Carbonate Lab Write Up
- 1. 1) Ooids cvx e3333334 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.
- 8. 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
- 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. .
- 20. Diagenetic Features: 19) Compaction 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. ABX (left) 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.
- 21. 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. 27) Dolomite