Heat transfer in the subsurface can vary with depth due to variations in the thermal conductivity of the geologic medium as well as variations in groundwater flow velocity. However, traditional thermal response tests (TRTs) do not allow for evaluation of the depth variability of heat transfer. We investigate the potential for using discrete-depth heat dissipation tests in open, water-filled boreholes to evaluate variations in heat exchange rate with depth. Heat dissipation tests were initiated at target depths in a test well using an electrical resistance heater. Heat dissipation was monitored by measuring borehole water temperature through time using a fiber-optic distributed temperature sensing system. Temperature data were used to compare the thermal response at different depths in the borehole. To account for both the thermal conductivity of the geologic medium and the groundwater flow velocity, we used a numerical groundwater flow model (MODFLOW) and solute transport model (MT3DMS) to simulate heat dissipation tests. Simulation results indicate the measured response to a heat dissipation test in an open borehole is strongly dependent on the measurement location within the borehole; thus, data are ambiguous when the measurement location is uncontrolled. However, modeling results also indicate that the thermal response of a heat dissipation test as measured at the center of the borehole is sensitive to variations in thermal conductivity and groundwater flow velocity, suggesting that heat dissipation tests are a potentially useful method for characterizing depth variability in thermal properties if a centralized temperature measurement method is used to monitor the tests.

ABSTRACT The deglaciation history of northeastern Minnesota and northwestern Ontario is outlined using geological and ichthyofaunal evidence from the continental to local scales. Both published and new data indicate the existence of eastern outlets drawing from pre-and-early Agassiz lakes. Part of the Arctic watershed was impounded between the Duluth Complex highlands and the retreating Rainy lobe. These outlets had their flows routed through the “Keating Complex” and the “Gunflint Arrow Lakes Corridor.” Discharges around the Duluth Complex’s northeast limb reached the Superior basin along Superior lobe ice, then exited down existing pre-glacial river valleys and a prominent, “valley-type” topographic bench between Hovland and Grand Marais, Minnesota.

Strong reasons exist to hypothesize that phyllosilicates, that is, clay minerals, played a critical catalytic role in the organic synthesis of prebiotic and possibly early biotic compounds and structures. Phyllosilicates would be expected to be abundant at the surface of early Earth (the Hadean) by the hydrous alteration of impact-generated silicate debris. The explorations of Earth, the Moon, and Mars permit reasonable inferences about physical conditions on prebiotic Earth. Also, currently available information allows the definition of necessary steps in prebiotic synthesis in which phyllosilicates may have participated. Consideration of these steps supports the plausibility that such minerals provided catalytic, substrate, and organizational functions for prebiotic and possibly early biotic development of organic structures, leading to formation and replication of ribonucleic acid (RNA) and, in turn, leading to a prebiotic RNA world. Ultimately, prokaryote cells may owe some of their functions to the inherent characteristics of associated phyllosilicates.

ABSTRACT This field trip examines a sequence of ejecta and deformed substrate resulting from the 1850 Ma meteorite impact. An impact origin for the Sudbury structure in Ontario has long been accepted, but knowledge of the corresponding ejecta was limited to fall-back breccia in the relict crater at Sudbury. The more distant ejecta blanket was discovered only recently near Thunder Bay, Ontario, and later in other parts of the Lake Superior region. Known informally as the Sudbury impact layer (SIL), it occurs at and near the stratigraphic top of Paleoproterozoic iron-formation. The impact-related deposits in the western Lake Superior region include (1) autochthonous material interpreted to be seismically folded and shattered iron-formation and carbonate rocks (breccia), overlain by (2) strata composed largely of allochtho-nous material (ejecta) derived in part from target rocks, and (3) irregular layers that appear to be mixtures of locally and distally derived material. Definitive microscopic evidence of an impact origin includes the occurrence of accretionary lapilli, ash pellets, spherules, devitrified glass, and quartz fragments marked by planar deformation features. The SIL exhibits extreme lithologic variability from place to place within each exposure area and between exposure areas. Nevertheless, the stratigraphic relationships that are presented by these exposures can be used to devise a sequence of deformation and depositional events that is largely consistent with experimental and empirical evidence of impact processes. This field trip will demonstrate that the stratigraphic arrangement of facies in the SIL has important temporal implications for understanding mechanisms of ejecta delivery and deposition.

Abstract Five depositional facies occur in five oriented cores obtained from the Bartlesville Sandstone (Cherokee Group, Middle Pennsylvanian) in the Madison Unit “B” enhanced recovery pilot project in Greenwood County, Kansas. By orienting the cores and studying the flow directions determined from high-angle crossbedding, a model for predicting the trend of the channel sandstone was constructed. Environments of deposition associated with the channels are (1) lnterdistributary Bay Siltstone, Shale, and Sandstone Facies; (2) Crevasse Splay Sandstone and Siltstone Facies; (3) Overbank Shale and Siltstone Facies; (4) Distributary Channel Sandstone Facies; and (5) Delta Plain Shale and Siltstone Facies. These facies may be recognized on gamma-ray logs as well as on permeability and grain-size plots. The Distributary Channel Facies is the major reservoir facies and overlies brackish to continental delta plain sediments. It is overlain by brackish and marine facies, including overbank and interdistributary bay shales and siltstones. The channel had low sinuousity, as is evident from the narrow trend of the sandstone and the blocky (rather than “bell-shape”) nature of the gamma-ray log. An overall fining-upward sequence typical of fluvial-deltaic sediments is suggested by an upward decrease in the grain size, a decrease in the scale of sedimentary structures, and an increase in the amount of interstitial clay matrix. Variations among the sandstone facies are also reflected by variations in permeability values. Controls on permeability that differ among the various facies are packing, the presence of ductile rock fragments, the amount and morphology of authigenic and detrital clay, and sorting. The permeability of the Distributary Channel Facies decreases from 60 to 100 md in trough crossbedded sandstones typical of the lower and middle part of the channel fill to 20 to 60 md in rippled sandstones, siltstones, and shales of the upper part of the channel. Low permeability values (0-20 md) are characteristic of facies composed primarily of siltstone and shale. The Distributary Channel Facies is continuous across the project area and its thickness varies in a predictable manner. Numerous and detailed dip directions measured primarily in trough crossbeds in four of the five oriented cores that were used in the study indicate that the channel flowed in a west-southwesterly direction across the project area. This flow direction is parallel to the maximum elongation direction of the sand body. The Overbank, Crevasse Splay, and Interdistributary Bay Facies are not continuous across the whole project area. These facies filled the topographically low areas adjacent to the channel.