi ne what these layers
represent when they look at associated facies and
fauna - the pelagic E unit of the turbidite, the shelf
to lagoonal fauna and trace fossils of tempestites,
and the terrestrial signature in fl oodplains cap-
ping meandering river sequences. In fact, copious
literature is devoted to characterizing the details
of these environments and deposits, but very little
