Facies model for a sandy ephemeral stream and clay playa complex; the Middle Devonian Trentishoe Formation of North Devon, U.K.

Abstract

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.

Depositional Environment and Petrography of Preglacial Continental Sediments from Hole 740A, Prydz Bay, Antarctica

“…This seems to have been most common during deposition of the upper part of the succession. The relatively low incidence of internal scouring and reworking of the channel sand bodies, clay drapes, and other evidence of bedform modification and emergence suggest that discharge fluctuations were not very great during their deposition, in contrast to the more ephemeral systems described by Turner (1978) and Tunbridge (1984), which are characterized by repeated episodes of scour and fill and abundant upper flow regime plane beds. This in turn implies that the climate was sufficiently wet to maintain discharge throughout the channel sand deposition.…”

Facies evolution and depositional model of an arid microtidal coast: example from the coastal plain at the mouth of Wadi Al-Hamd, Red Sea, Saudi Arabia

“…Similar features were reported by Morton (1978) in washover deposits on south Padre Island. The presence of evaporite deposits, though not pervasive at the top, goes well with arid climatic conditions (Tunbridge, 1984;Daniels, 2003;Hampton and Horton, 2007).…”

“…The channel fills were dominated by noncyclic alternation of fine and sharp based medium sands, suggesting deposition by intermittent supply under fluctuating flow speed and depth, similar to ephemeral channels that are common under arid to semiarid conditions (Tunbridge, 1984;Stear, 1985;Abdullatif, 1989;Luttrell, 1993). The thickness and attitude of medium sand beds are variable, indicating temporal variation in flow load and intensity.…”

“…The thickness and attitude of medium sand beds are variable, indicating temporal variation in flow load and intensity. It is followed by a generally massive, rarely rippled section displaying no cross-laminations that characterize perennial and ephemeral streams (Tunbridge, 1984;Abdullatif, 1989;Miall, 1996). The absence of crosslamination is attributed to the shallow rapidly waning flow.…”

Age and sedimentological features of fluvial series in the Toruń Basin and the Drwęca Valley (Poland)

“…The dipping of the laminas within the cosets Sl is insignificant. This may indicate that the sand sheets were deposited by overbank sheet flows (Tunbridge, 1981;1984), although the fine-grained sandy fraction indicates low energy of flowing water. On the other hand, however, the fact that there are large-scale lithofacies Sl, which are covered either with sands of ripple lamination or by scour-and-fill structure, might suggest the development of point-bars sensu lato (Olsen, 1988).…”