Cemendix Quarry Case Study

In many places rock units can be described, grouped together, given similar characteristics, and placed into lithofacies. These lithofacies are important for interpreting the past depositional settings of the rock units. At the Cemex Quarry, we went through three stratigraphic columns, describing the different rock layers, and placed them into distinct lithofacies. The first lithofacies we described was of an Imbricated Clast-Supported Conglomerate. The first thing that stood out was the orientation of all the clast, which ranged from gravels to cobbles. They were all oriented the same direction in accordance with flow direction. This can tell a lot about the flow strength, which had to be high in energy to move the large clasts. With further inspection of the facies, we determined the matrix to be feldspar rich. Because it is feldspar rich, it can be surmised that it is compositionally fairly immature. These two characters lead us to interpret this depositional environment as a braided river system. Braided rivers systems are capable of a high bed load of sediments, high flow energy, and still not too far from the
source, which would explain the feldspar rich matrix. To better constrain this interpretation, there should be more units similar to this in the area and stratigraphic level separated by braid bars. Lithofacies two is also of a clast-supported conglomerate.
However, unlike the above conglomerate, the clasts are not imbricated. The matrix supporting this conglomerate was a comprised of feldspar rich silt to sands but also included an abundance of clay sediments making up 50 percent of the matrix. These properties lead us to believe the deposition was rapid, most likely a flood event of a river system. A flood would bring in and randomly dump the large clast in no specific order. As the flood settles, the depositional energy lessens and finer sediments are deposited. It may also be possible that this formed in a basin. These conglomerate beds were all about a meter thick and would need plenty of room to build. Still some flood event would have to bring the large clasts and gaps would be filled in from finer sediments settling from
suspension. Another lithofacies we founded was Unlithified Lithic Arkose Sandstone. The sediments making up this facies comprised of 25 percent clays and 75 percent very fine sands to very coarse sands. Compositionally, the sediment was made up of more than 50 percent quartz, which includes micas, hornblendes, and pyrites: typical of a slow moving meandering river. There were also signs of faint laminations defined by alternating layers of layers of light and dark caused by differentiating flow velocity of the river. The final lithofacies identified was named Cross-laminated Subarksose Sandstone. These units are larger than other rock beds in the area and featured cross bedding which really made it distinct. The unit was also made up of mainly sands, which contributed to us interpreting the depositional setting as a low energy river. Possibly where the river meets a lacustrine environment, hinted by the inclined bedding. Going up section, all three stratigraphic columns showed repeating patterns of conglomerate and sandstone. This may be due to different climate periods of the Pleistocene. The sandstones would be deposited while the weather was colder. As glaciers melted it would contribute to the strength of flow and bring in the larger clasts of conglomerate.