Objective: In this laboratory exercise you will observe and describe features of diagenesis that are commonly seen in thin sections of the carbonate rocks. These features include cementation, dissolution, mineral replacement, and fracturing among others.
Please
thoroughly study the following:
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Micro-boring in a mollusk bioclasts. Thin section has been stained with alizarin red-s and was impregnated with blue dyed epoxy to enhance porosity. Neuse Formation (Late Pleistocene) Snows Cut, New Hanover county, North Carolina. Plain light. |
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Partial infilling of intrapartical porosity of an echinoderm bioclasts with glauconite (rusty red colored area in center of grain). No evidence of permineralization in this grain. If this grain were to become permineralized the presence of the glauconite would help to retain the original features of the grain. Modern, Onslow Bay, North Carolina. Polarized light with the 1st order red plate inserted. |
Unstable minerals like aragonite are frequently dissolved leaving behind a moldic pore in the shape of the original allochem. The pores than can be later infilled by various authigenic minerals (calcite be the most common but also gypsum, anhydrite, quartz and dolomite). Sometimes the dissolving material is immediately re-precipitated in adjacent pore spaces. Molds do not have to be of an allochem for even authigenic material can be later dissolved forming a moldic pore. If dissolution results not in a cavity of recognizable form then it is best to refer to it as a solution vug.
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Moldic pore after a mollusk bioclasts (pelecypod). Pelecypods produced aragonite so the aragonite was dissolved leaving behind only calcite. Castle Hayne Limestone (Eocene), southeastern North Carolina. Plain light. |
Any kind of pore space can be filled or as we will say ‘reduced’ by some filling material. If the fill is an authigenic precipitate like calcite then one can further specify the nature of the fill as being (1) isopachous, meaning it forms a layer of near uniform thickness around the pore, (2) blocky, referring to the mineral as forming a mosaic of crystals within the pore with no special relationship between crystal size and shape and pore wall, or (3) poikilotopic, meaning one single large crystal has engulfed the entire pore and adjacent pores and more than one allochem. If the material filling the pore was not totally filling or reducing the pore we say that such-and-such partially reduces the porosity. If more than one item is filling the pore then we list from the oldest to youngest using the principal of superposition.
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Complete reduction of moldic porosity after a mollusk bioclasts by isopachous bladed calcite. The bioclasts was probably initially aragonite; the material for the formation of the calcite could have been derived from the dissolution of similar bioclasts nearby. Castle Hayne Limestone (Eocene), southeastern North Carolina. Polarized light. |
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Moldic porosity after a mollusk (pelecypod) and porosity reduction by isopachous bladed calcite cement. The parallel lines in the center of the figure are all that remains of the pelecypod. These lines are the micrite envelope that formed post-mortem on the surfaces of the shell due to micro borings. Castle Hayne Limestone (Eocene), southeastern North Carolina. Polarized light. |
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Moldic porosity after bioclasts (mollusk) with moldic porosity and interparticle porosity partially reduced by isopachous bladed calcite cement. Note that the size of the calcite cement crystals outside of the molds is much larger than those within. This suggests that the interparticle cement was forming at the same time that the mollusk bioclasts was dissolving. Polarized light with the 1sr order red plate inserted. |
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Moldic porosity after bioclasts and interpartical porosity partially reduced by isopachous bladed calcite cement which was later partially dissolved. This interesting rock is now entirely calcite spar cement and pore space. Bioclasts were most likely all aragonitic. They were dissolved with the dissolved material being reprecipitated as calcite cement. Loss of the crystal points suggests a later dissolution of these. Polarized light. |
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Solution porosity completely reduced by anhydrite. The rock is otherwise dolomite of replacement origins (note the ghost textural features, especially the pisolite in the upper right area. Grayburg Formation (Permian), Ector County, Texas. Polarized light. |
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Intergranular porosity completely reduced by poikilotopic anhydrite cement. Allochems are dolomitized pelloids. Grayburg Formtion (Permian), Ector County, Texas. Polarized light. |
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Complete reduction of moldic porosity after dolomite euhedra by blocky calcite. The rhombohedral shape of this feature is the clue that it was originally dolomite. The rock is now entirely calcite. There is a three step paragenesis here: (1) formation of the dolomite euhedra, (2) dissolution of the euhedra, and (3) precipitation of calcite in the moldic pore. Polarized light with 1st order red plate inserted. |
In mineral replacements as secondary mineral (metasome) comes to occupy the special position of a former mineral (paleosome). The process involves two simultaneous chemical reactions, one dissolution of the paleosome and the other precipitation of the metasome, that proceed at the same volumetric rate. This is a very special condition that leads to a very specific type of fabric. In a replacement all material not involved in the dissolution-precipitation reactions maintains it special position and is transferred from being within the paleosome to the metasome. The consequence of this is that the some aspects of the original rock textural fabric is retained as ghost images in the replaced portions. Should dissolution volumetrically exceed the precipitation of the secondary then one simply has a moldic pore that is reduced by some type of cement, much like some of the above. If precipitation of the secondary proceeds faster than the primary is dissolved then the ‘force of crystallization’ does mechanical work on the rock fracturing it, twinning mineral grains, or simply plowing its way through the rock fabric. If you doubt that a growing crystal can do any significant work then think about what freezing ice can do. In both of these situations ghost images of the original fabric are not formed.
