--> ABSTRACT: A High-Resolution Seismic Survey of the Holocene Section Offshore Part of the Dutch Mesotidal Barrier Shoreline, by E. C. Kosters, T. McGee, B. Posthumus, J. Brouwer; #91003 (1990).

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ABSTRACT: A High-Resolution Seismic Survey of the Holocene Section Offshore Part of the Dutch Mesotidal Barrier Shoreline

E. C. Kosters, T. McGee, B. Posthumus, J. Brouwer

Genetic stratigraphy of the Holocene section offshore the Dutch mesotidal barrier islands Terschelling and Ameland was studied using a 220-mi-grid of both analog and digitally acquired high-resolution seismic records, obtained with a 300 Joule boomer and 103-in. air gun.

The southern North Sea basin was subaerially exposed during the most recent (Wurm) Pleistocene lowstand and covered by glaciers during the previous (Saale) glacial. The Pleistocene surface in the area is approximately 30 m below the sediment surface and shows strong erosional and glaciotectonic characteristics. Holocene inundation began after 10 Ka. The investigated area has been fully marine since approximately 8 Ka. The Holocene energy regime has maximum tidal currents of 10-15 knots and a semidiurnal tide range of approximately 2 m.

Pleistocene erosional relief may have been used for early Holocene tidal drainage, as evidenced by lateral accretion reflectors filling this relief. Locally, these reflectors become sigmoidal in expression, where they may represent early Holocene ebb-tidal deltas. This seismic expression is similar to that of recent ebb-tidal deltas in the area. In one location Holocene sediments offlap a Pleistocene topographic high.

In the western part of the study area, the sea floor offshore the barrier island Terschelling is relatively flat. The Holocene section appears to consist of two transgressive stratigraphic units. The lower unit contains a number of coast-parallel sediment bodies, measuring roughly 3 × 7 × 5 m. It is uncertain at this time whether these represent either part of an early Holocene barrier island chain, or tidal ridges of that age.

A number of approximately 2-3-m high sand ridges lies in a northwest-southeast direction at an angle of approximately 25°, with the present barrier shoreline offshore the barrier island Ameland in the eastern part of the area. Digitally recorded and processed seismic records reveal large-scale foresets within these sand ridges. Their origin is probably related to the present energy regime, although they seem to consist of two stratigraphic units, of which the upper unit shows lower angle foresets than the lower unit.

Although a transgressive Holocene section is broadly preserved here, its nature and expression is complicated by preexisting ice-influenced Pleistocene relief and by present high-energy shelf dynamics, Holocene development of which is unknown. Subtle changes in current regime preferentially preserved early Holocene sand bodies in the west, whereas the eastern area probably contains only a preserved late Holocene section.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990