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Paleomagnetism of Appalachian Paleozoic rocks offers potentially useful information regarding primary paleogeographic relationships between terranes and continents, if primary magnetizations can be demonstrated. Secondary "overprint" magnetizations commonly occur, however, and are usually related to later stages of orogenic evolution. We have collected a reconnaissance paleomagnetic sample suite from two localities in the Hanford Brook area of the Cambrian Ratcliffe Brook Formation, Caledonia terrane of New Brunswick to investigate its possible primary and/or secondary overprint magnetizations. Preliminary paleomagnetic results from the ca. 520 Ma Mystery Lake Member red-purple mudstone and siltstone strata carry a stable remanent magnetization that fails a fold test and has a combined in situ mean direction ofD = 165, I = 16°, consistent with it being a Carboniferous overprint magnetization of 300-310 Ma age. The overprint is similar to "Kiaman" superchron Carboniferous overprint magnetizations in Paleozoic strata throughout the Appalachians and adjacent basins, thought to be associated with the Alleghanian orogeny. Regionally, the apparent late Carboniferous overprint age is slightly younger than ca. 315-326 Ma metamorphic ages along the Fundy coast, which are possibly related to the arrival of the Meguma terrane as it collided with and overrode Avalonia in New Brunswick. Locally, no metamorphic ages are reported in published 40Ar/39Ar work on detrital muscovite from the Ratcliffe Brook Formation, indicating that the remagnetization likely reflects low temperature formation of new magnetic minerals. Overprint paleomagnetic directions in the Hanford Brook area appear to be slightly younger than recently reported ca. 320 Ma overprint magnetizations in Meguma terrane, and also do not show as much counterclockwise apparent rotation. Kiaman overprint directions in the Appalachians offer a tool for both dating the overprint acquisition timing (with reference to the North American Apparent Polar Wander Path) and also to detect post-magnetization rotations of portions of the orogen during the final assembly of Pangea. [ABSTRACT FROM AUTHOR]

A paleomagnetic study of subaerial volcanic rocks and associated siltstones of the Ediacaran Bull Arm Formation in the Avalon Zone of Newfoundland revealed a stable bipolar, hematite-borne primary remanence supported by positive conglomerate, contact, and fold tests. Mean remanence directions in two distal areas (Bonavista and Argentia) are similar, indicating a low paleolatitude position of Avalonia at ∼570 Ma. Redbeds of the overlying ∼550 Ma Crown Hill Formation also carry a primary bipolar hematite-borne remanence with moderate inclination, indicating that Avalonia remained at low to medium paleolatitudes through the end of the Ediacaran. Combining our results with previously published paleomagnetic data of Avalonia suggests moderate-scale drift of Avalonia through low southern paleolatitudes through the latter half of the Ediacaran, providing a paleogeographic context for the development of the first complex metazoan life. [ABSTRACT FROM AUTHOR]

The ca. 760 Ma Burin Group occurs in a 60 km long northeasterly-trending belt on the Burin Peninsula of southeastern Newfoundland and consists of low grade massive and pillowed basalts, abundant dykes and sills, with minor mafic pyroclastic rocks and limestone. The Burin Group preserves the oldest known magmatic event in the Avalon terrane, but its tectonic evolution and its relationship to the voluminous 635-570 Ma arc-related magmatism that typifies Avalonia is uncertain. Geochemical analyses confirm that the basalts are predominantly low-K tholeiites. They are characterized by high LIL/HFS, and display depletion to slight enrichment in LREE. Sm-Nd isotopic data reveal that most basalts have juvenile compositions, with εNd values similar to contemporaneous depleted mantle, indicating that high LIL/HFS was probably due to coeval subduction which contaminated the mantle source. Other basalts have lower εNd values, and the negative correlation of εNd with La/Sm, together with a positive correlation of εNd with 147Sm/144Nd suggest that their isotopic signatures have been modified by a Mesoproterozoic or older crust or sub-continental lithospheric mantle into which Burin Group mafic volcanics were emplaced. The isotopic signature of the Burin mafic rocks is similar to that inferred for the source of the main phase of Avalonian magmatism. These data, together with paleocontinental reconstructions for ca. 760 Ma, suggest that the Burin Group is a local representative of an ensimatic arcs within the peri-Rodinian ocean, possibly as a far-field response to the breakup of Rodinia. Vestiges of these arcs were accreted to the northern Gondwanan margin at about 650 Ma, and then recycled by subduction beneath that margin during the main ca. 635-570 Ma Avalonian event. Although its low metamorphic grade precludes it being the basement from which Avalonian magmas were extracted, the Burin Group may be representative of the geochemical and isotopic composition of that basement. [ABSTRACT FROM AUTHOR]

The Late Ordovician equatorial zone, like the zone today, had few hurricane-grade storms within 10° of the equator, as emphasized by the preservation of massive-bedded Thalassinoides ichnofacies in a trans-Laurentian belt more than 6000 km long, from the southwestern United States to North Greenland. That belt also includes nonamalgamated shell beds dominated by the brachiopod Proconchidium, which would not have been preserved after hurricane-grade storms. The belt lacks such storm-related sedimentary features as rip-up clasts, hummocky cross-stratification, or large channels. In contrast, other contemporaneous Laurentian Thalassinoides facies and shell beds on either side of the belt have been disturbed by severe storms below fair-weather wave base. The position of the biofacies-defined equatorial belt coincides with the Late Ordovician equator deduced from paleomagnetic data from Laurentia, thus providing both a high-precision equatorial location and an independent test of the geocentric axial dipole hypothesis for that time. [ABSTRACT FROM AUTHOR]

Impact-generated lithologies have recently been identified as viable and important microbial habitats, especially within cold and arid regions such as the polar deserts on Earth. These unique habitats provide protection from environmental stressors, such as freeze-thaw events, desiccation, and UV radiation, and act to trap aerially deposited detritus within the fissures and pore spaces, providing necessary nutrients for endoliths. This study provides the first culture-independent analysis of the microbial community structure within impact-generated lithologies in a Mars analog environment, involving the analysis of 44,534 16S rRNA sequences from an assemblage of 21 rock samples that comprises three shock metamorphism categories. We find that species diversity increases (H = 2.4-4.6) with exposure to higher shock pressures, which leads to the development of three distinct populations. In each population, Actinobacteria were the most abundant (41%, 65%, and 59%), and the dominant phototrophic taxa came from the Chloroflexi. Calculated porosity (a function of shock metamorphism) for these samples correlates (R 2  = 0.62) with inverse Simpson indices, accounting for overlap in populations in the higher shock levels. The results of our study show that microbial diversity is tied to the amount of porosity in the target substrate (as a function of shock metamorphism), resulting in the formation of distinct microbial populations. Key Words: Microbial diversity-Endoliths-Impact melt-rocks-Mars-Astrobiology. Astrobiology 16, 775-786.