Kean University Geology 3263
Structural Geology
Field Trip 2 Geology of Wissahickon Creek
Afternoon: Bell's Mill Road Area
Afternoon
In the afternoon we will map the area around Bells Mill road, and get a first-hand look at
the accretion of Avalonia and its forearc sediments to the Grenville Age Coast, forming
the Taconic Orogeny. Park at the parking lot east of Wissahickon Creek.
You will take at least one marked, fist-size or greater rock sample, strike and dip if
possible, a GPS reading, an outcrop photo, and a rock close-up photo, for every useful
(not floating) outcrop.
BEFORE YOU LEAVE THE PARKING LOT, BE SURE YOU HAVE A PENCIL,
NOTEBOOK, AND THESE PAGES. ALSO BE CERTAIN THAT THERE IS A
CLEAR PROTRACTOR AVAILABLE FOR YOUR USE.
Lunch discussion:
Here is a figure from an Introduction to Geology Textbook, Chernicoff et. al. Let’s
review the Taconic Orogeny.
Rodinia breaking up
Avalonia Terrane E. Cambrian
Plates reverse, in E. Ordovician
Laurentia moves toward Avalonia
Africa moves toward both
Mid – L. Ordovician, Avalonia
and island arc dock with
Laurentia, beginning the
new Appalachians
Lunch discussion continued:
Tectonic Setting. More pictures from Chernicoff et al. Earth
So far we have seen rocks we expect from a neararc basin ophiolites i.e. metamorphosed
sediments (Wissahickon), basaltic rocks (the Amphibolites we saw this morning), and
peridotites (here they are serpentinites due to hydrothermal alteration). The India and
Asia collision is an instructive starting analogy.
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2
3
4
Rocks thrust up over margin of Laurentia
CHECK your Brunton deviation. Prof. Smart will instruct you.
Start collecting data.
EAST SIDE OF STREAM
Walk south, carefully crossing Bell's Mill Road, and walk on the path to the first outcrop
of rock on your left (not right on Bell's Mill Road, your instructor will show you).
Stop 1. Wissahickon Formation: schist and quartzite. This originated as Flysch:
alternating mud and sands, formed as turbidites and tempestites in the ocean between
prior to the Taconic collision 400 to 450 mya. These lithified to interbedded layers of
shale and sandstone. The sands are often graded, allowing right-side-up
determinations. As Avalonia collided with North America along the eastern edge of the
Grenville highlands, the forearc sediments of the Wissahickon were compressed, folded
and heated, causing both structural and chemical changes.
Take a strike and dip of the general orientation of the quartzites. You will have to average
over the folds. Also obtain the latitude and longitude with the GPS. Plot everything on
your topo map, using landmarks.
STOP 2 is across Bell's Mill Road, back at the parking lot. It is the first outcrop on the
right. These are serpentine phyllites containing talc. NOTE THE "FEEL".
Chemically the rocks here are like lower ocean lithosphere, i.e. the upper mantle.
These peridotites, possibly from the base of an ophiolite suite, have been metamorphosed
by hot circulating ocean water.
Take a strike and dip for the phyllite,
and find the limits of the layer on this
side of the stream. Take GPS readings
for the limits of the serpentinites. Plot the
surface contacts with the serpentinite.
Mapping Suggestions: USE PENCIL and a protractor.
Once we have strikes and dips, show contacts between layers as a line on your topo
worksheet. Use land marks on the topo cutout below to place your strike and dip
symbols and a formation symbol your instructor will provide. For Stop 1, use the
formation symbol Czw for Wissahickon Schist and Quartzite. For Stop 2, a Serpentine
Phyllite, use Pzum. Get these as needed.
CONTACTS: Usually we are not certain where a contact is because the contact is not exposed.
We can make a good guess based on where we do see rocks and by using changes in topography
that reflect the differing ability of the rocks to withstand weathering. With the scale of the topo
worksheet given, the width of a pencil line is close to our uncertainty.
We can also infer the orientation of the contact from the strike for sedimentary and non-foliated
metamorphic rock. This usually doesn't work for igneous rocks.
STOP 3. If we continue north, we begin to see Wissahickon quartzite and schist again.
We have walked over another contact that we can put on our map. Get the strike and dip
and a GPS position, and mark a line along strike for the contact at some point between
outcrops that you choose.
Stop 4. Further along the stream a dike of granodiorite cuts into the Wissahickon
Formation. Trace the strike of this as it crosses the stream. We will see it again on the
other side of Wissahickon.
A close-up of the granodiorite
Stop 5. Past the Granodiorite are additional small outcrops of Wissahickon Formation.
Take GPS coordinates and note their position on your topo using landmarks. Collect
strikes and dips, rock samples if you find a good outcrop.
Stop 6. Next continue up toward the strong right bend in the stream. Note the shattered
rock on the floor of the path. The bend and the fracture aone should alert you to the
presence of a fault. Remember the shattered rock you saw on either side of the small fault
in the Allentown Dolomite on trip two. This fracture zone is much larger. This is the
Rosemont Fault, the suture zone between the lower Paleozoic Wissahickon Group, and
the Proterozoic
Grenville Craton of the Laurentian continent just to the north.
Take GPS coordinates. Also note your position on your topo map.
Follow the trail around the bend and turn left to cross the bridge over Wissahickon Creek.
Note Chestnut Hill College on your right. Turn left into the park: take the small path
along the north stream bank to the bridle path "Forbidden Drive" ahead. Turn left on the
bridle path and walk to the first quarry on your right.
WEST SIDE OF STREAM
STOP 7. Baltimore Gneiss. This rock is part of the Grenville Craton we saw on the
second field trip. Get the strike of the southern margin and a GPS position of the exposed
southern extent near the bridal path. Look for additional outcrops near the stream bank to
initially orient the suture. Note the limits on your topo using landmarks.
Discussion: Orthogneiss and paragneiss, stable isotopes (again)
We are north of the suture zone separating the Wissahickon Formation from the
Baltimore Gneiss, the Rosemont Fault. To further orient the fault, we could find the
location of the separation between the Wissahickon and Baltimore Formations along the
Schuylkill River to the west of here; the line for this fault strikes about S75W. Start at the
lat and lon of the southernmost extent of Baltimore Gneiss on this side, and draw a line to
show the southern boundary of the Baltimore Gneiss. Discussion.
Finish Mapping as a team:
Okay, now you are on your own. You should be able to re-find the rocks you saw earlier
on this side of the stream. For each rock bed you find, take GPS readings, a strike and
dip, and a rock sample carefully labeled. You can see some ridges of rock crossing the
stream; these will help you with the strike. Work your way back to the road. Do not cross
the road, wait there for your instructor. Don't be shy about strikes and dips for the
Wissahickon Formation, the quartzites will work fine. Plot all contacts, formation
symbols, and strikes and dips on your topo.