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Dolomitization of calcite. Here large dolomite crystals have replaced foraminifer and echinoderm bioclasts. The textural features that you see are ghost images of the original fabric of the allochems. The sample is entirely dolomite. Madison Group (Mississippian) Whiterocks River Canyon, Utah. Plain light. |
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Partial dolomitization of calcite. Ooids have been replaced with dolomite euhedra. Note the retention of the original textural character of the ooids within the dolomite. In this sample the dolomite has been largely selective for the ooids, a common feature of dolomitization. Thin section has been stained with alizarin red-s. Cambro-Ordovician, Spitzburgen. |
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Dolomitization of calcite where dolomite has partially replaced ooids and the sparry cement between the ooids. Note the ghost texture of both preserved in the dolomite. Two step paragenesis here, (1) interpartical porosity completely reduced by blocky calcite and (2) partial replacement of the calcite by dolomite euhedra. Madison Group (Mississippian), Utah. Thin section stained with alizarin red-s. Plain light. |
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Dolomite euhedra in limestone. This is possibly of replacement origin but we cannot tell for sure because no ghost texture is evident in the dolomite. The dolomite could be a displacement dolomite. Sample has been stained with alizarin red-s. Plain light. |
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Calcitization of dolomite. Here a dolostone, which appears to have had a xenotopic fabric (common euhedral crystals) has been entirely replaced with calcite. Note the rhombohedral shapes still evident. Davenport Breccia (Devonian), eastern Iowa. Sample has been stained with alizarin red-s. Plain light. |
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Calcitization of aragonite. Here a mollusk bioclasts, which was originally aragonite, is now calcite. The textural features that you see, the bars, are a ghost image of the original micro-texture of the bioclasts. The fine dark mosaic of lines is the crystal boundaries of the calcite crystals. Castle Hayne Limestone (Eocene), southeastern North Carolina. Plain light. |
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Partial Silicification of calcite with the development of radially fibrous or botryoidal quartz also referred to as chalcedony. Eocene, Aiken, South Carolina. Polarized light. |
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Silicification of calcite. Here an oolitic limestone has been completely replaced by quartz. Fabric, which is evident, is entirely a ghost image of the original rock fabric. The rock is now a chert. Polarized light. |
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Silicification of calcite with complete replacement of the limestone fabric with quartz. Note the ghost images of the foraminifers. Madison Group (Mississippian), Little Brushy Creek, Utah. Plain light. |
Grains or crystals being mechanically forced against adjacent crystals can result in dissolution of one or both. The result is that one grain may embay another or that they may form a common zig-zag like boundary called a micro-styolite. On opposite sides of this boundary may be the same mineral. The one being dissolved is for some reason slightly more soluble than the other. This may be due to trace element chemistry, crystal lattice orientation, or crystal lattice imperfections. Sometimes one observes on a macroscopic scale large zig-zag lines cutting the rock, these we call styolites.
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Pressure solution. The echinoderm bioclasts right of center has embayed the echinoderm bioclasts immediately above and below it. Also most other grain boundaries are sutured or micro-styolites. This sample lacks intragranual material (porosity, micrite, spar). Thin section was stained with alizarin red-s. Madison Group (Mississippian), Utah. Plain light. |
Rocks can deform in a brittle manner and fracture with the fracture pore being later filled with some form of cement. Generally the cement will have crystals that nucleate on the fracture wall and grow into the opening. Sometimes one observes an apparent fracture that is completely reduced by a prismatic or fibrous mineral that is oriented long axis normal to the wall. Here the force of crystallization of the ‘filling’ material may be the actual cause of the opening of the fracture.
Expect multiple sets of fractures and look for displacements from one side to the other of the fracture. Also look for what gets fractured especially in terms of other features of diagenesis. Apply the principal of cross-cutting relationships.
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Fracturing with fracture porosity completely reduced by isopachous saddle dolomite. Saddle dolomite is characterized by having a warped crystal lattice so that when the microscope stage is rotated there is a sweeping extinction. Saddle dolomite is a void filling dolomite. Here a fracture in a dolostone opens (note the dark sediment fill on the floor of the opening). Then the dolomite precipitates in the open space. Polarized light with 1st order red plate inserted. |
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Fracturing with fracture porosity completely reduced by equant calcite. Two sets of fractures here; the east-west set preceded the north-south. The north-south fracture exhibits dextral trans-tensional displacement (match the shape of the fracture walls). Four step paragenesis evident; two stages of fracture formation and two stages of infilling with calcite. Polarized light with 1st order red plate inserted. |
Exercise #1: You are to study the assigned set of thin sections. Please describe verbally and with a nice well drawn and ‘useful’ diagram each diagenetic feature that you see in each thin section. This is to be done in your lab book.