Juniata formation , geology reaserch paper .
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rose_hill_formation.docxAlmuhrej 10
Mohammed almuhrej
Dr. Jeffery
GEO 313
November 13, 2016
Rose Hill Formation
The Rose Hill Formation is a predominantly shale and sharp and placed at the top of the Tuscarora as the last quartz arenite. The formation is usually a hematic sandstone that normally has a metallic sheen; it has a dark maroon color, which is thick-bedded and indurated (Diecchio and Dennison 120). The reddish color forming the Rose hill formation was diagenetic, meaning that it had some elements of iron from hematite and limonite, but oxygen is what mostly caused the reddish color. The formation is bound by sandstones above and below it with a red to greenish-gray shale. Folk argued that it is a representation of deep water environments and is rich Brachiopod, Bryozones, and Crinoids fossils (78). The depositional environment, however, was a shallow marine environment that caused ripple marks due to the interbedded nature of the sandstone, causing a deltaic or a tidal flat environment.
The Rose Hill Formation can be found within the Silurian period, predominantly by its ostracode faunas. The Rose Hill Formation is about two hundred to four hundred feet deep with a thickness of about sixty-one meters to one hundred and twenty-two meters. It overlies the Tuscarora sandstone, and above it is the Keefer sandstone (Helfrich 560). The Rose Hill Formation is divided into three more parts, which include the upper shale with some purplish bands, the Cresaptown iron sandstone, and, finally, the lower shale and sandstone. The three divisions of the Rose Hill formation are not necessarily found in every part of the world, specifically in the highland parts of the country. Since the three layers are absent in the highlands, the formation retains the Lants formation only.
Sedimentary structures can be defined as the observable features within the sedimentary rocks whose formation came about during deposition and stand for the biological and physical processes that operate in an environment that undergoes deposition. The sedimentary structures found in Rosehill formation are termed as ripple marks. There are two types of ripple marks: asymmetrical and symmetrical (Diecchio and Dennison 99). It can be termed as asymmetrical because it helps geologist to ascertain the direction, in which water or the wind was flowing when cross beds were deposited. This is a possible and effective method because the shorter side of the ripple will always face the downstream direction.
Palaeontology is the branch of science that deals with fossils of both plants and animals. Correct age approximation or knowing the relative time, in which a palaeontologist holds prospect for it, can be the determiner of whether they will discover a fossil or not (Woodward 104). By having vast knowledge on the physical attributes of a rock unit (lithology), one is able to ascertain whether there is any fossil material. When a fossil is taken from its original position without temporal data, it does not maintain its ability to assist researchers with information. An environment, which undergoes sedimentation, allows for a fossil rock to be conserved for a period of time, as is with the Rosehill formation.
Stratigraphy is a part of geology that focuses on the rock layers and the actual process of layering of the rocks and is used primarily for the study of sedimentary rocks. Among the sub-disciplines to fall in the category of stratigraphy is relative dating. Layers formed on sedimentary rocks occur through a period of time. For the Rose Hill Formation, these layerings on the sedimentary rocks that have accumulated over time can be used by the researchers to interpret the historical events that have happened in the Earth’s past for as far back as the researchers can decipher from the layers in the sedimentary rocks.
A facies association can be defined as a cluster of sedimentary facies, which are used to define a certain environment with sediments. Rose Hill formation is characterized by sandstones, which are green and reddish brown in color, and have been embedded with green or brown siltstones (Swartz 24). The brown color can be linked to the high amount of iron oxide found in the rocks. These sandstones are normally ripple-marked and are the main characteristics, which identifies the Rosehill formation.
Diagenesis refers to all the processes that have been summed up that bring about change in a sedimentary rock after the deposition of water. The processes can either be biological, chemical, or even physical. This meaning has been filtered to best suit how diagenesis relates with paleontology. Diagenesis in the Rose Hill formation works hand-in-hand with paleontology to find the age of a fossil where it does the following:
1. Shows the chemical decrease during the organic phase.
2. Shows the chemical decrease during the mineral phase.
During the field trip, I was able to collect three samples of rocks and was also able to describe the outcrop of the samples, the rock types, and their thin selection. Since we went to see a Rose Hill formation, most of my samples were the same. All my rock type samples were Sub- litharenite belonging to the Silurian timeline. The formation had a barrowed outcrop and the grain mean size was fine medium sand which was very coarse; the range of sizes was between 0.1mm (0.7- average (0.25)), and it was concluded that the rock sample was very coarse. The matrix estimate of the rock was between zero and ten and comprised of cement only. The sample had an estimated rock size of a hundred percent which meant it had no matrix within it. The Rose Hill formation is mostly made up of quartz, which had the highest percentage taking up ninety percent, while lithaclast only took up less than ten percent, to be exact about four point nine percent. As a result of these mineral estimate percentages, I concluded that the rock type was a sedimentary rock, and its sedimentary structure was barrow. Moreover, the presence of coarse sand and sandstone was also another factor that led to that conclusion, and its sedimentary structure was lamination. The first sample had very well sorted sandstones, which had sub-rounded, sub-angular, and andhedral roundness and had low sphericity.
