Geology 3263
Structural Geology
Field Trip Report 2 – Chestnut Hill Taconic Suture
Name______
Each student attended at least one field trip. In this last part of the course we will use the data collected to map the structures seen on the second field trip.
Read the field trip 2 descriptions for both the morning and afternoon, then complete this exercise.
On the second field trip we collected samples and measured strikes and dips near a suture zone, part of the Taconic Orogeny. The Ordovician rocks are from the Wissahickon Formation, a metamorphosed Flysch, with contemporaneous and younger igneous intrusions. Along Wissahickon Creek at Bells Mill Road in Philadelphia, the complex is thrust northward over the Baltimore Gneiss. The Baltimore Gneiss is Grenville in age, similar to the Late Proterozoic metamorphosed rocks we examined on the first trip.
The purpose of this second report is to review plotting of data in map view.
A Google Earth plot of the data from November 2012 and April 2012 is attached. TODO: Make a map view of our findings. You may use a clear tracing paper or transparency overlay if you wish. Locate the local features (roads, etc.) if you use an overlay.
Step 1. Plot a strike and dip symbol for the Wissahickon Fm. south of Bells Mill Road, on the east side of the stream, marked N80E55S. Recall that we only used quartzite beds for the Wissahickon Fm. The schist foliates, so schists don’t show
aspects of structure we want at this point. We will use the symbol Owsq for this.
Step 2. Draw a double line between Serpentinite observations on the east and west sides of the stream. We will use the symbol Owsp for this.
Step 3. Draw a double line to connect Granodiorite East with Grnd West. This granodiorite cuts the Wissahickon Formation and is much younger. Radiometric dating shows that it is Permian in age. We will use the symbol Pmgd for this.
Step 4. Plot a strike and dip symbol for the Wissahickon Fm. marked N53E75N north of Bells Mill Road. It is on the east side of the stream.
Step 5. Draw a faint line from the Fracture Zone observation to a point below the Baltimore Gneiss (we will use symbol pCbgn). Use the spot marked “For” on Forbidden Drive as the west end of your faint line. This is the Thrust Fault.
Darken the east side as a solid line (we can see the fault zone there ), but just use dashes on the west (everything was covered with soil, we assume it is there).
The hanging wall is to the south on the Ordovician side, since the Ordovician Wissahickon was thrust over the Baltimore Gneiss. Draw in the barbs on the hanging wall side.
Step 6. On your map, make up your explanation blocks as before, in order by age. Label each block with the symbol marker given here. Select light colors for each. The serpentinite and granodiorite divide the Wissahickon, and the fault separates the pC Baltimore Gneiss to the north from everything younger to the south. Color in each region with the appropriate color, and mark each with the corresponding symbol.