Pinpointing Provenance Using Radiogenic Isotopes: Provenance of Deepwater Sandstone and Conglomerate from the Gualala Basin, California

Doebbert, Amalia C.¹; Carroll, Alan R.¹; Johnson, Clark ^1
1 Department of Geology and Geophysics, University of Wisconsin, Madison, Madison, WI.

Single-grain radiogenic isotope studies provide a unique means of linking downstream sediments to their upstream sources. Whereas detrital U/Pb or ⁴⁰Ar/³⁹Ar ages only constrain sediment sources if basement ages are unique, the addition of radiogenic isotope data such as Pb isotopes, εNd, εHf, and ⁸⁷Sr/⁸⁶Sr to age data provides a distinct basement signature which can be recognized in derivative sediments. However, due to analytical limitations their use has often been limited to conglomerate-based studies, or studies with very few detrital single-grain analyses. Developing rapid methods for the collection of single-grain isotopic data will provide a means of improved resolution in provenance studies, and will ultimately lead to better predictions about the distribution of reservoir facies in petroleum systems.

Deepwater sandstone and interbedded conglomerate from the late Cretaceous - Eocene Gualala Basin of northern California have a shared provenance history that is connected to the regional tectonic evolution and erosion from the Mesozoic Sierran arc. Based on U/Pb ages and isotopic data, several discrete populations have been recognized in conglomerate clasts and linked with possible sediment sources. These same populations are also present in U/Pb age spectra from associated sandstones. However, sandstones include additional age populations that are not represented by conglomerate clasts. Furthermore, sandstone populations with ages that match known conglomerate clast could represent sediments from different sources.

Femtosecond LA-MC-ICP-MS Pb isotope analyses of detrital k-feldspars can provide a new means of comparison between sandstone and conglomerate populations. They will also allow new constraints on the origin of sandstone populations that cannot be tentatively linked to conglomerate clasts. The use of a femtosecond laser allows rapid analyses of single grains, with significantly higher precision than more commonly used nanosecond lasers, allowing the same kind of multiproxy approach in sandstones that has previously been applied in conglomerates.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009