Beach sand from the coast of southwest Oregon, U.S.A.

- a little world of its own!

close-up photo of Coos Bay sample [349 kb]

Fig. 1: This little sample appears in homage to the January 2025 "sand of the month" on the lovely Splendid Sands calendar (Kenny et al., 2025: follow the link to their web site: Splendid Sands). More about that sample below. The sand in Fig. 1 is from the vicinity of Cape Arago, Coos Bay area, Charleston, Oregon, located some 138 km (86 miles) due north of the California state line. Individual grains, at hand lens scale, appear to include pale yellow to green epidote and a trace of olivine, abundant pale pink to red garnet, and numerous smaller, rounded grains of shiny black minerals with submetallic lustre, a mixture of chromite (iron-chromium oxide, "chrome spinel") and iron-(titanium) oxides (common examples being magnetite, hematite and ilmenite). In addition, there are a few shiny white shell fragments, a few colourless quartz grains, and a little (?) orange garnet. Sample from Kate Clover, Splendid Sands. The circular sand sample at the left, actually a polished thin section pared down from a small cylindrical puck, is 25 mm in diameter, prepared by Precision Petrographics ("up the coast" in B.C., Canada). A millimetre scale is just visible at the base, showing that the abundant grains of black oxides are generally <0.5 mm in diameter.

"Rock of the Month # 285, posted for March 2025" ---

Beach Sand from the Coast of Southwest Oregon, U.S.A.

Sand can tell many tales, of both the geological and zoological realms. Some varieties are very homogeneous, perhaps made largely of minerals from local rocks, such as quartz from sandstone or fragments of shells and coral on tropical beaches. Human artefacts such as wave-rounded glass fragments may be present. Sand is an important industrial mineral, used especially in construction, and it is strange but true that such a humble material is stolen and smuggled across seas to enrich criminal enterprises. Some sands, known as heavy mineral sands or black sands, have commodity-specific value, containing minerals such as rutile (titanium ore), garnet (abrasives) and monazite (rare earth elements and thorium). Geologist Michael Welland (2009) has written a detailed yet readable account of the world of sands, from deserts, rivers and beaches of the world. There are also many more technical works on sand and sandstone, the classic rock made of sand. Such rocks are composed of grains broken down from pre-existing rocks, and are known as clastic sediments. Geologists who specialise in these and other sedimentary rocks are known as sedimentologists.

Splendid Sands: a wonderful project, at once fun, artistic and informative, which illustrates and explains the many and varied sands of the world. Each year the three principals generate a photographic calendar of sand samples, with excellent close-up photography by Leo Kenney and detailed notes on the mineralogical and biological features of each sample, compiled by Kate Clover and Carol Hopper Brill, respectively.

The January sand in the Splendid Sands 2025 calendar is from Foramen Arch, on the headland of the Devil's Backbone on the coast of southwest Oregon, a winter sample rich in epidote and chromite (the heavy minerals concentrate with the heavy winter waves, as the less-dense minerals are washed into deeper water: in the calmer days of summer the quartz returns to bury the heavy minerals). That site is roughly 80 km (50 miles) north of the California state line, north from Ophir towards Port Orford, northwest of the Klamath national forest in California and the Siskiyou national forest in Oregon.

The Devil's Backbone sand seems to have more epidote that the Cape Arago beach sand. Chromite (and likely other oxides) and two colours of garnet are present in the sand at each locality. Possibly the more northerly, Cape Arago sand has less epidote because that site is further from the source ophiolite, and epidote is more easily broken apart than fellow travellers such as chromite and garnets? I would have imagined that the sand along this lovely coast is transported northwards along the Pacific margin by longshore drift. The reality is rather more complex. Barbara Katz and Stephen Gabriel (School of Oceanography, Oregon State University, 1982) wrote a lucid description of coastal processes along the Oregon shore. The coast is composed of headlands, between which are "pocket beaches", often with steep cliffs on the landward side. The beaches are dynamic features, changing from day to day and season to season. The longshore drift changes seasonally. It heads north in winter, with predominant southwest winds, and south in summer, with winds mainly from the northwest. Overall, net littoral drift is minimal, winter and summer seasonal changes cancelling out. Further, the headlands impede migration of sand, leading to unique sand assemblages in each bay (Kenny et al., 2025).

