Physical processes and sedimentary record of a modern, transgressive, lacustrine barrier island

Abstract

High water levels, storms, and a sparse sand supply have led to the formation and episodic migration of this 30 km-wide, foredune-lacking, lacustrine barrier. Transgression is dominated by overwash and by channels cut during storms that transport sand to the lagoon. The barrier system is characterized by five environments: near-shore, foreshore, backshore/washover terrace, channel, and lagoon. The nearshore consists of irregular bars that overlie a Holocene peat and/or clay. The bars are characterized by low-angle stratification, and ripples, and landward-dipping tabular sets. The foreshore consists of a smooth, plane surface and internally by lakeward-dipping (3° to 12°) laminations that truncate one another and thin towards the berm. The backshore/ washover terrace consists of a gently (1° to 2°) landward-dipping surface that changes at the lagoon margin to a steeply (30°) landward-dipping surface. Stratification consists of subhorizontal, landward-dipping laminations and thin beds that are thinnest nearer the berm and gradually thicken toward the lagoon, and by avalanche foresets. Washover fans coalesce to form the terrace. The channels, which are commonly less than a few tens of meters wide and a meter deep, form during storms when wave energy is concentrated by refraction around nearshore bars. The channel facies consists of nearly horizontal beds and laminations of gravel and coarse sand. The lagoon consists of mud that overlies a Holocene peat; the lagoonal mud is eroded during transgression. The vertical sequence which is 1–2 m thick, consists from the base up of horizontal laminations of coarse sand and gravel of the channel facies, the landward-dipping foresets of the washover fan margin, and the gently landward-dipping laminations and landward-dipping avalanche foresets of the backshore/washover terrace. The preservation potential is considered low, although Pleistocene and Holocene coastal deposits have survived for up to 15,000 years in the Great Lakes region.