Galena Creek rock glacier (GCRG), northwest Wyoming, exhibits most of the classic characteristics of rock glaciers. Clean ice with silty bands was found beneath a c. 1 m thick debris mantle by Potter. He inferred that the ice is glacigenic, originating in the small snowfield in the cirque at the head of GCRG. This view was challenged by Barsch, who asserted that the ice in GCRG is of "permafrost" origin. Since then GCRG has become a lightning rod for opponents and proponents of the glacigenic ice model for rock glaciers. We review evidence for that model here.Movement marks emplaced on GCRG in the 1960s were resurveyed in 1995 for a 30+ year record of movement. Maximum surface velocity is 45 cm/yr on gentle slopes and 80 cm/yr in a steep reach where GCRG spills out of the cirque. The less active, downvalley third of GCRG is moving at a maximum 14 cm/yr, and lobes formed between the more and less active parts have complex movement and are advancing down-valley over adjacent lobes at a maximum of 6.5 cm/yr. New refraction seismic profiles on GCRG were used to determine the thickness of the debris mantle over ice. On the up-valley, active part of GCRG, the debris mantle is a relatively uniform c. 1 m thick. On the down-valley, less active part, the thickness of the debris mantle is much more variable, but it is generally thicker. We cannot tell, on the basis of seismic data alone, whether the frozen material beneath the debris mantle is ice or a debris-ice mixture, but the results are not inconsistent with the glacigenic model for the origin of the ice. Two long-profiles in the cirque may identify bedrock at about 20-25 m depth.

We develop an analytical and numerical methodology for the analysis of large bottom‐hole temperature (BHT) data sets from sedimentary basins, and test the methodology using temperature, stratigraphic, and lithologic data from 411 boreholes in the Michigan Basin. Least‐squares estimates of temperature gradients in the formations and lithologies present are calculated as solutions to a large system of linear equations. At each borehole the temperature difference between the bottom and top of the hole is represented as a sum of temperature increments through the various formations or lithologies penetrated by the borehole. Quadratic programming techniques enable bounds to be placed on the gradient solutions in order to suppress or exclude improbable gradient estimates. Numerical experiments with synthetic data reveal that the estimates of temperature gradients for a given formation or lithology are sensitive to the degree of representation of that unit; well represented units have more stable gradient estimates in the presence of noise than do poorly represented units. The estimates of temperature gradients obtained for lithologies are more stable than those for formations and are believed to be good estimates of actual lithologic temperature gradients in the Michigan Basin. Formation temperature gradients obtained as a weighted sum of the estimates of the component lithologic temperature gradients appear to be good estimates of the average temperature gradients for the formations of the basin. At each borehole a temperature residual exists corresponding to the difference between the observed BHT and the BHT predicted by the estimated interval temperature gradients. Residuals are far more stable than estimated temperature gradients. The values of residuals change little regardless of whether lithology, formation, bounded, or unbounded gradient estimates are used to calculate them. Maps of residuals indicate well‐defined and spatially coherent patterns of positive and negative temperature residuals. Filtered subsets of large‐magnitude residuals alone show a pattern of negative residuals coinciding with the mid‐Michigan gravity high, a geophysical feature thought to delineate a Precambrian (Keweenawan) rift zone in the crust beneath the basin. Thermal models of the Michigan Basin and the crust and upper mantle beneath the basin indicate that the suspected rift beneath the basin can cause a variation in basement heat flow sufficient to produce temperature residuals of the magnitude observed in the sediments, with negative temperature residuals over the area of the rift.

No abstract

The University of Wyoming conducted seismic reflection profiling in the Laramie Mountains of Wyoming to investigate the nature of Precambrian crustal structure and Laramide deformation in the region. Here the imaging of layering in the Laramie Anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events. Moreover, the presence of prominent primary layering in the Complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush. Imaging of Laramide thrusts in the Laramie Mountains adds credence to the idea that brittle faults can be reflective in some situations. Migrated apparent dips on these faults are 30 ø westward. Basement-involved compressive structures, consisting of monoclinal folding and a fault triple junction, demonstrate that compression controlled the style of deformation during the Laramide orogen in Wyoming. West dipping Precambrian structures imaged in these data have a similar attitude to the Laramide structures, suggesting that Laramide deformation was influenced by Precambrian features. These Precambrian structures may be subthrust slices of exotic Proterozoic terranes. GEOLOGIC SETTING With the exception of the Black Hills of South Dakota, the Laramie Mountains, cored by Precambrian rocks, represents the easternmost extent of the exposed Laramide crustal uplifts. Within the Medicine Bow Mountains, 70 km to the west of the Laramie Mountains, the Cheyenne Belt separates the Archean Wyoming Province to the north from the Proterozoic Colorado Province to the south [Houston et al., 1979]. In the Laramie Mountains this structural belt is concealed by the intrusion of the Laramie Anorthosite Complex and the Sherman Granite. The seismic line traverses portions of the Laramie Anorthosite Complex and the Sherman Granite, both of mid-Proterozoic age. The Laramie Anorthosite Complex is divided into two bodies separated by a 1 to 2-km zone of granite gneiss [Newhouse and Hagner, 1957]. The northern anorthosite body shows abundant layering of anorthositic and gabbroic rocks [Newhouse and Hagner, 1957; Scoates, 1988; Scoates and Lindsley, 1989], and locally granitic layers or sills are present. The southern anorthosite body is not as well studied as the northern body; nevertheless, large-scale layering is evident on air photos, and this layering has a domal structure [Newhouse and Hagner, 1957]. A domal or antiformal structure for the complex also is suggested, based 417-425, 1959. Wong, Y. K., S. B. Smithson, and R. L. Zawislak, The role of seismic modelling in deep crustal reflection interpretation, 1, Contrib. Geol., 20, 91-109, 1982. Wu, F. T., Mineralogy and physical nature of clay gouge, Pure App. Geophys., 116, 655-689, 1978.

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