Migration, modification and merging in aeolian systems and the significance of the depositional mechanisms in Permian and Triassic dune sands of Europe and North America

Abstract

The main depositional mechanisms acting in aeolian systems can be assessed in two different and independent ways: first by the internal anatomical approach with analysis of the cross-stratification fabric, and second by the external morphological approach with study of shape and surface features of dunes. Anatomical investigations of various ancient aeolian systems in the Permian and Triassic of Europe and North America and an extensive literature review concerning both internal anatomy and external morphology of ancient and modern dunes reveal three main depositional mechanisms operating in aeolian systems: migration, modification and merging. Migration comprises downwind movement of isolated dunes or trains of aeolian bedforms under predominantly constant wind direction. The orientation of the interset surfaces partially depends on the angular relationship between the path of bedform climb and the depositional surface and also on subsidence. Simple systems are characterized by successive dunes of similar size, whereas compound systems originate by superimposition of smaller aeolian bedforms on larger dunes or draas. Compound migration gives rise to the hierarchy of diastems. Modification comprises overprinting of existing dunes by winds coming from directions other than those that govern migration of the original aeolian bedforms. The anatomical elements representing modification include both intrasets and extensive intersets, depending on duration and on the scale of the changes of the winds. Simple modification comprises occasional origin of either only cross-dipping erosional boundaries or both diastems and intraset laminae oriented in different directions. A special case of modification is the creation of intrasets dipping in the same direction as the foreset inclination, comprising either reactivation or relaxation surfaces, depending on whether the foreset slope is steepened or lowered in angle. Complex modification takes place under repeatedly alternating winds of changing effectiveness. Small-scale modification comprises minor alterations of existing dune types, and medium-scale modification is characterized by conversion of primary aeolian bedforms into secondary dune types. Small-scale and medium-scale modification take place during short-term to smaller-scale long-term changes of effective wind direction. Large-scale modification originates by large-scale long-term changes, accompanied by a certain subsidence, which result in burial of the older dunes by a younger train of aeolian bedforms within the same sand sea migrating obliquely to perpendicularly to the former wind direction. Very large-scale modification is induced by very large-scale long-term changes which result int deflationary erosion of older dune fields and subsequent superimposition of a new sand sea on top of the eroded remnants of the former erg. Merging comprises coalescing of bedforms with only negligible erosion during burial of the first bedform by subsequent dunes. Merging is induced mainly when migration of dunes is stopped by a morphological barrier or ceases as a consequence of weakening winds. Merging further occurs during coalescence of numerous isolated barchans in rows to parallel wavy barchanoid ridges as a consequence of increasing availability of sand. The main depositional mechanism in compound systems is migration of hierarchically superimposed bedforms with subordinate modification under predominantly unidirectional winds. In complex systems, migration, modification and merging take place in different amounts under unidirectional, bidirectional and multidirectional winds. Depending on direction and strength of both earlier and subsequent winds, complex systems are either charcterized by mainly migration with subordinate modification or by considerable modification with either conversion or dissection of older dunes. The effects of migration, modification and merging are discussed in the light of both internal anatomy of ancient cross-stratification fabrics and external morphology of recent dunes. A classification of dune types based on these mechanisms divides the aeolian bedforms into primary, secondary and tertiary types. Primary dunes originate by simple or compound migration with minor modification and comprise mainly transverse dunes. Secondary dunes originate by complex interaction of migration and modification or even predominantly by modification, with the best examples being longitudinal dunes. Tertiary dunes form by merging of transverse and longitudinal elements with varying amounts of migration and modification and are mainly represented by star dunes. The results of various anatomical case studies of ancient aeolianites indicate that simple migration was the dominant mechanism for construction of ergs in the northern hemisphere in Permian and Triassic times.