Late Ordovician climbing-dune cross-stratification: a signature of outburst floods in proglacial outwash environments?

Abstract

Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0AE3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/ subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddyrelated side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune crossstratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbingdune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

Glacial sedimentology interpretation from borehole image log: Example from the Late Ordovician deposits, Murzuq Basin (Libya)

“…The deposits can be formed of meter-thick sequences with a basal scour surface and a fining-upward trend with planar to trough cross-stratifications as the main sedimentary structures (Figure 10a and 10b). Characteristic of this glaciogenic succession, large climbing dune accumulations with coarse-grained reactivation surfaces can be identified (Figure 10c and 10d; Ghienne et al, 2010;Girard et al, 2012aGirard et al, , 2012b. Intervals with horizontal aggrading sand sheets mainly showing planar laminations with load casts and flame structures are also observed.…”

“…However, caution should be taken because step fractures form in other settings in association with compaction and gravity flows (e.g., the deltaic-fluvial transition in GS3; Figure 11). The proglacial sandstones can contain characteristic chute and pool facies related to very high energy flow conditions (Ghienne et al, 2010;Girard et al, 2012aGirard et al, , 2012b; Figure 7e and 7f). 3) Glaciomarine deposits (GS; Figure 8):…”

“…4) Deltaic deposits (GS; Figure 9): The facies association corresponds to stacked erosional fining-and thinning-upward sandstone sequences (containing successions of massive sandstones with outsized clasts, climbing megaripples, ripples, and planar lamination; Figure 9; Girard et al, 2012aGirard et al, , 2012b. This facies association is poorly developed in the Ghat area, but it is well exposed 100 km to the north (Ghienne et al, 2010;Girard et al, 2012aGirard et al, , 2012b. The deposits show alternations of turbiditic sequences and sandy mass flows.…”

“…However, in the outcrop, the equivalent rocks highlight a large panel of sedimentary facies and architectures, resolving the stratigraphy and the paleodepositional environments (Blanpied et al, 2000;Ghienne et al, 2003Ghienne et al, , 2007aGhienne et al, , 2010Le Heron et al, 2004;Moreau, 2005Moreau, , 2011Le Heron and Craig, 2008;Girard et al, 2012aGirard et al, , 2012b.…”

Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya?

“…A subaqueous interpretation is preferred by some workers (Hirst et al 2002;Hirst 2012), but a subaerial or deltaic interpretation is preferred by others (e.g. Ghienne et al 2010;Girard et al 2012). retreating ice sheets shed sediment toward the north during catastrophic collapse Girard et al 2012;Hirst 2012), and we interpret the climbing dune crossstratification facies association in the Jabal Eghei part of the uppermost Mamuniyat Formation as representing jökulhlaup outbursts following the main phase of the Hirnantian glaciation.…”