More generally, heavy mineral placers (0.5-3 km in length, with 20-100 wt.% heavy minerals) occur south of six prominent headlands on the 175 km of Pacific coast between Cape Lookout and Cape Perpetua (Peterson et al., 1986). Their survey covered Cape Lookout, Cape Kiwanda, Cascade Head, Cape Foulweather, Yaquina Head and Seal Rocks. Common light components are quartz, feldspar, rock fragments and mica, while heavy minerals in these beach placer deposits include hornblende, epidote, augite, hypersthene, garnet, zircon, ilmenite, chromite and magnetite.

Coos Bay sample [304 kb] Coos Bay sample [261 kb]

Figs. 2-3: Two photomicrographs of individual grains from the Cape Arago sand. Left: a translucent chromite grain. Chromite, like its related spinel magnetite, is generally opaque. The spinel family of oxides show great variation in composition, and this affects physical and optical properties. Image in transmitted, plane-polarized light, 100X magnification, long-axis field of view 0.8 mm. Right: Composite oxide grain. This grain is identified as brownish ilmenite withe exsolved lamellae of paler grey magnetite. Image in reflected, plane-polarized light, 200X magnification, long-axis field of view 0.4 mm.

The Source Region for the Sands

While quartz and the common kinds of feldspar may come from diverse sources, much of the heavy mineral content in the Oregon sands are from more specific, restricted geological sources. The following notes summarize some likely bedrock sources for the sand grains. Given the restricted migration of sand on the rocky coast, noted above, the following is really just an indicator of the source(s) of some of the heavy minerals along the Pacific coast in southwest Oregon.

Peridotite occurs on the west edge of the Klamath Mountains in the lower part of the Josephine Ophiolite. The well-studied Josephine ophiolite (Harper et al., 1988, 1994, 1996) was formed in a late Jurassic back-arc basin. It contains the classic seafloor assemblage of pillow lavas, cumulate gabbros, serpentinites, sheeted dykes and alteration forming rocks such as epidosites. The basal peridotite of the Josephine ophiolite, which is the youngest of the Klamath ophiolites, covers an area >1,000 km2. The local rocks display chromite zones in peridotite, several stages of dunite emplacement, and related ultramafic rocks such as harzburgite and orthopyroxenite (Evans, 1987). A suite of granulite-grade metagabbroic rocks are part of this assemblage in southwest Oregon, containing small amounts of sulphides (dominantly pyrrhotite, lesser chalcopyrite and pyrite). Sulphide globules occur in cumulus silicate grains, and some sulphide is remobilized along fault planes. Minerals in the metagabbros include clinopyroxene and orthopyroxene, olivine, plagioclase feldspar, amphibole, cordierite and the sulphides (Foose, 1986). Zoned, Alaskan-type ultramafic complexes in the Klamath Mountains contain resources of platinum group elements (PGE). Three such complexes in the region are dated at 163-142 Ma (Lower Coon Mountain, Tincup Peak and Chanchelulla Peak - Gray et al., 1987). The chromitites also contain grains of platinum-group minerals. These largely represent the more refractory PGE, such as platinum, ruthenium and iridium, rather than palladium and rhodium (Stockman and Hlava, 1984; Page et al., 1986). Podiform chromitite masses occur in the ophiolite, as at the Red Mountain outcrop, Tyson's mine and Brown's mine. The "podiform chromite deposits are found almost exclusively in dunite lenses in a larger groundmass of harzburgite" (Wynn and Hasbrouck, 1980). Dunite is an igneous rock composed mainly of olivine, while harzburgite is in essence an olivine-orthopyroxene rock. The seafloor sequence of rocks also contains some sulphide mineralization formed at hydrothermal vents. The Turner-Albright volcanogenic massive sulphide deposit is located at the base of the pillow lava sequence in the Josephine ophiolite. It contains a modest resource of copper-zinc-gold-silver mineralization (Zierenberg et al., 1990). The diverse ophiolite-related mineral occurrences are likely of academic rather than modern mining interest, though some chromite was mined in earlier days.

