Mudrock deposition in an ancient dryland system: Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales, UK

Abstract

Studies of the lowermost Old Red Sandstone of South Wales (Pridolian-Lochkovian) have revealed that the mudstones that constitute up to 80% of some of the formations have been deposited in a variety of environments. In addition there is a variety of lithofacies associations present in some mud-dominated units that suggests diverse mechanisms of deposition. This paper considers the Moor Cliffs Formation and examines four distinct mudrock facies.The overall depositional environment is considered to have been a multi-stage, multi-channel system, similar to a modern dryland river system, with mud-dominated, moderately sinuous ephemeral rivers that reworked muddy floodplain sediment over a proximal braidplain during seasonal flooding. The alluvial deposits were pedified to varying degrees as calcic Vertisols, which, due to seasonal wetting and drying, provided soil aggregates for reworking by wind and water as sand-or silt-sized pellets. Very infrequent flooding events (superfloods) resulted in extensive stripping of the floodplain surface and the introduction of allochthonous sand sheet deposits over wide areas in this normally sediment-starved system.Depressions (pans or waterholes) on the floodplain formed ephemeral lakes following floods, and acted as sediment traps for dust from the frequent dust storms occurring in the system.

Siluro-Devonian landscapes of southern Britain: the stability and nature of early vascular plant habitats

“…The siltstone units therefore might represent deposition and reworking of material within abandoned or isolated portions of the fluvial system, such as cut-off sections or waterholes. Similar settings have been assigned to siltstone units observed in the Moor Cliffs and Conigar Sandstone Formations of Pembrokeshire (Marriott & Wright 2004;). Waterholes are a common geomorphological component in the Channel Country, Australia and were created in localized areas of channel expansion and scouring (deepening) during flooding that subsequently retained water in drier periods (Knighton & Nanson 2000).…”

“…Sharp changes in grain size indicate rapid fluctuations in flow strength. Similar inclined and planar-bedded heterolithic units have been discussed by Marriott & Wright (2004) from the Moor Cliffs Formation (the lateral equivalent of the Raglan Mudstone Formation in Pembrokeshire), and by for intervals within the Conigar Pit Formation (the lateral equivalent of the St. Maughans Formation).…”

The Ridgeway Conglomerate Formation of SW Wales, and its implications. The end of the Lower Old Red Sandstone?

“…They are dm-m scale thick, with the only observed structures being weakly developed drab haloes. Similar mudrocks have been described in the Lower ORS Moor Cliffs Formation (Marriott and Wright 2004), the RIMM (Marriott et al 2005) and the CPSM (Hillier et al 2007). These have been interpreted as the product of bedload transport of sand-sized pedogenic aggregates, their massive nature resulting from stratification loss due to early compaction, or possibly from mud-grade loess deposition.…”

Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales

“…Marriott and Wright 2004), the Cosheston Group shows evidence of climate change in the form of the MBF. Its greater proportions of bedload deposits and primary grey-green colours, high sedimentation rates, relatively abundant meso-and macro-flora (Table 1), and a lack of calcrete (Figure 25) point to a pluvial interlude.…”

“…Allen 1974bAllen , 1986aWright 1993, 2004;Wright and Marriott 1996;Love and Williams 2000). Many are calcic Vertisols, indicating 'marked seasonal differences in soil moisture', characteristic of 'semi-arid climates with distinct wet and dry seasons', or 'periods of flood-related waterlogging followed by prolonged drought' (Marriott and Wright 2004).…”