Chert Textures
Introduction
Macro-Textures
Micro-Textures
The Rhynie chert, examined from float blocks, trench and cored material
exhibits a wide range of macro-textures (those large enough to be visible
in hand specimen) and micro-textures (those clearly visible using optical
microscope techniques). Studying both, together with the biota the individual
beds contain, can help elucidate the different palaeoenvironments that were
present in this hot spring complex at Rhynie 400 million years ago, useful comparisons
can be made with modern hydrothermal deposits and settings (see also the
section on The ancient environment and modern analogues).
The following gives a brief summary of the textures
observed within the chert beds together with some example images.
Five basic textures have been described from the chert beds (Trewin
1994).
However, within individual beds, particularly in composite beds of chert, more
than one textural type may be present and there are often gradations between the
textural types:
Laminated Cherts
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Cherts displaying irregular, sub-parallel, wavy to crenulated laminations on
a sub-millimetre to centimetre scale, consisting of variable thicknesses of
stacked chert laminae with fine sandstone partings (see inset right). Plants, if at all present,
are confined to specific chert bands. Locally fine organic detritus including
spores may be present. Under the microscope possible fossilised cyanobacteria
may be occasionally present in the chert laminae (see section on Cyanophytes).
This particular texture most probably originated by deposition from
successive inundations by silica-rich waters, with periods of silica
precipitation occasionally interrupted by the influx of detrital material.
Right: Tabular bed of laminated chert showing wavy and
crenulated laminae of chert (c) with fine sandstone partings (s). Notice also the syn-sedimentary
fracturing and brecciation in this bed (f).
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Vuggy to Massive Cherts
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Cherts with commonly well-preserved plants, the latter often autochthonous
with upright stems (see inset below), prostrate or flattened stems; or allochthonous,
inverted stems (see inset right). Sediment and silica cement forming geopetal in-fills are
often seen within plant straws and vugs or cavities within the chert (see
below). Vugs that
have not been totally occluded by silica cements are typically lined by euhedral
(well formed) quartz crystals.
The mode of formation of these cherts is variable, but examining the
micro-textures (see below) for further evidence may indicate silicification in
areas of terrestrial plant growth, suddenly flooded by silica-rich waters, or
silicification of the contents of small ponds.
Right: Bed of predominantly massive and vuggy chert (in
this example a block of the Windyfield chert) showing vugs (v) and
well preserved, inverted plant stems of Ventarura lyonii (p).
In this case these plants have been transported and were then preserved in
the chert upside down.
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Above: Polished slab of predominantly
massive and vuggy chert with exceptionally well preserved stems of the plant
Rhynia gwynne-vaughanii in growth position. The crude lamination in the
centre of the bed most likely represents a silicified microbial mat that in
life probably bound the plant stems.
Lenticular Cherts
| Cherts comprising irregular, laterally discontinuous, dark,
organic, often plant-rich lenses. These lenses display partings of
carbonaceous cherty sandstone (see inset right). The cherty sandstones
occasionally show organo-stylolites formed as a result of the
compaction of plant material within the sediment. The centres of the chert
lenses often grade into the massive texture described above (inset right).
Lenticular cherts often occur in thick composite beds.
The patchy silicification of these plant-rich lenses and their
gradation into cherty sandstone interbeds tends to suggest patchy
and poor silicification just beneath the ground surface.
Right: Bed of lenticular chert, showing dark
organic-rich lenses (l) with a milky, massive centre (m),
and bounding cherty sandstone (s). |
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Nodular Cherts
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Cherts occurring as nodules usually under 5cm in size, set within a cherty
sandstone matrix. The latter typically appears compacted around the nodules (see
inset right) and may display organo-stylolites. The chert nodules are often dark and may be plant-bearing,
however, cellular preservation is often poor. This textural type also often
occurs together with brecciated and lenticular chert textures (inset right).
As with the lenticular cherts, this texture appears to be associated with
patchy and rather poor silicification, some of which may have been formed at the
surface, but much probably occurring just below the sediment surface.
Right: Block of Windyfield chert showing nodular texture
(n) with variable plant preservation, enclosed in a cherty
sandstone matrix. Brecciated wavy laminated chert occurs towards the top of this bed
(b) (see below). |
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Brecciated Cherts
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In this case, cherts of all the textures described above may be extensively
fractured and resealed with chert or quartz. In a few instances brecciated
chert beds recovered form below the surface weathering
zone, fractures are cemented by calcite, baryte and rarely
fluorite. Timing of brecciation appears to vary, some is clearly related to
fracturing and faulting at depth in the sediment pile, the fractured chert being
cemented by late quartz cements. Other examples occurred at the surface as a
result of desiccation and weathering of the sinter, evidenced by the fractures
being in-filled with sediment (see inset right) and/or resealed with chert
containing later silicified elements of the biota, such as fungi and algae.
Right: Heavily brecciated laminated chert (b) and
brecciated nodular chert (n) in a cherty sandstone matrix.
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Apart from the studied large scale morphological features, optical microscope techniques
provide a wealth of further
information with which, for example, we can deduce the 'way-up' of the beds; and
the
burial history of the beds (in other words their diagenesis). Textural information
can be used to elucidate the palaeoenvironment in which
the individual beds were deposited. The following includes examples of
micro-textural information that can be used to solve these problems:
Geopetal Textures
| There are a number of methods used to determine the 'way-up' of a layer
of sedimentary rock. In the Rhynie chert one of the most common way-up
indicators are geopetal layers. These are in effect fossil spirit-levels
and although recognisable in hand specimen they are more clearly viewed using a
microscope (see inset right). They generally tend to form by the accumulation of
very fine infiltrated organic and detrital material as a layer within voids in
sediment, such as in the hollow straws of plant stems. Similarly, in the chert
beds geopetal layers may also form from successive generations of silica cement.
