Abstract: Structure of Coastal Washover Fans and Their Role in Internal Structure of Holocene Transgressive Sand Barriers
John C. Kraft, Evelyn M. Maurmeyer, Chacko John, Elizabeth A. Allen
Coastal washover fans of the large estuary, Delaware Bay, are generally tens of meters in lateral extent and less than 1 m thick. They consist of planar-bedded sands and gravels with uniform landward-dipping cross-lamination at high angles. They form as minor high-tide storm features in pulses during the upper flow regime or sheet flow. Smaller estuarine washovers contrast with much larger (hundreds of meters long and up to 2 m thick) storm washover features of the linear sand barrier of the Atlantic coast, which form only during major northeasters and hurricanes. Extremely variable internal structures such as ripples, megaripples, planar lamination, and antidunes(?) suggest a transition from lower to upper flow regime in continuous stream flow. During the extreme storm e ent, flow over the barrier into the lagoon occurs and stream flow and resorting lead to parallel lamination. Examination of barrier vertical sequence by core studies shows that, as the transgression proceeds, these sublagoonal sands are overlain by the barrier washover sands and ultimately by coast-parallel dunes up to 12 m thick.
The washover barriers vary in width from tens of meters in the estuaries up to 2.4 km along the ocean (including the sublagoonal part). Sediment supply and pre-transgression topography are important control elements. Migrating inlets (with sediment bodies up to 4 km wide and 11 m thick) and regressive spits form exceptions. Major downward flexures identified in the Tertiary subsurface nearby on the northwest flank of the Baltimore Canyon geosyncline, plus rapid early Holocene eustatic sea level change, provide potential mechanisms for preservation of the stratigraphic record of transgressive barrier sands.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC