Character of a major thrust zone in the central Brooks Range
Reia Chmielowski, MS student
Illite Age Analysis of Shaly Rocks from the Brooks Range, Alaska
Walt Munley, Ph.D. student.
Reia Chmielowski spent the 1995 field season studying a major thrust zone within the Endicott Mountains allochthon. The allocthon is exposed over much of the northern Brooks Range and in its lower part includes, from bottom to top, the Upper Devonian Hunt Fork Shale, the Upper Devonian to Lower Mississippian Kanayut Conglomerate, and the Mississippian Kayak Shale. The differences in structural competency of these units have resulted in a combination of large-scale detachment folds and thrust faults within the competent Kanayut during regional shortening in the mountain range. The thrust faults flatten in detachments within the incompetent shale units above and below the Kanayut, thus defining a duplex thrust system. The Hunt Fork Shale serves as the floor thrust and the Kayak Shale serves as the roof thrust.
Reia's field area is west of the trans-Alaska pipeline and is in a major thrust zone within the Endicott Mountains allochthon, theToyuk thrust zone. The Toyuk thrust zone marks the regional boundary between exposures dominated by Hunt Fork Shale to the south and structurally underlying Kanayut Conglomerate to the north. The thrust zone separates a thinner, finer-grained facies to the south from a thicker, coarser-grained facies to the north. These facies changes may be related to increased accommodation to the south that resulted from normal faulting before or during deposition. Reia's mapping shows that the thrust zone is defined by several detachment folds in Hunt Fork and Kanayut that step down to the north and are cut by relatively closely spaced thrust faults to form a duplex.
Previous workers have assumed that the Toyuk thrust zone is a single regionally extensive, large-displacement thrust fault. Reia's preliminary results suggest that the Toyuk thrust zone instead marks a northward decrease in structural relief within a duplex of thrust-truncated detachment folds. This change in relief and the associated close spacing of thrust faults may reflect a northward decrease in the thickness of section incorporated in the duplex, perhaps resulting from facies changes.
The next phase of Reia's M.S. study will be the construction of balanced kinematic models for the evolution of the thrust zone. These may provide clues to the controls on structural geometry and to the relative timing of structures. Reia also collected samples for 40Ar/39Ar and fission-track dating, which may help constrain the absolute timing of structures.
The generation of diagenetic illite correlates with hydrocarbon maturation, so the radiometric dating of illite provides a potential tool for dating hydrocarbon maturation, and for determining migration and trapping histories. Workers at Exxon have developed a method for K-Ar dating of diagenetic illite formed during burial or faulting. We plan to evaluate this technique using the more sensitive Ar-Ar method. Ages of diagenetic illite separated from shaly rocks will be determined, in part, as a function of the proportion of diagenetic-to-detrital illite within the samples. We will also evaluate the paleothermometry (or diagenetic grade) of the shales by measuring illite/smectite ratios using X-ray diffraction. Ideally, this procedure will provide insight into thermal and burial histories. We will evaluate our results by comparison with conodont alteration indices, vitrinite reflectance, and thermal alteration indices.
Samples to be analyzed were collected from outcrops of various formations and faults in the Brooks Range and its foothills during June, 1995. Formations sampled include the Hunt Fork, Kanayut, Kayak, Siksikpuk, Otuk, Okpikruak, Torok, and Fortress Mountain. Several clay experts from around the country have been contacted in order to define the optimal procedure for preparing these samples for X-ray diffraction. Upon integrating their suggestions into the procedure, we will proceed directly to X-ray diffraction.
In concert with X-ray diffraction, we will prepare the laser-based spectrometer for 40Ar-39Ar radiometric age determination of the illites extracted from these shaly rocks. Because illite crystal lattices are relatively small, argon generated within them during neutron irradiation may be ejected due to recoil. Such ejected argon will be trapped in a glass tube with its parent illite sample during irradiation. During each age measurement, the glass tube will be crushed, allowing the ejected argon to enter the spectrometer for measurement. We will then heat the parent illite sample by means of the focused laser to release all argon not ejected by recoil.
Measurement of released 39Ar and 40Ar will yield illite ages. These ages will then be plotted against percent diagenetic illite to yield ages of pure diagenetic illite and corresponding hydrocarbon maturation ages. These results, combined with X-ray diffraction data, should yield thermal history information as well as geochronological histories.