ABSTRACT

The Harthope Ganister is a fine-grained quartz arenite sand-body approximately 4.5 km long, 2.5 km wide, and up to 9 m thick which occurs within the Namurian (Arnsbergian) sequence of northern England. Deposition of this sand-body took place in a transgressive barrier island to shallow-marine sand-ridge setting. Barrier initiation occurred by reworking of deltaic sands during abandonment and transgression of a shallow-water, fluvially dominated delta lobe. Once formed, the barrier island migrated northwards towards an approximately east-west oriented shoreline by washover deposition of material eroded from the seaward (southern) flank. Washover sedimentation resulted in the formation of sharply or erosively based, parallel laminated and hummocky cross-stratified sands with low-angle n rtherly depositional dips. During fairweather periods, colonization and bioturbation of these backbarrier deposits occurred, and ripples and megaripples developed under the influence of wind-driven currents. Landward migration of the barrier over muddy lagoonal sediments produced a coarsening- and thickening-upward sequence up to 12 m thick.

Continued transgression gradually outpaced barrier migration. This resulted in submergence of the barrier, termination of washover sedimentation and development of a shallow-marine sand-ridge. Avalanche-faced bedforms initiated along the northwest flank of the barrier possibly as a recurred spit, migrated out across the sand-ridge flank into the surrounding deeper water. Migration of these bedforms produced sets of large-scale cross-bedding up to 6 m high. Increasing water depth over the sand-ridge eventually led to termination of bedform migration and abandonment of the sand-body as it subsided below wave-base. Colonization and bioturbation of the sand-ridge surface and reworking and rounding-off of the sand-ridge profile accompanied abandonment. Subsequent progradation of a shallow- ater, fluvially dominated delta across the region resulted in complete preservation of the sand-body by encasement in pro-delta/interdistributary mud.