Vertical changes in shoreline morphology at intra-parasequence scale

Abstract

It is commonly assumed in the high-resolution sequence stratigraphic analysis ofshallow-marine deposits (e.g., deltaic and shoreface settings) that the depositionalconditions of the system remain relatively constant during the transit of a shorelinethat would eventually produce a single parasequence. However, based on thedetailed sedimentary and architectural analysis of upper-shoreface and foreshorestrata of two Early Cretaceous shoreface-shelf parasequences (Neuquén Basin,Argentina), it was possible to document a vertical change through the stratigraphyfrom deposits representing wave-dominated barred shorelines to depositsinterpreted as representing a non-barred morphology. The presence of a welldefinedlimit between trough cross-bedded sandstones in the upper shoreface andplanar laminated sandstones in the foreshore (and the presence of a surf diastem)characterize the development of barred shoreline conditions. Instead, planarlamination is ubiquitous within non-barred deposits, where trough cross-beddingis restricted to the bottomsets of the large-scale inclined beds that characterize thisarchitectural style. Thickness, sediment composition and reconstructed shorelinetrajectory also seemingly change vertically within the investigated parasequences.Collectively, these pieces of evidence suggest that the vertical transition frombarred to non-barred deposits at this intra-parasequence scale could be relatedto wave-climate variations and the sequence-stratigraphic context. Specifically,changes in the prevailing wave behavior from dissipative to reflective conditionscould be a feasible explanation for the morphological transformation of coastalsystems through tens of thousands to hundreds of thousands years.