Miami oolites (left)
* The WAP‐BG TS, is an ooid grainstone according to Dunham’s classification,
A) Modern Miami oolites demonstrate two different types of microfabrics: some have tangential
and others have a random fabric of aragonite crystals.
In contrast, the Carboniferous ooids primarily have a radial concentric microfabric. Also note
that an intragranular fabric exists with <5% porosity.
B) In addition to the differences in microfabrics, Modern day ooids are usually composed
primarily of aragonite; whereas in carboniferous ooids, calcite has completed replaced the
aragonite, and their original texture preserved by both blocky and sparry calcite cement as
indicated on the diagram above.
C) In general, the environmental significance of ooids is that they indicate formation in a high‐
energy environment where water is supersaturated in respect to calcium carbonate, which
precipitates to form encrusting isopachous concentric layers. Ooids are generally formed in
agitated very shallow, tropical coastal environments and are favored by either intertidal or
subtidal conditions. However, ooids can form in wide variety of settings from shallow‐marine
settings to lagoons, lakes, rivers, caves and even calcareous soils. Also, the microfabric of ooids
can be indicative of the depositional environment. In low energy environments, such as
protected lagoons or troughs between oolite bars, aragonite ooids with a loose structure
predominate. In higher‐energy environments such as the crests of bars of tidal deltas the ooids
outer laminae have a tangential arrangement with tightly packed crystals. Tangential aragonite
ooids are preferentially found in areas of maximum agitation, however radial ooids can form
from turbulent environments as well.
AKA BAH Peloids
A) There are many possible modes of origin of peloids: fecal pellets, calcareous algae, micritised
grains, small intraclasts, mud clasts or a precipitate origin.
Deposit‐feeding animals produce fecal peloids. Peloids can also represent micritised grains such
as abraded shell fragments or ooids. They also can be remnants of fine‐grained algal remains or
form by the calcification of cyanobacteria in algal mats. Many peloids are simply sand‐sized
intraclasts or lithoclasts derived from pre‐existing micritic substrates. Finally they can also be
chemical in origin and represent cements in which the pellets are sits of small crystals of high
B) Peloids, composed of microcrystalline carbonate, are an important constitute of shallow
marine carbonate sediments and are typical of shallow, low energy, restricted marine
Maynes Hand Sample (below) WJM 608 TS (above)
A) Intraclasts formed from poorly sorted weakly consolidated sediment that is reworked from
within the area of deposition. This process consists of deposition in a river’s interchannel area,
followed by quick lithification of mud and then erosion by fluvial action.
PR‐10 Hand sample and Thin sample
A) According to my observations, these are vadose pisolites, since they are quite larger and have
several layers of isopachous coatings, many with stalactitic texture (due to gravity) as indicated
on the diagram above. To further support my claim, these pisolites share fitted laminae as shown
in the diagram as well.
6) Cyanobacteria: Algal Laminates, Stromatolites and Oncolites
A) As shown in the diagram above, DT‐20 shows evidence of algal filament molds, these white
calcified microbial filament molds are generally normal to bedding.
B) These laminations are formed by sediments such as sand deposited on tidal flats and trapped
by microbial algal mats which consist of layers of filamentous and unicellular micro‐organisms
mainly cyanobacteria. This alternating depositional pattern of sand and algae mats is then
repeated several times.
C) The algae laminations of DT‐20 and MI‐11 probably formed in a low energy environment such
as a restricted bay area, whereas the Kinblade and recent stromatolites probably formed in the
high‐energy environment such as the intertidal zone.
D) Oncolites usually form in a similar fashion as cyanobacteria laminations form except they
form radial layers around a central nucleus such as a shell fragment; calcium carbonate is
precipitated by encrusting microbes (cyanobacteria), causing a layered spherical growth
structure, facilitated by the cyanobacteria growth. Oncolites are very similar to stromatolites, but
instead of forming columns they form spherical structures.
7) Red Algae
A) The goniolithon red algae also called coralline have a distinguish branching shell structure,
note the fine scale polygonal cellular structure. Their multi‐layered coaxial skeletal structure
consists of a medulla (central part) extending upwards to the next segment composed of
partitioned filaments and peripheral cellular filaments surround the medulla. Other samples
show an encrusting form.