The second sample was also formed and collected from a Rose Hill formation. The rock type was Quartz arenite with a mean grain size of sand. The range size of the grains was 0.1-0.5mm and an average of 0.2 and had sand with a bedding. The rock matrix was between zero and ten percent and the estimated rock percentage between ninety-one and a hundred percent. For the outcrop, due to the matrix percentage present in the sample, it mostly appeared to have ripple mark due to erosion and effects from weather conditions. It had two dominant minerals, quartz with ninety-six percent and lithaclast with only four percent. Its particles were well sorted and traces of hematite and quartz cement were visible after the thin selection was carried out. It was also evident that the shape of the grains was rounded, sub-rounded, and andhedral with low sphericity. The grain to grain contacts were surrounded by matrix which was all cement and had a brownish color.
The third sample was still a Sub-litharenite. It still had a reddish brown color and, unlike the other samples, had a balanced rock and matrix percentage both at fifty percent. The sample had very coarse sand grains ranging from 0.1-0.6mm and had different averages of 0.3mm is some spots and in others 0.2mm. The distribution of the grains across the sample was lamination. It had mineral content, and some of the minerals present were quartz, which were dormant at ninety-one percent, and lithaclast at nine percent. For the grains, they were very well sorted and had some elements of hematite cement. The grains were also sub-rounded, sub-angular, and subhedral in shape with low sphericity and grains being surrounded by matrix. The sedimentary structure was lamination.
The thin selection for the first sample was carried out on a Quartz arenite rock type. To determine the petrography of the sample, several aspects were looked at; this included the grain size or range of the minerals and estimated percentages, a representation of the thin selection under a high-powered microscope, roundness, matrix, and cementation. The grain range size was about 0.1-0.5mm and an average of 0.2; this meant that the grain’s mean size was fine medium sand. The sample had fine medium sand, which meant that it had some elements of sandstone components in it and the grain surface had a barrow sedimentary structure. The grain size distribution was lamination, and the sample had a matrix of between zero to ten percent, which mostly comprised of cement only. Ninety to a hundred percent of the sample was made up of the actual rock, which qualified it as a Quartz arenite. For the mineral percentage, it had ninety percent of quartz, and less than four point five percent of lithaclast. The sample contained crystalline chemical cement and Quartz which occupied the first triangular panel, which is classified as a quartz arenite. The sand framework consisted of spaces and pores. The other ten percent consisted of lithaclast. For the cement, the sample consisted of a white opaque hematite cement; this kind of cement element can be said to have attributed to the rock having a pale brown color due to oxidization from the environment. The sample contained well sorted grains, which meant that the textural maturity was very high with low sphericity, and it was rounded, surrounded, and andhedral. The sample being a quartz arenite did not have fossil traces in its elements.
Since all samples were collected from the same place, the thin selection for the second sample was also carried out on a Quartz arenite rock type. For the second sample, the grain size range was about 0.1- 0.5mm with an average range of 0.2mm, meaning that the grain’s mean size qualified it to be sand granules. The sample had very coarse sand, and thus, indicated that it had many sandstone components in it, and the grain surface had ridges that looked like ripple marks as its sedimentary structure. The sample had the same matrix as the first sample of about zero to ten percent, which mostly comprised of cement. The sample was actually comprised of a ninety to a hundred percent of the actual rock. The grain to grain contact was surrounded by a cement matrix. For the mineral percentage, it had ninety-six percent of quartz, which means that the rock sample qualified to be quartz arenite. The sample contained crystalline chemical cement which were similar to the first sample and also had a white opaque color, another cement element present was also Quartz and occupied the first triangular panel. This kind of cement element can be said to have attributed to the rock having a pale brown color due to oxidization from the environment. The sand framework consisted of spaces and pores. The remaining four percent consisted of lithaclast. The sorting of the grains in the sample was very well, which meant that the textural maturity was very high with low sphericity; as for the roundness of the sample, it was rounded, surrounded, and andhedral. The sample being a quartz arenite did not have fossil traces in its elements.