They are perhaps most common in the vuggy to massive cherts described above.
Right: Geopetal layers (g) in a straw of Aglaophyton
major, denoting the image is the correct 'way-up'. Notice also earlier
'straw-lining' overlay of chert cement (c) and the later generation
of quartz cement (q) lining the remaining void space after the
geopetal fill (scale bar = 1mm). |
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Cement and Compactional Textures
A number of micro-textures combined with the mineralogy can be used to determine the
diagenetic history of the
chert beds. The order of different cement generations, any dissolution, compaction
and fracturing of the chert during deposition and later burial can be deduced to
a greater or lesser degree. In the image above right, for example, the
pore-lining quartz cement (q) clearly came after the geopetal layers (g) which in turn post-date an earlier generation of chert cement (c)
which lines the straw. The straw does not appear to be fractured and has
therefore not undergone any significant burial compaction suggesting early
silicification of the plant and surrounding matrix prior to burial.
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The image on the left is another example. Here is a thin section of a
brecciated nodular chert. The chert matrix (m) has been heavily fractured then
resealed by later very fine, cryptocrystalline chert (c). The framboidal pyrite (p) is
only present within and surrounded by the matrix chert and must therefore
predate this or was formed at the same time. Notice how the resealed fractures
stop where the nodule meets the sandstone (s), this suggests the sediment was
not fully silicified at the time the chert nodules were fractured, being more
ductile to compactional deformation than the brittle nodule.
Left: Brecciated nodular chert showing chert matrix (m)
with scattered framboids of pyrite (p). Fractures resealed by later
cryptocrystalline chert (c) have not affected the intercalated
sediment (s) (scale bar = 300µm). |
These are just two relatively simple examples of how micro-textures in the cherts can be used
to elucidate the diagenetic history.
Micro-Textures for Determining Palaeoenvironment
A number of micro-textures are found in the various chert beds that together
with the biota present help to determine the palaeoenvironments in which
individual chert beds were deposited. For the purposes of this resource we shall
only consider a couple of examples here:
| The image on the right is of a thin section taken from a
block of Windyfield chert. This texture is quite distinctive and
diagnostic. The amorphous, organic-rich, ellipsoidal, elongate and
spherical bodies are actually fossilised faecal pellets, termed coprolites.
It is most likely that the bed this sample came from was deposited in an
aquatic setting, perhaps a pond or ephemeral body of water. These
features, at least at this scale, are rarely preserved in terrestrial
settings. Secondly, it is likely that the silicification of this deposit
was very early because the coprolites are not squashed and compacted
together, in fact there is a very open 'framework' between the pellets. By studying coprolites, in terms of their size, geometry and content it
may be possible to determine what type of organism produced them. Thus by
understanding coprolites a lot of information can be also gained on the
interactions between fauna and flora (see Habgood
et al. in press). |
Above: Chert with loosely packed
coprolites (c). The bed this particular thin section came from must have
been deposited in an aquatic environment, possibly a pond. These
coprolites were probably produced by small crustaceans (scale bar = 500µm).
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An interesting point to note is that the coprolitic micro-texture (inset
above right) is often a good 'pathfinder' texture when prospecting for well
preserved arthropods in the chert.
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Above: Thin section of Windyfield chert,
from the same massive and vuggy block illustrated above, showing partially decayed stems and sporangia of Ventarura lyonii
enclosed in a patchy, organic-rich, clotted chert matrix (cl).
Open voids in the matrix have been lined with an overlay of chert
cement (c) and later in filled by geopetal sediment (g)
and cryptocrystalline chert. The preservation of this open framework
suggests initial silicification of the matrix and plants occurred whilst
still in an aquatic setting before exposure and/or burial (scale bar =
2mm).
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The image on the left shows a similar chert sample taken at
a lower magnification. The coprolites often occur together with a fine
meshwork of filaments (fungal and/or cyanobacterial), unicells and
aggregated fine
amorphous organic material forming an open, loose, plexus-like matrix that
often binds and coats floral and faunal remains, giving the chert a
'clotted' appearance in thin section (cl). Often associated with this
texture are fossils of aquatic biota such as charophyte
algae and branchiopod crustaceans,
though remains of terrestrial plants and arthropods may also be common. This distinctive clotted chert texture is directly comparable to
mulm, the amorphous organic material that is often found in modern
freshwater ponds (Anderson & Trewin 2003;
Trewin et al. 2003; Fayers
& Trewin in press). The presence of this texture in some of the chert beds is
useful in a number of respects. As mentioned above, it acts as a useful
'pathfinder' texture for finding well-preserved arthropods. It also
indicates that silicification in these particular beds took place in an
aquatic setting with no desiccation prior to preservation, since
desiccation would have led to the collapse and degradation of these fine,
open organic mesh-works (Fayers & Trewin
2003; Fayers & Trewin in press). |
Very often chert beds displaying these textures and containing some of the
most exquisitely preserved plants and arthropods also exhibit finely disseminated
pyrite in the chert matrix, occasionally occurring in framboidal clusters
(see above). The pyrite appears to be contemporaneous with the earliest stages
of silicification. Its presence indicates the waters from which it precipitated
were at least mildly reducing, suggesting that some of these small ponds were at
times stagnant. Such localised reducing conditions would inhibit the rapid decay
of organic matter and may in part explain the fantastic preservation of some of
the fossils (Fayers & Trewin 2003).
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