Abstract

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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