New insights on the sedimentology and stratigraphy of the glaciogenic Late Ordovician Sanamah Member, Wajid Sandstone Formation, southwest Saudi Arabia

Abstract

Recent field investigations in the Wadi Ad-Dawassir area, southwest Saudi Arabia, have provided new insights on the stratigraphy and sedimentology of the Late Ordovician Sanamah Member of the Wajid Sandstone Formation. Three main stratigraphic units were recognized and are characterized by considerable variation in thickness and lateral facies changes. These variations reflect the complex depositional and erosional processes associated with the evolution of the Late Ordovician icesheet margins through time. The lowermost unit (Unit 1) consists of a deformed heterogeneous assemblage and basal till overlain by a thick package of outwash deposits that recorded high sedimentation rates associated with high water discharges (climbing dunes, large-scale softsediment deformation). In outcrop, the top of Unit 1 is marked by a regionally extensive iron-cemented horizon, which is overlain by Unit 2. This second unit mostly consists of rain-out till made of red, poorly bedded, pebbly sandstone containing boulder-size drop-stones. This passes upwards to a coarser assemblage with thick beds of melt-out till, outwash cross-stratified and massive sands, and gravity-flow deposits, often showing large-scale soft-sediment deformation. The top of Unit 2 is also marked by an iron-cemented horizon overlain by Unit 3. The latter displays comparable sedimentological and compositional characteristics to Unit 2. In places, the bases of both Units 2 and 3 are characterized by thin, laterally discontinuous erosive, transgressive beach sand, which marks the onset of each sedimentation cycle. The top of Unit 3 is also characterized by an undulated erosional surface associated with coarse sands and conglomerates, which could represent either a base Silurian or pre-Khusayyayn unconformity.