Transcript BIOCHEMICAL SEDIMENTARY ROCKS

BIOCHEMICAL
SEDIMENTARY ROCKS
Prepared by Dr. F. Clark
Department of Earth and Atmospheric
Sciences, University of Alberta
August 06
INTRODUCTION
The two principle groups of biochemical sedimentary rocks
are carbonates and siliceous sediments called chert. We
will say more about chert in the presentation on
chemical sedimentary rocks, for reasons that will be
evident there. The bulk of carbonate sedimentary rocks
consist in large part of materials that are either directly
metabolized or secreted as solid carbonate minerals by
organisms as shells or other hard parts, or else their
precipitation from saturated marine or other waters is
aided by the influence of organisms on water chemistry.
It is important to note, however, that not all carbonate
production is organically influenced.
Calcite.
This common
carbonate
mineral, with
the formula
CaCO3, reacts
strongly with
dilute
hydrochloric
acid.
The other polymorph of CaCO3, with the same chemical formula but
different crystal structure, is called aragonite, and is often the form in
which this material is first precipitated. It reacts [fizzes/effervesces]
with HCl as well, and is generally not distinguished in hand specimen.
Dolomite.
This common
carbonate
mineral has Mg
as well as Ca,
with the
formula
CaMg(CO3)2. It
will also react
with HCl, but
weakly.
As seen in the crystals above, dolomite has rhombic cleavage, the
same as calcite. However, it is slightly harder than calcite (3 ½ to 4
instead of 3), and only reacts weakly with HCl when powdered. The
colour difference between crystals of the two is not consistent.
CARBONATE CONSTITUENTS
There may be three basic constituents in a carbonate rock.
The first, and often most obvious, consists of carbonate
grains, of which there are several types (some will be
illustrated). They generally, but not always, consist of
several smaller crystals of calcite or aragonite. The
second constituent is micrite, or microcrystalline calcite,
which is generally but a few microns in size. It is often
referred to as lime mud, which would be carbonate
material less than 1/16 mm. The third constituent is
sparry calcite, a fairly coarsely crystalline and clear (in
thin section) form. As a gross simplification, the latter
two occur in spaces between carbonate grains.
Carbonate Grains – Skeletal Grains
Both these samples consist almost entirely of skeletal grains, or fossil
shell material. The left sample has large fragments and intact shells
of the pelecypod mollusc Coquina, whereas the right sample is
chalk, comprising an almost pure collection of coccoliths, tiny
calcareous plates measuring only 5-10 microns (.005 to .010 mm).
These are from a group of planktonic algae called coccolithophores.
More Skeletal Grains
In the Paleozoic (that interval of time between 544 and 250 million
years ago), among the most common invertebrate groups, and thus
types of skeletal grains, were brachiopods [green arrows]. In the
right sample, the wide, straight hinge line of many specimens [light
blue arrows] is the only part of the shell clear of the lime mud
matrix. Purple arrows point to crinoids [next slide].
More Paleozoic Skeletal Grains
The prominent invertebrate fossils in the left sample are bryozoans
[blue arrows], colonial organisms whose individuals inhabited the
tiny openings, now filled with yellowish lime mud, in the branched
skeletons. On the right, purple arrows point to small circular discs
called ossicles that once comprised the stalk attaching the body of
these crinoids [a group of echinoderms] to the sea floor.
Carbonate
Grains –
Peloids.
These are
structureless
aggregates of
lime mud, most
commonly
invertebrate
fecal pellets.
In this core sample, blue arrows point to peloids, the green arrow is
parallel to lamination [best seen just below], the purple arrow points to
a stylolite [discussed later], and the yellow star highlights an area
where light acid etching has cleaned up “fuzz” created by the rock saw.
Carbonate
Grains –
Ooids.
Ooids are
concentricallylaminated
grains, the
laminae
consisting of
tiny crystals.
This sample, from a saline lake deposit, may be called an oolite. The
ooids [blue arrows] are sand-sized [between 1/16 and 2 mm], giving
the surface of the rock a pebbled look similar to a basketball or
football. They are usually formed in shallow, agitated environments.
Silicified
Oolite.
In this oolitic
rock [ooid
grainstone], all
the original
carbonate has
been replaced
by silica,
preserving
original details.
Carbonates are unstable, and often undergo changes after deposition.
All such changes, short of metamorphism, comprise diagenesis. This
sample presently is siliceous, but correct interpretation depends on
recognizing that it was deposited as a carbonate.
