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Permo-Triassic redbeds, consisting of mainly breccias and sandstones exceeding 6,000 feet in thickness, are well exposed in coastal cliffs in South Devonshire. They are disposed in semi-circular or elongate basins or cuvettes, margined by ridges of metamorphic rocks which supplied the bulk of the detritus.
The breccias show typical torrential bedding features. Low-angle cross-bedding units, festooned perpendicular to the direction of derivation, are interwoven with generally flat but lenticular beds of breccia and sandstone. Shallow channeling is common, deeper where sandstone is predominant. Mapping of pebble imbrication shows a basically centripetal transportation pattern in the cuvettes, consistent with derivations indicated by cross-bedding, channeling, and fragment composition. Roundness measurements of limestone fragments in the same rocks reveal sub-circular roundness contours increasing in value towards the cuvette centers, and approximately perpendicular to imbrication directions. All features indicate deposition on sub-montane alluvial fans in a semi-arid climate, with converg ng directions of sediment transport in semi-confined cuvettes.
Sandstones in the upper part of the sequence show clear eolian cross-bedding, with wedge-shaped units and some festoon cross-beds. Attitude measurement indicates deposition by a uni-directional wind from the SSW. Interbedded breccias show truncation and channeling of dune surfaces.
Quicksand injection structures occur in fine-grained silty breccias interbedded with sandstones (distal fan deposits). A saturated sand layer, sealed under an impermeable silty breccia, was mobilized and injected upward through desiccation cracks or other weak spots, forming sand dikes with elongate particles aligned parallel with the walls. Sun-cracked silt layers may show strongly upturned edges on desiccation polygons due to injection (extrusion) of quicksand, which may also be fragmented and dislocated.
Annelid burrows occur in some basal breccias, mostly fine-grained. Where elongate fragments constitute the breccia, the burrow filling shows a distinct internal fabric or "meniscus" particle arrangement. Commonly up to one inch in diameter with circular cross section, in coarser breccias they are ovoid and up to 7 inches wide. These large burrows branch, ascend through strata, and avoid large fragments; together with the consistent meniscus fabric, these features indicate organic origin.
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