Research Project 3: Paleocurrents in a synorogenic basin
Part 2 - Visher Ferry

Geology 21: Stratigraphy and Depositional Environments of New York

In this field exercise we will examine and characterize the paleocurrent directions in rocks that have been folded and faulted.  Unlike rocks at the Plotterkill Preserve, we will need to be concerned with the geometry of the folds and how we will restore the rocks (and their paleocurrents) back to horizontal.

The Visher Ferry Power plant is located on the Mohawk River at Lock 7.  These rocks belong to the Middle Ordovician Schenectady Formation (=Austin Glen Formation or Normanskill Fm.) and they are some of the westernmost folded and thrusted rocks in the area: deformation increases in intensity to the east towards Logan's Line.

PART 1: MORE PALEOCURRENT DATA

Many geologists accept the idea that the clastic material in these turbidites had a source to the east, the paleocurrent data from this unit does not clearly support this contention.  Using data collected from the Plotterkill Preserve, Visher Ferry, and data from several other sites in the Mohawk Valley (see additional data from Mohawk Valley) you will be able to partly reconstruct the paleoflow directions within the basin during deposition.

Age: The Schenectady Formation is Mohawkian in age (upper Middle Ordovician).  Most of the formation (and the Utica Shale) is restricted to the Nowadagan stage but the lower beds belong to the Canajoharian stage (i.e. the top of the Mohawkian).  One thing that you do not know, however, it the relative age of the units studied - they were not all deposited at exactly the same time, and we are not sure how they fit together in time.

Objectives: The objectives of this report are:  1) interpret the depositional environments of the Schenectady Fm.; 2) to identify and measure paleocurrent indicators in turbidites; and 3) to plot and present these data in a accurate and coherent manner.  The goal of this lab project is to understand the nature of the basin and basin infilling and source regions during the upper Middle Ordovician and relate this to tectonic events during this time.

Procedure: Describe bed thickness, sedimentary structures, sandstone to shale ratios, and bed coarsening and thickening trends.  In order to measure paleocurrent indicators, unidirectional and bidirectional sedimentary structures must first be identified.  We will be concerned primarily with flutes, grooves, ripple cross-lamination.

Field work: A Brunton compass is required to measure the orientation of any current structure.  Locate a sedimentary structure such as a groove.  Place your notebook on it such that the edge of your notebook is parallel to the current direction.  The azimuth of the Brunton when placed parallel to the edge of your notebook is the orientation of the paleocurrent indicator.  Carefully record the type of sedimentary structures measured, the lithologic association and other pertinent information.  Pay close attention to the thickness of each bed measured: record this information in your field notes.

Office work:  Summarize your data in a typed table (Table x).   Both your data table and the additional data (Tables 1,  2) should be included in your report as separate tables.  Plot unidirectional and bidirectional data separately for each location and place these plots on a map so that the spatial distribution of the paleocurrents can be inspected on one figure.  Make sure that bidirectional and unidirectional current indicators are plotted separately.  Photocopy the map with the different rose diagrams affixed to the map at their proper location.  Create one 'grand rose' which has all of the paleocurrent data.  Both the map with the paleocurrent information and the grand paleocurrent rose should be separate figures in your writeup.  You must use the program Rosy to plot this information.
 

Consider the following questions once you examine your data (make sure you look at the figure that shows the stratigraphy and the measured sections lines at the same time):
 

a) Just looking at rocks types in the regional stratigraphic chart, is there a deepening trend in this basin and in what direction?
b) Is there any facies control on the distribution of the rocks?
c) Aside from the water depth, what may determine the distribution of fossils in the strata?

Writeup:  Your typewritten lab report, which will be combined with analyses from last week, should include in some way, the following:
 

1) Introduction, including site locations, formation names and ages of the rocks studied, purpose of the trip and the approach to the problem.  Do not write this as a narrative!  Be factual in your introduction.

2) Presentation of the data.  Describe the sequence in full.  Refer to the data tables and the rose diagrams.  In general terms what does the data tell you the flow directions was?  Is there any significant variation in flow direction between different localities?  Is there any variation as you move upsection? Is there any variation between different types of current indicators?

3) Interpretation of the data.  A) What are the depositional environments of this section?  B) What does the paleocurrent pattern mean.  If the clastic sediments were derived from the Taconic Mountains, what was the probable path they needed take to arrive at their present position?  What implications do these data have for the shape and extent of the basin?  A sketch map may help you make your point.

4) Discuss the characteristics of the basin during Trenton time.  Describe the variation in basin depth from the east to the west.  Support your ideas with facts from both your stratigraphic section and from the distribution of fossils in the basin (you may want to include a diagram showing the distribution of different fossils).

5) Briefly discuss the broader implications concerning the paleogeographic setting of these rocks.  Specifically, how deep might this basin be?  Why is it getting deeper? Is a change in absolute sea level responsible for what is seen or do you need to invoke crustal subsidence? Why?  Be specific and support your ideas with facts.

6) Conclusion.  What is the significance of this unit with respect to the other units studied thus far?  Is clastic sediment with this composition present in any other units?

7) A map with all the paleocurrent information summarized (No paste ups!).

8) A rose diagram with all the paleocurrent information.

9) Data tables with your data and the additional data.  Table 1 is data from Plotterkill, Table 2 is the additional data, Table 3 is your data.

10) Optional.  A diagram showing the inferred depositional environment of the Schenectady Formation with paleoflow directions.
 

© Geology Department, Union College, Schenectady N.Y. 12308-3107.
All rights reserved. No part of the document can be copied and/or redistributed,
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John I. Garver, Geology Department, Union College, Schenectady NY, 12308-2311, USA.

Last Revised By: Geology Department, September 2001