SURFACE THERMAL MATURITY OF THE YUKON FLATS REGION, EAST-CENTRAL ALASKA

Surface thermal maturity (TM) was mapped at a scale of 1:500,000 in the Yukon Flats region of central Alaska (141 to 152.75 degrees W, and 65 to 68 degrees N). This mapping was done in support of a U.S. Geological Survey assessment of the undiscovered petroleum resources of the region.

Vitrinite reflectance (Ro) and conodont alteration index (CAI) values from approximately 1700 outcrop and core hole samples (including unpublished data) were used along with mapped geology to define mappable surface TM units for the entire Yukon Flats region. These TM units included: undermature (Ro < 0.6, CAI < 1.0), mature I (Ro 0.6 – 1.3, CAI 1.0 – 2.0), mature II (Ro 1.3 – 2.0, CAI 2.0 – 3.0), overmature (Ro 2.0 – 3.6, CAI 3.0 – 4.0), supermature (Ro 3.6 – 5.0, CAI 4.0 – 4.5), and igneous/metamorphic (Ro > 5.0, CAI > 4.5). In the few cases where Ro and CAI data conflicted, CAI data were given preference for Paleozoic samples and Ro data for Mesozoic and younger samples.

The surface geology of the Yukon Flats region is dominated by a large, central basin of unconsolidated to poorly consolidated, thermally undermature Tertiary and Quaternary nonmarine deposits, surrounded by bedrock highlands of primarily Mesozoic and older rocks that have reached TM levels of supermature to igneous/metamorphic. Exceptions occur along the Tintina fault zone, where several small areas of well-consolidated Tertiary nonmarine sedimentary strata exhibit moderate thermal maturities (mainly mature I). These units are interpreted as strata that were buried to moderate depths and then exhumed by uplift and erosion, perhaps associated with mid-Tertiary or younger tectonism along the Tintina fault.

In the Kandik region, located southeast of Yukon Flats, our TM mapping confirms previous work that showed a northwest-to-southeast gradational decrease in the TM of Mesozoic and older strata, from mostly igneous/metamorphic and supermature on the northwest to significantly less mature (mainly mature I) on the southeast. The northwest to southeast decrease in TM is related to a stack of thrust sheets which were transported to the southeast prior to about 80 Ma. The amount of tectonic burial experienced by each sheet decreases from northwest to southeast, resulting in the observed TM gradient.