THE ALONG-STRIKE CONTEXT OF THE RICHARDSON MEMORIAL CONTACT (RMC) IN THE TOWN OF CRAFTSBURY, NORTH-CENTRAL VERMONT

In Vermont, the RMC separates Pre-Silurian metamorphic rocks of the Rowe-Hawley Belt (west) from those of Siluro-Devonian age in the Connecticut Valley Trough (east). Richardson (1919) called this boundary the “Cambrian-Ordovician Erosional Unconformity/ Contact” and Currier and Jahns (1941) noted that it was marked by quartz pebble conglomerates and fossiliferous marbles (Shaw Mt. Fm). The RMC is interpreted as an unconformity throughout the southern half of Vermont (Ratcliffe et al., 2011).

In central Vermont, Westerman (1987) and Hatch (1988) proposed that the RMC was an Acadian (Devonian) fault, the Dog River Fault Zone (DRFZ). Walsh et al. (2010), however, argued that the RMC unconformity and DRFZ were not always coincident. Although the surface trace of the RMC correlates with the Acadian La Guadaloupe Fault in southern Quebec, inliers of Silurian rocks that unconformably overlie the Pre-Silurian Dunnage Zone to the west support an earlier stratigraphic origin.

In Craftsbury, the west side of the RMC is comprised of Pre-Silurian phyllitic quartzites (Moretown Fm) and black phyllites (Cram Hill Fm) whereas the Siluro-Devonian east side is composed of black phyllites interlayered either with quartzites (Northfield Fm) or sandy marbles (Waits River Fm). The Ordovician Taconian Orogeny deformed and metamorphosed the west side of the RMC, but the Acadian Orogeny affected rocks on both sides. Bedrock mapping defined two dominant foliations (Taconian =S₁, S₂; Acadian = S₃, S₄) for each orogeny.

Based on map-scale upper and lower plate truncations and the intensification of the fault-related S₃ foliation approaching the RMC, we believe the RMC is coincident with a high angle fault in Craftsbury. Strongly developed L₃ stretching lineations are found on both sides. Asymmetric quartz and mica tails on feldspar porphyroclasts in the Cram Hill Fm show west side up sense of shear, consistent with central Vermont kinematics. Within sandy marbles of the Waits River Fm, flattened grains of albite and calcite, bands of graphite, and alternating quartz- and mica-rich domains define S₃; quartz grains have strong shape-preferred orientations.

Ongoing work seeks to reconcile the tectonic vs. stratigraphic nature of the RMC along-strike. We are also pursuing ⁴⁰Ar/³⁹Ar ages and mica geochemistry for the Craftsbury rocks.