Stasis coupled to transgressive surfaces in Silurian sand-rich tide-dominated deltaic successions (Balcarce Formation, Argentina)

Abstract

Stratigraphic and ichnological evidence of synoptic surfaces developed during stasis periods from deltaic successions are poorly reported. Synoptic surfaces representative of prolonged stasis are typically highly bioturbated. A combined sedimentological and ichnological analysis undertaken along a down-dip-oriented transect of the Balcarce Formation allows the identification of true substrates within a Silurian tide-dominated deltaic succession. The lower interval comprises regressive packages of prodelta fines grading to sand-rich compound-dune or tidal-bar-dominated delta-front facies. True substrates developed during prolonged stasis are recorded along two types of transgressive surfaces (S1 and S2). S1 separates thicker packages (5–15 m thick) and is correlated seaward over longer distances (up to 20 km) than S2, showing that S1 is of a higher order of magnitude than S2. Synoptic surfaces with suites illustrating the Phycosiphon Ichnofacies are best preserved seaward, at the top of regressive cycles bounded by S1. Velocity fluctuations of tidal currents and the deactivation of migrating tidal bedforms during transgressions might have favoured stasis followed by hemipelagic sedimentation without erosion, and the development of true substrates. Although prolonged stasis mostly corresponds to transgressions (i.e. allogenic), some bioturbated layers may reflect short colonization windows opened at brief pauses in sedimentation during delta progradation (i.e. autogenic).