Structural and petrographic analysis of Cambrian rocks at Beavertail State Park, Narragansett Basin, Rhode Island

Carter, Matthew James
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Structural Mapping at 1:1000 scale of the Cambrian rocks within Beavertail State Park, Narragansett Basin, Rhode Island has revealed an early open to isoclinal fold generation (F₁) and axial planar foliation (S₁) followed by a later, more prominent, Evergent, N-trending non-coaxial fold generation (F₂) and associated foliation (S₂). A third map-scale folding phase is inferred from the NNE-trending, broad warping of S₂. Quartz veins are folded by and cross cut F₂ folds. N-S to NE-SW extension resulted in 1-10 m scale boudins that deform S₂. All of these structures are cross cut by NNE to ENEstriking faults and joints. Kinematic indicators show that the dominant motion on these faults was normal with minor sinistral and/or dextral motions. The Beaverhead Shear Zone that juxtaposes the Cambrian rocks with Pennsylvanian rocks of the Narragansett Basin deflects S₂ in a dextral sense, consistent with recorded motion elsewhere. Kink bands associated with faulting trend NNE to ENE and display WNW to NNW side up. Petrographic work reveals S₁ is a penetrative foliation defined by alignment of white micas. S₂ is a crenulation cleavage of S₁ that is enhanced by pressure solution. S₂ is the dominant foliation expressed in the field; however there are areas where S₁ is locally better developed. Siderite and quartz nodules overgrow S₁ and are wrapped by S₂. Mica, quartz and iron carbonate pressure shadows on these nodules are parallel to S₂. Fabric development and orientation, style, and sequence of structures found in the lower greenschist Cambrian rocks within and up to 4 km north of Beavertail State Park, RI are dissimilar to those associated with the Taconic and Acadian events, but identical to Alleghanian structures in Pennsylvanian aged rocks of the Narragansett Basin. These similarities indicate that the Cambrian rocks have exclusively recorded the Pennsylvanian-Permian Alleghanian orogeny, and could not have been in place in North America during the Ordovician Taconic and Devonian Acadian orogenies. Timing constraints indicate the collision of the Avalon microcontinent occurred during the late Pennsylvanian, whereas progressive deformation of the Pennsylvanian rocks of the Narragansett Basin and the Cambrian rocks took place during the Permian