Tsunami(?) Sculpturing of the Pebble Beach Wave-Cut Platform, Crescent City Area, California

Bedrock sculpturing of semilithified St. George Formation sandy mudstone exposed on a wave-cut platform has produced a variety of erosional forms that include surface scratches made by beach debris, ripple-like ridges that reflect differential erosion of bedrock, potholes, and scallop-shaped pockets and grooves, which may be straight or sinuous. Straight grooves form by preferential incision of regional joints. Sinuous grooves are not fracture controlled, are oriented parallel to wave run-up (orthogonal to the coast), and exist as closely spaced subparallel, nonconnecting, internally drained grooves that are best developed on higher platform ramparts and benches. Sinuous grooves have a mean length of 258 cm, mean maximum width of 14 cm, and mean width/length ratio of 0.08. They are not as deeply incised as straight grooves, do not serve as conduits for low-tide runoff during winter months, and typically terminate by shallowing and narrowing in both seaward and landward directions. Sinuous appearance results from trains of linked comma-shaped depressions, commonly with the blunt, highly curved end of each being most deeply incised and oriented seaward. Corrasion of bedrock highs and/or cavitation associated with turbulent vortices during tsunami run-up likely contributed to the genesis and/or enlargement of the sinuous grooves. Auguring and coring in a back barrier bog and diatom analysis reveals a landward-thinning, approximately 17 cm-thick, laterally continuous, clean, tsunami-emplaced sand layer with a sharp basal contact up to 125 m inland of the modern high-tide line. While seasonal cycles of beach aggradation and degradation combined with sediment transport and bedrock erosion accompanying low-tide runoff and high-tide wave motion undoubtedly accounts for form modification of sinuous grooves, it is unlikely to account for their origin.