The Journal of Geology

Brussels Hill in southern Door County, Wisconsin, is a localized area of faulted and brecciated bedrock in a region of otherwise undeformed lower Silurian dolostone that has been tentatively interpreted as an impact crater. The area of disturbed rock coincides with a nearly circular, 2-km-wide, flat-topped hill that stands about 40 m above the surrounding landscape. Glacial till and polished, striated surfaces on the brecciated bedrock indicate that the disturbance predated the last ice advance and suggest that the hill may have been even higher before glaciation. Anomalous, apparently intrusive bodies of glauconite-bearing sandstone within the brecciated dolostone were likely derived from Cambrian strata, which are normally 400 m in the subsurface in the area. A 103-m drill core into the center of the disturbance recovered 70 m of brecciated dolostone and shale, much of it vesicular, but with no features diagnostic of shock metamorphism. The lowest 33 m of the core sampled the upper Ordovician Maquoketa Formation, which appeared to be subhorizontal and only locally disrupted. This is puzzling, given that material from greater depth must have been brought up through this level. The fact that an area of such fragmented rock stands as a topographic high is also hard to explain. Brussels Hill may have been the site of supersonic impact, but an endogenous process such as violent fragmentation in the upper part of a diatreme cannot be ruled out.

The Chesterian (Mississippian) Hartselle Sandstone is a tar sand exposed in the Black Warrior Basin and southern Appalachian fold belt in northern Mississippi and Alabama. Previous studies disagree about the delivery direction and relative contributions of sediment from the cratonic interior, the Appalachians, and the Ouachitas. The goal of this research is to investigate lateral trends in sedimentary and geochemical properties to provide new details about the provenance of the Hartselle. Samples were collected along a west-to-east transect and analyzed using petrography, X-ray fluorescence (XRF) geochemistry, and U-Pb detrital zircon (DZ) geochronology. Point counting indicated a major cratonic interior source with a minor recycled orogen signal. Sillimanite in the easternmost sample narrows down the potential Appalachian sources to areas of high-grade metamorphism. Petrographic observations suggest both western and eastern sources. The XRF results showed high concentrations of Ti and Mo in the westernmost sample, suggestive of a nearby terrigenous source. All of the samples had similar U-Pb DZ age distributions except for the easternmost sample, which had statistically higher proportions of Paleozoic- and Archean-aged grains (Kolmogorov-Smirnov test: p < .02). For the first time, Eoarchean and Paleoarchean grains were found in the Hartselle, indicating a likely sediment source from the Minnesota River Valley gneisses of the Superior Craton prior to Grenvillian overprinting. Based on integration of all data, we conclude that a large fluvial system draining the continental interior provided a substantial volume of sediment from the northwest. At the same time, a more dispersed drainage basin associated with the Appalachian front contributed recycled orogenic material from the northeast.

Calcareous nannofossil, carbonate content, and δ¹⁸O and δ¹³C data were utilized to reveal the nature of the Paleocene-Eocene (P-E) boundary at the Gunnah section, Western Desert, Egypt. The uppermost Tarawan Formation (Fm.), Esna Fm., and the lowermost, Farafra Fm., were included in this study. Lithostratigraphically, the Esna Fm. was subdivided into the Hanadi, Dababiya Quarry, and Mahmiya Members. Three calcareous nannofossil zones (NP7/8, NP9, and NP10) and four subzones (NP9a, NP9b, NP10a, and NP10b) were recognized. The biostratigraphic significances of calcareoust nannofossil bioevents associated with the P-E boundary at the study section were discussed. The base of the Eocene was placed at the base of the Dababiya Quarry Member in coincidence with the base of subzone NP9b, which is coincident with the LOs of Rhomboaster cuspis, R. calcitrapa, R. bitrifidia, Pontosphaera pulchra, and Zygrhablithus bijugatus. However, Discoaster araneus first occurred about 3 m below the base of subzone NP9b. The global decreases of the δ¹³C and δ¹⁸O values and carbonate contents across the P-E boundary were documented in the Gunnah section, which represents one of the most expanded and complete P-E sequences in Egypt.