File:Graywacke turbidites in Bay of Pillars Formation.jpg

Summary

Stbg Bay of Pillars Formation graywacke turbidites (Silurian): Graywacke and mudstone turbidites, and subordinate limestone, conglomerate, and intermediate to mafic volcanic flow rocks. Turbidites are medium- to thick-bedded, and include graded beds and full Bouma sequences, representing proximal inner fan and channel depositional facies. Graywacke consists of poorly sorted angular clasts of carbonate rock, fossil debris, chert, felsite, volcanic rock fragments, volcanic shards, sedimentary rock, feldspar, quartz and rare detrital biotite. No white mica or metamorphic rock fragments have been observed (Brew and others, 1984). Graywacke clast composition varies locally, and may be dominantly calcareous, volcaniclastic, or quartzofeldspathic (Brew and others, 1984). Graywacke may have calcareous, chloritic, or clayey matrix. Limestone is thin- to medium- bedded, light gray, alternating with black, carbonaceous argillite. Representing slope or interchannel depositional facies. Conglomerate contains limestone similar to the Heceta limestone, volcanic rocks, and well-rounded pink syenite porphyry cobbles. Thickness uncertain, probably thousands of meters. Prince of Wales Island.

Copyright status:

This image is public domain because it was first published by the U.S. Geological Survey

Source:

http://pubs.usgs.gov/of/1999/of99-168/of99-168-pamphlet.html