LATE EDIACARAN(?) SYN-DEFORMATIONAL FLUVIAL SEDIMENTATION IN WEST AVALONIA: THE LILLY UNCONFORMITY REVISITED

Episodic late Ediacaran crustal deformation of West Avalonia records events of the enigmatic Avalonian Orogeny. On the Avalon Peninsula of Newfoundland, evidence of ca. 560 – 555 Ma deformation includes faulting, folding, soft-sediment deformation, and deltaic progradation of Signal Hill Group from exhumed sources to the north. By the latest Ediacaran to earliest Cambrian(?), Signal Hill strata were folded, faulted and unconformably onlapped by fluvial and alluvial fan strata of the Flatrock Cove Formation, forming the well-known “Lilly Unconformity” exposed in Flatrock, Newfoundland. Recent stratigraphic and sedimentologic investigations of the Flatrock Cove Formation further highlight details of floodplain reorganization during changes in local paleotopography resulting from progressive folding and faulting. The basal Knobby Hill Member consists of three unconformity-bound growth sequences of braided fluvial conglomerate and sandstone, recording the aggradation of wedge-shaped braided fluvial channel belts along an axial drainage path parallel to the axis of the growing Flatrock anticline. These rivers transported rounded volcanic clasts typical of the Harbour Main Group, ≥10 km to the north. The contact between the Knobby Hill Member and overlying Piccos Brook Member is gradational, intercalated, and characterized by progressive preservation of fine-grained floodplain strata with dewatering structures, interbedded with sheetflood conglomerate and sandstone. This contact records the reorganization of the Flatrock Cove floodplain during movement along the Flatrock thrust; to a sheetflood-dominated fan system with the progradation of locally derived, laterally fining sheetflood deposits depositing local angular sedimentary clasts from a point source along the Flatrock thrust. Three more unconformity-bound growth sequences within the Piccos Brook record episodes of alluvial fan sedimentation punctuated by movement along the Flatrock thrust. Although the exact depositional age of the Flatrock Cove Formation in unknown, it provides a record of the near-surface brittle compressional deformation from the enigmatic Avalonian Orogeny, and stands as a world-class example of fluvial basin-margin sedimentation and stratal evolution in response to progressive deformation.