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Carbonate flat-pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat-pebble conglomerate beds in shallow-water cyclic and non-cyclic strata. Several origins of flat-pebble conglomerate are inferred for these strata. In one case, all stages of development of flat-pebble conglomerate are captured within stormdominated shoreface deposits of hummocky cross-stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat-pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre-scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm-to dm-scale clasts. This transition was accompanied by the addition of sand-sized echinoderm-rich debris from local sources, which slightly lowered friction by reducing clastclast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat-pebble conglomerate bed occurs at the top of upward-coarsening, metre-scale cycles. The flat-pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron-rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat-pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third (2004) 51, 973-996 Ó 2004 International Association of Sedimentologists 973 type of flat-pebble conglomerate bed occurs within unusual metre-scale, shaledominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat-pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds....

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The composite Arctic Alaska-Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska-Chukotka has been proposed based on paleobiogeographic faunal affi nities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic-Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska-Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the composite Arctic Alaska-Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)-Carboniferous tectonism.

The name Lawsonian Stage is proposed as the highest stage of the Cambrian System (Stage 10). The base of the proposed Lawsonian Stage is at the First Appearance Datum (FAD) of the euconodont <i>Eoconodontus notchpeakensis </i>(Miller, 1969). That horizon, the base of the <i>E. notchpeakensis </i>Subzone of the <i>Eoconodontus </i>Zone, is 3mabove the base of the Red Tops Member of the Notch Peak Formation at the Steamboat Pass section in the House Range, western Utah, USA. The conodont fauna of the <i>Eoconodontus </i>Zone is widespread in North America and occurs in Asia, Australia, Europe, and South America in facies that include cratonic nearshore sandstones, shallow and intermediate-depth platform carbonates, deep-water ramp, continental slope deposits, and deep-ocean radiolarian chert. This horizon lies at or slightly below the top of the <i>Illaenurus </i>trilobite Zone of western North America and within the lower part of the <i>Saukiella junia </i>Subzone of the <i>Saukia </i>Zone in areas east of the Rocky Mountains. These trilobite and conodont faunas can be traced into slope deposits containing cosmopolitan trilobites. The horizon is nearly coincident with the boundary between the <i>Billingsella </i>and the <i>Finkelnburgia </i>calcitic brachiopod Zones; ranges of organophosphatic brachiopods also characterize the horizon. A high-amplitude, negative, carbon-isotope excursion, the HERB Event, occurs in the <i>Eoconodontus notchpeakensis </i>Subzone. This distinctive geochemical signal is known in western Utah, Australia, China, and in poorly fossiliferous slope deposits in Newfoundland, Canada. The proposed boundary fits between two closely spaced sequence-stratigraphic boundaries described from Utah and Texas, USA