Research Project 3: Paleocurrents in a synorogenic basin
Part 2 - The Shawangunks
 

Geology 21: Stratigraphy and Depositional Environments of New York
 

The Shawangunk Mountains in southeastern New York are a mecca for rock climbers in the east. The rock these climbers seek is the shallow-dipping Shawangunk Conglomerate that form dramatic towering white cliffs. At the same stratigraphic level to the south (Fig. 1) is the Tuscarora Quartzite, which forms the near vertical cliffs of Seneca rocks in west Virginia, another climbing hot spot in the east.  Not far to the west, both the Tuscarora Quartzite and the Shawangunk Conglomerate grade laterally into marine rocks of the Clinton Formation (Fig. 2). Farther to the west (Ohio and Pennsylvania), the Clinton Formation is a very important gas producer and it the target of many deep wells. Why are the best climbing areas in the east on the same strata? The Shawangunk Conglomerate and its equivalents to the south are very resistant to erosion, much more so than adjacent strata, and as a result they form prominent cliffs.

The purpose of this trip is to briefly examine some of the best and only exposed Silurian rocks in eastern New York in order to infer possible depositional environments and source terrains. As in the Plotterkill project, we are interested in collecting paleocurrent data and determining the geometry of the basin and the location of the source terrain. In both areas, it is necessary to describe the rocks, interpret depositional environments, and then analyzed the paleocurrent data. Please bear in mind that paleocurrent patterns for different depositional environments will have certain characteristic patterns typical of that environment.

Age: The interbedded sandstone and shale below the Shawangunk Conglomerate is equivalent to the Schenectady Formation, but is here referred to as the Quassaic Formation and the underlying Snake Hill Shale. Farther to the south, the correlative rocks belong to the Martinsburg Formation (Fig. 2). These flysch deposits are all Mohawkian in age (~ upper Middle Ordovician; circa 460-470 Ma). The overlying Shawangunk Conglomerate is poorly dated, but was probably deposited during the Clinton and Lockport stages of the Early/Late and Late Silurian (circa 420 Ma). (Note that there is no "Middle" Silurian; likewise there is no formal "Middle" Ordovician although the term is in use in North America)

Approach: We will examine two outcrops in detail, perhaps three if time permits. At each outcrop make sure you have a complete description of the rocks. At Minnewaska State park (Stop 2), were we will spend most of our time, you will be responsible for collecting paleocurrent data and interpreting the significance of the cross-bedding exposed in the ledges. Consider the scale of the cross beds, thickness changes from bottom to top in the outcrop, and the relationship between grain size and cross-bedding. As before, first describe the rocks and then partition you notebook into columns for data collection.

Summarize you data in a neat table form, and include the data in Table 1 (see attached). Plot the paleocurrent data on a rose diagram, using an interval width of 300. (Are you measurements bidirectional or unidirectional?) Think about the distribution of Silurian rocks in New York State. Examine the New York state geologic map in the hallway of the Geology Department. What happens to Silurian strata west of Schenectady? What is the principal rock type in these strata? Likewise, what happens to the distribution of Shawangunk Conglomerate? Were these two groups of rocks deposited it the same basin? Why or why not? The trend recognized on the NY State geologic map should help you interpret the significance of the paleocurrent data collected at Minnewaska State park.

Likewise, with your completed paleocurrent summary of Plotterkill and surrounding exposures of the Schenectady Formation, consider the distribution of age correlative rocks. What are the lateral equivalents of the Schenectady Formation to the east? To the west? Using thickness estimates alone, what could you infer about the source to the Schenectady Formation? Is this inference consistent with the paleocurrent data? Why or why not?

Finally, consider the big picture. First, you should be able to summarize and interpret the paleocurrent distribution of the Schenectady Formation and make inferences about the location of the source region, the composition of the source region (remember Wintergreen Park), and, considering the thickness of basin fill, the mechanism for basin subsidence. Second, you should be able to summarize and interpret the paleocurrent distribution of the Shawangunk Conglomerate and make inferences about the location of the source region, the composition of the source region (remember the factors that control composition). Third, what is the relationship between the Middle Ordovician flysch (i.e. Schenectady Formation, etc.) and the overlying Silurian rocks. What does this relationship (unconformity) mean, and what is its duration? In a concluding paragraph to your report, you should be able to summarize the fundamental events that effected the stratigraphy of eastern New York during this interval.

Writeup: Your typewritten lab report, should include the following:
 

1) Introduction. Include a brief discussion of each site location, formation names, ages, purpose of the field studies, and methods of your research. Avoid too much detail about each unit, the introduction should not be significanfly longer than one page.

2) Data presentation. Separate this part into two sections: Ordovician rocks (i.e. Plotterkill) and Silurian rocks (i.e. the Shawangunks). Start with a general description of the rocks in each location. Discuss the principal trends in the paleocurrent data from each location.

3) Interpretation of depositional environments. Again, separate this part into two sections:
Plotterkill and the Shawangunks. For the Schenectady Formation you should make an inference about not only depositional environments, but also for the mode of basin infilling. What do the paleocurrents tell you about how the basin infilled? What assumptions must be made in order to address the direction of the source region? For the Shawangunk Conglomerate, interpret a depositional setting and indicate how the paloeocurrents are characteristic of this environment.

4) Interpretation of the tectonic significance of this unit. Place these units in their tectonic setting. Address specifics such as the total thickness of the Schenectady Fm. For the Schenectady Formation what does the mode of basin infilling tell you about tectonism during deformation? Can the paleocurrents tell you anything about the structural evolution of the margin during this time? For the Shawangunk Conglomerate, what does the composition of the detritus tell you about the source terrain? What do lateral equivalents in both NY and PA tell you about depositional patterns during this time?

5) Reference list (optional if none cited).  This should include any references cited, including Boggs.
 
 

© Geology Department, Union College, Schenectady N.Y. 12308-3107.
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John I. Garver, Geology Department, Union College, Schenectady NY, 12308-2311, USA.

Last Revised By: Geology Department, September 2000