Abstract

A marine sand wedge intercalated between coastal plain sediments of an Upper Miocene Lignite Suite is described from an open pit mine in the Lower Rhine Embayment, Germany. The sand wedge has a thickness of 15 m at the inferred seaward side of the outcrop and thins landward amidst lagoonal clays over a distance of more than 2 km. Vertical and lateral facies changes were recorded in detail during the course of several years as the mine face shifted several hundreds of metres.

The wedge consists of two sand units separated by a continuous, thin clay bed. The basal interval of the lower unit, interpreted as a flood-tidal delta in its initial high energy stage, is characterized by unidirectional, tabular planer, crossbedded sets with numerous discontinuity (pause) planes reworked by subordinate wave and weak current action. The upper interval, consisting of strongly-bioturbated, essentially crosslaminated sands, developed in the sheltered platform environment behind a newly-formed spit or swash bar, part of which also was exposed. The upper unit of the sand wedge is dominated by an enigmatic type of crossbedded sets. These sets sometimes are hundreds of metres long, are consistently bundled and grade at the up- and downcurrent directions into flasered and biogenetically homogenized sands. These lateral sequences are interpreted as having been formed by aeolian sedimentation in a backbarrier wind tidal flat, which migrated into a shallow lagoon. At the boundary between the dry to damp area and the body of water, a front gradually developed, giving rise to the gradually-steepening, bundled crossbedding. Rises and falls of lagoonal water level caused the depositional system to shift back and forth. Sedimentation processes were found to be wave-dominated, the intermittency of sedimentation probably being governed by wind set-up at the shoreline. No sedimentary-structural features were found that reflected the astronomical tides. Landward from the lagoon, the flood-tidal delta bioturbated sand units join to form a terminal lobe of giant (> 4 m thick) delta type crossbedding that interdigitates with lagoonal clay.