Zarqa megafacies: widespread subglacial deformation in the Sarah Formation of Saudi Arabia and implications for the sequence stratigraphy of the Hirnantian glaciation

Abstract

AbstractThe late Ordovician glaciation in the Kingdom of Saudi Arabia is spectacularly represented in the rocks of the Sarah Formation, although relatively little has been published about this formation. This paper describes and discusses the Zarqa megafacies of the Sarah Formation, which crops out across a distance of 720 km in the NW of the Kingdom as a heavily deformed diamicton, representing an example of widespread subglacial deformation. At outcrop, this Zarqa megafacies consists of two major subfacies. Subfacies (M) consists of a deformed, pebble-bearing substrate of interstratified, argillaceous, very fine- to fine-grained sandstones and siltstones. It displays a glacio-tectonically derived internal stratigraphy consisting of two units. Unit (i) is a pebbly diamictite displaying recumbent folding and thrusting indicative of progressive simple shear and manifest in a number of glacio-tectonically stacked detachment sheets. Unit (i) is abruptly overlain by Unit (ii). This consists of a lower Unit (iia), which displays soft sediment folding, including common sheath folds, as well as dewatering phenomena, passing up gradationally into almost completely homogenized sandy siltstone of Unit (iib) with rare detached masses of contorted sandstone. Subfacies (M) represents subglacial deforming bed conditions that reflect variable, but overall upwardly increasing, pore fluid overpressure in the pre-glacial substrate as it was overridden by the advancing Hirnantian ice sheet. It has been identified in subsurface core material, displaying asymmetrical strain profiles that also indicate subglacial deforming bed conditions. Subfacies (N) is a heavily deformed, boulder-bearing subglacial tillite commonly expressed in a distinctive mounded landscape of irregular piles of debris up to 8 m high. Extremely poorly sorted, it contains outsize boulders of glaciogenically deformed Lower Paleozoic sediments and striated Precambrian crystalline rocks. This subfacies represents a subglacially uprooted substrate that was transported considerable distances by the overriding ice sheet. Aspects of the Zarqa megafacies have parallels in coeval glaciogenic rocks identified from across northern Gondwana and beyond. This megafacies is therefore considered to have considerable sequence stratigraphic significance, representing deformed lowstand substrate and subglacial deposits associated with the accumulation and advance(s) of the late Ordovician ice sheets.