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Late ordovician—early silurian glaciofluvial deposits preserved in palaeovalleys in South Jordan

Sedimentary Geology
Publication Date
DOI: 10.1016/0037-0738(94)90099-x


Abstract Periglacial, fluvial strata of Late Ordovician age, infilling incised palaeovalleys in the lower part of the shallow-marine, siliciclastic Khreim Group (Late Ordovician to Silurian) are described from outcrops in south Jordan. The palaeovalley fill predominantly comprises poorly sorted, medium- to coarse-grained, sub-arkose with subordinate beds of pebble- to cobble-size diamictite. Clasts include subangular rip-up sandstone and sparse shelly fossils derived from the local bedrock, together with exotic, rounded pebbles and cobbles derived from the Cambro-Ordovician sandstones and granitoid Precambrian rocks in the palaeohinterland, situated to the south. The palaeovalley fill is locally characterised by synsedimentary slump folds, micro-faulting and contorted bedding. These are attributed either to gravitational slumping at palaeovalley margins, or collapse due to melting of stagnant supporting ice. At some localities, the sandstones are typically trough cross-bedded, and exhibit upward-fining trends terminating in current ripples, suggesting episodic infill of the palaeovalleys. The few palaeocurrent measurements available from these sedimentary structures indicate a general palaeoflow to the north, coincident with the general orientation of the exposed palaeovalleys. The palaeovalleys may have been formed by glacial and/or fluvial processes; their predominantly sandstone fill is interpreted as fluvial and/or glaciofluvial in origin, and was probably deposited as proximal proglacial outwash at some distance from the ice-sheet which lay to the south, in present-day Saudi Arabia. The Batra Mudstone, which overlies coeval, non-channelised, glaciofluvial sandstones yields an Early Llandovery fauna, thus constraining the upper age of the palaeovalley infilling. By analogy with similar strata in Saudi Arabia, erosion of the palaeovalleys and infilling is likely to have taken place during Ashgill to Early Llandovery times. Erosion of the palaeovalleys and their subsequent infilling is associated with the Late Ordovician-Early Silurian glaciation of Gondwanaland, widely reported from central Saudi Arabia and, in the subsurface, from Jordan.

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