ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉Retrogressive slumping could accelerate sea-level rise if ice-sheet retreat generates ice cliffs much taller than observed today. The tallest ice cliffs, which extend roughly 100 m above sea level, calve only after ice-flow processes thin the ice to near flotation. Above some ice-cliff height limit, the stress state in ice will satisfy the material-failure criterion, resulting in faster brittle failure. New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs are prone to failure by slumping, unloading submarine ice to allow buoyancy-driven full-thickness calving. Full-Stokes diagnostic modeling shows that the threshold cliff height for slumping is likely slightly above 100 m in many cases, and roughly twice that (145–285 m) in mechanically competent ice under well-drained or low-melt conditions.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Mudstones hosting Burgess Shale–type preservation of soft-bodied fossils are commonly held to be characterized by little to no bioturbation. This has been taken as evidence for bottom-water dysoxia or anoxia, along with anaerobic conditions in the sediment, which favored preservation of soft tissues by hindering decay. Although invisible on fresh and weathered surfaces, laminated claystone comprising the middle Cambrian (Drumian) Ravens Throat River Lagerstätte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, is revealed by preparation of surfaces sawn parallel to bedding to exhibit extensive burrowing. Four types of burrows are distinguished: (1) rare large forms averaging 15 mm in diameter; (2) backfilled vertical burrows 3–6 mm wide; (3) oblique to horizontal burrows 2–4 mm wide and typically with meniscate backfilling; and (4) tiny, short, mostly vertical burrows 0.5–1 mm in diameter. The third group is the most common, locally completely bioturbating laminae and penetrating worm carcasses; it conforms to 〈span〉Planolites〈/span〉. A variety of ethologies is indicated, with the large type seemingly serving as a dwelling burrow and the smaller ones from deposit-feeding. Although dysoxic bottom conditions probably developed occasionally, the widespread burrowing argues for predominantly oxic conditions, and it indicates that restriction of bioturbation was probably not the most important factor leading to soft-tissue preservation. Bioturbation might be more common in other Cambrian Lagerstätten than is currently believed, and it is possible that low-oxygen conditions at the seafloor were not fundamentally necessary for Burgess Shale–type preservation.〈/span〉

Description: 〈span〉Interpreting the deposits of ancient epeiric seas presents unique challenges because of the lack of direct modern analogs. Whereas many such seas were tectonically relatively quiescent, and successions are comparatively thin and punctuated by numerous sedimentary breaks, the Mesoproterozoic Belt Basin of western North America was structurally active and experienced dramatic and continuous subsidence and sediment accumulation. The Grinnell Formation (ca. 1.45 Ga) in the lower part of the Belt Supergroup affords an opportunity to explore the interplay between sedimentation and syndepositional tectonics in a low-energy, lake-like setting. The formation is a thick, vivid, red- to maroon-colored mudstone-dominated unit that crops out in northwestern Montana and adjacent southwestern Alberta, Canada. The mudstone, or argillite, consists of laminated siltstone and claystone, with normal grading, local low-amplitude, short-wavelength symmetrical ripples, and intercalations of thin tabular intraclasts. These intraclasts suggest that the muds acquired a degree of stiffness on the seafloor. Halite crystal molds and casts are present sporadically on bedding surfaces. Beds are pervasively cut by mudcracks exhibiting a wide variety of patterns in plan view, ranging from polygonal to linear to spindle-shaped. These vertical to subvertical cracks are filled with upward-injected mud and small claystone intraclasts. Variably interbedded are individual, bundled, or amalgamated, thin to medium beds of white, cross-laminated, medium- to coarse-grained sandstone, or quartzite. These are composed of rounded quartz grains, typically with subangular to rounded mudstone intraclasts. Either or both the bottoms and tops of sandstone beds commonly show sandstone dikes indicative of downward and upward injection. Both the mudcracks and the sandstone dikes are seismites, the result of mud shrinkage and sediment injection during earthquakes. An origin via passive desiccation or syneresis is not supported, and there is no evidence that the sediments were deposited on alluvial plains, tidal flats, or playas, as has been universally assumed. Rather, deposition occurred in relatively low-energy conditions at the limit of ambient storm wave base. The halite is not from in situ evaporation but precipitated from hypersaline brines that were concentrated in nearshore areas and flowed into the basin causing temporary density stratification. Sandstone beds are not fluvial. Instead, they consist of allochthonous sediment and record a combination of unidirectional and oscillatory currents. The rounded nature of the sand and irregular stratigraphic distribution of the sandstone intervals are explained not by deltaic influx or as tempestites but as coastal sands delivered from the eastern side of the basin by off-surge from episodic tsunamis generated by normal faulting mainly in the basin center. The sands were commonly reworked by subsequent tsunami onrush, off-surge, seiching, and weak storm-induced wave action. Although the Grinnell Formation might appear superficially to have the typical hallmarks of a subaerial mudflat deposit, its attributes in detail reveal that sedimentation and deformation took place in an entirely submerged setting. This is relevant for the deposits of other ancient epeiric seas as well as continental shelves, and it should invite reconsideration of comparable successions.〈/span〉

