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The Soom Shale Member of the Cedarberg Formation (South Africa) is a key early Paleozoic Lagerstatte that directly overlies glacigenic deposits of the Late Ordovician glacial maximum; it was deposited on the Gondwanan craton during deglacial transgression. We show that it contains a substantial coarse silt to fine sand component that occurs in discrete laminae intimately associated with plankton-derived organic material. We interpret this component as loess, sourced from glacially derived debris and blown into the sea either directly or across seasonal sea ice, as happens today on the McMurdo Ice Shelf, Antarctica. Falling through the water column, this material likely stimulated production of phytoplankton, which then sank as sediment-loaded aggregates. In marked sedimentary partitioning, these silt and/or organic laminae alternate with fine, organic-poor mud laminae that likely represent (river-derived?) nepheloid plumes and that sporadically thicken into centimeter-scale mud turbidites. Sustained eolian input directly into the surface water may have been key to maintaining the high productivity of the Soom sea and then, via eutrophication and anoxia, to the exceptional preservation of its biota. doi: 10.1130/G31426.1

Despite the Burgess Shale's (British Columbia, Canada) paleobiological importance, there is little consensus regarding its taphonomy. Its organic fossils are preserved as compressions associated with phyllosilicate films ('clay templates'). Debate focuses on whether these templates were fundamental in exceptional preservation or if they formed in metamorphism, meaning that it is important to establish the timing of their formation relative to decay. An early diagenetic origin has been proposed based on anatomy-specific variations in their composition, purportedly reflecting contrasts in decay. However, we demonstrate that these films bear a remarkable similarity to those that occur on organic fossils in graptolitic mudrocks and form as a normal product of low-grade metamorphism. Such phyllosilicates may also occur within voids created by volume loss in maturation, a process that may have aided their formation. In bedding-plane assemblages from graptolitic mudrocks, different taxa are associated with distinct phyllosilicates. This likely reflects stepwise maturation of their constituent kerogens in an evolving hydrothermal fluid, with different phyllosilicates forming as each taxon progressively underwent maturation. These observations provide an analogue for the distribution and composition of phyllosilicates on Burgess Shale fossils, which we interpret as reflecting variations in the maturation of their constituent tissues. Thus, their clay templates seem unremarkable, forming too late to account for exceptional preservation.

We propose ending the distinction between the dual stratigraphic terminology of time-rock units (of chronostratigraphy) and geologic time units (of geochronology). The long-held, but widely misunderstood, distinction between these two essentially parallel time scales in stratigraphy has been rendered unnecessary by the widespread adoption of the global stratotype sections and points (GSSP--golden spike) principle in defining intervals of geologic time within rock strata. We consider that the most appropriate name for this stratigraphic discipline is 'chronostratigraphy,' which would allow 'geochronology' to revert to its mainstream and original meaning of numerical age dating. This in turn makes the little-used formal term 'geochronometry' redundant. The terms 'eonothem,' 'erathem,' 'system,' 'series,' and 'stage' would become redundant, in favor of 'eon,' 'era,' 'period,' 'ep-och' and (disputably) 'age.' Our favored geologic time units may be qualified by 'early' and 'late,' but not by 'lower' and 'upper.' These suggested changes should simplify stratigraphic practice, encompass both stratified and nonstratified rocks, and help geologic understanding, while retaining precision of meaning. Keywords: stratigraphy, chronostratigraphy, geochronology.

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by [sup.40]Ar-[sup.39]Ar infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 [+ or -] 3.1 to 397.8 [+ or -] 1.8 Ma (2[sigma]), with a mean age of 396.1 [+ or -] 1.4 Ma (2[sigma]). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by [sup.40]Ar-[sup.39]Ar techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2)[sup.39]Ar loss during the irradiation of illite mineral separates, and (3) thermally induced [sup.40]Ar loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision. Keywords: [sup.40]Ar-[sup.39]Ar laserprobe, low temperature, slate, deformation, geochronology, Acadian.

The Early Silurian graptolites Spirograptus turriculatus and Sp. guerichi are typically preserved in condensed graptolite shale lithologies as entirely flattened outlines; all whorls are visible and little or no sediment infills the originally cone-shaped rhabdosomes, as though the graptolites had been sealed in clingfilm, or plastic wrap, prior to burial and compaction. By contrast, the rhabdosomes of graptolites transported in turbidity currents typically are filled with sediment. The most likely reason for the clingfilm mode of preservation is encasement or covering of the graptolite rhabdosomes by marine snow and/or microbial mats prior to burial by clastic sediment and compaction on an anoxic seafloor. Experimental evidence reported herein supports this suggestion. The organic material that mediated the preservation of such graptolites was likely akin to, but probably physically stronger than, the delicate benthic flocculation layer of the current Black Sea floor. Like the latter, it probably formed significant microtopography at and just below the sediment-water interface and mediated geologic processes such as early diagenesis in graptolite shale lithologies. Keywords: graptolites, black shales, marine snow, microbial mats.

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by [sup 40]Ar-[sup 39]Ar infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 ± 3.1 to 397.8 ± 1.8 Ma (2σ), with a mean age of 396.1 ± 1.4 Ma (2σ). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by [sup 40]Ar-[sup 39]Ar techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2) [sup 39]At loss during the irradiation of illite mineral separates, and (3) thermally induced [sup 40]Ar loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision. [ABSTRACT FROM AUTHOR]