For the final sample, I did the thin selection for a Sublitharenite type of rock. To determine the petrography. The grain size was a little bit inconsistent and varied from 0.1mm-0.6mm with different average sizes of about 0.3mm and 0.2mm; the grain’s mean size qualified it to be a sand granule. The sample had very coarse sand, which meant that it had sandstone elements in it and the grain surface had large grains in laminations. The sample had a matrix of about zero to ten percent, which mostly comprised of cement. The other ninety to a hundred percent was made up of the actual rock. For the mineral percentage, it had ninety-one percent of quartz, meaning that the rock had quartz arenites elements in it. The sample contained crystalline chemical cement, which meant that it was classified as a quartz arenite. The sand framework consisted of spaces and pores and the grain to grain contact was surrounded by a cement matrix. The other nine percent consisted of lithaclast. For the cement, the sample consisted of hematite cement, and quartz this kind of cement element can be said to have attributed to the rock having a pale-brown color due to oxidization from the environment. The color of the cement was white and opaque. The sorting of the grains in the sample was well sorted which meant that the textural maturity was very high with low sphericity, and the sample was rounded, surrounded, and andhedral. The sample being a Sublitharenite did not traces have fossil in its elements.
The Sublitharenite sandstone facies is interpreted to form as interference ripple marks and linguold ripples formed by waves and currents occur locally (Folk 88). The texture, the grain size, and the mineral estimates make up the thin selection for this particular sandstone. The relationship between the facies and other layers, such as the Eagle Rock sandstone, present a very a close relationship with the Sublitharenite sandstone, which represents the thickening and merging of other varieties of sandstones like the unnamed sandstones and the closely related Keefer and Williamsport basins. The presence of minerals and the mineral composition ensure that there is provenance to make the study of the sandstone possible (Butts 193). The presence of quarts as the dominant mineral means that it is evident in both monocrystalline grains and polycrystalline grains and can, therefore, show the grain’s preexistence and those that were formed as a result of deformation. Even with the low percentage of lithic minerals, it acts as the most suitable proof of provenance existence.
The environmental disposition was a shallow marine environment, which caused ripple marks; it could also be a deltaic environment or tidal flat. The reason that the ripple marks were so clear was because of interbedded nature of the sandstone and shale. The burrows in the formation could have been a result of walking or hunting. Marine fossils present in this formation were Brachiopods, Bryozones, and Crinoids (Swartz 24). The reason the facies selected fitted well into the environment was the age that they were formed in. The mentioned formation originated during the Silurian period, which means that the sea levels were much higher at that time than now; this led to the change in climate. The effects of the climate change were that many faunal groups recovered from extinction, and due to the meltdown of the ice age several shallow seas flooded the large areas of some continents, creating some of the formations we have. Moreover, the earth’s magnetic fields contributed to the richness of some sedimentary rocks having iron-bearing minerals, consequently giving the colors present in the formations.
As mentioned earlier, the formation of sedimentary rocks is usually a result of sediment disposition. For one to fully understand the tectonic setting, it is essential that they are aware of the disposition and able to relate to past environments that relatively led to the formation. The tectonic setting is one of the factors determining what kind of sedimentary rock is to be formed. Woodward argued that tectonic setting occurs because of the deformities to some of the already formed rocks on active settings, which in turn can either cause sinking or elevation of some of the rocks, therefore, accumulating sediments (100). The elevated parts are known as a source, while the sinking parts are referred to as basins. There are several more factors that affect sedimentation, but the tectonic setting is the major one. Other factors may include climate changes and medium transport processes. Only by understanding how these factors affect the sedimentation process, can we be able to connect with tectonic setting.
Paleoclimatology refers to the study of past climates. The Rose Hill formation was created during the Silurian period. During this period, the earth experienced a lot of changes mostly in land mases; the most dominant change experienced during this time was the rise of sea levels (Swartz 18). The rise of sea levels exposed and covered different continental fragments ever since grouping has occurred from the Cambrian until now. In the case of paleogeography during the rose formation, the earth at that time experienced a lot of change in the continental elevation levels. The earth’s zonal climate was globally affected by the magnetic fields leaving certain sedimentary rock rich in metal elements with the magnetic signatures; an example of such mineral is magnetite.
It is conclusive to say that we create the world we live in. By actually doing things that change the climate and weather patterns, we are changing the natural forms of some of the oldest geographical formations. Using these formations, geologists are able to tell the disparities between certain ages and the present. If we do not create the right conditions for some of these geographical creations, then we leave the future generation not knowing about how their present came to be.
References
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Folk, R. L., 1960. Petrography and origin of the Tuscarora, Rose Hill, and Keefer Formations, Lower and Middle Silurian of eastern West Virginia: Journal of Sedimentary Petrology, V. 30, p.1-58.
Helfrich, C. T. 1980. Late Llandovery-early Wenlock conodonts from the upper part of the Rose Hill and the basal part of the Mifflintown Formations, Virginia, West Virginia, and Maryland. Journal of Paleontology 54(3): 557-569.
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Swartz, C. K. 1923. Stratigraphic and paleontologic relations of the Silurian strata of Maryland. In: Silurian, ed. C.K. Swartz, Maryland Geological Survey, Baltimore, Maryland, 25-52.
Woodward, H. P. 1941. Silurian System of West Virginia. West Virginia Geological Survey, Reports, v. 14, p. 326.
