Sedimentary architecture of an ancient linear megadune (Barremian, Neuquén Basin): Insights into the long‐term development and evolution of aeolian linear bedforms

Abstract

Linear aeolian bedforms are the most abundant bedform type in Earth's dune fields, and are very common in the Solar System. Despite their abundance, the long‐term development of these bedforms and its impact upon the resulting sedimentary architecture in the geological record is still poorly understood. The aim of this paper is to study the exposed record of an ancient linear megadune in order to discuss its development and the factors that impact the sedimentary architecture of aeolian linear bedforms. The outcrops of the ancient Troncoso Sand Sea (Barremian, Neuquén Basin, Argentina) provide a unique opportunity to study a preserved megadune record with an external body geometry that confirms its linear morphology. Architectural analysis reveals significant differences in cross‐stratified set bodies and bounding surfaces’ features and allows for the identification of three architectural complexes within the bedform's record. Analysis of deterministic models, sedimentary body relative chronology and distribution suggest that these architectural complexes result from distinctive phases in bedform development. It also clearly shows that construction of the megadune was achieved by expansion from a core, and that its development was characterized by sustained growth and strong longitudinal dynamics, without net accumulation. This study indicates how sustained bedform growth, rather than accretion, can be a critical factor conditioning linear bedform architecture towards a more ‘classic’ (bimodal bounding surface and cross‐bedding dip directions) concentric sedimentary architecture style. Furthermore, this research reveals how this style of architecture could only be relatively common in the geological record when related to bedform topography preservation.

Structure and chronology of a star dune at Erg Chebbi, Morocco, reveals why star dunes are rarely recognised in the rock record

“…Because sand dunes are preserved from the bottom-up, the strata from the arms that extend in the downdrift direction, as well as the arms that wrap around the dune and converge beneath the main body of the dune, have a much higher preservation potential than the sediments of the star dune peak. The base of a star dune may be marked by an unconformity that is an erosional contact on earlier dune deposits, as observed here at Erg Chebbi and in the Gran Desierto, Mexico the preservation of extending and reversing dune arms that resemble linear dunes 35 , 36 , as well as converging dune arms covered by larger sets of cross-strata, could be mistaken for strata deposited by barchanoid dunes 37 . Thus, star dune deposits in the rock record could be mistaken for deposits of other types of dune.…”

Why are star dunes rarely recognised in the rock record?

“…Because sand dunes are preserved from the bottom-up the strata from the arms that extend in the downdrift direction, as well as the arms that wrap around the dune and converge beneath the main body of the dune have a much higher preservation potential than the dunes characteristic peak. The preservation of extending and reversing dune arms that resemble linear dunes 33,34 , as well as converging dune arms covered by larger sets of cross strata could be mistaken for barchanoid dunes 35 showing how star dune deposits in the rock record could be mistaken for other types of dune, and provide a model for the identi cation of ancient star dune. The model presented here incorporates structures found within the main body of the dune as well as the dune arms and is different from that of 16 which emphasised coarse grained low-angle strata around the base of star dunes.…”