THE MAKING OF A SANDSTONE COLOSSUS: TECTONICALLY & CLIMACTICALLY INDUCED FLUSHING OF 'NUBIAN' SANDS IN THE EARLY PALEOZOIC

Massive ‘Nubian’ sandstones drape most of the Arabian and northern African tectonic plates, preserving a sensitive record of how continental margins evolve under greenhouse conditions. These strata contain important aquifers, petroleum reservoirs, and archaeological monuments such as Petra. They were formed by a geologically sudden and long-lasting influx of >500,000 km³of quartz sand that occurred sometime during Cambrian and/or Ordovician time. The cause and timing of this continent-scale sedimentation event are unknown.

By constraining the depositional history of an archetypal example of these sandstones from Jordan, we have identified a potential explanation for this massive sedimentation event. We hypothesize that poleward migration of the Gondwanan supercontinent out of the horse latitudes triggered a massive increase in sedimentation, and fostered deposition of thick sequences of sandstone stretching from Turkey to Ethiopia to Morocco.

Jordanian strata are well exposed and provide a paleoslope-axial transect through the Gondwanan continental margin during its early Paleozoic poleward movement. Sedimentologic, biostratigraphic, basement paleotopographic, facies, and tectonic dip analyses of these strata were conducted in order to model the geohistorical sedimentation and subsidence history of the region.

The region experienced a five-fold (~25 m/Ma) increase in sedimentation rate over ~30 Ma, concomitant with near-equilibrium plate subsidence response. Sedimentary rocks in the studied sequences also exhibit coeval compositional changes that suggest a switch in sedimentation style from immature to ultramature clastics. At this time, atmospheric pCO₂was ~12 times higher than today – together with the lack of terrestrial vegetation, this facilitated intense weathering and widespread production of mature clastics in continental interiors.

The observed upsurge in sedimentation rate coincides with movement of Gondwana from an arid subtropical high toward a wet subpolar low. This movement would have caused widespread flushing of hypermature sands sourced from the interior of the African-Arabian Shield toward the continent margin. The net result would be deposition of one of the largest epicratonic-pericratonic sand wedges in earth history.