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| The Upper Glen Rose Limestone is Lower Cretaceous in age. This sample was collected in Williamson, Texas, near the south branch of the San Gabriel River. The fresh surface of the hand sample is light gray in color and the exposed, weathered surface is a brownish gray. There are little to no visible grains present. Under the microscope, as you will see in thin section, there are however, fine grains of quartz to be classified as silt. Also in the specimen there are small holes, which are probably borings of some sort. Thin section reveals the presence of pelloids and a micritic matrix. Under the Folk classification system, this is called a silty pelmicrite. And under Dunham classification: silty mudstone. The likely depostitional environment is deep ramp. Evidence for this is provided with the following slides. |
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Figure 1. This slide of the Upper Glen Rose Limestone best shows the silt deposits. The white grains visible here are fine quartz silt. These would most likely be deposited and carried out by the storm wavebase. Width of view = 3.0mm (40x).
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Figure 2. This slide contains a good view of the pelloids present in the specimen. The pelloids are the brownish, darker, spots throughout. These pelloids are viewed as irregular spots, in contrast to the defined grains such as the quartz silt. Width of view = 3.0mm (40x).
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Figure 3. In this slide, it is clear that the sample is fine grained and well sorted. This supports the idea that the depositional environment is below the storm wavebase, but not in the deep marine because of the light color. Width of view = 3.0mm (40x).
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Figure 4. This slide shows one of the possible borings present. Around the hole is a visible oxidation ring. This oxidation ring further supports that this specimen came from deep ramp and not deep marine. The ring would be formed by oxygen levels lacking in the anoxic waters of the deep marine. Width of view = 3mm (40x). |