Exceptional preservation of a barred shoreline under forced-regressive conditions (Lower Cretaceous, Neuquén Basin, Argentina)

Abstract

Forced-regressive conditions have been widely described by sequence-stratigraphic conceptual models, and the resulting deposits have been widely documented in sequences and parasequences. However, how forced-regressive conditions might impact on the short-term to mid-term evolution and the high-resolution stratigraphic record (102 to 105 kyr) of barred shoreline environments represents an unexplored issue. This contribution presents a bedset characterization of Lower Cretaceous, barred shoreline deposits accumulated after a relative sea-level fall. Parasequence PS400 of the Pilmatué Member in the Neuquén Basin (Argentina) comprises several bedsets, most of them bounded by transgressive surfaces. However, one of them, the bedset BS400.5, is bounded below by a high-relief surface of truncation, here interpreted as an erosional regressive surface. This surface indicates incision of the underlying deposits and the onset of forced-regressive conditions at intra–parasequence scale. The internal architecture of the studied bedset exhibits seaward-dipping clinoforms that bound clinothems within the upper-shoreface deposits. These clinoforms were interpreted as the result of seaward-shifting events during the shoreline progradation. A sequence-stratigraphic conceptual model is proposed for the high-frequency evolution of a barred shoreline under forced-regressive conditions. The model explains how the intra–bedset evolution under low and stillstand sea-level conditions and cross-shore imbalances in the sediment budget controlled the exceptional preservation of bar and rip-channel deposits.