Generation, transport and preservation of armoured mudballs in an ephemeral gully system

Associate Editor -Nick LancasterABSTRACT Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid-Cretaceous outer back-erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain-size distributions, mud playa, ephemeral floodplain, pebble-sand and cobble-sand bedload stream, pebble-cobble-sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub-environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by winddriven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back-erg depositional system comprises an outer backerg close to the Variscan highlands, and an inner back-erg close to the centralerg area. The inner back-erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back-erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mudcracked surfaces, resulting in largely structureless sandstones with bouldersize mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser-grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back-erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back-erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization

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“…16F); locally, they contain armoured mud balls (cf. Mather et al. , 2008) and are covered by levels with mottling (arrows in Fig.…”

Section: Water‐laid Facies Associationsmentioning

confidence: 99%

The action of wind and water in a mid-Cretaceous subtropical erg-margin system close to the Variscan Iberian Massif, Spain

Rodríguez‐López¹,

Meléndez²,

Boer

et al. 2010

Sedimentology

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“…Mud rip-up clasts are common within the shell bed, including some with 'armoured mud ball' morphology (Fig. 4d;Bell, 1940;Kugler and Saunders, 1959;Dickas and Lunking, 1968;Stanley, 1969;Hall and Fritz, 1984;Mather et al, 2008). These rounded, mud-cored, shell-armoured rip-up clasts were presumably derived from erosion of exposed mudflat/marsh deposits along the shoreline of this area (Fig.…”

Section: Resultsmentioning

confidence: 96%

Shell bed tempestites in the Chenier Plain of Louisiana: late Holocene example and modern analogue

Williams

2011

J Quaternary Science

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Hurricane Ike's storm surge deposited a concentrated shell bed in the form of a series of coalescing washover fans over backshore sand and adjoining marshland in part of southwest Louisiana's Chenier Plain. The shell bed is a tempestite and has distinctive morphological, sedimentological and biogenic characteristics that provide a modern analogue to aid interpretation of older shell bed tempestites in the geological record. The shell bed has a wedge-shaped profile that thickens landward, is about 40 m wide, up to 27 cm thick and extends several hundred metres parallel to the shore. Shells are predominantly disarticulated valves of the common bivalve Mulinia lateralis, probably reworked and transported landward from skeletal remains offshore. The shell bed has an erosional base, is bioclast supported, normally graded and has common mud rip-up clasts. Similarities between the modern shell bed and another concentrated shell deposit, forming part of a sandy beach ridge some 1.5 km inland, suggest that the palaeo shell deposit is also a tempestite recording hurricane washover of a former shoreline 600 a ago. These findings demonstrate that the shell bed deposited by Hurricane Ike is a valuable analogue for palaeotempestological investigations and that hurricanes have likely contributed to the construction of both modern berm ridges and palaeo beach ridges on this coastal plain.

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“…Although the presence of mud clasts is sometimes viewed as a diagnostic of dryland settings (Tooth, ), actually they are found widely in other settings (e.g. Mather et al ., , table ).…”

Section: Discussionmentioning

confidence: 99%

Alluvial stratification styles in a large, flash‐flood influenced dryland river: The Luni River, Thar Desert, north‐west India

Carling

Leclair²

2018

Sedimentology

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Detailed descriptions of the architecture of dryland‐river deposits are few, which hinders the understanding of stratigraphic development in aggradational settings. The aim of this study was to obtain new insight into the depositional dynamics and resultant stratigraphy of steep, high‐energy dryland rivers in particular. The study site is within the Luni River, the largest river in the Thar Desert, India. A 700 m wide section, 5 m deep, was logged and the dated‐stratification related to the flow regimen. Seasonal discharges may reach 14 000 m3 sec−1 but the bed is usually dry. The deposits consist mainly of sandy upper‐stage plane bed lamination and low‐angle stratification deposited in supercritical flows with some strata showing gravel‐lense antidune signatures. Estimated hydraulic parameters indicate a dominance of the upper‐stage plane bed state. Less frequent dune trough cross‐sets occur at all depths, yet commonly as isolated sets with erosive upper boundaries. Localized deep scour causes slumping and soft‐sediment deformation. Scour holes adjacent to older bedded deposits are rapidly infilled, often by massive deposits. Repeated deep scour results in units of deposition of different ages (50 to 500 a bp) and different facies found at similar shallow depths (2 to 3 m). Older dates (1 to 2 ka bp) occur in lower strata (4 to 5 m depths) at which depths younger units also are preserved locally. The steep, high‐energy depositional environment promotes the unusual preservation and predominance of upper‐stage facies, in contrast to a relative paucity of lower‐stage facies. Frequent local deep‐scour during floods means that specific facies rarely can be traced across the river channel for more than a few metres, but are replaced by other facies in close proximity. These results should help interpretation of high‐energy sediment deposits widely and improve understanding of the stratigraphic development in modern dryland rivers, and in characterizing oil, gas and groundwater reservoirs in the dryland geological record more generally.

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Generation, transport and preservation of armoured mudballs in an ephemeral gully system

Cited by 16 publications

References 41 publications

The action of wind and water in a mid-Cretaceous subtropical erg-margin system close to the Variscan Iberian Massif, Spain

The action of wind and water in a mid-Cretaceous subtropical erg-margin system close to the Variscan Iberian Massif, Spain

Shell bed tempestites in the Chenier Plain of Louisiana: late Holocene example and modern analogue

Alluvial stratification styles in a large, flash‐flood influenced dryland river: The Luni River, Thar Desert, north‐west India

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