Development of a palaeovalley complex on a Late Ordovician glaciated margin in NW Saudi Arabia

Abstract

Late Ordovician glacial deposits are of great importance in North Africa and the Middle East as a result of their significance as reservoirs for hydrocarbons and groundwater. The sedimentary record of this glaciation in NW Saudi Arabia (the Sarah Formation) is generally preserved in meridionally oriented palaeovalleys cut beneath northward-flowing ice sheets. In the Tabuk region of NW Saudi Arabia, an apparently intersecting complex of north–south- and east–west-oriented palaeovalleys occurs in the Alwizam area. Field relationships show two generations of palaeovalley incision, suggesting that the north–south-oriented palaeovalley was cut subglacially, filled, subsequently deformed and then cross-cut by the east–west-oriented palaeovalley. Abundant facetted and striated quartzite clasts occur at the base of each palaeovalley, testifying to a subglacial origin. Detailed examination of the north–south-oriented palaeovalley shows it to be well-defined with symmetrical sides. Its fill is composed of nine lithofacies grouped into four facies associations. About 80% of the fill consists of three sandstone facies: a parallel-bedded massive sandstone, a stacked scoured sandstone and a massive sandstone. Centimetre-scale extensional faults developed in soft sediments are commonly found throughout the stratigraphy, along with a glacially striated surface seen mid-way through the succession. These features provide evidence for direct ice contact, synglacial fill, and consequent reworking, cannibalization and deformation by the fluctuating ice margin.

Scratching the surface: Footprint of a late Carboniferous ice sheet

“…Often, this information is missing, e.g., where transgressive deposits blanket glacial deposits, with extensive reworking suspected. However, reevaluation of comparable subglacial unconformities in other basins where a similar range of structures is recognized (e.g., the Sarah Formation of Saudi Arabia; Tofaif et al, 2019) may reveal critical steps in the evolving subglacial environment during retreat that have been hitherto overlooked. Reappraisal of similar surfaces of different ages with the approach adopted herein may help crack long-standing enigmas, such as whether they developed subglacially or through the grounding of drifting icebergs (Dowdeswell et al, 2016…”

“…Where cut into soft sediments, glacial unconformities may record the degree of basal coupling or changes in ice-flow velocity (Le Vesely and Assine, 2014). So-called "soft-sediment striated surfaces" are extremely common in the glaciogenic Late Ordovician (Deynoux and Ghienne, 2004;Le Heron et al, 2005;Denis et al, 2010;Girard et al, 2015;Tofaif et al, 2019) and Carboniferous-Permian (Visser, 1987(Visser, , 1990Assine et al, 2018;Dietrich and Hofmann, 2019) records alike. Although widely used to inform regional ice-sheet flow models (Ghienne et al, 2007;Le Heron, 2018;Visser, 1997), subglacial features in soft sediment are prone to later deformation and fluidization (e.g., Le Heron et al, 2005).…”

A slippery slope for Cryogenian diamictites?

“…The observation that some 73% of the clasts studied in the Kingston Range quadrat analysis are rounded to well‐rounded is perhaps counter‐intuitive, but can be understood in terms of recycling of clasts both through englacial, subglacial and proglacial processes. In other glacial records, rounded but beautifully striated clasts are very commonplace because they record several generations of recycling (Tofaif, Le Heron, & Melvin, ).…”

An introduction to glaciated margins: the sedimentary and geophysical archive

“…A trio of papers by , Tofaif et al (2018) and Hirst & Khatatneh (2018) provide detailed insights into the Late Ordovician glacial shelf and margin record of Arabia. These papers provide much needed data on the Arabian plate because most (although not all) research in the last 15-20 years has focused on the extensively exposed (and until recently comparatively easy of access) Saharan outcrops.…”

“…The other two papers on the Late Ordovician record focus on meltwater-related processes in both proximal and distal parts of the Arabian glaciated margin. Tofaif et al (2018) provide the first detailed description and interpretation of cross-cutting palaeovalley features in the Tabuk area, NW Saudi Arabia. Remarkable palaeovalleys, tens of kilometres long and hundreds of metres wide, are filled almost entirely with sandstone and contain little that the Quaternary glacial geologist would recognize as glaciogenic sediments.…”