Description: 〈span〉Retrogressive slumping could accelerate sea-level rise if ice-sheet retreat generates ice cliffs much taller than observed today. The tallest ice cliffs, which extend roughly 100 m above sea level, calve only after ice-flow processes thin the ice to near flotation. Above some ice-cliff height limit, the stress state in ice will satisfy the material-failure criterion, resulting in faster brittle failure. New terrestrial radar data from Helheim Glacier, Greenland, suggest that taller subaerial cliffs are prone to failure by slumping, unloading submarine ice to allow buoyancy-driven full-thickness calving. Full-Stokes diagnostic modeling shows that the threshold cliff height for slumping is likely slightly above 100 m in many cases, and roughly twice that (145–285 m) in mechanically competent ice under well-drained or low-melt conditions.〈/span〉

Description: 〈span〉Mudstones hosting Burgess Shale–type preservation of soft-bodied fossils are commonly held to be characterized by little to no bioturbation. This has been taken as evidence for bottom- water dysoxia or anoxia, along with anaerobic conditions in the sediment, which favored preservation of soft tissues by hindering decay. Although invisible on fresh and weathered surfaces, laminated claystone comprising the middle Cambrian (Drumian) Ravens Throat River Lagerstätte in the Rockslide Formation of the Mackenzie Mountains, northwestern Canada, is revealed by preparation of surfaces sawn parallel to bedding to exhibit extensive burrowing. Four types of burrows are distinguished: (1) rare large forms averaging 15 mm in diameter; (2) backfilled vertical burrows 3–6 mm wide; (3) oblique to horizontal burrows 2–4 mm wide and typically with meniscate backfilling; and (4) tiny, short, mostly vertical burrows 0.5–1 mm in diameter. The third group is the most common, locally completely bioturbating laminae and penetrating worm carcasses; it conforms to 〈span〉Planolites〈/span〉. A variety of ethologies is indicated, with the large type seemingly serving as a dwelling burrow and the smaller ones from deposit-feeding. Although dysoxic bottom conditions probably developed occasionally, the widespread burrowing argues for predominantly oxic conditions, and it indicates that restriction of bioturbation was probably not the most important factor leading to soft-tissue preservation. Bioturbation might be more common in other Cambrian Lagerstätten than is currently believed, and it is possible that low-oxygen conditions at the seafloor were not fundamentally necessary for Burgess Shale–type preservation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The alignment and age progression of volcanoes produced as a tectonic plate moves over a mantle plume can be used to reconstruct both the direction and rate of past plate motion assuming the plume remains in a fixed location. New 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dates for lavas from 15 volcanoes spanning the entire length (∼2800 km) of the Northwest Hawaiian Ridge (NWHR) facilitate improved age-distance relationships. These are used to constrain a significant kink in the NWHR at 25.3 ± 0.5 Ma as the Pacific plate experienced a brief episode of more northerly motion and rotated counter-clockwise. The age progression (i.e., velocity of the Pacific plate) increased markedly from 57 to 87 km/Ma following the plate motion change. This mid-Cenozoic tectonic reorganization has been previously identified in plate motion models, but has been poorly constrained temporally. We demonstrate that this event affected four seamount trails within the Pacific Basin, and had a significant impact on all circum-Pacific volcanic arcs.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Accurate and precise temperatures of metamorphism and magmatism can be obtained from O-isotope compositions of minerals using a system of linear equations that defines fractionation and mass balance of O isotopes among all minerals in a rock. This approach—linear-equation thermometry (LinT)—does not require that cooling rates or absolute O-diffusion parameters be known. If O diffusivities are known, closure temperatures for each phase can be calculated and inverted to determine a temperature-dependent cooling rate that is accurate to a factor of three. The highest and most precise temperatures will be preserved in rocks containing quartz and garnet, because O diffuses slowly in garnet and garnet-quartz O-isotope fractionation is relatively large. The technique will be most useful in rocks with low-variance mineral assemblages in which precise pseudosection analysis is inhibited by wide spacing of reactions in pressure-temperature space. Application of LinT to ultrahigh-temperature gneisses from Madagascar yields temperatures of 927 ± 31 °C, consistent with independent thermometry.〈/span〉