B) Red algae are a valuable paleo‐environment marker. Although they need sunlight to carry out
photosynthesis they are not always indicative of a shallow tropical warm water marine
environment. They have been found to flourish as deep as 250 meters, as long as light can reach
them. Red algae can grow in temperate and cold‐water climates as well. They prefer agitated
water environments, such as in the inner‐ramp. The reef frameworks and morphology of red
algae branches reflect different water energies. For example, red algae are the dominant
organism of most ancient carbonate rimmed shelves.
8) Green Algae
*According to Dunham classification, PSM is mostly an algal wackestone although there are some
areas that may be considered a algal packstone.
A) Since green algae can only exist in warm tropical waters in low latitudes they indicate a warm
tropical paleoclimate. Also they are unable to grow in deep water and therefore indicate a
shallow marine environment, such as in the outer ramp. They are common in quiet lagoons but
are also found in less turbulent parts of open shelves and reefs where sunlight penetration is
*According to Dunham’s classification, KLFI is a molluskan grainstone.
A) There are two types of shell preservation in Miami Mollusks; some are preserved as molds
(voids) since the aragonite shell itself has dissolved away. Other shells are preserved as casts
(mold filling) this calcitization preserves the relic micro‐architecture of the aragonite.
B) There are two major types of fabric:
The first is a primary structure that is indicative of the original biogenic shell, consisting of
fibrous crystal that are orientated perpendicular to the shell.
The second is a micritic fabric, since the aragonite shells are highly unstable and have been
diagenetically altered by leaching producing micritic envelopes.
There are also different porosity types present: There is intragranular fabric, a diagenetic
structure, the interior of some shells is dissolved away by leaching, some have been completely
dissolved causing a diagenetic mouldic fabric. There also is Intergranular porosity, a primary
structure. (I was confused by the word “fabric” so I gave two answers)
A) These brachiopods have a low angle fibrous wall structure which is different from than the
high angle (nearly perpendicular) lamellar wall structure (narrow bands of light/dark
extinction) and of the mollusks.
*According to Dunham’s classification, BURL‐1 is a echinodermian grainstone in most areas but
can be considered coarse grained crystalline in others.
A) Echinoderms have five‐fold symmetry, single crystal extinction and a holey fabric ( small
pores that appear as tiny black spots). As ancient shells lost magnesium calcite, the holes get
filled in and the cement takes on the same orientation of the echinoderms, this is called syntaxial
*According to Dunham’s classification 565018 is a bryozoan wackestone, although some areas of
the thin section can be classified as a bryozoan packstone.
A) The microstructure consists of chambers called zooecia that are arranged in either a radial
lacy pattern or an elongated stick pattern. Note the finely fibrous wall structure.
A) The tabular, and dendroid growth morphologies are labeled in the diagram above.
*According to Dunham’s classification scheme, HG 4‐4, is primarily a coraline boundstone.
A) The coral wall is made out of bundles of aragonite that have an irregular extinction pattern
under crossed polars. The microstructure is composed of a septa (vertical central dark line with
surrounding trabecular structure (horizontal fibrous or bladed crystals) and dissepiments
Visible in the CKL‐1 sample is the radial structure of the septa.
On a macrostructure scale, colonial corals have similar calcareous skeletons with basic skeletal
elements of aragonite or calcite fibers.
15) Trilobites and Ostracodes
A) The distinctive shell structure of trilobites is their long worm‐like shaped shell that may or
may not have hooks. Under crossed polars, extinction bands sweep across the grain as the stage
is rotated, this is called a uniform prismatic extinction. Ostracodes have a distinct eye‐shaped
B) Ostracodes are distinguishable from brachiopods or bivalves because they as smaller in size
and have a homogenous prismatic wall structure with calcite cement and chitin composition.
*According the Dunham classification, PR6‐10 most of the sample is a micritic mudstone but the
mid‐lower section is spongean boundstone.
