SHELTON AREA QUARRIES
near Shelton, Washington
LOCALITY IMAGES


GEOLOGY
Crescent Formation - Late Paleocene to middle Eocene (62-48 Ma). The Crescent Formation is comprised of a large volume of submarine/subaerial basalt. The Crescent basalts are among the oldest basalts in this region and makeup the basement of the Coastal range. Babcock et al. (1992) concluded that the Crescent Formation basalts erupted into a subsiding rift basin along the western coast approximately 60 mya. Exposure of the Crescent formation can be found as a continuous swath defining the northern, eastern, and southeastern edge of the Olympic Peninsula. Based on gravity studies, the Crescent Formation is thought to extend eastward beneath the Puget Sound to Seattle. Most of the Crescent Formation, especially on its eastern edge, is covered with Quaternary glacial till deposits.

Exposure of the Crescent Formation near Shelton is comprised primarily of dark black pillow basalts. The basalt has been altered to a low grade (sub-greenschist facies) metabasalt. Centimeter to decimeter scale zeolite filled vugs are found within the interiors of individual pillows and along planar trends parallel to flow contacts. Contact quarry operator for current collecting status.

MINERALS
*Analcime-
NaAlSi2O6 . H2O - Small colorless trapezohedrons line some amygdules. Transparent crystals turn opaque when removed from the ground due to dehydration.
*Apophyllite-
(K,Na)Ca4Si8O20(F,OH) - 8H2O - Translucent crystals to 1". Small crystals appear cubic due to absence or pyramidal faces. Crystals turn opaque white due to dehydration.
*Calcite-
CaCO3 - Beautiful tea colored rhombohedral crystals, some twinned, are found in vugs associated with natrolite or analcime.
Chlorite-
(Mg,Fe)3(Si,Al)4(Mg,Fe)3O10(OH)8 - Massive chlorite is found interstital to individual pillows and as matrix grains within the meta-basalt.
Mesolite-
Na2Ca2Al6Si9O30 - 8H2O - Fine fiber-like sprays with individual needles to 1"
*Natrolite-
Na2Al2Si3O10 . 2H2O - Radiating crystal groups line large pockets up to several feet long. Some natrolite pockets also contain a late stage clay mineral that coats the needles. Some pockets are completely filled with laminated clay.
Pyrite-
FeS2 - mm-sized disseminated grains are found within altered basalt adjacent to amygdules.

* photographed

Radiating NATROLITE

Radiating NATROLITE

Interpenetration twinned CALCITE


White APOPHYLLITE
and NATROLITE


ANALCIME

NATROLITE, CALCITE

NATROLITE, ANALCIME



REFERENCES
Babcock R. S., Burmester R. F., Engebretson, D. C., Warnock A., Clark K. P. (1992) A rifted origin for the Crescent basalts and related rocks in the northern Coast Range volcanic province, Washington and British Columbia. Journal of Geophysical Research, B, Solid Earth and Planets 97;5, 6799-6821.