Part II: Geological Observations by Land and Sea of the Champlain Thrust Fault at Lone Rock Point

This fall I visited Lone Rock Point, a rocky promontory just north of Burlington, Vermont, on the eastern shore of Lake Champlain. The locale displays a spectacular outcrop of the Champlain thrust fault. At the contact, the buff-colored Lower Cambrian Dunham Dolostone overlies the dark gray to black-bedded Middle Ordovician Iberville Formation.

 

In my previous post entitled "Part I: The Tectonic Genesis of the Champlain Thrust Fault at Lone Rock Point" I reviewed the tectonic events, both global and regional, leading to the creation of the thrust fault at the point. Also, I described how the reversed stratigraphic architecture of the thrust's component formations occurred.

It's time to visit the point, first by sea.  


Photo Below:

Seen from the water on the northern edge of Burlington Bay, we are looking down the south-face of Lone Rock Point towards the mainland (the sandy beach at the far end). On this side of the promontory, the cliff-face is formed mainly of Iberville Shale with a variable cap of Dunham Dolostone.

Photo Below:

Having just rounded the point, a segment of the Champlain thrust slice comes into view on the north side of the promontory. A massive block of dolostone has cascaded from the cliff-face, and a few others can be seen along the shore. Numerous light-colored, mass-wasting scars can be seen on the cliff-face. The shoreline is composed mostly of flat, highly polished, eroded pieces of shale (where the shore hasn't been drowned by the lake's recent high water), but relatively few massive, boulders of dolostone are found. This leads me to believe that the dolostone's erosion from the cliff-face and subsequent removal probably occurred much closer to the period of regional glaciation (ending about 10,000 years ago give or take) rather than recently. The scarp stands about 50 to 85 feet above the lake, which was running high, as mentioned, due to atypically heavy rains and Hurricane Irene. It is clearly evident that the erodable and fissile shale below the contact is basically undermining the overlying dolostone.

Photo Below:

We’re now looking back at the point. Burlington Bay lies on the far side of the promontory. Lake Champlain continues to the south for another 35 miles. The Green Mountains of central Vermont are on the horizon with the peaks of Camel’s Hump and Sugarbush barely visible. That’s dolostone holding up the point with shale at the base. A large section of "fresh" cliff-face can be seen with dolostone-talus at the shoreline. A "lone rock" island of dolostone hovers nearby, perhaps the namesake of the point.

 

Photo Below:

This is a straight-on view of the thrust slice at the north-face, the view seen in many geology textbooks. Viewing the thrust locality from the lake afforded me an excellent opportunity to gain a sense of the massivity and size of the slices. The thrust contact is sharp and surprisingly straight. In the early 1800’s, the thrust fault such as at Lone Rock Point was considered to be an unconformity between the underlying Ordovician shales of the “Hudson River Group” and the overlying dolostones of the “Red Sandrock Formation.” The Sandrock was thought to be of Silurian-age, since it bore a lithological similarity to the Medina Sandstone of nearby New York State. By mid-century, the noted geologist Edward Hitchcock and others correctly interpreted the contact as a major fault of regional extent based upon pre-Medina-age fossils in the Sandrock Formation. It is now recognized that the Champlain thrust fault is one of several important faults that floor major slices of Middle Proterozoic crust exposed in western New England.

Photo Below:

In addition to viewing the locality from the lake, I struck out on foot to view the fault and its component formations up close. The less resistant Iberville shale, further weakened by shearing at the fault, can be seen to have eroded from below the contact and undermining it in the process. The shale has developed a multitude of vertical joints and is gradually exfoliating from the cliff-face in an inverted-step pattern. The region of the contact is riddled with small grooves and larger corrugations within the resistant overlying dolostone, as it slid over the shale. The dolostone’s undersurface along the thrust reveals numerous fault mullions which plunge 15° to the southeast. The average southeastward dip of the thrust is 10°. Bedding in the shale can be seen to have been subjected to slight folding and deformation.

Photo Below:

This close-up of the contact shows the resistant planar fault surface on the under-aspect of the dolostone, the roof of the contact. The underlying black shale below the contact exhibits marked fissility, and slight deformation and folding. The intermediary region within the contact displays a very obvious brecciated zone of angular clasts distributed in a finer matrix. 

Photo Below:

This perspective is actually afforded from within the contact looking upward at the inferior surface of the dolostone at a distance of two feet. Indicative of movement along the fault surface, numerous straight lineations, striations and grooves called slickenlines are discernible running from upper left to lower right on the polished, inscribed slickensided-surface. Slickenlines are created as the fault surfaces abrade against each other. Mineral growth becomes aligned with the direction of slip along the fault recording the last direction of movement across the fault surface.

Photo Below:

In other areas along the contact, large slivers of dolostone and limestone embedded in a highly contorted matrix of shale can be found along the trace of the thrust in the brecciated zone. Some sources indicate that the slivers of limestone may represent pieces of the Beekmantown Group, a Lower Ordovician shallow marine, largely carbonate strata on Laurentia's passive continental shelf. 

Photo Below:

With the exception of jointing and karstic-induced irregularities on the uppermost surfaces of the Dunham dolostone, mostly minor structures were found in the body of the formation. On the other hand, the underlying Iberville Formation is riddled with deformed beds of gray and black shale, and white interbedded veins of carbonate that are finely bedded. Compression has caused the rock layers to bend plastically resulting in their deformation. The folds are essentially concordant with the plane of the contact. Deformation in the shale is most likely related to Taconic compression. Indeed, deformation seems to be more pronounced with increased proximity to the contact. Possibly some deformational overprinting exists, since the fault system is thought to have been reactivated during the ensuing Acadian Orogeny in the middle Paleozoic. Even some tectonic affect during the late Paleozoic Alleghenian Orogeny can not be ruled out, but seems less likely owing to its more southerly location. Asymmetrical drag folds with gently curved hinges were fairly abundant. I suspect that the presence of deformed, thin layers and lenses of white carbonate are related to the influx of water that was infused post-depositionally, since carbonates appear between shale beds but also cross-cut to the bedding. Close examination also reveals S and Z-folds and pull-apart structures in the shale beds, and small-scale boudin-like bodies in the carbonates. The fractal nature of the deformed bedding, evident even in the smallest of observations, has far greater tectonic implications.

Photo Below:

Occasional remnants of Early Cambrian bivalves can be found imbedded in the matrix of the dolostone. I found a few distributed in a narrow horizon. I suspected their presence in the formation but had to search hard to find them. Lichens cover many of the exposed dolostone's surfaces which is often pockmarked by karstic dissolution.

 

Photo Below:

This photo was taken at the end of the promontory high above the lake on a surface of heavily karstic-etched dolostone. At the day's end, the sun was getting low in the sky over the Adirondacks across the lake to the west. It was a glorious day in northern Vermont for October with blazing sun, rich autumnal colors, and perfect 85 degree weather, likely the last hurrah before cold winds foretell what's to come. 

Photo Below:

Back at the waterfront in the town of Burlington, the festive 4th Annual Giant Pumpkin Regatta was in full swing replete with watercraft of hollowed-out, 1000+ pound pumpkins!