Turbidites and rip-up clasts in Elder Creek

Elder Creek, part of the South Fork Eel River watershed, lies in the Franciscan Formation found underfoot in most of the Northern Coast Ranges of California. The rocks here were deposited in marine environments when the Farallon slab was still subducting under the North American plate at this latitude. Subsequent uplift following the passage of the Mendocino Triple Junction has elevated these rocks out of the sea.

Clastic sedimentary rocks found in Elder Creek record information about their depositional setting. Grain size, lithology, and shape all provide clues about the energy of the flow and the time spent in transit, sorting and abrading. The vast majority of the rocks in Elder Creek are turbidites, formed from turbidity currents: dense slurries of sediment sloughing off the edge of the continent, rushing off the continental slope to final resting places in deeper, still waters. These currents are thought to be triggered by earthquakes, among other things.

Turbidites contain sand and pebbles that were rounded in terrestrial rivers prior to their arrival at the ocean. They also contain small clay-sized particles that fall out of the ocean water column (the long snowfall, in Rachel Carson’s words). As numerous currents are laid down over time, they create a rhythmic sequence of grain sizes, with a fining upward sequence recording stratigraphic ‘up’ (left to right in the image from the bed of Elder Creek below.)

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Rhythmically bedded turbidite sequence (pebble to sand to clay size), Elder Creek. Photo credit: Jesse Hahm

Sometimes turbidity currents race over clay-sized mud deposits (shale). They pick up bits of the semi-lithified shale and carry them along. These shale bits are called rip-up clasts or intra-formational clasts. They are recognized by their darker color and angular shape, and are often much larger than the terrigenous sediment that surrounds them.

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek

Rip-up (intra-formational) shale clasts in sandy matrix, Elder Creek. Photo credit: Jesse Hahm