Research Project 4: Post-orogenic sedimentation in the Silurian and Early Devonian
Part 1 - Thacher Park
 

Geology 21: Stratigraphy and Depositional Environments of New York
 

Overview:  Last trip we examined the Shawangunk Conglomerate near New Paltz New York, and we came to some conclusions about depositional environments, source terranes, and the relief in the source region.  Additionally, using regional stratigraphic correlation charts, we discussed the large-scale depositional patterns in the Silurian.  This week we will measure a stratigraphic section at the Helderberg escarpment. The Helderberg Escarpment is a prominent geomorphic element of the Mohawk valley which is composed of Lower Devonian limestones that unconformably overly interbedded sandstone and shale of the Indian Ladder beds (similar to the Schenectady Formation).  From the edge of the escarpment one has impressive views (when it is clear) of the Adirondack, Green, and Taconic mountains to the north and east.  At the park, the Lower Devonian limestones are spectacularly displayed.  We will examine outcrops of the Rondout Dolostone, the Manlius Formation and the Coeymans Formation.  The relationship of the Devonian rocks to the underlying Ordovician rocks is an angular unconformity, but the contact is poorly exposed here.  Farther to the east (i.e. the Gunks for example) the angular discordance is pronounced.

Age:  All of the rocks that we will study at Thacher State Park belong to the Helderbergian stage (Early Devonian).

Objectives: The objectives of this lab are: 1) to describe lithologic and fossil content while measuring a thick stratigraphic section;  and 2) interpret the depositional environments for each section.  The goal of the lab is to understand interpret depositional environments and to determine the role of relative sea level in sequence development.

Procedure:  Examine the outcrop in its entirety as you descend the stairs and walk to the base of the  prominent waterfall in the center of the Indian Ladder trail.  We will be measuring a very thick stratigraphic section here so please plan your time accordingly!!   The total section is about 50 meters thick and you have about 3 hours to complete the section.

 Use a meter stick and work in pairs.  Both should decide where the prominent lithologic breaks occur.  Bear in mind that in order to measure the entire outcrop you will have to break the outcrop into units of between 1 and 3 meters thick.  Once the units, or individual units have been determined, one person should measure while the other records the data.  Both should then describe the unit together and look for fossils and sedimentary structures.  A formation should have a set of lithologic criteria upon which it is identified in the field.  Internally, a formation can have lithologic variability, but when the overlying rocks are quite distinct you should draw a formation boundary.

 We will be measuring carbonate rocks, so if you would like to brush up on their mode of deposition and occurrence please read pp. 75-86 in Boggs.  If you are do not understand the formation of dolomite, read those sections carefully.  We will see a number of sedimentary structures which may include cross-bedding (p. 156), ripple cross-lamination (p. 160), biogenic structures (p. 168), and trace fossils (p. 175); please read these sections or the lab will be relatively difficult to complete.

Writeup:  Your typewritten lab report, which is due in two weeks, should include the following.
 

1) Introduction.  This should include the site locations (Figs. 1,2), formation names and ages of rocks studied (Fig. 2), purpose of the trips and approach to the problem.

2) A brief discussion of the different lithologies and fossils encountered.  Put the detail in your stratigraphic section and do not repeat all the details from this important figure.

3) Interpret the depositional environments for each of the units measured using the data collected in the field.
 

[A] For the Gunks, make sure that you discribe the rocks and sedimentary structures.  Also present the paleocurrent data.

[B] For the Helderberg rocks, present the measured stratigraphic section.  Although you may put most data on the measured section, you should highlight important features of the different units in this part of your paper. As with the Manny Corners/Canajoharie project, a review of carbonate depositional systems would greatly enhance your understanding of the depositional environments of these rocks.  Your discussion concerning depositional environments, can be done after each section is briefly described, or, preferably, in a section entitled Depositional Environments which should immediately follow the description of all of the units.  Interpret the depositional environments of each formation, not individual measured units. When you interpret the depositional environments you must cite evidence for your interpretations.  For example, if you state "This unit was deposited in a tidal flat environment.", the following sentence(s) must support this assertion with facts from the outcrop.  For example, the following sentence tells the reader why a tidal flat environment is favored: "The presence of algal mats, abundant dolomitic rocks, a very limited fauna, which is characterized by gastropods, support this environmental interpretation."   Even a statement such as "...because the rocks are brown.", is fine because  you have stated your reasons why you think the rocks were deposited in a tidal flat environment. (Needless to say the latter example is totally inadequate from a geological perspective.)

4) An interpretation and discussion as to the transgressive or regressive nature of this sequence.  Which is it and why?  What might have caused this pattern?  If eustatic sea level is considered, what pattern should be seen throughout the entire Devonian?  Also ask your self how these two sections relate to one another.  What is the status of tectonism associated with the formation of the basin in which the Schenectady Formation accumulated in?

5) An interpretation as to the tectonism during this interval.  Again, use facts to support your assertions.  How has tectonism changed since the Ordovician?  What evidence do you have?

6) A reference list.  This list should be identical to the attahed references.

7) A neatly redrawn (ink) stratigraphic section with detailed lithologic descriptions and faunal list.(to the right of the stratigraphic column).  This should be Figure 3 of your report.

8) Any additional sketches, maps, of diagrams that you feel would improve the quality of your report.

Some helpful fossil names:

Favosites helderbergiae (tabulate coral)
Tentaculites gyracanthus (tentaculid in the Manulis Fm.)
Gypidula coemanensis (brachiopod)
Syringostroma barretti (stromatoporid in the Manlius Fm.)
ostracodes, bioherm, brachiopods, tabulate corals

References

Laporte, L.F. 1969.  Recognition of a transgressive carbonate sequence within an epeiric sea: Helderberg Group (Lower Devonian) of New york State, In Depositional Environments in Carbonate rocks, G.F. Friedman, editor, S.E.P.M. Special Publication 14. pp.98-119.

Vail, P.R., Mitchum, R.M. Jr., and Thompson, S., III, 1977.  Seismic stratigraphy and global changes of sea level, Part. 4: Global cycles of relative changes of sea level.  American Association of Petroleum Geologists Memoir 26, pp. 83-97.
 
 
 
 

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John I. Garver, Geology Department, Union College, Schenectady NY, 12308-2311, USA.

Last Revised By: Geology Department, September 2000