Ophiolitic peridotites are widespread, some examples being the Klamath Mountains and districts in New Caledonia and Papua New Guinea (Bodinier and Godard, 2003). Ophiolites, being the remnants of subducted and exhumed seafloor, are widespread around the world. The IGCP project 39, "ophiolites of continents and comparable oceanic rocks" produced maps for key areas in the northern hemisphere: the western USA, the Mediterranean area, the Urals of Russia, and the Himalayas and Tibetan plateau. (Working Group of Ophiolite Project, 1979).

REFERENCES

Bodinier,J-L and Godard,M (2003) Orogenic, ophiolitic, and abyssal peridotites. In "The Mantle and Core" (Carlson,RW editor). Treatise on Geochemistry volume 2 (Holland,HD and Turekian,KK editors), Elsevier- Pergamon, Oxford, 586pp., 103-170.

Evans,JG (1987) Deformation of the Josephine Peridotite, California and Oregon. USGS Prof.Pap. 1378, 45pp.

Foose,MP (1986) Setting of a magmatic sulfide occurrence in a dismembered ophiolite, Southwestern Oregon. USGS Bull. 1626-A, 23pp.

Gray,F, Page,NJ and Moring,BC (1987) Petrology and platinum-group element variation in zoned ultramafic complexes, Klamath Mountains, Oregon and northern California. In `USGS Research on Mineral Resources - 1987 Program and Abstracts', USGS Circ. 995, 82pp., 25-26.

Harper,GD, Bowman,JR and Kuhns,R (1988) A field, chemical, and stable isotope study of subseafloor metamorphism of the Josephine ophiolite, California-Oregon. J.Geophys.Res. 93 no.B5, 4625-4656.

Harper,GD, Grady,K and Coulton,AJ (1996) Origin of the amphibolite "sole" of the Josephine ophiolite: emplacement of a cold ophiolite over a hot arc. Tectonics 15, 296-313.

Harper,GD, Saleeby,JB and Heizler,M (1994) Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California-Oregon: U/Pb zircon and 40Ar/39Ar geochronology. J.Geophys.Res. 99 no.B3, 4293-4321.

Katz,BA and Gabriel,SR (1982) Oregon's ever-changing coastline. Oregon State University Extension Service brochure, 8pp.

Kenny,L, Clover,K and Hopper Brill,C (2025) Splendid Sands 2025. Exploring the Science and Beauty of Sand. See the Splendid Sands web site.

Page,NJ, Singer,DA, Moring,BC, Carlson,CA, McDade,JM and Wilson,SA (1986) Platinum-group-element resources in podiform chromitites from California and Oregon. Econ.Geol. 81 no.5, 1261-1271.

Peterson,CD, Komar,PD and Scheidegger,KF (1986) Distribution, geometry, and origin of heavy mineral placer deposits on Oregon beaches. J.Sed.Petrol. 56, 67-77.

Stockman,HW and Hlava,PF (1984) Platinum-group minerals in Alpine chromitites from Southwestern Oregon. Econ.Geol. 79, 491-508.

Welland,M (2009) Sand, the Never-Ending Story. University of California Press, 344+16pp.

Working Group of Ophiolite Project (1979) International Atlas of Ophiolites. GSA map folder MC-33, 15pp. plus 4 maps.

Wynn,JC and Hasbrouck,WP (1980) Geophysical studies of chromite deposits in the Josephine ultramafic complex of northwest California and southwest Oregon. USGS OFR 80-936, 47pp.

Zierenberg,RA, Koski,RA, Shanks,WC and Seyfried,WE (1990) Subseafloor deposition of sulfide at the Turner-Albright ophiolite-hosted sulfide deposit, southwestern Oregon. GAC/MAC Prog.w.Abs. 15, 144, Vancouver.

Graham Wilson, 27-28 February 2025, minor edits 01-03 March 2025

For further information, see:

Rock of the Month Thematic Index

or, visit the Turnstone "Rock of the Month" Chronological Archives!