Back-Barrier Sediment Dynamics: A Major Control on Modeling Sediment Properties, Cyclicity and Depositional Profiles: Examples from the Arid Coastline of Qatar

Jameson, Jeremy; Strohmenger, Christian J.

Building geological models based on modern analogue data relies on combining process-based observations with geospatial information. This paper describes the evolution of back-barrier sediments along the eastern coastline of Qatar to illustrate key processes that control deposition and diagenesis as they are captured in geological models.

Qatar barrier beaches have formed from coastal headlands with an oblique orientation to prevailing winds. Ridges have built 5-10 kilometers southeastward, by lateral accretion with minor washover. Landward of beach complexes are broad back-barrier areas with a range of surface sediment types ranging from evaporitic sabkha to open marine, subtidal lagoons.

Radiocarbon age dating indicates coastal sediments began forming approximately 8000 years before present, at the end of a period of rapid sea-level rise (1m/100 years). The rapid rise to 2-4 meters above present day formed a distinctive, 30-50 centimeter thick, basal transgressive lag composed of open marine, cardiid bivalves and mollusk hardground.

An initial, Lagoon Phase is marked by open marine circulation in back-barrier areas. Burrowed, thin-shelled bivalve sands generally fine upward to sea grass muds. Punctuated drops in sea level to present day have led to seaward stepping beach complexes.

A second, Inlet Phase is marked by restricted tidal flow. Generally concentric facies patterns result, proceeding from sabkha, stromatolite, mangrove, tidal flat and tidal channel facies. Confined exchange of tidal waters results in reworking of older beaches, distributing sand into sheets. Lateral migration of tidal channels produces a distinctive erosional unconformity mid back-barrier infilling cycle. Back-barrier erosion through sheet wash and confined channel flow redistributes linear sand bodies into sheets and creates a marker unconformity.

As barriers become continuous, tidal flow to back-barrier areas is closed off in a final, Evaporite Phase. Restricted flow and water depths lead to formation of a thin, muddy cap above the grainy intertidal and channel facies. Salt and gypsum flats form around the margins and deflation areas.

The model described here applies to wave-dominated, arid coastlines, lacking continental runoff. Although mud prone in final phases, tidal exchange and channeling create sand-prone facies throughout most of the back barrier depositional history.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013