A) While noting the relationship between the sponges and the other components of the rocks it is
important to notice how the sparry calcite fills the spicules. There also are meandering canal
structures and well‐preserved wall structures. Some canals are partly filled with hematite.
17) Benthic Foraminifers
*According to Dunham’s classification, 565022 is a foraminiferian packstone.
A) Characteristics of other fossils such as fragmented shells and skeletal fragments in 565022,
and the abundant pellets in AAX that also indicate a semi‐restricted shelf lagoon environment.
1035 TS: Chemical Compaction: dissolution of grains apparent
Burl 2: Chemical Compaction since there are stylolites
FT‐88: Mechanical Compaction: grain breakage an plastic deformation.
20) Marine Cements
BAH‐8: Fibrous marine cement along rounded oolitic and micritized grains with an isopachous
texture, since the fibers are circular and going all around.
ABX: Multiple isopachous layers of fibrous marine cement along elongated non‐spherical grains.
TR‐10: columnar isopachous cement is evident since the length to with ratio is greater than 6:1.
They form bladed rings around large massive crystals. Also sparry calcite cement fills voids and
holds the rock together.
21) Beach rock
**Shark Bay 1‐9 has intergranular and intragranular porosity.
22) Meteoric Cements: Vadose Cements
**Miami Oolite has three porosity types, primarily an intergranular fabric exists between ooids
and also the meniscus cement has intragranular porosity. In addition there is a very small
amount of mouldic porosity due to the leaching of grains.
A) The meniscus cement in the Miami Oolite sample is vadose in origin, evident by the low‐Mg
calcite whisker crystal cement. These thin, randomly orientated calcite crystals are produce in
the meteoric waters, probably in the vadose zone.
B) The cement in the KLFI sample also indicates an origin in the vadose zone evident by the
micrite envelopes, and finely crystalline meniscus cement. Also the pores themselves are more
rounded in outline since the cement prefers to grow more at the junction between grains, where
water is caught by capillary action. There is also some, but not much, evidence of calcite whisker
crystal cement as well.
23) Meteoric Cements: Phreatic Cements, Blocky Calcite
**Red algae has both intergranular and intragranular cement.
B) In the red algae 16‐203 sample, the blocky calcite cement surrounds the red algae and occurs
in between grains and also may replace grains.
C) In FT‐88, the block cement was not in place when the compaction occurred, evident by the fact
that the cement is not broken. However, cementation must of occur early and not too long after
compaction for the sample to be preserve like so.
24) Meteoritic Cements Inclusion free
Crinod fragments can be distinguished from syntaxial overgrowths even though syntaxial
overgrowths are cement that takes on the same crystal orientation of the crinod fragment grain
that it is growing. The syntaxial overgrowths are more massive and continuous than the crinoids.
Also, the boundaries are evident when the stage is rotated with crossed polars. Both syntaxial
overgrowths and Crinod fragment have unit (single crystal) extinction that will occur
simultaneously making the grain seem to be one whole grain while extinct. However, in when
rotated out of extinction the boundaries between crinod fragment and the syntaxial overgrowth
surrounding it are clear.
25) Cement Stratigraphy:
See Diagram below:
26) Neomorphic Fabrics
A) Petrographic features that differentiate neomorphic calcite from blocky calcite cement
because neomorphic spar have irregular embayed to curved intercrystalline boundaries,
contrasting with the commonly planar intercrystalline boundaries of blocky cement, and
neomorphic spar also has and irregular crystal size distribution and patchy development.
Gradational and irregular boundaries to the areas of neomorphic spar and the presence of
skeletal and other grains floating in coarse spar, also offer a means of telling one from the other.
** SR‐118 has three porosity types, intercrystalline, mouldic are visible in thin section and
fenestral in the hand sample
A) SW‐27 High degree of obliteration
B) SR‐118 Medium degree of obliteration, original fabric of fossils completely gone, but geometry
of original cement that supported grains is still evident.
C) B‐037 High degree of obliteration
D) P‐10, note high degree of fabric retention
E) Gibson TS: High degree of obliteration
Look a happy face in the Dolomite!!!