Weathering of Carbonates - 1
Both views are of an oolite from the Cayman Islands. The left view
shows the fresh surface, with its bright cream colour and pebbled
texture, whereas the right view shows the fresh surface toward the
left, and the darker, grey weathered surface toward the right, with
heavy corrosion pits [green arrows]. The irregular contact between
the two surfaces is highlighted in light blue arrows.
Weathering of Carbonates - 2
This sample of coquina [skeletal grainstone] shows the fresh surface on
the left, with spaces between the pelecypod mollusc shells [yellow
arrows] filled with sparry calcite cement. The weathered surface on
the right shows that the sparry cement has been selectively
dissolved or leached by corrosive meteoric [fresh] waters, exposing
the pelecypods in relief.
Diagenesis –
Recrystallization.
The instability
of carbonate
minerals
means that
when they are
buried, they
tend to
recrystallize.
When subjected to stress, a common response of minerals is to grow
as fewer, larger crystals, to minimize stress at grain boundaries. This
carbonate is presently characterized by coarse, sparry calcite, with
significant pores [light blue arrows] developed during crystal growth.
Recrystallization 2.
The coarse
individual
crystals can
easily be seen
[dark blue
arrows]; this
process tends
to obliterate
details.
One survivor of the recrystallization process is the ribbed shell of a
brachiopod [yellow arrow]. One common positive effect of
recrystallization is the development of pore spaces [light blue arrows]
as atoms are rearranged. Porosity enables a rock to hold fluids.
Diagenesis –
Stylolites.
Another effect
of stress at
grain
boundaries is
that grains may
dissolve by
pressure
solution.
Carbonate may be soluble, but clay and organics generally are not, and
these insoluble residues are concentrated at the solution front as
stylolites [purple arrow]. The amount of material lost can be gauged by
the offset of the distinctive laminae [stars] across the stylolite.
Stylolites 2.
Samples rich in
lime mud are
very vulnerable
to pressure
solution. In this
sample, several
stylolites are
developed
[purple
arrows].
A greenish layer [green arrow] of unknown original thickness has been
almost completely occluded or pinched out by this process. The fossils
highlighted by blue arrows are examples of Amphipora, a common
stromatoporoid in quiet waters of the Devonian of Western Canada.
DOLOMITE –PRIMARY SEDIMENT?
There is some debate about whether or not there is any
true primary dolomite, that is, precipitated directly from
saturated hypersaline [excess salinity] waters. What is
clear is that much dolomite, and in the case of the
Western Canada Sedimentary Basin, most dolomite,
forms as a replacement product, whereby Mg is added to
calcium carbonate precursor grains, and thus limestone
rocks, to produce dolomite crystals in dolostones. Just as
is the case with recrystallization, the process of
dolomitization tends to obliterate original textures, such
that interpretation of the rocks becomes problematic.
DOLOMITE – BIOCHEMICAL?
Some introductory texts will take the observation that there
may be no true primary dolomite and suggest that this
means that dolomite and dolostones are chemical
sedimentary rocks, formed by inorganic diagenetic
processes. However, the reality is that most dolostones
have biochemical carbonate precursor rocks. Thus the
interpretation of most dolostones only makes sense if
one considers them from a biochemical perspective. For
this reason, sedimentology texts will almost invariably
place dolostones in the biochemical camp.
Dolomitization – The Tyndall Formation
The Tyndall Formation is quarried for building stone, and adorns many
buildings in Western Canada, including the Tory Building and Telus
Centre at the U of A. The left view shows the bland weathered
surface, whereas the right view shows the fresh surface. The lighter
material is limestone, and the darker colour is dolostone mottling
[purple arrows]. The fossil is Receptaculites, possibly a fossil alga.
Control of
Dolomitization.
Dolomitization
of the Tyndall
Formation has
occurred next
to burrows
[purple
arrows].
These ancient burrows were formed in the sediment, before the
sediment was indurated/lithified. For whatever reason, the Mg-rich
fluids moved through, and out from, the burrows into the surrounding
limestone. Note the sunflower pattern to the Receptaculites skeleton.
Dolomitization and Dollars
Both core samples are from oil exploration wells, and illustrate the
Devonian Leduc Formation [left] and Grosmont Formation [right].
Both are fully dolomitized limestones with porosity [blue arrows]
that enables them to be prolific hydrocarbon producers. The
cylindrical plug [pink star] removed from the Leduc core was
analyzed for its petrophysical properties [reservoir characteristics].