Sediments in the Trentishoe Formation of the Middle Devonian Hangman Sandstone Group (North Devon, U.K.) provide the basis of a model for sandy ephemeral stream and clay playa deposition. Three types of sequence are found, representing proximal, medial and distal areas on an extensive alluvial plain. The Proximal sequence consists of cross‐cutting channel‐fill sandstones which represent the deposits of a network of low sinuosity sand bed streams. The Medial sequence comprises upwards coarsening cyclothems which start with relatively distal, thinly bedded sandstone and siltstone flood sheets cut by complexes of silt draped channel‐fill sandstones and single channel fill sandstones. The flood sheets coarsen and thicken upwards to more proximal multistorey sheet sandstones. The Distal sequences consist of laminated mudstone and sandstone, cut by desiccation and water escape features, alternating with wave rippled sandstones, and represent playa lakes occasionally incised by high sinuosity channels with laterally accreting sandstones. The three sequence types represent the downslope progression from a low sinuosity channel network which passed into an ephemeral flood deposit complex which in turn drained into clay playas.
A review of published data for the Palaeozoic of SE Turkey, together with facies distribution and palaeo‐fault maps derived from regional field data, are used to interpret the Palaeozoic tectonic history of the region.
Intracratonic rifting events in the Early Cambrian and the Early Ordovician led to syn‐rift deposition within fault‐bounded basins. Marine transgressions across the region in the mid‐Cambrian and mid‐to‐Late Ordovician were probably influenced by regional thermal subsidence after each rifting event, in addition to (glacio‐)eustatic mechanisms. Marine transgressions in the Early Silurian and Early Carboniferous are thought to have been entirely eustatic in origin.
Poorly‐constrained, Late Palaeozoic facies variations across the region can be related to uplift during the Caledonian and Hercynian orogenic episodes. Widespread tilting and erosion before the Cretaceous, which has removed much of the Late Palaeozoic record from this region, resulted from uplift at the edges of a major rift system which was initiated during the Triassic‐to‐Jurassic opening of the Southern Neotethys.
The Hollowbrook Formation is an early Middle Devonian regressive succession of sandstones and heterolithic facies marking a passage from shallow marine shelf muds, through a sandy shoreface to continental, ephemeral stream sheet sands. Ten facies are recognized, arranged in two sequence types. The shelf sequence consists of alternations of bioturbated muddy sandstone (fair weather sediment) and shelly concentrates or parallel laminated fine sandstone (storm deposits). The nearshore and foreshore sequences coarsen upwards, and pass from bioturbated muddy sandstone (shelf) to lenticular beds (inner shelf), then either wavy and flaser beds (fair weather lower shoreface) or hummocky beds and multistorey parallel laminated sands with bioturbated tops (lower shoreface storm shoals). Upper shoreface wave ripple cross laminated sandstones follow, then parallel and planar cross laminated sandstones (foreshore) and parallel laminated lithic sandstone (continental sheet flood).
The lower parts of the shoreface sequence and inner shelf are burrowed by
Skolithos. Arenicolites
and
Chondrites
. The sequence differs from the high energy 'Oregon Coast' shore in the absence of a megarippled facies and presence of much bioturbation, but closely matches the sequence found in modern low energy, wave influenced sandy shores such as Sapelo Island, Georgia.
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