The 400 m of Blomidon redbeds accumulated in a semi‐arid rift valley in the subtropics. At St Mary's Bay, these redbeds are 64% sandy mudstone (playa mudflats), 25% graded beds (sandflats at the toes of alluvial fans), 10% fissile claystone (playa lakes), and 1% channel sandstone (stream channels). Flash floods in mountains south of the valley flowed down alluvial fans to spread out to the north‐east as sheet flows on sandflats and playa mudflats. Deceleration of the sheet flows deposited graded beds 2–83 cm in thickness on the sandflats and thin layers of mud on the playas. Nine sequences, consisting of arrangements of six lithologies, compose 90% of the graded beds. In order of decreasing abundance, these are: ripple cross‐laminated siltstone → horizontally laminated mudstone; fining‐upward, ripple cross‐laminated siltstone; ripple cross‐laminated sandstone → horizontally laminated mudstone; cross‐bedded sandstone horizontally laminated mudstone; ripple cross‐laminated sandstone → ripple cross‐laminated siltstone; rippledrift cross‐laminated siltstone horizontally laminated mudstone; fining‐upward, ripple‐drift cross‐laminated siltstone; cross‐bedded sandstone → ripple cross‐laminated siltstone; and cross‐bedded sandstone → ripple cross‐laminated siltstone → horizontally laminated mudstone. The sheet flows, perhaps up to 1 m in depth, had a high concentration of suspended load. Deposition was dominantly during lower flow regime conditions and moderate to rapid flow deceleration. There are 32 thinning and fining‐up cycles where a sandflat package of graded beds is transitionally followed by a playa package of sandy mudstone. The cycles range in thickness from 1·3 to 13·3 m, averaging 4·6 m. Each cycle is initiated by avulsion to a new active channel network on a fan. Gradual abandonment of the channel network produces the thinning and fining‐up cycle. The cycles are grouped in three 60–70 m fining‐up megacycles. Upwards within each megacycle, the packages of sandy mudstone compose a progressively larger proportion of the cycles. Each megacycle evidently was initiated by a brief period of tectonic movement on the border faults that produced greater relief of the highlands relative to the valley floor. Subsequent erosion gradually lowered the relief to yield a fining‐up megacycle.

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The Triassic/Jurassic boundary, 208 million years ago, is associated with widespread extinctions in both the marine and terrestrial biota. The cause of these extinctions has been widely attributed to the eruption of flood basalts of the Central Atlantic Magmatic Province. This volcanic event is thought to have released significant amounts of CO2 into the atmosphere, which could have led to catastrophic greenhouse warming, but the evidence for CO2-induced extinction remains equivocal. Here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations, spanning a 20-Myr period across the Triassic/Jurassic boundary. Using a standard diffusion model, we interpret these isotopic data to represent a rise in atmospheric CO2 concentrations of about 250 p.p.m. across the boundary, as compared with previous estimates of a 2,000-4,000 p.p.m. increase. The relative stability of atmospheric CO2 across this boundary suggests that environmental degradation and extinctions during the Early Jurassic were not caused by volcanic outgassing of CO2. Other volcanic effects-such as the release of atmospheric aerosols or tectonically driven sea-level change-may have been responsible for this event.

The redbeds of the 220 m Blomidon Formation comprise more than 100 pairs of (i) sand-flat sandstone (distal alluvial fan) and (ii) playa sandy mudstone and (or) lacustrine claystone. These pairs are tectonic autocycles generated by rift-basin subsidence and sagging along the Glooscap fault zone that shifted loci of active fan sedimentation toward and away from the playa surface as fan lobes migrated into topographically low areas.Intervals of intense and prolonged aridity are recorded in many of the sand-flat sandstones and playa sandy mudstones by evaporite minerals, principally gypsum, precipitated by subsurface evolution of a CaS04-NaCI brine. Aridity is further demonstrated by intense disruption of sediment fabrics beneath evaporite crusts, deep mudcracks, eolian-sandstone layers and patches, and the presence of authigenic Ca-Mg-rich illite-smectite and analcime. During relatively wetter intervals, claystone beds accumulated in shallow lakes that filled the playa depression, lapping onto the sand flats. The claystones lack evaporite minerals and textures, and many are partially bioturbated.The long-term climate became progressively wetter from latest Triassic to earliest Jurassic time, as evidenced by an increase upsection in the ratio of lacustrine claystone to playa sandy mudstone and by less evaporite mineralization in the sand-flat sandstones.Les lits rouges de la Formation de Blomidon d'une puissance de 220 m incluent plus de 100 paires de bancs de (i) grks de platier tidal (cBne alluvial distal) et de (ii) mudstones grkseux de playa et (ou) d'argilite lacustre. Ces paires de bancs skdimentaires reprksentent des autocycles tectoniques crtks par la subsidence d'un bassin dans une zone de rift et par un flkchissement le long de la zone de failles de Glooscap; le jeu de ces failles a fait osciller le centre de la skdimentation active du cBne alluvial en directions convergente et divergente de la playa, au fur et i mesure que migraient les lobes des cBnes alluviaux dans les regions de faible relief.Les mintraux d'tvaporites, particulikrement le gypse prkcipitt par les changements en subsurface de la saumure CaS04-NaCI, prksents dans plusieurs lits de grks de platier tidal et de mudstones grtseux de playa sont les tkmoins d'intervalles d'une ariditk intense et prolongke. Le climat aride est corrobork par la fabrique skdimentaire kclatee des lits sous-jacents aux crofites d'kvaporites, par les fissures de desiccation profondes, par les lits et plages de grks kolien et par la prksence d'illite-smectite authigknique riche en Ca-Mg et de I'analcime. Durant les intervalles relativement plus humides, les lits d'argilite se sont accumulks dans des lacs peu profonds et ont comb16 la dkpression de la playa, chevauchant les grks de platier tidal. Les argilites ne contiennent pas d'kvaporites et n'exhibent pas de textures et plusieurs lits sont bioturbts.Entre la fin du Trias et le dkbut du Jurassique, la longue pkriode d'ariditk a ctdt la place h un climat progressivement plus humide, comme en tkmoignent I'augmentation...

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