1. 1) Ooids
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WAP‐BG
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
2. ooids are preferentially found in areas of maximum agitation, however radial ooids can form
from turbulent environments as well.
2) Peloids
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
magnesium calcite.
B) Peloids, composed of microcrystalline carbonate, are an important constitute of shallow
marine carbonate sediments and are typical of shallow, low energy, restricted marine
environments.
3.
3) Intraclasts
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.
4.
4) Pisolites
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.
7.
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.
12. 10) Brachiopods
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.
11) Echinoderms
13.
*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
overgrowth.
12) Bryozoa
14.
*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.
15. 13) Stromatoporoids
A) The tabular, and dendroid growth morphologies are labeled in the diagram above.
16. 14) Corals
*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
(curved plates).
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.
17. 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
thin shell.
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.
18. 16) Sponges
*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.
19.
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.
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