ABSTRACT

Oblique dunes have orientations that are intermediate between those of transverse and longitudinal dunes. The oblique dunes studied are reversing dunes which undergo no net annual migration when associated with normal meteorological patterns. From April through September, the dunes migrate northwestward under the influence of prevailing onshore winds. High velocity northerly winds associated with the passage of winter frontal systems (November through February), cause the dunes to rapidly migrate southward, despite accompanying rainfall. In October and March, frequent changes in wind direction cause many of the dunes to become flattened.

Hurricane Allen struck South Padre Island in August, 1980. Examination of trenches, profiles, and aerial photographs suggested that this catastrophic event caused only minor modification of the dune morphology and stratigraphic make-up.

Three major stratification types were observed in trenches and on etched surfaces: (1) translatent strata were deposited by wind ripples; (2) grainfall deposits accumulated when saltating grains settled on leeward slopes of the dunes; and (3) grainflow cross-strata were developed by avalanching on leeward slopes. Preservation of these stratification types occurred in zones of net deposition, predominantly leeward of the dune crests.

Strata deposited during the summer wind regime dip northwest, whereas the winter strata dip in a southerly direction. The winter deposits are best preserved in the central cores of the dunes. This indicates that either the high velocity winds of the initial winter frontal systems destroy large volumes of the summer deposits, or that there is a net migration southward during the dry northerly winds of droughts.

Oblique dune deposits should be difficult to discern in the rock record, because they may contain aspects of both transverse and longitudinal dunes. It is likely that some of the eolian stratification packages observed here are also present in ancient dunes.