Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)
Paper No. 1-10
Presentation Time: 11:40 AM-12:00 PM

COMPOSITIONAL VARIATIONS AND ORIGINS OF THE ORBS IN MORGAN HILL POPPY JASPER, SOUTHERN SANTA CLARA VALLEY, CA

HONESTO, Jenise Marie and FOX, L.K., Department of Geosciences, Univ of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211, j_honesto@pacific.edu

The purpose of this project is to determine the compositional variations and origin of the colorful spheres found in Poppy Jasper, a variety of chalcedony, found in the foothills of the Santa Cruz Mountains near Morgan Hill in the Southern Santa Clara Valley, CA. This is the first modern comprehensive study of the rock, which has not been studied since Dr. Albert Walcott’s 1939 article entitled “Origins of Orbicular Jasper.” This study utilizes many modern chemical and petrographic analyses of the rock.

Poppy Jasper was discovered in ridges trending East-West in the foothills of the Santa Cruz Mountains, a stark contrast to the North-South trend of the mountain range. The lodes of poppy jasper are part of the Franciscan Complex, and are found near belts of serpentine. There are four known locations of poppy jasper lodes near the town of Morgan Hill, two of which were used for this project. The first was a ranch owned by the Blair family, the second was Silviera Park.

The samples show distinct stages of formation. The samples collected from the ranch show the first phases of formation, while the samples from Silviera Park show the later stage. The first stage is characterized by the fracturing of red chert and the injection of quartz veins into the fractures. The second phase of poppy jasper formation is a period when the angular clasts begin to display a crescent-shaped appearance. The samples collected at Silviera Park show distinct orbs with color bands of red, yellow, and orange. Most of the orbs show perfectly concentric color banding, a spherical shape, and rest in a crystalline matrix.

There is believed to be another stage of formation between the crescent-shaped chert and the well-developed orb phases. However, no samples of this stage were found at either location, and therefore, were not studied.

The samples were polished and prepared for analysis by petrographic microscopes, and X-ray fluorescence spectroscopy (XRF). The petrographic method is used to determine the presence of microstructures or textures. The XRF is used to create an element map across the orbs and the matrix. Both of these analyses are being undertaken at present, and results are forthcoming in the spring of 2004.

Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)
Session No. 1
Volcanology, Igneous and Metamorphic Petrology
Boise Centre on the Grove: Salmon River
8:20 AM-12:00 PM, Monday, May 3, 2004

Geological Society of America Abstracts with Programs, Vol. 36, No. 4, p. 9

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