Sources of sediment in the Alberta portion of the Cordilleran foreland basin
Figure 1: Geological bedrock map of Alberta.
In recent years, the analytical techniques used to determine the source area of a sedimentary rock have changed significantly. While petrographic analysis and point counting of framework components still remains the stalwart first-order approach, increasingly sophisticated analytical techniques have been developed that allow for additional means of fingerprinting source areas from a geochemical and isotopic perspective. In this project we systematically sampled selected clastic units in the deformed portion of the foreland basin stratigraphy exposed in the Alberta Rocky Mountains and Foothills. Samples are being analyzed for the samarium-neodymium (Sm-Nd) isotope system and U-Pb dating of detrital zircons in selected siliciclastic strata of Devonian, Permian, Triassic, Jurassic, and Cretaceous ages. Application of both Sm-Nd analyses and U-Pb geochronology to the same sample of sedimentary rock constitutes a mutually complementary approach to determining provenance.
Samarium and neodymium are rare earth elements (REE) that are a parent-daughter pair linked by radiogenic decay. The Sm-Nd system is ideal for the study of sediment provenance because of its geochemical coherence and resistance to fractionation and disturbance during sedimentation and lack of geochemical redistribution during diagenesis and lithification.
Sm-Nd isotopic data provide an average of the isotopic composition of the source areas that fed a depositional system, whereas U-Pb dating of single grains of detrital zircon provide a more direct point-source fingerprint of the actual crystallization ages in the source area.
Over the last two decades, provenance studies have been initiated at different universities, but the work has been either selective or at reconnaissance scale, hence with a preliminary, sometimes highly speculative, character. For example, the widespread, over 1200 m thick Triassic stratigraphy has been discussed based on six samples. To date, a comprehensive regional-scale database for provenance isotope and mineral indicators (e.g., Sm-Nd, detrital zircon) in the province does not exist. Over the last 3 years, the AGS has undertaken a program of compiling existing provenance data, complemented by systematic sampling and analytical work of siliciclastic units—most of which are potential hydrocarbon reservoirs (e.g., Montney, Gordondale, Cardium)—in an attempt to better define their depositional environment and source areas.
The results of our studies establish constraints on possible (and impossible) sources for the sedimentary strata of interest and form a growing subset of data that characterize the Alberta Basin. Such data provide not only a measure of the evolution of the Cordilleran foreland basin, but also can be used as a template to evaluate the affinity of accreted/suspect terranes outboard of the North American margin.
Methodology and Preliminary Results
U-Pb analyses of detrital minerals such as zircon and monazite are gaining widespread application. Early studies of detrital minerals by the U-Pb method used large fractions of multiple grains in order to have enough Pb in the analysis. The advent of laser ablation–multiple collector–inductively coupled plasma mass spectrometry (LA-MC-ICPMS) has allowed for a larger number of single grains and parts of grains to be analyzed from each sample. The analysis of individual grains removes the effects of mixing of a potentially large number of different grains and results in a more accurate determination of the actual ages within a zircon population. The drawback is that the analysis of large numbers of grains by this method is time consuming and costly and that a bias is imparted by not analyzing poor-quality grains with obvious imperfections, which leads to discordant data of little value.
We have consistently collected a full bucket (~20–25 kg) of sandstone samples from selected stratigraphic units. Individual samples were processed with standard crushing and pulverizing procedures followed by density and magnetic separation. The U-Pb geochronology of detrital zircons was conducted by LA-MC-ICPMS at the University of Alberta, where 100 or so single grains from each sample were ablated with a laser spot of 10–50 μm diameter.
Detrital zircon U-Pb ages for each stratigraphic unit contain distinct signatures which can only be discussed and interpreted in the context of the adjacent older strata (see Figure 1); due to their extreme chemical and mechanical resistance, zircon grains can be recycled from old stratigraphic units into younger ones (incorporation of multicyclic zircon grains) so that the geographic significance of the crystallization age with respect to a discrete source area during a single sedimentary cycle is blurred.
Figure 2: Example of processing uranium-lead age dating of detrital zircon from Alberta Basin stratigraphy.