DETAILED ANALYSIS OF STRUCTURES IN THE FOOT WALL OF THE CHAMPLAIN THRUST AT LONE ROCK POINT, BURLINGTON, VERMONT

Between Burlington, VT and the Quebec border, the Champlain Thrust (CT) juxtaposed massive dolostones of the Cambrian Dunham Fm (hanging wall) with calcareous shales of the Ordovician Iberville Fm (foot wall), during the Ordovician Taconian Orogeny. After this orogeny, the CT was deformed by open folds of possible Devonian (Acadian) age. In southern Quebec, the CT maps continuously into the Rosenberg Thrust/ Logan’s Line. South of Burlington, the CT cuts up section into red quartzites of the Cambrian Monkton Fm via a presumed lateral ramp, and continues into New York. Published estimates of displacement along the CT range from 60-100 km.

Although the CT has been studied previously at Lone Rock Point, the multiple generations of ductile and brittle structure in shales of the footwall have never been systematically defined. We present the following relative chronology of structures:

1) Formation of bedding planes (S₀), characterized by thin layers of carbonate within black shale.

2) Formation of rootless isoclinal folds (F₁) of brittle carbonate layers and the development of an spaced pressure solution cleavage (S₁) that parallels the axial planes of the folds.

3) The S₁ cleavage is deformed into asymmetric S-C shear bands that merge into parallelism with, and are cut by, intraformational thrusts. The thrusts form oblate, eye-shaped structures that are stacked on top of one another forming thrust duplexes. A second cleavage (S₂) defines a part of the S-C fabric and is intensified in thrust zones. Calcite slickenlines on fault surfaces plunge to the SE and NW and slip directions fan up to 40 degrees with respect to one another in different thrust horses

4) Formation of sets of upright, north (F₃) and east-striking (F₄) folds of S₂ warping the CT.

5) Formation of conjugate sets of normal faults that record top-down-to-the–north and -south kinematics.

6) Formation of the steeply-dipping fracture sets (N-S and E-W striking) that cut across competent lithologies.

Preliminary interpretations suggest that simple and pure shear components of deformation in the foot wall are partitioned into thrusts and SE-plunging isoclinal folds, respectively. Ongoing research seeks to correlate footwall structures with those in the hanging wall. We will also seek additional CT sites along-strike to test our relative chronology.