Marine incursion in the lower Cretaceous Sanfranciscana Basin evidenced by ichnological data

The Lower Cretaceous deposits of the Sanfranciscana Basin (Brazil) were predominantly shaped by continental processes. Despite this, thin chert layers bearing marine microfossils indicate brief marine incursions. Various hypotheses have been proposed to explain these fossils, ranging from storm-induced remobilization to episodic incursions controlled by faulting. The short-lived nature of these events has been suggested by the absence of sedimentological and ichnological evidence of sustained marine influence in the basin’s interior. To explore whether these incursions left additional evidence beyond microfossil assemblages, this study integrates ichnological and sedimentological analyses in the southern Sanfranciscana Basin. Fieldwork focused on the transitional interval between the Quiricó and Três Barras formations. Notably, three sections within the basal Três Barras Formation preserve a marine ichnoassemblage, consisting of whitish, fine-grained stratified sandstones, with horizontally concentric-filled ( Asterosoma ), plug-shaped ( Bergaueria ), and spreiten-bearing ( Teichichnus ) burrows. The low-diversity ichnoassemblage suggests impacted salinity conditions, corresponding to a polyhaline-brackish setting. The studied successions show dominance of low angle stratified sandstone interbedded with trough cross-bedded sandstones and wave ripples, suggesting high energetic settings, locally reworked by waves. However, restricted exposition area and thickness, as well as low diverse marine signatures allow to identify a marine connection to the sea, precluding definition of an architectural element or depositional scenario. This record suggests that the marine incursions were long enough to allow benthic colonization of the substrate, as evidenced by at least 12 bioturbated beds. This data argues against episodic, short-lived colonization driven by storm events, at least for the here studied sections. • •Sedimentological and ichnological evidence of marine influence in the Lower Cretaceous of the Sanfranciscana Basin. • Integration of ichnology and sedimentology reveals brackish to polyhaline conditions. • Incursions might be tectonically controlled, generating estuaries during the Gondwana breakup