Leech River Fault at Sombrio Beach скачать в хорошем качестве

Leech River Fault at Sombrio Beach

Sombrio Beach, along Vancouver Island’s Juan de Fuca Marine Trail, lies between Jordan River and Port Renfrew. This scenic spot features sandy stretches, a popular surf break, a hidden slot-canyon waterfall, sea caves, and excellent low-tide tide pools. Its primary geological significance stems from its position directly within the Leech River Fault Zone, a major crustal boundary. The Leech River Fault is a northwest-striking thrust fault (with dextral strike-slip motion) that separates two distinct terranes. To the south lie Eocene (~50–54 million years old) volcanic rocks of the Crescent Terrane (Metchosin Igneous Complex), primarily pillow basalts and related volcanics. To the north are older Jurassic–Cretaceous (~130–160 Ma) metamorphosed sedimentary rocks of the Pacific Rim Terrane (Leech River Complex), dominated by schists and phyllites derived from deep-marine sediments. One of the most striking features is a well-exposed angular unconformity visible in the cliffs, particularly about 350 meters northwest from the southern beach access. The lower unit consists of deformed, foliated schist/phyllite (130–160 Ma). Above it sits an ~35-million-year-old (Oligocene) boulder conglomerate—representing an ancient beach or nearshore deposit—overlain by sandstone formed in shallow marine settings (5–15 m depth). This contact reveals a ~100–125 million year gap in the geologic record, erased by uplift, erosion, and tectonic events during terrane accretion. The beach itself is backed by wave-sculpted platforms of schist and phyllite, which supply most of the cobbles and pebbles. Upper cliffs expose more resistant sandstone and conglomerate. Differential erosion along joints, faults, and bedding planes creates tide pools, sculpted cliffs, and the popular waterfall slot canyon that cuts through the younger sandstone. Modern processes include ongoing tectonic uplift, post-glacial isostatic rebound, and high-energy wave action from the Juan de Fuca Strait, which continuously erodes the coastline. The fault zone also records evidence of prehistoric large earthquakes, linking to broader Cascadia seismic hazards.