Challenging the ‘sheetflood’ myth: the role of water-table-controlled sabkha deposits in redefining the depositional model for the Ormskirk Sandstone Formation (Lower Triassic), East Irish Sea Basin

Abstract

Two distinct units can be recognized in the Lower Triassic Ormskirk Sandstone Formation in the East Irish Sea Basin. Perennial fluvial channel, lacustrine and minor aeolian sandstones dominate the upper part, but the lower is characterized by thick (tens of metres) enigmatic wavy bedded deposits interbedded with aeolian and minor fluvial channel sandstones. Previous interpretations that invoked an overbank ‘sheetflood’ or ‘sandflat’ origin for the wavy bedding are difficult to reconcile with the sedimentary structures, which are more consistent with algal and evaporitic processes in modern sabkhas. These deposits contain widespread (km) ‘drying upward’ patterns 2–10 m thick, reflecting a sabkha to more aeolian dominated setting. Such thicknesses are consistent with an origin related to 23 000 year Milankovitch cyclicity. The lower, wavy bedded portion of the Ormskirk Sandstone Formation is regionally truncated by fluvial channel-belt sandstones. Their extent reflects a combination of high sediment supply, uniform subsidence and low basinal relief. Lacustrine units near the top of the formation can be correlated regionally, with one passing southward into a major aeolian unit that thickens to 60 m in the south of the basin. This transition demonstrates the diachroneity of the Mercia Mudstone Group transgression. The key to correlating the Ormskirk Sandstone Formation lies in identifying intervals dominated by fluvial, lacustrine or sabkha-aeolian deposits. Correlation of water-table-controlled ‘drying upward’ patterns within sabkha-aeolian deposits allows finer scale subdivision, and provides an estimate for the degree of fluvial truncation at the top of the unit.

The correlation and sequence architecture of the Ormskirk Sandstone Formation in the Triassic Sherwood Sandstone Group of the East Irish Sea Basin, NW England

“…correlation of the ormskirk sandstone, east irish sea basin 97 without the attendant fines indicative of waning flow and potential abandonment, indicates considerable variability in the flow regime and supports the suggestion that the fluvial system may have been perennial (Herries and Cowan 1997). Abandonment and partial abandonment deposits are, however, present in the form of claystones and siltstones ( Figure 3A) representing the lowest stages of flow to ponding and stagnation within channels, although they represent a minor component in the majority of preserved successions.…”

“…The various facies types have been previously discussed in some detail by, for example, Thompson (1969Thompson ( , 1970a, Meadows and Beach (1993), Thompson and Meadows (1997) and Herries and Cowan (1997) such that a summary will suffice here.…”

“…A further facies associated with the sandflats is termed 'clastic sabkha' and is used exclusively herein for sandstone-dominated intervals with abundant evaporite cement (mainly anhydrite) and primary anhydrite laminae. Such intervals are recorded principally in the cores from well 110/8a-5 in the East Irish Sea Basin and are considered to be synonymous with the 'damp sabkha' facies of Herries and Cowan (1997), discussed further below.…”

“…With a fall in the palaeowater-table relative to the sediment surface, aeolian processes would have been initiated and then progressively become dominant, first with the adhesion of wind-transported particles to a damp substrate giving rise to the characteristic irregular or wavy lamination of damp sandflats. Increased evaporation associated with such a change would also have increased the likelihood of evaporite cement precipitation in the near-sub-surface, potentially generating the irregular textures discussed by Herries and Cowan (1997) and the potential for the salt crust structures described by Goodall et al (2000) mentioned above. With further falls in the relative palaeowater-table, the drying transition would have continued with the development of thin wind-rippled laminae that represent the first stage of aeolian sandsheet formation.…”

Triassic palaeogeography and fluvial dispersal across the northwest European Basins

“…Aeolian facies have generally not been identified in the Central and Northern North Sea basins, although well rounded, millet seed grains in the Lomvi Formation (Nystuen et al 1989) suggest a 'cryptic' aeolian influence where some aeolian reworking of sediment occurred, but never in sufficient quantities to ensure stratigraphic preservation or protection from fluvial reworking. Damp sandsheet and sabkha facies occur in association with playa heterolithics, with fabrics indicative of widespread evaporite disruption (Herries and Cowan 1997;Goodall et al 2000), interbedded with wind ripple lamination and adhesion structures (Figure 10). 3.…”