Description
The California coast is divided into a series of geographical compartments called littoral cells, in which longshore current sand transport is predominantly north to south. It is thought that these littoral cells, being isolated by headlands and terminated at submarine canyons, have little to no transport of sand between them. However, there are still uncertainties regarding possible ‘leakage’ between cells. Most California beaches are dominated by monotonous quartz-feldspar rich sand, but on the central California coast, the Big Sur River delivers a distinctive garnet-magnetite-ilmenite rich sand into the system directly at Andrew Molera State Beach. This heavy mineral-rich sand is derived from Sur Series metamorphic rocks that are deep crustal exposures of the Cretaceous Salinian block batholith. This sand is presumably diverted offshore at the Partington submarine canyon at the south end of the Point Sur littoral cell. However, an isolated patch of distinctive garnet rich sand collected at El Capitan State Beach on April 8th 2017 from the uppermost part of the beach following a moderate amplitude spring high tide bears a strong resemblance to the Sur Series beach sand suggesting that this sand was transported from the Big Sur River ~300 kilometers to the north. This implies that the Big Sur sand bypassed Partington canyon and traversed the Morro Bay littoral cell to the south in order to arrive at El Capitan State Beach within the Santa Barbara littoral cell. The garnet-rich heavy mineral sand at El Capitan would not be expected to be derived from local marine sedimentary sections or Franciscan Assemblage rocks farther north. Samples collected from Andrew Molera and El Capitan State Beaches were separated into magnetite-ilmenite-garnet fractions using hand magnets and a Franz magnetic separator. The proportion of magnetite to ilmenite is nearly identical between the two (~1:2) but El Capitan has a higher proportion of garnet. The grain size of both sands were determined and document a unimodal well sorted medium to fine sand with a mean grain size of ~280 microns for both samples, although the El Capitan sample is slightly coarser-grained due to a component of feldspar we were unable to perfectly separate from the sample. Under the binocular microscope, El Capitan garnets are distinctly polished and rounded compared to Andrew Molera garnet which is more euhedral with well-developed crystal faces consistent with long-shore transport of the El Capitan garnet. Electron microprobe mineral analysis however demonstrates conclusively that at least a part of the El Capitan beach sand must come from another source. Gaviota Creek a short distance up the coast in back has been documented as a source of garnet delivered to the beach along this section of the coast. More work is required to determine whether or not the component of garnet at El Capitan that matches Andrew Molera is in fact Sur Series garnet from the same source.
