← Search
Sample Y-58N264
NavDat sample number 2912 · PYROCLASTIC-FLOW
Sample
| Sample ID | Y-58N264 |
|---|
| Rock name | PYROCLASTIC-FLOW |
|---|
| Rock type | VOLCANIC |
|---|
| Composition | FELSIC |
|---|
| Material | IGNEOUS |
|---|
| Collected by | Charles J. Vitaliano |
|---|
| Comment | Very fine-grained dark reddish-brown, densely welded ash flow tuff forming bold cliff-like outcrops even when not protected by a capping of more resistant rock. It consists largely of glass shards with sparse sanidine crystals and lithic fragments; occasional flattened vesicles are seen. The slightly devitrified glassy matrix contains grains of magnetite and hematite and needles of rutile?. The lithic fragments are largely collapsed pumice in the form of fiamme. The presence of tiny tridymite crystals throughout the rock and lining the vesicles indicates that the ash flow cooled in the vapor phase. According to Ransome et al 1909, p. 70-71 the Spearhead at Malpais Mesa consists of two distinct flows, with no evidence of any interval of erosion or deposition between them. Noble et al 1964, p. D26 found Ransome's Spearhead Rhyolite to be a simple cooling unit, part of a large composite sheet forming the lower part of the Thirsty Canyon Tuff; traced southward from Malpais Mesa, this cooling unit incorporates additional ash flows to become a thick compound cooling unit and is underlain by another compound cooling unit of similar composition and inasmuch as the break between the two compound cooling units becomes less distinct and disappears, they reassigned both to the Spearhead Member of the Thirsty Canyon Tuff. Kistler 1968, p. 254 has obtained a potassium-argon date of 7.5 m.y. for a sample of the basal member of the Thirsty Canyon Tuff from the Black Mountain region, about 40 miles southeast of Goldfield. For the uppermost member of the Thirsty Canyon Tuff Kistler obtained a date of 6.2 m.y. This neatly brackets our result of 6.7 m.y. for the stratigraphically intermediate Spearhead member. Discrepant dates of 7.8 and 9.5 m.y. were reported for glass from the Gold Flat member that overlies the Spearhead by Marvin and others 1970. |
|---|
Location
| Latitude | 37.6528 |
|---|
| Longitude | -117.111 |
|---|
| Location | Nye County, NV |
|---|
| State / province | Nevada |
|---|
| Volcanic field | Goldfield-Tonopah region, West-Central Nevada |
|---|
| Location comment | Spearhead Rhyolite; low cliff on SE side of a wash about 7 mi ESE of Rabbit Springs |
|---|
Age
| Age (Ma) | Error | Min | Max | Geologic age | Dated by |
|---|
| 7.3 |
|
6.8 |
7.8 |
|
|
Analyses
No chemical analyses are stored for this sample.
