AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
High Resolution Magnetostratigraphy as a Tool for Investigating Basin Evolution: Example from Upper Cretaceous (Santonian-Campanian) Marine Strata of Western Canada
(1) Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
(2) Department of Physics, University of Alberta, Edmonton, AB, Canada.
The third-order Santonian-Campanian Claggett Cycle generated marine sedimentation across the majority of western Canada. Resultant deposits constitute an important hydrocarbon bearing succession in the Western Canada Sedimentary Basin (WCSB) which includes the prolific, fine-grained Alderson Member. These widespread strata record a complex history of relative sea-level change as evidenced by regionally traceable unconformities associated with the Alderson interval. Understanding the timing of these unconformities is the key to understanding the evolution of the WCSB and the controls on accommodation during Alderson time, thus, it is also essential to producing a refined depositional model. To develop a chronostratigraphic framework, a magnetostratigraphic methodology was implemented using long, continuous cores from 9 control points across the WCSB. Rigorous paleomagnetic analysis of more than 2,000 samples yielded a high resolution dataset which captures dozens of polarity reversals, some with durations as short as ~50,000 yrs. Eight new 40Ar/39Ar ages of bentonite horizons coupled with a previously established biostratigraphic framework provide a means of calibrating the paleomagnetic data. The resulting chronostratigraphic framework establishes clear strike- and dip-oriented timelines across the entire WCSB for the 34n-33r and 33r-33n polarity chron boundaries (~83.5 Ma and ~79.5 Ma, respectively), thereby creating new genetic links between adjacent stratigraphic units basin-wide. These age boundaries serve as excellent time-stratigraphic datums for correlation, and in conjunction with more than 20 new empirical ages for the interval, provide much insight into basin dynamics by constraining the timing of event-significant surfaces. The data reveal that 3-4 m.y. of diachroneity are associated with the top of the Alderson Member, also known as the "Milk River Shoulder" unconformity-- an important regional marker horizon long associated with transgressive ravinement. Age correlation demonstrates the boundary to be youngest in the axial region of the basin. These observations give cause to invoke tectonism as the main driver of subsidence/uplift, as common eustatic cycles operate at much higher frequencies. When the "Milk River Shoulder" unconformity is viewed within the context of the magnetostratigraphic framework, a signature imparted by competing flexural tectonics and dynamic subsidence becomes apparent.