Duration of sedimentation of Creede Formation from 40Ar/39Ar ages

GSA Special Papers



The Oligocene Creede Formation was deposited in the moat of the Creede caldera, which formed as a result of eruption of ythe Snowshoe Mountains Tuff. The Creede Formation in the two moat drill holes contains ash layers that are considered fallout tuffs derived from Fisher Dacite volcanoes that were erupting during accumulation of the Creede Formation. The duration of sedimentation of the Creede Formation could hnot be determinted directly by measuring the ages of the ash layers because 40Ar/39Ar ages of biotite from the asj layers do not stack in the correct stratigraphic order, indicating that the ash layers have been contaminated by biotite from older units. The duration of sedimentation is constrained by the ages of volcanic unites that stratigraphically bracket the Creede Formation. Pooling all ages for the underlyinh Snowshoe Mountain Tuff yields an age of 26.92 ± 0.07 Ma for the unit. The age of the stratigraphically highest lavas of Fisher Dacite, which overlie the Creede Formation, is 26.26 ± 0.04 Ma. The two limits give a maximum duration for sedimentation of the Creede Formation of 0.66 m.y. Using the ages of older Fisher Dacite lavas, on which some beds of the Creede Formation were deposited, a more realistic maximum duration of 0.34 m.y. for sedimentation of the Creede Formation can be determined.

Study Area

Publication type Article
Publication Subtype Journal Article
Title Duration of sedimentation of Creede Formation from 40Ar/39Ar ages
Series title GSA Special Papers
DOI 10.1130/0-8137-2346-9.71
Volume 346
Year Published 2000
Language English
Publisher Geological Society of America
Contributing office(s) Earthquake Science Center
Description 6 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title GSA Special Papers
First page 71
Last page 76
Country United States
State Colorado
Other Geospatial Oligocene Creede Formation;San Juan Mountains
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