Masters Thesis

Marine inundation of a late Miocene forest: stratigraphy and tectonic evolution of the Saint George Formation, Crescent City, California

A paleosol developed on the Mesozoic Franciscan Complex is depositionally overlain by 48 m of late Miocene (diatom age ca. 6.0-6.4 Ma), mostly shallow-marine Saint George Formation. At the base of this sequence is a buried paleoforest of rooted tree stumps, found at three separate sites. Above the rooted stumps is a sequence of wave-reworked colluvium containing woody debris interpreted as a tsunami deposit, succeeded by beach deposits, and bioturbated, mollusk-rich mudstone having occasional hummocky cross-stratified sandstone interbeds. Regional correlations of the Saint George Formation with the Wimer Formation, preserved on the highland Klamath erosion surface immediately east of the study area (at elevations up to 680 m) and basal Pullen Formation at Scotia, California, 145 km to the south, suggest that these units are remnants of a single transgressive shelf sequence that blanketed northwestern California. These sediments have a likely Idaho batholith provenance and accumulated in response to rapid subsidence of the leading edge of North America during the late Miocene. The late Miocene Saint George Formation [units 1-3] dips easterly along Pebble Beach and is preserved in an open, northwest-trending syncline north of Point Saint George. The newly-described 'Pebble Beach thrust fault' has an north-northwest strike and west-southwest vergence and runs onshore from mid-Pebble Beach to north of Point Saint George, cross-cutting Pleistocene marine terraces and displacing a Holocene peat [radiocarbon date of 3000± 60 BP]. Saint George rocks are folded into an anticline-syncline pair disrupted by a smaller reverse fault. The folds are U-shaped, symmetrical, have widths of about 8 m and plunge north-northwest at about 10 degrees. They do not appear on the north side of Point Saint George. The reverse fault extends to the north side, for in its line of projection are two reverse faults that drop Pleistocene terrace deposits south side down against basal Saint George Formation in a step-like manner. North of Point Saint George, dip reversal across the lower plate (the Pebble Beach fault) syncline exposes a Franciscan Complex paleosol-basal Saint George conglomerate contact to the south, and paleosol with rooted stumps overlain by swash-cross-stratified sands to the north. Dips across this fold do not exceed 20 degrees and flexural slip formed bedding-parallel mudstone breccias. Fold and clustered fracture data record east-northeast-west-southwest crustal shortening in the lower, and north-northwest-south-southest extension in the upper plates. Offshore structures trend north-south and record east-west shortening. Contrasting orientations of structures along the Pebble Beach Fault reflect movement over an oblique thrust ramp and local thrust sheet rotation.

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