The “Beam” outcrop is located near
South Hero, Vermont in Middle
Ordovician (~450 million years old) calcareous shale of the Cumberland
Head Formation (Figure 1: Stanley,
1990, after Leonard, 1985). The
“Beam” is a band of micrite
(calcareous mud) roughly 20 cm (8in.) thick traversing the well-cleaved
Cumberland Head shale. The “Beam” has been cut into five imbricate thrust faults and associated ramps along the
13.7m (45ft.) exposure (Figure 2:
Stanley, 1990).
The deformation of the “Beam” mimics the larger-scale deformation of the foreland in Western Vermont. The ramp faults which cut the beam of micrite and the bedding-plane faults which occur on the upper and lower surface of the micrite are analogous to larger-scale thrust faults and decollment (floor) faults of northern and central Vermont.
Fault surfaces along the “Beam” are coated with sparry calcite and show slickenlines parallel with a N65W fault motion. Abundant veins of calcite both within the micrite and shale are isotopically similar to the host-rock shale and indicate that dissolution of carbonate shale was the likely source of sparry calcite (Stanley et al, 1996).
Take I-89 north to exit 17. Take
Rt. 2 north past the town of
South Hero.
The beam is in a driveway on your right.
References:
Leonard, K., 1985. Foreland folds and thrust belt deformation chronology, Ordovician limestone and shale, northwestern Vermont.
UVM Master’s Thesis, 120p.
Stanley, R.S., 1990. The evolution of mesoscopic imbricate thrust faults—an example from the
Vermont Foreland, U.S.A., Journal of
Structural Geology, Vol. 12, No. 2, p. 227-241.
Stanley, R.S., M. Abbott, T. Whalen, and A. Lini,
1996. Isotope analysis, systems dynamics, and the “Beam”—an interlude, Geol. Soc. of
America National Meeting Abstracts, Vol. ?,
No. ?, p. A244.
Stanley, R.S., K. Leonard, and B. Strehle, 1987.
A transect through the foreland and transitional zone of Western Vermont; in
Westerman, D., ed., NEIGC 79 th Annual Mtg.
Guidebook—1987, Vol. 2, p. 80-108.
For information about other geological teaching outcrops in Vermont, please contact the