Neogene strata of the Tanana basin provide a long-term record of a northward-propagating, transpressional foreland-basin system related to regional shortening of the central Alaska Range and strike-slip displacement on the Denali fault system. These strata are ∼2 km thick and have been deformed and exhumed in thrust faults that form the foothills on the north side of the Alaska Range. The lower part of the sedimentary package, the Usibelli Group, consists of 800 m of mainly Miocene strata that were deposited in fluvial, lacustrine, and peat bog environments of the foredeep depozone of the foreland-basin system. Compositional data from conglomerate and sandstone, as well as recycled Upper Cretaceous palynomorphs, indicate that the Miocene foreland-basin system was supplied increasing amounts of sediment from lithologies currently exposed in thrust sheets located south of the basin. The upper part of the sedimentary package, the Nenana Gravel, consists of 1200 m of mainly Pliocene strata that were deposited in alluvial-fan and braidplain environments in the wedge-top depozone of the foreland-basin system. Compositional data from conglomerate and sandstone, as well as 40 Ar/ 39 Ar dating of detrital feldspars in sandstone and from granitic clasts in conglomerate, indicate that lithologies exposed in the central Alaska Range provided most of the detritus to the Pliocene foreland-basin system. 40 Ar/ 39 Ar dates from detrital feldspar grains also show that two main suites of plutons contributed sediment to the Nenana Gravel. Detrital feldspars with an average age of 56 Ma are interpreted to have been derived from the McKinley sequence of plutons located south of the Denali fault. Detrital feldspars with an average age of 34 Ma are interpreted to have been derived from plutons located north of the Denali fault. Plutons located south of the Denali fault provided detritus for the lower part of the Nenana Gravel, whereas plutons located north of the Denali fault began to contribute sediment during deposition of the upper part of the Nenana Gravel. This age distribution documented in detrital feldspars of the Nenana Gravel is interpreted as representing a progressive northward exhumation of plutons that were located south of the Pliocene Tanana basin. In contrast to previous studies, we interpret the Usibelli Group and Nenana Gravel to represent a continuum in the evolution of a transpressional foreland basin that began during Miocene time on the north side of the Alaska Range.

Field instruction is a critical piece of undergraduate geoscience majors’ education, and despite its central importance, relatively little educational research has explored how students learn to solve problems during geological fieldwork. This study adds to work presented in previous studies by our group using global positioning system (GPS) tracking of students engaged in independent field examinations. We examined four students from our previous studies working in a new field area. We also applied a new variant of our polygon coding approaches for analyzing student navigation tracks to gauge the sensitivity of our method to the time scale of analysis. We captured position data at 1 min intervals and then coded the resulting data by generating 5 min and 15 min sequential polygons. Our analysis shows that the two methods are comparable at the coarsest scale, but that finer-scale coding reveals more detailed movements related primarily to identification of key features and lithologies, which lends insight into effective geologic problem solving in the field. Coherence of small-scale and large-scale coding is most useful for showing longer-range planning in problem solving as the large-scale movements average out small-scale investigatory moves. Our results also suggest that in detailed and difficult field areas with topography that permits easy reoccupation of critical areas, there is an optimum amount of relocation and back-tracking. Too much retracing indicates confusion, as found in our earlier study. However, too little reoccupation of key areas appears to accompany a failure to recognize important features. Our study offers additional refinement of instructional tools in gauging student skills in geologic field problem solving offered by GPS tracking.