Link to USGS home page.
Studies by the U.S. Geological Survey in Alaska, 2004
U.S. Geological Survey
Professional Paper 1709-F

Constraints on the Age and Provenance of the Chugach Accretionary Complex from Detrital Zircons in the Sitka Graywacke near Sitka, Alaska

By Peter J. Haeussler, George E. Gehrels, and Susan M. Karl


thumbnail of map
Gulf of Alaska area, showing location of study area and tectonostratigraphic terranes of Alaska, Yukon Territory, and British Columbia (from figure 1)

The Sitka Graywacke is the westernmost and youngest unit of the Chugach accretionary complex in southeastern Alaska. Using laser-ablation inductively coupled plasma mass spectroscopy, we obtained 492 detrital-zircon ages on seven typical samples of Sitka Graywacke turbidites, which were collected in a transect across much of the unit near Sitka, Alaska. Individual grains range in age from 66 to 1,802 m.y. The youngest peak ages on relative-probability plots of the western four samples (74, 72, 74, and 74 m.y., from west to east) are distinctly younger than the youngest peak ages of the eastern three samples (105, 103, and 97 m.y., from west to east). These youngest peak ages set maximum depositional ages for each sample. We suggest that these peak ages are not significantly older (within ~5 m.y.) than the depositional age of the Sitka Graywacke because the deposits accumulated in a trench along a convergent margin, where magmatic sources likely continuously introduced juvenile zircons. The differences in the youngest cluster of detrital-zircon ages between the eastern and western sample localities is likely due to both a change in provenance and a fault. The similarity of the youngest peak ages in the Sitka Graywacke to fossil ages in the Valdez Group, in Prince William Sound, implies that the western part of the Sitka Graywacke is correlative with the Valdez Group, as previously inferred. However, the eastern part of the Sitka Graywacke has youngest detrital-zircon ages older than fossil ages in the Valdez Group and younger than fossil ages in the McHugh Complex, which in south-central Alaska is the oldest part of the accretionary complex. The age distribution of zircons in the older, eastern sequence suggests sources along the British Columbia margin. The detrital-zircon ages in the younger, western sequence are similar to igneous ages from south-central Alaska to southern British Columbia. Right-lateral strike slip on various fault systems inboard of the Sitka Graywacke implies that it lay to the south when it was deposited and offscraped. Thus, although source areas as far north as the St. Elias Mountains and south-central Alaska are possible, they were most likely in coastal and interior British Columbia.

Download Professional Paper 1709-F as a 24-page PDF file (1.5 MB)

For questions about the content of this report, contact Peter Haeussler

Go to other chapters in this series: Haeussler, Peter J., and Galloway, John P., eds., 2005, Studies by the U.S. Geological Survey in Alaska, 2004: U.S. Geological Survey Professional Paper 1709.

Download a current version of Adobe Reader for free

| Help | PDF help | Western reports main page | Western Professional Papers |
| Privacy Statement | Disclaimer | Accessibility |
| Geologic Division | Western Mineral Resources Program |

This report is available only on the web.

| Department of the Interior | U.S. Geological Survey |
URL of this page:
Maintained by: Michael Diggles
Created: May 2, 2006
Last modified: May 2, 2006 (mfd)