Field Trip 1: Continental Passive Margin

Stop 1.1 - Mt. Holly Gneiss

The Mt. Holly Gneiss cooled 1.1 bya, and has not been reheated to any significant level since then. It has thin, tightly folded gneissic foliation, which is a Precambrian deformation fabric. The rock contains thick pegmatite bands that crystallized during the partial melting of meta-sediments which left behind mafic minerals. They contain biotite, quartz, feldspars, and some hornblende. There are also crystals of epidote that give the rock a green color. Some of the green color may also be due to the alteration of feldspars (Fig. 1.2).

Figure 1.1. Example of a thick pegmatite band (outlined by red dashed line) with a composition close to granite.

Figure 1.2. Thin, alternating light and dark bands that have been tightly folded.
Notice the green hue of the rock from the altered feldspars.

Stop 1.2 - Cheshire Quartzite

The Cheshire is a very homogeneous rock, mostly quartz (Fig. 1.4). This may be due to the removal of impurities during diagenesis. It can be found all along the eastern seaboard, from Georgia to Newfoundland. The provenance of this rock must be a marine continental shelf, as evidenced by the existence of cross-bedding (Fig 1.5). It must have been deposited at a time of little sea level variation, because there are no shale-derived metamorphic rocks interbedded with the quartzite.

Figure 1.3. Small anticline.

 Figure 1.4. The rock has very few impurities.

Figure 1.5. Some faint cross-bedding can be seen, highlighted here by black dashed lines. The cross-beds end asymptotically at the bottom (marked by solid black line) while truncating at the top, indicating that the bedding is currently right-side up.

Stop 1.3 - Monkton Formation thrust over Winooski Dolomite

The Monkton Formation is comprised of phyllite, quartzite, and dolomitic marble. The Monkton has been thrust on top of the Winooski, and the contact between the two is very visible (Fig. 1.6).

Figure 1.6. Contact between the Dunham Dolomite on bottom and the Monkton Formation on top, marked by dashed black line.

There also appears to be evidence of a fold underneath the fault, and is probably caused by the fault (Fig. 1.7).

Figure 1.7. Looking at the limbs from an oblique angle. The limbs are outlined in red and project out of the outcrop. the fold axis is marked by the black dashed line.

Stop 1.4 - Scolithus Tubes in Cheshire Quartzite

At this stop, bioturbation that occurred before lithification has eliminated original sedimentary structures. Therefore, the quartzite here is more massive than at Stop 2.

Figure 1.8. Tubes from worms burrowing vertically through the original sand. Some examples are outlined with boxes, but the entire rock is filled with tubes.

Stop 1.5 - Folded Bascom Formation

Figure 1.9. Massive folds of gray limestone beds and darker beds containing clay and silt.

There are a number off features associated with the larger fold deformation. There is a prominent fault that runs parallel to the axial plane of one fold (Fig. 1.10). This fault is associated with a space issue as the rocks were folded. On a smaller scale, we can see deformation within the darker beds created by flexural slip.Additionally, there are calcite veins that run parallel to bedding, indicating that they were deposited during deformation as the beds slid past one another.

Figure 1.10. Notice the fault offsetting the beds outlined in red.

Summary

 On this trip we traveled across the contact between the Grenville Basement (Mt. Holly Gneiss) and rocks that formed on the passive Laurentian margin, during the drift period of continental breakup. The some of the rocks on the margin were originally sediments that were transported from the continent and deposited on the shelf. Once deposited, the sediments were often reworked, as evidenced by the cross-bedding in the Cheshire quartzite. The limestone in the Bascom formation provides evidence for the formation of carbonates on the shelf during this period as well.