Controls on trace-fossil assemblages in a Cretaceous storm-flood-dominated delta (Kachaike Formation, Austral-Magallanes Basin, Argentina)

The paleoecological response of benthic communities to storm-flood events in deltas remains poorly constrained. This study presents a high-resolution sedimentological and ichnological analysis of the Lower Cretaceous Kachaike Formation in the Austral-Magallanes Basin, Argentina to evaluate colonization patterns and environmental controls on trace-making communities in a storm-flood-dominated delta. Facies analysis reveals five associations, including prodelta, storm-flood frontal splays, storm-flood channels, mouth bars, and storm-wave lower shoreface-distal delta front. Facies distribution and bounding-surface hierarchy reveal two deltaic units that differ in the relative dominance of depositional processes and in the expression of high-frequency deltaic progradational cycles, which are interpreted to reflect autogenic dynamics associated with delta-lobe evolution. Event beds exhibit generally low bioturbation intensity, and widespread unburrowed intervals suggesting rapid sedimentation and limited colonization windows. Low ichnodiversity and the occurrence of syneresis cracks, indicate episodic freshets. Elevated water turbidity during events and fair-weather times results in the dominance of deposit-feeding structures and absence of suspension-feeding strategies. Phytodetrital input likely contributed to reduced bottom-water oxygenation during inter-event periods, resulting in unbioturbated mudstone or Chondrites- and Trichichnus-dominated suites. Rapid mud accumulation via collapse of buoyant plumes or tractive remobilization formed soupgrounds, largely devoid of bioturbation, or bearing scattered biodeformational structures. Conversely, recurrent erosion promoted the exhumation of firmgrounds. These results show that, under storm-flood dominance, trace-making communities were shaped by recurrent environmental disturbances and multiple physicochemical stressors, producing distinctive trace-fossil suites of ichnological features that provide criteria for recognizing storm-flood-dominated deltas in the stratigraphic record. The ichnofaunal composition of the Kachaike Formation reflects evolutionary innovations resulting from the Mesozoic marine revolution.