STRUCTURAL AND PETROGRAPHIC ANALYSIS OF THE CAMBRIAN UNITS AT BEAVERTAIL STATE PARK, RI

Fabric development and orientation, style and sequence of structures found in the low grade Cambrian rocks within and up to 4 km to the north of Beavertail State Park, RI are identical to those observed in the Pennsylvanian rocks of the Narragansett Basin. These similarities indicate that the Cambrian rocks have exclusively recorded the Alleghenian orogeny and could not have been in place in North America during the Taconic and Acadian.

Detailed mapping on a 1:1000 scale has revealed an early tight to isoclinal fold generation (F₁) and axial planar foliation (S₁) followed by a later, more prominent, E-vergent, NE-trending non-coaxial fold generation (F₂) and associated foliation (S₂). A third map-scale phase may be inferred from the N-trending broad warping of S₂. Quartz veins are folded and cross cut by F₂ folds. N-S to NE-SW extension resulted in 1-10m scale boudins that deform S₂. All these structures are cross cut by NNE to ENE-trending faults. Kinematic indicators show that the dominant motion on these faults was normal with minor sinistral and dextral motion. The Beavertail Shear Zone that juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin deflects S₂ in a dextral sense, consistent with recorded motion on this zone. Kink bands associated with faulting trend NNE to ENE and display WNW to NNW side up. Detailed petrographic work has revealed that the penetrative foliation is S₁ which is defined by the alignment of fine grained muscovite. S₂ is a crenulation cleavage of S₁ and is enhanced by pressure solution. Iron carbonate and feldspar porphyroblasts overgrow S₁ and are wrapped by S₂. Muscovite, quartz and iron carbonate pressure shadows form off these porphyroblasts parallel to S₂. S₂ is the dominant foliation expressed in the field; however there are areas where S₁ is locally better developed.

These results have important relevance for the tectonic history of southeastern New England by establishing timing constraints on collision of an Avalon microcontinent, and will help constrain future tectonic models for the area.