| Abstract: | More than 50 ash-fall tuff beds which have altered to analcitized or feldspathized rocks have been found in the upper 500-600 feet of the Parachute Creek Member of the Green River Formation in the Piceance Creek Basin of northwestern Colorado. Similarly altered water-washed tuff occurs as tongues in the uppermost part of this member, and forms most of the lower 400-600 feet of the overlying Evacuation Creek Member of the Green River Formation.
‘The altered ash-fall beds of the Parachute Creek Member are all thin and show a characteristic pattern of alteration. Most beds range in thickness from a fraction of an inch to a few inches. One bed reaches a maximum thickness of 5 feet, and, unlike the other beds, is composed of several successive ash falls. The pattern of alteration changes from the outer part to the center of the basin. Most beds in the outer part of the basin contain about 50 to 65 percent analcite,with the interstices between the crystals filled mainly by microlites of feldspar, opal, and quartz, and small amounts of carbonate. At the center of the basin .essentially all the beds -are composed of microlites of feldspar, opal, and quartz, and small amounts of carbonate. The tongues of water-washed tuff in the uppermost part of the Parachute Creek Member and the similar rocks composing the lower 400-600 feet of the Evacuation Creek Mewber are feldspathized rocks composed mainly of microlites of feldspar, opal, and quartz, varying amounts of carbonate, and in some specimens tiny subrounded crystals of analcite.
The general trend in alteration of the tuffaceous rocks from analcitization near the margin to feidspathization near the center of the Piceance Creek Basin is believed to have taken place at shallow depth during diagenesis , as indicated by field observations and laboratory work. It is believed that during sedimentation and diagenesis the waters of the central part of the basin were more alkaline and following the breakdown of the original tuffaceous glass to a colloidal gel during diagenesis analcitized rocks crystallized near the basin margin and feldspathized rocks crystallized near the center of the basin. |
| Genre: | USGS Numbered Series |
| ProdID: | 13907 |
| Citation Author: | Griggs, Roy Lee |
| Citation Contributing Office: | |
| Citation Datum: | |
| Citation Day: | |
| Citation Edition: | - |
| Citation Editor: | |
| Citation End Page: | |
| Citation Issue: | |
| Citation Keywords: | |
| Citation Language: | ENGLISH |
| Citation Larger Work Title: | |
| Citation LatN: | |
| Citation LatS: | |
| Citation LonE: | |
| Citation LonW: | |
| Citation Month: | |
| Citation No Pagination: | |
| Citation Number Of Pages: | |
| Citation Online Only Flag: | |
| Citation Phsyical Description: | 38 p. ill., maps ;27 cm. |
| Citation Projection: | |
| Citation Public Comments: | |
| Citation Publisher: | U.S. Geological Survey], |
| Citation Series: | Open-File Report |
| Citation Series Code: | OFR |
| Citation Series Number: | 68-113 |
| Citation Search Results Text: | Altered tuffaceous rocks of the Green River Formation in the Piceance Creek Basin, Colorado; 1968; OFR; 68-113; Griggs, Roy Lee |
| Citation Start Page: | |
| Citation Volume: | |
| Citation Year: | 1968 |
| Type: | citation/reference |
| Text: | Altered tuffaceous rocks of the Green River Formation in the Piceance Creek Basin, Colorado; 1968; OFR; 68-113; Griggs, Roy Lee |
| URL (THUMBNAIL): | http://pubs.usgs.gov/of/1968/0113/report-thumb.jpg |
| URL (DOCUMENT): | http://pubs.usgs.gov/of/1968/0113/report.pdf |
| URL (PLATE): | http://pubs.usgs.gov/of/1968/0113/plate-1.pdf |
| URL (PLATE): | http://pubs.usgs.gov/of/1968/0113/plate-2.pdf |
| URL (PLATE): | http://pubs.usgs.gov/of/1968/0113/plate-3.pdf |
| Date Other: | Sat, 1 Jan 1994 00:00 -0600 |
| Publisher: | U.S. Geological Survey], |
