GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 96-17
Presentation Time: 9:00 AM-6:30 PM


FREELAND, Duncan L, Geology, Allegheny College, 520 N. Main St., Meadville, PA 16335 and CARTER, Matthew, Geology Department, Allegheny College, 520 N Main St, Alden Hall, Meadville, PA 16335

Conanicut Group Cambrian rocks exposed on the southern coastline of Beavertail State Park, Rhode Island contain trilobite fossils of Avalon affinity. The Avalon microcontinent is thought to have accreted to Laurentia during the Acadian Orogeny, but the exposed rocks only record Alleghanian deformation. Because these Cambrian rocks do not record any Acadian deformation, it remains unclear whether or not they indeed have an affinity to Avalon. To help resolve this problem, we investigated the stratigraphic units and paleoflow indicators of the fine-grained metasedimentary sequences to compare them to other Cambrian rocks associated with Avalon. Dunes, soft sediment thrust faults and folds, and localized channels were examined and measured along the SE and W coastlines to determine facing (i.e. “way up”) and paleocurrent directions. Thicknesses of all exposed sedimentary units were also estimated. Results indicate the Beavertail Point Member (> 65 m) is the oldest unit and is overlain by the Dutch Island Harbor Formation (130-160 m), Fort Burnside Formation (10-18 m thickness), and Lionhead Member (thickness unknown). The proposed stratigraphic succession and estimated thicknesses aided in reinterpreting a cross-section, revealing E-verging to recumbent folds from W to E across the southern shoreline. The Ft. Burnside Formation contains 20-70% sand, and has layers of 1-30 cm in thickness with dunes of 1-6 cm in amplitude and 20-150 cm in wavelength. Cross-laminations suggest a W to SSW paleocurrent direction at the SE locality, and at the W locality paleocurrent directions are W to SW at the bottom and NW to NE at the top. Soft-sediment deformed intervals contain local cm-scale thrust faults and associated folds that indicate E, S, and W-ward motion. Further, the unit thins from 18 m to 10 m, and sand content is reduced from the eastern to western coastal exposures. The Beavertail Point Member contains 8-15% sand and has layers that are 1-6 cm in thickness. Dunes of 1-3 cm in amplitude and 2-80 cm in wavelength have cross-laminations that suggest a W paleocurrent direction, and localized channels suggest a NW-SE flow axis. Results imply a low-density and/or distal sediment source from the E-SE, which does not appear to be consistent with other reported paleocurrent analyses from Cambrian rocks of the Avalon microcontinent.