Geology Reference
In-Depth Information
Fig. 3.6.
Field description of skeletal
concentrations
(Modified and completed
after Kidwell et al. 1986). Accumulation
of shells, echinoderm fragments and other
fossils are produced by biogenic (area 1 in
the triangle), sedimentologic (area 2) and
diagenetic processes (area 3). An example
of
biogenic concentrations
are oysters,
whose larvae preferentially colonize sites
with abundant adults causing reef-like
structures in subtidal shelf settings. Photo
at top right:
Placunopsis
, Middle Triassic,
Germany.
Sedimentological concentrations
form
through hydraulic reworking of hardparts,
and/or through non-deposition of sediment,
or selective removal of the sedimentary
matrix. These processes result in the
formation of conspicuous shell beds
(lumachelles, coquinas).
An example is shown in the foto at
bottom left: Accumulation of looseley
packed, bimodal sorted bivalve shells within
a storm layer originated in inner shelf
position; the inversely graded carbonate
mud was infilled sub-sequent to the
deposition of the shells. Late Triassic,
Northern Alps, Austria.
Diagenetic concentrations
originate from the increase of the spatial density of fossils by compaction and selective pressure
solution. The photo at bottom right displays a crinoidal limestone, formed on a carbonate ramp and strongly affected by
pressure solution. Early Carboniferous: Frank's Slide, Turtle Mountains, Alberta/Canada. The triangle can be used to estimate
the position of shell beds along an onshore-offshore transect: Tidal flats are characterized by a composition corresponding to
the areas 1 and 2; lagoons by the dominance of the areas 1 and 4; shallow shoals by the dominance of the area 2; inner shelf
shell beds by the areas 2 and 4; and subtidal outer shelf settings below storm wave base by the abundance of concentrations
that correspond to area 1.
and environments responsible for their origination. Fos-
siliferous carbonates yielding fossils or fossil fragments
larger than 2 mm can be readily categorized into mac-
roscopic fabric types using semi-quantitative scales for
close-packing and size-sorting of coarse bioclasts
(Kidwell and Holland 1991; Fig. 3.7).
The fabric types are useful to narrow the possible
modes of origin, e.g. accumulations of mollusk shells
or crinoidal fragments.
3.1.1.4 Field Logs and Compositional Logs
Field information originating from stratigraphic sec-
tions and mapping is commonly recorded by written
descriptions of observed features (field notes: Tucker
1982, Graham 1988).
Graphic logs aid in documenting of rock composi-
tion and texture and consist generally of a main col-
umn showing the lithological sequence (drawn as col-
umn or imitating a weathering profile) and adjacent
columns showing the occurrence and distribution of
sedimentological and paleontological data. These col-
umns may include only field data or a combination of
field and laboratory data.
Various standard forms for plotting graphic logs and
standardized coding systems have been proposed to
uniformly describe facies criteria, and to facilitate trans-
forming field and laboratory data into digital data (e.g.
Charollais and Davaud 1974; AmStrat-CanStrat sys-
tem: Miall 1990).
Reijers et al. (1993) recommend a hierarchically or-
ganized lithofacies coding system allowing macro-
Ichnofabrics and trace fossils
have a high potential
for interpretating of paleoenvironments and deposi-
tional processes (Donovan 1994; Bromley 1996). A semi-
quantitative field classification of bioturbation based
on pattern recognition of the percentage of original sedi-
mentary fabric disrupted by biogenic reworking facili-
tates the explanation of bioturbation structures seen in
thin sections (Droser and Bottjer 1986; see Sect. 5.1.4).
Distinctive distributional patterns of trace fossils in car-
bonate shelf environments are useful in delineating en-
vironments of deposition and determining sea-level
positions.
